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Abstracts of the 

Eighth International Conference on 

Geochronology, Cosmochronology, and 

Isotope Geology 

Edited by M.A. LANPHERE, G.B. DALRYMPLE, and 

B.D. TURRIN 

U.S. GEOLOGICAL SURVEY CIRCULAR 1107 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director 

For sale by 

U.S. Geological Survey, Map Distribution 

Box 25286, MS 306, Federal Center 

Denver, CO 80225 

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does 

not imply endorsement by the U.S. Government. 
PREFACE 

It all began in Canada in 1967 with The International Conference on the Geochronology 

of Precambrian Stratified Rocks. There followed, in 1969, The Colloquium on the 

Geochronology of Phanerozoic Orogenic Belts in Switzerland, at which time the name The _nth 

Conference on Geochronology, Cosmochronology and Isotope Geology (/COG) was adopted for 

future conferences. This now-quadrennial conference has become the most prestigious forum in 

the world devoted exclusively to the application of isotope research to solving problems in the 

physical and natural sciences. 

The /COG conferences arose because of a genuine need within the scientific community. 

The conferences have no formal organization behind them; there are no bylaws, no rules and 

regulations, no officers, and no permanent funding. They have been perpetuated by a vote of the 

/COG conference attendees, who choose the venue and the Chair for the succeeding meeting. It 

then becomes the responsibility of the new Chair to set about organizing the next meeting from 

scratch, although the outgoing previous chairs provide valuable records and advice. 

Since the first conference 27 years ago, the following I COGs have been held: 

1. Edmonton, Alberta, Canada, June 12-14, 1967 

2. Bern/Zurich, Switzerland, August 23-September 4, 1969 

3. Paris, France, August 26-31, 1974 

4. Snowmass, Colorado, USA, August 20-25, 1978 

5. Nikko, Japan, June 27-July 2, 1982 

6. Cambridge, UK, June 30-July 4, 1986 

7. Canberra, Australia, September 24-29, 1990 

And now, June 5-11, 1994, at the University of California, Berkeley, /COG returns to a 

North American venue for the first time in sixteen years. At this writing JCOG-8 by the Golden 

Gate, with 743 abstracts submitted, promises to be the largest of the /COG conferences to date. 

JCOG-8, of course, would not have been possible without the assistance and support of a 

great many people and other organizations and I would like to thank the Convening Institutions, the 

Co-Sponsors, and the Underwriters, listed on the following page, for their generous support. I 

would especially like to thank Pacific Gas and Electric Company for their substantial contribution 

to our Education Project and the Bank of America for their generous support of the Host 

Committee's program to bring scientists from developing countries. I am also deeply grateful to 

the Ann and Gordon Getty Foundation for providing matching funds for all of the moneys raised 

for ICOG-8. 

Finally, it was the wish of the Organizing Committee to provide the delegates with a 

referenceable abstracts volume and on their behalf I wish to express gratitude to the U.S. 

Geological Survey for publishing this volume as a USGS Circular, and for providing each delegate 

with a copy. 

Garniss H. Curtis 

Chair, JCOG-8 

June, 1994 

iii 
ICOG-8 Convening Institutions 

Institute of Human Origins 

University of California 

U.S. Geological Survey 

Lawrence Berkeley Laboratory 

Lawrence Livennore Laboratory 

Leland Stanford, Jr., University 

ICOG-8 Co-Sponsors 

American Geophysical Union 

Association of Women Geoscientists 

California Division of Mines and Geology 

Canadian Geophysical Union 

European Geophysical Society 

Geochemical Society 

Geological Society of America 

International Association of Geochemistry and Cosmochemistry 

International Union of Geological Sciences 

Subcommission on Geochronology 

Meteoritical Society 

Nevada Bureau of Mines and Geology 

ICOG-8 Underwriters 

Bank of America 

BHP Minerals International 

Charles Evans and Associates 

Chevron Corporation 

Finnegan Mat 

Ann and Gordon Getty Foundation 

Homestake Mining Company 

Interocean Oil and Gas Company 

John E. Kilkenny 

Mass Analyser Products, Ltd. 

Niles and Associates, Inc., Publishers of EndNote 

Pacific Gas and Electric Company 

Western Mining Corporation (USA) 

VMI, Inc. 

iv 
ICOG-8 Organizing Committee 

Gamiss H. Curtis, Chair 

Walter Alvarez 

Joseph Arth 

Tim Becker 

Scott G. Borg 

Marc Caffee 

Mark Conrad 

Robert E. Criss 

Brent Dalrymple 

AI Deino 

Donald J. DePaolo 

Robert Fleck 

Phillip B. Gans 

Brian Hausback 

Lynn Ingram 

B. Mack Kennedy 

Marvin Lanphere 

Michael McWilliams 

Michael Moore 

Syd Niemeyer 

Paul Renne 

John H. Reynolds 

Sharie Shute 

Doris Sloan 

Brian M. Smith 

Carl Swisher 

Brent Turrin 

Robert C. Walter 

Joseph Wooden 

v 
CONTENTS 

Preface 111 

Abstracts 1 

Author Index 3 7 4 

EXPLANATION 

Abstracts are organized alphabetically by first author 

and were printed as received from the author-prepared 

copy. The Author index is comprehensive and includes all 

authors. 

vi 
Abstracts of the Eighth International 

Conference on 

Geochronolgy, Cosmochronology, 

and Isotope Geology 

Edited by M.A. Lanphere, G.B. Dalyrmple, and B.D. Turrin 

BORON ISOTOPIC COMPOSITION OF ICELANDIC 

GEOTHERMAL SYSTEMS: MAGMA TIC DEGASSING 

OR WATER-ROCK INTERACTION ? 

AGGARWAL 1 K, PALMER, M.R., 

RAGNARSDOTTIR K.V, Dept of Geology, University of 

Bristol, Bristol, UK. BS8 IRJ. 

A variety of geothermal systems have been sampled a<..1·oss 

Iceland ranging from cold carbonate springs on Snaefellsnes, 

low temperature geothermal systems of the Southern 

Lowlands, the high temperature systems of Kratla, Bjamatlag, 

Nesjavellir, and the saline systems of Svartsengi and 

Reykjanes. Boron isotope ratios of the fluids have been 

determined using a new and rapid technique based on that of 

Leeman et al (1991). 

Samples collected from the Snaefellsnes Peninsular have 

also been analysed for carbon and sulphur isotopes. These 

elements show that the fluids contain signatures suggesting 

magmatic degassing. The variability in the boron isotope 

ratios ret1ects a primary magmatic signature with modification 

due to fractionation of boron into secondary mineral pha-;es. 

A detailed sampling strategy of both the Nesjavellir and 

Krafla systems, has allowed for a more complete evaluation of 

processes that fractionate boron isotopes, (Aggarwal et al 

1992). Samples have been obtained from both of these systems 

over the last ten years and the boron isotope ratios have been 

examined in conjunction wiU1 known magmatic episodes. 

1 

At all of high temperature geothermal wells, the tluid 

phase was separated into a stemn phase and a water phase. 

Examination of both of these phases showed that very little 

boron was partitioned into the steam phase. In these high 

temperature systems there is no evidence to suggest that boron 

is taken into secondary minerals. Therefore the 611 B of the 

water phase represents the boron isotope ratio of the fluid at 

depth. 

Preliminary results indicate that the boron in the high 

temperature fluids are derived from both water-rock reactions 

and also from magmatic degassing. In the lower temperature 

systems the boron 611 B values are likely to be further modified 

by the incorporation of 10B into secondary phases such as 

carbonates, silica or clay minerals. 

Aggarwal, J.K, Palmer, M.R., Ragnarsdottir, K.V., 1992, 

Boron Isotopic Composition of Icehmdic HydrotJ1ennal 

Systems. Water-Rock Interaction. Kharaka Y.F., Maest 

A.S., (ed), 893-895. Rotterdam: Balkema. 

Leeman, W.P., Vocke, R.D., Beary, E.S., Paulsen, P.J., 

1991, Precise Boron Isotopic Analysis of Aqueous 

Samples: Ion Exchange Extraction and Mass Specu·ometry: 

Geochim.Cosmochim. Acta, 55, 3901-3907. 
Oxygen stable isotope compositions of oceanic crust 

rocks exposed in the Hess Deep. Implications for the 

buffering of the seawater a 180 composition by 

seawater hydrothermal interaction in the oceanic 

crust. 

P. Agrinier, M. Javoy (Lab. G&>chimie Isotopes Stables, 

I.P.G.Paris, 2 place Jussieu, 75251 PARIS cedex 05, France) 

R. Hekinian and D. Bideau (IFREMER- Centre de Brest, 

BP 70, 29263 Plouzane, France) 

A large variety of rocks, consisting of basalts, 

dolerites, gabbros and ultramafics, are exposed at 

Hess Deep, representing a section of the oceanic 

crust formed at the East Pacific Ridge. We have 

conducted an oxygen stable isotope study in order to 

assess the nature and intensity of interaction of 

seawater with the oceanic crust in this region. 

Fresh basalt and dolerite samples have magmatic 

MORB values (a180:::: 6.0 and low H20+ (<0.3 

wt. %). The metabasalts are enriched in 180 (at so:::: 

7) and have H20+ contents higher than 2 wt. %. The 

metadolerites are depleted in 180 (alSO :::: 4.9), with 

H20+ contents (> 1.3 wt.% ). 

Most gabbros have atso around 5.6, essentially 

identical to their initial magmatic values although 

their mineralogy shows that they have reacted with 

fluids. In gabbros, displaying little or no evidence of 

alteration of their plagioclases and pyroxenes, 

~180plag-px is small (:::: 0.3 ) and consistent with 

magmatic values. The calcic plagioclases have 

magmatic atso (:::: 5.8), while altered plagioclases 

have variable atso values (4 to 8). It appears also 

that the tso modified gabbros are those in which 

plagioclase is transformed into albite or prehnite. 

The lack of calcic plagioclases depleted in tso 

suggests that most of the gabbros did not react 

extensively under amphibolite facies with seawaterderived fluids (T ~ 400°C) or that they were 

subsequently altered in the greenschist facies. 

The serpentinized peridotites were depleted in tso 

(3.3 to 4.9).In one sample, the 18 0serpentinemagnetite (:::: 5. 7) corresponds to a temperature of 

about 325 ± 50°C. These values are compatible with 

serpentinization temperature between 200 and 350°C 

and suggest that seawater-derived fluid may have 

been responsible for the serpentinization process. 

Another sample has a a 180 value of 10 

corresponding to a serpentinization temperature less 

than 50°C. 

The 18Q mass balance between seawater and oceanic 

crust seems to have attained steady-state where 

enrichment in 180 of metabasalts is balanced by the 

depletion in tso of metadolerites. The contribution 

of the gabbros is minor. 

2 

U-Pb EVIDENCE FOR PROVENANCES OF HOLOCENE 

SAND DUNES, NORTHEASTERN COLORADO AND THE 

NEBRASKA SAND HILLS 

ALEINIKOFF John N., MUHS, Daniel R., and 

WALTER, Marianne, U.S. Geological Survey, Box 

25046, DFC, MS 963, Denver, CO, 80225, USA 

Eolian sand seas (dune fields >125 km2) form in areas of 

abundant sand supply, strong winds, and suitable climatic and 

topographic conditions for long-term sand accumulation. 

Most sand seas occur in tropical or subtropical latitudes, but 

important exceptions are the large dune fields of the North 

American Great Plains. The Nebraska Sand Hills (>50,000 

km2) are comparable in size to many of the sand seas of the 

Sahara and central Asia. Knowledge of the sources of 

sediments in presently stabilized sand seas is important for 

understanding the conditions of reactivation due to possible 

future global climate change. 

U-Pb analyses of single zircons from Holocene eolian 

sand dunes of northeastern Colorado have yielded ages (1.06-

1.16, 1.38-1.5, 1.65-1.73, and 2.0 Ga) indicative of derivation 

from Middle Proterozoic crystalline rocks of the Colorado 

province. These data are supported by measurements of Pb 

isotopic compositions of multigrain and single microcline 

samples. All samples also contain a sanidine component, 

derived from Tertiary volcanic centers in the western U.S. 

These results support geomorphic, geochemical, and 

sedimentologic interpretations that the dunes formed by eolian 

processes that removed sand from fluvial deposits of the South 

Platte River. 

In contrast, no major modem river system is upwind 

(north-northwest) of the Nebraska Sand Hills, and most 

workers have considered the Tertiary Ogallala Group, in 

primary or reworked form, to be the main source of sand. UPb ages from single zircons (1.03-1.18, 1.4, 1.6-1.68, and 

2.5-2.75 Ga) and Pb isotopic compositions from multigrain 

fractions and single grains of microcline and sanidine indicate 

that these Holocene dunes have provenances in both the 

Colorado province and the Archean Wyoming province. Three 

possible sources are considered: (1) sediments of the North 

Platte River (which heads in north-central Colorado but flows 

north into Wyoming and then east along the southern margin 

of the Sand Hills), (2) the Miocene Ogallala Group, and (3) the 

Oligocene-Miocene Arikaree Group. The North Platte River 

sediments contain zircons that are Middle and Early Proterozoic 

and Archean, but lack zircons of the 1.4-Ga age group. 

Zircons from the Ogallala are solely derived from the Colorado 

province (no Archean grains found), in contrast to interpretations of Tertiary paleo-drainage patterns suggesting a 

source to the northwest in Wyoming, and thus the Ogallala 

could not be the sole source of the sand. Although the 

Arikaree has previously been ignored as a potential source, it 

contains zircons whose ages match the age distribution found 

in the Sand Hills. Thus, the source(s) of the Sand Hills 

sediments must be (1+2) or (3) or (1+2+3). We conclude that 

the Nebraska Sand Hills have multiple sources, including 

sources not previously recognized. 
PRESOLAR SIC: PRISTINE SAMPLES FROM 

OTHER STARS. 

ALEXANPER. Conel M O'D , McDonnell Center for 

Space Sciences, Washington University, StLouis, MO 

63130, U.S.A. 

Since its initial identification as the the long sought for 

carrier of Ne-E(H) (almost pure 22Ne), presolar SiC isolated 

from meteorites has become the focus of in-depth isotopic 

studies both of bulk samples and of individual grains. The 

results of this work have important implications for models of 

stellar, galactic and nebular evolution. 

The noble gases released from bulk samples are highly 

enriched in isotopes produced by s-process nucleosynthesis [1] 

and the emerging consensus is that most of the SiC formed in 

the envelopes of C-rich red giant stars which are known to be 

enriched in s-process material [2]. In apparent confirmation of 

this, the C isotopic compositions of individual SiC grains 

show a very similar distribution to those of C-rich red giants, 

Although a few SiC grains from super novae and massive 

Wolf-Rayet stars may also be present. The relative abundances 

of grains from these sources are in broad agreement with dust 

production estimates based on astronomical observations. 

Individual SiC grains provide a snapshot of a star's 

evolution at the time the grain formed. Most grains are 

enriched in refractory s-process elements and depleted in most 

other elements in accordance with the predicted volatility of 

their carbides [3]. However, Si and Ti isotopic compositions of 

individual grains do not conform to expectations of simplesprocess nucleosynthesis [3,4 ]. Rather it appears that the sprocess signatures arc superimposed on a general trend produced 

by galactic chemical evolution [3]. Thus the SiC found in 

meteorites must have been produced by many stars that formed 

at different times and/or places in the galaxy. 

The noble gases trapped in the SiC exhibit significant 

absolute abundance and isotopic variations with grain size [1]. 

Most of the 21 Ne in SiC is thought to have been produced by 

cosmic ray spallation reactions while the grains resided in the 

interstellar medium (ISM) [1]. The abundances of 21Ne in 

different SiC size fractions have been used to obtain average 

cosmic ray exposure ages and these vary from 13Ma in the 

finest fractions to 133Ma in the coarser ones [5]. This 

variation with size has been alternately interpreted as due to 

partial degassing in the solar system [5] or reflecting the 

average residence times of SiC in the ISM [6]. The latter has 

been used to develop a model that appears able to account for 

the variations of all the noble gases with size [6]. 

The use of interstellar grains to infer conditions in the 

nebula is just beginning to be explored. SiC is only preserved 

in the fine grained meteorite rim/matrix [7 ,8] confirming the 

long held belief that this is the most primitive material to be 

found in meteorites. The abundances of interstellar grains in 

rim/matrix prior to the onset of metamorphism appear to have 

been more or less the same in all the most primitive 

meteorites [8]. This observation presents some problems for 

the 'standard' hot nebula evolution model used to explain 

variations in the bulk compositions of the different chondrite 

classes. 

[1] Lewis R.S. et al. (1990) Nature, 348, 293. 

[2] Gallina R. et al. (1990) Nature, 348, 298. 

[3} Hoppe et al. (1994) Ap. J., in press. 

[4} Alexander C.M.O'D. (1993) G.C.A., 57, 2869. 

[5} Lewis R.S. et al. (1994) G.C.A., 58, in press. 

[6] Alexander C.M.O'D. (1994) L.P.S.C., XXV, submitted. 

[7] Alexander C.M.O'D. (1990) Nature, 348,715. 

[8] Huss G. (1990) Nature, 347, 159. 

3 

MELT IMPREGNATION OF HESS DEEP 

PERIDOTITES; CONSTRAINTS FROM SPINEL 

PETROGENESIS 

J. F. Allan, Ocean Drilling Program, 1000 

Discovery Drive, Texas A&M Univ., College 

Station, TX 77845 and Leg 147 Shipboard Party 

Melt-impregnated upper mantle peridotites, 

recovered by the JOIDES Resolution at Site 895, 

record melt-wallrock interaction processes during 

transport of MORB through the uppermost mantle 

beneath the adjacent East Pacific Rise. The 

recovered stratigraphy shows progressive sequences 

of harzburgite, dunite, troctolite, olivine gabbro and 

gabbro, troctolite, dunite, and harzburgite, with the 

troctolites representing basalt impregnation of 

peridotite on an intimate, sometimes intergranular 

scale. High precision microprobe analyses were 

obtained of Cr-spinel from these samples. 

Harzburgite Cr-spinels are unzoned and have low 

Ti02 (~0.07%) and Fe3+/(Cr+Al+Fe3+) (Fe3+#; 

<0.04 ). Dunite Cr-spinels have higher Ti02 (0.44-

0.48%) and Fe3+# (0.05-0.07), reflecting exchange 

during the metamorphic transformation of host 

harzburgite to dunite during melt intrusion. Large 

(to 1 em), skeletal Cr-spinels within the troctolites 

represent rapid crystallization associated with 

melt/wallrock exchange. They are individually 

unzoned but vary in composition within an 

individual thin section (Ti02, 0.7-1.5%; Fe3+#, 

0.08-0.11; Cr#, 0.48-0.61; and Mg#, 0.47-0.60). 

Their composition and spinel-olivine 

geothermometry show that they equilibrated at 870-

9000C with Ti-rich, evolved melts (Mg# of 0.43-

0.53). Olivine gabbros contain sub-rounded Crspinel (to 800 Jlm) as intergranular grains and 

inclusions; these have higher Ti02 (to 2.7%) and low 

Mg# (~0.58). Inclusions within plagioclase are 

armored and unzoned, but others show profound 

zoning in ZnO (0.2-0.75%) and Mg# (to as low as 

0.15) that reflects post-crystallization metamorphic 

exchange at temperatures of 600° C or less, perhaps 

with Zn-rich fluids. The troctolite and gabbro Crspinels show that pockets of highly evolved, Ti-rich 

melt (Mg#'s as low as 0.35) developed during 

impregnation of the Hess Deep peridotites, with melt 

impregnation occurring at the periphery of the EPR 

magmatic system in harzburgite wallrock having 

ambient temperatures of 900° C or less. 
STABLE ISOTOPES (O,H,S) DISTINGUISH SOURCES 

OF ACID DRAINAGE AT PENN MINE, CALIFORNIA 

Alpers, C.N. and Hamlin, S.N., U.S. Geological Survey, 

Water Resources Division, Sacramento, CA, 95825, 

U.S.A. and Rye, R.O., U.S. Geological Survey, Geologic 

Division, Denver, CO, 80225, U.S.A. 

Acid drainage with elevated concentrations of base 

metals and ·sulfate has contaminated surface and ground 

waters at the Penn Mine, Calaveras County, California. 

Contaminated surface and ground waters oq;ur less than 

100 m from Camanche Lake, a local water supply. The 

acid drainage results from oxidation of sulfide minerals in 

surface and subsurface environments: surface environments 

include waste rock and tailings piles, and the subsurface 

environment consists of unmined, mineralized rock in 

underground mine workings extending to more than 1 km 

depth. In this study, stable isotopes of 0 and H in water 

and 0 and S in dissolved sulfate are used to distinguish the 

surface and subsurface sources of acid mine drainage. 

Sulfide minerals found on waste-rock piles are pyrite 

(FeS2), sphalerite ((Zn.Fe,Cd)S), chalcopyrite (CuFeS2}, and 

bornite (Cu5FeS4). Wall rocks consist of greenschist facies 

intermediate-to-felsic metavolcanic rocks of Jurassic age. 

Acid surface drainage (pH 2.3 to 3.0) is formed by sulfide 

oxidation in waste-rock and tailings piles. The subsurface 

is drained seasonally by seeps (pH 4.0 to 4.2) associated 

with a shaft connected to the underground mine workings. 

Surface and subsurface drainage mixes in unlined 

impoundments that recharge contaminated, acid water (pH 

2.6 to 2.9) to a fractured metavolcanic-rock aquifer, which 

flows toward Camanche Lake. 

The impoundment waters are the most enriched in 180H2o 

and D, and contain the highest dissolved metal and sulfate 

concentrations. An eva~rative trend with slope 4.4 is 

revealed on a plot of 81 Omo vs. 8D for ground water (pH 

3.6 to 7.8) from the metavolcanic-rock aquifer plus surface 

water from the impoundments. The data span a range of 12 

o/oo in 8180H20 and 55 o/oo in 8D. Seeps and samples from 

the underground workings and uncontaminated ground 

waters plot near the meteoric water line, indicating minimal 

evaporation. Good correlations between 8180mo• 8D, pH, 

and dissolved S04, Fe(II) and Cu in the aquifer provide 

strong evidence that evaporated water in the impoundments 

is the major source of contamination of the metavolcanicrock aquifer. 

The highest values of 834S804 and 8180so4 correspond to 

water samples from the underground mine workings and the 

seasonal seep that drains the workings. Values of 834S804 

show a bimodal distribution with two ranges: + 1.0 to + 2.6 

%o and +4.1 to +4.8 %o. Values of 8180so4 range from -1.4 

to +2.5 %o. Possible explanations for variations in 834S are: 

( 1) preferential removal of 32S during sulfate reduction 

resulting in higher values of 834S in the underground 

workings, and (2) heterogeneity in primary ores. Values of 

8180so4 do not correlate With 8180mo and probably reflect 

differences in microbial activity and sulfide oxidation 

mechanisms (02 vs. Fe3

+ as oxidant). 

In summary, stable isotopes can provide a useful 

hydrogeochemical tool in mine drainage environments. 

4 

ISOTOPIC ANALYSIS OF 47 LOW DENSITY 

GRAPHITE GRAINS FROM MURCHISON 

AMARI S .. ZINNER E., McDonnell Center for the Space 

Sciences and the Physics Department, Washington University, 

One Brookings Dr., St. Louis, MO 63130, and LEWIS R. S., 

Enrico Fermi Institute, University of Chicago, 5630 Ellis 

Ave., Chicago IL 60637 

We have measured C-, N-, Si-, Mg- isotopic ratios of 

graphite grains in the low density graphite fraction KE3 (1.57-

2.12g/cm3, >3J.Lm, Amari et al., 1993a). 12ct13c and 

14Nt15N ratios in 47 grains show that more of the grains are 

enriched in 13c and 15N than are in the higher density graphite 

fractions (Amari S. et al., 1993b); seventeen grains have 

12ctl3C lower than 60 (2cr) and 16 grains have 14NJ15N 

lower than 240 (2cr). Grains in this density range have 

relatively high trace element concentrations, which enable us 

to measure isotopic ratios of several elements. 

Among 46 grains measured for Si, 14 have isotopic 

anomalies deviating from normal more than 2cr. Most of them 

have 28Si enrichments of up to 40% compared with normal Si, 

while 4 grains have either 29si or 29si and 30Si excesses. 

Twenty-six out of 40 grains have large 180 excesses of up tO a 

factor of 100. In contrast, 16ot17o raios are normal within 

relatively large errors. There is a tendency that grains enriched 

in 12c and 15N have larger excesses in 18Q. Twenty-seven 

out of 38 grains have 26Mg excesses with (26 Al/27 Al)o ratios 

ranging up to 0.15. Of the 7 grains in which we measured Ca 

and Ti, all have 49Ti excesses, ranging up to 1720±972%o 

(2cr). Grain 662, the grain with the largest 12ct13c ratio and 

largest 18Q excess has, in addition to a 49Ti excess of 

862±90%o, excesses of 175±72%o and 429±132%o (2cr) in 42Ca 

and 43ca, respectively. The low density graphite grains show a 

similarity in their C, N, Si and Ti isotopic compositions to 

SiC grains of type X, for which a supernova origin has been 

invoked (Amari S. et al., 1992). While Wolf-Rayet stars also 

can have 18o enrichments, it is doubtful that an 18o excess of 

more than a factor of 10 can be achieved (M. Amould pers. 

commun.). On the other hand, the He-burning shell in presupernovae has an 16QJ18Q ratio of about 5, which is 

comparable to the most exteme ratios observed in this study. 

AmariS. et al. (1992) Interstellar SiC with unusual isotopic 

compositions: Grains from a supernova? Ap. J. 394, L43 · 

AmariS. et al. (1993a) Interstellar grains in meteorites I. 

Isolation of SiC, graphite , and diamond; Size distributions 

of SiC and graphite. G. C.A., in press. 

AmariS. et al. (1993b) The isotopic compositions and stellar 

sources of meteoritic graphite grains. Nature 365, 806-809. 
SYSTEMATICS OF METAMORPHIC MONAZITE AND 

ITS BEARING ON THE PETROGENESIS OF THE 

KIGLUAIK GNEISS DOME, SEWARD PENINSULA, AK 

AMATO. Jeffrey M., Dept. Geological and Environmental 

Sciences, Stanford University, Stanford, CA, 94305-2115, 

USA; and J. E. Wright, Dept. Geology and Geophysics, 

Rice University, Houston, TX 77251, USA 

Monazite U-Pb isotopic data from upper-amphibolite-facies 

orthogneiss, metapelite, and partial-melt pegmatites document 

a 91 ± 1 Ma age for a high-temperature metamorphic event 

that overprints an earlier blueschist-facies metamorphism 

recorded in the mantling schists and gneisses of the granite 

cored Kigluaik gneiss dome on the Seward Peninsula, Alaska. 

Zircon U-Pb isotopic data from the compositionally diverse 

granitoid pluton in the core of the dome indicate an age of 

92 ± 2 Ma thus supporting a genetic relationship between Late 

Cretaceous magmatism and metamorphism. 

Monazite dates from orthogneisses range from 96 to 91 Ma, 

whereas monazite dates from metapelite and pegmatite cluster 

between 90 and 91 Ma. We interpret the older dates in 

orthogneiss as reflecting Pb inheritance from their original 

igneous crystallization at 555 ± 25 Ma as determined from 

U-Pb analyses of zircon. Monazite from the metapelite and 

pegmatite instead crystallized initially during Late Cretaceous 

metamorphism and thus do not record an earlier history. All 

monazite analyses vary systematically with grain size; 

75-150 J.U11 grains are 1-2 m.y. older than 45-75 J.U11 grains. 

Field relations together with Nd and Sr isotopic data indicate 

that the Kigluaik pluton contains a significant mantle derived 

component. The pluton consists of a -o.5-1 km biotite 

granite cap overlying quartz diorite to granodiorite whose root 

is not exposed. Mafic pillows with crenulate margins indicate 

mixing of two coeval magmas along the mafic/felsic contact. 

Geochemical data indicate mixing of mafic magma with an 

older, crustal source. Combined Nd and Sr isotopic data from 

the mafic root, the granitic cap, and from a related diabase dike 

swarm lie on a trend from initial ratios(@ 91 Ma) of 87Srj86Sr 

= 0.706 to 0.709 and ENd = -0.2 to -8.0. Isotopic ratios from 

the country rocks (calculated for 91 Ma) are 87Srj86Sr > 0.720 

and ENd > -8.0 and lie on the extension of the trend of the 

analyses from the pluton. Initial ratios from the pluton also 

correlate with silica content (positively for Sr, negatively for 

Nd) which would be expected if the magma were incorporating 

older crustal material. REB plots show mixing between a 

LREE-enriched source and a source with a flat REB 

distribution. 

The geochronological and geochemical data indicate that the 

igneous rocks and coeval metamorphism resulted from the 

emplacement of mantle-derived magmas which provided the 

heat necessary for upper-amphibolite-facies metamorphism and 

partial melting during a Late Cretaceous extensional event that 

post-dated crustal thickening in northern Alaska. The Kigluaik 

gneiss dome may help understand magmatic processes at the 

deeper levels of extensional core complexes such as those in 

the Basin and Range province of the western United States. 

5 

CONSTRAINTS ON THE EVOLUTION OF GRENVILLIAN 

LITHOSPHERE FROM Nd-Sr-Pb CPX AND GARNET AND 

U-Pb ZIRCON STUDY OF PYROXENITIC AND MAFIC 

GRANULmC XENOLITHS 

AMELIN. Yuri. Dept. of Geology, Royal Ontario Museum, 

Toronto, ON M5S 2C6, Canada, CORRIVEAU, Louise, and 

MORIN, David, Centre geoscientifique de Quebec, C.P. 

7500, Sainte-Foy, QC, G1V 4C7, Canada 

Clinopyroxene and garnet separates from 14 xenoliths of 

clinopyroxenites, websterites, garnet pyroxenites and mafic 

granulites, extracted from an 1.088±46 Ma (Sm-Nd internal 

isochron) ultrapotassic lamprophyre dyke in the Central 

Metasedimentary Belt, SW Grenville Province, Quebec, were 

studied for Sm-Nd, U-Pb and Rb-Sr. All 6 analyzed Gnt-Cpx 

pairs yielded similar Sm-Nd ages from 1091 to 1074 Ma, 

indistinguishable from the age of the lamprophyre. 

206pbJ204Pb-238UJ204pb and 207PbJ204pb_206PbJ204Pb ages for 

the same mineral pairs are less precise but also cluster around 

1.08 Ga, while the younger Rb-Sr ages between 630-840 Ma 

are indicative of post -crystallization resetting in garnets. 

Concordant U-Pb age of titanite from a clinopyroxenite 

xenolith 1072±12 Ma is consistent with Gnt-Cpx ages. 

The variations of initial isotopic values in the Cpx from -3.3 

to +5.7 for £N<f(l.08 Ga) and from 16.77 to 17.79 for 

206pbJ204Pb(l.08 Ga) are consistent with the values 

determined in coeval potassic plutons of the Central 

Metasedimentary Belt (Corriveau & Amelin 1994), but the 

87Srf86Sr(l.08 Ga) from 0.70398 to 0.70520 are distinctly 

higher, so the lithosphere region sampled by the studied 

xenolith suite is unlikely to represent the source of these 

potassic rocks. 

Zircons from two websterites and one garnet-bearing 

granulite have U contents between 60-200 ppm and model 

Th/U ratios between 0.2 and 0.4. Six single grain ID U-Pb 

analyses are nearly concordant with 207pb•J206Pb* ages 

between 1112 and 1182 Ma and together define a slightly 

scattered array with upper and lower concordia intercepts of 

ca. 1300 and 1110 Ma, respectively. The upper intercept age 

coincides with a major episode of tholeiitic volcanism in the 

Central Metasedimentary Belt, and we thus interpret it as the 

protolith age of the studied xenoliths, which could represent 

cumulates of a tholeiitic basaltic magma formed in the upper 

mantle or lower crustal magma chamber. The lower intercepts 

(and two concordant points) reflect the beginning of mantle reactivation at 1127-1110 Ma marked by new zircon growth in 

sub-solidus conditions. 

Two single grain zircon analyses from a two-pyroxene 

granulite sample have low U concentrations about 20 ppm and 

unusually high model Th/U ratios 9.4 and 22. These zircons 

are also nearly concordant but their 207Pb*J206Pb* ages are 

significantly younger, 1100 and 1091 Ma. The unusual 

chemistry and absence of older components may indicate that 

the formation of these zircons was related to the episode of 

mantle metasomatism. The garnet-cpx ages therefore reflect 

either the closing of isotopic systems in garnets during mantle 

cooling or heating by the host magma with possible partial 

resetting of isotopic systems in both garnets and zircons. 
U-Pb ZIRCON DATING OF ULTRA HIGHPRESSURE ECLOGITES WITH IMPLICATIONS 

FOR COLLISION OF THE NORTH AND SOUTH 

CHINA BLOCKS. 

AMES. L., Dept. of Geological Sciences, 

University of California, Santa Barbara, CA, 

93106; e-mail ames@magic.geol.ucsb.edu 

The collision zone between the North and South 

China Blocks is marked by subduction-related 

metamorphic rocks composed of crustal protoliths 

ranging in metamorphic grade from ultrahighpressure eclogite to low-greenschist facies. The 

paragenesis of the ultra high-pressure structural 

blocks (coesite and diamond-bearing) indicates 

subduction to a depth of approximately 120 km as a 

result of continental collision. The ultrahigh-pressure 

belts are largely composed of amphibolite facies 

quartzofeldspathic gneiss, but also contain boudins of 

mafic eclogites. 

The two belts of ultra high-pressure rocks (Dabie 

and Shandong) are offset from each other along the 

Tan-Lu fault by about 500 km, but the fault does not 

continue beyond the collision zone. The relationship 

between the timing of ultra high-pressure 

metamorphism in the two belts bears directly on the 

mechanism of collision of the micro continents and 

origin of the Tan-Lu fault. Comparison of 

preliminary U-Pb zircon ages from ultrahigh-pressure 

eclogites from the Dabie Mountains and Shandong 

Peninsula shows that the two areas have the same UPb concordia lower intercept ages, indicating that 

there may not be a significant difference in the timing 

of collision between the two areas. Although the 

timing of ultra high-pressure metamorphism in the 

Dabie belt is well constrained to be 209 ±2 Ma (U-Pb 

zircon), the preliminary ages from the Shandong area 

have large enough errors that an earlier collision 

cannot be ruled out. 

Despite extreme metamorphic conditions, zircons 

in ultra high-pressure rocks show significant 

inheritance from their original crystallization and 

pervasive, fine-scaled zoning. Single zircons from 

paragneiss indicate a range of protolith ages from 

670-800Ma. 

6 

AN ISOTOPE STUDY OF SOILS ON 

ALLUVIAL DEPOSITS IN THE MOJAVE 

DESERT: IMPLICATIONS FOR TIMING OF 

DEPOSITIONAL EVENTS AND 

PALEOCLIMATE CHANGE 

Ronald Amundson, Yang Wang, Department of 

ESPM, University of California, Berkeley, CA 

94720. 

Two soil chrono/climo-sequences in the 

Providence Mountain area in the Mojave desert, 

California were analyzed for their carbon (both 13C 

and 14C) and oxygen isotope composition. l4C dates 

of soil carbonate were interpreted in the light of a 

diffusion/reaction model, which takes into account 

various processes and factors controlling the 14C 

content of soil carbonate. Model ages calculated 

from t4c dates of soil carbonate are in correct 

relative order as suggested by geomorphic evidence, 

and are also consistent with model ages from l4C 

dates of soil organic matter. Stable isotope 

paleoclimate reconstruction in terms of model ages 

are consistent with results from other studies. 14C 

model ages suggest that the older geomorphic 

surfaces we studied are of Pleistocene age and the 

younger surfaces formed during Holocene. Alluvial 

deposits on limestone terrain consistently had 

younger 14c ages than equivalent deposits on 

granitic terrain. Stable carbon isotopic composition 

of soil carbonate indicates a slight increase in C4 or 

CAM plants, or a decrease in plant density in local 

ecosystems in the Holocene. Both carbon and 

oxygen isotopic composition of soil carbonates 

suggest that the climate in the eastern Mojave desert 

has become warmer and drier since early Holocene. 
BULK SEDIMENT COSMOGENIC RADIONUCLIDE 

CONCENTRATIONS CONSTRAIN RELIEF PRODUCTION 

RATES IN EVOLVING ALPINE LANDSCAPES 

ANDERSON RobertS .. Department of Earth Sciences, 

University of California Santa Cruz, Santa Cruz, California, 

95064, USA (rsand@bagnold.ucsc.edu) 

The production of topographic relief in evolving mountain 

ranges is the key to a large scale feedback between geomorphic 

and tectonic processes. Relief is produced when the erosion 

rates at mountain crests are lower than those in the valley 

bottoms, and vice versa. As relief grows, the effective density 

of the range diminishes, generating flexural isostatic uplift. 

Comparison of relief generation from basins of significantly 

differing tectonic and climatic settings may allow us to test 

existing hypotheses about the geomorphic processes involved, 

and their relative rates. 

I present a theoretical rationale for documentation of relief 

generation rates using cosmogenic radionuclide (CRN) 

concentrations of bulk basin sediments. I make use of the 

measurable probability distribution of elevations within the 

basin, D(z), and the known elevational dependence of in situ 

CRN production rates, P(z), in constructing an expression for 

the CRN concentration of basin sediments. 

I assume steady state erosion at all points within the basin. 

This allows the calculation of the CRN concentration in surface 

rock at all points from which sediments are derived. For the 

default case of spatially uniform erosion rates, a measured CRN 

concentration of basin-derived sediments, C, yields a mean 

. z•(P·-CA.) th b . ' fli . erosion rate of e = C . where e asm s e ecuve 

1

Zmax 

production rate, P• = PZmax D(z) e-(Zmax-z)/L dz, 

Zmin 

Z* is the length scale for decay of CRN production with depth 

below the rock surface (order 0.6m), L is the length scale for 

increase of CRN production with altitude (order 1.5km), Zmin 

and zmax are the minimum and maximum elevations within the 

basin, PZmax is the surface CRN production rate at Zmax• and A. 

is the radionuclide decay constant. The probability density of 

elevation and the dependence of in situ production with altitude 

serve to weight the CRN flux from each elevational slice of 

topography, as represented by the integral. When the basin 

relief (R=zmax-zmin) is much less than L, the expression for 

basin erosion rate reduces to that of Bierman and Steig (1992). 

The slightly more complex expression arising when erosion 

rates depend on elevation can be solved numerically for a 

number of possible erosion functions, e(z). If the mean 

erosion rate is known, for instance from modem sediment and 

solute fluxes, we can use departures from CRN concentrations 

expected from elevationally uniform erosion case (the above 

equation) to infer the relief generation rate, R = e~- e~ . Among other assumptions to be discussed, the analysis 

requires that there be no significant sediment storage within the 

basin, that steady state erosion be justified, and that fluvial 

transport times be short relative to weathering time scales. 

Bierman, P.R. and Steig, E., 1992, Using cosmogenic isotopes 

to measure basin-scale rates of erosion. GSA Abstracts with 

Programs, v. 24, p. A122. 

7 

WATER FLOWPATHS IN A SMALL CATCHMENT DURING 

STEADY, ARTIFICIAL RAIN AND A NATURAL STORM 

ANDERSON. Suzanne Prestrud, DIETRICH, W.E., TORRES, 

R., Dept. of Geol. and Geophys., U.C. Berkeley, Berkeley, 

CA 94720, MONTGOMERY, D.R., QRC, Univ. of 

Washington, Seattle, W A 98195, and LOAGUE, K., ESPM, 

U.C. Berkeley, Berkeley, CA 94720, USA 

The subsurface routing and residence time of water in 

catchments are fundamental controls on runoff chemistry. In a 

small, steep catchment with a thin, highly conductive soil 

mantle in the Coast Range of Oregon, we have found 

surprisingly long residence times for water and substantial flow 

through the bedrock. Our study site is an 860 m2 unchanneled 

valley, with 45° slopes, and an average lm thick colluvial soil 

mantling flat-lying sandstone. We have used deuterium and 

bromide tracers in addition to intensive piezometric, 

tensiometric, and geochemical measurements to document 

hydrologic processes during artificial sprinkler experiments and 

natural rainstorms. 

Deuterium spiked rain tracked in 34 lysimeters during a 

whole-catchment sprinkling experiment moved in a slow pluglike manner through the soil vadose zone. The spiked water 

travelled at approximately the rainfall rate during three days of 

steady 1.6 mm/hr rain, reaching a maximum depth of 0.45m. 

In contrast, bromide injected into saturated colluvium travelled 

at rates similar to the satuated hydraulic conductivity of Io-1 

cm/s. The bromide did not appear in all of the lysimeters 

down-gradient from the injection point because of flow into 

and out of the bedrock. In summary, flow through the vadose 

zone is controlled by the rainfall rate, while flow through the· 

saturated colluvium and bedrock is controlled by the saturated 

hydraulic conductivity. 

A two component mixing model for deuterium in the 

runoff shows that "old" water (water on the slope before the 

rain event) comprises 80-90% of the storm runoff in both our 

spiking experiment and a natural storm we analyzed. The slow 

flow of water through the vadose zone in these highly 

conductive soils leads to this non-intuitive result. The small 

proportion of "new" water comes from the shallow soil near 

the catchment exit. 

The water storage capacity of the colluvium is roughly 

0.25m. A mean annual rainfall of 2.5m implies that the 

average residence time for water in the soil is on the order of 1 

month. Storage of water in the soil can increase its dissolved 

organic carbon content and therefore substantially increase its 

weathering potential. Runoff chemistry is expected to be 

strongly buffered to that of the deep soil water chemistry. 
ISOTOPIC CONSTRAINTS ON AGE AND 

SOURCES OF SOME MINOR RAPAKIVI 

COMPLEXES, CENTRAL SWEDEN. 

ANDERSSON, U.B., Inst. of Earth Sciences, Uppsala 

University, Norbyvagen 18 B, S-752 36 Uppsala, Sweden 

NEYMARK, L.A., Inst. of Precambrian Geology and 

Geochronology, Russian Academy of Sciences, Makarov a emb. 

2, St. Petersburg 199034, Russia 

The suite of Fennoscandian rapakivi intrusions consists of 7 

major and> 10 minor plutons, distributed from Russian Karelia in 

the east to central Sweden in the west. We present here new 

isotope data on four minor intrusive complexes intruding the 

Svecofennian rocks in central Sweden, which on petrographical 

and structural grounds have been included in the rapakivi suite. 

Bimodal magmatism is typical and mafic to intermediate rocks 

dominate three of these. The granites are even-grained and lack the 

typical rapakivi texture, but show the characteristics of dry, bighT A-type granites. 

U/Pb zircon dating was performed on two of the complexes, 

Mlrdsjo and Mullnaset. Three single air-abraded zircon grains of 

the Mlrdsjo granite was measured, of which one was more 

discordant. The weighted average of the two almost concordant 

grains gives the most reliable age estimate, 1524±3 Ma. Three 

almost concordant abraded grains of the Mullnaset granite give an 

age of 1526±3 Ma. Age range for Fennoscandian rapakivi 

plutonics was until now 1645-1540 Ma. Our ages are thus about 

15 Ma younger than the previously youngest intrusion, but 

clearly put these complexes in this general age frame. 

Nd isotope data were collected for both mafic, intermediate and 

granitic rocks. They all have low initial (1.52 Ga) ENd-values, in 

the range -8.5 to -5.7, (av. -7.1), suggesting significant 

involvement of older crustal components. DM-ages range from 

2.16 to 2.64. However, no rocks of such ages are known in the 

Svecofennian (2.03-1.86 Ga). Metasediments in the region have 

identical ENd (1.52) values (av. -7.1) as the rapakivis, but an 

upper crustal peraluminous source is geochemically unlikely. 

Furthermore, 87 srf'~6Sr (1.52) values for rapakivi granites are 

much lower (0.703) than for the sediments (0.714). A 

metasedimentary protolith can thus be rejected. Instead the 

presence of a lower crustal basement beneath central Sweden, that 

contain a major Archean component, is indicated. The average 

EN:ll.52_value of early Svecofennian metaigneous rocks is -2.7, 

and similarly for Archean rocks (NE part of the Fennoscandian 

shield), -18.5. A generation of the felsic rocks from a mixed 

source requires ==40% Archean material. 

Perhaps most conspicuously, the mafic-intermediate rapakivi 

rocks show overlapping ENd-values with the granites. Since it is 

petrologically untenable to derive large amounts of felsic magmas 

from the mantle, we advocate that the mafic and felsic rapakivi 

rocks have different sources. The· following scenario for the 

formation of these complexes is suggested: mafic magmas 

generated from an enriched upper mantle intrude the lower crust. 

The lower crust is composed of a heterogeneous mixture of 

Archean and early Svecofennian calcalkaline metaigneous rocks. 

Lower crustal melts (rapakivi granite magmas) are generated by 

the heat from the mafic magmas from a 2:5 

Archean:Svecofennian source ratio. The mafic magmas are further 

enriched by assimilation in the lower crust. The magmas coexist 

and hybridize (forming intermediate magmas) and intrude to 

higher levels together. 

The present data strongly suggest that Archean basement rocks 

occur much further south than previously known ( =300 km to the 

north). This necessitates a revision of the tectonic models for the 

evolution of the shield. 

8 

EVOLUTION FROM TRANSITIONAL TO HIGHLY 

ALKALIC POTASSIUM-RICH MAGMATISM IN THE 

NORTHERN UDOKAN VOLCANIC FIELD WITHIN THE 

BAIKAL RIFT SYSTEM (RUSSIA). 

ANDRE. L., Dept. of Geology, Musee Royal de l'Afrique 

centrale, 3080, Tervuren, Belgium, RASSKASOV, S., Inst. 

of the Earth Crust, 664033 Irkutsk, Russia, IVANOV, V., 

Vinogradov Inst. of Geochemistry, 664033 Irkutsk, 

LIEGEOIS, J-P, Dept. of Geology, Musee Royal de 

l'Afriquc centrale, 3080 Tervuren, and BOVEN, A. Vrije 

Universiteit Brussel, Pleinlaan,2 1040 Brussels, Belgium. 

The Udokan volcanic field (UVF) occupies a large area at 

the Northeast of the Baikal Lake. The northern part of this riftrelated succession is composed of a sequence of 4 volcanic 

packages overtopped by recent cinder cones. The basaltic 

stratigraphic sequence range from transitional basalts (TB) 

(packages 1 and 3) through alkali olivine basalts (AOB) 

(packages 2 and cinder cones) and basanites (BAS) (package 

4) to olivine melanephelinites (OMN) and olivine 

melaleucitites (OML) (cinder cones). A time span from 17 Ma 

to 2.7 Ma is inferred for their emplacement by whole-rock KAr measurements. Representative samples from the whole 

succession were studied for major and trace (V, Cr, Co, Ni, 

Nb, Sr, Y, Zr, Nb, Ba, REE, llf, Ta, Th, U) elements and SrNd-Pb isotopes. The Mg number decreases from the OML (71-

67) and BAS (65-60) to AOB (59-54) and TB (62-49) showing 

that OML and BAS could represent rather primitive melts 

while AOB and TB correspond to differentiated magmas. 

The initial TB and AOB magmas have features (e.g. 

10<Lalfh<14; 13<Lalfa<30; 0.7040<87Srf86Sr<0.7051; 

0.51227<143Ndfl44Nd<0.51255; 15.37<207Pbf2°4Pb<15.39; 

17.83<206pbf204pb<17.95) which are compatible with a rather 

significant contribution of some Rb-U-Th depleted lower 

crustal material in their source. In contrast, the following AOB 

and BAS have typical OIB geochemical features except higher 

K!Yb, BalLa and Ce/Pb ratios in the range of those obtained 

for the basalts of the western branch of the African Rift 

System. The decrease with time of the crustal contribution is 

confirmed with the late eruption of the OML that show typical 

om signatures for all the parameters. 

From these preliminary data, we infer that the evolution of 

transitional to highly alkalic potassium-rich magmatism at the 

eastern Baikal Rift System was controlled by a gradual 

increase in the proportion of enriched sub-continental 

lithospheric derived melts (leucitites) over lower/intermediate 

mantle-derived melts (TB-AOB) through time. This is coupled 

with a decrease of the crustal contamination rate. 
HIGH COOLING RATES AND 1ECTONIC UNROOFING 

IN THE IN'IERNAL ZONE OF THE ALPINE BELT OF 

THE BETIC CORDILLERA, S SPAIN, DURING EARLY 

MIOCENE NAPPE EMPLACEMENT: ADDITIONAL 

CONSTRAINTS FROM FISSION TRACK ANALYSES 

ANDRIESSEN, P.A.M., M R0RHMAN, P. VAN DER 

BEEK AND C. SANDERS, Institute of Earth Sciences, 

Vrije Universiteit De Boelelaan 1085, 1081 HV 

Amsterdam, The Netherlands. 

The Betic Cordillera in southern Spain belongs to the 

western termination of the Alpine Belt of Europe and Mrica. 

The Internal Zone of the Betic Cordillera consists of three 

nappe complexes of variable metamorphic grade. The 

stratigraphic sequences of these nappes are roughly comparable 

and consist of pre-Permian-Triassic basement, PermianTriassic pelitic sequence, and Triassic carbonate rocks. On top 

or between nappes Early Miocene marine transgressive 

formations are found and they are interpreted as tectonic seals 

after emplacement of the Mesozoic nappe complex of the 

Internal Zone. 

Previous Sm-Nd and Rb-Sr analyses of the basement dated 

the emplacement of the Alpujarride nappe at 22 ± 2 Ma, 

Aquitanian-Burdigalian. K-Ar and Ar-Ardating of mica's of the 

(pre) Permian-Triassic mica schists and gneisses from the this 

nappe complex yielded cooling ages between 18.8 and 20.3 

Ma. All the geochronometers pointed to very high cooling 

rates of 200-300°C/Ma during nappe emplacement. However, 

the cooling trajectory could only be determined to temperatures 

down to some 350°C and no information is available on the 

low temperature side. 

A fission track analysis program was performed to 

constrain the cooling to (near) surface temperatures. Samples 

of the Torrox-gneiss body, occurring within the Alpujarride 

nappe complex, gave F.T. ages of 18.9 ± 2.1 Ma and between 

20-15 Ma for one zircon and nine apatites concentrates, 

respectively. The zircon age, representing cooling to below 

200°C, supports the previous conclusion of very high cooling 

rates during Early Miocene nappe emplacement. The apatite 

ages display a range, but a mean age of 16.6 ± 0.8 Ma is 

calculated for 126 individual apatite grains from 7 apatite 

samples with mean spontaneous fiSsion track lengths of 

between 14 and 14.7 J.lm. The mean apatite age of the Torroxgneiss of 16.6 ± 0.8 Ma is interpreted to represent the time of 

cooling to below 50°C. This conclusion seems to be in error 

with the paleontological dating of 19 ± 1 Ma of the marine 

transgressive Vinuela formation, that seals of the Triassic 

carbonate rocks of the youngest stratigraphic unit of the 

Alpujarride nappe complex. 

P-T -t determinations suggest that the Alpujarride basement 

rocks underwent medium to high temperature metamorphism 

of 575°C and 4-6 kbar, some 22 ± 2 Ma ago. The assumption 

of an ambient high thermal gradient of 50°C/Ma would still 

imply tectonic unroofing of at least 10 km at the end of the 

orogenic development in the Betic Cordilleras. Similar 

amounts of tectonic unroofing are not uncommon for active 

margins or continent-continent collision zones. 

Orogenic models explaining late Oligocene and early 

Miocene extension and compression regimes during the Alpine 

orogeny of the Betic Cordilleras in southern Spain, must take 

into account the constraints derived from the fission track ~malv~~ 

9 

ND- AND SR-ISOTOPIC CONSTRAINTS ON THB 

EVOLUTION OF THE MID GERMAN CRYSTALLINE 

RISE, VARISCAN FOLD BELT 

ANTHES. G. and REISCHMANN, T., lnstitut fUr 

Geowissenschaften, Johannes Gutenberg-UniversiUit, 55099 

Mainz, Gennany 

Granitoids and metamorphic rocks from the Mid German 

Crystalline Rise (MGCR), which represents a belt of ign~us 

and metamorphic rocks within the east-west trendmg 

Saxothuringian zone of the Variscan fold belt of central Europe 

were investigated in order to evaluate crustal residence ages and 

characterize Sr and Nd isotopic signatures of distinct crustal 

segments. 

The analysed metamorphic rocks comprising felsic gneisses 

and amphibolites yield £Nd(T)-values ranging from -4.8 to 

+2.7 and 87srJ86sr initial ratios from 0.7040 to 0.7523. The 

granodiorites, granites and some rare gabbros and diorites 

provide a large spread in £Nd(T)-values from -5.9 to +4.3 and 

in the 87srJ86Sr initial ratios from 0.7039 to 0.7543 which is 

similar to the range observed in the metamorphic samples. 

Calculated Nd model ages from the MGCR vary from 0.67 

to 1.54 Ga for the granitoids and from 0.88 to 1.81 Ga for the 

metamorphic rocks indicating a slightly older provenance. 

! 

TDM(Ga) 

Histogram plot of Nd model ages from the MGCR 

These data from the MGCR are in contrast to crustal 

residence ages of plutonic rocks from the Moldan~bian z~ 

such as granitoids of the Schwarzwald and Boheml3ll Massif 

and granites from the Harz mountains of the Rhenohercynian 

zone which show a very narrow variation between 1.4 and 1.7 

Ga whereas the metamorphic rocks from these regions vary 

between 1.3 and 3.0 Ga (Liew & Hofmann, 1988). The lower 

model ages of the MGCR can be explained by a larger amount 

of juvenile Palaeozoic material added to this segment of the 

Saxothuringian zone in comparison to the other parts of ~e 

Variscan belt with incorporated more Archean and Proterozotc 

material. 

The results of this isotope study indicate that the MGCR can 

be interpreted as a distinct terrane within the main structures of 

the central European Variscan fold belt. 

Liew, T.C., and Hofmann, A.W., 1988, Precambrian crustal 

components, plutonic associations, plate environment of the 

Hercynian Fold Belt of central Europe: Indications from aNd 

and Sr isotopic study: Contrib. Mineral. Petrol, v. 98, p. 

129-138. 
GEOCHRONOLOGY AND PALEOMAGNETISM OF 

THE QUATERNARY POTRILLO VOLCANIC FIELD, 

RIO GRANDE RIFT, NEW MEXICO 

ANTHONY. E. Y., Geol, Univ TX, El Paso, 79968; 

MCINTOSH, W.C., Geochron Lab, NM Inst Min & 

Tech, 87801; POTHS, J., INC-6, LANL, NM, 87545; 

WILLIAMS, W.J.W., Geol, Univ TX, El Paso, 79968; 

WHITELAW, M., Geol, Univ TX, El Paso, 79968; and 

GEISSMAN, J., E&PS, Univ NM, 87131. 

We report preliminary results on the chronology (using 

a combination of 3

He surface exposure and «> Arf39 Ar dating) 

and paleomagnetism of the Potrillo volcanic field·. This field 

includes cones, flows and maare and is considered here as 

three geographic sections: western, central and eastern. 

Data indicate that the western part of the field has had the 

greatest longevity, resulting in more than 200 cones and 

flows with some cones clearly more degraded than others. 

Our study shows that the majority of the field erupted during 

the Bruhnes chron; we have, however, documented reverse 

polarities for several flows emanating from more eroded 

cones in the western part of the field. A flow from this area 

yields an «>Arf39

Ar date of 823 ± 14 ka (determined using 

furnace step-heating). 

The presence of feldspar megacrysts in many flows 

allows us to investigate the suitability of pairing megacryst 

and host rock analyses for high precision 40 Arf39 Ar dating. 

One such pair gives dates of 521 ± 17 ka from a feldspar 

megacryst and 475 ± 23 ka for its basanitic host. Our 

research is also focussed on determining the oldest flows for 

which we can obtain reliable 3

He surface exposure dates. 

The oldest surface we have dated thus far is from a flow 

near Mal pais Maar, situated in the western part of the field. 

It yields an exposure date of 27 4 ± 15 ka; we are currently 

obtaining an 40Arf39 Ar date for this flow. Another flow in 

the vicinity yields an «> Arf39 Ar date of 249 ± 12 ka. 

A number of surface exposure dates for the central part 

of this field support the youthful geomorphology observed. 

The Afton flows, through which Kilbourne Hole maar 

erupted, are 101 ± 7, 114 ± 7 and 120 ± 8 ka for three 

samples from separate units. (Uncertainties on surface 

exposure dates represent fully propagated analytical errors; 

they do not reflect uncertainties in production rate.) Flows 

from within the Potrillo maar depression, located 10 km 

south of the Afton flows, yield dates of 69 ± 6 and 76 ± 

6 ka. The youngest dates in the Potrillo field are from 

Aden Crater, a spatter ring complex northwest of the Afton 

flows. Samples from an interior spatter cone, lava lake and 

breach flows yield dates of 20.9 ± 4.3, 21.4 ± 3.3, 23.3 

± 4.1, and 24.2 ± 3.5 ka. We are acquiring «>Arf39Ar 

dates for all of the features in this section of the field. 

The eastern part of this field is an alignment of several 

cones. Surface exposure dates on two separate flows from 

Black Mountain, the southernmost cone, yield 101 ± 9 and 

84 ± 7 ka. An 40 Arf39 Ar date on the first flow gives 127 ± 

9 ka. Also although we have not seen geomorphic evidence 

for polycyclicity at this cone, we have a second «> Arf39 Ar 

date of 184 ± 10 ka. 

10 

SR-ND ISOTOPIC AND GEOCHEMICAL 

CHARACTERISTICS OF THE TWO TYPES OF 

GRANITIC PLUTONS IN THE HIDA BELT, 

JAPAN 

ARAKAWA, Y., Earth Sci., Faculty of Education, 

Saitama University, Urawa, Saitama, 338, Japan 

Late Triassic to early Jurassic igneous activity 

occurred in the Hida belt, which is the northern-most 

geotectonic unit in the Inner Zone of Southwest Japan 

and is considered to have occupied a marginal part of 

the Sino-Korean craton before the Southwest Japan 

arc drifted away from the craton at 15 Ma. The 

igneous activity produced various sizes of calcalkaline granitic plutons which show wide varieties of 

lithology, chemistry and internal structures of the 

pluton. 

Based on the chemical compositions, and Sr and 

Nd isotope data, the plutons are grouped into two 

types (Type 1 and Type 2). The Type 1 is the pluton 

with a limitted range of variations in initial Sr and 

Nd isotope ratios (eSr=+l.4 to +15.9; eNd=-0.8 to 

+5.5) irrespective of the variety of rock compositions, 

whereas the Type 2 pluton has wide variations of 

isotope ratios (eSr=+17.8 to +89.0; eNd=-10.3 to 

+0. 7), forming a linear data array connecting a mafic 

rock and basement gneisses in the eNd-eSr diagram. 

These isotope data and element chracteristics 

(including REE) of the rocks suggest that there were 

contrasting differences in magma source and dominant 

process of magma formation between Type 1 and 

Type 2 plutons. The rocks of the Type 1 pluton are 

interpreted to have been formed mostly by fractional 

crystallization from a parental mafic (to intermediate) 

magam with lesser amounts of crustal contributions, 

while those of the Type 2 pluton are explained by 

the mixing between a parental mafic magma and the 

middle Proterozoic crustal materials (or crust-derived 

felsic melt). Mafic rock units (gabbro and diorite) 

which correlate to the parental mafic magma yield the 

lowest Sr and highest Nd isotope ratios within the 

pluton, and they possibly represent isotope 

compositions of their source regions (subcontinental 

mantle). Two contrasting types of plutons, being 

attributed to the differences in the magma source and 

the main mechanism of magma production, reflect the 

across arc difference in the Mesozoic crust-mantle 

structure beneath the Hida belt. 
THE KAAPVAAL CRATON- A CHRONOLOGY OF 1.6 

BILUON YEARS OF CRUSTAL GROWTH 

ARMSTRONG. RA., Department of Geological Sciences, 

University of Cape Town, Rondebosch noo, South Africa. 

The Kaapvaal Craton in southern Africa contains a near 

complete record of Archaean and Early Proterozoic Earth 

history. As one of the best-preserved and most intensely 

studied areas of the Earth's early continental crust and 

lithosphere, the Kaapvaal Craton has provided unique 

information which has contributed greatly to the formulation 

of fundamental global models concerned with the growth and 

evolution of continents over this time span. A number of 

these models have lacked the essential contraints on events 

and processes provided by precise modern geochronology and 

isotope geochemistry. In recent years, however, intensive UPb zircon geochronology in particular, together with 

integrated radiogenic and stable isotope, geophysical, 

structural, field and tectonic studies studies have shed new 

light on the development of the Kaapvaal Craton, including 

the recognition, definition and inter-relationships betwetm 

Archaean sub-terrains, the timing and correlation of major 

intra- and inter-continental events, the evolution of granitegreenstone belts and the development of the thick, stable 

continental lithosphere. This paper reviews these 

developments and presents new U-Pb zircon data from the 

craton. 

The earliest history of the craton is recorded in the Ancient 

Gneiss Complex (AGC) and the Barberton Greenstone Belt 

(BGB), where detailed U-Pb zircon geochronological studies 

have revealed a complex chronology starting some 3.65 Ga 

ago and continuing episodically for approximately 800 Ma. 

This core region (shield) of the craton comprises several welldefmed tectono-stratigraphic terrains, each with it's own 

history prior to amalgamation about 3.2 Ma ago. Xenocrystic 

zircons and Sm/Nd studies indicate the presence of 

substantially older, already evolved continental crust and the 

extent of recycling of this material in the growth of the shield 

region. 

Extensive intra-crustal melting and emplacement of a 

granitic basement marked the. consolidation of the crust into 

a stable continental platform by 3.1 Ga. The next 1.1 Ga is 

characterized by the craton-wide development of large 

mineralized volcano-sedimentary intracratonic basins (viz. the 

Dominion, Witwatersrand, Pongola, Ventersdorp and 

Transvaal sequences), features which have been modelled as 

being characteristic of the transition from the Archaean to 

the Proterozoic. New U-Pb zircon data show that they are 

Archaean features with previous dating underestimating the 

ages of these sequences by up to 500 Ma. Near-continuous 

chronostratigraphic records in some sequences show that 

these basins formed over extraordinarily long periods of time, 

of the order of several hundreds of millions of years. 

Precise U-Pb zircon geochronology has provided important 

insights into the inter-relationship between tectonic processes 

which operated and controlled continental crustal 

development in the Archean/Early Proterozoic in southern 

Africa. 

11 

DATING OF HIGH PRESSURE METAMORPHISM 

WITH A NEW HIGH RESOLUTION ULTRA-VIOLET 

LASER ABLATION MICROPROBE 40Ar/39Ar 

TECHNIQUE: DORA MAIRA, WESTERN ALPS, ITALY. 

ARNAUD, N.O., and KELLEY, S.P., Dpt. of Earth 

Sciences, Open University, Milton Keynes, UK 

Ultra-high pressure eclogite/amphibolite grade 

metamorphism of the Dora Maira Massif in the western Alps is 

a well established and intensively studied event. However, the 

age of peak metamorphism and early cooling remains 

controversial. 40 Ar/39 Ar step-heating and laser spot ages from 

high pressure phengites yield plateau ages as old as 11 OMa 

which has been interpreted as the time of early cooling after the 

high pressure event. Recent U/Pb and Sm/Nd results challenge 

this assertion, indicating a much younger age for the event, 

around 45 Ma and thus obviously a radically different tectonic 

evolution for the western Alps. In a new approach to the 

problem, samples from the undeformed Hercynian metagranite, 

Brossasco, were studied using an ultra-violet laser ablation 

microprobe technique for 40 Ar/39 Ar dating. The new technique 

allowed selective in situ analysis, at a spatial resolution of 50 

Jlm, of quartz, phengite, biotite and K-feldspar. The results 

demonstrate the frequent occurence of excess argon with high 

40Art36Ar ratios (1000-10000) and a strong relationship 

between apparent ages and metamorphic textures. The highest 

excess argon ratios are always associated with high closure 

temperature minerals or large diffusion domains within single 

mineral phases. The best interpretation of this relationship 

seems to that excess argon incorporated in all phases during the 

high pressure event, mixed with an amospheric component 

during rapid cooling and retrogression, producing the wide range 

of 40 Ar/36 Ar ratios. Step-heating of minerals which have 

experienced time integrated growth of this initial mixture will 

produce linear arrays on a 36 ArftO Ar vs 39 ArftO Ar correlation 

diagram, leading to geologically meaningless plateau ages, older 

than the real time of closure to argon diffusion. Though the 

results of in situ analysis and modelling do not rule out the 

existence of a cretaceous high pressure metamorphism in the 

Dora Maira, they strongly suggest that ages in the range 60-110 

Ma are best explained by the presence of excess argon 

incorporated around 45 Ma ago, reconciling argon 

geochronometers with other systems such as U/Pb or Sm/Nd. 

The common occurrence of excess argon in high-pressure phases 

from the Alps and other areas reflects crystallisation in the 

presence of deep crustal or mantle fluids followed by rapid 

exhumation and as such, Ar-Ar analysis of single separated 

phases, may be misleading. 
ABSOLUTE TIMING OF THRUST FAULTINGA THERMOCHRONOLOGICAL APPROACH 

ARNE, D.C., Fission Track Research Laboratory, 

Dalhousie University, Halifax, Nova Scotia B3H 3J5, 

Canada. 

The absolute timing of movement across thrust faults has 

important implications for both the tectonic and kinematic 

evolution of thrust belts. Absolute timing constraints provide 

a link between regional tectonics and structural development at 

a local scale, whereas kinematic models for fold and thrust 

belts generally assume a particular chronology for the 

formation of structures that is often untested. Apatite fiSsion 

track thermochronology has the potential to provide absolute 

timing constraints at shallow erosional levels in fold and 

thrust belts if it is accepted that distinct breaks_ in cooling 

history across discrete structures are related to the denudation of 

actively thrusting plates. In practice, fold and thrust belts are 

often deeply eroded, such that low-temperature 

thennochronometers often only provide evidence for regional 

cooling. 

An approach involving the integration of vitrinite 

reflectance data and apatite fission track thermochronology is 

proposed to assess this potential limitation. Both theoretical 

and empirical considerations suggest that fission tracks in 

apatite will be totally annealed at Ro values between 0.7 and 

0.9%. Thus in sedimentary rocks containing vitrinite, it is 

possible to evaluate the degree of regional cooling to have 

affected a fold and thrust belt, and to predict whether fission 

tracks in apatite were likely to have been totally annealed prior 

to regional cooling. Contrasts in cooling history across 

specific structures can also be recognized using vitrinite 

reflectance data allowing those sites favorable for 

thermochronometric analysis to be identified. In general, a 

situation in which fission tracks in apatite from the 

hangingwall of a thrust fault have been totally annealed 

(Ro>-Q.8%), whereas those in the footwall were only partially 

annealed {Ro<0.8%) is favored for detailed apatite fiSsion track 

thennochronology. 

Across the Burnt Timber Thrust in southwest Alberta 

(Rocky Mountain Foothills, Canada), Ro values vary from 

-Q.6% in the footwall to -1.5% in the hangingwall. Fission 

tracks in apatite in the hangingwall of this structure were 

totally annealed prior to the onset of track retention during the 

early Tertiary. By contrast, early Tertiary sedimentary rocks 

from the footwall of the Burnt Timber Thrust give Late 

Cretaceous apatite fission track ages, consistent with fission 

tracks in these samples having been only partially annealed 

following deposition. Apatite fission track data from the 

hangingwall of the Burnt Timber Thrust are therefore 

interpreted to indicate cooling during late-stage thrusting. 

Apatite fission tracks ages from western Sichuan Province, 

China vary dramatically from 100 to 200 Main sedimentary 

rock samples from the western Sichuan Foreland Basin, at 

elevations of -1 km, to less than -10 Ma in the southwest 

Songpan-Garz! Fold Belt, at elevations between 1 and 4 km. 

The observed reduction in apatite fission track age occurs 

rapidly across the intervening southern Longmen Mountains 

Thrust-Nappe Belt in which southeast-verging thrust faults 

have brought Proterozoic basement complexes to relatively 

high structural levels. Thus Late Miocene cooling of the 

southwest Songpan-G&rU Fold Belt is attributed to denudation 

driven by the reactivation of inferred Late Triassic structures. 

12 

THE COMPOSITION OF THE SOLAR NEBULA: NUCLEOSYNTHESIS CONTRIBUTIONS TO THE BULK 

MATERIAL AND TO THE ISOTOPIC ANOMALIES 

ARNOULD, M., Institut d'Astronomie et d'Astrophysique, Universite Libre de Bruxelles, B-1050 Bruxelles, 

Belgium 

There is now ample observational evidence that the solar 

system is made of material from compositionally different 

and imperfectly mixed reservoirs. 

One of them, which comprises the bulk solar system 

material, is considered to be made of the well-homogenized 

ashes of many nucleosynthesis events. Its composition can 

be studied through models of the chemical evolution of the 

Galaxy. The main nucleosynthetic agents responsible for 

that evolution are very briefly reviewed. 

The remaining, very minute, fraction of the solar system 

material exhibits anomalies (with respect to the bulk) in the 

isotopic composition of a variety of elements from carbon to 

neodymium (including the rare gases). It is carried in part 

by high-temperature inclusions of primitive meteorites that 

formed from solar reservoirs out of equilibrium with the rest 

of the solar nebula, and in part by various types of grains 

(diamond, graphite, SiC found in, mostly carbonaceous, 

chondrites) that are considered to be of circumstellar origin, 

and have survived the process of incorporation into the solar 

system. 

The isotopic anomalies provide new clues to many important astrophysical issues, and raise in particular the 

question of their nucleosynthetic origin. In fact, they offer 

the exciting perspective of confronting abundance observations with nucleosynthesis models for a very limited number 

of events. This situation is in marked contrast with the one 

encountered when trying to understand the bulk solar system composition. 

Some selected examples are given to illustrate the potential diversity of nucleosynthetic sources (Red Giant or 

massive mass losing stars, novae, supernovae) that might 

have produced the isotopically anomalous material. Some 

difficulties encountered by and uncertainties involved in the 

nucleosynthesis models that try to interpret the array of observed anomalies are stressed. 
ILLITE/SMECTITE, A VALUABLE K/AR CLOCK 

FOR THE SECONDARY HISTORY OF 

SEDIMENTARY BASINS 

ARONSON. James L . Department of Geology, Case 

Western Reserve University, Cleveland, OH 44106, USA 

and ELLIOT, Crawford ... Case Western Reserve University, 

Cleveland, OH 44106, USA. 

The mineralogic continuum between very thin particles of 

the clay minerals smectite (non K-bearing) and illite 

(K-bearing) is called Illite/Smectite (liS), the most comon 

family of clay minerals. The degree of illitization of liS is 

quantifiable by XRD from 0 - 100%. In mudstones, smectite 

exposed to diagenetic temperatures and fluids converts to liS, 

whereas in sandstones neoformed illite grows as a common 

cement in the pore space. 

I/S is an excellent clock by conventional K/Ar, e.g. 

equivalent to hornblende if not reheated after formation. 

Remarkable for sub-micron sized grains, its blocking 

temperature is at least 1800 C. 

The conversion of smectite to illite in mudstones accelerates 

at 800 - 1 ooo C, the same temperature range over which 

hydrocarbons form. Observing the degree of transformation of 

smectite to illite and dating the mean time of transformation 

quantifies the thermal history of sedimentary basins. The least 

ambiguous dates are obtained on K-bentonites, which begin as 

pure smectite, without the detrial illite present in shales. We 

have related K/Ar dates of K-bentonites to the mean time of 

deep burial diagenesis in sedimentary basins; to the time of 

overthrusting and tectonic burial of sediments in proximal 

parts of foreland basins; and to the passage of hot brines in 

distal parts of foreland basins which have been flushed there 

during tectonic collapse of the proximal basin. 

In the pore space of siliciclastic sandstones, neoformed 

whisker-like ribbons of liS have grown in response to the 

passage of brines accompanying oil and gas migration. The 

liS growth is inhibited in the oil and gas zones of these 

reservoir rocks. In ideal conditions, as for the Rotliegendes of 

the North Sea area, hydrocarbon emplacement can be dated. 

Conventional K/Ar is the method of choice for I/S. At 

some increased size, thicker 100% illite (approaching sericite) 

may be large enough to date by 40Ar;39 Ar. 

13 

PROPORTIONS OF JUVENILE AND RECYCLED 

CRUST IN A PALEOZOIC CONTINENTAL 

COLLISION ZONE, NORTHERN APPALACHIANS 

OF NEW ENGLAND, U. S. A. 

ARTH, JOSEPH G., US Geological Survey, Menlo 

Park, CA 94025 USA; and AYUSO, Robert A., US 

Geological Survey, Reston, VA 22092 USA 

Detailed petrologic, ~hemical, and isotopic-tracer 

studies of a suite of Acadian plutons in the northern 

Appalachians of Vennont provide insight into the 

proportions of magma that are added to the crust, or 

recycled from the deeper crust during a collision of 

cratons- in this case the North American and outboard 

terranes that collided during the Paleozoic. The plutons 

were intruded into metasediments of the Connecticut 

Valley- Gaspe synclinorium, and were chosen for genetic 

study because they fonn a complete suite of largely calcalkaline character, and range from gabbro through diorite, 

tonalite, quartz monzodiorite, and granodiorite, to oneand two-mica granites; and they were emplaced in a part 

of the Appalachians that has been little affected by postAcadian regional metamorphism. 

Rb-Sr isochrons indicate that most of the plutons were 

emplaced fratn 380 to 370 million years ago. Their initial 

Sr ratios have a wide range from 0.7040 to 0.7125. 

Oxygen isotopes also have a large range of delta values 

from +6 to + 14. Nd initial ratios range from .51177 to 

.51213, and initial206pbf204Pb from 18.13 to 18.46. 

The isotopic and chemical features of the plutons and 

zones within plutons suggest subdivision into three 

groups, each having a different origin. Group 1 consists 

of gabbro, diorite, and granodiorite that are generally of 

magmatic-arc composition. The magma sources were 

dominated by either a mafic to ultra-mafic assemblage in 

the mantle or deepest crust, or an Acadian subduction 

zone. Group 2, representing the largest volume of 

magma, consists of quartz monzodiorite, granodiorite, and 

granite that probably originated by melting of 

lithologically-diverse assemblages of Grenville gneisses in 

the deep crust. The sources for Group 2 were mostly 

orthogneisses, but paragneisses were also involved. 

Group 3 consists of a small volume of two-mica granite 

that has the features of melts from country-rock 

metasediments. 

Viewed collectively, the plutons preserve a record of 

melting in a significant vertical section extending from the 

mantle to the middle crust .. About 20 percent of the 

magma was generated in mafic to ultramafic mantle and 

deepest crust. Nearly 80 percent was generated in 

Grenville-age gneisses, and much of this was orthogneiss. 

Less than 20 percent resulted from melting of 

metasediments. Thus the Acadian collisional orogeny in 

this area largely represented a recycling of older crustal 

rocks. 
CONTINENfAL NATURE OF 87Sr/86Sr RATIOS IN TIIE 

NORTH CENTRAL PACIFIC SEDIMENTS 

Asahara.Y., Tanaka,T., Dept of Earth and Planetary 

Sciences, Nagoya University, Nagoya, 464-01, Japan, 

Kamioka,H. and Nishimura,A., Geological Survey of Japan, 

Tsukuba,305,J~ 

87Sr/86Sr ratios were examined for 171 deep-sea and 

continental-shelf surface sediments from the Pacific Ocean to 

estimate the provenance of them. Carbonates in sediments, 

which would inherit an isotopic signal from seawater, were 

leached with 0.25 to 1.0N-HQ for 1 hour. Sr isotopic 

compositions in the residues were determined by thermal 

ionization mass spectrometry after chemical separation of Sr. 

Major characteristics of spatial variation of Sr isotopes and 

their interpretations are summarized as follows ; (1) Sediments 

in the Pacific Ocean have a lower average of 87Sr/86Sr ratios 

(0.7110) and narrower range (0.703 to 0.723) than those in the 

Atlantic Ocean (0.7166, 0.704 to 0.743 ; Dasch, 1969). 

87Sr/86Sr ratios higher than 0.720 occur on the north side of 

30 degree south latitude in the Atlantic, while, in the Pacific, 

ratios higher than 0.720 ~ only in and around the Yellow 

Sea Large old continental crusts supply materials of high 

87Sr/86Sr ratio to the Atlantic, while many volcanos in the 

circum-Pacific volcanic belt, including the Izu-Ogasawara, 

Mariana and Japan island arcs, and young continental crusts 

supply those of low ratios. (2) 87Sr/86Sr ratios are high in 

the north central Pacific (0.711 to 0.719). The areas of higher 

ratios (>0.715) are limited on the abyssal plain. On the abyssal 

plain, the farther from the Im-Ogasawara and Mariana island 

arcs to east, the higher 87Sr/86Sr ratios are. This is due to 

relatively abundant supply of loess from the Asian contin~t 

(87Sr/86Sr=ca.O. 720) to the north central Pacific Ocean by 

middle-latitude westerlies, which is more predominant input 

to the area compared with volcanogenic input from the IzuOgasawara and Mariana island arcs (ca.O. 704) by ocean current 

A part of this work is Northwest Pacific Carbon Cycle 

(NOPACC) study consigned to Kansai Environmental 

Engneering Center (KEEC) by NEDO and a special program 

of MITI, studying on element cycles in the Oceans. 

e 87Sr/86Sr> 0.715 

0 <0.715 

14 

DATING ORDINARY CHONDRITE INCLUSIONS BY 

LASER 40 Ar-39 Ar AND LASER/RIMS I-Xe 

ASH R.D., GILMOUR J.D., TURNER G. Dept of 

Geology, University of Manchester, Manchester, M13 9PL, 

UK, BRIDGES J.C. and MUrCHISON R. Mineralogy 

Dept., Natural History Museum, London SW7 5BD, UK. 

Ar-Ar dating using laser probe techniques has been widely 

used in the analysis of small terrestrial samples. However 

fewer applications to extraterrestrial samples have been 

reported despite there being many potentially interesting 

problems to which this technique could be applied. We have 

used an IR (A=1060nm) laser for the extraction of gas for ArAr dating of clasts separated from ordinary chondrites. Clasts 

of this type have been described as xenoliths and may have an 

exotic origin, although an origin as reprocessed host material 

cannot be discounted. Although these samples are generally 

larger than chondrules the desire to characterise the samples in 

other ways led to the allocation of only 0.5-2 mg sized samples 

hence stepped heating analysis was impractical. The laser 

probe allows the extraction of multiple gas aliquots for the 

determination of 40 Ar/39 Ar ratios and the subsequent 

construction of an isochron. 

Barwell: The Ar-Ar age determined by laser extraction for 

a large H-group, re-equilibrated clast was identical (4.48Gyr) 

to that determined for whole rock sample by stepped heating 

(1). This supports the inference that the analysis of small 

samples by laser is applicable to meteoritic material 

Parnallee: Large porphyritic clasts, one silica and K-rich, 

the other containing Mg-rich olivine, gave Ar-Ar ages of 3.45 

(±0.06) and 3.57 (±0.13)Gyr whereas whole rock stepped 

heating indicates a highly disturbed system with an integrated 

age of ca. 2.9Gyr. 

Quenggouk: A large porphyritic clast gave an anomalously 

high Ar-Ar age of 4.63(±0.038)Gyr. The material in this 

inclusion includes mosaicised olivine with blebs of Fe-Ni 

metal and may have been shock produced. This can lead to 

anomalously old Ar-Ar ages as observed in shock melt veins in 

the ordinary chondrite Peace River (2) although the existence 

of a well defined isochron in Quenggouk would make this 

appear an unlikely explanation. 

Julesburg: An age of 0.95Gyr was determined for a large 

olivine crystal poikilitically enclosing anorthite, but Julesburg 

shows evidence for shock hence this young age could be due 

to shock resetting of the Ar age. The inclusion from Isoulane 

gives an age indistinguishable from that of Julesburg. 

Initial results from high sensitivity [3] I-Xe work on 

Parnallee inclusions suggest that some of the complexities 

observed in the whole rock Ar-Ar stepped heating analyses are 

attributable to a mixture of degassed material, apparently 

resulting from shock ca.1 Gyr ago, and material which has 

retained its full complement of gas during the shock event or 

did not experience a shock heating event. 

[1] Turner G. et al., 1978, The early history of chondrite parent 

bodies inferred from 40 Ar-39 Ar ages: Proc. Lunar Planet. 

Sci. Conf., 9, 989-1025. 

[2]McConville P. et al., 1988, Laser probe 40 Ar-39 Ar studies 

of the Peace River shocked L6 chondrite: Geochim. 

Cosmochim. Acta, 52,2487-2499. 

[3] Gilmour J.D. et al., 1994, RELAX: An ultrasensitive, 

resonance ionisation mass spectrometer for xenon: Rev .Sci. 

Inst. (in press). 
40 Ar/39 Ar SYS1EMA TICS IN A DIAGENETIC- TO 

CONTACT METAMORPHIC-GRADE CLAY MINERAL 

SUITE, NORTHERN NORTH SEA 

AWWILLER,12Ji, BURGESS, R., A YLIFFE, L. K., and 

TURNER, G., Department of Geology, University of 

Manchester, Manchester M13 9PL, United Kingdom. 

Clay mineral size-separates from a suite of diagenetic- to 

contact metamorphic-grade mudrocks (ca. 150-600 °C) have 

been studied in order to ascertain the extent to which illite and 

illite/smectite Ar systematics are dependant upon grain size 

and crystallinity (i.e. temperature of formation). All samples 

investigated are from late Cretaceous mudrocks, collected 

from a well drilled in the M0re Basin, located northeast of the 

Shetland Islands. Numerous late Cretaceous or Paleocene sills 

have intruded the sediments, and as a result the samples have 

experienced widely different maximum temperatures. 

Samfles were encapsulated prior to irradiation to contain 

recoil3 Ar. It was found that the fraction of 39 Ar which 

recoils out of the clay mineral lattice decreases not only with 

increasing particle size, but also with increasing mineral 

crystallinity (i.e. formation temperature). For example, the 

lowest diagenetic grade, <0.1 J.Lm clay fraction has lost ca. 

50% of its 39 Ar by recoil, compared with the 1-2 J.Lm size 

fraction from the same sample which has lost only 16% of its 

39 Ar. The 1-2 fJ.ffi size fraction from the highest-temperature 

sample has lost only 4% of its total 39 Ar from recoil, or 25% 

of the 39 Ar lost by the equivalent low-temperature size 

fraction. 

Step-heating experiments reveal the presence or absence of 

contaminating detrital material in diagenetic-grade clay 

fractions which would be undetectable by conventional K/Ar 

and total-fusion 40 Arf39 Ar dating techniques. Low 

temperature steps ($400 °C) release 40Ar* and 39 Ar which, 

when re-combined with recoil 39 Ar. appear to record 

reasonable diagenetic ages; this low-temperature Ar is 

interpreted to be released from fine-grained authigenic illite 

and illite/smectite. Subsequent higher-temperature steps (450-

600 °C) release more elevated 40Ar*f39 Ar, corresponding to 

much greater than depositional ages. These higher 

temperature-released ages are presumably from detrital illite or 

mica. Samples which have been heated to somewhat greater 

maximum temperature (ca. 300 °C) produce step-heated age 

spectra similar to those of the low-temperature samples, but 

'.vith !nv.u~r ~~.~imum ag~s, ·;.·hich suggi;st-5; thai. the h.t· 

systematics of the detrital component have been partially reset 

during the heating event. 

The samples which have been heated to the greatest degree 

yield consistent 40 Ar/39 Ar ages (64 ± 4 Ma) over 60-70% of 

their 39 Ar release range. The ages are in accordance with 

K/Ar ages reported for the adjacent siiJ complex, and their 

consistency over broad Ar-release temperatures suggests that 

they have not been reset since the time of sill intrusion. 

15 

STABLE ISOTOPE EVIDENCE FOR THE ORIGIN OF 

DIAGENETIC CARBONATE MINERALS FROM THE 

LOWER JURASSIC INMAR FORMATION, SOliTHERN 

ISRAEL 

AYALON A , Geological Survey of Israel, 30 Malkbe 

Yisrael Street, Jerusalem 95501, Israel , and 

LONGSTAFFE, FRED, J., Department of Geology, 

University of Western Ontario, London, Ontario Canada 

N6A5B7 

The oxygen and carbon isotope compositions of diagenetic 

minerals from the Lower Jurassic lnmar Formation, southern 

Israel are used to identify changes in pore-water composition 

during diagenesis. These changes can be related to major 

geologic events within southern Israel. Saline water has played 

an important role in the formation of diagenetic dolomite and 

ferroan dolomite/ankerite cements in these rocks on a basinwide 

scale during the Pliocene-Pleistocene. 

Diagenetic carbonate phases include early siderite 

(S180sMOW = +24.4 to +26.5 %o; 813CpoB = -1.1 to 

+0.8 %o) followed by dolomite and ankerite (8180sMOW = 

+18.4 to +26.3 %o; 813CPDB = -2.1 to +1.3 %o). Calcite 

shows a much larger range (8180sMOW = +21.3 to +32.6 %o; 

813CPDB = -4.2 to +3.2 %o). 

These results indicate that shallow burial, early in the 

diagenetic history included formation of siderite and early 

calcite cements. Further burial led to formation of dolomite and 

late calcite cements. Ankerite formation took place late in the 

diagenetic history. 

A significant negative correlation obtained between burial 

depth and the 8180 composition of the dolomitic cements 

indicates that dolomite formation postdated the tectonic activity 

and uplift that formed the present relief of southern Israel, most 

likely following Pliocene migration of subsurface brines. 

The 8180 composition of the dolomites in the northern 

Negev and Judea Mountains is in perfect accord with that of the 

present-day formation waters whereas formation waters in the 

central Negev have much lower salinity and 8180 values, 

indicating invasion of fresh water. Recharge of the Inmar 

Formation by low 18o meteoric water occurred in the 

Pleistocene, leaving no effect on the dolomite isotopic 

composition. 
PALAEOTEMPERATURES FROM NOBLE GAS 

ABUNDANCES IN SPELEOTHEM FLUIDS 

A YLIFFE, L.K., TURNER, G. and BURNARD, P.G., 

Department of Geology, University of Manchester, 

Manchester M13 9PL, U.K. 

Atmospheric noble gases have solubilities in aqueous 

solutions that display a strong temperature dependence. Ar, Kr 

and Xe are factors of 1.7, 1.8 and 2.4less soluble at 30 °C than 

at 0 °C. Determination of the absolute abundances of noble 

gases dissolved in speleothem inclusion fluids should therefore 

provide infotmation on cave formation temperatures. 

With this aim in mind a series of modern speleothem 

samples from caves with a range of tem~ratures were 

investigated for noble gas and fluid contents. Calcite samples 

(100-500 mg) were crushed in-vacuo to release inclusion 

fluids and associated noble gases. Pressure changes observed 

with a baratron (absolute high precision pressure gauge) 

provided estimates of the water released from crushing. 

Determinations of the absolute noble gas abundances were 

obtained by mixing the gases liberated during the crush with 

that of a mixed noble gas spike (enriched in 21Ne, 38Ar, 80Kf 

and 128xe). 

Speleothem samples analysed have Ne/ Ar, Kr/ Ar. and 

Xe/ Ar which generally lie between the values for air and air 

saturated water. This suggests that these two components are 

principally controlling the noble gases released during 

crushing. Noble gas concentrations measured for individual 

samples during successive crushes are moderately 

reproducible or show variations which correlate with 

variations in F-values. The implied concentrations of Ne, Ar, 

Kr and Xe are however, typically an order of magnitude 

greater than air saturated water concentrations. 

There are two possible explanations which could account 

for the above observations (1) water measurements are biased 

towards lower values by adsorption onto mineral and/or 

vacuum system surfaces and/or (2) another noble gas 

component is present in addition to that of trapped air and air 

saturated water in speleothems. Such a component could be 

fractionated air absorbed onto mineral surfaces and/or 

fractionated air trapped in the mineral lattice during the growth 

of the speleothem. 

It is clear from our data that meaningful estimates of 

speleothem formation temperatures using a simple two 

component noble gas mixing model are not plausible. 

Although the existence of at least one non-equilibrium noble 

gas phase in speleothems complicates the interpretation of our 

data, it should in theory be possible to disentangle up to four 

distinct noble gas reservoirs on the basis of a sample's Ne/Ar, 

Kr/Ar, Xe/Ar and H20/Ar ratios, possibly providing 

information on speleothem precipitation temperatures. 

16 

Pb/Pb DATING ON CARBONATE ROCKS FROM THE 

SAO FRANCISCO CRATON, BRAZIL. 

BABINSKI. M., Instituto de Geociencias, Universidade de 

Sao Paulo, SP, 05508-900, Brazil, and VAN SCHMUS, 

W.R., Geology Department, University of Kansas, 

Lawrence, KS 66045, USA. 

Pb/Pb dating was performed on Paleoproterozoic carbonate 

rocks from the Minas Supergroup, Sao Francisco Craton. 

Undeformed carbonates (Gandarela Fm.) yielded a 

depositional age of 2,420 ± 19 Ma, whereas marbles from the 

Piracicaba Group indicated the age of the metamorphism of 

the Transamazonian Orogeny (ca. 2,100 Ma). These results 

are in agreement with regional geology and previous 

geochronological data (Rb/Sr and U/Pb). 

Pb isotope studies were also carried out on Neoproterozoic 

limestones from the cover of the craton (Bambui Group, Sao 

Francisco Basin), which is bordered by Neoproterozoic mobile 

belts of the ca. 600 Ma Brasiliano Orogeny. The Pb in these 

carbonates showed complex behavior. Limestones displaying 

original structures, as well as very weakly deformed ones, 

showed four distinct types of Pb, here called types I, II, III and 

N. Type I Pb is present in samples with relatively low-Pb and 

high-U (U/Pb ratios> 1) and represents in situ growth of 

radiogenic Pb; it is variably radiogenic. Type II is found in 

samples with relatively high-Pb and low-U; it is nonradiogenic Pb with isotopic compositions (a= 18.8; ~ = 

15. 75) similar to those of average crustal Pb at ca. 600 Ma. 

Type III is also present in samples with high-Pb and low-U, 

but it is radiogenic crustal Pb and can be divided in Type Ilia 

(a= 36.0; ~ = 18.3), lllb (a= 34.0; ~ = 17.8), etc., according 

to isotopic ratios. Type IV is intermediate in composition 

between Type III and Type I Pb. 

Samples containing Type I Pb, the only one that is able to 

yield Pb/Pb isochron ages, showed values between 686 ± 69 

Ma and 520 ± 53 Ma. These ages are concordant to the 

metamorphic peak and post-tectonic phases of the adjacent 

Brasiliano mobile belts. Because of that, we believe that the 

isotopic system was disturbed by the Brasiliano Orogeny. In 

some areas the carbonates incorporated large amounts of 

average crustal common Pb (Type II Pb) so that they do not 

show sufficient radiogenic enrichment to be useful for age 

measurements. In other areas radiogenic Pb from the 

basement (Type III Pb) was incorporated into the carbonate 

rocks, probably during a large scale fluid percolation. 

Regression of Pb isotope compositions of types ~I and III 

yielded a Stacey & Kramers third stage geochron of 500 - 550 

Ma. Type IV Pb occurs in samples from the same outcrops 

where Type I and Type III were found, indicating that it is a 

mixture between those two types. 

According to our results, the Pb/Pb method was 

successfully applied to Paleoproterozoic Minas Supergroup 

carbonates. However, this method did not give conclusive 

geochronological results when applied to Neoproterozoic 

Bambui Group carbonate rocks, probably due to tectonic 

effects, which disturbed the U/Pb system. Despite this, the 

results obtained can provide good information about the Pb 

isotope geochemistry in such areas. 
VARIED MANTLE SOURCES AND MULTI-LEVEL 

CRUSTAL CONTAMINATION OF ARC MAGMAS AT 

CRATER LAKE, OREGON 

BACON Charles R , GUNN, Susan H., LANPHERE, 

Marvin A., and WOODEN, Joseph L., U.S. Geological 

Survey, 345 Middlefield Road, Menlo Park, CA 94025-

3591, U.S.A. 

Quaternary volcanic rocks of the Crater Lake area in the 

southern Cascade arc are characterized by a range of isotopic 

compositions. High-alumina olivine tholeiites (HAOT) have 

87Srf86Sr ratios of 0.70346 - 0.70364; basaltic andesite, 

0.70349 - 0.70372; shoshonitic basaltic andesite, 0.70374-

0.70388; and andesite, 0.70324 - 0.70383 .. Dacites of Mount 

Mazama have 87Srf86Sr ratios of 0.70348 - 0.70373. Most 

rhyodacites converge on 0.7037. However, rhyodacite of the 

caldera-forming, climactic eruption has S7SrJ86Sr = 0.70354 

because of an admixed low-S7SrJ86Sr component. Andesitic 

to mafic-cumulate scoriae of the climactic eruption and 

enclaves in pre-climactic rhyodacites show nearly the entire 

87Srf86Sr range of the data set, confirming previously 

suggested introduction of diverse parental magmas into the 

growing climactic chamber. Pb and Nd isotope ratios display 

less variation (206pbf204Pb = 18.838- 18.967, 207pbj204Pb = 

15.556 - 15.616, 20Spbj204Pb = 38.405 - 38.619; ENd= +3.9 -

+6.1) and generally covary with S7SrJ86Sr ratios. Radiogenic 

isotope data from Crater Lake plot with published data for 

other Cascade volcanoes on isotope ratio correlation diagrams. 

The isotopic data for the Crater Lake area require sources 

of primitive magmas to consist of depleted mantle and a 

subduction component, introduced in variable quantity to the 

depleted mantle wedge. Variable degrees of melting of this 

heterogeneous mantle, possibly at different depths, produced 

the diversity of isotopic compositions and LILE abundances in 

primitive magmas. Trace element ratios do not indicate 

presence of an OIB component that has been reported in lavas 

of some other Cascade volcanoes. 

Crustal contamination may have affected isotope ratios 

and LILE concentrations in evolved HAOT. where initial 

LILE concentrations were low. Contamination is more 

difficult to detect in the calc-alkaline lavas because of their . 

higher LILE concentations and the small isotopic contrast with 

likely contaminants. Crustal assimilation appears to be 

required for calc-alkaline rocks only by 51SO values, which 

range from lows of +5.6 to +6.0%o in HAOT and primitive 

basaltic andesites to +1.0%o in a dacite. Elevated 51 SO values 

may be attributed to interaction with relatively 1SO-rich, S7Srpoor deep crustal rocks such as igneous rocks of the Klamath 

Mountains and associated lower crust. Partially melted 

granodiorite, whose 1SOf160 ratio had been lowered by 

exchange with meteoric hydrothermal fluids, occurs as blocks 

in the climactic ejecta. Assimilation of such upper crustal 

granodiorite lowered 51SO values of rhyodacites without 

significantly affecting their magmatic composition in other 

ways. 

17 

TEXTURAL AND STABLE ISOTOPE DEVELOPMENT OF 

MARBLE ASSEMBLAGES DURING THE BARROVIANSTYLE M2 METAMORPHIC EVENT, NAXOS, GREECE. 

BAKER, J. Department of Earth Sciences, University of 

Cambridge, Cambridge CB2 3EQ, UK. and MATTHEWS, 

A. Institute of Earth Sciences, Hebrew University of 

Jerusalem, 91904 Jerusalem, Israel. 

The effects of fluid-rock interaction during metamorphic 

processes have been the focus of considerable attention over the 

past decade. The Miocene M2 metamorphic event on the island 

of Naxos in the Cycladic Archipelago provides a well known 

example in which fluid flow has been cited to be a primary cause of metamorphism. The purpose of this study is to 

examine this assumption through an analysis of the textural 

development and isotopic compositions of siliceous dolomite 

assemblages within high grade marbles. A detailed petrologic 

analysis of the siliceous dolomite assemblages shows that two 

distinct periods of mineral growth can be documented; the first 

associated with prograde M2 metamorphism and the second 

with high temperature retrograde M2 metamorphism occurring 

during the ductile extensional thinning of the complex. The 

textural and carbon and oxygen isotope characteristics of each 

generation have been determined, and the P-T-Xco2 conditions 

at which these two mineral generations were stable and the 

compositions of the fluids present during the metamorphism 

have been characterized. 

The P-T-Xco2 variations, low variance, and stable isotope 

compositions of dolomites and calcites of the prograde 

siliceous dolomite assemblages are consistent with internally 

buffered fluid evolution. In contrast the retrograde tremolitecalcite±talc mineral assemblages are shown to have grown as 

the result of the infiltration of a water-rich fluid phase that 

transported silica, Al203, Na20, and FeO into the host rocks. 

Carbon and oxygen isotope compositions of calcites of this 

second generation are in distinct isotopic disequilibrium with 

those of coexisting prograde dolomite (L\dol-cc(1So) varies 

from 5.5%o to 17.2%o); this precludes an origin through simple 

decarbonation reactions and requires the infiltration of an isotopically light fluid phase. The calculated oxygen isotope 

compositions of the waters in equilibrium with the retrograde 

calcites are consistent with their formation from crystallizing 

melts. The above observations, together with the fact that occurrences of veins of this type are limited to marbles in the 

highest grade areas (T>600°C) of the metamorphic complex, 

suggests that the fluids responsible for vein formation were 

generated during the crystallization of melts as the metamorphic 

complex cooled from peak metamorphic temperatures. 

The existence of this second generation of retrograde minerals 

has significant implications for previous studies of fluid 

transport on Naxos, because many of the unusually low alSo 

compositions of petites at high grades may be ascribable to the 

effects of interaction with retrograde M2 fluids rather than with 

prograde fluids. This view is strengthened by our observation 

of metapelitic assemblages whose quartz alSo values are lower 

by several permil than the quartz of coexisting prograde veins 

and adjacent whole-rock petites. Although the minerals within 

these petites show no obvious alteration or retrogression, the 

low quartz 51 So values suggest that they have been affected by 

high temperature retrograde infiltration 

Lowering of alSo in minerals of pelitic rocks of high grade 

metamorphic terrains down to values characteristic of igneous 

granites has been noted in a number of studies. The results of 

this study have identified a mechanism which could contribute 

to such a lowering, i.e., infiltration of water-rich fluids that have 

exsolved from melts generated during prograde metamorphism as these melts crystallize during cooling and ductile extensional 

tectonism. 
LASER FLUORINATION OXYGEN ISOTOPE 

EVIDENCE FOR CRUSTAL CONTAMINATION 

OF FLOOD BASALTS FROM YEMEN. 

BAKER. Joel A., MACPHERSON, C., MAITEY, D., 

MENZIES, M., and THIRLWALL, M., Department of 

Geology, Royal Holloway University of London, Egham, 

Surrey 1W20 OEX, UK. 

Laser fluorination (LF) oxygen isotope analysis of 

phenocryst phases from volcanic rocks has two principal 

advantages over conventional 0 isotope analysis: 1. small ca. 

1 mg samples can be rapidly analysed allowing easy replicate 

analysis of different mineral phases in a rock; 2. analysis of 

fresh mineral phases such as clinopyroxene bypasses the nearly 

ubiquitous effect of alteration on the 0 isotope composition of 

whole rock samples. 

0 isotope analysis of clinopyroxene, olivine and 

plagioclase phenocrysts from ca. 35 samples of flood and 

intraplate basalts from western Yemen suggests the basalts 

were derived from a homogeneous source with respect to 0 

isotopes. Clinopyroxene, olivine and plagioclase in 

equilibrium with crustally uncontaminated melts have restricted 

0 isotope compositions of 5.6, 5.2 and 6.0%c (±0.2%c). These 

values are identical to the range of o180 reported for mantle 

olivine and clinopyroxene analysed by LF. Basalts with Sr-NdPb isotopic ratios distinct from the Mar plume and Gulf of 

Aden/Red Sea MORB, in Rarticular those with low ENd and 

206pbf204Pb, high A207pbJ2 4pb and lithospheric trace element 

signatures, display more 0 isotope variation (5 .6-7 .O%c rel. to 

cpx), although this variation is small compared with the range 

commonly reported for conventional whole rock analysis. 

Clinopyroxene values upto 7 .O%c suggest the flood basalts 

assimilated continental crust en route to the surface. Moreover, 

the 0 isotope data supports evidence from correlations between 

indices of fractionation and radiogenic isotope data that AFC 

operated during magma storage and ascent. 

One high MgO basalt with low ENd and the most extreme 

A207pbf204pb might be interpreted as being derived from 

enriched lithospheric mantle (LM). However, it displays 

marked 0 isotope heterogeneity within zoned clinopyroxene 

phenocrysts. Clinopyroxene populations can be separated on 

the basis of colour, with zonation from deep green cores 

through to greenish brown, brownish green and finally dark 

brown/black types. Different clinopyroxene populations 

display the following correlated range in isotopic 

composition from early to late crystallizing clinopyroxene that 

nearly encompasses the entire isotopic range found in the 

Yemen flood basalts: 87Srf86Sr = 0.7035-0.7048; 143Ndf144Nd 

= o.51293-0.51273; 2°6Pbf2°4Pb = 18.65-17.97; o18o = 5.68-

6.87%c. Furthermore, olivine which crystallized before the 

brown, but after the green clinopyroxene, has a o180 of 6.3%c, 

which is not in equilibrium with either end-member pyroxene. 

The Sr-Nd-Pb-0 isotopic variation is attributed to phenocryst 

crystallization from a magma undergoing rapid assimilation of 

Pan-African continental crust whilst still hot and mafic. 

Modelling of the isotopic variation suggests the magma 

contains at least 20% continental crust. 

LF oxygen isotope analysis of mineral separates allows 

small differences in o 180 to be correlated with radiogenic 

isotope data, and allows rigorous identification of crustal 

contamination in suites of continental basalts. If the mantle is 

uniform with respect to 0 isotopes then this technique will 

prove a powerful test of the relative roles of enriched LM and 

crustal contamination in suites of continental flood basalts. 

18 

INTERCALIBRATION OF STANDARDS USED 

FOR 40 Arf39 Ar DATING. 

Baksi. Ajoy K., Dept. of Geology & Geophysics, 

Louisiana State Univ., Baton Rouge, LA 70803, 

USA), Archibald, D.A. and Farrar, E., Dept. of 

Geological Sciences, Queen's Univ., Kingston, 

Ont., Canada K7L 3N6). 

40 Arf39 Ar ages are reported by laboratories relative 

to a number of different standards. lntercomparison 

of such ages has proved problematical, and is 

perhaps most critical for geological time scale related 

studies. We report the results of two sets of 

experiments on a number of standards commonly 

used for argon dating work. The ftrst study utilized 

SB-3 Bio, LP-6 Bio 40-60#, FCT-3 Bio and MMHb1. The second used the same standards and also GA1550 Bio, Bern 4M and Bern 4B. Numerous splits of 

each sample were arranged within aluminium 

capsules, and irradiated in position 5C of the 

McMaster University Reactor. Samples were individually fused at 12000C and analyzed on a modified 

MS 10 mass spectrometer. 

Results were evaluated by plotting the J value 

versus height within each capsule, and yielded the 

best results with the following ages: SB-3 Bio = 

162.9 Ma (primary standard), LP-6 Bio = 128.1 Ma, 

GA-1550 Bio = 97.9 Ma, FCT-3 Bio =27.95 Ma, 

Bern 4M = 18.5 Ma and Bern 4B = 17.2 Ma. All 

these samples are homogeneous in replicate analysis 

in "age", K and Ca contents (based on manometric 

measurement of 39 Ar and 37 Ar, respectively). 

Individual splits of MMHb-1 yielded ages ranging 

from -512 Ma to -520 Ma, with an internal 1a 

precision of about 1.0 m.y.; though K and Ca 

contents appear reproducible from split to split, 

MMHb-1 is inhomogeneous in age at the -20 mg 

sub-sample level, and is unsuitable for for use as an 

interlaboratory standard. Splits of Taylor Creek 

Rhyolite Sanidine, Penn State OR-1A, Nancy biotite 

mica-Fe and Engels' Amphibole were also evaluated 

against these standards; attempts to flux the feldspar 

standards with degassed zero age basalt at 12000C 

were only partially successful. 

A step heating run on FCT-3 Bio yielded 100% gas 

on the plateau and an age of 27.9 Ma. LP-6 Bio 

shows a disturbed age spectrum, with step ages 

ranging between -120 Ma and -135 Ma; this appears 

to support a suggestion that LP-6 Bio 40-60# consists of two types of material ("blocky" and "flaky") 

with K-Ar ages of -119 Ma and -136 Ma. Our 

results suggest many of these standards are suitable 

for use as monitors in 40 Arf39 Ar dating work; SB-3 

Bio, LP-6 Bio 40-60# and GA-1550 at the -15-20 

mg subsample level, and FCT-3 Bio, Bern 4M and 

Bern 4B at the -30-35 mg subsample level. 
P-T-t PATHS OF AEGEAN METAMORPHIC CORE 

COMPLEXES: lOS, PAROS, AND SYROS 

BALDWIN. S.L , Dept. of Geosciences, University of 

Arizona, Tucson, AZ 85721, USA, and LISTER, G.S., 

VIEPS, Dept. of Earth Sciences, Monash University, 

Melbourne 3168, Australia 

In the Aegean, Miocene extension followed Eocene 

collision, and core complexes formed that resemble those in the 

Basin and Range Province. At least four metamorphic events 

have been recognized in the pre-Alpine gneissic basement (Mo), 

Eocene high-P, low-T metamorphic rocks <Mt). Miocene 

greenschists and amphibolites (M2), and contact metamorphic 

rocks (M3) associated with Aegean metamorphic core 

complexes. Thermochronologic and structural studies of shear 

zones and M 1 rocks of Paros, los, and Syros provide insight 

into the evolution of this polymetamorphic terrane, and the role 

of plutonism during the evolution of these core complexes. 

Undisturbed 40 Art39 Ar age spectra for white micas from 

Syros date M 1 at 50-54 Ma. However, the majority of white 

micas from Syros and los yielded disturbed age spectra which 

are interpreted to reflect variable partial outgassing and/or 

recrystallization ofMt white micas during subsequent M2 

metamorphism. The degree of overprinting varies considerably, 

but age spectra patterns are similar to those previously 

documented on Naxos and Sifnos. 

Biotite, muscovite, and K-feldspar from mylonitized 

granites on Paros indicate rapid cooling from 12-10 Ma. 

Results are concordant with previously reported Kl Ar ages on 

S-type granites, indicating cooling of granites occurred 

penecontemporaneously with shearing events which produced 

the spectacular S-C mylonites. 

Within the kilometer-scale Mylopotas ductile shear zone 

on los, 40 Art39 Ar ages record Mo-3 events. Boudins within the 

shear zone gave ages indicative of pre-Alpine basement, some 

mica schists record M 1 ages while others apparently record both 

M1 and M2 events. Analyses on biotite, muscovite and Kfeldspar from the S-C mylonites yielded Miocene ages with 

youngest apparent ages of 13 Ma. Preservation of Mo-1 ages 

suggest that temperatures were not sufficiently high to cause 

complete resetting of the schists and granite boudins caught up 

in the shear zone. The mylonites do not reflect a depthdependent transition but rather formed during a Miocene 

transient thermal pulse caused by the intrusion of granitoids. 

Modelling the effects of specific P-T -t histories on 39 Ar 

diffusion profiles obtained from white mica, K-feldspar and 

biotite in the shear zone on los suggests that ambient 

temperatures in the "basement" unit were lower than 250-270°C 

prior to the Miocene thermal event (i.e. less than the 

temperatures required to form greenschist facies mylonites), and 

that cooling subsequent to the thermal pulse may have involved 

timescales as short as 1o4- to5 yr. Rapid cooling continued 

after the Miocene thermal pulse as the core complexes were 

unroofed due to ongoing extension. The central granite gneiss 

terrane of los likely occupied a shallow crustal setting 

characterized by low ambient temperatures close to the foreland 

of the mountain belt that once defined the Aegean Alps. 

Subsequently the entire complex was stretched in a Miocene 

ductile shear zone resulting in recrystallization of some of the 

rocks to produce the S-C mylonites. 

19 

A HIGH-RESOLUTION LASER EX1RACfiON 

TECHNIQUE FOR STABLE ISOTOPE ANALYSIS OF 

CARBONATES: THE EFFECT OF LASER-SAMPLE 

INTERACTION ON THE COMPOSITION OF EVOLVED 

C02 

BALLJ D, Crowley,S.F., Marshall).D., 

~pt. of Earth Science, University of Liverpool, 

P.O.Box 147, Liverpool, L69 3BX, UK. 

A laser extraction system is under development at Liverpool 

for the ultra-fme scale analysis of carbonates. Using a highpower Nd:YAG laser, areas of carbonate -50 J.Ull in diameter 

are thermally decomposed to yield C02 gas. 

It is widely accepted that during the conversion of carbonate 

to C02 some isotopic fractionation occurs, and experiments 

performed to date confirm this. Hence, it is necessary to carry 

out a series of standardisation experiments, by analysing a 

range of isotopically homogenous materials using the laser 

extraction and traditional acid digestion methods, in order to 

establish a fractionation correction factor for the technique. 

The standardisation experiments carried out at Liverpool 

indicate that, while it is possible to establish a correction 

factor for a given material to a precision on 0.1 per mil, the 

implied correction varies depending on mineralogy, chemistry 

and physical composition of the sample. Therefore, no overall 

correction factor can be given for the technique. Recent 

experiments have shown that the difference in fractionation 

Jactors can be significantly reduced if close attention is given 

to the conditions under which the laser interacts with the 

sample and the C02 is extracted. 

An overview of the extraction system will be presented, 

including details of the laser, optical train, vacuum system and 

mass-spectrometry. The procedure for extracting, purifying and 

analysing the evolved C02 will also be discussed. 

Data from a range of standardisation materials of varying 

properties will be used to demonstrate possible problems 

associated with the technique and to support the fractionation 

correction factors quoted. 
ISOTOPE STUDIES OF THE SYSTEMATICS OF 

SULPHATE REDUCTION IN TWO CON1RASTING 

AQUIFERS 

BARKER. A.P., BOTTRELL, S.H., MONCASTER, S.J., 

Department of Earth Sciences, University of Leeds, Leeds, 

LS2 9JT, UK, LLOYD, J.W., and TELLAM, J.H., 

Hydrogeology Section, School of Earth Sciences, University 

of Birmingham, Edgbaston, Birmingham, B15 2TT, UK. 

Groundwater quality is controlled by many factors such as 

water-rock interactions and biological activity. Bacterial 

sulphate reduction is a process that can result in the degradation 

of water quality, and an understanding of the mechanisms of 

this process would be of great benefit to water resource 

evaluation. 

Data are presented on studies from two sites where sulphate 

reduction has occurred in groundwaters. One is in a PermoTriassic sandstone aquifer where sulphate reduction is believed 

to be a recent phenomenon. Here time-series data show increase 

in HC03- concentrations, decrease in SOi- and progressive 13C 

depletion in HC03- at certain depths. The difference in sulphur 

isotopic composition between s2- and SOi- (L\34Sso42--s2-) is 

18o/oo. The second site is in the deep confmed section of the 

Lincolnshire Limestone, a gently dipping Mid Jurassic 

limestone aquifer confmed by overlying clays. Here values of 

L\34Sso42--s2- of over 5<1Yoo are observed but HCQ3- is not 13C 

depleted. 

The data are modelled in terms of the effects of both bacterial 

sulphate reduction and groundwater mixing. At the sandstone 

site sulphate reduction appears to be proceeding at an 

appreciable rate, whereas in the limestone net rates of reduction 

are small as rates of 13C depleted HC03- production must be 

less than the rate of re-equilibration with CaC03 in the 

limestone matrix. Groundwater mixing best accounts for the 

observed chemical and isotopic distributions. However these 

very slow rates of sulphur redox reactions may be important in 

establishing large L\34Sso42--s2- values here, by effectively 

catalysing isotope exchange reactions promoting an isotope 

distribution close to equilibrium. 

Flow to wells in the Lincolnshire Limestone sites are 

dominated by fissure flow. Data on chemical and isotopic 

compositions of porewaters from a recently cored well will also 

be presented for comparison with fissure water compositions, 

as well as time-series data on the evolution of the new well 

since substantial perturbation of chemical and isotope 

compositions of groundwater were observed following drilling. 

20 

AN ISOTOPIC INVESTIGATION OF MAGMA 

CHAMBER PROCESSES AT THE TRANSITION 

BETWEEN TWO CYCLES IN THE BJERKREIMSOKNDAL LAYERED INTRUSION, S. NORWAY. 

BARLING J, WEIS, D. and DEMAIFFE D., Petrol. 

Geodyn. Chimique, CP160/02, Universite Libre de 

Bruxelles, Av. F.D. Roosevelt, 50, 1050 Brussels, 

Belgium. 

The Bjerkreim-Sokndal massif is a layered intrusion in 

the late Proterozoic Rogaland Anorthosite Complex of 

S. Norway 1 . The upper part of the intrusion is 

composed of mangerite and quartz-mangerite, whilst the 

lower part consists dominantly of plag-opx cumulates 

(anorthosite-norite) and comprises five macrocyclic units 

(MCU), subdivided in zones, which have been 

interpreted2 as reflecting repeated magma chamber 

replenishment events. The MCU III/IV boundary is 

marked by a strong compositional reversal of the main 

minerals (increases of An in plag, of En in opx and 

appearance of F075 olivine). Isotopic and mineralogical 

indicators of replenishment are displaced upward 

relative to the stratigraphically defined base of MCU IV. 

For this reason, Jensen et al. 3 propose that MCU IV 

started with a hybrid zone (IVa) before sufficient dense 

parental magma could pond on the floor of the chamber 

and crystallize as leucotroctolites (IVb). 

Plagioclase separates from a sequence of samples 

across the MCU III/IV boundary have been analysed for 

Sr, Nd and Pb isotope ratios and Rb, Sr, Sm, Nd, U and 

Pb concentrations. 87 SrJ86sn (940Ma) ranges from 

0.7048 (base of zone IVb) to 0.7061 (llle) together with 

systematic changes in 143Ndf144Ndi (0.51155 (ENdi 

=+2.5) to 0.51143 (ENdi =+0.1)) and Pb isotopic ratios 

(206pbJ204pbi =17.49 to 17.73; 207pbJ204pbi = 15.52 

to 15.54). The isotopic data are consistent with the 

influx of a batch of magma, whose isotopic composition 

is reflected by the plagioclases at the base of IVb into a 

magma chamber containing a resident magma whose 

isotopic composition is represented by plagioclases 

from zone llle. The positive ENdi value (+2.5) of 

plagioclase crystallized from the inferred replenishing 

magma indicates that this incoming melt came from a 

source either in the depleted upper mantle or recently 

derived therefrom. Whilst the higher 87srJ86sr, and 

206pbJ204pbi and lower 143Ndf144Ndi values of the 

resident magma suggest that it has been contaminated 

by continental crust. 

These data enable us to divide the transition zone at 

MCU III/IV boundary into two parts. The lower part, 

reflecting a magma chamber replenishment event (llle 

to the base of IVb) approximates to simple binary 

mixing, whereas the upper part (IVb - IVd) shows 

evidence for more complex processes. 

1. Michot, J., and Michot, P., 1969, New York State 

Museum Service Memoir 18, 399-41 0. 

2. Duchesne, J.-C., 1972, J. Petrology. 13, 57- 81 . 

3. Jensen, J.C. et al., 1993, Lithos. 29, 311-325. 
STRUCTURAL CONTROLS ON VOLCANISM IN THE 

NORTHERN KENYA RIFT 

BARREIRO Barbara, NERC Isotope Geosciences 

Laboratory, Keyworth, NG12 5GG, United Kingdom, 

and DUNKLEY, Peter and SMITH, Martin, British 

Geological Survey, Keyworth, NG12 5GG, United 

Kingdom 

Crustal thickness, topography, lithospheric 

heterogeneity, and magma plumbing systems all affect the 

chemistry of volcanic rocks in the Kenya Rift. The largest 

contrast is between volcanoes in the south, erupted through 

Archaean craton and its reworked margin, which exhibit 

many characteristics suggesting derivation from 

continental lithosphere, and those of the north, erupted 

through Proterozoic mobile belt. Volcanoes in the 

northern rift, between Lake Baringo and Lake Turkana 

(0°30'N to 2°30'N) show more subtle changes from south 

to north, due to structural controls. 

The shield volcanoes immediately to the north of Lake 

Baringo (Korosi, Paka, and Silali) are composed of 

bimodal, weakly-undersaturated transitional basalts and 

trachytes. Magma was stored and fractionated in large 

magma chambers, which were drained to form calderas. 

Paka and Korosi show Nd isotope evidence for two distinct 

magma sources (Paka, eNd =3 and 6, Korosi, eNd=3 and 5), 

similar to Silali (MacDonald et al.): trachytes have higher 87SrJ86Sr than basalts. Paka and Silali tend toward 

elevated 207Pbj204Pb relative to 206Pbj2°4Pb (15.53 at 18.4 

to 15.65 at 19.4), although extra-caldera flank eruptions 

have lower 207PbJ204Pb. 

At 1 °30'N, Emuruangogolak is situated on a major 

ductile-brittle shear zone within the Proterozoic basment. 

Between this shear zone and Lake Turkana. volcanoes are 

characterised by uniform but distinct eNd 

(Em uruangogolak=3 .8-4 .4, Namarunu=5 .6-7, Barrier=4. 8-

5.5) and low 207PbJ2°4Pb (15.57-15.62) relative to 

206pbj204pb (19-19.7). Trachytes and phonolites have the 

same Nd and Pb isotopic compositions as the basalts, but 

elevated 87Sr/86Sr, suggesting only small degrees of crustal 

input. 

The trend towards 'plume' isotopic signatures going 

from south to north in the Gregory Rift correlates with 

absence of evidence for large-scale crustal magma 

chambers, a decrease in elevation of the rift floor, 

narrowing of the rift, and decrease in depth to crystalline 

basement. 

21 

EVIDENCE FOR CLOSED-SYSTEM BEHAVIOR OF 

232.rb-208Pb IN ALLANITE UNDER HYDROTHERMAL CONDITIONS: A STUDY OF ALTERED 

RHYOLITE AND GRANODIORITE FROM THE 

ATESINA • CIMA D' ASTA VOLCANO-PLUTONIC 

COMPLEX (N ITALY) 

BARTH. Susanne*, OBERLI, Felix, and MEIER, Martin, 

Institut ftir Kristallographie und Petrographie ETH, 

Sonneggstr. 5, CH-8092 ZUrich, Switzerland 

* Now at: Centre de Recherches Petrographiques et 

Geochimiques, C.R.P.G.-C.N.R.S., BP 20, F-54501 

Vandreuvre-les-Nancy Cedex, France 

Magmatites from the Early Permian Atesina volcanic 

complex (A VC) and Cima d' Asta pluton (CAP) display 

variable degree of major/trace element mobility, open-system 

behavior of total-rock Rb-Sr, and shift in oxygen-isotope 

signatures induced by Triassic water-rock interaction. Totalrock REE and Sm-Nd isotopic signatures, however, are not 

affected by hydrothermal overprint. Oxygen isotopic 

evidence points to alteration temperatures of -210° C and 

160° C for AVC rhyolite and CAP granodiorite, respectively, 

assuming exchange with sea water (Barth et al., 1993). 

Single allanite crystals whose outer parts have been 

removed by abrasion prior to isotope analysis yield 232rh208Pb ages (95% c.l.) of 276.2 ± 3.2 Ma and 276.3 ± 3.0 Ma 

(n=2; 276.3 ± 2.2 Ma mean) and identical ages of 274.0 ± 

2.9 Ma to 277.0 ± 3.0 Ma (n=4; 275.5 ± 1.5 Ma mean) for 

hydrothermally altered rhyolite and granodiorite, respectively. 

The allanite Th-Pb ages conform to published Rb-Sr totalrock isochron ages of CAP granitoids (275 ± 9 to 274 ± 2 

Ma; 1a) as well as to published Rb-Sr and K-Ar biotite 

isochron ages of A VC rhyolites/rhyodacites. This indicates 

that the Th-Pb isotope system in allanite is virtually 

unaffected by hydrothermal overprint. In contrast, an allanite 

sample from the granodiorite showed considerably 

rejuvenated 238u-206Pb and 235u-207Pb ages (i.e., 264.3 ± 

1.2 Ma and 258 ± 18 Ma, respectively; ages not corrected 

for initial radioactive disequilibrium), pointing to differential 

loss of radiogenic 206pb and 207Pb as compared to 208Pb, 

presumably by exchange with hydrothermal fluids. This 

observation suggests the existence of domains in allanites 

characterized by distinct Th/U and varying stability with 

respect to hydrothermal fluids, giving rise to preferential loss 

ofU-derived Pb isotopes. This clearly underlines the superior 

stability of the Th-Pb system in allanite in comparison to its 

U-Pb systems and - in conjunction with the insensitivity of 

the Th-Pb ages to initial radioactive disequilibrium - makes 

the Th-Pb method the favorite choice for dating even 

hydrothermally altered magmatites. 

Barth, S., Oberli, F., Meier, M., Blattner, P., Bargossi, G.M., 

and Di Battistini, G. (1993): Geochim. Cosmochim. Acta, 

v. 57' pp. 4285-4300. 
THE GRANITE BELT ON THE EASTERN PART OF 

SOUTHERN BRAZIL AND URUGUAY 

BASEL M.A.S., Institute de Geociencias/Universidade de 

Sao Paulo, Caixa Postal 20899, 01498-970 Sao Paulo, SP, 

Brazil and HA WKESWORTH, C.J., The Open University, 

Department of Earth Sciences, Milton Keynes, MK7 6AA, 

U.K 

The eastern part of Southern Brazil and Uruguay is 

composed mainly by granitic rocks generated during the Rio 

Doce Orogeny whose ages are in the 620-440 Ma interval. 

This Granite Belt (GB) that is younger than the BrasilianoPan Mrican magmatism (700-620 Ma) observed in the 

adjacent Schist Belt (SB) is represented by granitoid rocks that 

can be roughly grouped in three major igneous suites: 

S.A.Imperatriz, S.P. Alcantara and Pedras Grandes. 

Geochemical and mineralogical data show several 

differences between the GB magmatism and that observed in 

the SB where alkaline and peraluminous granites predominate 

when compared with the more expanded calc-alkaline to 

alkaline suites of the GB. On Harker diagrams both domains 

have a small silica range with the high concentrations related 

to the two mica granites (SB) and the best trends related to the 

S.P. Alcantara Suite. The GB granites also show a smaller 

spread, steeper slope and higher concentration ofREE than 

the granites in the SB. 

The isotope data emphasize the differences pointed out by 

the geochemical data and strongly suggest the involvement of 

two different source areas in the generation of these 

magmatisms. In both cases important crustal component is 

recognized with &Nd ranging from -3.4 to -8.3 in the GB and 

from -10.1 to -20.4 in the SB. Model Nd ages are consistently 

different with values around 2.0 Main the GB and 1.6 Main 

the SB. 

The NE-SW contact between the two belts can be 

discontinuously recognized for more than 1200 Km, always 

parallel to a strong negative gravimetric anomaly. It is 

characterized by a set of low to high temperature shear zones 

where protomylonites to mylonites predominate. This 

megastructure of lithospheric scale is interpreted as a Lower 

Paleozoic (535 Ma) suture zone developed during the collision 

between the Granite Belt and the marginal cratonic 

volcanosedimentary sequences represented by the Brusque, 

Porongos and Lavaleja Groups. 

22 

DATING OF SPELEOTHEMS USING EXCESS 210pb: 

IMPLICATIONS TO THEIR USEFULNESS AS PROXY 

PALEOCLIMATIC RECORDERS 

BASKARAN. M., Department of Marine Sciences, and 

ILIFFE, T. M., Department of Marine Biology, Texas A&M 

University, Galveston, TX 77553-1675, USA. 

It has been shown recently that presently growing 

speleothems can be dated using excess 210Pb (Baskaran and 

lliffe, 1993). Subsequent study showed promise that these 

speleothems can be used as a proxy to retrieve a13c variations in 

almospheric C02 (Baskaran and Krishnamurthy, 1993). We have 

extended our study to many more speleothems, including 

stalagmites from caves in the San Saba and Travis Counties in 

Texas. This study provides information on the following 

aspects: (i) the uniformity on the growth rates at various 

segments of a speleothem (ii) the discontinuity of the growth 

rates, if any and (iii) the inventory of excess 21 Opb and its 

relationship to the water drip rate. 

The excess 210Pb (210pbxs) concentration in the uppermost 

layers of speleothems sampled varies between 51 and 200 dpm/g. 

The 210J>tlxs based growth rates vary typically between 0.01 and 

0.1 mm yr·l. A model discussing the growth rates of stalagmites 

and stalactites will be presented. 

cu 

.c 

.... 

E 

f .... _ 

E 

~.s 

:; cu 

;~ "0:::1 II) 

ii 

... 

cu 

.... 

l'll 

...J 

l.n (excess 210 Pb)- dpm/g 

0 , 2 3 4 5 

Or-----~r-----~~----~--------~--------~ 

--y - 6.1 32 + -1.1234x R- 0.97962 • 

2 

3 

4 

• 5 

Baskaran, M. and T. M. lliffe. 1993. A~e determination of recent 

cave deposits using excess 21 Pb- a new technique. 

Geophys. Res. Lett 20: 603-606. 

Baskaran, M. and R V. Krishnamurthy. 1993. Speleothems as proxy for the carbon isotope composition of atmospheric 

C02. Geophys. Res. Lett. 20: 2905-2908. 
TEMPORAL ISOTOPIC VARIATIONS IN THE 

HAWAIIAN MANTLE PLUME, THE LANAI 

ANOMALY, THE· MOLOKAI FRACTURE ZONE AND A 

SEAWATER-ALTERED LITHOSPHERIC COMPONENT 

IN HAWAIIAN VOLCANISM 

BASU, ASISH R. and FAGGART, B.E., Dept. of Earth 

and Environmental Sciences, University of Rochester, 

Rochester, New York, 14627, USA. 

Several recent studies have documented the temporal 

variations in major and trace elements and Nd, Sr, Pb and 

He-isotopic ratio variations in single Hawaiian volcanoes. In 

this study we examined Nd and Sr-isotopic variations on 

forty-five basaltic rocks (collected during the 1985 

ALCYONE expedition) from both the Hawaiian Islands and 

the leeward islands to the northwest. These data demonstrate 

relatively limited isotopic variations over most of the 1900 

km long and 27 million years of volcanism examined here. 

However, there is a short-lived but major shift in isotopic 

compositions towards enriched values that center about the 

island of Lanai. The Lanai rocks (ENd = 2.1-5.5; 87Sr/86Sr 

= 0.70388-0.70455) are similar to those ofKoolau, though 

with somewhat more radiogenic Sr. Lanai, Koolau and 

Kahoolawe all show extreme variation in Sr and Nd isotopic 

compositions relative to other Hawaiian tholeiites. These 

three volcanoes thus appear more closely related to one 

another and are of great importance in understanding 

Hawaiian volcanism. As the Molokai Fracture Zone crosses 

the Hawaiian trend near these volcanoes, we suggest that it 

has allowed a tremendous influx for about 3 Ma of a moreenriched lower mantle plume component to swamp the 

isotopic signatures of other source-components. 

The Hawaiian Sr, Nd-isotopic data are distinguished by 

stages of volcanism; the progression reported by Chen and 

Frey for the Haleakala volcanics is generally supported over 

the entire Hawaiian chain. The tholeiitic shield to post-shield 

alkalic to post-erosional alkalic volcanism correlates with 

decreasing 87Sr/86Sr and increasing E d" However, in the NdSr correlation plot as the MORB-fleld is approached, Srisotopic ratios are increasingly displaced to the right of the 

mantle array. This observation clearly suggests a seawateraltered ancient lithospheric component in Hawaiian 

volcanism. 

23 

NITROGEN AND CARBON ISOTOPIC 

COMPOSITION OF ORGANIC MATTER IN 

PRECAMBRIAN CHERTS. 

BEAUMONT, V, JA VOY, M., Geochimie des 

Isotopes Stables, Universite de Paris 7, Paris, 

75251 cedex 05, France, and ROBERT, F., 

Mineralogic, Museum National d'Histoire 

Naturelle, Paris, 75005, France. 

Carbon isotopes in sedimentary organic matter 

(kerogen) have been shown to reflect the isotopic 

composition of living organisms (Galimov, 1980), 

i.e., the signature of carbon photosynthesis via 

enzymatic processes. In Precambrian environments, 

the difference between carbon isotopic 

compositions of kerogen in cherts and in bulk 

carbonates is featuring the reality of photosynthesis 

as early as 3.5 Ga (Schidlowski, 1988). 

Nitrogen is included - during early diagenesis -

in stable sites of kerogen heterocycles (Behar et al., 

1987). This situation limits isotopic exchanges. 

Then, nitrogen isotopic compositions of organisms 

are preserved in kerogen, featuring the metabolism 

and /or the redox conditions of environments. Few 

previous data exist on the nitrogen isotopic 

compositions of Precambrian kerogen, primarily 

because of experimental difficulties. 

Both nitrogen and carbon isotopic compositions 

have been measured in HF-HCl residues from 

Precambrian cherts of ages ranging from 3.5 Ga to 

2.0 Ga. Data are compared to those obtained from 

Phanerozoi·c samples. 

.As documented by previous workers, bl3C values 

of Precambrian organic matter are depleted in 13C 

relative to those from the Phanerozoic. Possible 

reasons are discussed by Schidlowski et al. (1983). 

b15N values of Precambrian organic matter show a 

depletion in 15N for oldest samples (3.5 Ga), with 

b15N values as low as -6o/oo (exceptional in 

Phanerozoic environments). This depletion 

progressively disappears with decreasing age and as 

early as 2.0 Ga years ago, b15N distribution is 

similar to the modern one. Three hypotheses can be 

made concerning the origin of this geochemical 

variation : (1) a late diagenesis effect (previous 

works has shown this hypothesis improbable, as an 

opposite effect with burial is usually observed)~ 

and/or (2) a source effect (depletion in l5N of the 

early atmosphere)~ and/or (3) a metabolic effect (if 

so, the nitrogen isotopic composition is a criteria for 

studying the evolution of metabolic processes 

linked to atmospheric oxygen level). The likelihood 

of each aspect will be developed and discussed. 

Galimov, 1980, Kerogen, Ed Technip, 271-298. 

Behar eta/., 1987, Org. Geochem 11, p.15. 

Schidlowski, 1988, Nature 333, 313-318. 

Schidlowski et al., 1983, Earth's Earliest Biosphere. 

Princeton University Press, 149-186. 
APPLICATIONS OF NITROGEN ISOTOPES IN 

STUDIES OF HIGH-TEMPERATURE FLUIDROCK INTERACTIONS 

BEBOUT. G. E., Dept. Earth & Environ. Sci., 

Lehigh University, Bethlehem, PA 18015 

Many igneous and metamorphic rocks contain 

appreciable amounts of N, primarily as structurally 

bound NH4+; however, the N isotope system has not 

been extensively exploited as a tracer of high-T 

fluid-rock interactions and other mixing processes in 

the crust and mantle. Recent studies demonstrate 

that N isotope compositions of silicate mineral and 

whole-rock samples can be routinely measured with 

accuracy and precision approaching that of the other 

stable isotope systems (see Bebout and Fogel, 1992). 

Several characteristics of the N isotope system 

make it suitable for tracer studies. First, significant 

observed variation in o15N of igneous and metamorphic rocks is consistent with calculated N-isotope 

fractionations; the calculations predict significant Nisotope variation as a result of devolatilization, 

metasomatic alteration, and magma volatile release. 

Second, because N abundance in the crust is largely 

tied to its fixation by organic processes in sedimentary environments, the N isotope system may be particularly sensitive in tracing sediment-derived fluids. 

Third, because N is generally a trace element, N 

isotopes may exhibit differing sensitivity to fluidrock interactions relative to other isotope systems 

and may thus provide unique information regarding 

metasomatism in igneous and metamorphic settings. 

A study of the distribution of N isotopes in the 

Catalina Schist subduction complex, California, 

demonstrates significant N-isotope fractionation 

during devolatilization and metasomatic processes. 

The overall variation in o15N in the Catalina Schist 

(- 5.5%o) is easily resolved using combustion extraction techniques and conventional gas-source mass 

spectrometry; precision of the measurements (1a for 

n ~ 3) is typically near 0.1 o/oo. Study of devolatilization and large-scale fluid transfer in the Skiddaw 

Granite and Aureole (England; Bebout et al., 1993) 

reveals > 10%o variation in o15N. Large, systematic 

N isotope variations in these suites indicate great potential of theN system to yield information regarding sources of fluids and silicate melts, fluid-rock interactions, and crust-mantle exchange. Continuing 

improvements in techniques for routine extractions 

and mass spectrometry of N will make the N-isotope 

system yet more accessible in petrologic studies. 

Bebout, G. E., and Fogel, M. L., 1992, Geochim. 

Cosmochim. Acta, 56,2839-2849. 

Bebout, G. E., Bradley, A. D., and Cooper, D. C., 

1993, Geol. Soc. Amer. Abstr. Progr, 25, 78. 

24 

40ARf39AR DATING OF YOUNG LOW-K THOLEIITES: 

EXAMPLES FROM NORTHEAST CALIFORNIA, U.S.A. 

BECKER T A., SHARP, W.D., RENNE, P.R., Institute 

of Human Origins Geochronology Center, 2453 Ridge 

Road, Berkeley, CA, 94709, U.S.A., 

TURRIN, B.D. ,U.S. Geological Survey, MS 901, 345 

Middlefield Road, Menlo Park, CA, 94025, U.S.A., 

PAGE, W.D. Pacific Gas and Electric Co., Geosciences 

Dept., P.O. Box 770000, San Francisco, CA, 94177, and 

W AKABA Y ASm, J. ,Consultant, 1329 Sheridan Lane, 

Hayward, CA, 94544, U.S.A. 

Young low-K (CalK- 25-40) basalts pose challenges to KAr and 40 Arf39 Ar dating because of their low radiogenic Ar 

contents and resulting sensitivity to corrections for blanks, mass 

discrimination, nucleogenic Ar isotopes and trapped Ar of 

anomalous isotopic composition. 

We have developed an automated resistance furnace, 

employing an infra-red pyrometer, that provides reproducible 

blanks of <IQ-16 mol for m/e = 40 at 1200 °C. Heating time 

and residence time in the extraction system for gas purification 

are adjusted for minimizing blank contibution and optimizing 

isotope intensity measurement. Full automation of the furnace, 

along with the extraction line and air pipette, allows unattended 

operation and makes frequent characterization of procedural 

blanks and spectrometer discrimination feasible. 

Our M.A.P.215-50C mass spectrometer is configured for a 

resolution of 450, making 40 Arf3"9 Ar ratios of 100 or more 

acceptable. This allows neutron fluences to be reduced, and in 

conjunction with thermal neutron shielding and detailed 

characterization of production ratios, minimizes the effects of 

nucleogenic Ar isotopes. 

Incremental heating produces intermediate-temperature steps 

with enhanced radiogenic Ar contents and provides data for 

isochron analysis, revealing subtle anomalous trapped Ar 

(40Arf36Ar in the range of297-305) in many samples. Samples 

may yield sensu stricto plateaux, but have well-defined isochron 

ages that are significantly younger. Two samples, potentially 

from different flows, yield distinct plateau ages of 493 ± 22 and 

423 ± 7 ka, but give indistinguishable isochron ages of 409 ± 

18 and 415 ± 3 ka, with initial 40Arf36Ar of 304.6 ± 1.3 and 

301.2 ± 0.9, respectively. Another flow yielded well-defined 

plateau ages of 1113 ± 81, 921 ±51, and 907 ±51 ka from 

three closely-spaced samples, whereas isochron ages of 889 ± 

136, 834 ± 51, and 879 ± 40, and initial 40 Arf36 Ar of 297.8 ± 

1.4, 297.8 ± 1.0 and 296.3 ± 0.9, respectively, were obtained. 

Basalts with lower CalK (<15) are much less sensitive to 

slight departures in the trapped Ar from atmospheric air ratio. 

One such flow yields a plateau age of 68 ± 18 ka and an 

indistinguishable isochron age of 50 ± 16 ka with initial 

40 ArP6 Ar of 296.8 ± 1.0. 

These results indicate that detailed 40 Arf39 Ar incremental 

heating analyses are needed for precise and accurate dating of Late 

Pleistocene low-K basalts. 
DEPLETED 238U AND EXCESS 234U AND 230TH: 

ENHANCED DISEQUILIBRIUM OF U-SERIES ISOTOPES 

IN HYPERSALINE MEDITERRANEAN BRINES 

BEETS C J <Kay), WASSERBURG, GJ., and CHEN, 

J.H., Lunatic Asylum, Division of Geological and 

Planetary Sciences, California Institute of Technology, 

Pasadena, CA 91125, and DE LANGE, GJ., Institute of 

Earth Sciences, Department of Geochemistry, B udapestlaan 

4, 3584 CD Utrecht, The Netherlands 

Anoxic hypersaline brines in the Bannock Basin, Eastern 

Mediterranean Sea, show extreme physico-chemical properties. 

High density (1.21) due to salinities of more than 10 times that 

of normal seawater make the configuration of the brine stable, 

separated from the overlying waters by a well-defined interface. 

The brine is further characterized by lack of oxygen, a high 

concentration of H2S (-2.5 mM) and compositional layering. 

The upper brine (I) has a slightly lower salinity and contains 

more Ca and Sr and less Mg, S and S04 than the lower brine 

(II); however, both Brine I and II are believed to result from 

redissolution of Messinian late-stage evaporitic salts which 

outcrop in the basin wallsl. 

We analyzed unfiltered 0.5-1 liter samples of overlying 

Mediterranean seawater and of the two brines for U- and Thisotopic compositions and abundances by TIMS. Normal 

Mediterranean seawater has a 238U content of 3.56 ng/g 

(salinity of 38.68 psu) which is slightly higher than that of 

open ocean water ( 3.24 ng/g: North Pacific2), and a234U of 

151.8 ± 12.9 o/oo (vs. 144 ± 2 for North Pacific seawater2). 

Both Brine I at a waterdepth equivalent to 3330- 3470 dbar, and 

Brine II (3530 - 3784 dbar) show extreme depletion of 238U, 

with concentrations of 19 pg/g 238U and 17 pg/g, respectively, 

which are -200 times lower than seawater. The a234U in the 

brines, on the other hand, shows a large enrichment in 234U 

over seawater; a234U of Brine I is 223o/oo and that of Brine II is 

266o/oo, indicating a 234U enrichment over normal seawater of 

50% and 77%, respectively. The 23<>Th and 232Th contents of 

the brines are also highly enriched; with 0.15 fg/g 230Th and 

3.98 pg/g 232Th in Brine I and 0.06 fg/g 230Th and 2.01 pg/g 

232Th in Brine II vs. 0.004 fg/g 230Th and 0.12 pg/g 232Th 

for seawater. This results in -5 times higher 230Thf232Th 

ratio for the brines. In addition the 232Thf238U ratios of the 

brines are also much higher than that of seawater: 0.21 for 

Brine I and 0.12 for Brine II vs. 33.9 x IQ-6. Scavenging on 

particles alone cannot account for the relative 230Thf232Th and 

232Thf238U enrichments; an additional source besides seawater 

has to supply both 230Th and 232Th. Moreover, the Sr-isotope 

composition of the brines3 also shows a strong deviation from 

that of seawater (0.70917): 0.70861 for Brine I and 0.70866 for 

Brine II, both values close to those reported for Messinian 

evaporites (0.70801-0.70886). 

The excess of 234 U is consistent with enrichment due to 

recoil of 234U in ground waters with long residence times and 

high water-rock interaction, and suggests that outflow of 

hypersaline groundwater from the Messinian evaporites is a 

likely source for the brine. This would also provide the 230Th 

and 232Th to the brines, explaining the Th excesses, and would 

be consistent with the Sr-isotope values of the brines. 

Division contribution 5372 (846) 

1] De Lange et al., 1990. Mar. Chern. 31, 63. 

2] Chen et al., 1986. EPSL 80,241 

3] De Lange et al., 1990. Mar Chern. 31, 89. 

25 

CADOMIAN CONTINENTAL GROWTH RECORDED IN 

METASEDIMENTS AND HERCYNIAN GRANITOIDS OF 

THE IBERIAN MASSIF, N. PORTUGAL. 

BEETSMA, J.J. and ANDRIESSEN, P.A.M., Dept. of 

Petrology & Isotope Geology, Free University, 1085 HV, 

Amsterdam, The Netherlands. 

The Iberian Massif forms part of the European Hercynides, and 

is occupied by metasedimentary successions of Late Proterozoic 

to Paleozoic (LPP) age. The basal parts of the terrigenous 

sequence are affected by partial melting associated with the 

generation and intrusion of a variety of Hercynian granitoids, 

which include 350-310 Ma syntectonic (sub)alkaline granites 

and 320-280 Ma late/posttectonic calcalkaline granitoids. 

The Nd isotopic evolution recorded in the Late Proterozoic 

( ENd[i] -5.3), Late Proterozoic/Cambrian ( -1.8), Ordovician 

(-9.5) and Silurian (-9.0) metasediments is not compatible with 

derivation from a single crustal source. It signifies the addition 

of juvenile crustal material to the sedimentary mass during the 

Late Proterozoic/Cambrian, in an active tectonic setting related 

to the Cadomian/Pan-African orogenic period. The geochemical 

signatures including high Zr contents (220 vs. -200 ppm) and 

depressed Th/Sc (0.55 vs. >0.65) and La/Cr (0.55 vs. >0.7) 

ratios are typical for terrigenous sediments deposited in active 

continental settings, relative to passive marginal/cratonic 

settings for the remaining LPP sedimentary units. The 

abundance of Cadomian/Pan-African juvenile components in the 

metasediments contrasts with the scarcity of Paleozoic 

(Caledonian) juvenile components, which are commonly present 

in the Central European Hercynides. This results in a bimodal 

distribution, rather than a continuous range, of Nd isotopic 

signatures of the LPP metasediments and its basement 

constituting the Iberian upper and middle crustal protoliths for 

Hercynian partial melts. 

The Nd isotopic signature of the crustal-derived 

(sub)alkaline Hercynian granites shows a bimodal distribution 

with culminations at ENd[i] -3.0 to -5.5 and -7.0 to -10.0. These 

values are interpreted to represent two discrete protoliths and 

correspond to the Hercynian Nd isotopic signature of LPP 

metasediments containing and lacking juvenile components 

respectively. The calcalkaline Hercynian granitoids are 

characterized by a large but continuous range in trace element 

geochemistry and Nd-isotopic compositions (eNd[i] +0.1 to 

-7 .2). The (isotope) geochemical variation is attributed to 

hybridization of mantle-derived gabbroic magmas with crustalderived partial melts with similar (isotope) geochemical 

compositions as the (sub)alkaline Hercynian granites. 

The presence of Cadomian/Pan-African juvenile components 

in Hercynian granitoids of the Iberian Massif shows that crustal 

recycling is an efficient and important process. The potential for 

Nd isotopic systematics to constrain the relative I age of crustal 

protoliths for partial melts in a particular region thus depends 

on the rate of succession of earlier orogenic periods and the 

amounts of juvenile detrital components delivered to the 

sedimentary mass during these periods. 
lllGH DYNAMIC RANGE SIMS MEASUREMENTS USING 

THE ISOLAB-120 

Belshaw, N. S., O'Nions. R. K., von Blanckenburg, F., and 

Gibb, A. (Dept. of Earth Sciences, Univ. of Cambridge, 

Downing St. Cambridge, CB2 3EQ, U.K) 

Several isotope systems of importance in the Earth sciences 

require high dynamic range measurements which present a 

number of technical challenges. Examples of these are 

Beryllium and Thorium isotopes where in the case of 

Beryllium a dynamic range of -lo-7 to w-10 for 10Be/9Be in 

natural materials is required. Be is ionised efficiently by ion 

beam sputtering, with a yield of >= 0.1 %. Th on the other 

hand which requires a range of -lo-5 to w-7 for 

230Th/232Th can be measured by thermal ionisation but 

displays an improved ionisation efficiency of -1% by 

sputtering, a factor of 10 higher than for thermal ionisation. 

The high abundance sensitivity of the ISOLAB-120, <10-ll 

for Be and <lo-8 for Th, combined with its SIMS capability 

makes it suitable for these measurements. 

Techniques have been developed for the routine separation 

and measurement ofBe and Thin a range of natural materials. 

While the lOBe concentration of even the most enriched 

natural samples is insufficient for in-situ measurement, the 

study of Th indicates areas where the high mass resolution and 

high transmission capabilities of the ISOLAB-120 may allow 

in-situ measurement of 230Th/232Th. 

The technique for high dynamic range measurements using 

the ISOLAB-120 will be detailed, along with developments for 

in-situ applications. 

26 

THE MANTLE SOURCES OF BASALTS FROM CANARY 

ISLANDS WITH REFERENCE TO PB-SR-ND ISOTOPIC 

SYSTEMATICS 

BELYATSKY.B.V.,VNIIOkeangeologia, Maklina 1, 

St.Petersburg 190121, Russia, OVCHINNIKOVA,G.V.,and 

LEVSKY,l.K., Institute of Precambrian Geology and 

Geochronology RAN, Makarova 2,St.Petersburg 199034, 

Russia 

Canary islands are usually divided into three groups 

according to the distance from the African continent. 

This position is determined by the differences in the 

structure of the crust of each island. 

The basalts from ancient and young volcanic serieses 

from each group of islands as well as mantle xenoliths 

enclosed in these basalts have been investigated. On 

the 207Pb/204pb-206Pb/204pb and 208Pb/204pb-206Pb/204pb 

plots the isotopic ratios of Pb have trends with regular decreasing of 206Pb/ 204Pb values from the most radiogenic for the island of La Palma (19.846) up to the 

least radiogenic values for basalts from the island of 

Lanzarote (19.506-19.064). The obtained data in the 

frame of contamination model prove that the mantle 

source for the investigated volcanic rocks originated 

as a result of mixing of the substances from the reservoirs HiMu, OM and EM in different proportions. The 

most valuable contribution of HiMu have the basalts 

from the western is land of La Palma and on the eastern 

islands (Tenerife, Gran Canaria) the contribution of 

HiMu decreases with the increasing of the role of the 

depleted mantle enriched a Litle bit with EM. Basalts 

from the island Lanzarote which is situated more close 

to the continental boundary correspond to the source 

of depleted mantle contaminated with EM. The contribution of HiMU is very insignificant or doesn't occur 

at all. 

Sr-Nd isotopic data of the investigated basalts 

haven't such distinct evidences.The range of isotopic 

data for all islands is the following: 87srfi&sr: 

0. 703082-0.703410, l43 Ndt144Nd: 0.512802-0.513037. 

Data points are mainly in the field of MORB near the Hi 

Mu. Nd-Sr data of basalts from the island of Lanzarote 

unlike the Pb isotopic data do. not fall into the field 

of enriched DM. The contradiction can be explained by 

the fact that the material of the end member of the enriched mantle under the island of Lanzarote can be mel 

ted out of the continental lithospheric mantle of the 

West-African shield during the desintegration of Pangea. In case that contaminated lithosphere was rather 

young (-200 m.y.) and according to valuable difference in time of decay of 235u and 147sm the 143Nd/144Nd isotopic ratio in modern basalts mustn't be very different from MORB and206pb/204pb ratio must increase. 

Comparison of isotopic data for Canary with the same 

data for NES and Ahaggar evidences the similar mantle 

source under these volcanoes. At the same time the 

nearest analogue of the basalts from Canary is volcanics fromAzore islands connected with young hot spot. 
EARLY ARCHEAN HIGHLY DEPLETED 

MANTLE: Nd AND U-Pb ZIRCON EVIDENCE 

FROM SOUTHWEST GREENLAND 

BENN£TI, Vickie C., Allen P. Nutman and 

Malcolm T. McCulloch, Research School of 

Earth Sciences, The Australian National 

University, Canberra, ACT 0200, Australia 

The excellent exposures, good state of 

preservation and large areal extent of > 3700 Ma rocks 

combined with the wide diversity of rock types makes 

southwest Greenland a key area for the study of early 

Archean geochemical processes. Extremely positive 

initial eNd values determined for a suite of well dated 

3870 Ma to 3760 Ma metadiorites and tonalites from 

this area demonstrate the existence of a LREE 

depleted mantle reservoir with an eNd value of =::+4 at 

3800 Ma. Although initial Nd values for all 

Precambrian rocks, in particular early Archean ones, 

must be carefully evaluated owing to the possibility of 

secondary Sm/Nd fractionation, evidence for the 

accuracy of determined initial isotopic compositions 

include: high U (1000 ppm) zircons in some areas 

show no Pb loss indicating minimal fluid and/or 

thermal disturbance; some gneisses are from an 

extensive (>200 km2) 3800 Ma terrane minimizing 

the possibility of exchange with, or contamination by 

younger rocks; highly positive values are not 

observed in nearby slightly younger ( <3700 Ma) 

rocks which have almost identical post crystallization 

histories. 

We have also determined the 142Ndf144N d 

isotopic compositions of three of the high eNd 

gneisses The absence of measurable (> lOppm) 

complimentary 142Nd isotopic anomalies in the high 

eNd samples constrains the timing of LREE 

fractionation of the mantle reservoir to be younger 

than ==4300 Ma. Thus the Sm/Nd fractionation is not 

related to a primordial global differentiation event. 

Furthermore a 4300 Ma differentiation age requires 

the time averaged 147Smf144Nd of the mantle 

reservoir to be >0.26. It would be difficult to account 

for this degree of depletion for the whole upper 

mantle by extraction of continental crust. Rather we 

favor a model of isolation and depletion of limited 

portions of the upper mantle by extraction of felsic 

crust . The early Archean Nd database is currently not 

sufficient to determine whether the ultradepleted 

reservoir was a global phenomena, or restricted to the 

region of the North Atlantic Craton and thus related to 

the formation of the overlying continent. 

Owing to the significance of initial eNd values for 

constraining models of early Earth differentiation, it is 

necessary to validate the Nd isotopic compositions. 

To this end we are continuing isotopic and 

geochemical studies of additional ~3800 Ma gneisses 

and also of recently identified >3800 Ma large 

ultramafic enclaves. whose relict mineral compositions 

(aluminous spinels, Fo 90 olivine) suggest they were 

derived from the upper mantle. 

27 

THE H CHONDRITE PARENT BODY(IES): 

INTERNAL STRUCIURE AND TEMPORAL 

CONTRIBUTION TO THE METEORITE FLUX 

BENOIT, P.H. and SEARS, D.W.G., Dept. Chemistry 

and Biochemistry, University of Arkansas, Fayetteville, 

AR, 72701, USA. 

The Antarctic meteorite collection has made many 

contributions to the study of parent body structure and 

evolution. Among these is the discovery of a group of 

unusual ordinary chondrites which is not found among the 

modem falls. These H chondrites, referred to here as "Group A", have high induced thermoluminescence (TL) 

peak tempc;:ratures, high metallographic cooling rates, 

significantly lower abundances of certain REE elements and 

have cosmic ray exposure (CRE) ages of about 8 Ma and 

were apparently fairly smaJI during irradiation in space. 

Among the modem falls there are numerous H chondrites 

with CRE ages of about 8 Ma ("Group B"), but these 

meteorites have induced 1L peak temperatures, 

metallographic cooling rates and REE element abundances 

indistinguishable from H chondrites with larger CRE ages 

("Group C") from both the Antarctic and modem falls 

collections and were larger than Group A meteorites during 

irradiation (Benoit and Sears, 1992). 

Our original study of the Oroup A Antarctic meteorites was based large~ on H5 chondrites. We have since 

examined Antarctic H4 and H6 chondrites as well. We find 

that there are members of Group A among both the H4 and 

H6 chondrites. However, for the H4 chondrites the 

difference between Oroup A and the Group B and C 

meteorites is not as distinct as is the case for the H5 and H6 

chondrites. The H6 members of Oroup A appear to be rare relative to Oroup B, compared to the proportions observed 

for the H5 chondrites in the Antarctic collection. 

Considering only the Group B and C meteorites, there is a 

tendency for H4 chondrites to have higher metallographic 

cooling rates and somewhat higher TL peak temperatures 

than H5 and H6 chondrites. No such tendency is observed 

among the Group A chondrites of types 4-6, but this must 

be viewed with caution due to the relative insensitivity of 

both metallographic determinations and induced 1L 

response at the high cooling rates (about 100 K/My) 

observed. 

The existence of the unusual Group A meteorites 

among the H chondrites, which have generally been 

considered fairly homogeneous, suggests that: (1) The 

meteorite flux has changed in composition over the 

relatively short time interval represented by the Antarctic 

meteorite collection ( <800 ka), (2) the metallographic 

cooling rate data of the Group B and C chondrites can be 

explained by a simple stratified parent body model, with H6 

having slower cooling rates than H4, but (3) the unusual 

group A meteorites do not fit this simple model. The group 

A meteorites may be derived from a totally different parent 

body or may have been created by extensive thermal 

processing during the 8 Ma event which generated large 

numbers of meteoroid bodies. If this latter situation is the 

case, the rarity of H6 members of Group A may be evidence 

of, the survival of a stratified parent body up until the 8 Ma 

event. 

Benoit, P.H., and Sears, D.W.G., 1993, Breakup and 

Structure of an H-chondrite Parent Body: The Hchondrite flux over the last million years: learns, v. 101, 

188-200. 
CHRONOLOGY OF INTERIOR ALASKAN LOESSPALEOSOL DEPOSITS BY THERMOLUMINESCENCE 

BERGER. Glenn W., Dept. of Geology, Western 

Washington University, Bellingham, WA 98225, USA, and 

PEWE, T. L., Dept. Geology, Arizona State University, 

Tempe, AZ 85287, USA. 

The thick (25m+) and widespread loess deposits of interior 

Alaska have interbedded micro and macro fossils, organic beds 

and paleosols. They thus represent an important source of 

paleoclimatic and paleoecologic information. However, the 

chronology of deposition has only recently (1990) been 

quantified, in a broad sense, using fission-track dating of 

interbedded tephra beds. We applied thermoluminescence (TL) 

sediment dating methods to loess near Fairbanks thought to be 

younger than -200 ka, to provide a chronology of key 

deposits for an age range usually inaccessible to other dating 

methods. 

We have -20 TL preliminary ages (from the fme-silt totalbleach and partial-bleach procedures) for loess and buried soils. 

More samples are being analyzed. As an age check, we dated 

loess just above and below the 140± 10 ka Old Crow tephra 

(OCt) at the Birch Hill site, obtaining TL ages of 128±22 ka 

(1o) and 144±22 ka respectively. At the Sheep Creek Cut site 

loess 2 em above the regionally important Sheep Creek tephra 

(SCt) gave an age of 184±27 ka, in agreement with TL ages 

of 191±24 ka and 205±31 ka for loess 2 em above and 2 em 

below (respectively) this ash at the Dawson Cut site. These 

results support the recent suggestion of Preece that this tephra 

is older than the OCt. However, at site Eva Creek II loess 

above and below an ash also correlated to SCt gave ages of 

117±19 and 150±28 ka. 

At two sites we confirm that the significant erosional 

unconformity at the top of the Gold Hill Loess corresponds to 

the time of the Eva Interglacial (Sangamon). At Dawson Cut 

we obtained loess TL ages of 61 ±6 ka above and 177 ±30 ka 

below the unconformity. At Sheep Creek Cut, loess just below 

this unconformity gives a TL age of 115 ± 19 ka. We also 

obtain the first direct ages for two significant buried organic 

horizons at the Gold Hill 1 site. A 76± 10 ka age for a thick 

organic silt at 11 m depth could represent paleosol development 

during isotope stage 5a warming. A thick, laterally extensive 

organic silt at 3 m depth here is dated directly at 29 ±4 ka. In 

other parts of interior and northwestern Alaska there are 

prominent or recognizable "buried soils" in stratigraphically 

similar positions to our upper "soil", with radiocarbon ages 

near 30 ka B. P. at one site (Epiguruk), thus suggesting that a 

regional, late Wisconsin paleoclimatic stratigraphic marker may 

exist. 

At some localities there is evidence that post-depositional 

reworking or contamination (e.g., via translocation of fme silt 

through modem rootlet channels) have lowered TL ages. 

Future TL samples at certain sites would probably need to be 

excavated from at least 60 em behind "fresh" section faces. 

28 

STRUCTURE & CONSTITUTION OF INTERSTELLAR 

GRAPHITE SPHERULES. 

Bematowicz, T. and Amari, S., McDonnell Center for the 

Space Sciences, Washington University, St. Louis Mo. 63130 

and Lewis. R S., Enrico Fermi Institute, University of 

Chicago, Chicago, IL 60637. 

Transmission electron microscopy of 500A slices of presolar 

(Amari et al., 1993) interstellar graphite spherules (1-2 J.1Dl 

diameter) by direct imaging, convergent beam diffraction and 

energy dispersive x-ray analysis shows a range of crystallinaity 

and internal structure in the graphite as well as a variety of 

carbide inclusions 100s of A in diameter (Bernatowicz et al.., 

1991). Recent work (Bematowicz et a/.,1994) has extended 

observations of these polymineralic assemblages to include 

many more grains of the graphite separate KFC1 (Amari et al., 

1994) from the Murchison meteorite. Typically there is an 

amorphous carbon core surrounded by a well graphitized shell 

with interspersed carbides ranging from nearly pure TiC to Mo, 

Zr carbides with only a few % Ti. The Mo & Zr are enriched 

relative to Ti over solar abundances by factors as high as several 

thousand, evidently effects due to both nucleogenitic 

enrichments in AGB stars and chemical (condensation) 

enrichments in the stellar atmospheres. These grains are 

randomly dispersed throughout a large fraction, perhaps most, 

of the graphite spherules. Occasionally a nearly pure TiC 

crystal is found in the precise center of the graphite spherule, 

evidently a heterogeneous nucleation site, in which case the 

whole spherule is well crystallized graphite. These kinds of 

observations should provide evidence for stellar atmosphere 

condensation conditions . For instance (Lodders and Fegley, 

1992 & 1993; Sharp and Wasserburg, 1993) Ti, Mo, Zr 

carbides are calculated to condense under equilibrium 

conditions ahead of graphite for reasonable s-process elemental 

enrichments. The random distribution of grains of different 

composition within single spherules is consistent with occlusion 

of these precondensed grains within later growing graphite 

spherules. Hopefully, more refmed thermodynamic models, 

including e.g. solid solutions, combined with more extensive 

elemental, mineralogical, & isotopic data for these grains will 

determine chemical compositions & physical conditions of 

stellar atmospheres and the underlying stellar nucleosynthesis. 

Come and see the pictures. 

Amari, S., Hoppe, P., Zinner E., and Lewis, R.S. 1993, The 

isotopic compositions and stellar sources of meteoritic 

graphite grains: Nature 365 806-809. 

Amari, S., Lewis R.S., and Anders, E. 1994, Interstellar grains 

in meteorites: I. Isolation of SiC, graphite, and diamond; size 

distributions of SiC and graphite: Geochim. Cosmochim. Acta, 

in press. 

Bematowicz T.J., Amari, S., Zinner, E.K., Lewis, R.S. 1991, 

Interstellar grains within interstellar grains: Astrophys. J. 373 

L73-L76. 

Bematowicz, T. J., Amari, S., and Lewis, R.S. 1994, Refractory 

carbides in interstellar graphite: Lunar Planet. Sci. Conf. 25, 

submitted. 

Lodders K., and Fegley, B. Jr., 1992, Trace element 

condensation in circumstellar envelopes of carbon stars: 

Meteoritics 21250-251. 

Lodders K., and Fegley, B. Jr., 1993, Chemistry in circumstellar 

envelopes of carbon stars: the influence of P, T, and elemental 

abundances: Meteoritics 28 387. 

Sharp, C.M. and Wasserburg, G.J. 1993, Molecular equilibria 

and condensation sequences in carbon rich gases: Lunar Planet. 

Sci. Conf. 24 1281-1282. 
EXCITATION FUNCTIONS OF 31 TO 500 MEV PROTON 

INDUCED REACTIONS ON C, Mg, AI, Si02, AND Si 

BEVERDING, A.M, Englert. P.A.J., Gans, C. , Kim K., 

Chakravarty, N., Nuclear Science Facility, San Jose State 

University, San Jose, CA 95192-0163, Castaneda, C., 

Crocker Nuclear Laboratory, University of California, 

Davis, CA 95616, Young, J., Department of Physics, Chico 

State University, Chico, CA 95929, Sisterson, J., Koehler, 

A.M., Harvard Cyclotron Laboratory, Harvard University, 

Cambridge, MA, 02138, Jull, T., Donahue, D.J., NSF 

Arizona Accelerator Mass Spectrometer Facility, University 

of Arizona, Tucson, AZ, 85721, Vincent, J., TRIUMF, 4004 

Westbrook Mall, Vancouver, B.C. V6T 2A3, and Reedy, 

R.C., Los Alamos National Laboratory, Group SST-9, Los 

Alamos, NM 87545. 

Excitation functions for several cosmogenic nuclides were 

determined by experimental and theoretical methods. These 

data are essential for interpretation of solar-proton-produced 

radionuclides in extraterrestrial material. Targets of C, Mg, 

AI, Si~, and Si, major components of planetary surfaces, 

were irradiated with protons over the energy range of 31 to 

500 MeV. For the energies of31 to 67.5 MeV, 7Be, 22Na, 

and 24Na cross sections were determined by gamma ray 

spectroscopy. I4C cross sections were measured by accelerator 

mass spectroscopy for Si02 and Si targets and agree well with 

Sisterson et al. (1993), work on lOBe and 26AI is in progress. 

Experimental values for the short-lived isotopes compare well 

with those measured previously, where available. The 

theoretical calculations were optimized by choosing input 

parameters for the ALICE-91 code to best fit the measured 

benchmark excitation functions up to the pion production 

threshold. In addition, further insight is gained in modeling 

cross sections, especially for neutrons since these have been 

seldom measured. 

Sisterson et al., Proton Production Cross Sections for I4C from 

Silicon and Oxygen: Implications for Cosmic-Ray Studies: 

Proceedings 6th International Conference in Accelerator 

Mass Spectroscopy, NIM B, in press. 

29 

ISOTOPIC RECORDS OF GONDWANA GLACIATION IN 

TALCHIR SEDIMENTS 

Bhattacharya. S K, Physical Research Laboratory, 

Ahmedabad 380 009, India, and Chakravarti, A., Indian 

Institute of Technology, Kharagpur, 721 302, India. 

Talchir formation in eastern India represents early Permian 

glacial outwash deposits from the famous Gondwana 

glaciation and contains fine sandstone and siltstone in its 

upper part Spherical nodules of a few em size cemented by 

fresh-water carbonates have been found in siltstone horizon 

of Talchir sediments in West Bokaro, Bihar. These were 

analysed for oxygen and carbon isotopic composition to infer 

about the nature of the ice sheet and the environmental 

regime. 

The oxygen isotopic composition of these calcrete nodules 

ranges from -20% to -27.9% (w.r.t. PDB) and would 

correspond to similar values in SMOW unit for the nodule 

forming water. These values are extremely depleted 

compared to the expected meteoric water composition at 

50°S, the approximate palaeolatitude of this area during the 

Permian. Therefore, the glacial melt water used in these 

cements was derived not from local valley glaciers but from 

the edges of a continental ice sheet originating in higher 

latitudes (>80°S). The isotopic values of these cements are 

similar to those of the fresh-water early Permian cements in 

the continental tillites from Antarctic and South Africa and 

suggest that the glaciation was continental in nature and 

extended over a major part of t11e Gondwanaland. 

The carbon isotopic composition of these nodules ranges 

from -2% to -11% (w.r.t. PDB) and indicate vegetational 

source for the carbon in HC03. Since the deposition of these 

sediments took place in marginal areas like lakes supported 

by melt water, there may be organic production in summer. 

Decomposition of these organic carbon could have supplied 

13c depleted HC03 to diagenetichlly alter the original 

carbon isotope values of the sediments. 
ISOTOPIC STUDY OF THE KIMBERLITE DIKE 

OF MASONTOWN, PA, USA 

BIKERMAN. Michael. PRELLWITZ, H.S., 

Geology & Planetary Science Dept, University of 

Pittsburgh, Pittsburgh, PA 15260, USA, and 

SIMONETTI,A., BELL,K., Ottawa-Carleton 

Geoscience Center, Department of Earth 

Sciences, Carleton University, Ottawa, Canada 

The Masontown (Fayette Co) PA dike intrudes 

upper Pa and lower Pm (?) sedimentary rocks with 

little contact metamorphism. The phlogopite-bearing 

kimberlite was considered on mineralogic grounds to 

be formed from mixing of two magmas (Hunter & 

Taylor, 1984). K-Ar analyses on coarse and fme 

phlogopite, and on olivine, were dates of: 147.3 ± 

1.5 Ma, 353.2 ± 2.2 Ma and no detectable 

radiogenic argon, respectively. If the olivine result 

means no excess argon, then the two phlogopites 

may indicate different times of formation. 

Rb-Sr and Sm-Nd analyses on Cr-rich and Crpoor garnets, a calcite vein and a whole rock sample 

produced results which lie slightly above the field of 

South African Group 1 kimberlites, on an ENd vs E sr 

plot. The whole rock has the most radiogenic Sr 

isotopic ratio. 

Sr and Nd isotopic analyses on whole rock and 

mineral samples are: 

SSrjUSri fsr 14Nd/ 14l-Mi ENd 

calcite vein 

whole rock 

Cr-rich garnet 

Cr-poor garnet 

0.70365 -9.7 

0.70513 11.4 

0.70414 -2.7 

0.70437 0.5 

0.51287 

0.51280 

0.51279 

0.51290 

8.1 

6.8 

6.7 

8.8 

The garnets are not in Sr isotopic equilibrium 

with the whole rock or the vein calcite. A 

"scatterchron" Sm-Nd date of about 146 suggests that 

the garnets and fine phlogopites. are roughly coeval. 

This dike has had a complex history, perhaps 

involving mixing of mantle-derived magma with 

metasomatized Acadian "mantle" at about 146 Ma. 

The partially equilibrated mix was emplaced by gasdriven fluidization. The low initial Sr calcite vein may 

reflect the composition and source of some of the 

carrier gas. 

Hunter, R.H., & Taylor, LA., 1984, Magma mixing 

in the low velocity zone: American Mineralogist, 

69, p. 4-40. 

30 

CARBON ISOTOPE STRATIGRAPHY OF THE 

LIESBERG BEDS MEMBER (OXFORDIAN, 

SWISS JURA) 

BILL. Markus Institut de Geologie et Paleontologie, 

Universite de Lausanne, BFSH 2, Lausanne, CH-1015, 

Switzerland, and BAUMGAR1NER, P.O., same address, and 

HUNZIKER, J. C. and SHARP, Z. D. Institut de 

Mineralogie et Petrographie, Universite de Lausanne, BFSH 

2, Lausanne, 1015-CH, Switzerland 

The Liesberg Beds Member forms the transition between 

the lower to middle Oxfordian dark coloured marls (Renggeri 

Member and the Terrain a Chailles Member) and the middle 

Oxfordian reefal limestones (St-Ursanne Formation). Both 

litho-and biofacies are very diverse and evolve rapidly upsection. Isotopic studies of whole rocks are therefore 

excluded. In search for a convenient isotopic marker, we 

measured several fossil groups and chose crinoid stems of 

Millericrinus sp. and echinoid spines of Paracidaris sp., 

==~: :/ s=~f v~:J!::~~f~rp~~h~r~i~~n~e~~~~l ~~ between fossils of the same stratigraphic level. 

Values measured in echinoderms largely reflect earliest 

diagenetic conditions at the seawater-sediment interface. The 

porous stereome structure secreted as high Mg-calcite by 

echinoderms has a high reactive surface/volume ratio, which 

triggers the precipitation of very early syntaxial cements. 

In the four studied sections reproducible carbon isotope 

shifts were observed both for Millericrinus sp. stems and 

P aracidaris sp. spines. 

A negative shift of 1 - 1.5 %o ~ nepos was observed 

near the base of the section, just above the transition from 

the Terrain a Chailles Member, where the first corals occur. 

In the middle and upper part of the three sections, 

characterised by a stepwise increase of corals and the 

macrofossils, a positive shift of about 2 %o ~ l3Cpos was 

observed. 

Despite of the highly variable lithologic composition of 

the Liesberg Beds Member carbon isotopic shifts seem to be 

consistent and warrant an interpretation as an original signal, 

controlled by the isotopic composition of dissolved carbonic 

acid in sea water. 

In a platform area, local factors may cause a particular 

isotopic signature in restricted water masses. For instance, 

the compaction and dewatering of the lower Oxfordian marls 

could have released pore waters rich in light organic carbon 

(as methane and C02) explaining the negative shift at the 

base of the Liesberg Beds Member. 

The following positive shift, however, seems to 

correspond to a general trend of opening up of the platform 

and connection to open marine waters. In that case, they 

could be interpreted as a result of important global organic 

carbon burial, that occurred possibly during periods of warm 

and humid climate. 
OBSERVED 87Srf6Sr ISOTOPIC VARIATION IN PELAGIC 

MUDROCKS OF PLIOCENE AGE ARE THE RESULT OF 

ORBITALLY FORCED PERIODIC VARIATION IN 

CONTINENTAL RUNOFF 

Blenkinsop. John; Patterson, R. Timothy; 

Reinhardt, Eduard; Department of Earth Sciences 

and Ottawa-Carleton Geoscience Centre, Carleton 

University; Ottawa, Canada, KlS 5B6; and William 

Cavazza, Department of Mineralogical Sciences, 

University of Bologna, 40126 Bologna, Italy 

87Sr/86Sr ratios measured in planktic foraminifera 

extracted from rhythmically deposited Early Pliocene (4.5-4.3 

Ma) limestone-marl couplets of the Trubi Formation 

(Calabria, southern Italy) indicate that strontium isotopic 

analyses may record paleoclimatic and paleoceanographic 

inforn1ation in addition to age estimates. Previous hypotheses 

have correlated deposition of the Trubi couplets with changes 

in monsoon intensity driven by 23,000 ka Milankovitch 

precession cycles. 

Individual marl and limestone units yielded 87Sr/96Sr 

ratios oscillating in harmony with lithology. While the results 

from the limestones correlate well with open ocean strontium 

regression curves, the marls are characterized by lower 87Sr/96Sr ratios, probably the result of monsoon influenced 

increases in fluvial flow to the Mediterranean Sea. Based on 

present -day discharge rate, strontium concentration, and 

isotopic ratio of the major rivers discharging into the 

Mediterranean Sea, the observed 0.00003 shift in 97Sr/96Sr 

ratio between the limestones and marls could be generated in 

about 10,000 years, consistent with the known depositional 

rate. Although tentative, our results suggest that in restricted 

basins strontium isotopic ratios may be a useful indicator of 

past marine environments. 

31 

A SILICA1E WEA TIIERING MECHANISM FOR 

ELEVATING MARINE 87Sr/86Sr FOLLOWING 

GLACIAL-INTERGLACIAL TRANSITIONS 

.B.LliM, LJ:2., EREL, Y.t, and BROWN, K., Dept. 

of Earth Sciences, Dartmouth College, Hanover, 

NH, 03755, USA, talso Inst. of Earth Sciences, 

Hebrew Univ., Jerusalem, 91904, Israel. 

The isotopic composition of Sr in stream waters 

and exchangeable Sr in soils was investigated in 

recently glaciated granitoip terranes in the Sierra 

Nevada (SN), California and the Wind River Range 

(WRR), Wyoming. In both cases a negative 

correlation was observed between the S7Sr/86Sr ratio 

and the soil age, indicating that the S7Sr/86Sr ratio 

released in the early stages of weathering following 

glaciation is significantly higher than in later stages. 

In the SN, drainages that were glaciated -10 kyr 

ago (exposing fresh surfaces) have average stream 

water 87Sr/86Sr ratios 0.0004 to 0.0011 higher than 

drainages that were not glaciated in the past -100 

kyr. Based on 87Sr/86Sr ratios and major cation 

concentrations of bedrock minerals and stream water 

we have argued that stream water s7Sr/S6Sr is 

controlled largely by plagioclase and biotite 

weathering and that biotite is weathering -4 and -6 

times more rapidly than plagioclase in the recently 

glaciated and non-glaciated drainages, respectively 

(Blum et al., 1993). 

In the WRR exchangeable Sr was analyzed in 

soils developed on six moraines varying in age from 

-0.3 to -350 kyr as well as in stream water from 

drainages glaciated -20 and -100 kyr ago. Because 

granitoids in the WRR are much older than the SN 

(2.5 versus 0.1 byr) the contrast in the s7Sr/S6Sr ratio 

of the bedrock minerals is greatly enhanced due to 

the much longer time for radioactive decay of 87Rb to 87Sr. Using the -350 kyr old soils as a baseline, 87Sr/86Sr in younger soils is elevated by 0.0029, 

0.0053, 0.0127, 0.0240, and 0.0835 in the -100 

kyr, -20 kyr, -10 kyr, -1 kyr and -0.3 kyr old soils 

respectively. Similarly, stream water draining a -10 

kyr weathering surface has 87Sr/S6Sr elevated by 

0.0015 relative to a -100 kyr old weathering surface. 

The shifts we observe in the S7Sr/S6Sr ratio 

released by silicate weathering provide a previously 

unrecognized mechanism for elevating global riverine 87Sr/86Sr ratios following global glacial advances. 

This may help to explain the correlation between the 

marine 87Sr/86Sr ratio and global climate cycles over 

the past 450 kyrs (with -100 kyr periodicity) which 

requires increases in global riverine s'Sr/S6Sr by 

-0.0040 and/or dramatic increases in global riverine 

Sr flux following glacial to interglacial transitions 

(Clemens et al., 1993). 

Blum, J.D., Erel, Y. and Brown, K., 1993, 

Geochim. Cosmochim. Acta, 58,5019-5025. 

Clemens, S.C., Farrell, J.W. and Gromet, L.P., 

1993, Nature 363, 607-610. 
GEOCHEMISTRY OF LOWER OCEANIC CRUSTDATA FROM SITE 735B 

BLUSZT AJN .J.,HART,S.R. and DICK,H.,Woods 

Hole Oceanographic Inst., Woods Hole, MA 02543, 

USA 

Due to a lack of suitable dredged samples our 

knowledge of the geochemistry of the oceanic crust is 

mostly restricted to the upper crust. Site 735B on the 

southwest Indian Ridge, which recovered a 500m 

section of Layer 3 gab~ros, provides a unique 

opportunity to study the geochemistry of the lower 

oceanic crust. In order to get representative analyses of 

the whole core, we cut 1-2 meter long strip samples, 1-2 

em wide, from 22 sections, covering the whole range in 

lithologies and deformation styles. In addition, we 

analyzed carbonate, amphibole, diopside and felsic 

veins. Sr and oxygen isotopic compositions of carbonate 

veins indicate that sea water circulation at low 

temperature (<100C) ceased within 5-7m.y. of crust 

formation. Higher temperature plagioclase, amphibole 

and diopside veins show small sea water 87srJ86sr 

signatures (87sr JB6sr up to 0.7036), but primary igneous 

143Ndj144Nd (0.51312). The strip samples show a 

slight sea water signature at shallow depths 

(87sr J86sr=0.7030) which decreases down hole; 

143Ndj144Nd is virtually constant throughout the 

hole at 0.51312. The elevated 87sr J86sr, and Cs, U 

enrichment in the shallowest part of hole 735B 

indicate that only this upper 70m of the core was 

significantly modified by low temperature interaction 

with sea water. Excluding these shallow samples, the 

average Rb, Cs and U content is 0.32ppm, 5.5ppb and 

35.5ppb respectively. In conjunction with the lack of 

evidence for strong sea water alteration, this indicates 

that alkalies and U were not stripped from the gabbros 

by high temperature hydrothermal circulation. The 

putative hot smoker flux of Rb and Cs is thus still much 

too large to be supported by crustal inventories. The 

primary protolith has an isotopic composition of 

87sr J86sr=0.7029, 143N d 1 144Nd=0.51312 and 

206pbj204pb =17.5. Preliminary analyses document 

very low Os content (from 0.4 to 3ppt) with only one 

troctolitic unit having a high Os content (400 ppt) and 

eleveted 187osJ186os =1.217. The gabbros are quite 

enriched in Pb (average 0.52 ppm) causing the average 

238u j204pb ratio to be low (around 4.2). Without 

additional processes like dehydration removal of lead 

in subduction zones, the low J.1 value does not support 

existing models for generating the HIMU mantle 

component by recycling and long-term storage of oceanic 

crust. 

32 

SINGLE CRYSTAL LASER 40ARJ39AR AGES OF 

QUARTZ PROTOCRYSTS IN THE 0.761 MA BISHOP 

TUFF RHYOLITE (LONG VALLEY, U.S.A.) 

BOGAARD, P.V.D. and SCHIRNICK, C., Dept of 

Volcanology and Petrology, Geomar Research Center, 

D-24148 Kiel, F.R.G. 

The Bishop Tuff (BT) erupted 0.761 ± 0.001 Ma BP 

(weighted mean; MSWD=l.2; N=20; all errors 1o) according 

to laser 40 Ar/39 Ar analyses of sanidine phenocrysts from 

fallout and flow deposits. Fresh obsidian glass spherules from 

highly welded late flows yield a mean apparent age of 0.754 ± 

0.008 Ma. The BT rhyolite contains 5-25% phenocrysts, 

chiefly quartz, sanidine and plagioclase (3:3:1; Hildreth, 1979), 

with the quartz crystals showing abundant potassium-rich 

rhyolite glass inclusions, bipyramidal outlines and frequent 

embayed and resorbed surfaces. We have studied the Ar 

isotope composition of meticulously cleaned, inclusion-rich 

quartz crystals picked from essential lapilli of early airfall 

deposits (Type A) and from bulk samples of late highly welded 

ignimbrite cooling units (Type B). 

Individual Type A quartz crystals contain fresh glass 

inclusions which release up to 3.7E-13 ccSTP 36Ar, 6.7E-ll 

ccSTP 39Ar, and 1.9E-10 ccSTP 40Ar upon laser fusion 

(typical Sfstem blanks are 5.5E-14, 6.5E-14, and 1.8E-12 

ccSTP for 36Ar, 39 Ar, and 40Ar). They yield a mean apparent 

age of 1.89 ± 0.03 Ma (MSWD=2.2; N=25), and a decent 

isochron with an age of 1.93 ± 0.06 Ma (MSWD=2.3) and an 

initial 40 Ar/36 Ar ratio of 290 ± 7, indicating that the high 40 Ar 

contents are derived from in situ decay of 40K in the glass 

inclusions and not from excess 40 Ar in the quartz. Quartz 

crystals lacking inclusions yield argon signals similar to blank 

values. Type B quartz crystals contain more devitrified "glass" 

inclusions, abundant cracks and release much smaller Ar 

volumes (i.e. 39 Ar < 8E-12 ccSTP). Yet still, the data define an 

isochron with an age of 2.3 ± 0.4 Ma, and an initial 40 Ar/36 Ar 

ratio of 296 ± 2 (MSWD=l.4; N=7). 

Our study indicates that the age of individual quartz crystals 

can be determined by means of the Ar isotope composition of 

their glass inclusions, and that the vast majority, if not all of 

the quartz crystals in the BT rhyolite are neither phenocrysts 

nor xenocrysts sensu stricto. Instead, we interpret these 

crystals to represent remnants (protocrysts) of a crustal magma 

body that intruded the Long Valley basement, crystallized 

quartz phenocrysts trapping melt inclusions, and cooled below 

the Ar blocking temperature of quartz at 1.9 Ma BP. Glass 

Mountain rhyolites of the same age probably represent the 

surface projection of that intrusion. 1.14 Million years later, 

that magma had acquired a more evolved composition and 

erupted to form the Bishop Tuff. 

What about the magma in the meantime? Was it a melt at 

high temperature all along (Halliday et al. 1989), then why 

didn't the glass inclusions equilibrate isotopically? Was it a 

granite in-between (Sparks et al., 1990), then where are the 

remaining granite crystal assemblage and abundant granite xenoliths? We envisage a super-cooled, only partially crystallized 

and consolidated, glass-like state, permitting near-instantaneous transformation into a partial melt upon re-heating due to 

emplacement of mantle-derived basalt melts into the crust 
SM-ND AGE OF SUBOPHIOLITIC METAMORPHIC SOLES OF SEVANO-AKERA ZONE 

(LESSER CAUCASUS) BOGDANOVSKI O.G .. Vernadsky Inst. 

of Geochemistry. Moscow. 117975. 

Russia. JAGOUTZ E .. Max-Plank 

Inst. fur Chemie, Mainz D-6500. 

Germany, ZAKARIADZE G.S .. Vernadsky Inst. of Geochemistry. Moscow. 117975. Russia. KARPENKO 

S.F .. Vernadsky Inst. of Geochemistry, Moscow. 117975. Russia. 

and SILANTJEV S.A .. Vernadsky Inst. of Geochemistry. Moscow. 

117975. Russia. 

The ophiolitic zones of Lesser 

Caucasus reveal a prolonged period of tectonic accretion of paleooce- anic allochthones: 229+10 Ma. Sm-Nd 

for tholeiitic sequence (Bogdanov- ski et al .. 1992) and 160+4 Ma. U-Pb 

for boninitic sequence (Zakariadze et al .. 1990). Several blocks of 

garnet amphibolites were considered 

earlier as the most ancient (Early Carboniferous) paleooceanic frag- ments of the accretionary melange of Lesser Caucasus (330 Ma. Rb-Sr. 

Meliksetian et al .. 1984). Our recent studies of these garnet amphi- bolites have shown that: a) they 

were metamorphosed at T=600 -soo0 c 

and P=7-12 kb: b) their Rrotolith 

was a low-K and low-Ti aifferentiated tholeiitic series (basalts. andesites): (La/Sm) 0 =0.27-0.92. (Yb) 0 =9-19: and c)the age of metamorphism of these rocks (three-mine- ral Sm-Nd isochrone) is 155+11 Ma 

(epsNd (T)=4.5±0.3, MSWD=O.I75). Studied amphibolites are interpre- ted as subophiolitic series of metamorphic rocks. The obtained age value of metamorphism corresponds well to the age of obduction (J2 _ 3 ) 

of Mesozoic ophiolites along the 

Vardar-Ismir-Ankara-Erzingan-Lesser 

Caucasus belt of Eurasian margin. 

Bogdanovski O.G .. et al .. 1992. - Dokl. Ac. Nauk USSR. v.327. p. 566-569, (in Russian). Zakariadze G.S. et al., 1990, - Izv. Ac. i~auk USSR, Ser. Geol .. 3, p.17-30 (in Russian). Meliksetian B.M .. et al .. 1984,- Izv. Ac. Nauk ArmSSR. Nauki o 

Zemle. v.37, p.3-22(in Russian). 

33 

Nd ISOTOPES AND THE SOURCE OF SEDIMENTS IN 

THE MIOGEOCLINE OF THE CANADIAN CORDILLERA 

BOGHOSSIAN N.D., PATCHETT, P.J., 

Dept. of Geosciences, University of Arizona, 

Tucson, AZ 85721, U.S.A, and ROSS, G.M., Institute 

of Sedimentary and Petroleum Geology, Calgary, 

Alberta TIL 2A 7, Canada 

Nd isotopes in clastic sediments from the miogeoclinal 

sequence of Alberta constrain sediment sources and document 

continental input to crustal growth in the Cordillera. The 

samples range in age from ca. 750 to 70 Ma. The goal was (1) 

to develop a North American miogeoclinal reference for Nd 

isotopes and to provide a baseline for continental input to the 

terranes of the Canadian Cordillera; (2) to constrain the 

importance of proximal and distal North American basement 

sources for these sediments; (3) to constrain timing of arrival 

of juvenile terranes to the Canadian continental margin. 

Initial ENd values for Upper Proterozoic to Upper Ordovician 

samples range -14 to -22 and can be explained by derivation of 

clastics from a mixture of proximal Archean and Proterozoic 

basement. A positive shift of 6 ENd units occurs in Upper 

Devonian time, and this more positive signature persists until 

foreland basin formation in Upper Jurassic time. ENd values 

for this time slice of 3 70 to 150 Ma range from -6 to -9 and 

require involvement of a more juvenile source. After 

emplacement of terranes in the Cordillera beginning in Jurassic 

time, a foreland basin developed on top of the miogeocline. 

The foreland basin sediments show extreme heterogeneity, with 

ENd values as high as zero. 

Two hypothesis can explain the shift in ENd in Upper 

Devonian time. The higher ENd signature could represent a 

mix of Cordilleran juvenile component plus Precambrian 

basement. The ENd shift is coeval with tectonic activity along 

the western margin of North America in Devono-Mississippian 

time. However, in the southern Canadian Cordillera any 

western juvenile sources should have ceased to supply 

sediments to the miogeocline by mid-Mississippian time and 

thus cannot explain the persistence of the juvenile signature 

until Upper Jurassic. 

The second hypothesis requires transport of sediments across 

the craton from Appalachian sources The Appalachian belt 

(which includes Grenville age basement and sediments 

ultimately derived from this basement) provides the appropriate 

isotopic signature. It constituted the main mountain belt in 

North America during most of the Paleozoic and was a major 

source of detritus to the craton throughout that time. 

Pennsylvanian sediments of Appalachian origin with isotopic 

signature identical to our samples (ENd = -6 to -10) were 

documented as far into the interior of the craton as the central 

U.S.A. Our hypothesis would require the transport of 

Appalachian-derived sediments a distance around twice that 

documented by Gleason et al. (1994), maybe in a multi-step 

journey. 

Gleason, J.D., Patchett, P.J., Dickinson, W.R., Ruiz, J., 

1994, Nd isotopes link Ouachita turbidites to Appalachian 

sources: Geology, in Press. 
PRE- AND POST CRYSTALLIZATION CONTAMINATION 

OF Sll..ICIC PERALKALINE ROCKS BY SEA WATERDERIVED COMPONENTS: SOCORRO ISLAND. MEXICO 

BOHR.SQN. W.A • Dept of Geological Sciences. UCSB, 

Santa Barbara, CA 93106, USA. and REID, M.R .• Dept of 

Earth and Space Sci., UOA, Los Angeles, CA 90024, USA 

Peralkaline trachytes and rhyolites (63-70 wt% Si<h) 

dominate the last 540 ka of volcanism on Socorro Island, an 

ocean island situated in the Eastern Pacific on a mid-ocean 

ridge spreading center abandoned at -3.5 Ma. The dominance 

and peralkaline nature of silicic rocks on Socorro provide the 

ideal opportunity to evaluate magma chamber processes and 

silicic magmagenesis in the absence of continental crust 

Silicic whole-rock 87sr,S6sr (0.7034 -0.7086, Fig 1) are 

elevated compared to values for Socorro Island alkaline basalts 

(0.7031-0.7034, Fig 1) and approach that of modem seawater 

(0.7001). Low Sr concentrations ([Sr]) in silicic WR (<20 

ppm) render them susceptible to contamination; whole-rock 

alkali-feldspar (WR-AF) pairs (n=20) show pronounced 

87srf86sr disequilibria (AF = 0.7031-0.7041, Fig 1). 

Acid-leaching of 

silicic WR yield less 

radiogenic 87sr,S6sr 

(t.\=0.004) and lower .. 

[Sr] (t.\=12 ppm) with ta.-

increasing acid strength, '" '7QS 

demonstrating that a o. 

dominant component of 

the Sr is secondary. 

.. : .. • • • • • 

• • 

Leachates have 100 2110 • (b) 

87 Sr/86sr (0. 7086) similar to summit hydrothermal fluids 

(0.7079-0.7086). The remarkable correlation between age and 

unleached WR 87sr;86sr (Fig. 1) strongly suggests 

progressive, post-crystallization fluid-rock interaction where 

hydrothermal fluid/rock ratios increase from <1 in younger 

rocks to -10 for older rocks. 

Post-crystallization contamination had a minimal effect on 

AF phenocrysts; acid-leaching of AF produced only minor 

decreases in 87sr;86sr (t.\=0.0001) due to higher [Sr] and lower 

permeability compared to WR. Leached AF 87sr;86sr that are 

more radiogenic than alkali basalts and 87sr;86sr disequilibria 

between WR -AF pairs that persist despite acid-leaching 

illustrate that not all of the Sr isotopic variation is secondary. 

AF 87 Sr;86sr correlates with WR indices of fractionation 

(e.g., Zr/Nb, Fig 2), indicating that assimilation of a 

component with o.7042 o 

radiogenic 87Sr/86sr o.7040 Fi,- 2 

occurred during feldspar (_ o.7038 

growth and magmatic ~ o.

7036 differentiation within a ~ 0 o Cb 0

° 

magma chamber. The ~ 0.7034 eo 0 ° 0 0 0 identity of the assimilant 0.7032 r e 0 is tied to negative Ce 0.7030 6.0 65 711 7

.s 

anomalies and unusually WR7z/Nb 

0 

IIJ 

high ~bundances of other REE in many silicic rocks. 

Concomitant enrichments in HFSE such as Zr, Hf, Nb are not 

evident; rather, abundances are consistent with fractional 

crystallization. These trace element characteristics, together 

with radiogenic Sr, strongly favor assimilation of hydrothermal 

deposits, particularly Fe-oxyhydroxides, present in the shallow 

ocean crust. Isotopic and trace element data for Socorro silicic 

rocks provide unequivocal evidence that crustal contamination 

affects ocean island magmas and should be considered in 

petrogenetic studies of ocean island volcanic suites. 

34 

40 Ar/39 Ar GEOCHRONOLOGY OF ACCRETED 

1ERRANES FROM SOUTH-WES1ERN BAJA 

CALIFORNIA SUR, MEXICO. 

BONINI Jennifer. A., and BALDWIN, Suzanne, L., 

Department of Geosciences, University of Arizona, Tucson, 

Arizona 85721, U.S.A. 

A geochronologic study of Mesozoic oceanic rocks from Isla 

Magdalena and Isla Santa Margarita, Baja Calif?J'"ia Sur, 

Mexico provides constraints on the metamorphism and 

defonnation of this disrupted terrane. These iSlands are . 

comprised predominantly of ophiolitic an~ islan~-arc affimty 

rocks fanning an upper-plate, and blueschist-facies subd~ction 

complex rocks forming a lower~plate. Bet:v~en thes~ umts, 

within intervening fault zones, lS a serpentimte-matnx 

melange containing exotic blocks. 40 Arz39 Ar ~esults have 

been obtained for upper-plate metamorphic and tgneous rocks, 

and for exotic blocks from the serpentinite matrix melange. 

Results from hornblende separated from an amphibolite 

exotic block and muscovite from two greenschist facies exotic 

blocks indicate early-Cretaceous metamorphism. These data 

are consistent with previous work on exotic blocks from westcentral Baja California that indicate mid-Jurassic through earlyCretaceous subduction related metamorphism . 

40 ArJ39 Ar analysis of hornblende from upper-plate . 

amphibolite and garnet-amphibolite thrust sheets that ov~rhe 

the ophiolitic rocks yielded complex age sp~tra suggestmg 

late-Jurassic to early-Cretaceous metamorphism. A complex 

muscovite age spectrum from a sheared metavolcanic unit 

exposed at the contact between the upper ~l~te and the 

serpentinite matrix melange suggests a mmtmu~ age of lateJurassic shearing. Biotite from a lamprophyre dike that crosscuts the upper-plate ophiolite yielded an undisturbed age 

spectrum indicating mid-Oligocene intrusion, which we 

interpret to mark the end of extension within the ophiolite 

unit. Step heat experiments on whole rock samples from 

undeformed rhyodacites yield ages that suggest a more 

prolonged period of arc-related magmatism than has been 

previously proposed. 
IMPLICATIONS OF GROUNDWATER WEATHERED 

PROFILE INTERACTIONS TO THE MOBILIZATION 

OF RADIONUCLIDES 

BONOTTO, D.M., Departamento de Petrolo 

gia e Metalogenia, Institute de Geoci= 

encias e Ciencias Exatas, Rio Claro, 

Sao Paulo, 13506-900, Brasil. 

This study reports the nature and 

extent of open-system interaction between 

groundwater and a weathered profile 

developed in the high grade thorium and 

rare earth elements ore body in Morro do 

Ferro, Po~os de Caldas plateau. The 

radioelement mobility in the shallow 

oxidizing environment was considered on 

using chemical data in conjunction with 

U-234/U-238, Th-228/Th-232, Ra-226/Th-230 

and Th-230/U-234 activity ratios for 

borehole spoil and groundwater samples. 

Recharging groundwater from the studied 

borehole has low salinity values, with 

total dissolved solids content of 14.7 mg/ 

1 and total ionic strength of 0.00018. The 

ratio of the weight of dissolved radioelement per unit volume of solution to the 

weight of radioelement in solid phase per 

unit weight of solid phase showed that the 

radioelement solubility in the studied 

waters varied according to the following 

order: radium >>uranium >thorium. 

U-234/U-238 activity ratios less than 

1 were measured in solid phase and can 

justify the enhancement of U-234 in 

solution. Th-228/Th-232 activity ratio 

greater than 1 was found at about 18.75 m 

depth and is related to ingrowth of Th-228 

from Ra-228 held in this site. Ra-226/ 

Th-230 activity ratios greater than 1 and 

Th-230/U-234 activity ratios less than 1 

were evaluated between 20 and 27 m depth, 

where a 2.1-m thick magnetite dike was 

intersected. These ratios could be 

justified by deposition of U and Ra 

associated with ferric oxyhydroxides and 

kaolinite. Typical adsorption coefficient 

values for these phases and mineral 

saturation indices evaluated from the 

available data confirm this possibility. 

35 

K/Ar AND Rb/Sr AGES OF CELADONITES 

FROM THE TROODOS OPHIOLITE, CYPRUS 

BOOIJ. E. and STAUDIGEL, H., Instituut voor Aardwetenschappen, Vrije Universiteit, De Boelelaan 1085, 

1081 HV Amsterdam, The Netherlands and GALLAHAN, 

W.E., College of Oceanic and Atmospheric Sciences, 

Oregon State University, Corvallis, USA. 

Chemical interaction between oceanic crust and seawater 

begins immediately after the formation of new basaltic 

basement material at mid-ocean ridges. As long as the 

circulation provides fresh seawater and unstable (high 

temperature) volcanic phases are available for alteration, 

seawater reacts with these primary assemblages and new low 

temperature phases are formed. Little is known, however, about 

the duration of these secondary processes in the oceanic crust. 

We determined K/Ar and Rb/Sr geochronological data for 

celadonite, a predominant low temperature hydrothermal 

alteration mineral from the Troodos Ophiolite, Cyprus. 

K/Ar data yield ages for the celadonites between 54.8 ± 0.6 

and 90.9 ± 1.0 Ma, with the oldest in close agreement with 

the estimated 91- 92 Ma crystallisation age of the Troodos 

igneous complex. 

Rb!Sr model ages were calculated using 87 Srt86sr initial 

ratios of 0.7073 (90 Ma seawater) and 0.7037 (fresh basaltic 

glass), and assuming that the "true" Rb/Sr age is the mean of 

both model ages. These calculations yield ages varying between 

59.7 ± 0.6 and 90.9 ± 0.9 Ma. Together the K/Ar and Rb/Sr 

data establish that the low temperature alteration of the Troodos 

ophiolite continued for at least 30 Ma. 

When compared in a histogram (Figure 1), K/Ar and Rb/Sr 

data define a similar range of ages, whereby the mean of Kl Ar 

ages is lower than the mean of the Rb/Sr ages. This 

observation may be interpreted in terms of the possibility that 

the celadonites were not completely Ar retentive during the 

alteration process. 

Although there are some discrepancies between the Kl Ar and 

Rb/Sr ages, these results clearly demonstrate that the low 

temperature geochemical exchange between seawater and the 

oceanic crust is a long lived process, continuing for at least 30 

Ma. 

5 

"i 

~ CD 

- 4 .5 

.5 

(I) 3 CD 

., 

as II Rb/Sr ages 

-

0 2 .. • K/Ar ages CD 

.a 

E 

::::s 

z 

0 l{) 0 l{) 0 l{) 0 l{) 0 0 l{) 

~ co ";- ";- ~ ~ q> 0> I I 

..... co ..... co ..... co ..... co A l{) l{) co co ""'" ""'" CX) CX) 

Age Interval (Ma) 

Figure 1: Kl Ar and Rb/Sr age frequency distributions, using 

age intervals of 5 Ma. Note the shift of the Kl Ar ages towards 

lower values. 
PEDOGENIC CALCITE AS AN INDICATOR 

OF AN EARLY HOLOCENE DRY PERIOD IN 

THE SAN FRANCISCO BAY AREA 

BORCHARDT, Glenn, Soil Tectonics, Berkeley, CA 94705, USA, 

and LIENKAEMPER, J.J., U.S. 

Geological Survey, Menlo Park, 

CA 94025, USA. 

Rainfall at Union City, California during the early Holocene may 

have been half the current level. 

Bk (calcitic) horizons in paleosols 

in a well-dated alluvial fan indicate a relatively dry climatic 

period fro~ 10 ka to 7 ka (calendar 

corrected Cages). The welldrained paleosols and their associated Bk horizons developed mostly 

in thin, clayey overbank units 

associated with <2 m deep channel 

fills containing gravel, sand, and 

cobble. The pedogenic calcite 

exists primarily as vertically 

oriented filaments and fine, hollow 

nodules formed at ped-face intersections. Under toda~'s climate 

(MAP=645 mm, MAT=13.9 C) at this 

site modern soils and paleosols <7 

ka do not develop Bk horizons. The 

calcitic paleosols lie buried at 

depths over 1.5 m on the thick 

central portion of the fan where 

the alluvial deposition rate is 

highest. As the late Holocene 

alluvial cover thins.away from the 

fan axis, modern leaching has 

destroyed those portions of the Bk 

horizon within 1.5 m of the surface. The thickness of the paleoBk horizon thus decreases as a 

function of distance from the axis 

of the fan. No evidence for the Bk 

horizon was found >70 m from the 

axis. Thus, except at rapidly 

aggrading sites like this one, any 

Bk horizons formed elsewhere in the 

Bay Area during the dry period 

would have been subsequently destroyed. 

Some of the early Holocene 

paleosols in the thick part of the 

fan yield evidence for the effective percolation depth of rainwater 

during the dry period. Remnants of 

paleosol P3, for example, had 

leaching depths no greater than 0.6 

m for five widely spaced stations. 

This is about half the current 

depth of leaching. 

36 

TWO CONTRASTING INTRUSIVE SUITES IN THE SERlE 

DEI LAGHI (SOUTHERN ALPS): GEOCHEMICAL AND 

ISOTOPIC (Sr. Pb) INFERENCES ON THE GENETICAL 

ENVIRONMENTS 

BORIANI, A.,GIOBBI ORIGONI, E., Dip.to Scienze della 

Terra. University of Milano, Milano, Italy, 20100, and 

PINARELLI Laura. Istituto di Geocronologia e Geochimica 

Isotopica, C.N.R., Pisa, Italy, 55100. 

The Western Southalpine crust of Northern Italy and Southern 

Switzerland consists of high-grade (Ivrea Zone) and medium grade 

(Serle dei Laghi) metasediments and metabasites that underwent 

Hercynian metamorphism and which contain intrusive rocks of 

Ordovician and Permian age. This paper compares the geochemical 

and isotopic (Sr. Pb) characteristics of the Ordovician (OG) and 

Permian Granitoids (PO) of the Serle dei Laghi. 

The 00, which range in composition from gabbro-diorite to 

leucogranite, have a calc-alkaline meta-aluminous character. They 

show moderate ULE and HFSE enrichements, with negative spikes 

of Nb, Sr. P. and Ti, and plot in the VAG (Volcanic Arc Granites) 

field in the Pearce's diagrams. The felsic lithologies yelded a whole 

rock Rb/Sr isochron at 466±.S:Ma, with I.R. of 0.7087. The most 

mafic lithologies, instead, do not plot on the isochron, but arrange 

along a mixing line indicating mantle-crust interaction 

((87 Srf86Sr)i = 0.704-0.709). The initial Pb isotope ratios of the 

OG ((206Pb/204pb)i = 17.49-18.00~ (207pbf204pb)i = 15.55-

15.63~ (208pbf204pb)i = 37.66-38.12) fall within typical values of 

the lower crust. Linear trends in the Pb-Pb diagrams are in 

agreement with the mantle-crust interaction hypothesis. 

The PG range in composition from gabbro to leucogranite, and 

are calc-alkaline and weakly peraluminous. They are strongly 

enriched in UI.E and moderately-to-strongly enriched in HFSE, 

showing deep negative spikes of Ba, Nb, Sr. P. and Ti. They plot 

as WPG (Within Plate Granites) in the Pearce's diagrams. The 

felsic plutons give two whole rock isochrons at 276±6 and 277±8 

Ma, with the same I.R. of 0.710. The mafic dykes show Sr 

isotopic variations indicating mantle-crust interaction, as well 

((87 Srf86Sr)i = 0.705-0.710). The Pb isotope ratios of PG 

((206Pb/204pb)i = 17.88-18.53~ (207Pb/204pb)i = 15.60-15.71~ 

(208pbf204pb)i = 38.08-38.71) are distinctly higher than those of 

the 00, and are similar to those of the Ivrea Zone metasediments. 

For both the OG and PG, the following model can be 

suggested: the most mafic samples were generated by interaction of 

a mantle-derived magma with crustal material, while the most 

differentiated samples represent derivatives from the hybrid magma 

maintly through crystal fractionation. Nevertheless. OG appear to 

be depleted in both UI.E and HFSE, show less pronounced 

negative spikes of Sr. P, and Ti, have lower initial Sr and Pb 

isotope ratios than PO. It follows that the ultimate sources of the 

Ordovician and Permian magmatism were different both 

geochemically and isotopically, reflecting two different geodynamic 

environments: a convergent plate boundary in the Ordovician, a 

post-orogenic, extensional regime in the Permian. 
ROW INTENSE fS THE MONSOONAL 

SCAVENGING OF 2 0Pb IN THE WESTERN 

ARABIAN SEA? 

BOROLE. D. V. and NAIR R.R., 

Geological Oceanography Division, 

National Institute of Oceanography, 

Dona Paula, Goa,403004, India. 

21°pb concentrations determined 

from serial, sediment trap samples 

from the northwest Arabian sea 

during December 1986 to October 1987 

varied from 11.25±0.43 to 107.66± 

3.40 dpmjg and 57.98±1.81 to 587.78± 

17.32 dpmjg for the shallow and deep 

levels though the average flux 

increased from 0.21 to 0.58 dpm/ 

cm2Jy. The predicted 210Pb settling 

flux on the sediment particles differed by a factor of 3 to 4 when 

compared to those settled in the 

traps. 

The 210Pb flux via the settling 

particles exhibited strong seasonal 

pattern in tune with the variations 

of mass sediment flux and productivity. During NE monsoon season homo-

~Ineous conditions prevailed for 0Pb flux as seen by high correlation with that of mass flux and C0 r 

while heterogeneity, with poor corr2 

elation, underlined the rich, wet 

season. This is more conspicuous for 

the sediments from deeper trap than 

the shallower one. 

The observed non-uniformity 210Pb 

and C0 flux in this area is in 

contras~ to other world oceanic 

regions. Plausible causes for these 

departures have been detailed in 

terms of monsoon dominated effects 

such as i) simultaneous occurrence 

of intense aeolian input and productivity ('Prospera hypothesis'), ii) 

episodic variation of 210Pb input 

via atmospheric fall out as well as 

upwelling, and iii) bottom sediment 

re-suspensions as a result of bathymetric configuration (e.g., owen 

ridge) in the vicinity of the !8diment traps. The measured 2 Pb¥S 

inventories (148 to 160 dpmjcm2) 1n 

the bottom sediments from locations 

southwest of the sediment traps (at 

KL-74 and KL-87) could support boundar~ scaven~f8g making this area as 

a s1nk for Pb. 

The results of this study attain 

greater relevance to the ongoing 

JGOFS programme. 

37 

230Th/238U-AXIAL DEPTH CORRELATION: 

EVIDENCE FOR RIDGE-HOTSPOT 

INTERACTION NEAR THE AZORES 

BOURDON. B .. ZINDLER, A., and 

LANGMUIR, C.H., Lamont-Doherty Earth 

Observatory, Palisades NY 10964, USA. 

Geochemical modelling of hotspots and hotspotinfluenced ridges is often complex because source 

effects obscure the melting signature. For example, 

several hotspots fall off the global Fes.o-depth 

correlation of Klein and Langmuir (1987) and 

emphasize the difficulty inherent in the geochemical 

parameterization of plume. 

U-Th disequilibrium data should provide 

information about the melting process, independent of 

source composition, presuming that, at the inception 

of melting, the mantle source is in secular 

equilibrium. Thus, we have used 230Th-238U to 

investigate melting along plume-influenced segments 

of the MAR, north of the Azores triple junction 

(37°30 to 40°30N). We analyzed 22 hand-picked 

fresh glasses, collected during the F AZAR cruise for 

U-Th disequilibrium as well as major and trace 

elements. Our new data shows that Fes.o is 

negatively correlated with Ba/Ti02, perhaps 

indicating mantle heterogeneity. There is also a 

positive gradient in Ba/Ti02 towards the Azores 

platform, which can be taken as an index of 

enrichment due to the influence of the hotspot. 

Thorium concentrations range from 0.14 ppm to 

2.4 ppm and the Th/U ratios vary between 3.16 to 

3.64. 230Thf232Th ratios show remarkable 

homogeneity for samples from a single dredge. 

Additionally, samples with the highest Th 

concentrations (2.4 ppm) have the highest Th 

isotopes ratios. Taken together, these observations 

argue against assimilation of 230Th-rich sediment as 

an explanation the 230Th-238u systematics. 230Th 

excesses range from 20 and 35%, which is markedly 

larger than 230Th excesses reported for the EPR, and 

are positively correlated with axial depth. 

Additionally, in three of the ridge segments studied, 

230Th excesses are more extreme in the center of the 

segments. The 230Th excesses reflect larger extents 

of melting in the garnet field, and suggest greater than 

normal depths (-30kbar) for the initiation of melting 

beneath the MAR near the Azores. These results 

demonstrate the potential for U-Th systematics to 

constrain the nature of mantle melting, independent of 

source chemical heterogeneity. 
SUPPORT FOR STEADY -STATE CRUSTAL GROWTH 

FROM THE 4.0 GA ACASTA GNEISSES 

BOWRING. S A , HOUSH, T. B. * ISACHSEN, C. E. 

and COLEMAN, D.S. Department of Earth, Atmospheric 

and Planetary Sciences, MIT, Cambridge, MA 02139 *now 

at Dept. Geol. Sciences Univ. Texas, Austin, TX 78712 

Central to Armstrong's steady-smte model for crustal 

growth is early massive differentiation of the planet into crust, 

depleted mantle, and core. The lack of abundant pre-3.5 Ga 

crust, and detritus shed from it, is often cited as compelling evidence that large volumes never existed. However, as the geologic record may reflect an increasing probability of preservation with time, one cannot presume any relationship between 

the volume of crust preserved and the volume formed. 

The growth of Archean cratons involves amalgamation of 

dominantly oceanic tectonic elements (island arcs, accretionary 

prisms, etc.) and juxtaposition of these with older sialic basement. Much of the evidence cited against a steady-state model 

involves reconnaissance geochemical and isotopic analysis, 

such as REB abundances and Nd isotopic compositions, of sedimentary and volcanic rocks of greenstone belts which assume 

that sedimentary rocks "average" exposed crust. If the juxtaposition of greenstone belt rocks with sialic basement is tectonic, the geochemistry and geochronology of the greenstone 

belts reveals little about the presence of older sialic crust, and 

may even obscure its presence. 

Nd isotopic data from Archean to Recent rocks have been 

used to constrain the isotopic evolution of the mantle and to 

conclude that the crust and depleted mantle are complementary 

to one another, forming at the expense of "primitive" mantle. 

The retarded isotopic evolution of the mantle over earth history 

is generally attributed to progressive growth of the depleted 

mantle and crust with limited recycling of crust. This interpretation is not supported by the Nd isotopic database for early 

Archean rocks. 

The 3.4 to > 4.0 Ga Acasta gneisses are a compositionally 

diverse suite of tonalitic and granitic orthogneisses, amphibolites, and ultramafic gneisses. Nd isotopic data show considerable variability: 3.9 to 4.0 Ga gneisses have a range in initial 

ENd of +4 to -4; the ca. 3.4 to 3.6 Ga gneisses have a range of 

initial ENd of +1 to -4. Tonalites and granites record a protracted history of evolution with the formation of LREE-enriched magma from sources characterized by both long term 

LREE depletion and enrichment. These features require the involvement of an older, enriched crustal component; abundant 

inherited zircon cores in most of the gneisses support this interpretation. An ENd of +4 at 4.0 Ga represents a minimum value 

for the depleted mantle and the coexistence of tonalites that 

have interacted with enriched crust (ENd=-4) suggests that 

rapid recycling of older crustal material was buffering the isotopic evolution of the crust/mantle system at 4.0 Ga. The Nd 

isotopic evolution of Archean rocks in the context of the data 

from the Acasta gneisses is consistent with progressive homogenization of large volumes of enriched crust and a complementary depleted mantle. The lack of evolution of the depleted 

mantle during the Archean and the small volume of rocks 

greater than 3.5 Ga are testimony to vigorous reprocessing of 

early-formed crust, not progressive growth. Early formation of 

a core, depleted mantle and crust followed by nearly steadystate evolution is consistent with the recent suggestions that 

plumes sample a largely depleted reservoir. 

38 

REGIONALITY OF ()34s IN SEDIMENTS, GRANITES 

AND RELATED MINERALIZATION IN THE 

CALEDONIAN TERRANES OF NORTHERN BRITAIN. 

Boyce A J 1, Lowry, D.2.3, Fallick, A.E.1, Stephens, 

W.E.3 & Hall, A.J.4 

1 Isotope Geosciences Unit, S.U.R.R.C., East Kilbride, 

Glasgow, G75 OQU, Scotland 

2 Dept of Geology, Royal Holloway College, Univ. of 

London, Egham Hill, Surrey, TW20 OEX, England 

3 Dept of Geography & Geology, Univ. of St. Andrews, 

Fife, KY16 9ST, Scotland 

4 Dept of Geology & Applied Geology, Univ. of Glasgow, 

Glasgow, G12 8QQ, Scotland 

Extensive sulphur isotopic analysis of sulphides in 

sediments, granites and related mineralization in the Lower 

Palaeozoic and older terranes of northern Britain show a 

remarkable, distinctive though complicated regionality, which 

can be related to the development of the area during opening 

and closure of Iapetus. 

With the exception of the Northern Highlands Terrane, 

sulphides in the sediment-hosted veins of each segment appear 

to represent principally a homogenised value of local 

sedimentary sulphide. Sedimentary sulphide is strikingly 

enriched in ()34s in most of the Lakesman-Leinster and 

Dalradian Terranes (~+42%o), a feature we believe relates to 

the palaeogeography of each terrane during Iapetus 

development. In contrast, the Southern Uplands Terrane is 

characterized by relative 34s depletion, with values of -17%o 

to -1%o (more typical of diagenetic sulphide). 

Granite-related sulphide in each terrane, while being 

dominated by a lower crustal or magmatic sulphide source 

(0±4%o), show small deviations from O%o towards the ()34s of 

the host or underlying sediments (ranging from -12%o to 

+12%o). We suggest that this was a result of crustal 

contamination during granite genesis and/or development of 

related mineralization. 

The Northern Highlands Terrane is distinctive because 

granite-related mineralization is dominated by a limited 

sulphur reservoir, and because oxidation is uniquely prevalent 

in this terrane. Rapidly increasing fo 2 is evidenced by a 

paragenesis in which sulphides are closely associated with 

sulphates and late Fe-oxide. Such an fo2 increase is associated 

with sulphides displaying ()34s from -1%o (== ()34s of initial 

magmatic sulphur source) to -21%o, and sulphates with ()34s 

from + 7%o to + 12%o. The ultimate source of sulphur in these 

systems is thought to be the underlying North Atlantic 

Lewisian craton. 

Comparisons with the better developed ores in the 

Appalachians of Eastern Canada, in terms of mineralization 

styles and associated ()34s characteristics, bear fruit regarding 

terrane correlation. 
TRACE ELEMENT AND ISOTOPIC CONSTRAINTS FOR 

THE ORIGIN OF MESOZOIC GRANITOIDS IN THE· 

SOUTHERN CANADIAN CORDILLERA 

BRANDON A.D., EAPS, MIT, Cambridge, MA, USA, 

LAMBERT, R.STJ. (Deceased), Geology, U. Alberta, 

Edmonton, Alberta, Canada. 

Trace element and isotopic compositiOns of Mesozoic 

granitoids in southeast British Columbia constrain mechanisms 

for granitoid production in the Cordilleran interior in W. North 

America. These granitoids were emplaced in Middle Jurassic 

and mid-Cretaceous pulses into Paleozoic-Mesozoic accreted 

terranes, Paleozoic metasediments, and Proterozoic 

metasediments. Middle Jurassic rocks range from quartz diorite 

to tonalite to granite, with relative high Ba and low Rb and Nb 

abundances characteristic of granitoids found in Mesozoic arc 

complexes of western North America. Initial Esr range from 

27 to +33, ENd from 0 to -7, and initial Pb isotopic ·ratios lie 

between the NHRL, lower and upper crust Pb compositions. 

These isotopic relations can be produced by addition of 5 to 

25% Precambrian crust to basalt. These granitoids were part of 

an arc magmatic complex during subduction of ocean crust and 

as accretion of outboard terranes occurred west of the North 

American continent. In contrast, mid-Cretaceous batholiths are 

composed of high-K20, weakly peraluminous hornblendebiotite granitoids and strongly peraluminous 2-mica granites. 

These granitoids have lower ENd, higher ESr, and more 

radiogenic Pb isotopic compositions than the Middle Jurassic 

granitoids and completely overlap isotopic compositions of 

southeast British Columbia Precambrian crust. Oxygen, Sr, 

Pb, and Nd isotopes, REE modelling, and phase equilibrium 

constraints are consistent with crustal anatexis of Precambrian 

basement gneisses and Proterozoic metapelites exposed in 

southeast British Columbia. The sequence of intrusion in one 

of the the mid-Cretaceous batholiths (White Creek) constrains 

the melting sequence, and can be modelled as a zone of anatexis 

migrating upwards through the crust, first melting basement 

gneisses followed by melting of overlying metapelites. 

Basaltic magmatic underplating cannot be a primary cause of 

anatexis as there is an absence of Early Cretaceous basalt in the 

southern Canadian Cordillera. Rather, crustal anatexis was 

likely a response to crustal thickening in association with 

terrane accretion and collision along the western margin of the 

North American continent. 

The evidence from this study constrain the origins of the 

compositional distinctions between the predominantly 

tonalite/granodiorite belts along the Cordilleran margin and the 

mostly high-K20 granitoid belts in the interior to be the result 

of distinct processes rather than wholly from melting different 

crustal sources. In subduction zone tectonic settings where 

basalt magma is produced, this juvenile component is recorded 

in the isotopic compositions of the granitoids, and has played a 

key role in their petrogenesis. To generate granitoids in the 

interior regions without contemporaneous mantle input, an 

alternate tectono-thermal control is required to induce crustal 

anatexis. In southeast British Columbia, this control can be 

constrained to terrane accretion and collision resulting in crustal 

thickening. 

39 

PROVENANCE AND DISCORDANCE MECHANISM OF 

ZIRCONS IN THE AUSTRALIAN REGOLITH USING 

SHRIMP AND ELECTRON .MICROPROBE MAPPING. 

BRIMHALL ~H. OONOV AN, John, and SINGH. 

Balbir, Dept of Geology and Geophysics, Berkeley, 

California, 94720, USA. 

SHRIMP study of zircons in the Jarrahdale bauxite deposit 

in the Darling Range of Western Australia shows that rounded 

concordant zircons dominating the gravel and duricrust portions 

of the profile are a mixed exotic suite derived ultimately from 

several younger orogenic belts surrounding the Yilgam Craton. 

Euhedral zircons, often discordant and ubiquitous in all parts of 

the profile but most common in the saprolite, are indigenous, 

chemically-resistant minerals surviving weathering of the local 

granodiorite parent material. These results in combination with 

aluminum mass balance, require that the in situ residual 

concentration theory of cratonal bauxite fonnation be modified 

to incorporate long-distance, probably eolian, transport of a 

chemically-mature surficial detrital suite as an important part of 

regolith mineralization. 

The genesis of a concordant abraded zircon suite requires 

further explanation. An important question is: How much of 

the selectivity occurs below ground in the weathering profile of 

the source region ? Combined SHRIMP and electron microprobe mapping of euhedral zircons shows that the range of 

discordance is related to the presence and frequency of intracrystalline micro crack defects exposed in the SHRIMP craters. 

In areas free of micro cracks, zircons are concordant while 

micro-cracked areas are highly discordant 

Rather than "recent lead loss" explaining zircon discordance, intracrystalline electron microprobe mapping proves that 

it is controlled instead by introduction of excess uranium and 

accumulated along the micro crack networks. Uranium is 

introduced into penneable network in zircon along with other 

soil solution components (AI, Fe, Ti, Pb, Th and Y). Uranium 

comes from the advance of the incipient weathering front where 

sphene containing uranium-rich micro-inclusions such as brannerite is replaced pseudomorphically by porous anatase with 

release of Ca and Si. The other soil solution components 

deposited along with U arise from weathering of other minerals 

besides sphene including feldspars and other major, minor and 

trace minerals. The principal feature in common between U and 

the other mobilized elements is simply their migration through 

the same fracture network. 

The internal micro structural architecture of zircon and the 

crack network emanating from metamict and presumably 

expanded zircon, ensures that uranium mobilized from sphene 

sites eventually will enter the penneable portions of cracked 

zircons by ground water circulation. It also provides a means of 

mixing soil solution components such that they are localized 

together in cracks though derived from distinct mineral sites. 

Bedrock zircon metamictization and the micro-structural 

architecture of the granitic parent material offer insights as to 

the dual selectivity mechanisms of preservation of the exotic 

concordant rounded zircon suite and the destruction of imperfect 

indigenous euhedral crystals in the local weathering environment. The concordant zircon suite represents the most durable, 

structurally perfect and uncracked portion of a spectrum of cogenetic perfect to imperfect zircons. At depth within the Jarrahdale bauxite profile, we see the incipient stages of zircon 

weathering providing structural fragmentation and chemical 

alteration controlled by micro cracks. These effects increase 

upwards leaving severely damaged euhedral zircons in the 

pisolitic gravels. 
RELATIONSHIPS BETWEEN ISOTOPE 

GEOCHEMISTRY OF BASIC LAVAS, TIME, 

LOCATION AND MANTLE UPWELLING : 

BASALTS FROM THE FRENCH MASSIF 

CENTRAL (FMC) 

BRIOT, D., NERC Isotope Geology Laboratory, 

Keyworth, Nottingham NG125GG UK, and URA10 

CNRS, Dpt Sci. Terre, Univ. B. Pascal, 5 rue 

Kessler 63038 Clennont-Ferrand Cedex, France 

Classical isotopic intercorrelation diagrams show 

that depleted (DM), enriched (EM I and/or EM II) 

and HIMU-type mantle signatures must exist in the 

upper mantle beneath the Massif Central (France) to 

account for the Sr-Nd-Pb-isotope geochemistry of the 

FMC basalts and basanites. The main questions we 

address here are : (1) are these mantle components 

parts of the lithospheric mantle, the asthenospheric 

mantle or both; (2) or is there a mantle plume beneath 

western Europe. Such questions may be answered by 

looking carefully to how the mantle components have 

been sampled by the rift related-volcanism. Taking 

location and time constrains into account, we show 

that relationships exist between isotope and trace 

element features of the basalts and their chronological 

and tectonic setting. 

Considering the complicated history of western 

Europe, particularly the possible effects of the 

Hercynian orogeny on the subcontinental mantle, we 

claim that all the components that are needed to 

account for the isotopic variations in the mafic 

magmas exist in the subcontinental lithospheric mantle 

beneath the FMC without any intervention of 

upwelling asthenospheric (rift-related) material or deep 

mantle plume substances. 

In our proposed model, differences in the trace 

element geochemistry of volcanic districts which are 

located at the same place but have not the same age can 

be explained by differences in the mineralogical 

composition of the residual mantle for slightly 

different degrees of partial melting. The differences in 

these proportions and in the amount of melts produced 

reflect changes in the thermodynamical conditions 

and/or depth of melting and are to be related with 

isotopic heterogeneities in the subcontinental mantle. 

This would place important constrains on the 

geochemical structure of the subcontinental lithosphere 

beneath the french Massif Central. 

40 

SEPARATION OF THE NATURAL AND BOMBTESTING CONTRIBUTIONS TO OCEANIC 

RADIOCARBON 

BROECKER. W.S., Lamont-Doherty Earth 

Observatory, Palisades, NY 10964, and Stewart 

Sutherland, Lamont-Doherty Earth Observatory, 

Palisades, NY, 10964, and Tsung-Hung Peng, 

Oak Ridge Natl. Lab., Envl. Sciences Div ., Oak 

Ridge, TN 37830. 

The distribution of radiocarbon in the ocean offers 

the most powerful constraints on general circulation 

models used to estimate the uptake of anthropogenic 

CDl by the sea. The utility of this tracer will be far 

greater if a separation between its natural and bombtesting components can be made. We discuss a new 

attempt to do this which involves the use of tritium 

and dissolved silica data. 
CONSTRAINTS ON THE HISTORY OF THE 

SCANDINAVIAN ICE SHEET USING IN SITU 

COSMOGENIC NUCLIDES. 

EDWARD J. BROOK, Graduate School of Oceanography, 

University of Rhode Island, Narragansett Bay Campus, 

Narragansett, RI, USA 02882, ATLE NESJE, Department 

of Geography, University ofBergen, N-5035, BergenSandviken, Norway, GRANT M. RAISBECK and 

FRAN<;OISE YIOU, CSNSM, Batiment 108, 91405, 

Campus Orsay, France . 

The age and origin of in situ weathered bedrock surfaces 

and associated weathering boundaries on mountain summits in 

western Norway and other areas of Scandinavia have been 

controversial for over a century. The weathering boundaries are 

important because they appear to delineate the vertical limits of 

an ice sheet and may constrain vertical ice limits at the Late 

Weichselian maximum. 

To further constrain ages of these features we made 10J3e, 

26 AI, and 3He measurements in a number of surface samples of 

quartz veins from four different bedrock weathering zones in an 

altitude transect at SkAla in western Norway. 1 <13e exposure 

ages calculated assuming no erosion are 8.8 kyr for a Younger 

Dryas surface at 1145 m, 12.7 kyr for bedrock at 1420 m, above 

the Younger Dryas limit but below a "trimline" at 1350 m, 20.1 

kyr above that trimline, at 1500m, but below a trimline defined 

by the lower limit of the autochthonous weathered blockfield at 

the mountain summit (1740 m), which has an exposure age of 

53.6 kyr. Assuming the oldest surface is at erosional steady 

state the erosion rate is 1.2 cmlkyr and corresponding ages for 

the three younger samples are 9.6, 18.6, and 36.3 kyr. 

The data clearly show relative age differences between all 

four surfaces and that the blockfield surface is pre-Late 

Weichselian in age. Uncertainty in the true erosion rate 

precludes absolute determination of the location of the Late 

Weichselian vertical limit and therefore the exposure ages 

suggest three possible scenarios. 1) If erosion is negligible the 

Late Weichselian limit may be at the upper trimline. 2) If the 

erosion rate is- 1 cmlk:yr the Late Weichselian limit may be at 

the lower trimline. 3) The upper trimline may represent a 

thermal boundary in a Late Weichselian ice sheet where older 

topography was preserved between cold-based ice and the lower 

trimline may be the Late Weichselian limit. Both of the former 

two scenarios must explain the existence of two weathering 

boundaries above the younger Dryas limit. 

3He concentrations were determined in two samples; 

3He/ 1 <13e ratios are 0.2±0.04 and 1.1±0.6, indicating significant 3He loss (the production ratio is -20-25). In thin section the 

vein quartz is extremely fine-grained; rapid loss from small 

grains is the likely explanation for the low ratios. 26 Al/1 OBe 

ratios range from 6.1±0.8 to 9.5±5.4, consistent within error 

with predicted production ratios ( -6), indicating that these 

surfaces have not been subject to long-term burial since 

exposure .. 

41 

APPLICATIONS OF Hlse AND 26AJ IN EXAMINATION 

OF THE DEVELOPMENT OF TROPICAL SOILS AND 

LANDFORMS 

BROWN. Erik T., BOURLES, Didier L., RAISBECK, 

Grant M., and YIOU, Franyoise, (all at Centre de 

Spectrometric Nucleaire et de Spectrometric de Masse, 

CNRS-lN2P3, Batiment 108, 91405 Campus Orsay, 

France) and COLIN, Fabrice, SANFO, Zakaria, and 

DESCARCEAUX, Sophie (all at ORSTOM and Lab. 

Geoscience de I'Environnement, UMGECO, AixMarseille III, 13397 Marseille Cedex 13, France) 

We have investigated the development of iron crust 

lateritic systems in arid tropical environments and of rain 

forest soil profiles using lOBe and 26At produced in situ in 

quartz veins and pebbles. 

In the stable West African Craton in northern Burkina Faso 

a series of sequential and connected iron auriferous crust 

systems (including an active river drainage, and a series of 

lateritic surfaces which are presumed to represent relict 

drainages dating from as early as the Tertiary) were examined. 

They may have formed either through in situ chemical 

alteration or colluvial transport. The first scenario requires 

that the lateritic material has always been in its present 

position, while the other implies previous surface exposure of 

material currently emplaced at depths up to several meters. 

Systematic study of distributions of cosmogenic nuclides 

provides criteria for distinguishing between the proposed 

chemical and physical processes. In addition such results may 

be used to constrain the histories of erosion and burial, and 

potentially, of climatic conditions associated with formation of 

lateritic systems. 

Results from outcropping quartz veins suggest that the 

mean erosion rate in this region is about 3 to 7 m per Myr. 

Depth-dependent distributions of lOse and 26Al in quartz 

cobples and pebbles incorporated in iron crusts were used to 

distinguish surfaces undergoing burial from those undergoing 

erosive loss. Results from sections of the lowland fan .are 

consistent with mean accumulation rates of a few meters 'per 

Myr. Quartz cobbles, presently at depths of a few meters in a 

paleochannel filled with rapidly deposited fluvial-colluvial 

material, have lOse distributions suggesting that formation 

ofthe lowland lateritic surface may have been initiated during 

an erosive episode, presumably associated with wetter climate, 

roughly 300 ky B.P. 

A contrasting environment, the rain forest at Dimonika, 

Congo, has also been examined using lOse and 26At. 

Preliminary measurements indicate very low erosion rates ( -3 

m Myr-1) at ridgecrests, consistent with the stability and great 

age of the regional surface formations. Samples from soil pits 

and road cuts provide the possibility of determining whether 

quartz cobbles found at discrete horizons in soil profiles 

"stonelines" are alluvial deposits or relicts of chemical 

weathering. In conjunction with estimates of collapse these 

data may also be used to study rates of penetration of 

weathering fronts. 
VISUALISING THE STRUCTURE OF CONTINENTAL 

LANDSCAPES: AN AFRICAN VIEW 

BROWN. Roderick. Fission Track Research Group, 

Department of Geological Sciences, University College, 

Gower Street, London, WC1E 6BT, United Kingdom, 

FOSTER, David and GLEADOW, Andrew, Victorian 

Institute of Earth and Planetary Sciences, La Trobe 

University, Melbourne, 3083, Australia 

Regional apatite fission track analysis (AFTA) studies 

within eastern and southern Africa (Kenya, Namibia and 

South Africa), and indeed other parts of the world, have 

all revealed a close temporal and spatial association 

between the tectonic and denudational histories of 

continents. In particular, two Late Mesozoic tectonic 

episodes, characterised by major intracontinental 

deformation involving rifting and basin inversion, have 

been identified in Africa: one during the Early Cretaceous 

(140-120 Ma) associated with opening of the South 

Atlantic and a second during the late Cretaceous-early 

Tertiary (80-60 Ma), associated with a major change in the 

spreading geometry within the Central and South 

Atlantic ocean basins. Both these episodes were 

accompanied by enhanced rates of crustal denudation 

and cooling. 

A third episode of crustal cooling (at 90 ± 10 Ma) has 

been documented along the eastern margin of southern 

Africa, and its effects extend well inland (westwards) into 

the Kaapvaal craton region. This episode is accompanied 

by a dramatic increase in the rate of sediment 

accumulation (from -30 m.Ma-1 to over 300 m.Ma-1) and 

basement subsidence within the offshore basins along the 

continental margin of Mozambique. The timing and 

magnitude of cooling of this event is difficult to reconcile 

with the view that geomorphic stability is a characteristic 

of cratonic regions. 

More interesting though, is the remarkable similarity 

between the chronology of landscape change determined 

from AFTA within northern Namibia and eastern Kenya. 

Taken in conjunction with the recent interpretations of 

the regional tectonic history of West, Central and East 

Africa, this similarity suggests that reactivation of major, 

pre-existing crustal structures (such as regional shear 

zones and fault systems) played a significant role in 

determining the pattern of landscape development across 

the African continent. 

Importantly, the chronology and style of landscape 

evolution revealed through fission track analysis 

challenges some of the fundamental concepts of previous 

models of long-term, continental landscape evolution put 

forward for Africa, and also widely applied to other 

continents. The new long-term landscape history for 

southern Africa revealed through AFTA also accords 

well with the record of clastic sedimentation within the 

offshore basins and is easier to reconcile with current 

models of rifted continental margin development. 

42 

DATING OF GEOMORPHOLOGIC SURFACES IN THE 

ANTARCTIC DRY VALLEYS WITH IN SITU PRODUCED 

COSMOGENIC 3HE AND 21NE 

BRUNO. L.A.l; BAUR, H.1

; IVY, S.D.2; SCHLUCHTER, C.3; 

SIGNER, P.1; WIELER, R.1 

1

Isotopengeochemie, ETH ZUrich, 8092 ZUrich, Switzerland; 

2Ingenieurgeologie and Institut fiir Teilchenphysik, ETH 

Hooggerberg, 8093 ZUrich, Switzerland; 3Geologisches 

Institut, Universitat Bern, 3012 Bern, Switzerland 

Abundances of cosmic-ray-produced nuclides have been 

detennined in different rock types exposed in the Dry Valleys, 

North Victorialand, Antarctica, to provide constraints oo 

geomorphologic, glaciologic and climatologic histories. 

Emphasis was put on samples from erratic boulders within the 

Sirius Formation and on bedrocks from the Table MOuntain 

(Schliichter et al., 1994). Foc comparison, aliquots of quartz 

separates from Table Mountain were also analyzed foc their 

concentrations of cosmo~enic 1<>& avy et al., 1994). 

The stable nuclides He and 21Ne were extracted from 

pyroxene- and quartz-separates from dolerites, granites as well 

as sandstones and measured in a mass spectrometer resolving 

3

He- from HD-ions. Noble gases were released either in a 

single melting step at 1800°C foc 30 min. or by step-wise 

heating to separate cosmogenic and atmospheric Ne. In our first 

measurements it turned out that He is always partially lost from 

quartz. Thus, we analyzed only Ne in later quartz fractions. All 

samples showed clear contributions of cosmogenic 21Ne besides 

variable concentrations of atmospher:ic Ne. 

Presently available data of the eHef1

Ne)008 ratios in the 

pyroxenes point to a value of 4.5±0.2, whereby no cocrections 

for possible variations tt the chemical compositions tt the 

minerals have been possible at present. 

To calculate 3He and 21Ne exposure ages, we use the 

production rates and altitude/latitude adaptions proposed by Lal 

(1991). The ~e and 21Ne ages obtained from the pyroxenes are 

generally identical within a few %. Preliminary exposure ages 

from a doleritic erratic boulder within the Sirius Formation are 

4.9 Ma and 4.8 Ma as deduced from 3

He008 and 21Ne008, 

respectively. An origin of the Sirius Formation in the Upper 

Pliocene/Lower Pleistocene is not C<XTobocated by these ages. 

References: 

S. Ivy et al. (1994), this volume. 

D. Lal, Earth Planet. Sci. Lett. 104, 424-439 (1991). 

C. Schliichter et al. (1994), this volume. 
SULFUR RICH ENVIRONMENTS IN THE INITIAL 

TRANSGRESSIVE PHASE AT LEG 144 GUYOTS (OCEAN 

DRILLING PROGRAM, WESTERN PACIFIC): SULFUR 

ISOTOPES AS INDICATOR FOR SULFATE REDUCTION. 

BUCHARDT. B .. and ISRAELSON, C. Institute of 

Geology, University of Copenhagen, 0ster Voldgade 10, 

DK-1350 Copenhagen K, Denmark. 

The Initial Transgressive Phase (ITP) at the Leg 144 

Guyots is characterized by a typical association of sedimentary 

facies. From bottom to top: I) in situ weathered volcanic rocks 

(flow basalt and volcanoclastics), II) variegated clays, partly 

pyritic, III) grey clay, pyritic, homogeneous, with plant roots, 

IV) black clay, peaty, laminated or bioturbated, V) muddy 

grainstone, black to grey, bioturbated, with well preserved 

shell remains, VI) lagoonal limestone. Site 877 at the 

Wodejabato guyot represents the typical development of the 

ITP. At Sites 871, 873, 874 and 879, one or several of the 

ITP facies can be observed. 

The black clay facies (facies IV) is highly carbonaceous 

(TOC up to 40%) and sulfurous (TS up to 25% ). The organic 

matter is dominantly of terrestrial origin and contains plant 

fragments, spores, charcoal and disseminated plant material. 

The sulfur occurs as pyrite, markasite, organically bound 

sulfur and probably also as free elemental sulfur. 

The variegated clays (facies II) typically consist of a red 

lower part and a blue to grey-blue upper part. The lower part 

is free of sulfur, while the upper part contains an increasing 

amount of sulfur developed as sub-millimeter euhedral grains 

of pyrite. Organic carbon is not observed in this facies. 

The high content of sulfur in facies II to IV has been 

interpreted either as a result of hydrothermal activity or of 

bacterial sulfate reduction. In order to clarify this, sulfur 

isotope determinations have been performed on sediments from 

the ITP. Hydrothermal sulfides would either have a magmatic 

composition (~34S ~ 0 o/oo) or sea water composition (~34S ~ 

+ 10 to +20 o/oo). H2S produced by bacterial reduction of sea 

water sulfate would be strongly depleted in heavy sulfur (~34S 

~ -50 to 0 o/oo). 

The isotope composition of sulfur from facies II, III and IV 

varies from -50 o/oo to 0 o/oo. fhese numbers clearly point 

to a bacterial origin. This implies the following model for 

sulfur enrichment in ITP-sediments: 1) Marine flooding of an 

organic-rich back-reef mangroval swamp represented by facies 

IV and V. 2) Intense bacterial reduction of marine sulfate 

within the black clay of facies IV. 3) Downwards diffusion of 

H2S into the underlying clays, reduction of red iron oxides to 

blue-grey iron sulfides and growth of pyrite. 

43 

DIAMOND FORMATION AND ERUPTION AGES BY 

LASER 40 AR-39 AR DATING OF SYNGENETIC 

INCLUSIONS 

BURGESS R., Dept. of Geology, University of Manchester, 

Manchester, M13 9PL, UK, KELLEY S.P., Dept Earth 

Sciences, Open University, Milton Keynes, MK6 7 AA, UK, 

TURNER G. Dept. of Geology, University of Manchester, 

Manchester, M13 9PL, UK and HARRIS J.W., Dept of 

Geology, University of Glasgow, Glasgow, G12 8QQ, UK. 

Laser dating of individual syngenetic clinopyroxene 

inclusions in diamonds can potentially give both the diamond 

crystallisation and host kimberlite, or lamproite, eruption ages. 

Inclusions within diamonds that pre-date eruption will lose 

40 Ar* at the diamond/inclusion interface during mantle 

residence, this process will cease at eruption. Therefore, use of 

a laser to drill into diamonds in which inclusions are 

completely encapsulated, will initially release the interface gas, 

followed by Ar from within the inclusions. The latter can be 

used to calculate the eruption age and, when combined with the 

interface Ar, should give the age of diamond formation. 

An UV laser (A=266nm) has been used to drill into 

diamonds which contain clinopyroxene inclusions, to release 

the interface 40 Ar*. Following this, the inclusion was melted 

with an IR laser (A=1064nm). Samples include two stones from 

the Argyle lamproite and one stone from the Finsch kimberlite. 

The Finsch diamond was cleaved prior to analysis, to expose 

the inclusion at the diamond surface, in order to directly 

compare the UV and IR extraction methods. The UV laser 

experiments on the Finsch sample gave apparent ages that 

decreased from >1Ga to 121 ± 33 Ma, the latter age is identical 

with those obtained subsequently by IR laser (113 ± 6 Ma) and 

are also identical with the eruption age of the Finsch kimberlite 

(z120 Ma, [1]). The high age for the initial extraction using the 

UV laser probably resulted from release of some residual 

interface gas that was not released by cleaving. This is also 

reflected in the integrated age of 205 ± 30 Ma indicating that 

the diamond is older than the kimberlite, however a formation 

age could not be obtained because some of the interface 40 Ar* 

was lost during cleaving. 

The UV laser extracted about 50% of the total 40 Ar* from 

one of the Argyle samples (Al), together with negligible 39 Ar 

above the blank level. IR laser extraction of A1 gave an 

apparent age of 1091 ± 42 Ma which is the eruption age [2]. 

However, the integmted age of 1521 ± 80 Ma is essentially 

identical to the crystallisation age obtained previously of 1580 

± 80 Ma [3]. The second Argyle sample (A4) yielded 

negligible 40 Ar* using the UV laser, but gave two IR laser 

extractions with high apparent ages of ;;:::2 Ga. Besides 

indicating the possibility of an old crystallisation age, these 

high ages may also result from either incomplete melting of the 

inclusion (i.e. 39 Ar not totally extracted), or the presence of 

excess 40 Ar* (e.g. in volatile-rich inclusions). Experiments are 

underway on a suite of garnet and clinopyroxene inclusions in 

diamonds from the Ompa kimberlite to further test the method. 

[1] Smith C.B., Gurney, J.J., Harris J.W., Robinson D.N., Shee 

S.R. and Jagoutz E., 1989, Proc. 4th Int. Kimberlite Conf., v. 

2, p. 853-863. 

[2] Skinner E.M., Smith C.B., Bristow J.W., Scott-Smith B.H. 

and Dawson J.B., 1985, Trans. Geol. Soc. S. Mrica, v. 88, 

p.335-340 

[3] Richardson, S.H.,1986, Nature, v. 322, p. 623-626. 
LASER PROBE STUDY OF HE, AR AND C01 

TRAPPED IN A DUNITE XENOLITH, 

REUNION ISLAND. 

BURNARD, P.G. and TURNER, G., Department 

of Geology, Manchester University, Manchester M13 

9PL, and STUART, F.M., S.U.R.R.C., East Kilbride, 

Scotland 

The 'Dupal' island of Reunion in the southern 

Indian Ocean is characterised by exceptionally 

consistent 87srJS6sr (0. 70397 - 0. 70436), 143NdJ144Nd 

(0.51282- 0.51286) and 3He/4He (12.9 ±0.05) ratios. 

In contrast, 40 ArJ36 Ar ratios vary widely (300 - 8000). 

Specific fluid inclusion generations in a dunite 

xenolith were analysed using the Ar-C~-He (ARCHE) 

analysis system in Manchester, England, in order to 

determine mantle C~IHe ratios and place limits on the 

36 Ar concentration of high 3He/4He mantle volatiles. 

Laser decrepitation permitted particular inclusion 

generations to be targeted, vital to distinguish volatiles 

trapped in 'primary' inclusions from those associated 

with fractures or secondary melt features. 

Normalised plots indicate a constant 4He/40 Ar of 

1.92 for all inclusion generations. Fluid C~/He (0.38 

- 1.7 x10S) and C~/40Ar (0.38 -7.3 x1oS) 

compositions did not show any systematic variation 

with inciusion morphology. 40 ArJ36 Ar ratios average 

higher in 'primary' inclusions (4600) than in planar or 

fracture associated inclusions (1900). Linear 

correlations between co2J36 Ar and 40 ArJ36 Ar are 

consistent with atmospheric contamination reducing 

40 ArJ36 Ar and c~J36 Ar ratios in fracture associated 

inclusions. 

The range in C~/He observed in Reunion is 

similar to that of hotspots globally. These variations 

occur on a millimetre scale and are independent of HeAr fractionation or inclusion generation. The C~/He 

variations cannot be attributed to fractionation during 

fluid-melt partition as there is no concomitant He-Ar 

fractionation (Ar being less soluble in basaltic melts 

than both He and C~ [Lux, 1987]). It is possible the 

c~- noble gas fractionation is related to solid c (i.e. 

COJ2-) - C~ partitioning. 

Sediment recycling to the mantle would increase 

CJ3He but reduce C/4He over time. The similarity in 

C~J3He and C~/4He in Reunion and non-Dupal 

islands (Hawaii, Iceland) may indicate a separate, 

uncontaminated, high 3He/4He niantle source exists 

(the PHEM) which dominates mantle volatiles. 

Lux., G., 1987, The behaviour of noble gases in 

silicate liquids: solution, diffusion, bubbles 

and surface effects, with applications to 

natural samples, Geochim. Cosmochim. Acta, 

v51, 1549-1560. 

44 

PREUMINARY PETROLOGIC AND 

SEDIMENTOLOGIC COMPARISONS BETWEEN 

PALEOCENE CONGLOMERATES OF POINT REYES 

AND POINT LOBOS, CALIFORNIA, U.S.A 

BURNHAM, Kathleen and Anderson, Thomas B., 

Sonoma State University, Rohnert Park, Ca. 94928, 

U.S.A 

Preliminary studies reveal many similarities 

between the Point Reyes Conglomerate at Pt. Reyes and 

the lower conglomerate section of the Carmela 

Formation at Pt. Lobos, which are on opposite sides of 

the San Gregorio Fault. Outcrop appearance, 

sedimentary sequence and structures, distinctive clasts, 

abundance of clast types, stratigraphic and structural 

relationships to bedrock, and interpreted facies of 

origin are virtually identical. 

Both conglomerates rest nonconfonnably upon and in 

fault contact with granitic basement of the Salinian 

Block. Both are clast supported, poorly sorted, and 

occur in lensoidal beds up to several meters thick. Both 

include beds of conglomerate grading upward into 

sandstone, with sharp basal definition and scour 

channels. Matrices are angular and poorly sorted, and 

are very coarse in conglomerate beds and finer in 

sandstone beds. Flame structures, shale rip-up clasts, 

and fine laminae are locally abundant in the sandstone 

beds. The most abundant and largest (up to 2.5 m) clasts 

are granitic, and match the basement rock Second in 

abundance and size are well-rounded distinctive purple 

K -spar porphyry clasts. All other clasts are smaller, 

and most are well-rounded, fine-grained, and very 

hard. Both conglomerates are interpreted as having 

formed in a submarine canyon. Previous workers have 

used paleontological evidence to establish a Paleocene 

age for both the Pt. Reyes Conglomerate (Galloway, 

1977) and the Carmela Formation (Bowen, 1965). 

Several of the fossil species and genera they identified 

occur in both units. 

These results indicate the Point Reyes Conglomerate 

could be a part of the Carmela Formation, separated by 

180 km of lateral movement on the San Gregorio Fault 

since the Paleocene. However, K-Ar dates on feldspar 

concentrates of the purple porphyry clasts yielded 

disparate dates. Three clasts of the Pt. Reyes 

Conglomerate registered 82 Ma to 89.5 Ma, and one clast 

from the Carmela Formation registered 102 Ma. Thus, 

superficially, the limited geochronologic comparisons 

do not support a simple correlation between the two 

units. Further study is planned. 

Bowen, 0. E., 1965, Stratigraphy, Structure and Oil 

Possibilities in Monterey and Salinas Quadrangles, 

California. in: Rennie, E. W., Jr., ed., Symposium of 

Papers, AAPG, Pacific Sect., 4oth Ann. Mtg., 

Bakersfield, Calif., 1965, p. 48 - 67. 

Ganaway, A J., 1977, Geology of the Point Reyes 

Peninsula, Marin County, California. Calif. Div. 

Mines and Geology Bull. 202, 72 p. 
TRACING THE WATER MASS EVOLUTION OF Sr, Nd 

AND Pb IN THE CENTRAL PACIFIC OCEAN 

BURTON. K.W., O'NIONS, R.K., and MARTEL, D.J. 

(Dept. ofEarth Sciences, Univ. of Cambridge, Downing 

St., Cambridge, CB2 3EQ. U. K.; E-Mail kwblO@esc.cam. 

ac.uk) HEIN, J.R . (U.S. Geological Survey, Pacific Marine 

Geology, 345 Middlefield Road, Menlo Park, California, 

CA 94025, USA) 

The residence time of Sr is much greater than the mixing 

time of the major oceans. Consequently, the Sr isotopic 

composition of seawater is spatially uniform on a global scale. 

However, there are significant variations in the Sr isotopic 

composition of seawater over time. This combination of spatial 

uniformity and temporal variation enables the use of this 

system for high-resolution dating. By comparison, the 

residence times of Nd and Pb are less than the mixing times of 

the oceans, and the Nd and Pb isotopic compositions of 

seawater are likely to reflect spatial variations in the sources of 

those elements. Thus, in principle, the combined use of Sr, Nd 

and Pb isotopes provides both temporal and spatial 

information on water mass evolution. 

Hydrogenous Fe-Mn crusts are considered to grow directly 

from seawater, and potentially preserve a record of the 

chemistry of the seawater from which they grow. This study 

presents high-precision Sr, Nd and Pb isotopic data for a 

single Fe-Mn crust sampled from the South Karin ridge, at a 

water depth of 2390-2290 m, in the Central Pacific Ocean. Sr 

isotope compositions indicate that growth of the crust started 

some 20 Ma ago. Initial growth during the period 20 to 15 Ma 

appears to have been rapid, about 12 mm!Ma. Whereas, 

growth rates from 15 Ma to the present day were lower, 

between 3 and 6 mm!Ma. The Pb isotopic composition of the 

outermost 1 mm of crust gives a 206pbf204pb = 18.624±0.012. 

Whereas, Nd for the same sample gives an ENd value of 

-3.12±0.11, in good agreement with present day seawater 

compositions at similar depths (Piepgras & Jacobsen, 1988). 

Both, Pb and Nd isotopes show significant variations during 

growth. Nd shows a systematic decrease in ENd to a value of 

-5.79±0.12 at the base of the crust. While Pb shows an 

increase in 206pbf204pb to a value of 18.855±0.012 over the 

same interval. 

These results indicate that there have been significant 

changes in the sources of Nd and Pb at this locality and that 

the Nd isotopic composition of seawater supplying the crust 

some 20 Ma ago was significantly less radiogenic than the 

present day, whereas Pb was more radiogenic. 

Piepgras, D.J. & Jacobsen, S.B., 1988, The isotopic 

composition of neodymium in the North Pacific: Geochim. 

Cosmochim. Acta, v. 52, p. 1373-1381. 

45 

PEDIMENT FORMATION INTERVALS IN THE GRAND 

CANYON: A TEST CASE FOR THE USE OF COSMOGENIC NUCLIDES 

CAFFEE, MARC W., L-237, Livennore, CA 94550; LUCCHITI A, IVO, USGS, 2255 N. Gemini Dr., Flagstaff, AZ 

86001; FINKEL, ROBERT C., SOUTHON, JOHN R., 

LLNL, L-237, Livermore, CA 94550; DAVIS, SIDNEY, 

and DAVIS, MARIE, P. 0. Box 724, Georgetown, CA 

95634, USA. 

The Quaternary history of the Colorado River in the eastern 

Grand Canyon is one of overall downcutting punctuated with 

periods of backfilling. These activities were probably controlled by climatic fluctuations in the Rocky Mountain source 

region. Washes tributary to the Colorado show similar downcutting and backfJ.lling controlled by the grade established by 

the Colorado River. The processes involved on the tributaries 

are debris-flow emplacement and pedimentation. Continued 

downcutting of the Colorado river results in multiple terraces 

and pediments in which the lowest levels are the youngest ~d 

have the lowest number in our stratigraphic nomenclature. 

These surfaces are mantled by locally derived rocks, many of 

which contain quartz and chert. 

An obvious way to detennine the conditions under which 

these features are formed is to compare their ages with those of 

known climate fluctuations (e.g. glacial events), but there are 

few direct means of dating these debris flows. One promising 

method for obtaining qwmtitative data is to measure nuclides 

produced in-situ in surface rocks by cosmic-ray bombardmenL 

The production rates of ~ and 26 AI are well known in quartz. 

We have measured 10Be and 26 AI from four stratigraphically 

distinct debri$-flow levels in the Tanner Creek area and from 

one pediment in the Comanche Creek area. Samples were collected from several boulders at each level to test the reliability 

of the technique. Samples from the same surface should give 

concordant ages and samples from the lowest level (3) should 

be younger than those from higher levels (5 and 6). Measurements of 26 AI in Comanche-3 and Tanner-3 indicate minimum 

exposure ages of 1590±1000 and <2600 yr., respectively. Measurements of 26 AI and 1

0se in cherts indicate minimum exposure ages of 42±5 Ka from Tanner-4 (1 sample) , 75-94 Ka for 

Tanner-S (3 samples), and 142-180 Ka for Tanner-6 (2 samples). We have analyzed 10Be and 26 Al in nine samples from the 

Commanche-4 level and the apparent exposure ages are 70-100 

Ka. 

0~ results thus far indicate that there is a good qualitative 

correlation between apparent exposure age and stratigraphic 

level for the pediments up through Tanner-6. We also see a 

reasonable qualitative correlation between apparent exposure 

age and our observations of carbonate development, although 

these ages appear young from the perspective of soil 

morphology. One possible explanation is that the surfaces have 

been deflated. 

This work was performed under the auspices of the U.S. D.O.E. 

by LLNL under contract W-7405-Eng-48. 
TRACING STABLE ISOTOPES IN THE GRAPE 

PLANT ENVIRONMENT TO UNDERSTAND THE 

COMPOSITIONS OF VINTAGE WINES 

CALDWElL. E. A and INGRAHAM N. L., Water 

Resources Center, Desert Research Institute, 

University of Nevada System, Las Vegas, Nevada 

89132. 

Precipitation, vapor, ground water, soil water, 

grape leaf and berry water were collected in Napa 

Valley, California for stable isotopic analysis. In 

addition grape must and selected vintage wines 

from the region were analyzed for their oxygen 

isotopic compositions. The grape leaf water falls 

on a transpiration line of slope 2.3 with the 

composition of the soil and ground water, while the 

grape berry water falls on a transpiration line of 

3.0. These waters are up to 18 per mil more 

enriched in oxygen-IS than the soil waters, 

showing the largest observed isotopic shift from 

their source. The different transpiration slopes 

observed are controlled by differing isotopic 

kinetic effects imparted by the different surface 

transmissivities of the beny and leaf. 

In addition, the leaves and berries responded to 

a small rain shower of a more depleted isotopic 

composition, the leaf water being most responsive 

shifted 8 per mil in a I8o while the grape berries 

showed a delayed shift of only 3 per mil in a 18o. 

These systems behave differently due to their 

different surface area/volume ratios of water and 

may be modeled as a terminal flow system. 

The compositions of .the vapor fluctuate some 

but appear to be in isotopic equilibrium with the 

soil and ground water (as controlled by 

precipitation) without direct contact. And, a 

correlation is observed between the temporal 

variations in the compositions of the vapor, and 

grape leaves and berries as controlled by direct 

contact by kinetic equilibrium. 

The isotopic differences between the grape 

waters, and the grape must and wines are more 

controlled by transpiration (as affected by 

atmospheric conditions) from the grape prior to 

harvest than they are by the isotopic compositions 

of annual rainfall. Nevertheless, climate 

information may be obtained by the analysis of 

wine as fermentation does not significantly alter 

the isotopic composition of the grape juice. The 

isotopic compositions of selected wines from Napa 

Valley, California show variations of more than 5 

per mil in a I8o in wines produced in the last 30 

years and indeed represent variations in 

environmental conditions. 

46 

EVALUATION OF C AND N INVENTORIES, a13C AND 

A14C IN DEEP SOll..S FROM MATURE AND 

REGROWING FOREST (CAPOEIRA), AND PASTURE 

OF EASTERN AMAZONIA. 

CAMARGO P.B.DE*, Martinelli L.A., Pessenda L.C.R., 

Centro de Energia Nuclear na Agricultura, CP 96, 

Piracicaba, SP 13400-970, Brazil; TRUMBORE S. E., 

Dept. of Geosciences, University of California, Irvine, 

CA 92717-3100; Davidson E.A., and Nepstad D.C., The 

Woods Hole Research Center, Woods Hole, MA 02543. 

To better understand bow tropical forest clearing, pasture 

management, and forest regrowth affect deep soil C cycling 

we investigate: C and N inventories to estimate belowground C and C/N dynamics, a13C analysis to calculate C 

inputs to pasture soils from C-4 grasses and A14C analyses 

of soil C02 to estimate residence times of soil organic 

matter. 

Much of the eastern Amazon Basin, including our study 

area in Paragominas, Para, is characterized by deep oxisols 

(more than 8m). This region experience a significant dry 

season, although most of its forests retain leaf canopies 

continuosly. Deep rooting of trees in mature forests support 

evapotranspiration during the dry season and impact C and 

N inputs at depth. 

Measurements of the 0-8 m soil layer show that C and N 

decrease from the surface (2.00 - 0.15%) to 3m (0.20 -

0.01%) respectively, then remain constant to the bottom. 

The C/N ratio varies between 10 to 20 and usually increases 

below 2m. There are no significant differences below 2m 

between the three ecosystems. 

In the upper 1 Ocm the bulk density increases from mature 

forest and "capoeira", to pasture (0.96, 1.07, 1.24g/cc) 

respectively. Below that, bulk density is roughly constant 

(1.3±0.5g/cc). Large bulk density at the surface in pastures 

is due to compaction because of loss of organic matter and 

physical disturbance. 

The a13c values in the upper 10cm are -27.3, -26.7, 

-24.4o/oo, for forest, "capoeira" and pasture. These results 

indicate that only 20% of the SOM from pasture has been 

added since the change of vegetation (1969). The influence 

of C-4 vegetation can be seen in pastures to a depth of 

25cm, but not below. The values of a13C increase with the 

depth (by 1 to 4o/oo) in the soil proflle. No difference is 

observed at depth among mature, regrowing forest and 

pasture, probably because the deep pasture and "capoeira" 

profiles retain the characteristics of the primary forest before 

vegetation change. This increase could reflect fractionation 

due to decomposition or movement of organic matter down 

the profile. 

The few A14C data that we have now, show higher positive 

values in the surface of forest and pasture ( + 149 and 

+ 130o/oo) and indicate that turnover of soil C in this layer is 

rapid (less than 30 years). A14C values decrease with depth 

quickly in the frrst meter to -100%o and -630% in forest and 

pasture and remain between -750 and -800 at greater depth 

in both profiles. 

Our observations, together with C02 flux in the soil will 

be used to construt models with predict changes in C and C 

isotopes accompanying forest-pasture vegetation change. 

*Current address: Depart of Geosciences 

UC Irvine, Irvine, CA 92717 
MAGMATIC UNDERPLATING IN THE SOUTHERN 

BASIN AND RANGE DURING CENOZOIC 

EXTENSION: A XENOLITH PERSPECTIVE 

CAMERON.K.L., Earth Sciences, Univ. of California, 

Santa Cruz, CA 95064; McMILLAN, N.J., Geological 

Sciences, New Mexico State Univ., Las Cruces, NM 

88003; SMITH, R.D., Earth and Space Sciences, SUNY, 

Stony Brook, NY 11794; McDOWELL, F. W., Geological 

Sciences, Univ. of Texas, Austin, TX 78713. 

The xenolith localities of Kilbourne Hole (KH), New 

Mexico, USA, and La Olivina (LO), northern Mexico, occur 

in areas that experienced substantial mid- to late Cenozoic 

volcanism and both lie in areas affected by Basin and Range 

faulting (KH is in the southern Rio Grande rift). Mafic 

rocks are a major component of the granulite suite at each 

locality. In order to test the hypothesis that these xenoliths 

represent new crust formed by underplating during Cenozoic 

extension, we analyzed -15 representative mafic granulites 

from each locality for Sr-Nd-Pb isotopes, and we compared 

their isotopic compositions to those of igneous rocks of 

known age exposed nearby. 

At least four isotopically distinct pulses of magmatism 

occurred in the LO area during the past 40 Ma. With one 

exception, the isotopic compositions of the LO mafic 

granulites, closely match those of 32-31 Ma intermediate to 

silicic volcanic rocks ~Srf86Sri .. 0. 705; eNd .. + 1 to -1; 316Pbf204Pb • 18.6), and the xenoliths are isotopically distinct 

from all other age groups of exposed igneous rocks. 

In contrast, few mafic granulites from KH have isotopic 

compositions in the range of Cenozoic volcanic rocks from 

the southern Rio Grande rift. About half have eNd values 

between -7 and -11, and this group plots in the field of exposed early Proterozoic granites and gneisses on a Sm/Nd 

isochron diagram. The remaining xenoliths have eNd values 

ranging from +2 to -10, and most of these plot near a 1.04 

Ga reference Sm/Nd isochron defined by Grenville age 

granites and mafic lavas exposed in the region. The xenoliths have lower n;Pbf204Pb (16.2 to 17 .2) than all but a very 

few Cenozic volcanic rocks (17.0 to 19.7) 

Few if any of the KH or LO mafic granulites appear to 

represent lower crust formed by magmatic underplating 

associated with Cenozoic extension. The great majority of 

KH mafic granulites are most reasonably interpreted to be 

Precambrian basement. We can not rule out the possibility 

that the protoliths of some were Cenozoic gabbros that contained a much larger component of ancient crust than any of 

the rift-related volcanic rocks, but the data do not require or 

support that interpretation. The LO mafic granulites represent one specific magmatic event at 32-31 Ma that preceded 

Basin and Range faulting. The isotopic data indicate that the 

protoliths of both the KH and LO mafic xenoliths are related 

by differentiation processes to identifiable silicic rock types. 

This stands in contrast to underplating models where basaltic 

~gmas pooled near the crust-mantle boundary and generated 

silicic melts principally by melting the overlying crust. The 

KH and LO studies demonstrate that information on the 

isotopic and trace-element compositions of exposed igneous 

rocks is most useful in interpreting the age and tectonic 

significance of deep crustal mafic xenoliths. 

47 

CONS1RAINING ATMOSPHERIC INPUTS AND IN SITU 

WEATIIERING IN SOILS DEVELOPED ALONG A 

a..IMATE GRADIENT USING STRONTIUM ISOTOPES 

CAPQ R. C., Division of Geological & Planetary Sciences 

170-25, California Institute of Technology, Pasadena, CA 

91125, USA, CHADWICK, 0. A., JPL-Caltech, MS 183-

501, Pasadena, CA 91109, and HENDRICKS, D. M., Soil 

and Water Sci., Univ. of Arizona, Tucson AZ 85721, USA 

Soils developed on the Hawi Lava ( -180 Ky) along the 

western slope of Kohala volcano were sampled along an 

elevational transect (77 to 1254 m). Detailed morphological, 

mineralogical, geochemical and isotopic analysis of soil, basalt 

parent material, and weathered basalt clasts have been 

undertaken to provide a better understanding of the influence 

of climate on mineral weathering rates and to study the 

relationship between in situ weathering processes and 

atmospheric inputs under widely varying climatic conditions. 

Pedogenic response to climatic variability was determined 

using a paleoenvironmental model based on present rainfall 

and past orographic effects. The time-weighted mean rainfall 

along the transect increases with elevation and ranges from 18 

to 350 em; the increasing amount of rainfall corresponds to an 

increasing intensity of weathering. 

Strontium isotopes can be used to examine the relationship 

between the weathering of primary minerals and atmospheric 

addition. The Sr isotopic composition of the endmembers of 

the soil-atmosphere system along the Kohala transect are wellconstrained. The 87 Sr/86sr ratio of the parent material Hawi 

lava ranges from 0.7035-0.7036 [1, 2]. Atmospheric input 

consists primarily of rainfall, which would have an 87srJ86sr 

ratio similar to seawater (0. 7092), and dryfall. Dust over 

Hawaii is predominantly derived from Asia; eolian micas from 

Hawaiian soils have 87srt86sr ratios of0.713- 0.727 (3, 4]. 

Eolian.input to the soil is recognized by increased amounts of 

quartz and mica in the <2mm soil fraction, generally in the 

upper soil horizons. 

Preliminary results suggest considerable isotopic 

inheritance from atmospheric sources even where present dust 

and rainfall inputs are low. At Site B (18 em rainfall ). 

87Sr/86sr ratios of bulk soil decrease from 0.70422 in the dustrich A horizon (0-10 em depth; 2.9 wt % qtz; 3.8 wt % mica) to 

0.70405 in the Bw2 horizon (37-65 em depth; 0.2% qtz; 0.2% 

mica). These values are significantly higher than Hawi lava 

values. and suggest mixing of parent material with the more 

radiogenic atmospheric components. Along the climate 

transect, the 87srf86sr ratio of the exchangeable Sr of the 

dust-poor lower B horizon (leached with Nf4Cl [5]) is similar 

for Sites B. E and I (18-148 em rainfall; 87Sr/86sr =0.7040-

0.7041) but increases to 0.7089 at Site L (300 em rainfall). 

This most likely reflects the much greater intensity of 

weathering at Site L which has leached away parent material Sr 

and left Sr derived primarily from rainfall on the exchange 

complex. 

References: [1) Lanphere, M.and Frey, F., (1987) Conlrib. Mineral. 

Petrol. 95: 100; [2] Hofmann, A., Feigenson, M. and Raczek,l. 

(1987) Conlrib. Mineral. Petrol. 95: 114; [3] Dymond, J., Biscaye, P. 

and Rex, R. (1974) GSA Bull. 85: 37; [4] Nakai, S., Halliday, A. and 

Rea, D. (1993) EPSL 119: 143; [5] Jackson, M. (1969) Soil Chemical 

Analysis, 2nd ed., Madison, 895 p. 
DYNANICS OF MELTING AND MAN1LE 

SOURCESFORQUATERNARYCO~NTAL 

ALKALINE MAGAMS IN THE NORTHERN 

CANADIAN CORDILLERA. 

Cari~nan. J. and Ludden, J., Departement de 

Geologie, Universire de Montreal, Montreal 

Quebec, H3C 3J7, Canada and Francis, D.,' 

Department of Earth and Planetary Sciences, 

McGill University, Montreal, Quebec, H3A 

2A 7, Canada. 

Five Quaternary to recent volcanic centres in the 

Northern Canadian Cordillera comprising alkaline 

lavas ranging in composition from olivine 

nephelinite to By-normative basalt, have been 

analysed for their Sr, Nd and Pb isotopic 

compositions. Previous studies on these lavas 

indicate that the compositional spectrum is best 

explained by the melting of variable proportion of 

amphibole-rich veins hosted in a lherzolite 

lithospheric mantle. Samples covering the entire 

magmatic spectrum were selected to document the 

isotopic composition of endmembers and evaluate 

the hypothesis of a single heterogeneous reservoir as 

the source of alkaline magmas in the Northern 

Cordillera. 

Three mantle components have been identified as 

potential source endmembers. These are: 1) an 

unradiogenic Sr, Nd and Pb component from which 

olinine nephelinite formed; 2) a radiogenic Pb - non 

radiogenic Sr component (HIMU -type) represented 

by Mt.Edziza lavas; 3) spinel lherzolite mantle 

characterized by high 207Pb/2°4Pb (enriched mantle-II 

type), that is involved in the formation of basanites 

and alkali olivine basalts from some centres. 

The distinct isotopic character of the Edziza centre 

relative to most of the alkaline magams indicates that 

the Mt.Edziza reflects upwelling of sub-lithospheric 

Pacific mantle below central British Columbia. 

Incipient melts from amphibole-garnet-pyroxenite 

veins in the subcontinental lithospheric mantle were 

gener~ted. in response to the thermal upwelling and 

resultmg m the small (<1Km3) nephelinite to AOB 

centres characteristic of the Northern Cordilleran 

Province. The nephelinite source is slightly more 

radiogenic than present day Pacific MORB and is 

best represented by the most depleted component of 

the Aleutian magmas perhaps indicating an 

enrichment of subcontinental lithosphere in northern 

Cordillera by melts of this composition during 

Cretaceous subduction. The AOB magams of the 

Fort Selkirk volcanic complex appear to lie on a 

binary mixing trend between the nephelinite 

component and spinel lherzolite xenoliths from the 

region, whereas the Alligator lake complex lavas are 

highly radiogenic, display evidence for crustal 

contamination, but may nonetheless represent a 

highly radiogenic lithospheric mantle. 

48 

PLUMBOTECTONICS OF THE LACHLAN FOLD 

BELT OF EASTERN AUSTRALIA: CRUST-MANTLE 

INTERACTION IN THE FORMATION OF 

PALAEOZOIC MINERALIZATION 

CARR. G.R.. DEAN, J.A., CSIRO Division of 

Exploration and Mining, North Ryde, NSW, 2113, 

Australia, SUPPEL, D.W., Geological Survey of New 

South Wales, St Leonards, NSW, 2065, Australia, and 

HEITHERSAY, P.S., Geopeko, Melbourne, VIC, 3004, 

Australia. 

In contrast to most large Phanerozoic oro~enic systems, 

the Lachlan Fold Belt (LFB; > 500 000 km ) has similar 

lithological assemblages and structural style throughout. 

This relative homogeneity is mirrored by the Pb isotope 

systematics of magmatic units and minerals formed from 

magmatic and metamorphic hydrothermal activity. A study 

of over 100 mineral deposits in the LFB ranging in age from 

Ordovician to Permian has indicated a three fold subdivision 

based on the sources ofPb: (1) deposits containing almost 

exclusively mantle derived Pb, (2) deposits containing Pb 

which has had a very long crustal residence time (-1 Ga) 

and, (3) deposits containing Pb of mixed mantle and crustal 

origin. The distribution through time of these source-types 

has implications for the tectonic development of the LFB. 

In Ordovician sequences, both Besshi-style 

mineralization with crustal Pb isotope signatures and 

porphyry to epithermal Cu and Au mineralization with 

mantle signatures occur. This latter mineralization is 

associated with mafic igneous rocks, commonly shoshonitic 

in character, and the ores and corresponding unaltered host 

rocks have similar Pb isotopic compositions suggesting Pb 

is magmatic in origin. On each of the conventional Pb 

isotope diagrams, the data for these deposits plot on a very 

precise line which is not a typical arc-like mixing trend but 

represents mixing of Pb from two or more mantle reservoirs. 

It is postulated that the position of each deposit along this 

line is an indication of the timing of metallogenesis within 

the Ordovician magmatic cycle, and possibly also the degree 

of mixing of melts derived from a more primitive 

asthenosphere and a more enriched lithosphere, potentially 

fertile for Cu and Au. 

Magmatic rocks and large scale magmatic hydrothermal 

mineralization of Silurian and Devonian age are dominated 

by crustal Pb, but evidence of hydrothermal mixing of 

crustal Pb, and mantle Pb mobilised from Ordovician mafic 

units, is common. There is very little evidence for direct 

mantle input of Pb. Similarly, large scale incorporation of 

mantle Pb in Carboniferous and Permian granites and 

associated mineralization probably represents recycled 

Ordovician Pb. 

The homogeneous nature of the crustal and dominant 

Ordovician mantle source regions in the LFB allows the 

development of a plumbotectonic model which yields 

mineralization model ages with precision better than ± 15 

Ma based on mixing isochrons between a crustal growth 

curve (J.L = 13.3) and a mantle growth curve (J.L = 10). The 

model also appears to be valid for older (Kanmantoo Fold 

Belt) and younger (New England Orogen) units of the 

Tasman Fold Belt System. 
APPLICATION OF INVERSE 

NUMERICAL TECHNIQUES IN 

RADIOISOTOPE GEOCHRONOLOGY 

CARROLL. J., ABRAHAM, J. D., CISAR, D. J., RUST 

Geotech, Grand Junction, CO, 81502 USA, and 

LERCHE, 1., Dept. Geological Sciences, University of 

South Carolina, Columbia, SC 29208, USA. 

Many methods for determining sedimentation rates in 

lacustrine and marine environments use naturally 

occurring radioisotopes as geochronometers (e.g. Pb-210, 

C-14, Th-230, Ra-226). These methods work best when 

both adsorption to sediments and sedimentation rate are 

constant. Most often, neither process is constant, 

producing radioisotope activities which vary 

nonexponentially with depth in sediment cores. Without 

techniques to separate the effects of source from 

sedimentation variations, such profiles do not yield useful 

sediment ages. 

We have developed a procedure to analyze these 

complex isotopic profiles. The procedure uses inverse 

numerical analysis techniques to produce a model of 

appropriate resolution and matching the degree of 

complexity of a given isotopic profile. We demonstrate 

the procedure using synthetic profiles of known source and 

sedimentation components, which represent isotopic 

behavior in a variety of sedimentary environments. This 

new procedure removes limitations on the application of 

radioisotopes as geochronometers in paleooceanography, 

as well as providing variable source and sedimentation 

rates with time. 

49 

COSMOGENIC 3HE AND 21 NE FROM TIOGA AGE 

SURFACES, SIERRA NEVADA, CALIFORNIA 

CERLING. T. E., Department of Geology and Geophysics, 

University of Utah, Salt Lake City, Utah 84112 USA, 

POREDA, R. J., and KU, T., Department of Geological 

Sciences, Univesity of Rochester, Rochester, NY, 14627 USA 

Tioga age glacial surfaces in on the east side of the Sierra 

Nevada can be dated using cosmogenic 3He and 21Ne. At 

June Lake and at Devils Postpile in the Mammoth Lakes 

region volcanic rocks which were glacially scoured during the 

Tioga glaciation provide excellent surfaces for cosmogenic 

dating using 3He and 21Ne. In addition, the close proximity 

of glaciated Bishop Tuff and basalts allows comparison of 

2l Ne production rates in olivine and quartz, without the 

complication of 18o(a,n)21 Ne production problems 

associated with glaciated Sierra Nevada granites. Cosmogenic 

dates on volcanic flows of the Big Pine field in Owens Valley, 

which are overlain by glacial outwash, provide additional 

constraints on the Tioga glaciation. 
TIMS U-SERIES STUDY OF THE GROWTH HISTORY 

OF FERROMANGANESE CRUSTS 

CHABAUX. F., COHEN, A.S., O'NIONS, R.K. Dept. of 

Earth Sciences, Cambridge University, CB2 3EQ, UK, and 

REIN, J., US Geological Survey, Menlo Park, CA) 

Hydrogenous ferromanganese crusts grow from sea water, 

at rates which would be related to climatic variations 1. In this 

case, the detailed determination of growth rates together with 

details of chemical and isotopic composition could provide 

important information on sea water composition through 

climatic cycles. However this record is only easily accessible 

if the growth processes are uniform over an useful portion of 

the crust. 

In order to assess these g_uestions we present TIMS 

measurements of 234u and 23UJn excesses along different 

profiles of the outermost 2 mm rim of a single Mn crust, from 

Lomilick seamount in the Marshall Islands. Our results are as 

follows: 

1-Within a single profile, the variations of 234u- and 230-rhexcesses provide generally consistent growth rates or ages for 

Mn oxide layers. Small discrepancies between these two 

chronometers are nevertheless observed, which may indicate 

local disturbance of Mn layers since deposition, or 

alternatively contamination by detrital minerals incorporated 

into the Mn crust and partially dissolved during chemical 

treatment in the laboratory. 

2-Comparison of two profiles, obtained only a few 

centimetres apart on the same crust, shows that one has an age 

of 400,000a older than the other. This indicates that growth 

stopped during this interval of time, or that local abrasion or 

dissolution of the Mn oxides occurred after deposition. 

Similar observations were made by von Blanckenburg et al. 

(pers. comm.) using lOBe data from the same crust. In 

addition Sharma et al. 2 reported spatial variability on a scale 

of a few centimetres of U-Th series nuclides in surfaces layers 

of deep-sea ferromanganese encrustations. These results 

demonstrate significant variability in the growth process of 

Mn crust, which will make the observation of fine-scale 

growth histories difficult in many cases. 

!-Eisenhauer et al. E.P.S.L., 109, 25-36, 1992 

2-Sharma et al. E.P.S.L., 67, 319-326, 1984. 

50 

ISOTOPIC TRACING OF SONG BIRD 

MIGRATORY PATTERNS AND 

PALEOENVIRONMENTS OF FOSSIL BIRDS 

CHAMBERLAIN. C.P., BLUM, J.D., 

HOLMES, R.; POULSON S., Dartmouth 

College, Hanover, NH 03755; GRAVES, 

G., Dept. of Vertebrate Zoology ,Smithsonian 

Institute, Washington, D.C. 20560; SHERRY, 

T., Dept. of Biology, Tulane Univ., New 

Orleans, LA. 

We are exploring the isotopic variations in 

modem song birds as a possible tool for tracing 

migratory patterns as well as determining 

paleoenvironmental conditions of fossil birds. The 

motivation for this research is two-fold. First, song 

bird populations in North America are in decline, but 

the exact cause is unknown and requires a detailed 

understanding of the birds' migratory patterns. 

Unfortunately, at present there are no methods 

available to determine these migration routes with any 

certainty. Second, isotopic studies of modem birds 

are needed prior to using fossil birds to reconstruct 

paleoenvironmental conditions. 

We determined the H, C and Sr isotopic values of 

feather and bone material of Black-Throated Blue 

Warblers (Dendroica caerulescens). Our studies 

show that isotope values of birds collected in their 

nesting grounds correlate with isotope variations in 

the hydrosphere and lithosphere. This correlation 

demonstrates that is possible to use isotopes to 

determine both migration patterns of modem birds 

and paleoenvironmental conditions of fossil birds. 

The BD and al3C values of feathers from over 

200 birds collected from nesting sites vary in a 

predictable and systematic fashion. The BD of feathers· 

vary by over 30%o from the southern range 

(BD=69±10%o) to the northern range 

(BD=101±14%o). The variations in BD values of 

feathers mimics the observed variations in BD of rain 

water with !attitude. al3C of feathers vary from a low 

of a13C=22.0±0.6%o in the southern breeding range 

to a high of a13C=24.2±0.7%o in the northern 

breeding range. Although the variations in a13c are 

small they are extremely systematic with no overlap 

occuring between the B 13C of northern and southern 

birds. We determined the 87Srf86Sr values for bone 

for 9 birds from 7 localities. These values show a 

wide range between localities (87Srf86Sr=0.71155 to 

0.71852) and a much narrower range within single 

localities reflecting unique 87Srf86Sr signatures 

dependent on soil protolith and atmospheric 

deposition within each nesting site. 
GEOCHRONOLOGIC EVIDENCE FOR EPISODIC 

STRUCfURAL AND MAGMATIC REACfiVATION OF A 

MAJOR CONTINENT-ISLAND ARC SUTURE OVER A 

400 MYR PERIOD 

CHAMBERLAIN K R, SCOATES, J.S., and FROST, 

C.D., Dept. of Geology and Geophysics, University of 

Wyoming, Laramie, WY 82071, USA 

The Cheyenne belt in southeastern Wyoming, a 

Proterozoic (ca 1.78 Ga) suture between the Archean Wyoming 

Province to the north and Proterozoic island arc rocks to the 

south, has been the site of multiple periods of magmatism and 

deformation, and may have responded to orogenies occurring 

tOO's of km to the south. Recent discoveries and isotopic 

results that bear on the 400 myr tectonic activity of the 

Cheyenne beltinclude l)recognitionofthick-skinnedbasement 

deformation within the craton during collision, 2) evidence for 

transtension during late stages of suturing, and emplacement of 

the 1. 76 Ga Horse Creek anorthosite complex, 3) widespread 

1.66-1.60 Ga magmatism, metamorphism and uplift along the 

Cheyenne belt, and 4) direct age of crystallization of 1.43 Ga 

from anorthosites located within the pre-existing zone of 

transtension. 

Thick -skinned deformation of the craton during collision 

is manifested by a minimum of 10 km vertical uplift across the 

mylonitic Laramie Peak shear zone 60 km north of the trace of 

the Cheyenne belt. U-Pb apatite dates from both sides of the 

Laramie Peak shear zone constrain movement to ca 1. 78 Ga. 

Lack of a metamorphic gradient within the block between the 

shear zone and the Cheyenne belt suggests nearly vertical uplift 

without rotation. South of this block, the Horse Creek 

anorthosite complex was emplaced at 1. 76 Ga, coincident with 

the timing of late, dextral strike-slip motion along the 

Cheyenne belt, and thus in an inferred transtensional tectonic 

setting. Isotopic data from the Horse Creek complex imply a 

stratified crustal section with a gently dipping suture between 

Proterozoic upper crust and a rifted Archean lower crust. 

Additionall.76-1.74 Ga syn- to post-tectonic plutonism occurs 

in a broad, 200 km long, triangular-shaped region along the 

southern edge of the Cheyenne belt and may demarcate the 

extent oftranstensionally weakened crust. U-Pbmineral dates, 

including zircon, monazite, sphene, and apatite, indicate later 

1.66 to 1.60 Ga magmatism. metamorphism and uplift within 

this transtensional zone at a time that island arc collision was 

occurring along the Mazatzal orogeny at least 300 km to the 

south. Finally, this zone was the site for the generation and 

emplacement of a second anorthosite complex, the 1.43 Ga 

Laramie complex, suggesting renewed extension. The 

anorthosites within this younger complex have been directly 

dated by U-Pb baddeleyi te and zircon analyses. 

The 400 myr tectonic activity of the Cheyenne belt 

demonstrates the influence of pre-existing structures and zones 

of crustal weakness on later tectonic events. There may be 

strong structural controls on the generation of anorthosites, on 

deformation of the hinterland during orogeny. and on the 

locations of 'anorogenic' 1.4 Ga magmatism. 

51 

PALEOMAGNETIC CONSTRAINTS ON EPISODES OF 

BASALTIC VOLCANISM 

CHAMPION. D.E., Isotope Geology, U.S. Geological 

Survey, 345 Middlefield Rd., Menlo Park, CA 94025 

Field-based paleomagnetic studies, when coupled with 

geologic mapping, petrology, and chronometric techniques 

such as 14c, K-Ar, or 40 Ar/39 Ar, repeatedly demonstrate that 

basaltic volcanism is characterized by brief episodes of 

eruption and that the products of these eruptions may be large 

in volume and may also be compositionally zoned despite 

being erupted in short periods of time (<100 years). This type 

of volcanism occurs in the Cascade arc and adjacent Basin and 

Range province. Within the Shasta-Medicine Lake segment of 

the Cascades, paleomagnetic studies confirm the suggestion 

from field mapping that as few as 7 brief eruptions, each at 

least 2 km3 in volume, produced basaltic lavas which cover 

about 1500 km2. This represents almost 25% of the entire 

Quaternary volcanic axis between Mt. Shasta and the Medicine 

Lake Highlands. One well-documented example is the Giant 

Crater lava field on the Medicine Lake volcano. Detailed field 

and petrologic studies (Donnelly-Nolan et al., 1991), 

demonstrate compositional variation from basaltic andesite to 

high--alumina basalt (Si~ 53~8%), in 4.4 km3 oflava 

from at least 10 vents. Paleomagnetic data from 41 sites show 

no more than 2° of direction variation among the average 

directions of 6 mapped compositional variants. By analogy 

with modem geomagnetic records, all the Giant Crater 

eruptions, and approximately one half the chemical evolution, 

took at most a few decades. The basalt of Mammoth Crater, 

which covers about 250 km2 on theN. flank of Medicine Lake 

volcano shows even more compositional variation (Si02 55-

48%). Paleomagnetic data from 27 sites again indicate all 

Mammoth Crater lavas share the same paleomagnetic direction 

( <3° of variation). 

Studied alkalic basalts of the Basin and Range in southern 

Nevada also erupted in brief eruptions such that only single 

directions of remanent magnetization were recorded within 

individual cinder cones and lava fields. This is not surprising 

as historic observations at Paricutin in Mexico showed that 

cinder cones typically erupt in brief {S9 years) and sometimes 

compositional varying (Si02 55~60%) lavas. Lavas of the 

3.7 Ma and 1.1 Ma cinder cone eruptions at Crater Flat record 

similar, but individually distinguishable reversed polarity 

directions, while also showing chemical and trace element 

variations. Four different cinder cone centers of the 1.1 Ma 

episode record the same reversed polarity direction indicating 

all erupted within 100 years. Alkalic basalt of the Lathrop 

Wells center shows minimal trace element variation, but does 

record two different but allied directions of magnetization. 

Petrologic arguments which insist that these very small 

chemical differences are inconsistent with monogenetic origin, 

and are necessary evidence of independence of magma evolution 

and eruption, are probably incorrect. 

Donnelly-Nolan, JM. et al., 1991, The Giant Crater Field: 

Geology and Geochemistry of a Compositionally Zoned, 

High-Alumina Basalt to Basaltic Andesite Eruption at 

Medicine Lake Volcano, California, Jour. Geophys. Res., 

v. 96, p. 21843-21863. 
THE GEOCHEMISTRY AND CYCLE OF LITHIUM 

ISOTOPES IN THE MARINE ENVIRONMENT 

CHAN L H , Dept. of Geology and Geophysics, Louisiana 

State University, Baton Rouge, Louisiana 70803, U.S.A. 

Unlike other light isotopes, the geochemistry of Li isotopes 

has not been well known. An improved mass spectrometric 

technique permits measurement of Li isotopic ratios to a 

precision of 1 per mil (Chan, 1987). Using this method, the Li 

isotopic compositions of river water. seawater, fresh and 

variously altered oceanic basalts, submarine hot springs, marine 

sediments including hydrothermally altered and subduction zone 

sediments, pore waters, and island arc volcanics have been 

measured. This report presents an overview of the oceanic 

distribution of Li isotopes with respect to (1) the basic 

geochemistry of Li isotopes; (2) the isotopic constraints on the 

Li balance in the oceans; and (3) the tracer capability of Li in 

the study of oceanic processes. 

Seawater has a o6Li value of -32.3 per mil (relative to NBS 

standard L-SVEC). The o6Li values ofMORB range from -3.4 

to -4.7 per mil. These distinct endmember values make Li 

isotope ratios a valuable tracer for seawater-basalt exchange. 

Hydrothermal solutions from various ridge crest systems show 

isotopic values ranging from :.6 to -11 per mil, which can be 

understood in terms of a path-dependent dissolutionprecipitation process in response to progressive changes in 

temperature and water/rock ratio. The similarity of the Li 

isotopic compositions in fluids from fast and slow spreading 

ridges argues against contribution of recycled Li from older, 

weathered crust to the large hydrothermal flux as calculated from 

the 3He/heat ratio. Fluids from sediment-covered spreading 

centers (e.g., Guaymas Basin) exhibit Li concentrations and 

o6Li values similar to those of the mid-ocean ridge systems, 

implying an open flow-through system with rapid water 

recharge that largely obliterates contributions from the 

sediments. 

Seafloor basalts weathered at low temperatures show Li 

enrichment and more negative o6Li values as a result of 

addition of seawater Li. Metabasalts usually have lighter 

isotopic compositions as a result of retention of basalt-derived 

Li. The secondary minerals in these altered basalts show 

preferential incorporation of the lighter isotope with an apparent 

temperature dependency of the isotopic fractionation factor. 

The Li isotopic ratios of marine sediments vary with source 

and composition. The hemipelagic sediments of Guaymas 

Basin have a o6Li value of -10 per mil while the trench 

turbidite of Nankai Trough has -2 per mil. The strong 

depletion in Li associated with lighter o6Li values in the 

greenschist facies sediments of Guaymas Basin demonstrates 

high mobility of sedimentary Li at elevated temperatures. In the 

decollement zone of the Nankai Trough, a maximum in o6Li 

with corresponding low Li concentration in the pore water has 

been observed. The anomaly can be taken as an evidence of 

migration of low-Li fluid from deeper in the accretionary 

complex where mineral dehydration occurs. These results 

indicate the usefulness of Li isotope in the study of subduction 

zone processes. 

The isotopic values of rivers range from -15 to -29 per mil. 

Thus seawater is enriched in 7u relative to the inputs from 

riverine and hydrothermal sources and 6Li must be preferentially 

removed to maintain a steady state composition. Isotope 

balance considerations favor a hydrothermal flux that is 

comparable to the river flux unless more important 6u sinks 

than the authigenic clays exist. 

52 

CONSTRAINTS ON METAMORPHIC FLUID FLOW 

FROM STRONTIUM AND OXYGEN ISOTOPIC 

PROFILES ACROSS CARBONATE - SILICATE 

CONTACTS, LIZZIES BASIN, NEVADA. 

CHAPMAN. H. J, BICKLE,. M. J., Dept. of Earth 

Sciences, University of Cambridge, Cambridge CB2 

3EQ, U.K. and WICKHAM, S. M., PETERS, M. T., 

Dept. Geophysical Sciences, University of Chicago, 

Chicago, Illinois 60637, U.S.A. 

Strontium, oxygen and carbon isotope profiles were 

measured across two marble layers, and into adjacent 

calcsilicate gneiss and leucogranite, in the migmatitic 

amphibolite-facies gneiss complex at Lizzies Basin in the 

East Humbolt Range. Displacement of the isotopic tracers 

are used to assess the relative significance of advective and 

diffusive transport, porosity (and thus permeability) 

contrasts between lithologies, together with the fluid Sr and 

H20/C02 ratio. 

The Neoproterozoic and Palaeozoic metasediments from 

Lizzies Basin were metamorphosed to sillimanite grade 

during the Tertiary, overprinting earlier Mesozoic 

metamorphic events. Wickham and Peters (1992) have 

previously ascribed major oxygen isotope mobility in this 

sequence to infiltration of hydrous fluids accompanying 

Tertiary metamorphism. Both oxygen and strontium isotopic 

profiles indicate minimal advective transport across the 

marble bands. Modelling of oxygen-isotope diffusion 

distances indicates porosities in the calc-silicates were at 

least a factor of 10 more than porosities in the marbles. In 

contrast one of the strontium isotopic profiles (Fig 1) 

exhibits symmetrical diffusive broadening about the contacts 

consistent with nearly equal porosity between the 

calc-silicate and the marble. This discrepancy between the 

oxygen and strontium isotopic profiles may result from 1) 

a marked contrast in fluid Sr contents between marble and 

calc-silicate, 2) 0 and Sr diffusion responding differently in 

the several metamorphic events or 3) significant porosity 

variation within calc-silicates, (the Sr isotopic profile is 

spatially limited). Comparison of Sr and 0 isotopic diffusion 

distances indicates fluid Sr concentrations between -50 -

500 ppm. Comparison of ot8Q-o13C profiles indicates that 

fluids were water-rich (Xco2 < 0.02) and were probably 

derived from adjacent crystallising leucogranites. 

0.725 calc-silicate marble 

0.715 f0.705 ...__ _ __,__......__.__ __ .__ _ ___J 

0 0.2 Metres 0.4 

Figure 1. Sr-isotopic profile across upper contact of 

marble band. Line is least-squares fit assuming differing 

diffusivities in marble and calc-silicateand implies porosity 

contrast (calc-silicate to marble) of 1.7 ± 0.25. 

Wickham, S.M. and Peters, M. T., 1992, Oxygen and carbon 

isotope profiles in metasediments from Lizzies Basins, East 

Humbolt Range, Nevada: Contrib. Mineral. Petrol. , v. 112, 

p. 46-65. 
BORON ISOTOPE VARIATIONS IN CHONDRULES: 

CONSEQUENCES ON CHONDRULE FORMATION AND 

BORON COSMOCHEMISTRY 

CHAUSSIDON M., CRPG-CNRS, BP 20, 54501 

Vandoeuvre-les-Nancy, France and ROBERT F., Museum 

d'Histoire Naturelle-CNRS, 61 rue Buffon, 75005 Paris, 

France. 

A boron isotope study of chondrules was undertaken in 

several chondrites (Semarkona LL3, Allende CV3, Hedjaz L3 

and Estacado H6) to (1) evaluate the potential of B isotopes for 

constraining the mechanism of chondrule formation and to (2) 

document the possible B isotope heterogeneity of nebular 

precursors and of chondrites. Measurements were made with a 

IMS 3f ion microprobe at CRPG (Nancy) on =25J.Lm spots, 

following the procedures developed for mantle rocks[1,2J and 

with careful attention paid to problems of surface 

contamination. o 11 B values are given versus NBS 951 

(11BJ10Jl=4.04558) with an accuracy varying between ±5%o to 

±10%o according to the B contents. Elemental ratios of B, Na, 

Mg, AI, K and Ca versus Si were determined simultaneously 

on the same spots by ion probe. 

Large discrepencies exist between the only two existing B 

isotope studies of bulk chondrites which report 3 o 11 B 

measurements between -35±4 and -57±4%o for the oldest 

one£3] and 6 o11 B measurements between + 1.8 and -8.5o/oo for 

the most recent[41, while they both report B concentrations in 

the same range, between 0.4 and 0.9 ppm. These differences 

are un-explained, but large isotopic heterogeneities may have 

existed in the solar nebula since calculations of the 11 B/lOB 

ratios, produced during the synthesis of B via the inter-action 

between high energy cosmic rays and the interstellar matter, 

can theoritically vary between 2.5 and 4. I [5,61. 

The o11 B values found in this study are extremely variable 

between =+40±10 and -50±10%o in chondrules from 

Semarkona, Hedjaz and Estacado, and more homogeneous at -

25±15%o in chondrules from Allende. Strong linear 

correlations are observed in several cases between the o11 B 

values and some major element ratios (e.g. Na/Si, Mg/Si, 

Ca/Al). In addition, o11B variations in some barred chondrules 

follow mineralogical structures. These results imply that 

several processes concur to control the o11 B variations in 

chondrules, among which the most important are (1) isotopic 

fractionations occuring during heating (e.g. due to 

volatilization loss of B) and the cooling (e.g. due to 

disequilibrium partitioning of B between the different phases) 

of the chondrule and (2) B isotopic heterogeneity of the nebular 

precursors. The mean o11 B value of chondrules, similar to 

mean solar compositions in terms of their B/Si, Mg/Si, AI/Si, 

K/Si and Ca/Si ratios, is of -25± IOo/oo, significantly lower 

than the mean o11 B value of the Earth mantle (between =-

7 .5%o and Oo/oo) [2]. 

l.Chaussidon M. and Libourel G. (1993) Geochim. 

Cosmochim. Acta, 57, 5053-5062 

2.Chaussidon M. and Jambon A. (1994) Earth Planet. Sci. 

Lett. (in press) 

3.Shima M. (1963) Geochim. Cosmochim. Acta, 27, 911-913 

4.Agyei E. K. and McMullen C. C. (1978) Geol. Surv. Op. 

File Rep. 78-701, 3-6. 

5.Reeves H. (1974) An. Rev. Astr. Astrophys., 12, 437-469. 

6.Curtis D. et al. (1980) Geochim. Cosmochim. Acta, 44, 

1945-1953. 

53 

ORE DEPOSITS DATING THROUGH 40 Ar/39 Ar 

LASER SPOT ANALYSIS 

CHEILLETZ A .. GIULIANI, G., CRPG-CNRS, 

BP 20, 54501 Vandoeuvre-les-Nancy, France, 

and ORSTOM, TOA, UR 1H, 213 rue La Fayette, 

75480, Paris, France, FERAUD, G., and 

RUFFET, G., URA CNRS 1279, Universite de 

Nice-Sophia Antipolis, Pare V alrose, 06034 Nice 

Cedex, France. 

Three emerald deposits in Brazil and Colombia 

have been dated by using 40 Ar/39 Ar techniques. Due 

to large amounts of excess argon in beryl-type 

minerals like emerald, particular laser procedure have 

been applied to obtain formation ages. Firstly, 

emerald occurring inK-metasomatic rocks developed 

at the contact of the Carnafba leucogranite (Bahia 

state, Brazil) has been dated[ 1 l combining step 

heating and laser spot fusion experiments on (1) 

individual phlogopite crystals from enclosing hostrock and (2), syngenetic solid inclusions precipitated 

along growing zones of the emerald host crystals. 

Reliable ages of 1951 ± 8 rna and 1944 ± 8Ma have 

been measured in spite of a polyphased history 

during the Transamazonian (2 Ga) orogenesis. 

Secondly, two emerald deposits of the Eastern 

Cordillera of Colombia have been dated for the first 

time[2] by 40 Ar/39 Ar induction and laser-probe 

methods on contemporaneous Cr-V -rich K-mica 

aggregates consisting of muscovite as a dominant 

phase associated with ± kaolinite, ± paragonite, ± 

quartz, ± albite, ± pyrite and calcite. Contamination 

of the ~-mica aggregates by wall-rock impurities is 

eliminated by in situ 40 Ar/39 Ar analysis. Two 

distinct plateau and spot fusion ages of 35-38 Ma and 

31.5-32.6 Ma were obtained for the Coscuez and 

Quipama mines respectively. Concordant 

conventional K-Ar ages show that in spite of the 

small size of the K-micas, they did not suffer 

significant 39 Ar loss due to recoil during irradiation 

of the samples. Internal 39 Ar recoil may explain the 

slight disturbances observed on the age spectra. 

These results allow the reconstitution and timing of 

hydrothermal circulations inducing emerald veins 

formation with regard to the Eastern Cordillera 

tectonic evolution. 

l.Cheilletz 'A., Feraud G., Giuliani G. and Ruffet 

G., (1994), Earth Planet. Sci. Lett., 120, 471-483. 

2.Cheilletz A., Feraud G., Giuliani G. and 

Rodriguez C.T. (1994), Economic Geology (in 

press). 
COMPARISON OF LATE CENOZOIC BASALTIC 

MAGMA GENERATION IN RYUKYU ARC, 

OKINAWA TROUGH AND ADJOINING AREA (NW. 

KYUSHU,JAPAN ANDN. TAIWAN) 

Chang-Hwa Chen, Inst. of Earth Sci., Academia Sinica, 

P.O.Box 1-55, Taipei, Taiwan, ROC, and Nakada 

Setsuya, Dept. of Earth and Planet. Sci. , Kyushu Univ., 

Fukuoka 812 Japan 

The Philippine Sea plate started to subduct under 

the Eurasia plate at about 7 Ma. This subduction produced 

the Ryukyu volcanic arc from SW. Kyushu to eastern 

offshore Taiwan. The Okinawa Trough is considered as a 

back arc basin behind the R yukyu volcanic front. The 

basaltic samples were collected from the northwestern 

Kyushu (NWK), central Kyushu (Aso volcano)(CK), 

northern Ryukyu volcanic arc (RV A) and northern Taiwan 

volcanic zone (NTVZ). The data of central Okinawa 

Trough (COT) also was included to discuss the systematic 

variations (lshizuka et al., 1990) 

The NTVZ has been considered as a part of R yukyu 

Arc, but the depth of the Benioff zone present more than 

200 km under this belt. From the spider diagram of 

incompatible elements of basalts for NTVZ, it reveals two 

different patterns. The one shows no obvious Nb negative 

·anomalies like as ocean island basalt (OIB) trend, the other 

shows a medium Nb negative anomalies a little like as 

island arc basalt (lAB) trend. The Nb/Zr vs. K/Zr and 

Nb/Zr vs. Ba/Zr diagrams also show that the magma 

source in NTVB distribute between OIB and lAB. 

Comparison ofthe basalts from COT, CK, RVA and the 

adjoining area (NWK and NTVZ) reveals that the magma 

source beneath NTVZ is similar to that of NWK and 

CK. Otherwise, it exists a distinct difference with RVA. 

The trend of COT in above diagrams reveals the transition 

between NWK and RV A. 

The Sr isotopic study of basaltic rocks indicates that 

the range in NTVZ are much narrower than that of RV A, 

which is 0.7039-0.7045 and 0.7041-0.7053, respectively. 

Whereas, the Sr isotopic values of NTVZ are shown the 

similar range with that in NWK basalts. The Nd isotopic 

compositions ofNTVZ also show the same situation. 

These relationship indicates that the mantle wedge was 

contaminated by the slab-derived materials in a subduction 

system, but this influence has gradually decrease with the 

depth of slabs. When this subducting slab did not reach 

under volcanic zone or go downward to more than 200 km 

beneath volcanic belt, the island arc behaviors in volcanic 

rocks might be obscured. 

Ishizuka H., Kawanobe Y. and Sakai H. (1990) Geochem. 

J., 24, 75-92. 

54 

ORIGIN OF MESOZOIC CONTINENTAL 

BASALTS FROM SOUTHEASTERN CHINA: 

EVIDENCE FROM ND ISOTOPE AND TRACE 

ELEMENT COMPOSITIONS 

CHEN JIANGFENG, ZHOU TAIXI, CHENG 

ZHONGQI, Department of Earth and 

Space Sciences, University of 

Science and Technology of China, 

Hefei, 230026, China,and YU YUNWEN, Regional Geologic Survey 

Party of Zhejiang Province, 

Xiaoshan, Zhejiang, 311200, China 

It is found recently that except 

for majority of intermediate and 

felsic volcanic rocks there are 

basalts in the Mesozoic volcanic 

sequences in southeastern China. 

They are 110-100 Ma in age. These 

basalts together with felsic volcanic rocks form bimodal volcanic 

rock suits in some volcanic basins. 

Initial Nd isotope ratios for these 

basalts range from +1.2 to -7.1 and 

are negatively correlated with initial Sr isotope ratios which might 

be affected by secondary alteration 

and with Si02 contents suggesting a 

mixing b~tween mantle-derived parent magma and crustal materials. ~n 

addition, Nd initials are correlated with trace element parameters 

and the thickness of the basalt 

flows. Basalts from thick flows 

show positive epsilon Nd values 

with low La/Nb ratios of about 1 

suggesting the dominance of the 

mantle-derived materials, while 

rocks from thin layers have more 

negative epsilon Nd values and high 

La/Nb ratios up to 3. This suggests 

a more significant crustal contamination for basalts from thin layers 

because of a large surface area. 

Extrapolation from known trace element and {sotope relations indicates that the mantle source region 

for these basalts is similar to the 

ones of many oceanic island basalts 

and basalts from continental rifting areas, that is a slightly depleted mantle. The bimodal rock association also suggests an extension tectonic setting which is different from previously commonly accepted conclusion that Mesozoic 

volcanic rocks from southeastern 

China were formed in an active continental margin or arc like environment. 
A SEARCH FOR 107 Ag AND 205Pb IN METEORITES 

CHEN. J. H., and WASSERBURG, G. J. The Lunatic 

Asylum, Division of Geological & Planetary Sciences, 

California Institute of Technology, Pasadena, CA 91125, USA 

The nuclear astrophysical processes and stellar sources 

responsible for short-lived nuclei in the early solar system 

have been the subject of considerable discussion. Of 

particular interest are 107Pd and 205Pb which were produced 

by an "s" process. We have reported the Pd-Ag data on metal, 

sulfide and phosphide phases in a wide variety of meteorites. 

There is a good correlation between 107 Ag/109 Ag and 

108Pd/109 Ag for metal samples from Gibeon (IVA). The 

sulfides lie near the origin but are disturbed. The slope of this 

correlation line corresponds to a 107 Ag*/108Pd ratio of 2.4 x 

w- 5 and an initial 107 Ag/109 Ag near normal silver. The 

107 Ag* is inferred to be due to the in situ decay of 107Pd (tlh 

= 6.5 x 106 y). However, Pd-Ag data from Santa Clara 

(IVB) and several other IVB and "anomalous" groups irons do 

not lie on the isochron and require a more complex history, 

possibly reflecting different formation times. To date, 107 Ag* 

is present in meteorites from group II, III, IV irons, and in 

pallasites and mesosiderites. These results constitute strong 

evidence for the presence of live 107Pd in the early solar 

system and early planet formation. 

Assuming (107Pd/108Pd)0 = 2x10-5 after dilution with the 

ambient ISM, Wasserburg et al. (1994) have estimated the 

isotopic abundances of s-process nuclei for an AGB star and 

predicted a value of e05Pbt204Pb) 0 = 6.5 to 11 xl0-4 at the 

onset of solar system formation. We previously carried out 

Pb-Tl studies on two iron meteorites which have primordial Pb 

and thus have not suffered terrestrial Pb contamination. A 

hint of 205Tl * ( - 4 %o) was found for sulfide from an iron 

meteorite. It was evident that samples with higher 

e 04Pbt2°5Tl)/e04Pbt2°5Tl)0 were needed to obtain 

unambiguous results. We have determined Pb and Tl 

abundances and Pb isotopic compositions in the metal and 

sulfide phases of seven iron meteorites: Canyon Diablo (CD), 

Bogou, Derrick Peak, Cape York, Trenton, Nantan, and 

Gibeon and two stones: Mezo-Madaras (MM) and Angra dos 

Reis (ADOR). The primordial 204Pbt2°5n ratios in both 

metal and sulfide of iron meteorites and two stony meteorites 

range from 0.15 to 22. Assuming 205Pbt204Pb = 5.4 X w-4 

in the meteorite parent bodies, we have estimated the amounts 

of excess 205Tl to be from 0.1 to 12 %o. In summary, we 

found a general enrichment of Pb relative to Tl in most iron 

meteorites, achondrite and an achondrite, as compared to the 

e04Pbt2°5Tl) 0 . However, only a few samples may be 

suitable candidates for detecting 205Tl * at > 4 %o level. 

Thermodynamic condensation studies showed that Pb and Tl 

gas in equilibrium down to 5oo·K with Fe-Ni metal should 

have e 04Pbt2°5TI)/e04Pbt2°5TI)0 enhanced by a factor of -40. 

This suggests suitable candidates may be found for this work, 

but it is not evident that the fractionation found here in a 

chondrite and in iron meteorites can be simply related to a 

condensation model. Div. Contrib. 5365(839) NASA NAGW3337. 

55 

ARGON IN THE CRYSTAL STRUCTURE OF MICA AND 

ITS MUL TI-PUL~E RELEASE IN VACUO: IMPLICATIONS 

FOR 40 ArP9 Ar GEOCHRONOLOGY 

CHEN Yanshao, Department of Geological Sciences, 

Queen's University, Kingston, Ontario K7L 3N6, Canada 

and Department of Physics, University of Toronto, 60 St. 

George St., Toronto, Ontario M5S 1A7, Canada, 

FARRAR, Edward and CLARK, Alan H., Department of 

Geological Sciences, Queen's University, Kingston, 

Ontario K7L 3N6, Canada. 

An hypothesis for argon distribution in micas is presented 

to explain their in vacuo release patterns and age spectra. It 

is proposed that radiogenic and irradiation-derived argon is 

located in vacancies in interlayer and octahedral sites, 

corresponding to the cleavage plane and octahedral layer 

positions suggested by Melenevskiy et al. (1978). Upon 

heating in high vacuum, argon in the octahedral site is 

expelled at low temperatures (< 850°C), probably due to the 

destruction of the hydroxyl located on octahedral anion 

junction planes, especially those connected to vacant 

octahedra. However, argon located in the interlayer site 

vacancies is released above 850°C. 

Argon release in biotite commonly shows a "two-pulse" 

pattern. The lower-temperature pulse ( -700°C) may be 

correlated with the dehydration of the vacant octahedral site 

but only broadly with the dehydrogenation related to the 

oxidation of the octahedrally-coordinated Fe+2

• The remaining 

argon is released at -950°C with complete dehydroxylation. 

In nearly entirely trioctahedral phlogopite, dehydroxylation 

begins above 1 000°C and the absence of vacant octahedral 

sites results in only a single release pulse above 1 000°C. 

Dehydroxylation of muscovite begins at -700°C and, because 

of the overlap in the release of argon from octahedral and 

interlayer vacancies, most muscovites yield one-pulse release 

patterns with maxima at -900°C, although some release argon 

in two closely-separated pulses, at -800°C and -950°C. The 

difference in temperature between the argon release pulse of 

phlogopite and the high-temperature pulses of biotite and 

muscovite suggests that the distortion in crystal structure 

caused by the dehydroxylation may speed up argon release 

from the interlayer site. 

Octahedral and interlayer sites are distributed uniformly in 

the mica structure, and thus argon release from these sites at 

different temperatures offers a novel explanation for the "twofaced" spectra of both biotite (York and Lopez-Martinez, 

1986) and muscovite. This model provides reasonable 

explanations for the diverse mica age spectra and permits the 

deduction of geologically meaningful information even from 

complex spectra. The presence of argon in two uniformly 

distributed sites also prompts caution in the interpretation of 

laser spot fusion results, owing to possible low temperature 

contributions from the areas adjacent to the spots. 

Melenevskiy, V .N., Morozova, I.M. and Yurgina, Ye.K., 1978, 

The Migration of Radiogenic Argon and Biotite 

Dehydroxylation: Geochem. Int., v. 15, p. 7-16. 

York, D. and Lopez-Martinez, M., 1986, The Two-faced Mica: 

Geophys. Res. Lett., v. 13 , p. 973-975. 
PRECISE ZIRCON GEOCHRONOLOGY ON 

ARCHEAN ROCKS SAMPLED BY THE WORLD'S 

DEEPEST CONTINENTAL BOREHOLE, SD-3 

SUPERDEEP WELL, KOLA PENINSULA, RUSSIA 

CHEN, Y. D., KROGH, T. E, Geochronology Lab, 

ROM, I 00 Queen's Park, Toronto, Ont., Canada 

M5S 2C6; VETRIN, V. R. and MITROFANOV, F. P. 

Institute of Geology, Kola Science Center, Russian 

Academy of Sciences, 14 Fersman St., Apatity 

184200, Russia 

Low procedural blank/single zircon techniques have 

been used to date Archean rock samples (ranging in depth 

from 6.8 to 12 km) recovered by the world's deepest 

continental borehole, SD-3 Superdeep Well, drilled on 

Kola Peninsula. Zircon analyses of two tonalite samples 

(seven analyses for sample #42 and six for #48) from the 

Amphibolite-Tonalite-Plagiogranite Complex (ATPC, 

11708-12066 m) define upper intercept ages of 2832 +6/-

5 Ma (#42) and 2835 +6/-4 Ma(#48). Four analyses of 

three pegmatite samples (#40 from 11747-11750 m, #37 

from 11570-11575 m, and #22533 from 6850 m) define 

an upper intercept age of 2740 +10/-9 Ma. All tonalite 

and pegmatite zircon data point to a similar lower 

intercept of 1790±80 Ma as a result of thermal diffusion 

or new growth at this time. Four analyses from three finegrained porphyritic granite dykes (#56 fr~m 9665-9700 m, 

#54 from 9354-9370 m and #55 from 9088-9100 m) are 

concordant or less than 1 percent discordant and indicate 

emplacement during the thermal event at 1766±3 Ma. 

Eleven analyses from a biotite gneiss sample (#28 from 

10837-10844 m) show a complex age pattern; ages 

obtained by projecting from 1766 Ma range from 2730 to 

2836 Ma. New growth or resetting within this rock during 

the pegmatite growth is implied. 

In addition to the age information, the following facts 

are observed: (1) analyses of the 1766 Ma granite dyke 

zircons are almost concordant with or without abrasion; 

(2) the air abrasion technique helps create more 

concordant data points on the primary discordia line but 

recent Pb loss i~ absent even in unabraded grains. This 

indicates that recent surface-related Pb loss from zircons 

at depth is insignificant, probably due to spontaneous 

annealing of radiation damage in these ziroons. 

Investigation of this phenomenon as a function of ambient 

temperature is under way. A full understanding has 

strong implications for understanding zircon discordance 

mechanisms. 

56 

SEA-LEVEL CHANGE OVER PAST SEVERAL 

MILLENNIUMS AROUND PENGHU ISLANDS 

IN THE TAIWAN STRAIT 

CHEN. Yue-Gau and LIU, Tsung-Kwei., Dept. ·of 

Geology, National Taiwan University, Taipei 107, 

Taiwan, ROC 

Taiwan is an active mountain belt created by a collision 

between Luzon arc and Asian continent since 12 Ma (Teng, 

1990). The region of Penghu Islands may be the only area 

around Taiwan, which has not yet been tectonically affected 

by the collision. Geologic and neotectonic evidence indicates 

that this region is stable for time intervals not only in a longterm (several million years) but also in a short-term (several 

thousand years). Therefore, it can provide important 

information for the local Holocene sea-level change. Due to 

the falling of the sea-level in the western Pacific since several 

thousand years ago (Kidson, 1982), the ancient shorefaces 

have emerged and become coastal plains in Penghu Islands. 

Abundant coarse-grained detrital bioclastics (i.e., shells and 

corals) occurring in these plains were collected to study the 

Holocene sea-level change. Among them, twelve carbonate 

samples were dated by the 14C method. 

Our radiocarbon age data show that a progradational 

deposition model is applicable to Penghu region. In this 

model, bioclastics occurring near the shoreline angle are 

considered as deposits formed earlier at the highest waterstand 

period whereas bioclastics occurring around the outer edge 

should be younger products. Because in Penghu Islands most 

coastal plains are associated with vast reef flats, the surge 

energy would be hence greatly diminished and these plain 

surfaces should be regarded as integrated tracks of the paleoHTL (high tide line). If the tidal range during past several 

thousand years can be assumed to be the same as that of the 

present-day, ground altitudes of the sampling localites may 

represent the paleo-HTL position. The paleo-HTL can be 

reasonably converted into its contemporaneous M1L (i.e., 

mean tide line) by substracting a component of the modern 

tidal range. Consequently a sea-level change curve of the late 

Holocene in Penghu region can be constructed by a timealtitude plot. 

This sea-level curve suggests that the highest sea-level in 

Penghu region took place from 4300 to 4500 yrBP 

(conventional 14C age) with a magnitude of about 2.3 meters 

above the modern sea-level. Subsequently, the sea-level fell 

gradually to the recent state without observable fluctuation by 

this approach. 

Teng, L. S., 1990, Geotectonic evolution of late Cenozonic 

arc-continent collision in Taiwan: Tectonophysics 183, 57-

76. 

Kidson, C., 1982, Sea-level changes in the Holocene: Quat. 

Sci. Rev., 1, 121-151. 
LEAD ISOTOPES AS INDICATOR TO 

CONSTRAIN THE MARINE SEDIMENTARY 

PROVINCE IN SOUTH CHINA SEA 

CHEN YU-\VEL Guangzhou Branch, Institute of 

Geochemistry, Chinese Academy of Sciences, 

Wushan, Guangzhou 510640, P.R. China; and 

CHEN SHAO-MAO, South China Sea Institute of 

Oceanography, Chinese Academy of Sciences, 

Guangzhou 510301, P.R. China 

Several significant features of Pb isotopic 

compositions of the marine sediments from South 

China Sea (SCS) can be observed as following: 

l.Pb isotopic compositions of sediments vary 

spacially, reflecting the nature of the source 

materials. In the northern region of SCS, the 

sediments have lower 206pbf204Pb ( 18.6), but 

slightly higher 207pbf204Pb (15.7) and 208pbf204Pb 

(39.0) than those of recent marine sediments. Such 

Pb isotopic compositions are significantly different 

from those of sea-floor basalt in this region. The Pb 

isotopic compositions along the vo]canic chain in 

the Central Nanhai Basin can be divided into two 

groups: ( 1) eastern group with Pb model ages of 

100-125 Ma; and (2) western group with Pb model 

ages of 0-20 Ma. 

2. The South region of SCS can be divided into 

six geographic domains, based on the 207pbf204Pb 

ratios of the sediments. Each domains are correlated 

with the surrounding continental materials. 

3. The Pb isotopic compositions of carbonates in 

marine sediments vary not only spacially, but also 

with the depth of sea water. The 206pbf207Pb ratios 

increase with decreasing depth of sea water. 

4. The (206pbJ207Pb )CD.3 =/(206pbJ207pb )siO) = of 

marine sediments can be used as an isotope 

indicators for investigating sedimentary 

environment, i.e., the ratios of carbonate reef, 

pelagic sediments, and inner shelf and relitic 

sediments are eaqual to 1, graeter than 1 and less 

than 1, respectively. 

57 

NEW GEOCHRONOLOGICAL CONSTRAINTS ON 

THE BIMODAL VOLCANISM IN THE NORmERN 

MAIN ETHIOPIAN RIFT - SOUmERN AFAR 

TRANSITION REGION 

CHERNET T., HART W. K., Geology Dept., Miami 

University, Oxford, Ohio 45056. and ARONSON J. L., 

Department of Earth Sciences, Case Western Reserve 

University, Cleveland, Ohio 44106. 

Thirty new K-Ar ages from the Northern Main 

Ethiopian Rift (NMER) - Southern Afar transition region are 

presented and their implications to the volcano tectonic 

evolution of this region are discussed in light of about one 

hundred flfty new major and trace element analyses. Our 

studies show that unlike other areas on the northwestern 

plateau, flood basalt volcanism in the study area initiated as 

early as 30Ma (Oligocene) and continued episodically until 

about IOMa (Middle Miocene). At the culmination of the 

flood basalt volcanism initial subsidence of the NMER 

started around 1 OMa. 

Sedimentological and other lines of evidence suggest 

that most parts of the Afar Depression started forming 

earlier than the NMER (Upper Miocene). Episodic and 

occasionally localized flood basalt volcanism with 

subordinate silicic volcanism continued in the evolving Afar 

rift floor through the Middle Miocene and into PlioPleistocene. Post/syn subsidence volcanic activity in the 

NMER is on the contrary dominated by silicic volcanic 

products (Nazret Group) from centers which are now deeply 

covered by Quaternary volcanoclastic sediments. This MioPliocene silicic volcanism is associated with subordinate 

fissural basaltic products more prominent in the NMER-Afar 

transition region than other parts of the MER The presence 

of ignimbrites of Pliocene age on the northwestern and 

southeastern plateaus as well as in the NMER floor indicates 

that the NMER gradually attained its present form with the 

escarpments as high as 1.5km over the entire IOMa time 

period. Mio-Pliocene silicic centers (5-6.5 Ma) along the 

southern Afar and the eastern NMER margin are proposed 

to have been formed in a protorift which ran parallel to the 

southern escarpment of the Afar on trend with the structures 

defining the Gulf of Aden. This was followed by the 

formation of the Addis Ababa rift embayment and the 

gro\\1h of silicic centers (3-4 Ma) of the Addis Ababa area at 

the point of intersection of the Gulf of Aden structure to the 

Yerer-Gugu transverse structure. 

The evolution of the rift in the transition region is 

therefore characterized by a complex history of flood basalt 

volcanism, graben (protorift) formation, volcanoclastic 

and/or volcanogenic sediment deposition, dextral 

(transverse) crustal movements and isostatic adjustments. 

Since the initial subsidence of the rift started each stage of 

volcanic activity has been accompanied by major episodes of 

rift subsidence. It is probably the volcanism along the Wonji 

Fault Belt in the Quaternary that marked the beginning of an 

end to major tectonic movements along the escarpments 

and/or subsidence of the rift floor. 
RADIOGENIC ISOTOPIC INVESTIGATIONS OF THE 

ESKA Y CREEK VOLCANIC HOSTED MASSIVE 

SULPHIDE DEPOSIT, BRITISH COLUMBIA, CANADA 

CHILDE. F. C., Mineral Deposit Research Unit, 

Department of Geological Sciences, University of 

British Columbia, Vancouver, B.C., V6T 1Z4, Canada. 

The Eskay Creek deposit is a precious metal enriched 

massive sulphide deposit hosted within a sedimentary and 

bimodal volcanic sequence of the Jurassic Hazelton Group 

in the Iskut River area of northwestern British Columbia. 

Current mineable reserves for the deposit are 1.2 million 

tons grading 1.9 oz/t Au, 86 oz/t Ag, 5.6% Zn and 0.77% 

Cu (The Northern Miner, October 4, 1993). 

Stratabound mineralization of the 21A, 21B and HW 

zones consists of bedded clastic sulphides and sulphosalts 

within an impure mudstone. The immediate footwall to 

stratabound mineralization and host to discordant 

mineralization is an intensely altered (quartz-sericitechlorite) massive rhyolite and rhyolite breccia. 

The Pumphouse, Pathfinder and 109 mineralized zones 

are discordant to stratigraphy and interpreted as feeder pipes 

to the overlying stratabound mineralization (Edmunds and 

Koran, 1991). The discordant mineralization is characterized 

by quartz-sulphide veins within intensely altered wall rocks. 

Analyses of sulphide minerals from discordant and 

stratabound mineralization from this and previous studies 

(Godwin et al., 1991) show a considerable range of 

compositions (206Pbt2°4Pb = 18.779-18.899; 207pb;204pb = 

15.565-15.677; 208Pbt2°4Pb = 38.264-38.598) despite a very 

limited spatial distribution, suggesting mixing from two 

compositionally distinct reservoirs. 

A U-Pb zircon date of 173.6 +5.6/-0.5 Ma has been 

determined from an unaltered flow-banded rhyolite from 

the eastern limb of the Eskay Anticline, this date is 

consistent with paleontological data but is 5-10 Ma younger 

than intrusive rocks within the footwall. Trace element 

lithogeochemistry indicates that this rhyolite is 

compositionally distinct from the altered rhyolite which 

hosts mineralization. However, field relationships suggest 

that the two rhyolites lie within the same stratigraphic 

interval (H. Marsden, pers. comm., 1993). 

Work in progress includes U-Pb dating of rhyolite from 

the western limb of the Eskay Anticline, and of 

hydrothermal monazite from the 109 zone. 

Britton, J. M., Blackwell, J. D., 1990, #21 Zone Deposits, 

Eskay Creek, Northwestern British Columbia: 

Exploration in British Columbia 1989, B.C. Ministry 

of Energy, Mines & Petroleum Resources, p. 197-223. 

Edmunds, C., and Koran, D., 1991, Tok-Kay and GNC 

1991 Exploration Program Geological and Diamond 

Drilling Results: Internal Report for International 

Corona Corporation. 

Godwin, C. 1., Pickering, A. D. R., Gabites, J. E., and 

Alldrick, D. J., 1991, Interpretation of Galena Lead 

Isotopes from the Stewart-Iskut Area (1030, P; 104A, 

B, G): in Geological Fieldwork 1990, Grant, B. and 

Newell, J. M., B.C, Ministry of Energy, Mines and 

Petroleum Resources, Paper 1991-1, p. 235-243. 

58 

CHLORINE-36 IN AUSTRALIAN RAINFALL AND 

PLAYAS AND TilE RESIDENCE TIME OF CHLORIDE 

IN TilE AUS1RALIAN LANDSCAPE 

CHIVAS A R. KISS, E., KEYWOOD, MD., Research 

School of Earth Sciences, The Australian National 

University, Canberra, ACT 0200, Australia; 

FIFIELD, L.K., ALLAN, G.L., Research School of 

Physical Sciences and Engineering, The Australian National 

University, Canberra ACT 0200, Australia, and DAVIE, 

R., BIRD, J .R., Australian Nuclear Science and 

Technology Organisation, Lucas Heights, NSW, Australia. 

Bulk deposition for a series of t5 collectors sampled at 

seasonal (3-monthly) intervals over four years (September 

1988-December 1992) has been analysed for 3CJcl, and major 

and trace ions. Several collectors of varying construction were 

deployed at each site to monitor evaporation (ie. collection 

under paraffin oil), and to allow for analysis of S34S and au a. 

Ten sites comprise the southeastern array that forms an arc 

from the eastern coast to central New South Wales to the 

south coast near Adelaide. The remaining 5 sites near 

Birdsville in central Australia sample the driest part of the 

continent Minor constituents, particularly P04-, NOf, 

~-.and S04

2- are used to assess 'baseline' conditions, and 

reject collections with agricultural contamination (soil, 

fertilizer). 

The east coast site (Kioloa, lat 35.5°S) shows a range of 

36cVCl ratios for individual collection periods from 6 x to-ts 

to 115 x t0- 1S, with an inverse relationship to total chloride 

content (13.5 to 1.2 mg/L); and a mean weighted fallout of 35 

atoms 36CVm2/s. Inland stations from the sou~eastern array 

(-34°S) have a mean, but variable fallout of -25 atoms 

3CJcym2/s, whereas those from central Australia ( -25°5) have a 

mean flux of -13 atoms 36CVm2/s. 

Modem salt lakes (playas) are abundant in the western twothirds of the continent. Halite from these lakes has 3CJcVCl 

ratios of 28 x to-ts to 150 x to-•s. The 3CJcVCl values in 

lakes from the Lake Eyre Basin (coincident with the central 

rain-collector array) are low even though this is an area where 

bulk atmospheric deposition has 36cVCl ratios of 150 to 500 x 

to-ts. The majority of chloride in these salt lakes has a long 

residence time and ancient ultimate sources, ie. the constantly 

redissolving and reprecipitating salt-crust of Lake Eyre and 

discharge/leakage from the underlying Great Artesian Basin. 

By contrast, halite from modern salt lakes in Western Australia 

shows progressively higher 36cVCl ratios away from 

coastlines that mimics (at lower ratios) the trend of modem 

atmospheric 3CJcl fallout (Keywood et al., ICOG-8 abstract). 

This correspondence indicates that a principal source of surficial 

chloride in the western half of the continent is of meteoric 

origin with an integrated residence time in the landscape of 

-lMa. 
DUPAL ISOTOPE SIGNATURE IN MAN1LE DOMAINS 

OF EASTERN ASIA: IMPLICATIONS FOR DYNAMIC 

PROCESSES OF MANTLE CONVECTION AND 

LITHOSPHERE EROSION 

CIDJNG Sun-Lin, Dept. of Geology, National Taiwan 

University, Taipei, Taiwan, ROC, and SUN, Shen-su, 

Australian Geological Survey Organisation, P.O. Box 378, 

Canberra ACT 2601, Australia 

Cenozoic intraplate basalts are widespread in eastern China 

and adjacent areas. Numerous published data show that these 

rocks have incompatible element patterns and radiogenic isotope 

compositions similar to OIBs. Moreover, they exhibit overall 

higher 208pbf204pb and 207pbf204pb ratios than the Northern 

Hemisphere Reference Line (NHRL) indicative of the Dupal Pb 

isotope anomaly commonly observed in the southern 

hemisphere. Mixing of three mantle components, i.e., the 

depleted mantle (DMM) and two types of the enriched mantle 

(EM1 and EM2), can account for their geochemical signatures. 

It is generally agreed that the depleted mantle component 

corresponds to the asthenosphere whereas both the enriched 

mantle components are derived from the continental lithosphere 

(e.g., Tu et al., 1989). Because all these intraplate basalts show 

208 Pb/204 Pb ratios above and parallel to the NHRL, the 

depleted mantle source is further required to have Pb isotope 

characteristics resembling the Indian Ocean MORBs in the 

Duapl region. In this sense, Pb-Sr-Nd isotope systematics of 

the basalts suggest that there may be a Dupal convecting 

mantle domain underlying the entire eastern Asian continental 

margin. Isotope compositions of basalts from the South China 

Sea (Chung and Sun, 1992) and from the Sea of Japan 

(Cousens and Allan, 1992) strongly support such a suggestion. 

These marginal basin basalts show Dupal Pb isotope signature 

of MORBs from the Central and the Southeast Indian Ocean 

Ridges. A coherent Dupal isotope feature is also observed in 

MORB-type basalts from the Philippine Sea (Hickey-Vargas, 

1991). It appears that the Indian Ocean geochemical province 

may have extended to eastern Asia, and have a boundary with 

the Pacific mantle convection delineated by trench systems 

extending from NE Japan through the Izu-Bonin to the Mariana. 

A northward transport of the Dupal isotope anomaly by 

entrapment of subduction zones has been proposed for the 

Philippine Sea (Hickey-Vargas, 1991). A larger scale and long 

lasting mantle process, such as convective influx of the 

asthenosphere with Indian Ocean geochemical characteristics, 

however, is necessary to explain the distribution of Dupal 

signature in the convecting mantle up to the Sea of Japan. On 

the other hand, paleomagnetic studies reveal that many Asian 

continental terranes were once parts of the disintegrated 

Gondwanaland. They drifted northward from the southern 

hemisphere and amalgamated during late Paleozoic-early 

Mesozoic time. Our preferred interpretation for the Dupal Pb 

signature of Cenozoic intraplate basalts in eastern Asia is that 

continental rifting and thermal erosion of the lithospheric 

mantle with Dupal properties have played a major role. 

Chung, S.L., and Sun, S.-s., 1992, Earth Planet. Sci. Leu., 

109,133-145. 

Cousens, B.L., and Allan, J.F., 1992, Proc. ODP,1271128, 

805-817. 

Hickey-Vargas, R., 1991, Earth Planet. Sci. Lett.,107,290-304. 

Tu, K., Flower, M.F.J., Xie, G., Carlson, R.W., Wang, Q., 

and Zhang, M., 1989, Abst. 28th IGC, Washington, D.C. 

59 

AFC MODEL FOR SINGLE VOLCANO PLOSKIE 

SOPKY (KAMCHATKA) 

CHURIKOVA Tatjana G., Institute of Volcanic Geology and Geochemistry, Peep av. 9, PetropavlovskKamchatsky 683006 Russia, and 

KOSTITSYN Yu.A., Institute of Mineralogy, Geochemistry and Crystallic Chemistry of Rare Elements 

(IMGRE), Moscow 121357 Russia. 

Ploskie Sopky is largest basaltic volcano of Kamchatka disposed within the Central-Kamchatka depression. 

Volcanic rocks from basalts to andesites with different 

alkaline (normal and subalkaline) are united in it. The 

volcano includes the rocks with different structures from 

aphyric to megaplagiophyric. It was found by radiocarbon 

method, that the volcano has passed through three stages 

of evolution, interrupted with long-term periods: 45-50 Ty 

- the origin of shield volcano, 30 Ty - the formation of 

strata volcano, 9-10 Ty - the development of superimposed zone, which embraces slag and slag-lava cones, arranged along the traverse north-eastern fault zone, and 

two contemporary calderas. 

The Sr isotope compositions and concentrations of Rb 

and Sr were determined for 20 rock samples, collected 

from all three structural floors. They constitute the representative set, ranging from calc-alkaline basalts to subalkaline andesites, attributed to both aphyric and megaplagiophyric varieties. Furthermore, 5 plagioclase phenocrystals from megaplagiophyric rocks were analyzed. 

Both central and border parts of the crystals were analyzed separately. 

Overall variations of 87Sr / 86Sr in the lavas are from 

0. 70340 to 0. 70370. Basalts of shield and stratovolcano 

have almost constant 87Sr/86Sr (0.70342±5) but wide 

range of · Sr concentrations (320-57 4 ppm) that might 

reflect plagioclase .fractionation from primary basaltic 

magma that is typical for island arc mantle. 

The strata volcano and the superim~sed zone rocks 

show a negative correlation of 87Sr / Sr (0.70340 -

0.70367) with Sr concentrations (421-282 ppm). These 

results are compatible with a model [ 1) of concurrent 

assimilation and fractional crystallization (AFC), when 

crystalls not fully removed from a liquid. The best fitting 

was achived for mixture of 30% cumulate plagioclase 

with the melt. Initial melt in the model has [Sr)=350 and 

87Sr / 86Sr=0.7034 - the average values for aphyric basalts from shield volcano and stratovolcano. The volcanic 

structure assumed to be developed over the metamorphic 

basement, so the model contaminant isotope composition 

(0. 7070) taken out from data [2] on plagiogneisses and 

metamorphosed granites of Kolpakovskaya and Kamchatskaya series. The maximum quantity of the assimilated 

matter are 5-6%. 

The assimilation process was of considerable importance in the late stage of volcano evolution, probably, due 

to within-crust intermediate magmatic chamber appearance, that is implied by two large calderas existence. 

1. D.J .DePaolo. Earth and Planetary Sci. Letters. 1981. 

V.53. P.189-202. 

2. V.I.Vinogradov, V.S.Grigoriev, V.M.Kastrykina. Reports of Academi of Sciences. 1991. P.58-64. 
EARLY PROTEROZOIC DEVELOPMENT OF THE 

WESTERN PART OF THE EAST EUROPEAN CRATON 

CLAESSON S., Swedish Museum ofNatural History, S-104 

05 Stockholm, Sweden; BIBIKOV A E., Vernadsky Institute 

of Geochemistry, 117 334 Moscow, Russia; MANSFELD, 

J., Stockholm University, S-106 91 Stockholm, Sweden; 

BOGDANOV A, S., Lund University, S-223 62 Lund, 

Sweden; REINFRANK, R., Australian National University, 

Canberra ACT 2601, Australia 

The Precambrian crust of the East European Craton between 

the Fennoscandian (Baltic) and Ukrainian Shields is covered by 

platform sediments, but is known from thousands of boreholes. 

Structurally, it is divided into a series of generally N-S trending 

belts of varying lithologies, characterized by alternating 

granulite and amphibolite facies metamorphism. Towards the 

southeast, it is delimited by the NE-trending Osnitsk-Mikashevichi belt of igneous rocks, which has been interpreted to form 

the NW margin of the Ukrainian Shield (Bogdanova et al., 

1993). Traditionally, most of this crust has been considered to 

be Archaean, but recent results (Bogdanova et al. 1993, Huhma 

et al. 1993, Sundblad et al. 1993) have shown that much of it is 

Proterozoic in age. In this contribution, we present new U-Pb 

zircon and reconnaissance Sm-Nd whole-rock data from the 

basement between the Fennoscandian and Ukrainian Shields. 

A metasediment in the structurally oldest unit in eastern 

Lithuania has a bulk U-Pb zircon age of2.3 Ga, similar to that 

of Svecofennian metasediments. An undeformed granitoid in 

northern Latvia has a U-Pb zircon age of 1820±9 Ma, suggesting that no orogenic event has affected that area after 1.8 Ga. 

Single zircon crystals from a basement gneiss in NE Poland, 

previously considered to be >2 Ga, have been analysed with the 

SHRIMP II ion microprobe and yielded ages between 1.4 and 

2.0 Ga, interpreted to reflect the rock ages in the local crust. 

A metadacite from western Belorussia has a U-Pb zircon age 

of 1999±6 Ma, while an orthogneiss has a U-Pb age of 1903±2 

Ma. A U-Pb age of 1855 Ma for an enderbite is interpreted to 

give the age of granulite facies metamorphism. In the area north 

of the Ukrainian Shield, U-Pb data on zircons from a metapelitic granulite and a pegmatitic granite give ages of2.1-2.0 Ga. 

Various rocks from Lithuania and Belorussia analysed so far 

invariably yield £Nd(1.9 Ga)-values between +3 and -2. 

Together with recently published data, the results presented 

here show that most of the Precambrian basement between the 

Fennoscandian and Ukrainian Shields is Proterozoic, with no 

major Archaean component. Significant parts of the crust in the 

Baltic states and Belorussia are comparable to the Svecofennian crust in the Fennoscandian Shield, but the orogenic development seems to have started somewhat earlier in the SE. The 

granulite facies· metamorphism appears to be c. 1.8 Ga old. 

Bogdanova, S., Bibikova, E. and Gorbatschev, R., 1993, Paleoproterozoic U-Pb zircon ages from Belorussia: New geodynamic implications for the East European Craton. Precambrian 

Research, in press. 

Huhma, H., Puura, V., Klein, V., and Manttari, I., 1993, Ndisotopic evidence for Paleoproterozoic crust in Estonia. Geol. 

Surv. ofFinl., Spec. Paper 12: 67-68. 

Sundblad, K., Mansfeld, J., Motuza, G., Ahl, M., and Claesson, 

S., 1993, Geology, geochemistry, and age of a Cu-Mobearing granite at Kabeliai, southern Lithuania. Mineralogy 

and Petrology, in press. 

60 

ISOTOPE ARRAYS AND PLUME SOURCES: 

THE NINETYEAST RIDGE & COMORO PLUMES 

Class. C. and Goldstein, S.L., Max-PlanckInstitut fiir Chemie, D-55020 Mainz, Germany. 

It is still a matter of debate whether isotope arrays 

of oceanic island basalts reflect heterogeneity within 

the rising mantle plumes, or mixing between plume 

and non-plume components. Recently Class et al.l 

argued that the isotopic evolution of the Ninetyeast 

Ridge hotspot track indicates a distinct composition 

for the plume, associated with one end of the isotope 

arrays formed at each point in time. Here we show 

that isotope arrays of the Ninetyeast Ridge and 

Comoro hotspots reflect mixing of plumes having 

singular compositions with lithospheric components. 

For the Ninetyeast Ridge hotspot, trace element 

and isotope systematics indicate that the Heard Island 

isotope array reflects contamination of plume magmas 

with the Kerguelen Plateau lithosphere, on which the 

island is situated. Isotope ratios of lavas from the 

Kerguelen islands are consistent with this interpretation. New helium isotope data2 confirm the deep 

mantle origin of the mixing endmember we previously identified as representing the plumel. 

Grande Comore Island is a volcanic doublet. 

Karthala is a shield volcano consisting of alkali 

basalts. La Grille is smaller, covered by strombolian 

cones, and has erupted mainly basanites. The isotopic 

variation of Grande Comore indicates mixing of three 

components. One, almost exclusively in Karthala 

lavas, forms the high-Sr, high-Ph and low-Nd end of 

the array (K-endmember). The others are at the lowSr end of the La Grille array (G-endmember) and at 

the intersection of the Karthala and La Grille arrays 

(X-endmember). The spatial relationships of these 

endmembers can be deduced from the following 

considerations. ( 1) Karthala has produced much more 

magma, and lavas are related by shallow level 

fractional crystallization in a magma chamber. La 

Grille eruptions are fissure controlled, with lavas 

reflecting variable low degrees of partial melting, 

precluding significant residence in a magma chamber. 

(2) Both G and K mix with X, but mixtures of all 

three components are not present, indicating they are 

not all part of the plume. (3) Few Karthala samples 

show mixing with G, therefore G is separated in 

space from K. Trace elements in G-rieb lavas indicate 

a metasomatized source, probably of continental 

origin. (4) X and G are similar in composition, but 

X-rich samples show higher degrees of melting and 

have lower Tb/Yb ratios than G-rieb samples. 

Therefore, X is located at a shallower level. It is 

added to Karthala magmas in the shallow magma 

chambers. Thus, K clearly represents the plume and 

G and X are in the lithosphere. 

tc. Class et al. (1993) Nature 362, 715-721. 

2D. Hilton et al. ( 1994) this abstract volume 
ISOTOPIC CONTOURS OF AUTHIGENIC CLAYS IN 

SOILS: INSIGHTS INTO THE MINERAL-FLUID 

INTERACTIONS IN SOIL ENVIRONMENTS AND 

IMPLICATIONS OF ISOTOPIC DATING OF 

CONTINENTAL EROSIONAL DEBRIS 

CLAUER, N., Centre de Geochimie de la 

Surface, C.N.R.S., 67084 

Strasbourg,France, and CHAUDHURI, S., 

Kansas State University, Manhattan, 

Kansas 66502, U.S.A. 

Authigenic clays in a given soil 

environment can be widely varied in their 

Sr and Ar isotopic· compositions. The newly 

formed clays have been found to inherit 

from their feldspar and biotite precursors 

varied amounts of the radiogenic isotopes, 

causing very often the apparent isotopic 

dates to be excessively high in comparison 

to the age when the parent materials 

weathered to form the clay minerals. The 

variations in the content of radiogenic 

isotopes for the authigenic clays across a 

soil profile signify that the growth of 

individual clay particles occurs often 

under the influence of a small scale 

environment or microenvironment. The idea 

of a spatially limited restricted 

environment or a microenvironment for the 

growth of clay particles in soils also 

receives support from oxygen isotope 

evidence of clays in soils, as clays in 

soils have been found with significant 

variations in their oxygen isotope 

compositions which do not relate to the 

composition of local meteoric water. 

Changes in the Rb-Sr sytematics 

produced in crystalline rocks as a result 

of progressively increased alteration of 

feldspar and biotite may be described in 

terms of models depicting the displacement 

of the Sr isotopic and the Rb/Sr ratios 

along a curvilinear trend below the 

theoretical isochron line corresponding to 

the age of the rock. The model 

illustrations for the isotopic changes 

have supports from examples of several 

natural occurrences of weathered products. 

The models also show, by depicting that 

the soil clays are high in Sr isotopic 

ratios relative to ambient waters, why the 

Sr isotope ratios of runoffs are not 

significantly elevated due to the effects 

of weathering of feldspars and biotites. 

61 

a18o OF DEEP CONTINENTAL CRUST: COMPARISON 

OF GRANULITE XENOLITHS WITH THE MAFIC 

COMPLEX OF THE IVREA ZONE, ITALY 

CLEMENS-KNOTT. Diane. Dept. Geol. Sci., California 

State University Fullerton, CA 92634 USA, TAYLOR, Hugh 

P., Jr., Geology 170-25, Pasadena, CA 91125 USA, and 

SINIGOI, Silvano, Inst. Min. Pet., Univ. Trieste, Italy. 

The Ivrea Zone is arguably the best exposed cross section 

through deep continental crust on Earth. The Mafic Complex, 

a thick intrusive series emplaced into a granulite- and amphibolite-facies volcanosedimentary section, can be divided into the 

ultramafic-mafic Basal, Intermediate and Upper Zones (BZ, IZ, 

UZ), overlain by the Main Gabbro (MG), in tum overlain by 

normal and roof "Diorites" and chamockites. The Mafic 

Complex serves as a geochemical reference for in situ deep 

crust, because the values of 18 0 and Mg number (Mg') are 

similar to those of nonmetasedimentary granulite xenoliths 

collected worldwide. Possible petrogeneses of granulite 

xenoliths are revealed by classifying Mafic Complex samples 

by Mg# and Si02 according to the xenolith classification 

scheme of Kempton and Harmon (1992; 'roups M1, M2, M3, 

"intermediate", "silicic"). M1 rocks (Mg > 0. 7) of the Mafic 

Complex are found only in the BZ and IZ (~ 180 = 6.3 to 8. 7; 

n= 16), which are interpreted as being formed from initial 

intrusions of mantle-derived magma into relatively cool 

(subsolidus) crust. The M2 group contains samples from all 

units of the Mafic Complex except the chamockites. Most UZ, 

MG, and normal "Diorite" samples plot in the M3 field. These 

units are believed to have formed within a thermally-buffered 

magma chamber undergoing RTF and AFC processes. Subsequent modification of the UZ and normal "Diorite" cumulates 

resulted from the upward percolation and intercumulus 

crystallization of ( 1) chamockitic melt derived from a 

metapelitic septum at the UZ-IZ boundary, and (2) fractionated 

and contaminated magma concentrated in the late-stage, MG 

chamber. Five roof "Diorite" samples (~ 18 0 = 8.9 to 10.6) 

that contain melt derived from amphibolite-facies paragneiss 

overlap a high-180 subset of the "intermediate" xenolith class 

(~ 18 0 = 8.9 to 12.5). Absent from the Mafic Complex are 

"intermediate" rocks with 18 0 < 8.4. Mafic Complex rocks 

of the "silicic" class are represented by 7 chamockites (~18 0 = 

9.6 to 11.2) formed in part from melts of granulite-facies 

paragneiss. Thus, essentially all types of granulite xenoliths 

have counterparts in the Ivrea Zone. This correlation implies 

that many deep-crustal xenoliths could be generated during 

"Mafic Complex-type" events, and that generation of high-180 

mafic xenoliths does not necessarily require metasomatic 

reequilibration with mantle-derived fluids. 

Based on the observed covariation of 18 0, Si~ and 

<

87Sr/86Sr)i there appear to be petrogenetic similarities between 

Ivrea Zone lithologies and approximately coeval, mid- to 

upper-crustal gabbroids and granitoids found in the neighboring 

Strona-Ceneri Zone and in the Massif Central of France. The 

Mafic Complex thus may be interpreted as a snapshot of 

magmatism in the lower European crust during Permian crustal 

growth events manifest at all crustal levels. The generation of 

highJ8o lithologies in the Ivrea Zone very likely took place 

during synmagmatic extension of the deep continental crust, 

and did not require a highJ8o mantle source. 

Kempton, P.D, and Harmon, R.S., 1992, Oxygen Isotope 

Evidence for Large-Scale Hybridization of the Lower Crust 

During Magmatic Underplating: GCA, v. 56, p. 971-986. 
ION PROBE PARAMETERS FOR VERY HIGH 

RESOLUTION WITHOUT LOSS OF SENSITIVITY 

CLEMENT. S.WJ. and COMPSTON,W. Australian 

National University, Canberra, A.C.T. 0200 

The original SHRIMP ion probe at ANU uses a secondary 

mass-analyser corrected for aberrations to second order 

(Matsuda, 1974). A new generation of designs is now 

possible in which a large ratio of mass-dispersion I image 

magnification, combined with aberration correction to third 

order, permits the achievement of much higher mass 

resolution without loss of sensitivity (Matsuda, 1990). 

The main features of this new family of designs are a 

'reverse geometry' layout, i.e. the ions pass through the 

magnetic analyser before reaching the electrostatic analyser, 

and the addition of several more electrostatic quadrupole 

lenses. For an ion probe the same size as SHRIMP I or II, 

such a design will achieve mass resolution greater than 

20000 with flat-topped peaks for the same sensitivity as 

SHRIMP II operating at resolution 5500. 

An ion probe with this capability will open a new range 

of applications that are not presently practical because of 

hydrides and other isobars which cannot be separated on 

existing instruments without unacceptable loss of sensitivity. 

Examples areCa isotope dimers such as 44ca43ca on 87sr 

(m/L\m 16000) which discourage serious 87sr f86sr studies 

inCa rich minerals; 206pb1H on 207pb (m/L\m 29000) in 

hydrous minerals such as pitchblende, and separation of 58Ni 

and 58pe (m/L\m 29000) for studies of meteoritic isotope 

anomalies in the Fe-group of elements. 

In addition, such an instrument could be operated with a 

very wide source slit for other applications such as stable 

isotope analysis, which do not demand high mass resolution 

but are especially susceptible to variable mass discrimination. 

Use of very wide slits and apertures will reduce beam 

truncation during the transfer of ions from the analysed spot 

into the mass analyser, leaving the sputtering process itself 

as the main remaining source for variable discrimination. 

The prototype for such an instrument, named SHRIMP 

7000, is now under development at the ANU. 

Matsuda, H., 1974, Double Focussing Mass Spectometers of 

Second Order: Int. J. Mass Spect. Ion Phys., 14,219-233 

Matsuda, H., 1990, Noel. Inst. and Meth. in Phys. Res., 

A298,199-204. 

62 

Rb-Sr DATING OF WHITE MICA- NEW POTENTIAL IN 

MET AM ORPHIC GEOCHRONOLOGY. 

Cliff. RA, Department of Earth Sciences, University of 

Leeds, LEEDS, LS2 9JT, UK 

Despite being one of the oldest methods in geochronology 

and having a T c ~500°C, Rb-Sr dating of white micas has 

been little used in recent studies of young metamorphic belts. 

At least in part this stems from unsatisfactory experiences in 

earlier studies in the Alps. These showed that the relatively 

low Rb/Sr ratios of most metamorphic white micas yielded 

rather poor precision with the mass spectrometric techniques 

then available and, moreover, demonstrated major problems in 

defming initial isotopic compositions in many samples. 

Modem techniques result in analytical errors significantly 

less than 1Ma for white micas in the 10-50 Ma age range and 

because of the relatively high Sr concentration in many white 

micas it is possible to analyse sub-milligram samples. Using a 

21Ma white mica previously dated with 100mg samples, 

reproducibility better than 1Ma has been demonstrated on 

0.1mg samples. 

The initial ratio problems were accentuated by concentration 

on the analysis of micas from basement gneisses. Recent 

results from the Tauem Window in the Austrian Alps show 

that, despite lower Rb/Sr ratios, better results can be obtained 

from metasediments and in favourable cases from intensely 

deformed/recrystallised basement rocks. In the Central 

Tauem, Inger & Cliff (1993) find white mica ages are 

reproducible to 1 Ma in once-metamorphosed metasediments 

over a 6 km area. Reddy et al. (in press) find a factor of 5 less 

scatter in white mica ages from intensely mylonitised gneisses 

than in other gneisses from the Sonnblickkem. 

The petrological study of Dempster (1992) shows 

preservation of major element growth wning in white micas , 

confirming the potential to preserve aspects of prograde 

history. If white mica and a low Rb/Sr phase could be 

extracted from known microstructural settings then this 

prograde history could be dated. Techniques have been 

developed to extract microsamples from sub-millimetre 

domains under the petrological microscope using 100f.Ull thick 

sections. Sr concentrations in typical phengitic white micas 

are sufficiently high for such samples to be analysed 

isotopically. Results of such analyses on 30Ma metasediments 

and gneisses from the Tauem Window will be presented. 

Dempster,T.J., 1992 

Zoning and recrystallisation of phengitic micas: 

implications for metamorphic equilibration. Contributions 

to Mineralogy and Petrology 109, 526-537. 

lnger,S. & Cliff, R.A.1993 

More precise dating of tectonometamorphic evolution in the 

Eastern Alps; Rb-Sr white mica and U-Pb sphene and 

allanite ages from the Tauem Window. Terra Nova 5 

Abstracts Suppl.1, 388. 

Reddy,S.M., Cliff,R.A. & East,R. in press 

The thermal history of the Sonnblick Dome, SE Tauem 

Window, Austria: Implications for heterogeneous uplift 

within the Pennine basement. Geologische Rundschau 
ISOTOPIC ANALYSIS OF BOTH ()18() AND ()17o 

IN ATMOSPHERIC NITROUS OXIDE. NEW 

INSIGHTS FROM AN OBSERVED MASS 

INDEPENDENT ANOMALY 

CLIFF. S.S .. and THIEMENS, M.H., Dept. of 

Chemistry, University of California, San Diego, La 

Jolla, CA 92093-0356, U.S.A. 

Nitrous oxide is an important trace species in the 

earth's atmosphere due to its role in stratospheric ozone 

depletion, and its contribution to global greenhouse 

warming. The global N20 budget remains poorly 

defined due to a lack of source identification, 

presumably anthropogenic. Resolution of all significant 

sources is critical because of its role in the atmosphere 

coupled with a long (150 year) atmospheric lifetime. A 

system has been developed for the collection and 

analysis of air N20 which allows measurement of three 

oxygen isotopes. The use of stable isotope ratio 

measurements is established as a method for obtaining 

budgetary and transformation mechanism information. 

Stable isotope ratio (18Q 16(), 17a 16Q) measurements 

of tropospheric N20 have been performed in order to 

better constrain its budget. Tropospheric N20 shows a 

clear mass independent isotopic signature ranging from 0 

to +1.3o/oo. The ()lSQ in air N20 ranges from ca. 38 to 

43o/oo (SMOW), showing isotopic variability between 

collections. 

Stratospheric N 20 samples have been collected and 

analyzed from the southern hemisphere. From the data a 

correlation between mass independent magnitude and an 

index of altitude is observed. It has been shown that 

reaction with electronically excited atomic oxygen 

produces a small (ca. 6o/oo} strictly mass dependent 

fractionation. Nitrous oxide photolysis produces no 

observable fractionation (<0.1o/oo). Based upon the 

laboratory experiments and field observations, a 

stratospheric source or sink of N 20 exists that has yet to 

be identified. The observation of a yet undefined 

stratospheric N 20 process could not have been detected 

from single isotope ratio or concentration measurements. 

It is the mass independent character which provides this 

specific insight. 

63 

SHORT TERM VARIATIONS IN THORIUM AND 

URANIUM-SERIES ISOTOPES IN MAUNA LOA 

COHEN. A. S., O'NIONS, R. K., (Dept of Earth Sciences, 

University of Cambridge, Downing Street, CB2 3EQ, UK), 

KURZ, M. D. (WHOI, Woods Hole, MA 02543, USA) 

During the shield-building phase of Hawaiian volcanoes, 

the· major element composition of erupted lavas is relatively 

uniform (having allowed for olivine fractionation). However, 

significant variations do exist in the isotopic compositions and 

trace element ratios of material from different volcanoes. More 

recently, it has been shown that short term (1-10 ka) variations 

in He, Pb, Sr and Th isotope compositions and in Th!U ratios 

are present within a suite of tholeiitic lavas collected from a 

single shield volcano, Mauna Loa (Kurz and Kammer 1991; 

Cohen et al. 1993). A minimum of two distinct sources is 

required to explain these observations. The crucial question is 

whether the range in composition of erupted lavas results from 

melt/matrix interaction during extraction and transport, from 

the mixing of more than two distinct sources, or from both. 

Precise determination of disequilibrium within the 226Ra230Th_238u decay series forms a useful approach to this 

problem. (230Th/238U) ratios (measured by TIMS) in 

samples of young lavas from both Kilauea and Mauna Loa are 

frequently close to unity, and Th!U ratios have a mean value of 

2.96 (± 0.03). Interpretation of these results using a dynamic 

melting model constrains the rate of melting, which must be at 

least an order of magnitude greater than for MORB. In 

contrast, the Th!U ratios of samples from Mauna Loa which 

are older than 8 ka are around 10% larger than the ThiU ratios 

of samples less than 2 ka in age (ages determined by 14c 

dating). The e30Thf238U) ratios of those samples which are 

either older than 28 ka, or younfer than 2 ka, are also close to 

unity. However, 230Th and 38u are not close to secular 

equilibrium in some samples in the age range 2-28 ka, as these 

are found to possess small 230Th excesses of up to 10%. This 

observation indicates that there has been a transient departure 

from radioactive equilibrium in the erupted liquids. 

Furthermore, the data presently available show that the change 

in Th isotope composition occurred at constant Th!U ratio, 

and that this change preceeded the 10% decrease in Th!U ratio 

which took place between 8 ka and 2 ka ago. It is very 

difficult to produce these transient disequilibria by mixing 

multiple sources. It is also unlikely that there has been any 

appreciable disturbance to the U series nuclides in the lavas 

after their eruption, because the measured (234u;238U) ratios 

in all the samples are very close to unity. A more plausible 

explanation of these observations is that they reflect the 

consequences of the interaction of melt and matrix during melt 

extraction and transport. 

Kurz M.D. and Kammer D.P., EPSL 103 (1991), 257-269. 

Cohen A.S., O'Nions R.K., Kurz M.D., EOS 74 (1993), 629. 
CID...ORINE STABLE ISOTOPES AS A TRACER OF 

WATERS EXPELLED FROM MUD-ROCKS 

COLEMAN. Max L., BP Exploration, Sunbury-onThames, TW16 7LN UK & Postgraduate Research Inst. for 

Sedimentology, Reading Univ., RG6 2AB, UK and 

EGGENKAMP, Hans, G.M., Geochemistry Dept., Utrecht 

Univ., 3508 TA Utrecht, The Netherlands. and CURTIS, 

Chas, D., Geology Dept., Manchester Univ., Manchester 

M139PL, UK 

Formation waters produced from Forties Field (U.K.C.S.) 

appear to be a range of mixtures consisting of a typical North 

Sea saline formation water and a much more dilute brine from 

a mudrock source. 

Samples are uncontaminated, cover the full geographical 

extent of the field and were taken over a five year period. 

Analyses of ~37Cl, 02H, ~. 87Sr, Na, K, Mg, Ca, Sr, Ba, Cl, Br 

and S04 were made. There are linear correlations of all 

chemical components (except Ba and S04) which is the same 

for between-well and within-well variations. These data imply 

that all produced waters are the result of mixing on a fieldwide scale of two NaCl brines, the end member compositions 

are less than 26,000 ppm Cl and about 60,000 ppm Cl. ~2H, 

~1so give a line of negative slope, unlike most other formation 

water plots, thereby confmning that the waters are mixtures 

rather than products of evolution by water-rock interaction. 

The stable isotope values prove that there is no sea-water 

contamination in the samples. ~7Cl plots linearly against Cl 

concentration, which is surprising and implies a complex batch 

mixing process; diffusion cannot explain the data distribution. 

There is vertical zonation of salinity of oil zone waters, which 

increase in salinity from the top of the reservoir down to the 

oil water contact. This pattern extends over the whole oilfield. 

Thus, there is a range of fossil formation waters trapped by oil 

emplacement, forming mixtures with a water which 

approximates to the aquifer beneath the oil-water contact. 

~37Cl values also show that the more saline brine resulted 

from dissolution of evaporite salts and that the more dilute one 

contains salts affected by a physical process (diffusion or ion 

filtration). The most likely origin for such a water is a shale, 

probably the source rock for the oil, since the over-lying seal to 

the reservoir would not have given the observed spatial 

distribution of compositions. The hypothesis which best 

explains all these observations is that less saline brine (warmer 

and thus lower-density fluid than the pre-existing saline 

formation water) "migrated" from the shale associated with oil 

migration. However, preliminary ~37Cl data from shales 

confound this idea. We hope they are anomalous. 

60000.------------------. 

50000 

[ 40000 0. 

30000 

Cl = 57,600 + 7510 x &7CI 

r2 = 0.972 

-4 -2 

Fig. I. (Pseudo?) linear correlation of ~37Cl and Cl 

concentration, implying a complex mixing process 

-1 0 

64 

Nd AND Pb ISOTOPES OF ALLEGHANIAN 

GRANITES FROM THE GOOCHLAND TERRANE, 

SOUTHERN APPALACHIANS: EVIDENCE FOR A 

JUVENILE SOURCE COMPONENT. 

COLER. David G., SAMSON, Scott, Dept. of Earth 

Sciences, Syracuse University, Syracuse, NY 13244-

1070 and SPEER J. A., Dept. of MEAS, North 

Carolina State University, Raleigh, NC, 27695-8208 

The Goochland terrane of Virginia and North Carolina 

has been considered to be the easternmost piece of 

Grenville crust in the Southern Appalachians. This terrane 

and its southern extension, the Raleigh Belt, were intruded 

by numerous granitic magmas during the Alleghanian 

Orogeny(327-265 Ma). The resulting plutons are 

dominantly metaluminous biotite- granites with major 

element compositions of Si02 (69.0-76.6%), 

A1203(14.3-16.1 %), Ca0(0.62-1.8%), Na20(3.3-4.6%), 

K20(3.8-5.3%), although some volumetrically minor 

peraluminous bodies also occur. These compositions are 

most similar to plutons intruded in a continental 

collisional to post-collisional tectonic regime. 

Detaiied Nd isotopic studies of these plutons were 

undertaken because they serve as excellent probes of the 

lower crust in the Goochland terrane. The plutons analyzed 

include the Rolesville Batholith, the Wise, Wilton, 

Louisburg and the Archer's Lodge plutons and yield initial 

ENd values between +3.0 and -1.9, with associated ToM 

ages between 600-950Ma. These values clearly indicate 

the bulk of the source material is juvenile and could not 

contain a large percentage of recycled Grenville crust. The 

Wyatt pluton, however, has ENd values of"-2.6 and -4.4 

and therefore could include a larger component of recycled 

Grenville crust. Overall, however, the Goochland terrane 

cannot be entirely composed of Grenville crust, or the 

plutons were formed from a different crustal source located 

below the Goochland terrane. 

The least radiogenic leaches of K-feldspars from the 

Wise, the Archer's lodge and the Rolesville batholith yield 

uniform initial common lead ratios of 206pb;204Pb = 

18.36-18.44, 207pbj204pb = 15.53-15.58 and 

208pbf204Pb = 38.02-38.09. These data are similar to 

those reported from other Alleghanian granites, but are 

slightly less radiogenic. This agrees with the higher ENd. 

of the Goochland terrane granites compared to most other 

Alleghanian granites in the southern Appalachians. 

Melting of crust like that in the Carolina terrane, a late 

Proterozoic-Cambrian volcanic terrane that lies to the west 

of the Goochland terrane, could produce the range in Nd 

isotopic composition observed in the Alleghanian 

granites. Thus we infer that crust like that of the Carolina 

terrane must structurally underlie the Goochland terrane. 

The generally high 143Nd/144Nd ratios of the 

Alleghanian granites in the Goochland terrane indicate that 

the contribution of Grenville, or older, crust must be quite 

limited. 
U-Pb AGE CONSTRAINTS ON EARLY PROTEROZOIC GOLD 

DEPOSITS, PINE CREEK INLIER, NORTHERN AUSTRALIA, 

BY HYDROTHERMAL ZIRCON, XENOTIME AND MONAZITE. 

COMPSTON Dayid M, and MATTHAI, S. K., Research 

School of Earth Sciences, Australian National University, 

Canberra, A.C.T., 0200, Australia. 

Recent ion-microprobe U-Pb age determinations on 

hydrothermal minerals extracted from a gold-bearing quartz 

vein from the Goodall mine in the Early Proterozoic Pine 

Creek Inlier, northern Australia, identify two occurrences of 

hydrothermal activity in the area. 

At the Goodall deposit, gold mineralisation occurs in a wide 

halo of potassic alteration (K-spar+biot+and+cord+qtz) which 

hosts quartz stockworks and bedding-concord.:'lnt veins, formed 

at 550 - 620°C and 200 MPa. In general, gold deposits in the 

region are concentrated within the inner contact aureole of the 

1820 - 1835 Ma Cullen Batholith (Stuart-Smith et al, 1993), 

which is considered to be a potential source of the mineralising 

fluids. The Goodall deposit is hosted by greywackes and 

metasiltstones (with local carbonaceous intercalations) of the 

Burrell Creek and Mount Bonnie Formations which have a 

maximum 207pbf206pb age of 1885 ± 2 Ma, obtained from the 

underlying Gerowie Tuff (Needham et al, 1988). Mineralised 

veins elsewhere in the region contain pronounced Zr anomalies 

(to 2000 ppm), suggesting the precipitation of zircon during 

vein formation. 

Zircon, xenotime and monazite were extracted from a 

mineralised quartz-sulphide vein (20 - 40 ppm Au) and 

isotopically analysed by ion-microprobe. The 207pbf206pb age 

distribution of the analysed zircons results from the presence of 

several distinct populations, the ages of which suggest U1at the 

majority of zircons are mechanically incorporated into U1e vein 

from the host environment. Minor populations, with 

207pbf206pb ages of ca 2.50, 1.95 and 1.90 Ga, probably 

represent minor detrital populations within the sediment. The 

largest group has a mean age 1863 ± 7 Ma, which is interpreted 

as the maximum depositional age. About 30% of U1e analyses 

have a younger mean 2°7Pbf2°6Pb age of 1817 ± 16 Ma, 

within error of the age of the Cullen Batholit11. Although there 

are no chemical distinctions between these populations, the 

younger analyses were mostly of grain rims. 

This age of vein formation is supported by preliminary 

207pbf206pb ages of xenotime ru1d monazite, which, although 

with excess scatter, have a mean age of 1810 ± 10 Ma. 

In addition, a small number of analyses are much younger, 

with a slightly discordant mean 206pbf238u age of 536 ± 38 

Ma. This unexpected result does not represent sample 

contamination. The age and higher U contents of U1ese grains 

suggests that they represent new zircon growth. Some of the 

older zircons appear to have lost some Pb at this time, as their 

isotopic composition lies on a discordia tr~jectory between 2.5 

Ga and 0.5 Ga. The geological significance of these data is not 

yet fully understood, but, at the least, they are recording the 

occurrence of previously unknown fluid activity in the 

basement of a Cambrian basin. 

Needham, R. S., Stuart-Smith, P. G. and Page, R. W., 1988, 

Tectonic evolution of the Pine Creek Inlier, Northern 

Territory: Precambrian Res., v40/41, p. 543-564 

Stuart-Smith, P. G., Needham, R. S., Page, R. W., and 

Wyborn, L.A. I, 1993, Geology and Mineral Deposits of the 

Cullen Mineral Field, Northern Territory, Northern 

Territory: AGSO Bulletin 229, pp 145 

65 

TIME POINTS WITHIN THE VENDIAN BY ION PROBE 

COMPSTON. W. Australian National University, 

Canberra. A.C.T. 0200 and JENKINS, R.J.F. Dept 

Geology, University of Adelaide, South Australia, 5001 

Few Vendian and Cambrian ages are based on volcanogenic 

sediments within sections where key fossils indicate useful 

biostratigraphic constraints. We report two new U-Pb zircon 

ion-probe ages for tuffs associated with occurrences of the 

'Ediacara assemblage' at Mistaken Point on the Avalon 

Peninsula, SE Newfoundland, and at Mogilev-Podolsky in the 

Ukraine. We also describe a new assessment procedure for ion 

probe 206pbf238u ages based on mixture-modelling, both for 

age variation within the SL13 standard zircon, as well as for 

multiple detrital age populations. 

Ages for euhedral zircons from the Mistaken Point tuff can 

be resolved into three age groups in about equal proportions: 

607 ± 4 Ma (2a), 591 ± 5 Ma and 571 ± 4 Ma. The last is the 

most plausible for the tuff eruption and is close to a much-cited 

isotope dilution age by G. Dunning. It allows ca. 15 Ma for 

the accumulation of the 4.2 km thick pile of the Conception 

Group, and implies a rate of sedimentation of ca. 290 m I Ma. 

If the same (maximal) rate persisted in the Avalonian Orogen, 

the age of the upper exposed part of the Signal Hill Group 

would be 544-540 Ma. The two older groups are consistent 

with known ages in volcanogenic complexes that underlie the 

Conception Group. The Mistaken Point soft-bodied megafossil 

assemblage represents a biota not known elsewhere. However, 

elements with a wider distribution, Charnia masoni and 

Charniodiscus concentric us first appear in the fossil record near 

the top of the Mistaken Point Formation, so that the 571 Ma 

age applies closely to this important first occurrence. 

Four age groups are found in the Ukrainian tuff: a minor 

detrital I xenocryst group at 560 Ma, the biggest group (46%) 

at 541 ± 4 Ma inferred to represent the likely eruption age of 

the tuff, the zircons of which include a large proportion of 

finely acicular crystals, and a further large (35%) group 530 ± 

4.0 Ma and a small group at 512 Ma, together suggested to 

reflect Pb loss after deposition. The 530 Ma age is indistinguishable from present estimates of the base of the Cambrian. 

Interpretation of the 541 Ma age as the tuff deposition requires 

explanation of the 530 and 512 Ma groups undergoing Pb 

leaching, possibly through circulation of ground-waters during 

terminal Proterozoic and Cambrian exposure of the still 

presently unconsolidated sediments. 

A tuff high in the Slawatycze Fmn on the East European 

Platform, SE Poland, has given a SHRIMP U-Pb zircon age 

551 ± 4 Ma. Elsewhere the Slawatycze Fmn grades upwards 

into a shale-siltstone-sandstone sequence transitional from the 

Neoproterozoic (Vendotaenia and Sabellites Zones) into the 

Early Cambrian. The Slawatycze Fmn iLo::elf is believed 

equivalent to the Volhyn Series which underlies the early 

Valda ian or Redkino Horizon through a large area of the 

adjacent Moscow Syneclise. The 551 Ma age therefore relates 

to the close of the Early Vendian. 

Regardless of which interpretation is used for the Ukrainian 

zircon age groups, the age difference between 571 Ma from the 

Avalonian Orogen and either 530 Ma or 541 Ma for the Vendian of the Ukraine, the fossil record of soft-bodied late Precambrian 'metazoans' spans a time interval of at least 30 Ma, or 

virtually the same duration as the whole of the Cambrian itself. 
THE USE OF DEUTERIUM-ENRICHED WATER 

AS A TRACER FOR HYDROLOGIC TESTS. 

CONRAD. M.E., KARASAKI, K., and 

FREIFELD, B., Earth Sciences Division, 

Lawrence Berkeley Laboratory, Berkeley, CA 

94720. 

D20 is a relatively inexpensive, non-polluting 

tracer that may present an altenative to other tracers in 

environmentally-sensitive areas. To test the viability 

of water artificially enriched in deuterium as a tracer 

for hydrologic tests, D20 was used in a tracer test at 

the Raymond Field Site (a site dedicated to the 

characterization of ground-water flow in fractured 

rock). Along with deuterium, four other 

"conservative" tracers (fluorescein, bromide, fluoride 

and micro-spheres) were also used. 

The Raymond Field Site is located in the Sierra 

Nevada foothills of California, approximately 100 km 

north of the city of Fresno. It consists of a series of 

nine 90m-deep boreholes drilled into an equigranular, 

non-foliated granodiorite. The boreholes are 

arranged in V -shaped pattern at increasing distances 

of 7 .5, 15, 30, and 60m from the central hole. 

Hydrologic and geophysical tests have demonstrated 

that most of the flow between the wells is confined to 

two fracture zones. 

For a radial convergent tracer test, the upper of the 

two fracture zones was isolated using pneumatic 

packers. Before the tracer test, water was pumped 

from the packed interval in the central well at a 

constant rate of 11 liters/min for several days in order 

to establish a steady flow field. Solutions containing 

the tracers were injected into the packed interval in 

one of the boreholes at 30m from the central well. 

Increased oD values were measured in samples 

collected from the central (pumping) well 10 hours 

after injection. The arrival times for fluorescein and 

bromide were identical to that of deuterium. No 

micro-spheres were detected and the fluoride 

concentration was too low to give meaningful results. 

Breakthrough curves for fluorescein, bromide and 

deuterium have the same form, but level off at C/Co 

values of 0.0006, 0.0004 and 0.00035, respectively. 

Fluorescein has been reported elsewhere to react with 

certain minerals causing it to fluoresce more, 

resulting in an apparent increase in mass. The 

difference between deuterium and bromide is small, 

but significant (corresponding to a difference in oD of 

about 3%o). The reasons for this difference are not 

clear at this time. 

66 

MANTLE METASOMATISM VERSUS CRUSTAL 

CONTAMINATION IN THE GENESIS OF K-RICH MAGMAS: 

Sr & Nd ISOTOPES OF PRIMITIVE ULTRAPOTASSIC AND 

POTASSIC ROCKS FROM THE ITALIAN PENINSULA 

S. CONTICELLI. Dip. Sci. Terra, Firenze, Italy, L. CIVETIA 

and M. D'ANTONIO, Dip. Geofisica & Vulcanologia, Napoli, 

Italy, A. PECCERILLO, Ist. Sci. Terra, Messina, Italy. 

The Tyrrhenian border of the Italian peninsula has been the site of 

an intense magmatism from Pliocene to recent times. Although CalcAlkaline, Potassic and Ultrapotassic volcanisms overlap in space and 

time, a decreasing of alkaline character with time and southward is 

observed. Alkaline ultrapotassic and potassic volcanic rocks are 

characterized by a variable enrichment in K and incompatible 

elements, but with consistently uniformous high LILEIHFSE values, 

similar to those of calc-alkaline volcanic rocks. 

Initial 87Sr?Sr and 143Nd/144Nd values for ultrapotassic and 

potassic rocks range from 0.70506 to 0.71649 and from 0.51272 to 

0.51202 respectively. The general Ssr vs. ~d array along with 

correlations among initial isotopic vaJues and incompatible trace 

elements abundance, clearly indicate that a crustal component has 

played an important role in the genesis of these magmas. The further 

question as to where this crustal component has been acquired by 

magmas. Volcanological and petrological data indicates crustal 

contamination to be a leading process along with fractional 

crystallization and magma mixing during magmatic evolution (e.g. 

Franca/anci eta/., 1989; Civet/a eta/. 1991; Conticel/i eta/., 1991; 

D'Antonio & Di Girolamo, 1994). 

Taking into account only selected samples believed to represent 

"primary" magmas in equilibrium with mineralogy of their mantle 

source a clearer picture is displayed. A large variation of Ssr vs. ~d is 

still observed, with a general array running from Ssr = -20, ~d = +5 

to Ssr = +180, s,11 = -12. A biforcation of this array is observed in the 

samples plotting in the lower right quadrant. In this light samples 

from the Northernmost part of the peninsula point to an upper crustal 

end-member, whereas samples from the Central sector of the 

peninsula point to Ssr and ~d values similar to those of Miocene 

carbonatic sediments. Major and incompatible trace elements have 

consistently a behaviour similar to that of isotopes. 

In this light we may envisage that the source of magmas contain 

at least two different geochemical crustal components mixed with a 

double mantle component as well. One crustal component might be 

generated by melting of silicatic upper crustal rocks or sediments 

from an ancient subducted slab. The second one might be represented 

mostly by C02 rich fluid released more recently by carbonate 

sedimentary rocks incipiently subducted. 

Consistently with the increase of alkaline degree Northward, an 

increase of the amount of the upper crustal components in the 

inferred mantle source is observed. This may be due to the occurrence 

of a lower degree of partial melting in a heterogeneus mantle. 

Metasomatism in the mantle may have resulted in the formation of 

intense veins pattern cutting the mantle country rock. This physical 

heterogeneity and diverse degree of partial melting of the peridotitic 

country rocks may have generated the trends of mixing observed. 

Civetta L, Carluccio E, Innocenti F. Sbrana A & Taddeucci G 

(199J).Europ.J.Minera/., 2:415-428. 

Conticelli S, Francalanci L & Santo AP (1991). J. Vo/can. 

Geoth.Res., 46: 187-212. 

Francalanci L, Manetti P & Pecceril/o A (1989). Bull Volcano/., 51: 

355-378 

D'Antonio A & DiGirolamo P (1994). Acta Vocanol.ln the press. 
ON PLATEAUS 

COPELAND.~. Dept of Geosciences, Univ. of 

Houston, Houston, Texas 77204-5503, U.S.A. 

(copeland@uh.edu), and SPELL, Te!fr. Res. School of 

Earth Sciences, Australian National Umversity, Canberra, 

ACT 0200, Australia (tls150@cscgpo.anu.edu.au) 

The concept of the 40 Arf39 Ar age plateau has been a common 

approach in reporting the results of 40 ArP9 Ar analysis. We believe, however, that the use of "plateau analysis" has disadvantages which promote either over- or underinterpretation of data. 

The frrst problem with 40 ArP9 Ar age plateaus is that any definition of a plateau is always subjective. There are many defmitions of "plateaus" in the literature but it is not clear if one is 

better than any other. All of these have been made up in order 

to describe the age spectrum with a single number. What 

makes this worse is that even if a single plateau definition 

could be shown to be superior, the manner in which the sample 

is heated can have a significant effect on the value that is calculated for the plateau or even wheather or not a plateau is 

defined. Thus, although experience can be a guide in the choice 

of heating schedule, this choice still introduces another element 

of subjectivity. A third choice that must be made is how to 

calculate the single number and associated uncertainty which 

represents the steps on theflateau; most workers chose either 

to weight by the amount o gas present or by the precision in 

the age of each step. To illustrate the variability introduced by 

these obstacles we have run five splits of a K-feldspar from a 

granite from Tibet with different heating schedules and applied 

five published plateau definitions to the results. This sample 

exhibits increasing ages (from -20 Ma) in the initial-25% gas 

release followed by steps with ages which vary around 30 Ma 

for the remainder of gas released. Of the 25 possibilities, only 8 

defined plateaus. Weighting by gas fraction, the 8 calculated 

plateaus are 29.26±0.22 (2X), 29.43±0.29 (3X), 29.06±0.28, 

29.15±0.22, and 29.28 ±0.22. Weighting by the uncertainty in 

the age the values are 29.25±0.12 (2X), 29.42 ±0.08 (3X), 

29.52±0.17, 29.27±0.08, and 29.26± 0.12. These data show that 

for this sample the choice of heating schedule and plateau definition effect the calculated plateau age at the 0.1 Ma level. 

Unfortunately, some workers attach geologic significance to a 

single plateau age at this (or an even smaller) level of precision. 

Since there are at least 3 subjective choices that go into this 

formulation, we feel such overinterpretation is not warranted. 

On the other hand, it is entirely reasonable to say that the bulk 

of the gas in this sample corresponds to an age of "about 29.4 

Ma" (this can also be said of the 16 other examples which 

failed to defme "plateaus"). To put a fmer point on it is not reasonable, particularly since this age corresponds to a temperature which is not known nearly as well as might be suggested 

by any one calculated "plateau" age. Moreover, to assign a 

single number to this sample is to ignore important 

information. The structure of this age spectrum is a reflection 

of a complicated cooling history that can be unraveled through 

the multi-diffusion-domain model. These results indicate that 

this sample cooled from -350 to 200°C from 30 to 29 Ma, 

followed by an interval of slower cooling, reaching -190°C at 

20 Ma. While the cooling history of volcanic rocks is much 

simpler, the concerns about the subjectivity of plateau analysis 

still hold. In order to justiftably claim precision of better than 

-1%, the uncertainties in all relevant factors should be 

propagated. These factors include the age and 40 Ar* P9 ArK of 

the flux monitor (which are commonly not included), the 

values of the correction f~ctors for interfering reactions, the 

value of the initial 40 ArP Ar, and the discrimination of the 

mass spectrometer. All of these become more important as the 

age of the sample decreases. It seems unreasonable that the 

precision of any plateau age should be less than the uncertainty 

in the most important of these factors ~which in most cases will 

either be the uncertainly in J or in the ( 9 ArP7 Ar)ea). 

67 

U-Pb GEOCHRONOLOGY FOR MAGMATISM AND 

METAMORPHISM iN THE ENGLISH RIVER 

SUBPROVINCE, SUPERIOR PROVINCE 

CORFU. F., Dept. of Geology, Royal Ontario 

Museum, 100 Queen's Park, Toronto, Ont., M5S 2C6, 

Can., STOTT, G.M. and BREAKS, F.W., Ontario 

Geological Survey, Willet Green Miller Centre, 933 

Ramsey Lake Road, Sudbury, Ont., P3E 6B5, Can. 

The English River Subprovince (ERS) is an Archean 

metasedimentary belt in the northwestern Superior Province that underwent plutonism, low-pressure I hightemperature metamorphism and anatexis in the period 

2700 - 2670 Ma. Thick turbiditic sequences were 

deposited just prior to and during the terminal stages of 

the compressional orogeny that quickly followed the 

episodic assembly of the volcanic-plutonic Uchi 

Subprovince (US) to the north, mainly between 2710 and 

2700 Ma. The ERS was subsequently intruded by a suite 

of dioritic, tonalitic and granodioritic plutons at 2698-

2696 Ma. These are essentially coeval with late-tectonic 

plutons intrusive into the adjacent US. Several high-grade 

metamorphic events caused widespread anatexis in the 

ERS and in adjacent domains of the Winnipeg River 

Subprovince to the south at 2692-2690 Ma, 2680-2678 Ma 

and 2670 Ma. The earliest metamorphism accompanied 

and followed regional deformation. The youngest events 

occurred under more static conditions and appear to have 

been of more local significance. In general, the multistage metamorphic and magmatic evolution caused 

repeated growth of zircon and monazite leading to the 

common occurrence of multiple age populations and 

complex U-Pb relations within individual units. The 

relative moderate pressures (0.4-0.6 GPa) reported for 

the metamorphic assemblages of the ERS, the rapid 

sequence of events, and the heterogeneous distribution of 

metamorphic and anatectic assemblages suggest that 

metamorphism was not primarily the consequence of 

tectonic thickening by thrusting but that it was caused by 

magmatic underplating mechanisms. The pattern of early 

calc-alkalic magmatism followed by complex, multistage 

metamorphism and the gradual increase in the abundance of S-type granitoid rocks observed in the ERS is 

akin to that observed in the other metasedimentary belts 

of the Superior Province and in recent low-pressure I 

high-temperature metamorphic belts. 
SOURCES OF PROTEROZOIC K-RICH ALKALINE AND 

SHOSHONITIC MAGMATISM IN THE SW GRENVILLE 

PROVINCE, QuEBEC: Nd-Sr-Pb ISOTOPIC STUDY 

CORRIVEAU. Louise. Centre geoscientifique de Quebec, 

C.P. 7500, Sainte-Foy, QC, G1 V 4C7, Canada, and 

AMELIN, Yuri, Dept. of Geology, Royal Ontario Museum, 

Toronto, ON M5S 2C6, Canada. 

The 1089-1076 Ma potassic (K) and shoshonitic (S) plutonic 

suites in the Central Metasedimentary Belt (CMB) of the SW 

Grenville Province, Quebec, are close geochemical analogs of 

modem subduction-related K-rich alkaline magmas. A small 

but distinguishable difference in initial Nd and Sr isotopic 

ratios exists between individual plutons and between K and S 

rock series. This difference is not a result of contamination of 

magmas by older continental crust but rather could have 

originated from slightly different degrees of enrichment of 

mantle sources of K and S magma series. 

Initial Pb and Nd ratios are consistent between whole rocks 

and feldspars, but Rb-Sr systems of whole rocks and feldspars 

and all isotopic systems in apatites were modified to variable 

degrees by post-crystallization alteration. 

Unaltered rocks of both K and S series show a range of 

isotopic variations from + 1.2 to +3.3 for £Nd(l.08 Ga), and 

from 0.7031 to 0.7038 for 87Srf86Sr (1.08 Ga). The range of 

initial Pb ratios, obtained for both acid-leached feldspars and 

U decay corrected whole rocks, is 16.65 to 16.90 for 

206Pbf2°4Pb, 15.36 to 15.41 for 2°7Pbf204pb and 36.25 to 36.50 

for 208Pbf2°4Pb. This range is more restricted than that 

observed in most of the modem potassic rock series (Nelson 

1992) and demonstrate that there was no significant 

involvement of Archean or Paleoproterozoic enriched 

components in sources of the CMB potassic magmatism, in 

contrast to contemporary potassic rocks of Australia and 

W. Greenland. 

Based on the two-stage modelling using the Pb, Nd and Sr 

isotopic data, we can infer the following scenario of evolution 

of mantle sources of the CMB potassic magmas: 

1) Enriched component (upper continental crust?) was 

separated from depleted mantle with J.L=8.0 at ca. 1.45 Ga. 

This component was characterized by J.L=18±3, Th/U=3.56± 

0.13 and 87Rbf86Sr=0.40±0.05, 2) Sediments derived from this 

crustal source were transported into the mantle during 

orogeny at ca. 1.2 Ga, with lowering of U/Pb and Rb/Sr ratios 

during sediment formation and subduction, 3) Potassic 

magmas were generated by partial melting of this 

metasomatized mantle at ca. 1.08 Ga. 

68 

THERMAL AND TECTONIC EVOLUTION OF THE DENT 

BLANCHE NAPPE (SWISS AND ITALIAN ALPS) 

COSCA. Michael A .. Sharp, Zachary D., and Hunziker, 

Johannes C., Institut de Mineralogie, Universite de 

Lausanne, 1015 Lausanne, Switzerland 

The Dent Blanche nappe, which contains the Matterhorn, is 

located high in the Swiss and Italian Alps. It comprises two 

Austroalpine units, the Valpelline and Arolla series, which are 

in thrust contact with the underlying Combin zone. The 

Valpelline series comprises high grade metasedimentary and 

metavolcanic rocks, and on the basis of similarities in 

lithology and isotopic cooling ages appears to correspond to 

Austroalpine units of the Ivrea zone. Rocks of the Arolla 

series are similar to the high grade rocks of the external Sesia 

zone, comprised predominantly of orthogneisses and gabbros, 

and overlain in places by a Mesozoic cover (e.g., Mont Dolin 

and the Roisan zone). Published Rb/Sr and K/Ar dates 

indicate that these Austroalpine units escaped pervasive 

Alpine metamorphic overprinting, thus offering the potential 

of accurately reconstructing its pre-Alpine thermotectonic 

evolution as well as providing insights into the early 

geological history of the Ivrea and Sesia zones. 

Marbles containing the primary assemblage calcitemuscovite-quartz - phlogopite - tremolite - graphite with secondary phengite-talc-chlorite have been studied from the 

contact between the Arolla and Valpelline units. 

Metamorphic temperatures determined by stable isotope 

thermometry yield the following results: calcite-graphite 

...... 700°C~ calcite-muscovite-phlogopite -5 90°C~ calcite-quartzphengite -400°C. Large muscovites and phlogopites 

separated from the marble yield 40 Art39 Ar plateau and total 

fusion dates between 170 and 190 Ma. Smaller phengites 

occurring as rims on muscovites and phlogopites, and as 

discrete neocrystallized grains, have saddle-shaped age spectra 

consistent with excess argon. Argon isochron calculations 

clearly indicate trapped argon reservoirs of atmospheric and 

non-atmospheric composition in the phengites. Isochron ages 

calculated with steps containing trapped atmospheric argon 

vary between 112 and 136 Ma. Collectively we interpret 

these data as recording two tectonometamorphic stages: 

middle Jurassic cooling following high temperature 

metamorphism and a later, lower temperature, deformation 

episode during the early Cretaceous. The early Cretaceous 

episode dates the juxtaposition of the Valpelline and Arolla 

units and marks the early sta~es of Eoalpine high pressure 

metamorphism. A phengite 4 Art39 Ar plateau date of 40.8 ± 

0.3 Ma from near the base of the Arolla series dates the 

emplacement of the Dent Blanche nappe to its current 

structural position. Combined with published dates, we 

propose a new tectonic model for the evolution of the Dent 

Blanche nappe. 
DEPLEfED AND ENRICHED COMPONENTS IN THE 

UPPER MANTLE OF THE NE PACIHC: EVIDENCE 

FROM THE NORTHERN JUAN DE FUCA RIDGE 

COUSENS, B.L., Dept. Earth Sciences, Carleton 

University, Ottawa, K1S5B6, Canada; ALLAN, J.F., ODP, 

College Station, TX., 77845, USA; LEYBOURNE, M.l., 

Dept. of Geology, University of Ottawa, Ottawa, Canada, 

K1N6N5; CHASE, R.L., Geological Sciences, UBC, 

Vancouver, B.C., V6T2B4,Canada; VANWAGONER, 

N.A., Dept. of Geology, Acadia University, Wolfville, 

N.S., BOP1XO, Canada. 

Young basalts dredged from ocean ridges, near-ridge 

seamounts, and intraplate volcanoes of the NE Pacific are 

extremely heterogeneous in trace element content and character, 

yet are fairly homogeneous in isotopic composition. There is 

also no simple relationship between tectonic setting and 

geochemistry: ''hotspot" basalts are isotopically MORB-like, 

while at some the spreading centres E-MORB's predominate. 

West Valley, at the northern end of the Juan de Fuca Ridge, 

may be a microcosm of the NE Pacific, within which is found 

a variety of basaltic rocks encompassing the geochemical 

spectrum of the NE Pacific. 

West Valley is a young immature spreading centre of 3200 

m depth, dominated by extension over volcanism, which is 

volcanically active only within its southern half (50 km). 

Eruptions form small, isolated cones that emit small-volume 

lava flows. Basalts recovered from this area are tholeiites, 

ranging in Mg# from 0.69 to 0.55, K20 from 0.02 to 0.76%, 

La/Smch-n from 0.29 to 1.85, 87SrJ86sr from 0.70235 to 

0.70260, and 206pbJ204pb from 18.2 to 19.2. Together, the 

lavas form near-linear arrays on trace element ratio vs. 

concentration, ratio vs. ratio, ratio-isotope ratio, and isotopeisotope plots, suggesting that they are mixtures of a depleted 

and an enriched component. Further evidence for magma 

mixing comes from large phenocrysts in West Valley floor 

basalts which are complexly zoned and contain spinel and melt 

inclusions (R. Neilson, pers. comm.) that are not in 

equilibrium with host glasses. 

The incompatible-element-enriched basalts (from a single 

cone and lava flow) can be modeled as melts of amphibolespinel peridotite (5% modal amph), as has been shown for 

alkaline basalts from the Tuzo Wilson Volcanic Field at the 

Explorer Ridge-Queen Carlotte-Cascadia RFT triple junction. 

Amphibole in the source may also control the composition of 

intraplate alkaline rocks from the Pratt-Welker and other 

seamount chains in the NE Pacific. The most incompatibleelement-depleted basalts, from the near-ridge Heck Seamount 

Chain, West Valley rift walls, and one young flow on West 

Ridge, could be second-stage melts of mantle that produced 

MORB at older segments of the Juan de Fuca Ridge. Basalts 

from the floor of West Valley are mixtures of varying 

proportions of these two end members. 

NE Pacific basalts from all tectonic environments form 

mixing arrays between DM and HIMU mantle components. 

Our work suggests that the HIMU -like component in the NE 

Pacific may be amphibole. If the amphibole formed ,..., 1 Ga 

ago from depleted mantle, it would "age" isotopically to the 

ratios now observed in the enriched basalts from West Valley. 

Alternatively (and perhaps more likely), amphibole may have 

formed more recently from metasomatic fluids derived from a 

less depleted source. 

69 

HSSION-TRACK INVESTIGATIONS ON SPHENE FROM 

THE KTB DEEP DRILLING PROJECT (GERMANY): 

POST-PERMIAN COOLING HISTORY AND IN SITU 

ANNEALING 

COYLE David A., and WAGNER, GUnther A. 

Max-Planck Institut fiir Kemphysik, 69117 Heidelberg, 

Germany. 

The German Kontinentale Tiejbohrung (KTB) will reach 

a total depth of 10 kilometres in 1994, with a maximum 

bottom-hole temperature likely to be in excess of 300°C. Thus 

it is expected to completely cross the predicted partial 

annealing zone for fission tracks in sphene, the upper 

temperature limit of which has been estimated to be ca. 240°C 

[1]. 

The lengths of confined fission tracks indicate that at a 

depth of 7000 metres and a temperature of ca. 200°C, the top 

of the sphene partial annealing zone may now have been 

reached. At this depth the mean confined track length is less 

than 10 micrometres, compared to a mean length of ca. 13.5 

micrometres in rapidly cooled sphene from the sub-volcanic 

Mt. Dromedary standard (Australia). 

The apparent sphene ages do not decrease linearly with 

depth, but exhibit structure at ca. 2.5 km depth which may 

represent an uplifted partial annealing zone. In addition, there 

is evidence that more than a kilometre of section has been 

duplicated as a result of post-Cretaceous thrusting along the 

Franconian Line where it intersects the KTB below 6800 

metres. 

As drilling is still continuing in the depth range of the 

partial annealing zone for sphene, the latest data from these 

depths will be presented. 

[1] T.M. Harrison, R.L. Armstrong, C.W. Naeser and J.E. 

Harakal. Geochronology and thermal history of the 

Coast plutonic complex, near Prince Rupert, British 

Columbia. Can. J. Earth Sci., 16, pp 400-410. 
THE DISCOVERY OF ARGON: 100 YEARS AGO 

CRAIG, H., Isotope Laboratory, Scripps Institution of Oceanography, University of California San Diego, La Jolla CA 

92093, USA 

The year 1994 marks the 100th anniversary of the discovery 

of Argon by Sir William Ramsay and Lord Rayleigh. This 

remarkable research led to the modern understanding of the 

Periodic Table of the elements and the further discovery by 

Ramsay of terrestrial helium, and of neon, krypton, and xenon, 

thus contributing an entirely new group of elements to science. 

The discovery of the Noble Gases probably aroused more 

interest than any other scientific discovery (possibly excepting 

Fire), and as is generally the case, it was marked by extensive and 

bitter criticism and disbelief, now largely forgotten. The names of 

many distinguished scientists are involved in these discussions, including van't Hoff, Dewar, Crookes, Cavendish, and of course 

Morris Travers who carried out much of the experimental work in 

Ramsay's laboratories. Rayleigh and Ramsay stand alone from 

their contemporaries in physics and chemistry as two men of 

genius who recognized from the beginning the significance of the 

problem and who were able both to carry out the experimental 

research and correctly understand and extend the results. The 

well-known story of Ramsay's request to Lord Rayleigh for 

permission (!) to work on the problem of the atomic weight of 

atmospheric nitrogen, and Rayleigh's generous invitation to 

collaborate, are relics of a bygone gentility in science marking 

happier and more leisurely days, but the entire story of this 

·research and its reception by scientists provides a metaphor for the 

process of scientific discovery that is as relevant today as it has 

been for a century. 

70 

NOBLE GASES IN THE MANTLE AND ATMOSPHERE 

CRAIG, H., Isotope Laboratory, Scripps Institution of Oceanography, UCSD, La Jolla CA 92037, USA 

This paper summarizes recent work in the Isotope Laboratory 

([ 1]; Craig, Farley, & Wiens, submitted), plus literature data, with 

the aim of constructing a unifying model for rare-gas isotope 

systematics in the Earth's mantle and atmosphere. Our own work 

is based on studies of He, Ne, and Ar isotopes in olivine 

phenocrysts in Ocean Island Basalts (OIB) and in MORB olivines 

and glasses. We proceed audaciously in the style of George 

Danton [2], without, it is to be hoped, reaching his fmal conclusion. The following results are axiomatique: 

We show that ill Ar-36, Kr and Xe (minus radiogenic and 

fission isotopes), and - 1/2 the Ne and Ar-40 in OIB phenocrysts 

and glasses are uncorrelated with He-3, and are, in fact, derived 

from air contamination. Further, the Ar-36 is trapped in fluid 

inclusions in the phenocrysts [ 1], and was therefore derived from 

Air/Seawater (ASW) injection into a magma dJu:iDi crystallization. Thus the "Air-like 40/36 ratio" Ar component in OIBs il 

Air, rather than the "Lower Mantle component" conjured by the 

Ecole Parisienne. Ar 40/36 ratios in OIBs are => 8000 in Juan 

Fernandez "High-He-3" basalts [1). He-4/Ar-40* (the Armantle 

component) ratios are 1.1 in JFZ basalts, and 2.0 in Reunion and 

Grande Com ore, showing the presence of different K/U domains 

in the lower mantle. 

Neon in the mantle was long ago shown to be of the "solar" 

persuasion in MORB and OIBs, with added radiogenic Ne-21 and 

ASW components [ 3]. We now show that (1) no planetary Ne or 

Ar components exist in the mantle, (2) Solar Ne has a 20/22 Ne 

ratio of 12.3 in both upper and lower mantle basalts, and (3) 

Atmospheric Ne, Ar-36, and Kr are mixtures of solar and planetary gases, in which the solar component is due to outgassing and 

the planetary component was derived from impact degassing after 

the primary accretion. (Xe has been lost from the atmosphere to 

an unknown sink, as generally assumed). Further ( 4), this is true 

of both Ne-22 and Ne-21 in Offis (except that atmospheric Ne 

contains a slight degassing addition of Ne-21 from the upper 

mantle). Ne-21 is not a radiogenic component in the lower 

mantle, but simply represents the original solar neon composition. 

"Low-He-3" OIB phenocrysts show a different Ne and Ar 

isotopic pattern: they crystallized in magmas that had fmt been 

flushed with ASW (to the air 20/22 ratio), followed by degassing 

loss from the magmas prior to eruption, resulting in Ar, Kr, and 

Xe depletion due to their lower solubilities in silicate melts. These 

observations provide a method for reconstructing the details of 

air injection and degassing in sub-oceanic magma chambers on a 

fme scale, and for deconstructing the observed isotope ratios. 

Ocean-basalt magmas thus possess two unique rare-gas 

isotope tracers for studies of mantle provenance and of ASW 

injection and magma degassing: He-3, derived from the lower 

mantle (in OIBs) or from MORB, and Ar-36, derived from air, 

probably by release from seawater trapped in lithospheric basalts 

during their soujourn at seafloor spreading centers. 

1. Farley, K.A. & Craig, H., 1994, Geochim. Cosmochim.Acta 

(in press). 

2. Danton, G.: "De l'audace, et encore de l'audace, et toujours 

de l'audace!", Le Moniteur, 4 September, 1792 

3. Craig, H. & Lupton, J., 1976, Earth Planet. Sci. Lett. 31, 369. 
OSMIUM ISOTOPE SYSTEMATICS OF MODERN ARC 

MANTLE LITHOSPHERE. 

CREASER. R. A., Dept. of Geology, University of Alberta, 

Edmonton, Alberta, T6G2E3, Canada, and BRANDON, A. 

D., Dept. Earth Atmospheric and Planetary Sciences, 

Massachusetts Institute of Technology, Cambridge, MA, 

02139-4307, USA. 

The subduction of oceanic crust is a fundamental process 

which influences the geochemistry of the mantle and arc 

magmas. Element transport from subducted oceanic crust to the 

overlying mantle wedge is well-established, but significant 

amounts of material are also returned to the deeper mantle. The 

Os isotopic system presents an interesting geochemical case as 

a result of the vastly different geochemical behavior of Os 

relative to other isotopic tracers (Sr, Nd, Pb). Because both 

sediments and basalts of oceanic crust have very high ReiOs 

ratios relative to the mantle, their Os isotopic compositions 

.evolve very rapidly, and subducted oceanic crust will have 

highly radiogenic Os isotopic compositions. The fate of this 

radiogenic Os is at present poorly constrained. This Os, if 

removed from the oceanic crust in subduction zones, could lead 

to radiogenic Os isotopic compositions in the overlying mantle 

wedge and I or in arc magmas. If the Os is not removed from 

the subducted oceanic crust then radiogenic Os could be returned 

to the deep mantle. Recent studies of Os isotopic compositions 

from some .ocean island basalts have invoked a recycled crustal 

component to account for radiogenic Os isotopic compositions 

observed, suggesting return of crustal Os to the deep mantle via 

subduction of oceanic crust. 

In order to investigate the geochemical behavior of Os in 

subduction zones, we have analyzed spinel peridotite xenoliths 

from the Japan arc (lchinomegata) and the Cascade arc (Simcoe) 

for Os isotopic composition. These peridotite xenoliths have 

also been analyzed for sulfur isotopic composition, and showS 

isotopic evidence for interaction with and metasomatism by 

subduction-derived fluids I melts. Spinel peridotites (Type I) 

from lchinomegata, Japan, show values of 187os 1t88os from 

0.1240 to 0.1267 ('Y Os -3 to -1). A single peridotite sample 

from the Cascade arc shows. a similar Os isotopic composition 

with 187os 1t88os of 0.1247 (yOs -2.5). These Os isotopic 

compositions are less radiogenic than an average mantle value 

of 0.1278 and are similar to the average Os isotopic 

composition for modern depleted mantle determined from 

abyssal peridotites. 

These unradiogenic Os isotopic compositions determined 

from arc peridotite xenoliths place constraints on the 

geochemical behavior of Os, and possibly other platinum group 

elements, during subduction of oceanic crust. The preliminary 

data obtained here suggests little or no transport of radiogenic 

Os present in the oceanic crust to the overlying mantle wedge 

during subduction. It is therefore likely that radiogenic Os is 

returned to the deep mantle, and subducted oceanic crust is a 

viable source of radiogenic Os observed in some ocean island 

basalts. 

71 

ISOTOPIC CHARACTERIZATION OF 

GROUNDWATER RESOURCES IN THE 

SACRAMENTO VALLEY, CALIFORNIA: 

A NEW TYPE OF GROUNDWATER MINING 

CRISS. Robert E., Geology Department, 

University of California, Davis, CA 95616 USA; 

DAVIS SON, M. Lee, Lawrence Livermore 

National Lab L-237, Livermore, CA 94550 USA; 

CAMPBELL, Karla R., Geology Department, 

University of California, Davis, CA 95616 USA. 

The coupled use of atso and an values of water 

and of the apparent 14C ages of dissolved 

bicarbonate provide a powerful means of 

characterizing and imaging different bodies of 

groundwater, and of quantifying their sources, 

apparent ages, migration paths, and the effects of 

pumping and degradation. An important 

conclusion derived from these data is that a 

heretofore undefmed type of groundwater "mining" 

is widespread in the Sacramento Valley, whereby 

the recharge of young surface waters is induced by 

pumping of ancient pristine groundwaters. In 

intensively irrigated areas, as much as 90% of the 

total recharge is dominated by agricultural return 

water characterized by elevated a 18 0 and an 

values on evaporation trajectories, very young 

14C ages of < 1 ka, and commonly elevated 

contents of nitrate and possibly other 

anthropogenic constituents that pose a threat to 

water quality. This type of groundwater 

degradation occurs in many municipalities 

surrounded by agricultural activity in the Central 

Valley, and commonly causes 2 to 4x increases of 

N03 concentrations over time periods of 20 years 

or less, and in cases has resulted in non-potable 

water and production shutdown. This type of 

groundwater mining is not necessarily associated 

with a permanent decline in the water table, in 

contrast to the traditional concept of groundwater 

mining. Both processes are occurring in the City 

of Sacramento where pumping has induced the 

infiltration of Sacramento and American River 

water over length scales of 3 km, and also resulted 

in the development of two -30m deep cones of 

depressions in the water table. Even though the 

city of Sacramento pumps 6 times less 

groundwater per km2 than intensely irrigated areas 

to the west, the heavily urbanized areas of 

Sacramento are not as conducive to the natural or 

artificial recharge to the groundwater as the floodirrigated agricultural lands, so the water table has 

declined more than it has further west. 
OLIGO-MIOCENE EXTENSION DURING ARC-CONI'INENT 

CONVERGENCE CREATING THE SEPIK-RAMU AND 

AITAPE BASINS IN NORTIIERN PAPUA NEW GUINEA. 

Crowhurst. P Y Hill, K.C, & Foster, D.A., 

Victorian Institute of Earth and Planetary Sciences, 

School of Earth Sciences, LaTrobe University, 

Melbourne, Victoria, Australia 3083 

Papua New Guinea was the northern 'passive' margin of the 

Australian craton in the Mesozoic, but now comprises the 

northwest margin of the Indo-Australian Plate and is tectonically 

active due to ongoing convergence with the Pacific Plate. 

Convergence since at least the Eocene bas resulted in collision 

with an island arc, creating a typical 'Rocky Mountain type' 

structure across Papua New Guinea. The structure can be divided 

into three major zones; the Fly Platform in the southwest is a 

flat-lying foreland adjacent to the mountainous Papuan Fold and 

Thrust Belt. This is bound to the northeast by a distal sediment, 

igneous, metamorphic and ophiolitic province, the New Guinea 

Mobile Belt The Mobile Belt is of interest because it is an 

active collision zone from which a working model for the 

evolution of convergent boundaries can be generated. Such a 

model can then be applied to and tested against ancient 

collisional zones. The information obtained in recent studies 

indicates the northern New Guinea collision zone is very 

complicated which implies that models describing older terrains 

may be oversimplified. 

Tbermocbronological techniques are now being used to help 

unravel the complicated tectonic history of northern Papua New 

Guinea. Fission track and 40 Ar/39 Ar thermocbronological 

analysis of the igneous and high-grade metamorphic basement 

around the Sepik-Ramu basins reveals regional Pliocene cooling 

from temperatures of > 1 oo<>c, consistent with the deformation 

in the Fold Belt to the south and due to Late Miocene-Pliocene 

arc-continent collision. 

An Early Miocene collision bas been previously proposed, 

primarly based upon cooling ages from metamorphic basement 

rocks. However the data suggests very rapid cooling of these 

rocks from temperatures of >5000C and -10-15km of erosion, 

which is clearly inconsistent with Early Miocene starved graben 

and carbonate deposition. We suggest that increased subduction 

beneath New Guinea in the Late Oligocene caused extension 

above the slab as well as the Maramuni volcanic arc. Early 

Miocene extension may have been manifested locally as low 

angle detachment faults which exposed mid-crustal rocks in 

metamorphic core complexes, with negligible erosional 

denudation. Extension caused rapid Miocene subsidence of the 

whole PNG margin, enabling deposition of thick limestone and 

creating the Sepik and Ramo basins. 

Thus, Northern Papua New Guinea comprised a broad 

'passive' margin in the Miocene, the site of local deep 

extensional graben and carbonate deposition until the BewaniTorricelli Mountains were upthrust in the Pliocene to form the 

current boundaries of the Sepik, Ramo and Aitape basins. 

72 

CLOSING IN ON ARGON CLOSURE TEMPERATURES?: 

AN EXTENDED "IONIC POROSITY" MODEL FOR 

THERMOCHRONOMEfRIC MINERAlS, WITH 

APPUCATION TO AMPIDBOLES 

DAHL. Peter S., Department of Geology, Kent State 

University, Kent OH 44242, USA 

Refined estimates of argon closure temperatures (T c) for 

compositionally diverse, thermochronometric minerals like 

amphiboles and micas are important in establishing precise 

cooling histories of orogenic terranes. This paper presents an 

extended "ionic porosity" approach for quantifying 

compositional effects on Tc within a given mineral family, 

then applies the model to calcic/subcalcic amphiboles. The 

model rests on the demonstration that ionic porosity (Z; 

defined as the percentage of unit cell volume not occupied by 

ions) uniquely links mineral composition (X) to the 

volume-diffusion parameters E and D0 for both 0 and Ar in 

the Dodson (1973) equation for Tc· As applied to amphiboles, 

the model is buttressed by observed Tc·Z-X trends that are fully 

consistent with predictions from basic crystal-chemistry. 

Values of Z24oc calculated from published compositional and 

unit cell data for 25 synthetic clino- amphiboles exhibit a 

continuous range from 36.5% to 39.7% (end- member edenite 

to ferro-actinolite, respectively). Corresponding model Tc 

values for Arrange from 562°C to 443°C (SO,um grain radius; 

10°C/Ma cooling rate; infinite sphere). Natural hornblendes 

amenable to 40Ar/39 Ar dating yield a somewhat narrower, yet 

still significant, range of model Tc (i.e., -480-550°C). 

Three compositional effects on Ar retentivity modeled from 

the synthetic-amphibole Z-X data are readily attributable to 

Mg#, A-site occupancy, and AI-Tschermak's substitution. 

Specifically, the model generates Tc values some 60±10°C 

higher for Mg end-member calcic/subcalcic amphiboles than 

for their Fe analogues, and values some 40±15°C higher for 

amphiboles with full A-sites relative to those with empty 

A-sites. A -35°C increase in model Tc may accompany the 

increase in AI-Tschermak's substitution in the progression 

from end-member tremolite to end-member tschermakite. 

Parallel extension of the model to micas is in progress. 

The model results for argon in amphiboles (and micas) 

have important implications for interpretation of 

thermochronologic data. First, a 35 Ma 40 Ar/39 Ar age 

discordance among compositionally diverse hornblendes in a 

terrane cooled at 1 °C/Ma may arise from a 1% absolute 

variance in Z within the sample suite. Such variance in Z is 

well within the 2% absolute range demonstrated among 

natural hornblendic amphiboles. Thus, ionic porosity effects 

constitute a potential alternative to strictly tectonic 

explanations for age discordance. At l0°C/Ma, the model age 

discordance is only 3-4 Ma, such that the inherent 

compositional effects become lost within the -±3 Ma 

uncertainty common in 40 Ar/39 Ar ages. Second, previous 

field-based and experimental efforts to isolate Mg!Fe effects on 

Ar retentivity in hornblende have led to varying conclusions. 

This ambiguity may largely reflect the simultaneous influence 

of yet other compositional effects (such as those isolated above 

by empirical means), and/or variable tectonic and 

micro-textural factors both within and among cooling terranes. 
40ArJ39 Ar lASER SYSTEM DATING OF PHENGITE 

FROM DABIE MOUNTAIN, CIDNA 

Dai Tongmo Xie Jinlong and Pu Zepin(All at 

Guangzhou Branch,Institute of Geochemistry,Chinese 

Academy of Science. Guangzhou,Box 1131 code 

510640 P.R.Cbina) 

A laser fusion system in combination with a low 

volume high vacuum inlet system and VG 1200 mass 

spectrometer with an electron multiplier has been established in our Lab. The laser fusion system consists of a 

He Ne laser and a YAG continuous laser ( -1 Ollm diameter laser spot sizes at single mode mode--10 watt and 

<1000 j.lm diameter laser spot sizes at multiple mode--

70 watt). 

Measurement of 40 Ar/39 Ar age spectra on single 

white mica crystals is a new test of obtaining detailed 

geochronological information. The three samples of 

phengites with different sizes were picked from eclogites on the deferent zone located on the southern part of 

Dabie mountain, Anhui province, China. The mineral 

gmin were checked optically with a petrographic microscope for crystal class, and every sample were measured 

by bulk step heating and single grain step heating in 

order to obtain three pairs of date for comparing each 

other. 

Two samples of phengite (D 1, 02) yields two 

plateau age of 661.4± 3.4 Ma and 212.2± 2.0 Ma, 

respectively. But a third sample of phengite (03) appears a saddle-shape spectrum and an elevated initial 

40Arf39

Ar ratio of 810, suggestive of excess argon. Our 

results indicate that there were multiple times of the tectonic evolution in the Dabie mountain. The data reveals 

that the time of previous origin was no later then 661Ma 

ago, and the last once was finished in the time of IndoChina movement. 

73 

40 Arf39 Ar DATING ON A MICROSCOPIC SCALE WITH A 

CONTINUOUS LASER SYTEM: AN ENLIGHTENING 

APPROACH TO GEOCHRONOLOGY 

DALRYMPLE, G. BRENT, Branch oflsotope Geology, 

U.S. Geological Survey, Menlo Park, CA 94025 

Since its introduction more than four decades ago, KAr dating has undergone two significant advances in 

methodology. The first occurred in the mid-1960s with 

the development by Merrihue and Turner ( 1966) of the 

40 Arf39 Ar technique , which allowed K-Ar dating to be 

applied to open systems. The other was the extension by 

York and others ( 1981) of the 40 Arf39 Ar method to single 

crystals, which was made possible by the availability of 

ultra-clean, ultra-sensitive, and very precise rare gas mass 

spectrometers, and by the application of a continuous 

laser for fusing and heating very small samples. The 

addition of very low blank furnaces have significantly 

extended the applications of these very sensitive 

40 Arf39 Ar dating systems. 

Today, the 40 Arf39 Ar method using lasers and ultralow blank furnaces is being used to solve problems in 

geochronology that were intractable only a decade ago. 

Using these systems, it is now possible to routinely date 

single crystals of less than a milligram in mass, obtain 

detailed age spectra on samples weighing as little as 0.05 

mg, differentiate between indigenous crystals and 

xenocrysts, determine impact ages on both terrestrial and 

lunar samples using very small fragments of melt rocks, 

precisely date samples as young as 0.1 Ma, extract 

thermal history information from disturbed samples, and 

obtain detailed "age contours" showing the distribution of 

radiogenic Ar within a single crystal. 

These new methods have numerous applications in 

geochronology, thermochronology, tectonics, stratigraphy, 

impact dynamics, and studies of sedimentary provenance. 

Merrihue, C., and Turner, G., 1966, Potassium-argon 

dating by activation with fast neutrons: Jour. Geophys. 

Res., v. 71, p. 2852-2857. 

York, D., Hall, C.M., Yanase, Y., Hanes, J.A., and 

Kenyon, W.J., 1981, 40Arf39Ar dating ofterrestrial 

minerals with a continuous laser: Geophys. Res. Lett., 

v. 8, p. 1136-1138. 
PROTEROZOIC CRUSTAL HISTORY OF WESTERN 

IRELAND AND ROCKALL: PRECAMBRIAN 

TECTONIC RECONSTRUCTIONS IN THE NORTH 

ATLANTIC REGION 

DALY. J S , FITZGERALD, R.C., MENUGE, J.F., 

Department of Geology, University College Dublin, 

Belfield, Dublin 4, Ireland, BREWER, T.S., University of 

Nottingham, England, HEAMAN, L.M., Royal Ontario 

Museum, Toronto, Canada and MORTON, A.C., British 

Geological Survey, Keyworth, England. 

Because of their geographic position, Precambrian rocks in 

Britain, Ireland and on the adjacent continental shelf and 

Atlantic plateaux are important in tectonic reconstructions for 

the north Atlantic region over a long time period. Until 

recently our understanding of such regions has, of necessity, 

often been based on poor quality geochronology, Nd model 

ages or geological inferences alone. Moreover large volumes 

of submarine continental crust in the north Atlantic region are 

either inaccessible, or limited by the small sample sizes 

available from drill or grab sampling. 

Some previous studies of recently discovered Proterozoic 

continental crust in Britain, Ireland and the submarine Rockall 

Plateau have over-emphasised Sm-Nd toM ages of c. 1.9 Ga 

throughout the region. 

Single grain and small fraction U-Pb abraded zircon 

geochronology were essential in order to establish a wellconstrained chronology for the Annagh Gneiss Complex of N 

Mayo, Ireland, where the pitfalls of bulk fraction zircon dating 

can be amply demonstrated. Here juvenile Palaeoproterozoic 

crust is represented by c. 1.75 Ga calc-alkaline orthogneisses. 

Late Mesoproterozoic granitoids with A-type geochemistry and 

Palaeoproterozoic toM ages, emplaced at c 1.26 Ga, were 

followed by thermal disturbance and deformation during both 

the Grenville and Caledonian orogenies. A small volume of 

juvenile c. 1.25 Ga crust is present as a tectonic sliver, 

incorporated along a Grenville high strain zone. 

Single grain and small-fraction U-Pb zircon data from the 

southern Rockall region indicate close comparison with the 

Irish Proterozoic but with a very different crustal history. 

The new age data provide the basis for testing current 

tectonic models for the North Atlantic Precambrian. PreGrenville refits have a beguiling simplicity in spite of the 

complex Grenville and Caledonian history of the region. 

74 

K-Ar DATING OF WEATHERING-RELATED 

CRYPTOMELANE AND ALUNITE FROM WESTERN 

AND NORTIIERN AUSTRALIA 

DAMMER D, CHIVAS, A. R. and McDOUGALL, 1., 

Research School of Earth Sciences, The Australian National 

University, Canberra, ACT 0200, Australia 

K-Ar dating of K-bearing weathering products developed 

on very old bedrock (Archean and Proterozoic in this case) is 

commonly invalidated by the presence of small amounts of 

fine-grained micaceous minerals inherited from bedrock. The 

larger the differences in their K-Ar ages the smaller the 

proportion of mica that causes measured ages to be quite 

unrelated to the age of weathering. To quantify the effect of 

inherited K-silicates on K-Ar age, the weathering phase was 

selectively dissolved and radiogenic Ar and Kin the silicate 

residue were determined. Using material balance equations, 

estimates of the age of the weathering mineral were obtained. 

In this study more than 90 samples of weathering-related 

cryptomelane ((K,Ba)Mn016.xH 2 0) and alunite 

(K,Na)Al3(S04h(OH)6 were dated using the approach 

described above. The samples of cryptomelane come from the 

Groote Eylandt Mn deposit (Northern Territory), the Eastern 

Pilbara Province (Woodie Woodie, Mt. Sydney) and the Peak 

Hill Province (Horseshoe). In addition there are small 

occurrences of locally concentrated Mn oxides scattered over 

the Kimberley, Pilbara and Yilgam Cratons. Alunite comes 

from the southern Yilgam and Pilbara Blocks. 

This study is unique in that it compares the results of 

extensive dating of two minerals formed during weathering 

from the same general area. Results from cryptomelane, 

corrected for silicate residue, form three distinct age groups 

(see figure): 6-20 Ma (Middle to Late Miocene), 30-40 Ma 

(Oligocene) and 46-54 Ma (Early to Middle Eocene). The 

results obtained suggest that there were three major periods of 

intense chemical weathering in N and NW Australia 

corresponding to more humid climatic conditions. All corrected 

ages of alunite fall into the range of 0-14 Ma recording the 

youngest weathering event. It is probable that each new 

weathering event caused complete recrystallisation and 

resetting of the K-Ar clock in the case of alunite. Measured 

ages are interpreted as marking the time of cessation of intense 

weathering, and beginning of drier climatic conditions. This is 

compatible with essentially zero K-Ar ages measured on 

alunite being formed at present under humid climatic 

conditions in the very south of western Australia. 

16~------------------------------~ 

14 

= 12 

~ 10 ftl 

Ill 8 .... 

0 6 

d 

z 4 

2 

0 

• Cryptomelane - N, NW Australia 

~ Cryptomelane - SW Australia 

D Alunite 

0 5 10 15 20 25 30 35 40 45 so 55 60 

Age (Ma) 

In further work, 40 Arf39 Ar laser microprobe dating will be 

used to date banded void fillings and thin veinlets of 

cryptomelane free of silicate residue. This will enable us to 

estimate the rate of growth, as well as further refine the timing 

of periods of intense chemical weathering. 
SCHEELITE Sm-Nd AND FLUID INCLUSION Rb-Sr 

DATING OF SHEAR ZONE- HOSTED GOLD 

DEPOSITS, AFRICA. 

DARBYSHIRE. D.P.F .. NERC Isotope Geosciences 

Laboratory, Keyworth, Nottingham NG12 5GG, UK and 

SHEPHERD, T.J., British Geological Survey, Keyworth, 

Nottingham NG 12 5GG, UK. 

Information concerning the timing of mineralization for 

many world class ore deposits is scant, or even unknown, 

preventing objective appraisal of their genesis. Direct 

determination of the age is hampered by the lack of ore 

minerals that can be dated by conventional radiometric 

methods. An additional problem is the effect of prolonged 

mineral-fluid interaction at moderate to low temperatures 

subsequent to mineral deposition and the likelihood of open 

system behaviour. However two techniques have been 

developed which allow constraints to be placed on the age 

and evolution of hydrothermal fluids. 

Scheelites from the auriferous lode deposits of the 

Midlands Greenstone belt, Zimbabwe have been dated by an 

application of the Sm-Nd mineral technique. Two phases of 

scheelite deposition and gold mineralisation have been 

established which correlate with Late Archaean and Early 

Proterozoic magmatic events in the region. 

An alternative approach to the dating of shear zone-hosted 

gold deposits involves the use of fluid inclusion Rb-Sr 

geochronology which we have successfully applied to 

Palaeozoic mineral deposits. A suite of gold-bearing, vein 

quartz samples from the Ashanti Mine, Ghana, have yielded 

an age of c. 1850 Ma, thus providing the first direct 

geochronological evidence for the age of auriferous 

mineralization. This permits the Ashanti deposit to be 

placed in a regional setting and suggests that the 

mineralization is an end-Eburnian event (2400 - 1800 Ma), 

possibly linked to the emplacement of late orogenic granites. 

The data also reveal evidence of multiple reactivation of the 

shear zone as suggested by complementary fluid inclusion 

and mineralogical observations. 

75 

SPATIAL AND TEMPORAL VARIATIONS IN THE 

GEOCHEMISTRY OF THE ETIDOPIAN "FLOOD" 

BASALTS PROVINCE 

DAYIES Gareth R. Vrije Universiteit, De Boelelaan 1085, 

1081 HV Amsterdam, The Netherlands, NORRY, M.J., 

Leicester University, LEI 7RH, UK and Jones P.W., Leeds 

University, LS2 9JT, UK. 

Tertiary magmatism in Ethiopia originally covered an 

area of750,000 kffi2 and produced a volume in excess of 

350,000 km3 . In order to constrain the geodynamic setting of 

the Ethiopian "flood basalt province", K-Ar ages, major and 

trace element and Sr-Nd-Pb isotope data have been determined 

from > 15 volcanic successions from the north, central, southwest, south-central and east of Ethiopia. 

At least four distinct stages of volcanism occurred 

during the Eocene-Miocene: i) Initial magmatism between 49-

45 Ma appears confined to SW Ethiopia (although pre-1980 KAr ages of- 50 Ma have also been reported from NW 

Ethiopia); ii) Volcanic activity between 45 and -32 Ma, 

although of more regional extent, was mainly limited to the S 

and SW. In the SW 3000m of volcanics were produced prior 

to 30 Ma. The third and fourth volcanic stage -30-20 Ma and 

-20-15 Maare the main periods of magmatism inN, C and E 

Ethiopia. Significant crustal extension is not apparent prior to 

the fmal magmatic event. Development of the Main Ethiopian 

Rift began between 18-14 Ma. Regional uplift appears to postdate the earliest magmatism. 

Basalts from the province as a whole record large 

variations in both trace element and isotope ratios (2°6PbJ2°4Pb 

= 17.9 to 20.8). Within a single volcanic succession, trace 

element ratios can be correlated with variations in Sr and Pb 

isotopes. However, successions of the same age, even within 

100 km of each other, have distinct isotope ratios (206PbJ204Pb; 

SW, 20.8-20.4; SC, 18.0). In the SW and SC successive 

volcanic episodes have distinct trace element and isotope 

compositions (e.g. SC; 206PbJ2°4Pb -18, 18.7, 19.7). 

At high levels in the lava pile in theN, NW and E, 

correlations between fractionation indexes, Sr, Pb and K, Rb 

and Ba/Nb establish that crustal contamination was extensive. 

Significant volumes of rhyolites are associated with these 

basalts. In contrast, basal flows in N and SC are characterised 

by unradiogenic 206Pbf2°4Pb and high 207PbJ204Pb, marked Ba 

enrichment and positive Eu anomalies. The lack of K and Rb 

enrichment argues against an origin due to crustal interation. 

The trace element characteristics, particularly the positive Eu 

anomalies, are not characteristic of oceanic volcanism 

implying a contribution from the subcontinental lithosphere. 

The Eu anomaly argues for a source that has at some stage 

suffered shallow melting while retaining residual plagioclase. 

In contrast, basal lavas in other regions are characterised by a 

HIMU-like isotope signature. 

Early basalts (50-32 Ma) record a signature from the 

subcontinental lithosphere whereas later flows (30-15 Ma) 

have a large crustal signature implying that isotherms have 

been raised over the -30 Ma of igneous activity. The temporal 

variations in the magmatism and extremely heterogeneous 

mantle sources imply that Ethiopian volcanism was not related 

to a single large scale thermal event (plume). Equally, 

volcanism is not caused by lithospheric extension which does 

not occur until after the bulk of the volcanics have been 

formed. The extreme isotopic heterogeneity demonstrates a 

marked local control over volcanism but does not explain the 

nature of the thermal anomaly that caused the mantle melting. 
EVAPORATION OF ROCKS AND MINERALS IN THE 

LABORATORY AND IN NATURE 

DAVIS Andrew M 1, WANG, Jianbua2, and CLA YfON, 

Robert N.1•

2•3; 1 Emico Fermi Institute, 2Department of 

the Geophysical Sciences, 3nepartment of Chemistry, 

University of Chicago, Chicago, a 60637. 

In recent years, high temperature evaporation has come to 

be recognized as an important mechanism for isotopic mass 

fractionation as well as chemical fractionation. We present 

three examples of natural chemical and isotopic fractionation 

effects that have been reproduced in evaporation experiments. 

Wiistite-magnetite spherules collected from deep-sea 

sediments are believed to have been Fe-Ni particles that were 

oxidized during atmospheric entry. We have shown that these 

particles contain mass-fractionated Fe and Ni; each spherule 

has uniform isotopic composition, but among 10 spherules, 

heavy isotope enrichments of 8.6-19.2 %o/amu (avg. 14.5 

%Jamu) for Fe and 6.8-49.0 %o/amu (avg. 22.9 %o/amu) for Ni 

were found. 0 isotopic analyses of several aggregates of many 

spherules give isotopic composition enriched by 12-16%o/amu 

compared to fusion crust from iron meteorites. We evaporated 

synthetic wiistite in a vacuum furnace at a variety of temperatures above the melting point for a variety of durations. 

Analysis of the evaporation kinetics indicates that the evapora-

~on r~ction is FeO~l) = Fe(g) + 1/2 02(g)· Residues are ennched m the heavy tsotopes of Fe and 0. Isotopic compositions for both elements follow the Rayleigh fractionation law. 

The gas/solid fractionation factor for Fe is identical to that 

expected for the inverse square root of the masses for Fe(g); 

the value for 0 is somewhat lower than that expected from 

Dl(g), indicating some kinetic hindrance to 0 evaporation. 

Natural magnetite-wiistite spherules have experienced 

substantial mass loss during atmospheric heating. 

A number of refractory inclusions from carbonaceous 

chondrites are enriched in the heavy isotopes of 0, Mg and Si. 

We have shown that these mass-fractionation effects are consistent with Rayleigh fractionation during high temperature 

evaporation and that, for Mg2 SiO 4, the residue must be molten 

to allow isotopic equilibration in the residue during fractionation, as required by the Rayleigh law. 

A group of hibonite-rich inclusions (HAL-type inclusions) 

in carbonaceous chondrites contain mass-fractionated 0, Ca 

and Ti and REE patterns with large negative Ce anomalies. 

We recently conducted evaporation experiments on a melt 

whose starting composition consisted of chondritic proportions 

ofMgO, AI20J. Si02, CaO, Ti02 and FeO, doped with 100 

ppm each of the REE. The residues showed chemical fractionations of major elements similar to those found previously: Fe 

was the most volatile element; Mg and Si had similar volatilities and AI, Ca and Ti did not evaporate under the conditions 

of our experiment. Most of the REE did not evaporate either, 

but Ce was much more volatile than the other REE. A 2440x 

depletion in Ce was found in the most extreme residue (95% 

mass loss). Enrichments in the heavy isotopes of 0, Mg and Si 

were similar to those found in the liquid Mg2 SiO 4 evaporation 

experiments. The results of our experiments on chondritic 

composition samples support earlier conjectures that the HALtype inclusions are evaporation residues. · 

76 

COMBINED USE OF 14C AND STABLE 

ISOTOPES TO MAP AND MODEL 

GROUNDWATER SYSTEMS 

DAVISSON. M L. and VELSKO, C.A., Lawrence 

Livermore Lab L-237, Livermore CA 94550, USA, 

CRISS, R. E., and CAMPBELL, K.R., Geology 

Depart, University of California, Davis, CA 95616, 

MACFARLANE, P.A., Kansas Geological Survey, 

Lawrence, KS. 66047 

AMS techniques make available abundant and 

detaile~ 14C measurements that, when coupled with 

stable Isotope data, can define spatial images, 

apparent ages, and fluxes in hydrologic systems at 

dtff~rent s~~les. A new extraction technique 

routtnt?lY utiltzes 35 ml ~ater samples that require 

only stmple field samphng methods, making 14C 

analyses for hydrologic samples straight forward 

and convenient. Such easy access of these data 

make possible generation of detailed quantitative 

models of groundwater systems or validation of 

previously derived groundwater models. For 

example, groundwater beneath the City of 

Sacramento, California ( 100 km2) has two large 

groundwater cones of depression (-30m below sea 

level) that are reflected by 1SQ variations. 

Progressive groundwater depletion causes ancient 

(to 16 ka) groundwater beneath the city to be 

replaced by recharged modern American and 

Sacramento River waters (o18Q = -11.0, and 14C = 

>100% modern carbon) into the aquifer at a rate of 

-5% of the annual groundwater consumption. 

Lateral groundwater flow rates adjacent to the rivers 

calculated from the o18Q isopleths are 30-60m/yr 

where vertical flow rates calculated from downward 

migration of bomb-pulse 14C is about one order of 

magnitude lower. These same rates are observed in 

other parts of the southern Sacramento Valley, 

where groundwater cycling from intensive 

agricultural irrigation is calculated to be six times 

greater than the natural groundwater flow rates and 

have led to degradation of the water quality. 

As another example, large scale (450 km xsection) 14C and stable isotope characterization of 

groundwater systems in the Great Plains regions 

differentiates groundwater of young local-flow 

systems from older intermediate-scale flow systems. 

Local flow systems dominate areas of hill and 

valley-type topography, where apparent ages vary 

between 2-15ka and the o18Q values (-8.5 to 

-11.5%o) reflect local recharge. Intermediate-scale 

flow systems vary between 10 to >40ka and have 

o18Q values (-11.5 to -14.0%o) that reflect recharge 

from higher elevations just east of the Front Range 

of Colorado. These data are used to validate and 

calibrate a previously derived finite difference 

model. 

Funding for this work was performed under the auspices of the 

U.S. DOE by LLNL under contract No. W -7405-Eng-48. 
GROUND-WATER RECHARGE FROM RIVER WATER IN 

NORTHWEST HUNGARY 

DEAK. J, .Research Center for Water 

Research Institute, Budapest, Hungary H-1095, 

REVESZ, K. ,u.s·.Geological Survey, 

Reston VA. 22092, 

DESEO, E., Research Center for Water 

Research Institute, Budapest, 

Hungary, H-1095, 

BOHLKE, J.K., U.S.Geological Survey, 

Reston VA. 22092. 

The largest potential drinking water 

reservoir in Hungary is located in a 100 

to 500-meter-thick Pleistocene gravel aquifer (deposited by the Danube River) 

underlying the Szigetkoz area of northwest 

Hungary. Recharge conditions and 

potential contamination of this aquifer 

were investigated by use of environmental 

isotopes. 

Oxygen(O) and hydrogen(H) isotope ratios of Danube river water are 

significantly different from those of 

local infiltrated ground water, because 

the Danube water is recharging from the 

high Alps. The so-called •altitude 

effect• makes the Danube water 

isotopically more negative than the 

locally infiltrated ground water. 0 and H 

isotope ratios in the gravel aquifer are 

approximately equal to river values, 

indicating that river water is the 

dominant source of recharge to the entire 

aquifer. Infiltration of locally recharged precipitation water is 

negligible. 

High tritium concentrations (>40 

Tritium Units) in the ground water 

correspond to the mid-1960's atmospheric 

tritium-deposition peak in the Danube. 

This tritium peak can be found at flowpath distances of 5 to 15 kilometers from 

the river, indicating that horizontal 

ground-water-flow velocities are about 150 

to 450 meters per year. 

Tritium concentrations in the 

deepest parts of this aquifer are higher 

than the detection limit (1 Tritium Unit), 

indicating that recharge flushes out the 

entire aquifer in < 100 years. 14c data show that ground water 

beneath the Pleistocene gravel aquifer is 

much older (20,000 to 30,000 years), and 

it was recharged not by Danube water, but 

30 to SO kilometers west of Szigetkoz. 

77 

14C AND 40 ARf39 AR DATING OF THE CAMPANIAN 

IGNIMBRTIE, Plll..EGREAN FIELDS, ITALY 

DEINO A L, and CURTIS, G.H., Geochronology 

Center, Institute of Human Origins, Berkeley, CA 94709, 

USA; SOUTHON, J ., Center for Accelerator Mass 

Spectrometry, Lawrence Livermore National Laboratory, 

P.O. Box 808, L-397, Livermore, CA 94551-9900, USA; 

TERRASI, F., and CAMPAJOLA, L., Dipartimento di 

Scienze Fisiche, Universita' 'di Napoli "Federico II", Pad. 

20 Mostra d'Oltremare, 80125 Napoli, Italy;.and ORSI, 

G., Dipartimento di Geofisica e Vulcanologia, Universita 

di Napoli "Federico II", Largo S. Marcellino, 10, 80138, 

Napoli, Italy. 

We have carried out a series of geochronologic 

measurements of the Campanian Ignimbrite, a widespread and 

prominent ash-flow tuff in the Phlegrean Fields and outlying 

area in the Naples region of Italy. Precise ages for this 

eruption have some important volcanological and stratigraphic 

implications, but we will focus here on intercalibration with 

the 14C time scale. 

We have previously reported on single-crystal laser-fusion 

40 Arf39 Ar dating of three samples of the ignimbrite with an 

average isochron age of 36.8 ± 0.5 ka (Deino et al .• 1992). We 

have reproduced this age within error redating one of the 

original samples, but using a conventional resistance furnace 

and the incremental release 40 Arf39 Ar dating technique, 

obtaining an isochron age of 37.4 ± 0.5 ka. The overall mean 

age obtained by the 40 Arf39 Ar method for this eruption is 37.1 

± 0.4 ka. 

New radiocarbon measurements were carried out on charcoal 

from a carbonized bnmch exposed within the ignimbrite on the 

wall of an active quarry. The sample was split and analyzed at 

both the Naples AMS facility and the Center for AMS at 

LLNL. The samples at both laboratories received routine acidbase-acid treatment, while the samples at LLNL also received a 

subsequent bleaching/oxidation treatment, followed by 

additional base washes. The final steps of oxidation to C02, 

reduction to graphite, and AMS measurement were similar in 

both labs. Coal blanks were measured following the same 

pretreatment. 

The mean background-corrected radiocarbon age obtained at 

the Naples AMS and LLNL facilities were 32.3 ± 0.6 ka and 

34.60 ± 0.15 ka, respectively, calculated using a half-life of 

5568 years. Recalculating to the true 5730-year half life yields 

33.2 ± 0.6 ka and 35.60 ± 0.15 ka. As these results are 

discrepant further studies are underway that will hopefully 

resolve this conundrum. 

The radiocarbon age determined for the ignimbrite thus 

ranges between 1,500 ± 400 years to 3,900 ± 700 years 

younger than the 40 Ar (39 Ar age, or 4-11%. This range 

encompasses the shift of -2,300 ± 300 ka toward young 

radiocarbon ages for this interval predicted by Mazaud et al. 

(1991) based on geomagnetic field variations. 

Deino, A.L., et al., 1992, 40 Ar/39 Ar dating of the 

Campanian Ignimbrite, Campanian Region, Italy: 29th 

Abstracts, International Geological Congress, Japan, 3:2654 

Mazaud, A., et al., 1991, Geomagnetic field control of 14C 

production over the last 80 ky: Implications for the 

radiocarbon time-scale: Geophys. Res. Let. 18: 10: 1885-1888. 
EARLY PALAEOZOIC BIOTITE Rb-Sr DATES IN 

WESTERN AUSTRALIA 

DE LAETER J.R., Curtin University of Technology, GPO 

Box U1987, Perth, Western Australia, 6001, Australia and 

LIBBY, W.G., Geological Survey of Western Australia, 100 

Plain Street, East Perth, Western Australia, 6004, Australia. 

Biotite from granite and gneiss from the Yilgarn Craton in 

Western Australia, dated by the Rb-Sr technique, 

indicates that much of the margin has been tectonically 

active during post-Archaean time, whereas most biotite 

dates in the main body of the Craton are Archaean in 

age. 

A belt at the western margin of the Yilgam Craton gives 

ages of 430-500 Ma, which merges into a transition zone 

to the east some 15-40 km wide, which separates the 

young dates from the eastern chronological plateau where 

biotite dates range from 2300-2600 Ma, and in general 

are slightly younger than regional Rb-Sr whole rock 

dates which average about 2550 Ma. The 430-500 Ma 

dates probably represent resetting during uplift in the 

Early Palaeozoic. 

At the northwestern comer of the Yilgam Craton biotite 

dating gives three distinctive dates: 750-800 Main the 

Capricorn Orogen to the North, 1650 Main the west and 

2400-2600 Main the east. The 1650 Ma dates probably 

represent tectonic loading which resulted from the 

collision of the Yilgarn and Pilbara Cratons, the dates 

being set by cooling associated with subsequent erosional 

rebound. 

Biotite dating has also been carried out along a number of 

traverses to the south of the Yilgam Craton. Dates of 

450-500 Ma at the margin of the Craton suggest early 

Palaeozoic uplift and sets an upper limit for emplacement 

of the Albany-Fraser Province. 

78 

HYDROGEN ISOTOPIC COMPOSITION IN THE 

EARLY SOLAR SYSTEM: IN SITU ANALYSIS 

OF METEORITES BY ION MICROPROBE. 

DELOULE Etienne, CRPG-CNRS, BP 20,54501 

Vandoeuvre les Nancy cedex, France, and 

ROBERT Fran~ois, Museum d'Histoire Naturelle, 

Lab. de Mineralogie, 61 rue Buffon, 75005 Paris, 

France. 

The D/H ratios of 3 meteorites (Renazzo CR, A bee 

E3 and Semarkona LL3) have been measured with 

the CRPG Nancy ion-microprobe. These meteorites 

are well known for their large deuterium enrichments 

found in the organic macro-molecules [ 1 ,2]. The 

observed in situ variations of the D/H ratio cover the 

range of 7.10-5 to 8.10-4, i.e. -600 to +5000%o. 

Other secondary ions (C, Mg, Si, K,' ... ) were 

measured together with Hand D to characterize the 

hydrogen carriers. The internal correlations observed 

between the D/H ratio and the hydrogen carriers 

pointed out that, contrary to previous interpretations, 

the organic macromolecules are not the only carriers 

of the deuterium-rich hydrogen but are in close 

association with deuterium-rich phyllosilicates. 

These correlations also show that solar-type 

hydrogen (OD ~ -700o/oo) is present in organic 

macro-molecules of E3 meteorites. 

The high D/H ratios measured in water bearing 

minerals (~ 7.10-4 in Semarkona and~ 3.10-4 in 

Renazzo) could not have been reached thermally 

within a dense solar nebula. Ion or radical chemistry 

is certainly necessary to provide this heavy 

hydrogen, which therefore must have originated in 

the interstellar medium or in the outer part of the 

nebula. Until now, only organic macro-molecules 

have been identified in the interstellar medium [3]. 

These new data suggest that 1) interstellar water 

should also be enriched in deuterium (its D/H ratio 

has not been yet measured spectroscopicaly) 2) water 

condensed in a interstellar environment has been 

preserved during the formation of solar system 

solids. 

[l]Robertt & Epstein, 1982, Geochim. Cosmochim. Acta 46, 

81. 

[2] Yang & Epstein, 1983, Geochim. Cosmochim. Acta, 47, 

2199. 

[3] Erhenfreund et al., 1991, Astron. Astrophys. 252, 712. 
A SINGLE KIMBERLITIC EVENT ON THE KUNDELUNGU 

PLATEAU (SHABA, ZAIRE): A SA, NO AND PB ISOTOPIC 

STUDY OF THE KIMBERLITES AND THEIR 

MEGACRYST SUITE. 

DEMAIFFE D , WEIS, D. Petrol. Geodyn. Chimique, 

CP160/02, Universite Libre de Bruxelles, 

Av. F.D. Roosevelt, 50, 1050 Brussels and KAMPATA, 

M., MOREAU, J., Lab. Mineralogie, Univ. de Louvain, 

1348 Louvain-La-Neuve, Belgium. 

Twenty four diamond-poor kimberlite pipes of probably 

late Cretaceous (70 Ma) age are known on the Kundelungu 

Plateau over an area of 4,500 km2. They intruded a 1 .8 Ga 

gneissic basement. Ten pipes have been studied in detail, 

most of them are made of unbrecciated fresh kimberlites 

which contains two generations of olivine (Foas-91). 

monticellite, ilmenite, spinels, perovskite and calcite. 

Phlogopite is absent. Typical megacryst suite (Cr-diopside, 

pyrope, olivine, ilmenite, orthopyroxene) occurs as well as 

both crustal (felsic gneiss) and mantle (lherzolite, eclogite) 

xenoliths. 

The kimberlites are geochemically very similar. They 

have high MgO (28-34 %), Ni (950-3,000 ppm) and Cr 

(1,400-1,750 ppm) contents, low K20 (0.05-0.6 %) and 

appear very enriched in LREE (LaN:3-400) but depleted in 

HREE (YbN<3) when compared to other 'group 1' 

kimberlites. Chondrite-normalized spidergrams show 

enrichment (200 to 3,000) for most LILE (except K). Two 

altered samples show extreme enrichment of all REE 

(LaN:3-9,000; YbN:70-200) except Ce resulting in an 

apparent negative Ce anomaly. 

Initial (at 70 Ma) Sr and Nd isotopic compositions fall 

within a narrow range of values, between 0.70393 and 

0.70487 for 87SrfB6sr and 0.51266 and 0.51277 (ENd: +2.1 

to + 4) which is comparable to the values of diamond-rich 

kimberlites from Mbuji Mayi (2d kimberlite province in 

Zaire). These typical 'group 1' kimberlite values indicate a 

time-integrated slightly LREE-depleted mantle sourceregion comparable to an 018-type source. Initial Pb isotopic 

data are: 206pbf204Pb: 18.51-20.35, 207pbf204Pb: 15.64-

15.75. 

Crustal contamination is not detected in the Sr and Nd 

isotopic systems (except in one altered kimberlites with ENd 

= -1) but is visible in Pb ratios. The (207pbf204Pb)i ratios 

are significantly higher than MORSs and OIBs suggesting 

crustal contamination. 

The megacrysts (Cr-poor cpx, opx, pyrope) and the 

mantle xenoliths (one lherzolite, one eclogite) have ENd 

values (+1.1 to +2.6) comparable to but slightly less 

positive that their host kimberlites. Cpx have low Sr isotopic 

ratios (close to 0.7040) while opx and garnet have higher 

Sr isotopic ratios, up to 0. 71 0. Initial Pb ratios of cpx are in 

the kimberlite range but the opx and lherzolite show much 

more scatter, suggesting crustal disturbance. 

Mineralogical, geochemical and isotopic homogeneity of 

the fresh kimberlites implies an homogeneous mantlesource region and very uniform magmatic processes 

(degrees of melting and/or fractional crystallization) from 

one pipe to the other. This in turn suggests that the whole 

kimberlite province may have been formed by a single 

magmatic event. 

79 

STABLE CARBON AND OXYGEN ISOTOPES IN AN 

ALKALINE BASALTIC SUITE: PRIMARY 

COMPOSITIONS AND LOW TEMPERATURE REEQUILIBRATION PROCESSES 

DEMENY, A., Laboratory for Geochemical 

Research, Hung. Acad. Sci., Budapest, 

Budaorsi ut 45, 1112, Hungary, 

HARANGI, Sz. , Dept. of Petrology and 

Geochemistry, Eotvos Lorand 

University, Budapest, Muzeum krt. 4IA, 

1088, Hungary 

The Mecsek alkaline basaltic suite 

formed in a continental rifting process 

during the lower Cretaceous and comprises 

mainly pyroxene-olivine basanites and 

alkaline basalts with subordinate amounts 

of tephri tes, trachi tes, phonolites. The 

fluid-rich nature of these magmatic rocks 

is indicated by the presence of amphiboles 

and micas. The basaltic rocks contain 

calcite amygdales that are divided to two 

main types: recrystallized limestone 

xenoliths enclosed by lava rocks at magmasediment contacts and calcite amygdales 

with fibrous texture often containing 

analcime. The latter type occurs in dyke 

rocks. 

Oxygen isotopic compositions of mineral 

separates show positive shifts from 

primary magmatic values presumably 

depending on the degree of low temperature 

isotope exchange: pyroxenes ( 6. 0 to 6. 5 

0 1oo), amphiboles (4.7 to 7.1 °loo), 

biotites (7.1 to 7.4 °1oo), feldspars (7.6 

to 15.0 °1oo), analcimes (17.5 to 19.0 

0 I oo) . Calcite amygdales with carbon 

isotopic compositions (-8.4 to -6.3 °loo) 

characteristic for mantle-derived 

carbonates gave o18o values ranging from 

12. 3 to 16. 5 ° I oo. Those amygdales that 

show low temf~rature recrystallization 

gave higher c5 0 values (15.1 to 16.5 

0 1oo) than those without signs of 

recrystallization (12.3 to 13.4 °1oo). 

Calcites amygdales formfd at magmasediment contacts have o1 C values ( -1 . 0 

to 0.6 °1oo) characteristic for 

sedimentary carbonates. 

The most controversial data were 

obtained on analcimes. The C and 0 isotope 

compositions and textural characteristics 

of calcites coexisting with analcimes 

suggest high temperat~re formation 

conditions, thus the o18o values of 

analcimes (17.5 to 19.0 °1oo) indicate the 

sensitivity of this mineral toward 

retrograde isotope exchanges. 
SPATIAL AND TEMPORAL EVOLUTION OF MAN1LE 

SOURCES DURING CONTINENTAL RIFTING: THE 

VOLCANISM OF AFAR. 

DENIEL C CNRS, Universite Blaise Pascal, ClermontFerrand, 63038, France, PIK, R. and COULON, C., 

CNRS, Universite Aix Marseille Ill, Marseille, France, 

and VIDAL, Ph., CNRS, Universite Blaise Pascal, 

Clermont-Ferrand, France. 

Afar is a very favorable area for studying the interactions 

between lithosphere and asthenosphere during the opening of 

a continental rift by progressive stretching and thinning of 

continental crust and lithosphere, leading to oceanization. 

Magmatism occurred almost continuously over the past 25 

m.y. and 50 m.y. in the Republic of Djibouti and in Ethiopia 

respectively. Only basaltic rocks, mainly of transitional type, 

are considered in this study. Large chemical and isotopic 

variations among the volcanic series are interpreted in terms 

of mantle source heterogeneity. In Djibouti, a clear evolution 

through time of the mantle sources is observed in relation to 

rifting (Vidal et al., 1991; Deniel et al., 1994). Three sources 

were involved in the genesis of these lavas: (1) an old 

subcontinental lithospheric component (87Srj86Sr=0.706, 

206fbf204Pb= 17 .9), mainly observed in the oldest lavas (25 to 

10 Ma), (2) an IDMU (high U/Pb ratio)-type reservoir, and 

(3) a depleted mantle. As rifting goes on, there is an 

increasing contribution of an HIMU-type mantle source. It is 

attributed to the influence of a mantle diapir (Afar plume). 

The geochemical characteristics of 9 to 1 Ma old lavas, 

erupted after the strong increase of spreading rate in Afar, 

reflect this evolution of mantle sources. The influence of the 

mantle plume is most prominent in the northern youngest 

lavas (<1 Ma), particularly Manda, characterized by the 

stron&_est HIMU signature (87Sr/86Sr=0.7035, 

206pbf"'04pb= 19 .2). The contribution of the depleted mantle 

component originating from the asthenosphere is best 

recognized in the young ( <4 Ma) lavas, particularly Tadjoura 

and Asallavas (3 to 1 Ma). Preliminary results on Quaternary 

lavas from Ethiopia indicate that they have rather 

homogeneous Nd isotopic ratios, similar to those of <4 Ma 

Djibouti lavas. Although some lavas exhibit Sr and Pb 

isotopic compositions similar to those of Manda and lnakir 

lavas, two groups of lavas plot below the Sr-Pb trend defined 

by Djibouti lavas. Manda Hararo lavas with less radiogenic Sr 

and Pb isotopic compositions than Asallavas and Tana lavas 

with lower Sr isotopic ratios than Inakir lavas, although with 

similar Pb isotope ratios. All these data will be discussed in 

teims of spatial and temporal evolution of mantle sources in 

relation to rifting and mantle plume dynamics. 

Deniel, C., Vidal, Ph., Coulon, C., Vellutini, P.J., and 

Piguet P., in press, Temporal evolution of mantle sources 

during continental rifting: the volcanism of Djibouti 

(Afar): J. Geophys. Res. 

Vidal, Ph., Deniel, C., Vellutini, P.J., Piguet, P., Coulon, 

C., Vincent, J., and Audin, J., 1991, Changes of mantle 

sources in the course of a rift evolution: the Afar case. 

Geophys. Res. Lett., 18, p. 1913-1916. 

80 

NUMERICAL SIMULATIONS OF COSMOGENIC 

NUCLIDE PRODUCTION IN ROCKS FOR VARIOUS EXPOSURE GEOME1RIES 

DEP. Linus, ELMORE, D., Department of Physics, 

Purdue University, West Lafayette, IN 47907, U.S.A., 

FABRYKA-MARTIN,MASARIK,J.,and REEDY, R.C. 

Los Alamos National Laboratory, Los Alamos, NM 87545, 

U.S.A. 

In-situ produced cosmogenic nuclides in terrestrial rocks 

provides a means of determining exposure ages and erosion 

histories of landforms. To obtain useful quantitative information requires a good understanding of nuclide production 

systematics. The build-up of nuclides is sensitive to physical 

and chemical processes such as weathering, uplifting, and 

covering by snow and soil. Often more than one of these effects are present making experimental study of nuclide production difficult, requiring theoretical methods to understand 

production fundamentals. 

A model based on a low-energy neutron Monte Carlo 

transport code, MCNP, has shown a great promise in predicting the systematics of nuclide production by neutron capture. 

Incorporating a high energy nucleon-meson Monte Carlo 

transport code, LAHET, the model can include systematics of 

high-energy spalogenic nuclide production. 

We will present the MCNP-based model predictions for 

the depth dependence of thermal neutron fluxes in a concrete 

bloc~ covered with a water layer and compare them with experimental measurements. We will present predicted depth 

profiles of cosmogenic neutron-capture produced 36CI for 

rocks covered with snow and soil using the MCNP-based 

model. We will also present the predictions of production dependence on rock size for both spalogenic and neutron capture 

products using a model based on both LAHET and MCNP. 
FILTERING OF MANTLE ISOTOPIC 

HETEROGENEITY BY MAGMA GENERATION 

PROCESSES 

DEPAOLO, D.J., Berkeley Center for Isotope 

Geochemistry, Department of Geology and 

Geophysics, University of California, Berkeley, 

CA 94720 

The geochemistry of basaltic lavas is useful as a 

means of characterizing mantle convection. The 

"sourcing" of heterogeneities puts qualitative 

constraints on large scale convection, and the 

lengthscale spectrum of heterogeneity can yield 

quantitative constraints on flow properties. Stacks of 

lavas yield a time series of geochemical properties that 

need to be converted to a spatial pattern in the mantle 

source. Improving the ability to do this mapping is 

necessary to fully utilize the data on lavas for 

understanding the interior of the earth. Recent 

development of models that describe the magma 

generation and transport in the mantle, coupled with 

analysis of the effects of residence in magma 

chambers or melt collection zones, allow construction 

of analytical expressions that relate, for instance,' 

isotopic ratio-versus-time data from lava sequences, 

to isotope ratio-versus-depth in the mantle source 

material. A simple analysis for a steady-state melt 

zone continuously resupplied with heterogeneous 

material at its lower boundary suggests that diffusive 

effects through the melt zon.e will attentuate 

heterogeneities of wavelength 3m for incompatible 

elements and 30m for compatible elements. At 

1 Ocm/yr upwelling rate these correspond to periods of 

30 and 300 years in the lava sequence. Another way 

of stating this is that variability in lavas on ca. 100 

year time scales represents a highly attenuated version 

of the heterogeneity in the mantle source at lOrn 

wavelength. Magma storage at the top of the melt 

zone or in shallower chambers has a further, and 

larger, attentuating effect on the expression of mantle 

heterogeneity in the lavas. For magma residence 

times of 1000 to 100,000 years, mantle heterogeneity 

wavelengths of 300m to 30km are effectively 

attentuated. Phase shifts of elements with different 

compatibilities are also expected, and their observed 

magnitude places limits on the melting parameters. 

The definition of homogeneous versus heterogeneous 

magma sources needs to be reevaluated in these 

terms, as do inferred mixing relations between source 

components. 

81 

INTERPRETING MARINE TRACER RECORDS: A 

NEOGENE RECORD OF CHANGING RIVER 

87Srf86Sr FROM THE HIMALAYAN REGION 

DERRY L.A, Cornell U., Dept. of Geol. Sci., Ithaca, 

NY 14853-1504, USA (derry@geology.cornell.edu) 

and FRANCE-LANORD, C., CRPG-CNRS, BP20 

54501 Vandoeuvre, France. 

The ability to interpret variations in marine 

tracer records such as 87Srf86Sr and Ge/Si is limited 

by our ability to quantify the tradeoff between 

changing river compositions and/or fluxes. Here we 

present data from secondary minerals derived from 

chemical weathering in the Ganges-Brahmaputra 

watershed. atso -an measurements show that these 

minerals have been equilibrated in the G-B 

floodplain. Sr-bearing phases include smectites and 

vermiculites. Recrystallization during formation of 

these minerals implies extensive exchange of 

interlayer cations and thus isotopic reequilibration. 

Thus these minerals can constrain the 87srf86Sr of 

ambient waters. 87Srf86sr of smectites and 

vermiculites are near modern G-B river values 

prior to 7 Ma and after 0.8 Ma. From 7 to 0.8 Ma, 

smectite/illite ratios are much higher, and smectites 

are much more radiogenic. We infer that during 

this period of intense chemical weathering, 87 Srf86sr 

in G-B river water was significantly greater than at 

present. An increase in river water 87Srf86Sr is also 

suggested by measurements from coeval Siwalik 

carbonates (Quade, 1993). Shifts in late MiocenePliocene seawater 87Srf86Sr and Ge/Si may be 

related to changes in the composition of the dissolved 

load of the G-B system, driven by both climate-related 

weathering intensity changes and tectonics. 

smectite/illite 87 Sr/ ~r 

0 0.2 0.4 0.6 0.8 1 0.72 0.74 0.76 0.78 

100 

• 200 • • • 300 • 

0 • .. • • ""' 400 T""" . ,. ""' Q) 0 500 

-o 

~:; 600 

~ 700 

31: 

..Q 800 a> 

.0 500 

~ 

0 .s 600 co a> T"""E 

""' 700 

Q) 

0 800 

:::c 900 

• Smectites and 

Vermiculites 
ORIGIN AND RATE OF FLUID 

CIRCULATION IN CONVERGENT 

MARGINS 

PIA. A.N., Laboratoire de Geochimie, Geosciences Rennes, Universite Rennes 1, Campus Beaulieu, 35042 Rennes cedex - France, AQUILINA, 

L., BRGM, 45060 Orleans cedex 2 - France, 

CASTREC, M. and BOULEGUE, J., Laboratoire de Geochimie et Metallogenie, Tour 26 -16, E5, 

4 place Jussieu -75252 Paris- France. 

Fluid circulation in subduction zones has been 

studied in two areas: Barbados and Peru . Pore fluids have been recovered from an active 

diapiric field located in the accretionary complex of 

Barbados (13°5N}. The chemical analyses show that 

a and other major element -depleted fluids are rapidly 

expelled through the diapiric structures. The structures characterized by the strongest thermal and 

chemical anomalies correspond also to the highest 

expulsion rates: up to lm/yr for the central zone of 

the "Atalante" diatreme. The 87Srf86Sr ratios of the 

fluids suggest that the fluids cannot firstly originate 

from the South American basin of Orinocco, and 

secondly result from a seawater/sediments interaction. 

A comparable study has been undertaken on 

samples collected off Peru (5-7°S). These fluids 

show Cl, Ba and 87 Sr enrichments as compared to 

common bottom seawater. These enrichments can be 

related to a mixing between meteoric waters 

circulating within the metamorphic basement and 

sedimentary brines from the upper part of the margin. 

Reactions between seawater and continental-derived 

material can be suggested to explain such high 

87Srf86Sr ratios in the fluids. The Sr isotopic compositions of large authigenic barite deposits only located 

close to the fluid venting imply a continental origin 

for the fluids responsible for the barite deposit. The 

fluids responsible for the deposition of the barite can 

be proposed as good candidates. The occurrence of 

such barite deposits implies an increase of the fluid 

temperature in the past without any evidence today. 

This paroxysmal event has to be probably related to 

important tectonic events, even earthquakes. 

This study shows that if faults are always efficient 

conduits for focusing fluid flow and fluid expulsion 

in subduction zones, the origin and the nature of 

fluids inferred using geochemical and isotopic analyses is not unique, and the expulsion sometimes 

violent. 

82 

THE SILICON ISOTOPE VARIATIONS 

IN GEOCHEMICAL CIRCULATION 

DING, T., JIANG, S., WAN, D. and LI, Y., 

Institute of Mineral Deposits, CAGS, 

Beijing 100037, P.R. China. 

The silicon isotope compositions of over 1,000 

terrestrial samples have been determined, and a 

general picture of Si isotope variation in 

geochemical circulation has been configured in recent years. 

In magmatic processes, thermodynamic Si 

isotope fractionation is significant, causing remarkable difference between Si isotope composition of mafic rocks and that of acidic rocks. The 

fomer has a mean c5 30Si value of -o.6, and the latter has a mean c5 30Si value of -o.l. 

In the wheathering processes, the cJay mineral 

products have normally lower c5 30Si value (-Q.l to 

-1.9) than that of the original rocks, and the 

silicon dissolved in solution has higher c5 30Si value. 

In the biochemical processes; the Si isotope 

fractionation trends are very complex: the diatom, 

sponges and bamboo intend to have lower c5 30Si 

values than those of dissolved Si in the water, but 

the equiseta intend to have higher c5 30Si values 

than those of dissolved Si in the water. 

The Si isotope compositions are quite variable 

in the ocean water. The deep water near the middle ocean ridge and in the ocean trough has c530Si 

values similar to those of submarine volcanic 

rocks. However, the nearshore sea water has higher c5 30Si value for the input of river water silicon 

having higher c530Si value. Accordingly, the 

radiolaria of shallow water has higher c5 30Si values 

(0.2 to 0.8) than those of deep sea radiolaria (-Q.3 

to -o.6). The c5 30Si values of siliceous carbonate 

(0.1 to 2.8) and stromatolite (2.4 to 3.4) are even 

higher. 

In the hydrothermal processes, the silica precipitates have always lower c5 30Si values than those of" 

Si remaining in the solution, as a result of kinetic 

isotope fractination. For example, the silica sinter 

has c530Si values from 0.2 ·to -3.4, the siliceous 

. rocks from black smoker in the Mariana Trench 

have c530Si value from -Q.4 to -3.1, and the quartz 

in BIF have c5 30Si value of -o.8 to -2.2. 

As to the metamophic processes, no large scale 

Si isotope fractionation bas been found yet. We 

can only see the sma11 sca1e Si isotope 

fractionation among coexisting silicate minerals. 
MODELING AND MEASUREMENTS OF 

RADIOGENIC NUCLIDES IN URANIUM MINERALS: 

IMPLICATIONS FOR SPENT FUEL STABILITY 

DIXON. P R, FABRYKA-MARTIN, J., CURTIS, 

D.B., ROKOP, D.J., Chemical Science and 

Technology Division, Los Alamos National 

Laboratory, Los Alamos, NM 87545; CRAMER, J., 

AECL Research, Pinawa, Manitoba ROE lLO; 

CORNEIT, J., Environmental Research Branch, Chalk 

River Laboratories, Chalk River, Ontario, KOJ lJO; 

KILIUS, L., Isotrace Laboratory, University of 

Toronto, Toronto, Ontario, M5S 1A7; SHARMA, P., 

PRIME Lab, Physics Laboratory, Purdue University, 

West Lafayette, IN 47907 

Among the most persistent radioactive constituents of 

spent fuel are long-lived 239Pu, 99Tc, and 1291. The extent 

to which these nuclides will be retained by a waste 

repository will be in large part a function of their retention 

by the waste form and retardation by the natural geologic 

environment along any flow path. Uranium minerals offer 

a means to evaluate the geochemical transport and 

retention properties of these nuclides in natural materials 

analogous to spent fuel under a variety of geochemical 

conditions. 

Measured abundances of natural occurrences of these 

nuclides in uraninite and other U-bearing mineral phases 

are compared to abundances predicted using the Monte 

Carlo neutron transport code MCNP to evaluate the degree 

of retention of these radionuclides by the host minerals. 

Measurements of neutron-capture product 36Cl are used to 

assess the validity of the code predictions and to constrain 

production rate estimates where modeling uncertainties are 

excessively large. 

Release rates are shown to be immeasurably slow at 

the Cigar Lake uranium deposit (Canada), which is a 

reducing environment isolated by a clay barrier from the 

surrounding aquifer. In contrast, large deficiencies of 1291, 

and small ones of 99Tc, are observed in ore samples from 

the shallow Koongarra deposit (Australia), which is subject 

to relatively fast-moving, oxidizing groundwaters. No 

deficiencies of 239Pu are observed in any of the samples 

studied to-date. 

83 

PLUMBOTECTONICS OF THE ABSAROKA-GALLATIN 

VOLCANIC PROVINCE, WYAND MT 

DOE, B.R., U.S. Geological Su"ey, 923 National 

Center, Reston, VA 22092 

Thirteen new analyses of Pb isotopes in igneous whole 

rocks and ore minerals from the Absaroka-Gallatin volcanic 

province (AG) of Wyoming and Montana help fill out the 

Pb isotope picture of AG and the Yellowstone Plateau 

volcanic field (YP). The Quaternary bimodal YP field 

overlaps with the western !:rt of the Tertiary andesite-series 

AG. Values of 206pb/ Pb (cr), 207Pb1204Pb (/J), and 

208Pb/204Pb (y) for the new data range from 15.66 to 18.11, 

15.24 to 15.50, and 36.43 to 37.95, respectively. These 

isotopic ranges lie within those previously reported for the 

region although some samples are at the isotopic extremes. 

The new data, however, have no clear representative for the 

previously reported data from the Independence volcanic 

center (IV C) in the northeastern AG involving a 3,800 Ma 

source. 

Most new data lie close to a 2,800 Ma regression noted 

previously that encompasses both the YP rhyolite and the 

Archean igneous rocks of the Beartooth Mtns. on an afJ 

plot. The new data are only slightly higher in fJ than YP 

basalts but significantly higher than most rocks of the IVC. 

In spite of the similarity among the YP rhyolites, AG 

andesite-series, and Beartooth Mtns. surface Archean rocks 

in afJ plots, surface Archean rocks tend to have much 

higher values of y than the Cenozoic igneous rocks. This 

difference precludes major involvement of shallow Archean 

in any Cenozoic igneous melts, as previously noted for 

rhyolites. 

Unlike the values of fJ, values of y in the AG andesite 

series tend to be lower than YP rhyolites and more similar 

to those of YP basalts, the AG IVC, and other mafic 

igneous centers to the northwest. Therefore, these three 

series could not have solely generated the rhyolites. Two 

main trends are apparent on afJ and cry plots for the 

andesite series and basalts •• one from a previously 

proposed Archean mix of mantle Pb with subducted 

sediment Pb of high initial fJ Oithospheric mantle(?)) 

toward a Plumbotectonic II modem mantle; the other from 

the lithospheric mantle(?) that evolved until the Cenozoic 

with reduced 238ut204Pb ( -3-6) and 232n./204Pb ( -19-30), 

compared to "normal" values (9.7 and 37, respectively). The 

trend towards a Plumbotectonic II mantle also trends 

toward regional Phanerozoic sediment Pb isotopes, but the 

Pb concentration balance obviates sediment involvement. 
LASER 40Arf39Ar DATING OF BENTONITES AND 

SHALE FROM WALES 

DONG. H., HALL, C.M., PEACOR, D.R. and HALLIDAY, AN., Dept. of Geological Sciences, U. of 

Michigan, Ann Arbor MI, 48109-1063, USA 

Three bentonites and one shale were studied using the 

laser probe 40 Arf39 Ar method. This was a preliminary test 

prior to the use of vacuum encapsulation experiments to 

directly determine the extent of 39 Ar loss due to recoil from 

these clay-dominated rocks. The sub-millimeter clumps of 

clay were irradiated in a vacuum and thermal contact with 

the reactor's cooling water was optimized to reduce Ar loss 

due to self-heating. 

An epizone-grade Cambrian bentonite ( ~29 =0.21) from 

S. Wales yields plateau ages averaging to 421±1 Ma (2a) 

and an average integrated age of 410±2 Ma. The integrated 

age is consistent with an Rb-Sr whole-rock age of 409±11 

Ma for epizone-grade Paleozoic mudrock suites from 

Snowdonia (Evans, 1989) and probably represents the 

blocking age of late Silurian to early Devonian Acadian 

regional metamorphism. The 421 Ma plateau age is consistent with Sm-Nd ages of 420±20 Ma for epizone-grade 

Ordovician slates (Ohr et al., 1994) and possibly records the 

peak of metamorphism. Neither diagenetic (~29=0.52) nor 

low-anchizone ( ~29 =0.38) Ordovician bentonite (Roberts 

& Merriman 1990) gives meaningful ages, presumably due 

to 39 Ar recoil loss from imperfection-rich smectite, 1/S and 

illite. 

Since the previous clay samples released Ar at extremely 

low temperatures, subsequent runs were performed on 

samples which had not been baked prior to analysis. Three 

size fractions ( <5J.Lm, <1J.Lm and <0.2J.Lm) of one diagenetic grade Ordovician shale (depositional age 449-461 Ma) 

yielded integrated ages of 481±3, 465±2 and 466±3 Ma 

respectively. This agrees well with Sm-Nd ages of 500±26, 

468±25 and 463±26 Ma on identical size fractions from a 

slightly older shale (464-470 Ma). The Ar age for the <0.2 

J.l.m fraction is identical to the Sm-Nd result on the same 

size fraction from the same shale (Ohr et al. 1994). 

The excellent match between the 40Arf39Ar .and Sm-Nd 

ages suggests that there has been little net loss of 39 Ar due 

to recoil, although later experiments using vacuum encapsulation will measure this directly. It is also encouraging that 

the ages from both the <0.2 J.l.m and <1 J.l.m fractions, 

which are rich in authigenic clay, correspond well with the 

known depositional age of the shale. However, most of the 

clay age spectra have a characteristic "hump" shape with 

large apparent age swings both above and below the 

integrated age, suggesting that there is significant 39 Ar 

redistribution due to recoil. Both TEM observations and 

vacuum encapsulation experiments are underway to clarify 

these features. 

Evans, J.A 1989, A note on Rb-Sr whole-rock ages from 

cleaved mudrocks in the Welsh Basin, 1 GeoL Soc. Lond. 

146, 901-904. 

Ohr, M., Halliday, AN. and Peacor, D.R., 1994, Mobility 

and fractionation of rare-earth elements in argillaceous 

sediments ... , Geochim. Cosmochim. Acta. in press. 

Roberts, B and Merriman, R.J., 1990, Cambrian and Ordovician metabentonites ... , GeoL Mag. 127, 31-43. 

84 

INCOMPLETE EXTRACTION OF RADIOGENIC ARGON 

FROM HIGH-SILICA ANDESITES: IMPLICATIONS FOR 

K-ArDATING 

DONNELLY -NOLAN, J.M., IURRIN. B.D. (1), GRAY, 

L.B., and CONRAD, J.E., U.S. Geological Survey, 345 

Middlefield Rd., Menlo Park, CA, 94025, USA. 

( 1) also affiliated with Institute of Human Origins, 

Geochronology Center, 2453 Ridge Rd., Berkeley, CA, 

94709, USA. 

K-Ar and 40Ar;39Ar dating of rocks from Medicine Lake 

volcano, California, indicate that incomplete quantitative 

extraction of radiogenic 40 Ar (40 Ar*) can occur in high-silica 

(59-63% Si02) andesites (HSA), resulting in K-Ar ages that 

are too young. Provided that the system is undisturbed, 

incremental 40 Ar/39 Ar heating analysis obviates this problem 

because absolute concentrations of potassium and argon are not 

required to calculate an apparent age. 

40 Ar/39 Ar analysis from two Medicine Lake volcano HSA 

units (north and south rim andesites) yield undisturbed plateau 

ages ~ 95% of the total 39 Ar) of 100 ±3 ka and 124 ±3 ka, 

respectively. These ages are significantly older (as much as 

25%) than the corresponding K -Ar ages of 7 4 ±12 ka and 96 

±12 ka. The original K-Ar dated samples were heated to fusion 

temperatures ( -1300 to 1400 °C) for 30 min. Replicate 

extractions of these samples using longer fusion times show a 

regular increase in percent extracted 40Ar* (see Figure). Even 

with fusion times of 50 min, apparently 10 to 20% of the 

40 Ar* was still not extracted from the samples based on 

comparison with the 40 Art39 Ar plateau ages. 

'0 

.s 80 

e 

~60 

0 

• South Rim Andesite 

(Plateau Age 124±3 ka) 

• North Rim Andesite 

(Plateau Age 1 00±3 ka) 

10 20 30 40 50 

Time at Fusion Temperature (min.) 

60 

This phenomenon, well documented for sanidine K-Ar ages 

(McDougall et al. 1980; McDowe111983), was first observed 

for K-Ar ages on HSA in the Clear Lake volcanic field 

(Donnelly-Nolan et al., 1981). As in this case, increased 

extraction time demonstrated a consistent increase in age. This 

suggests that K-Ar ages of other HSA should be considered 

suspect. 

McDougall, 1., Brown, F.H., Cerling, T.E., and Hillhouse, 

J.W., 1992, Geophys. Res. Lett., v. 19, p. 2349-2352. 

McDowell, F.W., 1983, Chern. Geol., Isotope Geosci., v. 1, 

p. 119-126. 

Donnelly-Nolan, J.M., Hearn Jr., C., Curtis, G.H., and Drake, 

R.E., 1981, U.S Geol. Surv. Prof. Paper 1141, p. 47-60. 
GEOCHEMICAL MORPHOLOGY OF THE NORTH MIDATLANTIC RIDGE: NEW DATA (10°-24°N) PUT IN THE 

PERSPECTIVE OF THE 10°-70°N SECTION 

DOSSO Laure, URA 1278,Ifremer,B.P. 70,29280-Plouzane, 

France, BOUGAULT, Henri, Ifremer, B.P. 70, 29280 -

Plouzane, France, JORON, Jean-Louis, Laboratoire P.Siie, 

C.E.N.S., B.P. 2, 91191- Gif/Yvette, France. 

The new data presented here from the 10-24°N MAR ridge 

segment 

-show that the 10° - 24 °N segment is the most depleted from the 

10° - 70°N section of theN-MAR . 

- show the existence of a geochemical gradient from the 14 °N 

anomaly to l7°10'N: decreasing Sr and Ph isotopic ratios, 

increasing Nd isotopic ratios while K/Nb ratio stays constant 

with latitude. 

- show the existence of a very depleted mantle source (lowest Sr 

isotopic ratios found so far in the North Atlantic), located at 

about l7°N. 

- show the existence of a geochemical «deep» located at about 

17°10'N, with no relation to a structural feature. The l5°20'N 

fracture zone does not form a geochemical boundary. 

-show a constant K/Nb ratio from 12 to l7°N for basaltic glasses 

with widely different XREE patterns and Sr, Nd, Ph isotopic 

ratios (very depleted to enriched). This emphasizes the strong 

coherence of behaviour between the three elements K-Nb and 

Ta during the petrogenetic processes involved in the generation 

of these mid-ocean ridge basalts. 

- lead us to favour the existence of local individual mantle 

sources at the scale of a ridge segment over interpretations in 

terms of binary mixing between mantle sources of types described 

in far away places all over the globe. 

- and allow to talk about different «chemical memories» as 

recorded by different geochemical tools whether they are trace 

element ratios or isotopic ratios. 

The compilation of data from 10 to 70°N shows a generally 

good correlation between trace element and isotopic ratios. 

However only the Sr isotopic ratios reveal the Azores superstructure whereas both Sr isotopic ratios and trace element ratios 

(La/Sm-Nb/Zr) trace the second order structures (Azores Triple 

Junction and 45°N) superimposed on the superstructure. 

TheN-MAR section between l7°10'N and 33°N (Hayes F.Z.) 

and the South East Indian ridge section between 25°N and 33°N 

display comparable depleted trace element ratios (La/Sm and 

Nb/Zr) typical of N-MORBs but variable Sr isotopic ratios 

ranging from 0.7022 to .7030. This suggests that the mantle 

domains involved suffered fractionation events of comparable 

extent but at different times. 

85 

LAKE ONTARIO AND LAKE ERIE SEDIMENT 

INTERSTITIAL WATER- AUTOCHTHONOUS OR 

ALLOCHTHONOUS 

DRIMMIE. R.J.. FRAPE, S.K., and THOMAS, R.L .. 

Department of Earth Sciences, University of Waterloo, 

Waterloo ON, N2L 3Gl Canada. 

Land based groundwater geochemical surveys in Ontario 

and New York State have found highly saline fluids in the 

shallow environment. These fluids are believed to have their 

origin in the deep sedimentary rock environment. Recent 

investigations have shown a direct link between groundwater 

input and possible pollution sources to the Great Lakes. Over 

sixty cores up to eighteen meters long have been collected from 

Lake Ontario courtesy of the Canada Center for Inland Waters. 

Interstitial pore water from every meter in a number of 

cores has been analyzed for major ions, oxygen-IS, deuterium 

and chlorine 37/35. Ostracod and pelecypoda were separated 

and analyzed for oxygen isotope content. The results show 

progressive increases in Na and Cl with depth at all sites. The 

general increase towards depth indicates ion movement from a 

higher concentration under the sediments. The isotope 

signature indicates mixing of at least four different sources, one 

of which appears to be a relict glacial melt water. Comparison 

of shell and interstitial water oxygen isotope values indicate 

zones were groundwater has moved into the sediment. Chlorine 

isotope content helps delineate the bedrock source. 
NOBLE GASES IN GRANULITES FROM THE NILGIRI 

HILLS, SOUTHERN INDIA 

DUNAJ, TJ.1.2 and HILTON, D.R.2; 1) Isotope 

Geochemistry, NO-CO 61.5, ETH Ziirich, 8092-Ziirich, 

Switzerland, 2) Earth Sciences, Isotope Geology, Free 

University, 1081 HV Amsterdam, The Netherlands. 

Mantle-derived noble gases contained in primary carbonic 

fluid inclusions were reported recently fer a 2.5 Ga granulite 

sample from the Nilgiri Hills, Southern India (Dunai and 

Touret, 1993). Because this finding has important implications 

for regional granulite formation, our aim in the present work 

was to validate this finding utilising an extensive suite of lower 

crustal rocks from the area. We report new He, Ne and Ar 

isotopic and elemental results fer enderbites, charnockites and 

meta-sediments from the Nilgiri Hills and fer a meta-gabl:ro 

from the adjacent Bahvani Shear Zone. The samples come from 

localities scattered over an area of 650 km2• 

We analyzed coarse-grained (1-3mm) garnet separates as the 

primary carbooic fluid inclusions were present only in the 

garnets or in quartz inclusions in the garnets (Srikantappa et al. 

1992). The garnets had inclusion free rims but the cores were 

distinctive and contained abundant mineral- and primary 

fluid-inclusions. Only garnets from a meta-gabbro from the 

Bahvani Shear Zone contained abundant secondary fluid 

inclusions. 

The analysis of the noble gases hberated by aushing of the 

garnet separates confirm the findings of Dunai and Touret 

(1993). The fluid inclusions of most garnets contained helium 

with 3HefHe ratios between 0.8 and 1.7 times the atmospheric 

ratio <R_). Only in garnets from localities in the immediate 

vicinity of the Moyar Shear Zone and from within the Bahvani 

Shear Zone we did find more radiogenic 3HefHe ratios, with 

values of 0.2 and 0.6R. respectively. The highest 3HefHe 

values (>0.8 R.) were preserved in meta-intrusives as well as 

meta-sediments: thus, the carrier of the high 3HefHe signature 

- mantle-derived carbonic fluids - were probably ubiquitous in 

the lower crustal segment now exposed in the Nilgiri Hills. 

The presence of nucleogenic Ne in the fluid inclusions of 

some garnets (

21NePNe up to 0.16±0.02) indicates that a 

substantial proportion of the 4He in the fluid inclusions has 

grow-in by radiogenic decay. Therefoce, the original 3

HefHe 

ratio of helium trapped in the garnets was considerably higher 

than today thereby substantiating the presence of a mantlederived carbooic fluid. 

Dunai, TJ. and Touret, J.L.R., 1993, A noble gas study of a 

granulite sample from the Nilgiri Hills, southern India: 

implications for granulite formation: Earth Planet. Sci. Lett., 

v. 119, p. 271-281. 

Srikantappa C., Raith, M. and Touret, J .L.R., 1992, Synmetarnorphic high-density carbonic fluid inclusions in the lower 

crust: evidence from the Nilgiri granulites, southern India: 

Jour. Petrol., v. 33, p. 733-760. 

86 

RADIOMETRIC DATING OF YOUNG MORBs: 

INITIAL RESULTS USING A 40 Ar-39 A r 

INCREMENTAL HEATING PROCEDURE 

R A DUNCAN and L G HOGAN (COAS, Oregon State 

University, Corvallis, OR 97331; rduncan@oce.orst.edu) 

Determination of reliable crystallization ages by K-Ar 

methods for young (<1 Ma), fresh basalts from the seafloor has 

been frustrated by several issues. The small amounts of 

radiogenic 40Ar developed over these timescales in such low-K 

rocks are difficult to resolve from predominantly atmospheric 

40Ar. An additional concern is that mantle-derived 40Ar may 

not be totally outgassed when magmas quench at seafloor 

hydrostatic pressures, and the initial composition of Ar may 

not be atmospheric. However, developing quantitative 

chronologie methods is essential for determining the sequence 

and rates of volcanic and tectonic events associated with 

seafloor spreading. 

We report initial results of an exploratory program of dating 

MORBs using 40 Ar.39 Ar incremental heating experiments. 

Our new analytical system employs a high-sensitivity mass 

spectrometer (model MAP 215-50) with low-blank, all-metal 

gas extraction line. Samples (100-200 mg mini-cores and 

ground rocks) are dropped into a Mo-lined, Ta crucible and 

heated in a double vacuum resistance furnace in which T is 

thermocouple-controlled. 

Samples from detailed transects across "zero-age" pillow 

blocks, from glassy margins to holocrystalline interiors, show 

that excess 40 Ar is confined to the outermost few em, in 

agreement with previous studies. We have developed a 

successful strategy for partially separating sample 40Ar(atm) 

from 40Ar(rad), following Hall and York (1978). Pre-heating 

samples to 400°C removes surficial 40Ar(atm). Subsequent 

heating at 600°-1 000°C produces favorable proportions of 

40 Ar(rad), especially at T -800°C. Ar extracted at higher 

temperatures is largely atmospheric. We have calibrated our 

procedure using "zero-age" samples, Quaternary standards, and a 

suite of stratigraphically-controlled basalts from Iceland which 

we have dated previously using conventional K-Ar methods. A 

geologically-controlled suite of MORB samples from the East 

Pacific Rise near the Clipperton Fracture Zone has been dated 

using our procedure. These come from three dredge transects 

run perpendicular to the EPR, each out to the BrunhesMatuyama magnetic polarity boundary. In general, measured 

ages fall very close to those extrapolated from dredge location 

relative to the B-M boundary and the spreading ridge. 

1200 

1000 

800 

~ ~ 600 .._. 

< ~ 400 

200 

0 .A. B-M Boundary 

C MORBAges 

-~o+---~~~-r~~+---~~~-.~~ 

-60 -40 -20 0 20 40 60 

Distance from EPR axis (km) 
Fission track evidence on the Pliocene-Quaternary 

uplift of the margin of the Pannonian basin 

(Hungary) 

DUNKL, I., Lab. for Geochemical Research, 

Hungarian Acad. Sci., Budapest, H-1112, 

Hungary 

The deposition of the sediment filling of the 

Pannonian basin started in Middle Miocene (- 17 

Ma). The deepest water and the hig~est 

sedimentation rate formed around 7-5 Ma. Dunng 

the last 5 Ma the evolution of the Pannonian realm 

has changed. The margins and some zones show 

significant emergence related to the newly formed 

compressional tectonics. The evidences of the 

uplift are (I}: fo~ding, erosion and overt~t on 

Late Miocene sedtments, (II): remnants of Mtocene 

sediment on raised basement highs, above the top 

level of the sediment filling of the basin -at the 

altitude of 7-800 m, (III}: basalt flows of Pliocene 

age preserved as monadnock above the surrounding 

relie£ 

The apatite fission track analysis is the . most 

sensitive method to detect and evaluate the stze of 

this young uplift. Basement rocks of Paleozoic a~d 

Mesozoic age, Paleogene and Neogene volcamc 

tuff layers and synrift clastic sediments were 

chosen for fission track thermochronology. 

The apparent fission track ages and the 

shortening of almost all the tracks prove the young 

termination of the burial. The different zones of the 

margins of the basin have been exhumed during 5-

2 Ma. The thickness of the eroded sequences in 

some zones exceeded 1000 m. 

87 

A Major Element, Trace Element, and Isotopic 

Study of the Large-volume, Chemically-zoned 

Eruption of Baitoushan, 1010 AD 

C E Dunlap and J B Gill (Earth Sciences 

Department, Universitry of California, Santa 

Cruz, CA 95064; cdunlap@cats.ucsc.edu) 

Baitoushan volcano (located in the Changbai 

mountains on the China/N. Korea border) erupted 

more than 150 km3 of bulk rock (50 km3 dense 

rock equivalent) in 1010 ±10 AD. The eruption is 

compositionally zoned from comendite in 

pumiceous airfall and proximal lag deposits to a 

mixture of comendite and trachyte in pyroclastic 

flow units. 

Trachyte compositions range from 61 to 69 wt. % 

Si02, comendites range from 72 to 73 wt. %. In 

Harker diagrams (eg. K20 vs. Si02 and Ah03 vs. 

Ti02) the rocks form two distinct groups: one 

related by crystal fractionation, the other by 

magma mixing. Trace element concentrations vary 

by factors of less than 3, in the case of Rb, to over 

100, in the case of Sr. Compatible vs. incompatible 

element diagrams separate the deposits into groups 

identical to those identified using major elements. 

Trachytes contain phenocrysts of sanidine, 

hedenbergite, fayalite and apatite. Comendites 

contain Na-sanidine, ferroaugite, apatite and 

chevkinite. Least squares modeling of 

differentiation from trachyte to comendite suggests 

that > 70 relative wt. % of the fractionating 

assemblage is sanidine. This is consistent with the 

sense of the fractionation trends identified in both 

major and trace element plots. Rocks apparently 

related by mixing exhibit black and grey banding 

and intermingling in outcrop. Further evaluation of 

the nature, causes, and timescale of chemical 

zonation will be presented using elemental and Nd, 

Pb, Sr, and U-Th isotopic data. 
40 Ar/39 Ar DATING OF DEFORMATION: 

COMPETING CLOSURE MECHANISMS 

Dunlap, W. J ., UCLA, Earth and Space Sciences, 

Los Angeles, California. 90024 USA 

Resolving the timing of lower greenschist facies 

deformational events by 40 Art39 Ar geochronology bas been 

aided by 1) kinetic studies of argon diffusion in K-feldspar and 

2) the identification of dominant mechanisms of closure in 

synkinematic and relict white micas (i.e. neocrystallization, 

cooling through closure temperature, cessation of 

intracrystalline glide, etc). By combining the thermal history 

infonnation from multi-diffusion-domain modeling of Kfeldspars with isotopic, geochemical and microstructural data 

from synkinematic and relict white micas it is possible, in 

special cases, to date specific defonnational events (as opposed 

to cooling). 

Depending on the mechanism by which argon is 

quantitatively retained, white micas from rocks defonned at 

-150-350 °C will yield various types of tectonic information. 

For example, fine grained white micas are expected to close to 

diffusion in this temperature range, depending strongly on grain 

size. For grains with infinite cylinder geometry, activation 

energy of 42 Kcal/mol, frequency factor of 3x 1 Q-4 cm2fsec, and 

dT/dt=5 °C/Ma, closure temperatures of 326 OC, 293 OC and 

251 oc can be calculated for grain radii of 150 f.UD, 50 J.liD and 

10 J.UD, respectively. Alternatively, synkinematic white micas 

may preserve ages of neocrystallization if the grains grow 

below their closure temperature. In addition, white micas 

consisting of intergrowths of more than one phase may yield 

disturbed age spectra if the intergrowths have different closure 

temperatures, or if re-equilibration has resulted in argon loss 

below the closure temperature of the host. 

In vaccuo room-temperature defonnation experiments on 

biotite standard Fe-Mica yields 40 Ar/39 Ar ages identical to the 

age of the standard. 40 Ar/39 Ar ratios are unaffected by the 

deformation suggesting that both isotopes are similarly sited 

and that argon loss is facilitated by cleavage-perpendicular 

cracking and likely dislocation glide along K interlayers. 

Distortion of the lattice by dislocation glide may change the 

diffusivity of argon and promote outgassing in the sheared 

portions of grains. 

The cooling history of exhumed midcrustal rocks can be 

constrained to ± 25 °C by multi-diffusion-domain modeling of 

K-feldspar, which provides a continuous cooling curve from 

-150-350 °C. When combined with age data from white micas 

from defonned rocks, the temperature history during 

defonnation and/or neocrystallization of the white mica can be 

constrained, and the mechanism of white mica closure can be 

assessed. If, for example, an apparently synkinematic white 

mica gives the same age as the associated K-feldspar when it 

closed through about 200-300 °C then it is possible that the age 

of the white mica approximates the time that neocrystallization 

and ductile defonnation ceased. Thennal modeling of white 

mica and K-feldspar from shear zones in central Australia 

indicates that the synkinematic white mica neocrystallized while 

the K-feldspar closed through -240 °C. This is below 

calculated closure temperatures for the white micas, 261-281°C. 

Relict white micas may have lost argon during dislocation 

glide, but further work is needed to conflnn this. In addition, 

the results of thermal modeling of the K-feldspar are not 

unexpected in that relatively rapid cooling immediately 

postdates the neocrystallization age of white micas from shear 

zones which have accommodated significant uplift . 

88 

GEOCHEMICAL AND ISOTOPIC CHARACfERISTICS 

OF AN EARLY PROTEROZOIC ACCRETED 

MAGMATIC ARC (NARSAJUAQ TERRANE), 

NORTHERN QUEBEC, CANADA 

DUNPHY. J.M., and LUDDEN, J.N., GOOsonde Research 

Group, Departement de GOOlogie, Universi~ de Montreal, 

C.P. 6128, Succursale A, Montr~ Qu~bec, H3C 317, 

Canada 

The Narsajuaq terrane is a major component of the Ungava 

(Trans-Hudson) orogen of northern Quebec. It is a well 

exposed mid-crustal block interpreted to be the plutonic root of 

a magmatic arc which is preserved in oblique cross-section 

within the Ungava orogen. The Ungava orogen represents an 

arc - collision belt of Early Proterozoic age recording more than 

200 Ma of divergent and convergent tectonic activity. The 

Narsajuaq terrane comprises a suite of granulite grade plutons 

and supracrustal rocks which were accreted to the Archean 

Superior Province basement during Early Proterozoic time. 

The predominately calc-alkaline plutonic complex of the arc 

is comprised of an older suite (1898-1861 Ma) consisting 

primarily of well layered (em- tom-scale) tonalite and quartz 

diorite, intruded and interlayered with granitic veins, and a 

younger suite (1850-1826 Ma) of plutons ranging in 

composition from pyroxenite to granite which fonn Ian-size 

bodies intrusive into the older suite. Collision related granites 

(1-type) of even younger age (ca. 1803 Ma) occur as Ian-size 

sheets within the other two magmatic suites. The overall 

composition and volume of the granitoid rocks of the Narsajuaq 

plutonic complex is analogous to that exposed in the massive 

Mesozoic batholiths located along the western margin of North 

and South America. 

Although geochemically variable, the plutonic complex is 

characterized by high concentrations of Ba. Th and U and other 

LILE's, pronounced negative Nb and Ti anomalies and 

fractionated REE's ((La/Yb)N up to 73).The major and trace 

element chemistry of the Narsajuaq plutonic complex is similar 

to that previously reported for other volcanic arc granites. ENd(t> 

and 87Srf~r< > values for the complex range from juvenile ( +4.0, 

0.7020, respectively) to very enriched (-18.4, 0.7062, 

respectively) but show no consistent trends with age of 

emplacement, although a strong negative correlation exists 

between initial ENd and ToM ages. The most isotopically 

enriched granites require the involvement of anomalously 

radiogenic crust which is present in the region as very old crust 

(documented by inherited zircons up to 3.1 Ga). 

The geochemical and isotopic data for the Narsajuaq 

magmatic arc indicate a complex petrogenesis involving 

variable contamination of juvenile mantle-derived magmas by 

crustal sources (both continental crust and sediments derived 

from it). The magmas of the Narsajuaq terrane represent a 

significant addition of crustal material to the North American 

continent during the Early Proterozoic. 
ISOTOPIC RECORD OF BALTIC GLACIATION IN 

CURIOUS CALCITE PRECIPITATE FROM A CAVE IN 

DEVONIAN MASSIF, CENTRAL POLAND 

DURAKIEWICZ, T., Mass Spectrometry Laboratory, 

Institute of Physics, Maria Curie-Sklodowska University, 

Lublin, 20 - 041 Poland. 

Speleothems are commonly used for paleoclimate reconstruction. The unique "calcite grpats", found during the geologic exploration of the Chelosiowa cave in the Holy Cross 

Mts., preserved a record of climatic event that occurred during the Baltic glaciation period. The entrance of the cave is 

located in the quarry of Ja\Wrznia, close to the city of Kielce. 

The geologic column of the site consists of Devonian limestones. Beginning from the South-Polish glaciation time the 

cave was cut off from the surface due to filling of the adjacent valley. The present-day entrance was exposed by open 

excavation of the bedrock (a deep quarry) in the sixties of 

this century. 

The "calcite groats" samples \Wre examined by petrographic and isotopic methods. The t\\0 basic morphological 

forms were recognized : ( 1) aggradational (pin- and fanshaped, grainy concretions) and (2) degradational (isometric 

grains, skeletal forms, pins). 14C dating revealed the model 

age of 25 ky. The realistic estimation yields younger age, 

coinciding with the Baltic glaciation period. C and 0 stable 

isotope measurements were performed on 64 samples of the 

"calcite groats". The values of o13C range from -9.89%o to 

-3.67%o, whereas of o1SO from -20.98%o to -10.85%o (relative 

to PDB). In general, the "calcite groats" are enriched in 13C 

as compared to the speleothems, and differ significantly from 

other calcites (bedrock, veins, speleothems) because of their 

extremely negative o180, locally exceeding -20%o. There is a 

significant negative correlation (r = -0.86, n = 64) between 

the o13C and o180 in the ''calcite groats". The regression line 

is: o1SO = -1.43o13c- 25.41. 

The following model of "calcite groats" origin is proposed. Isotpically light water (d180 = -15%o SMOW) infiltrated through the cave bedrock, dissolving limestone and 

calcites, and was finally stored in shallow pools. The pools 

were partially covered by ice sheet, what caused isotope 

fractionation, driving off the heavier oxygen isotope from the 

remaining water. In a closed system, the carbon dioxide 

reached the saturation level. The partial pressure of C02 in 

cave atmosphere was increasing. This specific isolated environment of the cave may have been "frozen" for thousands of 

years. "Calcite groats" formed when the cave became suddenly open to the atmosphere. The probable reasons for such 

an event could be the gravitational collapse or thawing. 

With the decompression, the C02 was rapidly expelled from 

the water. Calcite concretions started to crystallize very fast, 

what caused several fractionation mechanisms, discussed in 

details elsewhere [ 1]. 

[1] Durakiewicz, T., Halas, S., Migaszewski, Z. M., and Urban, J., 1993, Origin of the "calcite groats" from Chelosiowa cave near Kielce (Holy Cross Mts. ), Poland, inferred from petrographic and isotopic investigations : 

submitted to Quat. Res. 

89 

THE NATURE OF THE ARCHAEAN CRUST IN NIGERIA 

A NEW DISCOVERY 

Ekwueme, B.N., Dept. of Geology, University of Calabar, 

Nigeria and Kroner, A., Institut fur Geowissenschaften, 

Universitat Mainz, Germany. 

The Nigerian basement complex is situated within a PanMrican mobile zone extending between the West Mrican 

craton in the west and the Gabon-Congo craton in the 

southeast. Previous geochronological studies indicate the 

dominance of Pan-African (600±150ma) ages in the region 

though ages ranging up to 2700 Ma had been reported. The 

area is therefore, characterized by mixed ages which reflect 

the imprints of at least four tectonothermal events; Liberian 

(2700±20Ma), Eburnian (2000±200ma), Kibaran 

(1100±200Ma) and Pan-Mrican (600±150Ma). 

Recent geochronological studies have, in addition shown 

that the Nigerian basement contains relicts of the oldest rocks 

in the West African shield. Zircons extracted from 

migmatitic orthogneisses in the Kaduna area of northern 

Nigeria yielded Zincon 207pbf206pb ages of 3460±9Ma, 

3437±7Ma and 3416±8 Ma respectively which indicate that 

early Archaean crust is exposed in the Nigerian basement 

complex. These ages reflect the time of emplacement of the 

gneiss pre-cursors which were granitic and granodioritic 

plutons. The Kaduna gneisses are the oldest rocks known so 

far from the West African Shield, and rocks with similar 

zircon ages were recently reported from the Sao Francisco 

craton of Brazil, suggesting a possible link between these two 

terrains in early Archaean time. 
RAPID REDOX SHIFfS IN GOLD-DEPOSITING 

HYDROTHERMAL SYSTEMS; THE SHRIMP 

SULFUR ISOTOPIC EVIDENCE FOR BOILING 

ELDRIDGE. C.S., Geol. Dept. and R.S.E.S., 

Australian National University, Canberra, 

A.C.T., 0200, Australia; AOKI, M., Geol. Surv. 

Japan, Higashi 1-1-3, Tsukuba Science City, 

Ibaraki, 305, Japan; AREHART, G.B., 

Geosciences/CMT-205, Argonne Natl. Lab, 

Argonne, IL 60439, DANTI, K., Western Mining 

Corporation, Olympic Dam Operations, P.O. Box 

150, Roxby Downs, S.A., 5725, Australia; 

HEDENQUIST, J.W., Geol. Surv. Japan, 

Higashi 1-1-3, Tsukuba Science City, Ibaraki, 

305, Japan; McKIBBEN, M.A., Dept. of Earth 

Sci., Univ. of California, Riverside, CA 92521 

SHRIMP microanalyitcal sulfur isotopic 

determinations on pyrite crystals from several 

gQld-bearing precincts have a common theme of low 

~4S values in high-grade gold regions. These data 

derive from gold-bearing rocks as diverse as those 

from the altered tuffs of the Valles Caldera in the Rio 

Grande Rift, to the sediment-hosted disseminated 

gold deposit of the Carlin Trend at Post/Betze in 

central Nevada, to the giant copper-gold-uranium 

orebody at Olympic Dam in South Australia. The 

534S values found in some pyrite from these deposits 

can be 20 to 40%o isotopically lighter than the typical 

pyrite found elsewhere in the system. In many cases 

such isotopically light pyrite is also internally zoned 

by tens of per mil over distances of tens to hundreds 

of microns. It is easiest to explain such isotopic 

heterogeneity and shifts to low o34S values in terms 

of rapid oxidation, or boiling, of the hydrothermal 

fluid. Such a process might preferentially oxidize 

34S, leaving more 32S-rich H2S to precipitate 

successive pyrite generations. Moreover, it would 

provide an effective mechanism to destabilize 

gold-bisulfide complexes and deposit native metal. 

Sulfur isotopic variability of a similar magnitude 

(10%o) has been discovered in single hydrothermal 

alunite crystals and crystal aggregates from the 

Philippines and Japan, suggesting that the oxidized 

caps of hydrothermal systems may well record the 

same process. 

Clearly, the sulfur isotopic microanalytical data 

offer a new perspective on the workings of 

hydrothermal systems and may help predict zones of 

elevated gold concentration. It seems likely that these 

sorts of isotopic anomalies would remain undetected 

in conventional isotopic studies of the same material. 

90 

SLAB DEHYDRATION BENEATH THE 

MARIANA ARC: CONSTRAINTS FROM 

U-TH ISOTOPE SYSTEMATICS 

ELLIOTT. Tim, LDEO (see below) and Faculty 

for Earth Sciences, Vrije Universiteit, Laboratory 

of Isotope Geology, De Boelelaan 1085, 1081 HV 

Amsterdam, Netherlands, ZINDLER, Alan, 

Lamont-Doherty Earth Observatory, Palisades, 

NY, 10964, USA, and PLANK, Terry, Dept. of 

Geological Sci., Cornell University, Ithaca, NY 

14853, USA. 

U-Th systematics, together with other geochemical 

tracers, have been measured on an extensive suite of 

recent lavas from the Mariana arc, West Pacific. A 

very wide range of disequilibrium is observed with 

(238Uf230rfh) activity ratios varying from 0.98-1.52. 

Notably, the lavas that show the greatest U excesses 

also have the most "depleted" trace element 

characteristics (i.e., low La/Sm and Ba!Zr). All 

lavas show low Nb/U (3 to 5), compared to the mean 

of 47 for Oms and MORBs. However, there is no 

correlation of Nb/U with inferred U excess, and 

more than one process must be invoked to explain 

combined Nb/U and U/I'h systematics. 

These results are consistent with addition of U to 

the arc-magma source mantle by dehydration of the 

down-going altered ocean crust near to the zone of 

melting. This aqueous fluid enriches U over Th and 

results in isotopic disequilibrium in the mantle 

source. Because Pb isotopic compositions and Ce/Pb 

ratios correlate well with U-Th disequilibrium in the 

Marianas lavas, the fluid is probably rich in Pb as 

well. 

Two end-member scenarios are proposed to 

explain the anomalously low Nb/U. The first, which 

would have to occur long before the above 

dehydration event to permit re-equilibration of the UTh system, would involve enrichment of the mantle 

overlying the down-going slab by a fluid or melt 

derived from the subducting sediment column. U 

would be strongly enriched over Nb during this 

process, but, due to either primary sediment 

chemistry or partitioning during fluid or melt 

separation, the U/Th ratio of the affected mantle 

wouid not be dramatically altered. In the second 

scenario, the Nb/U fractionation would occur in the 

mantle wedge, due to preferential retention of Nb in 

the residual source mantle during melting, or during 

subsequent mantle-melt interaction. Further results 

for trace elements and isotope ratios, including those 

in the sediments being subducted at the Mariana 

trench, will be presented at the meeting and used to 

evaluate the viability of these two scenarios. 
DEPTH PROFILES OF COSMOOENIC RADIONUCLIDES 

IN SURFACE ROCKS 

ELMORE. David, DEP, L., SHARMA, P., Department of 

Physics, Purdue University, West Lafayette, IN 47907, 

LIPSCHUIZ, M.E., VOGT, S., Department of Chemistry, 

Purdue University, West Lafayette, IN 47907 USA, and 

PHILLIPS, F.M., ZREDA, M. Department of 

Geosciences, New Mexico Tech, Socorro, NM 87801 USA 

The build-up of cosmogenic nuclides in rocks is a new and 

important method for determining exposure ages and erosion 

rates of landforms. Measurements of multiple samples, multiple nuclides in each sample, and depth profiles help to separate the effects of exposure and erosion and to determine the 

exposure history. The depth distribution of cosmogenic 

nuclides within a rock depends on the composition (in particular, water content) of the rock, the energy distribution of the 

cosmic rays, the erosion rate, and the geometry of the rock. 

The production of nuclides produced by high energy neutron 

spallation decreases approximately exponentially with depth. 

The production of nuclides by low energy neutron capture increases to a maximum and subsequently decreases exponentially with increase in depth. 

For many geologically interesting problems, such as dating 

sequences of glacial moraines, samples are collected from 

small rocks and large boulders of various geometry. Especially for neutron activation products such as 36c1. the production rate on the surface and at depth depends on the rock 

size and geometry. To determine surface effects and to test 

our model, we are measuring depth profiles of 1 0Be, 26 AI, and 

36c1 in rocks having simple exposure histories. 

We will frrst present the latest rates for ~uction of 36c1 

from K, Ca, and Cl. Then we will present 3 Cl, 10ae, and 

26 AI depth profiles in a silicious dolomite boulder from Meteor Crater, AZ, in a granodiorite moraine boulder from 

Bishop Creek, CA, and the 36c1 depth profile in a glacially 

polished basalt column at Devils Postpile National Monument, CA. We will compare the results with predictions of a 

nuclide production model based on Monte Carlo neutronproton transport codes. 

91 

40 Arf39 Ar CALIBRATION OF THE MID-CRETACEOUS 

TIME SCALE: ODP RESULTS FROM THEW. PACIFIC 

ERBA, E., Dipartimento di Scienze della Terra, Universita di 

Milano, I-20133 Milano, Italy; PRINGLE M.S., and 

WIJBRANS, J.R., Faculty of Earth Sciences, Vrije 

Universiteit, 1081 HV Amsterdam, Netherlands; DUNCAN, 

R.A., COAS, Oregon State University, Corvallis, OR 

97331, USA; and the ODP Leg 143& 144 Scientific Staffs. 

Drilling in the western Pacific during ODP Legs 130, 143, 

and 144 recovered samples suitable for integrated studies of 

critical mid-Cretaceous sections near the beginning of the long 

Cretaceous Normal Polarity Superchron (K-N), one of the 

most poorly calibrated portions of the Geologic Time Scale. 

All of the basalts at Site 807 on the Ontong Java Plateau 

(ODP Leg 130) are normally magnetized; their weighted age is 

121.2 ±1.2 Ma. Based on Early Aptian upper C. litterarius 

nannofossil zone assemblages found within the basalt 

sequence, these basalts must have been erupted shortly after the 

beginning of the K-N Superchron. Polarity Chron MO occurs 

entirely within the lower part of the C. litterarius zone, above 

the first occurrence of R. irregularis. We estimate that the 

basalts at Site 807 are between 0.30 and 0.55 m.y. younger 

than the top of Chron MO, based on Milankovich-driven 

cyclostratigraphy (Herbert, EPSL, 1992) and tephra- and 

biostratigraphy from DSDP Site 463 (Erba, Paleoceano., in 

press). Thus, the age of the top of Chron MO, i.e., the 

beginning of the K-N Superchron, is within analytical 

precision of the 121.2 ±1.2 Ma age of the basalts at Site 807. 

As Chron MO is only 0.3 to 0.4 m.y. long, and as both the 

first occurrence of R. irregularis and the base of Chron MO 

occur very near the Barremian I Aptian boundary, 121.2 Ma is 

a good minimum estimate for the age of that boundary, too. 

The volcanic basement at MIT Guyot (ODP Site 878) may 

be divided into an upper, normally-magnetized section and a 

lower, reversely-magnetized section of subaerial alkalic basalts. 

The weighted age of the four basalts on either side of the 

reversal is 122.9 ±0.6 Ma. The oldest dated sediments, platform carbonates located approximately 25 m above volcanic 

basement, are attributable to the lower portion of the C. litterarius nannofossil zone. If the lowermost shallow-water 

carbonates were deposited quickly (faster than about 75 

m/m.y.), and if there was no significant hiatus between the 

eruption of the basalts and the deposition of the first 

sediments, then the magnetic reversal seen in the basalts could 

be the top of polarity Chron MO . However, assuming slower 

sedimentation rates, the possibility of a reversal in the lower 

limestone unit, and a minimum hiatus of 1 m.y. between the 

limestones and basalts, the reversal seen in the basalts is more 

likely the top of polarity Chron M1. The 122.9 Ma age of the 

basalts would then provide a precise age for the top of Chron 

M1, constrain the maximum age of the beginning of the K-N 

Superchron, and suggest a significantly younger Barremian I 

Aptian boundary than the 124.5 Ma of Harland et al. (1990). 

The age of the oldest lavas at Resolution Guyot (ODP Site 

866) is 127.6 ±2.1 Ma. A complete mid-Cretaceous magnetostratigraphy down to at least polarity Chron M5 or M7 has 

been documented in the sediment section; the majority of the 

basalts are also reversed. If these basalts represent true 

volcanic basement, then the Hauterivian benthic foraminifer 

and Sr isotopic age of the basal limestone suggest that the 

131.8 Ma age for the Hauterivian I Barremian boundary of 

Harland et al. (1990) should be revised significantly downward. 

(Sites 807 and 878 ages relative to TCR 85G003 sanidine at 

27.92 Ma; Site 866 ages relative to FCT-3 biotite at 27.7 Ma) 
THE SEAWATER OSMIUM ISOTOPE RECORD AND THE 

MARINE CARBON CYCLE IN THE NEOOENE OCEAN. 

Esser. Bra<fley K., L-396, Lawrence Livermore Nat'l Lab., 

Livermore, CA, 94550, and Ravizza, Gregory E., Dept. of 

Geology and Geophysics, Woods Hole Oceanographic lost., 

Woods Hole, MA 02543. 

Temporal changes in the Os isotopic composition of 

Neogene seawater are recorded in North Pacific pelagic clay 

(Pegram et al., 1992) and in East Pacific Rise metalliferous 

carbonate (Ravizza, 1993). These seawater Os isotope records, 

which are similar, exhibit a pattern of change which differs in 

detail from the seawater Sr isotope record. From 28 to 15 Ma, 

seawater 87Sr/86sr increased rapidly while 187ost186os 

remained relatively constant. Sometime between 15 and 17 Ma, 

seawater187ost186os began to increase rapidly to the modemday ratio of 8.5 (Ravizza and Turekian, 1992), while the rate of 

87 Sr/86sr rise diminished. For both Sr and Os, continental crust 

is more radiogenic than the oceanic lithosphere. Their 

decoupling suggests that an increase in terrigenous silicate 

weathering cannot be solely responsible for secular change in 

seawater Os. We suggest that accelerated weathering of old 

organic-rich sediment accounts for the rapid increase in 

seawater 187ost186os over the past 15 Ma. 

Relative to average continental crust (Esser and Turekian, 

1993), organic-rich sediments have high Os concentrations and 

high Re/Os ratios (Ravizza and Turekian, 1989). Because of 

the decay of 187Re to 187os, old organic-rich sediments also 

have high 187ost186os ratios and represent an important 

crustal reservoir for excess 187 Os*. During weathering of 

organic-rich sediments, liberation of soluble 187os* should be 

coupled to organic C oxidation. Bulk carbonate ~13c profiles 

from the South Atlantic display a negative shift of 2o/oo over the 

past 15 Ma, which Shackleton (1987) interpreted as indicative 

of net oxidation of the global organic C reservoir. Whether net 

oxidation occurred by enhanced weathering or by diminished 

burial of organic C cannot be distinguished with the ~13c data 

alone. Although diminished organic matter burial or enhanced 

weathering of young marine organic matter will decrease 

seawater ~13 C, they will have no effect on seawater 

187ost186os. Enhanced weathering of old organic-rich 

sediment is compatible with both the observed increase in 

seawater 187ostt86os and decrease in bulk carbonate ~13c. 

Both the ~13c shift to more negative values and the 

187ost186os shift to more radiogenic values commence at -15 

Ma. We suggest that the correlation is not coincidental and 

reflects an increase in the weathering rate of old organic matter. 

If the Shackleton model is used, in which riverine organic ~13c 

is assumed to have remained constant, the excess 187os* influx 

integrated over the past 15 Ma is -13% greater than the pre-15 

Ma steady-state flux. An alternative interpretation is that the 

~13c shift was driven by a change in riverine organic ~13c; in 

this model, the size of the global organic C reservoir does not 

change. Lighter riverine organic ~13c values may have resulted 

from an increase in the mean age of riverine organic C over 

the past 15 Ma-Cretaceous marine organic Cis lighter than 

modem marine organic C (Dean et al., 1986). An increase in 

the mean age of riverine organic matter would also result in an 

increase in riverine 187 Os/186os. In both models, increased 

weathering of old organic C is more consistent with the Os data 

than a change in organic C burial. 

92 

40ArJ39Ar DATING OF ANORTHOCLASE, 

MOUNT EREBUS, ANTARCTICA: PROBLEMS 

OF EXCESS ARGON IN MELT INCLUSIONS 

·ESSER R P HEIZLER, Matthew, T., KYLE, P.R. and 

MciNfOSH, W., at: N.M. Bureau of Mines and Dept. of 

Geoscience, New Mexico Tech, Socorro, NM, 87801, USA. 

Mt. Erebus is a 3794 m high active phonolite volcano on 

Ross Island, Antarctica. Anorthoclase phonolite lava flows are 

exposed around the flanks and infill a major caldera. The 

summit cone is mainly composed of anorthoclase phonolite 

bombs erupted over the last 20 years. Mechanical 

disintegration of the bombs has left a lag of anorthoclase 

crystals up to 10 em in length. In the bombs and lavas 

anorthoclase is abundant (-30-40%) and is often riddled with 

melt (glass) inclusions trapped during rapid growth. The high 

K20 cpntent (3-4% ), abundance and large size potentially 

makes the anorthoclase ideal for 40 ArJ39 Ar dating. 

CQilventional Kl Ar ages of anorthoclase, glass and whole 

rock samples from Erebus ranged from 0.94 to 0.15 Ma. 

Historically erupted anorthoclase have an age of 0.44 Ma, 

indicating excess 40 Ar is a significant problem. 

40 Ar/39 Ar age spectrum analyses on pure and impure (1 to 

5% glass contaminated) anorthoclase were obtained on summit 

(1984 eruption) samples. A clear correlation between apparent 

age and degree of glass contamination is observed for duplicate 

analyses of summit phenocrysts. Nearly pure anorthoclase 

gave total gas ages of 68 and 41 ka whereas glass 

contamination of about 1% yielded total gas ages of 184 and 

165 ka and 5% contamination produced total gas ages of 283 

and 276 ka. Isochrons for each of the summit samples yield 

no apparent linear correlation and therefore suggest a 

heterogeneous trapped 40 Arf36 Ar composition. 

Mt..Erebus glass typically contains -1600 ppm Cl. The 

measurement of 38 Ar formed from a nuclear reaction on 37Cl 

provides a measure for sample a during age analysis. As the 

anorthoclase is presumed Cl-free, the glass content is directly 

proportional to the Cl signal. Unfortunately, there is not a 

straight-forward relationship between Cl and excess 40Ar for 

the summit samples, thus it may not be possible to 

quantitatively correct for excess argon. 

Geologically older samples from the flanks were also 

analyzed in duplicate (non-contaminated versus glass 

contaminated). Unlike the historic samples, the flank samples 

yield nearly identical apparent ages for both pure (isochron age 

= 88 ± 2 ka) and glass contaminated (isochron age = 84 ± 9 

ka) anorthoclase. The glass causes a significant drop in 

radiogenic yield as well as a large increase in chlorine. 

However, there appears to be minimal excess argon associated 

with the glass for these samples. Therefore we believe that 

the apparent ages from the flank samples are geologically 

relevant. 

Additional, older samples were also analyzed. Although 

minor amounts of glass were found to be present within these 

samples, the apparent ages were all less than 100 ka. Because 

glass enriched duplicates were not analyzed for these samples 

the presence of excess argon is difficult to assess. These ages, 

however, do represent a maximum age that are substantially 

younger than previously determined conventional Kl Ar ages. 

Therefore, we conclude that while excess argon resides in the 

glass, the presence of glass itself does not confirm the 

existence of excess argon. However analysis of nearly clean 

versus glass contaminated samples does provide a technique 

whereby non-anorthoclase excess argon may be verified. 
DATING THE S:MECITE ILLITE TRANSITION USING KAr AND Rb-Sr WHOLE-ROCK TECHNIQUES. 

1 A Evans NERC Isotope Geosciences Laboratory, 

Keyworth, NOTTM NG 12 5GG, UK. 

The different behaviour of the Rb-Sr and K-Ar systems at 

low temperatures makes it possible to show that mudrock RbSr whole-rock ages from Palaeozoic mudrocks date the 

smectite-illite transition. 

Rb-Sr whole-rock regressions and K-Ar data from the < 2m 

illite fraction of samples of Ordovician and Silurian mudrocks 

are presented. K-Ar ages from the< 2m fractions generate a 

normal distribution with a mean and quadratically combined 2cr 

error giving a Devonian age of 399 ± 4 Ma. This dates the 

passage down-temperature of the illite through 260 ± 30 o C 

during Acadian metamorphism. Rb-Sr whole-rock ages from 

the same rocks do not show the same consistency of ages, 

giving 431 ± 10,414 ± 6 and 403 ±15 Ma. The Rb-Sr wholerock isochron ages are interpreted as dating the time of passage 

of the rock through the smectite-illite transition because the 

radiometric ages correspond well to the estimated time of this 

reaction calculated from sediment thicknesses. Other 

explanations of the ages, such as the possibility that they were 

generated by mixing, or that they date sediment provenance 

ages, peak metamorphism, or cleavage, are rejected on 

geological grounds. 

The discrepancy between the K-Ar ages and Rb-Sr wholerock ages is caused by the different behaviour of the two 

systems at low temperatures. K-Ar records thermal events, 

whereas the Rb-Sr whole-rock system is trolled by mineral 

stability and is de~endent on water for moving and 

rehomogenising 7 Srt86sr over sufficient distance to create a 

common isotope composition in mudrocks at a specific time. 

This occurred in the mudrocks of this study during the smectiteillite reaction which is a major, prograde, dehydration reaction. 

93 

DISTRIBUTION OF CHLORINE-36 IN THE 

UNSATURA1ED ZONE AT YUCCA MOUNTAIN: AN 

INDICATOR OF FAST TRANSPORT PATHS 

FABRYKA-MARTIN. J., Group CST-10, Los Alamos 

National Laboratory, Los Alamos, NM 87545; 

BODV ARSSON, G.S., Earth Sciences Division, 

Berkeley National Laboratory, Berkeley, CA 94720; 

WIGHTMAN, S., MURPHY, W., WICKHAM, M., 

Hydro Geo Chern, Inc., 1430 North 6th Ave., Tucson, 

AZ 85705; CAFFEE, M., NIMZ, G., SOUTHON, J., 

Center for Accelerator Mass Spectrometry, Livermore 

National Laboratory, Livermore, CA 93550; 

SHARMA, P., PRI:ME Lab, Dept. of Physics, Purdue 

University, West Lafayette, IN 47907 

The 36CVC1 ratios for Cl extracted from soil and rocks 

are being used to provide information on characteristics of 

water movement through the unsaturated zone at Yucca 

Mountain. Deep penetration of the bomb-pulse signal in 

alluvium provides evidence for the variable effectiveness 

of alluvium and associated vegetation in attenuating 

infiltration, possibly as a result of different runoff 

characteristics of the watersheds. Fast transport of water 

via fractures through the Tiva Canyon welded unit is 

indicated by detection of elevated levels of 36Cl in the 

underlying Paintbrush nonwelded unit. 36Cl data for one 

deep borehole indicates average residence times of 5 to 7 x 

105 yr in the deeper Topopah Spring welded units, and 1 

to 3 x 105 yr in the underlying Calico Hills nonwelded 

unit. 

The apparently younger ages observed in the deeper 

unit suggests that moisture in this unit may not derive 

predominantly from vertically-downward matrix flow, but 

rather that a faster transport path may be involved such as 

lateral transport downdip from the direction of the Solitario 

Canyon Fault Zone. Numerical modeling studies are 

underway to evaluate possible moisture migration 

pathways that are consistent with the observed 36Cl 

distributions. 
C AND 0 ISOTOPE RECORDS OF THE 

ADRIATIC SEDIMENTS: INFLUENCE OF 

ALLOCHTONOUS 

INPUT AND TEMPERATURE 

FAGANELI, J., Marine Biological 

Station, Piran, 66330, Slovenia and 

DOLENEC, T. and PEZDI .... , J. , Jozef 

Stefan Institute, Ljubljana, 66000, 

Slovenia 

The C and 0 stable isotope 

composition of bulk carbonates and C 

isotope composition of sedimentary 

organic matter in the Adriatic Sea 

were used as an indicator of their 

origin and condition of deposition. 

The areal distribution of surficial 

carbonates depleted in 13C was 

restricted to the NW and ~ parts of 

the Adriatic strongly influenced by 

the river Po and other Italian river 

inflows and to SE part of the 

Adriatic affected by local riverine 

inputs. These are isotopically 

lighter carrying 13C depleted organic 

matter. This areal distribution is a 

consequence of the general water 

circulation system and 

sedimentological properties of the 

Adriatic Sea. The vertical profiles 

of d13C values of organic C in short 

cores from the northern and central 

Adriatic matched the organic C 

content, indicating this distribution 

as a function of allochtonous organic 

C input. Lower cJ13C value and high 

organic C content observed in the 

central Adriatic in the horizon from 

ca 15000 years B.P. during the last 

postglacial period was therefore a 

consequence of a larger terrestrial 

contribution in the past by increased 

riverine flow. The area of higher 13C 

and 18 contents in carbonates appears 

along the NE Adriatic coast due to 

the sediment enriched in dolomite. 

The carbonates with the highest 13C 

and 180 contents are located in the 

deepest basins of the Adriatic 

Jabuka and south Adriatic Pits, due 

to the ingression of colder and 

isotopically havier water from the 

eastern Mediterranean. These areas 

are also characterized by higher 13C 

content in sedimentary organic matter 

indicating the provenance from the 

autochtonous biological production. 

94 

PRESOLAR OXIDE GRAINS IN METEORITES. 

EVIDENCE FROM OXYGEN AND MAGNESIUM 

ISOTOPES. 

FAHEY. A. J. and HUSS, G. R, Lunatic Asylum, Division 

of Geology and Planetary Sciences, California Institute of 

Technology. Pasadena, CA 91125 

Carbon rich phases were the first materials of interstellar 

origin to be separated from meteorites and identified as having 

formed outside the solar system. Several oxide grains have 

recently been found in the ordinary chondrites Bishunpur and 

Tieschitz and in the carbonaceous chondrites Orgueil and 

Murchison that clearly are of extra-solar-system origin. Most 

of the grains are corundum (AI20 3) (e.g. Huss et a/., 1994, 

Nittler eta/., 1994). 

We have measured the oxygen isotopic ratios in -100 

oxide grains from Orgueil and Bishunpur, 57 of which were 

corundum, and 2 of these 57 were found to be presolar. The 

oxygen isotopic composition of Orgueil corundum B is: 

16Qfl7Q = 1028 ± 11, and 160/180 = 502 ± 10 (Hutcheon et 

a/., 1994), and ofBishunpur corundum B39 is 160/170 = 385 

± 9, and 16Qf18Q = 854 ± 25 (Huss et a/.,1994)((160/170)0 = 

2610, and (16Qf18Q)0 = 499). The oxygen isotopic 

compositions of these corundum grains set them apart from 

oxygen bearing phases that formed in the solar system. The 

measured ratios in these grains is similar to those observed in 

red giant or AGB stellar atmospheres (Smith and Lambert, 

1990). Stellar evolution calculations show that the oxygen 

isotopic compositions of the grains to be dominated by the first 

dredge-up event in such stars. Nittler et al. (1994) have 

observed isotopic compositions similar to the corundum grains 

that we have measured, but have also found two (Al20 3) 

grains from Tieschitz with more extreme 160/180 values 

(-7000 and -20000). The authors suggested Wolf-Rayet stars 

as a possible source for these two grains. 

The Mg isotopic composition Orgueil corundum B has 

been measured and shows (26Atf27 Al)0 = 8.9±0.1 x 104 . This 

is -18 times higher than the canonical solar system value of 

-5 x 1 o-s. Similar ratios have been observed in other 

corundum grains with values as high as 8 x 10·3 (Nittler et a/., 

1994). This is also consistent with formation of these grains 

in red giant or AGB stars that have ex-perienced the first 

dredge-up event. 

The isotopic compositions of Orgueil corundum B and 

Bishunpur B39 do not uniquely identify a stellar source for 

their formation since even the envelope of a Type II supernova 

is a possible source. Rather, they distinctly show the products 

ofH-buming and point to the first dredge-up event to produce 

their isotopic signatures. Division Contribution 5367(841) 

Russ, Fahey, Gallino and Wasserburg, 1994,Presolar Al20 3 with a 

Large Excess of 110 and Depleted 180, LPSC 25, submitted. 

Nitt1er, Alexander, Gao, Walker, and Zinner, 1994, Interstellar 

Corundum and Spinel from the Tieschitz Ordinary Chondrite, 

LPSC 25, submitted. 

Hutcheon, Russ, Fahey, and Wasserburg, 1994, Extreme 26Mg and 

110 enrichments in an Orgueil corundum: Indentification of an 

interstellar oxide grain, Ap. J. Let., submitted. 

Smith and Lambert, 1990, The Chemical Composition of Red Giants 

ill. Further CNO Isotopic and s-process Abundances in Thermally 

Pulsed Asymptotic Giant Branch Stars, Ap. J., 72, 387 
ND-SR ISOTOPIC CHARACTERISTICS OF 

HERCYNIAN POSf-RIFTING GRANITES AND 

XENOLITHS IN HAINAN ISLAND, S. CHINA: 

IMPLICATIONS FOR GENESIS OF THEM 

FANG,Zhong, ZHANG,G-D. ,TAO,X-C., 

LI, H-M. , and YANG, J-D. , Department of 

Earth Sciences, Nanjing University, Nanjing 

210008, P.R. China. 

The great granitic batholites, Hercynian (320......, 

270Ma) porphyroblastic granite-monzonites of ca. 

7000Km2 , are main part of granitoid rocks in 

Hainan Island, South China. They intruded into 

Pre-Cambrian gneiss of Baoban Group , and either 

intruded into or transferred from Carboniferous 

( 340 ......, 330Ma) lower metamorphic flysch with 

volcanics of Shilu Group. Authors have studied 

and Confirmed the Upper Palaeozoic rifting on 

Hainan lsi. There are similar REE patterns both of 

pre-granitic basement rocks and the granitoids. 

The latter is belonged to crustal source type, that 

is based on -E-Nd(t)-17. 2""-5. 2,-fSr(t)+23 

......,+ 141, and o180+6. 02......,+ 12. 97, an average 

value of the o18 0 + 10. 66, etc •• T~ft ages of the 

granitoids are from 1351 ......, 1797Ma, which are 

compatible with an U -Pb concordant age 

(1352Ma) of clastic zircons from Shilu Group, and 

a Sm-Nd whole rock isochron age (1702Ma) of 

meta-basalts from Baoban Group. The N d and Sr 

isotopic characteristics of the granitoids suggest the 

primitive origin rocks of them could be Proterozoic 

metamorphic rocks. It is very interested in that the 

rhyolites , a member of bimodal volcanic rocks in 

Shilu Group, keep their own -fNd(t) -6. 73......,-

6. 48, T~ ages 1562......,1610Ma, those are like the 

corresponding values of the granite near by. Their 

ages and petrographical facies are different, but it 

was possible that they were developed from an origin. 

A lot of xenoliths in the granitoids are belonged 

to restite ones. Their -E-Nd ( t) - 12. 9......,- 10. 2, 

-E-Sr(T)-86......,+6, and especially high ratios of 

Sm/N d 0. 38......, 0. 52. The elemental and isotopic 

features of the xenoliths show a drastic N d-Sr fractionation between melt and the restites. There are 

some variations on chemical components between 

the xenoliths and the pregranitic meta basalts, 

which could be identified source rocks of the former, their Si02t KzO and Rb are higher, and 

MgO, CaO, Sr, Ni are lower than those in the 

meta-basalts indicate some assimilation and contamination from Hercynian granitic melt. 

95 

VARIABLE U-Pb TIME DOMAINS IN A DEVONIAN 

LIMESTONE 

FARQUHAR. Ronald M., SMITH, Patrick E., Dept 

of Physics, University of Toronto, Toronto, Ontario, 

Canada, M5S 1A7 and HANCOCK, Ron 9· V., 

Slowpoke Reactor Facility, University of Toronto, 

Toronto, Ontario, Canada, M5S lAS. 

Significant variations in U and Pb concentrations are 

present on length scales of a few meters in limestones from 

the mid-Devonian Lucas Formation in the Michigan basin, 

southwestern Ontario. Pb is correlated with the elements 

Fe, AI and Sc, which are predominantly associated with 

non-carbonate (clay) minerals in the samples. Isotope 

analyses of carbonate and associated non-carbonate 

fractions in the samples suggest that the rocks have 

experienced exchange reactions during an Acadian 

diagenetic event, where significant amounts of radiogenicPb 

were sequestered by clays at 333 ±5 Ma (la). 

Total-rock subsamples of the limestone in an area 

slightly enriched in organic carbon show wide variations in 

U/Pb over distances of a few mm. Combined, these give 

isochron ages of 380.2 ±4.4 Ma for 206Pbf238U and 368 ± 15 

Ma for 2°7Pbf235U. The results agree with the stratigraphic 

age, and demonstrate that the use of total-rock samples 

from this site can effectively compensate for the 

radioelement redistributions. However, less radiogenic data 

for other stratigraphically equivalent sites are not entirely 

concordant with the above results. We believe this is 

related to partial or total resetting of the U-Pb isotopic 

systems in these samples during diagenetic events. 

Apparent ages determined by selective leaching for samples 

from sites characterized by high Pb are 332 ±6 (206Pb,f238U) 

and 320 ±39 (2°7Pbj235U), identical to the internal isochron 

ages for the samples. 

An extensive sequential leaching of another sample 

reveals that redistributions of U and Pb have taken place in 

smaller U-Pb domains which appear to result from 240 and 

45 Ma secondary events. The apparent secondary ages can 

be correlated with (1) regional fission-track, K-Ar and RbSr cooling/diagenetic ages in strata of the North American 

mid-continent; and (2) local U-Pb ages of secondary calcite 

spar. 
DETECTION OF 1-129 IN ENVIRONMENTAL 

SAMPLES ASSOCIATED WITH THE NUCLEAR 

FACILITY AT WEST VALLEY, NY 

FEHN.U., RAO,U., MORAN,J.E., and TENG, 

R.T.D., Department of Earth and 

Environmental Sciences, University of 

Rochester, Rochester, NY, 14627 and 

Nuclear Structure Research Laboratory, 

University of Rochester, Rochester, NY, 

14627 

West Valley is a former civilian reprocessing 

plant located in Western New York State. 

Although reprocessing activities stopped in 

1975, significant amounts of radioactive 

materials are still present there. For a preliminary study of the level of radioactive materials 

currently released from this site we collected 

water samples and plant material from the 

watershed near West Valley. We report here 

initial results for 1291/1 ratios determined using 

the AMS system at Rochester. 

For background determination of 1291/1 ratios 

in this region, samples were collected in two 

of the Finger Lakes. The ratios, around 1 o-s, 

are somewhat higher than typically found for 

anthropogenic I, probably reflecting a higher 

atmospheric ratio prevalent in New York 

State. 

The measurements around West Valley 

show a striking pattern of 1291 release from 

this site. The highest values were found in 

samples taken downstream from the site in 

Buttermilk Creek with 1291/1 ratios in excess of 

1 o-e. These values are more than two orders 

of magnitude higher than those found 

upstream from the site in the same creek. 

The signal is also clearly visible in Cattaraugus 

Creek, and can be followed at least into Lake 

Erie. Ratios found in the water and in the plant 

material were generally in good agreement. 

The results suggest continued release of 

radionuclides from buried waste in West 

Valley entering the surface waters. At the 

same time they demonstrate the sensitivity of 

the 1291 system for the detection of the release 

of radionuclides from buried nuclear waste. 

96 

APATITE FISSION TRACK (AFT) AND ORGANIC 

MATURITY (OM) PALEOTHERMOMETRY OF THE 

EASTERN MARGIN OF THE GULF OF SUEZ 

FEINSTEIN, S., Dept. of Geology, Ben Gurion 

University of the Negev, Beer Sheva 84105, Israel, 

Kohn, B.P., Victorian Institute of Earth and Planetary 

Sciences, School of Earth Sciences, La Trobe University, 

Bundoora, Victoria 3083, Australia, Steckler, M.S., 

Lamont- Doherty Earth Observatory of Columbia 

University, Palisades, New York 10964, and Eyal, M., 

Dept. of Geology, Ben Gurion University of the Negev, 

Beer Sheva 84105, Israel. 

Time-space relationships between vertical movements 

and thermal history of the lithosphere relative to riftrelated extension provide essential constraints for the 

relative role of different possible rifting-subsidenceuplift mechanisms. Large rift-flank uplifts, such as 

observed along the Gulf of Suez (>4 km in southern Sinai 

mountains), result from tectonic effects of lithospheric 

thinning and heating, and/ or the isostatic response to 

unloading associated with extension. Erosion of the rift 

flank can greatly augment the depth of exhumation of 

rocks for either mechanism. 

Miocene strata at -2-2.5 km in deep boreholes within 

the eastern Gulf of Suez graben yield a relatively wide 

AFT age range between 31 to 269 Ma and mean tracklength (TL) of -10.5-11.5 mm. AFT ages exceed the 

stratigraphic age of the syn-rift host rocks indicating 

that rift-related thermal effects were not sufficient to 

totally reset AFT clocks. However, the range of 

measured AFT data is similar to that previously 

recorded in the Precambrian crystalline basement 

flanking the Gulf of Suez which is believed to be the 

provenance for the rift-filL 

In the southeastern part of the Gulf of Suez AFT ages 

are younger than those recorded to the north and appear 

to become younger with depth, possibly reflecting a 

partial thermal overprint related to burial in the 

present rift setting. However, the TL data do not show 

such a clear relationship. In addition, both present day 

bottom-hole temperatures and paleotemperatures 

indicated by the level of OM in the Miocene sediments 

appear too low to account for the level of AFT shortening 

recorded, particularly for the relatively short time 

involved. Thus, the AFT data do not record any major 

rift-related thermal effects, but rather, they mainly 

retain a pre-rift provenance signature which reflects the 

order and depth of erosion at the uplifted flanks. 

The level of heat flow at the surface indicated by 

AFT and OM paleothermometry, and by BHT data in 

the Gulf of Suez appear too moderate to account for the 

vertical movements observed at the flanks. Hence, we 

conclude that either the full effects of the rift-related 

thermal anomaly have yet to reach the near surface 

environment, or that the isostatic response to denudation 

is the primary source for the flank uplift. 
THE D/H RATIOS OF TREE-RINGS IN 12 

TREES FROM WORLD WIDE WCATIONS 

AND THE IMPLICATIONS FOR CLIMATE 

CHANGE OF THE PAST 100-200 YEARS 

E.BtKi, Xiahong and EPSTEIN, Samuel, 

Division of Geological and Planetary Sciences, 

170-25, Caltech, Pasadena, CA 91125 

It has been shown that the ~D values of 

nonexchangeable hydrogen in cellulose of tree-rings are 

systematically correlated with the source water used by 

trees in photosynthesis. A highly significant correlation 

between ~D of tree-rings and mean annual temeerature 

has also been reported for trees of widely distnbuted 

geographic locations where the ~D values vary by more 

than 100 permit. However, the ~D-temperature 

relationship has not been well established for a time 

series of any location. In this case, we must deal with m~ch smaller variations and smaller signal to noise 

ratios. 

Detailed studies on the relationship between the ~D 

values of tree-rings and climatic parameters, such as 

temperature and precipitation, have been carried out for 

three locations. These locations vary from a flat area 

with simple topography, such as Manitoba, Canada, to 

locations in Southern California where the climatic 

regime is rather complicated due to the variable 

topography. The results show 1. that correlations 

between ~D of tree-rings and temperature are much 

more siBnificant in a flat area in comparison to a 

mountamous area, 2. that in Manitoba, the ~D values of 

tree-rings can be correlated to a temperature record of a 

meteorologic station 270 miles away, indicating that one sin~le tree may be used as a proxy temperature indicator for a large area, 3. that correlations between 

the ~D of tree-rings and temperature are particularly good for data sets that are smoothed using moving 

average. 

Twelve ~D records show that from the 1850's to 

approximately the 1920's, the ~D values continuously increased in all trees. In a few cases where the time 

series go further back in time, the warming seems to 

have started as early as 1820, 1800 or even 1780. The 

rate of warming shown by a given tree roughly correlates with the mean ~D value of that tree, meaning that a greater rate of warming occurs at colder 

locations. ·The trees of AlaslCa and Siberia are two 

exceptions. After about the 1920's, the trends become 

variable among the trees. Some series show continuous 

warming, some show little warming, and some show 

significant fluctuations. This time period also seems to 

be characterized by increased temporal fluctuations 

shown in the individual ~D time series. Overall, the 

warming trends are much more pronounced and 

consistent in the 19th century than in the 20th century when the climate forcing by anthropogenic C02 is 

stronger. In addition, the warming seems to have 

started as early as in the 1820's and continued until 

1920's in most of the locations. The question remains, 

therefore, whether the forcing mechanism is natural 

(recovery from Little Ice Age), anthropogenically induced, or a combination of the two. The observations 

also raise a question as to whether the increased spatial and temporal variations in the ~D climatic records are 

related to the anthropogenic activities. 

97 

40AR/39AR PLATEAU AGES ON PHENGITES 

AFFECTEDBYEXCESSARGON 

FERAUP. G., RUFFET G., DE JONG K., URA CNRS 1279, 

Universite de Nice, 06108 NICE Cedex 2, France, 

BALLEVRE M., Laboratoire de Tectonique, UPR CNRS 

4661, Universite de Rennes I, 35042 Rennes Cedex, France, 

AMOURIC M., CRMC2 CNRS, Universite Marseille, 

13288 Marseille Cedex 9, France. 

40Ar/39Ar phengite ages are commonly used for 

reconstruction of Pressureffemperature/time paths of regional 

metamorphism. It is therefore necessary to know if the 

40Ar/39Ar plateau age criterion used to test the validity of the 

measured ages is reliable in all cases for phengites or if 

unsignificant plateau ages may exist as for biotites affected by 

excess argon. Two distinct cases showing 40Ar/39Ar plateau 

ages obtained on single grains and bulk samples of phengites 

affected by excess argon are presented. 

(1) Fresh phengites from two restrictive areas of the Sesia 

Zone (Western AJps, Italy) display either plateau ages ranging 

from 104 to 180 Ma, geologically unsignificant (Mucrone area), 

or well defined plateau ages ranging from 69.3 ± 0.7 to 76.9 

±0.6 Ma on rocks sampled on a 50m wide outcrop (Marine 

area). These last ages which may be geologically significant, 

show discordant but reproducible plateau ages with a maximum 

difference of 10%, which may be only explained by the 

existence of undetectable excess argon. 

(2) Partially recrystallized phengites separated from a 5 km2 

part of a gneiss body of the Mulhacen Complex (Betic Zone, SE 

Spain), display plateau ages ranging from 15.8 ± 0.4 Ma to 90.1 

± 1.0 Ma. These phengites are characterized by a high 

atmospheric Ar contamination, an easy detachment over the 

basal plane, and different populations of recrystallized micas as 

observed by HR'IEM. These characteristics may make these 

micas susceptible for the uptake of variable quantities of excess 

argon from metamorphic fluids during a Miocene thermal event 
A TERRESTRIAL ISOTOPIC RECORD OF LOWER 

CRETACEOUS CLIMATIC CHANGE FROM 

SOUTHEASTERN AUSTRALIA 

Kwt M. Ferguson, and Robert T. Gregory; Dept. of Geol. 

Sci., Southern Methodist Univ., Dallas, Texas 75275; 

Andrew Constantine, Dept. Earth Sci., Monash Univ ., 

Clayton, Victoria 3168, Australia. 

Organic carbon isotopic data from insoluble residues, and 

oxygen isotope data from carbonate cements, in concretions 

taken from fluvial sediments at twelve sites in the Otway and 

Strzelecki Ranges of southeastern Australia, coupled with 

palynological dating, provide a record of high latitude 

continental climatic change from the early Aptian to the late 

Albian. Average calcite oxygen isotopic values (SMOW) 

increase from a minimum of +4.3 in the middle Aptian (=120 

Ma) to a maximum of +17.7 in the late Albian (=100 Ma). 

Meteoric water alSO values inferred from the calcite data 

range from -24.8 to -11.3. Using the modern correlation 

between mean surface temperature and the mean isotopic 

composition of precipitation indicates that surface temperatures 

increased by"" 10°C from 120 to 100 Ma, with a mean surface 

temperature of = 0°C ± 5° over that time interval. The surface 

temperature record derived from the concretion oxygen isotope 

data correlates well with an increase of =3°C in temperature 

over the same time interval derived from the oxygen isotope 

composition of marine foraminifera. Comparison of the 

deviations from the mean of the maximum and minimum 

values, normalized to ± 1, of the temperature records is shown 

below. 

Carbon isotopic values from the insoluble organic carbon 

residues of the concretions define a trend from a low value of 

-25.5o/ooat =120 Ma to a maximum value of -22.5 at the AptianAlbian boundary, and then decreasing to -25 in the late Albian 

at == 100Ma. The shape of this trend and the stratigraphic 

position of the maximum carbon isotopic value mimic the 

carbon isotopic trend defined by pelagic marine limestones. 

The correlation of the terrestrial and marine carbon isotopic 

records suggest that the concretions are recording changes in 

the global carbon reservoir. 

Comparison of the terrestrial concretion carbon and oxygen 

isotope data with the corresponding marine data suggests that 

the concretions provide an accurate record of global changes in 

climate and in the exogenic carbon reservoir . 

. 2 

c: 

~ 1.0 ---1---f-----+---+---+---e::::!~~--~ 

c:: 

~ 0.5 --+---+--+~!!l-+-~~---+---1 

Q) 

c: 0---t---f-----~~~--4---4---1 ·c:: 

ctS 

E -0.5 --t---+-~~-+--+----+-~ 

I-

~ -1.0 

<1 

-1. -0.5 0. 1.0 

.d MAT terrestrial concretions 

98 

BORON ISOTOPIC COMPOSITION OF FUMAROLIC 

FLUIDS, VULCANO CRATER. AEOLIAN ISLANDS. 

ITALY. 

FERRARA G. 1st. Geocronologia, CNR, 56127 Pisa, 

Italy LEEMAN. W., P. Keith-Wiess Geological 

Laboratory. Rice University, Houston, TX 77251, USA~ 

PENNISI, M. and TONARINI. S .• 1st. Geocronologia, 

CNR, 56127 Pisa, Italy. 

New boron isotopic data are presented bearing on the origin 

of fluids from Vulcano crater. Changes in composition and 

temperature of fumarolic emissions in this century. and 

correlations of these data with seismicity indicate that 

periodic reconfigurations of the hydrothermal system 

occasionally occur involving the development and/or the 

sealing of permeability connecting different reservoirs. An 

increase of steam output since 1987 reflects an increased 

input of water into the system. Evidence for contributions 

from a degassing magma body at depth remains inconclusive. 

Boron isotopic measurements were undertaken to 

characterize the sources of boron in the fumarolic fluids. 611B 

data are presented as permit deviation of 11B/10S relative to 

NBS SRM 951. Typical values are +40 for seawater, ± 5%o 

for volcanic rocks, from 0 to -20%o for sediments. 6 11B 

values in condensates from a single fumarole (F5) have 

generally increased with time (-4 to +3%o) since 1978~ this 

trend is interrupted by significant decreased (up to 3%o) in 

1985, 1989, and early 1993. The reversal in 1985 was 

associated with intense sismic activity and increased C02 

content of the fluid, which suggests an increased magmatic 

contribution. 

These trends suggest mixing between fluids having at least 

two distinct compositions: (1) a high 611B fluid like seawater 

(possibly modified by extensive interaction with igneous 

rock). and (2) a low 6 11B component linked to direct 

magmatic fluid or fresh-water/rock interaction processes. 

Analyses of 1993 fluids from higher temperature fumaroles 

(FO. FA. F5AT) show only minor differences in 611B (<1%o) 

from the F5 fluids. Significantly lower 611B values (-8 to 

-10%o) were measured in sublimates (sassolite) collected in 

quartz tubes from the same fumaroles. The difference between 

sublimates and aqueous fluids suggests that isotopic 

fractionation may be significant between these phases. 
Sr, Nd, 0 AND S ISOTOPIC SIGNATURES AND THE 

MANTLE SOURCE FOR UL TRAPOTASSIC SYENITIC 

MAGMAS IN NORTHEASTERN BRAZIL 

FERREIRA. V.P.; SIAL, AN., Stable Isotope Lab., Dept. 

of Geology, UFPE, C.P. 7852, Recife, PE, 50732-970, 

Brazil; LONG, L.E., Univ. Texas, Dept. of Geological 

Sciences, Austin, TX, 78713, USA; PIN, C., Univ. Blaise 

Pascal, 5, rue Kessler, 63038, Clermont-Ferrand, Cedex, 

France 

Elongate plutons consisting of alkali-feldspar syenitic rocks 

containing alkalic pyroxenite as enclaves and syn-plutonic 

dikes, form a unique Late Proterozoic plutonic ultrapotassic 

province, along the southern boundary of the CachoeirinhaSalgueiro foldbelt, northeastern Brazil. Coexisting syenite 

and pyroxenite have same mineral phases (aegirine-augite, 

microcline, apatite and sphene) at different modal amounts. 

Field and textural relationships indicate that they were mostly 

in liquid state when came into contact with each other. 

Angular to sub-rounded mica pyroxenite xeno1iths, up to 10 

em-long, are found in some of the plutons. They consist of 

pale-green pyroxenite containing major diopside-salite, that 

forms "mosaics" in which phlogopite, and occasionally calcite 

and feldspar, are interstitial, replacing pyroxene along its 

grain boundaries, a textural relationship which suggests metasomatic replacement. 

Syenite and pyroxenite have high pyroxene-corrected 8180, 

with limited variation (+8.1 to + 8.5%osMow in syenite and 

+7.6 to +7.7%o in pyroxenite), 834S (+ 12.3%o in syenite and 

+ 11.2%ocnT in pyroxenite), and initial 87Sr/86Sr ratio 

(0.7098, syenite and pyroxenite lying on the same Rb-Sr isochron), and low sNd (-15.3 to- 17.2 in syenite and -16.1 in 

pyroxenite). Whole-rock 8180 for mica pyroxenite xenoliths 

varies from +7.5 to +9.4%osMow· Syenite and pyroxenite 

have similar LREE-enriched chondrite-normalized patterns 

and are large-ion lithophile element-enriched. Xenoliths show 

significant incompatible element concentrations when compared to normal mantle compositions, particularly Ba, La, K, 

Ce, Nd, Sm, Sr andY. 

Geochemical and isotopic signatures for the ultrapotassic 

syenite magma suggest that it derived from an enriched mantle source. Such mantle enrichment likely resulted from hybridization by addition of crustal material via subduction, 

producing mica pyroxenite masses, at around 2.1 Ga, as estimated from Nd crust-formation model ages. Reactivation of 

extensive, deep shear zones seem to have triggered melting of 

these mica pyroxenites producing ultrapotassic magma. Shear 

zones were the pathway for emplacement of this magma, 

during the late stage of the Brasiliano (= Pan-African) orogeny, around 580 Ma, as indicated by the Rb-Sr isochron age 

for the syenites. 

99 

STABLE ISOTOPE ANALYSES OF LATE-GLACIAL TO 

HOLOCENE LACUSTRINE SEDIMENTS FROM 

DIFFERENT LAKES IN THE JURA (SWITZERLANDFRANCE) 

FILIPPI M. L., Inst. de Mineralogie et Petrographie, 

Universite de Lausanne, BFSH2 Lausanne, 1015-CH, 

Switzerland, and HUNZIKER, J. C., same address. 

Stable isotope analyses were made of whole-rock lacustrine 

chalks and shell materials from three lakes in Switzerland and 

in France for the purpose of obtaining paleoclimatic 

information. 

Detrital carbonate components can mask or obliterate the 

isotopic record of a lake, particularly a large lake (e.g., Lake 

Leman, SW Switzerland) where carbonates outcrop in the 

hydrological catchment area. In addition, tests must be made for 

the presence and quantity of aragonite in the composite samples 

(e.g., Lake Ilay, France). In the present study, X-Ray analyses 

was used to identify and quantify such components. In 

favourable cases, it may be possible to separate authigenic 

calcite from detritic phases. 

The temperature of bottom waters of the lakes is relatively 

constant. A comparison was therefore made between the oxygen 

isotope composition of bulk carbonates and benthic mollusks 

in order to distinguish between temperature and isotopic 

composition of the environmental water. 

From the lack or presence of covariance between 5180 and 

a13c values, it was possible to distinguish between open- and 

closed-system behaviour of the lakes. 

Coupled X-Ray and isotopic analyses were made of a core 

12 m long from the peri-Jura Lake Loclat (Neuchatel, 

Switzerland, Rolli et al., in press). The bottom part of the core 

is rendered useless for paleoclimatological work because of the 

presence of a large detrital component which is reflected by 

abnormally high values of o18o and o13c. In the upper part of 

the core, there is a well-delineated lowering of about 2 °/cx> in 

o 18o of bulk authigenic carbonates that corresponds to the 

Younger Dryas biozone. Two other smaller shifts in o18o are 

related to the "Gerzensee" (Aller0d-Younger Dryas) and 

"Aegelsee" (B0lling-Aller0d) oscillations. There is a rather 

abrupt decrease in 013C ValUes from -2.5 tO -8.0 °/cx> after the 

Boreal. This large change is interpreted to reflect a major 

change in bio-productivity and conforms with sedimentological 

data. 

The covariation observed between o18o and ol3c (R=0.83) 

over the interval 380 em to 0 em is interpreted to reflect 

precipitation of carbonates from a closed lake that experienced 

no fundamental change in basin hydrology over this period. 

Absence of a similar trend before the Younger Atlantic suggests 

an open lake system complicated by climatic changes. 

Rolli, M. et al., (in press), Late-glacial climatic oscillations 

recorded in lake sediments of Loclat (Neuchatel, 

Switzerland): sedimentology, palinology, mineralogy, 

geochemistry and isotopic evidences. Preliminary data: II 

Quatemario 
lOBe and 36cl in the GISP2 Ice Core 

R FinkeJl, K Nishiiiumi2, M Caffee1, C Kohl,3 and 

J Southon I 

1 LLNL, PO Box 808 L-237, Livermore CA 94550 

2Space Sciences Laboratory, UC Berkelely, 94702 

3UCSD, La Jolla, CA 92093 

Cosmogenic radionuclide concentrations in ice cores 

can be used to study the history of solar activity, of 

the geomagnetic field and of climate. With. these 

objectives in mind, we are currently measuring a 

continuous profile of lOBe and 36Cl in the 3.05 kmlong GISP2 ice core, from the ice cap summit in 

central Greenland. The. time resolution of our 

sampling varies from annual in the upper part of the 

core to centennial in the lower part. The annual 

samples show variations correlated with the 11-year 

Schwabe solar activity cycle. The lOBe and 36Cl 

concentrations also show significant climate-related 

variations. These variations are correlated with the 

climate swings (Oldest Dryas - B~lling-Aller~d and 

Younger Dryas - Preboreal) which occurred at the 

end of the last glacial period. Using the high 

resolution depth-age relation determined by annual 

layer counting on the GISP2 core[1], we have 

calculated radionuclide fluxes on a sample by 

sample basis in order to examine the role of 

changing annual layer thickness in determining the 

changes in radionuclide concentration. We have also 

revisited the variabiity in the 10Bef36Cl ratio in ice 

cores [2, 3]. We find that the ratios observed in these 

samples are significantly less variable than has been 

observed in cores from other sites in Greenland. 

References: [1] Alley, R.B., et al. Nature 362, 527-

529 (1993); [2] Elmore, D., et al., Nucl. Inst. Meth. 

B29, 207-210 (1987). [3] Suter, M., et al., Nucl. 

Instr. Meth. B29, 211-215 (1987). 

This work was performed under the auspices of the 

U.S. Department of Energy by the Lawrence 

Livermore National Laboratory under contract W7405-Eng-48 and supported by the NSF Division of 

Polar Programs. 

100 

THERMOCHRONOLOGIC CONSTRAINTS ON POSTPALEOZOIC TECTONIC EVOLUTION OF THE CENTRAL 

TRANSANTARCTIC MOUNTAINS, ANTARCTICA. 

FITZGERALD P.G . Dept. of Geosciences, University of 

Arizona, Tucson, AZ 85721, USA 

The development of topography is one of the most visible 

and obvious effects of the forces that drive plate tectonic 

processes throughout geologic time. Determining the age of the 

development of topography is important as it has fundamental 

implications for regional geological history, as well as a myriad 

of subsidiary processes, one of the most obvious being the 

effect that topography has on the world climatic systems. The 

-4500 km long Transantarctic Mountains (TAM) bisect 

Antarctica dividing it into two distinctly different geologic 

terrains, and acting as a buttress against which the East 

Antarctic Ice Sheet rests. The roots of the mountains are a late 

Proterozoic-early Paleozoic orogenic belt (Ross Orogen), but 

the present day range is a rift shoulder stemming from uplift 

episodes begun in the Cretaceous. 

Apatite fission track thermochronology on samples collected 

from the central TAM records a complex thermo-tectonic 

history for this region over the past 350 Ma. This part of the 

central TAM can be divided into a number of semi-independent 

fault blocks on the basis of topography and sub-glacial 

morphology. Apatite ages in the Miller Range vary from -250 

to -350 Ma and record an uplifted fossil apatite partial 

annealing zone formed subsequent to cooling of Ross Orogen 

granitic intrusives, peneplanation of those granites to form the 

Kukri Peneplain, and deposition of Devonian through Triassic 

Beacon Supergroup sediments. A period of Cretaceous uplift 

( -1.5-2 km), beginning at about 115 Ma, is recorded in data 

from Moody Nunatak on the inland side of the Queen Elizabeth 

Range. Uplift at this time may be related to the formation of a 

broad, southward plunging syncline in the Kukri Peneplain. 

Timing of the formation of this syncline is problematical and it 

may only be an apparent syncline formed by differential and 

variable block uplift associated with uplift of the mountains. 

Along the Beardmore Glacier, samples record rapid cooling 

indicative of uplift initiated in the early Cenozoic (-50 Ma). 

The amount of uplift -70 km inland of the coast in the Queen 

Alexandra Range since the early Cenozoic is -7 km, with the 

likelihood of -3 km more at the coast. Eastward facing 

topographic escarpments in the Queen Alexandra Range mark 

the likely position of steeply dipping normal faults, which 

offset apatite ages. Ages on the east side of the Beardmore 

Glacier mouth are generally younger (average 27 Ma) than on 

the west side (average 33 Ma) interpreted to reflect a greater 

amount of denudation. Assumptions made regarding the use of 

an assumed paleogeothermal gradient and paleo-land surface 

elevation are tested with available geologic evidence. Fission 

track data do not constrain whether there has been significant 

surface uplift of the TAM since the Pliocene as suggested by 

other evidence. The results build upon the available fission 

track database along the TAM and emphasize the subtle 

variability of uplift along the TAM due to episodic uplift 

involving differential block movements, both of which are 

important constraints on models proposed for the uplift of the 

TAM. 
CHRONOLOGY OF MULTIPLE INTRUSION IN THE 

TUOLUMNE INTRUSIVE SUITE, YOSEMITE 

NATIONAL PARK, SIERRA NEVADA, CALIFORNIA 

FLECK. Robert J. and KISTLER, Ronald W., Branch 

of Isotope Geology, U.S. Geological Survey Mailstop 937, 

345 Middlefield Road, Menlo Park, CA 94025, U.S.A. 

The Tuolumne Intrusive Suite is a zoned intrusive 

complex within the Sierra Nevada batholith. The zonation is 

lithologic, mineralogic, chemical, and isotopic, forming an 

irregular, ~ 1500 km2, plutonic complex elongate parallel to 

the regional structure. From the margin to the center of the 

suite, lithologically and mineralogically distinct zones are 

mapped as the Sentinel Granodiorite, granodiorite of 

Yosemite Creek, granodiorite of Kuna Crest, Half Dome 

Granodiorite, Cathedral Peak Granodiorite, and Johnson 

Granite Porphyry. Published age data are consistent with 

emplacement of the entire intrusive suite during a 

geologically brief interval at about 87±4 Ma. 40 Arf39 Ar and 

Rb-Sr mineral ages reported here, however, decrease from 

about 89 Ma at the margin of the intrusive complex to less 

than 81 Ma near the center, requiring either a prolonged, 

variable-rate, inward-progressing cooling or a much more 

complicated, multiple-intrusion history. Geobarometry 

indicating emplacement of the intrusive suite at ~ 7 km does 

not permit holding temperatures in a single intrusion above 

300°C for an 8-m.y. period, even with a 10-km plutonic 

radius. Miarolitic cavities in the youngest unit of the 

intrusive suite, the Johnson Granite Porphyry (80.5 Ma, K-Ar 

on biotite), suggest a shallow emplacement and raise further 

questions on the efficacy of an extended, lateral cooling rather 

than more rapid, vertical heat-loss to the surface. 

Patterns of mineral-age discordance indicate that most of 

the Tuolumne Intrusive Suite formed during three major 

intrusive pulses with maxima at about 89 Ma, 86 Ma. and 84 

Ma with the Johnson Granite Porphyry emplaced at 81 Ma. 

Coeval intrusive units defined by the distribution of mineral 

ages are consistent with chemical, isotopic, and geophysical 

data, but not all correlate with previously-mapped lithologic 

contacts. Based on zircon U-Pb and hornblende 40 Arf39 Ar 

ages, the 89-Ma group includes the outer three units and the 

outer part of the Half Dome Granodiorite. The inner part of 

the Half Dome Granodiorite and the outer part of the 

Cathedral Peak Granodiorite were emplaced sequentially at 

about 86 Ma with little or no disturbance of hornblende 

isotopic systems in the older units. 40Arf39Ar, K-Ar, and RbSr ages of biotite within 1-2 km of the 86-Ma group, however, 

are reset. U-Pb ages on sphene and 40 Arf39 Ar age spectra of 

hornblende from the younger, inner part of the Cathedral 

Peak Granodiorite indicate that it was emplaced at about 84 

Ma. Biotite ages of the 86-Ma group reflect similar patterns 

of discordance due to thermal effects of this 84-Ma body. 

Interpretation of an 80.5-Ma biotite K-Ar age as the time of 

emplacement of the Johnson Granite Porphyry is consistent 

with its shallow intrusion and with similar ages for Rb-Sr and 

K-Ar on biotite and microcline in adjacent rocks of the inner 

part of the Cathedral Peak Granodiorite that it intrudes. 

Minerals interpreted to be partially reset by younger phases of 

the suite commonly yield discordant 40 Arf39 Ar age spectra. 

101 

Isotope Geochemistry of Antarctic Flood Basalts 

(Ferrar Group): establishing the effects of 

alteration and upper crustal contamination and 

constraining the source signature. 

FLEMING. T.H., FOLAND, K.A., ELLIOf, D.H. and 

HEIMANN, A. Byrd Polar Research Center and Department of Geological Sciences, Ohio State University, 

Columbus, OH, 43210, USA. 

Jurassic tholeiites of the Ferrar Group, with volume of 

more than 5 x 1 os km3

, crop out in a linear belt extending 

over 3000 km along the Transantarctic Mountains. They are 

represented by flood basalts, sills and a large layered basic 

intrusion and are characterized by enriched crustal-type traceelement ratios and radiogenic isotopic compositions (

87Sr/86Sr 

= 0.709 to 0.713, fNd = -4.0 to -5.9). Considerable uncertainty and debate has revolved around the age, the significance of variations in Sr and 0 isotopic compositions, and the 

meaning of the apparent elevated initial 87Sr/86Sr ratios. Our 

recent work on the basalts has demonstrated: the formation of 

most of the province in a restricted interval ( < 1 m. y.) at 

177 Ma from 40 ArP9 Ar measurements on plagioclase; alteration in the mid-Cretaceous is based on several lines of evidence, including direct 40 ArP9 Ar and Rb/Sr dating of secondary minerals; and, the production of o180 variations by low 

temperature alteration at the surface. New Sr and Nd isotopic 

study of both rocks and separated minerals from north Victoria Land and the central Transantarctic Mountains further 

clarify: the effects of alteration; the role of upper crustal 

assimilation; and, constrain the apparent isotopic composition 

of primitive magmas. We demonstrate that some of the 

previously observed variations in calculated initial 87Sr/86Sr 

ratios reflect alteration but that 143Nd/144Nd ratios are not 

similarly affected. Many calculated initial 87Sr/86Sr ratios are 

simply incorrect due to post-formation changes in Rb/Sr; this 

has masked petrogenetic relationships. Good correlations 

exist between initial 87Sr/86Sr, fNd, and major and trace 

element chemistry. Isotopic-chemical relationships are consistent with upper crustal evolution by an assimilationfractional crystallization process involving a crustal end 

member similar to ~ 500 Ma basement granitoids of the Ross 

Orogen. This mode of evolution appears remarkably uniform 

over much of the province. The composition of the most 

primitive end member, representing the least contaminated 

magma, is 87Sr/86Sr = 0.7087, €Nd = -4.0, Mg# = 70, and 

Zr = 50 ppm. The earlier (e.g., lower crustal and upper 

mantle) petrogenetic history of the primitive magma remains 

uncertain. Their isotopic and geochemical characteristics 

could potentially be attributed to several processes. Coincidence of the outcrop belt and chemical and isotopic similarities to the Ross Orogen granite suite suggests that the enriched isotopic characteristics of these rocks are either inherited by earlier assimilation or maybe derived from an enriched 

source that was impacted by material from the Ross Orogeny. 
PHYSICAL CONTROLS ON THE COMPOSITION 

GRANITIC MAGMAS: THE ROLE OF OROGENIC 

STRAIN HISTORIF..S AND THE STATE OF THE 

LITHOSPHERE. 

FODEN, J.D., Dept. of Geology and Geophysics, 

University of Adelaide ,Box 498 GPO Adelaide, South 

Australia, Australia, 5001. 

Orogenic belts often experience early syn-tectonic 

intrusion of granites of 1- to S-type composition and may have 

undergone rapid transition from previous extensional 

sedimentary ba~ins with active mafic magmatism. They may 

also abruptly end their active deformation with a phase of rapid 

uplift and the post-tectonic appearance of bimodal magmatism 

in which the granites (or their volcanic equivalents) have Atype characteristics. 

These granites are related to influx of heat carried by 

mafic magmas and their chemistry reflects the degree of 

interaction between crust and mantle-derived melts. The 

proportion of crustal conlribution is maximal during maximum 

crustal thickening, but is minor in pre- and post-deformational 

regimes. 

Simple calculations based on the conductive cooling 

of initial sill-like mafic intrusions were a basis for the 

modelling, as a function of time, of the following parameters: 

residual melt composition, crystallisation and fractionation, 

wa11-rock heating and melting, assimilation, density, viscosity, 

Stokes Law buoyancy, Nd-Sr isotopic composition and zircon 

inheritance. Results imply that the cooling history of mafic 

intrusions falls into two classes: 

1. Those which heat significant volumes of their roof rocks to 

temperatures > crustal minimum melting temperatures (700 degree.~). 

2. Those for which the crustal minimum melting isotherm 

always falls within the margins of the intrusion. 

These distinctions are strongly depth dependant and are fairly 

independent of geothermal gradienL 

Type 1 magma chambers arise when mafic magmas 

stall at the density step at the Moho. These will evolve to 

produce I-S type granite melts with Si02 contents> 67% with 

masses equivalent to > 70% of the initial mafic body in 

between 0.3 and 1.5 Ma (±0.5 Ma). They will have Pb-, Srand Nd isotopic compositions which are strongly influenced by 

the crust and contain significant proportions of inherited 

zircons. 

Type 2 magma chambers arise when the crust is, 

undergoing extension, and mafic magmas experience only 

lithostatic loads and travel via dilatant fractures to levels 

approaching their levels of neutral buoyancy. Intrusions 

experience only slight crustal contamination and may develop 

tohighly fractionated melts (A-type granite, rhyolite, dacite, 

latite or trachyte). They experience rapid decline in melt mass 

reaching 70% silica with between 20 and 40% of their original 

mafic parent mass and have few inherited zircons. 

This model fits well with recent descriptions of Atype granites developed during advancing Tertiary crustal 

extension prior to the development of the Red Sea. It is also 

interesting to note that the Australian Palaeo- to 

Mesoproterozoic is characterised by extensive intmcratonic Atype granite magmatism evolution from mid- to high level 

crustal mafic intrusions and by a pervasively thinned 

lithospheric structure. By contrast the Palaeozoic of eastern 

Australia is dominated by 1-S type granites suggesting more 

crustal interaction and thicker , cooler lithosphere. 

102 

INTERPRETATION OF 40 Arf39 Ar AGES AND 

INCREMENTAL-HEATING DATA IN THE 

CONTEXT OF Ar VOLUME DIFFUSION 

FOlAND, K.A., Department of Geological Sciences, 

Ohio State University, Columbus, OH, 43210, USA. 

Transport of Ar in minerals by volume diffusion 

processes is generally viewed as important to Ar losses in 

nature and also during laboratory experimentation. Volume 

diffusion transport leads to the concept of a closure 

temperature and, as well, to straightforward modeling. 

However, there are several reasons and factors that make 40 Arf9 Ar incremental-heating relationships much more 

complicated than described by elementary theory. These 

include: (1) the mechanical degradation of mineral grains 

and reactions that during laboratory heating; (2) the 

heterogeneities on various scales in the physical and 

chemical characteristics of almost all natural samples; (3) 

sample reaction and the formation of grains below the 

characteristic blocking temperature in nature; ( 4) the 

occurrence of Ar in inclusions and other microstructural 

features; (5) the effects of neutron irradiation especially 39 Ar recoil and redistribution; and, (6) trapping of Ar in 

defects and such features as embedded inclusions, sealed 

micropores, or healed cracks. These factors are important 

not only for the hydrous minerals (micas, amphibole) but 

also anhydrous ones (feldspars). They affect both the ages 

and the kinetics of Ar release. Further, they combine to 

make the classic "Ar loss" incremental-heating spectrum 

basically a concept that is virtually never applicable to 

natural samples. Such spectra appear to result from the 

temperature dependence oflaboratory degassing as a 

consequence of phase instability or particle size. As a 

result, spectra are commonly convolutions of gas released 

from heterogeneous samples. The physical processes 

operating during laboratory heating as well as in nature 

must be realistically taken into account in the interpretation 

and modeling of incremental-heating age and kinetics data. 
SR-ND-PB ISOTOPE AND TRACE ELEMENT 

CONSTRAINTS ON ASSOCIA1ED HFSE VARIABLY 

DEPLE1ED SHOSHONITIC VOLCANISM IN THE 

HALMAHERA ARC, INDONESIA 

FORDE E.J., THIRLWALL M.F., and VROON P.Z., 

Dept. of Geology, Royal Holloway, University of London, 

Egham, Surrey, U.K, and HALL R., Dept of Geological 

Sciences, University College, London, U.K 

The Halmahera arc is an intraoceanic calc-alkaline arc with 

a wide range in K20 contents, ranging from medium-K to 

shoshonitic values. Volcanism dates from the early Neogene in 

the south of the arc to the Quaternary in the north, where it is 

still currrently active, due to the ongoing subduction of the 

Molucca Sea plate. 

A relationship between K content and depth to the Benioff 

zone is not seen; instead shoshonitic volcanism is restricted to 

the south-western area of the island of Bacan at the arc front, 

where lavas have been dated at 6.7Ma- 0.9Ma. They are 

characterised by large enrichments in LILE and LREE, 

particularly Th, Pb, Sr, Ba, La, Ce and Nd. Sr-Nd-Pb isotopic 

systematics have been used to divide the shoshonites into three 

groups, and to constrain the sources contributing to their 

geochemical characateristics. 

Group I has low 87Sr/86Sr (0.7031- 0.7036), in 

comparison with the rest of the arc (0.7038- 0.7044), despite 

similar 143Nd/144Nd (0.5129). They also show the greatest 

enrichments in LILE mtd LREE (eg Sr 1700-2600ppm, La 50-

80ppm). Group II, however, has relatively radiogenic 

87Sr/86Sr (0.7050- 0.7055) and 206Pb/204Pb (18.7- 18.8) and 

lower 143Nd/144Nd (0.5127). LILE and LREE enrichments are 

also lower than Group I (eg Sr 850-1400ppm, La 35-45ppm). 

Various different sources and/or magmatic processes are 

therefore considered to account for the isotopic variation of the 

shoshonites themselves, and relative to the rest of the arc. 

Isotopic variation between Group I and II can only be 

explained by mixing with a large proportion (up to 50%) of a 

crustal component, due to the high Sr-Nd-Pb concentrations in 

the most depleted end-member. However, this crustal 

component is not recognised elsewhere in the medium to high 

K volcanics. The isotopic and trace element variation between 

the shoshonites relative to the rest of the arc must be due to 

the involvement of different sources. Elevated BalLa (20-40) 

for the major part of the arc suggests a dominant influence 

from fluids derived from the subducted slab, whereas reduced 

BalLa (10-20) and extreme La/Nb values (10-22) favour a more 

LREE-enriched source for the shoshonites, within the mantle 

wedge. 

Finally Group III shoshonites have intermediate 87/86 

(0.7041), 206/204 (18.6) and high 143/144 (0.5129), 

comparable with Group I. These volcanics however show 

relative enrichment in HFSE, (eg Nb=10-18ppm). This 

suggests a controlling influence on the HFSE concentrations 

by a suitable mineral phase, such as phlogopite in an enriched 

source in the mantle wedge. Residual phlogopite in the mantle 

wedge thus produces Group I and II, whilst melting of 

phlogopite within the source produces the high Nb shoshonites 

of Group III. 

103 

THERMOCHRONOLOGIC CONSTRAINTS ON THE 

DEVELOPMENT OF METAMORPHIC CORE 

COMPLEXES IN THE LOWER COLORADO RIVER 

AREA 

FOSIER. D. A., VIEPS, School of Earth Sciences, La 

Trobe University, Bundoora, Victoria 3083, Australia, 

HOWARD, K.A. USGS, Menlo Park, CA 94025, JOHN, 

B.E., Department of Geology and Geophysics, University 

of Wyoming, Laramie, WY 82071. 

The recent literature abounds with models for the 

development and exhumation of metamorphic core complexes. 

Thermal histories of footwall and hanging wall rocks 

determined by 40 Arf39 Ar and fission-track thermochronology 

provide valuable constraints on the nature of extension and 

evolution of metamorphic core complexes. In addition to 

dating the timing of extension in core complexes, 

thermochronologic data reveal information about rates of 

cooling, exhumation, and fault displacement, as well as the 

geometry of faults and paleogeothermal gradients. 

Major extension in the lower Colorado River area, 

California and Arizona, occurred between -24 and -14 Ma, 

based upon geologic and thermochronologic data. Biotite and 

K-feldspar K-Ar and 40Art39 Ar apparent ages, and zircon and 

apatite fission-track apparent ages from the Chemehuevi, 

Sacramento, Whipple, Buckskin, and Harcuvar Mountains 

metamorphic core complexes consistently young in the 

displacement direction (southwest to northeast) of exposed 

detachment faults. An abrupt change in the age/distance 

gradients at 24-20 Ma for biotite and high temperature Kfeldspar data marks the onset of rapid extension, while linear 

trends with slopes of -3-8 km/m.y. for low temperature Kfeldspar and fission-track data in these complexes estimate 

displacement rates on detachment faults. 

Gradients of age vs. slip direction across tilted upper plate 

fault blocks reveal information about the temperature variation 

with depth at the onset of extension, when the amount of tilting 

is known from paleohoriztonal indicators. These trends can be 

used to estimate pre- to syn-extensional geothermal gradients 

when more than one mineral curve yields the characteristic 

"break-in-slope", indicating the depth at which pre-extensional 

ages were zero. In the Mohave Mountains (tilted -90.) the 

paleodistance between intercepts of biotite and K-feldspar (low 

temperature steps) apparent age trends with the time of tilting 

(a difference of -1oo·c) is -4 km, suggesting a synextensional gradient of -25.C/km. Similarly, apatite and 

zircon fission-track data from the Tortilla Mountains 

(southeast of the lower Colorado River area) give a gradient of 

-18"C/km, and apatite and K-feldspar ages from ·the Piute 

Mountains reveal a paleogradient of 25" -40"C/km. 

The dip at which brittle detachment faults operate in the 

upper crust is still highly controversial. In the Chemehuevi 

Mountains the thermochronologic data reveal a trend of 

increasing temperature in the slip direction from -200• to 

350" -400"C over an exposed distance of 23 km, at the onset of 

extension. Assuming the geothermal gradients listed above, 

the relatively gradual increase in temperature with original 

depth, suggests only gentle southwest titling of the footwall 

and constrains the Chemehuevi detachment fault to have had 

an original dip of -15" -30". 
A NEOGENE RECORD OF olSC OF TERRIGENOUS 

ORGANIC CARBON BURIED IN THE BENGAL 

FAN: IMPACT ON CARBON CYCLING 

FRANCE-LANORD C .. CRPG-CNRS, BP20 54501 

Vandoeuvre - France (cfl@crpg.cnrs-nancy.fr) 

and DERRY L.A., Cornell U., Dept. of Geological 

Sciences, Ithaca, NY 14853-1504 - USA. 

Sediments deposited in the Bengal fan and 

recovered by ODP Leg 116 contain an 18 Ma record of 

erosion from the Himalaya. Estimates of organic 

carbon burial rates suggest that the Bengal Fan has 

been a sink for 10-15% of global OC burial during the 

Neogene. Previous studies including gas 

chromatography and pyrolysis imply that the organic 

carbon is dominated by terrigineous sources 

(Poynter & Eglinton, 1990, ODP Sc. Res. 116). Thus 

the isotopic composition of organic carbon in the 

Bengal Fan should reflect the POC load of the 

Ganges-Brahmaputra rivers. We report ol3c 

analysis of total organic carbon (TQC) buried in the 

Bengal fan, in order to obtain a time series of the 

river inputs. TOC ranges from 0.5- 2.0 wt. %. ol3C 

values are near -25%o between 18 and 7 Ma. Values 

shift dramatically to -19%o by 6 Ma, reaching a 

maximum of -15%o at 2.4 Ma. ol3c in Pleistocene 

samples returns gradually to depleted values. 

The > 10%o positive shift beginning at 7 Ma 

mirrors similar magnitude changes in the ol3c of 

paleosol carbonates from the Siwaliks (Quade et al., 

1989). This change reflects the radiation of C4 

plants. Our data show that the export of terrigineous 

carbon from India was dominated by C4 material 

during the Pliocene. Mass balance calculations 

suggest that the isotopic change in river OC can 

account for a +0.5%o shift in the late Mio. marine 

carbonate ol3c record. 

0 

2 

.-.. 

ctS 4 

~ 

-Q) 6 

C) 1.\ <( '=/ (. 

8 .t. 

' 10 (. 

-27 -25 -23 -21 -19 -17 -15 

o13CTOC 

104 

OXYGEN ISOTOPIC COMPOSITION OF ORDINARY 

CHONDRITES -A MINERALOGICAL CONTROL? 

FRANCHI. I.A., PILLINGER, C.T., Planetary Sciences 

Unit, Dept. of Earth Sciences, Open University, Milton 

Keynes, MK7 6AA, UK, BRIDGES, J.C. and HUTCHISON 

R., Dept. of Mineralogy, Natural History Museum, London, 

SW7 5BD, UK. 

The oxygen isotopic variation of. ordin~ chondrites is 

limited to only 2-3o/oo in both ol70 and olSo, with the H, L 

and LL groups occupying distinct fields in the oxygen three 

isotope plot (1). However, Clayton and co-workers have also 

shown that the range in o17 0 and o18o of chondrules from 

unequilibrated ordinary chondrites (UOCs) is the same for all 

three groups and define a mixing line of slope approximately 

0.75, with no clear variation with host meteorite type or 

chondrule composition (eg. 1,2). However, one parameter 

which was found to influence oxygen isotopic composition of 

UOC chondrules was their size, the smallest chondrules falling 

at the lighter end of the mixing line, the largest towards the 

heavier end. The explanation favoured by Clayton and coworkers is that larger chondrules are more processed and have 

undergone a higher degree of exchange with a relatively heavy 

gas phase. 

Recently we reported the highest ever o 1 7 0 and o 18o 

values in cristobalite-rich material separated from chondrules in 

UOC Parnallee (LL3.6) using a laser micro-analysis technique 

(3). These chondrules define a mixing line (dependent on the 

proportion of cristobalite) which is an extension of the mixing 

line defined by chondrules from all UOCs, but with a o18o 

5o/oo heavier than any other chondrules. A cristobalite-bearing 

sample from the L5 ALH76003 ordinary chondrite has an 

enrichment approximately half that seen in the Parnallee 

chondrules (4). Clues to the origin of this enrichment may 

come from density fractions of the Mez6-Madaras L3 meteorite 

which have oxygen isotopic compositions which also fall 

close to the same mixing line, with the densest fractions 

(olivines and pyroxenes) at the isotopically lighter end and the 

least dense fractions (feldspar with glass) towards the heavier 

end (5). The relative positions of the various mineralogical 

components of chondrules in UOCs along this mixing line 

starting at the isotopically light end are olivine, pyroxene, 

feldspar (with glass) and finally cristobalite. 

There are a number of processes which could have produced 

mineralogical ordering of oxygen isotopic compositions. 

Perhaps one of the most intriguing is that the order of 

increasing o17o and olSo is essentially that of increasing 

diffusion co-efficients of oxygen in the various mineral phases . 

This would support the view that the oxygen isotopic 

composition of these phases was controlled by some exchange 

process between isotopically light solids and an isotopically 

heavier gas reservoir (1,6). 

We are currently separating additional phases from Parnallee 

and other UOCs to determine how common or variable these 

effects are in UOCs in order to explore further mineralogical 

controls responsible for this isotopic variation. 

(1) Clayton, R.N. et al. 1991, GC.A. 55, 2317-2337. (2) 

Gooding, J.L. et a/. 1983, 65, 209 - 224. (3) Bridges, 

J.C. et al. 1993, Meteoritics 28, 329 - 330. (4) Olsen, 

E.J., et a/. 1991, Lett. 56, 82-88. (5) Mayeda, T.K., 

Clayton, R.N. and Sodonis, A. 1989, Meteoritics 24, 

301. (6) Clayton, R.N. 1993, Ann. Rev. Earth Planet. 

Sci. 21, 1l5 - 149. 
THE STABLE CHLORINE ISOTOPIC 

SIGNATURE OF GROUNDWATERS IN 

CRYSTALLINE SHIELD ENVIRONMENTS 

FRAPE. S.K., BRYANT, G., Dept. of Earth 

Sciences, University of Waterloo, Waterloo, 

Ontario, N2L 3G I, Canada, and 

BLOMQVIST, R., Geological Survey of 

Finland, Betonimiehenkuja 4, SF -02150 Espoo, 

Finland. 

The evolution and origin of groundwaters in 

crystalline shield environments has been an issue 

for a number of years. Recent studies have 

expanded the use of stable chlorine isotopes to 

evaluate the evolution of saline waters and brines in 

the Canadian and Fennoscandian Shields. 

Groundwaters in ancient Shield environments are 

composed of Ca or Na and Cl; often with dissolved 

loads in excess of I 00 g 1-1. The origin of 

salinities in excess of sea water salinity has been 

attributed to a variety of processes; but the effects 

of water /rock interaction are generally assumed to 

overprint any original fluid signatures. 

The 37/35 Cl signatures for each Shield are 

quite different. Canadian Shield brines are almost 

all isotopically negative with values ranging from -

I.5 to SMOC. Fennoscandian Shields brines are all 

positive, with values ranging from SMOC to + 2.0. 

Most field sites have distinctive isotopic signatures 

and generally more mafic, volcanogenically open 

systems tend to have more positive isotopic 

signatures. 

As the major anion, chlorine has a limited 

number of possible sources. The seawater stable 

chlorine isotopic signature is assumed to be constant 

through time, although this is not proven. 

Therefore changes in chlorine isotopic ratios within 

shield rocks can be attributed to various processes 

such as boiling - volatilization rock-water exchange 

and mixing of very different waters through time. 

The data available from our studies suggests that all 

of these processes play a role in modifying the 

isotopic signatures observed. 

105 

RB-SR DATING OF HYDROTIIERMAL nLITES - PoTENTIALS, 

LIMITATIONS AND POSSIBLE CONSTRAINTS ON THE GENESIS OF A 

MINERALIZING FLUID; AN ExAMPLE FROM THE K.ASSANDRA 

MINING AREA (GREECE) 

FREI R, Group for Isotope Geology, MineralogicalPetrographical Institute, University of Berne, Berne, 3012, 

Switzerland 

The Kassandra mining district in Northern Greece consists of 

porphyry copper mineralizations, proximal copper skarns and 

distal high-temperature carbonate hosted Pb-Zn-Ag-Au 

replacement ores. Polymetallic mineralization is temporally 

related to the emplacement of granodioritic to quartz dioritic 

porphyries (24-25 Ma; Gilg and Frei (1993)), Tompouloglou, 

1981). Andesite porphyry dikes, which crosscuts the Pb-ZnAg-Au ores and associated alterations, represent the last 

magmatic phase in the area (19.1±0.6 Ma, Gilg and Frei 

(1993)). The Pb-Zn (Ag, Au) mineralization forms complex, 

structurally controlled replacement bodies in marbles and 

meta-pegmatites/ meta-aplites at a distance of 2-3 km from 

the porphyry copper system. Rb-Sr analyses were performed 

on different illite size fractions of two samples from the 

altered contact zone. Considerable amounts of adsorbed Sr (up 

to 92 wt.% of the total Sr) was removed by an ammonium 

acetate treatment. The 87 Sr/86sr ratios of the leachates from 

both samples are similar and vary between 0.7081 and 0.7090. 

These values are indistinguisable from the 87 Sr/86sr ratios of 

hydrothermal carbonates, a galena-hosted fluid incluison 

leachate from the mineralization and ~rphyry co~r-t~ 

vein fluids (0.7079 - 0.7089). 87sr/8 Sr versus 8 Rb/8 Sr 

plots of ammonium acetate-treated clay size fractions from 

each sample show a linear data array. However, data from 

coarse fractions (>2J.1ffi) exhibit an enhanced scatter due to 

significant amounts of feldspars, which have apparently not 

been completely homognized by the illitization process. The 

Rb/Sr age (21.6 ± 1.2 Ma) of the ammonium acetate-treated 

<2 J.1ffi fraction of one sample is significantly lower than the 

K/Ar isochron age of the same sample (24.3 ± 1.0 Ma, Gilg 

and Frei (1993)), which is interpreted as the formation age of 

the clay. Consequently it is assumed, that in this case the 

Rb/Sr system has been partly open and disturbed by 

retrograde alteration after formation of the illites. The second 

sample, which was taken in the vicinity of the crosscutting 

andesite porphyry, yields a Rb/Sr isochron age of 20.4 ±0.4 

Ma, well in accordance with the corresponding K/Ar age (19.7 

± 0.6 Ma). Thus both systems in clay fractions <2 J.1ffi can be 

reset by reheating events. Initial 87 Sr/86sr values in the range 

of 0.7072-0.7096 point to a contribution of both, Sr from the 

marbles (0.7071 - 0.7076) and from the meta-aplite (0.7103), 

to the mineralizing fluid. 

Gilg, H.A., and Frei, R., 1993, Chronology of magmatism and 

mineralization in the Kassandra mining area (Greece): The 

potentials and limitations of dating hydrothermal illites, 

Geochim. Cosmochim. Acta, in press. 

Tompouloglou, C., 1981, Les mineralisations tertiaires, type 

cuivre porphyrique, du massif Serbo-Macedonien 

(Macedoine grece) dans leurs contexte magmatique, Ph.D. 

thesis, Ecole National Superieure des Mines de Paris. 
JUVENILE CRUST IN THE SOUTHERN COAST BELT OF 

BRITISH COLUMBIA: EVIDENCE FROM A REGIONAL Nd-Sr 

ISOTOPIC STUDY OF THE SOUTHERN COAST PLUTOi'IIC 

COMPLEX BATHOLITH 

Friedman. R M and Cui, Y ., Department of Geological 

Sciences, University of British Columbia, 6339 Stores Road, 

Vancouver B.C., V6T lZA, Canada. 

New Nd and Sr isotopic data are reported for 26 Coast Plutonic 

Complex plutons and 2 volcanic flows from the southern Coast Belt, 

which collectively range from Early Jurassic to Miocene in age. 

These data are integrated with previously published Nd and Sr data 

from the area. Initial Nd (eNd(T)) and Sr (87sr,S6sr(i)) isotopic ratios 

vary from +9.8-+3.5 and 0.7029-0.7048, respectively, consistent with 

formation of southern Coast Belt igneous rocks within a juvenile 

magmatic arc, with minimal incorporation of old or isotopically 

evolved crustal material. There are no significant systematic isotopic 

trends on the basis of geographic position or age. The probable main 

sources for these igneous rocks are mantle and juvenile terrane 

material. 

Several Middle Jurassic through Late Cretaceous igneous rocks 

from the southeastern part of the study area contain zircon with 

Precambrian inherited components, which is viewed as inconsistent 

with the regional juvenile Nd-Sr isotopic signature in the area. 

Restoration of 80-140 km of dextral strike-slip motion along the 

Fraser-Straight Creek fault system aligns these rocks with 

Precambrian rocks of the Cascade Metamorphic Core to the east Two 

end-member scenarios which could account for the old inheritance and 

relatively primitive Nd-Sr isotopes in these rocks are: 1) incorporation 

of small volumes ( <5%) of old material, such that old zircons were 

imparted, without causing a significant shift in Nd-Sr values, or 2) the 

assimilation of sedimentary rocks with a relatively primitive Nd-Sr 

signature, that contain old detrital zircons. 

Exclusively juvenile Nd-Sr isotopic ratios for igneous rocks of 

the southern Coast Belt contrast with those of the northern Coast 

Belt, which in some cases contain significant proportions of evolved 

material. Igneous rocks from the southernmost Coast Plutonic 

Complex are viewed as juvenile material on the threshold of the more 

evolved Cascade Metamorphic Core crustal block to the east 

106 

TWO MECHANISMS OF GRANITE FORMATION, 

LARAMIE ANORTHOSITE COMPLEX, WYOMING 

FROST, C.D., SCOATES, J.S., CHAMBERLAIN, K.R., 

and EDWARDS, B.R., Dept. Geology and Geophysics, 

University of Wyoming, Laramie, WY 82071, USA. 

Nd, Sr and Pb isotopic compositions provide evidence of 

multiple origins for the 1.43 Ga granites and monzonites of the 

Laramie Anorthosite Complex (LAC) and Sherman batholith. 

These K-rich rocks may incorporate residual liquids to 

anorthosite or may be solely products of crustal melting 

induced by intrusion of high temperature anorthositic magmas. 

The LAC was intruded across the boundary between the 

Archean Wyoming Province and the Proterozoic Colorado 

Province, so crustal sources vary both in age and rock type. 

We have isotopically analyzed over 160 samples from the 

LAC, Shennan and country rocks in order to characterize the 

various magmas and their potential source materials. 

The Sherman batholith extends south from the LAC for 

more than 60 km into the Colorado Province. It contains both 

hydrous and water-undersaturated granites, aplites, 

granodiorites, and small volumes of monzodiorite. The 

contemporaneity of these different rock types is confirmed by 

commingling monzodiorite and both granite and aplite. Initial 87SrfB6

Sr ratios for the batholith range from contemporary 

depleted mantle values to values in excess of 0.725, but initial 

ENd is a uniform 0 ± 1. Feldspar Pb isotope ratios are 

similarly uniform and plot slightly above model mantle values, 

ruling out substantial involvement of Archean crust. The 

isotopic data constrain the Sherman batholith to be largely the 

product of melting of Proterozoic volcanic, pelitic and gneissic 

crust, although the monzodiorites may be related to anorthositic 

magmas. 

Granitic rocks also occur in three predominantly monzonitic 

satellite plutons to the LAC: Red Mountain, Sybille, and 

Maloin. The Red Mountain and Sybille plutons are in direct 

contact with Archean crust, and partially digested xenoliths in 

Sybille provide clear evidence for the involvement of Archean 

assimilants. Each has a characteristic initial ENd that records 

varying amounts of Archean assimilant in the parental magma. 

Extensive differentiation of this parental magma accompanied 

by further assimilation of up to 30% Archean granitic and 

pelitic crust yields the monzonite to granite suite composing 

these plutons. 

The major element chemistry of the granitic endmembers 

of the Red Mountain, Sybille and Maloin plutons are indistinguishable from that of the Sherman batholith granites. 

However, their origins are almost completely independent, 

demonstrating that multiple petrogenetic processes can produce 

granites that are geochemically similar. 
CORAL-BASED ISOTOPIC RECORDS OF THE AUSTRALIAN MONSOON, INOONESIAN TIIROUGHFLOW, AND SYNCHRONIZED MASS SPAWNING 

GAGAN. M K , CHIV AS, A.R., Research School of 

Earth Sciences, The Austmlian National University, 

Canberra, ACf 0200; ISDALE, P.J ., Australian 

Institute of Marine Science, Townsville, Qld 4810. 

Pandora Reef provides nearshore Great Barrier Reef corals 

which accurately record river runoff as a proxy for the 

timing and magnitude of the Austmlian monsoon, as well 

as the far western Pacific precursors to the east Pacific El 

Niflo. A high-resolution coral S180 and S13C time-series 

(1978-84), having near-weekly sample intervals, was 

produced using the consistent arrival-time(± 2 weeks) of 

the mid-winter minimum sea-surface temperature, recorded 

by the S18Q. The dependable chronology and highresolution sampling demonstrate that the arrival-time of 

monsoonal floods is precisely preserved (± 1 week) within 

the skeleton of Porites. Weekly sampling resolves a subtle 

warm-to-cool sea-surface temperature anomaly which 

preceded, by more than one year, the much larger warm-tocool anomaly associated with the 1982-83 El Nifio in the 

east Pacific. 

The strength of the anomalously warm, southwardflowing Leeuwin Current, Western Australia is thought to 

closely reflect the strength of the Indonesian throughflow. 

Initial observations of a near-weekly coral S18Q record 

(1978-1993) for Ningaloo Reef, Western Austmlia suggest 

that Indo-Pacific tradewind velocity may periodically be 

important in driving the throughflow, particularly during El 

Niflo-La Nifia transitions. For example, prior to El Nifto 

events when the tradewinds relax (or reverse), the Leeuwin 

Current response is much weaker than would be expected if 

it were driven by thermal forcing from the western Pacific 

alone. The opposite response (stronger than expected 

Leeuwin Current) occurs prior to La Nifta events, when the 

tradewinds are enhanced. By studying Ningaloo Reef corals, 

as well as corals in the western Pacific, it should be 

possible to obtain multi-century records of the interannual 

variability of the Indonesian throughflow and defme the 

balance between thermal- and wind-forcing (Southern 

Oscillation controlled) effects. 

It is essential to identify new chronometers, having 

much greater precision than coral density bands, to fully 

utilise the high-resolution information being extracted from 

corals. The annual synchronized mass-spawning event may 

provide a way around the chronology stumbling block. 

Every year, a few days after the full moon in OctoberNovember (Great Barrier Reet) or March-April (Western 

Austmlia), most corals synchronously release their gametes. 

The early worlc at Pandom Reef showed a pronounced l3C 

enrichment in the coral S13C values that corresponds 

closely to the October-November spawning event in the 

Great Barrier Reef. A similar signal appears in the new 

Ningaloo Reef S13C record that corresponds closely to the 

March-April spawning event in Western Australia. We 

believe this signal reflects the rapid build-up of gamete 

tissue which would form a significant sink for 12C 

resulting in 13C-enrichment of the skeleton up to the 

spawning date. A new mass-spawning chronometer would 

be particularly useful because its timing is reliable and, at 

present, it offers the only known means for constructing 

precise chronologies that are independent of temperature. 

107 

GENETIC ALGORITHMS : A POWERFUL NEW METHOD 

FOR MODELLING FISSION TRACK DATA AND 

THERMAL HISTORIES. 

GALLAGHER KERRY School of Geological Sciences, 

Kingston University, Kingston, KTl 2EE, England & Fission 

Track Research Group, University College London, England. 

Recent advances in our understanding of fission 

track annealing have led to empirical predictive 

models such that, given various assumptions, it is 

possible to predict the expected fission track 

parameters (age, track length distribution) for a given 

thermal history. In practice, however, it is the fission 

track parameters that are measured, and the thermal 

history is unknown. In sedimentary basins, the 

thermal history is intimately linked to the burial 

history and a reasonable reconstruction of both is 

often possible when a suite of downhole samples is 

used. However, in situations where only surface 

samples are analysed, the thermal history and 

cooling/exhumation rates must often be constrained 

from the fission track data alone. 

In principle, analytical inversion methods 

could be used to determine the thermal history 

directly from the data, but the mathematical 

formulations are non-linear, may be unstable and a 

rigorous resolution analysis of the solution is 

difficult. Alternatively, more robust optimisation 

methods can be used and genetic algorithms 

represent one such approach which has the 

advantages of being rapid, easy to invoke and the 

ability to test many different thermal histories at the 

same time. Unlike the more conventional random 

Monte Carlo methods, genetic algorithms actually 

learn the form of the better data-fitting thermal 

histories and the progressive sampling is 

automatically geared to finding the more optimal 

thermal histories. Genetic algorithms therefore 

provide an extremely efficient method to assess the 

information on the thermal history contained in the 

measured fission track data. 

An overview of the methodology will be 

given, using both synthetic and real data examples. 

The fact that many (> 1 000) thermal histories can be 

tested rapidly allows the spread in good data fitting 

solutions to be examined directly to assess the 

characteristics required to fit the observed data. The 

methodology may be applied to any type of 

thermochronological data, although fission track data 

is particularly useful because of the information on 

cooling rates contained in the length distribution. 
THORIUM-230 DATING OF GLACIAL-INTERGLAOAL 

CYCLES BY TIMS: RESULTS TO DATE 

GALLUP, C. D., EDWARDS, R. L., and JOHNSON, R. 

G., Minnesota Isotope Laboratory, Department of Geology 

and Geophysics, University of Minnesota, Minneapolis, MN 

55455 

The main application of thermal ionization mass 

spectrometric (TIMS) techniques for measuring 234U (1) and 

2301b (2) , has been the dating of carbonates. The 

chronometer's great advantage is its potential range of 3 to 

500,000 years. Thus, it is uniquely suited to examine 

chronologies on glacial-interglacial time scales. One exciting 

problem has been the extent to which glacial-interglacial cycles 

have followed changes in earth's orbital geometry, an issue that 

could not have been addressed with earlier measurement 

capabilities. Ironically, the high-resolution data have led to 

renewed controversy about the relationship between orbital and 

climate changes (3). The TIMS dated Devils Hole calcite vein 

appears to indicate that the oxygen isotope shift associated with 

the penultimate deglaciation preceded a rise in northern 

hemisphere summer solar insolation (3). Thus, the record is 

inconsistent with direct orbital forcing of climate change. 

Another TIMS dated chronology that bears on this issue is sealevel as recorded in corals. As of yet, no corals recording the 

penultimate deglaciation have been dated; however, many last 

interglacial corals have. The age of the oldest last interglacial 

coral places constraints on the timing of the penultimate 

deglaciation. However, on the basis of 234uj238u ratios, 

there is clear evidence for diagenetic alteration of most corals 

older than about 1 ()5 years. We have found that diagenesis in 

Barbados corals shows systematic trends. We generated a model 

that reproduces this correlation between 234uf238(J ratios and 

23Qn ages and provides a criterion for distinguising between 

accurate and inaccurate ages. By applying this criterion to both 

Barbadian and Bahamian coral data we fmd that the oldest date 

for the last interglacial (that is accurate to within 2 ka) is 130.4 

ka (4). This postdates most of the rise in 65°N summer 

insolation and is consistent with orbital forcing. The Devils 

Hole record may also be consistent with orbital forcing if a 

non-linear growth rate is considered (the date for the 

penultimate deglaciation is based on linear interpolation 

between two 23Qn ages). 

When we generalize our analysis to include other sea level 

events (see 5), we fmd that data from Barbados and the Bahamas 

constrain the timing of 5 high sea-level events during the l~t 

200 ka. The timing of these high sea levels also supports 

orbital forcing of climate change, as each one occurs at or after 

a major peak in northern hemisphere summer insolation (5). 

Furthermore, a several ka lag between insolation and sea level 

drop appears to be a general characteristic of transitions from 

interglacial to glacial states (5). 

(1) Chen, J.H., R.L. Edwards, and GJ. Wasserburg, 1986, 

Earth Plan. Sci. Lett. 80, 241-251. 

(2) Edwards, R.L., J.H. Chen, and GJ. Wasserburg, 1987a, 

Earth Plan. Sci. Lett .. 81, 175-192. 

(3) I. J. Winograd et al., 1992, Science 258, 255; K. R. 

Ludwig et a/.,1992, Science 258,284. 

(4) Chen, J.H., H.A. Curran, B. White, and GJ. Wasserburg, 

1991,Geol. Soc. Amer. Bull. 103, 82-97. 

(5) Gallup, C.D., Edwards, R.L., and Johnson, R. G., 1994, 

Science, in press. 

108 

U-Th-Pb SYSTEMATICS IN MOLYBDENITE 

FROM THE EASTERN CANADIAN SHIELD: 

IMPLICATIONS FOR METAL SOURCES AND 

TIMING OF MINERALIZATIONS. 

Gariepy, c., GEOTOP/Departement des 

Sciences de Ia Terre, Universite du Quebec a 

Montreal, Montreal H3C 3P8, Quebec, 

Canada and Carignan. J., Departement de 

Geologie, Universite de Montreal, Montreal, 

Quebec, H3C 3J7, Canada. 

U-Th-Pb isotope data were obtained on 

molybdenite (MoS2) of various ages and 

environments (skarns, pegmatites, granites) in the 

Eastern Canadian Shield (Late Archean Wabigoon 

and Abitibi greenstone belts and Late Proterozoic 

Grenville Province) in order to document the 

geochemical behavior of the U-Pb system and its 

dating potential. A number of leaching experiments 

have been done on single MoS2 crystals using acids 

like CH3COOH, HF, HBr, HCI and HN03 in order 

to remove preferentially radiogenic Pb located in 

cationic sites damaged by the radioactive decay. This 

was done to generate secondary Ph-isochrons. Bulk 

MoS2 crystals generally yield radiogenic Pb isotopic 

compositions (2°6PbP04Pb=20-35 for Proterozoic 

samples and 206PbP04Pb=40-127 in Archean ones). 

Leachates and residues from 8 MoS2 occurrences out 

of 12 define linear arrays in the Pb diagrams 

corresponding to ages in good agreement with those 

of their host rocks. The Th/U ratios of MoS2 

calculated from the linear arrays in the 208Pb-206Pb 

diagram are always low (0.7-1.7). This either 

reflects an input of meteoric water sources or a solidliquid partition coefficient for Th in MoS2 lower than 

that of U. K-feldspars from these 8 deposits regress 

well with MoS2 data in Ph-diagrams and are 

systematically more radiogenic than U-poor sulfides 

from other base metal deposits in respective areas. 

This suggests that Pb, and possibly other metals, 

came from rocks having a residence time in the crust 

long enough to produce Pb more radiogenic than the 

residual mantle. Three deposits have yielded 

significantly younger ages than their host rocks, but 

the reset ages correspond exactly to well dated postdepositional thermal events (Early Proterozoic) in the 

respective areas. The resetting induced calculated 

loss of 30-40% of uranium parent isotopes. Ucontent measured by isotope dilution in selected 

samples indicates also an actual deficit of U ( 45-

95%). For example, a MoS2 crystal with a 

206PbP04Pb=127, has yielded a 238Uf2° 4

Pb of 0.15. 

207PbP06Pb ages obtained preclude any continuous U 

diffusion so that recent changes in redox conditions 

must be responsible for U-losses. 230Thf234U activity 

ratios of 1. 7-2.9 measured in MoS2 crystals indicate 

an excess of 230Th, which also suggests a recent 

departure of U. Work is in progress to better 

constrain chemical fluxes in MoS2. 
A P-T-t PATH FOR SOME HIGH-PRESSURE ULTRAMAFIC/MAFIC ROCK-ASSOCIATIONS AND THEIR FELSIC 

COUNTRY-ROCKS BASED ON SHRIMP-DATING OF 

MAGMATIC AND METAMORPHIC ZIRCON DOMAINS. 

EXAMPLE: CENTRAL SWISS ALPS 

GEBAUER, D., Department of Earth Sciences, ETH ZUrich, 

CH-8092 ZUrich, Switzerland 

Numerous boudinaged garnet-peridotite bodies occur in gneisses 

of the Alpine Adula-Cima Lunga nappe system. Geothermobarometric data (e.g. Evans and Trommsdorff, 1978 or Heinrich, 1986) demonstrate a significant P-T increase to the S s~ggesting low, i.e. subduction related geothermal gradients. 

SHRIMP-ages are reported for various rock types from the two 

classical areas of Cima di GaB!lone (CdG, - 20 kbar, 750 °C) and AI~ Arami (AA,- 25 kbar, 850 °C). All U-Pb data were obtained 

Within distinct zircon domains that - based on their 

cathodoluminescence patterns - indicat~ either magmatic or various !}'pes ofmetamorphic origins. At i".A zircons from two 

gar-peridotites record pre-Alpine · ~es (1. 72 Ga, - 650 Ma and 456 

Ma) that probably reflect inter-granular HP-melting and formation 

of trap_Red mantle liquids during orogenic processes. Although the 

peridotites have suffered extreme Alpine P-T conditions, zircon 

aomains that registered this episode, could not be found in one sample. The otlier one, however, contains metamo!1JhiC zircon 

domains that average 36.3±1.2 Ma. Zircons extracted from various 

types of gar-p~oxenites that occur as layers and boudins within 

the gar-peridotites have formed magmatically under still HPconditions 35 Ma ago. The same was found for magmatic zircons 

of an eclogite that probably also formed magmatically under HPconditions. The zircons of the pyroxenites, one eclogite and one peridotite contain variably small amo•::Jlts of inherited cores of 

Phanerozoic, Proterozoic and . probably also Archean ages. 

Between these old cores and the Alpine metamorphic, respectively 

magmatic zircon domains metamorphicallY. recrystallized areas in 

the range of- 40 Ma to 45 Ma can occur. The latter formed most 

likely during prograde metamorphism in the course of deep 

sulxfuction of tlie peridotite. 

A coun~-rock gneiss at AA was found to be a Hercynian ~te 

(297 Ma) proving tectonic intermingling of the mafic/ultr8.mafic 

rocks witli the felsic country-rocks. Similarly, at CdG tectonic 

introduction of mafic/ultramafics occurred much later than 

formation of the sedimen!af}" protolith of the present countty-rocks as the .Youngest detrital zircon is of Caledonian age and no Hercyman detrital zircons were found so far. Thus, there is now good evidence that the peridotites were emplaced tectonically into 

typical Hercynian basement rocks found all over the Eur~ean 

Hercynides. The probable peak of Alpine metamorphism in the 

metagranitic and metasediriientary countty rocks at AA and CdG 

occurred at 33.0±0.6 Ma as detected in metamoryhic domains 

within detrital zircons at CdG and magmatic zircons from a deformed leucosome inside the Her_cynian orthogneiss at AA. This 

33 Ma event is also registered in U-poor, metamorphic domains 

rimming only 2 Ma older domains of zircon from the garnetpyroxenites, one eclogite and one ~ridotite at AA. The youn$est 

event, post-dating the last of the four deformation phases ~~.g. 

Mocker, 1969), was the emplacement of numerous pegmatite dikes 

(26.5±0.8 Ma). This event was probably coeval with both 

reheating during intrusion of intracrustal granites and introduction 

of large amounts of fluids. Some metamoq>hic zircon domains 

within mainly the felsic co~-rocks record this event 

The data obtained so far indicate that subduction of continental 

lithosphere, JlOst-dating subduction of Mesozoic oceanic crust, was active at - 40 Ma to 45 Ma. The solidus of the peridotitic sources above 1000 oc was passed at - 35 Ma, possibly auring isothermal ~lift of this heterogeneous and polymetamorphic mantle material. 

Tectonic intermingling of the coolmg peridotite and its enclosed 

partial melting prOducts with the felSic country-rocks must have 

occurred during rapid uplift between 35 Ma and 33 Ma. Both the 

age signatures m the 33 Ma old metamorphic zircon domains of 

tlie felSic and ultramafic/mafic rocks as well as the structural data on these rocks (e.g. Mackel, 1969) are in line with this conclusion. 

There are at present no constraints on the further cooling path as -26 Ma ago considerable magmatic and fluid activity must have 

existed under still amphibolite-facies conditions. This Upp~r 

Oligocene event is prooably responsible for complete or at least 

partial rejuvenation of numerous mineral ages (RJ>:Sr, K-Ar, U-Pb 

monazite and Sm-Nd). Geodynamically, the comer flow or orogenic wedge model - in combination with uplift caused by 

buoyancy, extensional faulting and erosion - seems to fit the 

observed age pattern best as it accommodates both subduction and 

uplift processes within a single P-T -t loop at plate tectonic speeds. 

109 

DETRITAL ZIRCON GEOCHRONOLOGY OF 

CAMBRIAN TO TRIASSIC STRATA OF THE 

CORDILLERAN MIOGEOCLINE, WESTERN 

NORTH AMERICA 

GEHRELS. G.E., DICKINSON, W.R., Dept. of 

Geosciences, Univ. of Arizona, Tucson AZ 

85721, ROSS, G.M., Geological Survey of 

Canada, Calgary, STEWART, J.H., HOWELL, 

D.G., U.S. Geological Survey, Menlo P~k 

Shelf-facies siliciclastics have been sam pled from 

Cambrian, Ordovician, Devonian, Penn-Permian, and 

Triassic units of the miogeocline along transects in 

northwestern Sonora, central Nevada, southern British 

Columbia, northern British Columbia, and east-central 

Alaska. 5 86 abraded single zircon grains have been 

analyzed from these units by isotope dilution and TIMS. 

Of these, 499 grains yield high-precision, concordant to 

slightly discordant ages. The zircon ages, combined with 

stratigraphic relations and the known distribution of 

basement provinces, suggest the following provenances. 

Grains from Cambrian, Devonian, and PennPermian units consistently record derivation from 

cratonal rocks that were exposed over a broad area 

inboard of each sample site. The only exception to this 

is a Cambrian quartzite in Sonora which was sourced 

almost entirely from nearby(?) Grenville-age plutons. 

Triassic strata in B.C. and Alaska show the same cratonderived provenance, whereas the Chinle Formation in 

Nevada records derivation almost entirely from Cambrian 

plutons exposed in Ancestral Rocky Mountain uplifts, 

and the Barranca Group of Sonora was sourced mainly 

in Permian and Triassic arc rocks developed along the 

Cordilleran margin. Ordovician quartzites record a very 

different provenance, with detritus in all units coming 

primarily from the Peace River Arch area of 

northwestern Canada. Longshore currents apparently 

transported detritus southward along the margin at least 

as far as Sonora. 

Except during Ordovician time, the detrital zircon 

ages can be divided into a northern versus southern 

Cordilleran provenance with a boundary between central 

Nevada and southern British Columbia. The northern 

provenance region is characterized by 1.84-2.0 Ga and 

2.0-2.4 Ga grains which are not present to the south. 

The southern provenance region has grains that are 1.65-

1.76 Ga, 1.4-1.45 Ga, and, at least locally in Triassic 

time, 525 Ga, which are not seen to the north. This 

latitudinal zonation provides a powerful tool for 

determining the paleolatitudinal position of displaced 

terranes outboard of the Cordilleran miogeocline. 
LATE MESOZOIC GRANITES OF THE SIBERIAN 

NORTHEAST 

Gelman. M. L., Northeastern Interdisciplinary Research 

Institute,Far Eastern Division, Russian Academy of 

Sciences.Portovaya 16, Magadan 685000, Russia 

Late Mesozoic plutonic complexes of the Siberian Northeast 

represent the most extensive belt of Phanerozoic granites 

known on Earth. Their emplacement occurred over 70 m.y. 

from the Middle Jurassic to the Late Cretaceous. This 

magmatic event is separated by 300 m.y. from earlier granitic 

magmatism. General trends in composition are controlled in 

part by the nature and age of the basement, specifically whether 

this basement is Precambrian continental crust or Phanerozoic 

accreted terranes. Along the Paleo-Pacific boundary the ancient 

continent has been rimmed by the Taygonos island arc system 

and then during the mid-Cretaceous by the Okhotsk-Chukchi 

volcanic belt (OCVB). The "inner zone" of the OCVB lies 

proximal to the Paleo-Pacific and its "outer zone" closer to the 

continent. Younger accreted, ophiolitic terranes lie SE of the 

OCVB. The plutons are referred to as types 1a and 1b where 

they intrude the OCVB and its basement, and types 2 and 3 

where they intrude epicratonic or inboard rocks. 

1a pluton emplacement is controlled by major faults and rifts 

at high angles to the belt. The representative rock types are 

tonalite to granodiorite and also trondhjemite. The inner zone 

of the OCVB contains layered anorthitic gabbroid similar to 

those plutons in neighboring outboard ophiolitic province. 

Gabbroic intrusion is generally separated from granitic 

magmatism by regional metamorphism, but emplacement 

overlaps in age in many places. Granitic plutons range from 

epizonal where emplaced into volcanic rocks and mesozonal in 

metamorphic culminations. K/Ar ages range from 120 to 84 

Ma, 40Ar/39Ar ages from .102 to 96 Ma, and a Rb/Sr isochron 

date is 108 Ma. Sri= 0.7036. 

In the flanks of the OCVB, quartz monzonite to granodiorite 

and quartz syenites (type 1b) predominate. Gabbroic intrusions 

are associated with silicic rocks and are less common than in 

the inrier zone. Granites are associated with voluminous midCretaceous ignimbrites. Rb/Sr isochron ages are 119, 94, 77 

and 70 Ma. Sri ranges from 0.7029 to 0.7085. Granites of the 

1a and 1b types show features of theM- and I-type granites. 

In the largest batholiths, 1a- and 1b-types are found together. 

Batholith zonations are similar to those in the Sierra Nevada, 

Bajil, and Idaho Batholiths. 

Type-2 granites range from rare gabbrodiorite and mozonite 

to leucogranite. Within the craton, these rocks form the cores 

of granite-metamorphic domes as well as numerous high-level 

stocks, laccoliths, and minor intrusions. K/Ar and Rb/Sr ages 

are 140 to 135 Ma and 95 to 90 Ma. Sri is >0.7010.These are 

A-type and similar to plutons in the Boulder-San Juan belt. 

2""mica adamellite with cordierite, garnet, and ilmenite 

constitute type 3 plutons. Voluminous epizonal and 

epimesozonal batholiths are intruded along deep-seated faults. 

Rb/Sr isochron ages are between 154 and 146 Ma. Younger 

subalkalic biotite granite has a Rb/Sr age of 89 Ma and Sri 

from 0.7045 to 0.7086. These more unusual granites areS-type 

and have no North American analogues in the Mesozoic. 

110 

ISOTOPE SIGNATURES OF VARISCAN GRANITOIDS AND THEIR POTENTIAL PROGENITORS IN 

THE HERCYNIAN FOLD BELT OF WESTERN 

CENTRAL EUROPE 

Heinz Gerstenberger, GeoForschungsZentrum Potsdam, D-14473 Potsdam, Germany 

The isotope signatures (initial values) of the Variscan granitoids in Western Central Europe are in many 

cases readily distinguished from those of pre-Variscan 

ortho-gneisses and other low and medium-grade metamorphics. Initial Sr isotope ratios of the granitoids of 

Variscan age occupy the field between bulk earth/-

depleted mantle and an extended ortho-gneissic Sr 

isotope evolution line (0.714 - 0.718 between 350 and 

250 Ma). This line results from a diagram of initial Sr 

isotope ratios vs. Rb-Sr-ages of orthogneiss samples. 

The best fit line in this diagram representing the Sr 

isotope evolution in the gneisses with time correspond 

to 87Rb/86Sr = 3.3. The highest frequency of data points 

of initial Sr isotope ratios of the granitoids is between 

0.706 and 0.711 with a trend of higher values for 

younger granites. 

The granites of the Saxothuringian zone of the 

Hercynian Fold Belt are the oldest among the Variscan 

granitoids, these exhibit lower initial Sr isotope ratios 

and tend to have higher &Nd(T) values than the Moldanubian and Rhenohercynian granitoids. This is in full 

agreement with the I-type character of the Saxothuringian granitoids ( Liew & Hofmann, 1988). 

Data demonstrates the broad coincidence of isotope signatures of granulites occurring in the Hercynian 

Fold Belt and the Variscan granitoids.This might be 

well the result of an initial mantle diapirism triggered 

the Variscan orogenesis and producing a magma 

chamber in the lower crust. Progressive melting of 

lower crustal material enhanced the proportion of the 

crustal component in the ascending magma. Thus the 

magma contained at the beginning a higher portion of 

a lower crustal component and got an increasingly 

upper crustal signature as the melting of lower crustal 

material proceeded. However the formation of the 

Variscan granitoids from mixed upper mantle and 

upper crustal source rocks cannot be completely 

excluded. 

Liew, T.C. & Hofmann, A.W. (1988) Contrib. Mineral. Petrol.,98, 129-138 
MODELS OF CHEMICAL EXCHANGE IN 

REACTIVE FLUID-ROCK SYSTEMS; 

GEOCHRONOLOGIC APPLICATIONS IN 

METAMORPHIC ROCKS 

GETTY, S.R., Berkeley Center for Isotope 

Geochemistry, Dept. of Geology and Geophysics, 

University of California, Berkeley, CA, 94720, 

USA, and DEPAOLO, D.J., same address. 

Models for chemical exchange in reactive fluidrock systems are used to relate spatial patterns of 

isotopic ratios (Sr,Nd,O ... ) to bulk ionic diffusivities 

(D,/K; '=porosity, K=solid:fluid partition coeff.), 

rates of mineral reaction (R), wavelength(s) of 

isotopic variation <Lo), durations of metamorphic 

conditions (~t), and rates of radioactive decay (A). 

The objective for geochronology is to combine 

solutions to the chemical transport equations with 

field, petrologic, and isotopic measurements to 

understand the isotopic evolution of whole rock 

samples or growing porphyroblasts. The model 

demonstrates that many of the tacit assumptions of 

geochronology are difficult to defend 

The exchange model is one-dimensional and 

emphasizes diffusive transport through pore fluids 

rather than advective flow. Isotopic communication 

between fluid and ·rock occurs by solutionreprecipitation. For a profile of initial isotopic 

variations, two dimensionless parameters dictate the 

pattern of isotope redistribution, a Damkohler number 

(DeK) relating reaction rate and wavelength(s) of 

isotopic variation to pore fluid diffusive transport, 

and a diffusion-decay parameter (Dd) relating 

diffusive transport to radioactive decay. The 

radioactive decay constant A describes the in situ 

increase of the daughter isotope with parent/daughter 

ratio, whereas the parameter a(L) describes the rate 

of decrease of the amplitude of isotopic variation 

along the profile due to exchange between adjacent 

rock layers: 

a(L) = R ' DeK = R Ps(l-') Ln2 K. 

l+DeK 4x2pfl D~ 

These parameters allow one to estimate the effects of 

chemical exchange between layers on their isotopic 

evolution, leading to new rules for sampling and data 

interpretation for whole rock and mineral isochrons, 

and studies of porphyroblast chronology. 

In fossil fluid-rock systems, vein minerals in 

tensile fractures may proxy for fluid isotopic 

compositions. The model shows that measurements 

of isotopic profiles across interlayered rock units and 

differences between fluid and host rock ratios, 

coupled with time-scales derived from porphyroblast 

studies, can be inverted for effective bulk ionic 

diffusivities and reaction rates. 

111 

STUDIES OF EXTRATERRESTRIAL XENON 

SAMPLES USING RELAX 

J D GILMOUR and G TURNER 

Geology Department, Manchester University, Manchester, 

M13 9PL, Unnited Kingdom 

RELAX 1 (Refrigerator Enhanced Laser Analyser for 

Xenon) is an ultrasensitive time-of-flight mass spectrometer 

designed to analyse xenon components. It combines a species 

selective resonance ionization ion source with a cryogenic 

sample concentrator. Sensitivites in excess of 1cps/500 atoms 

have been achieved. When combined with the 

'multicollection' inherent in the time-of-flight technique this 

corresponds to an increase in sensitivity by a factor of more 

than 100 over conventional, single collector mass 

spectrometers. 

The ultrasensitivity of RELAX allows higher resolution 

stepped heating experiments to be conducted than previously 

possible. A series of analyses of acid residues from the 

Murchison meteorite2 in a filament microfurnace shows 

evidence for separation of two sub-components of Xe-HL: one 

with a different ratio of light to heavy isotopes from that of the 

other. This was most marked in samples from which a 

preliminary purification step had removed the majority of a5 

(the microdiamond component believed to carry Xe-HL). 

A continuous wave laser microprobe has been developed and 

used to perform in-situ stepped heating experiments on 

irradiated chondrules from the Bjurbole U and Parnallee 

LL3.6 meteorites. Six Bjurbole chondrules exhibit good 

separation of 128Xe* (produced during the irradiation from 1271) 

between components correlated and uncorrelated with I29Xe, 

suggesting that the host phases are well separated. Initial 

I29J/I27I and I29Xe/I32Xe values are consistent with that of Caffee 

et al3. However, in these experiments 131Xe* correlates with 

t28Xe* suggesting that its parent isotope (probably tellurium) 

and iodine are predominantly in the same phase, possibly 

sulfide. 

The two Parnallee samples were a 'macrochondrule'4 and a 

cristobalite bearing chondrule of unusual oxygen isotope 

signatures, termed here 'chondrule 1'. In chondrule 1 the 

correlated and uncorrelated t28Xe* components were less well 

separated than in Bjurbole, possibly due to more intimate 

mixing of the host phases. However, an age of 4.6±0.4 

million years after the mean Bjurbole age is indicated. 

Although 12sxe* is present, the macrochondrule shows no 

evidence of t29Xe excess, suggesting a resetting event after the 

decay of t29I but presumably before its incorporation into the 

meteorite. 

1. Gilmour J.D., Lyon I. C., Johnston W.A. and Turner G. 

1994, (accepted for publication) Rev. Sci. Inst. 

2. Gilmour, J.D., Johnston W.A., Lyon I. C., Arden J. and 

Turner G. (1993) Meteoritics 28, 353. 

3. Caffee M W, Hohenberg C M, Swindle T D and Hudson B 

(1982) J. Geophys. Res. 87, A303. 

4. Dodd R T. (1969) Min. Mag. 37, 231-237 

5. Bridges J C, Franchi I A, Hutchison Rand Fillinger C T 

(1993) Meteoritics 28, 329. 
TOWARDS AN APATITE FISSION TRACK 

MORPHOTECTONIC IMAGE OF THE SNOWY 

MOUNTAINS, SE AUSTRALIA. 

GLEAOOW AJ.W , Kohn, B.P., Victorian Institute of 

Earth and Planetary Sciences, School of Earth Sciences, La 

Trobe University, Bundoora, Victoria, 3083, Australia, and 

Cox, S.J.D., Victorian Institute of Earth and Planetary 

Sciences, Dept. of Earth Sciences, Monash University, 

Clayton, Victoria, 3168, Australia. 

The Snowy Mts of SE Australia, centred around the highest 

peak on the Australian continent (Mt. Kosciusko - 2228 m), 

largely consist of Paleozoic granitic rocks of the Lachlan fold 

belt. A striking feature of the region is the contrast between 

the stepped fault block morphology of the uplifted Kosciusko 

massif and the old, gently dipping, erosion surface of the 

Monaro Tableland to the east. The mountain blocks rise up to 

-1 km above the Tableland surface. Remnants of the erosion 

surface are also preserved at higher elevations in the Kosciusko 

uplift. 

In this geomorphic setting, vertical and areal variations of 

apatite fission track (FT) parameters in basement rocks 

potentially provide unique thermochronological markers. Such 

markers can be used for reconstructing the neotectonic 

"disruption" of the old erosion surface. In the area of the 

Kosciusko massif five separate vertical profiles, covering -1700 

m of relief yield apatite Ff ages ranging from -90-290 Ma. 

Such an age range has only previously been revealed in the 

vicinity of the SE Australian margin where it is exposed 

laterally over some 150 km inland from the present coast. 

A positive correlation between age and elevation describes a 

regional master reference profile which preserves a pre-uplift 

partial Ff annealing zone. The slope and form of the profile in 

the Snowy Mountains suggests a pre-uplift geothermal gradient 

of -25-30°C/km. An inflection point in the age-elevation 

profile on the western side of the massif indicates the area of 

greatest uplift, and records the position of the pre-uplift -11 0°C 

isotherm. The inflection point at 90 ± 10 Ma approximates the 

onset of the Kosciusko massif uplift and establishes a firm 

temporal link between Snowy Mt. tectonism and the formation 

of the SE Australian passive margin during Cretaceous rifting. 

The relatively simple relationship between elevation and age 

may be used as the basis of an interpolation using a map 

algebra. Starting with a gridded digital elevation model, we have 

divided the massif and Monaro Tableland into a set of blocks on 

the basis of mapped and inferred faults. The age-elevation 

profile determined for each block is then applied to predict the 

local apatite Ff age. The result is a complete image of Ff age 

at the surface, tied to the set of control points. The age 

mismatch across each fault may in turn be used to infer vertical 

offset based on the master reference profile. The resulting image 

may be visualised in a variety of ways, including shaded relief 

maps and pseudo perspective views with various overlays of 

faults and two dimensional profiles showing relative offset of 

blocks. The use of a GIS in combination with apatite Ff data 

provides a new and invaluable means of visualising and 

modelling tectonism in the SE Australian Highlands. 

112 

Nd-Sr ISOTOPIC STUDY OF THE PALEOZOIC 

OUACHITA SEQUENCE: IMPLICATIONS FOR 

SEDIMENTATION WITHIN COLLISIONAL BELTS 

GLEASON. J.D., PATCHETT, P.J., 

DICKINSON, W.R., and RUIZ, J., Department of 

Geosciences, University of Arizona, Tucson, 

Arizona, 85721, USA 

We have completed a Nd-Sr isotopic provenance 

study of sediments from the Ordovician through Pennsylvanian 

Ouachita sequence of Arkansas-Oklahoma and west Texas. 

Correlated Nd-Sr relations are typical of evolved upper crust 

throughout the sequence but Nd isotopes distinguish three 

distinct groups: 1) lower to middle Ordovician strata with ENd(t) 

= -13 to -16 (Tc1m = 1.8 to 2.1 Ga); 2) upper Ordovician 

through Pennsylvanian with ENd(t) = -6 to -10 <Tc~m = 1.4 to 

1.7 Ga); 3) Mississippian tuffs with ENit) = -2 <Tc~m = 1.1 

Ga). These data document a rapid late Ordovician shift in 

sedimentary sources within the off-shelf passive-margin 

sequence of deep-marine cherts and shales of the Ouachita 

sequence. The lower to middle Ordovician strata are 

interpreted to have a dominantly older provenance of mixed 

Archean and Proterozoic cratonal sources. Early Silurian 

turbidites (ENd = -7 to -8) may represent the arrival of distal 

fan facies of an Appalachian Taconic clastic wedge. Middle 

Ordovician turbidites of the Sevier basin (Tellico Formation) 

in eastern Tennessee, representing sediment shed from the 

Taconic highlands, have the same Nd isotopic signature (ENd = 

-7 to -8), lending support to the hypothesis. 

Pennsylvanian non-marine sandstones and shales from 

the Arkoma basin, Illinois basin, and Black Warrior basin, in 

addition to 14 samples representing the thick (> 10-12 km) 

Ouachita Pennsylvanian turbidite flysch sequence, have ENd = 

-8 to -10. Of nine Pennsylvanian sandstone-shale pairs in our 

data set, all exhibit differences in initial ENd of < 1, and four 

have differences of < 0.5 with uniform 147Sm/144Nd ratios 

(0.10-0.12). The remarkable isotopic homogeneity of 

sediments delivered to the Ouachita-Appalachian region over 

this period implies extremely effective mixing and dispersal 

processes on a large (continent-wide) scale, consistent with a 

collisional belt provenance. 

Ouachita Mississippian silicic ash-flow tuffs have Nd 

isotopic compositions (ENd = -2) consistent with a continentalmargin arc source, and can be modelled as mixing of depleted 

mantle and an older (Precambrian) crustal component. 

Though arc material represented by the tuffs could be a 

significant component (up to 30%) of a small proportion of 

Mississippian turbidites (ENd = -6 to -10), this material is not 

a significant component of Pennsylvanian sediments analyzed. 

We suggest that the Appalachian collisional orogen of 

Pennsylvanian age produced a large, continent-scale 

sedimentary dispersal system which effectively overwhelmed 

all other available sources within the Appalachian-Ouachita 

region of North America, flooding the surface of the continent 

and its margins with sediment of a single homogenized 

isotopic signature. 
MIXED-AGE MOLLUSK ASSEMBLAGES IN 

HOLOCENE MARINE DEPOSITS: ANALYSIS BY 

AMINO ACID RACEMIZATION 

GOODFRIEND, GLENN A., Geophysical 

Laboratory, Carnegie Institution of Washington, 5251 

Broad Branch Rd., NW, Washington, DC 20015 

Amino acid racemization analysis (OIL aspartic acid 

and 0-alloisoleucine/L-isoleucine ratios) of individual 

shells is used to examine the age distribution of shells in 

several Holocene marine deposits: a relict salt marsh 

along the coast of Georgia, sediments of the Nile Delta, 

and cheniers (beach ridges) in the northern Gulf of 

California. Racemization rates were determined by 

calibration against radiocarbon dates. Racemization 

analyses require only very small samples (5-40 mg of 

shell) and are much less tim~ consuming than 

radiocarbon analyses, thus making it practical to carry out 

very detailed chronostratigraphic studies. 

On St. Catherines Island, Georgia, relict salt marsh 

muds presently exposed in the intertidal zone contain an 

assemblage of both shallow marine and salt marsh bivalve 

species in life position. Aspartic acid racemization 

analysis of individual shells shows that different species 

date to different periods. The muds were populated by 

several salt marsh and tidal channel species several 

hundred years ago. In the last few decades, the muds 

were briefly recolonized by two other marine species as 

well as one species that had previously lived there. 

Aspartic acid racemization analysis of two species of 

bivalves in a 5-m core from the Nile Delta shows that 

mixed-age assemblages are present at many levels. The 

time of sedimentation at each level was determined by the 

age of the youngest shells at that level. 

Several generations of late Holocene cheniers along 

the Baja California coast in the northern Gulf of 

California record variations in the discharge of the 

Colorado River. Amino acid epimerization analysis of 

individual shells shows that each chenier consists of shells 

of a wide variety of ages. The time of deposition of the 

cheniers was determined by the age of the youngest shells. 

113 

UNDERSTANDING LATERITISATION PROCESSES 

THROUGH COMPOSITIONAL, U DECAY SERIES 

ISOTOPES AND Sr-87/Sr-86 STUDIES. 

GOP ALAN, K., PANTULU, G. V., National 

Geophysical Research Institute, 

Hyderabad 500 007,India,RADHAKRISHNA, 

B.P., Geological Society of India, 

Bangalore 560 019, India, and 

RENGARAJAN,R., SARIN,M.M., and 

SOMAYAJULU,B.L.K., Physical Research 

Laboratory, Navrangpura, Ahmedabad 

380 009, India 

Three vertical profiles of laterites 

each comprising of three to five samples 

have been analysed for thirty elements 

(major and trace) including eleven 

REE by ICPMS and AAS, U-234, 238, Th230, 232, Ra-226 and Pb-21 0 by radiochemistry and alpha spectrometry and 

Sr-87/Sr-86 using mass spectrometric 

techniques. Two of the profiles were 

collected from coastal town Mangalore 

(annual rainfall=250 em) and one from 

inland Bangalore City (annual rainfall= 

75 em) both in the State of Karnataka, 

Southern India. All measured parameters 

in the laterites at various stages 

of evolution· are compared with those 

of the basal archaean rocks. 

Al, Fe, U and Th are enriched during 

lateritisation by a factor of about 

5 to an order of magnitude whereas 

Mg, Ca, Mn and Cu are depleted ..by the 

same factors. The LREE get depleted 

more than the HREE and the positive 

Eu anomaly in the basal rock is lost. 

The whole rock Sr-87/Sr-86:0.70172-

0.751138 increase to 0.72353-0.75182 

indicating the retention of resistant 

K and Rb rich minerals during lateritisation. 

The daughter/parent activity ratio 

ranges are as follows: 

U-234/U-238=1.0-1.1; Th-230/U-234: 

0.2-1.1; Ra-226/Th-230:0.8-4.5; 

and Pb-210/Ra-226:0.8-1.1 

The U-U disequilibrium is not very 

significant whereas the Pb-Ra one can 

be attributed to gaseous radon loss. 

However, the gross disequilibrium in 

the Th-230/U-234 and Ra-226/Th-230 

which have about 400,000 and about 

10,000 year equilibrium time scales 

respectively, suggest that lateritisation 

is ongoing in the Karnataka. 
87Sr/86Sr INITIAL RATIOS IN RIPHEAN, VENDIAN AND 

CAMBRIAN CARBONATES OF SIBERIA: EVALUATION OF 

CATAGENETIC IMPACT 

GOROKHOV.I.M •. Inst.of Precambrian Geol.& 

Geochronol., RAS,St.Petersburg, 199034 Russia, 

SEMIKHATOV,M.A.,Geological Inst.,RAS, Moscow, 

109017 Russia, BASKAKOV,A.V., KUTYAVIN, E.P., 

MELNIKOV ,N.N. ,and TURCHENKO, T.L., Inst.of Precambrian Geol. & Geochronol., RAS, St.Petersburg, 

199034, Russia. 

Secular variations in the Sr isotopic composition in 

carbonate sediments are a powerful tool for decoding 

relative roles of the continental and mantle influx 

into paleooceans. However, post-sedimentary transformation and neoformation of the carbonate minerals 

hamper interpretation of the 87srJ86sr ratios in preCenozoic whole-rock samples. The latter are often 

found to contain more than one generation <Pbase) of 

the carbonate minerals, each having its own 87Sr/86Sr 

and Rb/Sr ratios. 

Chemical treatment of the carbonate samples was used 

for discriminating between uncogenetic mineral 

phases. The technique included leaching with 1N ammonium acetate, the dissolving of a residue in acetic 

acid, and study of these readily soluble carbonate 

(RSC) and less soluble carbonate (LSC) for the Rb and 

Sr contents and the 87sr/86Sr ratio. 

Fifty one samples of Lower-Upper Riphean, Vendian 

and Lower Cambrian limestones and dolostones from the 

Olenek, Turukhansk and Ura uplifts, Siberia were 

studied. The leaching leads to RSC extraction from the 

whole rock sample (usually, <10% of the total weight). 

This RSC phase appears to be a secondary calcite. Sr 

concentration in that tends to be in dolostones higher 

than, and in limestones similar to the concentration 

in the LSC. The RSC is characterized by increased .,.iUP 

to tenfold) Rb content and a much higher 87sr~Sr 

ratio in comparison with the LSC. The LSC is considered as consisting of "primary" (early diagenetic) 

carbonate minerals. The transformation of the Rb-Sr 

systems in the whole rock sample scale was associated 

with an enrichment of limestones and dolostones in Rb, 

Fe and Mn that can be regarded as an indicator of a 

freshwater catagenesis. The primary and secondary 

carbonate minerals (the LSC and RSC) have also different 87Sr/86Sr initial ratios. 

Thus, whole rock samples of limestones and dolostones, contrary to the routine practice, should not be 

used for determination of the Sr isotope composition 

in paleooceans because such determinations potentially may be in error due to presence of uncogenetic 

carbonate phases. The magnitude of this error de~nds 

on the differences in both the Sr content and the B7sr/ 

86sr values in the primary and secondary phases. The 

leaching technique provides the better understanding 

of the postsedimentary processes affecting carbonate 

rocks and their Rb-Sr systems and opens a possibility 

to separate uncogenetic mineral phases. This techni· 

que proves to be more effective for limestones than 

for dolostones. 

The obtained values of the 87Sr/86Sr initial ratios 

in primary phases of the studied carbonate rocks are 

used for chemostratigraphic purposes and for evaluating the presently accepted pathway of the 87srJ86sr 

variation in the sea water through the Late Proterozoic time. 

This work was supported by the Russi an Foundation of 

Basic Research (Grant 93-05-9404). 

114 

PRECISION DATING OF GLACIAL EVENTS BASED ON 

AMS MEASUREMENTS OF COSMOGENIC 10SE 

PRODUCED IN BOULDERS 

GOSSE, J.C.l), KLEIN. J.2\ EVENSON, E.B.l>, LAWN, 

B.R.2>, and MIDDLETON, R. 2> 

1 )Dept. of Earth and Environmental Science, Lehigh 

Univ., Bethlehem, PA 18015-3188 

2)Dept. of Physics, Univ. of Pennsylvania, Philadelphia, 

PA 19104-6396. 

Cosmogenic nuclides (especially lOse, 26At, 14c, 36cl, 

and the stable nuclides 3He and 22Ne) are produced in rocks 

at the Earth's surface by secondary particles produced by 

cosmic rays. Geologists are rapidly recognizing the potential 

of cosmogenic nuclides to solve Quaternary chronological 

problems. We provide a formal field test of the technique to 

date glacial events using boulders deposited on the crests of 

moraines. 

We have dated more than 60 samples from Pinedale, 

Wyoming, the Wisconsin (marine ()18o stage 6) type locality 

for Rocky Mountain glaciation. Based on samples from the 

terminal moraines around Fremont Lake, Soda Lake, and 

Mud Lake, the last glacial maximum commenced 21±1.5 ka, 

lasted 5.3 ka, and ended 15.7±1.1 ka. ago. Eleven 

measurements on five nested recessional moraines around 

Mud Lake have an average age of 15.5±0.6 ka (internal error 

includes non-instantaneous time of deposition and variations 

in transport of boulders to moraine). We dated a moraine 

high in the Wind River Mts. in Titcomb basin, - 1 km from 

the head wall of the cirque and -35 km from Fremont lake, to 

10.9±0.5 ka (10.6 ka including anomalous sample), thus 

establishing the retreat rate for 5.1 ka of 6.9 m/a. 

Extensive dating of the inner Titcomb Lake moraine ( 10 

samples) allows us to estimate the precision of the technique. 

Nine samples cluster with a single sample-standard deviation 

of 3.6% (370 a); one sample is anomalously young (by -2 

ka). Dates from the recessional moraines around Mud Lake 

suggest a similar precision. Estimates of boulder surface 

erosion were possible from samples collected from moraines 

deposited during the Bull Lake (Illinoian, marine ()18o stage 

6) glaciation. Our best estimate for erosion is 1.3 mmlka; an 

absolute maximum is 3.1 mmlka. The effect of this erosion 

was to reduce the apparent age of glaciation by 2% (ages 

quoted have not been corrected for the effect of erosion). 

Production rates were calibrated using glacially striated 

bedrock surfaces with associated 14c dates of 11,000 a 

(calendar years)[l 1. Quoted uncertainties include both 

analytic errors and estimates of systematic uncertainties due 

to erosion, calibration, and changes in production rate. 

The dates we have determined for the Pinedale 

maximum are in general agreement w/ dates determined by 

other methods-although there is a suggestion that ages for 

the past 20 ka may be -1 ka too young. 

1. Nishiizumi, K., et al., Cosmic ray production rates of 1<>se 

and 26 AI in quartz from glacially polished rocks. J. Geophys. 

Res., 1989. 94(B12): p. 17,907-17915. 
SINGLE ZIRCON AGES FROM THE PRECAMBRIAN BASEMENT OF THE SOUTHERN 

ARA VALLI MOUNTAINS (RAJASTHAN ,INDIA) 

Goswami.J.N., Wiedenbeck, M., Physical 

Research Laboratory, Navrangpura, Ahmedabad 

380009, India, and Roy, A.B., Department of 

Geology, M.L. Sukhadia University, Udaipur 

31300 1, India. 

We are conducting a detailed survey of 

zircon ages from the Precambrian basement of 

the Aravalli Mountains 2ffl ~~thwestern India. 

Here we report our Pb/ Pb age results 

obtained using a Cameca ims 4f ion probe 

(technique described by Wiedenbeck and 

Goswa.ni; companion abstract, this volume). 

The Mewar Gneiss is a heterogeneous 

assemblage of orthogneisses and paragneisses 

containing inliers of metasedimentary rocks 

and amphibolites. This assemblage experienced 

multiple deformation and metamorphic events 

followed by the emplacement of voluminous 

granites and basaltic dikes. Based on 

previsouly available Sm-Nd whole rock data 

(Gopalan et a/., 1990) this activity spanned 

from 3.3 to 2.4 Ga. Subsequently, this entire 

assemblage served as the basement on which 

both the Aravalli and Delhi Supergroups were 

deposited. 

Zircons from a sample of orthogneiss 

collected near the village of Jhamarkotra 

yielded a minimum age of 3281±3 Ma (1cr), which 

we interpret as the time of emplacement of the 

igneous protolith. A unique, 30 J.Un wide 

overgrowth on a zircon from this population 

gave an age of 2536±20 Ma, which we 

tentatively attribute to a period of in situ 

growth. Zircons from a second sample, a 

leucocratic gneiss collected from Vali River 

15 Km to the SE, gave a minimum age of 2.5 Ga, 

thereby attesting to the presence of multiple 

age components within the basement gneisses. 

We have now conducted age measurements on 

five granitic bodies from this same region: 

the Ahar River Granite (minimum age 2.56 Ga), 

the Gingla Granite (2.4 to 2.5 Ga), the Berach 

Granite (2.44 Ga), the Untala Granite (2.50 

Ga) and an unnamed foliated granite from near 

Jhamorkotra (2.53 Ga). These data point to the 

gross stabilization of the Aravalli Craton 

during the earliest Proterozoic. 

Gopalan, K., Macdougall, J.D., Roy, A.B. and 

Murali, A.V. (1990) Sm-Nd evidence for 3.3 

Ga old rocks in Rajasthan, northwestern 

India: Precamb. Res., v. 48, 287-297. 

115 

COSMIC-RAY -PRODUCED NEON IN TERRESTRIAL 

QUAR1Z 

GRAF, Th., KIM, J.S., MARTI, K., and 

NIEDERMANN, S., Dept. of Chemistry, University of 

California, San Diego, La Jolla, California 92093-0317, 

U.S.A. 

We have developed new techniques for studies of:$; 1o6 

atoms of cosmic-ray-produced noble gases in terrestrial rocks, 

which are expected to provide important geomorphological and 

glaciological information such as surface exposure ages, 

erosion rates, and extent of glacial cover. In the case of 21Ne, 

the cosmic-ray-produced component represents one of several 

components. A component resolution based on three-isotope 

correlations and stepwise heating is essential. Our new 

techniques permit accurate corrections for all interfering ions, 

including corrections for doubly charged species. The 

assumption that charge state ratios are constant is not 

satisfactory. The spallation ratio (

22Nef21Ne)c from Si was 

calibrated using a quartz separate from an Allan Hills sandstone 

and yields a value of 1.243 ± 0.022. This ratio is expected to 

be essentially constant for terrestrial quartz samples and is 

useful in component resolutions. The stepwise release of Ne at 

increasing temperatures showed that cosmic-ray-produced Ne is 

released from quartz at rather low temperatures (Niedermann et 

al., 1993). 

At present the cosmic-ray production rates of stable 21Ne 

are not very well known. We determined the production rate 

ratio P21IP26 of 21Ne and 26AI. The 26Al concentrations in our 

samples were measured by Nishiizumi et al. (1989). The 

quartz was separated from Sierra Nevada rocks which were 

exposed to cosmic rays since the Tioga period of the last ice 

age. Ne in these rocks represents mixtures of several 

components: trapped Ne, nucleogenic 21Ne and 22Ne produced 

by (a,n) reactions in oxygen and fluorine, respectively, and 

cosmic-ray-produced Ne. The trapped component was 

substantially reduced by crushing, grain size, and density 

separations, permitting the isolation of cosmic-ray spallation 

and (a, n) components. 

Two P21IP26 determinations are consistent within error 

limits, and the value 0.65 ± 0.11 agrees with ratios observed in 

extraterrestrial matter. The observed production rate ratio is 

substantially larger than theoretical estimates for Si targets, 

reflecting poorly known neutron excitation functions. The 

above P21IP26 value, coupled to the observed 26AI production 

rate, corresponds to a 21Ne production rate P21 = 21 atoms g-1 

a-1 in quartz or to P21 = 45 atoms (g Si)-1 a-1 (at sea level and 

high latitudes). 

Niedermann, S., Graf, Th., and Marti, K., 1993, EPSL, 118, 

65. 

Nishiizumi, K., Winterer, E.L., Kohl, C.P., Klein, J., 

Middleton, R., Lal, D., and Arnold, J.R., 1989, J. 

Geophys. Res., 94, B12, p. 17907. 
ISOTOPIC AND CHEMICAL VARIATIONS IN 

HISTORICAL LAVAS FROM MOUNT ETNA 

GRAHAM. DAVID, College of Oceanic & Atmospheric 

Sciences, Oregon State University, Corvallis, OR 97331, 

GIACOBBE, A TTILIO and SPERA, FRANK, 

University of California, Santa Barbara, CA 93106, U.S.A. 

We have conducted a detailed survey of the major, trace 

element and isotope chemistry (Sr-Nd-Pb and He) of a suite of 

32 historical and 17 prehistoric lavas spanning the -200,000 

year subaerial history of Mt. Etna. Historical lavas are 

alkalic, while prehistoric lavas include both alkalic and 

tholeiitic types. Major element variations primarily reflect the 

multiple saturation with olivine, plagioclase, clinopyroxene 

and oxides. Analyses of minerals and glasses suggest that 

some crystal sorting has occurred, usually by addition of 

plagioclase and/or subtraction of olivine+clinopyroxene. 

Thermochemical modelling indicates a polybaric history 

dominated by low pressure (1-3 kbar) crystallization, but 

higher pressure (5-8 kbar) crystallization also occurred in some 

cases. Incompatible element ratios and isotope compositions, 

such as Rb/Nb-87srt86sr, Rb/La-87srt86sr and Zr/Nb208pbf206pb, show strong positve correlations for both 

prehistoric and historical lavas. 

Within the resolution allowed by our sampling, from 1329 

A.D. until the 2oth century there was a continuous increase in 

87srt86sr, from 0.70335 to 0.70343. The 1669 eruption, by 

far the largest in historical time, showed a small 87 Srt86sr 

increase and a relatively large decrease in 206pbf204Pb. Since 

1971 the 87srt86sr ratio has increased dramatically, to 

0.70359 in 1989. This increase since 1971 was accompanied 

by enrichment in K and Rb, a corresponding decrease in 

206pbf204pb from 19.95 to 19.87, and increased 226Ra 

disequilibrium [1]. 143Ndt144Nd and 3He/4He ratios are 

within analytical uncertainty for all historical lavas studied, 

with average values of 0.51288 and 6.6 ± 0.4 RA, 

respectively. Single stage Kpb values deduced from the 

208pb_206pb systematics are -3.8, greater than the present 

day 'K'fh of -3.3 [2]. Basalts which are isotopically similar in 

all respects to these Mt. Etna lavas also occur at some ocean 

island localities, such as in the Canary Islands. 

The rapidity of the isotopic changes since 1971 suggests 

that magma mixing plays an important role. A simple 

mixing model of a constant volume magma reservoir, using 

the known effusion rates of historical eruptions and the 

observed isotopic changes, indicates that the magma body 

beneath Mt. Etna is small, presently -0.3 km3. During the 

period 1329-1900 this model suggests a larger body of -1 to 4 

km3. Our observations indicate that the volcanic plumbing of 

Mt. Etna has changed during historical times, consistent with 

inferences from previous work [3-6]. The different magmas 

originate either by changes in the melting dynamics of a 

mantle source which is isotopically heterogeneous on a small 

scale (km?), or by varying degrees of interaction with the 

lithosphere or crust [7]. 

1. Condomines et al. [1987] Nature 325, 607 

2. Condomines et al. [1982] Geochim. Cos~ochim. Acta 46, 

1397 

3. Guest and Duncan [1981] Nature 290, 584 

4. Cristofolini et al. [1984] Neues Jahrb. Min. Abh. 149, 267 

5. Tanguy and Clochiatti [1984] Bull. Vole. 47, 879 

6. Armienti et al. [1989] J. Vole. Geother. Res. 39, 315 

7. Clochiatti et al. [1988] J. Vole. Geother. Res. 34, 241 

116 

ESTIMATING CATCHMENT-WIDE DENUDATION 

RATES FROM COSMOGENIC IS010PE 

CONCENTRATIONS IN ALLUVIAL SEDIMENT: FORT 

SAGE MOUNTAINS, CALIFORNIA 

GRANGER DE, and KIRCHNER, J.W., Dept of 

Geology and Geophysics, University of California, 

Berkeley, CA, 94720, USA. 

Cosmogenic isotope concentrations in alluvial sediment 

can be used to infer catchment-averaged denudation rates, 

despite individual mineral grains' complex cosmic ray 

exposme histories. We have mathematically modelled how 

bedrock erosion, soil mixing, and sediment transport affect 

in-situ cosmogenic isotope accumulation in mobile 

sediment. Our analysis shows that cosmogenic isotope 

concentrations yield reliable estimates of catchment 

denudation rates under the conditions that (1) the sediment 

grains' cosmic ray exposme times during both exhumation 

and transport are much less than the isotope's radioactive 

meanlife, (2) the stream sediment is well-mixed, and is 

derived from the entire catchment, and (3) the sampled 

mineral is distributed throughout the catchment. Variations 

in erosion rate, soil depth, sediment storage, and isotope 

production rate can potentially confound interpretations of 

cosmogenic isotope concentrations. We have quantified the 

sensitivity of inferred denudation rates to these factors -- thereby delineating a practical range of field conditions over 

which cosmogenic isotope concentrations in alluvial 

sediment may be interpreted in tenns of catchment denudation 

rates. 

We have inferred denudation rates from 26 AI and 10& 

concentrations in quartz sand from streams draining two 

adjacent catchments in the Fort Sage Mountains, California. 

Material eroded from these catchments has been deposited as 

alluvial fans overlying Lake Lahontan shorelines dated at 

12.5 ka 1. We have measured the volumes of these fans to 

estimate the catchments' erosion rates since lake retreat. 

Preliminary data from these mass balances indicate erosion 

rates of 3.5 ± 0.9 cm·ka-1 and 8.0 ± 2.0 cm·ka-1 for the two 

catchments. These estimates agree with denudation rates of 

4.8 ± 1.2 cm·ka-1 and 9.9 ± 2.5 cm·ka-1, respectively, 

inferred for the two catchments from cosmogenic isotope 

concentrations. 

This study shows that denudation rates inferred from 

cosmogenic isotopes in stream sediment can be independently 

verified over timescales comparable to cosmic ray exposure 

times. Further verification over a range of field conditions 

should permit long-term, catchment-scale denudation rates to 

be confidently estimated from cosmogenic isotope 

concentrations. These can be contrasted with modern 

sediment fluxes, which may reflect short-term rates of 

sediment mobilization rather than long-tenn denudation rates. 

1Benson, L.V., and Thompson, R.S., 1987, Lake Level 

Variation in· the Lahontan Basin for the Past 50,000 Years: 

Quaternary Research, v.28, p. 69-85. 
NEW RADIOMETRIC ARGON AGES FOR 

VOLCANIC ROCKS FROM THE NORTHERN LINE 

ISLANDS, CENTRAL PACIFIC OCEAN 

GRAY, L. B. and DAVIS, A. S., both of Branch of 

Pacific Marine Geology, U.S. Geological Survey, 

MS 939,345 Middlefield Rd., Menlo Park, CA, 94025, 

USA. 

The Line Island chain is a major bathymetric feature in 

the central Pacific Ocean consisting of many seamounts, 

atolls, and linear ridges that extend from H~zon Guyot in 

the northwest to the Tuamotu Islands in the southeast. The 

Line Islands chain has been proposed to be a major hot 

spot trace, analogous to the Hawaiian-Emperor chain. 

However, available radiometric ages indicate a complex 

history not compatible with the age progression that would 

be associated with a simple hot spot trace. 

We report 23 new ages for 10 volcanic edifices along the 

Line Islands trend between 5°N and 20°N latitude. Age data 

were produced using primarily 40 Arf39 Ar laser fusion 

analyses of mineral separates, but also include 40 Arf39 Ar 

laser incremental heating, 40 Arf39 Ar induction incremental 

heating, and conventional K-Ar analyses of mineral 

separates and whole rock samples. Volcanic rocks analyzed 

range from alkalic basalt and hawaiite to mugearite and 

benmoreite. Mineral separates are mostly feldspars, 

including plagioclase and sanidine, and some kaersutite 

amphibole. If the evolutionary stages developed for 

Hawaiian volcanoes were applicable to these edifices, then 

the samples would be classified as being from the postshield and/or rejuvenated stage. 

These data establish two parallel trends of differing ages 

within the northern Line Islands chain. Karin Ridge forms 

the easternmost, older trend, with three dredge sites located 

on the ridge yielding ages between 93 and 82 Ma, and a 

sample dredged from an unnamed seamount southeast of 

Karin Ridge along the same trend at 10°N latitude yielding 

an age of 83.7 Ma. Ages of 82.1 to 81.4 Ma for 

plagioclase from Horizon Guyot indicate that this large 

edifice, trending nearly perpendicular to Karin Ridge, was 

volcanically active at the same time. 

The younger, western trend is parallel to Karin Ridge and 

extends from Johnston Island at 17°N to Kingman Reef at 

6°N latitude. Samples from seven locations yielded ages 

ranging from 72 to 67 Ma. An additional dredge site on a 

seamount to the northwest of Johnston Island, near Hess 

Rise, yielded an age of 73.4 Ma, although this seamount is 

apparently part of the Mid-Pacific Mountains. 

In agreement with previous studies, these new ages do 

not show an age progression, but rather indicate two 

episodes of widespread, synchronous volcanism in the 

northern Line Islands chain, occurring about ten million 

years apart. 

117 

STABLE ISOTOPE STUDIES OF THE ARCHEAN 

PILBARA BLOCK: IMPLICATIONS FOR THE 

ARCHEAN HYDROSPHERE. 

GREGORY, Robert T., Department of Geological 

Sciences & Stable Isotope Laboratory, S.M.U., Dallas, 

TX 75275, U.S.A. 

The Pilbara block, Western Australia provides an 

opportunity to examine a well-preserved, essentially 

undeformed -3.5 Ga greenstone-granite succession. The 

isotopic compositions of the rocks reflect the conditions of 

hydrothermal alteration that is ultimately tied to exogenic 

cycles which are coupled to climate. Over 140 samples 

from whole-rocks, vesicles, and veins have been analyzed 

for either oxygen, carbon or hydrogen isotopic 

composition. The range in greenstone silicate fraction, 6 < 

o18Q < 12, is the same as that of the carbonate fraction, 

7.5 < olSQ < 13.5. The similarity of the ranges in the 

o18Q values of coexisting carbonate and silicate fractions 

of greenstones (70 pairs) is consistent with an approach 

to isotopic equilibrium with greenstone carbonategreenstone silicate £\18Q- +2. Over 50 chert analyses give 

an average o18Q = 13.8 and chert- greenstone silicate 

£\180- +3, which would correspond to an approach to 

isotopic equilibrium at- 300 oc. The isotopic 

compositions of these cherts, therefore, have no simple 

direct paleoclimatic significance. The lowest o18Q values 

for mafic rocks are found within the horn felsic contact 

aureoles (3.4 < o18Q < 7.2). Hydrogen isotope ratios on 

pillow basalt whole-rocks are given by -33 < OD < -84. 

The o13C values of secondary carbonate (average modal 

calcite> 10%) from the North Pole dome greenstones 

show limited variability, -3.7 < o13C < 1.6 (mean o13C= 

-0.8 ± 1.2). The spread in o13C values for carbonates in 

these Archean greenstones is virtually the same as 

carbonates formed during the seafloor weathering of 

modern basalts suggesting no significant difference 

between modern and Archean dissolved seawater 

carbon~te o13C values. The similarity of the alteration 

assemblages, the inferred temperatures of alteration, and 

the ranges of o18Q and oD values of -3.5 Ga Archean 

greenstones to modern greenstones, all suggest that the 

isotopic composition of seawater was not significantly 

different even back to the earliest preserved sections of the 

Archean. Finally, the most 180-depleted feldspar and the 

most D-deplcted hydrothermally altered rocks associated 

with the intrusion of granitic magmas suggest that 

meteoric fluids at 3.5 Ga were close to the isotopic 

composition of modern meteoric fluids near the 15 oc 

cutoff surface temperature necessary to produce 

isotopically-depleted meteoric fluids. 
HOTSPOT HELIUM IN GEOTHERMAL 

WATERS FROM ETHIOPIA AND DJffiUTI 

(EAST -AFRICAN RIFT SYSTEM) 

E.Griesshaber *, S. Weise ** and G.Darling # * MPI ftir Chemie, 6500 Mainz, FR Germany, · ** Institut ftir Hydrologie, GSF Miincchen, FRG, 

#British Geological Survey, Wallingford, U.K. 

3He/4He compositions are presented for 

groundwater samples from the Ethiopian segment of 

the East-Afrikan Rift and from its northern extension, 

the adjacent Afar region (Djibuti). Helium isotope 

data are compared to those obtained previously from 

the Gregory Rift, south of Ethiopia. The distribution 

pattern of mantle-derived volatiles along the entire 

East-African-Rift (-from south Kenya to Djibuti-) is 

discussed and their sources are identified. 

Helium isotope ratios (R) for samples from the 

Ethiopian part of the Rift range from 6.3 to 16.0 times 

the atmospheric ratio (Ra=1.4 x 10-6) and thus show 

together with a MOR component a considerable hot 

spot helium component. These mantle helium 

concentrations are comparable to those observed in 

ground waters and volcanic rocks from the Afar plume 

region in Djibuti. Here R/Ra values range from 9 to 

13 times the atmospheric composition, with mantlederived helium concentrations being higher than at 

spreading ocean ridges. Helium isotope compositions 

from Ethiopia and Djibuti are entirely different from 

those observed in groundwaters at the southerly 

extending Gregory Rift in Kenya, where R/Ra values 

scatter between 0.5 and 6. At the northernmost part of 

the Gregory Rift, close to Ethiopia mantle helium 

contents are slightly higher, with R/Ra-values varying 

between 6.5 and 8.0. 

The distribution pattern of primordial helium along 

the East-African-Rift clearly shows an increase in 

mantle helium contents from south to north, 

culminating at present at the central rift zone in 

Ethiopia and at the Afar region in Djibuti. Thus, the 

obtained helium isotope compositions clearly 

demonstrate that, in contrast to other major rift zones, 

volatiles in groundwaters that issue at the EastAfrican-Rift derive from three distinct sources 

regions: a crustal, a MOR and a hotspot source. 

Whereas crustal helium is a dominent component in 

groundwaters in the southern part of the East-AfricanRift along its northern segment it is almost 

negligiable, where MOR and hotspot sources are the 

prominent components. The transiotion zone between 

a MOR and a hotspot source can be located between 

Lake Turkana in Kenya and the southen edge of the 

Lakes District in Ethiopia. The observed geochemical 

observations are in good agreement with 

seismotectonic and magmatic models along the EastAfrican-Rift. 

1. Darling G., Griesshaber E., Andrews J, Annannssson Hand 

O'Nions K. (1993): Composition and sources of outgasing 

from the Kenya Rift Valley. Submitted to Geochim. 

Cosmochim.Acta 1992, reviewed 1993. 

2. Griesshaber E. (1990): Helium and carbon isotope systematics in groundwaters from W.Germany and E.Africa. Ph.-D. 

Thesis submitted to the University of Cambridge in 1990. 

118 

STABLE ISOTOPE EVIDENCE FOR CRUSTAL 

CONTAMINATION DURING THE FORMATION OF 

DIFFERENT TYPES OF LAYERED BASIC INTRUSIONS 

GRINENKO. L.N., Dept. of Geology, Moscow State 

University, Moscow, 119899, Russia, and KROUSE, 

H.R., Dept. of Physics and Astronomy, The University of 

Calgary, Calgary, AB, T2N 1N4 CANADA 

Sulphur and carbon isotopes and contents and oxygen 

isotope data for whole rock samples collected over crosssections of intrusions were used to investigate the formation 

conditions of two types of layered mafic-ultramafic intrusions: 

I - normal stratigraphic sequence (Norilsk and Pechenga 

regions, Russia) and II- rhythm-layered (Karelia, Preebaikal, 

Russia; Rhum, Scotland; Fox River, Canada). 

Type I intrusions have high sulphur contents ranging from 

0.2 to 3% and 34S values from 0 to + 12 °/oo. The graphite 

carbon content is low (0.006%) with 13C values from -34 to28 °/oo. The 180 values are near + 8 °/oo. Type II intrusions 

have low sulphur contents (less than 0.2%, usually -100 

ppm). Most 34S values are 0 ± 1.5 °/oo, but some intrusions 

have anomalous values, e.g., -7.5 °/oo, Rhum, and +8.5 °/oo, 

Fox River. The carbon content is higher than for Type I and 

the 13 C values range from -25 to -16 °/oo. The 18 0 values 

are close to the mantle value (- +6 °/oo) or lower. The 

vertical distributions of sulphur isotopes and contents are 

different in the two type of intrusions. 

Not only do the data identify specific crustal sources of 

magmatic contamination, but they also provide information 

about its incorporation and peculiarities of the ore body 

formation. Crustal contamination for Type I intrusions took 

place in intermediate magma chambers and for Type II during 

emplacement. The former were formed by a single magmatic 

pulse, the latter during several magmatic pulses. 
STABLE ISOTOPE ABUNDANCES IN PLANTS 

GROWING IN THE ARAL SEA REGION 

GRINENKO. V.A., MINEEV, S.D., V.I. Vernadsky 

Institute of Geochemistry and Analytical Chemistry, 

Russian Academy of Science, Moscow 117975, Russia and 

KROUSE, H.R., Dept. of Physics and Astronomy, The 

University of Calgary, Calgary, AB, T2N 1N4 

CANADA 

Little is known about sulphur metabolism during plant 

development on highly saline soils. Data were obtained for 

contents and 634S-values of sulphate and organic -S, and 613C 

and 6D values for 5 species of plants at 4 locations in the 

Aral Sea area. The 634S values and concentrations were also 

measured for soil sulphate. 

Tamarix, Halocnemum strobilaceum, and Salicornia sricta 

had 613C values ranging from -30.0 to -23.3 °/oo consistent 

with ~ photosynthesis. Haloxylon aphyllum sampled at three 

of the locations, had higher and more uniform 613C values 

(-13.0 to -12.8 °/oo) consistent with c4 photosynthesis. In 

contrast, Atriplex tatar, growing in soil with less than 5000 

mg/kg sulphate had a o13C value of -23.3 °/oo whereas that for 

a specimen at another location with much higher soil sulphate 

concentration, was -12.8 °/oo. 

The soluble sulphate concentrations in soil, ranged from 

300 to 12,000 mg/kg and the o34S values from -7 to + 15 °/oo 

dependent upon location. For many plants, the total sulphur 

634S values were close to those of soluble sulphate in the soil 

and the differences between o34S values of plant sulphate and 

organic-S were small. However, there were also anomalies 

where adjacent different species differed in o34S value by as 

much as 25 °/oo. Also, in some plants, the organic-S had 634S 

values which were 10 °/oo lower than those of sulphate. The 

data suggest that even if sulphate is the dominant form of 

sulphur in soil, sulphide and perhaps intermediate oxidation 

states of sulphur may be metabolized by these plants. 

119 

ARGON LOSS FROM F-OH PHLOGOPITE 

GROVE M., and HARRISON, T. Mark, Department of 

Earth & Space Sciences, UCLA, Los Angeles, CA 90024 

marty@argon.ess.ucla.edu 

Replacement of OH- by p- and/or Cl- has a significant 

effect upon the thermal stability and other physical properties 

of trioctahedral micas. Previous studies have exploited the 

high temperature stability of phlogopite (>900°C for OH-Phl; 

>1300°C for the F-Phl) to measure diffusion coefficients over a 

wider temperature range than that possible with biotites 

containing appreciable iron and octahedral aluminum. 

Overlooked, however, has been the opportunity to measure Ar 

diffusivities as a function of hydroxyl content. Ionic porosity 

model considerations predict lower argon diffusivities for 

halogen-rich trioctahedral micas. This expectation is based 

upon observations that lattice dimensions within the interlayer 

region of trioctahedral mica decrease, and interlayer bond 

energies increase, when proton-potassium repulsion is reduced 

by replacement of hydroxyls by F (or Cl). In addition, the 

potential for argon loss to be enhanced by structural failure 

driven by dehydroxylation processes is mitigated by uptake of 

halogens in the hydroxyl sites of mica. 

Previous radiogenic 40 Ar diffusion experiments performed 

with F-rich phlogopite (68% F-Phl) yielded argon diffusivities 

1.5 orders of magnitude lower than those obtained from 

halogen-poor (5-20% F), intermediate Fe-Mg-Al biotite at 

700°C. Hydrothermal annealing experiments (650-850°C; 500 

bars Pu 2 o) carried out in the present study using 

comparatively F-poor phlogopite (35% F-Phl), yield 40 Ar 

diffusivities about one half an order of magnitude lower than 

those determined for intermediate biotite. Relative short run 

durations needed to achieve 20-30% loss coupled with the low 

confining pressures permitted by the broad stability field cause 

only minimal recrystallization(<< 1 %) and grain comminution 

of run products. Diffusion coefficients calculated by assuming 

infinite cylinder geometry and using diffusive length scales 

equivalent to grain dimensions yield Arrhenius parameters of 

Ea= 50.1 kcal/mol and D0 = 0.12 cm2/s (based upon available 

results from four experiments from 750°C to 850°C). Very 

short duration (- hour) experiments to 850°C yield low degrees 

(<2-5%) of fractional loss. Additional hydrothermal 

experiments are in progress to examine other compositions 

along the F-OH phlogopite join. 
Sr ISOTOPE GEOCHEMISTRY OF MARINE SEDIMENTS FROM SOUTH CHINA SEA 

GUI XUN-TANG, YU JIN-SHENG, LI XIANHU A, Guangzhou Branch, Institute of Geochemistry, Chinese Academy of Science, Wushan, 

Guangzhou, 510640, P. R. China, and CHEN 

SHAO-MAO, South China Sea Institute of Oceanology, Chinese Academy of Science, 

Guangzhou, 510301, P.R. China. 

The range of our investigation is limited in the 

Nansha area of 3-12°N and 105-l18°E. The Sr isotope compositions of silicate-facies of surface sediments from 36 stations were analyzed. The main 

lithological characters are pelagic clay, arenaceous 

clay, pelagic mud, metargillitic mud, arenaceous 

mud, as well as mid-fine grain sand and sand. 

The geographic isogram of 87Sr/86Sr ratios of silicate-facies surface sediments in the N ansha area is 

clearly constructed. 

The major axis of isogram is NE-SW direction. 

The closed circle of 0. 710 of 87Sr / 86Sr ratios , as a 

center of isogram is constructed in the area between 

7-10°N and 112-l17°E. The increase tendency of 

87Sr/86Sr ratio (> 0. 710) is in the SW direction of 

isogram, which can be divided into 0. 714; 0. 718; 0. 

719 and 0. 720 four isolines. The decrease tendency 

of 87Sr/86Sr ratios (<O. 710) is in the NE direction of 

isogram. The distribution feature of 87Sr/86Sr isogram 

have proved that the source materials of surface sediments in west and south parts are different from that 

of northeast part. The former is controled by the 

continental clastic constituents from Sunda Shelf and 

the washings from delta of Mekong River and the latter is controled by the mantel derived volcanic materials from surrounding tectonic environment. 

The dynamic ocean current picture in the research 

area is quite similar to the above isogram of 87Sr / 86Sr 

ratios , and perhaps it aslo has some correlation between them. 

120 

IMPLICATIONS OF Sr, Nd, 0, AND COMMON-Ph 

ISOTOPIC VARIATIONS FOR THE ORIGIN AND 

EMPLACEMENT OF THE LAGUNA JUAREZ PLUTON, 

BAJA CALIFORNIA, MEXICO 

GUNN, S. H., Branch of Isotope Geology, U.S. Geological 

Survey, MS 937, 345 Middlefield Road, Menlo Park, CA 

94025, USA. 

Isotopic evidence indicates the normally-zoned, 700 Km2, 

90 Ma Laguna Juarez pluton of Baja California, Mexico is a 

composite body formed by the nested emplacement of 

sequential batches of magma. From margin to core, the pluton 

ranges from tonalite to monzogranite, and is characterized by 

four nearly concentric mineralogic zones: hornblende, biotite, 

medium-grained muscovite-biotite, and fine-grained 

muscovite-biotite. 

Isotopic ratios display a systematic trend from margin to 

core: 0 isotopic ratios increase, initial Sr ratios become more 

radiogenic while Nd ratios become less radiogenic. Initial 

87Srf86Sr ranges from 0.7043 to 0.7074, 8180 from +7.3 to 

+ 12.1 and ENd(90 Ma) from+ 1.8 to -4.9. Common Phisotopic ratios of a feldspar separate from each of the four 

ririneralogic zones demonstrates the core has been derived 

from a Pb reservoir different from the three outer zones. 

A low-magnesium, high-alumina basalt (HAB) is 

hypothesized to be the parental magma by extrapolating trends 

from the major-element variation diagrams back to a basaltic 

composition. HAB's are common subduction zone magmas, 

and represent a feasible composition for modeling purposes. 

AFC processes can model all samples using isotopic data from 

W.P. Leeman on a Cascadian HAB and the averaged 

composition of two host metasediments, a quartzite and a 

sillimanite schist, for the end members. 

The results suggest wet, high-alumina magmas, 

differentiates of hydrous, primary melts from the mantle 

wedge, migrated to mid-crustal levels where they ponded and 

established a wall rock interaction zone (WRIZ). The 

accumulation of these magmas in the mid-crust provided the 

heat necessary to melt the wall rock, while simultaneously, 

exsolution of aqueous fluids from the basalts forced their 

crystallization and differentiation. Initially, wall-rock melts 

mixed with relatively undifferentiated basalt and rose through 

the overlying crust, establishing a pathway for succeeding 

magmas. Episodically, batches of magma, each more 

fractionated and containing greater quantities of assimilant, 

rose and nested within the previous batch creating the pattern 

of normal zonation. 

Relative to samples from the outer mineralogic zones, 

core samples are characterized by discrete geochemical 

signatures on trace-element and Sr, Nd, 0, and common Pb 

isotopic diagrams. Although all samples can be modeled by 

AFC processes using a crustal end member represented by a 

composite of the analyzed host-rocks, the distinctive chemical 

and isotopic character of the core may indicate the 

involvement of a different crustal component. The core may 

have incorporated melt from an exotic block of crust situated 

in the WRIZ or possibly from a crustal source located along 

the path of emplacement or near the final level of 

emplacement. 
RAPID EMPLACEMENT OF YOUNG OCEANIC 

LITIIOSPHERE 

HACKER, B. R., Geological & Environmental 

Sciences, Stanford University, Stanford CA 

94305 

Emplacement of oceanic lithosphere onto 

continents results from a rare phenomenon that has 

long sparked interest and yet remained poorly 

understood. Speculations on the age of most 

ophiolites at the time of emplacement range from 5 to 

20 m.y. Consequently, potential tectonic 

mechanisms for ophiolite emplacement have faced 

few rigid temporal constraints. The Oman ophiolite 

is the best exposed and most studied mid-oceanridge-like piece of oceanic lithosphere and is of 

considerable importance for understanding ophiolites 

worldwide. Our 40 Arf39 Ar study reveals that sudden 

cooling deep beneath the Oman ophiolite began 

within 2 m.y. of igneous crystallization, and 

therefore, the emplacement process must have 

initiated very near the ridge crest. 

Eleven U/Pb zircon ages from the mafic, crustal 

intrusive sequence are in the range 95.4-93.5 ± 0.5 

Ma (±2cr), although two are slightly older (96.9 and 

97.3 Ma) [Tilton et al., 1981]. Amphibolite-facies 

metamorphic rocks from the thrust zone 17 km below 

the top of the ophiolite yield 40 Arf39 Ar hornblende 

ages of 94.9-92.6 ± 0.6 Ma (±2cr) and muscovite 

ages of 93.4-91.9 ± 0.6 Ma (±20'). Combined with 

biotite K/Ar ages of Gnos and Peters [ 1993], these 

data indicate cooling rates of -100 K/Ma for the frrst 

2 m. y. after igneous crystallization and -30 K/Ma for 

the next 4 m.y. This dramatic cooling 17 km below 

the sea bottom must be related to emplacement, 

which in turn must have already been underway for 

some time for cold material to reach such depths so 

soon after igneous crystallization. 

These results are compatible with 

thermomechanical modeling which indicates that the 

heat necessary to sustain metamorphism beneath the 

Oman ophiolite likely dissipated within 5 m.y. 

[Hacker, 1990, 1991], and with buoyancy 

calculations [Sacks, 1983; Cloos, 1993], which 

indicate that oceanic lithosphere younger than 5-10 

m.y. is less dense than underlying asthenosphere. 

121 

LASER 40Arf39Ar DATING OF TEPHRA IN MARINE 

SEDIMENTS: CALIBRATING THE OPTS 

HALL ChrisM .. Dept of Geol. Sci., U. of Michigan, 

Ann Arbor, MI 48109-1063, USA, and FARRELL, John 

W., Dept of Oceanography, U. of British Columbia, 

Vancouver, BC V6T 1Z4, Canada. 

Tephra from ODP site 758 have been examined for the 

purposes of radiometric dating of marine sediments. Site 758 

has both well documented magnetic stratigraphy and several 

distinct ash layers which erupted from the Indonesian Arc 

(1 ]. Ash D dates the Brunhes-Matuyama boundary at 

781±16ka (all errors 2a) as reported in [2], in excellent 

agreement with modern astronomical and radiometric 

estimates [3,4]. Ash I, which erupted during the Nunivak 

event, yielded enough biotite for 5 laser step-heating 

analyses. Each age spectrum showed high apparent ages at 

low T, levelling to a plateau at high T. An Ar isochron 

diagram reveals the presence of at least 3 components of Ar. 

We interpret the 

diagram as: A) atmospheric Ar mixed ·.. I _ + 

with initial 40Arf36Ar ·.I t- ·~-·02 

~348 based on a ~ o.002o 

ragged "isochron" 'l... LaaerP>400mw · .. 

through medium tO ~~ n•20 MSWD-2.6 '' •• 

high T points; or B) o.oo1o ~~~~i;.~!13 

atmospheric Ar 

mixed with small 

amounts of excess 40 Ar and/or small 39 Ar loss due to 

recoil (this assumes an atmospheric ratio initial Ar correction for the plateau points, giving the "plateau reference 

line"); or C) internal redistribution of 39 Ar due to recoil 

(integrated age 4.59±.04 

Ma). ~----------------~ Cande& 

The second figure ft~> 

shows our three models 

compared with age esti- B~ 

mates based on linear (1985) 

interpolation between ages L-L-.,-,.-~---.&:-~~­

assigned to the onset and 

termination of the Nunivak 

,......c 

event from various recent time scales. Our preferred interpretation is the relatively conservative "B" model, agreeing 

exactly with astronomically derived ages of the Nunivak 

event in [5,6], although "A" matches [7] and cannot be ruled 

out We feel that "C" is the least likely because of the 

freshness of the biotite, the survival of glass shards in the 

tephra and the lack of chlorite in XRD. In any case, all 

models are significantly higher than ones based upon either 

[8] or (9]. We feel that this is strong evidence for revising 

the OPTS and it further confirms the accuracy of astronomically derived time scales. 

(1] Farrell & Janecek, Proc. ODP Sci.Res. 1991121297-355. 

(2] Hall & Farrell, 1993 EOS 14 110. 

[3] Shackleton et at 1990 Trans R. Soc. Ed. 81 251-261. 

[4] Baksi 1992 Science 256 356-357. 

[5] Hilgen 1991 Earth Planet. Sci. Lett. 107 349-368. 

(6] Shackleton et at. 1994 ODP Leg 138 Sci. Res. in press. 

[7] Baksi 1993 Geophys. Res. Lett. 20 1607-1610. 

[8] Cande & Kent 19921 Geophys. Res. 97 13917-13951. 

(9] Berggren et at. 1985 GeoL Soc. Lond. Mem. 10 211-260. 
MODIFICATION OF THE LITHOSPHERE DURING 

CONTINENTAL RIFTING: GEOCHEMICAL AND 

ISOTOPIC EVIDENCE FROM THE SOUTHEASTERN 

COLORADO PLATEAU TRANSITION ZONE, NEW 

MEXICO 

HALLETT R.B. and KYLE, P.R., both at: Dept. of 

Geoscience, New Mexico Tech, Socorro, NM, 87801, USA, 

MUKASA, S.B., Dept. of Geological Sciences, University 

ofMichigan, Ann Arbor, Ml, 48109, USA. 

The elemental and isotopic signatures of both asthenospheric and lithospheric portions ofthe mantle (acknowledged 

to be source reservoirs during continental extension-related 

magmatism) are well documented for the Rio Grande rift and 

adjacent provinces. We have analyzed Pliocene-age alkaline 

basalts from the Rio Puerco Valley (RPV), located just east of 

Mount Taylor volcano in west-central New Mexico, in the 

transition zone between the southeastern Colorado Plateau 

and Rio Grande rift tectonic provinces. This setting offers an 

unique opportunity to monitor the evolution of mantle source 

regions associated with continental rifting. 

Over 40 volcanic necks occur in the RPV as small volume 

plugs, flows, and dikes. RPV basalts are low Si02 (42-49 

wt.%), ne-normative, high in Ni, Cr, and Mg#, and have REE 

and spider-diagram trends similar to OIB. Nd and Sr isotopic 

data define a homogeneous group; ENci = +3.5 to +4.8; 

87sr;86sr= 0.70361- 0.70407), also within the compositional range of Om. Pb isotopes are fairly radiogenic 

(206pb;204 = 18.84- 19.26; 207pb;204pb = 15.56- 15.61; 

208pb;204pb = 38.64- 39.00), plot above the NHRL and 

overlap both Om and MORB fields. There is no geochemical 

or isotopic evidence for crustal assimilation. 

Alkaline basalts from highly extended portions of the 

southern Rio Grande rift and Basin and Range provinces, 

believed to have originated in the asthenosphere, typically have 

low 87 Sr;86sr (~0. 7035) and high ENci (~ +5), close to 

MORB or a depleted (asthenospheric) mantle end member. 

Less extended portions of the northern Rio Grande rift (e.g. 

Taos Plateau and Cerros del Rio in New Mexico; Yampa 

province and Elkhead Mts in NW Colorado) tend towards 

enriched (lithospheric) mantle sources (e.g. 87 Sr;86sr ~ 

0.7035, ENci ~ +2). By comparison, 87sr;86sr and ENci values for RPV basalts lie between depleted and enriched mantles, indicating a source contribution from both reservoirs. Pb 

isotopes further suggest the enriched component is an old ( 1. 8 

Ga ?), radiogenic (with respect to Pb only) subcontinental 

lithosphere. When placed in a tectonomagmatic evolutionary 

framework for the Rio Grande rift we believe the RPV basalts 

represent a lithospheric source that has been modified by the 

asthenosphere by the following processes: (a) asthenospheric 

upwelling brought on by continental rifting and resulting in 

thermal, mechanical, and chemical transformation of the 

lithosphere, (b) clockwise rotation of the Colorado Plateau 

since the late Miocene, and (c) preexisting Laramide structure 

facilitating the transport of alkaline magmas by channeling 

melts along crustal weaknesses. 

122 

INDUCTIVELY -COUPLED PLASMA MAGNETIC 

SECTOR MULTI-COLLECTOR MASS 

SPECTROMETRY 

A. N Halliday, J. N. Christensen, C.M. Hall, C. E. 

Jones, D.-C. Lee, D. Teagle (All at: Dept. Geol. Sci. 

Univ. of Michigan, Ann Arbor, MI 48109) 

A. J. Walder (FI Elemental, Winsford, Cheshire, U.K.) 

P. A. Freedman (Fisons Instruments, VG Isotech, 

Middlewich, Cheshire, U.K.) 

Inductively-coupled plasma (ICP) sources offer 

considerable advantages over thermal sources because of the 

high ionization efficiency, facilitating relatively high 

sensitivity measurements for elements such as Hf. The mass 

discrimination (bias) is larger than for thermal ionization 

(typically about a percent per a.m.u. for Pb), and favors the 

heavier ions, decreasing with increasing mass. However, 

unlike for TIMS, this discrimination is largely independent of 

chemical or physical properties of the element or duration of 

the analysis. This can be demonstrated at high precision with a 

double focusing multi-collector magnetic sector mass 

spectrometer with an ICP source (ICPMCMS). For example, 

the reproducibility of Pb isotopic data using Tl to correct for 

mass bias is superior to that achieved by TIMS. An additional 

feature of such an instrument is that it offers the possibility of 

accurately measuring parent/daughter ratios by direct 

comparison with standards of known composition, critical in 

the development of laser probe techniques. A new wide flight 

tube and modified magnet capable of measuring U and Pb 

simultaneously have been incorporated into an ICPMCMS. 

Measurements on accurately prepared standard solutions show 

that for elements with high first ionisation potential (e.g. Sm, 

Nd, U, Tl, Pb) the element ratios are affected by the adjustment 

ofRF power to the plasma. However, at constant and high RF 

power (1.5 kW) and using the wide flight tube, U/Pb ratios can 

be accurately determined by running a similar standard and 

comparing relative ion beam intensities with those of the 

unknown. Correction is possible despite mass bias effects over 

this large mass range and differences in the ionisation efficiency 

of U and Pb. Isobaric interferences from molecular species are 

usually minor and simple because complex molecules are 

unstable in an ICP source. Use of a dry plasma, as with laser 

ablation or a desolvating nebulizer, minimizes hydride and 

oxide formation. 
THE DYNAMIC RANGE OF ARGON RETENTION IN 

SINGLE CRYSTALS, EXAMPLES FOR MUSCOVITE 

ACROSS THE ACADIAN METAMORPHIC FIELD 

GRADIENT IN WESTERN NEW ENGLAND 

HAMES, W.E., Department of Earth, Atmospheric, and 

Planetary Sciences, M.I.T., Cambridge, MA, 02139, USA. 

This study tests the practical limits of closure theory for 

laser 40Ar/39Ar muscovite age measurements by evaluating 

their relationship to peak metamorphism and unroofing along 

the Acadian metamorphic field gradient (MFG) of western New 

England. Peak metamorphic conditions vary by 500-700° C at 

5-10 kb from west to east on the MFG. The unroofing path is 

characterized by isothermal decompression, rapid cooling, and 

water-saturated melting of granitic rocks. Geochronologic data 

published in several sources provide the following regional 

constraints: ca. 385 Ma U/Pb and Nd/Sm crystallization ages 

for garnet and accessory minerals in schists; ca 375 Ma U/Pb 

crystallization ages for accessory minerals in post-kinematic 

granite dikes; 390----360 Ma hornblende 40Ar/39Ar plateau; 

and, 365--340 Ma muscovite and biotite 40Ar/39Ar plateau. 

New data indicate the integrated closure of 40Ar in 

muscovite, determined by laser fusion of entire crystals, 

occurred over 380-360 Ma in rocks of lower metamorphic 

grade; the range in intercrystalline 40Ar/39Ar ages was 

generally minimized at higher metamorphic grades by 

recrystallization and more rapid unroofing paths. However, 

large muscovite crystals from the highest-grade rocks (as that 

shown below) record retention of radiogenic argon during most 

of their metamorphic and unroofmg history. Laser spot fusion 

age measurements (with size and placement indicated by the 

apparent ages) for this muscovite phenocryst from a granite 

dike indicate initial retention of 40Ar at 371±4 Ma (very near 

to the timing of crystallization), and loss of 40Ar by volume 

diffusion over the ensuing 50 m.y. 

The new data are compatible with volume diffusion 

models (Dodson, 1973; Dodson, 1986) and geophysical models 

for metamorphic paths (e.g., England and Thompson, 1984). 

These data demonstrate that relatively complete metamorphic 

histories can evaluated with 40Ar/39Ar dating of muscovite 

crystals selected on the basis of size and paragenesis from a 

single exposure, and that the dynamic range of argon retention 

in muscovite can be extended to the range of U/Pb and Nd/Sm 

systems. 

lmm 

Ages inMa; 

10 m.y. contour interval. 

123 

AN EMPIRICAL STUDY OF FACTORS CONTROLLING 

40AR RETENTION IN MUSCOVITE FROM THE 

GASSETTS SCHIST, VERMONT APPALACHIANS 

HAMES, W.E., Department of Earth, Atmospheric, and 

Planetary Sciences, MIT, Cambridge, MA, 02139, USA. 

Variations in the size, major element composition, 

deformation characteristics, and relative crystallization history 

of minerals are evident in most metamorphic rocks. 

Microanalytical techniques permit age determinations to be 

made in the direct context of these variations, and thus 

empirical studies can readily test their influence on 40Ar 

retention during metamorphism and cooling. 

An exposure along the western flank of the ChesterAthens domes in Vermont (the locality for classic studies of 

the "Gassetts schist") contains feldspathic gneiss with high KFe muscovite and aluminous schist with high K-Na 

muscovite, and thus provides a setting to test the influence of 

several parameters on 40Ar retention in white micas. 

Penetrative metamorphic fabrics were developed in these 

lithologies during the Acadian (Devonian) event, a single 

schistosity is dominant within the exposure, and 

porphyroblasts are pre- to synkinematic with respect to this 

schistosity. Regional geochronologic and petrologic 

constraints indicate peak Acadian metamorphic temperatures 

(ca. 600° at a pressure of 8 kb) were reached at about 385 Ma. 

Early formed muscovite crystals from both the schist and 

gneiss (selected for their comparable size and lack of internal 

cleavage or deformation features) yield maximum laser spot 

fusion 40Ar/39Ar ages of 380±4 and 386±4 Main their cores. 

These crystals exhibit similar, radial patterns of age 

discordance that suggest loss of 40Ar via cylindrical volume 

diffusion over ca. 50 m.y. The population distribution of 

argon ages among single muscovite crystals does not vary 

appreciably for the schist and gneiss (based on 50 single 

crystal age determinations for each of two samples). These 

relationships indicate that retention of 40Ar in muscovite is 

strongly a function of physical grain size (consistent with 

previous laser studies), however, K-Na substitutions have 

relatively minor affects on mica retention characteristics. The 

data also indicate that argon retention can initiate in the cores 

of millimeter-diameter crystals at relatively high temperatures. 

Large early-formed muscovite crystals with obvious 

internal cleavage boundaries and mineral intergrowths yield 

ages that do not vary systematically with crystal size and are 

no older than about 360 Ma. In these crystals the effective path 

length for volume diffusion has been reduced from the physical 

crystal size to a distance controlled by the spacing of defects. 

In situ laser analyses of neoblastic, millimeter-diameter 

muscovite which defmes the dominant schistosity indicate that 

the cores of these crystals initiated retention of 40Ar at ca. 360 

Ma, and experienced volume diffusion loss of 40Ar over a 

shorter interval of 20 m.y. Regional geochronologic and 

structural data indicates these crystals grew after the peak 

conditions of Acadian metamorphism. 

Bulk sample methods for determining muscovite 

40Ar/39 Ar age in these lithologies would homogenize the 

observed age discordance; the resulting age would not reflect 

the integrated closure of a specific mica, but rather the mass 

balance of different mica sizes and structural types represented 

in the mineral separate. However, rather than emphasizing the 

problematic and complex nature of age discordance, 

microanalytical 40Ar/39Ar studies offer the potential to 

develop new standards for determining age discordance and 

representative estimates of closure age. 
DETAILED EMPLACEMENT CHRONOLOGY OF BASIC 

MAGMAS OF THE MID-PROTEROZOIC NAIN 

PLUTONIC SUTIE, LABRADOR: INSIGHTS FROM U-Pb 

SYSTEMATICS IN ZIRCON AND BADDELEYITE 

HAMILTON. Mike A., EMSLIE, Ron F., and 

RODDICK, J. Chris, Continental Geoscience Division, 

Geological Survey of Canada, Ottawa, Ontario, Canada. 

The Elsonian Nain Plutonic Suite (NPS) consists of a 

composite array of predominantly granitoid intrusions and 

large plutons of massif anorthosite, emplaced anorogenically 

over 19,000 km2. Associated with the largely adcumulate 

anorthositic rocks are smaller, but numerous bodies of 

troctolite and gabbro. Ferrodiorite also occurs, probably as 

late-stage differentiates from anorthosite crystallization. We 

report U-Pb data on zircon and baddeleyite which permits 

examination of the duration and high-resolution emplacement 

chronology of basic magmatism in the NPS. All cases yield 

concordant or only weakly discordant (<0.5%) U-Pb results, 

which help strengthen models ·involving genetic ties between 

anorthositic and ferrodioritic magmatism, in contrast to 

models favouring association of the latter with granitoid rocks. 

Zircons in NPS leucotroctolites and anorthosites (1R-AN) 

are typically clear to pale pink-brown euhedral prismatic grains 

exhibiting few fractures or inclusions, no overgrowths, and 

low U (-30 ppm) contents. Rare, individual acicular euhedra 

may reach up to 30 em in length. Preliminary U-Pb data from 

AN in the eastern and southern flanks of the complex indicate 

that the majority of magma batches crystallized over at least 17 

Ma from 1322±1 Ma (leuconorite, Makhavinekh) to 1305±2 

Ma (anorthosite, Koliktalik I.). Massive anorthosite of the Paul 

Island intrusion yields an age of 1319±1 Ma. Anorthosite and 

leuconorite of Kikkertavak Island and Tabor Island, and 

leucotroctolite cumulates from the Jonathan intrusion have 

identical ages of 1311±2 Ma. The entire magmatic interval for 

AN is doubled by inclusion of an Ukpaume intrusion 

leuconorite (1331±2 Ma) and leucotroctolite from 

southernmost Sango B'ay (1294±1 Ma). 

Fe-Ti oxide-rich, silica-poor differentiates such as 

ferrodiorites may contain appreciable amounts of both zircon 

(8-200 ppm U) and euhedral baddeleyite (-70 ppm U). The 

duration offerrodioritic (FD) magmatism (-24Ma) appears to 

mirror that described above. FD rocks at the margin of the 

mostly troctolitic Barth Island intrusion yield a concordant age 

of 1322±2 Ma. Isolated diorites exposed in the Jonathan 

intrusion crystallized simultaneously, within error (1312±3 

Ma) of adjacent troctolites. Immediately west, Tigalak 

intrusion ferrodiorites were intruded at 1306±3 Ma, while 

some sheet-like intrusions were emplaced in the southern 

quadrant of the NPS (Cabot Lake) as late as 1298±2 Ma. In 

some instances, large and often corroded or em bayed zircons 

have the appearance of being xenocrystic, potentially from 

NPS monzonitic to granitic magmas, some of which are 

known to have crystallized earlier. By example, zircons from 

the Satorsuakullik ferrodiorite dyke exhibit such morphology 

and yield a 1315±2Ma age, in contrast to a 1301±2 Ma age 

from magmatic baddeleyite in the same sample. Diorite from 

the Ukpaume intrusion has an age of 1333±2Ma, and extends 

the known duration ofFD magmatism to 35 Ma. 

The coincidence of ages between TR-AN and FD 

magmatism supports contemporaneous development of FD 

magmas from less evolved precursors, as is suggested by 

mineral compositional trends and tracer isotopic (Nd, Sr) 

compositions linking the two groups petrogenetically. 

124 

STABLE ISOTOPE COMPOSmON OF CONCRETION IN 

LOESS AS TRACERS OF THE PAST CLIMATE CHANGE 

HAN, .Iiamao and JIANG, Wenying, Institute of Geology, 

Chinese Academy of Sciences, P. 0. Box 634, Beijing 100029, 

China 

About 700 carbon and oxygen isotopic measurements were 

carried outon the samples from concretions of paleosols (S 1 -

Sl4) in Lishi Loess Formation. It shows that there are clear 

differences in both carbon and oxygen isotope composition 

between different kinds of paleosols. Usually the more 

developedthepaleosolis, the less negative the S0valueand 

the more negative the 13C value will be. 180 values vary 

within a range of about 1.3%o, ~13C, however, within a relative 

large range of 5.3o/oo. 

The isotopic record of the continental deposits is also a very 

useful tool in reconstruction of past environment. Based on our 

knowledge to the isotopic composition of modem carbonate in 

soil, an attempt was carried out using the isotope results semiquantitatively to estimate the environmental information from 

the time of each paleosol formation. 
IDGHL Y CORRELATED Pb - He - Sr ISOTOPES IN 

MID-ATLANTIC RIDGE BASALTS FROM 33°S 

B B HANAN San Diego State University, San Diego, CA 

D W GRAHAM Oregon State University, Corvallis, OR 

P J MICHAEL University of Tulsa, Tulsa, OK 74104 

We report Pb, He, and Sr isotopic analyses of MORB 

glasses from four southern MAR ridge segments from 31 o to 

34°S. The 33°S ridge segment (32.52°-33.47°S) is the central 

of three segments that are separated by left lateral offsets and 

bounded by the Cox and Meteor fracture zones. The remaining 

segment is located just north of the Cox fracture zone. 

Maxima in ridge axis elevation, Kffi, 206pbf204pb and 

87srt86sr, minima in Nas.o and 3He/4He, and a pronounced 

"bulls-eye" Bouguer gravity low occur simultaneously near 

33°S. 

Previously we have shown that the asthenosphere from 2°S 

to 47°S has been polluted by the off-ridge hotspots of Circe, 

St. Helena, and Tristan da Cunha, Gough, and Discovery 

(TGD). Variations in isotopic and incompatible element 

ratios along the ridge coincide with anomalous elevations of 

"zero age" crust [1-3]. Basalts from the spike-like 

geochemical anomalies define distinct binary-mixing vectors 

in Pb/Pb isotopic space between the off-ridge hotspots and the 

LILE-depleted asthenosphere. Basalts that would be 

considered "normal" MORBs by their (La/Sm)N <0.63, from 

ridge-segments between these anomalous segments, are also 

contaminated with re~ect to Pb isotopes. There is a broad 

maximum in 206pbf20 Pb and minimum in 3He/4He mposite 

the 206Pb-rich St. Helena plume, while 208pbf2 ~Pb is 

maximum opposite the 20"8Pb-rich TGD hotspots. We 

interpret this discrepancy between (La/Sm)N and radiogenic 

Pb and He to result from broad pollution of the 

asthenosphere by dispersion of these plumes during the time 

when they were intraplate, prior to being over-ridden by the 

migrating MAR. 

The 33°S geochemical anomaly has a short wavelength, 

high-amplitude spike-like profile superimposed on the broad, 

long wavelength trend. The anomalous enrichment shown by 

Pb and He isotopes, relative to the regional trend, is restricted 

to segment 4 ( < 100 km along strike length), where 

206pbf204pb increases to 19.3, 87srt86sr to 0.7029 and 

3He/4He decreases to 6.3 RA. The 206pbf204pb (18.0-19.3) 

and 3He/4He (8.0-6.3 RA) variations encompass much of the 

global range for MORB erupted away from the influence of 

known hotspots; 87srt86sr ranges 0.7025-0.7029. The 

206pbf204pb, 87srt86sr, and 4He/3He ratios are highly 

correlated, show good correlation with Kffi, poor negative 

correlation with Nas.o. and lack correlation with Fes.o or 

Mg#. All of the other ridge segments have distinct isotope 

signatures, suggesting that their magma sources are isolated 

from each another. Although the 33°S anomaly is similar to 

other MAR anomalies caused by hotspot-ridge interaction, 

the correlations in Pb, Sr, and He isotopic space are not 

consistent with binary mixing between the MORB 

asthenosphere beneath the 33°S segment and the off-axis TGD 

hotspots. Instead, an enriched source with 206pbf204pb 

greater than 19.3 is required. The combined Pb, Sr, and He 

isotope signature of the 33°S anomaly is distinct relative to 

the South Atlantic hotspots, and may reflect the drawing up 

by the migrating ridge system of a relatively small scale 

passive mantle domain, apparently unrelated to the South 

Atlantic hotspots at this time. 

1. Schilling et al. (1985) Nature 313, 187 

2. Hanan et al. (1986) Nature 322, 137 

3. Graham et al. (1992) EPSL 110, 133 

125 

COOLING AND UPLIFT IN A TERTIARY 

PLUME AREA: FISSION TRACK 

EVIDENCE FROM THE 

KANGERLUSSUAQ AREA, EAST 

GREENLAND. 

HANSEN .K. Geological Institute, 

University of Copenhagen, 0ster 

Voldgade 10, DK-1350 Ksbenhavn K, 

Denmark. 

In the early Tertiary, the Icelandic plume 

was located under the east coast of 

Greenland. Rifting across the plume was 

accompanied by extensive magmatic activity 

including flood basalts. The morphological 

features show that central uplift occurred and 

that the continental rims were uplifted 

subsequent to the rifting. 

New fission track (FT) investigations have 

been undertaken to yield information on 

cooling and uplift history in the region. 

Earlier FT investigations were performed for 

intrusive rocks confirming that Tertiary 

doming occurred in the area. Further 

information about the Tertiary cooling and 

uplift history can be obtained from 

sedimentary, metamorphic and pre-Tertiary 

intrusive rocks using the FT method. Mean 

track lengths range between c. 10.5 ~-tm and 

c. 14.0 ~-tm (the highest means were obtained 

for Tertiary intrusive rocks) while mean ages 

and single grain ages range from Permian to 

Tertiary. The present investigation shows the 

presence of several cooling paths in the area. 

Intrusive rocks in the center of the dome 

first cooled to the temperature of the 

surroundings and then followed the ~ooling 

paths of their surroundings determined by 

exhumation and uplift. The temperature 

histories of the surroundings show a variable 

amount of Tertiary overprint. Their 

surviving pre-Tertiary tracks record the early 

cooling/erosion history. FT analysis has 

shown late fast cooling to occur along the 

east coast of Greenland between 66° and 73° 

and thus no relaxation along rift shoulders 

occurred as would be expected. 
182Hf-l82W: A NEW EXTINCT RADIONUCLIDE 

CHRONOMETER OF ACCRETION & CORE FORMATION 

HARPER. C.L. Jr., and JACOBSEN, S.B., Dept. 

of Earth & Planetary Sciences, Harvard Univ., 20 

Oxford Street, Cambridge, MA, 02138, USA. 

The 182Hf-182W chronometer is likely to become a 

powerful tool in studies of the accretion and very 

early differentiation histories of planets. Careful 

investigation of the 182Hf-l82W system is also 

important in astrophysics because a determination of 

the initial abundance of 182Hf will provide a key 

constraint on models of the molecular cloud 

environment within which the sun formed. The 9 Ma 

half-life of 182Hf provides an excellent timescale for 

studies of both star-forming and planet-building 

processes. Several geochemical aspects also make 

the 182Hf-182W systematics particularly well suited 

for dating planetary accretion-related (as opposed to 

nebular) processes, especially core formation. 

182W Measurements: W isotopic measurements 

using WOa- beams were developed by Heumann and 

co-workers in Regensburg, and the first high 

precision measurements of meteoritic W made there 

were reported by Harper et al. (1991). Relative to a 

183Wfl84W normalization, an apparent shift of 

about -3.0±0.9 epsilon-units in 182W fl84W was 

observed in Toluca (iron meteorite) W relative to a 

standard. Radiogenic growth of 182W in the silicate 

portion of the Earth (high-Hf!W) after core formation 

would explain the anomaly, (with iron meteorites 

representing initial 182Wfl83W). To confirm the 

effect we developed a high precision multicollector 

technique utilizing a 7 -cup configuration with comonitoring of 18,17Qfl6Q fractionation and ReGsinterference. Reproducibility for 182W fl83W 

measurements (normalized to 184W fl83W) is shown 

indicating an -±1£-unit precision for standards. 

Three new preliminary measurements appear to 

confirm the previously-reported -3±1 e-unit anomaly. 

HARVARD NTIMS DATA, W ISOTOPES 

~ ~ .~T+~~~~~~~~~~ 

g ·1 

e Standards (18 runs) 

• Toluca Iron Meteorite _, ..... ___________ ,_J 

126 

MAGMATIC PROCESSES AND ENVIRO~S 

AS REVEALED FROM NOBLE GAS ISOTOPES 

-A CASE STUDY FOR UNZEN VOLCANO. JAPAN 

HANYU, T. md KANEOKA,I., Earthquake Research Institute. 

Univ. of Tokyo, Bunkyo-ku, Tokyo. 113, Japan. 

Noble gas study of coexisting minerals in volcanic rocks 

might provide a new approach for the magmatic processes and 

environments. It is commonly assumed that minerals, except 

for xenocrysts. observed in a volcanic rock are formed under 

an equilibrium state and the occurrence of an unequilibrium 

state among some kinds of minerals in a volcanic rock has not 

always been emphasized. Isotope studies for each kind of 

minerals would give us more information concerning the 

magmatic processes and environments where minerals 

crystallized. Among them. noble gas analyses would be quite 

effective, because noble gases are chemically inert and include 

both radiogenic and stable isotopes, which make them to be 

sensitive for small changes of magmatic environments. 

As a case study. we have performed noble gas analyses of 

volcanic rocks of Unzen Volcano, Japan. In November. 1990. 

it started to erupt and dacitic lava domes are still growing. It 

also poured out dacitic lava flows in 1792. Rocks in both cases 

contain large phenocrysts (up to about lOmm in size) of 

plagioclase. hornblende and biotite. Phenocrysts of different 

kinds of minerals were prepared for noble gas analyses by 

handpicking. Phenocrysts of plagioclase and hornblende were 

sorted out further according to their size. Noble gases were 

extracted from samples by using an induction heater at 700t 

and t600t, respectively and measured on a quadrupole mass 

spectrometer. 

The most remarkable result is that plagioclase samples show 

systematically higher 40Arf36 Ar ratios (31Q-360) than hornblende 

and biotite samples (296-310) in the 1600t fraction. In the 

700t fraction. most samples show 40Ar/ 36Ar ratios similar to 

the air value (296), representing the high air contamination in 

this fraction. On the other hand, the gases extracted at 1600'C 

can be interpreted to represent magmatic ones. 40 Art' Ar ratios 

of hornblende and biotite samples are a litde higher than the air 

value and obviously distinct from those of plagioclase samples. 

It indicates that the magmatic en~ironment where plagioclase 

crystallized was different from that where hornblende and biotite 

did, implying the crystallization at a different place and/or at a 

different time in the magma. It is also notable that plagioclase 

and hornblende of larger grain sizes show higher 40

Ari~ Ar ratios 

than those of smaller grain sizes. respectively. which might be 

directly related to the crystallization process of minerals. 

The variation of 40ArP6Ar ratios among different kinds of 

minerals and among samples of different grain sizes in the 

same kind of minerals might have been caused by mixing of 

original magmatic Ar and atmospheric Ar. The carrier of 

atmospheric Ar is considered to be groundwater. We infer that 

the 40Ar/36Ar ratio of the magma chamber was originally high 

(35o-400) and it gradually decreased as contamination of 

groundwater into the magma proceeded. Thus. it becomes 

possible to infer the time when each kind of minerals crystallized 

and we will get information of magmatic history. 
PRODUCTION OF COSMOGENIC NUCLIDES BY 

BOMBARDMENT OF THICK TARGETS WITH 

60,90, AND 120 MEV ALPHA PARTICLES· II. 

MEASUREMENT 26AJ AND lOse BY AMS. 

LI Harrjs1, R.L. Paul1, P.AJ. Englert1, E.B. 

Norman2, K.T. Lesko2, R.M.Larimer2, I. 

Goldman2, C. Jackson2,B.Napier2, B. Sur2, M.A. 

Caffee3, R.C. Finkel3, J.R. Southon3• 1) San Jose 

State University, Nuclear Science Facility, San Jose, 

CA 951922) 2)Lawrence. Berkeley Laboratory, 1 

Cyclotron Rd. Berkeley, CA 94720. 3) NCD/CAMS 

Lawrence Livermore National Laboratory, PO Box 

808, Livermore, CA 94550. 

We determined the average thick-target cross 

sections for the production of the long-lived cosmogenic 

nuclides 26Al and lOBe by alpha particle bombardment of 

cosmochemically important targets [1]. These crosssections are needed to interpret measurements of cosmic 

ray induced nuclear reactions in terrestrial and 

extraterrestrial material [2]. Understanding the 

production parameters of cosmogenic nuclide levels in 

extraterrestial matter (meteorites, cosmic dust, and 

planetary surfaces) provides a means of studying the 

history of both the irradiated matter and the cosmic 

radiation itself [3]. 

Pure targets of C, Mg, AI, Si, Si02, Fe, Ni, Bi, 

and Ge and Bi4Ge3012 were bombarded with 60, 90, and 

120 MeV alpha particles at the Berkeley 88 inch 

cyclotron. For all of these alpha energies the single-foil 

technique was used [4], where each target was 

individually wrapped or sandwiched between two high 

purity gold foils which are used as beam monitors. 

Following the separation chemistry [5] 26Al and lOse 

were measured using accelerator mass spectrometry and 

the experimental cross sections were calculated [6]. The 

experimental cross sections were compared with earlier 

published data where found and agree to satisfaction. 

REFERENCES. [1] Arnold, J.R. et al; J. of Geophys. 

Res 66, (1961) 3519-31.[2] Mahoney, W.A. et al; 

Astrophsics J., 278, (1984) 784. [3] Reedy, R.C. et al; 

20th Lun. and Plan. Sci. Conf. LA-UR-89-87 Hus. Tx, 

1989. [4] 0. Chamberlain. Phys. Rev. Let. 15 (1969) 485. 

[5]Theis, S. and Englert, P. et al, J. of Radioanal. and 

Nucl. Chern. 100 (1989). [6] Dittrich, B. et al; Nucl. Instr. 

and Meth. in Phy. Res. B52 (1990)588-94. 

127 

OXYGEN ISOTOPE GEOCHEMISTRY OF 

MESOZOIC RHYOLITES AND GRANITES 

OF NW NAMIBIA 

HARRIS, Chris, Department of Geological 

Sciences, University of Cape Town, Rondebosch 

7700, South Africa 

The oxygen isotope ratios of unaltered phenocrysts 

from rhyolites are particularly useful in constraining 

the nature of the source from which the rhyolites 

were derived. The Etendeka/ParanA "flood" 

rhyolites which erupted just prior to the rifting of 

Africa and South America, can be divided into a 

high-6 1SO type (6 1S0-10%o) and a low-6 1SO type 

(6 1S0-7%o). In Namibia, the high-6 1SO rhyolite type 

occurs in the south and the low-6 1

SO type occurs in 

the northern part of the province. The high-6 1SO 

rhyolite type appears to be the more abundant 

throughout the Etendeka/Parana province. Analyses 

of fresh mineral phases from the major Mesozoic 

complexes of NW Namibia show that the 6 1

SO 

values of the original magmas are to some extent 

related to the nature of the crust through which they 

were intruded. Granites from the Spitzkoppe and 

Klein Spitzkoppe complexes and rhyolites from the 

Erongo complex, which are situated in the middle of 

the Central Zone of the Pan-African Damara 

orogenic belt, have the highest magmatic 6180 values 

of 11 and 12%o respectively, whereas granitic rocks 

from the Brandberg and Messum complexes, to the 

north of the Central Zone, have lower 61SO values. 

Although they are not petrogenetically related, it 

seems probable that the granitic rocks from the 

Central Zone complexes and the southern rhyolites 

share a common origin; that is they formed by 

partial melting of Damara S-type granite restite. H 

the source(s) of the northern rhyolites and the 

Brandberg and Messum granites is within the crust, 

then that source must be dominantly Archaean 

lower crust of the Congo Craton. Alternatively, these 

rocks might have originated by partial melting and 

subsequent crustal contamination of underplated 

Mesozoic basalt. 

The eruption of large volumes of crustally-derived 

magmas during the breakup of southern Africa and 

South America contrasts with the situation during 

the rifting of southern Africa and Antarctica where 

large volumes of rhyolite produced by partial melting 

of underplated basalt were produced. This contrast 

may be related to differences in the size and/ or 

position of the underlying mantle plumes relative to 

initial rifting in the two areas. 
PROGRADE THERMOCHRONOMETRY USING SIMS 

ANALYSIS OF MONAZITE 

HARRISON, T.M., McKEEQAN. K.D, AKERS, W., Dept. 

of Earth and Space Sciences & IGPP, UCLA, Los Angeles, 

CA, 90024, U.S.A., RYERSON, F.J., Lawrence Livermore 

Nat. Lab., Livennore, CA, 94550, U.S.A. 

It is now well known that although monazite can retain Pb* 

at sustained temperatures of >700°C, it is unstable during low 

grade metamorphism in petites. Detrital monazite dissolves 

under diagenetic conditions and does not reappear until upper 

amphibolite facies. During this period, the components of 

monazite appear to be held in LREE/fh oxides, apatite, and 

allanite. This unusual behavior suggests the use of monazite as 

a thennochronological record for prograde metamorphism. 

Three related developments are required to exploit this 

approach; isotopic microanalysis of monazite, knowledge of 

the stability and reaction kinetics of LREE oxides in pelitic 

assemblages, and a Pb diffusion law for monazite. 

Measurements of the U-Th-Pb systems in monazite using 

the UCLA CAMECA ims 1270 ion microprobe suggest three 

properties that make it analytically less demanding than 

zircon; 1) the concentrations of U and Th (and thus Pb*) are 

typically 102-104 times higher, 2) the mass resolution required 

to separate Pb isotopes from their principal interferences 

(mostly LREE(POv+) is -4000 compared to -7000 to resolve 

HfSi+ in zircon, and 3) the yield of Pb ions from monazite is 

-2x higher than that from zircon. Tests on a standard monazite 

indicate a sensitivity of 40 cps/ppm Pb/nA 0 2-. Spot analyses 

of Tioga monazite yield (207Pbf206Pb )*=0.0536±0.0008 (±2 

<r_} which agrees with the Roden et al. (1990) TIMS 

detennination of (207Pbf206Pb)*=0.05442±0.00008. We are 

investigating several relationships for detennining interelement ratios: 208Pb+tp+ vs. Th02 +tp+, 208pb+f232Th+ vs. 

ThO+ ffh+, and direct comparison of 208Pb+ f232Th+ between 

sample and standard. The energy distributions of Pb+, u+, 

Th+, ThO+, and Th02 + ions are markedly different, with Pb+ 

behaving in a fashion more similar to molecular ions. 

Ion imaging of monazites known to contain inherited Pb* 

show clear core-rim relationships similar to the well-known 

zircon inheritance. Spot analyses (-15 J.ll11) of the rim of a 

monazite grain from the Sweetwater Wash pluton, SE 

California, yield 208Pbf232Th ages of 71±3 Ma, in accord with 

the known crystallization age of the pluton. The core yields a 207PbP06Pb age of 1 ,546±9 Ma which agrees with estimates of 

early crustal growth in the SE Mohave. 

Our hydrothennal experiments preclude rhabdophane from 

being the immediate precursor to neofonned monazite in 

lower amphibolite facies, but rather suggest that equilibria 

involving the reaction apatite + LREE oxides = monazite + 

calcic phase are responsible for its appearance. We estimate 

the monazite isograd in petites to be 450°-500°C at midcrustal pressures. 

Preliminary diffusion measurements using a La-monazite 

synthesized in a Ph-bearing flux heated in a reservoir of Phfree La-monazite powder yield np., = 4· x IQ-18cm2/s at 

1000°C. 

Roden, M.K., Parrish, R.R., and Miller, D.S., 1990, The 

absolute age of the Eifelian Ash bed: J. Geol., 98, 282-285. 

128 

EMINENT DOMAINS: K-FELDSPAR 

THERMOCHRONOMETRY 

HARRISON T M . LOVERA, O.M., GROVE, M., Dept. of 

Earth and Space Sciences & IGPP, UCLA, Los Angeles, 

CA, 90024, U.S.A., HEIZLER, M.T., New Mexico Bureau 

of Mines, Socorro, NM, 87801, U.S.A. and FITZGERALD, 

J.P., Research School of Earth Sciences, Australian National 

University, Canberra, A.C.T., 2601, Australia. 

The observation of correlated inflections between K-feldspar 

age spectra and Arrhenius plots is compelling evidence that 

this phase behaves in a broadly similar fashion during 

laboratory heating as it did during slow cooling in nature. To 

assess the degree to which this approximation holds, we have 

employed three kinds of probes to image K-feldspar samples; 

electrons, X-rays, and Ar isotopes. In general, we find 

lengthscales for Ar diffusion ranging from -~ to 100 J.l.ffi, a 

low density of extended defects, and no significant changes to 

cell dimensions, Or content, or Si/ AI ordering upon heating to 

1100°C or from irradiation doses corresponding to J-Q.Ol. 

Madagascar orthoclase reveals only small deviations from a 

single Arrhenius relationship relative to most perthites 

restricting intrinsic mechanisms to less than -15% of the total 

observed variation in log(r/rJ. Experiments to assess the effect 

of laboratory heating on K-feldspar microstructures show 

excellent reproducibility of Dtr2 and reveal no significant 

changes in diffusion properties up to 80% argon loss. Relative 

differences in domain size determined from sizing experiments 

indicate that domains have effective sizes between ~ and 100 

J.l.ffi. Numerical analysis indicates that the effect of nonunifonn 4~ distributions is small. For example, modeling 

perthite-like modulations as a cosine distribution results in an 

initial drop in D/r2 of only a factor of -3. 

In vacuo crushing experiments of K-feldspars typical of 

those used for thermochronometry indicate that only -Q.1% of 

the 39 Ar can be accessed by crushing. Anomalous age spectra 

of samples crushed below the largest domain size indicate 

subtle differences in the siting of 40 Ar* relative to 39 Ar. 

Identification of 40 Ar E/Cl component via isothennal step 

replicates (MR CLEAN) permits correction of excess 40 Ar from 

most K-feldspar age spectra. Ion imaging identifies -100 nm 

sized inclusions as containing the Cl-correlated excess Ar. 

The existence of discrete argon retentivities in basement Kfeldspars is a fact that any K-Ar geochronologist can verify. 

We find that assuming these retentivities are due to a 

distribution of diffusion domain sizes reconciles otherwise 

problematic aspects of age spectra and Arrhenius re$ults and 

pennits recovery of high resolution thennal histories. The 

results of the experiments described above indicate that most of 

the contrast in argon retentivity in K-feldspars is diffusion 

domain size related. Although continually testing the 

underlying assumptions of the multi-diffusion domain model, 

we have yet to encounter evidence seriously challenging the 

basic approach. 
LASER EX'IRACTION AND ANALYSIS OF 

EX1RA TERRESTRIAL He AND Ne ISOTOPES 

HARROP Paul J , Department of Geology, University of 

Manchester, Manchester, M13 9PL, United Kingdom, 

STUART, F.M., S.U.R.R.C., East Kilbride, Glasgow, 

G75 OQU, United Kingdom, and TURNER, G., 

Department of Geology, University of Manchester, 

Manchester, M13 9PL, United Kingdom. 

Ultra-sensitive noble gas mass spectrometry (MAP 215) 

combined with laser gas extraction with extremely low He and 

Ne blanks, has been used to analyse He and Ne in minute 

extraterrestrial samples ( < 1J..Lg), e.g. micrometeorites, rich in 

noble gases. The laser technique has a number of advantages 

including high spatial resolution of 10's J..liil, increased 

sensitivity by reducing extraction line volumes, low blank 

levels by localised heating of the sample surface and a rapid 

throughput of samples by reducing the time spent removing 

active gases. 

A pulsed Nd-glass laser has been used to extract noble 

gases, either by vaporising pits in the surface of mineral grains 

or by melting/step heating individual grains. 

Initial tests were performed on troilite from the Grant 

meteorite and individual glass spherules from a lunar clod 

(15426.41). The Grant troilite was tested as a possible 

calibration standard for laser extraction of extraterrestrial helium 

because it has a known helium abundance (5x10-6 cm3STPg-l 3He) and a very well characterised and consistent 3Het4He of 

0.2635 ± 0.0034 (Schultz & Kruse, 1983). 

Helium and neon was analysed from 12 individual basaltic 

glass spherules (100-150J..LII1) from lunar clod 15426.41. In two 

cases step-heating experiments were performed by initially 

defocusing the laser beam from the sample surface and 

gradually bringing the beam into focus until the sample was 

totally degassed, the remaining 10 grains were fused 

individually as single laser extractions. Total helium released 

from individual spherules varied from< lx1Q-l4 to 3x1Q-l0 

cm3STP 4He (< 3xto-9 to lx1o-4 cm3STPg-l 4He). 3He/4He 

values of single spherules varied from (4-200)xi0-4. The 

observed 3He/4He is the result of radiogenic 4He production 

since formation of the volcanic spherules and later addition of 

cosmogenic helium during near-surface exposure. If the 

spherules have existed as a coherent clod from soon after 

formation, and having a similar chemical composition, they 

would be expected to have similar 3He/4He values and helium 

abundances. The variations observed in the individual spherules 

can be explained by differential amounts of devitrification 

causing partial loss of radiogenic 4He tor to near-surface 

exposure, thus affecting the resulting He/4He value observed. 

Preliminary analyses of cosmic dust particles (50-400J..LII1) 

collected from the Antarctic ice sheet revealed that the majority 

of 50-100J..Lm particles have 3Het4He between (2-4)xi0-4, i.e. 

between SW- and solar energetic particle-derived helium. Only 

2 larger cosmic dust particles (100-400J..Lm) had detectable 

helium(> 2.5xto-t6 cm3STP 3He), both showing significantly 

higher 3Het4He values indicating the presence of cosmic ray 

spallation helium. 

Schultz, L. and Kruse, H., 1983. Helium, Neon, and Argon 

in Meteorites: A Data Compilation. Max-Planck-Institut filr 

Chemie, Mainz. 88pp. 

129 

Sr ISOTOPIC CHARACfER OF ACCRETED TERRANES IN 

THE NORTHERN CANADIAN CORDILLERA: 

IMPLICATIONS FOR BASEMENT ARCHITECTURE 

HART. Craig J.R., Canada/Yukon Geoscience Office, 

Yukon Territorial Government, Box 2703 (F-3),Whitehorse, 

Yukon, YlA 2C6, Canada. 

Stikinia, Cache Creek Terrane and Nisling Terrane are 

dissimilar crustal blocks that accreted to the western edge of 

ancestral North America in Early Jurassic time. Stikinia, north 

of 59°N latitude, is composed of Triassic arc volcanics and Late 

Triassic to Jurassic forearc (Whitehorse Trough) clastics. 

Cache Creek Terrane is an oceanic terrane composed of 

Mississippian to Triassic basalt, ultramafite, carbonate, chert 

and clastics. Nisling Terrane, part of the composite YukonTanana Terrane, comprises metasedimentary schist, marble, 

orthogneiss and amphibolite of Cambrian and older age. These 

terranes (and their components) have characteristic Sr isotopic 

signatures. 

Cache Creek basalts yield expected primitive Sr initial ratios 

(SIR; 0.7039±5) while the carbonates give higher values 

(0.7081±5) reflecting Upper Paleozoic sea water composition. 

Clastic sediments give mixed signatures reflecting variable 

percentages of detrital carbonate and continentally derived 

material. Stikinian Triassic arc volcanics give low initial ratios 

(0. 7040±5), and arc-derived sedimentary rocks yield similar, 

but locally anomalous values (0.706). 

Nisling Terrane metasedimentary schists have high present 

day SIR (0.725±10) and 87Rb/86sr ratios (0.5-5). Although 

modified by Mesozoic and Cenozoic thermal events, most data 

plot within Rb/Sr boundary isochrons of 800 to 1600 Ma (with 

an SIR of ca. 0. 706) supporting an ancient provenance for these 

rocks. Marble gives present day values of 0. 711. Felsic 

orthogneiss yields high modem values (ca. 0.7085±25), but give 

geologically reasonable initial ratios (0.7058±15) when 

corrected for 380 Ma of elapsed time. Similarly, amphibolite 

yields geologically reasonable initial ratios of 0.7046±10 

assuming a 550 Ma age. 

Primitive values from the Triassic Stikinian arc preclude 

suggestions that this arc was built upon Nisling Terrane 

basement. Locally evolved signatures in Stikinian sediments 

indicate a local and minor component of continental detritus, 

probably from Nisling Terrane, in Late Triassic time. Nisling 

Terrane comprises isotopically evolved crust of Middle 

Proterozoic age (or is composed of detritus of that age), Early 

Paleozoic amphibolite and Silurian-Devonian orthogneiss. 

SIR of Late Triassic to Early Eocene plutons that intrude 

these terranes occupy a large range (0.735-0.707) that spatially 

defines a region of low Sr initial ratios bounded west, north and 

east by higher values marked by the >0.7061ine. A sharp SIR 

gradient coincident with the Nisling-Stikinia contact argues for 

a structural juxtaposition. Lower SIR values in more southerly 

(60°N) Nisling Terrane granites result from decreasing amounts 

of continental contamination and suggests that Nisling Terrane 

here is thin, without basement and possibly allochthonous. The 

disposition of rocks that yield continental values outboard 

(west) of accreted (and exotic) terranes is enigmatic. 
THE GEOCHEMISTRY OF MCMURDO GROUP 

VOLCANIC ROCKS, ANTARCI'ICA 

HAKL S. R., BLUSZTAJN, J. S., Dept. of Geology and 

Geophysics, Woods Hole Oceanographic Institution, 

Woods Hole, MA 02543, and KYLE, P. R., Dept. of 

Geoscience, New Mexico Tech, Socorro, NM 87801, 

Young Cenozoic volcanic rocks from Marie Byrd Land 

(Futa and LeMasurier, 1983) and Scott and Balleny Islands 

(Hart, 1988) are unusual in showing 87srJ86sr ratios lower than 

most oceanic island basalts, with some values approaching 

those typical of mid-ocean ridge basalt (MORB). To assess the 

geographic extent of this low 87/86 Sr province, and to decide 

whether a MORB-type mantle is involved, we have analyzed a 

reconnaissance sampling of basalts from the Hallett volcanic 

province, Northern Victoria Land, for Sr, Nd and Pb isotopic 

ratios. A number of Hallett Province basalts show 87/86 Sr 

ratios less than 0. 7030, comparable to the low values reported 

earlier for Marie Byrd Land and Scott/Balleny islands. 

However, the Pb isotope data for all of these areas is quite 

radiogenic, with 206/204 Pb generally greater than 19.5 and 

ranging up to 20.4. This is in marked contrast with MORB, 

which rarely show 206/204 Pb ratios higher than 19.0. It is 

clear that many of the young Cenozoic volcanics from 

Antarctica are showing a strong involvement of the HIMU 

mantle component. 

In addition to the prevalent HIMU signature of these 

volcanic provinces, there is also a radiogenic 87/86 Sr 

component evident, especially in the Hallett Province, with 

87/86 Sr values as high as 0.7049. This suggests possible 

involvement of the EM2 endmember. However, many of the 

Antarctic basalts are erupted through continental crust, and 

contamination with crust typically produces an EM2-like 

isotopic signature. In the Hallett Province basalts, it is clear 

that the low 87/86 Sr (HIMU-type) samples all come from 

oceanic or continent-edge localities and the samples with a high 

87/86 Sr (EM2-type) signature occur farther inland. Sr isotope 

data from the Mt. Melbourne area (Worner et al., 1989) show 

values ranging from 0.7029 to 0.7039, so this area may be 

bridging the transition from "oceanic" to "continental" type 

isotopic signatures. 

The boundary marked by the isotopic data is consistent 

with the general location of the West Antarctic rift shoulder of 

Behrendt and Cooper (1991), or locally the Terror Rift of 

Cooper and Davey (1985). Basalts on the seaward side of this 

rift edge are uniformly low in 87/86 Sr, whereas higher values 

are encountered in basalts on the inland side of the rift edge. 

We hypothesize that volcanics which erupt through significant 

continental crust become variably contaminated with 

radiogenic Sr, whereas those erupted through extensionallythinned continental crust, or oceanic crust, are not significantly 

contaminated. In addition, the HIMU component appears to be 

present to some degree in virtually all of the Antarctic 

Cenozoic volcanic rocks thus far analyzed; the lateral extent of 

this HIMU volcanic belt is enormous (>4000 km). Verification 

of the isotopic "lineage" of this belt and illumination of the 

underlying causes and processes remains as a significant 

challenge for the future. 

130 

MEASURED AND CALCULATED DEPTH 

PROFlLES OF 26AL IN QUARTZ MINERALS OF 

THE LITHOSPHERE 

J. HARTMANN, B. HEISINGER, 

G. KORSCHINEK, E. NOLTE, E. STRACK, 

Faculty of Physics, Technical University of Munich, 

D-857 47 Garching, Germany 

S. NEUMAIER, CERN, CH-1211 GENEVE 23, 

Switzerland, 

C, PETIT-JEAN, PSI, CH-5234 Villigen, 

Switzerland 

The natural long-lived radioisotope 26AI (T112 = 

716,000y) is produced in quartz minerals by cosmic 

ray induced spallation reactions on Si and by cosmic 

muon capture reactions Si (J..L-, v~ xn). Depth profiles 

of cosmogenic 26 AI in quartz minerals of the 

lithosphere were calculated and measured. For the 

calculation, the branching of the reaction 

Si (J.L-, v~ xn) 26AI was measured by irradiating a 

quartz sample with slow negative muons at PSI in 

Villigen (CH) and by determing the produced 26AI 

with accelerator mass spectrometry (ams). For the 

measurement of the depth profile, 26 AI concentrations 

of quartz samples from pre-drill cores of the 

continental deep-drill core in N orthem Bavaria 

(Germany) from depths between 10 and 260 m were 

determined with ams. The comparison between 

measeured and calculated depth profile allows to set 

limits for the erosion rate. The agreement between 

measured and calculated depth profile is excellent for 

vanishing erosion rate. 

Other combinations of radioisotopes and minerals, 

such as 53Mn (3.7 million years) in pyrites, are 

discussed. 
THERMOCHRONOLOGIC VIEW ON THE 

EVOLUTION OF ACCRETIONARY COMPLEXES 

HASEBE N. TAGAMI, T. and NISHIMURA, S. 

Department of Geology and Mineralogy, Faculty of 

Science, Kyoto University, Kyoto 606, Japan 

Fission-track thermochronology was applied to Shimanto 

accretionary complex, Southwest Japan, to place thermotectonic 

constraints on the evolution of an accretionary complex from a 

viewpoint of material transport within the wedge. Sandstone 

samples were collected from coherent turbiditic sequences and 

blocks in melange matrix. Age spectra for individual samples 

were correlated to their depositional ages to judge the degree of 

thermal affection during accretionary processes (Hasebe et al., 

1993). 

Apatite ages show a good agreement around 10 Ma, in 

spite of their variation in depositional ages and rock facies. In 

consideration of their depositional ages consistently older than 

-10 Ma, this demonstrates that the Shimanto accretionary 

complex was heated higher than -125°C (apatite total annealing 

temperatme) and subsequently cooled regionally below -100°C 

(apatite closure temperature) at -10 Ma. Zircon ages and track 

length distributions indicate that the materials with higher 

maximum temperatures are cropped out at the rear of the prism 

with some local variation, showing no difference in thermal 

parameter values for both coherent and melange facies. Hence 

melanges and surrounding coherent rocks are considered to have 

followed the similar time-temperature paths through their 

histories after accretion. Therefore the melange formation 

should generally be explained by the processes without long 

vertical transport of melange forming materials. 

On the basis of the thermal history characteristics 

mentioned above, material transportation paths within 

accretionary prisms are proposed in consideration of following 

two aspects: First, the thermal structure of the wedge is parallel 

to the surface at shallow levels. Second, accreted materials 

move toward the rear of the wedge during successive accretion 

when the position of trench is flxed. The history of the accreted 

material is divided into two phases; the burial and uplift. The 

position of phase boundary in a wedge would be controlled by 

some accretionary tectonics; the formation of imbricated 

structure at the frontal part of the prism would contribute to the 

material burial, and underplating at depth that would disturb 

further burial could force the material uplift. When the accreted 

materials arrive at the boundary, the maximum temperature is 

recorded, and begin to uplift. Earlier accreted materials reach the 

boundary earlier, resulting in the exposure of the deeper buried 

materials at the rear of the wedge because they stayed longer in 

the uplift phase. The conflguration of melange and coherent 

units would be attained during the local internal deformation 

caused by burial and uplift processes. 

Hasebe, N., Tagami, T. and Nishimura, S., 1993, Evolution of 

the Shimanto accretionary complex; a fission-track 

thermochronologic study: Geol. Soc. Am. Spe. Pap., v. 273, 

p. 121-136 

131 

COMPLEX U-PB SYSTEMATICS OF 

PALEOPROTEROZOIC MONAZITE FROM THE 

GRAND CANYON, ARIZONA, USA 

HAWKINS. David P., and BOWRING, Samuel 

A., Dept. of EAPS Massachusetts Institute of 

Technology, Cambridge, MA 

Complex U-Pb monazite data are often explained 

by the presence of variable amounts of excess 206Pb, 

inheritance, and/or high temperature Pb loss. We 

present data from more than 65 clear, inclusion-free 

single monazite grains, or fragments of single 

grains, that suggest these monazites contain domains 

characterized by contrasting age and/or discordance. 

The monazites were separated from seven samples 

of ca. 1. 7 Ga muscovite granites, pegmatites and 

biotite schists that have undergone a period of slow 

cooling from ca. 1. 7 to ca. 1.6 Ga. 

Within individual rock samples U-Pb and Pb-Pb 

ages of single monazite grains vary considerably and 

range from 40% reversely discordant to 10% 

normally discordant. Reverse discordance cannot be 

explained by excess 206pb alone. The U-Pb ages of 

essentially concordant grains vary by as much as 20 

Ma in a single sample. Backscattered electron 

imaging reveals no evidence for inheritance. 

No relationship is observed between degree of 

discordance (and age) and: 1) grain size (long 

dimension ranging from 200 to 30 J..lm); 2) grain 

shape (prismatic, equant, and platy); and 3) U 

concentration (250 ppm to >350,000 ppm). Several 

single grains were broken into a number of 

fragments, some estimated to weigh as little as 100 

nanograms. Within one grain, two fragments are 

nearly concordant with Pb-Pb ages of 1676 ± 1 and 

1682 ± 1 Ga. Two other grains exhibit little U-Pb 

age variation between fragments, but show a large 

variation in the degree and sign of discordance. In 

one of these grains, the center part is 1% reversely 

discordant whereas part of the tip is 40% reversely 

discordant. The relationship between ThiU ratio and 

calculated excess 206pb suggests that the reverse 

discordance in the tip resulted from loss of U 

relative to Th and Pb. In another grain, five 

fragments yield similar ages but range from being 

1% reversely discordant (tip) to 1% normally 

discordant (center) with no correlation between 

position within the grain and either age or 

degree/sign of discordance. 

In the absence of physical evidence for 

inheritance, the range of single grain ages remains 

problematic. However, the discordance behavior 

can be explained if single monazite grains comprise 

complex mixtures of domains which have exhibited 

open system behavior with respect to U, Th, and Pb, 

including excess 206Pb, during cooling. Concordant 

analyses of single grains may represent fortuitous 

mixtures of these domains. There is no evidence for 

preservation of simple diffusion gradients within 

single grains. 
2.45 Ga GLOBAL MAFIC MAGMATISM: EARTH'S 

OLDEST SUPERPLUME? 

HEAMAN. L.M., Dept. of Geology, Royal Ontario 

Museum, 100 Queen's Park, Toronto, Ontario, Canada. 

M5S2C6. 

Radiometric dating, including new high precision U-Pb 

baddeleyite/zircon ages, of Paleoproterozoic flood basalts, 

dyke swarms, and layered mafic intrusions worldwide 

indicates that a substantial volume of mafic magma was 

produced on Earth in the interval 2.48 to 2.42 Ga with 

vestiges preserved in virtually all major Archean cratons. In 

North America, this event is well documented in the Superior 

(2.45 Ga Huronian flood basalts, 2.45 Ga Hearst-Matachewan 

dyke swarm, 2.47 Ga Mistassini dyke swarm, 2.48 Ga gabbro 

plutons) and Hearne (2.45 Ga Kaminak dyke swarm) cratons. 

Although poorly dated, other candidates exist in the 

Wyoming craton (e.g. leopard dykes) and together all this 

magmatism could be related to a common mantle plume 

system - the Matachewan plume. Elsewhere, temporally 

equivalent magmatic activity has been identified in the 

Lewisian craton, Scotland (2.42 Ga Scourie dyke swarm), the 

Karelian craton, Finland and Russia (2.44 - 2.45 Ga layered 

mafic intrusions, flood basalts and dyke swarms), the Yilgarn 

craton, Australia (c. 2.42 Ga Widgiemooltha dyke swarm and 

Jimberlana intrusion), Rhodesian craton (c. 2.46 Ga Great 

Dyke), Vestfold craton, Antarctica (c. 2.42 Ga Napier dyke 

swarm), and Dharwar craton, India (c. 2.42 Ga Bangalore 

dyke swarm). 

The potential volume, magnitude and extent of this 

magmatism eclipses many of the better known Mesozoic flood 

basalt provinces and represents the earliest recognized 

superplume event on Earth. Although the style of the 2.45 Ga 

magmatism, such as the development of giant radiating dyke 

swarms and coeval flood basalts, has been subsequently 

repeated many times throughout the Proterozoic and 

Mesozoic, it is clearly distinct from the dominant style of 

mafic magmatism in the Archean (i.e. greenstone belts). This 

dramatic difference in style likely reflects a fundamental 

change in the mechanism of heat transfer in the mantle and 

possibly signals the initiation of a new mantle convection 

mode in Earth evolution. Where preserved, the stratigraphic 

record accompanying the 2.45 Ga magmatism clearly 

demonstrates the concomitant development of rift basins 

(Huronian/Snowy Pass basin in North America and the SumiSariola basin in Karelia) that may herald the breakup of a 

pre-2.5 Ga supercontinent. 

132 

URANIUM-SERIES DATING OF CLOSEDSYSTEM PEAT FROM LONG TERRESTRIAL 

RECORDS. 

HEIJNIS. H., Environmental Radiochemistry 

Laboratory, Australian Nuclear Science and 

Technology Organisation, P:MB 1, 2234 NSW, 

Australia. 

The successful dating of peat was reported in 

1992 (Heijnis & van der Plicht, 1992 and Heijnis, 

1992). In principle, this method can be used to date 

peat to - 3 50 ka. The application of the U/Th 

disequilibrium method provides us with the 

probability of constructing a new independent 

chronology for the Late Pleistocene Palaeoclimate 

record. The problems with regard to detrital 

Thorium and suspected open system behaviour 

were solved. Since 1992 much of the attention has 

been focused on the dating of the terrestrial 

equivalents of sub-stages of stage 5 of the deep-sea 

record. Long Terrestrial Records from Europe and 

Australia were sampled, analysed and dated. 

Individual isolated peat layers in Europe with an 

unknown or correlated on basis of the 

palaeoecological information were also sampled, 

analysed and dated. Equivalents of all three sub 

stages Se, Sc and Sa were found in Europe as well 

as in the Long Records from Australia. 

Heijnis, H., and van der Plicht, J., 1992, 

Uranium/Thorium dating of Late Pleistocene peat 

deposits in NW Europe, Uranium/Thorium 

isotope systematics and open-system behaviour 

of peat layers: Chemical Geology (Isotope 

Geoscience Section), 94 p. 161-171. 

Heijnis, H., 1992, Uranium/Thorium dating ofLate 

Pleistocene peat deposits in NW. Europe. 

Published PhD thesis, University of Groningen, 

The Netherlands. pp. 149. 
40 Ar/39 Ar RESULTS OF INCOMPLETELY DEGASSED 

SANIDINE: AGE OF LATHROP WELLS VOLCANISM 

HEIZLER Matthew T., MciNTOSH, William C., New 

Mexico Bureau of Mines and Mineral Resources, Socorro, 

NM 87801, USA,. PERRY, Frank, V., and CROWE, 

Bruce, M., Los· Alamos National Laboratory, Los Alamos, 

NM 87545, USA. 

Ash flow tuff xenoliths were collected froiJl several of the 

basaltic flows and cone at Lathrop Wells volcano in southwest 

Nevada, as part of an on-going volcanic hazard study for the 

DOE's Yucca Mountain Project. Xenoliths ranged in size from 

-1 to 30 em in diameter. Pristine to variably partially melted 

sanidines were separated from 4 individual xenoliths from the 

QL-2 flow and cinder·cone. A glassy matrix was separated 

from a nearly completely melted xenolith from the QL-1 flow 

and a pristine sanidine was recovered from a large xenolith 

occurring in a conduit Also, a 1 em xenolith from the Little 

Cone cinder cone north of Lathrop Wells provided a pristine 

sanidine separate. 

To help characterize the sanidines prior to furnace stepheating, 5 single crystals from each xenolith were step-heated 

with a C02 laser. 

An 40 Ar/39 Ar age spectrum from the Little Cone xenolith 

shows a flat portion for the frrst 15% of argon released and 

yields an isochron age of 904 ± 11 ka. Tiiis is in excellent 

agreement with the 0.9-1 Ma whole rock basalt argon ages 

reported for the Little Cone flow. 

Pristine sanidines from the xenolith from the conduit just 

south of the main cone yields an age spectrum with a flat 

portion for the flfSt 40% of gas released and gives an isochron 

age of77 ± 2 ka. 

Three xenolith sanidines (samples A, G, and 0) were 

analyzed from the QL-2 flow and reveal at least 95% argon 

loss. The age spectra have the same general form and are 

relatively flat over the frrst 40 to 60% of argon released, but in 

detail they can be interpreted as having significantly different 

ages. lsochron ages of 106 ± 3, 78 ± 2 ka, and 60 ± 5 are 

obtained for xenoliths A, G and 0, respectively. 

Single crystal laser analyses for cone xenoliths show that 

two samples have experienced very minor argon loss, one 

(sample J) is 90% degassed and one glassy sample is 

significantly contaminated with air. Sample J sanidine yields 

an age of 166 ± 4 ka for the frrst 15% of gas released during 

step-heating. The cone is geologically younger than QL-2 and 

thus the -160 ka age may be problematic. Perhaps this 

xenolith was degassed during an older eruptive episodic and 

recycled, but not significantly heated, during cone formation. 

An additional possibility for all of the xenoliths is that, due 

to the partial melting, and thus disruption of the diffusion 

domain distribution of these crystals, argon is lost from both 

non-retentive (young) and retentive (old) sites simultaneously 

in the laboratory. This could account for the variability of ages 

from QL-2 as well as the apparent 166 ka age for the. cone. 

This hypothesis is being experimentally tested by partially 

melting Fish Cailyon Tuff sanidines in a muffle furnace. 

Since several factors could cause xenolith apparent ages to 

be anomalously old, the youngest age for any sweet of 

xenoliths may be the best age estimate for a given unit. 

Despite some complications, these data indicate that dating 

xenoliths will yield useful age constraints on Lathrop Wells 

volcanism and may provide a useful calibration for production 

rates of cosmogenic nuclide systems over the time interval ca. 

50 to 100 ka 

133 

Pb ISOTOPE SYSTEMATICS OF A LARGE IDGH 

GRADE QUARTZ VEIN, PIKWITONEI GRANULITE 

DOMAIN, MANITOBA, CANADA 

HEMMING, S. R, Dept. of Earth & Space 

Sciences, SUNY Stony Brook, NY, 11794, USA, 

MEZGER K .. Max-Plank-Institut fur Chemie, Postfach 

3060, D55020 Mainz, Germany, McLENNAN. S. M, 

and HANSON. G. N., Dept. of Earth & Space Sciences, 

SUNY Stony Brook, NY, 11794, USA. 

A Pb-Pb age of 2977±54 Ma and a J.Li of 8.5 for a 

large quartz vein 'in the Pikwitonei granulite domain of 

the western Superior Province in Manitoba records one of 

the oldest metamorphic events known in this area. The 

Pb isotope data reported here are based on 5 to 500 mg 

samples of quartz from a hand specimen collected from an 

island in Natawahunan Lake that is composed almost 

entirely of a large quartz vein. Pods ( -1 m) containing 

garnet-orthopyroxene-biotite gneiss also occur on this 

island. Large biotite inclusions (up to several mm) are 

common in the quartz vein. Dark red rutile inclusions 

(-50 to 500 J.LID) occur, but are less common. Crushed 

quartz fragments were picked to avoid visible inclusions. 

The Pb concentrations in these samples range from 0.179 

to 0.548 ppm and measured U concentrations range from 

0.0197 to 0.087 ppm. 206Pbf204Pb ranges from 16 to 40. 

Using K-feldspar from a metapelitic rock for the initial Pb 

isotopic composition (14.08, 15.03, 33.82), calculated 

238Uf204Pb ranges from 6 to 44, and calculated 

232'fh/238U ranges from 0.58 to 2.9. This variability in 

U-Th-Pb ratios may be the result of very small mineral 

inclusions within the quartz, that have different 

distribution coefficients for these elements. 

Other evidence for the metamorphic event 

recorded by the quartz vein comes from a xenocrystic 

garnet within a nearby graphic granite (2984±2 Ma). In 

addition, the Pb isotope systematics of the quartz vein, 

notably the high J.Li, implies the presence of even older 

crust in the region. Other evidence for older crustal 

history in the area is found in the Pb isotopic composition 

of K -feldspar from a nearby granulite grade metapelitic 

rock (see composition above), and in the Sm-Nd isotope 

systematics of some rocks from the area (T cHUR as high 

as 3.6 Ga). There is considerable evidence for 

metamorphism as high as granulite facies recorded by UPb garnet and zircon ages between 2.7 and 2.6 Gain the 

Pikwitonei granulite domain. Thus, the U-Pb system of 

this quartz vein appears to preserve the age of an old 

metamorphic event that has not been reset by a. second 

high grade metamorphic event. The island that this 

quartz vein forms is about 100m on a side. Accordingly, 

these data provide evidence that whatever contains the UPb system within large quartz veins may be armored and 

thus undisturbed through upper amphibolite to granulite 

grade metamorphism. 
TRACE ELEMENT AND ISOTOPE GEOCHEMISTRY OF 

FRIDAY AND DOMINGO SEAMOUNTS, JUAN 

FERNANDEZ HOTSPOT 

HEMOND Christophe, Geologisk Institut, 0ster Volgade 

10, 1350 Copenhagen K, Denmark, DEVEY C.W. and 

STOFFERS P. Geologisch-Palaontologisches Institut, 

Olshausenstr. 40, 24118 Kiel, Germany) 

The Juan Fernandez hotspot trace lies on the Nazca plate 

in the south-east Pacific. Westward from the two islands of the 

archipelago (Robinson Crusoe and Alexander Selkirk), two 

young seamounts (Friday and Domingo) have been dredged 

using the German research vessel "Sonne". Quite fresh 

samples containing glass were recovered. 

All samples are alkali basalts with a relatively narrow 

range of MgO (4-7%); most of them are basanitic in 

composition and some trachy-phonolites were also recovered. 

All basaltic samples are highly enriched in incompatible trace 

elements for both seamounts, with Friday being slightly more 

enriched (e.g. C1 norm. La/Sm::::: 2.6-3.0) than Domingo (e.g. 

La/Sm ::::: 2.2). Ratios of incompatible immobile elements like 

Nb/Zr confirm this observation (Nb/Zr ::::: 0. 21 and 0.18 in 

Friday and Domingo respectively). A ratio involving mobile 

and immobile incompatible elements like Ba/Zr shows a 

negative trend when plotted versus Ba concentration although 

Ba should be more incompatible than Zr during partial melting. 

REE patterns show that if Friday samples are more enriched in 

incompatible elements, they also have lower HREE 

concentrations than Domingo samples. The low SiD2 samples 

from Friday have lower Ba and Nb contents but much higher 

Ba/Zr ratio while Nb/Zr remains constant and La/Yb decreases. 

This cannot be explained by fractionation of the mineral phases 

present in the samples (plagioclase+olivine) and the constancy 

of the Ba/Rb ratio excludes phlogopite fractionation. We also 

tried to calculate degrees of melting using source compositions 

in the spinel and garnet stability zones. This leads to 

unrealistic degrees of melting for alkali basalts. These 

observations together with the very steep trend defined by our 

data in SiD2 vs MgO lead to the conclusion that the melts may 

have been produced in presence of a very undersaturated 

component probably nephelinitic in composition as seen in the 

Society and Austral plumes (Hemond et al., in press, Chern. 

Geol.). This component appears to be depleted in most 

incompatible elements, but particularly HFSE, producing 

negative mixing trends in Ba/Zr vs Ba in which low Si02 

samples have lower Ba/Zr ratios than higher SiD2 samples. 

Pb isotope data on samples from both seamounts (e.g. 

206pbt204pb::::: 19.10- 19.15) as well as Sr and Nd isotopes 

are homogenous and included in the range of literature data 

from the islands. They are very similar to our data from Moua 

Pihaa seamount, an active seamount of the Society plume. 

This emphasizes the homogeneity of the source region in the 

mantle beneath the Juan Fernandez chain; it underlines also that 

similar magmatic processes can have taken place in various 

plumes in the Pacific. 

134 

40ArP9

Ar CHRONOLOGY OF LACCOLITH-CALDERA 

DEVELOPMENT, THE SOLITARIO, TRANS-PECOS TEXAS 

HENRY, Christopher D1

., Bureau of economic Geology, 

University of Texas, Austin, TX, 78731, and KUNK. 

Michael J., U.S. Geological Survey, MS 981 Reston, 

VA 22092 1

now at Nevada Bureau of Mines and Geology, University of 

Nevada, Reno, NV 89557-0088 

The Solitario is a mid-Tertiary laccolith-caldera complex in 

southern Trans-Pecos Texas. The 16 km diameter laccolith is 

among the worlds largest. The laccolith apparently intruded at 

a depth of -4 km into folded Paleozoic rocks that are 

unconformably overlain by -1 km of Cretaceous rocks. Detailed 

geologic mapping reveals that the Solitario was formed by a 

complex sequence of laccolith, sill, and dike injection; doming 

during intrusion of the main laccolith; ash-flow eruption; 

caldera collapse; and intracaldera :volcanism and sedimentation. 40ArP9 Ar ages of various igneous units were determined by 

incremental heating of aliquots of alkali feldspar phenocrysts. 

The ages are consistent with all field relationships and indicate 

that Solitario evolution occurred in three distinct pulses over an 

interval of - l million years. 

Small volumes of magma intruded as abundant rhyolitic to 

trachytic sills and small laccoliths during the first pulse, at 36.0 

± 0.1 Ma. Emplacement of the main laccolith, doming, ashflow eruption, and caldera collapse occurred at 35.4 ± 0.1 Ma 

during the most voluminous pulse. Although the main 

laccolith is not exposed, dated rocks include a dike emplaced 

along radial fractures generated by doming, an ash-flow vent, 

and caldera-fill tuff. A complex sequence of debris-flow and 

debris-avalanche deposits, megabreccia, trachyte lava, and 

small-volume ash-flow tuff subsequently filled the 6 x 2-km 

caldera that developed over the laccolithic dome. The final 

magmatic pulse at 35.0 ± 0.1 Ma consisted of several small 

laccoliths or stocks and numerous dikes in caldera fill and 

along the ring fracture. Solitario rocks appear to be part of a 

broadly cogenetic metaluminous suite. 

A series of peralkaline rhyolite lava domes was emplaced 

around the northern periphery of the Solitario at 35.4 ± 0.1 Ma, 

contemporaneous with laccolith emplacement and the main 

pulse. We interpret the temporal and sparse geochemical and 

Pb isotopic data to indicate the peralkaline rocks are crustal 

melts related to the magmatic-thermal flux represented by the 

main pulse of Solitario magmatisim. 

The size, duration, and episodic behavior of Solitario 

magmatisim are more typical of large caldera systems than of 

most laccoliths. Although age data are sparse, current models 

of laccolith emplacement and evolution suggest a continuum of 

magmatic activity from initial sill emplacement through growth 

of the main laccolith. In contrast to existing models, our 40ArP9 Ar results suggest that repeated injections of magma, 

separated by intervals of little or no magmatic activity, are 

required. 
THE THERMOCHRONOLOGICAL POTENTIAL 

OF AGE/GRAIN-SIZE RELATIONS 

HESS. J.C. and LIPPOLT, H.J., Laboratorium 

fiir Geochronologie der Universitiit, 69120 

Heidelberg, Germany. 

The temperature of chemical mineral closure in a 

cooling geochronological system as well as 

diffusional loss out of minerals in the course of 

thermal overprinting strongly depend on the 

diffusion properties of the mineral and on the 

, diffusion length. Accordingly, relations~ips between 

isotopic age and diffusion parameters on the one 

hand and between age and diffusion length on the 

other hand have to be expected. The dependences of 

isotopic ages on the diffusion properties had been 

subject of investigations frequently. Although of 

great thermochronological potential relations betwen 

age and diffusion length (or physical grain-size) 

have been investigated only occasionally. 

To ascertain age/ grain-size effects and to 

establish their thermochronological possibilities we 

performed 40 ArP9 Ar age measurements on carefully 

separated mica concentrates from granites from 

three regions with different geological histories. 

On biotites from the 2.5 Ma old Eldzhurtinskiy 

granite (Caucasus) distinct age/grain-size 

dependences could be detected. They enable us to 

calculate cooling and uplift rates and a geothermal 

gradient at the time of biotite closure, which are in 

good accordance with independently obtained 

geophysical results. 

Age/ grain-size effects could also be found on 

biotite and muscovite of the 30 Ma old Bergell 

granitoids (southern Alps). Together with literature 

data the newly obtained mica results allow a detailed 

reconstruction of the temporal and spatial cooling of 

the Bergell pluton from intrusion time down to 

surface temperatures. 

The Variscan Mittagfluh granite (western Alps) 

was thermally overprinted by Alpine metamorphism 

which partially rejuvenated the biotites of the rock. 

The portions of radiogenic Ar loss strongly depend 

on the size of the biotites and can be used to 

estimate time, duration and peak temperature of the 

metamorphic event. 

All the examples prove the existence of age/ 

grain-size effects and clearly demonstrate their 

applicability and usefulness in thermochronology. 

135 

TIMESCALE OF MAGMA PRODUCTION AND 

CRYSTAL GROWTH IN SILICIC MAGMA 

CHAMBERS, KENYA RIFT VALLEY 

HEllMANN Amd, DAVIES, G. R., and STAUDIGEL, H. 

Faculteit der Aardwetenschappen, Vrije Universiteit, De 

Boelelaan 1085, 1081 HV Amsterdam, The Netherlands. 

Magma chamber processes are of fundamental importance 

in understanding the genesis of highly fractionated silicic 

magmas. Quantifying the kinetics and the timing of magma 

production and fractionation rates helps to explain the rate at 

which silicic magma systems form and evolve. 

The rhyolite suites under study contain highly fractionated 

magmas with high Rb/Sr ratios (100-700). These high Rb/Sr 

ratios result in rapid changes in 87Sr/86sr ratios potentially 

allowing resolution of events on time scales as low as 1000 

years. Strontium contents are low (<1 ppm) for these rocks and 

can only be explained by fractional crystallization involving 

substantial volumes of cumulate feldspar. 

At the low crystallization temperatures (700-750.C) of 

silicic magma systems the diffusion of strontium in feldspars is 

slow (l0-17 cm2/s). Mineral analyses of core/rim distributions 

of strontium in alkali feldspar may therefore provide 

information on the time and rate of crystal growth in slowly 

cooling magma chambers. Mineral growth rates in silicic 

magma chambers are still unknown under typical magmatic 

cooling rates in the upper continental crust. 

In this sudy, we present analytical data from fresh obsidian 

glasses and minerals from the most recent eruptions of the 

bimodal Naivasha volcanic complex in the Kenya Rift Valley. 

Our samples comprise peralkaline rhyolites (comendites) from 

silicic domes within the Naivasha complex which form seven 

different chemostratigraphic groups with distinct trace element 

and isotope systematics. The generation of the high viscosity 

melts forming the domes can be attributed to spatially 

separated, but genetically related small-volume(= 0.1 km3) 

magma chambers at the roof of which highly evolved 

differentiates accumulated. A young eruption age of the domes 

is confirmed by a 14C radiometric age determination(< 15,000 

y) of lake sediments. 

The investigated comendites are from two of the seven 

chemostratigraphic groups. Relative high concentrations of 

halogens ( = 1.2 %) in the comendites are coupled with extreme 

enrichment of certain incompatible elements (e.g. Nb=500 ppm 

and Zr=2000 ppm) for the different groups. The rocks show 

Rb/Sr ratios ranging from 240 to 508 and have strontium 

contents between 1 and 2 ppm. Present day strontium isotope 

ratios range from 0.70691 to 0.71189. The ENd values prove 

that the rllyolites are not simply melts of old continental crust, 

but have the signature of young mantle-derived material. In 

general, such melts have strontium contents of>> 100 ppm in 

contrast to the low contents of the rhyolites. This requires a 

marked chemical differentiation in a relative short period of 

time. 

Preliminary Sr data on obsidian glasses suggest an isochron 

at 70,000 years which implies that the magma chamber system 

has been in existence for = 60,000 years. Based on this age, 

production rates can be estimated. Microdrilling of alkali 

feldspars is in progress to investigate core/rim relationships of 

strontium isotope systematics and to estimate mineral growth 

rates. 
ESTIMATION OF NEUTRON FLUENCE FROM THE 

ISOTOPIC COMPOSITIONS OF SAMARIUM AND 

GADOLINIUM IN METEORITES AND URANINITES 

IDDAKA, H. and EBIHARA. M., Dept. of Chemistry. 

Tokyo Metropolitan University, Hachioji, Tokyo 192-03, 

Japan.. T AKAHASID, K.. Institute of Physical and 

Chemical Research (RIKEN), Saitama 351-01, Japan., 

HOLLIGER, P., CEA, Centre d'Etudes Nucleaires de 

Cadarache, 13108-Saint-Paul-les-Durance, France, and 

HONDA, M., Dept. of Chemistry, Nihon University, 

Setagaya, Tokyo 156, Japan. 

Isotopic deviations of Sm and Gd in geological and 

cosmological samples are useful for the estimation of neutron 

dose the samples acquired, because 149sm,155od and 157Gd 

have fairly large values of cross sections for thermal neutron 

capture reactions. Some meteorites were reported to show the 

depletion of 149sm, 155od and 157od isotopic abundances 

due to the capture reactions of cosmic ray-induced neutrons. 

Similar effect on isotopic variations has been observed in 

terrestrial uranium ore samples. In order to extend our 

understanding of the effect on isotopic compositions due to 

neutron capture reactions, we have started to measure Sm and 

Gd isotopic compositions of meteoritic samples (Norton 

County, Allende, Bruderheim and St. Severin) and terrestrial 

uraninites (sampled at Oklo (outside fission reactor zone, 

Gabon, 2.0Ga old), Labrador (Canada, 1.8Ga), Morogoro 

(Tanzania, 0.80Ga), Shinkolobwe (Zaire, 0.78Ga) and 

Madagascar (0.55Ga)). 

Thermal ionization mass spectrometry (TIMS) potentially 

offers precise isotopic measurements for Sm and Gd with 

reproducibilities of less than 0.01 %, which must be sufficient 

enough to investigate the irradiation effect due to cosmic rayinduced neutrons for meteorites having long cosmic ray 

exposure ages such as Norton County. The effect yielded from 

spontaneous fission of uranium can also be investigated for 

terrestrial uraninites with old formation ages. After acid 

digestion of each sample, Sm and Gd were sequentially 

separated on a cation exchange column with a-hydroxy 

isobutyric acid (0.15M and 0.20M, pH=4.5). Significant 

isotopic variations of 155odf160Gd (-0.9o/oo for Norton County, 

and -5.00/oo for Oklo) and 157odt160od (-1.5o/oo for Norton 

County, and -3.3o/oo for Oklo) could be observed in some 

samples. The isotopic decrements of 155odt160od and 

157odt160od seem to quantitatively correspond to the 

increments of 156odf160od and 158odt160od, respectively, 

suggesting that these isotopic variations resulted from neutron 

capture reactions of 155od(n,y)156od and 157Gd(n,y)158od. 

Moreover, geologically old uraninites sampled at Oklo and 

Labrador showed a slight positive anomaly in 152odf160od, 

possibly derived from the neutron capture reaction on 151Eu, 

namely 151Eu(n,yf3-)152od. 

Besides the analyses of natural samples, we also analyzed 

REE reagents irradiated in a reactor. On the basis of on the 

preliminary results obtained, it is inferred that the isotopic 

systematics on Sm and Gd can be utilized for the estimation of 

neutron fluence of to15- to16wcm2 in nature. 

136 

ISOTOPIC STUDY OF SEARCH FOR FISSION 

PRODUCTS IN OKLO NATURAL FISSION REACTORS 

HIDAKA. H., Dept. of Chemistry, Tokyo Metropolitan 

University, Hachioji, Tokyo 192-03, Japan, and 

HOLLIGER, P., CEA, Centre d'Etudes Nucleaires de 

Cadarache, 13108-Saint-Paul-les-Durance, France 

Isotopic measurements of Oklo natural fission reactor 

samples play an important role as an ideal natural analogue for 

nuclear waste disposal. This study has implications for 

clarification of the behaviour of (fissiogenic) radioactive 

nuclides in geoshere through stable isotopic measurements of 

Oklo samples by mass spectrometric techniques. 

The samples used in this study are from one of drilling 

cores SF84 of Oklo reactor zone 10: This zone has been kept 

in geochemically good conditions with little weathering and 

alteration, because it locates underground. Isotopic 

compositions ofRb, Sr, Zr, Mo, Ru, Pd, Ag, Te, Ba, REEs and 

U for whole rock samples have been determined by thermal 

ionization mass spectrometry (TIMS) and inductively coupled 

plasma mass spectrometry (ICP-MS). The behaviour of longlived radioactive nuclides such as 90sr, 99Tc, 107pd, 126sn, 

135cs and 137cs can be deduced from the present stable 

isotope abundances of90zr, 99Ru, 107 Ag, 12~e, 135Ba and 

137Ba, respectively. Moreover, in-situ isotopic analyses were 

performed with secondary ion mass spectrometry (SIMS) on 

uranium oxide grains and some inclusions in reactor core 

samples for the purpose of detection of fissiogenic nuclides on 

JJ.m scale. 

The combination of isotopic results from whole rock 

analyses and in-situ observation provides a comprehensive 

information on the geochemical characters of radionuclides: 

(1) Fissiogenic Ru, Pd and Te have been well preserved in 

uraninite whole rock, but some part of them have 

microscopically localized and formed metallic inclusions in 

uraninite grains. (2) In general, fissiogenic REEs have been 

well retained in uraninite. But fissiogenic light REEs such as 

Ce are relatively mobile to heavier REEs. Apatite minerals in 

reactor core samples contains larger amount of fissiogenic Ce 

than uraninite matrices. (3) The existence of fissiogenic Rb, 

Sr, Cs and Ba were confrrmed from the significant isotopic 

deviations, although retentivities of them are very low and do 

not exceed 5% to the best retention. 
HELIUM ISOTOPE VARIATIONS IN HEARD ISLAND 

LAVAS: IMPLICATIONS FOR MANTLE SOURCES. 

WLTON DR 1,2; 1) Earth Sciences, Vrije Universiteit, 

1081 HV Amsterdam, The Netherlands; 2) FR Geochemie, 

Freie Universitat Berlin, 14195-Berlin, Germany; 

BARLING, J. Universite Libre de Bruxelles, Belgium; 

WHELLER, G.E. Mineral Resources Dept., Fiji. 

We report 3 He/4 He analyses of olivine and/or 

clinopyroxene phenocrysts from a suite of 16 lavas from Heard 

Island in the Indian Ocean. These lavas display the greatest 

range in radiogenic isotope compositions yet encountered for a 

single oceanic island (Barling & Goldstein, 1990; Barling et 

al., 1994). Eleven samples come from the Big Ben Series 

which are characterised by high 87Srf86Sr and low 

206Pbf204Pb and 143Ndfl44Nd: these samples have been 

interpreted to contain the greatest proportion of an "enriched 

Heard" component ultimately derived from continental crust. 

Five samples come from the Laurens Peninsula Series and 

have lower 87Srf86Sr but higher 206Pbf204Pb and 

143Ndfl44Nd ratios: the origin of this "depleted Heard" 

endmember is considered to be either the Heard-Kerguelen 

mantle plume component (Barling & Goldstein, 1990; Class et 

al., 1993) or lithospheric mantle (Barling et al., 1994). In all 

cases, helium was released from the phenocrysts by crushing. 

The Big Ben lavas are characterised by a wide range in 

3He/4He ratios which appears related to helium concentrations 

([He]). Six olivine samples with the highest [He] (43-217 

nccSTP/g) have MORB-like 3He/4He values (-8RA where 

RA =air 3He/4He) whereas only 1 of the remaining 5 lower 

[He] olivines (13-28nccSTP/g) plots within error of MORB. 

The other 4 olivines have more radiogenic 3He/4He ratios of 

3.9-7.5RA. For co-existing clinopyroxenes, 4 high [He] 

samples (17-52 nccSTP/g) plot close to MORB whereas the 

lower concentration group ([He]= 1-20 nccSTP/g) range from 

0.4-4.7RA. The results are consistent with a MORB-like 

3He/4He signature for the Big Ben Series with both low-[He] 

olivine and clinopyroxene phenocrysts being variably 

susceptible to contamination by radiogenic helium (cf. Hilton 

et al., 1993). No correlation is observed between 3He/4He and 

87 Srt86sr indicating that the radiogenic helium addition is a 

shallow-level phenomena. The results have a strong bearing 

on the nature of mantle sources for the Heard Island lavas and 

for the origin of low3He hotspots. 

The same patterns of 3He/4He vs. [He] are seen for the 

Laurens Peninsula samples: 4 olivine samples of varying 

concentration (26-82 nccSTP/g) have a narrow range in 

3He/4He - 17.8-18.3RA. Both co-existing and a single 

clinopyroxene have lower [He] (8-9 ·nccSTP/g) and slightly 

lower 3He/4He- 16.2-17.8RA· Again, no correlation between 

3He/4He and other (radio)isotope tracers is evident. The 

overlap between olivine and clinopyroxene samples indicates 

that .radiogenic-He contamination of these samples is minimal. 

The high 3He/4He of the Laurens Peninsula thus supports the 

view that the low 87Srf86Sr, high 206pbf204pb endmember is 

representative of the Heard-Kerguelen plume. 

Barling, J. & Goldstein, S.L. Nature .31R pp 59-62 (1990). 

Barling, J., Goldstein, S.L. & Nichols, I. J. Petrol. in press 

(1994). 

Hilton, P.R., Hammerschmidt, K., Teufel, S. & Friedrichsen, 

H. EPSL 120. pp265-281 (1993) 

Class, C., Goldstein, S.L., Gaier, S.J.G. & Weis, D. Nature .3.62 

pp 715-721 (1993). 

137 

·STATIC MASS SPECTROMETRY OF NANOMOLE CARBON WITH A QUADRUPOLE-TYPE 

MASS SPECTROMETER 

HIYAGON, H~, SUGIURA, N. and MIYAZAKI, A., Department of Earth and 

Planetary Physics, University of Tokyo, 

Bunkyo-ku, Tokyo 113, Japan. 

We are constructing a vacuum system 

equipped with a quadrupole-type mass 

spectrometer (MSQ400, ULVAC Co., Japan) 

for analyzing isotopic composition of 

nano-mole carbon in a static mode. 

We analyze cabon isotopes as carbondioxide and measure mass 42, 44, 45 and 

46. Peaks are electrically scanned and 10 

sets of data are obtained in 160 seconds. 

Carbondioxide tends to decompose partly 

into CO and partly into C to form possibly 

metal-carbide during the analysis due to 

interaction between gas and hot filament. 

In order to minimize this effect, we 

replace first a tungsten filament with a 

yttria(Y 2o3)-coated iridium filament which 

has lower work function. This lengthen 

the "half life" of co2 in the mass spectrometer by a factor of about 5. Secondly, we lower the emission current from 1.0 

mA to 0.1 rnA. In this condition, the "half 

life" of co2 becomes longer than 120 sec. 

The ion source we are using is an "open~ 

type" (Prosser et al., 1990). However, the 

observed "half life" of co2 is much longer 

than that for other "open sources" (-35 

sec, Carr et al., 1986) and only a factor of 

three shorter than that for the "closed 

source" ( -450 sec, Prosser et al., 1990). 

We found more than 80% of the gas existed as co2 even at the end of the analysis. 

At present, the 13c;12c ratios observed for repeated analyses of a standard gas (about 1 nano-mole C02) are 

reproducible within +0.5%. However, 

there seems to be some pressure depen~­

ence in the discrimination factor for 1 C 

and 13c. The lower limit of the sample 

size for carbon isotope analysis with this 

system Is about 0.1 nano-mole. 

Prosser, S. J. et al., 1990: Chern. Geol., 

v .83, p. 71-88. 

Carr, R.H. et al., 1986: J. Phys. E. Sci. 

Instrum., v.19, p.133-149. 
MASS INDEPENDENT FRACTIONATION IN 

TROPOSPHERIC CARBON MONOXIDE 

HODDER. P.S .• Department of Chemistry, 

0356, University of California- San Diego, La Jolla, 

CA 90293-0356, BRENNINKMEIJER *, C. A.M., 

National Institute of Water and Atmospheric 

Research, Lower Hutt, New Zealand, and 

THIEMENS, M.H., Department of Chemistry, 

0356, University of California - San Diego, La Jolla, 

CA, 90293-0356. * Present address: Max Planck 

Institute for Chemistry, Airchemistry Department, 

Mainz, Germany. 

The measurement of multi -isotope, mass independent 

kinetic isotopic fractionation effects has previously been 

applied with notable success in the field of atmospheric 

chemistry as a sensitive technique to better lUlderstand the 

global budget of several atmospheric species. Multiisotope measurements have provided constraints on 

sources and global budgets of certain atmospheric species 

(e.g., ozone and methane) to a considerable degree, and 

also have helped lUlderstand the nature of the kinetics of 

the production mechanisms. The use of multi-isotope 

(b170, l)lSQ) ratio measurements of atmospheric nitrous 

oxide have elucidated the possible importance of new 

stratospheric reactions. In the case of nitrous oxide, this 

technique provides a resolution that could not be obtained 

from concentration or combined b15N and b1SO 

measurements. 

Carbon Monoxide (CO) is a trace gas in the 

atmosphere (4>pbv) with as much as 2/3 of its source 

signal influenced by anthropogenic activities. In addition, 

CO plays a key role in the budgets of important 

atmospheric species (ozone, OH, methane, NMHC's), 

necessitating a better understanding of the otherwise 

poorly constrained CO cycle. Considerable effort has 

been expended on measuring the kinetic isotope effects of 

t3C and 18() to characterize the different source strengths 

and to model the observed trends in the highly variable 

concentrations of atmospheric CO. 

Recently, a mass independent fractionation of large 

magnitude has been measured in the stable isotope ratios 

(18Q/16Q, t70f160) of tropospheric CO from the 

southern hemisphere. Based upon our present physicalchemical knowledge of mass independent fractionations, 

we are able to eliminate several sources as being 

responsible for this effect. This discovery may help 

better constrain the major source strengths, as well as 

different production pathways and circulation of CO. 

138 

U-Pb AND 40ARJ39AR THERMOCHRONOLOGY OF 

VERY SLOWLY COOLED SAMPLES 

HQDGES K v., BOWRING, S.A., and HAMES, 

W.E., Dept. Earth, Atm., and Planetary Sciences, 

MIT, Cambridge, MA, 02139, USA 

Reconstructing the cooling history of very slowly cooled 

terrains from geochronologic data from is not 

straightforward. At cooling rates of S 1 K/m.y., the 

integrated closure temperatures of mineral-isotopic systems 

are strongly dependent on cooling rate. Moreover, slow 

cooling can lead to the development and maintenance of 

large age gradients in minerals tha~ are com~only 

overlooked in the course of conventional analysts of 

multigrain samples. Recent 40 Ar/39 Ar and U-Pb studies of 

samples from the Proterozoic orogenic belt of the 

southwestern United States provide examples of 

complexities caused by slow cooling. 

The Crazy Basin plutonic complex of central Arizona 

yields U-Pb zircon ages ranging from 1690 to 1710 Ma. 

Incremental heating of single - 700 Jlm crystals of 

muscovite and biotite from a Crazy Basin monzogranite 

using a defocused Ar-ion laser resulted in simple, relatively 

flat 40 Arf39 Ar release spectra indicating closure ages of 1412 

± 5 and 1410 ± 10 Ma, respectively. However, analysis of 

the same samples by laser microprobe revealed large age 

gradients. Within one carefully mapped { 001 } cleavage 

fragment from a 740 Jlm-diameter muscovite crystal, total 

fusion ages ranged from 1650 ± 10 Ma in the core to 1270 ± 

10 at the rim in broadly concentric fashion. An adjacent 

cleavage fragment from the same crys~. though mappe~ in 

less detail, yielded an identical concentric age pattern .. Ftve 

cleavage fragments from a smaller (550 Jlm) crystal ytelded 

statistically indistinguishable core ages between 1560. and 

1530 Ma. Rim ages were much younger but more vanable 

(1440 to 1350 Ma), due at least in part to steep near-rim age 

gradients. An {001} cleavage fragment from a 1 mm biotite 

crystal also exhibited a concentric age zoning pattern from 

1420 ± 10 Ma in the core to 1150 ± 10 Ma at the rim. The 

lack of evidence for such large and reproducible age 

gradients in the incremental heating data suggest caution in 

the interpretation of simple release spectra from very slowly 

cooled samples. . 

U-Pb titanite data from the -1690 Ma Horse Mountain 

granite from the same area provide additional evide~ce .for 

very slow cooling. Cores of abraded - 300 Jlm titant~es 

cluster along concordia from > 1660 to 1650 Ma. R1m 

fragments and multigrain separates of smaller(- 100 Jlffi) 

titanites give substantially younger 206pbJ238u ages ranging 

from 1459 ± 6 to 1339 ± 5 Ma. 
PB ISOTOPE SYSTEMATICS OF GOLD 

MINERALIZATIONS AT ASHANTI (OBUASI, 

GHANA) 

HOEHNDORF. A., OBERTHUER, T., 

SCHMIDT MUMM, A., VETTER, U., Federal 

Institute for Geosciences and Natural 

Resources, Hannover, Germany. 

The Ashanti gold mine at Obuasi (Ghana) is the 

largest gold producer in West Africa having 

produced in excess of 700 t of gold historically. 

The mine workings are hosted in Palaeoproterozoic metasediments of the Birimian 

Supergroup. 

Gold mineralization is found in two major 

types of primary ores, namely (i) sulfide 

mineralization in the wall rocks with auriferous 

arsenopyrite as the dominant ore mineral, and (ii) 

quartz veins containing free gold associated with 

Pb, Sb sulfides. Both ore types are controlled by 

the same structural patterns and occur together or 

alternate with each other in the major ore zones. 

To get age information for the mineralization, 

Pb isotopic investigations were carried out on 

concentrates of gold-bearing arsenopyrites from 

sulfide ores in the host rocks, Pb sulfides (galena 

and bournonite) occurring in quartz veins, and 

native gold from quartz veins. 

For the arsenopyrites a Pb/Pb isochron age of 

2224 ± 40 Ma (2s) was obtained which may 

correspond to the age of the Birimian 

sedimentation. Geological and mineralogical 

evidence, however, implies an epigenetic nature of 

the arsenopyrite formation. 

Model ages around 2.1 Ga, calculated (using the 

Stacey-Kramers model) for Pb sulfides and Pb 

minerals intergrown with native gold from quartz 

veins, are assumed to mark the age of the 

epigenetic gold mineralization. 

139 

STABLE WATER ISOTOPES IN THE ATMOSPHERIC GENERAL CIRCULATION 

MODELECHAM 

G. Hoffmann (Max-Planck-Institut fiir Meteorologie, D-20146 Hamburg, Germany) 

M.Heimann (Max-Planck-Institut fiir Meteorologie, D-20146 Hamburg, Germany) 

We have established a water tracer model in the 

atmospheric general circulation model (AGCM) 

ECHAM. Water tracers like the stable water isotopes 1 H2 

180 and 1 H2H 160 pervade the whole hydrolocical cycle, i.e. evaporation from oceans, atmospheric transport, condensation inside clouds, 

precipitation as rain or snow and finally seepage 

into groundwater and rivers. We calculate the 

atmospheric transport of both water tracers and 

"normal" water 1 H2

160 by a semilagrangian advection scheme. During any phase transition water 

isotopes undergo a fractionation relative to normal water. We have incorporated both kinetic and 

equilibrium fractionation according to the wellknown experimental results. The ECHAM is a 

spectral model which is run for our tracer studies in T42 resolution (spectral truncation at wave 

number 42 and a physical grid of about 2.5° x 2.5°). 

We present here the results of a five year simulation under both todays climate conditions and 

the boundary conditions of the last glacial maximum(LGM). In a first step we compare the simulated isotopic composition of precipitation, soil 

moisture, snow etc. with observations (I.A.E.A. 

precipitation data, .. ) in order to assess the quality 

of the hydrological cycle of the ECHAM model. 

Afterwards regarding our LG M experiment we focuss on the question if the linear relation between 

temperature and 8 180 is insensitive against climate change, which is the basic assumption reconstructing paleotemperatures from the isotopic 

composition of ice cores and paleowaters. 
CARBON ISOTOPE CHARACTERISTICS OF C02 AND 

Cf4 IN GEOTHERMAL AND MINERAL WATER 

SPRINGS FROM THE CEN1RAL ANDES. 

HOKE, Leonore, Dept of Earth Sciences, University of 

Oxford, Oxford, OX1 3PR. SPIRO Baruch and 

CHENER Y, Carolyn, NERC Isotope Geosiences 

Laboratory, Keyworth, NG125GG, U.K. 

We report results of carbon stable isotope analyses on 

methane and carbon dioxide extracted from 12 gas and water 

samples collected from geothermal and mineral water springs 

across the Central Andes of northern Chile and Bolivia. The 

Central Andes provide an unique tectonic setting for the study of 

terrestrial degassing processes in an active plate boundary zone. 

This zone is chracterised by an active volcanic arc and unusually 

thick continental crust ( -70km) which extends beneath the arc 

and the high plateau region of the Altiplano to the east of the 

arc. Furthermore, helium isotope evidence suggests active 

mantle degassing in a wide zone beneath the thick continental 

crust in the Central Andes (Hoke et al, 1994). 

The results show a wide range of a 13Cco2 (-14.9 to 

-0.6%o) and a surprisingly narrow range of a 13CcH4 (-20.9 to 

-13.4%o ). The difference ~al3cc02-CH4 for individual samples 

varies between -1.7 and +13.4%o. The al3Cco2 results show 

wide and overlapping ranges in the samples collected from the 

volcanic arc, the Altiplano and the Eastern Cordillera, though 

they tend to show the widest range and most negative values in 

the Eastern Cordillera (-15.0 to -4.8%o.). The a 13Cco2 results 

of the geothermal samples from the volcanic arc f,ave -8.0%o. 

(Surire) and -0.6%o (Abrade Nappa), whereas a1 Cco2 

measured in gases collected from C02 bubbling mineral water 

springs from the Altiplano ran~e from -6.1 to -10.1 %o. 

The relationships between He,t4He, a 13Cc02 and 

a 13CCH4 will be discussed and compared with other 

geothermal areas. Geothermal geothermometry based on 

~a13ccH4-C02 fractionation will be discussed. It suggests 

very high equilibrium temperatures in excess of 530'C for 

some geothermal systems analysed. 

It is concluded that the formation of methane is neither 

bacteriogenic, nor by thermal decomposition of organic matter, 

but rather abiogenic by high temperature reaction of H2 and 

C02. The a13cCH4 results are similar to those reported from 

hydrothermal fluids emitted from the East Pacific Rise 

(Welhan, 1988), suggesting a mantle derived carbon component 

in the methane.The wide, 15%o range in the a 13Cco2 is 

interpreted to reflect contributions from different C~ sources 

that include crustal organic and inorganic carbon as well as 

mantle carbon. 

Hoke, L., Hilton, D.R., Lamb, S.H.,Hammerschmidt, K., 

Friedrichsen, H., 1994, 3He evidence for a wide zone of 

active mantle melting beneath the Central Andes: 

(E.P.S.L.,in press). 

Welhan, J.A., 1988, Origins of methane in hydrothermal 

systems. In: Origins of Methane in the Earth ( ed. M. 

Schoell): Chern. Geology 71, 183-198. 

140 

t8ott6o DATA ON CONTRASTING CRUSTAL-SCALE 

AQUEOUS FLUID REGIMES BEFORE AND DURING 

EOCENE EXTENSION IN THE SOUTHERN OMINECA 

CRYSTALLINE BELT, BRITISH COLUMBIA 

HOLK, Gregory J. and TAYLOR, Hugh P., Jr., Geology, 

170-25NM, Caltecb, Pasadena, CA 91125 USA 

The southern Omineca belt contains 3 fault-bounded zones: 

1) Precambrian gneiss basement, 2) mid-crustal amphibolitefacies metamorphic rocks and leucogranites, and 3) an upper 

zone ofbrittlely deformed rocks. Deep-seated E-W convergent 

shear zones separate zones 1 and 2, and zone 3 is bounded by 

giant N-trending detachment faults. This large area was chosen 

for stable isotope study because: (a) it bas great structural 

relief; (b) its structures are well mapped and dated; and c) its 

high-latitude location insures that meteoric waters were very 

low in o18o and oD. In the early Tertiary, two major episodes 

of water-rock interaction are recognized: (I) a pre-extensional 

metamorphic-hydrothermal event mainly affecting the midcrustallayer; and (II) a synextensional meteoric-hydrothermal event 

affecting detachment faults (e.g. Columbia River fault). 

Episode I is best documented in the banging wall of the 

Monasbee Decollement (MD) in the Thor-Odin core complex, 

where it resulted in large-scale anatexis. Concordant 

Ieucogranitic segregations in migmatites (o180Q = + 12.2±0.5 

and o

180F = +10.9±0.5) and petites (o180Q = +12.1±0.9 

and ol8oF = +11.0±0.9) are remarkably uniform in 180, 

suggesting equilibration with metamorphic fluids (o180_ = + 11.0) at T :::::: 650°C. Similar o180 values from an overlymg, 

coeval Ladybird-Series pluton (LSP) (o180Q = + 12.2±0.4 

and o180F = + 10.4±0.3) indicate it formed during the same 

anatectic event. Paleozoic marbles ( > 95% calcite) show a 

12%o downward shift in 180, but are unchanged in 13C, further 

evidence for exchange with H20-rich metamorphic fluids. 

Below the LSP, calcite from calc-silicates is uniform in 180 

(o18o = + 15.4±0.2), but variable in 13C (o13C = -7.6 to 

-3.5). However, calc-silicate calcite from above and within 

the LSP is not uniform in 180 or 13c (o18o = +9.6 to +20.3, 

013c = -6.4 to +0.3), suggesting contrasting styles of waterrock interaction fixed by position relative to the LSP. Data 

from another Ladybird granite pluton (o180Q = + 11.2±0.5) 

and associated hybrid gneiss (o180Q = + 11.4±0.9) in the 

Valhalla metamorphic core complex farther south are also 

compatible with deep-seated fluid-driven anatexis. 

Episode II occurred during Eocene detachment faulting associated with rapid unroofing of the metamorphic core complexes. Disequilibrium quartz-feldspar o180 values (~-F = 1.5 

to 16.0, with o180F low as -5.0) are diagnostic of tills event. 

On a op-OQ plot, values. typical_ly defin~ a steep array, 

suggesting that water-rock mteractton was mtense and shortlived (:::::: 1o5yr) during extensional faulting at T = 300-350°C. 

A systematic o180F decrease occurs in both upper and lower 

plates as detachment faults are approached, suggesting that 

fluid-flow was channelized by faults and occurred when faults 

were active. The tectonic juxtaposition of a hot lower plate 

(T > 500°C) against a cold upper plate (T < 300°C) was the 

likely driving force for circulation of meteoric water to great 

depth. o-0 arrays are less steep at detachment faults associated 

with late-stage Coryell alkalic intrusions, indicating that more 

beat was advected toward the surface where these plutons are 

present than along more distant faults. Remarkably, these 

effects are also seen along the MD, implying that meteoric 

H20 locally migrated deep into the lower plate. 
LITHOSPHERIC MAGMA SOURCES DURING 

TERTIARY CONTINENTAL BREAK-UP IN EAST 

GREENLAND: ELEMENTAL AND SR-ND ISOTOPIC 

GEOCHEMISTRY OF BASALTIC DYKES FROM THE 

KANGERLUSSUAQ AREA 

HOLM. P.M .• Geological Institute, University of 

Copenhagen, 0ster Voldgade 10, DK-1350 Copenhagen, 

Denmark, and N.-0. PR£GEL, University Library, Nmre 

Alle 37, DK-2200 Copenhagen, Denmark. 

Early Tertiary basaltic dykes intruded in the Kangerlussuaq 

area during continental rifting in East Greenland both as a 

coastal swarm parallel to the continent/ocean transition and 

along the Kangerlussuaq fjord stretching c. 70 km inland. 

The area was uplifted c. 6 km in a domal structure during 

the Oligocene and its probable cover of flood basalts was 

eroded away. Elemental and isotope geochemistry of 65 

picrites, tholeiitic basalts, basaltic andesites, transitional 

basalts, alkali basalts and basanites is presented. These 

rocks may represent the magmas generated over the central 

part of the protp-Icelandic mantle plume. In all sub-areas, 

both alkaline and sub-alkaline compositions are present. The 

liquid lines of descent of the dyke magmas are rather 

similar: ol+chr, ol+cpx and late in the basaltic interval 

ol+cpx+plg. The dykes cover the same range of elemental 

and isotopic compositions as the related CFBs but also 

extend to highly alkaline and extreme isotopic compositions 

not seen among the lavas. 87Srf6Sr50Ma range from 0.7030 to 

0.7202 and 143Nd/ 144Nd span 0.5118-0.5131. The 

geochemistry of the majority of dykes can largely be 

explained as mixing between an Icelandic type components 

(MORB and plume) and a highly LILE and LREE enriched 

component with 87Srf6Sr = c. 0.7055 and 143NdJ144Nd = 

0.5125, Ce/Y N = c. 25, Ti/Y = c. 600. This component is 

not seen in Icelandic lavas and is suggested to be derived 

from the lithospheric mantle. The group of dykes trending 

towards very high 87Srf6Sr and low 143NdJ144Nd comprises a 

wide range of rock types only excepting the most alkaline 

but including all basaltic andesites. Although more than one 

component is involved, the overall elemental character is 

relatively well-defmed: Ce/YN = 2-4, Zr/Nb ... 5-10, Ti/Y = 

300-400 and K{fi lower than in the proposed lithospheric 

mantle component. There is no suggestion that this 

component may be derived from the lower crust. It is seen 

especially in late dykes and is also the dominant component 

in an early central intrusion in the area and may be related 

to the ending of an extensional tectonic regime in the fjord 

area. Upper crustal contamination seems to be of relatively 

unimportant in most dykes magmas. Modelling the 

evolution of the sources for the magmas is facilitated by the 

strong isotopic contrast between asthenosphere and Archean 

lithosphere. 

Early basaltic magmas derived mainly from MORB 

and Icelandic plume material interacted with the 

subcontinental mantle increasingly with distance to the 

coastal rift. Later magmas more frequently paused and 

interacted with the lower crust which at that time had an 

enhanced geotherm. 

141 

STABLE ISOTOPE (C,S,N) DISTRIBUTIONS IN COALS 

SPANNING THE CRETACEOUSffERTIARY BOUNDARY 

IN THE RATON BASIN, COLORADO AND NEW 

MEXICO 

Charles W. Holmes and Bruce E....Bohor. U.S. Geological 

Survey,Box 25046, MS 972, Denver Federal Center, 

Denver,CO. 80225 

The distribution of the stable isotopes of carbon, sulfur, 

and nitrogen was measured in coal beds and their associated 

carbonaceous shales and the Kff boundary claystones at five 

sites in the Raton basin. S13C, measured on 35 carbonatefree samples, ranged from -24.9 to -27.2%o. S34S%o, 

measured on 15 sulfide and sulfate-free samples, ranged from 

4.6 to 8.0%o. S15N%o, measured on 15 coal samples, ranged 

from 1.9%o to 2.7%o. 

The Sugarite site is eocm..r- 0 ----=----" 

particularly well suited for this 

study because the Kff 

boundary claystone is 

completely enclosed in coal. 

Thus, the influences of preand post-impact lithologic 

variations are minimized. 

Previous studies of the Sugarite site documented the presence 

of the palynologically defined Kff boundary within the coal 

bed at the level of a thin, iridium-bearing claystone-shale 

unit (Nichols and others, 1985). This unit contains 22-25% 

carbonaceous material; the coal below it has a low ash ( -7%) 

and the coal above it is high· in ash content (-15%). 

The S13C value is -2%o heavier in the boundary claystone 

unit than the surrounding coal and the S34S value also 

increases by 3.5%o (Figure). In contrast, the S15N value 

decreases through the boundary interval. By comparison, the 

S13C and S34S across tonsteins within a Cretaceous coal 

shows no change. 

It has been suggested that the event that caused the mass 

extinctions at the Kff boundary was an asteroid impact near 

Chicxulub on Mexico's Yucatan peninsula. O'Keefe and 

Ahrens (1989) modeled the results of a large bolide 

impacting a carbonate/evaporite sequence and pointed out that 

the increased flux of C02 into the atmosphere would be 

significant, on the order of a 2-10 fold increase. In addition, 

a large volume of anhydrite would be vaporized, releasing 

>1016 moles of so2 into the atmosphere. The impactgenerated carbon and sulfur from vaporized marine target 

rocks would be isotopically heavier than their pre-impact 

equivalents in the Late Cretaceous paleoatmosphere. Our 

analyses verify the presence of heavier S and C isotopes in 

the boundary ejecta unit at Sugarite, thus appearing to 

validate the impact model of O'Keefe and Ahrens (1989). 

Nichols, D.J., Fleming, R.F., Upchurch, G.R., Tschudy, 

R.H., and Pillmore, C.L., 1985, SEPM Midyear Meet., v. 

2, p. 68. 

O'Keefe, J.D., and Ahrens, T.J., 1989, Nature, v. 338, p. 

247-249. 
CORRELATION BE1WEEN MANUE NEON, OXYGEN 

AND RADIOGENIC ISOTOPES IN OCEAN ISLAND 

BASALTS FROM THE PITCAIRN SEAMOUNTS, 

SOtrrHEAST POLYNESIA . 

HONDA M, WOODHEAD, J.D., McDOUGALL, 1., 

Research School of Earth Sciences, Australian National 

University, Canberra, ACT 0200, Australia, and DEVEY, 

C.W., Geologische Institut der Universitlt IGel, 40-60 

Olshausenstrasse, 2300 Kiel, Germany. 

Fresh basaltic glasses recently have been dredged from sites 

of youthful volcanism on the Pitcairn Seamounts, Polynesia. 

Radiogenic isotope results (Sr, Nd and Pb) suggest that the 

mande source for the Pitcairn samples is strongly influenced 

by an enriched mande component of S<H:alled EM-I type. The 

oxygen isotope compositions of the Pitcairn samples appear to 

require the presence of subducted crustal material for the EM-1 

component (Woodhead et al., 1993). 

In order to further understand the characteristics of the EMI component we have measured all five noble gas elemental 

and isotopic abundances in eleven fresh basaltic glasses from 

the Pitcairn Seamounts, whose radiogenic and oxygen isotope 

results were previously obtained by Woodhead et al. (1993). 

The 3Hef'He ratios observed in the Pitcairn samples range 

from 13.5 x to-6 (9.8 RJRA) to 2.3 x ut6 (1.6 RJRA). Neon 

isotopic ratios measured on the Pitcairn samples are enriched 

in 20Ne and 21Ne by as much as 19% with respect to 

atmospheric ratios. The neon isotopic results do not show any 

obvious correlation when they are plotted in ~eJ22Ne21NeJ22Ne space, suggesting that mantle neon in the source for 

the Pitcairn samples is extremely heterogeneous on a small 

scale. 

In general, mantle neon appears to comprise a mixture of a 

primordial solar component together with variable (timeintegrated) amounts of nucleogenic neon. Further, based on 

the calculated nucleogenic 21Ne (21Ne*) I solar ~e cWNes) 

ratios, generally it is possible to predict 3Hef'He ratios in the 

mantle source (e.g., Honda et al., 1993). For the Pitcairn 

samples there are weak correlations between calculated 

21Ne*f20Nes ratios and 206Pbfl04Pb, 143Nd/144Nd 

(positively), and 87Sr/86sr, &18o (negatively). At least two end 

members are required to account for the correlations, one of 

which is a MORB component. In contrast, there are no 

apparent correlations between 3Hef'He ratios actually 

observed in the samples and isotopic compositions of 

radiogenic elements and oxygen, suggesting that helium in the 

samples has been influenced by secondary processes such as 

loss of helium from the magmas and interactions with 

seawater. 

These observations from the Pitcairn samples suggest that 

the EM-I component is enriched in a primordial-solar noble 

gas ~mposition relative to the MORB componenl 

Honda, M., McDougall, I., Patterson, D.B., Doulgeris, A, and 

Clague, D.A., 1993, Geochim. Cosmochim. Acta, v. 57, 

p.859- 874. 

Woodhead, J.D., Greenwood, P., Harmon, R.S., and Stoffers, 

P., 1993, Nature, v. 362, p. 809-813. 

142 

THORIUM ISOTOPES AND SR-ND-PB TRENDS IN 

VOLCANICS AND SEDIMENTS ALONG THE SUNDABANDA ARC (INDONESIA) 

HOOGEWERFF, J.A., VAN BERGEN. M.J., VROON, 

P.Z., Faculty of Earth Sciences, Utrecht University, P.O. 

Box 80021, 3508TA Utrecht, the Netherlands, and 

HER TOG EN, J. , Div. of Physicochemical Geology, 

University of Leuven, Celestijnenlaan 200C, B-3001 

Leuven, Belgium. 

The oceanic and continental domains of the IndianAustralian plate exert a different influence on magma genesis 

in the Sunda-Banda Arc which can be evaluated with 

currently available isotope data on volcanics and sediments. 

Along-arc correlations between Sr, Nd, Pb isotopes in the 

volcanics reflect variations in the tectonically controlled 

supply of continental material to the trench. Strongest 

isotopic deviations from mantle values are not restricted to 

the Banda Arc but are found in the eastern part of the Sunda 

Arc and SW Banda Arc, near a volcanically inactive sector 

close to the location of earliest collision between the 

Australian passive margin and the arc. An important finding 

is that changes in Pb isotopic compositions of the volcanics 

along the East Sunda-Banda Arc correspond to similar 

changes in the sediments entering the trench. In addition, 

kace-element characteristics of the volcanics closely 

resemble those of detrital sediments derived from the 

Australian continent. All these observations are compelling 

evidence for source contamination by recent sediment 

subduction. 

New and compiled (230'fh)/(232'fh) ratios of recent lavas in 

the Sunda-Banda arc are all < 1. The values are low 

compared to those in other island arcs, are close to the ratio 

in the terrigenous fraction of the sediments near the collision 

zone, and argue against a contribution from altered oceanic 

crust. The (230'fh)/f32'fh) ratios do not exhibit a clear alongarc trend as do Sr, Pb, Nd and He isotopes, and correlations 

between Th and the other isotope tracers are generally poor. 

In a transect across the collision zone, however, lowest 

{230Jb.)/(232'fh} ratios were found at the volcanic front in 

combination with lowest 3

He/4He, relatively high 143Nd/144Nd, and strong enrichment of 238U and 226Ra over 232'fb. The cases of U enrichment (and high 143Ndf1 44Nd) are 

restricted to low-K centres at the front in the collision zone. 

It is concluded that the along-arc changes in the nature and 

amount of subducted source components are a dominant 

control in generating the isotope characteristics of the 

volcanics. The influence of terrigenous sediments and 

continental crust is widespread around the domain of the 

Australian margin. Mantle sources in this region were 

contaminated largely by bulk addition of continental material, 

although signatures of low-K frontal volcanoes locally favour 

the role of fluids as a more selective transfer medium. 

Selected refs: Whitford et al. (1977) Geology 5:571-575; Whitford and 

Jezek (1982) Geoi.Soc.Am.Bull. 93: 504-513; Vame and Foden (1986) In: 

Wezel, The origin of arcs, Elsevier: 159-189; Ben Othman et at. (1989) 

EPSL 94:1-21; Hilton and Craig (1989) Nature 342: 906-908; Rubin et al. 

(1989) JVGR 38: 215-226; Gill and Williams (1990) GCA 54:1427-1442; 

Stolz et al. (1990) CMP 105: 585-601; Vroon (1992) Geoi.Uitrai. (Utrecht) 

90:205pp.; Hilton et at. (1992) GCA 56: 851-859; Edwards et al. (1993) 

Nature 362: 530-533; Van Bergen et at. (1993) Tectonophys. 223: 97-116; 

Vroon et al. (1993) JGR 98:22349-22366; Condomines and Sigmarsson 

(1993) GCA 57: 4491-4497. 
SALT EFFECTS ON STABLE ISOTOPE 

PARTITIONING BETWEEN BRINES, AND VAPOR 

AND MINERALS AT ELEVATED TEMPERATURES 

HORITA J .. COLE, D. R., and WESOLOWSKI, D. J., 

Chemical and Analytical Sciences Division, Oak Ridge 

National Laboratory, Oak Ridge, TN 37831-6110, U.S.A 

A series of hydrothermal experiments have been 

conducted at elevated temperatures, in order to determine 

the effect of dissolved salts in water on equilibrium oxygen 

and hydrogen isotope partitioning between brines and other 

coexisting phases (vapor and minerals). The equilibrium 

fractionation factors of oxygen and hydrogen isotopes 

between pure water and vapor, and the effect of dissolved 

salts have been precisely determined from 25 to 350° C. In 

all of the single salt solutions (0-6 molal NaCl at 50 to 

350° C, 0-5 molal CaC12 at 50-200° C, and KCl, MgC12, 

Na2S04, and-MgS04 at 50-100°C), values of Hflna:1_v(D) for 

the salt solutions are appreciably smaller (up to 20%o) than 

those between pure water and vapor in the order pure 

water > Na2S04 > KCl == NaCl > MgS04 > MgC12 <!: 

CaC12• Except for KCl solutions at 50° C, values of 1cflna:1

_ ~ v 

( 0) for the salt solutions were larger (up to 4%o) than, or 

very close to, those of pure water in the order MgS04 == 

MgC12 > CaC12 > Na2S04 > NaCl ==pure water <!: KCI. The 

observed oxygen and hydrogen isotope salt effects in the 

single salt solutions are all linear with molality of the salt 

solutions, and were regressesed to simple equations as a 

function of temperature and molality. The isotope salt 

effects in several mixed salt solutions of the system Na-KMg-Ca-Cl-S04, including those resembling natural brines, 

were found to be exactly additive of the effects of single 

salts separately between 50 and 100° C. Thus, the simple 

empirical equation generated in this study can be used to 

predict the isotope salt effect in complex natural brines. 

In order to determine salt effects on isotope partitioning 

between brines and minerals, experiments of partial isotope 

exchange were carried out on calcite-water at 300° C and 1 

kbar for a duration of 126 days. Calculated equilibrium 

oxygen isotope fractionation factors, 1cflna:calcite-waler' were 

5.9±0.3%o for pure water, and 5.6±0.2, 5.0±0.15, and 4.9±0.2%o 

for 1, 3, and 5 molal NaCl solutions, respectively. The 

observed oxygen isotope salt effects ( -0.3 to -1.0%o) from the 

calcite-water experiments are consistent with those obtained 

from the above liquid-vapor equilibration method at 300° C 

( -0.2 to -0.6%o for 1 and 5 molal NaCl solutions). 

Our study demonstrates that salt effects on stable isotope 

partitioning between brines and other phases (vapor and 

minerals) indeed persist up to at least 350°C, but that the 

complex temperature and molality dependence reported in 

the literature is almost certainly an experimental artifact. 

The isotope salt effects observed in this study must be 

taken into account for isotopic studies involving brines, 

both experimental and natural. 

This study was supported by the Geoscience Program of 

the Office of Basic Energy Sciences and the Geothermal 

Technology Program of the Office of Energy Efficiency and 

Renewable Energy, U. S. Department of Energy, under 

contract number DE-AC05-840R21400 with Martin 

Marietta Energy Systems, Inc. 

143 

RAPID EXTENSION RECORDED BY 

COOLING-AGE PATTERNS AND BRITTLE 

DEFORMATION, NAXOS METAMORPHIC 

CORE COMPLEX, GREECE 

HOWARD K A., U.S. Geological Survey, Menlo 

Park, CA 94025, USA, and JOHN, B.E., Dept. of 

Geology and Geophysics, University of Wyoming, 

Laramie, WY 82071, USA 

The island of Naxos in the Aegean Sea records an 

unusually complete sequence of Miocene structures 

developed as a result of continental extension. The 

structures formed during rise and cooling from ductile, 

upper amphibolite-facies and anatectic conditions to nearsurface conditions beneath the Naxos extensional fault 

system. Top-to-the-north ductile shear fabrics, which 

initially developed during amphibolite-facies prograde 

metamorphism in the footwall, were overprinted by a 

succession of north-directed lower temperature brittle 

structures as the footwall was tectonically unloaded and 

unroofed: pseudotachylite, cataclasis and brittle faulting, 

alteration, tectonic denudation, erosion of the footwall, 

and continued slip. Intrusion of a granodiorite pluton at 

-12.5 Ma coincided with a lull in development of 

extensional microstructures, possibly when shear strain 

abandoned one level and transferred to a higher level in the 

crust. 

Geographic patterns of cooling ages in the footwall, 

derived from published geochronologic studies, provide a 

basis for interpreting the unroofing history of the domal 

footwall (at different mineral closure temperatures for 

hornblende, muscovite, and biotite) as the footwall moved 

from under the north-slipping hanging wall. Analysis of 

the published K-Ar and 40 Art39 Ar ages lead us to 

estimate the fault slip rate as the footwall was tectonically 

denuded and cooled. Contoured values of mineral age 

decrease northward in the slip direction in two stages. The 

southern stage (17-50 Ma) corresponds to areas of low 

metamorphic grade where pre-extension argon was partly 

retained. 

The northern stage (10-16 Ma) records cooling of 

high-grade rocks during the period of rapid extension. 

Northward-younging is seen for each mineral, at rates that 

depend little on the criteria used for selecting data. Ages 

for biotite, muscovite, and amphibole each decrease 

northward at rates between 0.13 and 0.22 m.y./km, to 

about 10 Ma. Assuming that these rates represent cooling 

during extension, they can be used to interpret the slip rate 

at which footwall rocks moved >20 km relative to their 

hanging wall along the base of the north-dipping Naxos 

extensional fault system. Extensional slip calculated this 

way accelerated from 16 to 14 Ma (from amphibole ages), 

and reached a rate averaging 5-8 mm/yr (5-8 km/m.y.) 

from 14 to 10 Ma (from muscovite and biotite ages). 

Maximum rates reported from the Basin and Range 

province, derived from lower closure-temperature minerals, 

are comparable at 7-9 mmlyr. 
SOIL-WATER OXYGEN ISOTOPE PROFILES 

ALONG A CLIMATE TRANSECT 

HSIEH. lean C.C .. Div. of Geol. and Plan. Sci., 

Caltech, Pasadena, CA, 91125, USA, KELLY, 

Eugene F., Dept. of Agronomy, Colo. State Univ., Ft. 

Collins, CO, 80523, USA, CHADWICK, Oliver A., 

JPL-Caltech, Pasadena, CA, 91109, USA, and 

SAVIN, Samuel M., Dept. of Geol. Sci., Case 

Western Reserve Univ., Cleveland, OH 44106, 

USA. 

Understanding soil-water oxygen isotope profiles in 

relation to rain water and evaporation is critical to 

interpreting the isotopic composition of pedogenic 

minerals. Theoretical models of variations in the 

isotopic compositions of soil-water as a function of 

depth, such as Barnes and Allison (1983), predict that 

there should be a maximum value near the surface 

with nearly no change below. These models are based 

on initial filling of pore space with water of a uniform 

isotopic composition. Subsequent evaporation near the 

surface results in tso enrichment. However, under 

natural conditions, substantial amounts of antecedant 

moisture create greater comglexities. 

We have measured the 8 so values of precipitation 

and soil-water along an elevational transect in 

Hawaii. As elevation increases from 77 to 1250 m, 

mean annual temperature ranges from 23 to 17°C and 

median annual precipitation ranges from 20 to 300 em 

with corresponding rain water atso values ranging 

from -4.41%o (SMOW} to -11.43%o. With increasing 

elevation, evaporative demand decreases and the 

amount of vegetative cover increases. Our 

measurements of the isotopic composition of soil water 

are used to determine the relative proportion of water 

lost through evaporative vs. transpirative processes. 

Although evaporation is highest at low rainfall and 

high temperature, our data indicate that even in 

areas of high rainfall and low temperature, 

significant evaporation occurs. 

Most techniques for extracting soil water remove 

water from pores, water adsorbed on mineral surfaces, 

and water fixed in mineral structures which have 

isotopically distinct compositions. For isotopic 

analyses of soil water, we have determined that it is 

important to measure the atso value of pore water 

which we have operationally defined as liquid water 

(Hsieh et al., 1993}. 

Barnes, C.J. and Allison, G.B, 1983, The Distribution of 

Deuterium and Oxygen-18 in Dry Soils:!. Theory: J. 

Hydrology, v. 60, p. 141-156. 

Hsieh, J.C.C., Savin, S.M., Kelly, E.F., and 

Chadwick, O.A., 1993, A New Method for Oxygen 

Isotope Analysis of Soil Water, GSA Abstracts, v 

25(6), p.315. 

144 

3.3GA SM-ND AGE AND THE CRUSTAL EVOLUTION OF XINJIANG OF 

NORTHWESTERN CHINA 

HU Aiqin,Institute of Geochimstry,Chinese Academy of 

sciences, Wu shan,Guang 

Zhou,Guang Dong Prov., 

510640,P.R.China 

The northern part of xinjiang, as a major region of 

study, is belonged to the 

paleoplate of Tarim,Junggar 

and Siberia respectively~ 

Recently, 3263±129Ma of Sm~ 

Nd i sochron age with tNd ( t) = 

+3.2±0.7 fo~ 10 amphibolites, 

3050Ma of the average TnM for 

8 acid rocks have been obtained, which indicated the existence of Early-Middle Archaean continental basement in 

the Tarim Block firstly. the 

events of crustal growth were 

happened during the precambrian period(3.3-3.0Ga,2.2Ga, 

1.8Ga and 0.7Ga)mainly,but 

some continental crust,may 

be, was formed during the 

phanerozoic period in the 

northern Xinjiang. 

Based on the different characterastics of Nd-Sr isotopic evolution and statistical 

age of ToM•the north Xinjiang 

continent can be devided into 

four continental blocks,those 

are:the Tarim Block is of Archaean basement;the TianshanEarly-Middle Proterpzic; the 

Junggar-Upper Proterozic and 

Phanerozoic;Artai-Middle Prorozic. There were different 

geological tectonic histories 

in the four blocks, especially, during the phanerozoic 

it was very obvious that the 

Tarim block was in the relative stable tectonic environment, but in the north continent to the Tianshan there 

was occured extensive and 

vast orogeny. 
PRECISE MEASUREMENT OF RUTHENIUM ISOTOPES 

BY THERMAL IONIZATION MASS SPECTROMETRY 

HUANG, M., and MASUDA A. Dept. of 

Chemistry, The University of ElectroCommunications, Chofu-shi, Tokyo 182, 

Japan 

Ruthenium is a very interesting element 

whose seven stable isotopes are related to 

p, s and r nuclear processes. A basic study 

to search for isotopic abundance anomalies 

in Ru by TIMS is tried in this work. 

Poths et al. employed a V-shaped filament with silica gel technique to measure 

the isotopic compositions of Ru in the 

Allede and Leoville carbonaceous chondrite. 

The potenial interferences on masses 100 

and 104 were found, although the sensitivity was improved considerably. To get 

accurate result without any additional interferences, no modifier is used in our 

work. 10 /..l g of Ru (Aldrich) is loaded on 

Re filament to create a strong ion beam. 

And the zone-refined Re filament is carefully outgassed at 5A over 3 hours beforehand in order to reduce isobaric interferences by Mo emitted from the filament. 

After corrections for interfering impurity ions, the results are normalized 

using 102Ru/ 99 Ru=2. 47404 for direct comparison of the data with those of Poths et 

al.. The deviations about +0. 5 e and +10 e 

on masses 100 and 104 are found, respectively. The interfering species are most 

probably to be 4 °Ca 28 Si 16 02 and 88 Sr 16 0 

due to the silica gel tecnique used in 

their work, although they carefully tried 

to eliminate the interferences. 

It is interesting to find that there 

appears to be about -10 e deviation in 

mass 98 as compared with the result of 

Poths et al.. However, further research 

work is necessary to make it sure. 

Poths, H .. Schmi tt-Strecker, S .. and Begemann,F.. 1987, Geochim. Cosmochim. 

Acta, v.5l,p.1143. 

145 

IMPLICATION FOR THE EVOLUTION OF THE 

LITHOSPHERE IN SOUTHERN AFRICA FROM ISOTOPE 

SYS1EMATICS IN GRANULITE XENOLITHS 

HUANG Y-M VANCALSTEREN, P. AND 

HA WKESWORTH, C. J ., Department of Earth Sciences, 

Open University, Milton Keynes MK7 6AA, U.K. 

The nature and evolution of the lower crust around the 

margins of Archaean cratons is not well constrained. New data 

are presented on granulite facies xenoliths from kimberlite 

pipes inN. Lesotho and the Markt area, Namaqualand, southern 

Mrica, close to the margins of the Kaapvaal Craton. Granulites 

from both areas have similar major element compositions with 

Si02 between 46-51%. Sr and Nd isotope ratios are 0.70479-

0.70723 and 0.51251-0.51149 respectively in the Markt 

granulite xenoliths, which have, on average, higher Sr and 

lower Nd isotope ratios than those in the N. Lesotho 

granulitesl. Whole rock Sm-Nd isotope data from the Markt 

granulites scatter on an errochron, corresponding to 1.18±0.2 

Ga, within error of the age of 1.24±0.17 Ga for the N. Lesotho 

granulites. Selected mineral (cpx, gt, plag)-whole rock 

isochrons from Markt yield ages of 851±56 and 604±36 Ma, 

and one from Lesotho gives an age of 595±10 Ma. 

Measured 206Pbf204Pb ratios from the N. Lesotho granulites 

range from 16.25 to 17.58, and 207Pb/204Pb from 15.19 to 

15.58. All but two of the granulites from N. Lesotho fall on an 

array which yields an age of 1.49±0.36 Ga. This age is 

indistinguishable within error from their Sm-Nd whole rock 

age. In contrast, the Markt granulites have 206Pb/204Pb ratios 

in the range 15.17 to 17.67, with relative low 207Pb/204Pb 

ratios between 14.87 to 15.10. These data fall on a trend 

corresponding an age of 3.19±0.04 Ga, which is much older 

than either the Pb-Pb age for theN. Lesotho samples and their 

own Sm-Nd age. Mineral-whole rock Pb-Pb isochrons from 

two Markt granulites with low whole rock Pb isotope ratios 

give ages of2.93±0.07 and 2.91±0.08 Ga, respectively. 

Thus, both Sm-Nd and Pb-Pb whole rock ages indicate that 

theN. Lesotho granulite xenoliths were mostly derived from a 

Proterozoic lower crust. However, taken at face value, Pb 

isotope data for the granulite xenoliths from Markt indicate 

Archaean ages. Correlations between major and trace element 

data indicate that the granulite xenoliths from Markt are 

genetically related and preclude simple mixing models for the 

Pb-Pb trend. Contamination from kimberlites is also unlikely 

because there is no correlation between Pb contents and isotope 

compositions. There are two explanations for the Pb and Nd 

isotope systems, one is that the Pb-Pb trend from the Markt 

granulites was been inherited from an Archaean lithospheric 

mantle source and the Sm-Nd whole rock age represents the 

time of the lower crust generation. Alternatively, ages of 2.9 

Ga yielded by the mineral-whole rock isochrons from two Udepleted samples represent early metamorphic ages, and the 

Markt granulites are inferred to have been derived from the 

mantle in the Archaean, probably at 3.2 Ga as indicated by the 

whole rock Pb-Pb age. In this explanation, the Nd whole rock 

age may reflect re-equilibration in an Archaean lower crust 

during the generation of Proterozoic crust, which in tum 

implies that Sm and Nd have higher diffusion rates than U and 

Pb, consistent with the conclusions of recent geochronological 

studies2.3. 

1 Rogers and Hawkesworth, 1982, Nature 299, pp. 409-413. 

2 Burton et al., 1993, Terra Nova 5, pp. 382. 

3 Mezger et al., 1992, Earth Planet. Sci. Lett. 113, pp. 397-

409. 
ISOTOPIC CHARACTERIZATION OF THE 

SVECOFENNIAN DOMAIN 

HUHMA, Hannu, Geological Survey of Finland, 02150 

Espoo, Finland 

The continental crust in the Svecofennian 

Domain of the Fennoscandian Shield was mostly formed 

between 1.91 and 1.86 Ga. The U-Pb zircon ages on gabbros 

and synorogenic granitoids are generally 1.87-1.89 Ga. Some 

felsic volcanics are slightly older at ca. 1.91 Ga. The initial 

e-values for mafic rocks range from 0 to + 3, and the felsic 

rocks have a range from -1 to + 3. Most detrital zircons 

from the Svecofennian metasediments analysed by SIMS are 

1.9-2.05 Ga old (Huhma et al., 1991'"). Due to mixtures 

between Archaean and Proterozoic crystals apparent 

conventional multi-grain U-Pb zircon ages of these samples 

are about 2.2-2.3 Ga, which are also close to the Nd-model 

ages TDM. As the ion micro-probe results are quite similar in 

eight samples throughout the Svecofennian domain, it is likely 

that all metasediments (including palaeosomes in migmatites) 

with an average zircon age of 2.2-2. 3 Ga represent analogous 

mixtures of Archaean and Proterozoic components. There are 

quite a few such zircon data from various parts of the Shield 

(including samples from southern Sweden, Estonia, southern 

and central Finland, "upper Kalevian" in eastern Finland, and 

even the Lapland granulite belt). All this calls for large 1.9-

2.05 Ga crustal blocks in erosion position at about 1.9 Ga 

ago, and an effective mixing and transport processes that 

filled the Svecofennian basins after the formation of the 1. 97 

Ga Jormua and Outokumpu ophiolites. The site of this ca. 2 

Ga felsic crust remains unknown. 

Excluding detrital materials, the oldest zircon 

ages from the Svecofennian domain have been measured from 

gneissose tonalites near the palaeosuture in Central Finland 

(ca. 1.93 Ga). These rocks have been sometimes described as 

domes and considered as possible candidates for reworked 

older (Archaean) crust. The Sm-Nd data on several samples, 

however, provide ENd( 1. 93) values of about + 3, which clearly 

show that these rocks represent new Proterozoic crust. 

Geochemical data on some of these rocks suggest that they 

probably originated through melting of a low-K island arc 

tholeiite source leaving gabbroic to amphibolitic residue. In 

the Pyhisalmi area also a 1.92 Ga felsic volcanic rock 

adjacent to the gneissic tonalites provides a positive E of 

+3.4. These Sm-Nd data together with analyses on slightly 

younger (1. 88-1.89 Ga) granitoids suggest that juvenile rocks 

with positive ENd :::=::: +3 occur mainly in a "belt" adjacent to 

the Archaean craton. The Pb isotopes from the Svecofennian 

galenas show an analogous pattern: The wvalues get higher 

further away from the Archaean crust. 

There are thus several felsic rocks which yield 

initial isotopic signatures similar to the convective upper 

mantle. In fact initial E-values for many mafic rocks (eg. 

Haveri and Jormua) are less positive, suggesting involvement 

of older LREE enriched lithosphere in their genesis. 

*Huhma, H., Claesson, S., Kinny, P.D. and Williams, I.S., 

1991. The growth of Early Proterozoic crust: New evidence 

from Svecofennian detrital Zircons. Terra Nova, 3, 175-179. 

146 

HELIUM AND CARBON ISOTOPE SYSTEMATICS OF 

TARANAKI BASIN, NEW ZEALAND, NATURAL GASES 

HULSTON. J. R., Nuclear Sciences Group, Institute of 

Geological & Nuclear Sciences, PO Box 31-312, Lower 

Hutt, New Zealand; KAPLAN, I. R. and JEFFEREY A., 

Global Geochemistry Corporation, 6919 Eton Avenue, 

Canoga Park, CA 91303, USA; HILTON, D. R., Isotope 

Laboratory, Scripps Institution of Oceanography, University 

of California, San Diego, La Jolla, CA 92093, USA 

(Present address: Labatorium Isotopen-geologisch 

Onderzoek, Vrije Universiteit Amersterdam, De B0elelaan 

1085, 1081 HV, Amsterdam). 

The chemical and isotopic compositions of the gases from 

Maui, Kapuni and Kupe South hydrocarbon fields of the 

Taranaki Basin of New Zealand display distinct differences. 

The Maui gas contains almost 10% ethane and higher 

hydrocarbons, with small quantities of C~ and N2• The 

iso/n-butane ratio indicates a mature gas, and the methane is 

isotopically rather heavy compared to deep sedimentary gas 

from areas such as the Permian Basin of West Texas. The 

Kapuni gas is wetter than the Maui gas, and is much richer in 

C02, which appears to be derived from decomposition of 

organic matter, based on the light carbon isotope value (813C 

-14.5%o). The Kupe S gas is also wet but contains virtually no 

C02• The Kapuni and Kupe S gases are both less mature than 

the Maui gas as shown by the higher iso/n-butane ratio and the 

lighter methane 813C value (-42.6%o). The two groups of gases 

when plotted on the genetic characterization diagram of Schoell 

fall within the regime of gas associated with condensate. 

The most striking difference between the two groups of 

gases is the helium content and its isotopic ratio. The Maui 

gas is over an order of magnitude higher in helium 

concentration (up to 190 ppm) and its 3He/4He ratio (3.8RA 

where RA = the air 3He/4He ratio of 1.4 x 1Q-4i) is 

approximately half that of upper mantle helium issuing from 

volcanic vents of the Taupo volcanic zone. In contrast, ·the 

Kapuni and Kupe S natural gases have little or no mantle input 

(0.08-0.19RJ and low helium concentrations (12-19ppm). 

This indicates that up to half of the helium in the Maui gas 

field could be of upper mantle origin and is most likely related 

to the subduction system that generates the volcanic arc to the 

east. 

The isotopic ratios of helium and carbon in methane and 

carbon dioxide are found to vary consistently and significantly 

from NE to SW across the Maui field. With the exception of 

carbon dioxide content, the differences between Kapuni and 

Kupe S are less significant than those across the Maui field. 

Two major processes can be envisioned to account for the 

hydrocarbon-rich, 3

He rich gas in the Maui field: (1) Mixing 

of a hydrocarbon-rich gas with a 3He rich gas of deep origin. 

(2) Intrusion of 3

He bearing magmas into shallower than 

normal petroleum source sediments. 

The evaluation of these processes needs to consider that 

release of 3He enriched helium from magmatic intrusions 

usually involves generation of considerable quantities of 

volatiles, either from the magma itself or by heating of crustal 

rocks. 
IMPLICATIONS OF MANTLE HELIUM IN TARANAKI 

BASIN, NEW ZEALAND, NATURAL GAS WELLS 

HULSTON. J. R., Nuclear Sciences Group, Institute of 

Geological & Nuclear Sciences, PO Box 31-312, Lower 

Hutt, New Zealand; KAPLAN, I. R. and JEFFEREY A., 

Global Geochemistry Corporation, 6919 Eton Avenue, 

Canoga Park, CA 91303, USA; HILTON, D. R., Isotope 

Laboratory, Scripps Institution of Oceanography, University 

of California, San Diego, La Jolla, CA 92093, USA 

(Present address: Labatorium Isotopen-geologisch 

Onderzoek, Vrije Universiteit Amersterdam, De Boelelaan 

1085, 1081 HV, Amsterdam). 

The chemical and isotopic compositions· of the gases from 

Maui, Kapuni and Kupe South hydrocarbon fields of the 

Taranaki Basin of New Zealand display distinct differences. 

The Maui gas contains almost 10% ethane and higher 

hydrocarbons, with small quantities of C02 and N2• The 

iso/n-butane ratio indicates a mature gas, and the methane is 

isotopically rather heavy compared to deep sedimentary gas 

from areas such as the Permian Basin of West Texas. The 

Kapuni gas is wetter than the Maui gas, and is much richer in 

C02, which appears to be derived from decomposition of 

organic matter, based on the light carbon isotope value (613C 

-14.5%<1 ). The Kupe S gas is also wet but contains virtually no 

C02• The Kapuni and Kupe S gases are both less mature than 

the Maui gas as shown by the higher iso/n-butane ratio and the 

lighter methane 613C value (-42.6%o). The two groups of gases 

when plotted on the genetic characterization diagram of Schoell 

fall within the regime of gas associated with condensate. 

The most striking difference between the two groups of 

gases is the helium content and its isotopic ratio. The Maui 

gas is over an order of magnitude higher in helium 

concentration and its 3

He/4He ratio is approximately half that 

of upper mantle helium issuing from volcanic vents of the 

Taupo volcanic zone. It is over two orders of magnitude 

higher than in Kapuni and Kupe S natural gases which have no 

apparent mantle input. This indicates that up to half of the 

helium in the Maui gas field could be of upper mantle origin 

and is most likely related to the subduction system that 

generates the volcanic arc to the east. 

The isotopic ratios of helium and carbon in methane and 

carbon dioxide are found to vary consistently and significantly 

from NE to SW across the Maui field. With the exception of 

carbon dioxide content, the differences between Kapuni and 

Kupe S are less significant than those across the Maui field. 

Two major processes can be envisioned to account for the 

hydrocarbon-rich, 3

He rich gas in the Maui field: (1) Mixing 

of a hydrocarbon-rich gas with a 3He rich gas of deep origin. 

(2) Intrusion of 3

He bearing magmas into shallower than 

normal petroleum source sediments. 

The process of release of 3

He enriched helium from 

magmatic intrusions appears to involve generation of 

considerable quantities of volatiles, either from the magma 

itself or by heating of crustal rocks. Mixing of such a gas with 

a shallower hydrocarbon gas should result in considerably 

higher quantities of carbon dioxide, nitrogen or hydrogen than 

is observed. The only sizable quantities of diluting components 

in the Maui gas are hydrocarbons. 

147 

THIRTY -1WO YEARS OF GEOCHRONOLOOICAL WORK 

IN THE CENTRAL AND WESTERN ALPS: A REVIEW ON 

SEVEN MAPS 

HUNZIKER. J.C., lost. de Mineralogie, UNIL BFSH2, 

Lausanne, 1015-CH, Switzerland, and DESMONS, J., 

C.N.R.S. C.R.P.G., 15 r. N.D. des Pauvres, B.P. 20, 

Vandoeuvre-les-Nancy Cedex, 54501, France, and 

HURFORD, A. J., Dept. of Geological Sciences, 

University College, London, WC1E6BT, UK. 

This paper represents a review of the last 32 years of 

geochronological work in the Central and Western Alps. Some 

of these data, distributed in over 240 papers, are poorly 

accessible, thus starting to fade away from the view of Alpine 

geologists. The data have been represented on 7 maps, 3 of 

which, at a scale of 1:800'000, cover the area from the 

Alpine/Apennine boundary near Genova, through to the western 

Silvretta. Map I contains Rb-Sr data, map II summarizes the KAr and Ar-Ar mineral data. Map III shows U-Pb and whole rock 

data, and therefore represents our knowledge of the pre-Alpine 

history. The ages on maps I and II are represented by a colour 

code, the symbol on the maps represents the mineral concerned. 

A number, printed near the symbol leads to the references, 

which are given both in numerical and alphabetical order. 4 

maps of the Central Alps are at a scale of I :635 000, Maps IV 

and V give the fission track data for apatite and zircon 

respectively. On maps VI and VII, the K-Ar and Rb-Sr data are 

given respectively at the same scale for direct comparison. All 

ages are calculated using the lUGS recommended constants. 

The present compilation of joint geochronological research 

in the Central and Western Alps reveals surprising 

inhomogenieties both in regional and temporal distribution. 

The Western Alps contain practically no Rb-Sr and U-Pb data. 

A majority of the data deals with the Alpine metamorphism and 

with the cooling after the Alpine orogeny. From pre-Variscan 

times, only little can be said with certainty, no pre-Variscan 

mica ages having been found so far. In the 

Austroalpine/Southalpine units, mica ages between 240 and 

140 Ma mark the slow uplift and cooling of these zones, from 

lower to upper crustal conditions. 

The Eoalpine orogeny has been subdivided into an early 

eclogitic stage (140-85 Ma) and a later blueschist and cooling 

stage (85-60 Ma). The coincidence of two or more different 

chronometers facilitates the interpretation of the Eoalpine age 

data. 

The Tertiary has been subdivided into 2 phases, the ±30 Ma 

calcalkaline magmatism subdividing the Cenozoic era into the 

Mesoalpine and Neoalpine. 

The 45-30 Ma period marks the Mesoalpine metamorphic 

phase, found dominantly outside the area. therefore The term 

Lepontine phase (as used by the Bern group) for this age range 

implies a more local distribution and is thus inapropriate. 

Ages between 30 and 0 Ma mark the Neoalpine event, 

subdivided (arbitrarily at 15 Ma- a more or less quiet period) 

into early and 

Late Neoalpine. 

The Alpine movements of major tectonic lines, such as the 

lnsubric line, the Simplon line and the Aosta-Ranzola line, 

have been dated both during their ductile as well as their brittle 

movements, by means of mineral ages. Thus periods of 

polyphase Alpine movements, ranging from as early as Jurassic 

and lasting throughout the whole Tertiary have been identified. 
REGIONAL THERMO-TECTONIC HISTORIES OF THE 

RHINE GRABEN AND ADJACENT HERCYNIAN 

BASEMENT: A KEY TO ASSESSING THb ALPINE 

INFLUENCE IN NORTH-WEST EUROPE? 

HURFORD, Anthony J, CARTER, Andrew, Research 

School of Geological Sciences, Birkbeck College and 

University College London, London WC1E 6BT, U.K. and 

BRIX, M., Institut fUr Geologie, Ruhr-UniversWit Bochum, 

D-4630 Bochum-1, Germany. 

Published and unpublished apatite fission data for UK 

crystalline and sediment samples have identified a regional late 

Cretaceous to early Tertiary cooling event which, in part, is 

associated with important basin inversions in the North and 

Irish Sea hydrocarbon fields. Plausible mechanisms considered 

for this major cooling event relate to either rifting of the northeast Atlantic and/or the synchronous Laramide phase of 

deformation in the Alpine domain. The extent and magnitude 

of Alpine thermo-tectonic effects in north-west Europe is 

poorly defined. Although the Massif Central, Rhenish Shield 

and Bohemian Massif were most probably exhumed at around 

kiT boundary times, the precise amounts and mechanisms are 

subject to much debate. The formation of the European 

Cenozoic Rift System coincident with Alpine deformation, and 

with identical principal stress directions suggests, it should 

provide a sensitive record of the effect and extent of Alpine 

deformation in the European plate. In a major study, 150 

samples from the Rhenish Massif and Rhine Graben have 

provided new apatite fission track data as part of a wider study 

throughout north-west Europe from the Alps to the Atlantic 

margins. Interpretation of initial results from the Rhine Area 

confirm the Upper Cretaceous timing of initial rifting, and 

associated flank uplift and high heat-flow. Results from 

outcrop and borehole samples in sediments in the rift itself 

provide clear evidence that the present-day high heat-flow is a 

very recent phenomenon. Discussion will be offered of the 

significance of the Rhine data to our wider understanding of the 

thermal and tectonic histories of north-west Europe. 

148 

IN1ERSTELLAR DIAMONDS IN METEORITES 

HUSS G R., Lunatic Asylum, Division of Geological 

and Planetary Sciences, California Institute of Technology, 

Pasadena, CA 91125 

Nanometer-sized diamonds were the first objects isolated 

from meteorites that could be shown with reasonable certainty 

to have originated outside of the solar system. They have been 

found in the least metamorphosed members of all classes of 

chondritic meteorites. The extra-solar-system origin of the 

diamonds is shown by the isotopic compositions of several 

trace elements (e.g., noble gases, H, N, Ba), which differ from 

bulk solar system values in ways not accounted for by known 

solar-system processes. Their presence within chondrites, 

which date from the formation of the solar system, indicates 

that the diamonds pre-date the solar system. 

Interstellar diamonds are very small, -2 nm (20A) in 

diameter, and each contains only a few thousand carbon atoms. 

Diamonds are the most abundant interstellar grains with 

abundances in unmetamorphosed 'matrix' material of0.1-0.3% 

(vs. 10-14 ppm SiC, 2-10 ppm graphite, 5-10 ppb Al203). 

Trace elements in diamonds exhibit large isotopic anomalies. 

For example, Xe-HL is enriched 2x in the lightest and heaviest 

isotopes relative to solar-system Xe. Nitrogen is highly 

enriched in 14N (815N = -345%o). Ba has excesses of r-process 

isotopes. In combination, these isotopic signatures point 

toward supernovae as the nucleosynthetic source. Interestingly, 

the isotopic composition of the carbon (8 13c = -32 to -38o/oo) 

falls within the range of terrestrial carbon. 

The process by which diamonds form is not understood, 

but three basic mechanisms are under consideration. 1) 

Diamonds may condense directly in stellar outflows. H-rich 

stellar ejecta is similar in composition to the gases used to 

produce diamond films by chemical vapor deposition (CVD). 

2) Diamonds may be formed from graphite through shock 

pressure during grain-grain collisions in interstellar space. 3) 

Diamonds may be produced from graphite or organic material 

either through annealing of particles by ultra-violet irradiation 

or through restructuring induced by collisions with energetic 

particles. Of these ideas, direct condensation of diamond seems 

to have the fewest problems. 

Because diamonds contain so few atoms, isotopic compositions can be measured only on bulk samples and it is not 

known how many sources contributed to the measured 

compositions. Measurements of individual SiC and graphite 

grains, which apparently provide 'snapshots' of conditions in 

the stellar atmosphere where they formed, show that these 

grains come from many stars. Thus, it is likely that the moreabundant diamonds also come from more than one star. If so, 

then the isotopic characteristics of diamonds do not represent a 

distinct source, which helps explain why such a source has not 

been identified. 

The abundance of diamonds indicates that either diamond 

production is much more efficient than production of other 

presolar grains or that the diamonds are less subject to 

destruction in interstellar space. Division Contribution 5366 

(840) Supported by NASA NAGW-3040, NAGW-3297. 

Anders E. and Zinner E. (1993) Interstellar Grains in Primitive 

Meteorites: Diamond, Silicon Carbide, and Graphite: 

Meteoritics, v. 28, p. 490-514. 

Huss, G. R. and Lewis, R. S. (1994) Noble Gases in Presolar 

Diamonds I: Three Distinct Components and Their 

Implications for Diamond Origins: Meteoritics, in press. 
CARBON AND NfiROGEN ISOTOPIC COMPOSITION OF 

MOISSANITE IN Y AKUTIAN KIMBERLITES 

I.D Butcheoo, Lunatic Asylum, Caltech, Pasadena, CA 91125 

and Lawrence Livermore Laboratory, Livermore, 94551; 

E.A. Mathez, R.A. Fogel, American Museum of Natural 

History, New York, NY 10024; V.K. Marshintsev, National 

Centre for Research, 677892 Y akutsk, Russia 

Trace quantities of moissanite (a-SiC) occur in mineral 

concentrates of diamond-bearing kimberlites and lamproites 

but little information about the chemical and isotopic composition, stability in upper mantle rocks and origin of SiC is 

available. We present new data on C and N isotope abundances, minor element chemistry and inclusion mineralogy of 

moissanite from the Mir and Aikhal kimberlite pipes, Yakutia. 

Moissanite was recovered from -700-800 kg of kimberlite 

drill core and occurs as elongate (0.5-1 mm), gemmy crystals 

varying in color from a characteristic blue-green to pale green 

to nearly colorless to blue-black [1]. Iron (<940 ppm) is 

present in nearly all crystals; AI (<920 ppm), Ca (<100 ppm), 

V ( <270 ppm), Cr ( <330 ppm) and Mn ( <200 ppm) were also 

detected. No correlation between color and chemistry is 

apparent. Moissanite contains a variety of mineral inclusions. 

The most common is metallic Si (up to 100 Jlm in size). 

Several, compositionally distinct Fe- and Fe-Ti-Zr-silicides 

were observed, typically at Si metal-SiC contacts. The Ti-rich 

silicides contain high levels of LREE (up to 16% Ce) and Th 

(up to 4%). Two 0-bearing phases occur in SiC: sinoite 

(Si2N20, previously found only in enstatite chondrites) and a 

rare earth silicate containing -75% LREE-oxides. This 

unusual inclusion mineralogy is unknown in synthetic SiC and 

is further evidence the Mir and Aikhal moissanites are natural. 

Ion probe analyses of 23 individual SiC crystals reveal a tight 

distribution of C isotope compositions: -32%o <S13C< -26%o; 

individual SiC are homogeneous in at3c within ±1.5%o. One 

micro-diamond recovered from the Mir SiC fraction has l)13C= 

-30.7 ± 2.1%o, while the mean of all SiC analyses is l)13C= 

-28.3 ± 3.4%o. In contrast, most "large" diamonds from Mir 

and Aikhal have anc in the normal mantle range of -8 to -3%o 

[2]. Moissanite in the Fuxian kimberlite is similarly enriched 

in 12c relative to diamond [3]. Two moissanites contain, 

respectively, 53 and 140 ppm N with l)15N of 9.7 ± 4.0 %o and 

5.6 ± 2.2%o. The difference in C isotope composition between 

moissanite and diamond within kimberlites is not plausibly 

attributed to fractionation of a homogeneous mantle C reservoir 

and appears to require isotopically heterogeneous mantle C 

sources. Based on the C and N isoto~ abundances and the 

requirement of an f(02) more than 106 times lower than the 

IW buffer, we suggest moissanite formed by metamorphism of 

reduced, carbonaceous sediments during subduction. 

~: [1] Marshintsev, V.K. (1990) Mineral Mag . .12. #3, 17-

26 (in Russian); [2] Galimov, E.M. (1991) ~. 1697-1708; 

[3] Leung, I.S., et al. (1990) Nature~. 352-354. 

149 

DATING HIGH PRESSURE METAMORPffiSM IN TIIE 

WESTERN ALPS; REFINING ISOTOPIC AND TECTONIC 

INTERPRETATIONS 

INGER, Simon, Department of Earth Sciences, University of 

Leeds, Leeds, LS2 9JT, UK, and CLIFF, Bob, Department of 

Earth Sciences, University of Leeds, Leeds, LS2 9JT, UK. 

TheW estern Alps record a collision orogeny in which 

basement rocks of both continents, and oceanic units, preserve 

eclogite facies assemblages. Understanding the timing of their 

polymetamorphic histories is crucial to geodynamic 

reconstructions of the belt, which have long assumed "Early 

Cretaceous" eclogite metamorphism in all such units. These 

models may need revision in light of increasing isotopic 

indications of high pressure conditions persisting into the 

Tertiary in some units. As with all metamorphic 

geochronology, meaningful age determinations rely upon 

adequate understanding of the relationships between mineral 

formation, thermal history and chemical equilibrium, which are 

not always clear in eclogite assemblages. 

Polymetamorphic marbles in the Eclogitic Micaschist 

Complex of the Sesia Zone basement have well-preserved 

eclogite facies assemblages which post-date amphibolitegranulite facies relics. High-alumina titanite yields U-Pb ages 

from 140 to 107Ma, while phengite Rb-Sr ages range from 58-

61Ma. A cover sequence believed to have experienced only the 

high pressure event gives a U-Pb titanite age of 66Ma, 

indistinguishable from the Rb-Sr ages on coexisting phengites 

and Sm-Nd garnet ages from eclogites in the underlying 

ophiolitic Pi~mont Unit. Titanites in polymetamorphic 

samples may therefore contain inherited lead, whereas those in 

the monometamorphic marble crystallised at the eclogitic stage. 

The lower portion of the Sesia Zone - the Gneiss Minuti 

Series - has been pervasively metamorphosed under greenschist 

facies conditions at 38-39Ma, and may have suffered a common 

earlier history with the upper unit despite the absence of 

convincing eclogitic relics. Titanite from a calc-silicate rock 

within this unit yields a U-Pb age of 249Ma, the age of the 

Variscan magmatism and metamorphism which generated the 

protoliths of these units. By contrast with those in the EMC, 

these titanites have therefore survived greenschist-facies, and 

possibly eclogite-facies, overprinting without lead exchange or 

new crystal growth. 

Underlying the Sesia Zone, the Pi~mont Unit contains 

metasedimentary and metabasic rocks of oceanic affinity whose 

lowermost subdivision records eclogite conditions. 

Greenschist-facies regional metamorphism in the eclogitic and 

non-eclogitic sections is dated at 38-39Ma by white mica Rb-Sr 

analyses. These fabrics are reworked by a top-to-SE-directed 

greenschist shear fabric which may be dated by 34Ma Rb-Sr 

ages from micas throughout the zone. This fabric defines a 

regionally significant shear zone which may backthrust more 

external units into the high grade nappe pile, or may have been 

responsible for the extensional unroofmg of the eclogitic 

Pi~mont units. In either case, constraining differences in 

thermal (and baric) histories across such structures by 

structurally-focussed isotopic studies offers the best means to 

distinguish between geodynamic models. 
STRONTIUM, OXYGEN, AND CARBON 

ISOTOPIC RECORDS OF PALEOSALINITY 

AND PALEO-STREAMFLOW IN ESTUARINE 

SEDIMENTS 

INGRAM, B.L., Dept. of Geology and 

Geophysics, University of California, Berkeley 

CA, 94720 

Strontium, oxygen, and carbon isotopic 

compositions of molluscan and foraminiferal 

carbonate from estuarine sediments cored beneath 

San Francisco Bay are used to derive a record of 

salinity and average freshwater inflow to the estuary 

for the past several thousand years. The large 

difference in the 87Srf86Sr ratio between seawater 

(0.7092) and the freshwater sources to the estuary 

(0. 7065) produces a correlation between 87Srf86Sr 

and salinity in bay waters that can be detected with 

high-precision measurements. Similarly, the oxygen 

and carbon isotopic composition varies between 

otso and o13C values of about -12o/oo and 14o/oo for 

river water, and O%o and lo/oo in seawater. 

Paleosalinity is inferred from the 87Srf86Sr, 180J160, 

and 13CJ12C ratios measured in calcite from fossil 

bivalves (Macoma balthica and Mytilus edulis) and 

foraminifera (Ammonia becarrii and Elphidiella 

hannai) preserved in the sediment. Because salinity 

in San Francisco Bay is primarily controlled by the 

freshwater inflow from the Sacramento and San 

Joaquin rivers, the paleosalinity record can be 

converted to a paleo-discharge record using a transfer 

function derived from historic data. 

The Sr isotopic data indicate there were extended 

periods (several hundred years long) when salinity 

was significantly lower than the modern, prediversion salinity, indicating higher freshwater 

inflow, particularly prior to 2500 years ago. There 

were also periods of high salinity, indicating low 

freshwater inflow, between 2500 years and the 

present. Modern freshwater inflow appears 

exceptionally low, less than one-half of the 4300-

year average. High resolution 8180 indicates cyclical 

variations in salinity in San Francisco Bay with 

perodicities of 500, 200, 90, and 55 years, similar to 

those seen in the .1,14C production rate, implying a 

common forcing mechanism linked to fluctuating 

solar activity. The data generally agree with other 

proxy climate records in California, such as lake 

level records from eastern California and Nevada, 

tree-ring records, and treeline fluctuations. 

150 

OXYGEN AND CARBON ISOTOPIC RECORDS 

OF PALEOSALINITY IN SAN FRANCISCO BAY 

INGRAM. B.L., Dept. of Geology and 

Geophysics, University of California, Berkeley 

CA, 94720, INGLE, J. C., Dept. of 

Environmental and Geological Sciences, Stanford 

University, Stanford, CA 94305, and CONRAD, 

M. E., Earth Science Division, Lawrence Berkeley 

Lab, Berkeley, CA, 94720. 

Oxygen and carbon isotopic compositions of 

molluscan shells separated from San Francisco Bay 

estuarine sediments are used to derive a highresolution record of salinity variations over the past 

5000 years. The large difference in the 8180 value of 

seawater (-O%o) and the freshwater inflow (-11o/oo) 

produces a simple mixing relation between otso and 

salinity in bay waters that is recorded in fossil 

carbonate shells. Similarly, the 813C varies between 

about -14o/oo for river water, and 1 o/oo for seawater. 

The 818Q of river water entering the Bay through 

the Delta over a three-year period (1991-1993) varied 

by about 1-2o/oo between wet and dry years, with 

average values during wet years of about -12o/oo. 

Because salinity in San Francisco Bay is primarily 

controlled by the freshwater inflow from the 

Sacramento and San Joaquin rivers, the paleosalinity 

record can be converted to a paleo-discharge record 

using a transfer function derived from historic data. 

The otso of the mussel (Mytilus) shells vary 

between -1 o/oo and -4o/oo (averaging -2.6o/oo), 

corresponding to salinities of between 22o/oo and 28o/oo 

(averaging 25.5o/oo). Because modem salinity at this 

location is 28o/oo, we infer that the modem average 

river inflow into the Bay is significantly lower than 

that over the past 5000 years. The 813C correlates 

with the 818Q variations downcore, indicating salinity 

(not temperature) is the primary control. 

The 818Q data indicates cyclical variations in 

salinity and freshwater inflow in San Francisco Bay 

with perodicities of 500, 200, 90, and 55 years, 

similar to those seen in the L\ 14C production rate. The 

data generally agree with other proxy climate records 

in California, such as lake level records from eastern 

California and Nevada, tree-ring records, and treeline 

fluctuations. 
ZIRCON AGE DISTRIBUTIONS IN GRANITES, 

GREYW ACKES, AND GNEISSES FROM THE 

SOUTHWEST PACIFIC GONDWANA REGION. 

IRELAND· T.R, Research School of Earth Sciences, 

Australian National University, Canberra ACT 0200 

Australia, BRADSHAW, J.D., MUIR, R., WEAVER, S., 

Department of Geology, University of Canterbury, 

Christchurch, New Zealand, and ADAMS, C., Institute of 

Geological and Nuclear Sciences, Lower Hutt, New Zealand 

The southwest Pacific region consists of New Zealand, 

eastern. Australia, and western Antarctica which were once 

contiguous on the margin of Gondwana prior to continental 

fragmentation and dispersal in the Cretaceous. Ion probe 

zircon dating offers a powerful method for the rapid 

characterization of terranes and sedimentary provenances in 

this region and allows correlations to be made across 

submerged areas. The ages yield direct chronological 

information on the source rocks and the zircon age profile can 

be used as a fingerprint for a given unit In addition, U and Th 

concentrations can be used to place constraints on protoliths. 

New Zealand has been broadly divided into two provinces 

on the basis of regional geology of the South Island. The 

western province consists of sediments, gneisses and granites 

of continental (Gondwana) affinity, while the eastern province 

consists predominantly of Permo-Triassic arc margin rocks that 

were accreted onto New Zealand 

In the western province, two major phases of granite 

plutonism are recognized in the Carbo-Devonian and 

Cretaceous. S -type granites have incorporated large 

components of Ordovician-Devonian Greenland Group 

greywacke. Metamorphic core complexes in the western 

province reveal Cretaceous gneisses that represent 

metamorphosed granite and greywacke protoliths. The 

Greenland Group greywackes have a characteristic zircon age 

profile consisting of a dominant peak at 500-650 Ma, a smaller 

peak at 1000-1200 Ma, a poorly defmed peak at 1500-1600 Ma 

and a scattering of ages out to 3500 Ma. This is the same age 

profile as has been determined for zircons from Lachlan Fold 

Belt Ordovician sediments in southeastern Australia. This 

signature has also been traced into North Victoria Land and 

Marie Byrd Land, Antarctica, as well as Campbell Island to the 

south of New Zealand and indicates that these areas once 

formed a single contiguous sedimentary horizon. 

The eastern province of New Zealand is dominated by the 

Torlesse terrane composed mainly of quartzofeldspathic 

greywackes and schists. Torlesse greywacke in the South 

Island is characterized by a major peak of Permian age, and a 

small but significant contribution of zircons with the western 

province greywacke signature. The Torlesse signature has 

been found on outlying Chatham Island and indicates an 

extensive horizon of this greywacke as well. 

The sources of the greywackes still remain a matter of 

conjecture although the age profiles will allow better 

constraints to be placed on prospective source areas. The 

Torlesse terrane remains enigmatic in that it apparently 

contains western province material, but it is separated from the 

western province greywacke by intervening volcanogenic 

terranes which show no evidence of any western province 

contribution. The identification and further documentation of 

the Permian granites in the Torlesse will also be an important 

key to the source area. 

151 

DIVERSE ORIGINS OF FLUID IN MAGMATIC 

INCLUSIONS AT ASCENSION 1., BINGHAM, BUTTE and 

ST. AUSTELL, INDICATED BY LASER MICROPROBE 

ANALYSIS OF Cl, K, Br, I, Ar, Kr and Xe. 

IRWIN. J. J, Scripps Institution of Oceanography 

UCSD, La Jolla, California, 92093, ROEDDER, E., 

Department of Earth and Planetary Science, 

Harvard University, Cambridge, Massachusetts, 

02138, U.S.A. 

Saline fluid inclusions (FI) trapped at close to the 

temperature of solidification of a granitic melt 

occur at Bingham (Utah), Ascension Island (mid 

Atlantic Ocean) and St. Austell, (Cornwall). 

Slightly lower temperature Fl occur at Butte 

(Montana). Argon, Kr and Xe were extracted from 

Fl by laser microprobe decrepitation of minute 

portions of samples after neutron irradiation and 

measured in a low blank, high sensitivity mass 

spectrometer. Results enable measurement of 

abundances of Ca, Cl, K, Br, Se, Ba + Te, I and U 

simultaneously with 36 Ar, 40 Ar, 84Kr and 129x e 

in Fl within less than 1 o-5 cc of quartz. 

In all samples, the absolute and relative 

abundance of halogens and gases varies on a scale of 

less than a millimeter, reflecting different 

generations of Fl. In most Fl, 40Art36Ar is greater 

than atmospheric, :>3,000 in some Fl at Bingham. 

Average K/CI values are consistent with high 

temperature fluid-rock equilibria. Overall, 1/CI 

and Br/CI indicate a restricted range, with 1/CI in 

Ascension and Bingham Fl between ... 1 and 8 x 1 o-5 

and Br/CI between 1 and 3 x 1 o-3, similar to the 

composition of the Earth's mantle as indicated by 

halogens in basalts and diamonds. Slightly higher 

1/CI and lower Br/CI in Fl from St. Austell and 

possibly Butte may indicate a crustal source of 

salinity. 

Concentrations of 36Ar and 84Kr in Fl at Butte 

are similar to fresh waters equilibrated with the 

atmosphere (ASW), with some Ar and Kr added 

from magmas. In contrast, many Fl from Bingham, 

Ascension and St. Austell indicate Cl/36 Ar > 1 os, 

and 84Kr and 36 Ar concentrations an order or 

magnitude less than fresh ASW, reflecting an 

unmodified magmatic origin 

Systematic variations in 40 Ar 8 /CI, 1/CI and 

Br/CI between Fl in different quartz veins at 

Bingham indicate trapping of fluids derived from an 

evolving magma system. The importance of 

multiple-component, elemental and isotopic 

analysis in defining fluid types is illustrated by 

these samples, where some Fl groups have similar 

halogen abundances and different noble gas isotope 

ratios and vice-versa. 
STRONTIUM ISOTOPE VARIATIONS AS AN 

INDICATOR OF SEDIMENT REWORKING: THE 

UPPER OLIGOCENE -NEOGENE INTERVAL FROM 

ODP SITES 871 AND 872 

ISRAELSON, C., Institute of Geology, University of 

Copenhagen, 0ster Voldade 10, DK-1350 Copenhagen K, 

Denmark, PEARSON, P. N., Dept. of Earth Sciences, 

University of Cambridge, Downing Street, Cambridge, 

CB2 3EQ, U.K., and BUCHARDT, B., Institute of 

Geology, University of Copenhagen, 0ster Voldade 10, 

DK-1350 Copenhagen K, Denmark 

Sr isotopes together with biostratigraphy are used to evaluate 

the subsidence and sedimentation history of the Limalok 

(ODP Site 871) and Lo-En (ODP Site 872) guyots in the 

western Pacific. Samples consist of remarkable well preserved 

planktonic foraminifera. 

It is found that the Sr isotope data from pelagic sediments 

deposited after the drowning of the guyots not only reflect the 

variations in seawater isotopic composition during the 

Neogene but also the amount of sediment reworking. Limalok 

subsided and started to accumulate pelagic sediments about 20 

rna BP and Lo-En about 27 rna BP. 

The sediment accumulation was not continuos and sediments 

were considerably reworked at several intervals. Reworking 

may be recognised by: 1) a disparity between the 

biostratigraphic age and the "Sr age"; 2) an inconsistency 

between a bulk sediment sample and a single species sample 

and 3) by unexpected variation and reversals in the measured 

Srcurve. 

In general, sediment reworking occured during the initial 

pelagic sedimentation immediately after carbonat platform 

drowning. On Lo-En, another interval of moderate sediment 

reworking between 13 and 15 rna BP has been identified. 

Reworking could be caused by ocean current which lead to 

migration of large carbonate sand dunes on top of the guyots. 

152 

MODEL FOR NEW VOLCANISM IN CENTRAL JAPAN 

ITAYA, T and OKADA, T., Hiruzen Research Institute, 

Okayama University of Science, Okayama 700, Japan 

In central Japan, the arc volcanism has been controlled by 

the two overlapping subducted slabs of the Pacific and Philippine 

Sea plates at least since 7 million years ago. The evolution 

from E-W toN-S trending volcanism in the Ryohaku-Hida 

mountains was due to the change of direction of movement of 

the Philippine Sea plate from northward to west-northwestward 

4 million years ago and the resulting disappearance from the 

area of the subducted slab of the Philippine Sea plate which 

had previously obstructed the ascent of the magma derived from 

the underlying subducted slab of the Pacific plate (Shimizu and 

Itaya, 1993). This suggests that the furthor western region 

may have important implications with respect to the recent 

geotectonic evolution of central Japan and the potential in new 

volcanism. 

The Kinki region including Osaka plain 200km SW of the 

Ryohaku-Hida mountains has many hot water springs which 

have high 3HefHe ratios similar to the active geothermal gases 

(Wakita et a., 1987) though no active volcanism has been 

identified so far in the region. Deep bore holes sunk recently 

in the Osaka plain for the exploitation of underground water 

reservoirs allow sampling of deep waters unaffected by surficial 

processes. Okada et al. (in contr.) presented 1fe, ~e. 36 Ar, 8

"Kr and 132Xe abundance patterns and He and Ar isotope ratios 

of the deep bore hole water samples to study the behavior of 

noble gases in undergroud waters. The elemental abundance 

patterns of representative noble gases show a progressive 

enrichment of the heavier noble gases (e.g., 8"Kr, 132Xe) compared 

with the composition of atmospheric air and that helium was 

found to be strongly enriched in deep bore hole water samples. 

Bore holes in sedimentary rocks that overly the pre-Tertiary 

basement contain He enriched in radiogenic 1fe (e.g., 3

HefHe 

= 0.5x1o·~. In contrast, ~efHe values in the bore holes 

reaching the Ryoke granitic basement were higher than the 

atmospheric value (1.4xl0-6), suggesting a release of mantle 

He through the basement. The highest recorded value (e.g., 

8.2x 10"6

) is in the range of arc volcanism. 

These facts suggest a presence of shallow magma body 

beneath the Kinki district, which was derived from the mantle 

wedge on the 400km ofWadati-Benioffzone of the subducted 

slab of the Pacific plate because of recent disappearance of the 

subducted slab of the Philippine Sea plate underneath the Kinki 

region. 

Shimizu, S. and Itaya, T., 1993, Plio-Pleistocene arc 

magmatism controlled by two overlapping subducted 

plates, central Japan, Tectonophysics, v.225, p.139-154. 

Wakita, H., Sano, Y. and Mizoue, M., 1987, High 3He 

emanation and seismic swarms observed in a nonvolcanic 

forearc region. J. Geophys. Res., v.92(Bl2), 

p.12539-12546. 
A LONG P ALEOMONSOON RECORD FROM NOR GAl 

PLAIN, EAS1ERN TIBET 

.ll:Q...a, KELTS, K., HAN, Y., Dept Geology and 

Geophysics, Univ. of Minnesota, Minneapolis, MN 55455, 

USA; WANG, Swnin., SID, Yafeng, Nanjing Inst of 

Geography and Limnology, Academia Sinica, PRC. 

The Qinghai-Tibetan Plateau is a key-climatic region 

controlling Asian monsoon and possibly as a trigger to global 

climate change. Clearly, long continuous Quaternary records 

are valuable for reconstructing timing, magnitude, and rate of 

change of the paleoenvironments associated with monsoon. 

We recovered a long (120+ meters) core spanning at least 

400,000 years from the Norgai Plain in 1992, and selected 150 

mollusk and rare authigenic carbonate samples from zones 

representing maximum variability between marsh-like to 

carbonate lacustrine muds. Norgai Plain is a large, flat, 

marshy, intermontane basin at 3500 m elevation along the 

eastern margin of the Tibetan Plateau where it currently 

receives monsoon moisture annually. Throughout the 

Quaternary the plain has been a lake basin with alternating 

warm-interval marsh facies and cold-interval green-gray, 

calcareous shallow lacustrine muds. 

Preliminary results of stable isotope analysis of mollusk 

fragment samples revealed a alSo variation of -5.5 to -9.5 %o 

(PDB) which is roughly covariant with a13c with a range of 

-5.0 to -10.5 %o. There are four positive isotope excursions 

which correlate with marsh facies, consistent with stronger 

monsoonal periods. The timing of these excursions is under 

study, but appears to be between 100 to 150 kyrs which would 

place marsh facies equivalent to interglacials. Because the 

Norgai record of paleohydrology represents a SE Asian 

monsoon signature, it may not necessarily correlate with loess 

stratigraphy which tracks ice sheet dynamics. 

153 

lOsE AND 26AL EXPOSURE DATING OF THE SIRIUS 

FORMATION .• TABLE MOUNTAIN, ANTARCTICA 

IvY S D., Ingenieurgeologie and Institut fUr 

Teilchenphysik, ETH HOnggerberg, CH-8093 ZUrich, 

Switzerland, SCHLUCHTER, C., Geologisches 

Institut, UniversiUlt Bern, CH-3012 Bern, Switzerland, 

KUBIK, P.W., Institut fUr Teilchenphysik, ETH 

HOnggerberg, CH-8093 ZUrich, Switzerland, BRUNO, 

L., Isotopengeochemie, ETH ZUrich, CH-8092 ZUrich, 

Switzerland, and BEER, J., EAWAG, CH-8600 

Diibendorf, Switzerland. 

We are measuring the concentrations of lOse and 26AJ in 

a suite of samples from the Sirius Formation and associated 

bedrock surfaces from Table Mountain, Antarctica 

(Schliichter et al., 1994). Exposure dating of the Sirius using 

these isotopes has been thwarted by the rareness of quartzbearing lithologies, although 3He and 21 Ne have been 

successfully measured in clinopyroxenes from Sirius dolerite 

boulders (Bruno et al., 1994). 

So far, we have measured lOse on quartz from sandstones 

and quartzites from Beacon Supergroup surfaces, and 

quartzite and granite boulders possibly associated with the 

Sirius drift Exposure dates were calculated using the 

production rates and scaling factors of La1 (1991). Our 

preliminary exposure ages in the million-year-plus range for 

the granite clasts, which had not been previously recognized 

as belonging to the Sirius, are intriguing. Saturation lOse 

values obtained on the quartzites of the Beacon surface 

(outside Sirius Formation and topographically higher) indicate 

that this landscape was carved at least 5 Ma ago. These 

speculations are being checked with our ongoing 26 AI 

measurements. 

Interpretations about the age of the Sirius drift must be 

constrained by comparison of 1 Ose and 26 AI to elucidate 

erosion rates, as well as by combination with the noble gas 

results (Bruno et al., 1994). The full set of isotopes (lOBe, 

26Al, and 21Ne in quartz and lOse, 3He, and 21Ne in 

clinopyroxene, whose closed system behavior for 1 Ose is still 

being checked) are necessary to draw real conclusions about 

the exposure ages of these rocks. 

Bruno et al., 1994, Dating of geomorphologic surfaces in the 

Antarctic Dry Valleys with in situ produced cosmogenic 

3He and 21 Ne (abstract this volume). 

Lal, D., 1991, Cosmic ray labeling of erosion surfaces: in situ 

nuclide production rates and erosion models: Earth and 

Planetary Science Letters, v. 104, p. 424-439. 

Schliichter et al., 1994, Ancient surfaces in the Transantarctic 

Mountains and the age of the Sirius Formation (abstract 

this volume). 
ONE-TO-ONE CORRELATION OF FISSION TRACKS 

BETWEEN SPHENE AND MICA DETECTORS 

IWANO .Hideki and DANHARA,Tohru, 

Kyoto Fission-Track Co., Kyoto 615, Japan. 

One-to-one correlation of induced fission-tracks 

is a useful technique to investigate the difference 

in track registration efficiencies among minerals. 

This technique was successfully applied to zircon 

and apatite (Iwano et al., 1992, 1993), and 

different track counting efficiencies (zircon/mica= 

0.75, apatite/mica=0.90) were found. It was also 

found that the presence of unetchable track ranges 

plays an important role in the track counting 

efficiencies of minerals. We report here further 

experimental results concerning sphene. 

The Fish Canyon Tuff and the Mount Dromedary 

Complex sphenes were used for the experiment. 

Each sample was pre-annealed to erase 

spontaneous tracks and pre-etched to reveal 

background defects. The sphenes were attached 

with mica detectors and irradiated with thermal 

neutrons. Sphene grains with "scratched surface" 

were selected in this study. Induced tracks in the 

sphene and mica were traced on transparent sheets 

on a high-resolution video monitor. Tracks in 

sphene and mica were spatially correlated one-toone by superimposing the reversed images of the 

mica tracks onto the corresponding images of 

sphene tracks. Preliminary one-to-one 

correlation data are as follows. 

Sample Sphene Mica 

tracks tracks 

F511Canyon 463 512(49)* 

Mt Dromedaty 261 306(45)* 

*Numbers in parentheses are the 

uncorrelated tracks 

Ratio 

(sphene/mica} 

0.90 

0.85 

numbers of 

The results revealed that the track counting 

efficiencies in the sphenes were about 0.85-0.90 

Urnes that in the mica and that uncorrelated 

tracks in the mica were remarkably short. 

Analogous to the situation found in zircon and 

apatite, the lower counting efficiency in sphene 

can be attributed to a higher registration threshold 

required to produce an etchable track in sphene. 

Detailed track counting efficiency and unetchable 

track ranges in sphene will be discussed. 

154 

SPATIAL DISTRIBUTION OF K-AR AGES OF MICA 

INTIIEKAMUIKOTAN:METAMORPIDC BELT AND 

1HE GROWTH OF 1HE :MESOZOIC-PALEOGENE 

ACCRETIONARY COMPLEX, HOKKAIDO, 

NORTHERN JAPAN 

IWASAKI. I.. WATANABE, T., YAMAZAKI,M., 

Dept. of Geol. & Mineral., Hokkaido Univ., Sapporo, 

Japan, and ITAYA, T., Hiruzen Res. Inst., Okayama 

Univ. of Sci., Okayama, Japan. 

Recent K-Ar age studies on mica in the central and 

southern parts of the Kamuikotan belt reveal that the 

Kamuikotan rocks span an age range of lOOMy from 

145Ma to 45Ma and the Kamuikotan belt is divided into 

several units comprised of rocks with an age-cluster, i.e., 

50-65Ma,60-75Ma,80-85Ma,90-110Ma, 120-145Ma. 

Some of the rocks have a similar Ar-Ar age plateau 

(fakigami, unpublished data). PIT gradients of the 

metamorphic rock units become low with descending 

age-clusters, i.e., glaucophane schist facies to intermediate 

pressure type or lower pressure type as indicated by 

pumpellyte-biotite assemblage known in the Tanzawa 

low-pressure terrain. The metamorphic rock-units with a 

similar age-cluster are often bounded by strongly sheared 

serpentinite and form nappe-sheet stmctures. Some of 

serpentinite bodies form serpentinite melange including 

metamorphic rocks with diverse ages and P/T gradients. 

Structural studies combined with the K-Ar age study 

enable us to explain that the Kamuikotan rocks continue to 

the accretionary complex of the Upper CretaceousPaleogene called the ldon'nappu belt, which is situated to 

the east of the Kamuikotan belt. The youngest uplift 

process of the Idon'nappu belt occurred at 3540Ma which 

corresponds to the stage of bending of the western margin 

of the Paleo Kurile arc-trench system due to initiation of 

westward migration. 

These dating studies mentioned above indicate that the 

accretionary complex along the NE margin of the Asian 

continent had successively grown with NNW -directional 

movement of the Pacific Plate and ceased immediately after 

the direction of movement of the Pacific Plate changed to 

WNW. 
#/JAr-39 Ar DATING OF FLOWS AND DYKE SWARMS IN 

TilE DECCAN PLATEAU, INDIA 

IWATA, N. and KANEOKA, 1., Earthquake Research Institute. 

University of Tokyo, Bunkyo-ku, Tokyo, 113, Japan 

In the Deccan Plateau, mafic dyke swarms distribute in the 

Nannada river region, northern part of the Plateau and in the 

west coast areas. The northern swarms show E-W trend, but 

dykes with N-S trend are observed in the west coast regions 

(Deshmukh and Shegal, 1988). Such difference would reflect 

the orientation of tectonic stress fields when they were made. 

Hence, the age results of both dyke swarms might give a 

significant constraint on the change of the stress field related 

to the Deccan volcanism. 

iftJAr-39 Ar dating results of the swarms and flows in the 

Deccan Plateau indicate that dykes with E-W trend might be 

overlap or older than the main Deccan volcanism which has 

been suggested by previous workers (e.g. Venkatesan et al., 

1993). Rows, intrusives and dykes in the west coast areas 

yidd distmbed age spectra in many cases. Major part ofiftJ Ar- 39 Ar 

total ages in this region show overlap or younger ages of the 

main Deccan volcanism, but some samples show obviously 

old total age, as old as 'T1 Ma, for example. 

Based on our results and ages reported by previous workers, 

it is inferred a tendency seems to exist that ages of flows 

and/or dykes in the northeastern part of the Plateau might be a 

little older than these of the west coast areas. Since the age 

values of flows and dykes in the west coast areas are scattered 

in many cases, however, this point should be further examined 

under careful consideration. 

Investigation to clarify the relationship between radiometric 

age and compositional, stratigraphical, regional features of the 

Deccan Volcanic Province are further required to reveal the 

origin of the Deccan volcanism. 

Deshmukh, S.S. and Sehgal, M.N .• 1988, Mafic dyke swanns 

in Deccan Volcanic Province of Madhya Pradesh and 

Maharashtra: Mem. Geol. Soc. India, 10, 323-340 

Venkatesan, T.R.. Pande, K. and Gopalan, K .• 1993, Did Deccan 

volcanism pre-date the Cretaceousffertiary transition?: Earth 

Planet Sci. l..ett:,ll9, 181-189 

155 

GEOGRAPHIC TRENDS IN STABLE ISOTOPE 

COMPOSITIONS OF CONTEMPORARY HUMAN HAIR 

SAMPLES FROM SOUTH AMERICA 

IYER. S.S., KROUSE, H.R., STOPFORTH, S. and 

WIESER, M, Department of Physics and Astronomy. 

The University of Calgary, Calgary, AB, TIN 1N4 

CANADA 

Stable isotope compositions (oD, o13C, o15N and o34S) 

of contemporary hair samples from different localities in 

Argentina, Bolivia, Brazil, Chile, and Paraguay were 

measured. The geographical distribution of the sampled 

populations ranged from l2°S (Salvador, Brazil) to 53°S 

(Punta Arena, Chile). Sampling (all ages and both sexes) 

included the El Chaco Indian tribe of Paraguay, as well as 

residents of the most populated industrial city of the Southern 

hemisphere (Sao Paulo, Brazil). A clear latitudinal trend in 

o13C values was found (more negative with higher southern 

latitude, consistent with proportionately greater C, 

photosynthesis), except for the El Chaco Indians. The latter 

had more negative o13C values than urban dwellers at the 

same latitude, reflecting more reliance on C3 plants in their 

diets. A geographical trend in oD values of the hair samples 

was found for all groups, including the El Chaco Indians. It 

parallels that observed for modem precipitation. The o15N 

and o34S values did not show consistent trends. For the latter, 

localized lithospheric sources of soil sulphur should be a 

significant factor. 
MODELING THE DIFFERENTIATION OF THE EARTH 

USING COUPLED ISOTOPIC SYSTEMS. 

JACOBSEN, S.B., & HARPER, C.L., Jr., Department 

of Earth and Planetary Sciences, Harvard University, 20 

Oxford Street, Cambridge, MA 02138. 

The coupled 146·

147Sm-142•143N d systematics are a powerful tool for investigating the history of the Earth. The 

bulk Earth evolution of 143N dj144N d ( ..... 116 !-units over 

the history the Earth) is relatively well determined while 

142Ndj144Nd increased only by about 2.8 !-units, however, 

its evolution in the Earth is not well understood. The coupled U-Th-Pb and I-Pu-Xe chronometers, have large uncertainties in their bulk Earth evolution parameters, and in 

both cases pre-accretionary elemental fractionations tend to 

obscure the signature of post-accretionary processes. The 

coupled Sm-N d systematics are particularly attractive as a 

tool for tracing the differentiation of the silicate portion of 

the Earth. 

We have developed a multi-reservoir transport model 

to take advantage of such coupled isotopic systems. The 

rate of crustal growth and recycling may be inferred from 

143N djl44N d evolution in the depleted mantle over Earth 

history using such a model if detailed knowledge of the 

fm/Nd evolution of the depleted mantle is available. The 

constraints from the 143N d evolution may be used to calculate the average 142N dj144N d evolution of depleted mantle. 

Such a continuous model would not allow the survival of 

142Ndj144Nd >+10 ppm (w.r.t. bulk Earth) at any time 

in the depleted mantle, unless recycling of crust to this 

reservoir was insignificant during the first 0.5 Ga of Earth 

history. In that case enrichments of up to 30-40 ppm in 

142N dj144N d are possible in the early depleted mantle. 

142N dj144N d heterogeneity in the Earth is generated 

only as a result of early (i.e. > 4.3 Ga) differentiation 

{plausibly protocrust formation). Subsequent to this time 

a progressive 'erasure' of the initial heterogeneity by geodynamic remixing of the complementary reservoirs may take 

place. For the post-146Sm-decay the mass transport coefficients may be obtained directly from the !142Nd evolution without any estimate of the Sm/N d fractionation in 

the source region. After ..... 4.3 Ga, therefore, the evolution 

of 142N dj144N d in the Earth's depleted mantle reservoir 

becomes a tracer for geodynamic mixing and can be employed to quantify the 'global' geodynamic history of the 

crust-mantle system throughout geological time. Coupled 

chronometers, where one of the radioactive parents have a 

half-life much shorter than the age of the Earth (i.e, SmNd, U-Th-Pb) are potentially useful tracers of geodynamic 

mixing. 

156 

ISOTOPIC SYSTEMATICS OF METAMORPHIC 

ROCKS. 

E. Jagoutz, Max Planck Institut ftir Chemie 

D55122 Mainz Saarstra8e 23 Germany 

Garnet and clinopyroxene form a common 

mineral association in metamorphic rocks. Sm/Nd is 

partitioned between these two minerals in a way, that 

this isotopic system should give ideal possibilities for 

the age measurements. Only in a few cases geologically meaningful ages were obtained, in the majority 

of cases the ages are clearly off and the data disappear 

in a lab-datafile. In most cases the garnetclinopyroxene age appears younger. Other authors 

discuss this effect as a late closure of garnets in a 

slowly cooling system. 

In young terrains as the Alps the garnet may 

even contain less-radiogenic Nd than the clinopyroxene. This is in contradiction with the higher 

Sm/Nd ratio in garnet compared to clinopyroxene. 

The negative-slope-isochrons ("futurechrons") clearly 

demonstrate that garnet and clinopyroxene were not 

equilibrated. When studying such cases, we found 

that garnet replaced another mineral such as plagioclase or amphibole. The garnet inherited the Ndisotopic composition of the precursor mineral, but had 

changed the Sm/Nd ratio. Since most of the precursor 

minerals had low Sm/Nd ratio, the inherited Nd in 

garnet is unradiogenic compared to the clinopyroxene, 

which results in younger ages or even "futurechrons". 

Evidence for this is found even in some highly 

metamorphed diamond-bearing eclogites. It seems 

that garnet is growing on the prograde path and the 

system is frozen after dehydration. The kinetics of 

metamorphic processes are commonly attributed to 

temperature and might effectively depend on dehydration, which is the dominant prograde reaction. 

In some cases texture and composition of garnet 

and clinopyroxene give clear evidence for the replacement, while often only the iso~pic relicts clearly 

demonstrate that garnet and clinopyroxene never 

equilibrated and other evidence of equilibrium is 

enigmatic. 
STABLE ISOTOPE COMPOSmON OF BIOOENIC 

CARBONATE AND ORGANIC CARBON 

IN HACKBERRY ENDOCARPS: 

POTENTIAL AS A TERRESTRIAL PALEOCUMATE 

INDICATOR 

JAHREN, A. H., and AMUNDSON. R G , Both at: Soil 

Science Group, University ofCalifomia, Berkeley, CA, 

94720, USA; M. Gabel, Department of Biology, Black 

Hills State University, SpearfiSh, SD, 57799, USA; and, L. 

Tieszen, Department of Biology, Augustana College, Sioux 

Falls, SD, 57197, USA. 

Fossil endocarps of the Hackberry (Celtis sp.) are common 

in Cenozoic sediments of North America. Both the modern 

and fossil endocarps have been reported to be composed of up 

to 84.3 wt% calcium carbonate, initially in the form of 

aragonite. This opens the possibility that a stable substrate 

exists for isotopic analysis and paleoclimate reconstruction 

using the Hackberry endocarp. Selective dissolution 

experiments followed by SEM revealed that modern endocarps 

have mineral frameworks of opal, which contains an 

"occluded" organic C fraction. The location and distribution of 

the occluded organic C is evaluated using fluorescence 

microscopy. 

Stable C isotope analyses of organic matter in whole 

modem Hackberry seeds, cellulose from whole seeds, 

mesocarp, endosperm, whole endocarp, and opal-occluded 

endocarp c are presented; also presented are at3c values of 

endocarp carbonate. Our objectives are the following: (1) 

determine the isotopic fractionation factor between the 

carbonate and the organic portions, and (2) to determine 

whether occluded C fractions, which may be most resistant to 

diagenesis (and therefore most likely to be preserved in fossil 

endocarps), reflect the iSotopic composition of the whole 

Hackberry plant. 

The sensitivity of the ol3c values of the various organic 

fractions, the ol3c values of the carbonate, and the o18o 

values of the carbonate to climate is evaluated by measuring 

isotopic ratios in modern endocarps from a large portion of 

central North America. Measured C isotope ratios are 

interpreted in light of environmental effects on photosynthesis; 

measured o18o values of the carbonate are interpreted in 

relation to the o18o values of precipitation, and associated 

values of temperature and relative humidity. 

Preliminary morphological and mineralogical observations 

of Quaternary and Tertiary fossil endocarps are presented, and 

the preservation of the various endocarp components in the 

geologic record is considered. 

157 

Carbon and Helium fluxes from Mid Ocean Ridges. 

M. Javoy, (Lab. Geochimie Isotopes Stables, I.P.G. Paris, 2 

Place Jussieu, 75251 PARIS cedex 05, Fance). 

- Mantle helium fluxes have been estimated to 

between 800 and 1100 moles 3Helyear ( -8 107 to 11 

107 moles/year of total helium). This rather robust 

estimate (1) corresponds to a mean ocean tum-over 

timeT of -1000 years and varies as 1/f, discounting 

the possibility of transient buoyant flows during 

eruptions. 

Corresponding carbon fluxes are then derived from 

the assumption that C;3He ratios in MORB glasses 

are constant, the pseudo-canonical value being 

generally taken at 2 109. This assumption is far from 

true, the C;3He ratio varying by more than one order 

of magnitude (from 0,5 to 6 109 at least), correlated 

to carbon content and vesicularity. 

This situation derives directly from the fact that 

helium is more soluble than carbon and that C!He 

ratios decrease in outgassing melts, whether under 

global equilibrium conditions during slow ascent 

from the source region or during distillation on the 

way from shallow magma chambers (-1 -3 km depth 

below sea floor) to the ocean bottom. (2) 

The above range (0.5 to 6 109) of C!He then 

corresponds to outgassed systems, with the lowest 

ratios corresponding to gases still dissolved in glass 

or to hydrothermal fluids, and the highest ratios to 

abundant vesicles. Modelling C!He ratio evolution 

according to the information obtained from carbon 

isotopes by Pineau & Javoy (2), we calculate that 

the average original C;3He is more likely between 8 

to 12 109, corresponding to an oceanic mantle 

carbon flux of 6.5 to 1.3 1013 moles/year. The aerial 

flux, corresponding essentially to subduction zone 

volcanoes and major aerial hydrothermal fields is 

probably similar in size. 

1) H. Craig, W.B. Clarke and M.A. Beg, E.P.S.L. 

26, 125, 1975. 

2) F. Pineau and M. Javoy, E.P.S.L. 1994. 
ISOTOPE AND FLUID INCLUSION STUDIES AS AN 

EXPLORATION GUIDE FOR GREENSTONE GOLD 

DEPOSITS AT LINGQIU NORTH HILL DISTRICT, SHANXI 

PROVINCE, NORTHERN CHINA 

Zhiyu Jiang1>, Research Institute of Geology for Mineral 

Resources, CNNC, Guilin, 541004, China 

Junming Chen and Youlu Guo, Research Institute of Geology 

of Shanxi Province, Taiyuan, China 

The greenstone sequence comprising the J ingangku Formation 

in the Lingqiu North Hill district was regarded as a prospective 

area for gold mineralisation. 

A recently published Sm-Nd age of 2599.2±49 Ma (Wang 

Rujing, 1989) is regarded as the age of the host sequences. Orebearing whole rocks and an apatite intimately intergrown with 

native gold yield a Rb-Sr isochron age of 2214.5±61 Ma and 

are believed to date the deformation and metamorphism related 

to the main phase of gold mineralisation in this area. A K-Ar 

age of 1780.5 Ma shows the next magmatic-tectonic episode 

with local limited gold mobilisation and enrichment associated 

with the metamorphosed basic dykes. 

The calculated isotopic compositions indicate that the initial 

mineralising fluid was metamorphic in origin (o180=6.3-

10.7%o, oD =-29.7--24.4). The re-crystallisation of the rocks 

resulted in a marked increase in oD (from -29.7 to+ 7.6%o). The 

relationship between this variation of oD in the residual fluid 

and the gold contents in the rocks indicates that gold was 

derived from the older rock units to the younger rock units 

during the metamorphic process. The spatial distribution of 

temperatures calculated from the values of 8180 for coexisting 

mineral pairs supports this concept. An obvious decrease in oD 

from -24.4 to -219.8%o which occurs in the major quartz veins 

is attributed to fluid boiling. Fluid inclusion studies indicate 

lower homogenisation temperatures for fluid boiling in the 

quartz veins (range 334°C to 398°C). This pressure decrease 

and consequent boiling also resulted in an increase in the 

salinity of the fluid up to 48.2 wt% NaCl equivalent and 

concentration of gold in the veins. 

The homogenisation temperatures for secondary fluid inclusions range from 156°C to 2l3°C, which represent the temperature of the second stage of stratabound mineralisation in 

laminated quartz veins formed during retrograde metamorphism. 

The 0 and H isotopic compositions of the second fluid may 

represent a mix of metamorphic water with meteoric water. 

A positive correlation between the o34S values and gold 

contents in pyrite points to a derivation of sulfur for the 

mineralisation in the Mesozoic structures from the underlying 

rocks. 834S values of sulphides from all kinds of mineralisation 

are restricted to a narrow range and are near O%o, which can be 

explained by the homogenising effect of metamorphism for the 

greenstone terrane. 

The lead isotopic data from native lead, whole rock and pyrite 

samples lie on an anomalous line and illustrates that the lead 

and the gold mineralisation in this area have experienced a 

multistage evolution. 

Eight ore bodies have been determined by this isotope study 

and companion petrological, geochemical and structural studies. 

'>Present address: Department of Geology, The University of Newcastle, 

NSW, 2308, Australia 

158 

BORON AND SILICON ISOTOPE SYSTEMATICS OF 

SEDEX-TYPE ORE DEPOSITS 

JIANG. S-Y., and PALMER, M.R., Dept. of 

Geology, University of Bristol, Bristol, 

BS8 IRJ, U.K., and Ding, T -P., Institute 

of Mineral Deposits, Beijing, 100037, 

P.R.China 

We have conducted a study of the boron and 

silicon isotopic systematics of some giant 

submarine hydrothermal exhalative ore 

deposits, such as, the Sullivan stratabound PbZn-Ag deposit (Canada), the Qinling 

stratabound Pb-Zn deposits (China) and the 

BIF-type Gongchangling iron deposit (China), 

in an effort to determine the B- and Sisources and the implications for ore genesis. 

B isotope analyses of tourmaline from the 

deposits indicate that B in the hydrothermal 

fluids is leached from the underlying clastic 

sediments. Differences in the o 11 B values in 

individual deposits may reflect seawater 

ingression into the submarine hydrothermal 

system which of significances for generation 

of the ores and may have exploration 

potential. 

Siliceous rocks with negative o30si values 

are related to submarine hydrothermal 

processes. The Si isotope compositions of 

tourmalinites from the Sullivan deposit 

support the genetic model whereby 

tourmalinites from the feeder zone formed by 

replacement of clastic sediments which 

inherited their o30Si values (0 tO 0.1 °/oo), and 

the stratiform tourmalinites and garnet-rich 

tourmalinites within or near the massive 

sulphide orebody formed by hydrothermal 

exhalative processes with negative o30s i 

values (-0.3 to -0.5 °/00). It is possible to use Si 

isotopes to distinguish between Si derived 

from alteration of clastic sediments and that 

derived from hydrothermal fluids, despite 

post-depositional alteration and metamorphism of the deposit. 
MODELS FOR THE EVOLUTION OF ISOTOPE 

RAnos IN REACTIVE GROUNDWATER SYSTEMS. 

JOHNSON Thomas M .. and DEPAOLO, Donald J., 

Dept. of Geology and Geophysics, U.C. Berkeley, 

Berkeley, CA 94720, and Earth Sci. Div., 

Lawrence Berkeley Laboratory, Berkeley, CA 94720. 

Isotopic measurements are valuable tools in 

hydrologic investigations, especially when water-rock 

interaction is to be investigated or in systems where 

physically-based predictions of water flow are highly 

uncertain. Usually, isotopic data take the form of 

spatial patterns formed by arrays of wells or other 

sampling points, and/ or temporal patterns from samples 

taken in time series. These patterns result from the 

interplay of transport effects (advection, dispersion, 

mixing) and reaction effects (dissolution, precipitation, 

exchange, sorption). The interpretation of the patterns 

amounts to an inverse problem where the values of the 

parameters that control the system are to be extracted 

from the output of the system. 

We have developed a modeling approach that 

relates the evolution of isotope ratios in the water as a 

function of time and position to the critical transport 

and reaction parameters. The model applies to any 

isotope ratio in any system that can be treated as a 

porous continuum. In an advective system where 

reaction with the rock or soil is important, one 

dimensionless parameter, a Damkohler number, most 

strongly defines the system. For the isotopic ratio 

problems discussed here, it can be expressed: 

N _lMRdcs 

nvcr 

where l is the characteristic length seal~ for the 

system, M is the solid-to-fluid mass ratio, Rd is the 

reaction rate, cs and Cf are the concentration of the 

element of interest in the solid and fluid, respectively, 

and vis the advection velocity. 

The value of this parameter ranges widely among 

the chemical elements considered in isotopic studies, 

and a range of behavior, from conservative to reaction 

dominated, is to be expected in natural systems. When 

reaction effects are important, the key to extracting 

information from isotopic data is to characterize 

spatial variation in both the fluid and the rock or soil 

matrix. The relationship between the two carries 

information on advective velocity, reaction rates, and 

dispersion. We present field data and numerical 

modeling from several studies in which this approach 

was applied. 

159 

DIFFERENTIATING THE TECTONIC AND EROSIVE 

COMPONENTS OF DENUDATION USING FISSION 

TRACK ANALYSIS: A CASE STUDY FROM S.E. SPAIN 

JOHNSON. KIT Research School of Geological Sciences, 

Fission Track Research Group, Birkbeck & University 

Colleges London, WClE 6BT, England. 

The erosive (i.e. surface processes) and tectonic 

(i.e. structural processes) components of any 

denudational history are generally difficult to 

differentiate because they possess a complex 

interdependence. That we should want to do so is 

largely because an appreciation of the means by 

which rocks are brought to the earth's surface helps 

constrain our understanding of the driving 

mechanisms that are responsible for denudation. In 

particular it will help clarify the hitherto poorly 

understood role played by extension in convergent 

orogens. 

It will be argued that it is possible to recognise the thermoc~onological consequences or 'signature' of 

both erosive and tectonic denudation and that such 

signatures may act as diagnostic tests for specific 

denudational regimes. In addition they can be used 

to constrain both the relative importance and timing 

of each component. 

The Betic Cordillera in southern Spain is the 

westernmost portion of the European Alpine 

mountain belt. Evidence exists for initiation of 

northerly directed compressive deformation in the 

late Eocene and the Oligocene which continued 

throughout the Lower and Middle Miocene. Fission 

track analysis (FT A) reveals that broadly east - west 

extension within the Internal Zone probably 

commenced during the late Oligocene and that by the 

early Miocene mid-crustal rocks of the Alpujarride 

Complex had been tectonically exhumed and brought 

into contact with essentially unmetamorphosed rocks 

belonging to the Malaguide Complex in the eastern 

Betics. Continued extension resulted in the 

emergence of lower crustal rocks of the NevadoFilabride Complex beginning in the (Langhian ?) 

Serravallian and ending abruptly in the early 

Tortonian ( -9 Ma). It was not until the cessation of 

extension, caused by a change in relative plate 

motions, that important erosive denudation 

commenced. 

Quantitative modelling of both tectonic and 

erosive denudation suggest that cooling and relative 

foot-wall uplift rates of -200°C.Ma-l and -4mm.yr-1 

respectively were experienced during the former and 

that subsequently, foot-wall erosive denudational 

rates of 0.6mm.yrl were experienced in the Sierra 

Nevada during the upper Miocene. 
ISOTOPIC (~18() and ~170) STUDIES OF 

TROPOSPHERIC OZONE 

JOHNSTON. I. C. and THIEMENS, M. H., Dept. of 

Chemistry, University of California, San Diego, La 

Jolla, CA. 92093, USA 

Although only 10% of atmospheric ozone is located 

in the troposphere, this relatively small amount plays a 

crucial role in maintaining the chemical composition and 

oxidation capacity of the atmosphere. Additionally, 

tropospheric 0:3 is an important greenhouse gas because 

of its strong infrared absorption at 9.6f.tm. 

Measurements indicate that background tropospheric 03 

concentrations are increasing at - 1 %/yr over northern 

midlatitudes. Thus, there is considerable interest in a 

better definition of the tropospheric 0:3 budget and 

transformation mechanisms. The budget is complex, 

with significant involvement of both photochemical and 

transport processes on differential spatial and temporal 

scales. 

The isotopic characterization of tropospheric ozone 

will provide important information regarding the 

seasonal variations in photochemical and surface 

destruction mechanisms. Models incorporating 

photochemical and transport processes are in need of 

direct observations to validate their predictions; isotopic 

measurements of tropospheric 03 may provide an 

important new source of direct information. The 

isotopic data may also be helpful in defming the relative 

importance of various vertical and horizontal transport 

mechanisms, both within the troposphere and between 

the stratosphere and the troposphere. 

The interpretation of isotope measurements of 

tropospheric ozone requires knowledge of the 

fractionation factors associated with 03 formation and 

destruction; fortunately, many of these values have been 

determined. For example, 0:3 formation and 

decomposition processes both produce distinctive mass 

independent isotopic fractionations. 

Stratospheric 03 is known to possess a large (400 o/oo) 

mass independent isotopic composition. These 

enrichments have proved difficult to interpret, due in 

part to the limited amount of chemical and 

meteorological data available to correlate with the 

isotopic measurements. In the troposphere, 

measurements of chemical and meteorological 

parameters are routine, thus it may be possible to 

interpret and model relations between these parameters 

and a given 03 isotopic enrichment. 

Cryogenic techniques are used to collect and isolate 

0:3 from ground level air. The sample is then converted 

to 02 and the B1So and B17o values measured on a 

Finnigan MAT 251. We will report results from ground 

level collections in La Jolla, CA. 

160 

THE MAKE-UP AND MELTING OF A SUBDUCTIONACCRETION COMPLEX: GENERATION OF S- AND ITYPE GRANITOIDS 

JONFS Catrin & TARNEY, J., Dept. Geology, U. 

Leicester, UK and BARREIRO, B., NERC Isotope 

Geoscience Laboratory, Keyworth, Nottingham, UK 

It has been argued that subduction-accretion processes have 

made a major contribution to continental development during 

the Phanerozoic1• Where super-continent land-masses are 

rimmed by active margins, major river systems discharge their 

upper crustal sedimentary load onto the shallow-dipping downgoing plate, which is then tectonically intermixed with other 

oceanic components (ocean islands, arcs, plateaus, carbonate 

banks, abyssal oozes, etc.) andre-accreted onto the continental 

margin. The resulting schistose or gneissose complex is thus a 

variable mixture of "crustal" metasediment and "mantle" 

amphibolite components that have been dragged down to deep 

crustal levels in the subduction complex, and have then 

suffered varying extents of metasomatism, migmatisation and 

stages of melt generation during decompression through uplift 

and exhumation. 

Using the well-exposed Rhodope Massif (Mesozoic) inN. 

Greece, where lithological packages with different proportions 

of mantle-derived (MORB, om, plateau) ahd crust-derived 

(sedimentary) components, usually separated by tectonically 

weak and buoyant marbles, we document the isotopic and trace 

element characteristics of different components of a 

subduction-accretion complex. 

Migmatitic granitoids within totally metasedimentary 

segments are muscovite- or biotite-bearing, with essentially Stype chemical and isotopic characteristics: high K/Na and Rb/ 

Ba ratios, low K/Rb ratios, low Sr abundances, moderate Y and 

HREE depletion, 87Sr/86Sr ratios ranging 0. 708 - 0. 716 and 

£Nd -7 to -2. 

However, where there is a small to significant amount of 

tectonically-intercalated amphibolite, the granitoids are all 

hornblende-bearing with clear 1-type characteristics: low K/Na 

and Rb/Ba ratios, high K/Rb ratios, high Sr abundances, strong 

Y and HREE depletion, 87Sr/86Sr ratios ranging 0. 705 - 0. 709 

and £Nd -5 to 0. This implies that the chemical and isotopic 

influence on melt products (particularly LILE and Sr relative to 

Nd systematics) of the amphibolites is disproportionate to their 

volume. This has important implications for the genesis of 

magmas with mixed 1- and S-type characteristics. 

Coarse grained migmatitic gneisses in central Rhodope, 

despite having all the visual characteristics of Archaean high 

grade grey gneiss terrains, have peak-metamotphic (U-Pb 

monazite) ages of ca. 55Ma, signifying that such characteristics 

can be produced through Cenozoic subduction-accretion 

processes. 

North to south increasing Nd crustal residence ages of 

gneisses from Rhodope to the Cyclades suggest that 

subduction-accretion has been an important mechanism of 

crustal growth throughout the Hellenides. 

1Sengor et al., 1993. Evolution of the Altaid tectonic collage 

and Palaeozoic crustal growth in Eurasia. Nature 364, 299-307. 
STRONTIUM ISOTOPE STUDIES OF CHLORIDE 

CONTAMINA1ED GROUNDWATER FROM DENMARK 

J0RGENSEN. N.O. & HOLM, PM., Geological Institute, 

University of Copenhagen, DK-1350 Copenhagen, 

Denmarlc. 

The occurrence of chloride-contaminated groundwater is a 

well-known phenomenon in Denmark and is a serious 

problem in water resources management Previous isotopic 

investigations of Danish aquifers have shown that isotopic 

hydrochemical criteria provide a strong tool to characterize 

groundwater qualities that are affected by naturally occurring 

sources such as water-rock interaction, intrusion of seawater 

and other sources of saline groundwater. The aim of the 

present case study is to demonstrate the applicability of 

strontium isotopes as natural tracers in a chloridecontaminated near-coastal aquifer in eastern Jutland, 

Denmarlc. 

The study area includes a confined aquifer at Stautrup 

Waterworks south of Arhus. The aquifer consists of c. 70 m 

fluvio-glacial gravels, sands and silts covered by 10-20 m of 

boulder clays and resting on a pre-Quaternary basement of 

Oligocene marine mica clay. Two boreholes were drilled in 

the fall of 1993 and depth specific groundwater sampling was 

undertaken in order to investigate a chloride contamination of 

the aquifer. Water samples from the upper part of the aquifer 

show relatively low Cl contents in the range of 30-50 mg/1. 

However, the Cl contents increased significantly with depth 

to a maximum of 850 mg/1 in the lower part of the aquifer. 

Selected freeze-dried groundwater samples, and seawater 

from the nearby Arhus Bay, were analysed for their Sr 

isotopic composition using standard ion-exchange techniques 

and a VG Sector 54-30 mass spectrometer. 

The 87 Sr/86Sr ratios and the Sr concentrations obtained 

from the groundwater samples show a well-defined mixing 

hyperbola with 87Sr/86Sr ratios varying from 0.7088 in the 

low-Cl samples to 0. 7084 in the high-Cl samples. The 

87Sr/86Sr ratio at 0.7092 for the seawater from Arhus Bay 

falls significantly outside the mixing hyperbola for the 

groundwater samples and, therefore, excludes the ~ssiblity 

of any seawater infiltration. The remarkably low Sr~ Sr 

ratios at c. 0. 7084 of the high-Cl groundwater samples 

indicate that the chloride-contaminated groundwater is most 

likely heavily influenced by saline formation waters forced 

into the Quaternary aquifer from the underlying Oligocene 

marine mica clay. 

161 

NEW U-Pb DATA FOR SINGLE SHOCKED ZIRCONS 

FROM THB KIT BOUNDARY, SASKATCHBWAN: 

FURTHBR PROOF FOR A CHICXULUB SOURCE 

KAMO. S.L., KROGH, T.E., Geochronology 

Laboratory, Royal Ontario Museum, 100 Queen's 

Park, Toronto, Ontario, M5S 2C6; 

HILDEBRAND, AR., Geophysics Division, 

Geological Survey of Canada, 1 Observatory 

Crescent, Ottawa, Ontario, K1A OY3. 

Recent U-Pb dating of single shocked zircons from 

impact-related sites has proven extremely useful in 

"fingerprinting" the target source for KIT boundary ejecta 

and for correlating distal ejecta from widely-spaced 

localities1•

2• Characterization of the target site using this 

method can uniquely link ejecta to its source because 

zircon, which is extremely refractory, represents a residual 

fragment of the target itself that records both its primary 

crystallization age and the time of impact. 

New U-Pb data for five single shocked zircons from 

the KIT boundary near Rock Creek, south-central 

Saskatchewan, define a colinear array with an upper 

intercept age of 548 ± 6 Ma and a lower intercept age of 

59.0 ± 10.0 Ma. These data points are 7, 19, 30, 59, and 

98% discordant with a 52% probability of fit. A sixth grain 

has a younger upper intercept age of 363 ± 26 Ma (20% 

discordant), assuming lead loss occurred at 65.5 Ma. This 

age is within error of that for a single unshocked zircon 

reported previously1 from the KIT boundary in the Raton 

Basin, Colorado, and represents a minor target component 

in the fireball layer that has so far not been observed at the 

Chicxulub crater. The data points show a correlation 

between the intensity of shock metamorphism and the 

amount of discordance. These results from the Rock Creek 

site, located ==3500 km from the Chicxulub crater, are 

remarkably consistent with zircon ages found at other KIT 

boundary sites in Colorado, Haiti, and the Chicxulub crater 

breccia, all of which give a predominant U-Pb primary 

zircon age of ca. 545 Ma (e.g. the primary source age for 

KIT Colorado zircons is 545 ± 5 Ma ). 

While arguments for a terrestrial origin for the 

Chicxulub breccia and Caribbean tektite units have recently 

been proposed, U-Pb data and shock-related textural 

features of zircon unequivocallysupporta meteorite impact 

origin. Zircons with primary ages of ca. 545 Ma that display 

polycrystalline shock metamorphic textures3 (that have 

never been reported in explosive volcanic rocks) that are 

isotopically reset to ca. 65 Ma, found in sites from across 

North America at the KIT boundary, cannot have been 

derived from a volcanic eruption (which would yield 65 Ma 

zircons) and are best explained by derivation from a single 

large bolide impact. 

1. Krogh, T.E., Kamo, S.L., Sharpton, V., Marin, L., & 

Hildebrand, Nat. 366, 731-733, 1993. 

2. Krogh, T.E., Kamo, S.L, & Bohor, B., Earth & Planet 

Sci. Lett. 119, 425-429, 1993. 

3. Bohor, B., Betterton, W., & Krogh, Earth & Planet. Sci. 

Lett. 119, 419-424, 1993. 
HOW HETEROGENEOUS ARE NOBLE GAS SIGNATURES 

IN THE EARTH'S MANTLE ? 

KANEOKA, I., Earthquake Research 

Institute, University of Tokyo, 

Bunkyo-ku, Tokyo 113, Japan 

Noble gas state in the Earth's interior 

has been inferred from many kinds of materials such as pillow glasses, olivine phenocrysts, ultramafic nodules,etc. Each 

material reflects both the primary signatures and secondary ones, the latter of 

which should be properly eliminated to 

infer the former ones. Based on such procedures, MORB sources are inferrred3to ~e 

characterized by relativly uniform He/ He 

4otio~ (8±1RA) and sysytematically high 

Ar/ Ar ra~ios (more than S,OOO)together 

y~gh the frequent occurrence of excess 

Xe. Plume samples show more varia~le 

~ignatures including higher or lower40He/ 

3

He ratios and systematically lower Ar/ 

Ar ratios than those of MORB, but there 

are still controversies about the characteristic vt~~es for them. The occurrence 

of excess Xe has not been confirmed in 

the plume sources. 40 36 Although relatively low Ar/ Ar ratios for plume samples are often attributed to the secondary atmospheric contamination, stepwise degassing experime~ts 

~nd the correlation 8etw~gn the high He/ 

He ratios and low Ar/ Ar ratios in 

the higher temperature fractions of mantle 

materials sugge~ 0 thj~ the occurrence of 

relatively low Ar/ Ar ratios4bn t~g 

Earth's mantle are real. Even Ar/ Ar 

ratios of less than the atmospheric value 

has been observey2gogether with the occurrence of excess Xe in olivine megacry-

·sts contained in South African kimberlites. 

Such information implies that noble gas 

signatures of the Earth's mantle might be 

rather heterogeneous for heavier noble 

gases at least in the lithosphere. Apparent variation of noble gas signatures 

for plume materials can be ascribed to 

the mixing of different sources. Among 

them, recycled materials would play an 

important role. Uniform noble gas systematics of MORB might reflect that of 

the source and/or the secondary processes 

to homogenize it. Detailed r2~mination 

of the occurrence of excess Xe would 

reveal the degree of heterogeneity of 

noble gas systematics in the mantle by 

correlating it with the other isotopes. 

162 

DETERMINING THE EFFECTIVE NEUTRON 

FLUX FOR 40Ar-39Ar DATING OF FINE 

GRAINED CLAY MINERALS 

KAPUSTA, Y., STEINITZ, G., KOTLARSKY, 

P. , Geological Survey, Jerusalem, Israel. 

The neutron fluence and its variation as utilized 

in 40 Ar-39 Ar dating is determined by using a mineralmonitor which is irradiated in a nuclear reactor 

together with the sample. It is assumed that the 

efficiency of the 39K(n,p)39 Ar reaction in the monitor 

and the sample is the same. Using standard monitors 

(LP-6, HD-Bl, MMhb-1 etc.) for 40Ar-39Ar dating of 

clay minerals yields ages significantly too high. The 

conventional explanation assumes a 39 Ar deficit 

within the mineral, due to its "recoil" from the fine 

grains, as the cause for the phenomenon. 

Two different aliquats (0.028, 0.351 gr) of the 

same clay fraction ( <0.2p.) irradiated together 

contained different amounts of 39Ar (1.3*10-7

, 

1.96*10-7 cc/gr). The dependence of the 39 Ar 

concentration on the mass of the sample indicates a 

varying neutron flux in the sample. In the irradiated 

samples no loss of 40 Arrad was observed nor could 

implanted 39 Ar be detected in the aluminum foil used 

for packing the samples. 

The measured 39 Ar content in different clay 

fractions irradiated together shows a linear 

correlation between the calculated 39 Ar deficit (using 

their K-Ar age) and their H20 content (determined 

by DTA). This implies that the water in the clay 

minerals moderates and/or absorbs the neutrons and 

thus is the source of the 39 Ar deficit relative to the 

monitors which are devoid of water. 

It is suggested to use clay fractions with 

calibrated K-Ar ages and known water contents as 

additional neutron flux monitors for 40 Ar-39 Ar dating 

of fine clay minerals. A first test using this approach 

yielded 40 Ar-39 Ar ages of H20 bearing clay fractions 

close to their K-Ar ages. The deviations may be due 

to difference in size and geometry between the clay 

monitors and the samples. The results open the 

possibility of extending the approach to step-wise 

heating experiments. 
PALEOPROTEROZOIC CARBON ISOTOPE RECORDS 

FROM THE FENNOSCANDIAN SHIELD: IMPLICATIONS 

FOR THE GLOBAL CARBON CYCLE 

KARHU. J.A., Geological Survey of Finland, FIN-02150 

Espoo, Finland. 

s13c and s18o values of calcite and dolomite have been 

determined for 229 sedimentary carbonate samples from 

Paleoproterozoic supracrustal belts of the Fennoscandian 

Shield (Karhu, 1993). Generally only those sedimentary 

carbonates estimated to contain more than about 80% 

carbonate were chosen for analysis, and in addition, a few 

carbonate samples were rejected, if the measured 8180 values 

remained below 13%o (SMOW). 

The distribution of 813C values of total carbonate is 

clearly bimodal, showing a pronounced maximum at about 2%o 

and another less distinct maximum at about 10%o. Relying on 

formations for which the time of deposition could be 

estimated from radiometric age data, a carbon isotope 

evolution curve was constructed for the time period from 2.5 

to 1.9 Ga. The most conspicuous and stratigraphically useful 

feature of the curve is the interval from about 2.2 to 2.1 Ga, 

which is represented by highly 13C-enriched sedimentary 

carbonates with 813C values in the range 10 ± 3%o. These are 

followed between 2.11 and 2.06 Ga by a sharp, almost 1 O%o 

drop in the 813C values of carbonates, while from that time 

untill.9 Ga, the 813C values remain in the range 0 ± 3%o. The 

813C evolution pattern is also roughly compatible with the 

813C results from other well dated Paleoproterozoic carbonate 

formations, and it appears that these 13C-enriched carbonates 

represent a global perturbation in the carbon cycle. 

Carbon isotope compositions for organic carbon in black 

shales associated with the t3c enriched carbonates show large 

variation. Most of the 813C values fall in the range -19 ± 3%o, 

suggesting a comparative 13C-enrichment in the coexisting 

organic matter, but some samples show more negative al3C 

values, down to as low as -43%o, possibly resulting from local 

predominance of processes producing extremely light 

kerogens through methylotrophic pathways. 

It is suggested that the positive carbon isotope shift 

recorded in the Paleoproterozoic sedimentary carbonates was 

related to an increased relative rate of burial of organic matter 

on a global basis. The shift was possibly enhanced by local 

burial of isotopically unusually light organic matter. As high 

burial rates of organic carbon are probably followed by a 

large flux of free oxygen, the positive carbon isotope shift 

may be fundamentally connected with the significant rise in 

atmospheric oxygen levels at about 2.0 Ga. 

Karhu, J.A., 1993, Paleoproterozoic evolution of the carbon 

isotope ratios of sedimentary carbonates in the 

Fennoscandian Shield. Geol. Surv. Finland, Bull. 371, 87p. 

163 

39Ar-40Ar DATA ON EXCESIVE 40Ar IN NEPHEliNE 

FROM THE KOVOOR .MASSIF, KOlA PENINSULA 

KABPENKO Michael I. and IVANENKO, V. V., 

Institute of Ore Deposit Geology, Petrography, 

.Mineralogy and Geochemistry, Russian Academy 

of Sciences, 109017 Moscow, Russia 

At the Gomoozero intrusion, the age of nepheline in the 

earliest nepheline-pyroxene rocks is 630 Ma, while the 

age of the main intrusive phase is 450 Ma and for the 

nephelinites is 400 Ma. The Kovdor intrusion has been 

examined in the most detail. Here the nepheline ages for 

five rock groups, distinguished on the basis of their 

structural position and composition, range from 600 Ma 

(nepheline from the oldest pegmatoid, ijolite dikes cutting 

through core olivinites) to 400 Ma (418 Ma nepheline 

and 370 Ma from mica in nepheline-syenite dikes in the 

Malyi Kovdor Intrusion). Such discrepancies within a 

intrusion raise doubts as to whether the values reflect 

breaks in the time of formation of the alkali rock or 

whether there are other causes. 

Two samples of nepheline with maximal K-Ar ages of 700 

Ma have been examined by stepwise heating and by laser 

40 Ar-39 Ar methods. 

It is evident that these samples contain 30% excess 40 Ar 

in gaseous form. When the samples are 

ground, the inclusions containing the excess argon are 

destroyed. Then, the calculated age is 500 Ma, which 

approximates the crystallisation age, but may exceed it if 

the excess 40Ar is present not only in gaseous form but 

also as single atoms trapped in the lattice. 
HIGH-RESOLUTION MEASUREMENT OF 

STRONTIUM/CALCIUM RATIOS IN MARINE 

BIVALVES USING PIXE 

KASHGARIAN. Michaele. Graham BENCH, Center 

for Accelerator Mass Spectrometry, Lawrence 

Livermore National Laboratory, ·po Box 808, L-397, 

Livermore, CA 94551, and Chris WIEDMAN, Dept. 

of Marine Geology and Geophysics, Woods Hole 

Oceanographic Institution, Woods Hole, MA 02543 

We have calibrated a Sr/Ca paleothermometer 

in two bivalve mollusk species using the high 

resolution non-destructive method Proton Induced Xray Emission (PIXE) to measure Sr concentration in 

thin sections of shell material. Using a spot size of 

10-30 J.Lm we have been able to obtain continuous 

high resolution measurements of Sr concentration 

across annual growth bands of ~1 mm and we have 

measured Sr/Ca ratios in shell carbonate to better 

than 1% precision. 

SrI Ca variability in scleractinian corals and 

mollusks has been shown to correlate with seasonal 

temperature changes (e.g. Becket al., 1992, and 

Schifano and Censi, 1986). We have found greater 

than 10% variability of SrI Ca in growth bands from 

individual specimens of Mercenaria mercenaria and 

Arctica islandica. Specimens were collected from 

Long Island Sound and Vineyard Sound off the US 

East Coast from areas with long term continuous 

temperature records in order to correlate the Sr/Ca 

in the shells with seasonal temperature variability. 

We have also correlated our Sr/Ca with ()180 

measurements in one specimen of Arctica islandica. 

Beck, J.W., et al., 1992, Sea Surface Temperature 

from Coral Skeletal Strontium/Calcium Ratios, 

Science, v.257, pp. 644-647. 

Schifano, G., and P. Censi, 1986, Oxygen and Carbon 

Isotope Composition, Magnesium and Strontium 

Contents of Calcite from a Subtidal Patella 

coerulea Shell, Chemical Geology, v. 58, pp. 

325-331. 

164 

URANIUM ISOTOPE DISTRIBUTIONS WITHIN 

MERCENARIA SHELLS: GEOCHRONOMETRIC 

POSSffiiLITIES 

KAUFMAN. Aaron, Dept. of Env. Sciences, 

Weizmann Inst. of Science, 76100 Rehovot, 

Israel, GHALEB, Bassam, GEOTOP, Univ. du 

Quebec a Montreal, C.P. 8888, Montreal, H3C 

3P8, Que., Canada, and WEHMILLER, John, 

F., Dept. Geology, Univ. of Delaware, Newark, 

DL, 19716, U.S.A. 

Uranium concentrations and 234UJ238U ratios 

were measured as a function of depth within the shell 

for 3 live and 10 fossil specimens of the marine 

bivalve, Mercenaria, from the U.S. mid-Atlantic 

coastal plain. The samples were prepared by 

grinding the shell into 5-6 portions, each of which 

represents a layer parallel to the shell surface. As 

expected, the live specimens had much lower wholeshell uranium concentrations (mean of 40 ppb) than 

did the fossils (mean of 300 ppb). However, the 

concentration gradients were much steeper than 

expected, especially for the live-collected specimens 

where the exterior typically had about 200 times the 

concentration of the interior; in the fossils this 

proportion was typically about 12. 

Apparently, the uranium uptake process starts 

very shortly after shell formation and thoug.h the 

fossil continues to take up much more uramum, there 

are two lines of evidence which suggest that this lasts 

for only a relatively short period, especially in the 

shell interiors: 1) the 234UJ238U ratios of9 of the 10 

fossil shell interiors were such as to indicate that the 

uranium uptake ended before the shells left the 

marine environment which probably occurred less 

than 10 ka after deposition, though most of the shells 

probably formed 80-100 ka ago. ~le some of. the 

specimens undoubtedly took up contmental uramum, 

hence after emergence, this was restricted to their 

exterior layers. 2) Assuming that the process by 

which the uranium enters the shell is diffusioncontrolled, one can estimate the relative amounts of 

time over which the diffusion occurred in the fossil 

vs. the live specimens by comparing their 

concentration gradients. This suggests that the 

diffusion typically occurred over a period of the 

order of less than 1 ka. 

One consequence of the present findings is that 

though the apparent 230Th ages of whole mollusk 

shells are usually unusable because of complicated 

patterns of post-mortem uranium uptake, those of 

their interior layers may generally be too low by only 

a small amount. 
EVOLUTION OF FORMATION WATERS 

IN THE ILLINOIS AND APPALACHIAN 

BASINS AS INDICATED BY CHLORINE 

STABLE ISOTOPE COMPOSITIONS 

Ronald S. Kaufmann, RUST Environn1ent 

& Infrastructure, 1515 N. Federal Highway 

Boca Raton, FL, 33432, Siegel, Donald I., 

Department of Geology, Syracuse 

University, Syracuse, NY 13244-1070, 

Prinos, Scott, and Dickey, Neil, Department 

of Geology, Northern Illinois University, 

DeKalb, IL, 60115. 

Chlorine stable isotope composition was 

determined for waters from Paleozoic 

formations in the Illinois and Appalachian 

Basins to identify similarities in the origin of 

these waters and diagenetic processes. 

Br-Cl relationships in both basins suggest 

the presence of Ca-Cl saline formation 

waters that have evolved from residual 

evaporated ocean water with the initial 

Illinois Basin formation waters below halite 

saturation and Appalachian waters evolving 

from a water that has precipitated some 

halite. Plots of N a and Ca for both basins 

show a trinary mixing pattern, evidence of 

mixing among meteoric, Ca-Cl, and Na-Cl 

waters derived from dissolution of halites. 

The chlorine isotope ratios of the Illinois 

Basin range from -0.2 to +0.89 per mil, 

(precision 0.1 per mil) with respect to 

SMOC and correlate with most major ion 

distributions and ratios of ions. In the 

Appalachian Basin, the range is from -0.11 

to +0.98 per mil and correlated 

relationships for the chlorine isotope ratio 

among major ion is limited to Na, Cl and 

Br, and the ratio of Na/Cl. These 

correlated variations may be explained by 

diagenetic diffusion. Data from both basins 

suggest that if the formation waters were 

initially Silurian ocean water, than the 

isotope composition of the chlorine from the 

Silurian ocean is about +0.9 per mil as 

compared to modem oceans. 

165 

GEOCHEMICAL ESTIMATION OF THE HALF-LIFE 

FOR THE DOUBLE BETA DECAY OF 96zR 

K.A W ASHIMA, Atsumichi, Dept of Chemistry, 

Univ. of Tokyo, Tokyo, Japan (Present Address: Div. 

of Geological &. Planetary Sci., California Institute of 

Technology, CA91125, U.S.A.), TAKAHASHI, 

Kazuya. Earth Sci. Lab., Inst. of Physical and 

Chemical Research, Saitama, Japan, and MASUDA 

Akimasa, Dept. of Chemistry, Univ. of Tokyo, Japan 

The precise determination of the decay constants of 

double beta decay has put a constraint on the mass of 

the neutrino. Their half-lives are estimated to be of the 

order of I018.}()21 yr. Accordingly, it is difficult to directly detect the radioactivity accompanying double beta 

decays using counting methods. An alternative approach 

is the detection of the decay product accumulated in very 

old natural minerals. In the present work, we have attempted to estimate the half-life of double beta decay of 

96Zr from the amount of radiogenic 96Mo found in the 

Zrmineral. 

Natural zircon usually contains a few ppm of Mo. 

Mo, however, is believed to be present as a mineral inclusion (perhaps molybdenite) and the proportion of Mo 

substituting for Zr ions in the crystal lattice is anticipated to be low. Thus it is expected that radiogenic 

96Mo is not present in molybdenite inclusions but can 

be detectable in intrinsic portions of zircon. In order to 

reduce the Mo background and to find out the isotopic 

effect on Mo due to double beta decay of96zr, molybdenite inclusions must be removed with aqua resia. 

Zircon is not eroded by this treatment, and the Mo produced by the decay of Zr is considered to be maintained 

in the pure zircon crystal grains. 

After Mo was separated from Zr matrix, HCI solution 

of ascorbic acid and N'J4l was added to the sample in order to reduce the oxidation state of Mo from +6 to + 3 

and depress the volatility of the Mo during the TIMS 

measurement. A Re single filament technique was used 

for TIMS. Small amounts of a mixed powder ofRe and 

Pt (Re:Pt=1:2) were added to enhance the ionization efficiency of Mo. (Pt was mixed because of its relative high 

work function, and Re powder was added to increase the 

surface area of the ionizing filament) 

The isotopic compgsition of Mo in zircon showed the 

anomalies of 95Mo, 97Mo, 98Mo and HX>Mo caused by 

the spontaneous fiSsion (SF) of238U, and 96Mo caused 

by the double beta decay of 96zr. The content of Mo in 

the zircon was calculated from the content of U, the 

half-life of the SF of 23Su and the yield of Mo on the 

SF of 23su. Therefore, the half-life of 96zr was calculated with the anomaly of 96Mo and the age of the zircon (1.7xl09 yr), and the half-life was estimated as 

3.9xi019 yr. The obtained half-life of96zr is consistent 

with those of other double beta decay nuclides, 82Se and 

lJOre in the view of the decay energy. 

Kawashima, A., Takahashi, K. and Masuda, A., 

Physical Review C, 47, R2452-2456 (1993) 
A UV LASER ABLATION MICROPROBE 

TECHNIQUE FOR ARGON ISOTOPE ANALYSIS. 

KEI I EX S P Dept of Earth Sciences, Open 

University, UK, ARNAUD N.O., Univ Blaise Pascal & 

CNRS, Oennont Ferrand, France, CARROLL, M.R. and 

D~ D.S., Univ Bristol, UK. 

A new ultra-violet laser ablation tedlnique for noble gas 

extraction yields both true high spatial resolution analyses 

and the ability to analyse not only "opaque" minerals 

available to infra-red and visible laser techniques but all 

important silicate minerals. Sample ablation using a 

quadrupled Nd-Y AG laser (A=266nm), is a slower 

extraction process than visible and infra-red lasers though 

the power density during tOns pulses focused to a lOJ.lm 

laser spot is extremely high (-3 x tolO W cm-2) and the 

light is typically absorbed in the top few microns of the 

sample. An advantage of this effect is that the average rate 

at which energy is dumped into the sample during analysis 

is typically 0.01-0.lW (1-lOmJ/pulse at 10Hz), over two 

orders of magnitude smaller than typical powers delivered 

by CW visible and IR lasers. Such low average powers 

yield insignificant sample heating during analysis. This 

opens the doors to new sample extraction procedures, 

including beam raster which can achieve a much higher 

spatial resolution than current techniques. 

Early experiments on natural rocks have demonstrated 

high spatial resolution in clear minerals such asK-feldspar 

and plagioclase. It has even proved possible to achieve 

high spatial resolution at biotite/quartz boundaries 

(typically the most difficult). As a result, the UVLAMP 

technique has been used to measure diffusive proflles in 

biotites in dyke wall rocks, heated during emplacement 

Natural argon diffusion profiles less than 50J..un deep have 

been measured which were combined with laboratory 

diffusion parameters for biotite to constrain thermal 

histories. 

In another application, controlled laboratory heating of 

gem quality k-feldspar and basalt composition glasses 

yielded argon concentration gradients up to 50JUD deep, 

which have been imaged with a resolution of as little as 2.5 

J.liD. by the lNLAMP using a beam raster technique. Such 

experiments measure not only diffusion parameters but 

also the solubility of argon. 

166 

THE PERFORMANCE CHARACfERISTICS OF THE WA 

SHRIMP II ION MICROPROBE 

Kennedy A K and de Laeter J. R., Curtin University of 

Technology, GPO Box U1987, Perth, 6001 Australia; 

Internet: ikennedy@info.curtin.edu.au 

The Sensitive High-Resolution Ion Microprobe (SHRIMP) at 

the ANU has produced numerous major advances in isotope 

geochronology and geochemistry. As part of the testing of the 

new SHRIMP ll ion microprobe at the Western Australia 

Center for Isotope Science we have measured the following 

performance characteristics at a mass-resolution of 5000 (1% 

defmition): (1) Mass discrimination for 208Pbf206Pb is 

respectively < 0.5%, < 0.3% and < 0.1% , in K-feldspar with 

4% Pb, SRM 610 glass with 426 ppm Pb, and zircon with 20 

ppm Pb. For 34sf32s in galena mass discrimination is 3%; (2) 

Ion transmission was measured using an Aluminium target. 

Approximately 11% of the total number of secondary ions 

emitted were recorded as AI+ ions on the faraday cup; (3) The 

abundance sensitivity at 1 mass unit offset was 5x1o-9 at 30si+ 

and 3x1o-s at 254uo+; (4) A 1 sigma precision based on 

counting statistics of 0.3% for 206pbf238u in a 570 Ma 

standard zircon, which contains 20 ppm Pb and 220 ppm U, and 

0.7% for 208pbf206Pb in SRM 610 glass were achieved with 

total count times of 70 sec for each Pb isotope and 35 sec for 

238u; (5) A 1 sigma reproducibility of 1.0% for 206Pbf238u 

for 15 analyses of the standard zircon and 0.5% for 

208pbf206Pb for 6 analyses of SRM 610 glass; (6) A 

sensitivity of> 20 cps/ppm Pb/nA of 02- is achieved during 

U/Pb analysis of zircon at the above mass resolution. 

As a test of day-to-day operation, we have analysed zircons 

from separates that give ages between 100 and 200 Ma by 

conventional mass spectrometry. These zircons are from the 

plutonic rocks of the Sierra Nevada Batholith (SNB) and have 

been classified as internally concordant (1), thermally disturbed 

(D), or possibly containing an inherited component (IC), 

Average 206Pb*f238u ages and the range of ages from 

SHRIMP analyses and conventional analyses are listed below 

along with the type of zircon. Apart from a bimodal age 

4istribution in the Coso-1 zircons, with 3 grains giving an age 

> 180 Ma, there was no evidence of inheritance. This is 

consistent with very little recycling of ancient material during 

growth of the SNB. 

Sample Type Conv. IP av. Range n 

SQ61 I 101 107 (5) 99-118 18 

MA9 D 156 151 (6) 142-162 11 

W32 I 103 107 (8) 105-112 6 

Coso-1 I 156 162 (10) 154-184 17 

MA17 D 201 202 (10) 190-227 11 

MA7 IC 156 153 (7) 139-159 7 

1 standard deviation for the the zircon sample population are 

given in brackets. 

Additional performance characteristics of the SHRIMP II and 

zircon data from other SNB granites will be shown. 
NEOPROTEROZOIC POST GLACIAL CARBONATE 

ROCKS: IMPLICATIONS FOR THE PRECAMBRIAN 

CARBON CYCLE. 

Martin J. Kennedy Department of Geology and 

Geophysics, University of Adelaide, S.A. 5001, Australia, 

and Malcolm W. Wallace, School of Earth Sciences, 

University of Melbourne, Parkville, Vic. 3052, Australia 

Thin (<15m) enigmatic dolostone units cap 

Neoproterozoic glacial sequences throughout the world and are 

particularly well represented in Australia's numerous 

Proterozoic Basins. The finely laminated and micro-crystalline 

nature of the Australian "cap dolomites" constitute deeper water 

(largely below storm wave base) post glacial transgressive 

deposits and are commonly overlain by deep water 

stromatolites representing the highly condensed downlap 

surface below prograding mudstones of the overlying highstand 

sediments. Stratigraphic profiles of stable isotopic values 

measured through these units in key Neoproterozoic sections 

across the Australian Craton indicate a consistently low values 

of ac13 (-2 to -5 PDB) with progressive depletion of up to 

-3%per/mil upsection. Texturally well preserved dolomite 

fibrous (marine) cements retain up to -1 %per/mil depletion of 

ac 13 relative to surrounding matrix material and overlying 

internal sediments. The retention of this trend within discreet 

sections, and lack of homogenisation of 8 C 13 values within 

textural components suggest primary preservation of 8C13 

values. The persistence of this trend in basins across Australia 

indicates a broad scale control of ac13, suggestive of secular 

variation of sea water during the post glacial period. 

The coincidence of major deglaciation, transgression 

and widespread carbonate deposition in marine environments 

implies a causal link between these processes. The continentwide and perhaps global nature of these post-glacial cap 

dolostones indicates that immense volumes of carbonate were 

precipitated during this period. Such hypothesised large-scale 

carbonate precipitation must have caused (or have been the 

product of profound changes in the carbon cycle and global 

climate at this time, which is likely reflected by the depletion 

of 8c13. In sharp contrast there are no equivalent carbonate 

horizons capping Phanerozoic glacial successions. This may 

be related to the transition from abiogenic to biogenic skeletal 

carbonate precipitation across the Precambrian-Cambrian 

boundary. The supersaturation of Ca and Mg needed to directly 

precipitate carbonate from Proterozoic sea water may now be 

taken up by biogenic precipitation in Phanerozoic oceans. 

Greater flux of bicarbonate to the oceanic reservoir may have 

been possible during the Proterozoic and modified the silicate 

weathering cycle by absorbing and releasing correspondingly 

greater amounts of atmospheric co2. 

167 

40Ar_39 Ar DATING OF ARCHAEAN GOLD 

MINERALISATION IN THE KALGOORLIE GOLDFIELD, 

WESTERN AUSTRALIA: EVIDENCE FOR MULTIPLE 

MINERALISATION EPISODES. 

KENT AJR and McDOUGALL, I., Research School of 

Earth Sciences, The Austtalian National University, 

Canberra, Australia, 0200. 

Mineralisation in the Kalgoorlie goldfield, in the 

Archaean Yilgam Craton of Western Australia, has been 

responsible for over 1300t of gold, making this region one of 

the most productive gold-mining districts in the world. 

Mineralisation is hosted within a ca. 2700 Ma volcanosedimentary sequence and occurs in two predominant 

structural settings: (i) along a complex series of anastomosing 

shear zones (Golden Mile style) and (ii) in stockwork zones 

adjacent to late dextral faults (Mt Charlotte style). Both styles 

of mineralisation exhibit many of the features typical of 

Archaean gold deposits, although field relations indicate that 

the shear-hosted Golden Mile style mineralisation occurred 

prior to formation of the Mt Charlotte stockwork systems. 

Muscovite is a common hydrothermal mineral 

paragenetically associated with gold deposition in both 

settings. 40 Ar-39 Ar geochronology of hydrothermal 

muscovite has been used to investigate the timing of each 

style of mineralisation. Two samples of muscovite from 

Golden Mile style shear. lodes have 40 Ar-39 Ar plateau ages of 

2627 ± 12 and 2628 ± 12 Ma (all errors are 2a). Three 

samples of hydrothermal muscovite associated with Mt 

Charlotte style stockwork veins have identical 40Ar_39 Ar 

plateau ages (within 2a errors) and these have a weighted 

mean of 2602 ± 7 Ma. 

It is considered that the 40 Ar-39 Ar ages from 

hydrothermal muscovites represent the timing of the Golden 

Mile and Mt Charlotte styles of gold mineralisation. 

Mineralisation occurred after greenschist facies 

metamorphism of the host sequence, and ambient 

temperatures in host rocks were probably much lower than 

the closure temperature of argon in white mica (ca 3000C) at 

this time. Further, fluid inclusion measurements indicate that 

gold mineralising systems cooled rapidly below this closure 

temperaure during progressive mineralisation, precluding the 

possibility of argon loss during slow cooling. There is also 

little evidence of subsequent thermal activity which may have 

produced argon loss in hydrothermal muscovites, although it 

is possible that muscovite associated with Golden Mile style 

shears may have lost argon during subsequent formation of 

Mt Charlotte style stockworks. . 

Muscovite 40 Ar-39 Ar ages indicate that shear-hosted gold 

mineralisation at the Golden Mile occurred at, or before, ca. 

2630 Ma, prior to stockwork mineralisation at Mt Charlotte at 

ca. 2600 Ma. This is consistent with the field relations 

between the two styles of mineralisation. Recent theories 

suggest that Archaean Gold deposits in the Yilgarn Craton 

formed broadly contemporaneously in response to large-scale 

crustal fluid circulation. Such theories must now also contain 

the caveat that discrete, and unrelated, mineralisation 

episodes may also produce significant gold deposits with 

features similar to other 'typical' Archaean gold deposits. 
METASOMATISM IN ISLAND ARC MANTLE CAUSED 

BY VARIABLY FRACTIONATED SLAB-DERIVED 

FLUIDS AND MELTS: CONSTRAINTS FROM 

ULTRAMAFIC XENOUTHS IN VOLCANIC ARCS 

KEPEZHINSKAS. P.K., DEFANT, M.J., AUSTIN, 

P., all at the Dept. of Geology, University of South 

Florida, Tampa, Florida 33620, USA, DRUMMOND, 

M.S, Dept. of Geology, University of Alabama at 

Birmingham, Alabama, USA, and MAURY, R.C., 

Universite Bretagne Occidentale, Brest, France. 

A common feature of the Kamchatka arc volcanoes is 

the presence of ultramafic xenoliths hosted within recent 

eruptions. Xenoliths are variably deformed under high P,Tconditions, consistent with their derivation from the mantle 

wedge below the volcanic arc. These xenoliths exlubit pervasive 

metasomatic features believed to record multi-stage reactions 

between fluids and melts and a sub-arc mantle. 

Ultramafic xenoliths from 4 frontal-arc and 1 rear-arc 

volcanoes can be grouped as 1) Cr-rich harzburgites, dunites, 

and lherzolites believed to represent a sub-arc lithospheric 

mantle prior to metasomatism and 2) Cr-poor pyroxenites, 

wehrlites, and websterites which are thought to be formed during 

a melt-mantle interaction. 

Cr-rich xenolith suites exhibit across-arc variations. 

The frontal arc xenoliths are harzburgites and dunites containing 

Cr-rich spinels and Na-/Al-depleted diopsides. The rear-arc 

xenoliths are lherzolites and Cpx-poor harzburgites with less Crrich spinels and moreNa- and Al-rich clinopyroxenes. Ion-probe 

results indicate that rear-arc mantle diopsides are both LREEenriched and depleted, while frontal-arc mantle clinopyroxenes 

are mainly LREE-depleted. Frontal-arc, Cr-poor xenoliths 

contain LREE-enriched, high-Sr, low-Y felsic glass veins with 

pyrope-almandine garnet, Al-rich spinel, and Na-rich 

plagioclase. Rear-arc, Cr-poor xenoliths include phlogopite and 

a LREE-enriched hornblende and clinopyroxene. 

The across-arc mantle heterogeneity in Kamchatka is 

consistent with greater degrees of melting below the arc volcanic 

front. The rear-arc tends to be less affected by flux -induced 

melting but carry metasomatic signatures introduced by slabderived melts and fluids. This suggests that the slab undergoes a 

major stage of melting and/or devolatilization at relatively 

shallow depths (45-55 km), but still is capable of generating 

siliceous melts and hydrous fluids at greater depths (65-85 km) 

below the rear-arc. Surprisingly, fluids introduced from the slab 

below the Kamchatka arc front are C02-rich and contain 

detectable amounts of CH4, F, and Cl based on fluid inclusion 

study. Rear-arc fluids are water-rich as suggested by amplnbole 

and phlogopite present in Bakening low-Cr xenoliths. Three 

types of metasomatism are recognized in the Kamchatka arc 

mantle: 1) related to siliceous, slab-derived melt-mantle 

interaction; 2) related to mafic melt-mantle interaction; 3) related 

to the introduction of slab-derived, C0 /~0 fluid into the subarc mantle. 

168 

LITHOSPHERIC CONTRIBUTIONS IN 1HE 

GENERATION OF ISLAND ARC MAGMAS: 

GEOCHEMICAL VARIATIONS IN ALONG 

STRIKE VOLCANOES FROM NORTHERN 

HONSHU, JAPAN 

KERSTING. A.B., Earth Science Dept. & 

Institute of Geophysics & Planetary Physics, 

Lawrence Livennore National Laboratory, 

Livermore CA 94550 email: Kersting@llnl.gov, 

ARCULUS, R.J. (Dept. of Geology and 

Geophysics, Univ. of New England, Armidale, 

NSW 2351, and GUST, D.A., Dept. of Geology, 

Queensland Univ. of Tech. QLD 4001 

The genesis of island arc volcanoes results from 

a complex interaction of many different sourcessubducted oceanic crust, sediments, fluids, 

overlying mantle, and arc lithosphere. 

Quantifiying the extent and mechanisms by which 

these reservoirs contribute to island arc magmas 

has been controversial and difficult. The 

approach taken in this study is to identify 

lithospheric influences in a transect of along-strike 

island arc volcanoes that crosscut different tectonic 

terranes. 

In northern Honshu, Japan, the Tanakura 

Tectonic Line (TTL}, a left-lateral transfonn fault 

reactivated during the opening of the Japan Sea, 

crosscuts the recent volcanic front. The TTL is a 

boundary between a package of terranes accreted 

in the Mesozoic (to the North) to the Sino-Korean 

craton (to the south). 

Preliminary results of a combined radiogenic 

isotope (Pb, Sr, & Nd), major and trace element 

investigation of a transect of Quaternary volcanoes 

in northern Honshu, Japan will be presented. 

Initial measurements of the Pb isotopic ratios of 

the Northern Honshu volcanic front lavas lie well 

above the MORB field in conventional 207/204pb 

and 208/204pb versus 206/204pb ratio diagrams. We 

interpret this to reflect crustal contamination in the 

generation of these magmas. In addition, elevated 

87Srf86Sr has also been interpreted to reflect 

crustal involvement in some of the volcanic 

centers. 
SAO RAFAEL BATHOLITH, RIO GRANDE DO NORTE, 

BRAZIT..: AN UNREGENERATE PLUTON 

KETCHAM, 12 . .H., LONG, L. E., Dept. of Geological 

Sci., Univ. Texas, Austin, Texas 78712, USA, and SIAL, 

A. N., Dept. of Geology, Fed. Univ. Pernambuco, Recife, 

PE 50732-970, Brazil 

During the Brasiliano (Pan-African) orogeny from 700 to 

550 Ma, the geologic province of Borborema in northeast 

Brazil was intruded by more than 80 granitoid bodies, one of 

them the primary epidote-bearing Sao Rafael Batholith located in the Serid6 foldbelt. This unfoliated quartz monzonite 

pluton is thought to be of very late Brasiliano age, perhaps 

between 600 and 550 Ma based on field relations. 

Twenty-four samples were analyzed isotopically, including 

4from a subtly different sub-facies of the pluton ("Serratype"), and 2 mafic enclaves. In general, the rocks have high 

Sr (655 ± 175 ppm) and low Rb (61 ± 9 ppm), with consequent low enrichment in radiogenic Sr. Three biotite-apatitewhole rock isochrons provide cooling ages of 0.49 ± 0.01 

Ga. Data of 18 main-facies whole rocks plot as an amorphous "blob" on a Rb-Sr isochron diagram. 

0~ • 

.... 

Cl) 

\0 

~0.71 

Cl) 

~ 

• •••• ,. ~.'1 .. 

• Main facies 

o Serra-type 

• Mafic enclave 

• 0 

• 

0.70'--------......._ _____ ___, 

0 0.3 

87Rbf86sr 

0.6 

Inhomogeneous initial Sr ratios preclude the determination 

of a Rb-Sr age of crystallization. Somewhat more systematic 

data from the Serra-type samples suggest an age of =0.6 Ga. 

Sr ratios of main-facies rocks, projected back to 0.6 Ga, are 

high (0.713 ± 0.001), pointing to an ancient radiogenic 

crustal source. 

Six whole-rock samples provide a Sm-Nd isochron apparent age of = 1.1 Ga, which is inconsistent with field evidence 

for late Brasiliano emplacement. Anomalous Sm-Nd data indicate that initial Nd ratios also were inhomogeneous, just as 

Sr initial ratios were. Strongly negative values of £Ndo.6Ga 

(-18 to -21) imply a source relatively enriched in Nd. Extrapolations of Nd evolution trajectories to intersection with 

the depleted mantle curve are consistent with minimum crustformation ages of 2. 7 ± 0.3 Ga. Poorly understood Precambrian basement in northeastern Brazil has been complexly reworked, and there is some evidence for presence of relict 

Archean crust. 

The only "normal" isotope data from Sao Rafael are 

sharply delimited whole-rock B180SMOW = +7.8 to +8.1%o 

(Sial et al., in prep), and consistent mineral cooling ages of 

=0.49 Ga. Formed at about 0.6 Ga apparently by partial 

melting of Archean crust, the Sao Rafael magma mixed sufficiently to attain a distinctively uniform physical appearance 

and chemical-mineralogical composition, but it failed to homogenize isotopically. Sao Rafael is an "unregenerate" pluton, consisting neither of simply remobilized source rock nor 

of completely re-equilibrated material. 

Sial, A. N.8 Ferreira, V. P., and Long, L. E., in preparation, 

High 51 0, High Al-Homblende, Epidote-bearing Granitoids in Northeast Brazil. 

169 

Clll..ORINE-36 IN Rl\INFALL FROM AUSTRALIA 

K£YWOOD. M.D., and CffiV AS, A.R., Research School 

of Earth Sciences, The Australian National University, 

Canberra 0200, Australia; FIFIELD, L.K., ALLAN., GL. 

and CRESSWELL, R., Research School of Physical 

Sciences and Engineering, The Australian National 

University, Canberra 0200, Australia. 

The 36cl contents of bulk depositional precipitation was 

determined from remote sites in Central and Western 

Australia. Sampling transects included a west-east array 

trending inland from the coast of Western Australia (latitude 

270S) and a north-south array through the Northern Territory 

and South Australia (longitude 1330E). Precipitation was 

collected at three monthly intervals in coincidence with the 

seasons, for a period of two years. 

36ClJCl ratios along the west-east array increase with 

increasing distance from the coast. This reflects a decreasing 

influence of marine Cl, (which contains negligible 36cl) at 

inland sites. Temporal variation of 36Cl fluxes across the 

west-east array reflect the dependence of 36cl fallout on 

rainfall amount This dependence explains the high fallout 

observed at coastal localities and suggests the importance of 

wet deposition for the removal of 36cl from the abnosphere. 

The extensive range of latitudes covered by the north-south 

sampling array allows an assessment of the predicted fallout 

rates of 36cl to the Earth's surface. Cosmic ray fluxes to the 

Earth's surface are dependent on the Earth's magnetic field 

(Lal and Peters, 1967). A discontinuity in the tropopause at 

latitude 35-40° allows maximum transfer of air from the 

stratosphere to the troposphere at 400. Thus a latitude 

dependence in the fallout rate is expected. The fallout rate 

along the west-east array of 15 atoms 36C1Jm2Js, averaged 

over six seasons, is similar to the predicted flux of Andrews 

and Fonte (1991) at this latitude. However preliminary 

results from the north-south array show fallout rates that 

differ from predicted values, suggesting that mixing of air 

masses in the troposphere may be another factor in 

determining the fallout rates of 36Cl to the Earth's surface. 

Andrews, J .N. and Fontes, J-Ch., 1991. Importance of in situ 

production of chlorine-36, argon-36 and carbon-14 in 

hydrology and geochemistry. Isotope Techniques in Water 

Resources Development, pp 245-269, IAEA, Vienna, 

1992. 

Lal, D. and Peters, B., 1967. Cosmic Ray Produced 

Radioactivity on the Earth. In K Sitte, ed. Handbuch der 

Physik, 4612, Springer Verlag, Berlin, 551-612. 
Natural etching and annealing of fission tracks 

in zircons during hydrothermal alteration. 

Nghiem Vu KHAI (University of Mining and 

Geology, Dongngac Tuliem, Hanoi, Vietnam) and 

WADATSUMI Kiyoshi (Osaka City University, 

Sigimoto Sumiyoshi, Osaka, Japan). 

The fission track ages obtained from the internal 

(4:n) and external (2:n) surfaces of the zircon samples, 

collected in an orderly manner from near contact zone 

of a hydrothermal ore vein outward into the host rocks 

show two contrasting tendencies. The internal ages . 

decrease toward the ore vein and, on the contrary, the 

external ages reveal an increasing for a limited 

distance. Consequently, the 2:n ages are consistently 

older .than 4:n ones. 

The decrease in 4:n ages is probably due to natural 

annealing of spontaneous tracks in zircon crystals 

during a high temperature ore forming stage. 

Considering the external tracks as an open system in 

term of physico-chemical interaction with surrounding 

environment, the behavior of 2:n ages can be explained 

as a result of the combined effect by both natural 

etching and annealing. It means that the external tracks 

underwent natural etching ,during pre-ore hydrothermal 

alteration, before annealing process took place, so that 

the etched tracks could not be annealed effectively. 

Closer to ore fluids channel both natural annealing and 

etching rates are higher and they decrease with distance 

from the vein. The 4:n- and 2:n ages merge when the 

etching effect disappears. 

This model demonstrates a real difference between 

4:n and 2:n ages obtained for the samples and indicates 

that careful consideration is needed before interpreting 

their thermochronological meaning. 

170 

OXYGEN AND SULFUR ISOTOPE STUDIES OF 

PRECAMBRIAN ANOROGENIC GRANITIC ROCKS IN 

THE MIDCONTINENT OF NORTH AMERICA 

KIM, Sun-Joon, Dept. of Earth and Atmospheric 

Sciences, Purdue University, West Lafayette, IN 47907, 

U.S.A. (Now at Dept. of Mining and Mineral 

Engineering, Hanyang University, Seoul, Korea), and 

SIDEH. Yuch-Nin&, Dept. of Earth and Atmospheric 

Sciences, Purdue University, West Lafayette, IN 47907, 

U.S.A. 

Oxygen and sulfur isotope compositions have been 

detennined for the granitic and related rocks from the Wolf 

River Batholith, Wisconsin; the Illinois Deep Holes; the Wet 

Mountains plutons, Colorado; and subsurface samples from 

illinois, Iowa, S. Dakota, Nebraska, Missouri, Kansas, and 

Oklahoma. Hydrothermal alteration resulting in a decrease of 

AQ-F values was observed locally throughout the Wolf River 

Batholith. Feldspars of different colors (pink, gray, and red) 

were separated whenever feasible and analyzed. Most red 

feldspars (An10-30) show the highest and constant g18o 

values (9.3-10.0 permit) suggesting near complete isotope 

exchange with hydrothermal fluid. Based on g18o values 

and alteration temperatures estimated from fluid inclusions 

(260-350°C), g18o of the fluid is calculated to be 5.0 ± 1.4. 

Sedimentary formation water is most likely the source of the 

fluid. Sulfur content and isotope composition of granitic 

rocks of the Wolf River Batholith range from 30 to 117 ppm 

and from 1.1 to 6.5 permit respectively and are considered to 

be magmatic. The positive correlation observed between 

oxygen and sulfur isotope data of granitic rocks may be due to 

assimilation of the Penokean plutonic rocks by a primary 

magma of deep-crustal origin, or to mixing at depth of a 

primary magma with another magma having higher g18o 

and g34s. Oxygen isotope fractionations among quartz, 

feldspar and biotite indicate two different isotope exchange 

processes at two different stages: 1) the continuous 

retrograde isotopic exchange shortly after emplacement, and 

2) the hydrothermal alteration at a much later time. 

When data from the present study are combined with 

data from previous studies on similar Precambrian anorogenic 

granitic rocks in the midcontinent of North America, a 

regular increase of g18oQ from NE to SW is observed: Wolf 

River Batholith, WI (mean g18oQ = 7 .9); Illinois Deep Holes 

(8.2); St. Francois Mts., MO (9.5); and Wet Mts., CO (9.8). 

The g18oQ values of subsurface samples range from 7.3-

10.5. Thus, all the granites may have formed by partial 

melting of a low-18o source material in the lower crust, with 

an increasing contribution of high-18o material such as 

metasedimentary and/or metavolcanic rocks toward the 

southwest 
MIDDLE AND LATE TRIASSIC TIME SCALE 

CALIBRATION OF 1HE MURIHIKU 

SUPERGROUP, SOUTHLAND, NEW ZEALAND 

KI!"fBROUGH, D.L., Department of Geological 

Sciences, San Diego State University, San Diego 

CA 92182, MAITINSON, J.M., Department of 

Geological Sciences, University of California, 

Santa Barbara CA 93106, RENNE, P.R., Institute 

of Human Origins, 2453 Ridge Road, Berkeley 

CA 94709, and CAMPBELL, J.D., Geology 

Department, Otago University, Dunedin, New 

Zealand. 

The New Zealand Triassic is divided into seven 

local stages based on subdivision of the Murihiku 

Supergroup and is representative of the time-span 

Scythian to the end of the Triassic. Faunal 

assemblages include hydrozoans, ammonoids, 

nautiloids, conodonts and halobiid bivalves and 

provide a fmn basis for interregional correlation of 

these strata with North American and other 

sequences. Zircon U/Pb and biotite 40Arf39Ar dates 

from three crystal-vitric tuffs in the Middle and 

Upper Triassic portion of the Murihiku Supergroup 

constrain local and international biostratigraphic 

time scales. The stratigraphically lowest sample 

occurs in upper Etalian strata, close to the EtalianKaihikuan Stage boundary which in tum is tightly 

correlated to the Anisian-Landian international 

boundary. Two individual grains of biotite from this 

tuff yielded indistinguishable laser step-heating 

40Arf39Ar plateau ages of 242.3 ± 0.8 and 243.4 ± 

0.8 Ma. The weighted mean of the two plateaus is 

242.8 ± 0.6 Ma. Zircon 206Pb*f238U dates from the 

same sample range from 236.8 ± 2 to 243.1 ± 2 Ma. 

The later result is from a nearly concordant finegrained fraction leached in HF prior to dissolution 

and is in good agreement with the more precise 

argon plateau age estimate. Zircon U/Pb dates from 

from Otamitan and Otapirian tuff beds higher in the 

section provide maximum (226 ± 2 Ma) and 

minimum (214 ± 2 Ma) ages, respectively, for the 

occurrence of Monotis in the New Zealand 

stratigraphic record. These two ages are based on 

207Pb*fl06Pb* dates from zircon exhibiting slight 

amounts of relatively recent lead loss but no 

inherited components. Ammonite-bearing Triassic 

strata continue upsection for approximately 1500 

meters above the 214 ± 2 Ma Otapirian tuff bed. 

171 

518 Ma SPHENE (TITANITE) FROM THE KHAN 

PEGMATITE, NAMIBIA, SOUTHWEST AFRICA: 

A POTENTIAL ION-MICROPROBE STANDARD 

KINNY. P D , Department of Applied Physics, Curtin 

University of Technology, Perth, WA, McNAUGHTON, 

NJ., Dept of Geology, University of Western Australia, 

Perth, WA, FANNING, C.M. and MAAS, R., Research 

School of Earth Sciences, ANU, Canberra, ACT, Australia. 

Large, euhredral crystals of black sphene (titanite) occur in 

copper-mineralized pegmatite at the Khan mine, Namibia. A 

12 mm crystal has been investigated as a potential standard for 

U-Pb isotopic and trace-element studies by microprobe. 

Sphene can be a useful mineral for dating, particularly when 

associated with mineralization. 

Six conventional U-Pb analyses of splits from two crushed 

fragments yielded concordant results with a 207Pbf206Pb age 

of 518 ± 2 Ma (95% conf.). U contents were in the range 682 

to 712 ppm, and 206Pbf204Pb ratios were ca. 2000. The 518 

Ma age is in agreement with a sphene- K-feldspar isochron of 

521 ± 27 Ma (MSWD 0.15) derived from the same material. 

Khan Pegmatite 

Sphene 

REE abundances detennined by isotope dilution on four 

chips agree to within 10% for each analyzed element. The 

pattern dips smoothly from La 1xlo4 chondrites to Gd-Lu 

2x103 chondrites, with a negative Eu anomaly, as shown. 

10000 

1000 

Khan sphene 

REE 

A series of ion-probe 25J.UD spot traverses across transverse 

and longitudinal sections through the sphene has so far failed to 

detect significant heterogeneity in U-Pb isotopic composition 

or in trace-element abundances. The ion-probe U-Pb analytical 

routine is the same as for perovskite, using the CaTi204 peak 

at mass 200 as a reference for concentrations, and 206Pb*/UO 

vs U02flJO covariation to calibrate the measured Pb/U ratios. 

Pb sensitivity on the Western Australia Isotope Science Centre 

SHRIMP II is comparable to that derived from zircons. 

The method has been applied successfully to date sphenes 

associated with mineralization in the Olympic Dam deposit of 

South Australia, and elsewhere. 
NEON DEGASSING OF THE SUBCONTINENTAL 

MAN1LE UNDER THE EIFEL REGION, GERMANY 

KIPfER R 1, AESCHBACH-HERTIG, W.1, BAUR 

H2, IMBODEN, D. M.1, HOFER, M.l, HOHMANN, 

R 1 and WIELER, R2 

1 Environmental Physics, Swiss Federal Institute of Technology, ETHIEA WAG, 8600 Diibendorf, Switzer land 

2 Isotope Geology, Swiss Federal Institute of Technology, 

ETH, 8092 ZUrich, Switzerland 

Phreatomagmatic eruptions formed the typical maar 

lakes of the Eifel region (Germany). The magmatic activity 

started 200'000 years ago in the western part and then moved 

towards east ending 11 '000 BP with the formation of 

Laacher See, the youngest maar. 

Recent degassing of fluids from the subcontinental mantle is common in this area as indicated by numerous mineral 

springs and C02 wells. 

We analysed the light noble gases in several C02 wells 

and natural gas discharges. All samples contain isotopically 

light helium from a mantle source and radiogenic helium in 

different proportions. All 3Het4He ratios are lower than the 

those found in fluid inclusions of xenoliths related to the 

magmatic cycle [Dunai and Baur, 1993]. Some well gases 

even contain non atmospheric isotopically light neon. 

2~ef22Ne ratios as high as 11.2 and 21 Net22Ne ratios up to 

0.048 have been measured. The neon results group the C02 

gases in two different sets, representing the eastern and the 

western part of the Eifel, respectively. Assuming a common 

mantle origin for the noble gases in the Eifel, one can correct 

the neon data for radiogenic production using the difference 

between the measured 3He!4He ratio and the initial mantle 

ratio conserved in xenolithic fluid inclusions. The corrected 

neon data define a mixing line between atmospheric neon 

and a mantle neon component which differs from MORB 

since it contains less nucleogenic neon. 

Our divers sampled a C02 gas plume entering Laacher 

See at a depth of 31 m. The neon in this high pressure fluid 

contains up to 4 % of mantle neon with a mantle 2~et3 He 

ratio of about 1.4. Combining the elemental ratio with the 

known 3He flux into the lake (6500 atoms/em 2ts, Kipfer et 

al., 1992) we estimate a recent mantle 20Ne flux of about 

9000 atoms/cm2/s. 

Dunai, T. J. and Baur, H., 1993, Noble gas and Sr-, NdSystematics of the European Subcontinental Mantle: 

supplement to European Journal of Mineralogy, v. 5, p. 

134 

Kipfer, R, Aeschbach-Hertig, W., Baur, H., Hofer, M. and 

Imboden, D. M., 1993, Helium and C02 Fluxes from the 

Earth Mantle in Laacher See, Germany: supplement to 

Annales Geophysicae, v. 11, part II, p.236 

172 

FLUID MIGRATION AND THERMAL HISTORY OF 

THE MORCLES/DOLDENHORN NAPPE (HELVETIC 

ALPS): alSo AND a13c ANALYSES OF QUARTZCALCITE VEINS AND MYLONITES 

KIRSCHNER. D.L. Institut de Mineralogie, 

Universite de Lausanne BFSH-2, CH-1015 Lausanne, 

Switzerland, SHARP, Z. D., and MASSON, H. (same 

address) (e-mail: dkirschn@imp.unil.ch,) 

The western Helvetic Alps is a classic fold/thrust 

belt in the external zone of the Alpine orogen. Stable 

isotope analyses of mylonites and veins are used to 

constrain the sources and pathways of fluids and the 

thermal history of the lowest nappe in this belt (the 

Morcles and structurally-equivalent Doldenhorn nappe 

exposed 50 km along strike to the east). 

One hundred mylonite samples were analyzed from 

five traverses at the basement-nappe thrust contact. alSo 

values decrease by -7%o depletion toward the basementDoldenhorn contact, while the a13c values do not vary 

systematically in proximity to the contact. The strong, 

oxygen isotope shift localized in a 5 meter zone about 

this contact is similar to alSo shifts documented for the 

overlying Diableretsl and Glarus2 thrusts, and are 

consistent with water-dominated fluids having entered 

the thrusts either from more deeply buried Mesozoic 

sediments or underlying basement massifs. Conversely, 

the alSo values at the basement-Morcles nappe contact 

do not vary systematically, while the a13c values are 

lowered by up to 8%o in a 5 m wide zone at the contact. 

Fluids entering the base of the Morcles nappe must have 

been from an isotopically distinct (water-poor?) reservoir 

relative to the fluids entering the Doldenhorn, Diableret, 

and Glarus thrusts. 

Additional analyses have been made of eleven 

syntectonic, fibrous quartz-calcite veins from Morcles 

nappe in order to constrain its thermal history during 

deformation. The temperature in the hinterland core of 

the Morcles fold nappe was 300°C during NW-extension 

(overthrusting?), it then increased to 350°C and 

subsequently dropped back down to 300°C during late 

orogen-parallel, NE-extension of the nappe. Farther to 

the foreland, analyses of crosscutting veins show that 

temperature during NW-extension was 330°C and 

decreased to 300°C during NE-extension. In the most 

foreland area samfled (Les Plans) the average 

temperature was 260 C. The alSo and a13c values of 

the calcite in all the veins analyzed are the same as those 

of their surrounding host marl and limestone, indicating 

that these isotopic systems were rock-buffered 

throughout most of the Morcles nappe. 

1 Crespo-Blane et al. (in preparation) 

2Burkhard et al. (1992). Contrib. Min. Petrol. 112, 293-311. 
INTERPRETATION OF U/PB AGES OF ZIRCONS 

FROM MESOZOIC PLUTONS IN THE SALINIAN 

BLOCK, CALIFORNIA 

KISTLE&. R W, WOODEN, J. L., U.S. Geological 

Survey, Menlo Park, CA, U. S. A. 

Mesozoic plutons exposed in the Salinian block to the 

west of the San Andreas fault in California commonly yield 

zircon fractions that have almost concordant to concordant 

206pbt238u_207pbf235u ages (James, 1992; Mattinson, 

1978). For most plutons dated, however, different zircon 

fractions yield different dates that form trends parallel to or on 

concordia on U/Pb plots. Complex explanations that rely on 

minor inheritance of Proterozoic zircons from source rocks of 

the plutons coupled with subsequent lead loss from newly 

formed zircons have been developed to interpret the U/Pb 

data. Interpreted and measured zircon ages in most cases are 

greater than ages from coexisting minerals dated by K-Ar or 

fission-track techniques and those ages have been interpreted 

to reflect cooling at the end of a prolonged high temperature 

post emplacement history of the plutons. 

We have dated four fractions of zircons and two of sphene 

separated from a biotite, hornblende tonalite dredged at a depth 

of about 210 feet in the Pacific Ocean from the southern end 

of the Cordell Bank about 15 km northwest of the Farallon 

Islands. All six mineral fractions yield concordant U/Pb ages, 

but they span the interval from about 91 to 99 Ma. SHRIMP 

analysis of seven spots on zircons from the concordant 94 Ma 

80-100 micron fraction yield concordant U/Pb ages that range 

from about 95 to 127 Ma. 

There is a positive correlation of age with U concentrations 

that range from 346 to 420 ppm in the four zircon fractions 

dated by conventional techniques. The SHRIMP data show a 

surprising range of U concentrations from 146 ppm to 7836 

ppm that do not correlate with age. There is obvious 

inheritance in the zircons, and it is impossible to select a 

meaningful age from the spectrum of concordant ages. 

206Pbt204pb vs 23Su1204pb and 207Pbt204pb vs 

235ut204pb isochron plots of the conventional zircon and 

sphene U/Pb data yield ages of 93.9 Ma and 94.3 Ma, 

respectively. The 206pbf204pb and 207pbf204pb intercepts 

on these diagrams indicate a small radiogenic lead inheritance 

in the minerals and that the feldspar lead isotopic 

composition should not be used as the common lead 

correction in the age calculations. The zircon and sphene 

isochron age is the same, within experimental error, as the 

Rb-Sr whole-rock-mineral isochron and 40 Ar/39 Ar biotite 

and hornblende incremental heating plateau ages. 

A similar treatment of U/Pb zircon, sphene and apatite ages 

determined for tonalites at Bodega Head (Mattinson, 1978) 

and Montara (James, 1992) in the Salinian block brings them 

into a~reement with Rb/Sr whole-rock-mineral isochron and 

40 Ar/ 9 Ar biotite and hornblende incremental heating plateau 

ages. 

James, E. W., 1992, Cretaceous metamorphism and 

plutonism in the Santa Cruz Mountains, Salinian block, 

California, and correlation with the southernmost Sierra 

Nevada: Geol. Soc.America Bulletin, v. 104, p. 1326-

1339. 

Mattinson, J. M., 1978, Age, origin, and thermal histories of 

some plutonic rocks from the Salinian block of California: 

Contrib. to Mineralogy and Petrology, v. 67, p. 233-245. 

173 

Comparison of FT, K-Ar and 

Ar-Ar age using JAS-Gl 

obsidian glass: 

Calibration of each method and 

laboratory 

Naoko KITADA*l, Kiyoshi WADATSUMJ*l, 

Gullo BIGAZZJ*2, Alan DEINQ*3, Marinella 

LAURENZJ*2, Bart KOWALLIS*4, Keisuke 

NAGAQ*S, Harue MASUDA*l (*1; osaka City 

Univ. *2; Instituto di Geochronologia e Geochimica 

Isotopic a *3; Geochronology Center of the Institute of 

Human Origin *4; Brigham Young Univ. *5; Earth's 

Interior Inst.) 

The glass age determinations using FT, 

K-Ar and Ar-Ar were carried out for the 

purpose of get independent ages and 

comparison each other. The glass sample 

is used JAS-G 1 obsidian that produced as 

a chilled margin of rhyolitic intrusive rock 

at Nagano prefecture in japan. 

On account of high K-content and 

obtain spontaneous fission-tracks enough 

to counting easily, JAS-G 1 is appropriate 

sample in this experiment. At the dating 

of fission-track age, both ambient and 

plateau age were determined. Plateau 

method was used in order to take away of 

fading effect that observed in spontaneous 

fission. 

At the result of this experiment, plateau 

ages indicate little bit old age rather than 

ambient one, and seemed to cancel of 

fading effect. In fact, plateau ages show as 

nearly same age as K-Ar and Ar-Ar age. On 

the point of comparison of K-Ar and Ar-Ar 

age, these do not necessarily coincide in 

1a-error each other. The reason of this 

result is indicate following problem: 

1) partial alteration may occur. 

2) sample is not homogeneous about Kcontent. 

3) at the comparison of each laboratory, 

calibration of each mass may have to 

use same standard sample. 
CARBON ISOTOPE THERMOMETRY IN THE 

ADIRONDACKS 

KITCHEN. Nami E. and VALLEY, John W., 

Dept. of Geology and Geophysics, Univ. of 

Wisconsin, Madison, WI 53706, USA 

The fractionation of carbon isotopes between 

calcite and graphite provides a temperature sensitive 

thermometer for high grade marbles. We have 

conducted a study of calcite-graphite fractionations 

(.6cc-gr) in amphibolite and granulite grade rocks of 

the Adirondack Mtns., NY, to evaluate the accuracy 

of the thermometer. Detailed thermometry in the 

amphibolite grade NW Adirondacks shows small 

systematic changes in Llcc-gr northwestwards from 

3.7 to 4.3 to 3.9 over a 30 km distance, reflecting 

transitions from amphibolite facies towards 

granulite facies to theSE and NW. These 

fractionations correspond to temperatures of 685, 

640, and 670°C respectively, based on the empirical 

cali6ration of Dunn and Valley (1992). Calcitegraphite pairs from the granulite grade Central 

Adirondacks give consistent! y smaller 

fractionations. 22 analyses from between the 725 

and 775°C isotherms (petrologic thermometry, 

Bohlen et al, 1985) yield L\cc-gr = 3.67 to 2.95, 690° 

to 750°C (average= 715°C). This suggests that the 

empirical calibration may be -0.4%o too low, in 

agreement with Chacko et al (1991), or that the 

calcite-graphite system has been reset from peak 

metamorphic conditions. 

7 graphite flakes from 4 different localities with 

an average size of 0.4 x 1.8 mm were delaminated 

perpendicular to the c-axis in order to measure ol3C 

variations across individual flakes, and thereby test 

for zonation. In every case the cores of the flakes 

were heavier than the rims, by 0.1 to 0.6%o (±0.05) 

across less than 0.25mm. Core ol3C values yield 

L\cc-gr values as low as 2.6 and 2.7 (785 to 775°C), 

while the rims of the same samples give .6cc-gr = 3.0 

to 3.2 (745 to 725°C). Although L\cc-gr(bulk) does 

not correlate with distance to the Marcy Anorthosite 

Massif (for 18 bulk analyses within 4 km of the 

contact, the average ~cc-gr = 3.33, 725°C), the 

highest .6cc-gr(core) temperatures are from within 0.5 

km of the anorthosite contact and may preserve 

early contact metamorphic temperatures. Late-stage 

overgrowths of graphite were not apparent in any of 

the samples, therefore diffusive exchange of C may 

be responsible for the zonation. 

Values of L\cc-gr(core) correlate very well with 

petrologic temperature data in the Adirondacks. 

Thus, despite minor zoning in graphite, the calcitegraphite system provides a sensitive and reliable 

thermometer. 

174 

NITROGEN ISOTOPE STUDY OF PRIMITIVE 

CHONDRITES BY A LASER PROBE METHOD 

KIYOTA. K. and SUGIURA, N., Dept. of 

Earth and Planetary Physics, Univ. of Tokyo, 

Tokyo, 113, Japan. 

In search for presolar grains, we have 

measured isotope ratios of nitrogen and noble 

gases in UOCs by a stepwise combustion method. 

Some types of isotopically anomalous nitrogen 

components which are probably due to unknown 

presolar grains have been found. Though some 

chemical and physical treatment have been 

performed, the carriers are not concentrated and 

they are not identified yet. UOCs are 

heterogeneous in mm scale, and in some UOCs, 

whole rock samples contain various amounts of 

anomalous nitrogen, according to the results of 

the stepwise combustion method. Therefore, to 

investigate where the carriers of anomalous 

nitrogen are, we measured nitrogen and rare gases 

by a laser probe method. 

A sample was cut from a chunk of UOC by a 

diamond saw and a face was polished. After 

washing with water and acetone in an ultrasonic 

bath, it was heated in a vacuum system at 500C 

for an hour to remove terrestrial contaminations. 

The polished face was heated by a focused CW 

Nd-YAG laser and the extracted gases were 

measured with a static QMS.A diameter of a pit by 

the laser is about 0.1 mm. 

Mezo Madaras (LL3.4/3. 7) was used for the 

preliminary experiments. Previously, we found by 

a stepwise combustion of bulk samples this UOC 

contains both light (delta 15N < -70 permit) and 

heavy (delta 15N > 230 permit) nitrogen 

components heterogeneously. 

The abundance and isotopic composition of 

nitrogen from some laser pits are about 0.1 ng and 

50 permit. 40Ar/36Ar ranges from 20 to 300. The 

sensitivity of N measurement of our laser 

extraction system is better than that reported by 

Franchi et al. ( 1986). Thus we will be able to 

analyze smaller regions in meteorites which will 

help us find where presolar grains are located. 

Franchi, I. A., et al., 1986, The laser microprobe: 

A technique for extracting carbon, nitrogen, 

and oxygen from solid samples for isotopic 

measurements: Journal Geophys. Research, 

v.91, No.B4, p.D514-D524. 
NICKEL-59 ACTIVITIES OF GRANT AND OTHER 

METEORITES MEASURED BY ACCELERATOR 

MASS SPECTROMETRY 

KLEIN, J., MIDDLETON, R., Dept. of Physics, Univ. 

Pennsylvania, Philadelphia, PA, 19104, HERZOG 

G.F., and XUE S., Dept. of Chemistry, Rutgers Univ., 

New Brunswick, NJ 08903. 

The radioactive nuclide 59Ni is made in meteorites 

when 58Ni, the major stable isotope of nickel ( -68% ), 

captures thermal neutrons produced during cosmic ray 

bombardment. The nuclide holds much promise as a 

monitor of the thermal neutron fluxes in meteorites for 

severat reasons: (1) the target element of the 

bombardment, nickel, occurs in convenient 

concentrations in many classes of meteorites; (2) the cross 

section crth for the reaction 58Ni(n;y)59Ni, 4.6 b, is 

appreciable; (3) the long half-life (76 ky) means that 

measurements need not be restricted to recent falls or 

very young finds. After a -20-year lull, experimental 

interest in 59Ni has revived with the realization that 

accelerator mass spectrometry (AMS) makes the 

measurements easier. We report 59Ni activities measured 

by AMS in several meteorites as a first step toward the 

systematic investigation of its production rate. 

After magnetic separation and HF etching where 

necessary, metal-rich phases were dissolved in 7% HN03 

to which Be, AI, and Ca carriers had been added for other 

measurements. Nickel was isolated by a series of ion 

exchange and precipitation steps. 59Cof58Ni ratios were 

typically less than 400 (atom/atom). AMS analyses on 

the FN tandem at the Univ. Pennsylvania were made 

relative to a 59Ni standard prepared by irradiating highpurity nickel metal in a reactor. Backgrounds, due 

primarily to 59Co, were less than 5% of the measured 

values. 

Measured 59Ni activities (dprnlg Ni) were as follows: 

Admire, 3.1±0.5; Charlotte, 0.58±0.17; Estherville, 4.1± 

0.5 and 3.4±0.3; Watson, 0.40±0.05; Zhaodong, 0.33± 

0.03; activities in four samples of the Grant iron meteorite 

had values between 1 and 2. Agreement with previously 

published values is satisfactory for Estherville, Grant and 

Admire. In contrast to the 26 AI and lOBe activities but as 

expected, the 59Ni activities in the Grant samples increase 

with increasing 4HeJ21Ne ratio, and hence depth in the 

meteorite. 

Nominal thermal neutron fluxes q, (n/cm2-s) calculated 

from the relation 59Ni = cr1h<pN where N is the 58Ni 

concentration range from 0.17 for Zhaodong, an L4 

chondrite with an unremarkable 22Nef21Ne ratio, to 2.1 

for the mesosiderite Estherville. The neutron fluences 

estimated for the Grant samples are - 2x1Ql6 n/cm2; 

published values for samples of the lunar regolith are 

similar. 

175 

EVIDENCE FOR THE REWORKING OF CADOMIAN 

CRUST IN VARISCAN GRANITOIDS OF CENTRAL 

EUROPE (SOUTH BOHEMIAN PLUTON, BOHEMIAN 

MASSIF, AUSTRIA): A ZIRCON PB-PB AND U-PB 

STUDY. 

Klotz1i. U.S., Dep. of Geology, Lab. for Geochronology, 

Univ. of Vienna, Franz GriJlstr. 9, A-1030 Austria, and 

Parrish, R.R., Geol. Survey of Canada, 601 Booth Street, 

Ottawa, K1A OE8, Canada. 

The existence of metamorphic and plutonic rocks of 

Cadomian (pan-African) age either consolidated with or 

reworked into the European basement during the Variscan 

orogeny is well documented for the middle and eastern part of 

the Bohemian Massif (i.e. "Moldanubikum" of Bohemia, 

"Bruno-Vistulikum"). For the composite South Bohemian 

Pluton in Austria and the Czech Republic such evidence has 

been lacking. 

A combined Pb-Pb evaporation and conventional U-Pb 

study of zircons from quartz monzonitic enclaves of the 

Weinsberg type granite and the Rastenberg type granodiorite 

gives the following age results: enclaves in Weinsberg type 

granite: 355 +1- 8 Ma (rim or long prism.) and 529 +/- 22 Ma 

(core or short prism.); Rastenberg type granodiorite: 339 +/- 5 

Ma (rim, short prism. or tabular habit), 623 +/- 22 Ma (core 

or long prism.). Upper intercept ages for both bathoJiths are in 

the range of 2000 - 2540 Ma. 

The Carboniferous ages are interpreted as the time of 

intrusion of the Variscan granitoids. 

The Cambrian and pre-Cambrian ages demonstrate the 

presence of inherited zircons from protoliths of Cadomian age 

in the investigated granitoids. As up to 50% of the 

investigated zircons show Cadomian ages, it is postulated that 

the amount of reworked Cadomian basement is quite 

substantial. 

All the zircons investigated show true magmatic 

morphologies. This and the limited scatter in age distribujion 

suggest that the reworked Cadomian protoliths were either 

intrusive rocks or orthogenic metamorphic rocks. 

Thus the zircon investigations in the South Bohemian Pluton 

clearly demonstrafe the presence of reworked Cadomian 

basement within or at least below the present Moldanubian 

upper crustal rocks. It is not yet clear whether the Cadomian 

rocks belong to the overridden basement complex of the 

"Bruno-Vistulikum" microplate or whether they belong to the 

Moldanubian microplate probably having been incorporated 

there during earlier stages of the Variscan orogeny. Sr isotope 

systematics favor the latter model. 
THE COSMOGENIC RADIONUCLIDES 7

Be, 'OSe, AND 36Cl 

IN PRECIPITATION 

KNIES D. L, ELMORE D., SHARMA P., Department of 

Physics, Pw-due University, West Lafayette, IN 47901 USA, 

VOGT S., LIPSCHUTZ M.E., Department of Chemistry, 

Pw-due University, West Lafayette, IN 47901 USA, and 

PETTY G., MONAGHAN M., Department of Earth and 

Atmospheric Sciences, Pw-due University, West Lafayette, IN 

47901 USA 

Two-thirds of the 7Be (t112 =53 d), 'OSe (t112 = 1.5 My), and 36Cl (t112 = 0.3 My) is produced in the stratosphere and one-third 

in the troposphere. The residence time of these radionuclides in 

the stratosphere is a few years and in the troposphere is a few 

weeks. Since 7Be's half-life is short compared to its residence 

time in the stratosphere and similar to its residence time in the 

troposphere, 7Be/10Se and 7Bef36Cl concentration ratios should not 

equal their production ratios, but should have distinct tropospheric and stratospheric values. For this reason, these isotopes 

can be used to study processes involving mixing of air from the 

troposphere and air from the stratosphere. 

We built a wet -only rain collector and located it 10 km west of 

West Lafayette, Indiana. The collector is equipped with an 

automatic roof and an automatic sample changer that can hold up 

to seven 8-liter bottles. Dw-ing the period April 18, 1992 to 

August 31, 1993, 350 samples were collected with -3 mm 

resolution. Three 125 ml aliquots were taken from every sample 

for cation (ICP-AES) and anion (IC) analyses and two 500 ml 

aliquots were taken from selected samples for future 2fl, 3H, and 

6180 measw-ements on selected samples. 

The 7Bef36Cl and '0Sef36Cl ratios varied by a factor of60. The 

mean 10Sef36Cl ratio was 8. 9. 7Be, 'OSe, and 36Cl concentrations 

covaried through a single event. Typically they decreased from 

high values at the beginning of an event to lower values at the end. 

Mean concentrations were 1.4 x 107 atoms/L 7Be, 1.8 x 107 

atoms/L 10Se, and 0.2 x 107 atoms/L36Cl. A strong washout effect 

is evident in the 7Be, 10Se, and 36Cl data, with up to an order of 

magnitude decrease from the first to the second sample. 

We are investigating relationships between the radionuclide 

concentrations and event type, air masses history, and season. 

Using back trajectories and standard atmospheric data bases, we 

will determine the sow-ce regions of the isotopes and their 

residence times in the lower atmosphere. Fw-thermore, we will 

investigate whether the different isotopes are scavenged by similar 

mechanisms as they move through the atmosphere. This knowledge is critical to understanding the large anomalies observed in 

the 10Sef36Cl ratio ice core data, where the measw-ed '0Sef36Cl 

ratio is lower than predicted by theory and varies with time in a 

way that cannot be explained by production rate variations alone. 36Cl has a high thermal neutron cross section for production 

from 35Cl and is usually present in reactors, reprocessing facilities, 

and low- and high-level nuclear waste. 36Cl can be released in 

gaseous, liquid, and solid phases and eventually reaches groundwaters where it is a highly mobile anion. Thus, 36Cl is a good 

tracer for studying radioactivity releases into the environment. 

Our studies of 36Cl in rain provide a monitor for atmospheric 

releases and a baseline for natural 36Cl concentrations. 

176 

THERMOTECTONIC HISTORY OF THE CAJON 

PASS DEEP DRILLHOLE, SOUTHERN 

CALIFORNIA: AN APATITE FISSION TRACK 

STUDY 

KOHN B.P .. Victorian Institute of Earth and Planetary 

Sciences, School of Earth Sciences, La Trobe University, 

Bundoora, Victoria, 3083, Australia, Saltus, R.W., USGS, 

Federal Center, Denver, CO 80225, USA, and Glcadow, 

AJ. W ., Victorian Institute of Earth and Planetary Sciences, 

School of Earth Sciences, La Trobe University, Bundoora, 

Victoria, 3083, Australia 

The 3.5 km Cajon Pass deep drillhole (CPDDH) was drilled 

to provide information on the absolute stress state of the San 

Andreas fault (SAF). High heat flow could imply frictionally 

generated heat and therefore high absolute stress ("strong fault"). 

Low heat flow would imply a weak fault. High heat flow was 

measured at the surface, but it fell off rapidly with depth, an 

effect attributable to the recent sedimentation and erosion 

history, thus supporting a relatively low average shear stress for 

the SAF. 

Apatite fission track (FT) analysis offers the potential to 

assess the thermo-tectonic history of the predominantly 

crystalline rocks in the CPDDH. Apatite FT data in 23 samples 

of surface and CPDDH core sediment and basement down to 

total depth reveal ( 1) a complicated sawtooth pattern of FT age 

versus depth relationships, with characteristic FT age and 

lengths providing distinctive thermochronological markers 

repeated down the section, an example of two such markers is 

indicated by the following data: 

FT Age (Ma) Mean ttack length Present temp Depth (m) 

46.8 ± 2.3 9.85 ± 0.24 JJ.m 58°C 1140 

51.5 ± 2.6 9.47 ± 0.19 JJ.m 90°C 2018 

9.8 ± 1.1 8.77 ± 0.31 J.lm 76°C 1656 

10.2 ± 1.0 9.08 ± 0.28 J.lm 111°C 2605 

(2) occasional fission tracks preserved at bottom hole 

temperatures of 142°C, and (3) bimodal track length 

distributions associated with younger FT ages. 

The distinctive markers preserved in the upper -2.0 km of the 

section yield relatively young FT ages and short mean track 

lengths which are clearly not in equilibrium with present 

downhole temperatures. This indicates that parts of the upper 

CPDDH section have been considerably hotter in the recent past 

and cooled by -32-35°C, which is equivalent to -o.9-1 km of 

vertical displacement under the present day geothermal gradient 

This cooling interpretation is also supported by previous zeolite 

mineralization studies. We attribute the observed sawtooth 

pattern to a complicated tectonic history whereby imbricate 

thrusting stacked slices of the pre-thrust partial annealing zone 

within the present CPDDH section. The repetition of markers 

through the section (over present day temperatures ranging from 

58° -116°C) and the presence of fission tracks at present bottom 

hole temperatures imply a very brief heating time of <1Ma. 

The bimodal track length distributions indicate some reheating 

of the section (possibly due to burial) prior to thrusting. 

The apatite FT data independently confrrm and quantify the 

tectonic scenario previously postulated to account for the 

anomalous heat flow of the Cajon Pass area; in addition they 

indicate that Pleistocene imbricate thrusting (at too fine an 

interval to be preserved in the present day geotherm) was a 

fundamental mechanism in the uplift/erosion history of the 

CPDDH site. 
OXYGEN ISOTOPE CONSTRAINTS ON 

METAMORPHIC FLUID FLOW, TOWNSHEND 

DAM, VERMONT, USA. 

KOHN. Matthew J and VALLEY, John W., 

both at Dept. of Geol. and Geophys; Univ. of 

Wisconsin; Madison, WI 53706 USA 

Fluid-rock interaction during amphibolite-facies 

metamorphism has been investigated for rocks 

exposed in a single 400 meter long, lithologically 

heterogeneous outcrop near Townshend, Vermont, 

USA. Oxygen isotopic compositions have been 

measured in profiles across coarse-grained garnets 

from 13 samples, and from hornblende and garnet 

separates from 33 fine-grained samples. This 

outcrop was previously studied by Chamberlain and 

Conrad (1991, 1993), who observed large oxygen 

isotope gradients (3%o) from core to rim in garnets 

from one sample, and inferred large amounts of 

pervasive fluid flow. None of the garnets that we 

have analyzed show strong isotopic zonation, and the 

mineral separate data imply both large gradients in 

peak metamorphic fluid isotopic compositions and a 

strong correlation of isotopic composition with rock 

type. Although devolatilization reactions in these 

rocks must have produced metamorphic fluids, the 

data preclude syn-metamorphic cross-foliation timeintegrated fluid fluxes greater than 200 cm3/cm2. 

The isotopic trends have significant implications 

regarding the physical transport of fluids in the crust 

during regional metamorphism. The data can all be 

interpreted in terms of closed system behavior (i.e., 

no lateral con1munication between adjacent rocks), 

channeled fluid flow (i.e., extremely limited 

advective communication between rocks), or 

diffusive transport of oxygen in an interconnected 

grain-boundary fluid (i.e., limited communication 

without cross-strike advection). This implies that 

any significant flow at this locality was dominantly 

either layer parallel or channeled out of the system in 

cracks or veins, and that pervasive cross-strike fluid 

flow was unimportant. If fast-transport channels 

were active, then the maximum fluid flux (-200 

cm3fcm2) and the amount of fluid generated during 

garnet growth restrict the spacing of fluid conduits to 

have been less than -300 meters. Veins of the 

appropriate size and spacing are present in the 

outcrop. The isotopic data do not support the models 

of Ferry (1992), Stem et al. (1992), and Chamberlain 

and Conrad (1993), who proposed that large volumes 

of metamorphic fluids (time-integrated fluid fluxes 

of 1x104 - 1x106 cm3/cm2) flowed pervasively 

across strike during metamorphism. 

177 

ISOTOPE GEOCHEMISTRY OF WEST PACIFIC 

CRETACEOUS SEAMOUNTS AND GUYOTS 

KQDpers. Anthony A P, Staudigel, H., Pringle, M.S., and 

Davies, G.R., Laboratory of Isotope Geology, Vrije 

Universiteit, Amsterdam, 1081 HV, The Netherlands. 

The mantle underneath the South Pacific is both isotopically 

and thermally anomalous (SOPITA). Within the last 20 Ma, 

eight currently-active SOPIT A hot spots formed a large number 

of seamounts on anomalously hot oceanic lithosphere. Their 

volcanogenic products display an extreme isotopic diversity 

that covers the entire range between the EMil and HIMU 

mantle components, including a small admixture of EMI. 

The studied West Pacific Cretaceous volcanic edifices of 

Limalok, Lo-En, Wodejebato (Marshall Sm's), Vlinder, HIMU, 

Hemler (Magellan Sm's), MIT, and Takuyo Daisan are situated 

on the Jurassic aged oceanic crust. 40 Arf39 Ar-ages (Pringle et 

al., this volume) were used to reconstruct their locations of 

formation, demonstrating that the West Pacific Sm's originated 

from the SOPITA region. The Marshall and Magellan Sm's 

originated close to the Rarotonga, Rurutu and MacDonald hot 

spots, whereas MIT formed near Society, and Takuyo Daisan in 

between Marquesas and Samoa. 

Sr-Nd-Pb isotope data (see Diagram) from Cpx-Plag-Hbl, 

and leached bulk rocks of Cretaceous 1UNES6 and ODP LEG144 samples, display isotope compositions comparable to 

SOPIT A volcanics: (1) intermediate EMIIIHIMU signatures of 

Limalok, Lo-En and Wodejebato are similar to that of MacDonald, (2) individual samples from Limalok and Wodejebato, 

and the Vlinder data are indicative of minor EMI admixture (cf. 

Rarotonga), and (3) MIT slightly extends the isotopic field of 

the Wake Sm's towards EMil and has isotopic correlations 

comparable to the more HIMU-like Society volcanics. 

In conclusion, the overall spatial and isotopic ranges for 

Cretaceous Sm's and SO PIT A hot spots are comparable. Within these ranges individual correlations are persistent, suggesting 

that the isotopically and thermally anomalous mantle source in 

the South Pacific has been active since the Cretaceous. Such a 

long-lived correlation supports the need for a coherent genetic 

explanation of both features. Recycling of eihter sediments, 

oceanic crust, or lithospheric mantle may fuel the SO PIT A 

mantle source with extreme radiogenic signatures, as well as 

with high abundances U, Th, K, etc. anomalously heating-up 

the mantle plume over periods >100 m.y. by radioactive decay. 

... 

r 

0.708 ...-----.... ~-..--------..., 

0.707 

0.706 

~ Umalok 

Lo-En 

Wodejebato 

MIT 

1::0.705 

. 

PJ 

0.704 

0.702 -.~--........... ----,,----.,---...,...---t 

17 18 19 20 21 22 208Pb /2«M Pb 
MAGMATIC CARBON DIOXIDE IN LOWER SILESIAN 

COAL BASJN, NW POLAND 

KOTARBA Macicj. Dept of Fossil Fuels, University of 

Mining and Metallurgy, Cracow, 30-059, Poland 

The Lower Silesian Coal Basin (LSCB) is situated within 

the ~Sudetic depression, in the Sudetes Mts. (SW 

Poland). The Sudetic Tectogen is cut by ntnTlerous deep 

fractures which may reach the upper mantle (e.g., 

Intra-Sudetic Deep-seated Fault). In LSCB ntnTlerous 

examples of the Variscan magmatism were fol.Dld Terti my, 

rnul ti phase basaltic magmatism has not been observed 

within the Upper Carboniferous coal-bearing strata of the 

LSCB but ocoured in its vicinity . Beneath the Paleozoic 

complex ofLSCB active magma chambers probably exist up 

to now. 

Carbon dioxide accumulated in the coal seams of the 

LSCB reveal variability of both the content and isotopic 

composition. o13C-values vmy from -26.6 to +16.8 •1 .. and 

CDMI* from 0. 12 to 99.96% (Kotarba, 1988, 1990). 

Stable isotope studies allowed distinction between 

magmatic, thermogenic and bacterial C02_. The o13C values 

of the endogenic (magmatic and/or upper mantle origin) 

C02 are close to the mean values for elemental carbon in 

the upper mantle and vary from -8 to -5 ·1.. (about -7 ·1.. on 

average). Carbon dioxide occuring in some faults zones of 

the LSCB reveals CDMI values from 88.2 to 99.96% and 

o13C-values from -10.5 and -5.7 ·1 ... These data indicate 

endogenic origin of carbon dioxide. Somewhat wider 

interval of o13C-values of C02 than for typical endogenic 

one can be connected with mixing of thermogenic and/or 

bacterial carbon dioxides, and/or isotope fractionation 

during col-migration through the microporous structure of 

coals. Carbon dioxide has migrated from the upper mantle 

and/or from the intermediate magma chambers along the 

Intra-Sudetic Deep-seated Fault and was re-distributed 

through the network of smaller dislocations. This migration 

continues even recently as is shown by the high content of 

carbon dioxide in coal seams in spite of conditions 

favourable for the releasing of gases during coal 

expl oi tati on 

Kotarba, M., 1988,Geochemical criteria for the origin of 

natural gases accumulated in the Upper Carboniferous 

coal-bearing formations in Walbrzych Coal Basin, 

Stanislaw Stas.zic Academy of Mining and Metallurgy 

Scientific Bulletin, 1199,119 p. (in Polish with English 

abstract) 

Kotarba, M,, 1990,Isotopic geochemistry and habitat of the 

natural gases from the Upper Carboniferous Zacl er coalbearing formation in Nowa Ruda coal district (Lower 

Sil esi a, Pol and), Durand, B., and Behar, F., eds., Advances 

in Organic Geochemistry, 1989, v.1, p.549-560. 

*Carbon dioxide-methane index, CDMI = [C02 /(CH4 + 

C02)] 100 (l'/o) 

178 

USE OF HELIUM AND TRITIUM 

ANALYSIS FOR CHARACTERIZING FLUID 

MOVEMENTS IN A CONFINED AQUIFER 

Katzer, T.G., AECL Research, 

Chalk River, Ontario, and 

Noack, M. and Cornett, R.J., 

AECL Research, Chalk River 

Ontario, KOJ lJO, CANADA. 

Helium isotopic compositions 

and tritium concentrations have 

been measured on groundwaters 

collected from multi-level wells 

drilled along a hydrologicallycharacterized, sand aquifer at 

Chalk River Laboratories, 

ontario, Canada to determine the 

migration path, residence times 

eH/ 3He ages) and flow velocities 

for waters in the aquifer. 

Currently, the aquifer is 

recharged by meteoric waters 

which have elevated tritium 

concentrations (-800 TU) due to 

the close proximity of the 

recharge site to a waste 

management area. Helium isotopic 

analyses on groundwaters taken at 

depths of approximately 5 to 20 m 

indicate the waters have 4He 

concentrations similar to that 

expected for a groundwater which 

equilibrated with atmosphere at 

approximately l0°C whereas 

measured 3He concentrations exceed 

solubility equilibrium by a 

faqtor of 10 suggesting a 

substantial tritogenic 3He 

component. The measured 3He and 3H 

concentrations yield 3H/ 3He ages 

of approximately <5 years and 

suggest flow velocities of <1 

mjday which are comparable to a 

flow velocity of o. 63 mjday 

calculated from independant 

tracer studies. The helium 

isotopic and tritium data on 

these groundwater samples 

indicate minimal diffusive loss 

of helium from the groundwaters 

which are taken from a relatively 

shallow, confined aquifer. 
U- Th- Pb SYS1EMA TICS, EARTH ACCRETION AND 

CRUST-~ EVOLUTION 

KRAMERS J D., Min.-Pet. Inst., Univ. of Berne, 3012, 

Switzerland, and TOLSTIKIDN, I.N ., DepL of Earth 

Sciences, University of Cambridge, CB2 3EQ, England. 

A combination of two transport balance models, for 

accretion and subsequent evolution of the Earth, is presented. 

The homogeneous accretion model is based on the scenario of 

Azbel et al (1993) which satisfies siderophile element 

constraints. It is extended by a protocrust. Assuming 60-90% 

impact volatilization Pb loss, Pb partitioning into metal (K0 c. 

6) forming the core, and U, Th enrichment in the upper 

mantle-protocrust system, U/Pb and Th/Pb ratios and U, Th 

concentrations of the upper Earth are reproduced. Formation 

of a protocrust during accretion is essential for this. 

After accretion the upper mantle, lower and upper crust 

are together considered as a closed system. Their estimated 

combined bulk present day Pb isotope composition yields an 

accretion time of 150- 220 Ma (dependent on the chondritic 

U/Pb ratio), in agreement with constraints from the most 

realistic version of the Pu-U-I-Xe model using the same 

scenario (Azbel and Tolstikhin, 1993). No two-stage or 

continuously open Pb evolution model involving an unknown 

low J.L reservoir is required to solve the classical Pb paradox. 

In addition to constraints imposed by present day Pb 

isotope ratios of crust and mantle reservoirs including IDMU, 

modelling of the crust-upper mantle system after accretion has 

to satisfy Pb isotope data from the crust and mantle in the 

geologic pasL Archean provinces show extreme heterogeneity 

in 207Pbf04Pb at similar 206pbf04Pb, which is not seen in 

younger crust. This "banana" aspect of terrestrial Pb history 

can be modelled postulating: (1) The syn-accretion protocrust 

was largely recycled into the upper mantle giving a minimum 

in crust mass at 4 Ga ago. (2) Crust formation dominated over 

recycling prior to about 3 - 2.5 Ga ago, and a near steady state 

total continental crust mass prevailed after that (the recycling 

fluxes used also yield the observed age distribution and the 

mean crustal age of 2 Ga). (3) The nature of crust forming and 

intracrustal processes changed at 3 - 2.5 Ga: Early intracrustal 

melting (large amounts of melt, bulk s/l K0 for U, Th around 

0.2) created high J.L upper crust; later i-c. melting (suspension, 

filter pressing, bulk s/l U, Th K0 closer to 1) differentiated J.L 

values much less. In early crust production processes a bulk 

Pb K0 of c. 0.04 is postulated (amphibole control) which is 

much reduced in later processes (eclogite formation). IDMU 

mantle can thus be produced since the Proterozoic. The interdependence of the set of assumptions is critically examined. 

Azbel, .Y., Tolstikhin, I.N., Kramers, J.D., Pechemikova, 

G.V. and Vityazev, A.V., 1993, Core growth and 

siderophile element depletion of the mantle during 

homogeneous Earth accretion. Geochim. Cosmochim. 

Acta, 57,2889-2898 

Azbel, I.Y. and Tolstikhin, I.N., 1993. Accretion and early 

degassing of the Earth: Constraints from Pu-U-I-Xe 

isotopic systematics. Meteoritics, 28, 609-621. 

179 

CHEMICAL AND ISOTOPIC EVOLUTION OF 

CRYOGENIC CARBONATES: ·A LABORATORY 

STUDY. 

KRISBNAMURTHY,R.V and MACHAVARAM, M 

both at Imtitute for'Water Sciences and Dept. of 

Geology, Western Michigan University, Kalamazoo, 

Michigan 49008, USA 

Calcium carbOnate is one of the most important minerals 

that fonns on the earth's surface at ordinary temperature 

and pressure conditions. The basic over all chemical 

process that drives the precipitation of calcium carbonate 

can be written as: CaCO. + B:zO + CO:z .,. ea++ + 

2HCO;. The CO:z is derived from the soil zones where it 

is generated by plant respiration and decay and the B;zO is 

derived from local precipitation. The carbonate rich 

waters, upon exposure to a region of low partial pressures 

of COlf precipitate CaC03 through degassing of C~ 

Precipitation is also possible through evaporation of water, 

as might be expected in arid regions. 

Yet another mechanism that is known but has received 

less attention is the precipitation of eaco. via freezing of 

carbonate rich waters. This is a significant process in 

many parts of the world which experience long seasons of 

sub-zero temperatures. In order to understand the 

chemi~ and isotopic behavior of carbon of solutions 

undergoing this process we carried out experiments using 

a natural ground water sample. The experiment involved 

freezing and thawing of the solution in succession. The 

CO:z that evolved during each of the thawing stage was 

collected and its carbon isotope ratio determined. The 

results for the evolution of CO:z is shown below (Fig.). The 

chemical and isotopic evolution of freeziDg carbonate rich 

waters would help us determine if the isotopic distributions 

are governed by: 1) Equilibrium fractionation process 2) 

Kinetic fractionation process 3) A Rayleigh type distillation 

process. 

14 

12 

'(i) 10 Q) 

0 

E 8 

::t 

~6 0 

(.) 

4 

2 

0 ~~~~~TT~~~~~~~~~~~ 

0 2 4 6 8 10 12 14 

Extraction sequence 

F~g. 1. CO:z Released During Successive FreeR-Thaw 

Cydes of a Carbonate-Rich Water. Results are 

for a Duplicate Run. 
IDENTIFICATION OF CONCORDANT ZIRCONS 

USING ETCH TECHNIQUES 

J<ROOH. T.E .. Geochronology Laboratory, Royal 

Ontario Museum, Toronto, Ontario, Canada. 

Discordance due to recent Pb loss must be eliminated 

to gain maximum precision in single-age populations and 

to allow primary and secondary ages to be defined in twostage populations. In the latter, grains with the least 

secondary loss are needed to define the primary age. 

Analyses of microgram quantities of selected high quality 

abraded grains (that are invariably low in U) can achieve 

both objectives but in many cases equally perfect grains 

are variably discordant A simple etch technique allows 

large numbers of grains to be screened for U level. 

Instead of a fiSSion track approach we use the grains as 

the detector of radiation damage. Disturbed U-Pb 

systems are avoided because we analyse only the highest 

quality material after abrasion, as documented below. 

Grains remaining from a previous abrasion study (Krogh 

1982), in which 4% discordant multigrain fractions were 

obtained from a 20% discordant population, were used. 

A repeat bulk fraction is 17% discordant (650 ppm U) 

and two abraded single grains are 1.0 and 3.4% 

discordant (230 and 175 ppm U). Two grains selected 

after 25 hr at l10°C in NaOH (room T saturated) are 

1.8 and -2.0% discordant (112, 41 ppm U); the latter with 

a large error due to small Pb content Four grains 

selected after a 3.5 hr, 220°C etch give data that are 

analytically concordant and identical to those previously 

published and are 0.4, 0.3, 0.3 and -0.2% discordant (152, 

20, 58, 154 ppm U). Most grains turned white during 

etching and are excluded. 

A two-stage circa 1.0-1.8 Ga discordia (Krogh 1973) 

defined by 80-90% discordant points (>800 ppm U) was 

used to try to isolate primary age components. Three 

selected high quality grains after abrasion are 59, 44, and 

32% discordant toward 1745 Ma ( 453, 269, 129 ppm U). 

A clear multigrain fraction picked from turbid grains 

produced after a few minutes in room temperature Hf is 

6% discordant (198 ppm U). Two clear fractions selected 

after a 2 hr 220° C Hf etch are 2.0 and 0.3% discordant 

(100, 80 ppm U). A third two-stage high angle discordia 

gave 8 abraded single grain results on a 1430-1740 Ma 

line bounded by 7/6 ages of 1560 and 1632 Ma. 

Progressively up the line U varies from 211, 218, 191, 156, 

and 88 ppm with the latter two selected by maximum 

lustre. The highest two grains were selected after 23 hr 

in a 220°C NaOH etch to maximize leaching but are still 

on the line. 

Etching is especially important in Archean samples 

where the percentage of concordant material is extremely 

low, and where 1000's of two-stage variably discordant 

grains are present 

Krogh, T .B. 1982. Geochimica et Cosmoschimica Acta 

46, pp.637-649. 

Krogh, T.B. and Davis, O.L 1973. Carnegie Institute 

Washington Yearbook 72, pp.601-610. 

180 

40ARf39 AR CONSTRAINTS ON REGIONAL THERMAL 

RESETTING OF ALKALI FELDSPARS FROM THE 

NEW ARK BASIN AND ADJACENT READING PRONG 

BASEMENT 

Krol, M.A. & Zeitler. P K. Department of Earth & 

Environmental Sciences, Lehigh University, Williams Hall 

31, Bethlehem, PA, 18015, USA. 

Alkali feldspar from the Triassic-Jurassic Newark Basin and 

surrounding Precambrian Reading Prong basement gneisses 

were studied using 40 Arf39 Ar thermochronology in order to 

evaluate the thermal evolution of the basin. Utilizing duplicate 

isothermal heating in the early portion of the step heating 

experiments enables us to see through the effects of excess 

argon typical in these early steps. Application of this 

technique provides additional information on the age and 

thermal history these samples have experienced. The use of 

alkali feldspar multidomain theory allows us to evaluate the 

cooling history of these rocks within the temperature range of 

150" -300"C and now possibly to lower temperatures. 

Detrital alkali feldspar from the 225 Ma Stockton Fm. 

yielded minimum ages younger than the stratigraphic age. 

Ages of 155-180 Ma are associated with closure temperatures 

between 115" -170"C and record cooling following a Mesozoic 

thermal event The stratigraphically youngest Boonton Fm. 

(198 Ma) yielded ages consistently older than the depositional 

age, suggesting either post-Alleghenian slow cooling or partial 

resetting due to Mesozoic heating. Ages ranged from 225 Ma 

to over 550 Ma with corresponding closure temperatures of 

110· to 24o·c. Small domains from samples of Precambrian 

Reading Prong gneisses with closure temperatures of 110• to 

25o·c record minimum ages between 131-260 Ma. These 

minimum ages may record cooling following a Mesozoic 

thermal event or may be consequence of erosional unroofmg. 

In addition, a low temperature alkali feldspar extracted from a 

calcite vein within the Cambro-Ordovician Beekmantown 

carbonate associated with the early Paleozoic Friedensville zinc 

deposit. Small domains are associated with closure 

temperatures of 135•-18o•c; corresponding to ages of 155-180 

Ma. These ages indicate cooling following the Mesozoic 

thermal pulse providing a minimum temperature and time of 

this event. 

The alkali feldspar data provide minimum estimates on the 

time and temperature of a Mesozoic thermal event. This 

thermal event probably occurred prior to the initiation of seafloor spreading ( -175 Ma) and was most likely the result of 

elevated heat flow due to lithospheric thinning and localized 

hydrothermal fluid circulation. Additional work is in progress 

in which we are evaluating the lateral and stratigraphic extent 

of this thermal event within the rift basin. 
PRECISE DATING OF GRANITOIDS BY SINGLE 

ZIRCQN GEOCHRONOLOGY: A NIGHTMARE! 

KRONER. A. and JAECKEL, P., Institut fiir 

Geo-wissenschaften, UniversiHit Mainz 55099 

Mainz, Germany. ' · 

The dating of granitoid rocks used to be relatively 

simple matter when Rb-Sr whole-rock and U-Pb 

multigrain analyses were used and when errors were 

i~ the o~der of seve:al tens of millions of years. 

Smgle zucon analysis has changed this belief by 

documenting the presence of variable amounts of 

inherited ~en<?c~sts in virtually every granitoid rock, 

thus m~mg 1t difficult to obtain the precise age of 

magmati~ emplacement. We have used single zircon evapo~atlon and the vapor digestion technique in the 

analysis of Archaean granitoids from southern Africa 

and late Proterozoic to early Palaeozoic granitoids 

from the eastern margin of the Bohemian Massif in 

east-central Europe to document extraordinaryly 

com~le~ zircon populati~ms. Both I-type and S-type 

gramtoids have older zircon xenocrysts, revealing 

precursor reservoirs which are more complex than 

commonly thought. Zircon morphology sometimes 

helps to distinguish between magmatic and 

xenocrystic zircons, but we also found perfectly 

euhedral and homogeneous varieties which were 

inherited. Mixture of such grains with magmatic 

zir~ons in .mult~grain analysis tends to produce ages 

which are Invanably too high. 

ITO-granitoids from the Barberton and Swaziland 

granite-greenstone terrain in southern Africa have 

ages between -3.55 and 3.2 Ga and contain zircon 

xenocrysts as old as 3.7 Ga. These rocks are derived 

from a -3.~-3.7 Ga old isotopically very homogeneous reseryorr, as do?ul?ented by Sm/Nd analyses. 

Volummous granitoids from the Lusatian terrane in 

eastern Saxony, Germany, have magmatic zircon 

ages between -540 and 570 Ma and contain xenocrysts varying in age between 706 and 2020 Ma. 

These rocks are derived from melting of older continental crust as also revealed by eNd(t) values of -4 to 

-19. One sample has more than 95% xenocrysts. 

I- and S-~ype gra~it?ids from the eastern margin of 

the Bohemian Massif m eastern Czech Republic have 

magmatic emplacement ages between 503 and 515 

Ma and contain a voluminous and variable xenocryst 

population with ages between 1100 and 2070 Ma. 

The~e granitoids. prob~bly formed along an active 

contmental margin dunng Caledonian ocean closure 

and microcontinent accretion. 

In all these cases conventional multigrain analysis 

would not haye enabled the precise emplacement ages 

to b~ ~stabhshed. I~ su~ary, precise dating of 

gramtmds frequently IS a mghtmare and requires caref~l study ?f zrrco~ morphology and time-consuming 

smgle gram or gram-domain analysis. 

181 

ISOTOPIC TRACING OF ANTHROPOGENIC 

SULPHATE & WATER/ROCK INTERACTIONS IN 

CONTAMINATED GROUND WATER 

KROUSE, H. R., Department of Physics & Astronomy, 

VAN OONKELAAR, C and HUTCHEON, I, Department 

of Geology & Geophysics, University of Calgary, 

Calgary, Alberta, TIN 1N4 Canada 

The environment surrounding sour (H2S) gas processing 

plants can be impacted by industrial sulphur loading. 

Objectives of this study were: 1) to assess whether o34s was a 

conservative tracer of anthropogenic sulphate; 2) to use &34s 

to trace anthropogenic sulphate in ground water surrounding a 

sour gas plant, 3) to understand how the ground water 

chemistry of the system had been altered by industrial sulphate 

loading. 

The presence of sulphate reducing bacteria (SRB's) in 

ground water was a concern because they cause ~nrichmcnt of 

sulphate o34Ssulphate due to preferential32s1604- reduction 

and would affect the use of o34s as a tracer. Bacteria were not 

active enough to shift significantly the o34s of the ground 

water sulphate, nor did o34s values of incubated ground water 

samples become enriched with o34s over seven months. 

Further, o180 enrichment in sulphate was not found. Finally, 

ground water at the site was oxygenated (dissolved oxygen 1.1 

mg/1). Under aerobic conditions, bacteria that use oxygen in 

water dominate over those that use oxygen from dissolved 

sulphate. 

Sulphate from gas processing enters ground water through 

runoff from the processing facility, precipitation and leaching 

from the sulphur storage area. At this site anthropogenic 

sulphate ~as a o34S of +16%o (CDT), while naturally occurring 

sulphate IS depleted to about -1 O%o. As the S04 concentration 

in ground water increases, &34s approaches that of the 

anthropogenic source. 

Water/rock interactions are dominated by the acid that 

originates from the gas plant and the minerals present in the 

soils. Shallow ground water at this site moves through a 

heterogeneous glacial deposit that contains clasts of limestone 

dolomite and shale and some water samples are shifted away • 

from the global meteoric water line. The enrichment in o18o 

that this shift represents is indicative of evaporation due to the 

aridity of this region and calcite dissolution, resulting from 

acidic conditions created by sulphate release. As S04 

concentration rises, and Ca increases due to calcite dissolution 

waters approach saturation with gypsum. • 

When the difference between natural and industrial &34s 

is larg.e and sulphate reducing bacteria are remittent, o34s is an 

effecuve tracer of anthropogenic sulphate in ground water. 

The chemical and isotopic composition of ground waters 

shows strong indications that the addition of sulphate to the 

water causes acidic ground water conditions that are rapidly 

buffered by dissolution of calcite or other carbonate minerals. 
THE ISOTOPIC CHARACTER OF THE BUSHVELD 

COMPLEX MAGMAS. 

KRUGER, F.Johan., Bernard Price Institute 

(Geophysics), University of the Witwatersrand, WITS 

2050, South Africa. 

0, Sr, Os, Nd and Pb isotopic data indicate that the 

magmas of the Bushveld Complex contained variable proportions of crustal material prior to intrusion into the 

magma chamber. 

Oxygen isotopic data indicates that there was a significant crustal content in the magmas higher in the succession and especially in the magma injected into the Main 

Zone. A strong positive correlation of very radiogenic 

87Sr/86Sr initial ratios with the 8180 values also supports 

this model. 

Magmas with both high and low Sr concentrations and 

Sr isotopic ratios were injected into the magma chamber. 

87Sr/86Sr ratios suggests that the first magma was less radiogenic (c. 0. 7044) and that subsequent additions were 

more radiogenic (0.7085-0.7090). In the Lower Zone to 

Main Zone interval, at least three magma types can be 

identified which were derived from the subcontinental 

upper mantle and a continuum of "crustal" sources. A 

fourth mafic magma is needed to account for the Upper 

Zone. 

Os isotopic data on the UGl, UG2 and Merensky reefs 

indicate a substantial crustal component in the magmas 

from which these rocks are derived. Nevertheless, the 

mantle remains the main source ofPGE. 

Nd isotopic data and REE profiles indicate a slightly 

enriched source for the Bushveld magmas. Bushveld rocks 

have LREE enriched profiles, but low absolute REE concentrations, and the Nd isotopic data also indicate a 

slightly enriched source. This is in contrast to the Sr and 

Os isotopic data which indicate a highly enriched but 

"unsupported" source. This is envisaged to be due to the 

compatible nature of Sr and Os during crustal melting so 

that the concentrations of these two elements are strongly 

enriched, whereas the incompatible REE were depleted in 

the residue after melting. Therefore the REE concentrations in the crustal residue were comparable to those of the 

mantle magmas and hence the crustal signature is subdued. 

Pb isotopic data is positively correlated with the Sr isotopic data, and the large discontinuity at the Merensky 

level to more radiogenic (crustal) values also argues for a 

strong crustal imprint in the Main Zone consistent with the 

other isotopic data. 

The available data indicates that there was minimal assimilation of roof rocks by the magmas in the chamber, 

except by the Lower Zone magma which was hot and undersaturated in silica. The lack of in situ assimilation indicates that the Bushveld magmas were stratified and that 

magma addition occurred mainly at the base of the chamber as fountains which caused minimal distl,Jibance of the 

interface between the layered mafic magmas and the overlying "granophyric" magmas or rocks. The process of 

magma addition thus continuously elevated the residual 

magmas in the chamber. 

182 

26AI AND lOse IN niE OCEAN 

KU. Teh-Lun&. WANG, Lei and LUO, Shangde, 

Dept. of Earth Sciences, University of Southern 

California, Los Angeles, CA 90089-07 40, USA 

SOUTHON, John, Lawrence livermore National 

Laboratory, Livermore, CA 94550, USA 

KUSAKABE, Masashi., Japan Marine Science 

and Technology Center, Yokosuka, 237 Japan 

The long-lived cosmogenic 26AI (half-life 0.7 Ma) 

and lOJJe (1.5 Ma) enter the ocean chiefly via wet/dry 

fallout. Their ratios in marine sediments can be used 

as powerful time and process tracers for problems 

such as marine geochronology and productivity 

changes in the past. A pre-requisite to the usage is the 

better understanding of the comparative oceanic geochemistry of these isotopes. 

In this presentation, we report the first watercolumn profile (upper 1 km) of 26 AIP7 AI in the sea. 

The 26AV10Be ratio in the surface Pacific near 12°S 

and 135°W is estimated to be approximately 3 x 104. 

This is about an order of magnitude lower than the 

atmospheric production ratio and the ratio measured in 

rain water. The low surface ocean ratio may reflect 

the relatively intense particulate removal of AI relative 

to Be. The source of 26 AI in the ocean, similar to that 

of lOBe, comes· mainly from production in the 

atmosphere rather than from production in surface 

rocks on the continents. 

We have also measured the authigenic 26 AIP7 AI 

and 10BeJ9Be ratios in deep-sea sediments using a 

NaOH-leach technique. The authigenic 10BeJ9Be 

ratios extracted from surface sediments are close to 

10Be/9Be measured in the overlying sea water, 

indicating that isotopic equilibrium for Be exists 

between deep water and authigenic sediment phases. 

The ratio of -4 x 10-14for26AIP7AI extracted from a 

Pacific sediment is similar to that found in the 

authigenic phillipsites. The results further indicate 

that ( 1) depositing deep-sea sediments incorporate 

26 AI and 1 OBe in a ratio close to their atmospheric 

production ratio, and (2) the authigenic 27 AI and 9Be 

constitute, respectively, -15% and -40% of the total 

27 AI and 9Be in the sediments studied. 

Our results point to the good feasibility of using 

26 Alfl OBe as an age-dating tool and as a proxy for 

paleoproductivity. 
ISOTOPIC STUDIES ON GROUNDWA TERS IN 

SOUTHWESTERN MAHANADI DELTA, ORISSA 

STATE, INDIA. 

KULKARNI, K.M., Isotope Division, 

Bhabha Atomic Research Centre, 

Bombay, MS, 400085, India, RAO, 

S.M., Isotope Division, Bhabha 

Atomic Research Centre, Bombay, 

MS, 400085, India, and NAVADA, 

s.v., Isotope Division, Bhabha 

Atomic Research Centre, Bombay, 

MS, 400085, India. 

As part of a drinking water 

supply project isotope geochemical 

investigation was launched in 

Delang - Puri sector of Orissa. 

These studies were aimed at 

determining the origin of salinity 

and evaluating groundwater recharge 

conditions in the coastal, 

alluvial, multi-aquifer system. 

In this system fresh, brackish 

and saline groundwaters occur in 

rather complicated fashion. The 

conditions change from phreatic to 

artesian flowing type with 

increasing depth. The groundwater 

flow is towards south, i.e., 

towards the sea. 

The environmental stable isotopes 

of hydrogen, oxygen, carbon, 

sulphur and radioisotopes of 

hydrogen and carbon as well as 

minor and trace ions were studied. 

The findings based on isotope 

geochemical, sedimentological and 

paleontological studies indicate 

that a) shallow groundwaters are 

relatively young and are recharged 

through floodplains and outcrops of 

basement rocks, b) the groundwater 

salinity in aquifers occurring in 

the intermediate zone (depth 50-100 

m) is largely due to Flandrian 

transgression during the Holocene, 

c) deep fresh and young groundwater 

forms a well developed aquifer 

which receives recharge through 

basement rocks. This could form 

main source of groundwater for 

exploitation, d) the deep saline 

groundwaters are marine waters 

entrapped due to eustatic level 

rise during the late pleistocene 

interglacial stage. 

The stratification of saline 

groundwaters is also observed. 

The study is expected to help in 

a better choice of well sites. 

183 

PROGRESS ON THE PREPARATION OF THE PROPOSED 40Arj3!'Ar STANDARD MMhb-2, PLANS FOR ITS 

CALIBRATION, AND INTERLABORATORY 

CALIBRATION OF ARGON FACILITIES. 

KUNK. M.J., U.S. Geological Survey, MS 981, Reston, VA 

22092, DALRYMPLE, G.B., U.S. Geological Survey, MS 

937, Menlo Park, CA 94025, and SNEE, L.W. U.S. 

Geological Survey, MS 963, Denver, CO 80225 

Intralaboratory precision of 0.1-0.25% for ages measured by 

modem 40Arfl9

Ar techniques is commonplace. However, 

comparison of high-precision ages between laboratories is 

made difficult because no high-precision interlaboratory 

standard is available and because no systematic interlaboratory 

calibration program has been designed. Because the supply of 

MMhb-1 (Alexander et al., 1978; Samson and Alexander, 1987) 

has been depleted, we are currently in the process of calibrating 

the conventional K-Ar age of MMhb-2, a new preparation of 

the international 40 ArP9 Ar hornblende standard from the 

McClure Mountain Syenite. More than 3 kg of high-purity 

hornblende, MMhb-2, has been prepared and split into 336 

vials, each weighing about 10 g. Calibration of the potassium 

concentration of MMhb-2 by isotope dilution is in progress at 

the National Institute of Standards and Technology (NIST). The 

precision and accuracy of the K concentration measurements 

are expected to be 0.1% and 0.25% or better, respectively. 

Calibration of the 40Ara concentration of MMhb-2 will be 

done jointly by NIST and the USGS. NIST will calibrate a 

pipette system that will deliver aliquots of high-purity 38 Ar with 

precision and accuracy of 0.1% and 0.25% or better, 

respectively, of the molar quantity of argon delivered. NIST 

will also calibrate three pipette systems containing artificial 

mixtures of 40Ar and 36Ar with ratios of 1:1, 30:1, and 300:1. 

The isotopic composition of the artificial mixtures will be 

known to a similar level of precision and accuracy as the 38 Ar 

molar quantity. The artificial 40Arfl6Ar mixtures will be used to 

measure the mass discrimination of the spectrometers to be 

used, prior to using the 38Ar pipette in measuring the 40Ara 

concentration of MMhb-2 in USGS laboratories. In the process 

of making these measurements, the isotopic composition of 

atmospheric argon will be re-measured. 

Upon completion of calibration, MMhb-2 will be certified by 

NIST for potassium and argon concentrations, and NIST will 

issue a Standard Reference Material Certificate for it. 

Distribution of the MMhb-2 standard will be through the NIST 

Standard Reference Material Program. 

The NIST-calibrated 38Ar and 40Arfl6Ar pipette systems will 

be constructed of all metal components and are designed to be 

transportable. These pipette systems, together with MMhb-2 

and the re-measured atmospheric 40 ArP6 Ar, could form the basis 

for a systematic interlaboratory calibration program that would 

dramatically improve 40 Arfl9 Ar interlaboratory precision. 

Alexander, E.C., Jr., Mickelson, G.M., and Lanphere, M.A., 

1978, Mmhb-1: a new 40 Arfl9 Ar dating standard, in Zartman, 

R.E., ed., Short papers of the fourth international conference, 

geochronology, cosmochronology, isotope geology 1978: 

U.S. Geological Survey Open-File Report 78-701, p.6-8. 

Samson, S.D.and Alexander, C.A., 1987, Calibration of the 

interlaboratory 40Arfl9 Ar dating standard, MMhb-1 :Chemical 

Geology (Isotope Geoscience Section) v. 66, p. 27-34. 
SURFACE CHRONOLOGY FROM COSMOGENIC 

NUCLIDES: THE ANTARCfiC GLACIAL RECORD 

KURZ, M.D., ACKERT, Robert, Dept of Marine 

Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, 

USA., and BROOK, Edward J., Graduate School of 

Oceanography, University of Rhode Island, Narragansett, 

Rhode Island 02882, USA. 

Glacial deposits in the Dry Valleys region of Southern 

Victoria Land, contain an important record of advance and 

retreat of the great Antarctic ice sheets. The unique climate 

and extremely slow erosion rates result in remarkable pre-

. servation of the moraine surfaces and make them ideal for 

surface exposure dating. We have measured surface exposure ages in different moraines and bedrock surfaces, using 

in situ produced cosmogenic 3

He, 10se, and 26 Al in olivine, 

quartz and clinopyroxene. The results demonstrate that the 

technique can be utilized for surfaces as young as several 

thousand years and as old as millions of years. We have 

evidence of surfaces greater than 5 million years in age from 

the Sirius formation on Mount Fleming. However, these 

ages must be viewed as minimum ages because of helium 

leakage from quartz, and the restrictive half lives of 1

0se 

and 26 Al. These limitations are being overcome by the use 

of 21Ne on these samples. Ages from pleistocene moraines 

are particularly important, as they can be used to directly 

constrain ice sheet reconstructions. This is demonstrated by 

new results from Mt. Morning, a Cenozoic volcano on the 

southern side of McMurdo Sound, where interbedded glacial 

drift and lava flows record past ice levels in the Ross Sea. 

By combining surface exposure dating with field mapping 

and stratigraphy we have found evidence of three different 

glaciations on the North-east flank of Mt. Morning. Absolute chronology provided by 3

He dates indicates that the 

deposits are marine isotope stage 6 (160 Ka), stage 4 (80 

Ka) and stage 2 (< 28 Ka). The youngest drift (stage 2) 

overlies a well preserved lava flow (dated at 28 Ka), and a 

well preserved stage 4 moraine. The upper limit of the stage 

6 deposit is the highest (approximately 456 meters), which 

is consistent with sea level records. The upper limit of the 

stage 2 deposit is at lower elevation than stage 6, but at a 

higher elevation than the stage 4 moraine, suggesting that 

the Ross Sea ice levels at this time were intermediate. As 

expected from this stratigraphy, exposure ages from the 

stage 4 moraines are scattered, because the surfaces have 

been partially reworked by the younger glaciation. These 

results demonstrate that surface exposure dating, combined 

with glacial stratigraphy, can be used to constrain past ice 

geometry, and hence to correlate the terrestrial glacial 

record with marine and ice core climate records. 

184 

Sr, Nd AND Pb ISOTOPIC VARIATIONS IN THE 

MARIE BYRD PLUME, WEST ANTARCTICA 

KYLE. P.R. 1

, PANKHURST, R.2, MUKASA, S.3

, 

PANTER, K.t, SMELLIE, J.2

, MCINTOSH, W.1 

1 Geoscience, N.M. Tech, Socorro, N.M. 87801 2 British Antarctic Survey, Cambridge, U.K. 3 Geol. Sci, Univ Michigan, Ann Arbor, MI 48109 

Alkalic volcanism in Marie Byrd Land (MBL), West 

Antarctica is dominated by 18 major and many minor 

volcanic centers ranging in age from 35Ma to presently 

active. The formation ages for the major volcanoes 

young progressively outwards in a radial circular pattern 

with a diameter of 600 km. The four youngest volcanoes 

Mt. Takahe (7-40ka), Mt. Waesche (<0.5Ma), Mt. Siple 

(<2Ma) and Mt. Berlin (active) are located 90° apart 

around the outer edge of the volcanic province. Ranges 

of volcanoes show well-defined linear younging trends. 

The elevation of a prevolcanic erosion surface, although 

poorly exposed, also shows an ill-defined radial pattern. 

We interprete the radial patterns of age and elevations as 

a single large stationary mantle plume beneath the 

stationary Antarctic plate. 

Basaltic samples from Mts. Aldez (19Ma), Takahe 

and 5 centers in the Executive Committee. Range (ECR) 

(13 to <0.5Ma), show a small spread in 87Srf6

Sri (0. 7027 

- 0.7038, n= 34) and 143Nd/144Ndi (0.51277 - 0.51292, 

n=25). There is no evidence of crustal contamination 

except in highly evolved comenditic and trachytic rocks. 

Seven samples from 3 different volcanoes show 

significant variations in Pb isotopes: 206Pbf04Pb (19.52 -

20.47), 207Pbf04Pb (15.65 - 15.72), 208Pbf04Pb (39.10 -

39.94). The data define a distinct linear array above the 

NHRL on a 207Pbf04Pb vs 206Pbf04Pb plot. 

The overall low 87Srf6Sri, distinct 87Srf6Sri vs 143Nd/144Ndi field and radiogenic Ph isotopes clearly 

demonstrate a mantle source dominated by a HIMU 

component. Mt. Takahe has higher 87Srf6Sri (0.7033 -

0.7038) and lower 143Nd/144N~ (0.51277- 0.51279) than 

other MBL volcanoes and is interpreted as mixing of 

plume-derived HIMU-rich mantle with lithospheric 

mantle as the plume head flowed horizontally along the 

base of the lithosphere. Similar processes could also 

account for some of the Sr and Nd isotopic variations at 

Mt Sidley (5Ma) in the ECR. 

The strong HIMU isotopic signature of the MBL 

volcanics is also found in the Late Cenozoic alkali 

volcanics in the western Ross Sea and the Balleny 

Islands. This suggests the presence of a large and distinct 

mantle reservoir under Antarctica and the Southern 

Oceans extending for over 3200 km. 
SEEING THROUGH THE EFFECTS OF HYDROTHERMAL ALTERATION USING Nd ISOTOPES: AN EXAMPLE FROM THE ABU KHRUQ COMPLEX, EGYPT 

LANJ)QLL J.D., and FOLAND, K.A., Dept. of Geological Sciences, Ohio State University, Columbus, OH 

43210, USA, and HENDERSON, C.M.B., Dept. of Geology, The University, Manchester, England, M13 9PL. 

The Abu Khruq complex is a multi-intrusion, alkaline ring 

complex located in the Eastern Desert of Egypt. This 89 Ma 

complex consists of a gabbroic body which was intruded by 

rings of nepheline-, alkali-, and quartz syenite. This association of both quartz and critically undersaturated syenites is 

interesting in that it is not explicable by closed-system fractionation. The complex experienced sub-solidus hydrothermal 

alteration by circulating meteoric waters that modified the 

syenites isotopically and apparently, in part, chemically. Both 

0 and Sr isotopes were so greatly affected that they cannot be 

used to address petrogenetic questions, in particular the role of 

crustal materials. The present study focuses on Nd isotopes in 

an effort to: examine the degree to which Nd may also have 

been modified hydrothermally; and, attempt to address the 

petrogenesis of Abu Khruq by avoiding the alteration that 

affected other isotope systems. Small, but significant variations are observed in calculated initial 14~d/ "'Nd ratios. The 

variations occur not only between rocks of different units, but 

also among rocks of the same unit, particularly the quartz 

syenites. Initial 14~d/ "'Nd ratios range from: 0.512721 to 

0.512748 for the gabbros; 0.512739 to 0.512750 for alkali 

syenites and trachytes; 0.512717 to 0.512755 for the nepheline syenites; and 0.512706 to 0.512732 for the quartz syenites. During hydrothermal modification, Nd appears to have 

been immobile as indicated, in part, by: a lack of correlation 

of 14~d/ ~d ratios with apparent degree of alteration as 

judged from petrographic and isotopic indicators; correlations 

of 14~d/ 4<Nd with major element variations; and, a limited 

range in initial 14~d/"'Nd ratios that would otherwise require 

uniform alteration of all rocks. The isotope ratios appear to 

represent primary magmatic signatures and thus can be used to 

address the petrogenesis of Abu Khruq. The parental melt for 

the complex had 14

~d/4<Nd of 0.512750 (£Nd = +4.4) and an 87Srf6

Sr of 0. 7030, indicating derivation from a depleted 

mantle source. The variations in initial 14~d/ 4<Nd ratios 

indicate that the parental magmas of the complex experienced 

input of crustal material, apparently by AFC processes. Furthermore, the quartz syenites are clearly more affected whereas the nepheline syenites experienced little or no contamination. This indicates an important a link between the contamination and rock composition and provides the explanation for 

the association of over- and undersaturated syenites. The 

results further demonstrate that Nd isotope ratios can provide 

important petrogenetic information for hydrothermally altered 

rocks even when other isotopes are severely affected. 

185 

A COMPARISON OF CONVENTIONAL K-AR 

AND 40ARJ39 AR DATING OF YOUNG MAFIC 

VOLCANIC ROCKS 

LANPHERE, MARVIN A., Branch of Isotope 

Geology, U.S. Geological Survey, 

Menlo Park, CA 94025 

Conventional K-Ar dating has been the typical 

method for measuring ages of mafic volcanic rocks 

younger than 1 Ma. The 40 Arf39 Ar technique was 

difficult to apply to these rocks because fairly large 

samples were required. However, the availability of 

ultra-clean, ultra-sensitive rare gas mass 

spectrometers and low-blank resistance-heated 

furnaces have significantly extended the application 

of the 40 Arf39 Ar technique to young volcanic rocks. 

Measurements to compare these two methods were 

made on andesite and basalt from Crater Lake, OR 

(CL) and Mount Adams, W A (MA) and on basalt 

from the Snake River Plains, ID (SRP). 

Conventional argon measurements were made by 

isotope dilution on samples of approximately 25 g; 

K20 was determined by flame photometry. 40 Arf39 Ar 

incremental-heating experiments were made on 

samples of approximately 100 mg. Representative 

results on eight lava flows from these areas are as 

follows: 

Age (ka) 

Conventional K-Ar 40 Arf39 Ar plateau 

CL: 106±7 117±3 

306 ± 7 303 ± 3 

SRP: 565 ± 14 567±6 

626 ± 67 596 ± 18 

619 ± 22 619 ±9 

MA: 33 ± 14 51± 3 

106 ± 25 107 ±3 

251 ±6 241 ± 12 

Ages determined by the two techniques agree in most 

cases within analytical uncertainties (1 cr). 

Conventional ages are the mean of two or more Ar 

measurements and four K20 determinations. The 

40 Arf39 Ar plateaus contained between 49 and 98 

percent of the 39 Ar released. 

The results indicate that high-quality ages can be 

measured on young, mafic volcanic rocks using either 

the conventional or 40 Arf39 Ar techniques. The 

precision of an 40 Arf39 Ar plateau age generally is 

better, but comparable precision probably can be 

obtained by pooling 4-6 conventional K-Ar 

measurements. 
HIGH RESOLUTION Ar/ Ar CHRONOLOGY OF 

PRFDAZZO MAGMA TIC CO~ (SOUTHERN ALPS, 

ITALY) 

LAURENZI. M.A., Istituto di Geocronologia e Geochimica 

Isotopica, CNR, 56127 Pisa, Italy, VISONA' D, Dip. Scienze 

della Terra, 35100 Padova, Italy, ZANfEDESCHI C., C.S. 

Oeodinamica Alpina, CNR, 35100 Padova, Italy. 

Predazzo magmatic complex comprises a volcanic sequence 

intruded by composite plutonic bodies at very shallow crustal 

level. These bodies, which display an anular shape, have been 

grouped into four families, corresponding to the supposed 

intrusive sequence (Lucchini,1967): 1) monzonites, by far the 

more widespread lithotype, monzodiorites and dior~:~ (and 

subordinate gabbros and cumulitic pyroxenites); 2) 

leucomonzonites; 3) syenites; and 4) biotite bearing and 

tourmaline bearing granites. A new detailed field work has 

shown a more complicate situation and this sequence is not 

strictly true in the entire area. 

A tentative to discriminate small differences in age among 

different pulse of the multiple intrusion is the aim of this 

research. Previous radiometric investigations gave an age of 

242 Ma for granites (Rb/Sr isochron on wr and related 

minerals, after Borsi and Ferrara, 1967, recalculated with the 

new decay constant). Amphiboles and biotites were taken into 

consideration for this purpose; all samples were irradiated 

together in nearby position to minimise source of errors which 

are of the order of 5 per mil. ' 

Biotite ages, obtained on few lithotypes, are restricted into a 

narrow interval of time (232-235 Ma).Amphiboles data, which 

cover the majority of the area, are slightly more widespread. 

Some of their age spectra are not ideal, but a massive presence 

of massive parentless 40 Ar can be excluded. The observed 

discrepancies are likely due to the presence of biotite exsolution 

in amphiboles, as shown by variation in the CalK ratios of 

different temperature interval. 

Older ages (237 and 236 Ma) are obtained for a 

leucomonzonite and a monzodiorite in different sectors of the 

intrusion; another group of data, referred to more than one rock 

type, is restricted to an interval between 232 and 230 Ma. The 

more basic members of the magmatic body, among the first to 

be intruded in the classical reconstruction of this area, show 

slightly lower ages. Since their intrusion as dike cannot be 

excluded at this stage of research, radiometric data seem to 

fulfill this hypothesis. 

Obtained ages display an emplacement of the composite pluton 

spanning over an interval of time of about 7-8 Ma. 

Borsi S. and Ferrara G; 1967, Determinazione dell'eta delle 

rocce intrusive di Predazzo coni metodi del Rb/Sr e K/Ar: 

Miner. Petrogr. Acta, 13,45-66. 

Lucchini, 1967, Studio petrografico degli affioramenti sienitici 

del M.te Mulat (Predazzo, ltalia settentrionale): Miner. Petrogr. 

Acta, 13, 195-215. 

186 

THE CONTINENTAL SHELF RECORD OF 

UPWELLING, SEA LEVEL AND ICE VOLUME FROM 

THE S.E. ATLANTIC 

LA YELLE. M,, ARMSTRONG, RA., HARRIS, C., 

DE WIT, MJ., Department of Geological Sciences, 

University of Cape Town, Rondebosch 7700, South Africa, 

and GIRAUDEAU, J., South African Museum, P.O.Box 

61, Cape Town 8000, South Africa. 

The cold water Benguela System, off the west coast of 

southern Africa, forms one of the world's major eastern 

boundary currents. Strong offshore winds, related to the 

South Atlantic anticyclone, drive intense upwelling on the 

inner and middle shelf. 

Six sediment cores, up to 6 m long, have been sited using 

· high-resolution seismic data in water depths ranging from 96 

m to 175 m in the southern Benguela System. 

Palael)ceanographic reconstructions are based on the analysis 

of K-Ar, strontium and oxygen isotope data, as well as the 

development of benthic and planktonic foraminiferal transfer 

functions. Data on bottom temperature, salinity, dissolved 

oxygen and organic matter, as well as surface temperature 

and salinity have been used to examine regional 

palaeoceanographic changes, as the South Atlantic responded 

to glacial fluctuations in the Quaternary. 

Strontium isotope, K-Ar and biostratigraphic dating of the 

basal carbonate and glauconitic sands indicates an age of 

24Ma. Oxygen isotope data obtained from bulk carbonate 

samples reveals a sea-bottom isotopic signature 2-3%o lighter 

than the present day. This carbonate unit has since been 

correlated around the entire sub-continent, and is believed to 

be a marker for the Late Oligocene - Early Miocene global 

warming event. 

A major erosional unconformity separates the basal 

Tertiary units from the overlying Quaternary deposits. 

Coccolith and oxygen isotope data reveal deposition of the 

lowest sequence taking place during the transition from 

isotope stage 10 to 9 ( -340Ka). Extensive storm erosion 

during low sea levels has resulted in the preservation of 

transgressive and interglacial deposits only. Glacial sequences 

are marked by distinct erosional surfaces and reworked shell 

horizons. 

Sea level variations have been investigated utilising 3 

independent techniques: (1) from an estimate of ice volume 

from our sea bottom calculations of 6180(seawater); (2) from our 

benthic foraminiferal transfer function; (3) from 

measurements of coastal onlap delineated by high resolution 

seismics. Sea level estimates obtained for the last 5 glacial 

cycles are all remarkably similar; sea level having dropped to 

between -120 m and -150 m during each regression. Sea level 

recovery during interglacial periods is estimated to be similar 

to modern mean sea level. 

Sea bottom temperatures during oxygen isotope stages 9 

and 7 (340Ka to 186Ka) were similar to modern Benguela 

values. Stages 5 and 3 (128Ka to 24Ka) however, show the 

Benguela System as being significantly warmer (by up to :ZOC) 

than previous or modern day values. Variable upwelling 

intensity, perhaps linked to the magnitude of the glacial cycle, 

is considered to be the driving mechanism of 

palaeoceanographic change in the Benguela System. 
40Ar;39Ar DATING OF EOCENE OBSIDIAN FROM THE 

BATZATENA ARCHEOLOGICAL SITE, WEST-CENTRAL 

ALASKA 

LAYER Paul W., Geophysical Institute, University of 

Alaska, Fairbanks, Alaska 99775, USA. 

The BatzaTena (Indian River) obsidian site in westcentral Alaska is thought to be the source for many obsidian 

artifacts found in northern Alaska. This site was first 

described by Patton and Miller (1970) who speculated that the 

deposit was mid-Tertiary in age based on stratigraphic 

relationships. The BatzaTena obsidian occurs as subround 

pelletal fragments up to 10 em long and are thought to have 

formed as residual inclusions ('marekanites' or 'Apache tears') 

in hydrated perlitic volcanic !Jass. Several fresh obsidian 

fragments were analyzed by 0 Ar/39 Ar step heating. The 

samples show no evidence of alteration and age spectra from 

the samples are flat with an average plateau age of 41 Ma. 

This age is consistent with stratigraphic controls and probably 

reflects the eruptive age of the obsidian. 

If 41 Ma obsidian is unique to this site, then these results 

may be used along with Instrumental Neutron Activation 

Analysis, X-Ray fluorescence, microprobe and hydration 

measurements (compiled by John Cook at the Bureau of Land 

Management) to provide a distinctive 'finger-print' to identify 

artifacts derived from BatzaTena. Laser 40 Ar/39 Ar dating 

techniques would allow small (<1 mm) fragments from actual 

artifacts to be dated without destroying the entire samfle. 

There are few accounts of reliable 40 Art3 Ar ages 

obtained from terrestrial glass samples older than -10 Ma. 

The BatzaTena study indicates that reliable ages may be 

obtainable from old obsidian elsewhere. 

~40 

< 

BatzaTena obsidian 

3 8 plateau age: 41.50 + 0.36 Ma 

0 0.2 0.4 0.6 0.8 1.0 

Fraction of 3 9 Ar released 

Patton, W.W. Jr. and Miller, T.P., 1970, A possible bedrock 

source for obsidian found in archeological sites in 

northwestern Alaska: Science, v. 21, p. 760-761. 

187 

779 Ma MAFIC MAGMATISM IN THE 

NORTHWESTERN CANADIAN SHIELD AND 

NORTHERN CORDILLERA: A NEW REGIONAL 

TIME-MARKER 

LeCHEMIN ANT. A.N ., Geological Survey of Canada, 

601 Booth St., Ottawa, Ont. KIA OE8, Canada, and 

HEAMAN, L.M., Department of Geology, Royal Ontario 

Museum, Toronto, Ont. MSS 2C6, Canada. 

Precise dating of Neoproterozoic diabase dykes and gabbro 

sheets and sills emplaced during formation of the western rift 

margin of North America can determine the timing, duration 

and sequence of rifting and can establish time-markers to test 

plate reconstruction models. U-Pb baddeleyite ages have been 

obtained for two gabbros that are representative of a parallel set 

of NE-striking sheets within crystalline rocks of the Great Bear 

magmatic zone in the northwestern Canadian Shield. 

Baddeleyite ages have also been obtained for a gabbro sill 

emplaced within the Tsezotene Formation in the Mackenzie 

Mountains and for a N-striking diabase dyke that cuts the 

Tuchodi Formation in the Muskwa Range, northeastern British 

Columbia. Results indicate that intrusion of all four mafic 

rocks was synchronous and occurred at 779 ± 2 Ma. This 

age establishes a new time-marker extending more than 1000 

km from the western edge of the Slave Province into the 

northern Canadian Cordillera. 

Hansen et al. {1993) suggest that the Neoproterozoic rift 

margin of western North America consists of alternating 

highly extended and less extended segments. The dated 779 Ma 

diabase and gabbros are within or adjacent to the proposed 

extended segment in the northern Cordillera. We interpret the 

779 Ma mafic magmatism as a response to large-scale crustal 

extension along this northern segment of the rift margin. In 

basement uplifts in Montana and Wyoming, WNW -striking 

diabase dykes yield 40 Ar/39 Ar hornblende ages of 760-770 Ma 

(Harlan, 1993), suggesting near-synchronous extension and 

rifting took place on the western edge of the Wyoming 

Province. The 0.76-0.78 Ga mafic magmatism along the 

developing western margin of North America occurred about 

40-60 million years before the abrupt onset of the voluminous 

0.72 Ga Franklin igneous events in the northwestern Canadian 

Shield and the Amundsen Basin. These discrete episodes of 

mafic magmatism establish precise markers to constrain the 

timing and sequence of break-up of the Neoproterozoic 

supercontinent Rodinia. 

Hansen, V.L., Goodge, J.W., Keep, M., and Oliver, D.H., 

1993, Asymmetric rift interpretation of the western North 

American margin: Geology, v. 21, p.1067-1070. 

Harlan, S.S., 1993, New paleomagnetic results from middle 

and late Proterozoic intrusive rocks of the central and 

southern Rocky Mountains: G.S.A., Abstracts with 

Programs, v.25, p.48 
SIMILARITIES AND DIFFERENCES IN ISOTOPIC AND 

CHEMICAL COMPOSITIONS BETWEEN CONTINENTAL 

AND OCEANIC BASALTS IN THE CAMEROON LINE 

LEE Der-Chuen, HALLIDAY, A. N. and HALL, C. M., 

Dept. of Geological Sciences, Univ. of Michigan, Ann 

Arbor, MI 48109, and FITTON, J. G., Dept of Geology and 

Geophysics, Univ. of Edinburgh, Edinburgh, EH9 3JW, 

UK. 

Basaltic lavas away from the major continental volcanic 

centers of the Cameroon line have been dated by 4C:.:\rf39 Ar 

step-heating and their Sr-Nd-Pb isotopic compositions 

determined. New 40 Arf39 Ar ages reveal that the basaltic 

volcanism of the Cameroon line extends back to ca 42 Ma, 

greatly extending the previous range of ca 30 Ma. The 

majority of basic lavas display little evidence of crustal 

contamination, plotting within the field of the Cameroon line 

ocean island basalts ((87Sr/86Sr)t = 0.7030-0.7035, 

( 143Ndfl 44Nd)t = 0.51285-0.51296 and (2°6Pbf204Pb)t = 19.2-

20.1). However, some samples from near the Bambouto and 

Oku areas are apparently affected by contamination with 

continental crust, showing more radiogenic Sr and less 

radiogenic Nd and Pb isotopic compositions, comparable to 

the crustal xenoliths from Bambouto area. Basic lavas from 

around Mt. Cameroon display temporal trace element and 

isotopic variations, analogous to those observed in the 

Cameroon line ocean island basalts. Samples > 1 Ma show 

trace element and isotopic compositions similar to the oceanic 

basalts, while samples < 1 Ma display lower Ba/Nb and 

enriched Nd and Pb isotopic compositions, similar to HIMU. 

In contrast to the oceanic basalts, no PREMA, like that of 

Pagalu, and no "St. Helena type" HIMU, like that of recent 

Principe volcanism, is observed in the continental sector. The 

HIMU isotopic signatures in the continental sector appear to 

be more radiogenic in Sr and less radiogenic in Nd, and may 

have been caused by an enrichment event beneath the 

continental lithosphere focused near the continent I ocean 

boundary during the Mesozoic. Isotopic variations in the 

continental sector are explicable through mixing between this 

modified HIMU and a second component that appears to be a 

mixture of "St. Helena type" HIMU and PREMA as found in 

the oceanic sector in their more distinct identities. Continental 

basalts are chemically similar to oceanic basalts with the 

exception that K/Nb is higher in the continental sector. This 

may be caused by amphibole in the source. 

188 

RAPID UPLIFT AND ROTATION OF MYLONITIC 

ROCKS, NORTHERN SNAKE RANGE, NEVADA: 

INSIGHTS FROM K-FELDSPAR 40Art39Ar AND 

APATITE FISSION 1RACK THERMOCHRONOLOGY 

LEE..L. Geology Dept., Central Washington University, 

Ellensburg, WA 98926, USA, MILLER, E.L., Dept. of 

Geology, Stanford University, Stanford, CA 94305, USA, 

GANS, P.B., Dept. of Geological Sciences, University of 

California, Santa Barbara, CA 93106, USA and BROWN, 

R.W., Dept. of Geological Sciences, University College, 

London WClE 6BT, United Kingdom 

Metamorphic core complex detachment faults have been the 

subject of much study over the last 15 years, yet the origin and 

movement history of these faults and the mechanisms by 

which they translate mid-crustal rocks to the surface are still 

topics under debate. Questions remain as to whether these 

faults initiated and moved at low angles or whether they 

initiated at high angles and rotated to a shallow dip during slip. 

To address this problem, we established the degree of 

fool wall rotation by characterizing the thermal history of lower 

plate mylonitic rocks. The thermal histories of mylonitic 

rocks from the footwall of the northern Snake Range 

decollement (NSRD), east-central Nevada, USA were calculated 

using multiple diffusion domain analyses of potassium feldspar 

Arrhenius data and 40 Ar/39 Ar age spectra, and apatite fission 

track studies to characterize the cooling (uplift) history of these 

mylonitic rocks. These data, along with published data on the 

structural and kinematic history of ductile extension, constrain 

the origin and movement history of detachment faults. The 

calculated thermal histories indicate three episodes of mpid 

cooling (10-55°C/Ma) related to extensional denudation as 

lower plate rocks cooled from -350°C to -900C: 1) Middle 

Eocene (48-41 Ma), 2) Late Oligocene (30-26 Ma) and 3) Early 

Miocene (20-16 Ma). The fU'St episode of rapid cooling may 

be related to the earliest period of normal faulting in eastcentral Nevada, initiation of slip along the NSRD and onset of 

ductile extension in the footwall. The second episode signals a 

pulse of slip along the NSRD, and the third episode marks the 

cessation of lower plate mylonitic deformation and the onset of 

the major pulse of uplift and rotation of lower plate rocks from 

beneath the NSRD and/or younger east-dipping normal faults. 

The thermal history data provide evidence for a temperature 

difference across the range within the same structural horizon 

(70-170°C); the northwestern flank of the range dropped below 

300°C by 46 Ma and reached 115°C by 20 Ma, whereas in the 

eastern parts of the range, lower plate rocks were at 

temperatures above 300°C until19 Ma and ductile extension 

was still ongoing. If isotherms were subhorizontal, these 

relations suggest either an eastward dip of lower plate units 

prior to the onset of Tertiary mylonitic deformation or that 

layering dipped eastward as a result of top-to-the-east shear 

within an east-dipping shear zone. Either case results in 

differential cooling (uplift) of lower plate rocks from beneath 

the NSRD. Simple palinspastic restorations of lower plate 

ductile extension based upon fmite strain data, along with the 

calculated thermal histories, show that moderate to steeply 

dipping units were rotated to shallow dips during ductile shear 

and brittle slip from beneath the detachment fault and/or 

younger east-dipping normal faults. These reconstructions 

show that detachment faults initiate at moderate to steep angles 

and rotate to a shallow angle during slip. Such a history is 

compatible with a rotating shear zone-detachment fault model 

or rolling hinge/isostatic rebound model. 
MULTIPATH DIFFUSION AND THE INFLUENCE OF 

MICROSTRUCTURES ON CLOSURE TEMPERATURES 

AND COOLING AGES 

LEE James K. w .. Research School of Earth Sciences, 

Australian National University, Canberra, A.C.T. 0200, 

Australia; e-mail: jld 152@ huxley .anu.edu.au 

Traditionally, the interpretation of geochronological data has 

involved, directly or indirectly, the application of solid-state or 

volume diffusion (VD) theory, which describes the migration of 

atoms or ions through a homogeneous crystal structure. 

Crystals in geological materials are far from perfect, however, 

and commonly contain extended defects (e.g. dislocations, 

exsolution lamellae, micropores, microfractures, fission tracks) 

which may act as pathways for much more rapid transport. 

Although it is known that the diffusion of a solute species 

along such high-diffusivity pathways (here termed "shortcircuit" [SC] diffusion) can enhance the bulk diffusivity of a 

mineral, the influence of microstructures on the diffusional 

behaviour of geochronologically important chemical species 

(e.g. Ar, Pb) has been largely ignored. 

The importance of microstructures in the interpretation of 

geochronological data may be seen in examples from two of the 

most commonly used isotopic dating methods. 40 Ar/39 Ar: 

Isothermal step-heating of Fe-homblendes in vacuo at 

temperatures ~ those at which structural decomposition 

takes place yields Ar release patterns which do not reflect a VD 

mechanism but are consistent with enhanced SC transport 

resulting from defects created during the oxidation of Fe (Lee 

1993). U/Pb: SHRIMP analyses of metamictization-induced 

and hydrothermally altered fractures in zircons from a granitic 

pluton yield U/Pb ages significantly less than the granite age, 

suggesting substantial Pb loss. Thus, metamictization-ioduced 

fractures can enhance Pb mobility by providing a network of 

pathways for fast transport (Lee and Williams 1993). 

The simultaneous transport of a solute species through both 

the crystal lattice (via VD) and high-diffusivity pathways (via 

SC) is termed multipath (MP) diffusion (Aifantis 1979, Lee 

and Aldama 1992). Using MP theory, the effective diffusion 

dimension, a, for many minerals appears to remain on the order 

of the grain radius, and the model adequately explains increases 

in the diffusion coefficient, D, with a as observed in the 

hydrothermal bomb data of previously published Ar diffusion 

studies on hydrous minerals. In addition, Arrhenius diagrams 

reflecting a MP mechanism will have "kinked" curves, 

indicating a continuous change in the relative importance of the 

different diffusion mechanisms with temperature. 

The most general and important consequence of MP 

diffusion is that the overall bulk diffusion coefficient of a 

mineral can be enhanced significantly above its volume 

diffusion coefficient. As a result, integrated ages and effective 

closure temperatures (Tc) can be much lower than those 

predicted assuming only a VD mechanism. Under certain 

conditions, it is even possible that minerals normally 

characterized by low "VD" Tc's (e.g. Ar in biotite) can have 

older integrated ages than minerals normally associated with 

higher "VD" Tc's (e.g. Ar in hornblende) - an enigmatic 

phenomenon reported in some previous studies. Thus, it may 

be essential to carefully characterize mineral samples to be 

dated, since microstructurally dependent transport mechanisms 

may ultimately control the migration and intra-grain spatial 

distribution of geochronologically important isotopic species. 

Aifantis EC (1979) Acta Metall27: 683-691. 

Lee JKW (1993) Chern Geol (lsot Geosci Sect) 106: 133-170. 

LeeJKW and Aldama AA (1992) Comput Geosci 18: 531-555. 

LeeJKW and Williams IS (1993) EOS 74:650-651. 

189 

Cs PRODUCTION RATIO IN FALWUT 

LEE. T)!Phoon. CHEN, Hsin-Wei, Inst. Earth 

Sci., Academia Sinica, and Geology Dept. Nat'l 

Taiwan Univ., Taipei, Taiwan, R.O.C, 

KU, Teh-Lung, Dept. Geol. Sci., Univ. 

South. Calif., Los Angeles, CA. U.S.A. 

Fallout Cs-137 and Cs-135 are present in soil 

and sediment with typical abundances of 0.1 to 1 

ppb of natural Cs. Their ratio is potentially a 

powerful tool to quantitatively estimate modern 

sedimentation and erosion rates. Lee et al 

(Geochim. Cosmochim. Acta (letter), 57, 3493, 

1993) has recently developed mass spectrometric 

techniques to measure all three Cs isotopes in 

coastal sediments. A pre-requisite of applying this 

tool is the knowledge of the production ratio of 

Cs-137 /Cs-135 in the nuclear fallout when they 

were initially deposited. We have measured Cs 

isotopic compositions of rain water samples 

collected in the 60's and in coastal sediments with 

well-established chronology from Santa Monica 

Basin (Table). Since weapon yields are dominated 

by tests around 1962, we consider the fallout Cs in 

both sediments to be essentially 30 years old. The 

rain samples have much higher 135/133 ratio. The 

two rain samples from southern California 

collected in 1963 and 1965 gave 137/135 ratios 

consistent with the sediment results within 

respective uncertainties. We thus believe that the 

Cs-137 /Cs-135 ratio was dominated by fallout Cs 

having an initial of about 0.70-0.75. This is 

surprisingly low since the reactor fission yield 

seems to be about 1. 

Table Cs Isotopes in Dated Samples 

----------------------------------- Sample 135/133* 137/135 Note 

----------------------------------- Santa Monica 

1-2 em 13.9±1.4 

2-3 ern 11.7±0.7 

4-5 em <0.4 

Rain 

Basin Sediment 

0.35±6 1964-1980# 

0.35±6 1946-1964# 

1903-1927# 

Water 

1992 

1965 

1963 

<6 Taipei 

107±15x1o2 0.41±9 La Jolla 

105± 7x1o2 0.37±4 Los Angeles 

----------------------------------- * x1o-10, # Pb-210 chronology 
INCISION RATES DETERMINED BY COSMOGENIC 

DATING OF BEDROCK STRATIIS ALONG TilE INDUS 

RIVER IN NORTIIERN PAKISTAN 

LELANP J F , Department of Earth and Space 

Sciences, University of California, Los Angeles, CA 

90024, BURBANK, D. W ., Department of Geological 

Sciences, University of Southern California, Los 

Angeles, CA 90089, REID, M. R., Department of Earth 

and Space Sciences, University of California, Los 

Angeles, CA 90024, FINKEL, R. C., and CAFFEE, M. 

W., Lawrence Livermore National Laboratory, 7000 East 

Ave., Livermore, CA 94550, USA. 

Abandoned river-cut bedrock terraces ranging between 10 

and 400 m above the modem Indus River were sampled to use 

cosmogenic nuclides such as lOBe and 26AJ to date the 

abandonment of these straths and determine local incision 

rates. Rapid incision and changes in gradient of the Indus 

River in northern Pakistan have occurred throughout the last 

100 ka, especially within the Middle Indus Gorge, a westflowing portion of the river located between the Skardu basin 

and the Gilgit-Indus River confluence bisected by a NNW 

trending axis of modem rapid uplift inferred from thermochronological studies (e.g., Zeitler, 1985). We report lOse 

exposure ages for four different straths located many 

kilometers apart within the Middle Indus Gorge. 

Sampled terraces generally showed excellent preservation 

of original fluvial features such as fluting and fluvial polish 

comparable to those found at the modem river bank implying 

very low erosion rates, especially along the very arid stretches 

of the river. Because much of the sample area sits in the rain 

shadow of the Himalaya and is at elevations below 2500 m, 

significant shielding by persistent deep snow cover in the 

past is unlikely. Regions showing evidence of prior 

glaciation such as the presence of moraines were avoided. 

Also, surfaces previously covered by landslide were avoided 

by sampling only on the lee side of or between large bedrock 

knobs or tOO's of meters away from existing landslide 

deposits. It is assumed that exposure during strath cutting is 

negligible compared to the total exposure history after 

abandonment. 

Preliminary lOse dates on nine leached quartz samples 

from four different strath terraces ranging from 65 to 185 m 

above the Indus in the Middle Indus Gorge show the 

following features. Pairs or triplets of samples from single 

straths show consistent ages within 1-a deviations. 

Preliminary strath exposure ages range from 12 ka to 80 ka. 

Bedrock incision rates vary by a factor of 3, from 2 mm/a 

upriver near the mouth of the Skardu basin to 6 mrnla 

downriver within the axis of inferred rapid uplift. We await 

26Al data to corroborate these findings; however, if accurate, 

our data suggest a differential uplift rate of 3-4 nun/a over the 

last 30 ka between the lower (rising faster) and upper portions 

of the Middle Indus Gorge. 

Zeitler, P. K., 1985, Cooling history of the NW Himalaya, 

Pakistan: Tectonics, v. 4, p. 127-151. 

190 

SOLID HOSTED TRACERS FOR USE AS 

STANDARDS IN ISOTOPE DILUTION 

ANALYSIS OF ARGON (AND NOBLE GASES) 

LEMPERT, G.D., Soreq Nuclear Research 

Center, Yavne, Israel, KAPUSTA, Y., 

STEINITZ, G., KOTLARSKY, P., Geological 

Survey, Jerusalem, Israel and BUKSHPAN, S., 

Soreq Nuclear Research Center, Yavne, Israel. 

A solid spike (SSP) is defined as a solid material 

selectively enriched with one or more isotopes of the 

noble gases, to be used as a tracer in: 1) precise 

mass-spectrometric isotope dilution (ID) analyses of 

noble gases utilizing mono-isotope SSP; 2) improved 

mass discrimination control in the analyzer, between 

and within sample measurement, utilizing a double 

isotope SSP. SSP is produced using neutron nuclear 

reactions and/ or ion implantation technology, 

enabling SSP production with predetermined 

concentrations of the tracer isotope. The rationale in 

using SSP in ID is identical to the classical one, 

avoiding the disadvantages of the pipette system, 

while providing significant advantages: a) flexibility 

is allowed with respect to an optimal spike quantity; 

b) simpler and automated analytical setups can be 

made; c) periodic calibration in not required. 

In an initial experimentl1

1 a 39 Ar SSP was 

prepared by neutron irradiation of a biotite standard 

(HD-B1). The relative error in the determination of 

the 40 Ar content was < 2%. In a second 

experimentl21 SSP was prepared by implanting argon 

ions, at 45KeV, into copper foil. Implanted Ar 

isotopes are retained in the Cu foil to a temperature 

of 150°C and completely released at 800°C. Thus 

ion implantation technology can be used for serial 

production of different SSP for mass-spectrometric 

analysis of Ar and other noble gases. 

[ 1] Kapusta, J. , Kotlarsky, P. and Steinitz, G., 

1992. Isr. Geol. Surv. Rep. GSI/20/92, 14p. 

[2] Kapusta, Y., Lempert, G.D., Bukshpan, S., 

Kotlarsky, P., and Steinitz, G., 1993. Isr. Geol. 

Surv. Rep. GSI/15/93, 15p. 
ISOTOPE STUDY OF THE PONGOM-NAVOLOK 

HYPERSTHENE DIORITES PLUTON, BALTIC 

SHIELD, NORTH KARELIA 

LEVCHENKOV,O.A., OVCHINNIKOVA,G.V., 

ZINGER,T.F., KOTOV.A.B •• LOKHOV,K.I., 

YAKOVLEVA,S.Z., MATUKOV,D.I., 

SINELNIKOVA,I.M., GOROKHOVSKY,B.M. 

Institute of Precambrian Geology and 

Geochronology, Russian Academy of 

Sciences, Makarov a emb. , 2, St.-

Petersburg, 199034, Russian Federation 

The hypersthene diorites of the Pongomnavolok pluton underwent granulite and 

amphibolite facies metamorphism and 

multiphase deformation. Isotopic 

composition of o2 corresponds to I-type 

granite (618o = 5.9-7.4%o). The variation of 

6 18o is independent of the degree and 

character of hypersthene diorites 

transformation. The fluid composition of 

the hypersthene diorites is similar to 

granulite-facies metamorphism rock; H/C = 

10-15; N/C less 0.15 while H/C is about 30 in 

samples from amphibolite facies shear 

zones. 

Three type zircons in pluton rocks have 

been established. The first type zircon of 

2725+/- 23 Ma age is magmatic. The second 

type zircon is considered to be the result 

of the transformation of magmatic zircon 

under the condition of granulite-facies 

metamorphism. The latest zircon of 2.4 Ga 

age occures in hypersthene diorites samples 

from amphibolite facies shear zones. This 

zircon is rich U and has Th/U ratio less than 

in two other types of zircons. 

The Pb-Pb isochron of whole rock samples 

yields the age of 2690+/- 80 Ma. Within the 

framework of plumbotectonic model the data 

of acid leaching residues favour the lowcrust source of primary magma. The results 

of acid leaching also indicate that the 

intense metamorphic event with the gain of 

cru-st lead in plutons rocks took place about 

2.4 Ga. 

191 

THE NEW INTERPRETATIONS OF SOME 

INCORRECT RADIOCARBON AND 

PALYNOLOGICAL DATING OF THE 

ARCHAEOLOGICAL SITES. 

LEVKOVSKAYA G.M. 

RUSSIA 

Some radiocarbon dates sometimes do not 

agree with the pol1en or archaeological dating. 

Sometimes different parts of one archaeological 

site are characterized with different types of 

pollen diagrams, 

As a result of the vertical and horizontal 

man's impact on the surface, sometimes it's 

difficult to distinguish the following pollen 

dating obtained from the level with 

archaeological findings (Levkovskaya, 1983, 

1987, Table2, p. 33): 

1 . the dating of the "floor" of 

archaeological layer (the level of inhabiting). 

2. of the "cellar" (different pits). 

3. of the "archaeological mainland" (the 

naturally sterile surface, where the 

archaeological site was springing up). 

4. of the "roof" (the sterile sediments 

directly over archaeological layer). 

According_ to the theory of the "floor" of 

buried stone age sites, archaeological layers 

could give (in some cases) the mixture of 3 _ 

different but correct radiocarbon dates (the 

dating of the "floor", "roof'' and "archaeological 

mainland"). 

The author's stratigraphical and 

planygraphical pollen materials on Gubsky cave 

with Neanderthal bones (Caucasus), on Upper 

Palaeolithic loess sites from Kostenki area 

(Russian plain), on Mezolithic, Neolithic, Early 

Bronze age pitbog sites with about 70 

radiocarbon dating from Latvia (Levkovskaya, 

1987) and others illustrate the conclusions, 

mentioned above. 
THE BEHAVIOUR OF BORON AND ITS' ISOTOPES IN 

THE YELWWSTONE HYDROTHERMAL SYSTEM 

LEWIS Anita J. and PALMER, Martin R., 

Department of Geology, University of Bristol, Wills 

Memorial Building, Queen's Road, Bristol, BS8 1RJ, UK. 

Boron concentrations have been measured in 35 hydrothennal 

w~ in and around Yellowstone National Park including hot 

spnngs and pools, snow melt and stream water. Eighteen 

samples were collected in the fall (1991), 15 of which were recollected in the spring (1993). Boron isotopic values have 

been measured on a representative sample of these waters. 

The boron concentrations range from 0 to 2.54 mM/kg and 

broadly correlate with chloride concentrations except for two 

samples (with the highest boron concentrations) which do not 

appear to belong to the same geothermal regime. The range of 

boron concentrations is considered to be due to various degrees 

of dilution of original hydrothennal waters and meteoric waters. 

Preliminary results indicate that there is no seasonal variation 

in boron concentration. 

Boron isotopic values range from -9.3%o to +4.4%o. The 

lower values are reported from extra-caldera watel'S and are 

consequently considered to represent input from fluids which 

have leached boron from a sedimentary source. Boron isotopic 

values reported from intra-caldera fluids range from -6.1%o to -

3 .6%o reflecting the value of the volcanogenic rocks with which 

they have reacted. 

Palmer, M. R. and Sturchio, N.C., 1990, The boron isotope 

systematics of the Yellowstone National Park (Wyoming) 

hydrothermal system: A reconnaissance. Geochim. 

Cosmochim. Acta, v. 54, p. 2811 - 2815. 

192 

PRISTINE PRESOLAR GRAPHTIE SPHERULES. 

Lewis Roy S., Enrico Fermi Institute, University of Chicago, 

Chicago, IL 60637. · 

Graphite grains are the most fragile identified presolar 

material that survives in relatively pristine condition in 

primitive meteorites (Anders and Zinner, 1993). It (along with 

SiC) caiTies Ne-E, nearly pure 22Ne, although in only -1/3 of 

the grains. The presolar nature of the caiTier was recognized by 

Black (1972, but largely ignored as being based on neon), the 

fmt such recognition, many years before its identification as 

graphite by Zinner et al. (1990). The grains are close to 

spherical, with most of the mass found in grains 1-4J.lm in 

diameter, but many grains are at least as small as 0.3J.UI1, a few 

as large as l5J.UI1. The grains typically have an amorphous core 

sUITounded by a better crystallized outer layer, display a range 

of surface morphologies, and are commonly found to contain 

grains of various refractory Ti, Zr, & Mo carbides, presumably 

trapped within the growing graphite grains in carbon rich stellar 

atmospheres. All elements measured to date have isotopic 

compositions distinct from solar system values. E.g., noble 

gases have Ne-E and s-process enrichments. Density separates 

of these grains yield distinct fractions in terms of noble gas 

abundances, release temperatures, and isotopic abundances of 

80,86K.r - isotopes reflective of the nucleogenic conditions due 

to temperature and neutron density sensitive branch points in 

the s-process chain. The carbon itself is wildly anomalous, with 

individual grains scattering within the range 12/13C = 6-3000 

(solar=89). Nitrogen ranges from solar to 14/1SN down by a 

factor of 3, oxygen from solar to 16/180 down by a factor of 

100. A clustering of some of the grains near the solar values for 

N & 0 may indicate that dilution with normal solar material is a 

common problem with these elements. Silicon has both 

enrichments and depletions at 29,30Si of up to 50%. Large 

quantities of 26Mg excesses are found, indicating 26/27 AI ratios 

as high as 0.1, far above the canonical primitive solar value of 

SE-5. Additional elements are being measured (Amari et al., 

1994). These reactive elements are measured on single grains, 

and the experimental difficulties have rapidly become 

formidable as low abundance trace elements are examined in 

these tiny grains. While some rough correlations are evident, 

e.g. in the oxygen and nitrogen isotopic compositions, each 

grain appears to be distinct, requiring the treatment of each as a 

unique sample from some particular stellar .environment, 

necessitating the measurement of multiple elements on each 

grain to disentangle the record. Isotopic abundances seem 

roughly consistent with an origin of these grains in a range of 

stellar environments, AGB stars, Wolf-Rayette stars, novae, and 

possibly (pre-)supernovae, but the view resembles that given 

not so much by a window as a kaleidoscope. 

Amari, S., Hoppe, P., Zinner E., and Lewis, R.S. 1993, The 

isotopic compositions and stellar sources of meteoritic 

graphite grains: Nature 365 806-809. 

Amari, S., Zinner, E., and Lewis, R.S. 1994, Isotopic analysis of 

low density graphite grains from Murchison, ICOG 8 abstr. 

Anders, E. and Zinner, E. 1993, Interstellar grains in primitive 

meteorites: diamond, silicon carbide, and graphite: 

Meteoritics 28 490-514. 

Black, D.C. 1972, On the origins of trapped helium, neon, and 

argon isotopic variations in meteorites. II. Carbonaceous 

meteorites: Geochim. Cosmochim. Acta 36,377-394. 

Zinner, E., Wopenka B., Amari S., and Anders E. 1990, 

Interstellar graphite and other carbonaceous grains from the 

Murchison meteorite: structure, composition, and isotopes of 

C, N, and Ne: Lunar Planet. Sci. 211379-1380. 
FLUID INCLUSION Rb-Sr ISOCHRON AGE DATING OF 

XIHUASHAN AND DANGPING TUNGSTEN QUARTZ 

VEINS DEPOSITS, JIANGXI PROVINCE, CHINA 

LI. Huaaing. LIU, Jiangji, Yichang, Institute of Geology 

and Mineral Resources, Chinese Academy of Geological 

Sciences, Yichang, Hubei Province, China; and LU, HuanZhang, Sciences de la terre, Universite du Quebec A 

Chicoutimi, Chicoutimi, Quebec G7H 2B 1, Canada. 

China has the major tungsten ore reserves and production in 

the world which occur mainly in the southern part of China, 

especially in the Jiangxi, Hunan, Guangxi and Guangdong 

provinces. The tungsten mineralization is hosted and associated 

with granites of Yanshanian age (70-195 m.a.). The major 

type of tungsten ore deposits is in wolframite-quartz veins and 

the well known are Xihuashan, Dajishan, Dangping, Piaotang 

and Shanhu mines. The fluid inclusions in these quartz vein 

are aqueous and H20-C02 inclusions with homogenization 

temperatures ranging from 250 to 400°C and salinities ranging 

from 1 to 10 wt.% 

This study presents preliminary results of Rb-Sr isochron 

age dating of fluid inclusions of these tungsten mines in order 

to understand metallogenetic timing. Samples for fluid inclusion analyses were carefully selected based on the affinity 

between quartz host related to tungsten mineralization. Other 

samples including granite, and mineral are chosen to compare 

with the results from fluid inclusion dating. The Xihuashan 

tungsten mine is a big quartz vein occurred in the two phases 

Y anshanian granites. Two isochron ages have been obtained 

for this granite: 156 ± 4 m.a., (87Sr/86Sr)i = 0.7186 ± 0.0008 

and 142.2 ± 3 m.a., (87Sr/86Sr)i= 0.70.7338 ± 48. The age of 

fluid inclusions in quartz is 139.8 ± 4.5 m.a., (87Srf86Sr)i = 

0.7437 ± 0.41. The age of wolframite dated by Sm-Nd isochron method: 139.2 ± 3 m.a., (143Ndfl44Nd)i = 0.511970 

which is concordance with the age dated by fluid inclusions in 

quartz. The results indicates that tungsten mineralization is 

about 16 m.a.later than the formation of the granite. The RbSr isochron age for Dangping mine are : 150 ± 2 m.a., 

(87Sr!86Sr)i = 0.7160 ± 0.0015 and t = 140.8 ± 3 m.a., 

(87SrJ86Sr)i = 0.7653 ± 4 m.a. for the granites; t = 130.3 

± 6.3 m.a., (87Sr/86Sr)i = 0.7485 ± 14 for the fluid 

inclusions. The Sm-Nd isochron dating for fluorite in 

Dangping quartz vein is 137.4 ± 3 m.a., (143Ndfl44Nd)i = 

0.511930 ± 5 which resemble the age obtained from the fluid 

inclusions. These show that the tungsten mineralization is 

about 10 m.a. later than the formation of the Dangping 

granites. Combining with other fluid inclusion age data on 

Dajishan, Piaotang and Shanhu mines, we conclude that the 

tungsten mineralization age in south China are the same as the 

Y anshanian age but with 10 or 20 m.a. later than that of the 

granite hosts. 

193 

THE EFFECT OF PHOSPHORUS ON CHEMICAL AND 

PHYSICAL PROPERTIES OF BASALTS: AN 

EXPERIMENTAL AND ION PROBE STUDY 

LIBQUREL. G., Univ. Nancy I. Lab de Petrologie, BP239, 

54506 Vandoeuvre and CRPG-CNRS, BP20, 54501 

Vandoeuvre, France), TOPLIS, M. J., CARROLL, M. R., 

both at: Dept. of Geology, University of Bristol, BS8 1RJ, 

UK, and DINGWELL, D. B., Bayerisches Geoinstitut, 

Universitiit Bayreuth, 95440, Bayreuth, Germany. 

Despite the abundant evidence for enrichment of phosphorus 

during the differentiation of natural basalts, the role of 

phosphorus on the chemical and physical properties of basaltic 

melts is poorly understood. The situation is especially acute in 

the case of iron-rich melts, where extreme P-enrichments are 

observed. 

To evaluate the effects of phosphorus on differentiation of 

evolved basaltic magmas, a series of one-atmosphere 

experiments on a ferro-basaltic composition were carried out, 

isothermally, over a range of P205 contents and at oxygen 

fugacities from 2 log units below to 2 log units above the 

FMQ buffer curve. The results confirm the high solubility of 

phosphorus in basaltic magmas and show that, at fixed 

temperature, the progressive addition of phosphorus causes: ( 1) 

the disappearance of olivine at reducing conditions, (2) the 

disappearance of magnetite at oxidising conditions, (3) the 

stabilisation of pigeonite throughout the studied range of f02, 

(4) an increase in the modal plagioclase/pyroxene ratio, and (5) 

concomitant changes in the composition of the coexisting 

liquids. 

In order to complement and clarify the role of phosphorus on 

the chemical and physical properties of basic magmas, the 

same starting ferro-basaltic composition was also used to 

determine the effect of phosphorus on the ferric-ferrous ratio, 

viscosity and density of the melt, over a large range of 

temperature and at two different oxygen fugacities. The results 

suggest that addition of P205 causes (1) a reduction of ferric 

iron, (2) an initial decrease and subsequent strong increase in 

viscosity, and (3) a decrease of density. The magnitude of the 

reduction of ferric iron is greater than that of all the quoted 

oxide components in the calculation scheme of Kilinc et al. 

(1983). The overall increase in viscosity at 1200°C is estimated 

to be of the order of 15% relative per wt% P205, which is of 

the same order of magnitude as Si02. A partial molar volume 

of 64.5 cm3 /mol has been determined for P20 5 in this 

ferrobasalt at 1300°C. 

Together with new phosphorus partition coefficients (D crystalliquid) measured with an ion probe on the same ferro-basaltic 

composition, we will show that these results provide insights 

into the role of phosphorus on differentition of both lunar and 

terrestrial basalts, as well as providing information regarding 

the solubility behaviour of phosphorus in silicate melts. 

Kilinc A .. et al. (1983) Contrib. Mineral. Petrol., 83, 136-

140. 
THE IMPORTANCE OF CATHODOLUMINESCENCE STUDY FOR ZIRCON U-PB DATING OF 

S-TYPE GRANITOIDS: AN EXAMPLE FROM 

THE SILVRETTA NAPPE, SWITZERLAND 

LIEBETRAU. V., POLLER, U., Instit. of Mineralogy & Petrography, Fribourg, 1700, Switzerland, 

SERGEEV, S.A., STEIGER R.H., Lab. for Isotope 

Geochemistry ETH, Zurich, 8092, Switzerland, 

FREI, R., Lab. for Isotope Geology, In st. of Min. 

& Petr., Bern, 3012, Switzerland 

The complex zircon U-Pb data for S-type granitoids 

in the Austroalpine Silvretta nappe called for a detailed study of the internal morphology of the zircons. 

A cathodoluminescence (CL) investigation was initiated which revealed the internal zircon structures and 

greatly aided in the interpretation of the data set. 

The following three clear zircon morphologies were 

found to be present in the population of each rock 

sample: 1. prismatic type, 2. bipyramidal type and 3. 

flattened species. For one granitoid type (Monchalp 

gneiss) the analyses of three different typological 

populations within the same size fraction suggest a 

discordia line with a lower intercept around 460 Ma. 

B ipyramidal single grains considered as last magmatic 

result in discordant data points which differ in age 

because of an optically invisible inherited component. 

But their combination with other grain size fractions 

leads to a discordia with an apparent lower intercept of 

510 Ma. Upper intercept ages of both discordias 

indicate an inherited 2.3 Ga old Pb component. In 

contrast to this, the single-grain data of an other 

orthogneiss (Urezzas) give concordant points around 

450 Ma. The data points close to the concordia from 

both rocks align to a new 450 - 800 Ma discordia line 

possibly indicating an additional inherited Pb component. 

CL imaging by scanning electron microscope show 

that among (magmatic) zoned grains of the same 

typology and size there exist some which bear 

inherited cores of which at least three types may be 

distinguished: 1. a conglomerate of several small cores, 

2. broken (detrital) well structured cores and 3. weakly 

structured resorbed cores. Grains bearing one of the 

first two core types, often exhibit a zone of metamorphic growth between core and rim. 

In light of the internal zircon structures the different 

discordia ages can be reinterpreted, in correlation with 

the S-type characteristics of the rocks and the metamorphic and magmatic evolution of the Silvretta nappe 

(Maggetti & Flisch 1993). Special sample preparation 

combining CL studies with high resolution zircon 

fragment analysis may eventually allow systematic 

dating of the mentioned zircon phases. 

Maggetti, M., & Flisch, M., 1993, Evolution of the 

Silvretta Nappe, in: v. Raumer, J.F., & Neubauer, F., 

Pre-Mesozoic Geology in the Alps, Springer Berlin, 

pp. 469-484 

194 

A ROBUST METHOD FOR ESTIMATING SURFACE 

EXPOSURE AGES AND LONG-TERM EROSION RATES 

IN BEDROCK USING IN SITU-PRODUCED 

COSMOGENIC RADIONUCLIDES 

LIFfQN Nathaniel A., Dept. of Geosciences, University 

of Arizona, Tucson, Arizona, 85721, USA, and JULL, 

A.J. Timothy, NSF-Arizona Facility for Radioisotope 

Analysis, University of Arizona, Tucson, Arizona, 85721, 

USA. 

Numerous ambiguities, limitations and uncertainties in 

geomorphic research have resulted from a historical inability 

to reliably and quantitatively estimate (1) the exposure age of 

a geomorphic surface, and (2) spatial variability of long-term 

erosion rates. Radioactive and stable nuclides produced in 

situ by cosmic ray bombardment of rock materials exposed at 

the earth's surface may enable us to estimate surface exposure 

ages and long-term average erosion rates directly. Recent 

development of techniques for measuring and interpreting 

concentrations of these nuclides may ultimately help to 

resolve many long-standing geomorphic debates. 

Cosmogenic radionuclide pairs such as lOBe and 26 AI have 

been used most frequently to solve the governing equations 

for both surface exposure ages and erosion rates. However, 

the applicability of these techniques has been limited to some 

extent by the number of assumptions which had to be made 

to interpret the measured concentrations - particularly those 

pertaining to steady-state or non-steady-state erosion 

conditions. 

We are developing a robust method for interpreting the 

depth distribution of cosmogenic radionuclides in bedrock 

which avoids the need for many of these assumptions. Monte 

Carlo methods are used to explore the range of erosion rate 

and exposure age conditions on a grid which would produce 

theoretical depth distributions of the radionuclide consistent 

with the measured depth profile. The X2 statistic is used to 

test the goodness of fit between the measured and theoretical 

profiles at each grid point. Scenarios were modeled to 

simulate potential bedrock environments, with no inherited 

cosmogenic nuclide component arising from prior exposure. 

Results suggest that a single erosion rate/exposure age pair 

may be found from the grid which would best fit the 

measured data. Thus, it should be possible, in theory, to find 

the most probable approximation for the exposure history of 

a bedrock surface using a single radionuclide. 

We plan to corroborate the model results with new 

cosmogenic 14c data from bedrock depth profiles collected 

from pristine shoreline features of well-known age and 

unambiguous exposure history associated with Pleistocene 

Lake Bonneville, Utah. These features include wave-cut 

bedrock benches at the Bonneville and Provo level 

shorelines, scoured basalt surfaces associated with the 

Bonneville Flood, and the Tabernacle Hill basalt flow, which 

was erupted into a Provo-level lake. Although unanswered 

questions and potential problems remain, this method appears 

to hold great promise for unraveling spatial variation in both 

erosion rates and exposure ages of a wide range of 

geomorphic surfaces. 
CHRONOLOGY AND SR IS010PIC STUDY OF THE 

TRANS USA WET EVENT IN GREAT BASIN 

LIN JO C , BROECKER, W. S., and 

RUBENSTONE, J. L., Lamont-Doherty Earth 

Observatory, Palisades, NY, 10964, USA, OORN, R. 

I., Department of Geography, Arizona State University, 

Tempe, AZ, 85287-0104, USA, PHILLIPS, F., 

Department of Geosciences, New Mexico Institute of 

Mining and Technology, Socoro, NM, 87801,.USA, 

HAJDAS, I., ETH-Ziirich, Institut fiir Teilchenphysik, 

CH-8093 ZUrich, Switzerland, and SMITH , G. I., U. S. 

Geological Survey, Menlo Park, CA, 94025-3591, 

USA. 

Closed basin lakes in the Great Basin of the western 

United States underwent a major pluvial event just prior to 

the BOlling warm period in Europe, i.e. 13,000 to 14,000 

years BP. Pollen records indicate that the Southeast United 

States also experienced unusually high precipitation at the 

same time. Better dating of the pluvial history of the 

Lahontan Basin and Searles Lake in Great Basin can help 

constrain the timing and duration of this trans USA wet 

event and its relationship to contemporaneous high 

frequency climate events in the North Atlantic Ocean. 

Lacustrine carbonates collected from the last high 

shorelines in these two basins are dated by the radiocarbon 

and uranium-series methods. In addition, radiocarbon ages 

from organic materials preserved beneath desert varnish on 

cobbles from the same shorelines give minimum ages for 

the high lake levels. Sr isotopic ratios on ancient 

carbonates and modern river waters in the drainage basin are 

measured to monitor any change in the source of water 

during the wettest period. 

The radiocarbon ages of lacustrine carbonates range from 

12,200 to 13,200 ka. 230nt_234u isochron results yield an 

age of about 16,300 ka, equivalent to 14c age of 13,800 

ka. Radiocarbon ages of the organic material from the rock 

varnish range from 12,700 to 13,600 ka. 

Rivers originating from the Sierra Nevada today have 

much lower 87Sr/86sr than rivers draining from central 

Nevada into the Lahontan Basin. Sr isotope ratios in 

carbonates decrease through this pluvial event suggesting 

relatively greater flux from the western rivers at this time. 

195 

DISTRIBUTION OF RADIOACTIVE NUCLIDES 

OF THE SEDIMENT CORES FROM SOME 

LAKES OF CHINA 

·LIN R.-F., CHENG Z.-Y., WEI K.-Q., 

Guangzhou Branch, Institute of Geochemistry, 

Chinese Academy of Sciences, Wushan, 

Guangzhou 510640, China 

The samples of five sediment cores were collected 

from Dianchi, Dongting, Bosten, Taihu and Kunming 

lake, respectively. 

According to the 210pb Constant Initial 

Concentration model (CIC), the sedimentation rates 

and the average fluxes of sediment were determined 

to be 0.41, 0.71, 0.43, 0.05 and 0.31 crnlyr for 

Dianchi, Dongting, Kunming, Taihu and Bosten, 

respectively. The average values of the sedimentation 

flux were 285, 497, 300, 79.5 and 136 mglcm2Jyr. 

Using the 210pb Constant Flux model (CF), a 

sedimentation rate and sediment flux can be obtained 

for different durations for each lake. There was a 

sedimentationratefrom0.16 crnlyr before 100 yrs to 

0.53crnlyr at present and a flux from 120 mgjcm2Jyr 

before 100yrs to 300 mgjcm2Jyr in 1960s for Dianchi 

lake and a rate from 0.40 crnlyr before 40 yrs to 0.85 

cm/yr at present, and a flux from 450 mgJcm2Jyr to 

800 mgjcm2Jyr for Dongting lake. These results 

reflect an increase in the processes such as 

deforestation, industry development, human 

activities, etc. since 1950s-1960s. The pollution of 

metal elements at different levels was found in these 

lakes since 1950s. 

The sedimentation rates and sediment fluxes based 

on 228'fh data for recent 10 yrs were calculated. 

The 239,240Pu contents in sediment samples from 

Dianchi, Dongting and Bosten lake were determined. 

The maximum contents of 239,240Pu occur at the 

sediment layer of 1963±2 dated with 210pb method. 

It fits very well with the peak period of atmosphere 

nuclear test during early 1960s. The depth obtained at 

239,240pu peak can be used to derive the 

sedimentation rate and sediment flux for recent forty 

years. The results are in agreement with those based 

on 21 Opb data. 

According to 3H data in pore water, the infiltration 

rate of lake water to sediment were estimated to be 

2.7 crnlyr and 2.3 crnlyr for Bosten and Taihu lake, 

respectively. The tum over time of lake water is 7 

years and 1 year for Bosten and Taihu lake, 

respectively. 
ZIRCON AGES IN YOUNG FELSIC VOLCANICS AND 

UNDERLYING BASEMENT IN NORTHERN NEW 

ZEALAND: IMPLICATIONS FOR RHYOLITE GENESIS. 

LINDSAY, J.M., Department of Geology, University of 

Auckland, Auckland, New Zealand, WILLIAMS, I.S., 

IRELAND. T.R., Research School of Earth Sciences, 

Australian National University, Canberra ACT 0200 

Australia, SMITH, I.E.M., and BLACK, P.M., Department 

of Geology, University of Auckland, Auckland New ~aland ' 

Late Cenozoic volcanic associations in the Taupo and 

Coromandel Volcanic Zones (TVZ and CVZ) contain a large 

proportion of felsic rocks. The TVZ in particular is unusual 

because of the large proportion of rhyolite relative to more 

mafic magmas (rhyolite:(andesite + dacite):basalt = 80:20:<1)). 

Models of rhyolite genesis in the TVZ generally involve either 

crustal anatexis or AFC processes (in particular basalt 

fractionation). The huge volume of erupted rhyolite, the 

relative scarcity of compositionally intermediate rocks, and the 

lack of geophysical evidence for complimentary cumulates 

pose a major problem for models invoking fractionation of a 

parent mafic magma. The alternative of crustal anatexis can be 

tested by searching for inherited relics of the crustal source. We 

analysed zircons from five North Island rhyolite samples in an 

attempt to fmd inherited grains, with the aim of constraining 

petrogenetic models. We also analysed zircons from basement 

rocks to determine age profiles for comparison with the 

inheritance in the rhyolites. 

Zircons from rhyolite samples were initially examined 

under cathodoluminescence to detect the presence of structural 

cores. Zircons showing promising structure were then analysed 

using the Sensitive High Resolution Ion MicroProbe (SHRIMP 

2). ~ecause of the young magmatic age (generally < 3 Ma) any 

~rams older than 10 Ma could be rapidly identified by 

06pbf207pb >> 1. Out of 5000 imaged grains, of which 300 

were analysed, only two old cores were identified. These were 

both from one TVZ rhyolite, with ages of 660 Ma and 2600 

Ma. No inherited cores were found in the Coromandel samples. 

The exposed geological basement in northern New Zealand 

contains two distinct greywacke assemblages (Torlesse and 

Waipapa). The zircon ages in the Torlesse sample are 

dominated by two peaks at 175-190 Ma and 230-275 Ma. 

These peaks can be resolved into six distinct components. 

Other age peaks are apparent at ca 350 Ma and 490-600 Ma, 

and two Precambrian grains at 1650 and 3000 Ma were present. 

One Waipapa sample gives a single broad peak around 213 

Ma that can be resolved into 5 components between 202 Ma 

and 255 Ma. Another Waipapa greywacke is dominated by 

peaks at 155-186 Ma, 211 Ma and 244-266 Ma, with minor age 

peaks at 350 Ma and 450-700 Ma, and a few grains at 900-

1050 Ma. There is no correlation between the inherited ages in 

the TVZ rhyolite and the age peaks in the greywackes. 

The lack of significant inherited zircon in the rhyolites 

places constraints on the amount of greywacke assimilation 

involved in the formation of TVZ rhyolite. If greywacke was 

involved, then assimilation proceeded to a degree where 

essentially all zircon was dissolved. This is highly unlikely in 

such felsic magmas, particularly as zircon can survive even 

when out of equilibrium. The rarity of inherited zircon and 

discrepancy between its age and that of zircon in the exposed 

greywackes indicates that greywacke assimilation, even during 

eruption, has been minimal. We suggest instead, generation of 

the TVZ rhyolites by partial melting of a zircon-poor 

(intermediate-mafic) lower crust. 

196 

ISOTOPE GEOCHEMISTRY AND METALLOGENIC 

REGULARITY OF THE HAIGOU GOLD DEPOSIT IN 

RLIN PROVINCE 

Yuqing Liu, Institute of Mineral 

Deposits, CAGS, Baiwanzhuang 

Road 26, Beijing 100037, CHIN A 

The Haigou gold deposit is located at the E end of 

the Huadian gold ore zone in Jilin Province, lying along 

the suture belt between the old Mongolian oceanic plate 

and the North China paleoplate. The gold deposit consists of >40 Au-bearing sulphide quartz veins which are distributed northeastward, 1600 m in total length, 

250m in width and >400 m downwards. Wall rocks 

inc1ude monzonitic granite, diorite porphyrite and 

metamorphic rocks of the middle Proterozoic Seluohe 

group, the Au abundance of which varies over the 

range 2.42-30ppb. Wall rocks on both sides of the 

Au-bearing quartz vein have undergone such alterations as silicification, sericitization and carbonatization. 

The isotope geochemical characterisitics of the gold 

deposit are given (S, C, 0, D and Pb). Sulfur in both ore veins and waH rocks is rich in ~, and average a

34S 

valuse of sulfide are -7.9%o and -4.1%o respective];;, 

exhibiting on obvious temporal -spatial variation. a

3 Sr s of ore fluids is -8.8%o. It is considered that sulfur might · have been derived from volcano- sedimentary 

-metamorphic rocks. aD and a180 values of Mesozoic 

meteoric water in this area are -149%o and -20%o 

respectively. During the ore- forming process, oxygen 

isotope composition of water and rocks varied considerably, and a180 value of the fluids tended to increase 

and drift. At the principal ore -forming sta¥;, aD val- ues of the fluids were -126- -1 OO%o , a 0 values 

2-6%o, and a13Cc0 , values -7.8--13.2%o. Ore fluids 

were mainly derived from magmatic source- hot meteoric water, Carbon came from deep carbon and organic 

carbon of strata. Lead in orcs and wall rocks is old lead 

from two- stage evolution, with t 1 being 2398Ma. 

Model ages oflead in single samples arc 1000-1200Ma, 

with p. values nearly 9.73. K I Ar Closed age of ores is 

143Ma, whereas Rb I Sr isochron age of rock body 

(waJI rock) is 181Ma. 

The da~a show that the greenschist -facies 

metamorphic rock of the middle Proterozoic Seluohe 

group served as the source bed of Au, monzonitic granite was formed by remelting of the source bed, and me- teoric water was the initial source of the ore fluids. It is 

thus inferred that during the early stage of the 

Yanshanian orogeny, due to the continuous expansion 

of the Mongolian oceanic plate and its subduction towards the base of the North China plate, strata in this area were intensely folded together with deep faulting 

and magmatic- hydrothermal activity, and this led to 

the formation of the remelting magmatic source-hot 

meteoric water type Au deposit. The rock- forming 

and ore- forming process might be divided into three 

epochs consisting of six stages, and the native goldsulphides- quartz assemblage marks the principal ore- forming stage of gold during which the Twas the range 

250-350t, ore fluids varied from acid to weakly acid, 

and the metallogenic environment was of the reduction 

type. 
MID-PALEOZOIC AGE OF PROVENANCE OF 

TRIASSIC (DOCKUM GROUP) SANDSTONE, TEXAS 

PANHANDLE, USA 

L.QNQ, I&Qn E. .• Dept. of Geological Sciences, Univ. of 

Texas, Austin, Texas 78712, USA, and LEHMAN, 

Thomas, Dept. of Geosciences, Texas Tech Univ., Lubbock, Texas 79409, USA 

Redbeds of the Upper Triassic (Carnian to Norian) 

Dockum Group are widely exposed around the Southern 

High Plains of the Texas Panhandle and adjacent eastern New 

Mexico. The Trujillo Sandstone and Cooper Canyon Formation in the upper Dockum contain abundant rock fragments and detrital mica flakes derived from metamorphic 

source(s). Paleocurrent data for these fluvial deposits indicate that streams flowed a direction opposite to that of modem streams, coming from a source lying to the south or 

southeast. Candidates for sedimentary provenance are 1.2-

1.3 Ga basement such as that in the Llano Uplift of central 

Texas, or metamorphic rocks in the subsurface of west Texas 

and northern Mexico belonging to the Interior Zone of the 

Ouachita orogenic belt. Ouachita rocks were exposed by uplift during late Triassic opening of the Gulf of Mexico. 

Muscovite is highly resistant to weathering, and to loss or 

exchange of Sr during mild thermal episodes. Fifteen detrital 

muscovite samples weighing 1 to 15 mg were separated 

from Dockum sandstones collected around the entire perimeter of outcrop, encompassing a vast tract that is 460 km 

(285 miles) wide in a direction away from the source region. 

Rb-Sr isotope data plot as a linear array corresponding to t = 369 ± 8 Ma (2a) and initial 87srt86sr = 0.7095 ± 0.0018 

(2a). If the array defines a mixing line, then mica from diversely aged sources would have had to be mixed in correlated proportions in each sample, which is possible but unlikely. Nevertheless a high MSWD (44.6) indicates a large 

geologic contribution to scatter of data, as expected for detritus from a source whose age is not perfectly single-valued. 

The indicated provenance is thus the Ouachita Interior 

Zone; Rb-Sr data for five muscovites from the Llano Uplift 

plot on a much steeper regression line. The new results and 

scant previous data suggest that metamorphic rocks of late 

middle Devonian age are present in the Ouachita orogenic 

belt. Metamorphism antedated the late Paleozoic collision 

that created the Ouachita Mountain foldbelt. 

Rb/Sr ratios (8.9 to 35.9) in nine muscovite samples 

from near the base of the stratigraphic section are much 

higher than Rb/Sr (2.8 to 8.1) of six samples from higher in 

the section. Regression of data grouped stratigraphically 

provides identical ages within error: 372 ± 36 Ma (lower) 

vs. 371 ± 7 Ma (upper). Apparent initial ratios, 0.707 ± 

0.016 (lower) vs. 0.709 ± 0.001 (upper), also overlap but 

with large uncertainties. Somewhat elevated initial ratios are 

consistent with metamorphism of a protolith that was Rbrich or ancient, or both. 

Suites collected from two geographically restricted areas 

hint of a fine structure of data. Four highly radiogenic muscovite samples collected from low in the Dockum section in 

the southernmost High Plains provide t = 330 ± 8 Ma, Ri = 

0. 732 ± 0.006. Four less radiogenic muscovites from high 

in the Dockum section in the northernmost exposure provide 

t = 384 ± 7 Ma, Ri = 0.706 ± 0.002. Possibly at different 

times the Triassic streams delivered detritus from sources of 

different age and initial ratio to different depocenters. 

Model ages of two rather unradiogenic biotite samples are 

=210 Ma, possibly registering Triassic volcanism. 

197 

1.1 Ga OLD ZIRCONS IN W-ARGENTINA: IMPLICATIONS- FOR A SEDIMENTARY PROVENANCE IN TilE 

DEVONIAN OF WES'IERN GONDWANA 

LOSKE, W .P ., Institut fiir Allgemeine und Angewandte 

Geologie, Ludwig-Maximilians-Universitllt 

80333 Mfinchen, Luisenstr. 37, Getmany 

The validity of conventional U-Pb dating of detrital 

zircons is still in discussion due to the general difficulty to 

select homogeneous and cogenetic zircon populations. Precise 

visual selection and accompanying cathodoluminescence 

analysis (CL) can help to elucidate the diversity of detrital 

zircons extracted from sedimentary rocks. 

The Argentine Precordillera, north of the city of 

Mendoza, delineates a Palaeozoic Orogen as a part of the 

western margin of Gondwana. Devonian sedimentary rocks 

consist mainly of turbiditic graywackes and mudstones. 

The following morphological zircon populations were 

recognized in the Devonian sandstones: 

(1) euhedral, magmatic 

(2) well rounded red coloured with transport marks 

(3) subhedral colourless, rich in inclusions 

CL analysis of the euhedral, magmatic zircons (1) testify a 

polyphase, but homogeneous, apparently oogenetic 

population. Conventional U-Pb analysis yielded a slightly 

discordant isotope pattern (ca. 20' %) for this population, and 

a calculated regression line intersects with the Concordia at 

ca. 1.1 Ga. The lower intercept is indicative for recent Ph-loss 

processes (fig. 1). 

Different stYles of luminescence (oscillatory and 

sandglass-like) from the well rounded red population (2) give 

hints for a presumably not oogenetic crystallization. The 

isotopic U-Pb ratios of this zircon population point to an 

uncertain, but significantly higher crystallization age (fig. 1). 

The subhedral population (3) often shows typical core -

rim patterns. Diffuse scatter of the U-Pb ratios supports the 

CL infotmation of a non-oogenetic zircon population (not in 

fig.1). 

Comparison of the well constrained crystallization 

interval! of 1.1 Ga with potential South American granitoid 

source areas of the same geological history at the western 

margin of Gondwana supposedly include the Rondonia Belt 

the Arequipa Massif and most likely the Sierra Pie de Palo as 

provenance areas of the detritus. 

0.26 

0.13 100 

1.0 
STABLE ISOTOPE AND CHEMICAL 

VARIATIONS IN DIAMOND FACIES MANTLE: 

EVIDENCE FOR FLUID INTERACTIONS 

ASSOCIATED WITH DIAMOND FORMATION 

LOWRY, D., MATTEY. D.P., MACPHERSON, C. G., 

Department of Geology, Royal Holloway, University of 

London, Egham Hill, Egham, Surrey 1W20 OEX, UK and 

HARRIS, J.W., Department of Geology, University of 

Glasgow, Glasgow, UK. 

New high-precision oxygen isotope data have been obtained 

for a large suite of mantle peridotites and eclogites by laser 

fluorination (LF) analysis. LF data for garnet (and spinel) facies 

peridotites reveal much uniformity in terms of mineral o18o 

values (notably o1 80olivine which avera~es 5.2±0.1, n=70) 

and inter-mineral fractionations. Garnet ol8o values in non- diamondiferous peridotites are 5.2-5.7%o (n=19) but lower 

o180g_arnet values in diamondiferous peridotites (4.9-5.3%o, 

n=9) mdicate that subtle disequilibrium effects may be 

associated with the presence of diamond. 

Eclogites possess whole rock o 18o values ranging from 

2.4-7 .6%o and ~ 18o garnet-cpx values measured by laserfluorination are consistently positive, averaging 0.35%o. All 

diamondiferous samples have o18o >4.7%o with 18 0garnet­ cpx values that are smaller than non-diamondiferous eclogites. 

All samples with fractionations <0.25%o (mostly 

diamondiferous) yield unrealistic isotopic temperatures 

(>1400°C) and again may represent disequilibrium assemblages. 

Syngenetic inclusions in diamond have remained isolated 

from diffusive exchange since encapsulation and record initial 

conditions of diamond growth. Oxygen isotope compositions 

have been obtained by LF for peridotitic (P) and eclogitic (E) 

inclusion suites from the Finsch Mine, RSA. P-type purple 

'G10' garnet inclusions have restricted o18o values falling 

between 4.6%o to 5.6%o (n=13) whereas E-tlf.e orange garnet 

inclusions have higher and more variable o 0 values (5.7%o 

to 8.0%o, n=11). Olivine inclusions in peridotitic diamonds 

have o180 of 4.9%o to 5.4%o (n=7). 

Most encapsulated 'G 1 0' garnets would not be in oxygen 

isotopic equilibrium with typical garnet peridotite and the data 

suggest a relationships between the magnitude of oxygen 

isotope disequilibrium of the P-type garnet o18o inclusions 

and inclusion chemistry. The change in o180garnet from 

equilibrium value of 5.6%o to lower, presumably 

disequilibrium, o18o values is related to both REE enrichment 

and Ca content. The enrichment in Ca in the garnets is also 

related to enrichment of Be in the host diamonds with 

diamonds bearing diopside inclusions having heaviest o13C and 

low o18odiopside (4.1%o). These changes may be consistent 

with interaction of garnet peridotite with Ca-deficient and REErich carbonate fluid with the main period of diamond growth 

associated with carbonation reactions. 

REE profiles for Finsch E-type garnet inclusions are similar to MORB and o180-o13c systematics of E-diamonds 

have affinities with carbonated (i.e. shallow) oceanic crust. Ca 

enrichment and Fe and Mn depletion in both garnet and 

clinopyroxene is related to decreasing inclusion olB"o (+8.0%o 

to +4.7%o) and increasing host diamond o Be ( -8.4%o to 

-3.0%o). The jadeite component in E-type clinopyroxenes 

decreases as host diamond o13c increases. 

Tendencies toward 18o depletion, Be enrichment and Ca 

enrichment during the major period of diamond growth in 

garnet and pyroxene inclusions of both peridotitic and eclogitic 

affinity at Finsch suggests a common process was involved, 

perhaps carbonation and exchange reactions between host rock 

and a carbonate-rich fluid. However it is less likely that the 

apparent disequilibrium observed in diamondiferous peridotites 

and eclogites is related to the diamond formation event, and 

may record a later fluid interaction, perhaps associated with 

movement and accumulation of protokimberlite melts. 

198 

FLUID INCLUSION Rb-Sr ISOCHRON AGE 

DATING OF LINGLONG GOLD DEPOSIT, 

SHANDONG, CHINA 

LU, HUAN-ZHANG, Science de la terre, 

Universite du Quebec a Chicoutimi, Quebec, G7H 

2B 1, Canada and LI, Huaqing and LIU, Jiangji, 

Yichang Institute of Mineral resources, Chinese 

Academy of Geological Science, Yichang, Hubei 

prov. China. 

The Linglong quartz vein gold deposit located in 

Shandong province, is one of the largest gold mine in 

China. The deposit is hosted by the Linglong 

gneissic biotite granite. Four mineralization stages 

have been recognized: 1. milky quartz-pyrite, 2. goldquartz-pyrite, 3. gold-polymetallic sulfides-quartz and 

4. carbonate stage. The ore minerals are native gold 

and electrum associated with pyrite and chalcopyrite. 

Associated alteration are mainly silicification, Kfeldspathization, sericitization and chloritization. 

These alternations are superimposed on the mineral 

assemblages of the granitic rocks. 

Four types of fluid inclusions :H20-rich (with 3-17 

wt. % equivalent NaCl), C02-rich, C02-H20, and 

C02-CH4 have been found. The homogenization 

temperatures of fluid inclusions in the main gold 

mineralization stages (2 and 3) range from 200 to 380 

°C. The present study is to date the gold 

mineralization age in fluid inclusion by using the RbSr isochron dating method. All samples are from the 

second mineralization stage of No. 55 vein , but 

located in different levels. Following 

microthermometry the quartz sample was heated until 

200 °C to decrepitate the secondary fluid 

inclusions. Then the Rb and Sr content were analyzed 

by mass spectrometer. 

This study presents preliminary results of Rb-Sr 

isochron age dating of fluid inclusions in order to 

understand the metallogenetic timing. Samples for 

fluid inclusion analysis were carefully selected based 

on the affinity between hosted mineral quartz related 

to gold mineralization. Other samples including 

granites are chosen to compare with the results from 

fluid inclusion determination. Two isochron ages 

have been obtained for these granites: 156 m.a. and 

154 m.a. The age of fluid inclusions in quartz is 

126.5+5.7 m.a., (87 Sr/86 Sr)i = 0.7111 +2. 

Based on these data we conclude that the Linglong 

granite formed from 156 m.a. to 154 m.a. and the 

gold mineralization is about 30 years later than the 

formation of the granite. 
A BETTER APPROACH TO 130rrh!U ISOCHRONS 

Ludwig. K.R .. U.S. Geological Survey, Box 25046, 

Denver, CO 80225, U.S.A., and Titterington, D.M., 

Dept. of Statistics, University of Glasgow, Glasgow 

G 12 8QW, United Kingdom. 

Most materials of interest for 230Th/U dating contain 

significant amounts of initial 230Th, 234U, and 238U, as evidenced by a measurable content of 232Th. To correct for 

the initial isotopes, linear trends are commonly fitted to the 

data on two X-Y isotope-ratio plots, such as 230Th/232Th238U/232Th and 234U/232Th 238U/232Th H · t' - . owever, ex1s mg 

treatments of 230Th/U isochron ages are inadequate, because 

they (1) ignore (usually) the analytical errors and (always) 

the analytical-error correlations of the input data, (2) do not 

take into account the fact that the data in the two isotoperatio plots are not independent, and (3) ignore the correlation between the errors of the isochron slopes (or intercepts) when calculating the isochron age. 

Expanding on the earlier work of one of us (D.M.T.), we 

have developed a Maximum-Likelihood Estimate (MLE) 

approach to determine the single linear trend that fits both 

pairs of ratios at once, with appropriate weighting for both 

analytical errors and error correlations, and that properly 

determines the errors, error-correlations, and goodness-of-fit 

of all derived isochron parameters. Equations for determining the appropriate input-error correlations for both alpha 

spectrometric and mass spectrometric data are also given, 

as well as the correct propagation of the errors and error 

correlations of the isochron parameters (that is, the 232Thfree isotope ratios) in the 230Th/238U age equation. 

Our method is equivalent to determining the best-fit line 

to the 230Th/U data in three dimensions, with X-Y-Z equal 

to either 230Th/23su_234uf23su- 232Th/23su or 23Sufl32Th234U/232Th 230Th/232Th · - . A smgle goodness-of-fit parameter 

(the familiar MSWD) permits quantitative evaluation of the 

validity of the assumptions of the single isochron. Arguments about the "best" choice of axes for 230Th/U isochrons 

are shown to be irrelevant (except for visualization purposes) because, if a correct method of line-fitting is used, any 

set of three axes that yields linear isochrons must (because 

they use the same data) yield essentially the same 230Th/U 

age and errors. 

This MLE method does not address the situation wherein 

significant scatter arising from non-analytical error exists. 

However, unless such scatter is gross, its presence cannot 

be evaluated until a realistic statistical evaluation (such as 

this procedure) has been done. Therefore, even if causes 

other than analytical error are responsible for data-scatter 

about an isochron, this procedure (or one similar) should be 

performed as a first step. Robust methods (which generally 

weight points according to their observed scatter rather than 

their analytical errors) are promising, but inefficient for the 

typically small 230Th/U-isochron data-sets. 

The 230Th/U isochron approach discussed here is implemented in current versions of Isoplot, as well as UISO (a 

small, stand-alone program that can be invoked by other 

programs). 

199 

GLOBAL ATMOSPHERIC C(h UPTAKE BY 

niEOCEAN: SEASONALANDGEOGRAPIDCAL CONSTRAINTS 

WO. Shan&@ and KU, Teh-Lung, Dept. of 

Earth Sciences, Univ. of Southern California, 

Los Angeles, CA 90089-0740, USA 

An analysis bas been done on the oceanic-C(h 

partial pressure (pC02) data observed in global 

oceans during the late 1980s to investigate the 

seasonal and regional variability of pC(h in the 

surface ocean. Two important findings in this 

study have made it possible to evaluate the global 

C02 uptake by the ocean from the observed pC02 

difference (~C02) between the surface ocean and 

the atmosphere. First, the aqueous C02 concentration [C02(aq)], expressed as pC (h at 20°C 

(pC02,2o), at a specific location in the ocean 

decreases exponentially with increasing temperature from winter to summer. This can be explained by the seasonally-variable, vertical convective 

mixing between the surface and deep waters and/or 

biological productivity in the surface ocean. 

Second, the temperature coefficient of C02(aq) fJd. 

defined as the relative change in C(h(aq) per °C, 

can be related to the y-intercept in a pC~20 vs. 

~C plot by the relationship: 

y-intercept = 195.6 + 1.50x104fJd- 4.67x104p; 

The data also show that there is a good correlation between winter pC~,20 and the temperature 

in surface waters. On the basis of our analysis, we 

have deduced global pC ~ distributions in the 

surface ocean for different seasons of the year and 

calculated the season-dependent, global air-sea 

C(h fluxes (in GtC/y, i.e., 1015 grams of carbon 

per year) during 1984-89 as follows: 1.8 (spring), 

0.5 (summer), 1.7 (fall), and 3.5 (winter). They 

give an annual uptake rate of 1.9 GtC/y. This 

uptake rate is in balance with the emission rate of 

fossil-fuel C~ and its accumulation rate in the 

atmosphere during the same period. It appears that 

the terrestrial ecosystems act neither as a net sink 

nor as a net source of anthropogenic C02. 
ISOTOPIC COMPOSmONS OF WARM SPRING AND 

NATURAL GASES FROM THE SOUTH ISLAND, NEW 

ZEALAND 

LYON. Graeme L., and GIGGENBACH, Werner F., 

Nuclear Sciences Group, Institute of Geological & 

Nuclear Sciences, PO Box 31312, Lower Hutt, New 

Zealand 

Analyses of seep gases from prospective petroleum areas and 

from warm and cold springs in the South Island are presented 

and compared with data from elsewhere. 

Several warm spring gases from fault zones are C02-rich or 

N2-rich and have methane that appears to have been microbially 

oxidised giving isotopically heavier methane. Hanmer Springs 

CH4 has a thermogenic composition that resembles that of gases 

from greywacke in the axial ranges of the North Island. 

Isotopic analyses of gases from the sedimentary basins show 

that CH4 from southern Marlborough is derived from mature 

sources, that from Wharf Stream is the most mature of the 

South Island hydrocarbon-rich gases. Three gases from the 

Murchison Basin show a trend in isotopic values indicating 

increasing maturity from W to E. Gases from Kotuku (Grey 

Valley) appear to be derived from thermogenesis at low 

maturity levels with variable additions of a microbial 

component. Gas from the Hindley Bum (Western Southland) is 

probably derived from a coal source as it is similar to that of 

the gas collected from a well drilled into the late Cretaceous, 

sub-bituminous Ohai coal deposit. Gases from three sites, 

Marlborough, the Murchison Basin and Kotuku, have high 

( > S%) ethane contents and high Ni Ar ratios confirming 

formation from mature sources. 

An unusual ~-rich gas from Fiordland contains CH4 that is 

similar in carbon isotopic composition to gases from greywacke 

but with very depleted hydrogen isotopic composition probably 

due to abiogenic reduction. In contrast, gas from an alluvial 

delta in Lake Te Anau shows that microbial reduction of C02 

is occurring as buried vegetation decomposes producing ca. 

that is isotopically light in both carbon and hydrogen. 

Ratios of 3

He/4He for most gases are generally above crustal 

values ( > 0.1 R.J implying addition mantle fluids. High values 

of > 2.5 R,. are observed in high CH4 gases discharged in the 

Murchison and Grey Valley basins, in Fiordland (H2-rich) and 

East Otago (C02-rich). 

200 

ISOTOPIC CHARACTERISTICS OF 

ULTRADEEP ULTRAMAFIC XENOLITHS 

FROM KIMBERLITES 

MACDOUGALL. J.D., Scripps Inst. 

Oceanography, La Jolla CA 92093, USA, and 

HAGGERTY, S.E., Dept. of Geology, U. 

Massachusetts at Amherst, Amherst, MA 

01003, USA. 

Ultramafic xenoliths containing garnet with 

exsolved pyroxene are believed to originate at 

depths > 300 km, possibly within the 

seismologically identified transition zone ( 1). If 

unaltered during subsequent residence at shallower 

depths, their isotopic compositions would be of 

obvious importance for understanding mantle 

chemical structure and evolution. We have 

analyzed pure mineral phases from three such 

xenoliths from Jagersfontein, South Africa. Sr and 

Nd isotopic compositions of garnets vary widely 

between xenoliths. Most fall within the known 

fields of lamproites and some Group II 

kimberlites, but are su~stantially different from 

their Group I kimberlite host. Analyses of leach 

solutions from pre-analysis cleaning of mineral 

separates suggest the presence of surface-sited 

"contaminants" which are also unrelated to the 

kimberlite host or analyzed phases. Three garnet 

separates analyzed show a good correlation 

between Sm/Nd and Nd isotopic composition but 

this relationship is probably without time 

significance. The single clinopyroxene separate 

analyzed to date exhibits strong isotopic 

disequilibrium with coexisting garnet, which could 

be interpreted in terms of the time of cpx 

exsolution (i.e. time of transfer to moderate 

mantle depths) at approximately 670 Ma, with an 

initial Nd isotopic ratio corresponding to ENd = 

+4 relative to CHUR. If correct, this would 

require that the (transition zone?) source of 

material in this xenolith was "depleted", at least in 

terms of its Sm/Nd ratio, before - 700 Ma. 

Alternatively, if continued isotopic exchange 

occurred, the time of cpx exsolution could be 

much earlier. 

1. Haggerty, S.E. and Sautter, V. (1990) 

Ultradeep (greater than 300 kilometers), 

ultramafic upper mantle xenoliths: Science 

248, 993-996. 
CHLORINE ISOTOPIC COMPOSmON OF OCEANIC 

ROCKSANDCARBONACEOUSCHONDRITES 

MAGENHEIM. A.J. , Scripps Inst. Ocean., UCSD, 

LaJolla, CA 92093, USA , Spivack, A.J., Dept. of Earth 

Sciences, University of North Carolina at Wilmington, 

Wilmington, NC, 28403, USA, and Michael, P. 

University of Tulsa, Tulsa, OK USA 

We have developed a method for the precise low level 

determination of the stable Cl isotope ratios (37cy35cl) of 

silicates. Cl is extracted by pryohydrolysis, purified by ion 

exchange chromatography, and isotopic abundances are 

determined by thermal ionization mass spectrometry of the 

dicesium chloride ion. Isotopic compositions are reported as 

per mil deviations relative to sea water. Based on replicate 

extractions and analyses the external reproducibility is 0.2 

per mil. 

Chloramphibole rich whole rocks dredged and drilled from 

the seafloor are enriched with ro37 Cl ranging between 0.5 

and 3.4. per mil. Oxygen isotope and fluid inclusion 

thermobarometry suggest that these rocks formed between 

450-500°C by reaction with Cl rich brines (up to 50 wt. %). 

In principle, the observed isotopic enrichment may be 

associated with phase separation and/or water/rock 

interaction. Brine and vapor pairs from supercritical phase 

separation experiments were analyzed for Cl isotope 

composition. Under the experimental conditions (301-397 

bars, 419-450 °C) isotopic differences between brine and 

vapor pairs are less than 0.2 per mil. Therefore, it appears 

that isotopic fractionation occurs during incorporation of Cl 

into solid phases. 

Fresh MORB and Marianas Trough basalt ro37 Cl values 

are between 1.0 and 4.9 per mil. This large of a range in Cl 

isotope ratios in fresh glasses is unexpected and supports the 

notion that Cl abundances are effected by assimilation of 

seawater or altered oceanic crust (amphibole) during 

eruption on a local scale. In addition, the MORB and 

chloramophibole data suggest that Cl isotopes may be a 

useful tracer of water released from amphiboles following 

subduction. 

The carbonaceous chondrites, Orgueil (CI) and Murchison 

(CM), have ro37cl of2.9 and 3.5 respectively. preferential 

volatilization of the light isotope from Murchison is 

consistent with the more evolved nature of CM meteorites 

relative to Cis. Assuming the isotopic composition of 

Orgueil as a constraint for the whole Earth and that the '0 

37cl of the hydrosphere is o.o, an enriched ro37cl reservoir 

in the mantle is inferred. By mass balance, we calculate that 

if60% of the Earth's Cl is in the mantle, its avera~e ro37cl 

is 4.8. This values is consistent with the MORB '03 Cl values 

which appear to be the least effected by assimilation. 

201 

He AND Ar ISOTOPES IN HOT FLUIDS AND ROCKS OF 

VULCANO (AEOUAN ISLANDS. ITALY): CONSTRAINfS 

ON MAGMA COMPOSITION 

MAGRO G. and FERRARA G • Istituto di Geocronologia e 

Geochimica Isotopica. CNR. 56127 Pisa. Italy 

The fluids of medium-high temperature (up to 650°C) 

fumaroles. issuing from La Fossa crater of Vulcano island are 

the result of a variable mixing between a deep magmatic 

component and a shallow one. The evolution in time of this 

mixing has been studied during a long period of observations 

(1979 up to now) either for the composition of the reactive and 

the inert gases. In a first time a two~omponent mixing was 

assumed between a deep magmatic fraction which does not 

seem to change and a shallow fraction carrying atmospheric 

components (ASW and/or seawater). as evidenced not only by 

Ar isotopes (Magro and Pennisi. 1991). but also by the bulk 

chemical and isotopic (CJD) composition (Chiodini et al.. 

1992). 

Actually. on the light of the important changes occurred in 

the 3Hef4He and 40 Ar/36 Ar ratios. a better definition of the 

deep component has been obtained. The increase of the 

3He/4He ratio (from 4.3 to 6 R/Ra). with the synchronous 

decrease of 40Arf36Ar (from 2000 to 300) points out that the 

deep component has changed in time the mixing between crust 

and mantle derived fluids. The crustal fraction. evidenced by 

40Arf36Ar high values. decreased towards a mantle-enriched 

fraction. marked by a high 3Hef4He ratio and a relatively low 

40Arf36Ar without an appreciable atmospheric contamination. 

Low 40Arf36Ar ratios (close to 320) have been found also in 

minerals (olivine) separated from lavas of Vulcano. The 

relatively low 40Arf36Ar and the high 3Hef4He suggests that 

the magma which had generated rocks with relatively low 

87srf86sr ratios (0.70425-0.70520; Ellam et al .• 1988; Del 

Moro, personal communication), is influenced by fluid-rock 

interactions with the surrounding basament rocks; this implies 

that low 40Arf36Ar ratios are representative of the magma 

originated in such a geological setting, where crust-mantle 

interactions occur. 

We recommend the importance of determining both 3Hef4He 

and 40 Ar/36 Ar in the hot fluids and in the volcanics rocks. and 

we emphasize the long-term observations as tools to 

discriminate between the events occurring in an active volcanic 

system. 

Chiodini, G., Cioni. R. and Marini, L., 1993, Reactions 

governing the chemistry of crater fumaroles from Vulcano 

island, Italy. and implications for volcanic surveillance: 

Appl. Geochem .• v. 8, p. 357-371. 

Ellam, R.M., Menzies. M.A .• Hawkesworth. C.J .• Leeman, 

W.P., Rosi. M. and Serri. G .• 1988. The transition from 

calc-alkaline to potassic orogenic magmatism in the Aeolian 

Islands. Southern Italy: Bull. Volcanol., v. 50. p. 386-398. 

Magro. G. and Pennisi M., 1991, Noble gases and nitrogen: 

mixing and temporal evolution in the fumarolic fluids of 

Vulcano. Italy: J. Vole. Geotherm. Res .• v. 47. p. 237-247. 
Nd-8r ISOTOPIC CORRELATIONS OF FINE-GRAINED 

CLASTIC SEDIMENTS IN WESTERN PACIFIC 

MARGINAL BASINS: A POTENTIALLY POWERFUL 

TOOL FOR S1RATIGRAPIDC CORRELATION 

Mahoney. J. Brian. Department of Geological Sciences, 

University of British Columbia, Vancouver, B.C. V6T 1Z4 

The Nd and Sr isotopic signatures of fine-grained 

sedimentary strata may provide a new tool applicable to 

stratigraphic correlation and basin evolution studies. The 

isotopic signature of a basinal sequence is a function of the 

weighted average isotopic composition of all the crustal 

domains providing sediment to a depositional basin. 

Thorough mixing of fine-grained sediments homogenizes the 

isotopic signal, thus providing a signature that may be unique 

to an individual basin. The basinal isotopic composition will 

vary within limits, reflecting changes in the sediment flux 

derived from isotopically distinct crustal domains within the 

source region(s). These temporally controlled compositional 

fluctuations impart a corresponding isotopic variation that 

may be used as a stratigraphic marker to correlate strata 

throughout a depositional basin. The isotopic signature of 

well-dated strata may be compared with sections with poor 

age constraints to provide stratigraphic control. In addition, 

fluctuations in the isotopic signature ultimately result from 

geologic events and paleoclimatic variations in the source 

region(s), and may therefore be useful in reconstructing basin 

evolution. 

Isotopic analyses of Plio-Pleistocene sediments from 

western Pacific marginal basins (Sea of Japan, Philippine 

Sea, Sulu Sea) are presented as a test case for Nd-Sr isotopic 

correlation and basin discrimination. Results indicate that 

the isotopic signature of these basinal sequences fluctuates 

within narrow, predictable limits, and that the signature is 

sensitive to changes in sediment flux from isotopically 

heterogeneous source regions. Deviations from predicted 

isotopic values suggest extrabasinal sediment influx; values 

from the Shikoku Basin (Philippine Sea) are significantly 

lower than expected (epsilon Nd ( -7) - ( -9) vs. expected 

epsilon Nd (-2) to (+2)). These values are interpreted to 

represent eolian sediment influx into the basin from eastern 

Eurasia, coincident with climatic change in Late PlioceneEarly Pleistocene time. 

These results indicate that the isotopic signature of finegrained clastic sediments may be an extremely useful tool in 

discriminating ancient, structurally disrupted basinal 

sequences, and placing constraints on basin history. 

202 

OS ISOTOPES AND RECYCLED OCEANIC CRUST 

MARCANTONIO. F .. ZINDLER, A., ELLIOTT, T., 

Lamont-Doherty Earth Observatory, Palisades, NY, 

10964, USA, and STAUDIGEL, H., Vrije 

Universiteit, Laboratory of Isotope Geology, De 

Boelelaan 1085, 1081 HV Amsterdam, The 

Netherlands 

The isotopic and trace element signatures of HIMU 

ocean island basalts (OIBs) have provided evidence for 

the role of subducted oceanic crust in their genesis [1). 

Os isotope analysis of OIBs has the potential to further 

constrain crustal recycling hypotheses. Recent Os 

isotopic measurements of HIMU-endmember OIBs [2,3] 

reveal large and intriguing variations in 1870sJ186Qs 

ratios that range from 1.12 to 1.72. Before attributing 

these variations to heterogeneous mantle source regions 

containing recycled crust, other possibilities, including 

contamination with radiogenic Os within the volcanic 

pile, or interaction between magma and lithospheric 

cumulates, must be ruled out [3]. 

In order to better distinguish between these 

possibilities, we have investigated Os isotope 

systematics at the island of La Palma, in the Canarian 

archipelago. We analyzed samples both from the older 

(4.0-2.9 Ma), altered, uplifted seamount section of the 

island, and the younger (1.6 Ma to Recent) subaerial 

lavas. Initial 1870sJ186Qs ratios for the older altered 

samples are extremely heterogeneous and range from 

1.21 to 3.36, while the younger subaerial samples are 

less heterogeneous in 187QsJ186Qs and range from 1.13 to 

1.59. Our results imply that significant contamination 

of the subaerial basalts with radiogenic Os from 

altered submarine basalts may occur. However, mixing 

calculations suggest that only the Os-poor subaerial 

magmas (with concentrations less than 30 ppt) are 

susceptible to observable isotopic modification by this 

process. 

Os isotopic signatures for uncontaminated La Palma 

samples range from 1.13 to 1.25. Pb isotope ratios for 

the same samples show no correlation with Os isotope 

ratios. When potentially contaminated low-Os OIBs 

are screened from the literature data set [2,3], HIMU 

and transitional-to-HIMU OIBs have the highest (up 

to 1.25) and most variable Os isotopic signatures. 

However, simple mixing between recycled oceanic crust 

and ambient peridotitic mantle is not demonstrated by 

the Pb-Os isotope systematics. These systematics may, 

however, provide information about the specific 

chemical characteristics of the recycled material 

sampled at individual ocean islands. 

[1] Hofmann and White, 1982, Earth Planet. Sci. Lett., 

57, 421-436. [2] Hauri and Hart, 1993, Earth Planet. 

Sci. Lett., 114 353-371. [3] Reisberg et al., 1993, Earth 

Planet. Sci. Lett., in press. 
PHOSPHORUS CYO..ING AND PHOSPHORUS SOURCES 

IN LAKE KINNERET; TRACING BY OXYGEN ISOTOPES 

IN PHOSPHA1E 

MARKEL Doron . KOWDNY Yehoshua. LUZ Boaz, The 

Institute of Earth Sciences, The Hebrew University of 

Jerusalem, Jerusalem, Israel, and NISHRI Aminadav, 

1~1 Oceanographic Limnological Research, Kinneret 

Lunnological Laboratory, Tabgha, Israel. 

Th~ isotopic composition of oxygen in phosphate (~180p) in 

sediments and suspended matter of Lake Kinneret (the Sea of 

Galilee, L.K.) serves as a tracer of the phosphorus sources 

and sinks. ~~~and the distribution of phosphate 

concentrations in different fractions of the sediments were 

measured at a number of stations in the lake and in its 

catchment basin. 

Sequential extracts of phosphate show that the major fraction 

of phosphorus in L.K. sediments is linked to calcium either as 

apatite or as a surface complex on calcite crystals. A ~nor fraction is adsorbed on clays and iron hydroxides. No ironphosphate or aluminum-phosphate minerals were detected. 

Approximately 70% of the particulate inorganic phosphate 

~ntering L.K. is ~m a basaltic source (~18<>p= 6%o). This 

mclu~es ~e detrital sand fraction (probably fluor-apatite,) most 

of which Sinks to the bottom immediately upon entering the 

lake. and is thus mechanically removed from the cycle. 

Only 40% of the phosphate in the silt and clay fractions is 

derived from a basaltic source; the rest is of sedimentary and 

antbro~genic origin (~18<>p= 18 to 25%o). 

Isotopic mass balance shows that authigenic apatite is 

precipitated in L.K., mostly at the Jordan inlet delta, due to 

high pH and intermediate dissolved phosphate concentration. 

This apatite, possibly hydroxy-apatite, reaches the deep part of 

the .lake and dissolves at relatively low pH ( -7 .5) mainly 

dunng the stratification period (April - November). 

A phosphate surface complex on calcite crystals, 

~3(HC03)3(P04) (CCP), or CaHP04 {PCP), is precipitated 

m ~e L.K. catchment basin and contributes one quarter of the 

particulate phosphate entering the lake from the Jordan. A 

large part of this complex is removed at the Jordan inlet delta 

at relatively high pH, but it is re-precipitated on calcite 

surfaces in the deep part of the lake. 

The phosphorus which is released from dissolution of 

authigenic apatite in the deep part of the lake and from the 

dissolution of the phosphate surface complex are both internal 

sources of bio-available inorganic phosphate. 

203 

PALEOHYDROLOGY FROM STRONTIUM ISOTOPES 

ATYUCCAMOUNTAIN,NEVADA 

MARSHALL B.D .. PETERMAN, Z.E., and 

STUCKLESS, J.S., U. S. Geological Survey, Federal 

Center MS 963, Denver, CO 80225 USA. 

Yucca Mountain, located in southwest Nevada, is being 

studied as the site for a potential nuclear waste repository. The 

host rock for the potential geologic repository, the Topopah 

Spring Tuff, is in the unsaturated zone. The elevation of the 

water table is fairly uniform beneath the site, but the depth to 

the water table ranges from approximately 480 to 780 m, due 

to topography, and the potential repository horizon is 180-

410 m above the water table 

Calcite and other secondary minerals are being examined for 

their paleohydrologic implications. Core samples from holes 

drilled through the Tertiary volcanic section show that calcite 

occurs in both the unsaturated and saturated zones as fracture 

coatings and infillings. The calcite ages could range from the 

period of volcanism (11-14 Ma) to the present-day, but at least 

some samples are Quaternary .1 

In the saturated zone, calcite 87Srf86Sr ranges from 0.7087 

to 0.7098, similar to values observed in the volcanic rocks. 

However, both the rocks and secondary minerals may have 

been influenced by lower carbonate aquifer water that may have 

saturated the volcanic section early in its history. In the 

unsaturated zone, 87Srf86Sr in fracture calcite ranges from 

0. 7114 to 0. 7127, similar to pedogenic calcite exposed at or 

near the surface. Six samples from 48 to 90 m above the water 

table have 87Srf86Sr significantly lower (0.7104 to 0.7112) 

than that of the pedogenic calcite, but similar to present-day 

groundwater in the region. There is a weak correlation between 

depth and 87Srf86Sr in the samples from the unsaturated zone, 

similar to the trend of 87Srf86Sr in the host volcanic rocks. 

This may indicate an additional influence of volcanic rock 

strontium on the infiltrating fluids from which the calcite 

precipitated. 

Locally, the basal vitrophyre of the Topopah Spring Tuff is 

relatively impermeable and water is perched above it. This 

perched water sampled from 90 m above the water table has 

87Srf86Sr = 0.7124, similar to average pedogenic carbonate, 

indicating its derivation as infiltration from the surface. Tuff 

samples from a nearby drill hole at the same stratigraphic 

horizon show extensive alteration; their strontium 

compositions are consistent with a fluid composition of 

0. 7124. The intense alteration of the volcanic section here, 

accompanied by a large increase in strontium concentration, 

indicates high water/rock ratios were attained locally. 

Based on strontium isotope data, three tentative and 

preliminary conclusions may be drawn: 1) the water table 

beneath Yucca Mountain may have been up to about 90 m 

higher in the past; 2) perched water derived its strontium from 

pedogenic sources; and 3) perched water may be responsible for 

localized alteration of the volcanic rocks. 

1 Szabo B.J. and Kyser T.K. (1990) Ages and stable-isotope 

compositions of secondary calcite and opal in drill cores 

from Tertiary volcanic rocks of the Yucca Mountain area, 

Nevada. Geol. Soc. Am. Bull., 102, 1714-1719. 
LIMITS OF IDGH PRECISION TIMS ANALYSIS IN 

PALAEO-OCEANOGRAPHY 

MARTEL. D. J., HENDERSON, G. M., BURTON, K. W. 

and O'NIONS, R. K. (Dept. of Earth Sciences, Univ. of 

Cambridge, Downing St., Cambridge, CB2 3EQ. U.K.; 

E-Mail DJM14@ESC.CAM.AC.UK) 

Modern palaeo-oceanographic research sets some 

particularly challenging aims for high precision TIMS isotope 

ratio measurements. These challenges are set by the scope to 

use isotopic tracers of varying residence times to constrain 

palaeo-oceanographic response to climatic and tectonic 

forcing. When studying systems with residence times that are 

long in comparison with the events being studied (for example 

Sr), the magnitudes of the isotopic shifts may be small, and 

approach the limits to which they can be measured. In such 

circumstances the factors which may affect the measurements 

must be fully and carefully evaluated to ensure that any 

measured isotopic shift incorrectly ascribed. 

Modern multiple collector mass spectrometers readily 

produce extremely precise ( -5ppm) single analyses of Sr and 

Nd using either static or multi-dynamic procedures. However 

the accuracy of the measurement (long-term reproducibility) is 

the main criterion of interest. 

Many factors affect the accuracy of analyses, some such as 

amplifier gain calibration, DVM linearity, and tau-correction 

may be measured accurately without using ion beams. Other 

factors such as uniformity of bucket efficiency, and 

fractionation behaviour may only be measured using ion 

beams, and are limited in accuracy due to beam instability 

during calibration, or the need to make many high precision 

isotope ratio analyses of a standard. 

In this contribution particular attention is given to 

limitations that arise from fractionation behaviour. High 

precision Sr standard analyses were made over a period of 

several months using triple filaments at 88sr beam currents of 

-3xlo-11amps. Observed 87srJ86sr ratios were best 

reconciled with 86sr/88sr by fitting an empirical fractionation 

correction, giving an external reproducibility of <lOppm. 

Conventional calculation assuming fixed (either exponential 

or power law) fractionation behaviour on the same data set 

degrades the reproducibility by a further 6 to 30 ppm 

depending on which law is used. Indications are that running 

conditions may affect fractionation behaviour. 

By careful control of analytical procedures, and machine 

calibration it is possible to achieve reproducibility of the order 

of lOppm using the current generation of mass spectrometers. 

Details are given of the procedures used for 87 Sr/86sr and 

progress with high precision 143Nd/144Nd ratio 

measurements. 

204 

SIMULATION OF COSMOGENIC-NUCLIDE PRODUCTION IN TERRESTRIAL ROCKS 

MASARIK. J. and REEDY, R. C., Astrophysics 

and Radiation Measurement Group, Los Alamos 

National Lab., Los Alamos, NM 87545, USA. 

Some secondary particles produced by the interactions of galactic cosmic rays (GCR) in the Earth's atmosphere react with nuclei in the surface. Detectable 

amounts of stable or radioactive isotopes not present 

or rare in the target material can be produced in these 

interactions. For the interpretation of measured cosmogenic nuclides in terms of the target's exposure history (e.g., age or erosion rate), good modeling of their 

production rates is necessary. 

We used a purely physical model that has been 

well tested with extraterrestrial materials [Masarik 

and Reedy, 1994] for the calculation of cosmogenicnuclide production rates in the Earth's surface. The 

nucleon spectra as a function of depth in the Earth 

were calculated by Monte Carlo simulations using the 

LAHET Code System for interactions of cosmic-ray 

particles with the Earth's atmosphere and surface and 

the subsequent production and transport of secondary 

particles. An isotropic GCR particle flux irradiated a 

sphere with the Earth's radius and having an average 

continental-crustal composition with a 2% water content. This irradiation applies to geomagnetic latitudes ~60° where there is no cosmic-ray cutoff. The model 

atmosphere has a thickness of 1030 gfcm2 (sea level) 

and observed composition and densities. Having calculated nucleon fluxes, the production rate of nuclides 

were calculated by integrating over energy the product of these fluxes and cross sections. for the nuclear 

reactions producing each nuclide that had been tested 

using extraterrestrial materials. Running 650,000 primary GCR protons, we calculated surface particle 

fluxes with errors of "'30%. We are doing more calculations to get better statistics and to study attenuation 

lengths and different atmospheric thicknesses. 

Our calculated surface production rates for high 

geomagnetic latitudes and sea level agree with those 

reported in natural samples. In quartz, our preliminary calculated production rates (atomsjyearjg-Si02) 

were 3He ~ 124, 10Be ~ 6, 14C ~ 19, 21 Ne ~ 19, and 

26 Al ~ 36. In CaO, the calculated 36Cl production 

rate was 47 atoms/yr/g-CaO. In olivine with Mg being 81% of the Mg+Fe atoms, rates per gram of olivine 

were 3He ~ 105 and 21 Ne ~ 40. These production 

rates were calculated with only neutron and proton 

fluxes; muon contributions were not calculated. For 

surface 26 AI, only "'6% of the calculated production 

was by protons. Better statistics are needed to get 

good depth profiles, but our preliminary calculations 

show the production-rate-versus-depth profiles relatively flat to a depth of "'20 gfcm2 and then decreasing 

roughly exponentially with attenuation lengths that, 

within large errors, are consistent with reported ones. 

Masarik J. and Reedy R.C. (1994) Geochim. Cosmochim. Acta (submitted). 
MAGMATIC PROCESSES AT A CONTINENTAL 

COLLISION ZONE: THE EAST CARPATHIAN ARC, 

ROMANIA. 

MASON P.R.D .. and DOWNES H., Dept Geology, 

Birkbeck College, University of London, Malet St, 

London, WC1E 7HX, UK, THIRL WALL M.F., Dept 

Geology, Royal Holloway, University of London, 

Egham, TW20 OEX, UK, SEGHEDI I., and 

SZAKACS A., Institute Geology and Geophysics, Str 

Caransebes, Bucharest, Romania 

The East Carpathians represent the final episode of 

subduction-related magmatism in Eastern Europe, at the 

south-eastern end of a Mio-Pliocene arc stretching from 

Hungary and Slovakia into Romania. South-westwards 

subduction beneath the Pannonian microplate produced 

mainly andesitic to dacitic stratovolcanism. Activity 

became progressively younger south-eastwards along the 

arc, terminating at 1-2Ma with contemporaneous high-K 

arc lavas and back-arc alkali basalts. 

Most volcanic centres are calc-alkaline to high-K calcalkaline. Lavas are LILE- and LREE-enriched by slab 

fluid/melt metasomatism (Ce/Yb=4-7) and exhibit a large 

range of silica contents (51-70%) and Mg#s (0.25-0.64). 

The wide range in trace element abundance and Eu/Eu* 

(0. 7 -1.0) can be related to fractional crystallisation 

processes. ESr (-6.2 to +75.5) and ENd (+5.0 to -3.9) 

isotope ratios vary widely. Mixing is suggested between 

different primitive magmas and crustal end members in 

different parts of the arc. Some suites show systematic 

Si02_87srt86sr relationships consistent with varying 

degrees of crustal assimilation. Pb isotopes are strongly 

enriched (~207pb=9.7-19.3, ~208Pb=35.7-112.3) and can 

be, in part, linked to crustal assimilation as indicated by Nd 

isotopic correlations. However the least contaminated rocks 

still have Pb displaced well above the NHRL. Laser 

fluorination oxygen isotope analysis will further constrain 

the roles of crustal assimilation and source enrichment. 

The youngest activity (1-3Ma) consists of small volume, 

high-K (2-4% K20) volcanic centres and intrusions of 

shoshonitic affinity. Trace elements in these magmas were 

strongly enriched due to smaller degrees of partial melting 

related to the waning of magmatism. Shoshonitic rocks are 

strongly LREE-enriched (Ce/Yb=18-34). Mineral 

disequilibrium assemblages indicate the existence of 

quartz+biotite- and Mg-olivine-rich end members. Mixing 

may have occurred between an evolved calc-alkaline magma 

and a contemporaneous alkali basalt magma. 

An older (8-9Ma) group of low-K dacites ( -1 %K20) with 

primitive Sr-Nd isotopic compositions (ENd=+5.0 to +2.5) 

are distinctive in the north of the region. Small positive Eu 

anomalies (Eu/Eu*-1.08) and the lack of negative 

strontium anomalies, are common indicating derivation 

without extensive plagioclase fractionation. LREE 

enrichment is similar to the main arc trend (Ce!Yb=4-6). 

205 

EVOLUTION OF CERIUM AND NEODYMIUM 

ISOTOPIC RATIOS CONSISTENT WITH 

MASUDA-MATSUI'S MODEL 

A. Masuda, 

Department of Chemistry, 

University of Electro-communications, Chofu, Tokyo-182, Japan 

Thirty years ago, Masuda and 

Matsui [1] succeeded in mathematically dealing with the chondritenormalized REE patterns. This 

treatment led them to a conclusion 

that the bulk partition coefficients (k) of REE between the coexisting equilibrated solid and 

liquid (molten) silicate phases is 

a linear function of difference in 

atomic number between REE. According to their analysis, the k values 

were estimated to be 0.119, 0.155, 

0.227 and 0.299 for La, Ce, Nd and 

Sm, respectively. (Another set of 

k values, i.e., 0.191, 0.249, 0.364 

and 0.480 has been proposed afterwards, but this does not affect the 

present discussion, because the 

relative magnitudes are given to be 

identical to each other of both 

sets.) The corresponding La/Ce and 

Sm/Nd abundance ratios for the 

earliest-stage solid phase separated from CHUR are evaluated to be 

0.297 and 0.424. The calculation 

indicates that a set of the numerical values of eCe = -1.4 and 

eNd +10.6 for T (separation 

time) = 2.2 Ga is consistent with 

Masuda-Matsui's model. The observed 

values for N-type MORB [2,3] are in 

pretty good agreement with the 

results mathematically inferred 

based on the above model. This 

agreement endorses empirically the 

validity of the important assumptions adopted in proposal of the 

model under consideration, for the 

evolution of the earth [1]. But 

other questions can be raised. 

1.Masuda, A., and Matsui, Y., 1966, 

Geochim. Cosmochim. Acta, 30, 

239-250. 

2 .Makishima, A., and Masuda, A., 

1994(under submission to a 

journal) 

3.Tanaka, T., Shimizu, H., Kawata, 

Y., and Masuda, A., 1987, 

Nature, 327, 113-117. 
EXPERIMENTAL STUDY OF 18ot16o PARTITIONING 

BETWEEN CRYSTALLINE ALBITE, ALBITIC GLASS, 

AND C02 GAS: IMPLICATIONS FOR MELTING AND 

DIFFERENTIATION. 

MATTHEWS, A. Institute of Earth Sciences, Hebrew 

University of Jerusalem, 91904 Jerusalem, Israel, PALIN, 

J.M., EPSTEIN, S., and STOLPER, E.M., Division of 

Geological and Planetary Sciences, California Institute of 

Technology, Pasadena, CA 91125, U.S.A. 

Oxygen isotope partitioning between gaseous C02 (1 bar) 

and crystalline albite and albitic glass has been measured at 

750° - 950°C. Convergence of oxygen isotopic values of C02 

to equilibrium and avoidance of surface-correlated fractionation 

effects is achieved in long runs (>200 days) using relatively coarse grain sizes. Equilibrium C02-crystalline albite oxygen 

isotope fractionation factors are: 4.74 ± 0.22 at 750°C, 3.77 ± 

0.23 at 850°C, and 3.36 ± 0.21 at 950°C. These values compare 

well with estimates made by combining the experimental 

calcite-albite data of Clayton et al. (1989) with C02-calcite 

fractionation factors. Our results thus provide independent 

support for the high pressure calcite-mineral fractionation 

factors of Clayton et al. (1989). 

C02-albite exchange experiments of relatively short duration 

give disequilibrium fractionation factors. Oxygen diffusion 

coefficients calculated from these experiments, however, are comparable with previous determinations of oxygen diffusion in 

feldspars under nominally anhydrous conditions. Diffusion 

coefficients calculated for albitic glass are slightly higher than 

those measured for silica glass by Stolper and Epstein (1991) 

and support the hypothesis that isotopic exchange is controlled 

by diffusion of molecular carbon dioxide in these glasses. 

Equilibrium oxygen isotope fractionation factors determined 

for C02 -albitic glass are identical within experimental 

uncertainty to those determined for C02-crystalline albite. 

Additional measurements of oxygen partitioning between C02 

and silica glass confirm the results of Stolper and Epstein 

(1991). A comparison of the silica glass-albitic glass 

fractionation from our work with the quartz-albite fractionation 

from Clayton et al. (1989) suggests that the silica glass-albitic 

glass fractionations are distinctly larger than the equivalent 

fractionations for the crystalline phases. Given that crystalline 

albite and albitic glass have essentially identical reduced 

partition function ratios , a clear implication is that the isotopic 

properties of silica glass differ significantly from those of 

crystalline quartz; i.e., silica glass is isotopically enriched with 

respect to crystalline quartz by 0.5 to 0.9 %o at 650° to 950°C. 

An analysis of consequences of the experimental data for 

melting and differentiation processes in silicic magmas suggests 

that for given constant crystal compositions, crystal-liquid 

fractionation factors will also change systematically with melt 

composition with the magnitude quartz-melt fractionations 

decreasing and feldspar-melt fractionations increasing with 

increasing silica in the melt. An example calculation of the 

melting of a model quartz-feldspar-biotite gneiss at 700° C 

indicates that the initial minimum melt will be 0.8%o heavier 

than the parent rock. Thus, although these changes are not 

likely to be much larger than 1 %o at magmatic temperatures, 

they are likely to be important in efforts to interpret and 

quantitatively model the evolution of oxygen isotopes during 

processes of melting and differentiation. 

Clayton R. N., Goldsmith J. R. and Mayeda T. K. (1989) 

Oxygen isotope fractionation in quartz, albite, anorthite, and 

calcite. Geochim. Cosmoch~m. Acta 53, 725-733. 

Stolper E. and Epstein S. (1991) An experimental study of 

oxygen isotope partitioning between silica glass and C02 

vapor. In Stable Isotope Geochemistry: A Tribute to Samue1 

Epstein. Special Publication No. 3, The Geochemical Society 

(ed. H. P. Taylor jr., J. R. O'Neil, and I. R. Kaplan), pp. 35-

51. 

206 

BASIC AND UL TRABASIC XENOLITHS FROM 

KERGUELEN: RECORD OF A DUPAL PLUME ACTIVITY 

MATTIELLI N., WEIS, D., Petrol. Geodyn. Chimique, 

CP160-02, Univ. Libre de Bruxelles, 50, 

Av. F.D. Roosevelt, 1050 Brussels, Belgium and 

SHIMIZU, N., WHOI, MA 02543, USA, GREGOIRE, M. 

and GIRET, A., Univ. J. Monnet, St.Etienne, France. 

In order to understand the geochemical evolution of 

Kerguelen plume and its interactions with the upper 

mantle, we have studied ultrabasic and basic xenoliths 

from Kerguelen Archipelago. 

Our samples come from pipes and dykes in the Upper 

Miocene Series of the Southeast Province. 

The xenoliths are distributed in 2 categories: Type1 -

spinel-2 pyroxenes bearing ultrabasic and basic series (all 

terms from lherzolite to gabbro) with a predominant 

granoblastic texture and numerous relics of cumulate 

magmatic texture - and Type 2 - harzburgites and dunites 

with protogranular - porphyroclastic textures. 

Trace element abundance patterns in cpx of Type1 are 

characterized by low overall abundance levels ( -1 times 

chondrite). Those of Type 2 show LREE enriched and/or 

LREE enriched convex down geometry (La/Sm)N=1 0. 

Sr, Nd and Pb compositions and concentrations have 

been measured on whole rocks of 10 xenoliths (sp/sa±gt 

bearing metagabbros; ilm/gt/rut bearing metagabbro; 

websterite; harzburgites and dunite) and on separated 

leached cpx and/or pi and/or gt from 2 harzburgites and 2 

sp/sa±gt bearing metagabbros. Xenoliths as a whole, 

cover the entire range of isotopic variations in plutonic and 

basaltic series of Kerguelen islands. Hence they carry a 

Dupal anomaly signature. Type 1 xenoliths isotopic 

signatures are very homogeneous (B7SrfB6Sr from 

0.70422 to 0.70447, ENd from 4.8 to 2.6, 206pbf204pb 

from 18.102 to 18.184, 207pbf204pb from 15.501 to 

15.570, 208pbf204pb from 38.136 to 38.592), similar to 

tholeiitic-transitional basalts, suggesting a common origin 

for those basalts and all xenoliths of this type. A 

preliminary Sm-Nd isochron on cpx and whole rocks 

suggests an age of 161 ± 73 Myr, i.e. contemporaneous 

with the Kerguelen Plateau formation. We speculate that 

Type 1 xenoliths are cumulates from basaltic magmas 

formed by underplating 1 beneath the plateau during its 

formation. Type 2 peridotites display the most Dupal 

signature (B7srtB6sr from 0.70506 to 0.70577, 

206pbf204pb from 18.390 to 18.456, 207pbf204pb from 

15.546 to 15.593, 20Bpbf204pb from 38.846 to 39.024, 

and ENd 3.9 to -0.1) comparable to the young (8 Myr) 

highly alkaline series signature. These isotopic ratios 

indicate that the Type 2 peridotites can correspond to 

residual products of the highly alkaline series. They have 

suffered a stronger enrichment (explaining their LREE 

enriched signature) through interaction with an alkaline 

magma and/or with a C02 enriched magma2. 

(1) Gregoire et al. (1993), in press in Nature. 

(2) Schiano et al. (1993), submited to EPSL. 
REVERSE DISCORDANCE IN THE U-Pb SYSTEM IN 

ZIRCONS: THE ROLE OF SMALL-SCALE ZONING AND 

DIFFUSION AS REVEAl .. ED BY STEP-WISE 

DISSOLUTION EXPERIMENTS 

MATTINSON. James M., PARKINSON, David L., 

McCLELLAND, William C., and GRAUBARD, Cinda M., 

Department of Geological Sciences, University of California 

Santa Barbara, California 93106 USA 

Reverse discordance (here used in its original sense to mean 

206/238 age> 207/206 age) is a relatively rare but welldocumented phenomenon in zircons. Most previous 

explanations of reverse discordance have invoked introduction 

of radiogenic Ph from outside sources, preferential loss of U, 

or breakdown of zircon into badellyite and silica phases, with 

sequestering of radiogenic Pb in the silica phase. 

Specially designed step-wise dissolution experiments reveal 

that some zircons contain zones that are highly insoluble and 

contain little or no U, but do contain significant amounts of 

essentially pure radiogenic Ph. The highly radiogenic nature 

of the Pb, together with its isotopic systematics, clearly reveal 

that the Ph is derived from within the zircon grains 

themselves, rather than from outside sources. The association 

of the "excess" Ph with the least soluble parts of the zircon 

argues against a silica host phase, which would be highly 

soluble in HF. 

We suggest that magmatic zoning on a fine (i.e., sub-micron 

to few micron) scale in zircons is an important key to 

understanding reverse discordance. We propose that the 

zoning results from the interplay between episodic zircon 

crystallization and the relative diffusion rates for Zr and U. 

Episodic crystallization can be understood as follows: 

initiation of crystallization, either of seed crystals or of a layer 

of zircon on an existing zircon crystal, requires a finite level of 

supersaturation as a driving force. Zircon growth then 

proceeds fairly rapidly until Zr drops to the exact saturation 

level in the magma immediately surrounding the growing 

zircon crystal. This produces a hiatus in crystallization until 

the local Zr level is restored to the supersaturation level 

required for renewal of crystallization, either by diffusion, or 

by continued crystallization of major minerals that exclude Zr, 

thus driving its concentration back up in the residual magma. 

During each cycle of crystallization, strong zoning of trace 

elements such as U will be produced if their diffusion rates in 

the magma are lower that of Zr. In particular, under highly 

oxidizing conditions, U may be in a higher valence state, 

leading to higher field strengths, a higher degree of 

complexing, and hence, much slower rates of diffusion. 

Small-scale strong zoning of U in zircon grains sets up a 

situation where, after radiogenic ingrowth, Ph need only 

diffuse sub-micron distances to be redistributed from high-U 

zones to adjacent low-U zones. This will produce "internal 

reverse discordance": the zircon as a whole might be slightly 

normally discordant or even concordant, but the low-U zones 

within it are reversely discordant. 

"External reverse concordance" can be produced if fluids 

moving through the rock during metamorphism, hydrothermal 

activity, or even near-surface weathering cause dissolution and 

removal of the highest-U zircon zones, together with their U 

and remaining Ph. This leaves the residual zircon enriched in 

the low-U, relatively insoluble zircon zones that contain 

parentless radiogenic Pb, thus producing an overall reverse 

discordance for the bulk zircon sample. 

207 

SULPHUR DYNAMIC STUDIES IN FOREST 

ECOSYSTEMS USING STABLE ISOTOPES 

MAYER, B., and KROUSE, H. R., Department of 

Physics and Astronomy, The University of Calgary, 

Calgary, Alberta, T2N 1N4, Canada 

The sulphur cycle in terrestrial ecosystems has 

previously been studied mainly by input/output balances of 

whole watersheds or sub-compartments therein. These studies 

limit conclusions about sulphur dynamics since the obtained 

sulphur fluxes represent only the net-effect of various 

transformation processes. Stable sulphur isotopes have been 

used to further delineate sulphur sources, especially where 

834S-values of lithogenic (pyrite, evaporites) or anthropogenic 

(SOJ sulphur differs from those of unpolluted atmospheric 

deposition. 

An isotope tracer technique with sulphate was tested 

as a new approach for understanding sulphur cycling in soil. 

It was used in a lysimeter experiment with five aerated 

German forest soils, where 15 soil cores of each soil type 

were irrigated with 34S- and 180-enriched sulphate derived 

from Silurian gypsum. Since isotope fractionation during 

sulphur transformations under aerated conditions is rather 

small, it was shown by concentration and isotope 

measurements in seepage water sulfate and different forms of 

soil sulphur, that the applied sulphate was retained primarily 

as inorganic sulphate, secondarily as organic sulphur, and very 

little sulphate was exported in the seepage water. It was 

concluded that the mean residence time of sulphur in the 

pedosphere is of the order of many years to a few decades, 

much longer than previously suggested. Since these results are 

in excellent agreement with those of radioactive 35S-tracer 

studies, stable sulphur isotopes prove to be a valuable tracer 

for long term studies under aerated conditions. 

The technique is currently being tested under field 

conditions in a Norway spruce forest near Munich (Germany), 

where isotopically enriched sulphate was applied to the forest 

floor. Preliminary concentration and isotope data for seepage 

water sulphate and soil sulphur have been obtained. 
AQUIFERS IMMUNE TO POLLUTION 

MAZOR, E:, Dept. of Environmental 

Sciences and Energy Research, 

Weizmann Institute of Sciences, 

Rehovot 76100, Israel 

An aquifer is a body of permeable rock that 

contains water in all its voids and can sustain 

economical wells. This definition leaves room for 

active aquifers (with recharge, through-flow and 

discharge) and passive aquifers (trapped, with no 

recharge, through-flow and discharge). The most 

common example of an active aquifer is the phreatic 

aquifer, whereas examples of passive aquifers are 

traps of oil, gas and brines. Passive artesian fresh 

water aquifers can be passive when they are deeply 

buried, sealed above and beneath by impermeable 

rocks, e.g. in rift valleys or in subsidence basins. The 

observed artesian pressure is induced in these cases by 

compaction. The pressure in a new well, drilled Into a 

passive pressurized aquifer, is expected to drop 

rapidly and not recover after shut down. 

Geological criteria can support the passive 

aquifer case, but the properties of the stored 

groundwater itself are needed as augmenting 

evidence: (a) high water ages, indicated by no 

measurable C-14 (i.e. no recharge in the last 

25.000 years), low {observed Cl-36/initial Cl-36} 

ratios, indicating water ages in the range of 1 ()6 

years or greater, and in- situ He-4 concentrations, 

indicating ages in the range of 1 Q4 to 1 Q8 years; 

(b) paleo-recharge, indicated by deuterium and 0-

18 concentrations that differ from the present 

values; (c) hydraulic isolation, indicated by 

dissolved ion concentrations and relative 

abundances that typify a given artesian aquifer, 

and differ from the composition of overlying and 

underlying aquifers. 

Passive pressurized aquifers containing potable 

water should be saved (a) for times of needextreme droughts, or large scale pollution events, 

and (b) for future generations. Artesian aquifers 

that have already been exploited and ceased 

flowing, may be artificially recharged and used as 

safe water storage reservoirs. 

Artesian aquifers that have been depressurized 

by exploitation, may be considered as potential 

sites for waste injection, for the following reasons: 

(a) they have been hydraulically well isolated by 

nature, for time periods equaling the age of the 

water, (b) no uncontrollable hydrofracturing is 

anticipated as a result of the waste injection, as 

long as the pressure is kept bellow the initial preexploitation pressure. 

Examples of passive pressurized aquifers will 

be presented. 

208 

RHENIUM-OSMIUM ISOTOPE ANALYSIS OF 

BASE METAL SULFIDES FROM LARAMIDE 

PORPHYRY DEPOSITS, SOUTHWESTERN 

NORTH AMERICA 

MCCANDLESS. Tom E .. RUIZ, Joaquin, and 

CHESLEY, John T., Department of Geosciences, 

University of Arizona, Tucson, Arizona, USA 

85721 

Base metal porphyry deposits represent 

major accumulations of chalcophile elements within 

the earth's upper crust. These deposits are related to 

subduction of oceanic crust beneath continental 

lithosphere, and the metals may originate from the 

mantle, lower crust, or intruded country rocks. Base 

metal porphyry deposits in southwestern North 

America have Re-Os mineralization ages that 

correspond to two Precambrian crustal domains 

which differ in age and lithology, suggesting that 

crust influences mineralization (McCandless and 

Ruiz, 1993); but the amount of crustal involvement 

in the formation of these deposits is still uncertain. 

The Re-Os isotope system is ideally suited to 

evaluate the crustal contribution to base metal 

porphyry mineralization, because both Re and Os 

are chalcophile elements, and the 1870sJ188Qs ratio 

of the crust (>3.6) is very different from that of the 

mantle (-0.1). Base metal sulfides from major 

porphyry deposits in southwestern North America 

have been analyzed for Re and Os isotopes using 

fusion/distillation and NTIMS methods. Preliminary 

results suggest that a very radiogenic (i.e. crustal) 

component of Os is recorded in the mineralization. 

Additional analyses will determine if these 

radiogenic signatures are unique to the Precambrian 

crustal domains jn which they occur. 

McCandless, T.E. and Ruiz, J., 1993, RheniumOsmium Evidence for Regional Mineralization in 

Southwestern North America: Science, v.261, 

p.1242-1246. 
RESOLVING HIGH PRECISION U/Pb AGES FROM 

TERTIARY PLUTONS WITH COMPLEX ZIRCON 

SYSTEMATICS: EXAMPLES FROM SE ALASKA 

McCLELLAND. William C and MA TTINSON, James M., 

Department of Geological Sciences, University of 

California, Santa Barbara, CA 93106, USA 

Complex zircon systematics due to the combined effects of 

inherited components and Ph-loss are relatively common in 

Mesozoic and Tertiary plutonic complexes. Interpreting 

slightly to strongly discordant conventional zircon data from 

such samples is difficult since the assumption of either simple 

Ph-loss or a singular inherited or entrained xenocrystic 

component necessary for conventional concordia analysis is 

invalid. In addition, commonly employed air abrasion 

techniques cannot decipher the effects of both inheritance and 

Ph-loss. In contrast, step-wise dissolution techniques 

entailing analysis of sequential partial digestion steps have 

proven instrumental in resolving useful age information from 

populations exhibiting complex discordance patterns. 

Utility of the partial dissolution technique is exemplified by 

comparison of conventional and step-wise analyses on 

relatively low-U (200-500 ppm) zircon populations from two 

Tertiary plutons in the Coast Mountains batholith, SE Alaska. 

Conventional analyses on abraded and unabraded fractions of 

differing size and magnetic properties yield moderately 

discordant to concordant results defining arrays that clearly 

indicate the presence of inherited components (Fig. 1). Age 

interpretations for these samples are limited by the non-linear 

variation in U/Pb and Pb/Pb ages and uncertainty in the 

magnitude of Ph-loss suspected from independent evidence (A: 

56.5 ± 1.1 Ma assuming no Ph-loss; B: 62 ± 3 Ma as 

reasonable estimate). Partial dissolution experiments tailored 

to relatively low-U, young zircons produced successive leach 

steps that define a trajectory of decreasing 207pbf206pb ages 

and, with exception of sample B residue, increasing U/Pb ages 

(Fig. 1 ). In both cases, the residues are concordant allowing 

for a Th correction uncertainty of -1 Ma in the 207pbf206pb 

age. Sequential leach steps are inferred to have preferentially 

removed outer high-U zones susceptible to Ph-loss as well as 

internal inherited or xenocrystic components. The resilient 

residues are interpreted to have been comprised of pristine 

magmatic zircon relatively free of these effects and thereby 

providing the best estimate of crystallization age for the 

samples (A: 57.8 ± 0.3 Ma; B = 62.6 ± 0.3 Ma). 

We suggest that the step-wise dissolution technique allows 

resolution of complex zircon systematics and high precision 

age determinations that are unobtainable with conventional 

thermal ionization or ion microprobe techniques. Although 

absolute age estimates for young samples are plagued by decay 

constant and Th correction uncertainties, this technique will 

allow resolution of relative emplacement and deformational 

chronologies for plutonic and metaplutonic complexes. 

.ceo 

a_ tO A .... ········ 0 

co ;g IX) 

0 . ~ 

q (\J 

-

.0~ ~ ,.... rE-r--. ~ 

0 ;g q (\J • 

100 120 

Figure 1. Terra-Wasserburg diagrams showing conventional 

analyses (open), partial dissolution steps (solid), and trajectory 

(dotted line) from initial steps to residues (R). 

209 

OXYGEN ISOTOPE MICROANALYSES OF SILICIC 

ROCKS IN LONG VALLEY CALDERA: UNCOVERING 

TRENDSINTHEPALEOHYDROTHERMALSYSTEMS 

McConnell. y.s.l, Valley, J.W.2, Spicuzza, M.J.2, and 

Eichelberger, J.C.l, !Geophysical Inst., Univ. of Alaska, 

Fairbanks, AK, 99775-0800, USA, and 2 ,Dept. of Geology 

and Geophysics, Univ. of Wisconsin, Madison, WI, 53706, 

USA. 

Microanalysis of oxygen isotope compositions in samples 

from the Long Valley Exploratory Well (L VEW) centered on 

the resurgent dome of 760 ka Long Valley Caldera and 

LV13-26 Well located 4 km to the SE provide a basis for 

understanding the complex hydrothermal history in the central 

caldera. Previous work on L YEW whole rock samples 

indicated depleted atso values as· low as -0.3%o for 

intracaldera volcanic rocks, but whole rock data do not 

sufficiently constrain water/rock interactions or degree of 

isotopic equilibrium. In this study, C02 laser probe 

microanalyses of 0.5-2.0 mg samples from the wells reveal 

considerable isotopic heterogeneity between pumice, sanidine, 

matrix, and quartz phases in the intracaldera Bishop Tuff and 

at so values as low as -6.5%o. Overall trends become evident 

when values are plotted relative to depth and location in the 

caldera. 

Oxygen isotopes in pumice and sanidine phenocrysts frQm 

the Bishop Tuff most readily exchange with hydrothermal 

fluids and display subparallel trends as atso decreases with 

depth in LVEW: 5.3 to -2.0%o and 6.9 to -6.5%o, respectively. 

Pumice (-0.1%o) approaches equilibrium with hydrothermal 

quartz ( -0. 7%o) where co-measurement was possible. Matrix 

samples of the Bishop Tuff in L YEW are also depleted in 

atso (3.5 to -3.7%o) but the trend with depth is obscure. 

Quartz phenocrysts remain relatively unaffected reflecting 

magmatic values (6.7 to 8.2%o). atso is also depleted in the 

Bishop Tuff samples from LV13-26 Well but the trend with 

depth is reversed, pumice (-1.1 to 8.2%o) and sanidine 

approach magmatic values by the bottom of the well. 

Basement metavolcanic and metasedimentary rocks display 

8180 depletion (-6.3 to 14.8%o) relative to Mt. Morrison Roof 

Pendant metamorphic rocks (8.8 to 19.5%o). Sill-like Early 

Rhyolite hypabyssal intrusions identified in the Bishop Tuff in 

LVEW are depleted in atso (-1.3 to 3.1%o) but the values do 

not vary with depth. These intrusions are thought to be 

partially responsible for resurgence. Hydrothermal vein quartz 

(2.7%o) measured in an intrusion (-1.2%o) at 784 m depth in 

LVEW was apparently deposited in a cooler hydrothermal 

domain assuming all hydrothermal fluids had similar atso 

values to the present day system (-14.00/oo). 

The trends in isotope data can be explained by a centrally 

located magma body beneath the resurgent dome heating 

hydrothermal fluids to >250°C at the base of the Bishop Tuff 

at 1800 m depth. Intrusions into the Bishop Tuff with lower 

local hydrothermal temperatures and isolated isotopic 

exchange may be related to this magma body or later 

post-caldera-collapse volcanic activity. Localized channel 

fluid flow and/or structural control on flow off the resurgent 

dome is a likely explanation for the pattern seen in LV13-26 

Well. 
TIMING AND SOURCE OF LODE GOLD IN THE 

FAIRBANKS DISTRICT, INTERIOR ALASKA 

McCOY. D., LAYER, P.W., NEWBERRY, RJ., Dept. 

of Geol. and Geophys., Univ of Ale., Fbks., 9977S 

BAKKE, A., Fairbanks Gold Mining, Inc., Fbks., Ale. 

99709 

MASTERMAN, S., Ryan Lode Mines, Fbks., Ale. 

99709 

GOLDFARB, R, USGS, Denver, Co. 8022S 

The Fairbanks Mining District bas produced over 7 

million ounces of placer gold. Recently two large bulkmineable vein/stockwork lode deposits have been discovered. 

The Ryan lode mine contains approximately 1 million 

ounces of gold grading .08 ounces per ton. It is hosted in 

greenschist facies schist P.•.id in a small granitic porphyritic 

intrusion. The ore is localized in brittle shear zones. 

Arsenopyrite is the main accessory mineral. The Fort Knox 

deposit contains approximately 4 million ounces of gold 

grading .028 ounces per ton. The deposit consists of 

stockwork veins and brittle shear zones hosted wholly in a 

multi-phase granitic porphyritic intrusive. Bismuthinite and 

tellurobismuthite are the main accessory minerals. 

The Ryan lode and Fort Knox Yield similar ages. 

Magmatic ages (U/Pb for zircon, Ar40/Ar9 for homeblende 

and biotite) are 91±1 Ma whereas hydrothermal micas give 

.Ar40/Ar9 dates of 89±1 Ma with minor resetting from an 

:early Tertiary heating event. 

Fluid inclusions (all secondary) indicate trapping 

temperatures and pressures of 310°± lS°C and .S-.1S kbar 

for the Ryan lode and 310°± 1S°C and 1-l.S kbar for Fort 

Knox. Aplite geobarometry gives pressure estimates of 1-l.S 

kbar for intrusive rocks. C~ contents are variable and 

average approximately 13 mole %for the Ryan lode lode 

and 22 mole % for Fort Knox; methane is a minor 

contaminant. Salinities are 1-3 wt. %for the Ryan and 3-6 

wt. % for Fort Knox. The variable C02 content is 

indicative of minor H20-C02 immiscibility. 

The dating, together with fluid inclusion and aplite 

geobarometry, indicate that the heat source for the deposits 

were intrusives emplaced at moderate to shallow depths. 

The similarity of fluid compositions and dates from both 

deposits indicate a common fluid genesis. 

Fluid compositions and temperatures are similar to 

!)secondary inclusions found in some porphyry molybdenum 

and porphyry tungsten deposits (implied magmatic origin) 

and 2}primary and secondary inclusions associated with gold 

districts identified as having a metamorphic fluid source. 

The high geothermal gradient, porphyry style veining, low 

methane content, low chlorine content of magmatic biotites, 

and reduced nature of the plutons presently point to a 

porphyry-type model with a probable magmatic source for 

both fluids and gold. This indicates that the fluids 

commonly found in schist -hosted mesothermal vein districts 

(low salinity, high C02, moderate temperatures) may have 

either magmatic or metamorphic sources. 

210 

IDGH FIDELITY Sr/Ca RECORD OF SEA SURFACE 

TEMPERATURES: 1982-83 El NINO AND MIS-5e' 

McCULLQCH M.T., and MORTIMER G. 

Research School of Earth Sciences, The Australian 

National University, Canberra, ACT, 0200. 

Both long and short-term changes in our climate are 

intimately associated with changes in ocean temperature. 

For example, the movement of warm water from the 

central western Pacific to the eastern Pacific, known as 

the El-Niii.o and the related Southern Oscillation (ENSO) 

occurs on average every 4 to 6 years. The El-Nifio is 

now known to be directly responsible for the droughts 

that occur periodically throughout Australia and for the 

catastrophic floods in western South America. 

Using high precision measurements of Sr/Ca ratios 

combined with high resolution sampling of Porites 

corals we have undertaken a detailed study of corals 

from the Great Barrier Reef (GBR) and the Huon 

Peninsula of New Guinea for the period 1979 to 1986. 

This period is of particular interest as it encompasses the 

1982-83 El Niii.o, the strongest event that has occurred in 

this century. The results are shown in the figure below. 

It can be seen that a very marked decrease in the sea 

surface temperature occurs in the GBR in the winter of 

1982, reflecting the influx of unusually cold water into 

this region. In New Guinea, a different behaviour is 

observed with the temperature decreasing steadily from 

mid1983 to 1984, nearly two years after the 1982-83 El 

Nino. It is not until the beginning of 1987 that 

temperatures again reach similar levels to the 1982-83 

period. This probably reflects the progressive buildup of 

the warm pool in the western Pacific. 

In order to extend this approach further back in time 

an exceptionally well preserved Porites from Reef 6 in 

the Huon Peninsula with a U-Th age of 120,000±500 

years has been analysed for Sr/Ca. This coral, which 

occurs -10m below the main interglacial Reef 7, has a 

surpringly low mean temperature of -20 to 22oc during 

this period (marine isotope stage 5e). This may provide 

confirmation of the findings of the Greenland ice core 

project for climate instability during the last interglacial, 

and implies that this is a global phenomenon. 

1982 1983 1984 1985 1986 

P1111dora (Great Barrier Reef) 

28 modem Huon PeninsiJa (New Guinea) 

120,000 120,002 120,004 120,006 120,008 

TIME (years) 
HOLOCENE CLIMATE CHANGE ON THE EASTERN N. 

ATLANTIC SEABOARD; A SPELEOTHEM RECORD 

McDERMOTT. F., Dept. of Geology, University 

College Dublin, Dublin 4, Ireland; SW ABEY, S.E.J., 

and HA WKESWORTH, C.J., Dept. of Earth Sciences, 

The Open University, Walton Hall, Milton Keynes, 

MK7 6AA, U.K; MATTEY, D.P., Department of 

Geology, Royal Holloway, University of London, 

Egham, TW20 OEX, U.K. 

Recently published stable isotope profiles for the 

Greenland ice-cores have highlighted unexpectedly large and 

rapid climatic fluctuations during the last glacial, but they 

indicate relatively stable conditions during the Holocene. The 

extent to which these high-latitude ice core profiles record 

global, or even regional, climatic patterns is still debated, 

although the correlation (Bond et al., 1993) with the midlatitude North Atlantic ( -44-55°N) sea surface temperatures 

(SSTs), is strong evidence for ocean-atmosphere coupling on 

a sub-millennium timescale. 

In order to investigate palaeoclimatic conditions on 

continents adjacent to theN. Atlantic ocean we have 

undertaken a stable isotope study ofU-Th dated speleothems 

(cave carbonates) from the mid-latitude eastern N. Atlantic 

seaboard (west coast of Ireland, 52-55°N). Speleothems have 

considerable potential as quantitative palaeoclimatic 

indicators; frrst because their 0 isotope ratios can reflect the 

temperature of calcite deposition (and the olSO of meteoric 

water) and second because they can be dated precisely using 

the U-Th technique. 

Mass-spectrometric U-Th ages for the 12 speleothems 

studied range from 76.51±0.98 kyr to <400 years, and 

continuous dated proftles have been obtained for the Holocene 

period. Here the calcite o180 varies from 26.10 to 28.61%o 

(SMOW) and o13C varies from 5.43 to -10.95%o (PDB). 

Typically, o180 ratios exhibit a pattern of slow smooth 

increases (cooling) with time (over time scales of -3000 

years), followed by rapid (decadal) returns to lower values 

(warming): Three such asymmetric cooling cycles, 

representing relatively small temperature fluctuations of -l30C have been detected during the Holocene, the oldest and 

most severe of which coincides with the Younger Dryas 

(Y.D.) event. 

Apart from the Y.D. event, when N. pachyderma (s.) 

abundances increased markedly in sediments from the 

N.Atlantic core V23-81, SSTs appear to have remained 

relatively stable during the Holocene. Thus, the relatively 

small (1-2°C) mid-Holocene temperature changes inferred 

from the speleothems do not appear to be accompanied by 

SST variations. However, the pattern and magnitude of the 

temperature fluctuations resemble closely those inferred from 

European continental pollen records. 

Bond, G. et al., 1993. Correlations between climate records 

from North Atlantic sediments and Greenland ice: Nature, v. 365, 143-147. 

211 

SOLAR NOBLE GASES IN THE EARTH- EVIDENCE 

FROM NEON AND HELIUM 

McDOUGALL 1., HONDA, M., and PATIERSON, D.P., 

Research School of Earth Sciences, The Australian 

National University, Canberra, ACI' 0200, Australia. 

The abundances and isotopic compositions of noble gases 

trapped in mantle-derived samples provide useful consttaints oo 

the origin and evolution of the Earth's atmosphere, crust, 

mantle and core. Identification of the noble gas composition 

of the primordial Earth is of particular importance in relation 

as to how and when the Earth acquired its volatiles and how its 

atmosphere evolved. 

Noble gas studies on mantle-derived samples from a wide 

range of environments indicate that neon isotopic ratios 

commonly differ significantly from atmospheric in a 

systematic way, and that a correlation is evident in many cases 

between neon and helium isotopic compositions. These results 

are most readily interpreted as indicating that there is a noble 

gas primordial component in the Earth that is solar in 

composition, providing a much fmner basis than previously 

for the idea since Craig and Lupton (1976) suggested it on the 

basis of measurement of neon isotopic ratios in some 

submarine basaltic glasses and volcanic gases. 

Neon isotopic ratios observed in some mantle-derived 

samples are enriched in ~e and 21Ne relative to 22Ne 

compared with atmospheric neon. In particular, Sarda et al. 

(1988) demonstrated correlated enrichments of~e and 21Ne 

in a suite of mid-ocean ridge basalt (MORB) glasses; similar 

relationships were found subsequently by other workers in 

samples from a range of environments. We interpret these 

results in terms of mixing between three neon isotopic 

components: solar, nucleogenic and atmospheric. In general, 

we fmd that the solar and nucleogenic neon components in 

mantle-derived samples are strongly correlated with measured 

3He (primordial) and 4He (dominantly radiogenic) in the same 

suite of samples. Such correlations are evident in MORBs, in 

MORB-like basalts from back-arc basins, from intraplate 

plume-related oceanic island basalts, mantle xenoliths, from 

ancient diamonds, from CCh well gases and from a fluid 

sample from Yellowstone. Indeed, the correlations are so 

strong as to provide much confidence in the hypothesis that the 

mantle contains a signifiCant primordial component of solar 

composition. 

In contrast to neon and helium, isotopic compositions of the 

heavier noble gases, argon, krypton and xenon, in mantlederived samples are less readily interpreted in terms of the 

presence of a primordial solar component, in part owing to the 

common dominance of atmospheric-like noble gas components 

in the samples. In addition, the hypothesis of a primordial 

solar component in the Earth is not yet fully reconcilable with 

the observed noble gas composition of the Earth's atmosphere 

derived from degassing of the Earth. 

Craig, H., and Lupton, J.E., 1976, Earth Planet. Sci. Lett., 

v. 31, p. 369-385. 

Sarda, P., Staudacher, T., and All~gre. CJ., 1988, Earth 

Planet Sci. Lett., v. 91, p. 73-88. 
GEOCHRONOLOGY OF MIDDLE TO LATE 

TERTIARY VOLCANISM ACROSS SOUTHCENTRALSONORA . 

MCDOWELL. Fred W., Dept. Geol. Sci., Univ. 

Texas at Austin, Austin, Texas 78712 USA, 

and ROLDAN-QUINTANA, Jaime, Estaci6n 

Regional del Noroeste, Inst. Geol. UNAM, 

Hermosillo, Sonora, 83000, Mexico 

During middle and Late Tertiary, plate 

interactions along western North America 

underwent a gradual and complex transition from 

Farallon plate subduction to Pacific plate 

transcurrent motion. Contemporaneous volcanism in 

northwestern Mexico should retain a record of this 

transition. A mapping and K-Ar dating survey 

across south-central Sonora between 28 and 29° 

north latitude identifies three regional components 

to that volcanic record. 

In eastern Sonora thick (>500 m), dominantly 

felsic and pyroclastic deposits comprise the western 

portion of the Sierra Madre Occidental (SMO) 

volcanic province, an arc related to Farallon plate 

subduction. Ages are from 32 to 27 Ma, and locally 

as young as 24 Ma. Flows of basaltic andesite occur 

just above or intercalated with the youngest felsic 

units. Abrupt thinning of these sequences marks a 

distinct western boundary to the SMO province. 

Much thinner ( < 100 m) volcanic exposures in 

central Sonora are associated with coarse clastic 

sediments deposited in linear fault-bounded basins. 

Basin formation and sedimentation were associated 

with Basin-and-Range extensional faulting. Mafic 

flows (27 to 20 Ma) occur at or near the base of the 

sedimentary sequences, viscous porphyritic domes 

and flows are found along some basin margins, and 

thin felsic lavas and distal ash flows (14-10 Ma) cap 

the thickest and coarsest portions of the sedimentary 

sections. Tilting of the youngest volcanic units 

indicate that faulting continued to < 10 Ma. 

In western Sonora, adjacent to the Gulf of 

California, Neogene (23-10 Ma) volcanic 

accumulations are thicker' (up to 1000 m) and lack 

intercalated clastic sediments. Although they are 

similar in composition to sections within the SMO 

province, they are younger and pyroclastic units are 

less prominent. Alkaline mafic lavas (10-8.5 Ma) 

are commonly the youngest volcanic units present. 

This volcanism could have been related either to the 

last stages of plate convergence and/or to proto-Gulf 

of California extension. 

Variations in the timing and intensity of 

volcanism indicate the operation of distinct tectonic 

controls in each region. These volcanic rocks are not 

components of a single homogeneous arc or an arc 

that underwent a continual westward migration of 

focus. 

212 

APPLI~A TIONS OF C02 LASER HEATING IN 40 Ar/39 Ar 

GEOCHRONOLOGY 

MciNfOSH William C. and HEIZLER, Matthew T., New 

Mexico Geochronology Research Laboratory, New Mexico 

Bureau of Mines and Mineral Resources, New Mexico 

Institute of Mining and Technology, Socorro, NM 87801, 

USA. 

A 10 Watt C02 laser in the New Mexico Geochronology 

Research Laboratory has proven useful for controlled 

40 Ar/39 Ar incremental heating analyses of optically 

transparent silicates ·such as sanidine and anorthoclase, and for 

heating of Ar-bearing inclusions in quartz and diamond. 

Near visible wavelength lasers have been successfully used 

for more than fifteen years in ultrahigh vacuum·(UHV) 

40 Art39 Ar extraction systems: yielding extremely low argon 

blanks, rapid heating and the capability of in situ analysis. 

Drawbacks of these lasers systems have included high cost and 

inability to achi~ve and maintain consistent te"mperawres in 

transparent mineral phases due to poor coupling with their 

crystal lattices. 

The 10.6J.1 wavelength C02 laser light has been known to 

couple extremely well with clear silicates, but its use in 

40 Art39 Ar analysis has been hindered by the lack of an 

appropriate bakable window material and window-to-metal seals 

for introduction of the laser beam into the UHV extraction 

system. This difficulty has been overcome by the use of a 

commercially available (lSI Corp.) 3.5 em diameter antireflective coated ZnSe window sealed by double metal gaskets 

separated by a void space which is continuously pumped with a 

mechanical roughing pump. A KBr cover slip is used to 

protect the ZnSe window from precipitation of vaporized 

sample material. This window and cover slip combination is 

repeatably bakable to temperatures in excess of 150 °C, and 

vields 40Ar blanks less than 5 X 10· 18 moles/min. 

· The chief advantage of the C02 laser over shorter 

wavelength lasers is enhanced coupling with silicate materials. 

Power levels of 1-2 watts are generally sufficient to fuse and 

repeatedly refuse sand-sized individual crystals ofK-feldspar. 

Additional advantages of the C02 laser include reduced cost 

( <S4k), small size (50 em long), and modest input power 

requirements (<8 amps@ 112 volts). 

The enhanced coupling of the C02 laser with clear silicates 

provides several capabilities useful for 40 Ar/39 Ar analysis; 

including, achievement of complete degassing of transparent Kfeldspar samples, the ability to fully degas fluid inclusions at 

very low power levels, removal of low radiogenic gas fractions 

prior to fusion and identification of excess argon associated 

with low temperature heating steps. As an example, sanidines 

from a pyroclastic fall tuff which is constrained to be between 

1.1 and 1.6 Ma, yields total fusion ages which range from 1.3 

to 2.1 Ma. Two step heating (0. 75 and 2.5 W) of single 

crystals reveal low-temperature gas fractions with radiogenic 

yields varying from 47-88% and model ages of 1.4 to 2.1 Ma. 

These steps yield an isochron age of 1.23 ± 0.01 Ma and a 

trapped 40Art36Ar = 623 ± 32. The second steps are extremely 

consistent with radiogenic yields of -99% and, give a mean age 

of 1.23 ± 0.01 Ma. Clearly, the step-heating capability 

provides a much more precise age determination and lends 

additional insight to the argon systematics of this sample. 
ACTIVE SEDIMENT DEGASSING AND C02 

FLUX FROM THE SALTON TROUGH RIFT 

McKIBBEN. M. A., and WILLIAMS, A. E., 

Dept. Earth Sciences, Univ. of California, 

Riverside, CA 92521-Q423. 

The Salton Trough rift is one of the most 

extensively explored areas of active crustal 

decarbonation. 6 km of deltaic-lacustrine sediments, deposited onto active spreading centers, 

are locally undergoing metamorphism and decarbonation. Initially containing ~12 wt % diagenetic 

and detrital carbonate {~5 wt % C02) with 513C ~ 

-3 °/ oo and 5180 ~ 22 o / oo, they are progressively 

transformed to carbonate-poor, epidote-rich 

hornfels at ~ 3000C. With increasing depth and 

temperature the diminishing carbonate undergoes 

progressive isotopic depletion' reaching values of 

513C ~ -8 o / oo and 51so ~ 2 o oo at 2-3 km depth. 

Coexisting oxidized brines 260-3100C) contain 

C02 with 513C ~ --{) o / 00. urface mud pots (~ 

750C) are aligned along faults above and peripheral to the system and presently degas CO2 

with 513C identical to C02 in the deep brines. 

Only hydrothermal dissolution of calcite to 

aqueous C02 can explain the isotopic depletions. 

A drilled volume of ~ 55 km 3 of metamorphosed sedimentary rocks contains ~ 11 km3 {1013 

kg) of hot (230-3300C) brine with 1000-5000 ppm 

dissolved C02: 10-50 Mt of C02 are stored in the 

hot brines. At least 50% of the original sedimentary carbonate has been lost upon metamorphism, 

corresponding to at least 3 Gt of C02. This is 100 

times the mass of C02 presently stored in the hot 

brines. The metamorphic 13C depletions of the 

deep carbonates and brines also require that a 

large mass of isotopically heavy C (513C > -3 

o / oo) has been lost from the rock volume. 

High sediment permeability, active rift tectonics and mudpot activity leave no doubt that 

most of this C02 was lost to the atmosphere over 

the lifetime of the rock volume's metamorphism. 

The heating duration of the rock volume is < 105 

years, so the minimum time-averaged decarbonation rate is 30,000 t C02/yr {~109 molesfyr). The 

average flux may be much higher if shorter 

heating duration and decarbonation of deeper 

unexplored sediments is included. 

Comparison of this mass balance based timeaveraged C02 flux with the independent presentday heat flow based flux is excellent: both yield ~ 

109 moles C02/yr. When the other less explored 

areas of high heat flow and likely sediment metamorphism in the Salton Trough are considered, 

the total C02 flux from this rift probably exceeds 

1010 moles/yr. 

On a global scale, rift sediment decarbonation 

may make a major contribution to the total nonanthropogenic C02 flux. 

213 

ISOTOPIC CONSTRAINTS ON EVAPORATION AS A 

CONTROL OF GROUNDWATER COMPOSITION IN A 

SEMIARID ENVIRONMENT, SOUTH-CENTRAL 

WASHINGTON 

MCKINLEY. J.P., Pacific Northwest Laboratory, 

Richland, Washington, 99352, U.S.A., and HOOVER, 

J.D., Westinghouse Hanford Co., Richland, Washington, 

99352, U.S.A. 

Understanding the natural compositional variability of 

groundwater is an important part of the characterization efforts 

associated with environmental restoration and remediation at 

the U.S. Department of Energy (DOE) Hanford Site, 

Washington. The range of analyte levels in groundwater can 

serve as the site-specific baseline for defining groundwater 

contamination and assessing risk-based cleanup levels. 

At Hanford, the unconfined aquifer is composed of 

Pliocene fluvial-lacustrine sediments but extends locally into 

overlying Pleistocene sands and gravels. The climate is 

semiarid, with little direct aquifer recharge. Surface recharge 

originates primarily from meteoric and spring-fed runoff in 

upgradient highlands that expose the underlying Columbia 

River basalts. Other sources include leakage from the 

underlying confined aquifer and side-bank recharge from the 

Columbia and Yakima rivers downgradient. 

Evaluations based on groundwater compositions and mass 

balance indicated that the compositional variability could be 

explained by either evaporation or water-rock interaction. An 

understanding of the extent to which each of these processes 

affects groundwater composition is necessary because the 

relative concentrations and spatial variability of dissolved ions 

resulting from water-rock interaction and evaporation are 

different. 

Experimental studies demonstrated that water-rock 

interaction could account for most of the observed 

compositional variation. Evaporation also could control 

compositions, and elevated concentrations of alkalis, halogens, 

nitrate, and sulfate in some parts of the aquifer have been 

interpreted by others as evidence of evaporative effects on 

groundwater composition. Therefore, the extent of evaporation 

was evaluated using oxygen and hydrogen isotopic data coupled 

with solute mass balance modeling. 

Groundwater Cl concentrations are generally less than 

10mg/L but range as high as 25 mg/L. Spring waters contain 

1-15 mg/L Cl. Spring and groundwaters define overlapping 

groups below the meteoric water line, with spring waters 

(()180=-17 to -14) more positive than groundwaters (()180=-18 

to -14; most are ()180=-18 to -16). These groups together 

overlie the range in compositions for regional surface and 

meteoric waters. Groundwater Cl concentrations of 25 mg/L 

imply 40% evaporation of spring waters with 15 mg/L Cl. 

Evaporation of more than 15% of spring waters would in all 

cases, however, result in an isotopic composition outside the 

boundaries of groups defined by spring and groundwaters. 

Isotopic constraints indicated that groundwaters having the 

highest Cl concentrations could not have derived solely from 

evaporation of spring or other runoff waters, and that waterrock interaction is the dominant process in controlling the 

natural composition of groundwater at the Hanford Site. 
PROTEROZOIC CRUSTAL EVOLUTION OF THE 

ADIRONDACK SECTOR OF THE GRENVILLE 

PROVINCE. 

McLELLAND, J., Department of Geology, Colgate 

University, 13 Oak Drive, Hamilton, NY 13346-1398, 

USA and DALY. J.S. , Department of Geology, 

University College Dublin, Belfield, Dublin 4, Ireland. 

Much of the Canadian sector of the Grenville belt 

comprises reworked older material ranging from Archaean to 

Palaeoproterozoic in terms of crustal residence age. By 

contrast, Sm-Nd data reveal major juvenile crustal additions in 

the Adirondacks and Grenville inliers of the northern 

Appalachians in the Mesoproterozoic. For example, 1.31 -

1.36 Ga old orthogneisses from the Green Mountains have 

initial ENd values of + 3 and tnM ages in the range 1.45 - 1.48 

Ga, while granitoids of less certain age from the Hudson 

Highlands have toM ages of 1.38 - 1.56 Ga. These data, 

combined with geochronology and petrology indicate a 

predominantly juvenile Mesoproterozoic origin for the entire 

region. Crust formation began about c. 1.36 Ga ago and U-Pb 

data suggest a possible progression in the age of~ of arcmagmatism from east (Green Mtns) to west (Adirondack 

Lowlands). This suggests crustal accretion in a (present) 

westward direction either onto the western side of an unknown 

ancient continent or an intraoceanic juvenile terrane. Eastward 

accretion onto the eastern margin of Laurentia is not easily 

envisaged. Metasediments from the Adirondack Highlands/ 

Lowlands and the Hudson Highlands have tnM ages of 1.44 -

2.08 Ga indicating that both juvenile (arc-derived?) and 

Palaoproterozoic (continental) material was present in the 

sourcelands. Following sedimentation and arc magmatism, 

further (and fmal) crustal addition took place in the form of 

massif anorthosite magmatism at c. 1.15 Ga. Associated 

voluminous granitoids of the AMCG suite comprise reworked 

crust and are not newly added material. Crustal assembly and 

accretion to Laurentia was accomplished during the Grenville 

orogeny. Lack of further tectonic activity (witness the 

disposition of the overlying Phanerozoic sediments) suggests 

that the bulk of the present day lower crust has been present 

for the entire Phanerozoic. 

214 

RELATIVE CHRONOLOGY OF EVENTS AT AND NEAR 

THE CRETACEOUS-TERTIARY BOUNDARY 

MCWILLIAMS, M., Geophysics Department, Stanford 

University, Stanford, CA 94305-2215 

40 Arf39 Ar age determinations are inherently relative because 

they depend upon the absolute age assumed for the neutron 

fluence monitor. To reduce the experimental uncertainty 

introduced by the use of different monitor minerals, differing 

irradiation parameters and other inter-laboratory differences, 

samples from five Kff boundary sequences in North America 

were co-irradiated with whole rock and plagioclase samples 

from Deccan Trap flood basalts. By monitoring neutron fluence 

in detail and by interfingering unknowns from different 

localities in the same irradiation capsule, relative ages of some 

important Kff boundary sections have been measured with 

considerable precision. 

Bentonite beds at 3 localities in western North America lie 

5-80 em above the Kff Ir anomaly and yield a mean age of 

64.71 ± 0.09 Ma from a combination of laser fusion, laser step 

heating and resistance furnace experiments (uncertainties 2 

standard errors; ages relative to sanidine from Taylor Creek 

Rhyolite at 27.92 Ma). Laser total fusion ages from the Beloc 

tektites in Haiti yield a slightly older mean age of 64.91 ± 

0.12 Ma. The magnitude of the difference is just statistically 

significant and its sense is in the correct stratigraphic order, 

assuming that the tektites date the event which produced the Ir 

anomaly. 

RF step heating of whole rock and plagioclase samples 

from the Igatpuri and Mahabaleshwar Formations of the Deccan 

sequence yield less well constrained ages, as might be predicted. 

The best results are constrained on the 67 to 63 Ma interval. 

Although the individual results are not as precise, it seems 

clear that Deccan volcanism began well before the Kff 

boundary as recorded by the Beloc tektites and associated Ir 

anomalies below the bentonites. This suggestion is consistent 

with the recent placement of the KIT boundary in Anjar 

intertrappean beds by Bhandari et al. (LPI 825:10). 

Laser and RF step heating of whole rock and plagioclase 

samples from the Chicxulub C1 well at the Kff impact site on 

the Yucatan peninsula produced a range of apparent ages 

between 62 and 66 Ma. To date, none of the more than 20 

follow-up experiments have reproduced either the age or quality 

of the 3 experiments published in 1992 by Swisher et al. 

(Science 257:954). The discrepancy cannot result from sample 

differences because some experiments used exactly identical 

sample splits. 
PRECAMBRIAN STABLE ISOTOPES EXCURSION: 

APPUCATION OF GLOBAL CYCLIC 

DEVELOPMENT MODEL 

MELEZHJK v A Geological Survey of Norway, 

Post Box 3006-Lade, 7002 Trondheim, Norway, and 

F ALLICK, A. E., Scottish Universities Research & 

Reactor Centre, East Kilbride, Glasgow G75 OQU, 

Scotland, U.K. 

A secular trend of the production and accumulation of 

the freely available 02 in the Precambrian still remains a 

matter of great discussion ( eg. Des Marais et al., 1992 

versus Schidlowski & Aharon, 1992; ). The main way 

for free-oxygen production is biological photosynthesis, in 

cases when the generated organic mass is subsequently 

buried 

C02 + ~0 ~ C~OJ. + 02i 

As a result there is an increase in the oxidizing power 

of the Earth's atmosphere-hydrosphere that causes a 

number of extensive alterations in the exogenic systems 

such as weathering, sedimentation, diagenesis. Therefore 

an investigation of mass balances within the carbon cycle, 

including isotope analyses of both ccarb and corg; the 

growth of the crustal reservoir of reduced carbon ; the 

diversity and abundance of biological oxygen producers; 

the grade of bacterial sulphate reduction; style of 

diagenetic transformations, all are parameters that should 

be studied to monitor the secular oxygen trend. 

All these parameters listed above have been used in 

our investigation applied to the 2.5 to 1.8 Ga-old 

Precambrian sections of the Baltic Shield with high 

stratigraphic resolution dated at 2.5, 2.45, 2.4, 2.2, 2.11, 

2.06, 2.0, 1.97, 1.8 Ga by a U-Pb (zircon) technique. 

Three main results in respect to the development of the 

secular oxygen trend can be made: (1) an irregular change 

all investigated parameters is observed; (2) almost all 

parameters are changed synchronously through time; 3-in 

the time interval from 2.5 to 1.8 Ga, at least one major 

cycle (Fig.), which reflects an episodic reiease of 

oxidising power, can be distingwished at 2.1±0.1 Ga ago. 

The isotopic excursions, pecularities of sedimentation 

and diagenesis can be understood within a framework of 

the irregular cyclic oxidation of the Precambrian 

hydrosphere-atmosphere-lithosphere terrestrial system. 

Ga 

1.8 

1.9 

2.0 

2.1 

2.2 

2.3 

2.4 

2.5 

Concret on abundance 

(not to scale) 

Corg •t/km2/Ma 

0 25 50 -20 0 +20 

Stromatolite 

abundance 

0 5 15 

d 13CCPDB) 

-40 -25 0 

215 

SPATIAL AND TEMPORAL PATTERNS IN TilE STABLE 

ISOTOPE{O,H) COMPOSffiONS OF WA1ERS AT TilE 

LAWRENCE BERKELEY LABORATORY, CA 

MENCHACA. L. B., SMI1H, B. M., and CONRAD, M., 

Lawrence Betkeley Laboratory, 1 Cyclotron Rd., Berkeley, 

California, 94720, USA. 

With the advent of recent laws governing waste 

management and cleanup of contaminated sites in the United 

States, it has become increasingly important to Wlderstand the 

intricacies of smface and subsurface water movement. The new 

hydrogeologic challenges are especially imposing for DOE's 

National Laboratories and require the development of new, 

environmentally innocuous hydrogeologic tools, including 

tracers for water movement. At the Lawrence Berkeley · 

Laboratory (LBL), we are developing and applying water tracer 

methods using the diverse stable isotopic signatures of local 

waters of different origins. 

We have measured the 0- and H-isotope compositions of 

groundwaters, su.r&.ce waters and rain at LBL, a 130-acre site 

located on the western flank of the Berkeley Hills, -8 km east 

of San Francisco Bay, California. Most groundwater samples 

show arestrictedrangeofb18o values (-6.0 to -7.0 %o vs VSMOW) over periods of several months to one year. 

Exceptions are three local areas (-8.0 to -9.5 %o) where facility 

water ( -13.0 %o) piped in from higher elevations is thought to 

have leaked from broken delivery pipes and a sanitary sewer. In 

addition, one well showed a higher b18o of -4.9 %o, perhaps 

due to local infiltration of cooling tower water. Local rain 

storms showed a much larger range in b18o ( -1.4 to -14.0 %o ), 

with a weighted mean value of -5.8 ± 1.0 %o for the rainy 

winter season of 1992-3. In general, tropical storms bring 

relatively 18o-rich rains to the site, while storms that originate 

in the Gulf of Alaska bring rain with more negative b18o 

values. It is possible to predict the stable isotopic signal of 

input rainfall in the region, based on satellite imagery of storm 

origin and storm track. 

Repeated analysis of ground waters from -45 monitoring 

wells before and throughout the rainy season provided a method 

to delineate permeable zones beneath the site. The 0-isotope 

compositions of ground waters from 30 of the wells were 

constant to within 05 %o over the time of sampling, implying 

that the wells are properly constructed and that the rocks in 

these areas are relatively impermeable. Most of the wells with 

constant b18o are l.Ulconta.minated. Fifteen of the wells showed 

changes in b18o of0.5 to 25 %o, indicating influx of 

isotopically distinct water and suggesting relatively high local 

permeability. Most of these wells are located along inferred 

faults, and many have shown very low levels of contamination 

with chlorinated hydrocarbons and petroleum products. 

The ability to characterize waters of differing origins and to 

identify permeable zones through repeated sampling and stable 

isotopic analysis will be particularly important in the 

characterization of contaminated sites with complex 

permeability structures and limited outcrop for direct study. 
EVOLUTION OF THE RED SEA VOLCANIC 

MARGIN - A MULTI-ISOTOPIC APPROACH 

MENZIES Martin A., YELLAND, A., BAKER, J., 

BLAKEY, S., CHAZOT, G., AL-KADASI, M., 

Department of Geology, Royal Holloway University of 

London, Egham, Surrey, England TW20 OEX & RUNDLE, 

C., NIG, Keyworth, U.K. 

Traditionally rifting in the southern Red Sea-Gulf of Aden 

has been associated with early uplift of the Afro-Arabian dome 

prior to extension and magmatism related to the Afar plume. 

However selected geotraverses, perpendicular to the Red Sea 

margin, provide evidence for early magmatism followed by 

extension, uplift and exhumation. Although very little evidence 

exists for pre-volcanic uplift or extension geochemistrychronostratigraphy indicates involvement of the Afar plume in 

magmatism for the last 30 m.y. TIMING : Magmatism -

K-Ar & Ar-Ar data reveal that volcanism peaked in the 

Oligocene-Miocene with minor amounts of post-erosional 

volcanism. Magmatism may have begun in the Eocene with a 

possible northward migration during the Oligo-Miocene. 

Extension - Dating of the uppermost flows on domino fault 

blocks indicates a major episode of extension in the late 

Oligocene-early Miocene that post-dated much of the basaltrhyolite volcanism. Little or no evidence exists in the prevolcanic Jurassic-Tertiary stratigraphy for widespread prevolcanic extension. Exhumation - Apatite fission track 

analysis of once deeply buried basement rocks indicates several 

distinct thermal domains on the rifted margin. While the rift 

shoulders have a complicated thermal history the area occupied 

by the Tertiary basalt-rhyolite province has experienced rapid 

exhumation in the Tertiary. Studies elsewhere indicate that 

reactivation of old & young basement lineaments may have 

played a fundamentally important role in the evolution of the 

rift margin. In Yemen rift development was dominated by 

Oligo-Miocene magmatism, late Oligocene extension & surface 

uplift followed by Miocene exhumation. CRUSTMANTLE RESERVOIRS : Afar plume - Sr-Nd-Pb 

isotopes in uncontaminated volcanic rocks in Yemen & its 

environs indicate that the Afar plume has a HIMU signature & 

that the plume has made a variable contribution to volcanism 

for some 30 m.y. In addition the spatial occurrence of this 

HIMU signature over several thousand kilometers may point 

to lateral movement of mantle material from the plume head, 

thus supporting deep seismic data. The presence of a broad 

asymmetric plume head in the Oligocene may account for the 

lack of early domal uplift & radial drainage patterns in western 

Yemen. Pan-African lithosphere : The temporal change 

from basalts to rhyolites-ignimbrites & the occurrence of 

granite-gabbro-syenite plutons/sheets indicates an important 

change in process. To help evaluate the role of shallow magma 

chambers, elemental & isotopic data have been linked with 

high precision 0 isotope data. In Yemen the majority of 

volcanic rocks have experienced high level contamination with 

continental crust. Detailed isotopic studies caution against the 

use of highly magnesian rocks as indicators of sub-crustal 

processes as even highly magnesian (>10%) volcanic rocks can 

be contaminated with crust in utilising. The precise 

contribution (if any) from the depleted lithospheric mantle is 

difficult to assess because of similarities in chemistry with sublithospheric components. 

216 

CFF - XENON IN METEORITES AND ON THE EARTIJ 

MESHIK. Alexander P., SHUKOLYUKOV, Yuri A., 

Vemadsky Institute of Geochem. & Anal. Chemistry, 

117975 .Moscow, Russia, JESSBERGER, E. K., .MPI 

fur Kernphysi~ Postfach 103980 Heidelberg, Germany 

Since 1963 (Butler,1963) xenon enriched in the isotopes 

132xe, 131 Xe, I34xe, 129xe has been observed in C02 

well gases, .MORB glasses (Bennett, I 970 ), anorthosite 

(Jeffery, 1971) and in meteorites (Jessberger,1992). The 

surprising sirnUarity of xenon in these distinctly different 

objects was pointed out but no explanation had been suggested (Kirsten, 1981). We have found out that the difference between solar (AVCC)- Xe and terrestrial atmospheric Xe has the isotopic pattern similar to the alien Xe. 

The alien Xe behaviour looks strange. The repeated Xe 

analysis in Greenland anorthosite (Azuma, 1993) did not 

confirm the undoubted Xe anomaly previously reported 

for the sample of the same locality (Jeffery, 1971). Some 

of 129xe anomalies in MORB glasses turned out to be 

"gone with the wind• (FISher, 1993). 

Hints towards the origin of the strange Xe component 

came from our Xe measurements in rocks from the Oklo 

natural nuclear reactor. Follow-up experiments - analysis 

of Xe evolved in the ampoule during n-irradiation of pitchblends and Xe analyses of sandstone from the epicentre 

of a nuclear test - proved that the enrichment in middle 

isotopes of Xe results from the migration of fl-radioactive 

Xe precursor produced during the fission of heavy nuclei. 

The migration of Xe precursors have been occurred at a 

relatively high temperature and/ or in external part of the 

samples, so this alien, Chemically Fractionated Flssion Xe 

(CFF-Xe) could be or not be observed depending on the 

grain size of sumple uner study. 

The existence of this CFF-Xe might be of great geochemicaJ and cosmochemicaJ importance. 

Azuma, S., Ozima, M. and Hiyagon, H.,l993, Anomalous 

neon and xenon in an Archaean anorthosite from West 

Greenland: Earth and Plan. Sci. Lett., 114, p. 341-352. 

Bennett, G. A. & .Manuel, 0. K., 1970, Xenon in natural 

gases: Geochirn. Cosmochim. Acta, 34, p. 593-610. 

Butler, W. A., Jefferey, P . .M. and Reynolds, J., 1963, 

Isotopic variation in terrestrial xenon: Joum. Geophys. 

Res. 68, p. 3283-3291. 

Fisher, D. E., 1993, The 129xe anomaly in .MORB: gone 

with the wind? : Meteoritics, 28, No.3, p. 112. 

Jeffery, P . .M., 1971, Isotopic Composition of Xenon from 

a Greenland Anorthosite: Nature, 223, p. 260-261. 

Jessberger, E. K., Jordan, J. L., Shukolyukov, Yu. A., 

Meshi.k, A. P., Dang Vu Minh, 1992, Widespread alien 

Xe and its formation: LPSC xxm, p. 615-616. 

Kirsten, T., Jordan, J. and Jessberger, E. K., 1984, Two 

unexplained Xe-components in meteorites and on the 

Earth: are they linked?: Terra cognita, v.4, No.I, p.75. 
A COMBINED PB-ND-SR ISOTOPIC AND TRACE 

ELEMENT STUDY OF MID-OCEAN RIDGE 

BASALTS FROM EXPLORER RIDGE, NORTHEAST PACIFIC 

MICHAEL. Peter J. (Dept. of Geosciences, 

University of Tulsa, Tulsa, OK, 74104; 

Chase, R. L. and Armstrong* R. L. (Dept. of 

Geological Sciences, Univ. British Columbia, 

Vancouver, BC V6T2B4 (*deceased) 

Incompatible trace element and Pb, Nd and Sr 

isotopic data have been obtained for 15 basalts and 

glasses from the mid-ocean ridge north of Juan de 

Fuca Ridge (JdF). Of these, 9 are from Southern 

Explorer Ridge (SER); 4 from Northern Explorer 

Ridge (NER); and 2 from Explorer Deep (ED). One 

sample from Tuzo Wilson Seamounts (TWS) to the 

north was analyzed. In addition, 2 samples from 

Endeavor Seamount and West Ridge, at the northern 

end of JdF, were analyzed and found to be the most 

highly depleted samples of the study. The most 

enriched samples are E-MORB from SER and ED 

and a hawaiite from TWS. 

There are good correlations between all isotopic 

systems and also between incompatible element 

enrichment (expressed as La/Sm or Nb/Zr) and 

isotopic ratios. Isotopic and trace element ratios are 

modeled as binary mixing of enriched and depleted 

end members. Hyperbolic arrays on isotope ratio 

diagrams of Pb vs. Sr, Pb vs. Nd, and Nb/Zr vs. Pb 

allow some characteristics of the mixing end members 

to be identified (see boldface type below). The 

enriched end member itself could be a mixture of 

DMM and HIMU. The depleted end member is 

nearly pure DMM. The sense of hyperbolic curvature 

suggests that the enriched end member has lower 

Nd/Zr and Sr/Zr ratios and higher Pb/Zr, Pb/Sr and 

Pb/Nd ratios than the depleted end member. 

A two-stage model of mantle enrichment (possibly 

during plume activity) followed by MORB formation 

is proposed to explain the isotopic composition and 

Pb/Zr, Pb/Sr etc. ratios of end members. 

The isotope-isotope and Nb/Zr-isotope arrays for 

the Explorer Ridge are distinctly different than 

published data for the East Pacific Rise. Our data 

substantiate the dearth of EM-I and EM-II 

components for the entire northeast Pacific, including 

perhaps in terrestrial volcanics. 

Depleted component Enriched component 

87Srf86Sr 0. 70233 0.70270 

143Ndf144Nd 0.51318 0.51303 

206pbf204Pb 18.10 19.20 

207pbf204Pb 15.47 15.56 

208pbf204pb 37.55 38.45 

217 

ISOTOPIC SYSTEMATICS OF ROCK-FORMING 

MINERALS OF LAMPROPHYRES AND LAMPROITES OF 

CENTRAL EAST ANTARCTICA 

MIKHALSKY,E. V. and BELIATSKY,B. V., VNIIOkeangeologia, Maklina 1, St.Petersburg 190121, Russia 

The central sector of Precambrian East Antarctic Shield 

(between 60 -80 E) has been known for a number of 

occurrences of alkaline rocks of different type and age. Two 

compositionally, spatially and temporaly distinctive dyke rock 

types of lamproites (Rubin Massif, southern Prince Charles 

Mountains, PCM) and lamprophyres (Vestfold Hills, VH) 

were studied. Both rock types are high-K and high-Mg 

alkaline rocks strongly enriched in LILE, HFSE and REE. 

VH lamprophyres represent a heterogeneous rock group 

ranging from alkaline to ultramafic varieties derived from 

somewhat different Grt-bearing source regions by varing 

degrees of partial melting. Phlogopite, clinopyroxene, 

feldspars, olivine are main rock-forming minerals. Rb-Sr 

isochron (phlogopite, clinopyroxene and WR) shows an age of 

last thermal reworking of a at c.800 Ma. Lamprophyres reveal 

a wide scatter of Sm-Nd data attributed to mantle source 

heterogenity with E(O) ranging between -11 and -18 and 

T(DM) between 1500 and 1950 Ma. However, three rocks and 

a clinopyroxene define a reference line of 1900±300 Ma, 

·which is likely to constrain the lower crystallization time 

limit. Clinopyroxene has 87Sr/86Sr (1900) = 0. 703, E(O)= -6 

and T(DM)= 1820 while the host WR has T(DM)= 1890 Ma. 

Similarity between these model ages implies: i) Sm-Nd 

system of the rock has ilot been reequilibrated since c.1800-

2000 Ma, and ii) the source region had isotopic characteristics 

akin of depleted mantle (E(l800)= +4- +5). 

PCM lamproites are composed of sanidine, phlogopite and 

K-richterite with microcline partly replacing sanidine. Rb-Sr 

isochron (phlogopite, microcline and WR) defines a 460±23 

Ma reference line attributed to the latest thermal reworking 

roughly co-eval with low-grade metamorphism in the area. 

Sanidine shows extremely low 87Rb/86Sr ratio, and its low 

87Sr/86Sr= 0. 702 is thought to reflect mantle source 

characteristics. Sanidine Sm-Nd data show extremely high 

143Nd/144Nd= 0.513357 consistent with derivation from a 

depleted (E(l800) =+5 - +6) rather than previously enriched 

source. Model ages for sanidine and the host rock are 1500 

and 1300 Ma respectively. The latter is co-eval with 

subduction-related igneous processes in the adjacent mobile 

belt and may reflect the age of lamproite dykes in PCM. The 

isotopic systematics of selected rock-forming minerals of two 

alkaline rocks reveal low 87Sr/86Sr and high 143Nd/144Nd 

ratios which are likely to reflect mantle source isotopic 

characteristics corresponding to those of depleted mantle. 

However, high LILE, HFSE and REE concentrations 

attributive of these rocks imply mantle sources to have been 

metasomatically enriched. To coinside the data presented the 

enrichment processes must not be supposed to predate the 

igneous processes for significant period of time being not only 

precursors to alkaline igneous activities but perhaps also 

triggering the partial melting of essentially depleted source 

regions. 
TIMING OF DEFORMATION ON EXTENSIONAL 

SHEAR ZONES IN NORTH-WEST PALMER LAND, 

ANTARCTIC PENINSULA. 

MILLAR. I L., British Antarctic Survey, c/o NIGL, 

Keyworth, Nottingham NG12 5GG, Great Britain, and 

VAUGHAN, A.P.M., British Antarctic Survey, High 

Cross, Madingley Road, Cambridge CB3 OET. 

Extensional shear zones deform basement gneisses and a 

Mesozoic subduction-related plutonic and volcanic complex 

in the north-west Palmer Land region of the Antarctic 

Peninsula magmatic arc. New zircon ages from basement 

and arc-related rocks, together with mineral ages from shear 

zones are presented in order to constrain the timing of this 

extensional deformation. 

Conventional zircon analyses of basement orthogneisses 

from several localities give consistent Late Triassic igneous 

emplacement ages. In particular, gneisses from Campbell 

Ridges, which were previously thought to be Palaeozoic in 

age, give an age of 227 ± I Ma. The gneissic basement is cut 

by a suite of Early Jurassic foliated granitoids, typified by a 

syn-metamorphic granodiorite at Auriga Nunataks, which 

was emplaced into pre-Late Triassic basement paragneisses 

at205 ± 3 Ma. 

Dextral transtensional ductile shear zones within the 

gneiss complex at Auriga Nunataks became active around the 

peak of upper amphibolite-granulite facies metamorphism 

(ca. 185 Ma). The extensional deformation may be related to 

the break-up of Gondwana, the high grade metamorphism 

resulting from high geothermal gradients during the 

initiation of break-up. An upper age limit for the timing of 

extensional shear deformation is given by a cross-cutting, 

unsheared gabbro, which is dated at 132 ± 4 Ma. 

Further shearing related deformation and amphibolite 

facies metamorphism occurred at around 150 Ma, with 

associated anatexis of the Campbell Ridges orthogneiss. 

Anatectic granite sheets from the gneisses have given an RbSr whole-rock isochron age of 153 ± 10 Ma. 

At Creswick Peaks, Early Cretaceous arc-related 

magmatic rocks show evidence of syn-magmatic crustal 

extension within a major dextral shear zone. Textures 

ranging from sub-magmatic phenocryst alignment to 

ultramylonite are preserved in granodiorite. The age of 

emplacement of the magmatic rocks has been dated at 141 ± 

2 Ma, while hornblende separated from a ductile shear fabric 

has yielded conventional K-Ar ages of 141 ± 4 Ma. 

The extensional structures described above often show 

partial overprinting during a mid-Cretaceous episode of 

ductile thrusting, which is contemporaneous with, and may 

be related to, increased rates of Pacific sea-floor spreading at 

that time. 

In conclusion, extensional shear zones were initiated in 

NW Palmer Land during the early stages of Gondwana 

break-up. Extensional deformation continued intermittently 

throughout the Jurassic. In Early Cretaceous times 

extensional shear zones developed during a major episode of 

magmatism and crustal growth. 

218 

ANCIENT CRUSTAL FLUIDS ASSOCIATED WITH 

HYDROTHERMAL PROCESSES, S W ENGLAND 

MILLER. M. F., Glenny Associates, 9 Oak Piece, 

Welwyn, Herts. AL6 OXE, UK; SCRIVENER, R. C., 

British Geological Survey, St Just, 30 Pennsylvania 

Road, Exeter EX4 6BX, UK; BANKS D. A., 

Department of Earth Sciences, University of Leeds, 

Leeds LS2 9JT, UK; PILLINGER C. T ., Planetary 

Sciences Unit, The Open University, Walton Hall, 

Milton Keynes, MK7 6AA, UK. 

Recent U-Pb and 40 Ar-39 Ar studies (Chesley et al., 

1993) have confirmed that the Comubian batholith, S W 

England, was emplaced diachronously over an interval 

extending from ca. 295 to ca. 275Ma before present, with 

long -lived multiple intrusive episodes and no systematic 

trend in the age of pluton emplacement. 

We present here the results of an investigation into the 

provenance of hydrothermal fluids associated with early 

pegmatitic quartz and lithophile mineralisation of the S W 

England granites. In particular, the chemical composition 

and isotopic characteristics of hydrothermal fluids 

associated with tourmaline -dominated and greisen 

mineralisation of the Dartmoor granite are compared with 

early- stage W±Sn oxide-associated fluids in the province. 

Methods of investigation included the use of highsensitivity techniques to characterise the stable isotopic 

composition of traces of palreofluid carbon and nitrogen 

species, in conjunction with measurements of the associated 

electrolyte composition (including halogen ratios), as well as 

liD and li180 values of the waters. 

Together with geochemical and isotopic constraints from 

the characterisation of Palreozoic metasediments of the 

region, the results of this integrated approach support the 

view that the early fluids were primarily of magmatic origin; 

there is little evidence for fluid interaction with the local 

metasedimentary rocks at a high level crustal. Very low 

levels of carbon and nitrogen in the Dartmoor- hosted fluids 

reflect the comparatively low 'S' -type character of the 

parent intrusive. The incorporation of sedimentary matter 

into granitic protoliths during anatexis, with subsequent 

transfer to an exsolved hydrous phase during pb·.ton cooling, 

is believed to be the most probable route by which a 

significant proportion of the palreofluid solutes entered the 

early hydrothermal systems. This has implications for the 

recycling of trace elements such as carbon and nitrogen 

from the sedimentary rocks. 

There is little evidence for the involvement of meteoric 

water in early mineralising fluids hosted by either the 

Dartmoor or the adjacent Hemerdon Ball granite. This is not 

universally the case throughout the batholith, however. 

Chesley J. T., Halliday A. N., Snee L. W., Mezger K., 

Shepherd T. J. and Scrivener R. C. (1993), 

Thermochronology of the Cornubian batholith: 

implications for pluton emplacement and protracted 

hydrothermal mineralization: Geochim. et Cosmochim. 

Acta, v. 57, pp.1817-1835. 
Hf ISOTOPE CONSTRAINTS ON ANCIENT 

DEPLETION AND ENRICHMENT EVENTS IN 

THE MANTLE BENEATH THE SW USA 

Milling, ME, Jr, Johnson, CM and Barovich, 

KM, Univ. Wisconsin, Madison, WI 53706, USA 

Hf isotopes may constrain the depths of 

ancient mantle depletion and enrichment due to 

the large contrast in Lu/Hf K0 ratios for garnet 

(>24) and clinopyroxene (-1); such a contrast is 

not found in Sm/Nd K0 ratios. 

New data from high-£ Nd volcanic centers that 

are thought to have been derived from 

asthenospheric mantle (Cima, SE CA; Lunar 

Craters, cent NV) have anomalously low 176Hf/177Hf ratios (AEHt<-4; AE Ht from Johnson & 

Beard, 1993, Nature) that are similar to those 

previously found in the southern Rio Grande rift 

and may reflect ancient depletion in the spinel 

peridotite stability field; these results suggest that 

large areas of the asthenospheric mantle 

beneath the SW USA may be sub-oceanic 

mantle that was overridden by the North 

American continent. The low Lu/HfMEASURED ratios 

of these rocks, however, indicate derivation from 

the garnet stability field and indicate a major 

change in mantle mineralogy in the last few b.y. 

Moderate AE Ht values (- -2) for high-ENd (>+9) 

basalts associated with the Mendocino Triple 

Junction (Sonoma, N CA) suggest ancient 

depletion with a garnet componet, and contrast 

with the high Lu/HfMEASURED ratios that are best 

modeled by melting spinel peridotite. 

New data for alkaline lavas from the Colorado 

Plateau show an extremely wide range of AE Ht 

(+6 to -6) at relatively constant ENd (-0); such a 

vertical E Ht-E Nd array at E Nd-0 has not been found 

before, and may reflect mixing between ancient 

depleted spinel- and garnet-peridotite mantle 

(similar to the present- day MOAB array), in 

addition to LREE metasomatism and isolation for 

1-2 b.y. beneath the stable plateau. 

Finally, new data from the highly extended 

Death Valley region indicate that low e Nd lavas, 

possibly reflecting ancient enriched lithospheric 

mantle, have positive AEHt values that are similar 

to those found in NW Colorado (Beard & 

Johnson, 1993, EPSL); these compositions may 

reflect ancient shallow metasomatism or 

contamination by pelagic sediments. 

The diversity of Hf-Nd isotope and elemental 

variations for subcontinental basalts greatly 

exceeds that found in the oceans, and provides a 

unique view into the processes that shape the 

subcontinental mantle. 

219 

PALEOTHEMOMETRY USING THE U/CA RATIO 

OF CORAL SKELETONS 

MJN. G R and EDWARDS, R.L. Minnesota Isotope . 

Laboratory,' Dept Geology and Geophysics, U. Minnesota, 

Minneapolis, MN 55455 USA; TAYLOR, F.W. Institute 

for Geophysics, University of Texas at Austin, Austin TX 

78759 USA; RECY, J., Laboratoire de Geodynamique SousMarine, B.P. 48-06230, Villefranche SIMer, France; 

GALLUP, C.D. and BECK, J.W. Minnesota Isotope 

Laboratory, Dept Geology and Geophysics, U. Minnesota, 

Minneapolis, MN 55455 USA 

Annual variations in the 238Uf40Ca ratio of Porites lobata 

coral skeletons have been discovered for both modem and fossil 

samples collected from Amedee Lighthouse, Noumea, New 

Caledonia (modem); Papeete, Tahiti (modem) and Espiritu 

Santo Island, Vanuatu (fossil). We sub-sampled coral on 

ttansects perpendicular to coral growth bands. The sub-sample 

size for U, Sr and Ca isotope analysis was about 3 mg, which 

re~resents an a~roximately monthly growth interval. 

23 Uf40Ca and Srf40Ca ratios were determined by isotope 

dilution thermal ionization mass ~trometry on aliquots of 

the same sub-sample. Errors for 2 8Uf40Ca were less than ±2 

%o (20') on several nanograms of U. Errors for 88Srf40Ca were 

less than ±1%o. All samples have annual variations in 

238Uj40Ca, which mimic and are in ~base with variations in 

sssrf40Ca. A plot of 88SrJ40Ca vs. 2 8Uf40Ca for sub-samples 

of 4 corals from different localities shows a high correlation 

between 88Srr<>ca and 238U/4Dca. This suggests that annual 

variations of 238U/40Ca in coral skeletons are largely controlled 

by the sea surface temperature. Using our correlation for 

modem sub-samples and the known dependence of 88Sr/40Ca on 

temperature, we calculated a temperature dependence for 

23suroca: 

Tee)= 43.703-17.477 x 1o6 x (

238Uf40Ca)(molar) 

The fractional shift per °C is about 7 times larger (5.3% per 

degree) for 238Uf40Ca than for sssrf40Ca (0.7% per degree). 

Using our 238Uf40Ca vs. T and 88Sr/40Ca vs. T correlations, 

we calculated temperatures for 8 and 10 Ky fossil corals from 

Vanuatu. Both thermometers give mean annual temperature 

that are well below the present mean annual temperature of 

26.5 °C. If these temperatures can be generalized in time and 

space, they suggest that tropical sea surface temperatures during 

deglaciation were 4 to 6 °C lower than present However, 

238Uj40Ca and 88Srt40Ca do not correlate perfectly; the mean 

annual 238Uf40Ca temr:.ratures for the fossil samples are about 

3 oc higher than the 8 Srf40Ca temperatures and the seasonal 

amplitude is somewhat smaller. It is possible that 238U/40Ca 

ratios may be sensitive to other paleoceanographic parameters, 

such as PCOl, pH or alkalinity. Because annual cycles in 

238Uf40Ca appear to be a general feature of primary coralline 

aragonite, the preservation of such features will be important in 

identifying unaltered corals for U-series dating studies. 

Furthermore, 238uj40ea thermometry rna~ have advantages over 88SrJ40Ca thermometry because the 38Uf40Ca ratio is 

more sensitive to temperature than the sssrf40Ca ratio. Once 

high resolution calibration is completed, it is possible that less 

precise, higher throughput techniques (such as ICP-MS) may 

be used in routine measurement of 238Uf40Ca. 
HIGH-TEMPERATURE WATER-ROCK INTERACTION: EXPERIMENTAL INVESTIGATION USING RADIO-ACTIVE TRACER METHOD 

MIRONOV A.G_, ZHATNUEV N.S. 

Buryat Geological Institute, UlanUde, Russia 

The radio-nuclides of gold (Au- 195, Au-198), silver (Ag-110m+110) and hydrogen (tritium-bearing water) were used for the study of 

the high-temperature wate~rock 

interaction. The use of the radioactive tracers method allowed to 

go into such problems the solution 

of which by traditional methods is 

diffieult or impossible. The expe- riments in hydrothermal (T=200- 5000C, P•200-1000 bar) and magma- tic (T•825°C, P=2100-2300 bar) systems on the study ot the water, 

gold and. silver behaviour under 

water-rook interaction were carried out. The tritium was used a· 

tracer in the water (3HHO+H20) 

with quantitative 10-15 MBc/g. Radio-isotopes of gold and silver 

were placed in basalt, andesite 

and riolite glasses melting beforehand with total concentration of 

gold 50 ppb, silver - 50-100 ppm. The mapping of the radio-isotopes distribution in glass samples was 

obtained by the beta radiography 

on the nuclear emulsion plate with 

the spatial resolution about 20 

micron for tritium. 

In process of the water-glass interaction (200-500°0) the hydra- tation, re-crystallization and leach zones were formed in the glass of different composition. Gold and 

silver were leached from hydrata- tion zone and were re-concentrated 

in re-crystallization zone. The 

high mobilization from riolite,andesite and basalt glasses in more 

typical for silver (60-95%), whereas the gold was leached from 

glasses only with increase of the 

oxigen fugasity in system. A conclusion was made that volcanic rocks may be a source of precious metals for the gold and silver depo- sits at hydrothemal,r~al.'bfd!•aiioll of 

host voleanie rooks depending on 

their composition, temperature, 

pressure and oxigen fugasity. · 

220 

RB-SR SYSTEMATICS IN RECENT SEDIMENTS, AND 

ITS BEARING FOR GEOCHRONOLOGICAL 

INTERPRETATIONS 

MISUZAKI, A.M.P., Petrobnls/Cenpes Cidade 

Universitaria/ Quadra 7, Ilha do FundAo/21949-900 Rio de 

Janeiro, RJ, Brazil; CORDANI. U.G., Instituto de 

Geociencias/ Universidade de SAo Paulo, C.P. 20899, 

01498-970 SAo Paulo, SP, Brazil; KAWASIDTA, K. 

Instituto de Geociencias/Universidade de Sao Paulo; and 

THOMAZ-FILHO, Instituto de Geociencias/Universidade 

de Sao Paulo. 

In many known cases, when dealing with fine grained and 

argillaceous sedimentary rocks, the age values obtained from 

Rb-Sr whole-rock isochron calculation are of geological 

significance, despite of the fact that the initial condition of Sr 

isotopic homogenization cannot be envisaged, in such low 

temperature environments where deposition and diagenesis 

occur. To explain it, Cordani et al (1978), as well as Cordani 

et al (1985), proposed complete mechanical mixing, leading to 

material with fairly uniform 87Sr/86Sr ratios in relation to the 

Rb/Sr ratios of the samples. 

In this work, in order to investigate the chemical mobility 

ofRb and Sr during sedimentation and early diagenesis, more 

than 60 samples of fine grained recent sediments from selected 

coastal localities in Brazil were studied by several different 

techniques, including conventional chemical analyses, X-ray 

difraction and fluorescence, scanning electronic microscopy, 

and isotope geochemistry through mass spectrometer. The 

samples were collected from the continental shelf at the 

Amazon river mouth (AM), the deltaic region of the A~u river 

(RN), the deltaic region of the Paraiba do Sul river and the 

Jacarepagua coastal plan (RJ), as well as the continental slope 

in the Campos basin (RJ). 

The results demonstrate clearly that, especially when 

smectites and illite predominate in the sediments, the initial 

conditions that may evolve to significant isochrones after the 

closure of the chemical systems to Rb and or Sr mobility are 

indeed fulfilled. The Rb/Sr ratios of the rock systems are 

modified since the chemical weathering processes, but possibly 

the more radical changes occur when the sediments reach the 

coastal plains, and interact with sea water. In these conditions 

halmyrolisis occur, inducing most Sr to be dissolved into the 

sea water, producing relevant increases in the Rb/Sr ratio. At 

the same time, interaction and mixing with marine strontium, 

during halmyrolisis and early diagenetic processes, makes the 

87Sr/86Sr of the sediment to approach the marine value of 

about 0.709. 

The above conditions, which confirm the existance of 

"horizontal" isochrons for fine grained recent sediments, seem 

to adequately justify the applicability of the Rb-Sr method to 

shales and related rocks .. 

Cordani, U.G., Kawashita, K., and Thomaz-Filho, A., 1978. 

AAPG Studies in Geology, v.6, p.93-ll7. 

Cordani, U.G., Thomaz-Filho, Brito-Neves, B.B. and 

Kawashita, K., 1985, Giomale di Geologia, v.47 (l-2), p.253-

280. 
THERMAL VARIATIONS IN A PASSIVE MARGIN, 

FROM APATITE FISSION TRACK ANALYSIS IN THE 

WESTERN OTWAY BASIN, AUSTRALIA. 

MITCHELL. M M., HILL, K.C., FOSTER, D.A., 

O'SULLIVAN, P.B. at VEIPS, School of Earth Sciences, 

LaTrobe University, Bundoora, Victoria, 3083, Australia, 

DUDDY, I. R. at Geotrack International, Melbourne, 

Victoria, 3052, Australia. 

The Otway Basin formed during rifting of Australia and 

Antarctica commencing in the Late Jurassic. A stable passive 

margin setting has been proposed for the basin since the midCretaceous breakup. The Gambier Embayment, the western 

section of the Otway Basin, consists of fault-bound structural 

troughs and highs, including the Merino Uplift, which defines 

the eastern margin. 

The apatite fission-track (AFT) thermochronology and 

vitrinite reflectance data for several wells in the Gambier 

Embayment indicate thermal variations in the Cretaceous and 

Tertiary. Data from the Penola-1 and Robertson-1 wells, 

within the Penola Trough at the northern margin of the basin, 

indicate different thermal histories. AFT ages of five samples 

from Penola-1 increase with depth from 143 to 279 Ma. Mean 

lengths (ML) for these samples decrease from 12-13 IJ.m to 

10.2 IJ.m with depth. Single grain ages indicate detritus from 

multiple source regions, with pre-Mesozoic age grains 

dominating the lower section at the expense of 

contemporaneous volcanogenic material. Mesozoic(?) 

trachytic basement intersected at the base of Robertson-1 

(presently at 97°C) has an AFT age of 104±11 Ma and a ML 

of 10.41J.m. A sample from the overlying Otway Group in 

Robertson-1 currently at 61 °C, records an age (283±20 Ma) 

older than the stratigraphic age (112-123 Ma). Intergration of 

vitrinite reflectance and AFT data suggests that Penola-1 

experienced a recent decline in heat flow, while Robertson-1 

underwent uplift and erosion prior to an increase in heat flow. 

On the Beachport-Kalangadoo High elevated heat flow 

during burial in the Cretaceous occurs at Kalangadoo-1, and 

not at Geltwood Beach-1 and Beachport-1. Kalangadoo-1 

penetrates Palaeozoic ('?) metasediments which yield AFT ages 

from 94 to 20 Ma and a paucity of lengths <9 IJ.m, for 

samples at 77-98°C. Three samples from the Otway Group 

give ages from 166-111 Ma with ML decreasing with depth 

from 13.5 to -12 IJ.m. AFT results from Geltwood Beach-1 

and Beachport-1 are consistent with present temperatures, with 

samples retaining Mesozoic syndepositional single grain ages. 

Mocamboro-11 on the Merino Uplift, intersects Otway Group 

sediments near surface and metasedimentary basement at 1374 

m. The basement gives an AFT age of 44±6 Ma and a 

bimodal length distribution with MI... of 8.9 IJ.m. The samples 

from the overlying Otway Group exhibit reduction in ML (11-

10.2tJ.m) indicative of higher palaeotemperatures. 

Variable thermal histories are also suggested for the Tertiary 

and are manifested as Miocene cooling at both Mocamboro-11 

and Kalangadoo-1, and a Neogene increase in heat flow at 

Robertson-I. Variable amounts of denudation are suggested, 

the most significant in the vicinity of Mocamboro-11 and 

Robertson-1 on the basin margins with -700 m and -1000 m 

of apparent denudation, respectively. This uplift along the 

basin margins may have resulted from lithospheric flexure as 

the main depocenters of the basin loaded with sediment, or 

block faulting. In summary, the results show that the western 

section of the Otway Basin did not become thermally and 

tectonically stable following 'break-up'. Episodes of uplift, 

erosion, and fluctuations in geothermal gradient continued 

throughout the Late Cretaceous and Tertiary in the basin. 

221 

DEGASSING HISTORY OF NITROGEN AND 

ARGON 

MIYAZAKI. A .. HIYAGON, H., SUGIURA, 

N., Dept. of Earth and Planet. Phys., Graduate School of Sci., Univ. of Tokyo, Tokyo, 

113 Japan. 

Degassing process of nitrogen from the mantle is considered to be mostly controlled by 

physicochemical properteies of nitrogen, such as 

gas/silicate melt and silicate/solid partition coefficients. Since there are very few data about 

these partition coefficients, we conducted an experiment to obtain them. We used 15N-labeled 

gas in synthesizing basaltic melt and melt/crystal 

pairs. From the obtained partition coefficients, we 

estimate the transport efficiencies of nitrogen and 

argon from the mantle to the atmosphere. 

Then we constructed a simple 2-reservoir ( atmosphere and mantle) and 2-stage (catastrophic and 

continuous) nitrogen and argon degassing model 

on the basis of the estimated transport efficiencies of nitrogen and argon and the N2 / 40 Ar flux 

ratio of f"oJ 50 from the mantle estimated from 

MORB glass data. The present result indicates 

that nitrogen has a degassing history different from 

that for argon; either nitrogen may be degassed 

less efficiently at the catastrophic degassing event, 

or considerable amount of nitrogen once degassed 

may have been returned to the mantle. Assuming N2/ Ar transport efficiency ratio being f"oJ 0.1 

estimated from the preliminary result of partition 

coefficients of nitrogen [1] and other published data 

[2,3], the amount of nitrogen in the mantle is caluculated to be 1.5 times as much as that in the 

atmosphere. 

1. Miyazaki, Hiyagon and Sugiura, 1993, 29th 

IGC abstract vol.3, 651. 

2. Hiyagon and Ozima, 1986, G.C.A. 50, 2045-

2057. 

3. Yurimoto and Sueno, 1984, Geochem. J. 18, 

85-94. 
ISOTOPIC EVIDENCE OF ARCHEAN CRUST IN THE 

REINDEER ZONE OF THE TRANS-HUDSON OROGEN, 

NORTHERN SASKATCHEWAN 

MOCK. Timothy D., and BICKFORD, M.E. 

Department of Earth Sciences, 

Syracuse University, Syracuse, NY 13244 USA 

The Reindeer Zone of the Trans-Hudson orogen is a 

collage of volcanic arcs and related plutonic rocks that were 

assembled during the Early Proterozoic. Recent seismic 

reflection studies show that the Glennie Domain is a 

culmination between east -dipping reflectors beneath the 

Superior craton to the southeast and west -dipping reflectors 

to the west. Archean crust is exposed in small structural 

windows in the Iskwatikan and Hunter Bay region of the 

Glennie Domain and farther to the east in the Sahli 

Charnockite in the Hanson Lake Block. K-feldspar common 

Pb and Sm-Nd whole-rock data indicate extensive 

involvement of Archean crust in the generation of postorogenic leucogranites but not in the formation of the 

"Hudsonian" plutons. 

U-Pb zircon ages of plutonic rocks in the Reindeer Zone 

fall into three age ranges. (1) "Hudsonian" granodiorite and 

tonalite gneisses that have crystallization ages between 1880 

and 1854 Ma. (2) Weakly deformed granites that were 

emplaced after peak metamorphism between 1833 and 1824 

Ma. (3) Post-orogenic leucogranites and pegmatites that 

have zircon ages between approximately 1780 and 1770 Ma. 

Pb K-feldspar analyses of the exposed Archean basement 

have f06Pbf204Pb)i = 14.615-14.769 and f07Pbf204Pb)i = 

15.167-15.378. Calculated f07Pbf206Pb) ages for the 

"Hudsonian" plutons are between 2580 and 2350 Ma 

indicating that these rocks evolved in an environment with 

high 238Uf204Pb (p= 11). The pegmatites that cut the Sahli 

Charnockite also formed in a high I' environment but have 

lower 207Pbf206Pb ages (2330-2250 Ma). "Hudsonian" Kfeldspar 207Pbf206Pb ages range from 1720-1650 Ma 

indicating that these plutonic rocks were derived from 

juvenile sources. Leucogranite and pegmatite K-feldspar 

data from the Glennie Domain and the Hanson Lake Block 

range from f06Pbf204Pb)i = 15.271-15.368 and f07Pb/ 204Pb)i 

= 15 .165-15.307. These rocks have 232Thf238U ratios 

between 4.72-6.19 consistent with derivation from a high 

Th/U source such as the exposed Archean basement. In 

contrast, pegmatites from the volcanic La Ronge Domain 

have high f06Pbf204Pb)i = 15.634-16.344 and f07Pbf204Pb)i 

= 15.228-15.307. This indicates that there was little or no 

involvement of Archean crust in the western part of the 

Reindeer Zone. 

The "Hudsonian" plutons have eNd<,> values ranging from 

+3.0 to +4.5 and ToM ages of approximately 1870 Ma, 

very similar to their crystallization ages. This indicates that 

little or no Archean crust was involved in the generation of 

the "Hudsonian" plutons. In contrast, the post-orogenic 

leucogranites have eN~t) values of -13 to -14 and T0M ages 

of 3400-3150 Ma. Both the Sm-Nd and K-feldspar common 

Pb data are consistent with an extensive region of buried 

Archean crust beneath the Glennie Domain and the Hanson 

Lake Block. 

222 

EROSION HISTORY OF THE CENTRAL SIERRA 

NEVADA USING COSMOGENIC lOsE AND 26AL 

MONAGHAN M C., Department of Earth and 

Atmospheric Sciences, Purdue University, West 

Lafayette, Indiana, 47907-1397, USA. 

The present physical geography of the central Sierra Nevada 

can be subdivided into regions of somewhat similar form. 

These are: i) deep valleys of large rivers that trend roughly 

orthogonal to the axis of the Sierra Nevada crest from the east 

to west or northeast to southwest; ii) broad, inclined, plateaulike interfluves between these deep valleys; iii) steeply-rising 

west- and south-west facing steps and step fronts found within 

some of these intertluve regions; iv) glaciated uplands; and v) 

weathered uplands that comprise west-dipping rough tables on 

mountain and ridge tops. 

Through the study of in situ produced cosmogenic lOse and 

26 AI we hope to constrain exposure ages and erosion rates of 

geomorphic surfaces found in these physiographic regions. We 

hope to determine whether or not our simple classification of 

physiography corresponds to a simple pattern of erosion rates 

and exposure ages. Two particular studies are presently 

underway. First, we are studying the weathered uplands of the 

Crystal Range in the Central Sierra Nevada that comprise westdipping rough tables on mountain and ridge tops to determine 

whether or not these surfaces are remnants of pre-Wisconsin 

surfaces. Second, we are studying the steeply-rising westfacing steps and step-fronts in the Shaver Lake region of the 

Central Sierra Nevada in an effort to determine whether or not 

the exposure ages of these surfaces increase with distance from 

the Sierra Nevada/Great Valley boundary. The first study 

addresses an age-old geomorphic question - Do uplifted erosion 

surfaces survive as flat regions along mountain and ridge tops? 

The second study addresses Warhafitg's hypothesis that the 

steps and step-fronts developed through differential weathering 

rates. 
ISOTOPIC EVIDFNCE OF HOLOCENE AND 

PLEISTOCENE PALEOFNVIRONMENTS IN THE 

CHIHUAHUAN DESERT, USA. 

MONGER H Curtis, Dept. of Agronomy and 

Horticulture, New Mexico State University, Las Cruces, 

New Mexico, 88003, USA, and COLE, David R., 

Chemical and Analytical Sciences Division, Oak Ridge 

National Lab., Oak Ridge, Tennessee, 37831, USA. 

As part of the USDA Desert Project, decades of effort have 

been invested in understanding the Quaternary soil 

stratigraphy in the Chihuahuan Desert of southern New 

Mexico (Gile et al., 1981). Because of the good chronologie 

control and because many calcic soils are buried beneath the 

zone of modem pedogenesis, the Desert Project is a good 

location for conducting stable isotope studies. For instance, 

o I3C values of pedogenic carbonates on alluvial fan soils 

decreased an average 6%o just prior to a major erosion episode 

about 8 ka (Fig. 1). This isotope shift indicates the decline of 

c4 grasses and an increase in c3 desertscrub vegetation. 

Concurrent with the carbonate o l3C shift was a decrease in 

organic o 13C values from- -17 to -25%o and a reduction in 

Gramineae pollen. 

Although the vegetation change is apparent at about 8 ka, 

o 18Q values remained relatively constant (Fig. 1), suggesting 

that a major temperature increase did not accompany the 

ecological change. This raises the possibility that increasing 

atmospheric C02 during deglaciation contributed to the 

decline of the c4 grasses growing on the ecologically fragile 

alluvial fan soils. In contrast, the deep sandy soils of the 

basin floors do not contain the dramatic o 13C shift. These 

results support the usefulness of stable isotopes for recording 

the timing and nature of terrestrial environmental changes. 

0 

5 

§1o 

~ 15 

a: 

m 20 2! 

as 

CD 

> 25 0 

;! 

30 

35 

co~i~c~ 

<Ill) '-8 

()I 

0 -

c~. ,...... 

~ 

0 ,....< .. " 

,...... 

'-' • 

~ ~ 

'-" 

( 

~ (~ i 

0 

r. ,...... 

~ '-' 

• 

-12 -10 -8 -6 -4 -2 0.8 -6 -4 -2 

S 1 3C POB S 180 PDB 

Figure 1. o 13C and o 180 values versus time for pedogenic 

carbonates from four alluvial fan soils. Solid circles 

indicate 14C-dated samples. Other samples are interpolated 

according to their stratigraphic position to dated samples. 

Gile, L.H., Hawley, J.W., and Grossman, R.B., 1981, New 

Mexico Bureau of Mines and Mineral Resources Memoir 

39, Socorro, 222p. 

DRC sponsored by the Geoscience Research Program, Office 

of Basic Energy Sciences, US DOE Under Contract DE-AC05-

840R21400. 

223 

INTEGRATED STUDY OF THE MIOCENE TIME SCALE 

MONTANARI A. (Osservatorio Geologico di Coldigioco, 

62020 Frontale di Apiro, Italy), COCCIONI R. (Istituto di 

Geologia, 61029 Urbino, Italy) and ODIN G.S. (Dept. 

Geologie Sedimentaire, Uniersire P. & M. Curie, 75252 

Paris, France) 

Under the acronym of "Miocene Columbus Project" 

(MICOP) we have organized in 1991, an interdisciplinary 

working group focusing on high-resolution stratigraphic 

calibration of the Miocene Epoch. 

The pelagic carbonate series of the Apennines (Italy) 

contains volcanoclastic layers which offer the rare opportunity 

to calibrate the biostratigraphic sequence with radiometric dates. 

Following petrographical and geochemical study, appropriate 

geochronometers have been selected. Radiometric ages are now 

available from 1- the Oligocene-Miocene boundary (Bosio) 

40Ar-39Ar step heating on hornblende at 22.7 Ma; 2- the 

Gubbio section: 40Ar-39Ar laser fusion dates from plagioclase 

from 2 layers yielding 21.2 ±0.5 (2a) Ma and 19.8 ±0.5 Ma 

which bracket the magneto- and biostratigraphically calibrated 

Aquitanian-Burdigalian boundary (Montanari et al. 1991); 3-

the Moria section: a biotite plateau age at 17.1, and 2 sanidine 

laser fusion ages at 16.2 and 15.5 Ma bracketting the 

Burdigalian-Langhian boundary; 4- other sections within the 

Apennines including an ash about 18 Ma old (upper 

Burdigalian, Santa Croce), a level about 15 Ma old (Langhian, 

L'Annunziata), and a biotite-rich level about 11.5 Ma old (base 

Tortonian, Monte dei Corvi); 5- Serravallian of Sicily: 40Ar39Ar step heating on biotite at 13.9 Ma; 6- a series of biotiterich layers delivering biotite and plagioclase consistent 

analytical ages between 7.34 and 8.25 Ma immediately below 

the Tortonian-Messinian boundary, (Faenza area) by Vai, et al. 

(1993) and supplemented in 2laboratories; 7- Late Messinian: 

40Ar-39Ar step heating on biotite at 5.5 Ma (Apiro, Italy). 

More data are prepared from samples collected across the 

Burdigalian-Langhian boundary and in the Langhian to 

Messinian pelagic sequence exposed on coastal cliffs. 

A group of datings is also in way in 3 laboratories from 

Japanese volcanogenic layers located within the marine 

Serravallian and Mio-Pliocene sequence with some already 

available under published (Kasuya, 1990) or preliminary form. 

Because of their qualities which include 1) applicability of 

bio-, magneto-, chemo-, and litho- stratigraphic tools, as well 

as geochronologic dating, allowing correlation with other 

sections in the world; 2) continuous and reasonably slow 

accumulation rate for accessible outcrops free of structural 

compexities; and 3) large quantity of diversified data from about 

40 researchers presently involved in this project, we propose 

some of these sections as potential "Global Stratotype Sections 

and Points" for the 6 Stages of the Miocene Epoch. 

The geochronological instrumental laboratories involved are 

Berkeley, Berne, Lausanne, Montpellier, Pisa, and Yamagata. 

The results of this interdisciplinary study will be gathered 

in two volumes in preparation. 

Kasuya, M., 1990. Quaternary Research, 33: 86-93. 

Montanari A., Deino A. et al., 1991, Newsl. Stratigr. 23: 151 

Vai G.B. et al., 1993. C. R. Acad. Sci. Paris, 316: 1407-1411. 
RADIOISOTOPIC CALIBRATION OF THE PALEOCENEEOCENE BIO-MAGNETOSTRATIGRAPHIC SEQUENCE 

AT GUBBIO (ITALY) 

MONTANARI. A. (Osservatorio Geologico di Coldigioco, 

62020 Frontale di Apiro -MC- Italy)~ and SWISHER, C. III 

(Geochronology Center, Institute of Human Origins, 2453 

Ridge Road, Berkeley, CA 94709) 

Radioisotopic dates from several volcanic ashes contained in 

Paleocene and Eocene marine sediments in Italy, DSDP 516F 

(South Atlantic Ocean), a deep sea core from the North S~. and 

on-land sections in Disko Island (Greenland}, and Agost (Spain) 

are here utilized to calibrate, with unprecedented precision and 

accuracy, the age of bio-, and magnetostratigraphic boundaries 

in the continuous and complete pelagic sequence of the 

Contessa highway, near Gubbio (Italy-- see figure). Details on 

mineral separation and purification techniques, instrumentation, 

and computation of the numerical ages with their standard 

deviations, will be given orally, along with the illustration of 

the bio-magnetostratigraphic characteristics of the marine 

sequences containing the dated volcanic material. 

NP 15 w 

NP14 0 

AGE 

Ill 

"C 

"C 

E 

RADIOISOTOPIC 

CALIBRATION 

POINT 

I Massignano 

-~~~:~b/Sr 

·~~~e zircon, monaz~e 

• r;,~~:S16F 

sanidine, biotite 

Greenland 

Ar/At 

basa~ glass 

Ill 

J----L~:.........L-NP 1 _._____....__a>__,. ~:· Spain 

biot~e 

224 

1291 AS A HYDROLOGIC TRACER: 

DETERMINATION OF SOURCE AGES AND 

MIGRATION PATTERNS OF BRINES IN 

SEDIMENTARY BASINS. 

MORAN. J.E. and FEHN, U., Department of 

Earth and Environmental Sciences, University 

of Rochester, Rochester, NY 14627 

129m ratios have been measured in brines from the 

Gulf Coast Basin, the Anadarko Basin in Oklahoma and 

Nevada oil fields. Results indicate that this isotope can 

be used to identify source formations of I and organic 

matter, and to traqk fluid migration from deeper parts of 

a sedimentary basin to present host formations. 

Interpretation of 129m ratios measured in brines from 

sedimentary basins is based on estimation of the relative 

contributions of cosmogenic and fissiogenic 1291. Ratios 

measured in brines from the Louisiana Gulf Coast basin 

can be compared to the decay of cosmogenic 1291 to get 

minimum source ages (when no fissiogenic contribution is 

included). These minimum ages are significantly older 

than present host formation ages, indicating migration of 

brine from older, deeper sources. Based on the measured 

ratios and conservative corrections for a fissiogenic 

contribtution, brines sampled from Pleistocene and 

Pliocene reservoirs have migrated from Paleocene-Eocene 

sources several kilometres deeper. Some ratios measured 

in Texas Gulf Coast brines show evidence of a greater 

fissiogenic component which indicates that the brines 

have resided in formations with locally high U 

concentrations. Brines with extremely high I 

concentrations from a Pennsylvanian host formation in 

the Anadarko Basin point to the Woodford Shale as the 

probable source formation, based on comparison of 

potential fissiogenic contributions from underlying 

sandstones and shales. Ratios measured in brines from 

Nevada oil fields are higher than would be predicted for 

deep sources, and indicate mixing with shallower, 

younger formation waters. 
DETERMINING THE CARBON ISOTOPE 

CHEMOSTRATIGRAPHY ACROSS THE 

PERMIANffRIASSIC BOUNDARY IN AUSTRALIA 

MORANTE. R, School of Earth Sciences, Macquarie 

University, North Ryde, NSW, 2109, Australia. 

A negative ol3c excursion at the palaeontologically 

determined Pennianffriassic boundary (Pffr) in marine 

sediment has been well established. A similar ol3c 

excursion to more negative values occurs in organic carl>on 

in pelitic whole rock samples from conventional or sidewall 

cores from seven Australian sedimentary basins. The ol3c 

excursion in marine and nonmarine sediment occurs within 

or near the base of the Protohaploxypinus microcorpus 

palynology zone. This zone is therefore interpreted as 

straddling the Ptrr. The size of the ol3c excursion, 

interpreted environment of deposition at the boundary and 

degree of biostratigraphic control based upon palynology in 

each basin is summarised in the table below. 

Basin ol3c %o Depositional Palynological 

excursion environment control 

Bonaparte ·8 Marine Good 

Carnarvon -4 Marine Spot 

Canning -8.5 Marginal Good 

marine 

Bowen -6 Fluvial Good 

Sydney -6 Fluvial Good 

Perth -5.5 Fluvial Spot 

Cooper -6 Fluvial Spot 

The ol3 C excursion in marine and nonmarine sediment 

alike is interpreted as reflecting a shift of up to -1 O%o in the 

ol3c of atmospheric co2 that masked expected variations 

in enclosed basins. The possible global extent of this ol3c 

excursion at the P ffr may provide a correlation tool 

enabling calibration of biostratigraphy schemes world wide 

at the Pffr across a ranye of sedimentary environments. The 

shape of the negative 5 3c excursion varies from sharp to 

gradual. A sharp ol3 C excursion indicates a lacuna, a 

condensed section, or a rapid change to more negative ol3c 

values in the Triassic. A gradual shift in ol3c indicates 

either a real, prolonged continuous ol3c excursion or a 

mixture of Permian and Triassic values. The ol3c excursion 

is explained by a flux of 12c enriched carbon as C02 from 

the reduced to the oxidised global reservoir. Possible 

sources of 12c enrichment suggested include, a) oxidation 

of organic facies and clathrate methane and b) volcanic 

eruption of mantle carbon as carbon dioxide. Depleted ol3c 

values until at least the Middle to Late Triassic may reflect 

the loss of 12c fixers having high preservation potential at 

the Pennianffriassic extinction. A large transfer of carbon 

from the reduced to the oxidised global reservoir and release 

of methane may have produced greenhouse conditions that 

contributed to the Permianffriassic extinction. 

225 

PB AND U ISOTOPE CHEMISTRY OF ACI'INOLITE-TALC 

ASSOCIATIONS FROM HIGH-PRESSURE METABASIC 

ROCKS. 

MOREE, Martijn, STAUDIGEL, H. and DAVIES, G.R., 

Dept. of Petrology & Isotope Geology, Free University, De 

Boelelaan 1085, 1081 HV, Amsterdam, The Netherlands. 

Chemical modification and open system geochemical 

behaviour in subduction zones plays an important role in 

producing the chemical components in the Earth's mantle and 

the sources of island arc magmas. Yet the fractionation of trace 

elements during metamorphic processes in this tectonic setting 

is very poorly understood, in particular for the Uffh/Pb 

systematics. We have begun to analyze the U and Pb 

abundances of metamorphic mineral phases from blueschist 

terranes in order to identify major mineral assemblages that 

control the distribution of U and Pb in accretion zones. 

Furthermore, we studied Pb isotopic compositions to determine 

the origin of any mobile Pb. 

Our first data from a actinolite-talc association within the 

melange of Catalina Island, California yielded: 

sample WR act 

208pb;204Pb 38.584 38.539 

207Pb;204Pb 15.625 15.618 

206Pb;204Pb 18.827 18.795 

11 1.99 1.97 

u (ppb) 70 60 

Pb (ppb) 2180 2090 

tic 

38.374 

15.631 

18.787 

0.49 

20 

2820 

A Pb mass balance shows that the rock consists of 90% 

actinolite and 10% talc, which is confirmed by thin section 

data. To balance the U, however, a U-rich third phase (e.g. 

zircon) is needed. 

Due to the very low 11-ratios, Pb isotopes do not display 

significant radiogenic ingrowth during the past 108 years, but 

they are reliable indicators for the fluid composition. Fluids 

released during low-grade metamorphism of the melange would 

be dominated by sediment derived Pb. The actinolite-talc 

associations which are commonly found as cap rock around 

metabasic rocks were apparently formed in equilibrium with 

fluids that are characterized by mantle-like Pb isotope 

compositions. We therefore conclude that the actinolite-talc 

association is an early metamorphic assemblage, reflecting fluid 

interaction during subduction rather than the emplacement of 

the melange. 
CARBON AND OXYGEN ISOTOPES OF TWO EARLY 

JURASSIC STRATIGRAPHIC SEQUENCES: 

POSSIBLE IMPLICATION FOR FORMATION OF 

RHYTHMIC MARL - LIMESTONE BEDDING 

(UMBRIA-MARCHE APPENNINES) 

MORETTIN!, E. Institut de Geologie et 

Palaeontologie, Universite de Lausanne BFSH-2, CH1015 Lausanne, Switzerland, and BAUMGARTNER, 

P., HUNZIKER, J.C. (same address). 

Differences in isotopic values of alternating marllimestone bedding provide new means for understanding 

the mechanisms responsible for the formation of such 

rhythmic sequences. Preliminary carbon and oxygen 

stable isotope analyses have been made on two basinal 

stratigraphic successions, the Fiuminatal and the Colle 

d'Orlando sections, from the Umbria-Marche 

Appennines. The results of this isotope study are also 

combined with results from micropalaeontological, 

sedimentological, palaeomagnetic2, and calciumcarbonate analyses to constrain the a13c curve through 

the early Jurassic (Pliensbachian-Early Aalenian). 

Carbonate isotope analyses from 50 samples of marl 

and limestone in the Fiuminata section (Marne e Calcari 

a Posidonia unit) show a fluctuation in a13c values 

related to the alternation of the two lithologies. The a13c 

of the marls is on average 0.3%o lighter than those of the 

limestones (the constantly observed difference between 

two alternating lithologies ranges between 0.2 and 0.9%o). 

It is not apparent in this preliminary data set whether 

oxygen values show any systematic fluctuations or 

variations in the sequence. 

Differences in values could be due solely to 

diagenesis if originally isotopically similar marl and 

limestone lithologies would have followed different 

diagenetic paths. However, it is likely that observed 

isotopic differences reflect original isotopic fluctuations 

in the precipitated carbonate. These differences may 

depend on local or regional variations such as basin to 

basin fractionation and fluctuating vital effects. Or, 

alternatively, the observed oscillations document shifts in 

the global carbon reservoirs, linked to climatic cycles of 

Milankovytch-type. 

1 Morettini, E. et al. (in prog.) 

2 Channel, J.T. et al. (in prog.) 

226 

GOLD-BEARING EPITHERMAL VEINS IN THE 

HOKUSATSU DISTRICI', SOUTHERN KYUSHU, JAPAN: 

CARBON AND OXYGEN ISOTOPIC EVIDENCE 

MORISHITA, Yuichi, Mineral Resources Department, 

Geological Survey of Japan, 1-1-3 Higashi, Tsukuba 305 

Japan 

The Hokusatsu gold district in southern Kyushu has 

produced more than 180 metric tons of gold. Production is 

continuing, with ore reserves of at least 200 tonnes of gold. 

The basement of the Hokusatsu gold district is the Cretaceous 

Shimanto Supergroup. Subaerial andesitic volcanism occurred 

in the late Miocene to early Pleistocene. Many of the 

epithermal gold deposits of the district, including the Hishikari 

deposit (1 Ma) and the Kushikino deposit (4 Ma), occur mainly 

in these andesitic volcanic rocks as fissure-filling veins 

consisting of gold- and silver-bearing quartz, calcite and 

adularia. Neutron activation analyses revealed that there are 

little gold concentrations in monomineralic vein samples of 

quartz or calcite in the area. 

The alSo distribution of vein quartz in the Kushikino mine 

area suggests that the hydrothermal fluid was supplied from 

below in the eastern part of the area, then flowed westward to 

the deposit, progressively mixing with a low-temperature 

meteoric ground water. Vein calcites from the Kushikino 

deposit show a a13c-a1So trend which forms the basis for 

interpreting the a13c and also values of vein calcites from 

other deposits in the Hokusatsu gold district. Most isotopic 

compositions of vein calcites in the district lie on the 

Kushikino a13c-a1So trend, suggesting that the hydrothermal 

fluids were isotopically similar to the Kushikino fluid. 

The a13c values of hydrothermal fluids in the Hokusatsu 

district during the mineralization stages were low (- -11%o), 

compared with that of average crustal carbon ( -7%o). The 

relatively low and uniform a13c value of the hydrothermal 

fluids in the district might have been regionally controlled by 

organic carbon in widely distributed sedimentary rocks of the 

Shimanto Supergroup basement. Although the quartz-calcite 

veins formed from ascenoing hydrothermal fluids, precipitation 

in some cases may have been caused by mixing with nearsurface waters having relativelyhigh a13c values. 

The ol3c values in the hydrothermal fluid responsible for 

the Hishikari deposit were also controlled by basement 

sedimentary rocks of the Shimanto Supergroup. Here it is 

inferred that the ascending ore-forming fluid mixed with nearsurface water at the unconformity between the basement and the 

overlying volcanic rocks. As a result, relatively high ol3c 

values are present in vein calcites which occur above the 

unconformity. 

A computer simulation analysis was carried out on the vein 

system in the Kushikino mine area in order to study the 

formation mechanism. The observed fracture (vein) system 

related to the Kushikino ore-forming hydrothermal system was 

proved to be formed by basement uplift as a consequence of 

intruding magma at depth. The localized sub-surface doming of 

magma is also thought to account for the positive Bouguer 

gravity anomaly over the area and which most likely provided 

the heat needed to drive the Kushikino hydrothermal system. 

According to the isotope data, incorporation of a large quantity 

of high olSo thermal waters is not plausible, however, the 

potential magma intrusion might have discharged some 

magmatic fluids in the early stage of the evolving hydrothermal 

system. 
Nd, Sr AND Pb ISOTOPIC CONSTRAINTS ON 

MECHANISMS OF CRUSTAL GROWTH IN THE 

YUKON-TANANA TERRANE IN YUKON TERRITORY, 

CANADA, AND EASTERN ALASKA, U.S.A. 

MORTENSEN. J. K., Dept. of Geological Sciences, 

University of British Columbia, Vancouver, B.C., 

Canada, V6T 1Z4 (jmortens@geology.ubc.ca) 

Yukon-Tanana Terrane (YTT) is a large, possibly 

composite, terrane whose constituent lithotectonic 

assemblages suggest a mainly continental affinity. Previous 

geological, geochemical, and U-Pb dating studies suggested 

that arc magmatism occurred across much of the YTT in 

latest Devonian to mid-Mississippian and in the central and 

southeastern parts of the terrane in Late Triassic to Early 

Jurassic time. These arc assemblages were built on a 

basement of Early Paleozoic and older passive margin 

sedimentary rocks. Broadly bimodal volcanic and plutonic 

rocks of mid-Permian age in western Yukon may be the 

product of either a more areally restricted intervening phase 

o.f arc magmatism or an anorogenic igneous event. Isotopic 

s1gnatures of each of the Paleozoic to Early Mesozoic 

igneous suites indicate a marked component of contamination 

from continental crust. Metaluminous plutonic and related 

volcanic rocks of the Devonian-Mississippian arc assemblage 

have ENdcn values of -10 to -13, Sri of 0.708-0.712, and 

feldspar Pb isotopic compositions that plot near the "shale 

curve" defined for evolution of Pb in strata of the 

miogeocline of western North America. Strongly 

peraluminous plutons, also of Devonian-Mississippian age, 

that were emplaced at deeper crustal levels within YTT have 

have ENd(f) values of -13 to -16, and highly radiogenic Sri 

(0.716-0.730) and feldspar Pb compositions. This plutonic 

suite may be entirely crustally derived and not directly 

subduction-related. Both of the Devonian-Mississippian 

igneous suites contain a trace to strong inherited component 

of older zircon (average ages 1. 7 - 3.1 Ga). Pb isotopic 

compositions for syngenetic sulfide deposits hosted by the 

Devonian-Mississippian volcanic arc rocks also fall on or 

near the "shale curve". The mid-Permian igneous suite has 

ENdcn values of -5 to -15 and Sri of 0. 706-0.711, and contains 

a relatively minor inherited zircon component. Compositions 

of Pb in feldspars and in sulfides from syngenetic base metal 

occurrences hosted by the mid-Permian volcanic rocks are 

slightly less radiogenic than the "shale curve". Available 

data do not permit distinction between a magmatic arc Y§. 

anorogenic origin for this suite. Early Mesozoic plutonic 

rocks in YTT are compositionally less evolved than the older 

igneous suites, have more juvenile Sr1 values (0. 704-0. 706), 

and typically contain only a minor component of inherited 

zircon; however ENd(f) values of -1 to -7 indicate significant 

interaction with older continental crust. 

Taken together, the isotopic and other data for the YTT 

indicate that the terrane is mainly a product of mid-Paleozoic 

to Early Mesozoic continental arc magmatism. The data are 

permissive of an origin for YTT as a distal portion of the 

western edge of North America. 

227 

THE CHEMICAL AND STABLE ISOTOPIC 

COMPOSITION OF MICROBIALLYPRECIPITATED SIDERITE. 

MORTIMER. R. J. G., *COLEMAN. M.L.. and 

RAE. J.E .• Postgraduate Research Institute for 

Sedimentology. Reading University. Reading. 

Berkshire RG6 2AB. U.K. 

*also B.P. Exploration. Sunbury-on-Thames. U.K. 

The composition of siderite is highly variable. The 

aim of the project is to show how well the chemical and 

isotopic composition of microbially-precipitated siderite 

reflects conditions of formation. 

Many early diagenetic carbonate concretions 

previously described (e.g. Curtis et a/1986) were 

probably precipitated by microbially-mediated reactions. 

The usc of stable isotopic data for such material is 

dependent on a knowledge of the effect of temperature 

upon fractionation processes. However. whilst 

equilibrium fractionation of oxygen isotopes between 

siderite and water has been measured down to 330C 

(Carothers et a/1988). extrapolation to lower 

temperatures may not be wholly valid. Furthermore. 

inorganic measurements may not be applicable tp 

microbiological systems. 

The recent isolation of a specific iron-reducing 

microorganism. Geobacter metallireducens. has 

provided a unique opportunity to study microbial 

siderite precipitation. Geobacter can be cultured 

anaerobically in the laboratory using acetate as an 

organic substrate and amorphous FeOOH as an electron 

acceptor. The acetate is completely oxidised to c~. 

with concurrent iron reduction and extracellular siderite 

precipitation. 

Rhombohedral siderite crystals up to 25 microns in 

size have been precipitated over a range of temperatures 

(18-400C). Chemical data produced from microprobe 

analysis to date show that the incorporation of trace 

elements such as Mn is dependent on the rate of growth 

rather than the temperature. This is compatible with 

results for inorganically precipitated ferroan calcites. 

Preliminary stable isotopic data show that 

microbially precipitated siderite. the closest analogue to 

ancient concretions. has a different fractionation pattern 

from the established inorganic data. Further work is 

necessary to fully understand the systematics involved 

and calibrate the system. 

Carothers. W.W .• Adami. L.H .• and Rosenbauer. R.J .• 

1988. Experimental Oxygen Isotope Fractionation 

Between Siderite-Water and Phosphoric Acid Liberated 

C~-Siderite. Geochim. Cosmochim. Acta. v.52. p. 

2445-2450. 

Curtis. C.D., Coleman. M.L., and Love. L.G .• 1986. 

Pore Water Evolution During Sediment Burial From 

Isotopic and Mineral Chemistry of Calcite. Dolomite 

and Siderite Concretions. Geochim. Cosmochim. 

Acta. v. 50. p. 2321-2334. 
PALEOPROTEROZOIC METAMORPHISM 

AT THE BASE OF THE NORTH AMERICAN 

CRATON REVEALED THROUGH U-PB 

ZIRCON DATING OF GRANUUTE-FACIES 

XENOUTHS 

MOSER. D.E. and HEAMAN, LM., Jack 

Satterly Geochronology Laboratory, Royal 

Ontario Museum, Toronto, Ont., Canada M5S 

2C6 

Granulite-facies xenoliths isolated from a 

Jurassic kimberlite pipe near . Kirkland Lake, 

Ontario represent fragments of the lower crust 

that underlies Archean rocks of the southern 

Canadian Shield. Sixteen U-Pb analyses of singlegrain and multi-grain zircon fractions from three 

xenoliths fall into two age groupings. The older 

group includes two metamorphic grains, from 

different xenoliths, that are concordant at 2580±2 

Ma. Metamorphic conditions in the lower crust at 

2580 Ma must therefore have been sufficient to 

cause growth of, or total Pb-loss from, zircon at 

this time. Most of the zircon grains define a 

second grouping of model ages that range 

between 2520 Ma and 2432 Ma, with the least 

discordant data clustering at 2495 ± 10 Ma. These 

are interpreted to result from a second, previously 

unrecognized, crustal-scale metamorphic event 

that is related to re-setting of K-Ar mica ages in 

the upper crust, the deposition of the lower 

Huronian supergroup, and the opening of the 

Matachewan ocean along the southern margin of 

the Superior Province. Sm-Nd dating of garnet 

and clinopyroxene in textural equilibrium with 

metamorphic zircon is in progress to examine 

whether these minerals preserve either Archean 

protolith ages or ages which indicate Proterozoic 

growth and/or re-setting. The ca. 2.5 Ga lowercrustal metamorphic episode identified in this 

study is coeval with high heat-flow and re-melting 

of continental crust in other Archean Provinces of 

North America as well as in six other Archean 

cratons. Together, this implies significant transfer 

of thermal energy from the mantle to Archean 

continental nuclei in the Paleoproterozoic. 

228 

K - AR AGES AMD GEOCHEMISTRY OF THE 

MARIANA TROUGH BASALTS 

Mu,Z. ,LIU,C. & HUANG,B. ,Dept. of Geol., 

Peking University, Beijing, 100871, P.R. China 

The nine Mariana Trough basalts were collected by 

the China - Germany cooperation ' SONEE' (Voyage 

59) in 7-8, 1990 from the places where 

located between 14°59. 16' -18°12. 76' N and 144°12. 

66' -144°50.17'£. The depth of sampling is from 3314 

m - 4498 m. K - Ar age -dating, geochemical analyses 

on major and trace element, REE and 

isotope composition have been done. The majority of the 

basalts are enriched in LIL elements and volatiles and 

depleted in HES elements like the rocks found 

throughout MORB, but there is a little difference from 

N- MORB both in trace elements and in isotope 

compositions. The results are consistent with published 

papers. The age ranging is from 2. 65 Ma to 10. 1 Ma 

belong to upper Miocene to Pliocene, that means the 

formed age of the Mariana back arc basin should be 

older than 10 Ma. A calculated half spreading rate of 

the Mariana back arc basin should be bigger than 

1 em I yr. In addition to oceanic tholeiite, both altered 

Peridotite and old oceanic crust are found in the 

Mariana Trough firstly. The former are enriched in 

Mg, Cr and Ni, but depleted in Si, AI, Ca and REE. 

The Ia ter has an age of 90 Ma around belong to upper 

Cretaceous period. 

Floyd, P. A., 1990,0ceanic Basalts, Blackie,Glasgow 

and London, P. 219 - 262. 

Hawkins, 1. W., Lonsdale, P. F., Macdougall, 1. D. 

and Volpe, A. M. , 1990, Petrology .of the axial 

ridge of the Mariana Trough backarc spreading 

center: Earth and Planetary Science Letters, V. 

100, p. 226-250. 

Volpe, A. M. , Macdougall, J.D. , Lugmair, G. W. 

and Hawkins, J. W., 1990, Fine- scale isotopic 

variation in Mariana Trough basalts: evidence 

for heterogeneity and a recycled component in 

backarc basin mantle: Earth and Planetary 

Science Letters, V. 100, P. 251-264. 
40AR/9 AR GEOCHRONOLOGY OF LATE CRETACEOUS 

VOLCANIC EVENTS IN THE CANADIAN ARCTIC 

ISlANDS: ARCTIC BIOTIC HETEROCHRONEITY 

REVISITED 

MUECKE Gunter K.. REYNOLDS, Peter H., Dept. of 

Earth Sciences, Dalhousie University, Halifax, NS, B3H 3J5, 

Canada, and MACRAE, R.A., NUNEZ-BETELU, L. Koldo, 

Dept. of Geology and Geophysics, University of Calgary, 

Calgary, AB, TIN 1N4, Canada. 

The Late Cretaceous depositional history of the northeastern 

Sverdrup Basin, Canadian Arctic islands, features episodic 

volcanism interspersed with an almost continuous succession 

of clastic sediments. Magmatism accompanied extensional 

phases of basin development in the Sverdrup Basin and the 

adjacent Canada Basin of the Arctic Ocean. Volcanic products 

include tholeiitic flood basalts, bimodal suites of alkali basalts 

associated with rhyolites and trachytes, as well as widespread 

ash horizons (bentonites). Sedimentary sequences include both 

marine and terrestrial organic-rich mudstones and shales that 

have yielded rich palynomorph assemblages for 

biostrati~aphic age determination. 40 Arf 'Ar plateau ages have been obtained for whole-rock 

basalt, rhyolite and trachyte samples, as well as single-grain 

and composite high-precision determinations on sanidines. 

Sample locations were chosen to provide unambiguous 

relations to palynologically studied sites. While argon spectra 

from the older volcanics (Late Albian) show some disturbances 

that may be attributed to alteration and later thermal events, all 

younger samples provide excellent concordant spectra. 

Comparison of the absolute radiometric dates with 

biostratigraphic determinations yields discrepancies of between 

2-5 Ma. For both the Cenomanian - Turonian and the 

Campanian - Maastrichtian boundaries radiometric dates 

appear to be older than biostratigraphic determinations. 

Previous investigators (Hickey et al., 1983) have noted age 

disparities, based on questionable paleomagnetic evidence, 

between Arctic and lower latitude biotas in this time span, and 

attribute it to biotic heterochroneity. We re-examine this 

hypothesis in the light of our new paleontological and 

radiometric data. Paleo floral provincialism of some of the Late 

Cretaceous age-diagnostic pollen genera may explain their 

earlier appearance at high latitudes. Other possible 

explanations for the discrepancy include an error in the 

currently accepted biostratigraphy, errors in the current time 

scale, or some combination of these. 

Hickey, LJ., West, R.M., Dawson, M.R., and Cboi, O.K., 

1983, Arctic terrestrial biota: Paleomagnetic evidence of age 

disparity with mid-northern latitudes during the Late 

Cretaceous and Early Tertiary: Science, v.221, p.1153-1156. 

229 

CALffiRATION OF TIME _SCALE AND SEDIMENTARY 

CYCLES: HIGH RESOLUTION SINGLE-CRYSTAL U-PB 

AGE DETERMINATIONS ON MID-TRIASSIC TUFFS OF 

THE SOUTHERN ALPS 

MUNDIL, R,l), BRACK, P.2), MEIER, M. 1), and OBERLI, 

F.n, 1) Isotope Geochemistry, 2) Department for Earth 

Sciences, ETH Zurich, CH-8092 Zurich, Switzerland 

Carbonate platforms of the Dolomites (Southern Alps) 

display conspicuously regular cyclic stratal patterns ("Latemar 

Limestone"). The platform successions can be correlated in 

detail with pelagic sediments ("Buchenstein Beds") on the basis 

of large scale geometries and biostratigraphy (Brack & Rieber, 

1993). The occurrence of multiple phenocryst-rich volcaniclastic layers in the pelagic successions offers a unique 

opportunity for a) time scale calibration and b) quantitative 

evaluation of cyclicities in the age-equivalent platform 

carbonates. High-frequency sea level fluctuations with 

Milankovitch characteristics have been suggested as primary 

control for these cycles (Goldhammer et al., 1987). This would 

imply a time span of 12 Ma for the lower part of the Ladinian 

alone. However, this is in conflict with current time scales, 

which suggest a duration of 4-7 Ma for the entire Ladinian stage. 

For the present study, samples of volcaniclastic layers were 

collected from biostratigraphically well documented and 

correlated South Alpine sections. High resolution single zircon 

analytical techniques (Oberli et al., 1989) have been applied so 

far on zircon grains from a single layer in the Buchenstein Beds 

at Bagolino (Brescian Alps). Biostratigraphically, this horizon 

belongs to the Ladinian Protrachyceras gredleri-zone. Selected 

zircons were carefully examined using transmitted light 

microscopy in order to exclude potential core bearing crystals. 

Abrasion techniques were applied to minimize discordance by 

secondary lead loss. Four well abraded zircon grains reproduced 

206Pbf238U ages within a narrow time interval of 238.6-239.2 Ma 

yielding a mean age of 238.8 ± 0.4 Ma (95% confidence level 

ext.). Slightly younger apparent U-Pb-ages (223-235 Ma) were 

obtained on non- to moderately abraded zircons and demonstrate, that age values can be strongly affected by secondary lead 

loss. Moreover, presence of an inherited core component in one 

zircon grain is indicated by its older apparent age (246 Ma). 

The age of 238.8 Ma for this Ladinian level is in contrast to 

most time scales. However, this age is compatible with an age of 

239.5 Ma for the Anisian/Ladinian boundary suggested by 

Harland et al. (1989). Interestingly, the database for the latter 

time scale differs only slightly from data used for previous time 

scales. Furthermore, the result is in conflict with K-Ar- and ArAr-ages of Hellmann & Lippolt (1981), who suggested a value 

of 232 Ma for the Anisian/Ladinian boundary based on data 

from slightly older volcaniclastic layers in the Southern Alps. 

Considering the platform carbonate cycles and their biostratigraphic calibration, the present result may imply a periodicity that is significantly shorter than suggested previously. 

Brack, P., and Rieber, H., 1993, Eclogae geol. Helv., v. 86, p. 

415-527. 

Goldhammer, R.K., Dunn, P.A., and Hardie, L.A., 1987, Am. J. 

Sci., v. 287, p. 853-892. 

Hellmann, K.N., and Lippolt, H.J., 1981, J. Geophys., v. 50, p. 

73-88. 

Oberli, F., Fischer, H., and Meier, M., 1989, Abstr. 28th Int. 

Geol. Congress, Washington D.C., v. 2, p. 536-537. 
HF-ZR INTERDIFFUSIVITY IN ZIRCON 

NAGASAWA,Hiroshi, Dept. Chem. Gakushuin 

University, Toshima-ku, Tokyo, -Japan. 

Hf-Zr interdiffusivity in zircon has 

been determined on hafnon-zircon doublelayered crystals using scattered electron 

image of scanning electron microscope. 

The double-layered crystals were prepared by epitaxial overgrowth of zircon 

on synthetic hafnon(HfSi04) seed crystal. 

After diffusion annealing diffusion profiles were determined by scattered electron image(SEI) contrast by calibrating 

with synthetic crystals of hafnon-zircon 

solid solution with known compositions. 

Diffusion profiles are shown in Fig. 1 

and the calculated diffusion coefficients 

are tabulated in Table 1. In a previous 

work only an upper limit value was obtained by the energy dispersive X-ray analysis which has a spatial resolution of 

a few pm(Suzuki et al, 1992). SEI with 

much better spatial resolution offered 

extremely low D values for zircon. 

The observed low D values for Hf-Zr 

indicate that the isotopes of U and Th 

with the same ionic charge and similar 

ionic sizes have similarly low D values 

in zircon. This implies the daughter isotopes of U and Th would also have low D 

values in zircon, although the lower 

ionic charges may result somewhat higher 

D values. 

100 IAI 

1600°C, 12day 

-2 -\ 

DISTANCE pm 

Figure 1. Diffusion profiles. 

(A) "'- c (a or b), (B) #C, (C) "'- c, before 

annealing. 

Table 1. Diffusion coefficients. 

Tem}2eratur~(°C) Axis u. cm ·s~c- 1 

1600 J.C 2 X 10- 15 

bC .-2x 10-16 

oi-C =:,lx 10-]. 5 (1) 

1550 .&-C ...,2x 10-16 

(1) Suzuki et al(l992) G.J. ~' 99. 

230 

PRODUCTION OF ACIDIC ROCKS BY PURE 

DIFFERENTIATION OF A DEPLETED MANTLE SOURCE: A 

COMBINED U-PB-ZIRCON, PB-PB-ND ISOTOPE STUDY ON 

THE MASIRAH OPIDOLITE I OMAN 

NAGLER. TH F .. FREI, R., Min. -Pet. Inst., Lab. of 

Isotope Geology, Bern, 3012, Switzerland 

Field- and sedimentary evidence within the Masirah 

ophiolite complex I Oman establish at least three different 

magmatic events: 

1) generation of oceanic crust > 150 Ma 

2) production of alkali volcanites and 

3) emplacement of acidic magmas, including real 

granites. 

£NdU50Ma] values of a variety of different rock types 

(i.e. troctolite, different gabbros, anorthosite, dolerite, 

pillow basalt, pegmatitic gabbro and granite) range from 

+7 to +9.7. The respective Nd-model age calculations 

indicate derivation from upper mantle somewhere between 

160 Ma and 300 Ma. The poor resolution is inherent in the 

system, as young model ages are always strongly 

dependent on the used depleted mantle model and error 

magnification is large. 

U-Pb zircon data of a trachyte from the alkali volcanic 

suite give a magmatic age of 123 ±2 Ma, whereas two 

granites from the third stage yield 112 ±10 Ma and 

113 ±11 Ma. Correspondingly corrected Pb-Pb data point to 

a uniform MORB-type source. 

The tectonic setting of the acidic rocks and their initial 

Pb and Nd isotopic composition as well as the lack of 

correlation between Nd contents and isotopic composition 

exclude a significant admixture of continental material, 

although the sensitivity of the isotopic data towards such a 

contamination is large due to the extremely low Pb, U and 

REE concentrations (Pb: 0.008 ppm to 2.55; U: 0.002 ppm 

to 1.74; Nd: 0.16 ppm to 65 ppm.). 

The absence of continental contribution is in sharp 

contrast to such contaminations in calc-alkaline plutonic 

suites within the Semail ophiolite (mainland of Oman; 

Briqueu et al., 1991). 

At the present state of the investigation, the evolution of 

the ophiolite is assumed to have taken place as follows: 

Partial melting of upper mantle material and generation 

oceanic crust occurred > 150 Ma. Different parts of this 

crust have been remelted at about 123 (probably due to a 

hot spot activity with possible addition of new oceanic 

crust) and/or at c. 112 Ma without addition of material 

from the continental crust. 

Moreover, the present study reveals the likelihood of 

granite genesis through pure remelting and subsequent 

differentiation of pre-existing oceanic crust. 

Briqueu, L., Mevel, C. & Boudier, F., 1991, Sr, Nd and Pb 

Isotopic Constraints in the Genesis of Calc-Alkaline 

Plutonic Suite in Oman Ophiolite Related to the 

Obduction Process, in: Peters, T. et al. (EDS). Ophiolite 

Genesis and Evolution of the Oceanic Lithosphere, 

Ministry of Petroleum and Minerals, Sultanate of Oman, 

p. 517-542 
40 Art39 Ar GEOCHRONOLOGY OF WHOLE-ROCK 

BASALTS. 

NAUERT, J.L., and GANS, P.B., Dept. of Geology, 

UCSB, Santa Barbara, CA, 93117 

Dating of mafic lavas is extremely important for a wide 

range of geologic problems (time scale, magnetic reversals, 

stratigraphic correlations, tectonics, etc.). The 40 Art39 Ar 

method has been increasingly applied to the dating of mafic 

lavas, but results have been mixed. K-contents of mafic lavas 

range from <0.1% to >2.0%; 40Aratm ranges over several orders of magnitude, generally around w-14 mollmg; and age 

spectra on both plagioclase separates and whole rock chips 

range from simple plateaus to hump-shaped, U-Shaped, or Sshaped spectra. During step heating of whole rocks, maximum 

radiogenic yields are c9mmonly obtained between 500° and 

1000°, with a dip between 700° and 800°. We are attempting 

to better understand this range of behavior and the systematics 

of Ar in whole-rock basalts by looking at the effects of initial 

composition, grain size, and degree of alteration, as well as the 

effects of various sample preparation and analytical procedures. 

Experiments were carried out on a variety of whole-rock 

basalts and basaltic andesites, ranging from 3 year old glassy 

sea-floor basalts to Miocene holocrystalline basalts, but focussing on holocrystalline groundmass concentrates. Not surprisingly, highest radiogenic yields are correlated with the 

highest K-contents and K/Ca ratios, but also depend critically 

on groundmass grain size, with coarse grained, holocrystalline 

samples from the interior parts of flows yielding substantially 

lower %40 Aratm. 

The size fraction of the whole rock separates may have a 

significant effect on the %40Aratm (e.g., a glassy basalt 

crushed to 90jl showed a fivefold increase in 40Aratm/mg over 

a split crushed to -600j..L, though the amount of 40 Ar*/mg 

remained unchanged). 

HCl etching to remove calcite and HF etching to remove 

gl~s and alteration products may substantially improve K/Ca 

ratios and remove unwanted phases, but this benifit appears to 

be ou~eighed by a dramatic increase in %40 Aratm, thereby 

lowenng overall and maximum radiogenic yields (e.g., HCl 

etchi~g. resulted in a sevenfold increase in 40 Aratm/mg). 

Bmlmg to remove loosely-held atmospheric argon, followed 

by air drying or vacuum drying yielded no significant reduction 

of atmospheric argon. 

Separation of the most magnetic one-third to one-half of 

whole .rock splits tends to decrease the amount of 40 Aratm by 

removmg low-K phenocrysts, calcite, and alteration products, 

and increase 40 Ar* by concentrating the K-rich groundmass, 

with significant improvement in analytical results. 

I~ addition to the sample preparation, correctly designed 

heating schedules can have a significant effect on the release of 

Ar. Cyclic heating schedules can greatly increase the radiogenic yield of the higher temperature (>800°) steps and help 

separate the atmospheric from the radiogenic Ar components. 

Although our experiments to date are limited, we have found 

that the best analytical results for whole rock basalts are obtained from fresh, coarse grained holocrystalline samples, 

coarsely crushed, magnetically separated to concentrate the Krich groundmass, and degassed using a cyclic heating schedule. 

Acid treatments generally do more harm than good. For example, when we follow these procedures on Miocene whole rock 

basalts from southern Nevada, we commonly get >95% radiogenic yields over much of the spectrum, and estimated uncertainties in plateau and isochron ages of <0.2%. 

231 

SHRIMP U - Pb ZIRCON DATING OF ARCHAEAN 

GREENSTONE SEQUENCES FROM THE EASTERN 

GOLDAELDS, WESTERN AUSTRALIA 

NELSON. D.R.. Geological Survey of Western Australia, 

East Perth, W A, 6004, and Curtin University of 

Technology, GPO Box Ul987, Perth, WA, 6000, Austrnlia, 

SWAGER C.P., WITT W.K., GRIFFIN T.J., WYCHE S. 

and SMITHIES H. Geological Survey of Western 

Australia, East Perth, W A, 6004, Austrnlia 

Detailed mapping by the GSW A in the Eastern Goldfields 

Province of the Archaean Y ilgarn Crnton has delineated a 

series of fault-bounded tectonostrntigrnphic terranes. The 

westernmost Kalgoorlie Terrane consists of ultrnmafic and 

mafic flows and felsic volcaniclastics with relatively simple 

and coherent regional strntigrnphy. The eastern terranes have 

diverse and complex strntigrnphic sequences comprised. 

predominantly of laterally discontinuous basaltic and 

ultrnmafic units and proximal volcaniclastic units. Highprecision U - Pb zircon dating using the recentlycommissioned Perth SHRIMP indicates that felsic 

volcaniclastic rocks within the Kalgoorlie and eastern terranes 

were deposited contempornneously at 2683 ±5 Ma Similar 

emplacement ages of 2690 - 2680 Ma have been reported for 

early (pre-02) grnnitoids (e.g. Hill et al. 1992). Geochemical 

and isotopic data indicate that the early grnnitoids and felsic 

volcanics were derived from similar sources and may have 

been cogenetic. Multiply-deformed monzogranite- to 

grnnodiorite gneisses are exposed at localities near the western, 

southern and eastern margins of the Eastern Goldfields 

Province greenstones. These gneisses typically preserve upper 

amphibolite facies mineral assemblages, are characterised by 

low zircon abundances, complex zircon age populations and 

contain zircons as old as c. 3300 Ma. Frngments of similar 

gneisses probably formed part of the basement to the 

greenstones and provided the source rocks from which the 

grnnites and felsic volcanics were derived. 

The following evolutionary model of the formation of the 

Eastern Goldfields grnnite-greenstone terranes is consistent 

with the available field and geochronological constrnints. At c. 

2710- 2690 Ma, asymmetric rifting, mainly by N - S-trending 

normal faulting, of pre-existing granitic and gneissic crust 

resulted in the development of a series of adjacent shallow 

basins, into which predominantly basaltic and ultrnmafic 

volcanics were deposited. At c. 2685 Ma, felsic volcaniclastic 

rocks were erupted from numerous volcanic centres and early 

{pre-D2) grnnitoids were emplaced mainly as thick sheets into 

the base of the greenstone sequences. This igneous activity 

may have been a consequence of heating of the base of the 

crust during crustal thinning. This was followed at c. 2675 -

2665 Ma by regional (D2) compression involving reactivation 

of early structures. Additional episodes of grnnitoid 

emplacement have been identified at c. 2665 - 2660 Ma and c. 

2630-2600 Ma (Hill et al. 1992). 

Tectonic models advocating the involvement of mantle 

plumes (e.g. Hill et al. 1992) cannot account for the 

compressional regime responsible for D2 structures and the 

volcanic rocks lack many of the diagnostic geochemical 

features found in the volcanic rocks of modem subduction 

zones. An active continental margin back-arc basin setting 

offers the closest modem analogous tectonic setting. 

Hill R.I., Chappell B.W., and Campbelli.H., 1992, Late 

Archaean grnnites of the southeast Yilgarn Block, Western 

Austrnlia: age, geochemistry, and origin: Trans. R. Soc. 

Edinburgh: Earth Sciences, v. 83, p. 211-226. 
AGE AND ORIGIN OF THE LATE ARCHAEAN DARLING 

RANGE BATHOLITH, SOUTH-WEST YILGARN 

CRATON, WESTERN AUSTRALIA 

NEMCHIN. A.A. and PIDGEON, R.T., School of 

Applied Geology, Curtin University of Technology, Perth, 

6001, Western Australia. 

A major problem of Archaean geology is the origin and 

mode of emplacement of enormous masses of granitoids. Our 

aim in this study was to investigate the genesis of a typical 

Late Archaean granite batholith, the Darling Range Batholith, 

located in the south western part of the Yilgarn Craton of 

Western Australia. This batholith, which occurs over an area 

of more than 100 x 300 hundred kilometres, varies 

transitionally from porphyritic to even medium and coarse 

grained varieties with late stage leucocratic and pegmatite 

dykes. Granite samples from the central area of the batholith 

show relatively small chemical variations. Trends observed 

were decreasing Ti02, Al203, MgO, CaO, Sr, Zr and 

increasing Rb, La/Yb, Eu/Eu* with increasing silica. Small 

variations in the chemical composition can be explained t>y 

crystallization of feldspars ant to a lesser extend some accessory 

minerals. 

Preliminary dating of the granites, using conventional UPb zircon and titanite analyses, gives the age of 2625±15 Ma. 

Epsilon Nd for 2.6 Ga calculated for 20 granite samples varies 

from 0 to -3.5, with the average -2. The average Depleted 

Mantle model age is 3.12 Ga, with a spread from 3.00 to 3.25 

Ga. This spread as well as the chemical variations limit the 

possibility of mixing of chemically contrasting source 

components with different ages and we interpret the age of 3.12 

Ga as close to the formation age of the granite source. 

The low initial 87srt86sr (0.7009±35, 2 sigma of the 

mean) suggests low Rb/Sr ratios in the source rocks. This is 

estimated to be less than 0.2, based on ages of 2.6 Ga and 3.12 

Ga for the granite and its source, and the value of mantle 

87srt86sr at 3.12Ga. This level of Rb/Sr ratio is found in 

tonalitic rocks, whereas most sedimentary rocks are generally 

much higher. One present evidence the large mass of post 

tectonic granites of the South-West Yilgam Craton appears to 

have formed at 2.6Ga by the melting of relatively 

homogeneous 3.12 Ga old tonalitic crust. 

232 

WHOLE-ROCK AND MINERAL U-Th-Pb SYSTEMATICS 

IN LOWER CRUSTAL XENOLITHS FROM THE BALTIC, 

ANABAR AND CANADIAN SHIELDS- EARLY ARCHEAN 

LITHOSPHERIC DIFFERENTIATION AND EVOLUTION. 

NFYMARK I A lnst. of Precamb. Geol. & Geochron., St. 

Petersburg, Russia, PACES, j.B. and ZARTMAN, R.E., 

U.S. Geol. Surv., Box 25046, DFC, Denver, CO, USA 80225 

Access to the lowermost continental crust is limited to 

crustal xenoliths in basalts and kimberlites. Xenolith suites 

from Archean cratons are noticably under-represented in 

published studies. In this paper, we summarize new data 

for the Archean lower crust represented by xenoliths from 

Paleozoic and Mesozoic kimberlites erupted through the 

Baltic (southern Kola Peninsula), Anabar (eastern Siberian), 

and Canadian (Superior province in northern Michigan) 

shields. Mafic granulites typically contain subequal 

proportions of Pig, Cpx, and Gt, but modes grade into both 

amphibolites and eclogites. Most granulites are mafic, 

and although some have major element._contents similar to 

basaltic liquids, others clearly do not have magmatic 

compositions. Occasional Plg-Gt granulites have intermediate compositions. Peak temperatures and pressures are 

estimated from 700 to 1 050. c and 8 -15 kbar. 

Whole-rock and mineral separates were analyzed for 

Pb isotopes and U, Th, and Pb concentrations. Most 

whole rocks have low U contents (0.05-0.5 ppm) and K/U 

ratios from 1.6 to 23.6x1 04. Despite the U depletion, 

observed Th/U ratios are not significantly elevated. Typically, xenoliths have low measured J.L (238Uf204Pb) values 

(0.5-8.5); however, J.L values as high as 20 are observed. 

The total range of whole rock 206Pbf204Pb is 14.7 to 23.3. 

All but one of the whole-rock Pb compositions from 

Michigan granulites plot above the Stacey-Kramers (S-K) 

206Pbf204Pb-207Pbf204Pb growth curve, in contrast to Baltic 

and Siberian xenoliths, which plot below the S-K curve. 

lead isotopic data from hand-picked, acid-leached 

mineral separates do not form well-defined linear arrays 

on Pb/Pb plots suggesting extended or complex cooling 

histories and variable blocking temperatures. Garnetdominated Pb/Pb mineral ages group around 2.6-2.7, 1.8-

2.1 and 0.9-1.5 Ga. Rutile U-Pb systematics date the host 

kimberlite ages. Some Siberian and Baltic garnets yield 

concordant U-Pb ages in the range of 1.4-2.6 Ga. 

Michigan garnets are discordant and reflect a thermal 

disturbance at about 1.1 Ga, also recorded by U-Pb ages 

for rims of metamorphic zircons. lead compositions of 

many Pig separates from all three localities plot above the 

S-K growth curve -- suggesting an early history in a high-J.L 

environment (J.L = 16 from 3.7 to 2.7 Ga in one case). 

These data require that protoliths originated from ancient 

crustal differentiation events and imply substantial time 

between protolith formation and the granulite-forming 

event (granite melt extraction) that resulted in U depletion. 

In contrast, some mafic granulites display mantle-like Pb 

isotopic characteristics and may represent younger 

additions of basaltic magma into the lower crust. 

Collectively, Pb isotopes and supporting data imply that 

both basalt underplating and intra-crustal differentiation of 

older crust are important processes affecting the formation 

and evolution of the lower continental crust. 
MASS SPECfROSCOPY AND THE EARLY 

DAYS OF ISOTOPE GEOLOGY 

NIER, A. 0., School of Physics and Astronomy, 

University of Minnesota, Minneapolis, MN 55455, 

USA, 

The application of mass spectroscopy to problems 

of geological interest owes its existence to the earlier 

application to basic problems in atomic and nuclear 

physics. and the simultaneous development of 

improved electronic, vacuum, and instrument 

techniques. 

Prior to 1936 the isotopic composition of lead and 

uranium were uncertain. Determinations of 

geological age by the uranium-lead method relied on 

chemically determined atomic weights to ascertain 

the common lead content of the lead found in 

uranium minerals. The precise determination of lead 

isotope abundances [1] showed that common lead 

varied widely in spite of a near constant atomic 

weight, and led to the conclusion that the age of the 

solar system was approximately 4.5 by.[2]. 

Determination of the 235UJ238U abundance ratio [3] 

made possible the development of the 207pbJ206Pb 

method for age determinations [4]. The discovery of 

401( and the subsequent identification of 40 Ar in 

potassium minerals led to the demonstration that the 

K/ Ar method was feasible for age determinations [5]. 

Whereas early investigators had to construct their 

own instruments, following WW II commercial in~truments became available, opening the field to 

many investigators and new applications, and mass 

specrometers became routine tools for geological and 

planetary research. 

[1] Nier, A.O. (1938) J. Am. Chern. Soc. 60, 1571. 

[2] Patterson, C. (1956) GCA 10, 230. [3] Nier, 

A.O. (1939) Phys. Rev. 55, 150. [4] Nier, A.O. 

(1939) Phys. Rev. 55, 153. [5] Aldrich, L.T. and 

Nier, A.O. Phys. Rev. 7 4, 876. 

233 

EVIDENCE FROM Nd ISOTOPIC SYSTEMATIC~ IN 

PERIDOTITES FOR THE TIMING OF MANTLE 

LITHOSPHERE FORMATION BENEATH NORTHERN 

MEXICO 

Njmz GJ , Isotope Geochemistry Section, Lawrence 

Livermore Laboratory, Livermore, CA, 94550 and 

Cameron, K.L., Department of Earth Sciences, 

University of Califmria, Santa Cruz, CA 95064 

Peridotite and orthopyroxenite xenoliths of the Cr-Diopside 

Group are found in -2 Ma basanitic cinder at the La Olivina 

locality in northernmost Mexico. The samples fmn continuous 

arrays on chemical and isotopic co-variation diagrams, have 

similar coarse-granular textures, and have similar calculated 

mantle equilibi'ation temperatures ( -950-10500C); all of which 

suggest that they existed in close proximity while within the 

upper mantle. Sr and Nd isotopic compositions of acid-washed 

diopsides (£Nd = +1.9 to +13.3; 87srJ86sr = 0.'10220 to 

0.70328) indicate long-term Rb and Sm depletion similar to 

asthenospheric (MORB) mantle.· However, Pb isotopic 

compositions are generally higher in 207pb relative to 206pb 

than is typical of MORB (206pbf204Pb = -18.53-19.40; 

207Pb/204Pb = -15.58-15.61). The peridotites contain veins 

of pyroxenite that appear to have resulted from mid-Cenozoic 

magmatic activity in the La Olivina region. Chemical and 

isotopic evidence suggests that this mid-Cenozoic magmatism 

also produced chemical alteration in several of the peridotites. 

It is therefore believed that the peridotites were in-place and 

attached to the continent prior to f)le Cenozoic, and certainly 

before regional late-Cenozoic Basin-and-Range extension. 

This is in distinction to recent studies that suggest oo the basis 

of basalt isotopic systematics that the region is underlain by 

recently . emplaced asthenospheric mantle. Modeled 

chronological data from the peridotites can be related to known 

events in the geologic record of southwestern North America. 

The peridotite suite as a whole is extremely depleted in 

both clinopyroxene and incompatible trace element 

concentrations (Sm/Nd = 0.5 to 0.8; [Nd] = 1.19-2.69 mg/g). 

Isotopic systematics require more than one depletion episode, 

but can be modeled as the result of just two melt extraction 

episodes. Calculations indicate that melt extraction during the 

second episode may have been about 10-15% of the existing 

mass. Chronologie modeling suggests that the second-stage 

melting· event occurred no longer than -280 Ma ago. 

Enrichment chronologies suggest the melting occurred prior to 

the mid-Cenozoic magmatism in the region, -30 Ma. Nd 

isotopic similarities ~een the peridotites and late-Paleozoic 

arc-related basalts reported to the southeast of La Olivina 

suggest that the large-scale melting event may have produced 

these basalts. This arc may have been related to the Ouachita 

collisional tectonism in the region. The calculated magnitude 

of the melting implies that the La Olivina mantle was 

asthenospheric at the time. Increased buoyancy of the newly 

depleted asthenosphere coupled with a delamination of 

previously-existing lithosphere, suggested by oth~r studies, 

may have led to the accretion of the La Olivina mantle to the 

crust by the end of the Ouachita orogeny. The La Olivina 

mantle at that time became lithospheric in character and the 

peridotites have apparently undergone little or no melting 

since. 
COSMOGENIC PRODUCTION OF lOse AND 26Al ON 

THE SURFACE OF THE EARTH AND UNDERGROUND 

NISHIIZUM1 K , Space Sciences Lab, Univ. of Calif., 

Berkeley, CA 94720, U.S.A., Finkel, R.C., Caffee, 

M.W., Southon, J.R., Lawrence Livermore National Lab 

Livermore, CA 94551, U.S.A., Kohl, C.P., Arnold, J.R.: 

Dept. of Chemistry, Univ. of Calif., San Diego, La Jolla, 

CA 92093, U.S.A., Olinger, C.T., Poths, J., Los Alamos 

National Lab, Los Alamos, NM 87545, U.S.A., Klein, J., 

Dept. of Physics, Univ. of Penn., Philadelphia, PA 19104, 

U.S.A. 

To ~pply in-situ produced cosmogenic nuclides to the study 

of the htstory of earth surface processes, production rates at the 

surface of the earth and the attenuation length of the production 

underground must be known. We have extended previous 

studies of production rates by using both geological and 

laboratory methods. 

(1) Depth profiles of 10se (tl/2=1.5 My) and 26Al (0.71 

My) along with 21Ne in quartz extracted from the vertical face 

of a quarry in Bandelier Tuff near Los Alamos show a tight fit 

to a decreasing exponential with depth down to a depth of 500 

g/cm2. The 1/e attenuation lengths are 159 g/cm'l for lOse 

and 166 g/cm2 for 26AI. lOse and 26Al produced by cosmic 

ray muon capture are observed below 500 g/cm2. The muon 

capture contribution are -80% of the total production of lOse 

and 26 AI in quartz at 965 g/cm2. The muon induced 

production ratio of 26Al I lOse is 5±1 in the sample. Surface 

erosion can yield samples with a high contribution of muon 

capture production. 

~2) Until now geological studies involving cosmogenic 

nuclides have assumed that production occurs primarily by fast 

neutron reactions. The existence of muon-induced reactions 

was r~lized •. b~t considered to be unimportant. Preliminary 

expenments mdtcate that lOse and 26Al production by muon 

are different than expected. We collected quartz from deep 

mines (Kamioka and Homestake) at depths of up to several 

~~ndre~ meters below the surface. The production of lOse and 

AI wtll be compared to the flux of high energy muons at the 

depth. For 1 0Be and 26 AI studies, if the surface erosion rate is 

lower than 1m/My oo-4 cm/y), neutron production dominates 

~ut if ~he erosion rate is higher than 10 m/My, muo~ mteractiOns become important. This problem has not been 

addressed in our earlier work. We will discuss the importance 

of the muon contribution for calculation of erosion rate. 

(3) Over 500 kg of H20 and -10 kg of Si targets were 

exposed to cosmic rays at high altitude (3246 m for H2o, 3246 

m and 4298 m for Si) and sea level for a number of years. Be 

was extracted from H20 and AI was extracted from Si. The 

7Be (tlt2=53 day) production rate is lower than previously 

reported values but the production rate ratio at 3246 m to sea 

level is in good agreement with the theoretical model. The 

production rate ratio of lOse I 7Be from Oxygen is 0.53±0.03 

(preliminary result). The concentration of 7se, lOse, and 26Al 

will be compared to the production rates obtained from 

measurements on geological samples and from theoretical 

calculation. 

234 

U-Pb GEOCHRONOLOGY OF THE CORDILLERA REAL 

ANDEL ORO PROVINCES, ECUADOR. 

NOBLE S R., NERC Isotope Geosciences Laboratory, 

Keyworth, Nottingham, NG12 500, UK., ASPDEN, J., 

JEMIELITA, R., and LITHERLAND, M., British 

Geological Survey, Keyworth, Nottingham, NG12 500, 

UK. 

The Cordillera Real (CR) and El Oro Provinces (EP) 

comprise part of the metamorphic "basement" of the 

northern Andes. The CR and EP consist mainly of 

metasedimentary rocks and migmatites cut by ca. 200 Ma Stype and later 1-type dioritic and granitic batholiths. Major 

structures in the EP are currently oriented orthogonal to 

structures in the CR. Prior to this study key unresolved 

questions were: (1) the crystallization age of the S- and 1-

type granitoids in the CR and EP, given there is widespread 

and variable resetting of K-Ar and Rb-'Sr systems; (2) the 

age of the Piedras metagabbros of the EP (published K-Ar 

ages range from 74 to 743 Ma); and (3) were the EP and CR 

originally part of the same terrane? 

Concordant zircon and monazite ages of 227.3 ± 2.2 Ma 

and 227.5 ± 1 Ma for the Tres Lagunas batholith (CR) and 

the Marcabeli pluton (EP), respectively, show that S-type 

granitoids in the CR and in the EP are contemporaneous. 

Multi-grain fractions of strongly abraded rounded Tres 

Lagunas > 10 % discordant zircons yield 454, 627 and 773 

Ma 207Pbj206Pb ages. Strongly abraded individual Marcabeli 

zircon cores are 0-10% discordant, giving 546 ± 3.3 Ma, 

2220 ± 2.1 Ma, and 2876 ± 1.8 Ma 207Pbf206Pb ages. The 

ages of inherited zircons together with chemical, Sr-, and 

Nd- isotope evidence indicate significant contribution of 

older crust in 227 Ma granitoid genesis. 

A 221 +181-16 Ma concordant zircon age is obtained from 

the Piedras intrusion. Hence, mafic intrusive activity is 

much earlier than anticipated, and accompanied granite 

emplacement. Latest 1-type intrusive activity in the CR is 

constrained by a 142.7 ± 1 Ma concordant zircon age for the 

Azafran batholith, revised from a 120 Ma Rb-Sr age. 

The similarity of field and geochemical characteristics of 

EP and CR lithologies suggested they were parts of the same 

terrane before ca. 140 Ma. These data together with new UPb isotope data support the following conclusions. (1) Stype intrusions and host metasediments in the EP and CR 

were likely contiguous at 227 Ma and that the two regions 

were subsequently tectonically separated. (2) Components of 

the crustal protoliths of the S-type granitoids may be derived 

from the Amazonian and Guyana Shields together with 

younger more locally-derived material. This is supported by 

Nd ToM ages of 1400-1600 Ma for metasediments from EP 

and CR suggesting sediment sources were mainly late 

Precambrian crust with some older components. Similar 

ToM ages published for suspect terranes in Mexico suggests 

a connection with the EP and CR prior to the break-up of 

Gondwana. (3) K-Ar ages are often profoundly disturbed in 

the EP and CR, e.g. Piedras. Rb-Sr isochrons are also reset, 

typically being 20 Ma younger than U-Pb ages for the same 

intrusions. 
EXTENSIONAL TECTONIC EVOLUTION OF 

EASTERN TANZANIA FROM APATITE FISSION 

TRACK THERMOCHRONOLOGY 

NOBLE W P, Foster, D.A. & Gleadow, A.J.W. 

Victorian Institute of Earth and Planetary Sciences, 

School of Earth Sciences, LaTrobe University, 

Bundoora, VictOria, 3083, Australia. 

The basement terrain of eastern Tanzania shows evidence of 

nascent rift development. Apatite fission track data elucidates 

the Mesozoic and Cenozoic structural evolution of the region, 

and has implications for processes involved in the initial stages 

of continental extension. 

Apatite fission track thermochronology from 25 samples of 

basement rocks along an east-west transect, from Dares Salaam 

to Dodoma, reveal a range in apatite fiSsion track ages and mean 

confmed track lengths from 48 ± 3 to 211 ± 11 Ma and 11.6 ± 

0.7 to 14.1 ± 0.2 J.11Il respectively. The broad range reflects a 

protracted low temperature cooling history for the region with 

tectonism from early Permian to Recent times. 

The youngest apatite fiSsion track ages range from 48 ± 3 to 

87 ± 6 Ma and occur in the vicinity of boundary faults between 

mountain blocks and Karoo to early Tertiary sedimentary 

basins. The relatively long mean confined track length of 14.1 

± 0.2 J.11Il for one sample indicates that rapid cooling (from 

temperatures > 120°C to < 60°C) occurred at 60 - 65 Ma. One 

sample with an age of 48 ± 3 Ma and mean track length of 13.8 

± 0.2 J.1IIl suggests a later stage of final cooling, probably 

during Miocene times. 

Samples collected away from the boundary faults show a 

range of ages from 91 ± 6 to 221 ± 11 Ma. The relatively long 

mean confmed track length of 13.7 ± 0.2 J.11I1 for a sample with 

an apatite fission track age of 103 ± 5 Ma indicates an episode 

of relatively rapid cooling during the Early Cretaceous. 

Samples from similar elevations along parts of the east-west 

transect through the Uluguru and Ukuguru mountains display 

vastly different apatite fission track ages. The pattern of ages 

suggests that high angle block faulting was a fundamental 

mechanism in the Mesozoic and Cenozoic tectonic development 

of eastern Tanzania. 

Apatite fission track age and mean confined track length 

information from the transect show periods of slow cooling 

punctuated by at least two periods of relatively rapid cooling 

from temperatures greater than 120°C to temperatures below 

60°C. The results suggest cooling events at -110 Ma and -65 

Ma, which were probably caused by denudation at rates 

> 30 m/m.y. Normal faulting related to extension throughout 

Africa during Cretaceous time led to the development of 

sedimentary basins and adjacent horst blocks, and caused 

extensive regional denudation. 

235 

ANNEALING CHARACTERISTICS OF SPONTANEOUS 

AND INDUCED FISSION TRACKS IN THE FISH CANYON 

TUFF ZIRCON 

O'BRIEN ALUN Research School of Geological Sciences, 

Fission Track Research Group, University College London, 

Gower Street, London, WClE 6BT, England. 

The aim of this study is to determine the annealing 

characteristics of fission tracks in zircon under 

laboratory conditions and experimental time periods, 

and to use this data to mathematically describe the 

annealing process. It may therefore be possible to 

determine and model temperatures needed to cause 

annealing over geological time periods. 

The Fish Canyon zircon was chosen for its 

relatively low spontaneous track density and since it 

is already used as an age standard. A number of 1, 10 

and 100 hour isochronal annealing experiments were 

carried out on both spontaneous and induced tracks. 

For the induced aliquot, zircons were heated to 

1 000°C for 1 hour to remove spontaneous tracks, and 

then irradiated with a fluence of 1xl015 ncm. The 

variation in track density with horizontal confined 

track length was measured at various temperatures 

between 400°C and 7 50°C. Both etching time and 

anisotropy increase for the spontaneous samples 

annealed above 550°C because of the loss of 

accumulated alpha damage. The etching time for the 

induced samples remains the same as that for the 

spontaneous samples annealed >550°C. A long term 

isothermal experiment was also carried out using 

both spontaneous and induced samples at 350°C. 

Significantly, no reduction in confined track length 

was observed after a 12 month period at this 

temperature. 
HISTORY OF EARTH'S DEGASSING 

O'NIONS R.Keith and TOLSTIKHIN Igor Department of 

Earth Sciences, Cambridge University Cambridge CB2 

3EQ, U.K 

The Earth is an highly degassed body depleted in 36 Ar 

and other noble gases by several orders of magnitude relative 

to the solar abundance. Attempts to reconstruct the timing 

and rates of gas loss from the Earth-atmosphere system have 

been made for more than 30 years (Ref.l). 

The degassing of the Earth during accretion is constrained 

by Pu-U-1-Xe systematics. Degassing was much more 

efficient during the first 100-200 Ma, than subsequently, and 

it was more complete for Xe than for the lighter gases. More 

than 90% of the degassed Xe escaped from the atmosphere 

during this period. The combination of fractional degassing of 

melts (Ref.2) and rare gas escape from the atmosphere (Ref. 

3) is able to explain the deficit of terrestrial Xe as a simple 

consequence of this early degassing history. 

By the time Xe was quantitatively retained in the 

atmosphere, the abundances of Kr and the lighter gases in the 

Earth's inferior were similar to or higher than the present-day 

atmospheric abundances. Subsequent transfer of these lighter 

rare gases into the atmosphere requires a high rate of postaccretion degassing and melt production. Considerations of 

Pu-U-Xe systematics suggest that relatively rapid postaccretion degassing was continued to ca 4.1 - 4.2 Ga. 

It is shown that the small amount of 3He and other 

primordial rare gases in the present-day upper mantle may be 

residual (Ref. 4) or entrained from the low mantle (Ref. 5). In 

the latter case entrainment of only -1% of the lower mantle 

mass per Ga is able to provide the 3He inventory in the upper 

mantle. In this case the residence time of He in this reservoir 

is estimated at - 1 Ga, which is very similar to earlier 

estimates for the highly incompatible lithophile elements, U, 

Th, Pb (Ref.6). However the amount of these lithophile 

elements that accompany the introduction of 3He into the 

upper mantle is too small to sustain their steady - state 

abundances. The continental lithosphere appears to be the 

major source of these lithophile elements. 

1 Turekian K.K., 1959, The terrestrial ~onomy of He and 

Ar: Geochim. Cosmochim. Acta v. 17,37-43. 

2 Tolstikhin I.N. and O'Nions R.K., 1993, The earth's 

missing Xe: a combination of early degassing and of Rare 

gas loss from the atmosphere: accepted by Nature. 

3 Pepin R.O., 1991, On the origin and early evolution of 

terrestrial planet atmospheres and meteoritic volatiles: 

Icarus v. 92, 1-79. 

4 Azbel I.Ya. and Tolstikhin I.N., 1990, Geodynamic, 

magmatism and degassing of the Earth: Geochim. 

Cosmochim. Acta, v. 54, 139-154. 

5 O'Nions R.K. and Tolstikhin I.N., 1993, Behaviour and 

residence times of lithophile and rare gas tracers in the 

upper mantle: submitted to Earth. Palnet. Sci. Lett. 

6 Galer S.J.G. and O'Nions R.K., 1985, Residence time of 

thorium, uranium and lead in the mantle with implications 

for mantle convection: Nature v. 3.16, 378-782. 

236 

APATITE FISSION TRACK STUDY OF A VERTICAL 

PROFILE THROUGH THE BOGONG HIGH PLAINS, 

VICTORIA, AUSTRALIA 

O'SULLIVAN A J , O'SULLIVAN, P.B. and GLEADOW, 

A.J.W., VIEPS, School of Earth Sciences, La Trobe 

University, Bundoora, Victoria, 3083, Australia. 

To help constrain the uplift and erosion history of the 

Bogong High Plains, in northeastern Victoria, the thermal 

history of a detailed vertical section in the Falls Creek area has 

been evaluated using apatite fission track data. Previous 

fission track studies of southeastern Australia have described a 

general decrease in apatite ages from -250-350 Main the 

highland areas located up to 200 km inland, to ages as low as 

-80 Ma towards the coast. It has been proposed that this age 

range was due to partial or complete resetting by a thermal 

pulse associated with the opening of the Tasman Rift to the 

east 

The Falls Creek area comprises the central part of the 

Wagga-Omeo Metamorphic Belt, a NNW trending zone of the 

Lachlan Fold Belt of southeastern Australia. The rocks in the 

region generally consist of Ordovician metasediments intruded 

by granitic rocks thought to have been emplaced during the 

Late Silurian to Early Devonian Bowning Orogeny. Further 

deformation was caused by the Middle Devonian Tabberabberan 

Orogeny. Subsequently, a long period of tectonic quiescence is 

thought to have occurred during the Late Paleozoic, resulting in 

peneplanation of the ancient land surface, which has since been 

uplifted and eroded. This uplift is considered to be the result of 

predominantly horst-like movements along steeply dipping 

normal faults on the eastern (seaward) side of the mountain 

range, and warping on the western flanks, with up to 800m of 

relative uplift along the eastern boundary faults. 

For this pilot study, sixteen samples were collected along a 

1280 m vertical profile. The results show a general decrease in 

apatite fission track age and mean confined track length with 

decreasing elevation, with the exception of two samples in the 

middle whose ages and lengths are greater than those 

immediately above and below, suggesting the presence of a 

previously unrecognised fault block. Samples from the top of 

the profile exhibited the greatest mean lengths (>13.31J.lll}, and 

ages of -270-280 Ma, while samples near the base of the 

section have ages of -230-240 Ma and mean track lengths of 

less than 12 IJ.lll. Detailed modelling of these data suggests the 

samples experienced an initial episode of rapid cooling at some 

time between -300 and 280 Ma, followed by a prolonged 

period of much slower cooling during the Late Palaeozoic and 

Mesozoic, before a fmal period of accelerated cooling beginning 

in the middle Cretaceous. 

The two samples from within the proposed fault block in 

the middle of the vertical profile have apatite ages of -373 and 

319 Ma and track lengths between 13.2 and 12.7 IJ.lll. Crosscutting relationships suggest that the main episodes of faulting 

in the region were associated with Paleozoic deformation, but 

several faults show evidence of having been reactivated more 

recently, possibly as a result of uplift and erosion of the region 

during the Mesozioc or Cenozoic. 

Based on these results, it is proposed that following 

peneplanation of the ancient land surface during the Late 

Paleozoic, the Falls Creek area started to cool in response to 

uplift and erosion during the Late Carboniferous to Early 

Permian. The subsequent episode of accelerated cooling 

beginning in the middle Cretaceous was probably related to 

uplift of the rifted margin in response to the opening of the 

Tasman Rift to the east and opening of the Gippsland Basin to 

the south. 
CENOZOIC UPLIFT AND THERMAL HISTORY OF THE 

NORTH SLOPE FORELAND BASIN, NORTHERN 

ALASKA AND NORTHWESTERN CANADA 

O'SULLIVAN P.B Victorian Institute of Earth and 

Planetary Sciences, School of Earth Sciences, La Trobe 

University, Bundoora, Victoria, 3083, Australia. 

Results from apatite fission track analyses of Mississippian 

through Tertiary sedimentary rocks and Devonian granitic rocks 

from the Alaskan Cordillera of northern Alaska and 

northwestern Canada, have provided important information 

about the timing, amount, and rate of uplift experienced within 

the region. The analyses, together with regional geological 

observations indicate that several episodes of rapid cooling have 

occurred in the northern part of the Alaskan Cordillera 

throughout the Tertiary. Most of these episodes of rapid 

cooling were due primarily to uplift and erosion, but at least 

one case, during the Miocene, must have occurred in response 

to a major regional decrease in the mean annual surface 

temperature. 

The northern part of the Alaskan Cordillera has long been 

interpreted as a component of a major Jurassic-Cretaceous 

convergent margin compressional orogen. The northern 

tectonic elements of this orogen include the North Slope 

foreland basin, and the Brooks Range fold and thrust belL The 

geologic relationships across much of the foreland basin have 

been complicated by continued advancement of the fold and 

thrust belt to present time. However, due to poor geologic 

control, the timing of the deformational events responsible for 

uplift and erosion during the Tertiary have been constrained, for 

the most part, only in a relative sense. 

Interpretations of apatite fission track analyses, suggest the 

region studied experienced rapid cooling due to uplift and 

erosion at various times during the Tertiary. This includes the 

Big Fish River area of the Mackenzie Delta region in Canada 

which experienced rapid uplift and erosion during the early 

Eocene at -53±4 Ma (all errors reported at ±2a). Sedimentary 

and granitic rocks exposed in the northeastern Brooks Range 

(NEBR) in northeastern Alaska, and along the Phillip Smith 

Mountains Front to the west of the NEBR, experienced four 

episodes of rapid uplift and erosion during the Tertiary. These 

occurred in the middle Paleocene at -60±4 Ma, in the middle 

Eocene at -43±3 Ma, in the early Oligocene at -34±3 Ma, and 

in the late Oligocene at -25±3 Ma. Fission track results from 

sedimentary rocks collected from both outcrop and subsurface 

localities along the northern foothills of the Brooks Range 

west of the Phillip Smith Mountains Front, and throughout 

the North Slope foreland basin, suggest the entire region 

experienced two episodes of rapid uplift and erosion, at -60±4 

Ma and -25±3 Ma. 

The interpreted fission track results also suggest that the 

region experienced a major decrease in the mean annual surface 

temperature of -l5°C during the Early-to-middle Miocene at 

some time between -20-10 Ma This decrease in mean annual 

surface temperature has resulted in an equivalent amount of 

subsurface cooling at shallow depths (less than -4-5 km), 

unrelated to either uplift and erosion or a decrease in heat flow. 

Therefore, the results suggest that there were two major 

episodes of deformation which have affected the entire foreland 

basin in the Tertiary; during the Paleocene at -60±4 Ma, and 

during the late Oligocene at -25±3 Ma. Deformation and 

resulting uplift and erosion also occurred on the North Slope 

during the mid-Eocene to mid-Oligocene, at -43±3 Ma and 

-34±3 Ma, but deformation was localised and only affected the 

NEBR. 

237 

HIGH-RESOLUTION SINGLE-CRYSTAL U-PB DATING OF 

ORDOVICIAN ECLOGITES - TRACERS FOR THE EARLY 

EVOLUTION OF A CENTRAL SWISS ALPINE TERRANE 

OBERLI. F. and MEIER, M., Isotope Geochemistry ETH, 

CH-8092 Zurich, and BIINO, G.G., Min. Petr. lost., Univ., 

Perolles, CH-1700 Fribourg, Switzerland 

Polymetamorphic mafic-ultramafic rock associations are the 

oldest igneous remnants in the pre-Variscan Helvetic basement 

of the central Swiss Alps. They can be divided into 1) a structurally older oceanic group consisting of lenses of meta E-N-MOR 

basalts, abyssal peridotites and minor metagabbros, and 2) a 

younger group of island arc (lA) - type intrusive metagabbros 

and ultramafic cumulates. Both groups together with their country rocks (accretionary wedge-type metasediments) were 

metamorphosed at eclogite facies (650-750 °C I P > 1.8 GPa), 

and later at granulite facies (600-700 °C I P - 0.8 GPa; Biino, 

1994). Granites emplaced at -440 Ma postdate the granulite 

stage. Partial re-equilibration at amphibolite and greenschist 

grade occurred during Variscan and Alpine times, respectively. 

Based on ion-probe U-Pb data of zircons from an IA-type metagabbro, Gebauer et al. ( 1988) postulated a proto lith age of -870 

Ma and -468 Ma for the eclogite event. The resulting time interval is large compared to the mean life of modem oceans. 

Zircons from four garnet bearing mafic rocks were analyzed 

by single-crystal U-Pb techniques to constrain origin and early 

metamorphic history and to correlate distinct basement segments. Complex evolution and prospect of multiple inheritance 

precluded application of analytically more precise but geologically less accurate multi-grain methods. Using microscopical 

criteria for rejection of inheritance (cores), abrasion and cathodoluminescence imaging, we arrive at the following results: 

1) Two intrusive metagabbro stocks belonging to the 

younger group yield almost identical upper Concordia intercept 

ages of 467 +5/-4 Ma (95% c.l. ext.) and 471 + 7/-6 Ma, respectively. U concentrations of 84-1087 ppm, euhedral magmatic 

morphology and multiple internal zoning suggest preservation 

of primary magmatic properties. The isotopic results are therefore interpreted to approximate the time of intrusion. Only rare, 

core-bearing zircons show pre-Ordovician inherited Pb components. Lower intercept ages of 298 ± 21 Ma and 272 ± 11 Ma and 

varying degree of discordance reflect Pb loss during the 

Variscan and, subordinately, Alpine orogenies. 

2) Zircons from a fine-grained MORB eclogite display uniform low U contents of 28-37 ppm and apparent U-Pb ages of 

334-474 Ma. Lack of magmatic zircon in undifferentiated 

MORB and absence of multiple internal zoning suggest an 

entirely metamorphic origin. Limited precision due to low radiogenic Pb contents ( 6-13 pg) precludes calculation of meaningful 

Concordia intercept ages. The highest U-Pb age, 474 ± 4 Ma, 

defines a minimum age for subduction-related metamorphism. 

In conclusion, our results are in support of a Caledonian evolutionary cycle of subduction, island arc magmatism, collision 

and uplift, occurring on an ordinary time scale of some 30 Ma. 

Biino, G.G., 1994, Schweiz. Mineral. Petr. Mitt. (in press). 

Gebauer, D., Quadt, A., Compston, W., Williams, I.S., and 

Griinenfelder, M., 1988, Schweiz. Mineral. Petrogr. Mitt., 

v. 68, p. 485-490. 
RECOIL REFINEMENTS: IMPLICATIONS FOR THE 

40ARJ39 AR DATING 1ECHNIQUE 

Onstott. I C., Dept. of Geological and 

Geophysical Sciences, Princeton University, 

Princeton, NJ 08544, and Miller, M., and Ewing, 

R.C., Dept. of Geology and Institute of 

Meteoritics, Univ. New Mexico, Albuquerque, 

NM 87131 

Integration of the neutron energy distribution 

for a water-moderated reactor with the most 

recent cross-section data yields mean recoil 

energies of 180 ke V for 39K (n,p) 39 Ar, 970 ke V 

for 4UCa (n,a) 37 Ar, and 140 eV for 37Cl (n,g) 3SC1 

(b) 38 Ar. Utilizing numerical simulations of 

collision cascades, we calculate a mean recoil 

range of 1620 A for 39K (n,p) 39 Ar, 3780 A for 

40Ca (n,a) 37 Ar, and 11 A for 37Cl (n,g) 38Cl (b) 

38 Ar. Integration of the recoil range distributions 

yield a mean depletion depth in a semi-infinite 

medium of 820 A for 39 Ar, 1950 A for 37 Ar, and 6 

A for 38 Ar. The concentration gradients generated 

by recoil redistribution between thin slabs were 

then incorporated into standard diffusion 

equations. These calculations indicate that the 

argon release rates and 40 Arf39 Ar age spectrum 

derived from incremental heating of exsolved 

minerals are significantly affected by recoil 

redistribution. 

If the exsolution lamellae are the effective 

diffusion dimension, the measured activation 

energy of diffusion will be greater than the true 

value and the age spectra will be discordant even 

if the feldspar has not experienced a complex or 

slow cooling history. Incremental step apparent 

ages will increase with the fraction of 3 9 A r 

released as the albite lamellae degas. The age 

spectra will exhibit decreasing apparent ages with 

increasing fraction 39 Ar released as the potassium 

feldspar lamellae degas. The overall profile of the 

age spectrum will be depend upon the composition 

of the feldspar and the size of the lamellae. 

Finally, the 11 A mean recoil distance 

calculated for 38 Ar indicates that it is not a proxy 

for anion-sited excess argon. Instead, published 

correlations of 38 Ar with excess 40 Ar probably 

reflect the degassing of fine-grained, Cl-rich 

inclusions. 

238 

EARLY IRRADIATION OF SILICON CARBIDE: 21NE 

PRODUCTION RATE AND RECOIL LOSS 

OTT, U. and BEGEMANN, F., Max-Planck-Institut fiir 

Chemie (Otto-Hahn-Institut), Postfach 3060, D-55020 

Mainz, F.R.Germany. 

The isotope abundance anomalies carried by interstellar 

silicon carbtde are highly important in their own right and 

with respect to the processes of nucleosynthesis in their 

parent stars (Ott, 1993; Anders and Zinner, 1993)- probably 

AGB stars for the majority of grains. However, a knowledge 

of their age is desirable in order to address additional areas of interest such as number of contributing AGB stars 

(Alexander, 1993) or a possible major contribution from a 

late AGB star. The latter may have contributed many of the 

extinct radionuclides the decay products of which have been 

identified elsewhere (Cameron, 1993; Wasserburg et al., 

1993). 

The approach that has been taken (Tang and Anders, 

1988; Lewis et al., 1993) is based on observed excesses of 

21Ne over the amount expected from mixing normal (Ne-A 

or solar) with Ne (Ne-E(H)) expected from the He shell of 

AGB stars. After correction for the recent spallogenic Ne 

produced during cosmic ray exposure of their meteorite 

host, this approach yields for the SiC grains nominal presolar 

cosmic ray exposure ages between a few and -130 Ma, 

correlating with grain size and Ne-E(H) content (Lewis et 

al., 1993). There are, however, a number of uncertainties 

involved in this approach. 

In order to address one of the problems - the extent to 

which cosmogenic 21Ne is lost from the grains due to recoil- we have irradiated size-separated terrestrial SiC in the range 

-0.3 to -5 J.lm with 1.6 Ge V protons to produce approx. 

10(-12) ccSTP 21Ne/mg SiC. To avoid as much as possible 

recoil 1m plantation into adjacent grains, the SiC grains (ca. 

2-5 mg of each size fraction) were suspended and 'frozen in' 

in paraffin wax (1-2 grams) before irradiation. Results of the 

Ne analyses will be presented at the Conference. Also a 

different approach using cosmogenic Xe rather than Ne will 

be discussed. 

Alexander, C.M.O'D, 1993, Presolar SiC in chondrites: How 

variable and how many sources? Geochim. Cosmochim. 

Acta, v. 57, p. 2869-2888. 

Anders, E., and Zinner, E.K., 1993, Interstellar Grains in 

Primitive Meteorites: Diamond, Silicon Carbide, and 

Graphite, Meteoritics, v. 28, p. 490-514. 

Cameron, A.G.W., 1993, Nucleosynthesis and Star 

Formation, in: Protostars and Planets III (E.H. Levy, J.I. 

Lunine and M.S. Matthews, eds.), p. 47-73. 

Lewis, R.S., Amari, S., and Anders, E., 1993, Interstellar 

grains in meteorites. II. SiC and its noble gases, Geochim. 

Cosmochim. Acta, in press. 

Ott, U., 1993, Interstellar grains in meteorites., Nature, v. 

364, p. 25-33. 

Tang, M., and Anders, E., 1988, Interstellar silicon carbide: 

How much older than the solar system? Astrophys. J., v. 

335, p. L31-L34. 

Wasserburg G.J., Busso, M., Gallino, R., and Raiteri, C.M., 

1993, Asymptotic Giant Branch Stars as a Source of ShortLived Radioactive Nuclei in the Solar Nebula, Astrophys. 

J ., in press. 
THE OCEAN GEOCHEMISTRY OF OSMIUM AS 

INFERRED FROM MARINE SEDIMENTS. 

OXBURGH. Rachel. MARCANTONIO, Franco 

and ZINDLER, Alan, Lamont-Doherty Earth 

Observatory, Palisades, NY 10964, USA. 

Osmium isotopes in ocean systems have exciting 

potential as geochemical tracers. However the marine 

chemistry of osmium is not well constrained. 

Concentrations of osmium in sea water are too low to 

permit direct measurement, although this is 

anticipated in the near future. Currently, marine 

sediments provide the sole source of information on sea 

water composition. 

In an attempt to establish the distribution of 

osmium in the modem oceans, we have analyzed surface 

sediments from a wide variety of locations as well as 

both shallow and deep dwelling corals. The surface 

oceans, as represented by analyses on planktonic 

foraminifera and surface dwelling corals appear to 

have a 187 Qsj186os ratios that are significantly lower 

than that of the deep ocean, as recorded by deep-sea 

corals and the authigenic phases of pelagic sediments. 

There are two important implications of this 

observation. First, for surface and deep isotopic 

signatures to be distinct, the residence time of osmium in 

the oceans cannot exceed the oceanic mixing time (1000 

years). Second, a significant flux of osmium to the 

surface oceans with low 187 Os;186Qs is required. We 

consider the most likely candidate to be the dissolution 

of ultra fine micrometeoritic material on contact with 

sea water. 

239 

NOBLE GAS STATE IN THE MANTLE 

OZIMA, M., Dept. of Earth & Space Sci., Fac. of 

Sci..Osaka Univ.,Toyonaka-City, 560 Japan. 

There are a variety of mantle degassing models 

based on noble gases. Because of different choices 

of noble gas data by the authors, however, their 

conclusions are often contradictory. To reduce such 

confusion, I examined all the published noble gas 

data available to me, and I present a mantle noble 

gas data base deduced from the recent high quality 

experiments. 

Because of various elemental fractionation processes, it is difficult to conclude definitely the noble 

gas elemental abundances in the mantle from the 

noble gas analyses of mantle-derived materials. In 

contrast to the elemental abundances, however, 

isotopic fractionation in geological processes can 

generally be neglected, and we can infer the noble 

gas isotopic state in the mantle from the 

experimental results on mantle-derived materials. 

We conclude the following characteristics in the 

mantle noble gas isotopic state. 

(1) Apart from the addition of-radiogenic isotopes, 

isotopic compositions of argon, krypton and xenon 

in the mantle (both degassed and less-degassed) are 

similar to those of the air noble gases within the experimental uncertainties of about 1 %. 

(2) 20Nej22Ne in the mantle (degassed and less- degassed) is distinctly higher (11 - 13) than that of atmospheric neon. The ratio resembles solar neon ( 

either SEP or SW neon ). There appears to be 

little difference in the isotopic ratio between the degassed and the less-degassed mantle. 

(3) Radiogenic isotopic ratios such as 3He/4He, 

21 Nej22Ne, 40 Ar/36 Ar which are in excess of the 

respective air ratios, are higher in the degassed 

mantle (except for 3Hef1-He) than in the less degassed mantle. 

( 4) 40 Arj36 Ar ratio in the less-degassed mantle is 

likely to be much higher (>3,000) than hitherto assumed (-500). 

(5) Xenon isotopic ratios containing radiogenic isotopes such as 129xeJ130xe and 131-136xeJ130xe 

are significantly higher in the mantle than in the air. 

However, there appears to be little difference in 

these isotopic ratios between the degassed and the 

less-degassed mantle. 

(6) Because of possible air contamination, all the 

excess isotopic ratios observed in mantle-derived 

materials relative to the air noble gas isotopic ratios 

should be regarded as a minimum. 

(7) Present observational data do not support a significant contribution of subducted extra-terrestrial 

noble gases, either helium or neon, in the mantle 

noble gas inventory. 
U-Pb ZIRCON AND Pb ISOTOPE S1UDIES IN RELATION 

TO PR01EROZOIC SEDIMENT-HOS1ED LEAD-ZINCSll..VER DEPOSITS IN NORTHERN AUS1RALIA 

PAGE R W., Sun, Shen-su, Australian Geological Survey 

Organisation, Canberra, A.C.T., 2601, Australia, and Carr, 

G.R., CSIRO Division of Exploration and Mining, North 

Ryde, N.S.W., 2113, Australia. 

Models for the tectonic and metallogenic evolution of major 

stratabound lead-zinc-silver mineral deposits are clearly enhanced 

by lmowlOOge of both the sediments' age(s) and the coherence or 

otherwise of Pb isotope patterns associated with the primary 

sulfides. In the last few years, our motivation to better 

understand such economically important sequences in northern 

Australia has coincided with the huge growth and success of 

exploration for base metals and gold, especially in the Mount 

Isa Inlier and McArthur Basin. 

The yo1Dlger cover sequences in these terrains that host the 

Pb-Zn-Ag accumulations, also contain sparse felsic tuff beds. 

New SHRIMP U-Pb zircon work on these rocks effectively 

defines the times of deposition of the associated stratabound 

ores. For example, work on tuffs from the Mount Isa and 

Hilton orebodies provides a depositional age for this part of the 

Mount Isa Group at 1653±7 Ma. Preliminary data on high-grade 

rocks equivalent to those hosting the recently discovered 

Cannington Pb-Zn-Ag deposit, show that this succession has a 

maximum age of 1677±9 Ma, comparable to the depositional 

age of the Broken Hill ore host sequence at 1690±5 Ma. 

Sequences similar to the Mount Isa Group are found in a 

number of basins and fault-bounded troughs containing the 

McNamara and McArthur Groups north of Mount Isa. 

Syngenetic or late diagenetic Pb-Zn-Ag deposition in these 

latter sequences has been dated, again by means of the SHRIMP 

U-Pb zircon work on concordant tuffs. These deposits are 

somewhat younger than the Mount Isa Group, ranging from 

1640±7 Ma (HYC orebody), to 1640±7 Ma (Walford Creek 

deposit), to 1595±6 Ma (Century orebody). This points to a 

diachronous development of similar extensional tectonic and 

metallogenic environments over a period of 60-90 million 

years. 

Comparison of these depositional ages with the abundant 

galena Pb isotope data for the respective deposits allows us to 

more fmely tune the Pb evolutionary model and test the 

significance of these Proterozoic Pb model ages. Subtle 

variation in 206pbJ204pb (16.0 to 16.3) and the relative twostage model ages for Pb from Broken Hill, Mount Isa, HYC, 

Lady Loretta, and Century broadly mimic the progressively 

younging stratigraphic ages of these deposits (1690 to 1595 

Ma). This nexus between relative Pb model ages and actual 

depositional ages of the sediment-hosted Pb deposits of the 

Proterozoic of northern Australia allows refmement of part of 

the Pb isotope growth curve for this particular region, but we 

concede that a universally applicable single growth curve may 

not be achievable. The connection between stratigraphic ages 

and primary Pb isotope signatures in northern Australian 

Proterozoic terrains supports a syngenetic or diagenetic origin 

for mineralization in the deposits studied. 

240 

40AR·39AR DATING OF QUATERNARY BASALT, 

WESTERN MODOC PLATEAU, NORTHEASTERN 

CALIFORNIA: IMPLICATIONS TO TECTONICS 

PAGE W. D , Geosciences Department, F22A, Pacific Gas 

and Electric Company, P. 0. Box 770000, San Francisco, 

CA 94177 and RENNE, P. R., Institute of Human Origins 

Geochronology Center, 2453 Ridge Road, Berkeley, CA 

94709. 

The Modoc Plateau has had low seismic activity, yet many 

faults have displaced the extensive Quaternary "Warner 

basalts" in this region. In order to help assess the seismic 

hazards and risks to PG&E's facilities in the region north and 

northwest of Mt. Lassen, we dated 15 samples from 10 flows 

by the 40Ar_39 Ar, step-heating method. Dating of the rocks 

proved critical to understanding the tectonics and timing of 

faulting in the region Discussion of the Ar-Ar results and 

tectonic implications for the area south of Mt. Lassen are 

presented in companion abstracts by Becker et al. and by 

Wakabayashi et al. 

Basalts in the Modoc Plateau were mostly contained from 

spilling to the west and south by the Klamath Mountains and 

Sierra Nevada as they built up the plateau. The oldest dated 

basalt flows (- 2.4 Ma) are small remnants that have been 

locally weathered to saprolite and are plastered on north flank 

of the Sierra Nevada. Geomorphic analysis suggests that the 

older flows may have locally spilled over the divide north of 

Mt. Lassen. The interconnected, north-striking McArthur, Hat 

Creek, and Almanor graben now separate the western Modoc 

Plateau from the mountains to the west and have been a major 

geomorphic feature for more than a million years. 

The west-facing Hat Creek fault zone on the east side of 

the Hat Creek graben north of the Pit River (Soldier Creek 

fault) displaces the 1.2 Ma basalt on top of the scarp > 260 m, 

giving a long-term vertical separation rate of> 0.22 mm/yr. 

South of the Pit River the Hat Creek fault zone displaces the 

flow (1.0 Ma) on top of the scarp > 390m (> 0.38 mm/yr). At 

the same locality the eastern two faults in the zone displaces 

the basalt flow west of Hogback Ridge (850 ka) > 85 m 

(> 0.10 mm/yr). These rates are less than the 1.3 mm/yr (post 

15 ka) presented by Muffler et al. (1993 manuscript) for the 

Hat Creek fault in the near Lost Creek to the south. The Sam 

Wolfln Spring (800 ka), Rocky Ledge(?) (150 ka) and other 

flows younger than 1 Ma were deposited within the Hat Creek 

graben. Extensive diatomite lake beds in the graben underlie 

the Wolfin Spring flow and hence appear to have, been 

deposited between about 1 Ma and 800 ka. 

The Yellow Jacket(?) (840 ka) and the Tuft Creek flows 

(150 ka) are northwest of Indian Spring Mountain on the 

northeast side of the McArthur graben. The Mayfield fault 

displaces the Tuft Creek flow 12 m, down west (0.09 mm/yr). 

The entire west-facing Mayfield fault zone displaces it about 

64 m (0.5 mm/yr). The Fall River Mills basalt ( -- ka) and 

other younger flows partly fill the McArthur graben. 

Northwest of Mt. Lassen the northeast-striking Battle 

Creek fault vertically displaces hypersthene andesite of 

Brokeoff Mountain flow (550 ka) about 43 m, down south 

(0.078 mm/yr). The scarp is overlain by the Black Butte basalt 

( -60 ka) that does not appear to be faulted. 

The results of this study strongly indicate that Ar-Ar 

dating of basalts as young as late Quaternary can provide 

significant constraints to neotectonic processes. 
PB, SR, ND ISOTOPE AND U-SERIES INVESTIGATION 

OF THE HOLOCENE INYO AND MONO HIGH SILICA 

RHYOLITE DOMES, E. CALIFORNIA. 

ZENON PALACZ JAMES GILL. Earth Science Board, 

University of California, Santa Cruz. CA95064. 

The Mono and Inyo rhyolite domes are the most recent 

products of prolonged silicic volcanism in E. California. 

They fonn a series of northerly trending domes between the 

Long Valley Caldera and Mono Lake. The age of volcanism 

varies between 25ka and 0.5ka in the Mono domes, with the 

most recent eruption in the Inyo domes occuring 600 yrs BP. 

Th-U disequilibria in the Inyo finely ~hyritic magma 

type shows a progressive decrease in 23 Th/232Th and 

238uf232Th with increasing fractionation (Ba decreases from 

1350 to 350ppm and Th increases from 13 to 23ppm). These 

data define a U-Th whole rock isochron of 55ka which is also 

shared by the high Ba Negit and Paoha island dacites and 

rhJolites within Mono Lake 30km to the north. Because 

(2 0Th)/(232Th) and 1{fh are positvely correlated, the 

isochron could be a two component mixing line between a 

hifh Ba magma with (230Th)/(238U)=l and 

(2 0Th)/(232Th)=1, and a more evolved low Ba low Zr 

m~ma with (230Th)/(238U) 1.15, (230Th)/(232Th) 0.91. 

14 Nd/144Nd increases from 0.51258 to 0.51264, and 

206pbJ204Pb decreases from 19.16 to 19.13 with increasing 

fractionation and increasing (230'fb)/(23Su) while 87sr,S6sr 

remains constant at 0. 7060. These data show that the change 

in Th isotopes is due to magma mixing and not time. The 

evolved low Ba m~ma may have fractionated zircon and 

lowered the (238u/ 2Th) of the magma perhaps within the 

past 1-10ka, and mixed with the high Ba magma during 

eruption. 

In marked contrast Mono rhyolite domes are all U enriched 

with (230Th)/(238U) 0.94-0.87 and (230Th)/(232Th) 0.86-

0.89. The younger (<1ka) aphyric domes have the lowest Th 

isotope ratios while the older biotite bearing (-25ka) domes 

have the highest. The Th-U data are consistent with continual 

fractionation of an allanite bearing assemblage over 1-50ka. 

All the domes have similar Pb ratios (206pbJ204pb 19.13). 

Nd isotope ratios vary between 0.51260 and 0.51264, 

87srJ86sr between 0.706 and 0.7064. The older biotite 

domes have the highest Sr and Nd isotope ratios while the 

youngest aphyric domes have the lowest. The possible 

relationship between Th isotope ratio and eruption age may 

be significant, though magma mixing cannot be ruled out. 

241 

PETROGENETIC INTERPRETATIONS OF Sr, Nd, 0 

AND Pb ISOTOPES FROM THE MOUNT SIDLEY 

VOLCANO, MARIE BYRD LAND, ANTARCTICA 

PANTER K. S. 1, PANKHURST, R J.2, KYLE, P.R. I, 

SMELLIE, J. L. 3, MUKASA, s.4 and MciNTOSH, w.I 

1 Geoscience, New Mexico Tech., Socorro, NM 87801, USA 

2 Iso. Geosci. Lab., NERC-BGS, Keyworth, NG12 5GG, UK 

3 BAS, High Cross, Madingley Rd, Cambridge CB30ET,UK 

4 Geol. Sci., Univ. Michigan, Ann Arbor, MI 48109, USA. 

Mt. Sidley (4281m) is a large (>200km3) alkaline 

stratovolcano within the Late Cenozoic Marie Byrd Land 

(MBL) volcanic province, Antarctica. Detailed mappingstratigraphy and 40 ArJ39 Ar geochronology reveals a complex, 

multicenter, polygenetic volcano constructed in successive 

stages over 1.5 m.y. (5.7 - 4.2 Ma). Rock compositions consist 

of strongly undersaturated sodic lavas, ranging from basanite 

to anorthoclase phonolite and trachyte with minor volumes of 

oversaturated trachyte. 

Major changes in composition coincide with a southward 

migration of volcanic activity(- 0.6 cmlyr). Four stages of 

eruptive activity are recognized and commenced with eruptions 

ofphonolitic lavas (Stage I: 5.7-4.7 Ma) followed by 

deposition oftrachytic lavas and tephras (Stage II: 4.6-4.5 

Ma). A highly explosive eruption of trachytic magmas and 

sector collapse of the volcanoes southern flank, was followed 

by the deposition of intennediate composition lavas and 

tephras (Stage III: 4.4-4.3 Ma) approximately 9 km south of 

Stage I activity. The final stage (Stage IV: 4.2 Ma) produced 

xenolith-bearing parasitic cones of basanite. 

Most samples display a narrow range in 87srt86sri (0.70283 

- 0.70315), 143Nd/144Ndi (0.51280- 0.51290) and BISo (5.2-

6.2%o), indicating their derivation from a primitive mantle 

source. Two basaltic samples have values of206pbf2°4Pb = 

19.5, 207pbf204pb = 15.7 and 208pbJ204pb = 39.1, suggesting 

a significant HIMU mantle source com~nent. Linear 

variation of 143Ndf144Ndi with 87srt8 Sri may be explained 

by interaction of a HIMU-rich source with an enriched mantle 

component (lithosphere or EMI). On the other hand, 

systematic variation of 143Nd/144Ndi with Si02 and Mg# 

(87srt86sri and Btso are relatively constant) may suggest 

evolution by AFC processes in a mafic lower crust. Four 

highly evolved trachyte samples show elevated 87Srt86sri 

(0.70363- 0.70422) and Bl8o (7.6- 8.4%o) indicating some 

assimilation of the upper crust by ascending melts. 

A significant number (13 or 39) of sam£les analyzed for 

oxygen isotopes display relatively low Bl 0 values of -1.4 to 

4.1%o. Low Bl8o values are not associated with a specific rock 

composition or lithology, however, the extent of 180 depletion 

does correlate with measured loss on ignition (LOI contents up 

to 2.4 wt.%). Although the majority of these samples appear 

"fresh" in thin section, the 18o depletion is considered a result 

of low temperature exchange-hydration with extremely light (B 

18o = -40 to -30%o) Antarctic meteoric water. 
HIGH ABUNDANCE SENSITIVITY MASS SPECTROMETER FOR U-Th STUDIES. 

PAPANASTASSIOU.D.A.,BERNIUS,M.T. 1 ,CHEN,J.C., 

and WASSERBURG, G.J.,The Lunatic Asylum, Div. Geol. 

& Planet. Sci. Caltech, Pasadena, CA 91125. 1Dow Corning 

Co., Central Research Bldg. 1712, Midland, MI 48674 

We report the abundance sensitivity (AS) and ion 

transmission characteristics of a modified Finnigan-MAT 

2628-RP mass spectrometer (Table). This instrument is an 

outgrowth of our joint effort with Drs. Habfast and Laue of 

Finnigan-MAT. The instrument is compact, relatively low 

cost, and versatile, enabling dual use: a)high precision (10-20 

ppm) with the 1st stage multiple collector; and b) high AS and 

transmission with the 2nd stage. The 2nd stage was 

configured to reduce the contribution to the beam of ions that 

have suffered collisions. The design includes a retarding 

potential (RP) lens; this serves the dual purpose: a) of 

stopping ions that have lost some energy due to collision or 

charge exchange and that would have formed the •tow-mass" 

tail of the ion beam, and b) permitting the introduction of the 

decelerated ions through a second ion selective device, chosen 

to be a dynamic electric quadrupole, with AS of -10-4• The 

latter design was not pursued by F/MAT. An ion-optical study 

was carried out at Caltech, which indicated that it was 

necessary to have the ion beam optimized in angular 

dispersion for introduction into the RF-quadrupole. We have 

completed a new 2nd stage corresponding to the ion-optical 

design. It includes a static electric quadrupole doublet for 

beam shaping prior to the RP lens. The RP lens was designed 

to provide for gradual deceleration of the ions and increased 

ion transmission. The results of the unmodified and modified 

system (without the RF quadrupole) are shown in the Table. 

The abundance sensitivity of this _system for a two mass unit 

difference, as is applicable for 23UJb, is found to be better 

than 5 x to- 10, obtained for 1.5 amu difference, for an 

accompanying transmission of 50%. Data have been obtained 

on U and Th gravimetric standards with 236ut238u = 10-8 

and 230rht232u = 10-7• The mass spectrometric data agree 

with gravimetry within counting statistics. Measurements of 

230nt/232Th on young volcanics and on mineral separates are 

routinely obtained. We have checked the RF-quadrupole on the 

mass spectrometer. We have obtained up to-84% transmission 

through this RF quadrupole used in a static mode. However, 

the RF-quadrupole showed evidence for electric discharge at 

the 10 kV operating voltage. The system has been improved 

and is now under test. It is our expectation that the increased 

mass selectivity by the RF quadrupole will allow a wider 

energy range of ions through the RP lens and, therefore, 

higher transmission. We hope to reach an AS close to 

10-14amu-1, which is now available only with accelerator 

mass spectrometry. Division Contribution 5368 (842). 

Unmodified MO<hfiedb 

Abundance senstttvtty (U-Th regton)1 

1 amu 1 X 10-S 5 X 10-9 

1.5 amu 4 X 10-9 5 X 10-lO 

Transmissionc 50% 50% 1For low mass stde. "New ton-optical deStgn. cSecond stage. 

242 

STABLE ISOTOPIC COMPOSITIONS OF OSTRACOD 

IN THE SOGWIPO FORMATION, KOREA 

Park, B.K., Lee, K.S., Korea Basic Sciene Center, 

Taejeon, 305-333, Korea, and Lee, E.H., Dept. of 

Geology, Korea Univ. Seoul, 136-701, Korea. 

The oxygen and carbon isotopic compositions of the 

ostracod species, Normanicythere sp., in the Sogwipo 

Formation exposed at the southern part of Sogwipo 

city of Jeju Island have been measured. This work was 

intended to study the relationship between oxygen and 

carbon isotopic compositions of the ostracod and 

paleotemperature of the depocenter of the formation 

and/or the global climate changes. The values B13CPD8 

varied from -6.77'Yoo to -4.86'Yoo with an average of 

-5.57%o, while the values of B1S0p08 varied from 

-2.26%o to +0.45'Yoo with an average of -0. 76%o. The 

values of B1

S0p08 were decreased gradually ·from sample 1 

to sample 19 except for sample 11 where the value was 

sharply increased. The minimum value of B1S0p08 was at 

sample 11. The values of B13Cp08 showed the similar trend 

of B1S0p08 values. The variations of B1S0pos and B13Cp08 

values seemed to be dependent on the variations of sea 

water temperature during deposition of the Sogwipo 

Formation. Also it seemed that the Sogwipo Formation 

was deposited during glacial period with a short 

interglacial epoch. 
ISOTOPIC CHARACTER OF CAMBROORDOVICIAN MAGMATISM IN SOUTH 

VICTORIA LAND ANTARCTICA, AND PRODUCTION OF SUB-CONTINENTAL MANTLE 

PARKINSON. D.L., Geol. Dept. Univ. of Otago, P.O. 

Box 56, Dunedin, New Zealand, and Dept. of Geol. Sci. 

U.C. Santa Barbara, CA 93106, and Cox, S., Geol. Dept., 

Univ. of Otago, P.O. Box 56, Dunedin, New Zealand 

The Dry Valley (DV) region of South Victoria Land (SVL) 

Antarctica is the site of voluminous granitoids that intrude the 

Late Proterozoic Koettlitz Group, a sequence of syn- to postrift deposits. These intrusives range from foliation parallel 

orthogneisses to cross-cutting slightly deformed and 

undeformed plutons. Cox and Allibone (Ant Sci 3, 1991) and 

Allibone et al. (NZTGG, 1993), using mapping and 

geochemical data, divided these intrusives into three categories: 

DV1a (calc-alkaline, metaluminous hbl-bi granodiorites); 

DV1b (calc-alkaline, meta- to peraluminous bi granodiorites 

and granites); and DV2 (alkali-calcic granites). 

Geochronologic and isotopic data from a variety of units 

support the division of SVL granitoids into the DV1a, DV1b, 

and DV2 suites. U-Pb zircon ages indicate a range from 530-

520 Ma for DV1a suite; a large DV1b body yields U-Pb 

monazite ages of 489 ± 5 Ma. Granites of the DV2 suite are 

probably correlative with Sil-Dev Admiralty intrusives of 

North Victoria Land (NVL, Borg et al., AJS 287, 1987). 

Pb isotopic data from DV1a yields lower 207/204 values 

for equivalent 206/204 values than DV1b. DV2 samples plot 

more radiogenic in both 207/204 and 206/204 than either 

DV1a or DV1b. Sq and tNdi from DV1a and DV1b indicate a 

crustal component in these arc related intrusives, plotting in 

the range 0.705-0.708 for Sri and -2 to -7 for tNdi· 

The DV1 event is correlated with the Ross Orogeny which 

has been identified from NVL through the Transantarctic 

Mountains. The DV1 intrusives correlate with the Granite 

Harbor Intrusives of Borg et al. (1987), and more specifically 

with the Gabbro Hills/Marsh Glacier intrusives in the Central 

Transantarctic Mountains described by Borg et al. (JGR 95, 

1991). DV1a intrusives are interpreted to represent 

construction of a Cambrian arc, based on isotopic and 

geochemical data. The higher 207/204 values in DV1b 

indicate an increasing crustal component with time. We 

interpret this Cambro-Ordovician arc to have been built on 

transitional crust to the East Antarctic craton, and to have 

constructed a geochemically distinct sub-continental mantle. 

The Jurassic flood basalts of the Ferrar Magmatic Province 

(FMP) are spatially coincident with DV1 and correlative 

Granite Harbor Intrusives. The FMP has uni-formly high, but 

variable, Sri (0.709 to 0.713), uniformly negative tNdi (-3 to6), and elevated 207/204 values with respect to the Northern 

Hemisphere Regression Line. Hergt et al. (EPSL 105, 1991) 

state that the source of the FMP and related flood basalts is the 

sub-continental mantle, and attribute the distinctive isotopic 

and trace element signature to incorporation of subducted 

sediment into this source region during a prior arc building 

event. 

Calculated isotopic values for the DV1 suite for Jurassic 

time produces similar or overlapping fields in all three isotopic 

systems when compared with FMP data. This isotopic data is 

interpreted to indicate that the sub-continental mantle produced 

during the Cambro-Ordovician Ross Orogeny is the source for 

younger FMP magmas. 

243 

U-Pb PROBLEMA TICS OF VERY HIGH-U ACCESSORY 

MINERALS: EXAMPLES FROM THE HIMALAYA AND 

CORDILLERA AND IMPLICATIONS FOR U-Pb 

GEOCHRONOLOGY 

PARRISH. R. R., Geological Survey of Canada, 601 

Booth St., Ottawa, Ontario, Canada, KIA OE8, and 

CARR, S.D., Ottawa-Carleton Geoscience Centre, 

Carleton University, Ottawa, Ontario, Canada, K1S 5B6 

Orogenic belts rich in clastic sedimentary protoliths give 

rise to anatectic leucogranitic rocks during high grade 

regional metamorphism. Both the leucocratic rocks and the 

metasedimentary rocks will generally contain zircon, 

monazite, and sometimes xenotime as accessocy minerals, 

and often these minerals will have vecy high uranium 

concentrations. Extensive studies in the Cordillera of British 

Columbia, Canada and the Pakistan and Nepal Himalaya 

indicate that the uranium concentration can exert an 

important control on Pb retentivity of these minerals, an 

effect not previously clearly documented. Dating of optically 

high qualitY igneous and metamorphic grains of these 

minerals indicates that when uranium concentrations exceed 

ca. 1000ppm in zircon or 4000ppm in monazite and 

xenotime, these minerals can act as substantially open 

systems during post-ccystallization cooling over the 

temperature interval >400° -650°C, despite their nominal Pb 

closure temperatures being regarded as in excess of 

650-700°C. 

In the dating of more than 15 leucogranitic rocks from 

the Himalaya, minerals with these "vel)' high" uranium 

concentrations inevitably yield a spread ofU-Pb ages which 

val)' between ca. 16 to 21-22 Ma, the actual ccystallization 

age being 21-23 Ma. U-Pb age clearly correlates with U 

content in several cases. These samples did not cool below 

ca. 400°C until ca. 5-15 Ma ago. It is possible that many 

other published U-Pb dates of Himalayan leucogranites may 

be too young. Similarly, in detailed dating studies in the 

Cordillera, some U-Pb dates on vel)' high-U minerals from 

deformed protoliths give ages which are younger than the 

rocks which crosscut them. We draw the conclusion that the 

notion that closure temperature is independent of U 

concentration is probably incorrect. 

The dependence of Pb retentivity on U concentration is 

clearly evident in vel)' young minerals where the effects ofPb 

loss caused by time-integrated radiation damage is 

insignificant, and where there was a period of relatively slow 

cooling of the rocks following metamorphism at upper 

amphibolite facies conditions. Cogenetic grains which lose 

some component of their radiogenic Pb immediately 

following ccystallization will retain "concordant" U-Pb 

compositions, but will differ in age from neighboring grains 

depending upon their size and U concentration. Analysis of 

single grains reveals this natural effect, and causes significant 

ambiguity in the interpretation of the U-Pb systematics. It is 

possible that this effect is much more widespread than 

acknowledged, and it may account for small amounts of 

measured scatter in U -Pb ages in accessocy minerals from 

rocks of older orogenic belts. 
CRUST -MANTLE DIFFERENTIATION AND R.L. 

ARMSTRONG'S NO-CONTINENTAL-GROwrn MODEL 

PATCHETT P Jonathan, Dept. Geosciences, University 

of Arizona, Tucson, AZ 85721, USA 

From the time of its fmt proposal in 1968 (Annsttong, 

1968), the hypothesis of no continent growth has been a key 

element of the debate on crust-mantle evolution. Initially, 

Annsttong viewed the continental crust as unchanged in mass 

since 2.5 Ga, but later advocated a steady state since 2.9 Ga 

and even as early as 3.5 Ga. The Annsttong hypothesis has 

been ignored by many, but accepted as truth by some. There 

have been quite a small number of attempts to evaluate and 

test the no-growth hypothesis, or build it into other models. 

The main reason for the varied responses and the paucity 

of real evaluation of the no-growth hypothesis has been that 

the vast majority of geochemical and isotopic data for mantle 

and crustal rocks can be fit to either growth-dominated or nogrowth models. This was of colirse repeatedly pointed out by 

Armstrong (1981a,b; 1991). As noted by Annsttong in his 

last paper of 1991, the response was often to proceed with 

growth-of -crust modeling and ignore the unsolved conundrum 

posed by the no-growth model 

Attempts to incorporate the Armstrong model into mantle 

evolution (e.g. DePaolo, 1983; Jacobsen, 1988) have shown 

how recycling of sediments into the mantle figures into such 

calculations. The same circularity remains, however, in all 

cases. Attempts have been made to limit the degree of continent destruction via recycling of sediments using geochemical 

and isotopic tests (e.g. Patchett et al., 1984; Stevenson and 

Patchett, 1990; see also Taylor, 1990). These tests have 

resulted in geochemical and geological plausibility arguments 

against a pure no-growth scenario, but have left the precise 

degree of continent-to-mantle recycling open. Modeling of 

crust-mantle evolution without significant recycling has certainly reached maximum sophistication (e.g. All~gre et al., 

1983a,b). 

A pure no-growth model involves recycling of a mass of 

continent equal to the present one over geologic time, as well 

as subduction into the deep mantle of most sediment on the 

ocean floor (Armstrong, 1981). It seems fair to say that 

present data and plausibility arguments weigh against the nogrowth hypothesis in its end-member form. The extent of 

continent recycling into the mantle could still be very large, 

however. Thus the conundrum of growth vs. no-growth 

remains unsolved, and this problem will certainly outlive the 

late Dick Armstrong by many years. 

All~gre, C.J., Hart, S.R., and Minster, J.-F., 1983a, Earth 

Planet. Sci. Lett., v. 66, p. 177-190. 

All~gre, CJ., Hart, S.R., and Minster, J.-F., 1983b, Earth 

Planet. Sci. Lett., v. 66, p. 191-213. 

Armstrong, RL., 1968, Rev. Geophysics, v. 6, p. 175-199. 

Armstrong, RL., 1981a, Geochim. Cosmochim. Acta, v. 45, 

p. 1251. 

Armstrong, R.L., 1981b, Phil. Trans. R. Soc. Lond., v. 

A301, p. 443-472. 

Armstrong, R.L., 1991, Aust. J. Earth Sci., v. 38, p. 613-

630. 

DePaolo, OJ., 1983, Geophys. Res. LetL, v. 10, p. 705-708. 

Jacobsen, S.B., 1988, Earth Planet. Sci. Lett., v. 90, p. 315-

329. 

Stevenson, R.K., and Patchett, P.J., 1990, Geochim. 

Cosmochim. Acta, v. 54, p. 1683-1697. 

Taylor, S.R., 1990, Nature, v. 346, p. 608-609. 

244 

APPUCATION OF 10Jie TO LOESS STRATIGRAPHY OF 

THE LOWER MISSISSIPPI VALLEY, U.S.A. 

Pavich, M. I., U.S. Geological Survey, Reston, 

Virginia, Markewich, H.W., U.S. Geological Survey, 

Atlanta, Georgia, and Millard, H.T., U.S. Geological 

Survey, J;>enver, Colorado 

Dating of continental glacial deposits beyond the range of 14C is needed. Correlations of continental and marine oxygen 

isotope stratigraphy must be tested prior to accepting 

•Milankovitch • orbital forcing as the driver of Quaternary 

glacial fluctuations. Richmond and Fullerton (1986) proposed 

correlation of late Quaternary glacial deposits to marine 

oxygen isotope •ages. • This study tests the pre-Late 

Wisconsin and IDinoian age assignments made for Mississippi 

Valley paleosols developed in loess. Inventories of 

cosmogenic 1"Be are used to test age assignments. 1

".Be concentrations range from 1 ()6 atom/ g to 109 atom/ g 

in soil samples measured by Accelerator Mass Spectrometry. 

Beryllium oxide targets are produced with a known 

concentration of 9_Be and 10/9 ratios are determined by mass 

spectrometry. Analyses presented here were obtained from 

the Lawrence Livermore National Laboratory Center for 

Accelerator Mass Spectrometry. 1

".Be inventories (atom/em~ 

can be calculated for Soil horizons from (atom/g) X (g/cm3

) X 

(em of thickness), and inventories can be integrated for the 

entire soil profile. Assuming no inherited 1

"Be and no surface 

erosion, soil inventories divided by 1

"Be deposition flux 

(average = 1.3 X 1e>6 (atomlcm2 yr for 100 cmlyr rainfall) 

approximate the exposure period for the soil. 

Analyzed sites in Arkansas and Mississippi include loess 

exposures at Phillips Bayou on Crowley's Ridge, Arkansas 

and Y ocona River, Mississippi. Each has multiple subsurface 

paleosols. 

General observations for tqese sites are as follows. There 

is strong correlation of 10 Be with the paleosols in all of the 

profiles. 1

"Be concentrations are highest in Bt horizons but not 

well correlated with percent clay. The depth distributions 

support evidence from other studies that 1

"Be is highly 

immobile in slightly acidic to neutral pH soils. The high 

values of 1

".Be in the basal parts of the loess sections at 

Phillips Bayou and Y ocona River indicate that loess parent 

material commonly contains a concentration of about 200 X 

1e>6 atom/g. This result is consistent with recent data from 

loess in China (Chengde and others, 1992) and supports the 

conclusion that loess is not derived directly from glacial •rock 

flour. • This indicates that in the Mississippi Valley periglacial 

or post-glacial soil erosion contributed to the provenance of 

loess. Exposure duration calculations based on the assumption 

of a constant deposition flux of 1"Be of 1.3 X le>6 (atom/em~ 

yr, and corrected for the inherited component in loess, are 

very consistent with available 14C and thermoluminescence 

data for Late Wisconsinan (-20 ka) to Early Illinoian (-300 

ka) deposits. The last interglacial soil, the Sangamon, was 

exposed to 1

"Be accumulation for at least 70,000 yr. This 

indicates that the continental interglacial climate at this latitude 

(35° N.) spanned all of oxygen isotope stage 5 and not just 

stage 5e. 
CENTRAL EQUATORIAL PACIFIC HOLOCENE 

SEDIMENTATION. RATES FROM RADIUM-226 

ACTIVITIES IN MARINE BARITE 

PAYTAN. A., KASTNER, M., Scripps Institution of 

Oceanography, UCSD, La Jolla, CA 92093, USA, and 

MOORE, W. S., Department of Geological Sciences, 

University of South Carolina, Columbia, S.C. 29208. USA. 

226Ra in marine barite from the Equatorial Pacific is 

enriched by at least an order of magnitude over 230ofb 

activities (Borole & Somayajulu, 1977). Ifbarite acts as a 

chemically closed system for Ra, the decay of unsupported 

226Ra would allow dating of these barites and estimate of 

accumulation rates of the associated sediments. 

The 226Ra /Ba ratios of marine barite separated from the 

upper 25cm of a box core from the central equatorial pacific 

(0.6°N, 140°W) was determined by gamma counting. The 

Ra!Ba ratios of the bulk sediment were also measured and the 

data compared with their excess 230Th activities. 

Prelimiruuy results indicate that indeed barite in this core 

behaves as a closed system with respect to Ra, indicating that 

barite has not recrystallize nor exchanged with the pore water. 

The exponential decrease of the Ra/Ba ratio with depth in the 

upper 25cm of the core yields sedimentation rates of 3-

4cm/1000y, a reasonable rate for the central equatorial Pacific. 

The Ra/Ba in barite separated from the upper 10cm of the core 

is rather uniform indicating bioturbation. This ratio is lower 

than the Ra/Ba ratio of particulate matter arriving at the 

sediment water interface in this area as measured in sediment 

traps (Moore & Dymond, 1991). This difference is consistant 

with the decay rate of Ra and the calculated "age" of the 

mixed-layer using the above sedimentation rate. 

Sedimentation rate calculated using excess 230ofh activities of 

the bulk sediments in the same core yields approximately 

0.3cm/1000yr, an order of magnitude lower than the 226Ra 

based rates. This may reflect higher fluxes of 230ofh during 

the past 5-10 ky. 

A second core with lower sedimentation rates is being 

analyzed for comparison. 

Borole, D.V., and Somayajulu, B.L.K., 1977, Radium and 

Lcad-210 in marine barite: Marine Chemistry, v.S, p.291-296 

Moore, W.S., and Dymond, J., 1991, Fluxes of 22~ and 

Barium in the Pacific ocean: the importance ofboundry 

processes: EPSL, V.107, p.SS-68. 

245 

GEOCHEMICAL VARIATIONS INVANUATU ARC 

LAVAS: THE ROLE OF SEDIMENT SUBDUCTION 

AND VARIABLE MANTLE WEDGE COMPOSITION. 

PEATE. David W .. 170-25 Division of Geological and 

Planetary Sciences, California Institute of Technology 

Pasadena, CA 91125, USA, PEARCE, J. A., Dept. of 

Geological Sciences, University of Durham, Durham, 

DHl 3LE, UK, HAWKESWORTH, C. J ., Dept. of 

Earth Sciences, Open University, Milton Keynes, MK7 

6AA, UK, and COLLEY, H.C., Dept. of Geology, 

Oxford Brookes University, Oxford, OX3 OBP, UK. 

Sr- and Pb-isotope and trace element compositions of 

Vanuatu arc lavas vary systematically along strike. 

The central islands (Efate to Vanua Lava) have high 

87Sr f86Sr (0.7037-0.7045) relative to the rest of the arc 

(0.7031-0.7036). ENd, in contrast, shows limited 

variation (+6 to +8) throughout the arc. Pb isotope 

compositions are controlled by mixing between three 

distinct components; attributed to 'Sediment', 'PacificMORB' mantle and 'lndian-MORB' mantle. 

New major and trace element and isotopic analyses of 

sediments from the North Loyalty Basin (DSDP Site 

286) should allow slab-derived elemental fluxes to be 

quantified for the Vanuatu arc. This arc differs from 

other arcs in that the subducted sediments are 

dominated by volcaniclastic material. The Pb isotope 

composition of the 'Sediment' component recognised in 

the arc lavas (206Pbf204Pb -18.60, 207Pbf204Pb -15.58, 

208Pbf204Pb -38.60) is consistent with an average bulk 

composition for the sediment column at Site 286. This 

component is strongest in the central and southern 

islands (Tongoa, Efate, Tanna, Erromango) which are 

adjacent to the main sediment thickness (North Loyalty 

Basin) on the downgoing slab. 

The presence of both 'Pacific-' and 'Indian-' MORB 

mantle in the Vanuatu arc lavas has implications for 

the location of the boundary between these two major 

mantle domains, and for the mantle dynamics within 

the Vanuatu arc-basin system. The 'Pacific-MORB' 

mantle component (206Pbf204Pb -18.80, 207Pbf204Pb 

-15.53, 208Pbf204Pb -38.36) is similar in composition to 

South Fiji Basin basement. It dominates the southern 

(Anatom, Futuna) and northern (Ureparapara, Vot 

Tande) islands and probably represents the unmodified 

mantle composition underlying most of the arc. The 

'Indian-MORB' mantle component (206Pbf204Pb -18.10, 

207Pbf204Pb -15.49, 208Pbf204Pb -38.20) is found in the 

central islands (Merelava, Vanua Lava, Aoba, Gaua, 

Ambrym), and also in the active North Fiji Basin backarc spreading axis. The on-going collision and subduction 

of the D'Entrecasteaux Zone (since - 2 Ma) has led to 

significant tectonic disruption of this central region of 

the arc, which may also have caused the recent influx of 

new, perhaps deeper, 'Indian-MORB' mantle material 

from the back-arc region. 
EPISODIC MAGMATISM IN THE PERMOCARBONIFEROUS OSLO RIFT 

PEDERSEN, L.E.a, HEAMAN, L.M.b, and HOLM, 

P.M.a,aGeological Institute, 0ster Voldgade 10, DK-1350 

Copenhagen, Denmark, bGeology Dept., Royal Ontario 

Museum, 100 Queen's Park, Toronto, Ontario, M5S 2C6 

Canada. 

The Permo-Carboniferous Oslo rift in southern Norway is 

an an aborted continental rift in which both extrusives and 

intrusives are exposed today. 

Precise U-Pb zircon and baddeleyite ages have been 

obtained for two intrusive complexes in the Vestfold Graben 

Segment. The Skrim-Mykle complex covers ca. 450 km2 and 

the composition of the intrusions range from monzonite 

(larvikite) over syenite to peralkaline granite. The SiljanHvarnes complex outcrop over ca. 85 km2 and comprises 

monzonitic and syenitic rocks. 

Due to the steep slope of concordia for young ages, even 

small uncorrelated variations in the 207Pbf35

U and 206Pbf2~ 

ratios will have a large effect on the calculated 207Pbf206Pb 

ages. Therefore, the ages discussed below are averages of 3-4 206Pbf2~ ages for each sample, and must be considered 

minimum ages in the U-Pb system. All analyses are 

concordant within errors. 

In the Skrim-Mykle complex 3 samples were analysed. Two 

are from the composite Skrim larvikite pluton and one is of 

the likewise composite Mykle peralkaline granite intrusion. 

The two Skrim larvikites yielded mean ages of 280.8 ±0. 7 

Ma (2o) and 281.2 ±0.6 Ma, while the Mykle peralkaline 

granite is dated to 279.8 ±0. 7 Ma. 

Further south, in the Siljan-Hvarnes complex, 4 samples 

were analysed; the Odberg larvikite, Siljan nordmarkite, and 

the Vierod and Ostvann syenites. The Odberg larvikite, which 

is also from field evidence inferred to be the earliest emplaced 

intrusion in this part of the area, yielded a mean 206Pbf38U 

age of278.5 ±0.8 Ma. Two of the analyses of this sample are 

of baddeleyite. The Siljan nordmarkite yielded a mean age of 

278.6 ±0.6 Ma, while the Vierod and Ostvann syenites 

yielded 278.4 ±0.9 and 277.3 ±0.8 Ma, respectively. 

On the basis of these ages, two periods of intense magmatic 

activity can be recognised at ca. 281-280 and 279-277 Ma. 

Each took place within less than 2 Ma, with the Skrim-Mykle 

complex being slightly older than the Siljan-Hvarnes complex 

In addition, it is an interesting point, that not only larvikite but 

also peralkaline granite were emplaced in the Skrim-Mykle 

complex before activity began in the Siljan-Hvarnes complex. 

246 

DEVELOPMENT OF PRECAMBRIAN CRUST IN THE 

SOUTHWESTERN PART OF THE BALTIC SHIELD 

PEDERSEN. S. and KONNERUP-MADSEN, J., Institute of 

Geology, University of Copenhagen, 0ster Voldgade 10, DK1350, Copenhagen, Denmark. 

Approximately 80 % of the Baltic Shield in Central South 

Norway consists of magmatic rocks emplaced 1500-900 Ma 

ago. The regional tectonic framework of the area was formed 

during the Sveconorwegian/Grenvillian Orogeny. 

Detailed field work within key areas in conjunction with 

geochemical and Sr isotope analyses have provided new 

information on the magmatic crust-forming processes in the 

Baltic Shield. 

Calc-alkaline rocks (s.s.) are rare and belong to the oldest 

period of magmatic activity with ages up to 1350 Ma. 

Renewed magmatic activity between 1200 and 900 Ma 

reflects a largely continuous progressive evolution of crustal 

amalgation. 

The earliest magmatism (ca 1120 Ma) in this period includes 

acid intrusives and extrusives (high in silica, anorogenic in 

character, high FeO/MgO ratios, and Sri=.706 +/-2) and associated basic magmas (Sri=.7028-.7024). Later, basic magmas intruded into the still warm crust in several pulses separated by 

periods of deformation. 

The magmatic activity terminated with emplacement into a 

warm crust of K-rich magmas of alkaline affinity which form 

granodioritic-dioritic composite bodies and bimodal monzoniticgranitic complexes (age: 1025-900 Ma; Srii=:.7037-.7044), with 

associated rare mineral bearing pegmatites. Emplacement of 

individual bodies forming these complexes was separated by 

periods of deformation. 

The magmatic activity collectively reflects an increasing 

cratonization of the SW Baltic Shield. 
PROVENANCE STUDIES OF AUS1RALIAN DUNE 

SANDS USING COUPLED RADIOGENIC AND STABLE 

ISOTOPIC TECHNIQUES 

PELL S D, CIDVAS, A.R. and WILLIAMS, I.S., RSES, 

The Australian National University, Canberra, ACT 0200, 

Australia. 

This study utilises both single-grain U/Pb (SHRIMP) 

analyses of zircon separated from dune sand, and the 180 

values of bulk quartz to investigate the provenance areas for 

sands presently located in the Australian Continental 

Dunefield (ACD). 

Quartz-rich continental dunes of varying type and areal 

density cover an area of approximately 3.1 x 1o6 km2 or 40% 

of the surface of Australia. The largest of the deserts making 

up the ACD is the Simpson Desert covering an area of 1.3 x 

UP km2 in the northeast comer of central Australia. 

Three samples believed on the basis of preliminary 

characterization to be representative of the Simpson Desert as 

a whole have been selected for analysis of their U/Pb age 

distributions using the SHRIMP Ion Microprobe. It is intended 

to use the migration patterns of zircons to establish the scale 

and orientation of quartz sand dispersal in the same system. 

There are distinct differences in the source regions for sand 

from the three areas. Samples from both the northwestern and 

northeastern comers of the desert show largely bimodal 

population distributions with peaks centred on 1125 and 1750 

Ma, and 550 and 1750 Ma respectively. The third sample 

from the southeastern comer displays a greater spread of ages 

ranging between 100 and 3100 Ma. Comparing the age 

distribution of the samples with known ages of crustal 

material some preliminary comment can be made on sand 

source areas. The age range of the major population (1600-

1900 Ma) from the northeastern and northwestern Simpson 

correspond well with that of the Arunta and Mt. Isa Blocks, 

while the minor population (1000-1300 Ma) may be related to 

the Musgrave Block. The 250-550 Ma population from both 

the southeast and northeast of the desert can be attributed to 

sediment derived from the northeastern Australian granitoids. 

The presence of this younger granitoid material is strong 

evidence for the fluvial transport of material from ultimate 

source regions 1000 km east of the dunefield and 

perpendicular to, or against the direction of aeolian transport. 

The ~tso values of bulk quartz have been determined not 

only to investigate the source areas of sand, but also to 

examine the mixing trends between the different source areas. 

Preliminary ~tSO results range between 10.2 and 12.6%o 

(SMOW) and show significant variation with location. Such 

isotopic studies provide the means of carrying out provenance 

investigations on a much wider scale than is possible using 

zircon age dating, although on bulk samples and not 

individual grains. 

Results obtained in the study point to there being numerous 

local sources of sand to the desert areas. The suggested 

mechanism for the transport of this material. into the desert 

areas is via rivers and drainage channels. It is suggested that 

the effect of wind is only to shape dunes and not to transport 

sand significant distances. 

247 

CHARISMA, A NEW MICROFOCUS SECONDARY 

NEUTRAL MASS SPECTROMETER EMPLOYING 

RESONANCE IONIZATION 

PELLIN, Michael J. 1, MA, Zhiliu 1 , THOMPSON, Rita 

N.1.2, DAYIS Anclrew M 3, LEWIS, Roy S.3 , and 

CLAYTON, Robert N.2.3.4; 1 Materials Science and 

Chemistry Departments, Argonne National Laboratory, 

Argonne, IL 60439; 2nepartment of the Geophysical 

Sciences, 3Enrico Fermi Institute, 4 Department of 

Chemistry, University of Chicago, Chicago, IL 60637. 

Despite the tremendously successful use of conventional 

secondary ion mass spectrometry in geochemistry and 

cosmochemistry, this technique suffers from two important 

limitations: (1) the presence of unresolvable isobaric 

interferences (between Zr and Mo or between many of the 

REE, for example) and (2) the low ionization efficiency of 

some elements, such as siderophiles. We have developed a 

new instrument, named CHARISMA (CHicago-Argonne 

Resonance Ionization Spectrometer for Micro-Analysis), that 

uses tuned lasers to resonantly ionize sputtered or laser-ablated 

neutral atoms, allowing elemental selectivity and useful yield 

(atoms detected/atoms removed from sample) in excess of 

10%. A microscope employing reflected-light optics is built 

into the instrument, permitting samples to be viewed with subJ.llll resolution and allowing introduction of focussed light 

from a UV laser for laser ablation of samples a few J.llll across. 

Secondary neutrals can also be produced by sputtering using 

an ion gun with a 30° incident angle. Lasers tuned to resonant 

frequencies ionize neutral atoms immediately above the 

sample surface. The resulting ions are extracted normal to the 

surface and passed through a 4 m flight path reflectron-type 

time-of-flight mass spectrometer capable of a mass resolution 

(M/AM) of 104. Testing of the recently-completed instrument 

has demonstrated sub-J.llll imaging, laser ablation on few-J.llll 

spots with photoionization by a second laser (MIAM ~ 3000) 

and laser photoionization of Ar+ -sputtered neutral Mo (MI AM 

~ 2500). We anticipate significant increases in mass resolution 

as we gain familiarity with the tuning characteristics of the 

instrument. Planned future improvements to the instrument 

include installation of tunable solid-state lasers for resonance 

ionization. 

Great strides have been made in understanding 

nucleosynthesis in the past few years through microbeam 

analysis of interstellar SiC and graphite grains isolated from 

primitive meteorites. Ti, Zr and Mo are important for 

understanding a variety of nucleosynthetic processes. SIMS 

analyses of large SiC grains have been done for Ti, but 

analyses of Zr and Mo have not been done because of mutual 

isobaric interferences. We plan to extend Ti isotopic 

measurements to smaller SiC grains as well as TiC-bearing 

graphite grains. We also hope to initiate Zr and Mo isotopic 

analysis of individual grains. 
FIELD, GEOCHEMICAL AND GEOCHRONOLOGIC 

EVIDENCE FOR POLYCYCLIC VOLCANISM AT THE 

LATHROP WELLS VOLCANIC CENTER, 

SOUTHWESTERN NEVADA 

PERRY Frank Y , CROWE, Bruce. M., Los Alamos 

National Laboratory, Los Alamos, NM 87545, USA, 

WELLS, Stephen G., University of California, Riverside, 

CA 92521, USA, and McFADDEN, Leslie, D., University 

ofNew Mexico, Albuquerque, NM 87131, USA. 

The Lathrop Wells volcano in southwestern Nevada is an 

isolated, small-volume ( <0.15 km3

) basaltic eruptive center that 

has been studied in unprecedented detail as part of 

multidisciplinary volcanic hazard studies for the DOE's Yucca 

Mountain Project. Volcanoes of the Lathrop Wells type are 

generally interpreted to be monogenetic, representing the 

eruption of a single magma batch over a period of weeks to 

years. Erosional unconformities and soil horizon development, 

however, indicate that the Lathrop Wells center formed during 

four main eruptive episodes separated by significant 

(> 104 years) periods of inactivity (polycyclic volcanism). 

Multiple dating methods {

40Arf39

Ar, U-Th disequilibrium, 

cosmogenic He, thermoluminescence) indicate that eruptive 

episodes occurred at > 100 ka to <4 ka ago, but these methods 

do not yield consistent results in some cases. 

All four eruptive episodes erupted compositionally similar, 

evolved hawaiite with Mg numbers that cluster tightly at a 

value of54±1. In detail, all ofthe eruptive episodes can be 

distinguished by small, but systematic, differences in 

incompatible-element concentrations and ratios (e.g., La/Sm, 

Th/K). The different incompatible-element ratios at the same 

Mg number for different eruptive units cannot be related to 

each other by simple fractionation of a single magma batch. 

We have modeled recharge of a more primitive magma into a 

fractionating magma as a mechanism for buffering the Mg 

number of a magma at a steady-state value, while allowing 

continued evolution of trace-element ratios. Recharge/ 

fractionation of a single magma may account for differences in 

highly incompatible/incompatible-element ratios (e.g., La/Sm), 

but cannot account for differences in highly incompatible/ 

highly incompatible element ratios (e.g., Th/K). 

From differences in incompatible-element ratios, we 

conclude that a minimum of 6-8 magma batches were involved 

in the formation of the Lathrop Wells center, representing 

separate partial melts of a heterogeneous lithospheric mantle 

source. Systematic enrichment of La/Sm, ThiK and Th, and 

systematic depletion of Ti of the eruptive units with time may 

be related to changes in the mantle source or changes in the 

amount of partial melting through time. 

The ascent and eruption of multiple magma batches over the 

last 105 years at a single persistent location may be related to a 

penetrating, northwest-trending zone of right-slip shear. The 

pattern of past eruptions suggests that the location of future 

eruptions is unlikely to change in the short term, which 

decreases the probability that magma will directly penetrate the 

proposed repository site at Yucca Mountain,.located 18 km to 

the north, during the next 104 years. 

248 

ISOTOPIC AND TEXTURAL STUDIES OF MINERALS 

FROM THE CORDILLERA BLANCA BATHOUTH, PERU: 

IMPLICATIONS FOR CRYSTALLISATION PROCESSES 

IN LARGE GRANITOID INTRUSIONS 

Nick PETFORD, School of Geological Sciences, University 

of Kingston, Surrey, KT1 2EE, UK, and Barbara BARREIRO, 

Isotope Geoscience Laboratory, Keyworth, Nottingham NG 12 

SGG, UK. 

New isotopic data (Pb, Sr, Nd, 0, DIH) are presented on 

mineral separates from five young (ca. 6 Ma) granitoid rocks 

from the Miocene Cordillera Blanca batholith, Peru. The 

samples, previously analysed for whole-rock 87 SrJ86sr and 

143Nd/144Nd (0.7041-0.7057 eNd +0.16 to -2.46), show 

evidence at the grain scale for large degrees of mineral 

disequilibrium, inconsistent with simple (monotonic) cooling 

models for large intrusions. All of the variation in isotopic 

composition within a particular rock is bracketed by muscovite 

and sphene, which act effectively as compositional end members. 

Thus, the range in 206pbJ204Pb between muscovite and sphene 

in one sample (17.84-22.33) is similar to that recorded for the 

entire Andean volcanic chain. 

Muscovite textures, along with relatively low Ti contents 

(< 0.18 wt%) show them to be secondary in origin, consistent 

with their low 206pbJ204Pb ratios and younger Rb-Sr and 

40Ar/39Ar ages. The development of secondary muscovite can be 

related to late-stage fluid influx from the country rocks along 

shear zones during the waning stages of batholith emplacement. 

More problematical are the consistently higher 207pbJ204pb 

ratios inK-feldspars from all rocks, and the high Nd contents (up 

to 94 ppm) of some biotite micas. However, detailed optical 

examination (TEM, CL) shows them to contain numerous tiny 

inclusions of accessory apatite and sphene. We consider these 

included phases to be the cause of much of the apparent isotopic 

disequilibrium within samples. 
He, Ar AND Rb-Sr ISOO'OPE SYSTEMATICS OF FLUID 

INCLUSIONS IN QUARTZ, VEIN MINERALS AND 

NATIVE OOID FROM EPIGENETIC LATE ALPINE 

Au-VEINS 

~ 

IN NW ITALY 

DIAMOND1, L. W.; VILLA1, L M.; 

TOLSTIKJDN2,3, L N. AND KAMENSKy3, I. L. 

(1) University, lnsL of Mineralogy, Erlachstr. 9a, Bern, CH3012, Switzerland. (2) Dept. of Earth Sciences, University, 

Cambridge CB2 3EQ, U. K. (3) Geol. lnsL, Kola Scientific 

Centre of Russian Acad. of Sci., Apatity, 184200, Russia 

He, Ar and Rb-Sr systematics were investigated in Au-qtz veins 

(Brusson, Monte Rosa district) on qtz (He, Ar), carbonates (carb; 

He, Ar, Rb-Sr), native Au (He), bulk qtz inclusion fluids (RbSr) and muscovites (Rb-Sr, 40 Ar.39 Ar) respectively. The aim 

was to constrain modifications of the ore bearing fluid dwing its 

ascent, its age, and to estimate characteristics of its somce. 

3HeJ4He (always in *10-6) of all qtz + carb range between 

0.035-0.238. Meta-ophiolite hosted veinlets have 0.201-

0.238, their Rb-Sr (qtz+cc+ms) yield the fonnation age of 30Ma 

(confmned by 40Ar.39 Ar on ms) and the lowest Sri= 0.708. 

3HeJ4He of thick veins cutting various rock types all cluster 

between 0.084- 0.134 (qtz) and 0.06 - 0.035 (carb) and show 

higher and variable Sri (0.7095- 0.729). He concentrations are 

higher for samples with a more radiogenic signature. 

One freely grown Au-sample (300mg) was degassed in 5 

steps. Most He came at •700°C (fluid inclusion decrepitation?): 

3HeJ4He = 0.34±9, [4He] = 5.7nl STP/g, significantly higher 

than all qtz 3HeJ4He (0.084- 0.107) from the same vein. 

4HeJ40 Ar * varies, even within individual veins, from 1.3 

(veinlets in meta-ophiolite, late qtz from thick veins in metagranite) up to 23 (qtz from thick veins in metasediments and 

metagranite). 40Arf36Ar range between 1450- 2290 for the 

700°C step of qtz (most gas because of final fluid inclusion 

breakup), for associated dol between 1530 - 2350. Later cc is 

markedly lower (312- 496). 

The bulk He-Ar signature for qtz + carb is taken as the trapped 

signature, being aware of diffusive loss and radiogenic production affecting trapped gases. Given 30Ma, the low [U], ['Ib] and 

[K] in qtz + carb contribute less radiogenic 4He, 3He and 40Ar 

than our analytical error. Further, the retentivity for in situ produced He is lower than for trapped He. If all 4He* produced from 

U/Th decay in 30Ma was retained in the gold we need a trapped 

3Het4He of .. 0.6. 

In summary: (1) The Au-veins have qtz + carb with a radiogenic (crustal) He - Ar signature. 3Het4He from thick veins cutting different rock types are indistinguishable, but the corresponding Sfi vary with host rock type and vein thickness. 

(2) Higher 3HeJ4He in qtz + carb from veinlets in meta-ophiolite are interpreted as a host-rock signature. (3) The elevated 

3HeJ4He found in native Au points to a 3He input of a meteoric 

(or mantle?) componenL (4) 40 Arf36Ar for qtz of the free Au 

bearing vein are the lowest of all veins and support the idea of 

formation water, but not mantle, input with Au deposition. 

The fluid signatures show important and variable modifications of He - Ar isotopic and Rb/Sr elemental ratios. The results 

allow to exclude the metagranites (wrong direction of Sri evolution) and the meta-ophiolites (they increase 3He,I4He) as the 

fluid source for qtz + carb. The paragneisses remain as its most 

likely source rock. 

He isotopic disequilibrium between native Au and qtz + carb 

questions the use of petrographically oogenetic vein minerals to 

estimate processes responsible for Au deposition. 3HeJ4He of 

the native Au does not argue against a meta-ophiolitic source for 

Au, in contrast to qtz + carb. 

249 

THE MARINE OS-ISOTOPE RECORD OVER THE PAST 

85MA 

PEUCKER-EHRENBRINK. B., MPI fiir Chemie, Geochemie, 

Postfach 3060, 55020 Mainz, Germany, RAVIZZA, G., 

WHOI, Woods Hole, MA 02543, USA, HOFMANN, A.W., 

MPI fiir Chemie, Geochemie, Postfach 3060, 55020 Mainz, 

Germany. 

In order to extend the previously reported marine Os record 

back in time, we have measured the Os concentration and Osisotopic compositions of 25 Pacific metalliferous sediments. 

Analyses of a few samples of Cretaceous and Jurassic age 

yield 187ost186os values between 3.5 and 5.5. Re concentrations of several of these old samples have been measured. 

Resulting 187Ret186os values are lower (<55) than average 

crustal material (about 400) and age corrections to the 

measured Os-isotope ratios usually can be neglected (<0.1). 

Throughout the Cenozoic the observed isotopic variations are 

similar to existing data. 

There is a pronounced 187ost186os minimum at the Kffboundary, probably reflecting the enormous input of cosmogenic input into the oceans by the Kff-impactor. Bulk- and 

leach-analysis of the Kff samples from DSDP site 596 yield 

nearly chondri tic Os isotopic compositions ( 187 Ost186os 

about 1.25). Low 187ost186os values persist for several 

million years after the impact at site 596 but analyses of 

sediment leachates from additional cores suggest that the Os 

isotopic composition of seawater recovered more rapidly, to a 

187ost186os of about 3.2 by 63 Ma ago. These data also 

indicate that some portion of the extraterrestrial Os in site 596 

sediments is soluble during peroxide-leaching and therefore 

complicates our efforts to reconstruct the Os isotopic 

composition of seawater through time. 

Data from the early and middle part of the Cenozoic show a 

continued gradual increase in 187ost186os to about 6 at 15 

Ma. The significant scatter in the data and the fact that bulk 

analyses generally show a smoother trend compared with 

leachates indicates that the peroxide-leach does not liberate 

exclusively hydrogenous osmium. Alternatively, this scatter 

could be interpreted as an indication that the Os isotopic 

composition of seawater is heterogeneous during this time 

period. 

The period from 15 Ma to the present is characterized by 

good agreement of bulk- and leach-analysis and a gradual 

increase in the 187ostl86os from about 6 to approximately 

8.5 in recent marine sediments. If the scatter in the data prior 

to 15 Ma is not due to analytical problems, and therefore 

reflects either short-term variations or regional differences in 

the isotopic composition of seawater, the change in slope of 

the seawater evolution curve at 15 Ma could coincide with a 

transition from a heterogeneous to a well-mixed ocean with 

respect to osmium. 
CHLORINE-36 CHRONOLOGY FOR THE LATE 

PLEISTOCENE LACUSTRINE HISTORY OF 

PANAMINT AND DEATH VALLEYS, CALIFORNIA 

PHILLIPS. Fred M., and ZREDA, Marek G., 

Department of Geoscience, New Mexico Tech, 

Socorro NM 87801, SHARMA, P., and 

ELMORE, David, PRIME Laboratory, Physics 

Department, Purdue University, West Lafayette IN 

47907-1396 

Panamint and Death Valleys were the last two lacustrine 

basins in the pluvial paleo-Owens River system. Both of them 

show clear evidence of occupation by large lakes, in the form 

of shoreline features and lacustrine/evaporite sediments in drill 

cores. However, the deeply eroded state of the high 

shorelines indicates that the lakes predated the last glacial 

maximum. In this study we have analyzed cosmogenic 36Cl in 

a variety of shoreline materials, including beach gravels, 

boulders from wave-cut shorelines, and tufas. In addition to 

these, we analyzed ~1 in halite deposited out of the lake 

waters. 

Of all these materials, the tufas appear to give the most 

reliable results. Tufas on the upper edge of the shoreline 

features in Panamint Valley gave exposure ages close to 90 

ka, those near the bottom edge closer to 65 ka. These ages 

are in good agreement with U/Th age estimates by other 

investigators. The 36Cl measurements on the uppermost halite 

bed yield a deposition age of 90 + /-70 ka. These ages Indicate 

an effectively wetter climate in the area during the last 

interglaciation (isotope stage 5) and early Wisconsin than 

during the late Wisconsin maximum. 

Other shoreline materials gave systematically older 36Cl 

exposure ages than the tufas. We attribute this to 36Cl 

accumulated during the alluvial history of the clasts prior to 

the lacustrine reworking. Caution should be used in 

interpreting cosmogenic nuclide exposure ages from any 

shoreline materials that were not precipitated from the lake 

waters. 

250 

IN SITU-PRODUCED 14c IN LA1E QUATERNARY LAVA 

FLOWS, WES1ERN UNITED STA1ES 

PHll.LIPS. W. M .. LIFTON, N. A., QUADE, J., JULL, A. 

I. T., Department of Geosciences, University of Arizona, 

Tucson.~ 85721 

The buildup oftn situ-produced radiocarbon (14cis) is 

potentially useful for dating geomorphic surfaces exposed over 

the last 2000 to 20,000 years. We have developed methods to 

remove contaminant 14c and quantitatively extract 14qs 

from basalts and quartzites (Lifton and others, 1993). These 

procedures do not require mineral separates and should work 

with any non-carbonate rock. We are testing our techniques 

on late Quaternary basalt flows from the western United 

States. These include the McCartys, Bandera, and Twin 

Craters flows, El Malpais field, New Mexico; Blue Dragon 

flow, Craters of the Moon field, Idaho; and the Tabernacle Hill 

flow, Utah. In addition, we are applying 14cis methods to 

the problem of dating the Lathrop Wells volcanic field, 

Nevada. Our samples are from pahoehoe flows with clear 

indication of original surfaces. Surface samples consist of the 

upper 2 to 4 em of unshielded, horizontal pahoehoe slabs with 

less than 1 em erosion and no evidence for shielding by soils, 

aeolian sediments, or nearby objects. Shielded samples greater 

than 2 m below flow surfaces were also taken from lava tubes 

and road cuts. The shielded samples test our ability to remove 

contaminant 14c and to detect subsurface 14c production 

mechanisms. Except for Lathrop Wells samples, the flows are 

dated by one or more conventional radiocarbon dates and 

range in age from about 17,800 to 2000 years old. Preliminary 

results indicate that shielded samples contain less than 7 x 1o4 

14c atoms gm-1. This suggests removal of nearly all 

contaminant 14c and that subsurface production mechanisms 

are unimportant. A surface sample from the McCarty flow 

contains 2.16 x 105 14c atoms gm-1. The McCarty flow has 

two AMS dates on charcoal from rootlets at the base of the 

lava that give ages in calibrated ye:ars bp of 3332 - 3292 and 

3271- 3074 (1 a). The apparent 14qs production rate is 37 ± 

10 14c atoms gm-1 Si02 (normalized to rock surface, sea 

level , and latitude > 60°; rock density 2.5 ± 0.2 g cm-3, 

absorption mean free path 160 ± 15 g cm-2, geographic 

latitude 35.08°). This is significantly greater than the 17,800 

year integrated rate of 19 ± 3 atoms gm-1 reported by Ufton 

and others (1993). Further work is needed to determine if this 

variation in production rate is real. 

Lifton, N. A., Phillips, W. M., Jull, A. J. T., Quade, J., 1993, 

Integrated late Quaternary production rates for in situproduced cosmogenic 14c from Lake Bonneville shoreline 

features using new extraction techniques: EOS, v. 74, no. 

43,p. 649. 
111Papl$U DISEQUILmRIUM IN VOLCANIC ROCKS 

PICKEIT. David A., and MURRELL, Michael T., CST8, MS 1514, Los Alamos National Lab, Los Alamos, NM 

87545. ·110096@incdp3.lanl.gov 

231Pa is the only long-lived (~12 =32.8 ka) intermediate 

nuclide in the 235U decay chain. Radioactive disequilibrium in 

this parent-daughter pair in magmas, i.e., PaJ235U) ¢ 1, 

indicates recent fractionation between two highly incompatible 

elements, as in the 2»fhf238U and 226Raf2»rb systems. We 

have measured (

231PaJ235U) in young volcanic rocks by IDTIMS in an attempt to better understand such disequilibria. 

Published data on MORB by Goldstein et al. (EPSL 115, 151) 

show large Pa enrichments, with PaJ235U) = 2.36-2.89, 

where ~U) = 1.13-1.25; these disequilibria have 

proven useful for age-dating (Nature 367, 157). We have 

recently reported (Eos 74, no. 16, 341) that Pa is markedly 

more incompatible that U in andesite, qualitatively consistent 

with Pa enrichment relative to U in melts. 

We present data on 31 rocks of various tectonic settings 

and compositions (included are three analyses by R. Williams). 

With few exceptions, young volcanic rocks have 1 PaJ235U) 

> 1, PaJ235U) > ~U) (i.e., Pa/U fractionation is 

greater than ThiU as in MORB), and show correlation 

between the two ratios. Compared to MORB, intraplate 

basalts have smaller, but still high, Pa!235U) ratios, 

indicative of fundamental differences in evolution. Ratios in 

Iceland tholeiites (1.9, 2.0) are much higher than in a Hawaii 

tholeiite ( 1.1 ), perhaps reflective of higher melting rates in the 

latter (see Beattie, Nature 363, 63). An Iceland basalt with 

other evidence for crustal contamination has appropriately 

lower PaJ235U) of 1.2. Among three Hawaii basalts 

ranging from tholeiite to basanite, there is a corresponding 

increase in (

231Paf235U) and {2»fh.J238U) (Sims et al., this 

volume) that is likely due to varying degrees of partial 

melting. Ratios of up to 2.0 in continental basalts argue 

against models invoking long (compared to 33 ka) crustal 

residence to explain their near-equilibrium (2»fh.J23~ values. 

Among arcs, island arc lavas tend toward higher 

Paf23 U) than those on continental margins, but both suites 

show a rough correlation between Pa!235U) and 

{2»fh.J238U). This could be explained by U enrichment 

relative to both Th and Pa, consistent with a slab-derived fluid 

component. A Tonga andesite is the only non-carbonatite to 

show Pa depletion (0.93), perhaps due to this component. It 

is notable, however, that arcs typically still have (231Paf235U) 

> 1 when ~) < 1, either arguing against U 

enrichment or arguing for the dominance of the melt-related 

(?) Pa enrichment over the fluid-related U enrichment. 

Regardless of the process, the patterns suggest that the wellnoted tendency of arc rocks for near-equilibrium (2»fh.J23~ 

(e.g., Condomines and Sigmarsson, GCA 57, 4491) is 

fortuitous and not due to a component in secular equilibrium. 

The generally high ~~ PaJ235U) ratios noted in most 

samples, and especially in MORBs, are difficult to reconcile 

with current melt evolution models. Nevertheless, the 

Paf23 U) system promises to greatly enhance studies based 

on {2»fh.J238

U) and ~aJ230rb) disequilibria for two reasons: 

it is more sensitive to magmatic processes than the former and 

approaches equilibrium less rapidly than the latter. 

251 

CALIBRATION OF ZIRCON STANDARDS FOR THE 

CURTIN SHRIMP II. 

PIDGEON R T., FURFARO, D., KENNEDY, A.K. 

NEMCHIN, A.A., VAN BRONSWJK,W., Curtin 

University of Technology, Kent Street, Bentley, 6012, 

Western Australia and TOOT, W.A., Max Planck Institute 

for Chemistry, Mainz, 55020, Germany. 

A SHRIMP II has recently been installed at the Curtin 

University of Technology. A most important task in developing 

a capability to analyse zircons using this ion microprobe is the 

preparation of zircon standards. We are approaching this task in 

two ways. Firstly we are investigating large homogeneous 

zircon crystals as suitable standards and secondly we are 

investigating the suitability of zircon populations from certain 

rock types The requirements for a standard are that it has a 

suitably high concentration of U and Pb and has concordant UPb systematics. Also there needs to be enough of the standard to 

allow three or more years ion microprobe analyses, assuming 

that a piece of the standard is included in every mount as is the 

present practice at the ANU. Our primary standard zircon CZ3 

has been prepared from a Sri Lankan gem zircon weighing just 

under one gram. It has no observed zoning and XRD and Raman 

spectroscopy analyses confirm that the structure of this crystal is 

relatively undamaged by long term radiation. Conventional 

isotope dilution measurements on chips taken from various 

parts of the crystal show it to have a concordant age of 564Ma. 

SHRIMP measurements on the polished surfaces of a number of 

chips show these are homogeneous in Pb/U ratio and has aU 

concentration of 530-560ppm. In addition to CZ3 we are 

investigating a number of ca 2.6 Ga zircon populations which 

have crystallised or recrystallised under granulite facies 

conditions as potential standards. 
NEOPROTEROZOIC ISLAND ARC ROCKS OF MARA 

ROSA, CENTRAL BRAZIL: PRELIMINARY U-Pb AND 

Sm-NdDATA. 

PIMENTEL,M.M., Institute de Geociencias, Universidade de 

Brasilia, Brasilia, 70910-900, Brazil, WHITEHOUSE, M.J., 

Dept. ofEarth Sciences, University of Oxford, Oxford, OXl 3PR, 

U.K., MACHADO,N., Geotop, UniversiteduQuebecaMontreal, 

Montreal, H3C 3P8, Canada, FUCK,R.A.,Instituto de 

Geociencias, Universidade de Brasilia, Brasilia, 70910-900, 

Brazil, VIANA,M.G., Institute de Geociencias, Universidade de 

Brasilia, Brasilia, 70910-900, Brazil. 

The Precambrian Mara Rosa sequence in northern Goias, central 

Brazil, includes metavolcanic (mafic to felsic) and 

metasedimentary rocks which have geochemical characteristics 

compatible with an origin in an island arc system. The sequence 

occurs in several NNE belts, separated from each other by 

tonalitic/granodioritic orthogneisses. In this study we report 

preliminary zircon and titanite U-Pb geochronological data and 

whole-rock Sm-Nd results for the Mara Rosa rocks. The results 

are as follows: (i) calc-alkaline orthogneissesyielded a zircon age 

of856 + 13/-7 Ma and ~(T) of +4.6, with T0M model age of ca. 

1.0 Ga; (ii) felsic metavolcanics from the Posse gold mine have 

a concordant zircon age of862 +/- 8 Ma. ~d (T) is positive ( + 3. 7) 

and the ToM model age is also ca. 1.0 Ga. Titanites from the same 

sample revealed a recrystallization age of 632 +/- 4 Ma which 

may represent a good estimate for the age of Au mineralization; 

(iii) a small syn-tectonic dioritic body emplaced into the Mara 

Rosa metavolcanics has a concordant zircon age of630 +I- 3 Ma. 

~d (T) forthisrockis +2.1 and T0Mmodel age is ca. 1.0 Ga; (iv) 

the Amadorpost-orogenictwo-micaleucogranite (<ca. 630 Ma) 

has a slighlty older ToM model age ( ca 1.2 Ga) and a negative ~d 

(600) of -2.1, compatible with a more "enriched", perhaps 

metasedimentary, source; (v) east of the Mara Rosa terrane, a 

sample ofthePaudeMel granite-gneiss, from the sialic basement 

of the Serra da Mesa supracrustals (pelite-psamite-carbonate 

association) yielded a zircon age of2175 +12/-9 Ma. 

The data show that the Mara Rosa rocks were accreted to the 

continental crust during the Neoproterozoic and do not represent 

Archaean granite-greenstone associations, as previously thought. 

The older age of the Pau de Mel granitoid suggests that it is part 

of the continental foreland to which the island arc terrane was 

accreted. 

The U-Pb and Sm-Nd isotopic/geochronological patterns 

obtained are very similar to those previously identified in similar 

metavolcanic-orthogneiss assemblages in western Goias (Pimentel 

&Fuck 1992), demonstratingthatthe products ofNeoproterozoic 

crustal accretion processes along island arc systems are exposed 

over very large areas in central Brazil. The data show that the 

Brasilia fold belt includes an extensive area of juvenile crust and 

does not represent an ensialic orogenic belt. 

Pimentel,M.M., and Fuck,R.A., Neoproterozoic crustal 

accretion in central Brazil: Geology,20(4):375-379. 

252 

GEOCHEMICAL AND ISOTOPIC (Sr. Nd) MODELUNG 

OF OROOVICIAN INfRUSIVES fROM SERlE DEI LAGHI 

(S-ALPS) 

PINARELLI. Laura. lstituto di Geocronologia e 

Geochimica Isotopica. C.N.R.. Pisa. Italy. 55100. and 

PEZZOTTA. F .• Dip.to Scienze della Terra. University of 

Milano. Milano. Italy. 20100. 

Granitoid rocks belonging to an Ordovician magmatic cycle 

are present in many areas of the European Paleozoic basement. 

Calc-alkaline intrusive rocks attributed to this cycle (wholerock isochron age of 466±.5Ma). occur in the central-western 

segment of the Alpine arc (Serle dei Laghi). Hercynian 

amphibolite facies regional metamorphism affected the Serle 

dei Laghi basement. 

This paper deals with the petrogenesis of the Ordovician 

intrusives of the Serle dei Laghi using mineral chemistry. 

major and trace elements. REE and isotope geochemistry (Sr 

andNd). 

Four lithologies are present: Horneblende-bearing 

Metagranitoids (HM); Biotite±_muscovite-bearing 

Metagranitoids (BM); (meta)Aplites and Microgranites (AM); 

Porphyritic Metaleucogranites (PM). 

The mineral chemistry data evidence incomplete 

mineralogical modifications. indicating that the whole rock 

geochemistry of the studied samples was unaffected by the 

metamorphism. 

As a whole. the intrusives studied range from gabbrodioritic to granitic in composition. Major elements. as well as 

Co. Cr. Ni. Sc. and V. show mostly linear trends in the 

variation diagrams versus Dl. UIE and HFSE. instead. do not 

show a regular distribution along the series. Indeed. most trace 

elements have bell shaped trends. the PM samples having 

opposite trends with respect to HM and BM. AM show largely 

scattered plots. 

REE patterns show moderate LREE enrichement. and a 

progressive increase of the negative Eu anomaly and HREE 

fractionation from gabbro-diorite to granites. AM show higly 

variable cross-cutting RFE patterns. 

Initial Sr and Nd isotope ratios vary considerably along the 

series from the more mafic ((87srJ86Sr)i = 0.70432; eNd = 

-0.72) to the more felsic ((87sr,s6Sr)i = 0.71232; eNd= -6.64) 

samples. These data indicate a mantle-derived parental magma. 

which evolved towords more crustal characteristics. 

On the basis of trace element distribution and Sr and Nd 

isotopic covariation. a model of combined assimilation and 

fractional crystallization (AFC) is proposed for the maficintermediate part of the series (HM and BM). For the 

geochemical evolution of the more felsic samples (PM). an 

incomplete unmixing model between crystals (Kfeldspar+plagioclase) and residual melt could be inferred. 
SR- 0 -ISOTOPE SYSTEMATIC OF 

ALKALINE ROCKS OF RUSSIA 

POKROVSKY B.G. Geological 

Institute, Russian Academy of 

Science, Pyzhevsky per.,7, 

Moscow 109017, Russia 

Rb-Sr systems and stable 

isotopes composition have been 

studing in the massifs of alkaliultrabasic rocks, alkali-gabbro, 

nepheline sienites, ultrapotassic 

pseudoleucitic sienites, rare metal 

alkaline granites and alkaline 

extrusives of different regions of 

Russia. The isotope data indicate 

that alkaline rocks parental magmas 

were derived in th~ cases from 

depleted mantle source and then 

contaminated by crustal material in 

the magmatic chamberes and 

conduits. Rocks of similar 

chemical composition (such as 

pseudoleucitic sienites for example) 

have different 0- and Sr-isotope 

compositions and cannot be connected 

with any uniform mantle source. 

Both high 180 (6180>7), and low 180 (6180<5) rocks were found. The 

former occurred in the 

sedimentary carbonate host, and 

the latter occurred in the 

precambrian metapeiites. The 

form of dependence between Sr- and 

0-isotope ratios, and between 

isotope and elemental composition 

of alkaline rocks suggest that 

interaction of mantle magmas with 

high Sr brines is responsible for the 

isotope variations. 

Examples of the strongest 

contamination are urtitesanchimonomineralick nepheline rocks 

of magmatic origin, widespread in 

the South Siberia. On their isotope 

composition: (

87Sr/86Sr) 0 = 0.7058 ~ . 

0.7075; 6180 = 9.4 ~ 12.1; w.r. · 

61 8 0 = 9 . 0 + 11 . 5 ur tit e s are px 

close to nepheline-mel il i te scarns 

which form 

at the contact of 

gabbro with sedimentary carbonates. 

Material balance calculation 

indicate that up to 60-70% of Sr 

and 40-50% of 0 in the urtites were 

extracted from surrounding rocks. 

253 

A STUDY ON CRUSTAL EVOLUTION AND 

TECTONIC SETTING APPLYING RB/SR, SMIND 

AND TRACE ELEMENT ANALYSES (SILVRETTA, 

SWITZERLAND) 

POLLER. U. & LIEBETRAU, V ., Instit. of Min. & 

Petrography, Fribourg, 1700, Switzerland, NAGLER, T., 

Lab. for Isotope Geology, Instit. of Min. & Petr., Bern, 

3012, Switzerland , MAGGETTI, M. Institute of Min. 

& Petrography, Fribourg, 1700, Switzerland· 

The Silvretta cristalline complex, one of the upper austroalpine nappes, is situated in the eastern part of Switzerland and 

the austrian regions Tirol and Vorarlberg. Beside amphibolites, 

gabbros, eclogites and paragneisses a large variety of orthogneisses exists. Generally they are devided into two groups: the 

Fliielagranitic Association and the so-called Older Orthogneisses including the Monchalpgneiss. 

The Monchalpgneiss is classified as metagranodiorite to 

metagranite and the Fliielagranitic Association as former 

monzo- to syenogranites. Conforming to the common definition of granitoids an A-type characteristic can be excluded for 

all samples by trace element diagramms. The analyzed Silvretta 

gneisses show S-type characteristics in the diagrams of Hine et 

al. (1978) as well as in the more recent ones of Castro et al. 

(1991). 

The original tectonic setting of the rocks was evaluated 

based on trace element-plots (e.g. Pearce et al. 1984). For most 

samples the within plate granites (WPG) and the ocean ridge 

granites (ORO) can be negated using Nb, Y and Si. Only a few 

orthogneisses of the Fliielagranitic Association plot inside the 

WPG field because of exceptional high ·yttrium contents. 

Plotting Rb against Y +Nb the Monchalpgneisses clearly 

occupy the volcanic arc (VAG) field, whereas the Fliielagranitic Association plots in VAG and syn-collision fields. 

REE patterns from all analysed Silvretta gneiss types give a 

good conformity with VAG. The tectogenesis of the 

investigated orthogneisses will be discussed with special 

regards to the geochemically different influences of the 

sedimentogenesis and the last crystallisation. · 

Sm-Nd model ages from petrographically different, but 

chemically similar gneisses of both groups cluster in a narrow 

range around 1.7 Ga. However, samples from the xenolith 

bearing and geochimcally exceptional type "Urezzas" show Nd 

model ages from 1.2 to 1.5 Ga. A xenolith sample again 

yielded= 1.7 Ga. These results most probably reflect a mixture 

of 1. 7 Ga old crust with variable amounts of younger crustal 

components. This model is supported by reinterpretated Rb-Sr 

whole rock data of granodioritic to gabbroic rocks and Nd 

model ages of metatonalites. 

At the present state of investigations the origin of the 

Silvretta orthogneisses is interpreted as sediments that were 

remelted - possibly in an Volcanic Arc environment - and 

formed granitic rocks with a mean crustal residence age of 1. 7 

Ga. A subsequent addition of juvenile crust is only reflected in 

parts of the Fliielagranitic Association. 

Castro, A. et al., 1991, H-type (hybrid) granitoids: a 

proposed revision of the granite-type classification and 

nomenclature, Eart Sci. Rev., v. 31, p. 237-253 

Hine, R. et al.,1978, Contrast between I- and S-type 

granitoids of the Kosciusko Batholith: J. Soc. Austr., v. 25, p. 

219-234 

Pearce, J.A., Harris, N.B.W., Tindle, A.G.,1984, Trace 

element discrimination diagrams for the tectonic interpretation 

of granitic rocks: J. of Petr., v. 5/4, p. 956-983 
NEW METHODOLOGICAL APPROACH IN 

INVESTIGATION OF EVOLUTION OF 

PRECAMBRIAN OCEAN ISOTOPE COMPOSITION. 

PONOMARCHUK, V.A., TRAVIN ,A.V.,. 

KISELEYA Valentina.Yu ,MOROZOVA,I.P., 

KUZMIN ,D.S.; United Institute of Geology, 

Geophysics and Mineralogy, Siberian Branch, Russian 

Academy of Sciences. University Ave.3, Novosibirsk, 

630090, Russia. 

Absense of an exact time coordinate for particular 

carbonate sample seems to be one of the base problems to 

be solved for investigation of the isotope-geochemical 

time-chronikle of the Precambrian ocean. For decision of 

this problem the direct isochrone K-Ar dating of 

carbonate samples (dolomite, limestone) was devised. 

For examination of this approach carbonate samples 

(stromatolites) from the Olenekskoe rise columns 

'North-East Siberian platform) were investigated. 

Moreover, using conventional methods, the o13 C, 

6180, 87 Sr/ 86 Sr isotope compositions on the same 

samples were determined. The Rb,Sr concentrations and 

the 87 Sr/ 86 Sr ratio were determined on dissolvable and 

undissolvable components of stromatolites. Coincidence of 

the 87 Sr/ 86 Sr ratio on dissolvable and undissolvable 

components was additional criterion for choice of samples 

siutable for reconstruction of the isotope-geochemical 

evolution of the Precambrian ocean. The ages measured 

are between 1100 Ma and 1400 Ma. 

The o13 C and o18 0 (for this age range) values are 

from -1.03 to 0.35%0 (PDP) and from -8.1 to- 9.4 %o 

(PDP) respectively. The 87 Sr/ 86 Sr ratio measured is 

equal to 0.70519+/-14(95%> forage 

1118+/-28Ma(95%), 0.70554+/-23 for 1143+/-20 Ma, 

0.70516+/-24 for 1151+/-20 Ma and so on. 

Thus, using the direct isochrone K-Ar dating of the 

most "clean" carbonate samples,the reconstruction of "fine 

structure" of the Precambrian ocean isotope record 

appears to be real. 

254 

ESR DATING OF BURNED FLINT FROM THE HOMINID 

BEARING T ABUN CAVE, ISRAEL - THE ISOCHRON 

METHOD 

PORAT, N., Israel Geological Survey, Jerusalem 95501, 

:rsraer;-scHW ARCZ, H.P., Geology Department, 

McMaster University, Hamilton, Ontario L8S 4M1, 

Canada, and RONEN A., The Zinman Institute of 

Archaeology, Haifa University, Haifa 31905, Israel. 

Two dating methods have been used in recent years for 

constructing a time scale for human evolution, electron spin 

resonance (ESR) and thermoluminescence (TL). Both methods 

are based on similar physical principles and are applicable to a 

wide range of materials from archaeological sites over the time 

range from 1 to 2,000 ka. TL has been used to date burned 

flint, while ESR has been applied mainly to tooth enamel. 

Recent studies have demonstrated the feasibility of applying 

ESR dating to burned flint, and here is a further example of its 

applicability. Both ESR and TL signals are reset by beating to 

T > 400°C and grow back with time due to natural radiation. 

ESR and TL ages are calculated from the ratio Dald. De, 

the equivalent dose, is the amount of natural radiation the 

sample had absorbed since the last resetting event and is measured in the lab using artificial irradiation. d, the annual dose 

rate, is the natural radiation emitted by the sample and its environment per year. It can be measured directly or calculated 

from U, Th and K contents in the sample and sediment. 

Tabun cave is situated at the foothills of Mt Carmel, Israel. 

Neanderthal hominids were found in Layer C and its archaeological sequence has been used extensively for Late Pleistocene 

correlations throughout the Levant. Burned flint from the site 

was dated by TL (Mercier et al., in press), and tooth enamel 

was dated by both ESR and U/Th methods (McDermott et al., 

1993). However, the TL ages are almost twice as old as the 

ESR and U/Th ages, which are in a good agreement. Different 

external annual dose rates (deJ used for TL and ESR age calculations can explain this discrepancy. Uncertainties in the dcx 

could be caused by variations over time, but should be the 

same for samples buried in the same environment. Also, lab 

measurements of small sediment samples may not represent the 

true des.• 

To overcome uncertainties in des. we used here the isochron 

approach. Different flint samples have different U contents, 

which would cause variations in the internal annual dose rate 

(dw) and as a consequence in the D8 • When De is plotted 

against din, coeval samples which were exposed to the same dex 

should fall on a line, whose slope is proportional to the age and 

its intercept is the dex• 

Five burned flint samples from the top of level E in Tabun 

were analyzed. The AI signal was used to estimate the De, and 

din was calculated from U, Th and K contents in the samples. 

The 5 samples form an isochron, giving the age of 177 ±34 ka 

and a dex of 400± 130 p,Oy/a. The age is close to ages obtained 

for level E using ESR on teeth (EU age of 154±34 and LU 

age of 188±31 ka) and U/Th on teeth (164±4 ka), but differs 

substantially from the average TL age for burned flint from the 

same level (296 ± 18 ka). des. is similar to that used in the ESR 

age calculations (495 p,Gy/a), which was evaluated using sediments adhering to the teeth. Our dex differs from that used for 

the TL age calculations (710 p,Oy/a), which was measured 

directly in the field. We conclude that the age of the top of 

level E is in the range of 160-190 ka, and that the isochron 

method can be used when dex is not well constrained. 

McDermott F., Griin R., Stringer C.B., and Hawkesworth C.J., 

1993, Mass-spectrometric U-series dates for Israeli 

NeanderthaVearly modem hominid sites: Nature 363, 252-255. 

Mercier N., et al., in press: Bar Yosef 0., and Kra R., (eds), Late 

Quaternary Chronology and Paleoclimates, Peabody Museum, 

Harvard. 
Xe ISOTOPES IN A STEADY STATE UPPER MANTLE 

PORCELLI. D. and WASSERBURG, G.J., The Lunatic 

Asylum, California Institute of Technology, Pasadena, CA 

91125 USA 

We present a model of steady-state transport of Xe through 

the upper mantle. The model is explored using the full mass 

transport equations for each isotope of Xe and is coupled with 

the steady state model for He [1]. Upper mantle Xe reflects 

the isotopic characteristics of external sources altered by 

internal production of radiogenic Xe, in contrast to the 

standard model [2] where upper mantle Xe is residual from 

atmosphere removal. The mantle is divided into two 

reservoirs; a lower mantle (P) that has evolved as an 

approximately closed system, and an upper mantle (D) of mass 

(Mn). Xe fluxes into the upper mantle are by mass transfer 

from Pat hotspots (Mpn) (where a fraction r is degassed) and 

from the atmosphere by subduction. Xe leaves the upper 

mantle at mid ocean ridges and at hotspots. The silicate Earth 

initially had uniform U, Pu, I, and Xe concentrations and 

(Pu/U)0 =0.0068. We consider the generation of 136xe 

excesses by both U and Pu in P and D. We assume that 1291 

was present in P (in unknown abundance) and is the source of 

the observed 129xe excesses in D. The lower mantle Xe 

concentration (13°Cp) and isotopic composition are unknown. 

Radiogenic 136xe in D is the sum of 136xe generated in P 

that is dominated by decay of now extinct 244Pu, and 136xe 

generated in D by the decay of U. For upper mantle Xe, the 

relative proportion of 136xe* derived from D to that derived 

from P is osp= 13 ~n uCnMn/[ 136*Cp(l - r)Mpn], with 

uCn=U concentration in D, 13 ~p = 1.4 x 104 atoms, 136Xe*/gU-yr, 136*Cp=3.3x107atoms, and uCnMn/(1-r)Mpn 

= 6.3x1o2 from consideration of steady state He fluxes in D 

[1], so that osp=0.27. Observed excesses of 129Xe and 136xe 

(relative to atm) in mantle-derived materials are positively 

correlated [2]; we interpret the range to reflect different 

degrees of atmospheric contributions to a single D component, 

and a value of 1361130Xen =2.45 is used here (along with 

1291130Xen=7.35). The Xe isotopic composition of D and P 

can be related by 

osF = [t36/130xen _ t36/t30xe ]/[136/130xep _ t36/130Xeo] 

Using 136113°xe0 = 2.075 [3], 1f61130Xep = 2.37. For closed 

system evolution of P, this requires 13°Cp = 1.2x108 atoms/g. 

One consequence of the model is that because of the 

differences in isotopic composition between P and the 

atmosphere, the atmosphere has been formed largely from a 

source other than P material. From an Earth originally 

uniform in Xe isotopic composition and concentration (equal 

to values obtained for P) 1.2 x 1035 atoms 130Xe were 

contributed by D to the atmosphere. This is 32% of the 

present atmospheric 130xe inventory. Simple mixing 

calculations indicate that the remaining Xe must have had 

136/130xe = 2.094 and 1291130Xe = 6.094. These values are 

similar to proposed initial atmospheric values of 2.075 +0.006 

and 6.061 +0.045 respectively, obtained from meteorite 

compositions [3], and suggest that the additional Xe was 

unradiogenic. Div. Contrib. 5364(838). 1. Kellogg & 

Wasserburg (1990) EPSL 99, 276. 2. Allegre et al. (1983) 

Nature 303, 762. 3. Pepin & Phinney (1976) Lunar Sci. VII, 

682. 

255 

3He SURFACE EXPOSURE AGES AT THE 

LATHROP WELLS, NV, VOLCANIC CENTER. 

POTHS. J.; PERRY, F.; CROWE, B.M.; 

Los Alamos National Laboratory, 

Los Alamos, NM, 87545. 

We have determined cosmogenic 3He 

surface exposure ages of samples from three 

eruptive units at the Lathrop Wells volcanic 

center. The units have been divided into four 

eruptive episodes from field relations, verified 

by trenching of contacts. Geochemical and 

petrology studies show that each episode is 

distinct and probably represents volcanic 

eruptions of short duration. Our goal is to 

determine the timing of these eruptive episodes. 

Dates are given in the table below, and 

generally fall in stratigraphic order (011 the 

oldest, 013 the youngest). Despite evidence 

for several meters of ash cover, surfaces from 

the 011 A flow yield reproducible dates within 

analytical uncertainty. Apparently the ash was 

stripped from these surfaces in a time that was 

short compared to the uncertainties ( s 5ka). A 

related (011 B) and a stratigraphically younger 

flow (012) are identical to 011 A · within 

uncertainty. The somewhat younger dates for 

011 D and Os 1 may reflect thicker ash cover at 

their locations. Dates for bombs and scoria 

collected from the cone show scatter, probably 

reflecting difficulty in collecting a preserved 

surface. The oldest date provides a lower limit 

of 57ka for the age of the cone. We are 

investigating the discrepancy between the two 

samples from 013. As a test of our method, we 

analyzed a sample shielded by > 5m. It contains 

s 7% of the concentration of cosmogenic He at 

the surface. 

Unit 

011A 

0118 

011D 

Osl 

012 

Dates# (ka) ± 1 u 

81 ± 7, 81 ± 6, 8 2 ± 5, 84 ± 7, 

85±5, 87±6 

88±8 

76±7 

61 ±8 

82±9, 88±6, 82±5 

013 61 ±6, 100±9 ?same unit 

Os3 (cone) 30±5.35±5.36±5.43±3.57±7 

#Prod. rate = 245 g-1 a-1 at 968m elevation 
1lfE MODElliNG OF XENON AND LEAD ISOTOPES 

MIGRATION IN ZIRCON AND PITCHBLENDE 

UNDER HYDROTHERMAL CONDIDONS 

PRAVDIVTSEVA Olga V., SHUKOLYUKOV, Yu. A., 

Vemadsky Institute of Geochemistry and Analytical 

Chemistry, Russian Academy of Sciences, 

117975 Moscow, Russia. 

Xes-Xen isotope dating and classical U-Pb methods 

have been used for the comparative study of diffusion 

properties of Xe and Pb under laboratory hydrothermal 

conditions. 

The first object was the metamict zircon from .Uros 

Lake metasomatic rocks (Central Karella). Even initial 

natural sam~! clearly showed discordant and too young 

U-Pb age (.tWpt,j238u- 2.1 Ga; 206IJb;238u- 2.3 

Ga) as compared to true age of this zircon - 2.5 Ga. 

Five Xes-Xen age spectra of initial and hydrothermally 

treated zircon were obtained. Two samples- initial one 

and treated under the extremely low conditions ( 1 kbar 

pressure of 2NlNaCl solution vapour, temperature 300'C, 

72 hours) showed Xes-Xen age to be 2.34+/-0.16 and 

2.73+ /-0.36 Ga close to the true age. Thus, under 

definite conditions U-Xe isotopic system turns out to be 

more robust then U-Pb one. 

This conclusion was confirmed by the experiments 

with the pitchblende selected from Shlema-Alberoda 

U-deposit (Erzgebirge, Germany). Thirteen aliquots of 

different grain size were treated with HCl solutions of 

various concentrations. Independently from the grain 

size and the treatment condition all aliquots as well as 

initial sample have concordant Xes-Xen plateau ages. 

Even baH dissolved sample did not lose radiogenic Xe. 

Nevertheless Xe release kinetics and fine structure of 

Xes-Xen age spectra depend on treatment conditions 

and grain size dramatically. This correlation helps to 

illuminate the diffusion mechanism and nature of high 

xenon retention. 

The obtained results aUow "to conclude that the Xe 

migration is controlled by volume diffusion with a weak 

Xe trapping during the diffusion. The mechanism takes 

place usuaUy at temperatures which are, lower then a 

half of melting point value. 

The Krs-Krn dating method (which is in progress 

now) would be a valuable adjunct to this study. 

256 

CRETACEOUS PACIFIC VOLCANISM: DISTRIBUTION 

IN SPACE, TIME, AND SOURCE COMPOSITION 

PRINGLE. M.S., , KOPPERS, A.A.P., STAUDIGEL, H., and 

WIJBRANS, J.R., Faculty of Earth Sciences, Vrije 

Universiteit, 1081 HV Amsterdam, Netherlands; 

CHRISTIE, D.M.: and DUNCAN, R.A., COAS, Oregon 

State University, Corvallis, OR 97331-5503, USA. 

Radiometric ages for almost 90 distinct sites of Cretaceous 

Pacific Seamount volcanism (shown below) have a bimodal 

distribution, with peaks between 80-90 and 115-125 Ma. We 

compare the original positions of the melting anomalies that 

formed these seamounts with the modem South Pacific 

Isotopic and Thermal Anomaly (SOPITA), and speculate on 

the implications of these observations for mantle dynamics. 

Cretaceous seamounts in the east-central Pacific formed 

between 100 and 64 Ma, with a peak near 85-80 Ma. The 

Musicians and South Hawaiian Seamounts had near-ocean 

ridge sources located to the north and east of the SO PIT A. 

Sources for volcanism in the Line Islands define the 

northeastern extent of the Late Cretaceous SOPITA. 

Volcanism in the Marshall Islands had an age distribution 

similar to the east-central Pacific seamounts. However, all of 

the Marshall Islands and the older seamounts to the northwest 

originated in the central and western SOPITA. Also, the 

geochemical signatures of these western Pacific seamounts are 

similar to the extreme compositional range seen in the modem 

SOPITA (c.f., Koppers, et al., this session). 

The plateau and flow and sill complexes of the western 

J;>acific formed mainly 110-125 Ma, with sources to the south 

of, and geochemical signatures distinct from, volcanoes of the 

modem SOPITA. Either these flood basalts were not products 

of the Cretaceous SO PIT A, or they were part of a more 

extensive and diverse precursor of the modem SOPIT A. 

Although the SO PIT A region has been erupting lavas with 

unique geochemical signatures for at least 126 Ma, individual 

hot spot tracks cannot be traced for more than 20 m.y. In fact, 

hot spot tracks world-wide seem to be either less than 20 m.y. 

or greater than 65 m.y. long. As thermal plumes originate at 

thermal boundary layers, we suggest that mantle plumes 

responsible for shorter hot spot tracks originate at the 

shallower mantle thermal boundary layer, i.e. the 670 km 

transition zone, while mantle plumes responsible for the 

longer hot spot tracks 6riginate at the deeper mantle thermal 

boundary layer, i.e. the core/mantle boundary. The Pacific 

SOPITA, consisting of individual hot spot tracks less than 20 

m.y. long, would then be mainly the result of relatively 

shallow, upper mantle dynamics. Longer-lived hot spot 

tracks, such as the Hawaiian/Emperor and Louisville Ridges of 

the Pacific, would be the products of deeper mantle processes. 

(1)14 

.. 

c 

::I 12 

0 

E 10 &a 

CD 

0 8 

-0 6 

.. 

j 4 

~ 2 

z 

0 

• Eastern Smnts 

Ill Central Smnts 

18 Marshall Is. 

0 0 0 0 0 0 0 0 0 

~ m m o - N ~ • ~ ~ ~ p ~ p p 

Age (Ma) 
ISOTOPE COMPOSITIONS OF Sr, Pb, Nd, Os AND 

REE DISTRIBUTION IN OILS, BITUMENS AND 

BLACK SHALES AS INDICATORS OF MATTER 

SOURSES, MIGRATION WAYS AND GENESIS OF 

HYDROCARBON FLUIDS 

PUSHKAREV,Y.D., Karpinsky Geol. Res. 

Inst. (VSEGEI), Sredny pr. 74, St.-

Petersburg, 199026, Russia; GOTIH,R.P., 

PISOTSKY,B.I.,(VNIIGEOSISTEM), 

Varshavskoye shosse 8, Moscow, 113105, 

Russia; ZURAVLEV,D.Z., (IGEM), 

Staromonetny per.35, Moscow, 109017, 

Russia; ACHMEDOV,A.M., (VSEGEI), Sredny 

pr. 74, st.-Petersburg, 199026, Russia. 

The problem of hydrocarbon fluids and oil 

generation is still widely being debated. 

In parallel with their low temperature 

generation in sedimentary basins high 

temperature generation of the deep-earth 

hydrocarbon fluids is assumed. However 

there are no evidenses of available deepearth organic fluid component. The results, 

obtained in this work, are the following. 

1. Initial 87 Sr t 86sr values are often 

higher than average 87srt 86 sr for 

continental crust (0. 720) and imply high 

temperature (>350°C) interaction with micas 

and potassium feldspars. 

2. On the diagram of Sr-Pb isotope 

systematics of recent mafic volcanites the 

oil compositions occupies a specific space 

typical of lamproites and kimberlites. The 

similar place but in Nd-Sr isotope systematics of the same volcanites is occupied by 

bitumens compositions. That suggests the 

same contamination sources both for the 

oils and bitumens, and for the lamproites 

and kimberlites, i.e. crystalline rocks of 

the upper and lower crust. 

3. High concentration of REE (up to 3000 

ppm), their relative distribution and Nd 

isotope composition (from -20 up to +4.2 

ENd) in some bitumens, unlike those in host 

rocks, indicate high temperature assimilation of these elemens. 

4. Os isotope com~osition in black shales 

of Onega Lake (18 Os/186os=l.06 +/- 0.04) 

corresponds to a purely mantle component. 

The data obtained show that there is a 

deep-earth component in the objects investigated that was assimilated by 

hydrocarbon fluids under temperatures too 

high to be interpreted in terms of the 

traditional biogenic hypothesis. It is left 

for further study to find out what role the 

deep-earth component played when major oil 

pools were being formed. 

257 

TERRESTRIAL MO[YBOENUM 

ISOTOPIC ABUNDANCE RATIOS 

QI-LU1.2 and A. Masudal, 1 Dept. of Chern., 

University of Electro-communications, 1-5-1 

Chofugaoka, Chofushi, 182 Tokyo, Japan 

2Dept. of Chern., Inner Mongolia University, 

Huhhot, China. 

Potentially molybdenum is a very interesting 

element in cosmochemistry and geochemistry l · 2. 

However, now the data on molybdenum isotopic 

abundance ratios regarded as reliable in terrestrial 

and meteoritic matters are sparse. We measured 

all of seven molybdenum stable isotopes of 12 

molybdenites collected from different locations 

over the world3. A Faraday collector of thermal 

ionization mass spectrometer (VG Sector 54-30) 

was applied in peak jumping mode for the 

isotopic measurement. All the ratios have been 

normalized to 94Mot98Mo = 0.3802. 

The normalized data provided no special 

evidence for large, mass-dependent isotopic 

anomalies of molybdenum isotopes, except for 

the depletion of 97Mo (-0.7 E) in Mulgine 

(Australia), the excess of 95Mo (+1.3 E) and the 

excess of lOOMo (+2.8 E) in China 110. 

Furthermore, some samples exhibit a hint of 

scatters in 92Mo and 1 OOMo. 

Judging from the precision of isotopic 

measurement of our standard (Mo0:3, 99.999%), 

· it is unlikely that these discrepancies are due to 

the mass fractionation effect during the 

measurement. However, currently, our knowledge 

can not give a reasonable explanation for these 

discrepancies, since the terrestrial matter has 

experienced much chemical fractionation, and it is 

assumed that terrestrial matter is isotopically 

homogeneous. 

lHutcheon, I. D., Armstrong, J. T., and 

Wasserburg, G. J., 1987, Geochim. Cosmochim. 

Acta, 51,3175-3192. 

2woosley, S. E., and Howard, W. M., 1991, 

Astrophys. J., 373, L5-L8. 

3QJ-LU and Masuda, A., 1994, Int. J. Mass 

Spectrom. Ion Processes, in press. 
COMBINED U-Pb-Xe CHRONOLOGY, PRECISE 

DETERMINATION OF THE PRODUCT OF THE DECAY 

CONSTANT AND FRACTIONAL YIELD OF 136Xe FROM 

238u SPONTANEOUS FISSION 

R Rageuli, Institut de Physique du Globe, F-75252 Paris 

and Isotopengeologie, ETHZ, CH-8092 Zurich, EH Hebeda, 

Ringstrasse 10, D-92331 Parsberg, R Wieler and P Signer, 

Isotopengeologie, ETHZ, CH-8092 Zurich'. 

Combined U-Pb and U-Xe analysis on aliquots of mineral 

separates offers a powerful approach for determining the time 

and nature of igneous and metamorphic processes. However, 

the application of the U-Xe technique requires the knowledge 

of the product of the decay constant (Asf) and the fractional 

yield of 136xe (136y sf) from the spontaneous fission of 

238u. Recent estimates of this product obtained using Uminerals (Eikenberg et al., 1993) are in conflict with the value 

derived from earlier studies of zircon and monazite 

(Shukolyukov & Mirkina, 1963; Kapusta & Matukov, 1982). 

In order to resolve this issue we have obtained precise U-Pb 

and U-Xe data from a carefully selected suite of zircons and 

monazites. All samples have precise and concordant U-Pb 

ages. Recoil losses of fissiogenic Xe were either corrected for 

numerically or were circumvented by abrading samples or by 

using very large minerals. Old crystal cores, that might have 

retained their fission Xe but not their radiogenic Pb, were not 

detected in any of the samples. For all these reasons the 

Asf*l36y sf value obtained of-(6.82±0.36)*10·18/a (1cr 

standard deviation) is considered to be correct, and is the bestestimate thus far. The value obtained is similar to that 

determined from earlier studies of accessory minerals 

(Shukolyukov & Mirkina, 1963; Kapusta & Matukov, 1982) 

but some 20% higher than the value recently derived from 

carefully selected pitchblendes, uraninites and coffinites 

(Eikenberg et al., 1993). These results suggest that such Uminerals systematically lose around 20% of their fission Xe, 

possibly due to regeneration of the crystal lattice or change in 

oxidation state. 

In addition, we will present U-Pb and U-Xe data for zircons 

and monazites from a diverse range of rock types, some having 

experienced varying degrees of metamorphism. Minerals from 

amphibolite facies gneisses from within the same geological 

unit show three different types U-Pb-Xe behaviour: 1) Zircons 

and monazites show minor lead loss and indicate an upper 

intercept age that is concordant with the U-Xe ages. 2) Zircons 

which have lost both fissiogenic and radiogenic daughters but 

where neither the U-Xe nor the U-Pb systems having been 

completely reset, neither of the systems give a meaningful age 

information. 3) Zircons which experienced 95-100% loss of 

radiogenic lead and yield a lower intercept age that is 

concordant with the U-Xe ages. Furthermore zircons from a 

granulite show minor lead loss and yield an upper intercept age 

younger than the corresponding U-Xe ages. These results 

illustrate the utility of combined U-Pb-Xe analysis in 

understanding the timing and nature of petrogenesis in 

complex geological terrains. 

YuA Shukolyukov & SL Mirkina, 1963, Geochemistry, v 7, 

p 729-731 

YaS Kapusta & DI Matukov, 1982, Soviet Radiochemistry, 

v 24, p 462-465 

J Eikenberg et al., 1993, GCA, v 57, p 1053-1069 

258 

DETRITAL ZIRCON STUDIES OF NEOPROTEROZOIC 

QUARTZARENITES FROM NORTHWESTERN 

CANADA:ADDITIONALSUPPORTFORAN 

EXTENSIVE RIVER SYSTEM ORIGINATING FROM 

GRENVILLE OROGEN 

RAINBIRD, R. H. and McNICOLL. V. J., Geological 

Survey of Canada, Ottawa, Ontario, KIA OE8, Canada, 

and HEAMAN, L. M., Dept. of Geology, Royal Ontario 

Museum, Toronto, Ontario, M5S 2C6, Canada. 

Provenance studies of quartzarenite from the Shaler 

Supergroup of Minto Inlier, Arctic Canada, suggested that 

during the Neoproterozoic a braided fluvial system with a 

reach of at least 3,000 km extended across Laurentia, a 

central block of the supercontinent Rodinia (Rainbird et al. 

1992). Consistent northwest paleocurrents and a large 

proportion of detrital zircons with U/Pb ages of 1.2-1.0 Ga 

(characteristic of the Grenville Province within Laurentia) 

are consistent with provenance from the Grenville orogen, 

formed during the initial assembly ofRodinia. To test the 

validity of this hypothesis, detrital zircon geochronology was 

completed on samples of fluvial quartzarenite from the 

Nelson Head Formation (Rae Group- Shaler Supergroup) of 

Minto Inlier and from unit K7 (Katherine Group -

Mackenzie Mountains Supergroup), a lithostratigraphically 

correlative unit located approximately 1,000 km to the 

southwest in the northern Canadian Cordillera. 

U-Pb ages of 24 single concordant zircons from the two 

samples are Archean (2.6-2.8 Ga), Paleoproterozoic (1.9-2.0 

Ga) and Mesoproterozoic (1.1-1.6 Ga). Mesoproterozoic 

zircons represent the majority of the analyses (18) and are 

distributed evenly between the two specimens. Eleven grains 

are between 1.08 to 1.2 Ga, reflecting significant 

contribution from a Grenville source. 

These results firmly support the original hypothesis and we 

interpret the Nelson Head Formation and the Katherine 

Group as distal deposits of the same huge braided fluvial 

system that originated from the foreland of Grenville orogen. 

Rainbird, R. H., Heaman, L. M. and Young, G. M., 1992, 

Sampling Laurentia: detrital zircon geochronology offers 

evidence for an extensive Late Proterozoic river system 

originating from Grenville orogen: Geology, v. 20, p. 

351-354. 
THE SHACHANG RAPAKIVI GRANITE COMPLEX, 

EASTERN CHINA: IMPLICATIONS FOR Sm-Nd, 

Th-U-Pb, AND Rb-Sr SYSTEMATICS OF THE 

ARCHEAN SINO-KOREAN CRATON 

RAMO. 0. Tapani, Dept of Geology, P.O. Box 11, FIN00014 University of Helsinki, Finland, HAAPALA, 

Ilmari, Dept of Geology, University of Helsinki, 

V AASJOKI, Matti, Geological Survey of Finland, FIN02150 Espoo, Finland, YU, Jian Hua, Geological Institute 

ofBeijing, Deshengmenwai, Beijing 100011, China, and 

FU, Huei Qin, Geological Institute of Beijing. 

The Sino-Korean Craton, located in eastern China and 

adjacent regions around the Yellow Sea, registers over one 

b.y. of continental crustal evolution, commencing at least 3. 7 

Ga ago, with major tectonothermal events between 2. 7 and 

2.5 Ga (Liu et al., 1990). In the northwestern part of the 

Craton, just north of Beijing, a 1. 7 Ga (U-Pb age) old anorogenic rapakivi granite-anorthosite suite transects Archean 

medium- to high-grade gneisses overlain by unmetamorphosed early Proterozoic and Phanerozoic supracrustal formations. This paper focuses on Nd, Pb, and Sr isotopes of one 

of the rapakivi intrusions (Shachang) and its metamorphic 

country rocks. 

The Shachang intrusion is strongly enriched in LREE 

(147SmJ144Nd 0.0758 to 0.1093) and has initial ENd(1.67 Ga) 

values averaging -5.9±0.3 (10', n=lO) and ToM model ages 

averaging 2.32±0.06 (10') Ga. Pb isotopic data on granites 

and feldspars define a 207Pbf206Pb isochron with an age of 

1693 ± 110 Ma (2a) and a 208Pbf206Pb errorchron with K 

(232Thf238U) of 6.3 ±0.6 (20'). S&K (Stacey and Kramers) ll2 

values of feldspars average at 8.92±0.17 (1a, n= 10). Sr 

isotopic data on the granites yield an errorchron corresponding to an age of 1511 ±67 Ma (2a) and initial 87Sr/86Sr of 

0.7068±0.0017 (20'). 

The country rocks of the intrusion show a U-Pb age of 

2.5 Ga and are strongly enriched in LREE (147SmJ144Nd 

0.0816 to 0.1315) . They have positive initial ENd(2.5 Ga) 

values averaging +0.9±0.5 (10', n=5) and ToM model ages 

averaging 2.63 ±0.06 (la) Ga. Whole rock and feldspar Pb 

isotopic ratios are very low (down to 614= 13.67, 7/4= 

14.55, 8/4=33.41) and show S&K ll2 values between 7.78 

and 9.36 (average 8.86) and~ values between 3.75 and 7.08 

(average 4.83). The country rocks are depleted in Rb and 

lield a Rb-Sr errorchron of 2190±350 Ma (20') and initial 

7Sr/86Sr of 0. 7011 ±0.0037 (2a). 

The Shachang granites are felsic and homogeneous and 

probably crystallized from anatectic melts generated at deeper 

levels within the crust. Hence they presumably preserve a 

record of the isotopic composition of the lower crust. Comparison of the isotopic data on the granites and their exposed 

country rocks suggests that: 

(1) The lower crust in this area has a higher apparent 

Sm/Nd (-0.22) than its exposed upper parts (-0.17). 

(2) This part of the Sino-Korean Craton is characterized 

by low overall U /Pb and high overall Th/U. There is, however, not much difference between the upper and lower parts 

of the crust in terms of these ratios. 

(3) The initial Sr isotopic ratios of the granites and their 

country rocks are both low and not indicative of long-term 

upper crustal Rb-enrichment. 

Liu, D.Y., Shen, Q.H., Zhang, Z.Q., Jahn, B.M., and 

Auvray, B., 1990, Archean crustal evolution in China: UPb geochronology of the Qianxi Complex: Precambrian 

Research, v. 48, p. 233-244. 

259 

Uffh DISEQUILIDRIA AND MAGMA MIXING AT SAN 

PEDRO VOLCANO, SOUTHERN VOLCANIC ZONE 

(SVZ), CHILE. 

RAMOS F C, REID, M.R., DAVIDSON, J.P., Dept. of 

Earth and Space Sciences, U.C.L.A., Los Angeles, Ca 

90024-1567, USA. 

San Pedro Volcano is a Holocene stratocone lying at 36°S 

latitude in the northern region of the Southern Volcanic Zone. 

This volcano has had an extensive volcanic history which 

includes the eruption of a composite flow of inferred Holocene 

age. This flow grades from basaltic andesite to dacite in 

composition and basaltic inclusions are contained within 

specific andesite and dacite portions of the flow. The variable 

compositions of this flow are postulated to be the result of 

magma mixing between dacite and basaltic andesite endmembers (Singer, submitted). 

Nd, Sr, and Pb isotopes (-0.5128, -0.7041, and -18.55, 

-15.58, and -38.44) in the composite flow show little 

variation and are therefore of limited utility in constraining 

magmatic mixing. (23~h/232Th) isotopic values in this flow 

also have a limited range of between 0.80 and 0.84, but 

(238uJ232Th) values range extensively from 0.73 to 0.91. 

These samples range from Th- to U-enriched and 

(230fh/232Th) and (238uJ232Th) are positively correlated. 

Basaltic inclusions found in host dacite and andesite define the 

end-members of the range, while host dacite and andesites fall 

between these end-members suggesting that magma mixing is 

generating the variations. If initial (23~hf232Th) values were 

similar at the time of eruption, mixing occurred at 

approximately 25-30 ka. This is older than the inferred 

Holocene eruption age based on field relations to glacial 

features and suggests that mixing and evolution of these 

magmas may have occurred in less than 15 ka. 

The basaltic inclusions (SiOz= 51-52%) analyzed suggest 

that U vs Th enrichment is magma dependent (individual 

magma characteristics) and not location dependent in the 

northern SVZ (as suggested by Sigmarsson et al., 1990). The 

U!Th isotopic data also indicate that the mixing array could be 

generated by mixing two basaltic end-members (as represented 

by the basaltic inclusions) with subsequent differentiation. 

This interpretation differs from Singer (submitted) which, based 

on major element considerations, suggests mixing between 

dacite and basaltic andesite. 

Sigmarsson, 0., Condomines, M., Morris, J.D., and 

Harmon, R.S., 1990, Uranium and lOBe Enrichments 

by fluids in Andean Arc Magmas, Nature, v.346, P.163-

165. 
EFFECT OF "BULK" ETCH RATE AS RECORDED 

IN ETCH PIT DIMENSIONS ON MEASURED 

FISSION TRACK LENGTH ANISOTROPY IN 

DURANGO APATITE 

RA VENHURST. C.E., Dept. of Earth Sciences, 

Dalhousie University, Halifax, N.S.,B3H 3J5, Canada 

and RODEN, M.K.,Dept. ofEarth and Environmental 

Sciences, Rensselaer Polytechnic Institute, Troy, 

N.Y., 12180 U.S.A. 

Observed fission track (FT) lengths are a function of 

annealing and etching, both of which are dependent on 

crystallographic orientation. We used etch pits (the 

intersection of tracks with prismatic surfaces) to study 

etching effects. Etch pit aspect ratios are dependent on 

HN03 etchant strength and apatite chemistry. However, 

etchant strength does not effect the measured mean track 

length nor the crystallographic anisotropy of FT lengths, 

so long as etch pit width is held constant (Roden et al., 

1991). 

Oriented slabs of Durango apatite containing 

identically annealed induced tracks were etched with 5N 

HN03 for 20, 30, 40, 50, 60 and 70 seconds at 21 °C. 

Prismatic section etch pits increase in width at about .016 

J,Lmlsec and in length at about .10 J.Lmlsec. In each slab at 

least 200 confmed track lengths were measured with 

respect to crystallographic orientation and then ellipsefitted. FT lengths perpendicular to the c-axis increase at 

.018 J,Lmlsec, similar to etch pit width, whereas those 

parallel increase at an order of magnitude less than the 

etching rate for etch pit length. Therefore, the etching 

effect on FT lengths is mainly "bulk" etching as 

determined from etch pit width. 

Few measurable tracks oriented approximately parallel 

to the c-axis could be found in Durango apatite at a 20 

second etching duration. Therefore the Laslett et al. 

(1987) annealing model is likely biased towards annealing 

of tracks oriented perpendicular to the c-axis, which 

anneal more readily than those parallel. In general, we 

suggest that etching conditions not be set at 5N HN03 

for 20 sec @ 21 °C, but instead that the etching time be 

adjusted for the dominant apatite composition in the 

sample so that etch pit widths of about 0.75 J.Lm are 

produced (5N HN03 for 25 sec@ 21°C is suggested 

initially). These sample-dependent etching conditions will 

allow measurement of confmed track lengths over all 

orientations, so that a more accurate mean length can be 

determined .... 

Roden, M.K.,Ravenhurst, C.E. & Miller, D.S., 1991, 

Effect of etching on apatite fission-track annealing 

anisotropy (Abstract): AGU, Baltimore. 

Laslett, G.M., Green, P.F.,Duddy, I.R., and Gleadow, 

A.J. W., 1987, Thermal annealing of fission tracks in 

apatite 2. A quantitative analysis: Chemical Geol. 

(Isotope Geoscience Sect.), v.65,p.1-15. 

260 

OSMIUM IN PELAGIC SEDIMENTS FROM THE 

N-E ATLANTIC: ISOTOPIC COMPOSITION AND 

DISTRIBUTION. 

RA VIZZA, G. WHOI, Woods Hole, Ma, 02543 

USA, COLODNER, D. C., L-DEO Palisades, 

NY 10964, USA and THOMSON, J. lOS Brook 

Road, Wormley, Surrey GU8 SUB, UK 

In order to evaluate the potential influence of 

redox changes on diagenetic redistribution of Os in 

slowly accumulating pelagic clays, the Os 

.concentration and isotopic composition of sediment 

samples from a well characterized turbidite-pelagic 

clay sequence have been investigated. In an 

otherwise homogeneous oxic brown clay sequence, a 

10 em reduction halo has formed due to organic 

matter degradation in a thin turbidite deposited 

immediately above this reduced horizon. 

Redistribution of Pt and Ir have accompanied this 

redox change with Pt, and to a lesser extent Ir, being 

depleted in the reduced horizon relative to the 

surrounding unaltered brown clay. In contrast, the 

Os concentration of the reduced horizon ( 116 ppt) is 

slightly elevated compared to surrounding sediment 

(= 100 ppt). These subtle concentration variations 

are suggestive of diagenetic PGE redistribution. In 

spite of these concentration variations, the Os 

isotopic composition of the bulk sediment is nearly 

homogeneous (1870sjl860s range: 8.21 to 8.54), 

except for the turbidite which is distinct (1870sfl86Qs 

= 7.02). 

Model calculations using the 230Th- based 

sediment accumulation rate ( = 0.3 cm/Ka) and the 

cosmic Os flux ( 2-3 pg.cm2 Ka-1) indicate that 

approximately 10% of the Os in this sediment is of 

cosmic origin. Assuming the silicates in the core 

carry an average crustal Os concentration (50 ppt), 

detrital Os accounts for an additional 50% of the 

sediment Os budget. The remaining 40% of the Os 

budget must therefore be attributed to a hydrogenous 

source. Leaching experiments intended to isolate a 

hydrogenous Os component liberate approximately 

30% of the total Os in the sediment, which is in fairly 

good agreement with the above model calculation. 

This result and the accompanying calculation indicate 

that post-depositional redistribution of the 

hydrogenous Os component within the sediment pile 

can easily produce the observed 16% increase in the 

Os concentration of the reduced horizon. The 

deepest sample from the core, has a Th-based age of 

= 550 Ka. The 1870sjl860s ratio of leachable Os 

from this sample is 8.47, a more radiogenic value 

than in Pacific sediments of similar age. This 

discrepancy may be an additional indication of 

complexities associated with inferring the Os isotopic 

composition of seawater from pelagic clay data. 
APATITE FISSION TRACK THERMOCHRONOLOGY OF 

SELECTED WELLS FROM THE BOWEN/SURAT 

BASINS, EASTERN AUSTRALIA 

RAZA Asaf. GLEADOW, A.J.W., HILL, K.C., 

O'SULLIVAN, P.B., VIEPS, School of Earth Sciences, La 

Trobe University, Bundoora, Victoria, 3083, Australia., and 

KORSCH, RJ ., AGSO, Canberra, ACT, 2601 Australia. 

As part of continuing research of the geologic evolution of 

the eastern margin of Australia, the thermo-tectonic history of 

the Bowen/Surat Basins in south-eastern Queensland, Australia, 

has been evaluated using apatite fission-track 

thermochronology and published vitrinite reflectance data. The 

Bowen Basin is a narrow. north-south basin containing up to 

10 km of Permian and Triassic sediments. It was initiated as 

an extensional basin in the Early Permian and bacame a 

foreland basin in the Late Permian. Unconformably overlying 

these Permo-Triassic rocks are Jurassic and Cretaceous 

sedimentary rocks of the Surat Basin, which developed along 

the eastern margin of Australia in response to regional thermal 

subsidence. Interpretation of regional reflection seismic data 

suggests that up to 3 km of section was removed in the Middle 

to Late Triassic to create the unconformity separating the two 

basins. 

Published vitrinite reflectance data from the Bowen/Surat 

basins suggest that rocks preserved had previously been 

exposed to higher temperatures, but give no indication of the 

time of cooling from these higher paleotemperatures. To 

constrain the thermochronologic history of the sedimentary 

rocks, 25 core samples from six wells were selected for apatite 

fission track analysis. The Cockatoo Creek-1, Burunga-1, 

Wandoan-1 and Cabawin-1 wells are situated west of the 

Burunga Leichhardt Fault, which is close to the eastern 

boundary of the Bowen Basin, and the Rockwood-1 and 

Cannan-1 wells on the up-thrown block east of the thrust 

The fission track results suggest that rocks within the 

basins were previously exposed to higher paleotemperatures, 

consistent with the vitrinite reflectance data, and that maximwn 

paleotemperatures were acheived during the Jurassic to Early 

Cretaceous in response to Surat Basin deposition. Therefore 

the Middle to Late Triassic thermal history is not recorded by 

the fiSSion track data. Subsequently, starting in the middle 

Cretaceous at -105 Ma, a maximum of -65°C of cooling has 

occurred. This cooling episode pre-dates continental rifting 

which commenced in the northern Tasman Sea in the Late 

Cretaceous (-85 Ma). Modelling of the data suggest that 

cooling from maximum paleotemperatures occurred in two 

stages, an episode of relatively rapid cooling between -105-85 

Ma which resulted in -1.5 km of denudation, followed by slow 

protracted cooling to present-day conditions, possibly due to a 

further -0.5 km of exhumation. Timing of cooling for host 

rocks from all wells is identical, regardless of their location 

with respect to the Burunga Leichhardt Fault suggesting that 

the thrust was not active in the middle Cretaceous. 

This abrupt regional inversion and denudation is difficult to 

explain in terms of the known regional tectonics. It may be 

related to a middlle Cretaceous (c. 105-95 Ma) deformational 

event, evidence for which is preserved in neighboring basins. 

It also coincides with similar middle Cretaceous regional 

inversion and denudation around the Bass Basin, -2000 km to 

the south, thought to be related to plate realignment and 

extension propagating south of Tasmania. Perhaps the Bowen 

Basin inversion was related to the same plate realignment that 

caused regional isostatic rebound following rapid Jurassic to 

Cretaceous deposition. 

261 

THE USE OF HETEROGENEOUS EXCESS ARGON AND 

ARGON LOSS TO CONSTRAIN THERMAL lllSTORIES: 

LASER 40 Arf39 Ar DATING OF DEFORMED MICAS 

FROM THE SESIA ZONE, ITALIAN ALPS 

REDDY. S.M., WHEELER, J ., Dept. of Earth Sciences, 

University of Liverpool, Liverpool, UK. and KELLEY, 

S.P., Dept of Earth Sciences, Open University, Milton 

Keynes, UK. 

A combination of detailed micro-textural analysis using 

scanning electron microscopy (SEM) and high-resolution laser 

40 Arf39 Ar dating provides insights into the complex behaviour 

of argon in deformed metamorphic rocks that have previously 

been unobtainable. Two samples from the Eclogitic Micaschist 

Complex (EMC) and the Seconda Zona Diorito Kinzigitica 

(IIDK) of the Sesia Zone have been studied in detail to 

investigate the relationship between microstructure 

development and argon isotope systematics in metamorphic 

rocks that in the past have proved difficult to date. 

Large phengites from the EMC sample record age profiles 

from Jurassic to end-Cretaceous which compare favourably 

with theoretically-derived volume diffusion profiles and suggest 

that the profiles probably developed at temperatures in excess 

of 400°C. The size of the profiles differ significantly between 

undefonned grain ends and those grain boundaries formed 

during boudinage of individual micas. Consequently we are 

able to use the asymmetry of these diffusion profiles to 

constrain the timing of defonnation and infer the temperatures 

at which defonnation was taking place. 

Within the IIDK biotite gneiss sample, biotites record a 

range of 40 Arf39 Ar ages from Permian to end-Cretaceous with 

the youngest ages preserved in fine-grained biotite within preAlpine micro-shear zones. Rb-Sr data from biotites within 

equivalent structural units yield pre-Alpine ages suggesting 

that excess argon cannot account for the observed range of 

40 Arf39 Ar ages. Comparing 40 Arf39 Ar ages with measured 

grain sizes shows a relationship that is best explained by 

partial resetting of biotites by an Alpine thermal event in 

which temperatures were less than 300°C. The sample lies 

only a few kilometres from metamorphic units that reached 

temperatures of around 600°C during the Alpine orogeny. 

These data therefore place important constraints on the Alpine 

tectonic evolution and the exhumation history of the Sesia 

Zone. 
THE TRANSANTARCTIC MOUNTAINS AND 

THE BREAKUP OF GONDWANA: 

UNDERPLATING, UPLIFT, AND FLEXURAL 

SUPPRESSION 

Redfield. I F Grimm, R. E., Dept. of Geology, 

Arizona State University, Tempe AZ 

85287, Fitzgerald, P. G., Dept. of Geosciences, 

University of Arizona, Tucson, AZ 

85721, and Stump, E., Dept. of Geology, Arizona 

State University, Tempe AZ 85287. 

25 new apatite fission track (AFf) ages from a 

vertical transect in the Admiralty Mountains of 

northern Victoria Land (NVL), Antarctica indicate that 

an uplift/denudation event occurred at -120 Ma, 

coincident with rifting between Australia and 

Antarctica (the last phase of Gondwana breakup). 

These and previously published AFT data from other 

areas in the Transantarctic Mountains (I AM) support 

our contentions that 1) growth of the entire I AM was 

episodic, with early Cretaceous, late Cretaceous, and 

early Cenozoic events, and 2) the I AM are broken 

longitudnally into discrete crustal blocks that act 

somewhat independantly. 

Coupled with regional geophysical data, the new 

AFT ages can be incorporated into a tectonic model 

describing the genesis of the I AM. Bouguer gravity 

data from the TAM show large mass deficiencies 

beneath the range. We attribute this anomaly to 

crustal roots, possibly emplaced by underplating and 

crustal thickening caused by Jurassic tholeiitic 

magmatism accompanying incipient Gondwana 

rifting, or alternatively by post-Ross subduction 

(suggested by volcaniclastic sediments in the Beacon 

Supergroup). The roots were flexurally suppressed 

until initial early Cretaceous rifting thinned the 

elastic lithosphere between Australia and Antarctica as 

well as along the present-day I AM range front, 

allowing a partial isostatic response. Late Cretaceous 

to early Cenozoic extension in the Ross Sea further 

thinned and fractured the lithosphere, allowing 

structural failure of discrete crustal blocks and 

initiating the main phase of uplift. Full isostatic 

adjustment has not occurred: the uncompensated 

portions of today's roots are restrained by regional 

flexure, warping the edge of the East Antarctic 

lithosphere and maintaining the TAM in a steadystate, largely aseismic equilibrium. 

262 

PRODUCTION OF COSMOGENIC NUCLIDES BY 

MUONS 

REEDY, R. C., Los Alamos Nat'l Lab., Los Alamos, NM 87545 USA; NISHIIZUMI, K., U. Calif., 

Berkeley, CA 94720 USA; KLEIN, J., DAVIS, 

R., and MIDDLETON, R., U. Penn., Philadelphia, PA 19104 USA; LAL, D., and ARNOLD, 

J.R., U. Calif., La Jolla, CA 92093 USA; KUBIK, P., PSI/ETH-Honggerberg, CH-8093 Zurich, 

Switzerland; JULL, A.J.T., U. Arizona, Tucson, AZ 85721,USA; ENGLERT, P.A.J., San Jose 

State U ., San Jose, CA 95192 USA; ELMORE, 

D., Purdue U., W. Lafayette, IN 47907 USA. 

Cosmic-ray interactions in the Earth's atmosphere 

produce a cascade of particles. Some neutrons and 

muons reach the surface and produce nuclides. Longlived radionuclides, such as 5730-year 14C, 0.3-Ma 

36Cl, 0.7-Ma 26 Al, 1.5-Ma 10Be, and 15.7-Ma 129I, and 

some noble-gas nuclides, such as 3He and 21 Ne, made 

in situ in certain materials can be used to study their 

recent exposure histories, but their production rates 

are needed. Irradiations at the Los Alamos Meson 

Physics Facility (LAMPF) have simulated the in-situ 

production of radionuclides. 

Below a few meters, muons become a major source 

of cosmogenic nuclides. Little work has been done 

on yields of long-lived radionuclides in muon-induced 

reactions. At the LAMPF Stopped Muon Channel, 

Si02, CaO, KN03, KCl, CaC03, and Te powders were 

mixed with nickel powder in plexiglas boxes and exposed to stopped negative muons (J.L- ), and a Si02 target was irradiated with ~ 70 MeV J.L- [Reedy et al., 

1988]. Low-Z materials such as paraffin, which produce few neutrons from muon reactions, were used 

to slow the muons and support the targets. The Ni 

was physically separated from stopped J.L- targets after each irradiation. Radionuclides were measured by 

{-ray spectroscopy in the Ni, in Fe, Ti, and Au foils 

used to study beam locations and neutron impurities, 

and in the targets (e.g., 7Be). Low-level gas counting 

was done for 37 Ar and 39 Ar. Accelerator mass spectrometry (AMS) was or will be used to measure 10Be, 

14C, 26 Al, 36Cl, and 1291. 

Radionuclides in the Ni showed that there were 

few neutrons in the targets and will be used to get the 

number of J.L- stopping in each target. Measured were 

7Be, 10Be, and 26 AI in all Si02 targets; 7Be and 10Be 

in CaC03; 14C and 36Cl in KN03 and CaO; and 37 Ar 

and 39 Ar in all K and Ca samples. The 10Bef26 AI ratio in Si02 exposed to stopped J.L- is 7.0±0.4, similar 

to that in neutron-irradiated Si02 (7.1) and natural 

surface quartz (6.0) and is 3.2±0.9 in the Si02 irradiated by ~70-MeV J.L-. The 10Bej1Be ratios in CaC03 

(19±6) and in Si02 exposed to stopping (23±2) and 

~70-MeV 11- (3.2±0.9) were much greater than for 

energetic neutrons and protons ( <1). The relatively 

large amounts of 10Be and 7Be from Si02 was unexpected as nuclide yields for stopped-11- reactions decrease rapidly for products farther from the target. 

Reedy, R.C. et al. (1988) in Progress at LAMPF 1987, 

Los Alamos report LA-11339-PR, pp. 148-154. 
SEPARATION OF CERIUM FROM OTHER RARE 

EARTH ELEMENTS WITH APPLICATION TO 

Sm-Nd AND La-Ce CHRONOMETRY AND 

TRACER STUDIES 

REHKAMPER. M., GARTNER, M., GALER, 

S.J.G., and GOLDSTEIN, S.L., Max-PlanckInstitut fiir Chemie, Postfach 3060, 55020 Mainz, 

Germany. 

The rare earth element (REE) pairs Sm-Nd and LaCe include three radioactive decay systems that are 

widely used in geo- and cosmochemistry for dating 

and tracer purposes. In addition to the familiar 

147Sm-143Nd isotope system, 146Sm a-decays to 

142Nd with a half-life of 1.03x108 a, allowing the 

study of processes that took place in the early history 

of the Earth (Goldstein and Galer, 1992) and the 

solar system. The 138La-138Ce decay scheme has 

been gaining increasing importance in geochemistry 

with the advent of high precision isotope analysis of 

Ce (Makishima and Nakamura, 1991). 

Efficient chemical separation procedures are 

necessary prior to TIMS in order to remove 

isobarically interfering elements. Particularly 

important is the interference of 142Ce ( 11% of Ce) on 

142Nd (27% of Nd). Mutual separation of the REE is 

commonly obtained by reversed-phase partition 

chromatography using HDEHP-coated Teflon 

powders. However, the separation factors for the 

elements Ce-Pr-Nd are generally inadequate for high 

precision 142Nd and Ce isotope studies. 

Here we report a simple but highly efficient solvent 

extraction technique for the separation of Ce from the 

other REE. The procedure involves the oxidation of 

Ce3+ to Ce4+ using NaBr03 and extraction of the 

tetravalent species into an organic solvent containing 

HDEHP while the remaining lanthanides are left in 

aqueous nitric acid. The solvent extraction technique 

can be used for (1) cleaning of Nd from Ce 

contamination and (2) the isolation of Ce from other 

REE prior to mass-spectrometric analysis. 

The technique has been applied to the cleanup of 

Nd fractions obtained by HDEHP chemistry as part 

of a 142Nd isotope study of geological samples 

(Goldstein and Galer, 1992). The extraction procedure is quick, increases the Nd-Ce separation factor 

by up to several orders of magnitude and reduces the 

isobaric interference of 142Ce (as expressed by the 

142Cef142Nd ratio) to <5 ppm. We are currently 

investigating the potential of the extraction technique 

for a simple, one-step separation of Ce from the other 

REE for use in Ce isotope studies. 

Goldstein, S.L. and Galer, S.J.G., 1992, Eos AGU 

Spring Meeting, p. 323. 

Makishima, A., and Nakamura, E., 1991, Chern. 

Geol. (Isot. Geosci. Sect.), v. 94, p. 1-11. 

263 

CHEMICAL DYNAMICS OF MAN1LE LITHOSPHERE 

FROM TH ISOTOPE SYSTEMATICS 

REID. MR. and RAMOS, F.C., Dept. Earth Space Sci., 

UCLA, Los Angeles, CA 90024. mary@argon.ess.ucla.edu 

Basalts of the western USA sample tectonically distinct 

mantle reservoirs. Th isotope characteristics of these basalts, 

considered in conjunction with other isotope systematics, 

permit evaluation of Uflb fractionation associated with 

melting and metasomatism, and identification of the source of 

the basalts. Our work shows that the Th isotopic 

heterogeneity exhibited by basalts in the western USA is much 

greater than that exhibited by basalts erupted in the oceans. It 

is unlikely that this simply reflects greater participation of 

crust in the origin of continental basalts. 

The eastern and western Great Basin (EWGB), as represented 

by eastern California and southwestern Utah respectively, are 

zones of Proterozoic continental crust thinned by Tertiary 

extension. Basalts erupted in these areas range from tholeiites 

to basanites, and their isotopic compositions typify the 

diversity of mantle reservoirs in the western USA. The range 

in (230Jbt232Th) is large in basalts of the EWBG (0.65-1.09) 

and includes the lowest value yet obtained in the western USA. 

No distinctions are evident between the Th isotope 

characteristics of silica-saturated and under-saturated basalts. 

Most samples lie close to the equiline but the range in 

(230Jbt232Th) is unlikely to be due solely to in situ decay: an 

unprecedented U/Th fractionation of >40% would be required 

for some of the tholeiites. Similarly, this isotopic 

heterogeneity is also.unlikely to be the result of crustal 

contamination: three case studies performed specifically to 

evaluate the effects of crustal assimilation yielded <12% 

variation in (230Jbt232Th) over the compositional range of 

basalt to andesite. 

Covariations of Th with other isotopic systems generally 

trend from depleted mantle compositions represented by lavas 

of the central Basin and Range and Mojave toward those of 

enriched mantle. In detail, the Th isotope composition of the 

enriched mantle endmember for the EWGB basalts is less than 

those of oceanic basalts with equivalent 208pb*f206pb* 

compositions. Th seems to follow Nd more closely than Sr, 

supporting previous interpretations about heterogeneity in the 

nature of metasomatic phases. 

Trace element characteristics of the enriched mantle reservoir 

in the EWGB are attributable to subduction-related processes. 

The paired Th-Pb isotopic compositions of the EWGB basalts 

are not those expected of relatively recent introduction of 

subducted components into the mantle indicating that 

Mesozoic to Tertiary plate convergence west of the Great Basin 

apparently did not modify the mantle under this region. 

Rather, the Th-Pb isotope systematics of the enriched mantle 

reservoir can be explained by evolution from an orogenic Pb 

reservoir that has been chemically isolated since the 

Proterozoic. Melting must have penetrated to <60 km depths 

to explain the origin of tholeiites, placing the compositionally 

enriched mantle reservoirs at relatively shallow depths under 

the margins of the Great Basin. The depleted mantle reservoir 

may also be Proterozoic in origin but invasion of the 

lithosphere by depleted mantle like that under the central Basin 

and Range is more consistent with crustal extension and 

asthenospheric decompression in these areas. 
OS ISOTOPIC RESULTS FROM EASTERN PYRENEAN 

LHERZOLITE MASSIFS 

REISBERG L., CRPG/CNRS, B.P. 20, 54501 

Vandoeuvre-les-Nancy Cedex, France, LORAND, J.-P., 

and PATTOU, L., Museum National D'histoire Naturelle, 

Laboratoire de Mineralogie, 61 rue Buffon, 75005 Paris, 

France. 

Orogenic lherzolites present an opportunity to directly 

examine portions of the earth's mantle that have been 

tectonically emplaced in the crust. A recent study 1 of the 

Ronda Ultramafic Complex demonstrated that a strong 

correlation exists between Os isotopic composition and 

major element content in this massif. This contrasts with 

the results of Sr and Nd isotopic studies, which displayed no 

such correlations. These observations suggest that Os 

isotopes may provide information about the evolution of the 

major element content of mantle rocks that is not available 

from other isotopic systems. 

To test whether similar correlations between Os isotopic 

ratio and major element composition are observed in other 

orogenic lherzolites, we have begun a Re-Os study of the 

Eastern Pyrenean ultramafic massifs. 187Qsf186Qs ratios 

from six peridotites analyzed to date range from 0.98 to 1.10 

and show some correlation with alumina content, but this 

relationship is considerably weaker--than that observed in 

Ronda. This difference may result partly from the fact that 

the Pyrenean massifs comprise many small, distinct 

peridotite bodies while the Ronda massif is large and 

contiguous. Furthermore, unlike the Ronda massif, some of 

the Pyreneean lherzolite bodies have experienced modal 

metasomatism. The point which plots furthest from the 

1870sf1860s vs. Al203 trend comes from the Cassou 

massif, where modal metasomatism was most developed. A 

somewhat stronger correlation exists between 1870sf1860s 

and sulfur content, and includes the sample from Cassou. 

In the coming months, we plan to determine the Re and Os 

compositions of numerous other peridotites, as well as 

pyroxenites and amphibolites, from the Eastern Pyrenean 

massifs. This study will be complemented by the analysis of 

platinum group element patterns from these rocks. 

lReisberg, L.C., Allegre, C.J., and Luck, J.-M., The Re-Os 

systematics of the Ronda Ultramafic Complex of southern 

Spain, EPSL, v. 105, p. 196-213. 

264 

ISOTOPE MAPPING OF THE ARABIAN-NUBIAN 

SHIELD: IMPLICATIONS FOR LATE PROTEROZOIC 

CRUSTAL GROWTH. 

REISCHMANN, T., Institut fUr Geowissenschaften, Johannes Gutenberg-Universitiit, 

Becherweg 21, D-55099 Mainz, Germany 

The Arabian-Nubian Shield (ANS) is 

considered to have formed by accretion of arc 

terranes in the late Proterozoic and thus to 

document a major portion of continental 

crust formed at that time. In order to monitor 

the growth and evolution of this part of the 

continental crust, regional variation in Nd 

and Sr isotopes were used to map and to 

distinguish isotopic provinces. The samples 

for this study were taken from the central 

part of the ANS, namely from the Red Sea 

Hills, Sudan and from the western Arabian 

Shield. 

The Nd isotopic variations observed in the 

selected areas of the central ANS of this study 

are eNdt between +5.4 and +8.0. These Nd data 

are considered to approximate late Proterozoic 

mantle values and to indicate a juvenile 

origin of this part of the ANS with no 

significant crustal contribution. 

The 8 7 S r j8 6 Sri of these samples are 

generally low, most of them in the range of 

0. 702 - 0.703 and like the Nd data, the Sr data 

document the ensimatic origin of the central 

part of the ANS. This part is built up by 

juvenile continental crust that formed in the 

late Proterozoic. 

A compilation of these new and previously 

published Nd and Sr isotopic data of the whole 

ANS is used to constrain the crustal growth 

rate in the late Proterozoic. The major part of 

that region is mantle-derived with eNd t of ca. 

+6.5 and Sri of ca. 0. 703 and thus defines the 

area of juvenile crust of the shield probably 

formed by subduction relatect processes. Based 

on these data the arc accretion rate for the 

ANS can be calculated to be 46 km3km-1Ma-1 

which is similar to recent rates. 

Taking 300 Ma as the whole time span for 

crust formation of the ANS the crustal growth 

rate for this· region can be calculated to be 

0.29 km3 a -1. This implies that about 30 % of 

the late Proterozoic crustal growth have to be 

assigned to the ANS, a number which is lower 

than those obtained in previous estimates and 

does not indicate disproportionate crustal 

growth in the ANS. Therefore, crustal growth 

during the late Proterozoic was probably the 

same as it is today. 
ORBITAL TUNINGOF40Ar./39Ar STANDARDS: THE 

CASE OF FISH CANYON SANIDINE 

RENNE P.R. (1), DEINO, A.L. (1), WALTER, R.C. (1), 

TURRIN, B.D. (2,1), SWISHER, C.C (1), BECKER, T.A. 

(1), SHARP, W.D. (1), and CURTIS, G.H. (1): (1) IHO 

Geochronology Center, 2453 Ridge Rd., Berkeley, CA, 

94709, USA; (2) U.S. Geological Survey, MS 901,345 

Middlefield Rd., Menlo Park, CA, 94025, USA. 

The 40 Ar/39 Ar dating method is well-established as one 

of the most precise and versatile methods in the 

geochronologic arsenal, but the accuracy of this method is 

limited by uncertainties as to the ages of neutron fluence 

monitors (dati~ standards). Conventional (K-Ar) age 

calibration of Ar/39 Ar standards has met with difficulties, 

and minerals yielding the most reproducible and correctioninsensitive 40 Arf39 Ar isotopic data prove problematic for 

measurement of absolute isotopic abundances. · 

The astronomically-calibrated geomagnetic polarity time 

scale (APTS) developed by Shackleton et al., (1990) and 

Hilgen ( 1991) offers a means to assess the ages of 40 Arf39 Ar 

dating standards that is independent of absolute isotopic 

abundance measurements. Seven published 40 Arf39 Ar dates 

for polarity transitions, nominally ranging from 0.78 to 3.40 

Ma, are based on the Fish Canyon sanidine (FCs) standard and 

can be compared with APTS predictions. All were based 

originally on FCs at 27.84 Ma except the Brunhes/ Matuyama 

datum, which is recalculated from Spell and McDougall 

(1992). Solving the 40 Arf39 Ar age equation for the age of FCs 

that produces coincidence with the APTS for each of these 

seven reversals yields mutually indistinguishable estimates 

ranging from 27.78 to 28.09 Ma, with an inverse variance 

weighted mean of 27.95 ±0.09 Ma. Normalized residuals are 

minimized at an FCs age of 27.92 Ma, indicating the 

robustness of the solution. 

The APTS-optimized age for FCs corresponds to an age of 

522.1 ± 1.6 Ma for Mmhb-1, based on inter-calibration 

between the two standards. 

Boundary APTS 4oArt39Ar Calculated 

or Age Age(Ma)±s AgeofFCs 

Event (Ma) (FCs=27.84 Ma2 (Ma) ±s 

Brunhes/Matuyama 0.78 0.778 ±0.010 27.91 ±0.50 

Top Jaramillo 0.99 0.992 ±0.039 27.78 ±1.53 

Cobb Mountain 1.19 1.186 ±0.008 27.93 ±0.26 

Top Kaena 3.04 3.042 ±0.039 27.82±0.50 

Bottom Kaena 3.11 3.095 ±0.027 27.97 ±0.34 

Top Mammoth 3.22 3.210 ±0.010 27.93 ±0.12 

Base Mammoth 3.33 3.300 +0.020 28.09 ±0.24 

Weighted Mean 27.95 ±0.09 

Hilgen, F. J., 1991, Earth Plan. Sci. Lett., v. 107, p. 349-366. 

Shackleton, N.J., Berger, A. and Peltier, W. R., 1990, Trans. 

Roy. Soc. Edinburgh, v. 81, p. 251-261. 

Spell, T. L., and McDougall, I., 1992, Geophys. Res. Lett., v. 

19, p. 1181-1184. 

265 

RATES CONSTRAINTS OF RIVER INCISION USING 

COSMOGENIC RADIONUCLIDES: EXAMPLES 

FROM FLUVIAL TERRACES ALONG THE 

FREMONT RIVER, UTAH 

REPKA. James L., ANDERSON, Robert S., DICK, Greg 

S., (Earth Sciences Department, University of California, 

Santa Cruz CA 95064, USA; jlrpk@bagnold.ucsc.edu) 

FINKEL, Robert C., and SOUTHON, John R. (Center for 

Accelerator Mass Spectrometry, Lawrence Livermore 

National Laboratory, Livermore CA 94550, USA) 

We are investigating the incision history of the Fremont 

River in south-central Utah by measuring concentrations of 

in-situ produced 26Al and lOse in fluvial gravels found on the 

surfaces of strath terraces near Caineville, Utah. This in turn 

constrains the baselevel history of tributaries to the Fremont 

River, whose landscape evolution we are studying. 

Downcutting of the Fremont River has created a series of 

distinct fluvial terraces. Strath surfaces cut into Ferron 

Sandstone and Blue Gate Shale members of the Mancos Shale 

are mantled by up to 3m of fluvial deposits that include 

boulders of basalt and cobbles of quartzite, sandstone, 

petrified wood and chalcedony. We are sampling siliceous 

cobbles, primarily quartzites, whose surfaces show no signs 

of post-deposition weathering. 

Interpretation of cosmogenic radionuclide (CRN) results 

from these cobbles is, however, complicated by the potential 

for inherited CRN's from exposure during weathering from 

the source area and during hillslope and fluvial transport to 

the site of deposition. We believe the siliceous cobbles to be 

derived from the Shinarump Conglomerate member of the 

Chinle Formation in Capitol Reef National Park, within 20-

30km of the sampling area. Such clasts weathering from this 

fluvial formation generally are already rounded and display a 

range of sizes similar to that found on the Fremont River 

terraces. This suggests a simple and limited exposure and 

pre-terrace depositional history. We present a numerical 

model used to generate a suite of synthetic cobble CRN 

concentration histories resulting from a range of hypothetical 

transport and burial paths. Measured CRN concentrations of 

cobbles obtained from a terrace surface and from several 

meters below the surface are contrasted in order to constrain 

the model parameters. 

Preliminary results from two of the most extensive 

surfaces yield ages of 75-85ka for the lower terrace and 145-

165ka for the highest terrace. These ages correlate well with 

isotope stages 4-5a and 6, respectively, corresponding to a 

relatively steady 0.6-0.8mm/yr long-term incision rate of the 

Fremont River. 
THE TRANSFER OF TERRANES FROM SOUTH TO 

NORTH AMERICA BASED ON THE PROTEROZOIC 

AND PALEOZOIC EVOLUTION OF COLOMBIA AND 

SOUTHERN MEXICO 

RESTREPO P.; RUIZ, J., Department of Geosciences, 

Universitv of Arizona, Tucson, Arizona 85721, 

COSCA. ·M.. Institut de Minerlogie University of 

Lausanne. Lausanne. Switzerland. 

The Precambrian and Paleozoic tectonic evolution of 

Colombia and southern Mexico is central to tectonic models 

related to the opening and closing of the proto-Atlantic and to 

the ultimate paleogeographic reconstructions of Pangea. 

Paleogeographic reconstructions based on paleomagnetic 

data suggest that parts of Colombia collided with southern 

Mexico at least twice during the Paleozoic. These models are 

supported by Ordovician trilobites of southern Mexico, 

which are very similar to those of Colombia and Argentina. 

Basement rocks in southern Mexico are Grenvillian in age 

based on U-Pb. Sm-Nd and Rb-Sr geochronology. Paleozoic 

tectonothermal events that affected parts of southern Mexico 

occurred at ca. 380Ma (Acadian Orogeny) and 270 Ma. 

There is no evidence for an early Paleozoic (Taconic) event 

(Yafiez et al.l991). Because of the fossil evidence, as well as 

the fact that the Grenville age rocks occur to the east of the 

Paleozoic rocks. which is opposite to the geometry in North 

America, it is likely that Southern Mexico was part of South 

America prior to the final opening of the Atlantic. In 

Colombia, Proterozoic and Paleozoic rocks are found in the 

Santander. Garzon and Santa Marta Massifs and the Puqui 

Complex. The oldest rocks found in the Santander and Santa 

Marta massifs. based on 40 Arf39 Ar are Grenvillian in age 

(ca. 1000Ma) We have no evidence of older rocks in these 

areas. There are only two Paleozoic tectonothermal events 

recognized: a pre-380 Ma and £a& 200 Ma. The 200 Ma 

metamorphic rocks are related to the Jurassic arc of South 

America, which extends north into Colombia. The preDevonian age may represent the Taconic orogeny, which is 

absent in southern Mexico. The field paleontologic, and 

geochronological data of Colombia and southern Mexico 

suggest that the collision represented by the Taconic orogeny 

occurred south of what is presently southern Mexico. The 

collision represented by the Acadian orogeny must have 

occurred D.Q!lh_of southern Mexico leaving most of Mexico 

behind in the final opening of the Atlantic. Thus during the 

Paleozoic, terranes that were part of south America were 

transferred to north America. These terranes are fingerprints 

of a geologic history but do not represent additions of 

continental crust during the Phanerozoic. 

Y afiez, P ., Ruiz. J., Patchett, P J ., Ortega-Gutierrez, F., and 

Gehrels, G .• 1991, The Acatlan Complex, Geological Society 

of America Bulletin v 103. p. 817-828. 

266 

EVALUATION OF ARGON STANDARDS WITII 

SPECIAL EMPHASIS ON TIME SCALE 

MEASUREMENfS. 

REX,D.C. and GUISE,P.G., Dept of Earth Sciences, 

University of Leeds, Leeds, LS2 9JT, UK. 

The urgent requirement for standards which have been 

analysed by K-Ar and other methods to calibrate Ar-Ar 

analyses is well known. The precision of the Ar-Ar method 

is as good as 0.1% but the accuracy depends on the K-Ar 

calibrated standards used to monitor the irmdiation 

parameters. 

The table below shows Leeds ( for method, see 

LeMasurier & Rex 1989) and published K-Ar ages for 

various standards . 

S1awJanl fllhli~d Lm11 

MMHb-1 514-525 ** 

Hb3gr 1064-1071 1064 

IID-B1 24.0 24.4 

LP-6 128.9 130.0 

Bem4M 18.6 18.5 

BSP-1 2060 2073 

133 Bio 1013 1033 

NancyMg 318 314 

RoM 424 

Tin to 410 412 

Fy12a 436 442 

Fy12b 435 430 

** Evidence of inhomogeneity of MMHb-1 has been 

noted. We bad insufficient material for K-Ar analys~s, but 

report a %K content of 1.552 ±0.002 (2 sigma). 

Aliquots of all the standards above were irradiated in the 

Ford Reactor at Ann Arbor. Using MMHb-1, Hb3gr and 

Fy12a as Roddick (1983) the flux gradient over the length 

of the silica tubes was 3% (full experimental details in Rex 

et al1993). Recalculating the 'J' values using the K-Ar 

values from the table, changed the fit of the flux curve and 

increased the 'J' error from 1% to 1.5% ( 2 sigma). 

Roddick has shown that hand picking of standards 

produces 'J' curves with little scatter and therefore high 

precision. However, if this approach is to be used for time 

scale and interlaboratory comparison of unknown samples, 

then unpicked standards with agreed K-Ar ages must be 

included so that the J calibration can be adjusted to an 

absolute framework. 

Le Masurier, W.E. & Rex,D.C. Evolution of linear 

volcanic ranges in Marie Byrd Land, West Antarctica. 

J .G.R., 94, (1989) No.B6 , 7223-7236. 

Rex,D.C.,Guise,P.G. & Wartho I.-A. Disturbed 40Ar39 Ar spectra from hornblendes: Thermal loss or 

contamination? Chem. Geol .(lso. Geo. Sec.), 103 

(1993) 271-281. 

Roddick,J.C. High precision intercalibration of40Ar_39Ar 

standards. G.C.A. 47 (1983) 887-898. 
ORIGIN OF LHERZOLITIC DIAMONDS SAMPLED BY 

SOUTHERN AFRICAN KIMBERLITES 

RICHARDSON, Stephen H., Dept. of 

Geological Sciences, University of Cape 

Town, Rondebosch 7700, South Africa. 

Diamonds with lherzolitic inclusions 

(olivine, orthopyroxene, garnet and 

clinopyroxene) constitute only a small 

fraction of inclusion-bearing diamonds 

worldwide. Nevertheless, there has been 

considerable interest in relating the 

conditions of origin of lherzolitic 

diamonds to those of harzburgitic diamonds 

(the dominant type of peridotitic diamond) 

or, alternatively, to those of eclogitic 

diamonds. 

In the case of the 1180 Ma old Premier 

kimberlite, all three types are well 

represented. They vary in age from >3000 

Ma (harzburgitic) to -1180 Ma (eclogitic). 

In addition, they show a range in 

precursor isotope signature from strongly 

enriched (high Rb/Sr; harzburgitic) to 

strongly depleted (high Sm/Nd; eclogitic). 

Lherzolitic garnet and clinopyroxene 

inclusions yield an intermediate Sm-Nd 

isochron age of 1930 ± 40 Ma with a mildly 

enriched isotope signature (Richardson et 

al., 1993). This age is -100 Ma less than 

that of the adjacent Bushveld Complex 

which represents one of the largest mantle 

melting events beneath the Kaapvaal craton 

during the Proterozoic. 

A similar age for lherzolitic diamonds 

from the Cretaceous Kimberley kimberlite 

cluster may be inferred by comparison of 

the above data with that for a 

'websteritic' clinopyroxene inclusion from 

a Kimberley diamond (Smith et al., 1991). 

A temporal, spatial and compositional link 

between Bushveld style magmatism and 

lherzolitic diamond formation can be 

developed, by analogy with that between 

komatiitic magmatism and subsequent 

harzburgitic diamond formation in the 

Archean. 

Richardson, S.H., Harris, J.W., and 

Gurney, J.J., 1993, Three generations 

of diamonds from old continental 

mantle: Nature, 366, 256-258. 

Smith, C.B., Gurney, J.J., Harris, J.W., 

Otter, M.L., Kirkley, M.B., and 

Jagoutz, E., 1991, Nd and Sr isotope 

systematics of eclogite and websterite 

paragenesis inclusions from single 

diamonds, Finsch and Kimberley Pool, 

RSA: Geochim. Cosmochim. Acta, 55, 

2579-2590. 

267 

ESR DATING OF TOOTH ENAMEL IN 

ARCHAEOLOGICAL SITES: INTERCOMPARISON 

WITH 14C DATES 

RINK. W .J., Dept. of Geology, McMaster 

University, Hamilton, ON, Canada LSS 4Ml, and 

SCHW ARCZ, H.P., Dept. of Geology, McMaster 

University, Hamilton, ON, Canada LSS 4Ml. 

Electron spin resonance. (ESR) dating of tooth enamel is 

based on the measurement of the growth of a radiationsensitive ESR signal present in hydroxyapatite of 

mammalian tooth enamel. This material is a ubiquitous 

component of all archaeological sites. In contrast, other 

methods such as U-series datmg of carbonates, TL dating 

of burned flints, or 40 Arf39 Ar dating of volcanics, all 

require samples which are relatively rare at Palaeolithic 

sites. The range of ESR da(ing is from about 10 ka to 2 

Ma, which extends the dating range for most sites beyond 

the limits of 14C (c. 40 ka). We report here on attempts to 

make intercomparisons with 14C and other methods over 

their overlapping ranges, in order to confirm the validity 

of ESR dating. The site of Das Geissenklosterle 

(Germany) contains a an archaeological sequence of 

Mousterian, proto-Aurignacian, Aurignacian, and 

Gravettian artifacts, whose enclosing sediments have been 

dated by 14C, and for which we will present comparative 

ESR data on teeth obtained from these deposits. The 

Spanish site of El Castillo contains a deep stratigraphic 

sequence including Acheulian, Mousterian and 

Aurignacian deposits. We have made comparative ESR 

dates on teeth from near the AMSJ14C-dated 39 ka 

boundary between Mousterian and Aurignacian; and for 

the Mousterian sequence whose age is constrained by a 

U -series isochron date for an intercalated flowstone. 
ISOTOPE SYSTEMATICS OF CONTAMINANT LEAD 

FLUXES IN THE SAN FRANCISCO BAY ESTUARY 

RITSON, P.R., Earth Science, WIGS, University 

of California, Santa Cruz, Calfirornia, 95064, USA, and 

FLEGAL, A.R., Earth Science, WIGS, University of 

California, Santa Cruz, California, 95064, USA. 

Measurements of lead in the San Francisco Bay 

estuary were initiated after isotopic composition analyses of 

lead in the northeast Pacific indicated that relatively little of 

the lead discharged in the estuary was advected into adjacent 

coastal waters. This was corroborated by the heterogeneous 

distribution of dissolved lead concentrations ( < 10 to 300 

pM) and its high partition coefficient (Kd • lOS) within the 

estuary, which both attest to the efficacy of biogeochemical 

scavenging processes in those waters. Conversely, the 

homogeneous distribution of lead isotopic compositions 

(107Pbf206pb • 0.8446) in the dissolved phase, suspended 

particulates and surficial sediments throughout the estuary 

attest to the extensive remobilimtion of lead from sediments 

deposited within the system. Those benthic lead fluxes were 

previouslyestimated to range from 1.4 x lOS to 2.2 x 106 kg 

yr1

, which is 5 to 6-orders of magnitude greater than the 

fluvial input of dissolved lead to San Francisco Bay and 

comparable to the anthropogenic input of industrial lead to 

that embayment (Rivera-Duarte and Flegal, 1994). With the 

contrasting distributions of lead concentrations and isotopic 

compositions, those estimates corroborate Turekian' s 

proposal that estuarine sediments are a trap for this particle 

reactive element even though it may be extensively 

remobilized by diagenic processes. 

Rivera-Duarte, I., and A.R. Flegal. 1994. Benthic lead 

fluxes in San Francisco Bay. Geochimica et Cosmochimica 

Acta (in press). 

268 

DEUTERIUM-RICH- WATER IN THE SOLAR 

SYSTEM. 

ROBERT F., Museum Histoire Naturelle, Lab. 

deMineralogie. 61 rue Buffon.75005.Paris.France & 

DELOULE E. CRPG-CNRS, BP20, 54501 

Vandoeuvre les Nancy Cedex France 

The 0/H ratio of the solar nebula H2 was close to 

20.10-6 (i.e. bD = -870 o/oo). Based on recent laboratory 

determinations on isotopic reaction rates (Lecluse & 

Robert), the classical exchange model HD + ~o <--> 

HDO + H2 has been re-investigated : D/H ratios in 

protosolar water, similar to those measured in 

meteoritic phylosilicates or on Earth (D/H < 180.10-6 

i.e. bD < +150 o/oo), can be reached during the life time 

of the nebula (106 years). 

However, the isotopic composition of the deuteriumrich water bearing minerals (bD +3500o/oo i.e. D/H = 

700.10-6) discovered in the Semarkona meteorite . 

(Deloule & Robert, 1994) is in clear disagreement with 

such a thermal model. These phyllosilicates are 

probably under form of deuterium-rich clusters where 

organic macromolecules are in close association with 

-OH bearing minerals. 

An alternative interpretation for this deuterium 

enrichment in water, is to consider that an isotopic 

fractionation occurred via photo-chemical or ionmolecule reactions. It is possible to model such an 

~nvironment by reactions such as : 

D + H3o+ <----> H 2oo+ + H & H3o+ + e ----> H20 + H 

Using the usual astrophysical approximations (Duley 

& Williams, 1984). reaction rate calculations show that, 

for a fractionational ionisation lying .between to-5 and 

1 o-6, the meteoritic phyllosilicate D/H ratio can be 

reached at temperatures between 120 and 80K. These 

results suggest that synthesis of water has been 

confined in the low pressure outer regions of the nebula 

(probably because of the high opacity in the mid plane 

of the nebula), where ions or radicals can be produced 

by uv molecular dissociation. The source of UV's in 

these regions is the interstellar radiation field. 

According to the present isotopic results, it seems 

impossible to admit that the solar system water is a byproduct of a condensation sequence in the solar nebula. 

DELOULE E. & ROBERT F. See the present volume 

abstract. 

DULEY W.W. & WILUAMS D.A. Interstellar chemistry. 

Academic Press 1984. 

LECLUSE C.& ROBERT F. Geochim & Cosmochim. Acta 

(in press) 
THREE REGIONALLY COHERENT LOWER CRUSTAL 

MAGMA SOURCES FOR MESOZOIC GRANITOIDS IN 

NEVADA AND EASTERN CALIFORNIA NORTH OF 37° N 

ROBINSON. A. C .. U.S. Geological Survey, Menlo Park, 

California, 94025 

Strontium whole-rock isotopic analyses of 2137 Mesozoic 

granitoid samples from the western three quarters of Nevada 

and eastern California north of 37° N latitude form three 

coherent fields on a measured 87 S rf86Sr versus 87R b/ 86sr 

diagram. Although the granitoids are of diverse age, these 

fields relate functionally to inferred source isochrons of three 

regionally coherent lower crustal magma sources: (1) an Early 

Proterozoic ( -1800 Ma) lower continental crust of basaltic 

composition, yielding granitoid magmas with initial 87Srf86sr 

(Sri) in the range 0.7048-0.7110; (2) a slightly alkalic basaltic 

(higher Rb/Sr) Late Proterozoic ( -760 Ma) rift-generated lower 

crust, emplaced during incipient rifting beneath uplifted and 

attenuated continental crust along the locus of the rift, yielding 

Sri in the range 0.7050-0.7072; and (3) a Late Proterozoic 

(-760 Ma) rift-generated lower crust, of composition similar to 

( 1) and emplaced as oceanic crust as the continental segments 

spread apart, yielding Sri in the range 0.7038-0.7052. 

Histograms of granitoid Sri are normally distributed for sources 

(2) and (3) and for source (1) over the range 0.7048-0.7080. 

Forty-two published Nd whole-rock isotopic analyses of 

Mesozoic granitoid samples from the same region form three 

coherent fields on a measured 1 4 3Ndf144Nd versus 

147Smf144Nd diagram that are analogous to the Rb-Sr fields, 

and that relate to inferred source isochrons of comparable Early 

and kate Proterozoic ages. The granitoid Ndi ranges are: 

source (1), 0.51180-0.51247; source (2), 0.51205-0.51245; and 

source (3 ), 0.51250-0.51278. This diagram illustrates that 

lower crustal sources (1) and (3) were derived from relatively 

depleted mantle, whereas lower crustal source (2) was derived 

from relatively undepleted mantle near CHUR in composition. 

The Sri and Ndi ranges for granitoids of source (2) lie 

within the granitoid Sq and Ndi ranges of source (1), so that 

other criteria are needed to distinguish between granitoids of 

sources (1) and (2). The slightly alkalic composition of source 

(2) serves this need. Normally-distributed histograms of Rb 

and Sr concentration in granitoids from sources (1) and (3) are 

coincident, whereas normally-distributed histograms of Rb and 

Sr concentration in granitoids from source (2) are offset to 

higher concentrations. Consequently, the overlapping data 

fields for granitoids from sources (1) and (2) on the measured 

87Sr/86Sr versus 87Rbf86sr diagram tend to separate into 

discrete fields on the measured 87Srf86sr versus Rb 

concentration diagram. 

On the ENdi versus Sri diagram the three coherent lower 

crustal sources are manifested as discrete linear trends, the 

regressed slopes of which are proportional to the ratios of the 

parent/daughter Rb/Sr and Sm/Nd ratios in the sources. This is 

demonstrated graphically by plotting source 147Smf144Nd 

versus 87Rbf86sr obtained by solving intersections of granitoid 

isochrons with inferred source isochrons. 

The geographic boundary between Sri < 0.7050 and Sri > 

0. 7050 conceptually locates the Late Proterozoic rifted 

continental margin. This boundary has two 90° ridgetransform-like bends, one north of Lovelock, Nevada and 

another a little south of Austin, Nevada. In eastern California, 

the boundary has been disrupted by at least 300 km of 

cumulative right-lateral offset along NNW-trending crustal 

shears of possible Early Jurassic age. 

269 

KINETICS AND CHEMISTRY OF THE ZIRCON 

EVAPORATION TECHNIQUE 

Roddick. J.C .. Geological Survey of Canada, 601 Booth 

St., Ottawa, Ontario, KIA OE8 Canada, and 

Chapman, H.J., Dept. Earth Sciences, University of 

Cambridge, Downing St., Cambridge CB2 3EQ, U.K. 

The zircon evaporation technique is increasingly being used 

to determine 207Pb • f 06Pb • (radiogenic Pb) ages. The process 

is now understood to involve the breakdown of zircon to 

porous Zr02 (baddeleyite ), with the associated loss of mainly 

silica, along a reaction front which progresses into the 

interior of a grain {Ansdell and Kyser, 1993; Roddick and 

Chapman, 1991). To further understand the kinetics and 

chemistry of the process a seri.es of experiments have been 

carried out. The rate at which the process takes place has 

been determined by heating a series of high quality nonmetamict zircon grains at different temperatures and for a 

range of times. Polished sections of these grains show the 

progress of the reaction front as a function of temperature 

(1470°C to 1670°C) and time (10 min. to 6 hr.). Optical 

pyrometers used in the experiments were accurately 

calibrated by melting pure metals (Au, Ni, Pt) in folded Re 

filaments with the same geometry as used in heating zircon 

grains. The process is strongly temperature dependent and 

follows an Arrhenius relation with an activation energy of 

130±10 {la) kcal.·mole-1 and a reaction constant at infmite 

temperature of about 4x1012 J.Lm·sec·• .. These parameters 

indicate that a 100 ~m diameter zircon grain is completely 

converted to Zr02 in 6 hr. at 1600°C and 1 hr. at 1700°C. 

The chemistry of the process has been determined by ICPMS analyses of residual Zr02 grains and ICP-MS laser 

ablation of companion deposits on filaments usually used for 

Pb isotopic analyses. Material vaporized and deposited 

consists mainly of a mixture of Si02 (70%) from the zircon 

and Re (30%) from the filament used to heat the zircon. 

Other trace elements in zircon are also vaporized to varying 

degrees. Most Pb, Ce and about 50%· of the U are 

quantitatively evaporated. Lesser amounts of Ba are 

transported. REEs (except Ce ), Hf, Y and Th are retained in 

the residual Zr02• In evaporation dating U readily ionizes as 

UO/ at a lower temperature than Pb+ and is useful for 

focusing and optimizing the beam both during the deposition 

stage and prior to Pb isotopic analysis. 

Ansdell, K.M. and Kyser, T.K., 1993, Textural and chemical 

changes undergone by zircon during the Ph-evaporation 

technique: Am. Min. 78, 36-41. 

Roddick, J.C. and Chapman, H.J., 1991, 207Pb*f06Pb* dating 

by zircon evaporation: Mechanisms of Pb loss: EOS, 

Trans. Am. Geophys. Union 72, 531. 
EXCHANGE BETWEEN C02 AND Ag3P04 AT HIGH 

TEMPERATIJRES: A NEW METHOD FOR 

DETERMINING THE OXYGEN ISOTOPE COMPOSITION 

OF PHOSPHATIC MATERIALS 

ROE LJ, Dept. Geosci., Univ. of Arizona, Tucson, AZ 

S571S, O'NEIL, J.R. and REINHARD, E.A., Dept. 

Geol.Sci., Univ. of Michigan, Ann Arbor, MI 4Sl09 

Here we report for the frrst time that C02 exchanges readily 

with Ag3P04 at high temperatures and propose controlled 

equilibrium exchange as a method of determining the oxygen 

isotope composition of phosphates. The advantages of this 

method are: (1) it is the simplest, fastest, and least expensive 

method available for the analysis of phosphate oxygen; (2) it 

requires neither fluorination nor graphite reduction and (3) it 

can be done in glass tubes at temperatures low enough to 

reduce or eliminate the problem of exchange between the 

oxygen in the tube and the reagents. 

In initial experiments done at the University of Arizona, 

fairly large (24-50 mg) samples of Ag3P04 precipitated from 

NBS-120C Florida phosphate rock were placed in 6mm Si02 

tubes and evacuated to 5*lo-4 torr. An aliquot of one of two 

different C02 reference gases was frozen into each tube, and 

the tube sealed. The alSo values of the two reference gases 

were+ 7 .6%o, and +34.0%o, and the alSo of the NBS standard 

was +21.S%o. Thus, equilibrium exchange was approached 

from opposite directions. The samples were then heated at 

S7s·c for 10 minutes, withdrawn rapidly from the furnace and 

quenched in a beaker of water. The C02 samples were 

expanded into the inlet system of the mass spectrometer 

without purification and ran very cleanly. The Ag3P04 

retained its original color, giving support to the conclusion 

that no chemical reaction had occurred. In all cases the 

resultant isotopic composition of the C02 was different from 

its initial value, and had a alSo value 1.95-3.25%o higher than 

the Ag3P04 (corrected for mass balance). C02-Ag3P04 

exchange was confirmed in separate experiments run at the 

Institut de Min~ralogie et P~trologie in Lausanne, Switzerland, 

using a C02 reference gas with a alSo value of 7 .74%o and 

30-35 mg samples of a Ag3P04 standard made from KH2P04 

at the Carnegie Institution and reputed to have a alSo value of 

11.9%o. These samples were heated at S75·c for 5 and 10 

minutes, with no significant difference in the resultant alSo 

value, indicating that isotopic exchange is complete within 

five minutes. The final alSo value of the C02 in these 

experiments was 3.5S%o- 3.79%o higher than the presumed 

value of the Ag3P04. 

We are now conducting experiments to determine the 

optimal conditions for the use of controlled C02-Ag3P04 

equilibrium exchange as a method for determining the oxygen 

isotope composition of phosphates and the fractionation factor 

over a wide range of temperatures. Initial results indicate that 

isotopic exchange does not require as high a temperature as did 

our earlier method of reacting graphite and Ag3P04 at 125o·c. 

It may therefore be possible to achieve oxygen isotope 

exchange between C02 and Ag3P04 at times and temperatures 

at which the glass tubes will not exchange their oxygen with 

the C02. Alternatively, this exchange reaction could be done 

in metal tubes. 

270 

LARGE SCALE OOMING AND ASSOCIATED BASIN 

EVOLUTION IN SOUTH NORWAY: EVIDENCE FROM 

APATITE FISSION TRACK THERMOCHRONOLOGY 

ROHRMAN. M., P.A. VAN DER BEEK and 

ANDRIESSEN, P.A.M., Dept. of Geology, Free University, 

1081 HV, Amsterdam, The Netherlands. 

We present Apatite Fission Track (AFT) ages and confined 

track length distributions of Caledonian and Precambrian 

basement rocks in Southern Norway and offshore commercial 

boreholes. As no post-Permian rocks are present on the South 

Norwegian mainland, the post-Permian evolution has been badly 

constrained. This in contrast with the bordering North sea basin 

as a result of hydrocarbon exploration. 

The combined results of AFT data from the mainland and 

reflection seismics and AFT data from the North sea basin 

constrains an unique combination of basin and margin 

evolution, which can be used to test rifting and erosion models. 

AFT thermochronology on more than 50 samples of the 

Precambrian/Caledonian South Norwegian mainland, including 

transects and elevation profiles up to 2469 m above sea-level, 

show isochrons of AFT ages which closely follow the summitheights and peneplanation surfaces. 

The AFT ages show a bending of the isochrons, with the 

youngest ages of around 100 Ma and mean track lengths around 

11.8 J.lffi with a broad (2pm) standard deviation lying deep in 

the fjords at sea level. Ages and mean track lengths increase 

towards the peaks of the mountains and the West coast to about 

160 Ma and 13.5 Jllil respectively and narrow standard deviation 

(1.2 J.lffi). No clear break-in-slopes are observed, this agrees 

with the low thermal gradient (20°C/km) for this area, which 

would require fast uplift of > 3km to become visible. In the 

Southeast the ages increase to more than 200 Ma, defining a 

structural dome with a wavelength of 400 km and an amplitude 

of around 4 km. The AFT ages indicate that doming must have 

taken place less than 100 Ma ago. 

AFT results from commercial wells offshore Norway show 

AFT ages of around 160 Ma for the 2 km thick Neogene 

sedimentary wedge. Mean track lengths are around 12.9 micron. 

They can be backstacked on the mainland and show a clear 

relation between basin evolution and hinterland erosion. 

Modelling of AFT ages and confined track length 

distributions suggest two major episodes of exhumation and 

cooling~ a Jurassic and Neogene event. We suggest a scenario 

in which Jurassic uplift coincides with active rifting in the 

North Sea and was followed by peneplanation during the 

Cretaceous and Paleocene. This is also shown in offshore 

carbonate and clay sedimentation during these times 

respectively. Furthermore the seismics and sedimentation 

patterns indicate sediment transport directions from the 

Shetlands in the Paleocene, argueing against uplift of the 

Norwegian margin around that time. Rapid Neogene domal 

uplift corresponds to the build-up of the Neogene sediment 

wedges in the Northern North Sea basin. 
BURIAL OF LIGHT SULFUR IN NEOPROTEROZOIC 

CONTINENTAL MARGIN STRATA, SOUTHERN 

CANADIAN CORDILLERA: IMPLICATIONS FOR 

UNDERSTANDING SEA WATER EVOLUTION 

ROSS. GERALD M., Geological Survey of Canada, 

3303 33rd St. N.W., Calgary, Alta. T2L 2A7; BLOCH, 

John D., Scealu Modus, 2617 Cutler Ave NE., 

Albuquerque, New Mexico; 87106 and KROUSE, H.R, 

Department of Physics, University of Calgary, Calgary, 

Alta. T2N 1N4 

The Neoproterozoic was a fundamental time in Earth 

history during which the intertwining of tectonic, climactic 

and geochemical processes culminated in the first 

appearance of metazoan life forms. Available B13C data 

from marine carbonates indicate dramatic variations in the 

record of organic carbon burial which affects the oxygen 

reservoir, a key control on evolution of metazoan 

respiration. A missing element from existing isotopic 

studies has been data from sedimentary rocks of deep water 

aspect, as well as a clear understanding of the role of sulfur 

in these changes. We present the results of the isotopic 

analysis of pyrite from deep water Neoproterozoic 

sedimentary rocks, thus providing a new perspective on 

evaluating paleoenvironmental changes in this time period. 

The Neoproterozoic record in the southern Canadian 

Cordillera (SCC) is characterized by sedimentary rocks 

formed on the slope of a passive continental margin. 

Isotopic data (170 analyses) from the analysis of authigenic 

pyrite that is common in these rocks indicate a large 

component of 32S-enrichment with values up to B=-31 

permil. This degree of 32S enrichment is unknown from 

Neoproterozoic rocks but is a necessity to balance the 

isotopic composition of evaporitic sulfur. The isotopic 

compositions of the pyrites span a broad range of ca. 50 

permil in each of two units examined, indicative of sulfide 

formation by bacterial sulfate reduction, perhaps in a 

euxinic marine basin, and influenced by diffusioncontrolled sulfate availability. The most 34S-enriched values 

of the sulfides (+18 and +32) are inferred to approximate 

the isotopic composition of sea water preserved as a _ 

consequence of the rate of sulfate reduction exceeding the 

rate of sulfate diffusion (i.e. effectively a closed system). 

A shift to 34S-enriched compositions (+32) oflate 

Neoproterozoic sea water is known from evaporites (e.g. 

Claypool et al.) but the 34S-depleted sulfide reservoir has 

never been found, leading to an enigma in the mass balance of 

sulfur. We suggest that the SCC is an analogue of the process 

of 32S burial that may have occurred on a more widespread, 

but rarely preserved, scale in the Neoproterozoic and 

eventually led to enrichment of sea water sulfate in 34S. Our 

calculations, made with reference to a Rayleigh distillation 

model, imply bacterial reduction of 20-30% of the global 

sulfate reservoir in order to produce the observed shift. Sulfate 

reduction on this scale would dramatically affect the burial of 

organic carbon through anaerobic oxidation, hence releasing 

abundant 12C into sea water. This mechanism provides an 

alternative explanation for the dramatic 12C enrichment 

observed in Neoproterozoic marine carbonates, consistent with 

available stratigraphic correlations. 

272 

OSMIUM ISOTOPIC COMPOSITION OF 

SULFIDES FROM BASAL TIC GLASSES 

ROY -BARMAN, M., WASSERBURG, G. J. and 

PAPANASTASSIOU, D. A., Div. Geol. PlaneL Sci. 

(170-25), Caltech, Pasadena, CA, 91125 USA 

Osmium isotopic composition is a potential tracer of 

old oceanic crust recycling and thus analysis of 

187Qsfl860s in oceanic basalts should provide a lot of 

information on mantle processes. Direct measurement of 

187ostl86os in MORBs is difficult due to the low osmium 

content of these rocks (1 - 200 ppt) and the possibility of 

contamination by sea-water [1]. Taking advantage of the 

highly chalcophile character of osmium (PGEs are scavenged 

from the magma by immiscible sulfide globules [2]), Martin 

et al. have demonstrated the possibility of analysing 

187 Osfl86os on a single large sulfide globule from a 

MORB [3). However, such large globules are rare and thus 

this simple procedure cannot be used for most MORBs. ln 

order to analyse 187ostl86os on a large variety of MORBs, 

it is necessary to concentrate sulfides from a large amount of 

sample. 

We have set up a procedure for extracting sulfides from 

up to several hundred grams of basaltic glass. This procedure 

is based on magnetic and heavy liquid separation. We obtain 

two sulfide fractions: a coarse grained fraction (>40 Jlm) 

which is an almost pure sulfide concentrate and a (ine 

grained fraction (<40 Jlm) which contains sulfides diluted 

with olivine+ chromite. We have tested this procedure with 

two osmium rich samples: a MORB from the FAMOUS 

area (ALV 526-1 also host of Martin's globule) and a lava 

from Loihi seamount (BENTH 4). Measurements are done 

by NTIMS. Results are listed below: 

ALV 526-1 

fine fraction 

coarse fraction 

BENTH4 

187os/186os = 1.1009 ± 0.0014 

lS7ostlS6os = 1.0894 ± 0.0011 

[Os] sulfide= 445 ppb 

fme fraction 187os/186os = 1.0961 ± 0.0014 

coarse fraction 187os/186os = 1.0956 ± 0.0018 

[Os) sulfide= 890 ppb 

These results are in good agreement with previously 

reported data [1,3-4]. Using theIr concentration [2] as a proxy 

for Os concentration of sample 526-1, we estimate that 

osmium recovery is around 40% and that the enrichment 

factor of Os in the coarse and the fine fraction is 2800 and 6 

respectively. This represents a substantial improvement in 

the analysis of osmium. It will permit the measurement on 

low concentration lavas with minimal sea-water 

contamination, very low blank contribution and possibly 

reduced effect of Re in situ decay. These preliminary results 

suggest that the sources of MORBs and Loihi are quiet 

similar in term of 187Re-187os systematics. We are now 

using this procedure to obtain a series of MORB isotopic 

composition. Division contribution 5369 (843) 

[1] Roy-Barman et al., 1993, EOS, vol. 74, p 120 

[2) Peach et al., 1990, Geochim. Cosmochim. Acta, vol 54, 

p. 3379-3389 

[3] Martinet al., 1993, EOS, vol. 74, p 121 

[4] Pegram & Allegre, 1991, Earth Planet. Sci. 

Lett., vol. 111, p. 59-68 
ISOTOPIC CONTENT OF C01 EXTRACTED FROM 

CARBONATES DURING LASER PYROLYSIS 

Romanek, C.S., Socki, R.A., and Gibson, E.K., Jr., 

SN4 AND LESC, NASA, Johnson Space Center, 

Houston, TX 77058, USA. 

Laser-assisted extraction of c~ was developed to provide a 

fast, high resolution, in-situ technique for the carbon and 

oxygen stable isotope analysis of carbonate minerals. This 

technique compliments established acid digestion procedures 

but eliminates the problems associated with the physical 

handling and reaction of micro-gram size samples. Like the 

acid digestion technique, the isotopic composition of the solid 

is determined from C~ using a correction factor. However, 

correction factors used in laser-assisted analysis can vary for a 

given mineralogy. Variability is controlled not only by the 

flux of energy reaching the target, but also by the intrinsic 

(i.e., laser-solid coupling) and extrinsic (e.g .• crystal defects) 

character of the solid. 

Thick sections of calcite and dolomite were analyzed at an 

identical spatial resolution using both the acid digestion and 

laser technique to demonstrate how sampling protocol 

influences the isotopic composition of laser-generated C02• 

Continuous wave and high repetition rate Q-switch mode 

irradiation (low peak power) promote target melting and the 

fonnation of a COrrich gas phase. Low repetition rate Qswitch mode irradiation (high peak power) promotes target 

sublimation and the production of a gas phase containing C02• 

CO, and 0 2, which is documented for the first time in lasergenerated gas. C~ concentration co-varies with repetition rate 

in Q-switch mode. Gas speciation can be described by a massbalance mixing of the gaseous products from the two 

reactions: 

CaC03<•> --> CaOc•> + C02W 

CaC03<•> --> CaO<•> + CO(&) + In. ~ 

which occur in cw and Q-switch mode, respectively. Minor 

sublimation, which occurs in cw mode, is the result of a 

transient energy flux which accompanies the initial release of 

irradiation from the laser. 

The isotopic composition of C02 increases with the 

percentage of C~ in laser-generated gas. Since modeled 

equilibrium compositions predict an inverse relationship with 

C02 yield, exchange solely among gaseous phases cannot not 

account for the observed trends. The S13C of C02 can be 

explained by isotopic exchange between C02 and recrystallized 

CaC03. Depleted S180 values, which generally occur at low 

repetition rates, are probably caused by kinetic effect during 

short-lived devolatilization events that occur at the target 

surface. Secular enrichments in 1BQ, sometimes observed 

during sampling in cw and high repetition rate Q-switch 

. modes, are always associated with large volumes of melt. 

Perhaps secular trends reflect isotopic fiactionation by 

adsorption/desorption reactions occurring on the surface of 

recrystallized solid by-product during sampling. 

271 

EUROPLUM PUDDING: FLUID AND MELT 

INHLTRA TION INTO Tiffi LITIIOSPHERE 

ROSENBAUM. J. M., Dept. of Earth Sciences, University 

of Leeds, Leeds LS2 9JT UK, WILSON, M., Dept. of Earth 

Sciences, University of Leeds, Leeds LS2 9JT UK, and 

DOWNES, H., Research School of Geological & 

Geophysical Sciences, Birkbeck College & University 

College London, London WClE 6BT UK. 

Widespread Tertiary-Quaternary mafic magmatism in 

western and central Europe has exhumed abundant spinel 

lherzolite facies mantle xenoliths which have been used to 

study the geochemical evolution of the lower part of the 

lithosphere. Detailed trace element and Sr-Nd-Pb isotopic 

studies of xenoliths from the Massif Central of France, the 

Rhenish Massif of Germany and the Pannonian Basin of 

Hungary, suggest that at least two distinct processes have 

modified the lithospheric mantle. There is widespread evidence 

across Europe of silicate melt infiltration from an enriched 

mantle source region, chemically similar to the source of 

IDMU ocean-island basalts (OIB), into a depleted mantle 

protolith. Within the mantle lithosphere beneath the 

Pannonian Basin there is also evidence of lithospheric 

enrichment by aqueous fluid infiltration into a rapidly 

extending back-arc basin tectonic regime, which can be related 

to Tertiary subduction along the Carpathian arc. The 

geochemical signature of fluid metasomatism has been partially 

overprinted by silicate melt infiltration with the same 

geochemical characteristics to that observed beneath the Massif 

Central. 

Lead isotopes appear to be the most sensitive indicator of 

the subduction-related fl~x which has a distinctively enriched 

207pb signal, with 207pbt204pb ratios greater than 15.74 for 

206pb;204pb ratios less than 18.5. While affecting the Pb 

isotopic composition of the lithosphere, the subduction fluid 

does not significantly alter the rare earth element (REE) content 

of the mantle. The plume-type component, however, affects 

both the radiogenic isotopic composition and the REE 

signature of the fluxed lithospheric mantle. Sr, Nd and 

206pbf204pb isotopic compositions change from those typical 

of mid-ocean ridge basalt sources to those resembling a lllMUOIB source. Samples throughout Europe show a correlation 

between shift in Pb, Sr, and Nd isotopic composition and the 

degree of light REE enrichment. 

Our data suggest that silicate melts, derived from partial 

melting of IDMU-type mantle plumes rising from the base of 

the upper mantle, are responsible for extensive modification of 

the mantle lithosphere across Europe during the NeogeneQuaternary. Based upon radiogenic isotope data, it is probable 

that the same plume activity is responsible for the mafic 

alkaline magmatism which exhumed the xenoliths studied. 

Integration of the geochemical data with geodynamic models 

for the evolution of the European lithosphere suggest that 

much of the heterogeneity seen in the mantle beneath Europe 

today is of recent, rather tlian ancient origin. 
OSMIUM ISOTOPE AND OTHER PLATINUM 

GROUP ELEMENT DETERMINATIONS BY 

LASER ABLATION ICP-MS. 

RUIZ. J. and McCANDLESS, T.E., 

Department of Geosciences, University of 

Arizona, Tucson, Arizona 85721, and 

TURNER, P.J., Finnigan MAT, Unit, Asher 

Court, Lyncastle Wa, Appleton, Warrington 

W A4 4ST, England. 

Some platinum group elements and their 

isotopes are notoriously difficult to measure by 

conventional wet chemical and mass spectrometric 

methods. In addition by dissolving large amounts of 

material, important spatial information found at the 

mineral scale is lost. Laser ablation inductively 

coupled plasma mass spectrometry (LA-ICP-MS) 

has been used in samples from/ the Bushveld 

Complex in South Africa in order to determine the 

origin and distribution of the noble metal 

mineralization in this ultramafic/mafic intrusion. In 

particular, a question that can best be addressed by 

LA-ICP-MS is the mobility of the platinum group 

metals in high temperature hydrothermal fluids. The 

instrumentation consists of a Finnigan MAT SOLA 

ICP-MS and Nd:Y AG Q-switched laser with 

frequency quadrupling hardware that allows the 

original infrared 1064 nm laser to operate in the UV 

part of the spectrum at 266 J.tm. The laser was 

operated at a repetition rate of 5 HZ and 2 mj power 

per shot. The beam diameter was 25 J.tm. The 

coupling of the laser beam with the sample when the 

laser operates in the UV is greatly superior to that 

operating at 1064 nm. The pits produced in the 

samples were smooth and of constant geometry and 

the sputtering around. the crater is reduced. More 

importantly, matrix effects, on the laser/sample 

coupling seem to be minimized when the laser is 

operated in the UV. Thus it seems possible to use 

glass standards to analyze trace metals in sulfides. 

The analytical procedure used in this study was to 

analyze a major element in the sulfide sample by 

electron microprobe. The quantified element was 

then used as an internal standard. The results of this 

work show that Re, Pt, Ir, Rh, and Pd can be 

measured by La-ICP-MS to better than 7% precision 

in samples that are 25 J.tm as long as the 

concentration of the platinum group element is 

greater than 100 ppb. Osmium isotopic ratios can be 

determined to better than 1% precision, based on 

multiple measurements of the same mineral grain, on 

samples that have more than 1 ppm Os. 

274 

Sr-ISOTOPE DISEQUILIBRIUM BETWEEN 

FELDSPAR PHENOCRYSTS AND MELT IN 

LARGE-VOLUME SILICIC MAGMA SYSTEMS 

RUIZ. JOAQUIN, Geosciences, University of 

Arizona, Tucson, AZ 85721 USA, and Duffield, 

W.A., U.S. Geological Survey, Flagstaff, AZ 

86001 USA 

Feldspar phenocrysts in many large-volume 

silicic volcanic deposits are in Sr-isotope 

disequilibrium with surrounding groundmass, 

which represents melt from which the crystals 

were growing just prior to eruption. We model 

such disequilibrium through contamination of 

phepocryst-bearing magma by melt derived from 

adjacent granitic (sensu lato) country rock. 

Essential features of the model are: (1) 

Incongruent melting of country rock introduces 

progressively less-radiogenic contaminant into the 

magma. (2) 'Diffusion of the country-rock derived 

Sr is orders of magnitude faster in magma than in 

feldspar phenocrysts. (3) Eruption quenches 

contaminated magma before Sr-isotopic 

equilibrium can be attained between crystals and 

melt. 

The model embodies temporal variations and 

spatial differences in isotopic disequilibrium 

conditions for affected magma. For example, in a 

system with contaminant from country rock more 

radiogenic than uncontaminated magma, growing 

feldspar phenocrysts are initially less radiogenic 

than surrounding magma but eventually become 

more radiogenic. Thus, depending upon the 

timing of eruption, a rock may contain feldspar 

more radiogenic or less radiogenic than 

surrounding groundmass. If the contamination 

process continues long enough, a magma body can 

produce an eruptive unit that varies from one 

relation to the other with stratigraphic position. 

The model predicts the shape of isotopic profiles 

across individual feldspar grains. 
THE YOUNGER DRYAS EVENT: COMPARISON OF 

HIGH-RESOLUTION ISOTOPE RECORDS FROM GREENLAND AND CENTRAL EUROPE. 

ROZANSKI K , International Atomic Energy Agency, 

Vienna, Austria, GOSLAR, T., Radiocarbon Laboratory, 

Silesian Technical University, Gliwice, Poland, KUC, T. 

Faculty of Physics and Nuclear Techniques, University of 

Mining and Metallurgy, Krakow, Poland, RALSKAJASIEWICZOWA, M., Szafer Institute of Botany, Polish 

Academy of Sciences, Krakow, Poland, ARNOLD, M., 

Centre des Faibles Radioactivites, CNRS-CEA, Gif-surYvette, France 

A high-resolution record of climate changes in central Europe 

during the final stages of the last deglaciation, derived from 

annually laminated sediments of Lake Gosciaz (central Poland), 

is presented and compared with the ice core data from central 

Greenland. 

The Lake Gosciaz has accumulated in its deepest part around 

18.5 meters of sediments covering the Late Glacial and Holocene. 

The sediment reveals a distinct lamination consisting of couplets 

of light and dark layers. The light layer is formed by authigenic 

calcite whereas the dark layer consists mainly of organic detritus 

and mineral fraction. 

The Late Glacial/Holocene transition left a distinct record in the 

sediments of the lake: the onset and termination of the Younger 

Dryas (YD) is well marked in the isotope composition of 

authigenic carbonates as well as in plant and animal microfossil 

assemblages preserved in the lake sediments (Ralska- Jasiewiczowa et al., 1992). The B180 variations preserved in lake 

carbonate reveal an analogous structure to the B180 profiles 

available from the GISP 2 and GRIP ice cores drilled in central 

Greenland (Johnsen et al., 1992; Taylor et al., 1993), showing an 

abrupt decline of 180 content at the onset of the YD, lasting about 

150 years and an even faster increase at the termination of the 

YD, completed within approximately 70 years. The characteristic 

structure of the YD/PB transition, with a very fast rise of 180 

content at the bound~, preceded by a relatively small, but 

distinct minimum of o1 0, and followed by a brief, temporary 

drop and further gradual increase of 0180 values is well recognized in all profiles. As in the Greenland profiles, the lowest 0180 

values in the Lake Gosciaz are observed in the early part of YD. 

The annual lamination preserved in the lake sediments allowed us to estimate the duration of the YD in central Europe to be 

1140±40 years, in good agreement with the ice core data. 

The striking similarity of the 0180 variations in sediment 

carbonates and that observed in Greenland ice cores confirms the 

prompt response of climate on the European continent to changes 

in the North Atlantic region during the last deglaciation. 

The AMS radiocarbon dates of terrestrial plant macrofossils 

from the Late Glacial and the early Holocene part of the sediment 

fit into the plateau at ca. 10,000 radiocarbon years BP. The 

difference between radiocarbon and calendar time scales at the 

ALIYD transition ( -11,000 radiocarbon years BP), derived from 

counting of varves and the AMS radiocarbon dates of plant 

macrofossils, is between 1150 and 1850 years. 

Ralska-Jasiewiczowa, M., Van Geel, B., Goslar, T., and Kuc, T., 

1992, The record of the Late Glacial/Holocene transition from 

the varved sediments of Lake Gosciaz (central Poland). in: 

Quaternary Stratigraphy, Glacial Morphology and Environmental Changes, Robertson, A.M., Ringberg, B., Miller, U., 

and Brunnberg, L., (Eds.): Svenska Geologiska 

Undersokning, v. C81, p. 257-268. 

Johnsen et al., 1992, Irregular glacial interstadials recorded in a new Greenland ice core: Nature, v. 359, p. 311-313. 

Taylor et al., 1993, The "flickering switch" of late Pleistocene 

climate change: Nature, v. 361, p. 432-436. 

273 

INCORPORATION AND WSS OF ARGON IN 

PHENGITES: A 40Art39 Ar LASER PROBE STUDY OF 

THE EFFECT OF THE BffiLLA INTRUSION IN THE 

"ECLOGIDC MICASCIDSTS UNIT" OF THE SESIA 

ZONE (WESTERN ALPS). 

RUFfET, G., FERAUD, G., Laboratoire de Geodynamique, 

URA CNRS 1279, Universite de Nice-Sophia Antipolis, 

06108 NICE cedex 2, France, BALLEVRE, M., Laboratoire 

de Tectonique, UPR CNRS 4661, Universite de Rennes I, 

35042 Rennes cedex, France and AMOURIC, M., CRMC2 

CNRS, Universite Marseille, 13288 Marseille Cedex 9, 

France. 

In an attempt to clarify the role· of the temperature in the 

isotopic redistribution process of argon in metamorphic 

minerals, some peculiar results were obtained from the 

40 Ar/39 Ar laser probe analysis of single grains of phengites. 

The studied rocks were sampled in the "eclogitic micaschists 

unit" (EMU) of the Sesia zone (Western Alps), in the contact 

aureole of the Oligocene Biella syenite. Our aim was to compare 

the results obtained on single grains of phengites at increasing 

distances from the intrusion. 

Strong disturbances of the isotopic distribution of argon in 

the phengites were observed up to more than 3km from the 

contact with the 8km diameter pluton. Single grains of phengites 

within the "contact aureole" displayed age spectra which show 

high apparent ages in the low temperature steps followed by a 

decrease down to a minimum age and an increase until reaching 

a flat age shape. The magnitude of the age spectra disturbances 

decreases regularly according to the distance to the pluton. This 

allow to distinguish two distinct subcontemporaneous diffusion 

phenomena. The first one is thermally activated and generates a 

total or partial radiogenic argon loss depending on the distance 

to the pluton. The second one could be linked to a fluid 

circulation process and wave of argon or argonami [1] which 

causes the incorporation of an excess argon component. We 

discuss the parameters (fluid circulation, age of the EMU, 

existence of several populations of phengite) as a function of the 

time after intrusion and the distance to the pluton to explain the 

observed shapes and the mechanism of excess argon 

incorporation. 

York, D., 1993, The Discovery and Significance of a Fossilized 

Radiogenic Argon Wave (Argonami) in the Earth's Crust: 

Geophys. Res. Lett, v.20, p. 61-64 .. 
DIRECT Pb/Pb AGE DETERMINATION OF 

PROTEROZOIC STROMATOLITES FROM THE 

ASHBURTON AND NABBERU BASINS, WESTERN 

AUSTRALIA 

RUSSELL, J., Shell Research, P.O. Box 60, 2280 AB 

Rijswijk, The Netherlands, GREY, K., Geological Survey of 

Western Australia, East Perth, Western Australia 6004, 

Australia, WHITEHOUSE, M., Dept. of Earth Sciences, 

Oxford University, Oxford OX13PR, U.K., and 

MOORBATH S • Dept. of Earth Sciences, Oxford 

University, Oxford OX1 3PR, U.K. 

A series of Proterozoic stromatolitic carbonates has been 

analysed from the Ashburton and Nabberu Basins, Western 

Australia. Samples of Pi£6aria perpfel(Jl (GSW A 88032) and 

hperia asli6urtonia (GSW A 46207) from the Duck Creek 

Dolomite of the Wyloo Group, Ashburton Basin have an 

inferred age of between 2.2-1.6 Ga. Nine analyses of GSW A 

88032 define a Pb/Pb errorchron date of 1717 ± 64 Ma and 

nine analyses of GSW A 46207 an isochron date of 1741 ± 53 

Ma. The results are in statistical agreement and indicate that 

the date of c.1.73 Ga is geologically significant. Petrographic 

evidence for carbonate recrystallisation and disruption of 

primary stromatolitic fabrics is interpreted as the age of 

diagenetic recrystallisation, possibly related to fluid activity 

from the greenschist facies metamorphism that can be seen to 

affect rocks in the vicinity. 

Samples of tyantfilla meeKP.tfulrrensis (GSW A 46589) and 

?Pi£6arlatftvere({a(GSWA 84767) were analysed from the 

Yelma Formation of the Earaheedy Group in the Nabberu 

Basin which has an inferred age of between 1.8-1.6 Ga. 

Sixteen analyses of GSW A 46589 define an errorchron date of 

2008 ± 68 Ma and six analyses of GSW A 84767 a concordant 

errorchron date of 1946 ± 71 Ma. Both are significantly older 

than the inferred depositional age from independent data. 

However, evidence to support a depositional/early diagenetic 

age of c.2.0 Ga exists in that, (1) no similar stromatolitic taxa 

are found anywhere in the world in rocks younger than 1.8 Ga; 

(2) microfossils from the Earaheedy Group carbonates show a 

distinct similarity with those from the 2.0 Ga Gunflint Chert 

of Canada; (3) petrographic observations do not suggest 

extensive disruptive recrystallisation of original sedimentary 

fabrics; and (4) stromatolites from the underlying Glengarry 

Group yield an older Pb/Pb isochron date of 2258 ± 180 Ma. 

It is concluded that sediments of the Earaheedy Group were 

deposited around 2.0 Ga. 

Other stromatolites, however, while demonstrating excellent 

preservation of primary sedimentary fabrics, have failed to 

produce meaningful radiometric results. This may result from 

a failure to homogenise Pb isotopic ratios at re-crystallisation, 

from particularly limited variation in U/Pb ratios or 

subsequent disturbance of closed system conditions. 

275 

AN ATTEMPT TO DETECT POSSIBLE 

VARIATION OF ISOTOPIC COMPOSITION 

OF ZIRCONIUM IN NATURE 

S. K. Sahoo and A. Masuda, 

Department of Chemistry, 

University of Electro-communications, Chofu,Tokyo-182, Japan 

Zirconium involves five stable 

nuclides~ These nuclides can be 

subject to slight variations in 

abundances due to a few nuclear 

effects . A relatidely long galflife ot EC decay of aNb 1.3x10 y or 

3.3x10 y g~ads us to an expectation of Zr variation. The variation observed by Minster and 

Allegre are marginal on account of 

emp~fical errors[l]. The half-life 

of Nb remained ambiguously "long" 

until recently, but a recent information in~icates a "short" halflife, 7x10 y. Considering N being 

a magic number 50, this half-life 

seems to be too short, being open 

to further examination. That is, it 

may be premature to rule out the 

potentiality ~f isotopic abundance 

var~ition of 9 Zr re~Mlting from EC 

of Nb. As regards Zr, there can 

be a possible variation related 

with the p~!~ordial spontan7ous 

fission of Pu (Tl12=8.1x10 y). ~~esides, it may mer t a note that 

Zr is a rapid process nuclide.) 

As a special phenomenon, ~t does 

not escape our notice that ~r can 

be produced by double EC of Mo in 

molybdenite. 

Except the last effect, our main 

purpose is to re-examine precisely 

the isotopic abundance ratios of Zr 

especially in relation with the effects of extinct nuclides. Our data 

are obtained for different sources 

of chemical reagents based on zr+ 

ion generated by a triple filament 

ion source. These data are scrutinized in theoretical view of mass 

fractionation and are compared 

with those published by other researchers [1,2]. 

!.Minster, J.F., and Allegre, C.J., 

1982, Geochim. Cosmochim. Acta, 

46, 565-573. 

2.Nomura, M., Kogure, K. and 

Okamoto, M, 1983, Int. J. Mass 

Spec. Ion Phys., 50, 219-227. 
K-AR AGE STUDIES ON SEVERAL 

PLUTONIC BODIES IN THE SOUTH 

FOSSA MAGNA ARC-ARC COLLISION 

ZONE, CENTRAL JAPAN 

§~lTQi ~~' SUGI, S., KATO, K. 

and OTOMO, I., Dept. Earth 

Sci., Yamagata University, 

Yamagata, Japan, 990 

The South Fossa Magna region 

is tectonically unique of its 

arc-arc collisional feature. The 

Izu peninsula, which is located 

at the northern edge of the Philippine plate, is just colliding 

to the Honshu (the main island of 

Japan) arc. Many geoscientists 

have discussed that this area 

experienced multiple arc-arc 

collisions. The Tanzawa and 

Misaka blocks are candidates of 

such exotic terrains. 

Several plutonic bodies exist 

in this area. The Kofu plutonic 

complex is the largest plutonic 

body in the area. Samples from 

the southern part of the complex 

(Ashigawa body) give well defined 

hornblende K-Ar age of 11 Ma. The 

southern part of the Tokuwa body, 

distributed to the north of the 

Ashigawa body, shows a systematic 

age decrease from south (12 Ma) 

to north (9 Ma). The MizugakiShosenkyo body, which is located 

in the northwestern part of the 

complex, shows a similar systematic decrease in ages from north 

(15 Ma) to south (11 Ma). The 

Tanzawa tonalitic body yields 

hornblende K-Ar ages from 7 Ma 

(the northwestern part) to 5 Ma 

(the southeastern part). We dated 

both biotite and hornblende for 

several specimens. Some of them 

yielded older ages for biotite, 

but most of them showed insignificant age differences between 

these minerals. 

These data suggest that the 

plutonic bodies in the area experienced considerably rapid cooling, but the spatial distribution 

of temperature in them were not 

uniform in the course of the 

cooling. 

276 

CONSTRAINTS ON THE MORB MELTING REGIME 

BASED ON THE LU-HF, SM-ND AND U-TH 

SYSTEMATICS 

SALIERS. YINCENIJ.M., BOURDON, B. and ELUOIT, 

T.E. Lamont-Doherty Earth Observatory, Palisades, NY 

10964,USA .. 

Lu-Hf and Sm-Nd isotope systematics place the 

onset for MORB melting in the garnet tiel~ and are 

indicators for degree and depth of melt~n~ .. In 

addition, recent results on U-Th part~ttontng 

indicates Th-disequilibri~ ru::e also explatned. ~y 

placing the onset of m~lttng tn the g~et stabthty 

field. And, thirdly, Kletn and Langmutr (1989) have 

shown that the normalized major element content of 

MORB also reveal information about the degree and 

the depth of melting. Especially. Nag .an~ Feg are 

important indicators whereby ~as ts. an.tndicator for 

the degree of melting and Feg 1s an Indicator for the 

average depth of melting. We have taken the 

constraints derived from the 176Hffl77Hf, 

143NdJ144Nd and (230rfhfl32Th) systematics one step 

further and combined the constraints derived from 

the isotope systematics with those derived from 

major elements. 

Two Atlantic Ridge segments with a clear 'local' 

trend also show positive correlations with both Feg 

and N as and the isotope melting parameters, 

B(Sm/Nd) and B(LufHf), indicatin& that the small~r 

degree of melting, the more ~el~ng tak~s place .tn 

the garnet stability field.. Thts ts con~t~tent wtth 

previous explanations of Incomplete mtxtng of the 

different melting columns in a triangular shaped 

melting regime, and the tapping of melts from 

different of-axis melting columns. 

Klein and Langmuir ( 1989) interpreted the 

'global' trend and its correlation with ridge depth as 

follows: the deep ridge segments have big~ Nag and 

low Feg indicating a low degree of relatively low 

pressure melting, while the shallow ridges represent 

a large degree of melting an~ a high av~ra~e 

pressure during melting. The Mid-Cayman Rise ts 

one of the deepest parts of the ocean, but MCR 

basalts have among the highest ~Sm/Nd) and Bo.:unm 

indicating they represent small degrees of relatively 

high pressure melts. Based on the presently 

available data the isotopic systematics for the 

'global' trend can be in!erpreted several '!lays: 1) the 

deeper the melting regtme the less efficiently melts 

can be collected at the ridge and thus the lower 

B(Sm/Nd) and B(Lu/HO, or 2) the onset of melting for 

all ridges is similar, however, the. d~pth of 

termination of melting varies, or 3) a stgntficantly 

older isotopic reservoir for the MCR basalts 

compared to other MORB. 

Further 176Hff177Hf and 143NdJ144Nd analyses of 

deep ridge (American-Antarctic Disco:danc~) and 

shallow ridge (Kolbeinsy) MORB combmed wtth UTh systematics will be presented. 
PHANEROZOIC CRUSTAL GROWTH: THE 

IMPORTANCE OF JUVENILE TERRANES IN THE 

APPALACHIAN AND CORDILLERAN OROGENS 

SAMSON. SCOTT D., Dept. of Earth Sciences, Syracuse 

University, Syracuse, NY 13244-1070 

Many crustal growth models show that the last - 1 Ga of 

Earth history was an interval of very slow crustal generation. 

The concept of insignificant crustal growth during Phanerozoic 

time has largely been based on isotopic studies of European 

orogenic belts and of the Himalayas. Nd isotopic studies of 

the Cordillera, however, have shown that much of that orogen 

is composed of juvenile continental crust. The Cordillera, 

formed by the accretion of tectonostratigraphic terranes, is 

therefore a major site of new crust generated during Phanerozoic time. 

The terrane concept has also been applied to the Appalachian orogen and it now appears that accretionary events 

within the Appalachians are similar to those that formed the 

Cordillera. However, because the Appalachians are . often 

considered to be an extension of the Caledonides and the 

Hercynides, and the crust in those orogens is composed 

predominantly of recycled material, it is often assumed that 

the Appalachians do not represent any significant contribution 

of new crust. New isotopic studies in the Appalachians, 

however, demonstrate that this view is incorrect and that there 

are significant regions of juvenile crust in that orogen. For 

instance, the largest southern Appalachian terrane, the 

Carolina terrane, has felsic igneous rocks with initial €Nd 

values ranging from + 1 to +6. Positive €Nd values have also 

been reported from the nearby Kings Mountain belt demonstrating that this crustal region is also juvenile. Granitic 

plutons of Pennsylvanian-Permian age, generated during the 

Alleghanian (Hercinian) orogeny, also document the presence 

of juvenile Appalachian crust. The range in initial ~d is 

from -8 to + 3, but the vast majority of the Alleghanian 

plutons have values between -2 and +2 indicating that a 

significant amount of juvenile material must have been 

involved in their petrogenesis. 

Mantle-derived crust also is present in the northern Appalachians. The Avalon terrane of Canada clearly contains a high 

percentage of juvenile crust as most ~d values are between 

+ 1 and + 5. Recycled crust is also present, however, since 

zircons from some late Proterozoic rocks have Archean and 

early Proterozoic components. Volcanic and plutonic rocks of 

Ordovician age from the Bronson Hill Anticlinorium, central 

Massachusetts, are similar to those in Avalon; initial €Nd 

values of +5 have been measured, although most samples 

have €Nd closer to zero. Unevolved crust is therefore an 

important part of this Appalachian terrane as well. 

Similar accretionary processes were responsible for the 

construction of the Appalachians and the Cordillera and both 

orogens are important sites of new continental crust. The 

formation, amalgamation and accretion of chemically evolved 

but isotopically juvenile terranes are thus fundamental 

processes of crustal growth. The isotopic results from these 

two major orogens show that crustal growth during Phanerozoic time is thus more important than has been previously 

recognized. 

277 

THE FIRST FISSION TRACK RESULTS OF THE 

TRANSYLVANIAN BASIN-CARPATHIAN MOUNTAIN 

BELT SYSTEM IN ROMANIA. 

SANDERS, C.A.E and P.A.M. Andriessen. 

Vrije Universiteit, Institute for Earth Sciences, Dept. of 

Petrology and Isotope Geology, De Boelelaan 1085, 

1081 HV Amsterdam, tel 020-5483521, fax 020-6462457. 

Recently an increasing amount of investigations of a variety 

of geological disciplines are concentrated on the CarpathianPannonian system in Central-Southeastern Europe. 

It resulted in the understanding that discrete regional tectonic 

blocks in the internal part of the mountain belt have undergone 

different histories during the early stage of Alpine convergence. 

These microplates, separated by oceanic crust during the 

Mesozoic, were assembled to a coherent unit in the Tertiary. 

Ongoing Alpine collision caused internal fragmentation and 

formation of various extensional basins coeval with 

compression in the outer Carpathians. The Tertiary event has 

been explained as an example of "escape tectonics" as a 

consequence of large scale Alpine contraction between the 

European plate and the African plate. 

Especially this last phase is suitable to study with the Fission 

Track method, because the apatite FT analyses record the last 

low temperature stage of the thermal history. 

We focus on the relation between thrusting and uplift in the 

outer Eastern Carpathians, subsidence in the Transylvanian 

Basin and tectonics in the more internal located Apuseni 

Mountains. FT results of volcanics, sediments and basement 

rock allow quantitative constraints on the amount of uplift, 

erosion, denudation, subsidence and sedimentation. Different 

thermal histories of various parts of the mountain belt are 

compared to each other. 

These fission track data are the first data presented on this 

part of the Alpine orocline. Due to the quantitative character, 

constraints can be put on the tectonic modelling of the 

dynamics of this region. 
MAGMA TIC CARBON FLUX FROM 

·SUBDUCTION ZONE 

SANO, Y ., Department of Earth and Planetary 

Science, Hiroshima University, Higashi 

Hiroshima 724, Japan and Williams, S.N., 

Department of Geology, Arizona State 

University, Tempe AZ 85287-1404, U.S.A. 

Carbon dioxide has been discharging naturally 

from the Earth's mantle to the atmosphere through 

volcanic and hydrothermal activity for long periods of 

geological time. The magmatic carbon flux at midocean ridge has been assessed by spreading rate of 

oceanic plate and carbon content or mantle 3He flux 

and C02/3He ratio in mid-ocean ridge basalt (MORB) 

glasses, while degassing rate at convergent plate 

margins is not well documented. We present here 

magmatic carbon flux from subduction zone based on 

reported 3He flux and observed C02/3He ratios of 

circum Pacific volcanic gases. 

We have collected 14 volcanic gases from Japan, 

Papua New Guinea, Central and South America and 

measured helium and carbon isotopes and C02/4He 

ratios. Corrected 3He/4He ratios for atmospheric 

contamination vary from 4.2 Ratm (where 

atmospheric 3He/4He ratio is 1.39x1o-6) to 8.4 

Ratm, agree well with those reported for circum 

Pacific volcanic gases. Observed ()13c values range 

from -9.5o/oo to -2.6o/oo, are again consistent wit_h those 

reported for magmatic carbon. The C02/3He ratios 

are from 6.6x109 to 3.3x1ol0. Taking reported 

C02/3He ratios for volcanic gases from New Zealand 

and North America, average of the ratios in circum 

Pacific volcanic zone is about 2 x 1010. 

Volcanic carbon flux of about 3x10l2 mol/year is 

estimated for subduction zone by the 3He flux in 

literature and the C02/3He ratio obtained in this work. 

This flux is larger than carbon flux of 1.5xlo12 

mol/year from mid-ocean ridge and may contribute 

significantly to natural global carbon cycle. 

278 

THE OXYGEN AND CARBON ISOTOPIC 

COMWOSITIONSOFCARBONATnES 

SANfos1.2, Roberto V.; CLAYTON I, Robert N.; Univ. 

of Chicago, Dep. of Geoph. Sci 1.; Chicago, IL 60637, USA; 

Univ. Federal de Ouro Preto, Dep. de Geologia2. Ouro 

Preto-MG, 35400, Brazil. 

We present an oxygen and carbon isotopic study of 

carbonatite and metasomatic rocks from South America 

alkaline complexes and discuss whether the observed isotopic 

variations are due to mantle heterogeneity or processes related 

to the evolution of the alkaline magma. The complexes of 

Jacupiranga, Arax8, Catalao, Tapira. and Mato Preto surround 

the Parana Basin and were emplaced dming theW ealdenian 

Reactivation of the South American Platform (between the 

Jurassic and the Eocene). Carbonatites usually comprise only a minor part of these complexes and occur as plugs, dikes, or veins intruding the alkaline silicate rocks. 

Table I summarizes the results of this study and shows that 

except for the samples from Mato Preto, which seem to have 

been contaminated by limestone from the country rock, the 

samples from Jacupiranga, Arax8, Catalao and Tapira have 

similar 613c values. The samples from Araxa, Catalao and 

Tapira also have a wide range in oxygen isotopic composition, 

which may be related to their degree of hydrothermal alteration 

and erosion level. For instance, while the carbonatites from 

Jacupiranga have a narrow range of 618o and have not been 

extensively affected by fenitization, the carbonatites fromAraxa, Catalao and Tapirahave a wide range in 618o and are 

accompanied by pervasive potash-fenitization of their host 

rock. 

The variations of oxygen and carbon isotopes in the 

carbonatites may be explained by three different kind of 

processes: (a) magmatic processes; (b) crustal contamination; 

and (c) fluid-related processes. We show that most of the 

variations of ()18o and ()13C in carbonatites may be related to 

low temperature processes (i.e. below 400 oC), and may be 

explained by isotope exchange between H20-C02 fluids and 

the carbonatites. We conclude that the absence of carbon 

isotopic differences among the complexes studied here 

suggests that there is no significant difference in the carbon 

isotopic composition of their mantle source regions. However, 

because carbonatites are derived from large volumes of the 

mantle, this conclusion does not preclude the existence smallscale heterogeneity in subcontinental mantle, such as those 

observed in diamonds and mantle xenoliths. 

Table 1: Isotopic composition of the analyzed samples. 

()18o (o/oo) ()13C (o/oo) 

MatoPreto 9.1 to 13.8 -6.9to0 

Jacupiranga 6.6to7.3 -6.6 to-6.2 

Araxa 8.7to 16.3 -7.2 to-4.8 

Catalao 7.9to 19.3 -7.1 to -5.3 

T~ira 9.7to 15.4 -8.6to -4.8 
TEMPORAL VARIATIONS IN LEAD 

CONCENTRATIONS AND ISOTOPIC 

COMPOSmON IN THE SOUTHERN 

CALIFORNIA BIGHT 

SAN1JDO-WILHELMY, S.A., Marine 

Sciences Research Center, State University of New 

York, Stony Brook, New York 11794-5000, USA, 

and FLEGAL, A.R., Earth Science, WIGS, 

University of California, Santa Cruz, California, 

95064, USA. 

Lead concentrations in surface waters of the 

Southern California Bight appear to have decreased 

three-fold (from > 170 to <60 pM) since the 

1970s. The decreased parallels a three-fold decline 

in anthropogenic inputs of industrial lead to the 

Bight over the past two decades. Moreover, mass 

balance calculations indicate that the primary 

source of lead to the Bight now is upwelling. This 

is evidenced by the lead isotopic compositions of 

surface waters in the Bight, which are most 

characteristics of Asian industrial lead aerosols 

(0.4793 ~ 206PbP08Pb ~ 0.4833) deposited in 

oceanic waters of the North Pacific. While the 

decreased in surface water lead concentrations in 

the Bight reflects the reduction in industrial lead 

emissions from the United States, the isotopic 

compositions of surface waters in the southern 

reach of the Bight reflect a concurrent increase in 

industrial lead emissions from Mexico (0.4852 ~ 206PbP08Pb ~ 0.4877). The isotopic composition 

f08PbP07Pb • 2.427) of elevated lead 

concentrations of surface waters in San Diego Bay 

indicate that lead is remobilized from contaminated 

sediments within that bay. 

279 

pH-ESTIMATION OF GLACIAL OCEANS BASED ON 

BORON ISOTOPIC COMPOSITION OF FORAMINIFERA 

SANXAL A 1, HEMMING, N.G.2, HANSON, G.N.2 and 

BROECKER, W.S.l 

1 Lamont Doherty Earth Observatory, Palisades, 

NY -10964, USA. 

2 Dept. of Earth and Space Sciences, State University of 

New York at Stony Brook, Stony Brook, NY 11794-2100, 

USA. 

The boron isotopic composition of foraminifera has been 

suggested as a recorder of ocean pH. The relative abundances of 

the two dominant aqueous boron species in seawater, B(OH)3 

and B(OH)4-, is dependent on pH. Isotopic fractionation 

between these two species enriches B(OHh in the heavier 

isotope of boron. Therefore the boron isotopic composition of 

each species is pH dependent. Based on previous studies an 

assumption can be made that B(OH)4- is the only species 

being incorporated into the carbonate phase with the result that 

carbonate shell material has the same boron isotopic 

composition as the aqueous B(OH)4- species. Hence the boron 

isotopic composition of the shell material could be used as a 

pH indicator for the fluid in which it grew. 

Boron isotopic compositions of planktonic foraminifera 

(Globogerinoides sacculifer) and of benthic foraminifera (mixed 

species) were analyzed to estimate the pH of surface and deep 

water respectively. Modem planktonic samples have an 

isotopic composition approximately 2%o heavier than that of 

benthic samples. This is consistent with the difference in pH 

between the surface and deep waters at the sampling sites as 

calculated by GEOSECS. Hence the foraminifera appear to be 

recording the pH conditions where they grew. The uncertainty 

in the estimation of pH corresponding to the uncertainty in the 

analysis of boron isotopic ratio of foraminifera is about 0.1 

pH units in the pH range of interest. 

The boron isotopic compositions of glacial benthic and 

planktonic samples from both Atlantic and Pacific oceans have 

been analyzed. The boron isotopic compositions of glacial 

planktonics from both equatorial Atlantic and equatorial 

Pacific oceans are similar and consistent with an increase in 

pH of 0.2 - 0.3 pH units for glacial surface waters compared to 

modem surface waters. This pH is consistent with a decrease in 

pC02 of surface water by 80-90 J.latm and an increase in 

alkalinity of the surface ocean by about 3%. The boron 

isotopic compositions of glacial benthic forams from both 

equatorial Atlantic and equatorial Pacific oceans are also 

similar and suggest that the pH of glacial deep oceans were 

about 0.3pH units higher than that of the respective modem 

deep oceans. This is in contrast to pH changes estimated from 

lysocline changes (between modem and last glacial periods) 

which suggest no change or a slight decrease in pH for the 

Atlantic and an increase of only about 0.05 pH units for the 

deep Pacific. Such an increase in pH of deep water (as 

suggested by boron isotopic composotion of benthic 

foraminifera) without a significant change in lysocline depth 

could be explained by a decrease in the calcite to organic 

carbon rain rate ratio during glacial period compared to that 

during modem period (Archer and Maier-Reimer, 1994). 

Archer, D. and Maier-Reimer, E., 1994, Nature (in press) 
RARE GAS SYSTEMATICS IN MANTLE 

MATERIALS 

Philippe Sar<la, Thomas Staudacher, and Claude 

J. Allegre, 

Laboratoire de Geochimie, Institut de Physique du 

Globe, 4 place Jussieu, 75252 Paris Cedex 05, 

France. 

We have continued to measure isotopic 

composition and concentrations of all the rare gases 

in glassy margins and olivines from oceanic basalts. 

We obtained high 40 Arf36 Ar of more than 28,000 in 

MORB basalts both by stepwise heating and 

crushing under vacuum. These high ratios coinCide 

with low 87SrJ86Sr. In contrast, the oceanic islands, 

including Loihi seamount, Rocard and McDonald, 

have 40Arf36Ar ratios generally (but not always) 

lower than 350. 

The high 40 Arf36 Ar ratios in MORB coincide with 

high 20NeJ22Ne, 21Nef22Ne, 129Xef130xe and 

134Xefl30Xe ratios, and, in a less systematic way, 

with high 4Hef3He ratios of = 86,000. Oceanic 

Islands glasses have shown atmospheric 

129Xef130xe and 134Xefl30Xe. Correlations for 

MORB in (Ar, Xe) or (Xe, Xe) diagrams indicate 

mixing. One end member is radiogenic, the other is 

close to atmospheric. The situation is less clear for 

Ne in this respect. 20Ne excesses do correlate with 

40 Ar, so that we tend to attribute them to the same 

radiogenic end member, the other end member of the 

neon mixing line being atmospheric. 

We have developed several models with common 

structure and different assumptions. The common 

structure is a two layered mantle, one upper layer, 

strongly outgassed, and one lower mantle, less 

outgassed (but probably 60 to 70% ). This point 

differs from our original model. The two different 

assumptions are: 

1) Loihi data are representative of the lower 

mantle, 

2) Loihi data are polluted by atmospheric gases (in 

situ or reinjection). 

Both assumptions are discussed quantitatively, with 

results which are not too different qualitatively. 

280 

EXCESS 129xE AND RELATIVELY LOW 

40AR;36AR RATIOS IN CARBONATITES FROM 

CANADA AND BRAZIL 

SASADA. T. and HIYAGON, H., Dept. of Earth 

and Planetary Physics, Univ. of Tokyo, Tokyo, 

113, Japan, WADA, H., Inst. of Geosciences, 

Shizuoka Univ., Shizuoka, 422, Japan and 

EBIHARA, M., Dept. of Chemistry, Tokyo 

Metropolitan Univ., Tokyo, 192-03, Japan 

Isotopic compositions of all noble gases 

(He,Ne,Ar,Kr,Xe) in one Brazilian carbonatite 

from Jacupiranga (130Ma), three Canadian 

carbonatites from Oka (llOMa), Prairie Lake 

( 1 030Ma) and Borden Lake ( 1870Ma) and one 

Canadian syenite from Poohbah Lake 

(2700Ma) were analyzed with a sector-type 

mass spectrometer VG5400. We separated 

forsterite, diopside and apatite from the 

samples and extracted noble gases by 

stepwise heating. We also measured the 

concentration of potassium by RNAA method. 

Primordial He (3He/4He=2. 74x 1 o-6, 3.52 

x1Q-6) and anomalous Ne (20Ne;22Ne=0.01-10, 

21 N e;22Ne=0.031-0.097) were observed. The 

4 0 A r I 3 6 Ar ratios varied from nearly the 

atmospheric ratio to 42420~ However, apatite 

samples from carbonatites showed the 

identical 40 Ar/36 Ar ratio of about 6200 at the 

highest temperature fractions. At the same 

fractions, we found the excesses in 131-13 6x e 

and 129xe. The excess 131-136xe are thought 

to be of 238u-fissiogenic Xe. The 129xe;130x e 

ratios observed in Jacupiranga and Borden 

Lake samples were 7.31 and 7.36, 

respectively, which are similar to the value 

observed in MORB samples. Pririe Lake 

sample shwos a higher , ratio (8.6). However, 

it is probably originated from 2 3 8 U -

fissiogenic 1 2 9 I judging from very high 

excesses in 131-136xe (136xet130xe=l400). 

The 40 Ar/3 6 Ar ratios of carbonatites 

(6200)are lower than that of MORBs (>28000). 

High 129Xe excess and relatively low 

40Ar/36Ar ratios of carbonatites suggest that 

the source of carbonatites is different from 

that of MORBs. The excess 129xe also suggests 

that volatiles of carbonatites are juvenile. 
IDGH PRECISION MEASUREMENTS OF 180fl60 AND 

170fl60 OBTAINED FROM GEOLOGICAL AND EXTRATERRESTRIAL MATERIALS USING AN ISOLAB 54 ION 

MICROPROBE. 

Saxton. J.M .. Lyon I. C. and Turner, G., Department of 

Geology, The University of Manchester, Manchester, M13 9PL, 

United Kingdom. 

Mass fractionation during ion probe measurements of oxygen 

isoto_pe ratios at high mass resolution can arise from a mass 

dependent angular and energy distribution of the secondary 

ions. We have sought to reduce this effect by sweeping the 

secondary beam at high frequency (2.5kHz) across the source 

slit using a saw tooth wave form and integrating the total beam 

for each isotope. A new detector type which uses a conversion 

dynode in front of a channeltron (CDS detector) has also been 

developed. Oxygen isotopes are detected by a multicollector 

using a Faraday detector for 160, and CDS detectors with 2000 

mass resolving power for ISQ and 6000 mass resolving power 

for 170. The CDS detectors have proved to offer extremely 

high gain stability. Figure 1 shows raw 18Qfl60 ratios obtained 

on polished sections of magnetite grains (LP204, courtesy of Dr 

J Valley) over a period of a inonth when the beam integration 

procedure was in operation. The ratios plotted have not been 

normalised relative to each other and exhibit a standard 

deviation (all points) of a=l.S0 /

00, the average within day 

reproducibility is a=1°/00 or better. The reproducibility for 1'0 

is intrinsically worse than 180/160 because of the low natural 

abundante of 170 and has varied between 3-4.6°/00 during a 

day. We have perfo~ed 180/160 and 170/160 measurements 

on small clusters or" micron sized magnetite grains from the 

Orgueil CI and Y82162 (CMl) meteorites. Results for Orgueil 

show a mean S180 (-1°/oo> and st7o (+3°/oo> which lie close to 

the terrestrial fractionation line but exhibit a real spread (stso 

= -6°100 to +5°/00• We have also undertaken work on olivine 

standards from the Brenham pallesite which exhibit a similar 

reproducibility 18Qfl60. 

~1171 .. 

;1170 

]; 1115 

... 1180 

t 1155 

~ 

Figure 1 

Meaaured (un-normallaadl "oi"O raUoa from 

magnetite atandard• taken over 1 month 

Std1 St111 St\11 Std1 Std2 Stcl2 Stcl2 Std1 Std1 Std2 Stc12 +5 •1f + ·~·uf 1 I f ~ 1 ,, =~j 2 3 

1• Dec 93 

4 5 I 7 I I 10 11 12 13 

Spot Number 

16 Dec e3 4 Jell 84 7 Jen u 3 Jill 84 I Jan U 10 J111 114 

281 

MULTI-EPISODIC LATE OROGENIC VOLCANISM AND 

PLUTONISM IN THE VARISCAN FOLD BELT OF 

CENTRAL EUROPE 

SCHAL TEGGER, U. and CORFU. F .. Dept. of Geology, 

Royal Ontario Museum, 100 Queen's Park, Toronto, Ont. 

MSS 2C6 Canada 

The end of the Variscan orogeny in Central Europe is 

marked by the transition from compressive to strike-slip 

tectonics, causing the formation of numerous elongated pullapart basins. They were filled with clastic sediments and 

volcanic rocks of upper Carboniferous age and were subsequently intruded by granitoid melts. 

In the Aar massif, one of the pre-Mesozoic basement 

windows of the Central Alps, late Variscan magmatic rocks 

were emplaced during three distinct magma pulses at 334, 310 

and 298 Ma, respectively. Two Carboniferous volcanosedimentary sequences can be distinguished in the same area: 

(I) an older series, which was strongly deformed and 

metamorphosed in upper greenschist facies during a late 

Variscan event, is intruded by a 334±2 Ma old granite. These 

rocks are unconformably overlain by a younger series (II), 

consisting of estuarine to lacustrine sediments, conglomerates, 

ignimbrites and tuft's. The younger sequence only shows weak 

deformation. It was deposited at 299±2 Ma and was subsequently intruded by microgranites and granites, whose U-Pb 

age is identical within analytical uncertainty to that of the 

volcanic rocks. 

The group I volcano-sedimentary sequence of the Aar 

massif may be coeval with volcanic and clastic sedimentary 

rocks in the southern Vosges area (Schneider, 1990). Zircon 

U-Pb ages of rhyolites from the bottom and the top of the 

Visean stratigraphic sequence indicate that the deposition 

occurred within a short time-span between 345±1 Ma and ca. 

342 Ma. Late volcanic activity at around 335 Ma covered the 

area with ignimbrites. 

The zircons of volcanic rocks from both the Aar massif 

and the southern Vosges show abundant zircon inheritance of 

Late Proterozoic age indicating contamination or even 

derivation from crustal sources, in contrast to the coeval 

granites that exhibit distinct mantle affinities. The latter melts 

are interpreted to be derived from an enriched subcontinental 

mantle source and underwent various degrees of crustal 

contamination (Schaltegger and Corfu, 1992) . 

Schaltegger, U. and Corfu, F., 1992, The age and source of 

late Hercynian magmatism in the central Alps: evidence 

from precise U-Pb ages and initial Hf isotopes. Contrib. 

Mineral. Petrol. 111: 329-344 

Schneider, J.L., 1990, Enregistrement de Ia dynamique 

varisque dans les bassins volcano-sedimentaires devonodinantiens: exemple des Vosges du Sud (zone 

moldanubienne). Unpubl. PhD thesis, Univ. Louis Pasteur, 

Strasbourg, France, 220 pp. 
THE VULKAAN ARGON LASERPROBE: INTER -

CALIBRATION, PERFORMANCE, AND NEW 

STANDARDS. 

SCHEVEERS R., WIJBRANS J.R., PRINGLE M.S., and 

KOPPERS A.A.P., Faculty of Earth Sciences, Vrije 

Universiteit, 1081 HV Amsterdam, Netherlands. 

The argon laserprobe facility at the Vrije Universiteit 

(VULKAAN) consists of an MAP215-50 mass spectrometer 

fitted with a Johnston MM1 SEM, an SP 2080 18W argon-ion 

laser, and a low volume (ca. 100 ml) UHV inlet line. Operating 

the SEM at gain 10000, we measure quite reproducibly 

backgrounds of 3.1Q-16,1.1Q-18,4.1Q-18,9.1Q-17, and 2.1Q-17A, 

at m/e 40, 39, 38, 37, and 36, respectively. The proportion of 

blank 40 from the mass spectrometer, getter cross, and sample 

house is 2:3:6. Virtually all 36 can be attributed to the mass 

spectrometer. Mass discrimination (1.0058 ± 0.3%) is 

monitored using pneumatic pipette yielding air aliquots of ca. 

10-8 cc STP. The sensitivity of the system (2.10-17 moles/m V) 

was determined using weighed single grains of GA 1550 biotite. 

The laser beam can be focussed continuously between <20 pm 

and ca 5 mm. Sample temperature can be monitored with an IR 

microscope (Minarad MR200), fitted with a fibre optic probe, 

and temperature controller. 

Sample irradiations were carried out at the OSU TRIGA 

reactor. Potassium correction factos were measured using K -

glass for both the unshielded and Cd shielded CLICKIT facility 

(40/39K=0.00422 ± 0.00049 and 0.00090 ± 0.00065, 

respectively). For irradiation of older minerals we are currently 

assessing the HFPIF facility of the 45 MW Petten reactor, with 

a neutron flux of 2.3.10-18 s-lm-2 thermal neutrons, and 

0.3.10-18 s-lm-2 fast neutrons. Fluxmonitor TCR Sanidine 

850003 (age 27.92 Ma) from irradiation VU1 (OSU93/30) was 

measured at the Amsterdam, Stanford and Menlo Park facilities, 

demonstrating that J-curves can be reproduced to the known 

reproducibility of 850003. Using the combined J-curve, biotite 

GA1550 yielded ages of 97.89 ± 0.30 Ma (multiple single 

grains), Fish Canyon biotite ages of 28.09 ± 0.09 Ma 

(replicate analysis of multi-grain samples), and MMhb-1 ages 

of 519.4 ± 1.5 Ma (multiple single grains). 

As new in-house mineral standards we have assessed the 

suitability of biotite 100-2 (10.68 ± 0.04 Ma), and sanidine 

DRA-1 (24.96 ± 0.08 Ma). 

282 

ANCIENT SURFACES IN THE TRANSANTARCTIC 

MOUNTAINS AND THE AGE OF THE SIRIUS 

FORMATION 

SCHLOCHTER. C., Geologisches Institut, UniversiUlt 

Bern, CH-3012 Bern, Switzerland, HELFER, M., 

Geologisches Institut, UniversiUlt Bern, CH-3012 Bern, 

Switzerland, BRUNO, L., Isotopengeochemie, ETH 

ZUrich, CH-8092 ZUrich, Switzerland, IVY, S., 

Ingenieurgeologie and Institut filr Teilchenphysik, ETH 

HOnggerberg, CH-8093 ZUrich, Switzerland, 

The occurrences of Sirius Formation sediments are related 

to a higher elevation ancient topography of the Transantarctic 

Mountains (e.g. Mt. Feather, Mt. Fleming, and Table 

Mountain in the Dry Valleys sector). The Sirius Formation 

itself is an assemblage of glacigenic and glacilimnic facies 

associations whose textural characteristics are strongly 

influenced by the local bedrock lithology. Detailed sediment 

analyses of samples from selected outcrops in the Dry Valleys 

now permit a more realistic interpretation of the paleoglacial 

parameters. 

The stratigraphic position of the Sirius Formation has been 

under considerable debate, some scientists have recently 

emphasized an Upper Pliocene I Lower Pleistocene origin. 

Exposure age determinations have been carried out on erratic 

boulders from the Sirius surface and on samples from Siriusfree bedrock below and above patches of Sirius drift on Table 

Mountain and Mt. Fleming. The exposure ages as reported by 

Bruno et al. (1994) and Ivy et al. (1994) cast serious doubt on 

the young origin. The bearing of these age determinations on 

the understanding of the paleolandscape evolution is 

presented: the Sirius Formation was deposited at least by midPliocene or, more likely, even earlier. 

Bruno et al., 1994, Dating of geomorphologic surfaces in the 

Antarctic Dry Valleys with in situ produced cosmogenic 

3He and 21Ne babstract this volume). 

Ivy et al., 1994, I Be and 26Al exposure dating of the Sirius 

Formation, Table Mountain, Antarctica (abstract this 

volume). 
A NEW TECHNOLOGY IN HIGH PERFORMANCE ICP-MS 

MIKE P.SCHMIDT, Finnigan MAT GmbH 

Barkhausenstr.2, 28197 BREMEN, Germany 

Spectral interferences by poly-atomic or multiply 

charged species are major limitations to analysts 

currently using quadrupole based ICP-MS instruments. 

The recognized advantageous characteristics of 

atmospheric plasma ionization in respect to quantitative multi-element analysis or isotopic analysis 

is significantly deteriorated by the limited . resolving power of these analyzers. The result is 

poor sensitivity for many elements and lack of 

selectivity. OVer the last few years considerable 

research effort was focused by many groups on improved sample preparation and sophisticated clean 

up techniques to overcome these limitations. 

However, common interferences remain an inherent 

problem in elemental plasma mass spectrometry, in 

particular at ultra trace levels. 

With respect to sensitivity and power of detection 

significant improvement can be achieved by using a double focusing magnetic sector based high 

resolution nalyzers instead of quadrupole 

analyzers. 

Unfortunately, up to now commercially available 

HR-ICP-MS systems have been derived from complex 

instruments originally designed to meet the 

requirements of organic mass applications. 

The design of such "hybrid" HR-ICP-MS compromised 

performance and operating convenience. For 

instance, significant, user accessible, parts such as torch and cone system is floating at high voltage. 

To overcome those problems, a compact high 

performance ICP-MS was designed from first 

principles, exclusively tailored to the characteristics of a plasma source at atmospheric pressure. 

An innovative analyzer design, combined with an expert mass control system resulted in a HR-ICP-MS 

which, on one hand, offers the speed, operating 

convenience and front-end accessibility of a quadrupole based ICP-MS and, on the other hand, 

provides the selectivity to resolve the most common spectral interferences. 

The resolving power of this analyzer can be switched 

in a second from 300, 3000 and 7500 times that of a quadrupole analyzer. The trapezoid peak shape, typical 

for magnetic sector analyzers, is of particular 

benefit to analysts having to perform fast precision 

isotope ratio determination. 

The dual-mode detector with more than 9 orders of 

detection dynamics, less than 0.2 counts/sec dark 

noise and no mass discrimination allows analyses 

of concentrations ranging from percent to ppq levels. 

283 

Geochronology of the proterozoic magmatism around 

Kristiansand, Nouth Norway. 

By: 

Geology, 

Denmurk. 

ULRICH A.S. SCHNELL. Institute of 

University nf Arhus, DK~SOOO Arhus 

K.ristiunsand is situated in the southernmost end of the 

Barnble linear belt in the Proterozoic gneiss region of 

Southern Norway. The Kristiansand area is a complex 

of para~ and ortho- gneisses. Some 48 samples where 

chosen for major element and isotopic analysis. Some 20 

samples were chosen for REE and U, Th analysis on the 

ICP-MS. Rb/Sr whole rock age determinations were 

made on the four major rock units. The foUowing 

geological history can be deduced: A sedimentary 

package is deposited on an unknown basement. The 

package contained among others a marl, a clay sediment 

and a quartz rich sandstone. A series of gabbroic or 

basaltic material can have been introduced. A deep 

burial of the sediments produced a banded gneiss, into 

which a highly evolved allanite bearing biotite-granite 

intruded (The Beg pink-gneiss) at 1256 Ma. Large 

volumes of gt'anitic to granodioritic water saturated kfeldspar megakrystic magma (augen~gneiss) intruded 

(1 158 Ma) in u plastic regime and formed "mushroom"· 

shaped intrusions, followed by upper amphibolite-facies 

metamorphose at 15 to 20 km. Simultaneously or 

immediately after the intrusion of this magma the entire 

area was deformed under an east-west compression 

leading to a marked north-south striking foHation and 

iso-clinal folding. The marble was mobilised und 

transported to the hinge zones. The folding was overprinted by an east-west trending folding wit.h a Ia:ge 

wavelength. During the lust pwt of the deformation 

(1 020 Ma). granitic magma (S~alen-Murtedalen pi~­

gneisses) intruded banded gnetss and augen gnetss 

discordantly. The granite sended concordant apophyses 

into the banded-gneiss. Finally at 1009 Ma a highly 

evolved granitic to alkali feldspar granJtic magma 

intruded t.he banded-gneiss discordanlJy. At 816 Ma 

hydro-thermul resetting effected the Jate pin~ -gneisse~ in 

fracturing zones. Indicating a po~lorogemc fracturmg 

and local mobilisation of alkali-elements 

As a whole the kristiansand area displays a record of 

svcconorwegian granitic magmatism extending from an 

initial formation of a granitic-granodioritic batholit thru 

late-kinematic intrusion of highly evolved granites to 

post kinematic fraturing and hydl'othermal resetting. 
MAGMA CHAMBER PROCESSES AND CONDUIT 

EVOLUTION IN THE lARAMIE ANORTHOSITES, 

WYOMING: Sr AND Nd ISOTOPES 

SCOATFS, James S , and FROST, Carol D., Dept of 

Geology and Geophysics, University of Wyoming, 

Laramie, WY, 82071, USA. 

A Sr and Nd isotopic investigation of anorthositic rocks in 

the 1.43 Ga Laramie anorthosite complex (LAC) of 

southeastern Wyoming reveals that Sr isotopes are effective 

monitors of magma chamber processes, while Nd isotopes 

constrain the influence of lower crustal structure on magma 

conduit evolution. The northernmost intrusion, the Poe 

Mountain anorthosite (PMa), contains a diapiric core of 

porphyroclastic and megacrystic anorthosite enveloped by a 

marginal layered intrusion composed of steeply-dipping 

leucogabbroic cumulates. Construction of the PMa involved at 

least three different resident magmas with distinctive initial Sr 

ratios: 0.7051, 0.7048, and 0.7040. 

The diapiric core was emplaced as a crystal-dominated 

mush with initial Sr=0.7050 to 0. 7051. Consolidation of the 

core involved extensive fractionation of liquid (Mg#=0.54 to 

0.30) interstitial to suspended isothermally crystallized 

plagioclase (All46+t-z). Towards the margin of the diapiric 

core, a shift to more radiogenic initial Sr (up to 0.7055) over a 

stratigraphic interval of 2000m records contamination by 

Archean roof rocks. Within the enveloping layered intrusion, 

chamber-wide recharges of 0.7048 and 0.7040 magmas are 

recognized by abrupt shifts in initial Sr at roughly 600m 

intervals. Magmas in the layered intrusion were relatively 

crystal poor, showing coupled fractionation of plagioclase 

(An56 to A114o) and interstitial melt (Mg#=0.50 to 0.27). 

Cognate xenoliths of anorthosite contained within the 

diapiric core and the layered cumulates have a restricted range 

of initial Sr compositions, 0.7043 to 0.7045, indicating an 

earlier, isotopically distinctive period of anorthositic 

magmatism. A single high-AI clinopyroxene megacryst 

(Mg#=0.68, Alz0.3=6.20 wl%) is much more primitive than 

the host layered anorthosite: initial Sr=0.7041 vs. 0.7051. 

Crystallization at high pressures ( -11 kbar) from a basaltic 

parent is favored for the megacryst, which was subsequently 

entrained in rising anorthositic magma. 

There is little resolvable variation in Nd isotopic 

compositions within individual anorthositic intrusions in the 

lAC, however significant variation exists between intrusions: 

£Nd=-5.0 to -4.4 in early, deformed porphyroclastic anorthosite 

in the centre of the complex. £Nd=-0.7 to -2.2 in the PMa and 

other composite diapiric and layered intrusions, and £Nd=-0.5 

to +0.9 in late undeformed troctolitic intrusions. The trend 

towards more primitive ENd values with decreasing relative age 

is considered to reflect the effects of an evolving magma 

conduit which becomes increasingly insulated from crustal 

contamination over time. ENd values as low as -5.0 also 

indicate contamination by Archean crust ( £Nd=-15) of the 

Wyoming Province to the north of the lAC, rather than 

accreted Proterozoic island arc terranes ( £Nd=+ 1 to -1) to the 

south of the LAC. This corroborates seismic reflection data 

for a southward-dipping terrane boundary. with Archean crust 

at depth, in the southern Laramie Mountains. 

284 

Sr AND 0 ISOTOPIC RECORD OF THE EARLY- MIDDLE 

CAMBRIAN OCEAN 

SEGEV A 1

, SASS, E.2, HALICZ, L.1, ST ARINSKY, A 2, 

1 Geological Survey of Israel, Jerusalem 95501, Israel. 2 Dept. of Geology, The Hebrew University of Jerusalem, 

Jerusalem 91904, Israel. 

The Timna Fm. in southern Israel and its stratigraphic 

equivalent in southwest Jordan (Burj Lst), whose age straddles 

the Early-Middle Cambrian boundary (530 ± 10 Ma), represent 

the first Phanerozoic marine sedimentation in the area. 

The analysis of the strontium and oxygen isotopic data of 

these rocks (limestones and dolomites) has a potential bearing, 

therefore, to the isotopic composition of the coeval Cambrian 

ocean. The 87Srf86Sr ratios of the limestones cover a relatively 

narrow range, 0. 7084-0.7089. The 87Srf86Sr ratios of the dolomites 

cover a wider range (0.7086- 0.7108), whose lower value falls 

within the limestone field The generally higher 87Sr/86Sr of 

the dolomites seem to be related to the presence of siliciclastic 

minerals of granitic origin, whose contents in the dolomites 

are significantly higher than in the limestones. It is suggested, 

therefore, that the higher 87Srf86Sr :. ..... , • in the dolomites reflects 

porewater modification during early diagenesis and oolomitization. 

It is quite posstble that the calcite in the limestones was similarly 

affected, even though to a lesser degree. It is argued, 

accordingly, that the 87Sr/86Sr ratio of the Early-Middle 

Cambrian seawater was equal to or smaller than 0. 7084. This 

value is significantly lower than the recorded (e.g. Burke et al, 

1982) Late Cambrian and Early Ordovician values (~0.7090), 

and may reflect a fast secular change between this value and 

the Late Proterozoic ratio (::::;0.7070; Derry et al., 1989). 

The 6180 values of the studied marine carbonates are all in 

the range of -2.0 to -7.5%o PDB. Such low values are commonly 

ascribed to isotopic exchange with freshwater. The present 

data suggest, however, that the 6180 values are not modified, 

as follows: (1) The 61

S0values of the dolomites (-2.4 to -5.6%o) 

are higher by-3 %o on the average than that of the limestones 

(- 6.2 to -7.6%o), which is consistent with the expected dilfurence 

between oogenetic calcite and dolomite. (2) The 87Srf86Sr ratios 

of the carbonates covary positively with the 6180. Assuming that 

the reason for the large variations in 6180 is due to a late 

exchange, it is expected that the more modified rocks would be 

lower in 6180 and higher in their 87Sr/86Sr ratios (due 

to interaction with the admixed siliciclastic minerals). 

The described trend is inconsistent with the predicted one, 

suggesting that the observed variations in the isotopic signatures 

are not due to late resetting. 

In summary, it seems that the 61SO values of the limestones 

are not modified to a large extent, and reflect the Lower - Middle 

Cambrian seawater. Assuming that the limestones formed 

from seawater at 30°C, the isotope composition of these waters 

was - - 4%o, which is compatible with other evaluations. 

Burke, W.H. et al., 1982, Variation of seawater 87Sr~Sr 

throughout Phanerozoic time. Geology, v. 10, p. 516-519. 

Derry, L.A et al., 1989, Sr isotopic variations in Upper 

Proterozoic carbonates from Svalbard and East Greenland 

Geochim. Cosmochim. Acta, v. 53, p. 2331-2339. 
COMPARISON OF RADIONUCLIDE AND NOBLE GAS 

DATA SUGGESTS POSSIBLE LIMITS FOR EXPOSURE 

AGE DATING 

SEIDL. M.A . Lamont-Doherty Earth Observatory of 

Columbia University, Palisades, NY, 10964, USA, 

HUDSON, G.B., CAFFEE, M.W., FINKEL, R.C., 

HARRIS, L., Lawrence livermore National Laboratory, 

livermore, CA, 94550, USA, and DIE1RICH, W .E., 

Department of Geology and Geophysics, University of 

California, Berkeley, CA, 94720, USA. 

We have calculated surface exposme ages by measuring 

multiple isotopic concentrations in single samples. 

Comparison of the data yields information on extraction 

techniques and nuc~de retention. Cosmogenically-produced 

Be-10, Al-26, 0-36, He-3, and Ne-21 were measmed in rocks 

collected from the 5.1 m.y. Napali member flows of the 

Waimea Canyon Basalt on Kauai, Hawaii. All analyses were 

made on olivine mineral separates, with the exception of 0-36 

concentrations measured in both olivine and whole rock 

samples. 

Several obstacles had to be overcome prior to 

measmement of the radionuclides. Be-10 and Al-26 are most 

commonly measmed in quartz. Meteoric Be-10 contamination 

of the olivine grains proved to be a large problem, and a series 

of leaching experiments were performed to remove the 

meteoric component from the mineral surfaces. High AI 

concentrations (on the order of 10,000 ppm) precluded 

detection of the Al-26 in several samples. Cl-36 is potentially 

an important isotope for this study, as it can be analyzed both 

in mineral separates and in bulk material. However, as is the 

case with Be-10, there are potential meteoric Cl-36 

contamination problems. Our initial Cl-36 results are 

systematically lower than the Be-10 and Al-26; the Cl 

concentrations were very low and not well determined in the 

analyzed olivines. We are presently repeating these 

experiments on bulk material. 

We have also measured cosmogenic He-3 (He-3c) and 

Ne-21 (Ne-21c) in the olivine separates. Other investigators 

have observed Hec loss in quartz samples and one of our goals 

is to determine whether He-3c is quantitatively retained in 

olivine. Our initial results indicate a deficit of He-3c relative 

to Be-10 and Al-26. It is tempting to infer that there has been 

diffusive loss of He-3, but until we have collected more data 

we are hesitant to draw a definitive conclusion. With one 

exception the Ne-21c concentration is low relative to the Be10 and Al-26. In the exceptional sample, the Ne-21c was 

released during the low temperature heating of uncrushed 1 to 

2 mm-sized grains. This suggests that over the course of 5.1 

m.y. the olivine structure may alter as the result of weathering, 

resulting in Ne-21 migration into other sites. 

These results have ramifications for analyzing 

cosmogenically-produced He-3 and Ne-21 in olivines from old 

flows, on the order of millions of years. Increased erosion 

may not account for discrepancies between expected and 

measured He-3 concentrations in weathered olivines. Of 

course, it is not the absolute age of the olivine that matters, but 

rather its "weathering age." However, absolute age may serve 

as a proxy for weathering age at this time. 

285 

TIME AND CONDIDONS OF CONfiNENTAL 

GROWTH PROCESSES IN THE GOTTHARD 

EXTERNAL MASSIF, CENTRAL SWISS ALPS 

SERGEEV. S.A. and STEIGER, R.H., Isotope 

Geochemistry ETH, Zurich, CH-8092, Switzerland. 

Granitoids, wide-spread in stable segments within mobile 

belts, are most sensitive indicators of changes in the geodynamic regime dwing continental development One of the 

best known External massives of the Alpine orogen, the 

Gotthard, exhibits three granitoid groups, different in age and 

mode of emplacement, allowing us to apply geochemical 

(REE and trace elements distribution, Sm-Nd and Rb-Sr systematics), petrographic and geochronologic (single zilcon UPb) methods for petrogenetic and geodynamic constraints. 

1. The Caledonian orthogneiss (Streifengneis, 440 

Ma old) which dominates the massif as core of a preVariscan anticlinorium-like structure, represents Proterozoic units (>760 Ma) mobilized during the anatexis 

closely following the regional HPT metrunorphism. 

2. The older Hercynian group (Fibbia, Gamsboden and 

Medels gneissose stocks, 298-302 Ma old) exhibits most of 

the S-type, collisional granitoid diagnostic features. It was 

formed by metasomatic recrystallization and partial melting of acid pre-Variscan rocks (-450 and >760 Ma old) 

slightly different in composition from body to body with 

some island-arc material added. The zircons from enclaves 

in the granitoids are also of Proterozoic age (-760 Ma). 

3. The younger Hercynian group (Rotondo{fremola, 

Qistallina, Cacciola and Saedelhom intmsives, 294-297 

Ma old), surrounding the gneissose stocks, are withinplate, post-tectonic homogeneous granitoids of crustal origin, containing inherited zircon grains about 450 Ma old 

Single zircons individually separated from rock-fonning 

minerals show presence in granitoids of oldest dettital (?) 

zircon grains (-1800 Ma) and allow recognition of an 

intensive Si-alkaline metasomatism (microcline porphyroblastesis) event 270 Ma ago which led to complete resetting of the rocks' Rb-Sr system. Alpine greenschist/chloritoid metammphism did not affect the U-Pb zircon systems. 

Om' results pennit to define tluee stages of Gotthard's geodynamic history from Late Ordovician to Carboniferous: 1) 

-440 Ma: completion of the Caledonian subduction regime; 

2) ..... 300 Ma: climax of the Heteynian plate collisional tectonics; 3) ..... 295 Ma: consolidation and uplift of Alpine basement 

The lack of ''Pan-African" (Cadomian) memory in zircon 

grains from the Gotthard granitoids derived from several crustal units suggest an insignificant role for Gondwana-incorporated material in this prut of the European continental mrugin. 
CAN SAND DUNES BE USED AS ARCHIVES OF 

OLD AIR? 

SEVERINGHAUS. J.P .. and BROECKER, W.S., 

Lamont-Doherty Eanh Observatory, Palisades, NY 

10964 USA, and KEELING, R.F., DECK, B., 

MILLER, B.R., and WEISS, R.F., Scripps Institution 

ofOceanography,_U.C. San Diego, La Jolla, CA 

92093 USA 

Measurement of the atmospheric 021N2 ratio several 

decades ago would reveal changes in the size of the 

biospheric carbon pool, after correcting for known 

fossil fuel burning, and show whether the "missing 

sink" of C02 is the biosphere or the ocean. How 

then to obtain a clean sample of several-decade-old 

air? Sand dunes are a porous media in diffusive 

contact with the atmosphere, somewhat analogous to 

the unconsolidated layer of frm atop glaciers. 

Recent attempts to use frm to measure OVN2 in old 

air show promise (Todd Sowers, personal communication, 1993). Unlike frm, sand dunes are incompressible and so remain permeable to much greater 

depths, and may extend the frm record into the past 

century. 

We drilled a 60 m deep test hole in the Algodones 

Dunes, Imperial Valley, California to see if the air in 

a sand dune is as old as predicted by a diffusion 

model, or if the dune is rapidly flushed by advective 

pumping during windstorms and barometric pressure 

changes. We dated the air with chloroflourocarbons 

(CFCs). Our preliminary data match the diffusion 

model well, suggesting that advection is negligible 

and the mean age of the air at 60 m depth is 11 years. 

Microbial metabolism is evident in elevated C02 apd 

subambient ~ concentrations, suggesting that 

samples for OVN2 analysis must come from dunes in 

a subfreezing environment. 

286 

NEOGENE EXTENSION IN THE WESTERN ALPS -

EVIDENCE FROM FISSION TRACK DATING 

SEWARD DIANE, and MANCKTELOW, 

NEll., S. Dept Erdwissenschaften, ETH 

Zenuum, 8092 Zfirich, Switzerland . 

Zircon and apatite fission track analysis has established the 

continuation of a zone of rapid change in mineral ages running 

from the Simplon Fault Zone in the Simplon Alps, through the 

Rhone Valley and along the south-eastern side of the Mont 

Blanc and Belledonne massifs. The jump in ages is interpreted 

to represent the position of a Neogene structure with a 

significant normal fault component. Apatite fission - track ages 

to the north-west of this zone are generally in the range 2- 4 

Ma, whereas those to the south-east range from 10 to 13 Ma. 

The corresponding values for zircon are 8 - 12 and 17 - 25 Ma 

respectively. Detailed sampling along a profile in Val de 

Bagnes suggests that the zircon age change may occur as a 

rapid gradient rather than as a single discrete discontinuity. The 

zone corresponds to the position of the "Frontal Pennine 

Thrust" in the Mont Blanc - Belledonne region. This sttucture, 

which is a prominent feature on recent deep seismic profiles in 

the region, may have been reactivated in Neogene times with an 

important normal fault component, as demonstrated by the 

markedly younger apatite and zircon fission- track ages in the 

footwall compared to the hanging wall. The normal fault 

component is also consistent with the observed elimination of 

the lower Pennine (or Simplon) nappes along this fault zone in 

the Rhone Valley - Mont Blanc - Belledonne region, resulting 

in the juxtaposition of the Valais zone and the Helvetic I 

Ultrahelvetic nappes at the current erosion level. 
niE GUANIDINE HYDROCHLORIDE METiiOD OF 

OXYGEN ISOTOPIC ANALYSIS AS APPLIED TO 

WINES. 

SHADEL. C., CALDWElL, E. A, and INGRAHAM 

N. L., Water Resources Center, Desert Research 

Institute, University of Nevada System, Las 

Vegas, Nevada 89132. 

Guanidine hydrochloride has been used for 

oxygen isotopic analysis of waters by the complete 

conversion to C02 (Dugan et al., 1985) in addition 

to the standard C02 equilibration method. In this 

method, ten J.1.l of water are heated for 8-10 hours at 

260°C with 100 mg of guanidine hydrochloride to 

produce ammonia and carbon dioxide. When 

cooled, the gases combine to produce solid 

ammonium carbamate. co2 is then produced by 

heating the ammonium carbamate with 0.5 m1 of 

1000A> H3P04 at 80°C for one hour. The co2 may 

then be purified and analyzed. The guanidine 

hydrochloride method has the distinct advantage 

of requiring only small amounts ( 1 OJJl) of water, 

allowing the measurements of, for example, 

indMdual leaf or berry water. 

Recently, this method has also been used for the 

quantitative determination of the a 18o 

composition of grape must and vintage wines, 

which is not possible with the the co2 

equllibration method. The C02 equilibration 

method may allow some ethanol (mass 46) to be 

trapped in the C02 sample, yielding spuriously 

high a 18o values as the co2 can not be purified of 

ethanol by normal slushing. With the guanidine 

hydrochloride method, the ethanol also reacts 

producing ethylene (mass 28), and water which is 

converted to co2. Evidence of this is not only the 

production of C02 but also an elevated mass 28 

signal on the mass spectrum above that normally 

obsetved as N2 background. Additionally, 

analyses of samples of varying ethanol content (0 

to 100% byvol.) and water show a systematic 

change in total a 18o composition, and a linear 

relationship between a 18o and volume % (or mole 

%) of ethanol. 

The standard process of baking water samples 

with guanidine hydrochloride and reducing the 

ethanol to C02 allows the quantitative isotopic 

analysis of oxygen in the closed system of the wine 

bottle. Such an analysis allows bottled wines to be 

better used as surrogate samples of precipitation 

for climate change studies. 

Dugan J.P. Jr., BorthwickJ., Harmon RS., Gagnier 

MA, Gahn J.E., Kinsel E.P., Macleod S., Viglino 

J.A, andJ.W. Hess, 1985. Guanidine 

hydrochloride method for determination of 

water oxygen isotope ratios and the oxygen-18 

fracUJ>naUon between carbon dioxide and water 

at 25 C. Anal. Chern., 57: 1734-1736. 

287 

POSSIBLE 142Nd EXCESS IN 3.8 )E ISUA 

SUPRACRUSTALS 

SHARMA, M., PAPANASTASSIOU, D. A., and 

WASSERBURG, G. I.~ The Lunatic Asylum, Div. of 

Geol. and Planet Sci., Caltech, Pasadena, CA 91125, and 

DYMEK, R.F., Depl of Earth and Planet. Sci., 

Washington Univ., St. Louis, MO 63130. 

Harper and Jacobsen (1992) have reported a 33 ± 4 J.l (J.l = ((Rock/ Standard)-I) x 106) 142Nd excess in a 3.8 iE felsic 

gneiss (IE 715-28) from Isua, Greenland. If this result can be 

substantiated it would indicate an early (between 4.5 and 4.3 

iE) planetary differentiation and generation of reservoirs with 

large fsm/Nd. Further, it would suggest the existence of 

mechanisms that preserved 142Nd excess in a much younger 

3.8 )E old rock. These observations prompted other workers 

to search for 142Nd excesses in other samples from early 

Archean terrains. So far no additional evidence for the 

excesses has been found. Our efforts have been directed toward 

addressing the central issue: high precision mass spectrometry 

that can resolve -30 Jll42Nd excesses. We used a F-MAT 

262 multicollector mass spectrometer, similar to the 

instrument used by H~r and Jacobsen. The data were 

obtained with 2 x 10·11 A 142Nd+ beams in static mode. Our 

experiments showed that position of the sample and ion source 

potentials can engender large (up to 60 Jl) changes in 

142Nd/144Nd ratios. These observations led to the 

development of an arbitrary but strict, standard operating 

procedure (SOP) with fixed focusing conditions. All data 

obtained by SOP are considered valid. 

Following the SOP a series of Nd ~ Standards were run for 

the 0 J.1 standard. Gravimetric standards were prepared with 

excesses in 142Nd; analyses of the 30 J.l standard were 

interleaved with the 0 J.l standard. Results were obtained 

concurrently on two Isua samples, including the one analyzed 

by Harper and Jacobsen. Figure 1 shows samples and 

standards run using SOP. The data show an internal (within 

run) precision of 6 ppm (2a) and an external reproducibility, 

following SOP of 12 ppm (2a). Figure 2 is a histogram of 

the samples and standards run using SOP. It is apparent that 

the 0 J.1 and 30 J.l standards are completely resolved with no 

overlap. Thus there is a strong indication of 142Nd excess in 

the two Isua rocks analyzed. In order to check the robustness 

of our data we ran OA and 3B by not following our SOP. 

These samples show -30 J.l increase with respect to OB and 3A 

(Fig. 1) We therefore consider the results to be still 

ambiguous given the fact that departures from the strict but 

arbitrary optimization· ·procedures can cause large shifts. 

Unless the running conditions are rigidly controlled and shown 

to yield good reproducibility, substantial artifacts can and will 

creep in the data. Division Contribution 5371 (845). 

60 3 SO 1B3B .t--.-r--, Fig 2 

40 ~ 30~~~~~~~~~~~~-+~~~~~ 

~20 

10I-f+-r.;pr--1.J.r--+-'----ll-o~o ~-I= 0 J.1 Std. 

o ~ ll=30J.1Std. 

m = Isua rocks 

4 8 12 16 20 24 

Run Numbers fi 

0 0 J.l Std • 1= ID 27-2A internal I external 

precision reproducibility D 30 J.l Std • 2-3 =IE 715-28 
OXYGEN AND HYDROGEN ISOTOPE GEOCHEMISTRY 

OF TALC 

SHARP. Zacharv D .. Hunziker, Johannes C., Institut de 

Mineralogie, UNIL BFSH-2, CH-1015 Lausanne, 

Switzerland 

Talc is generally formed by the metamorphism of either 

ultramafic rocks or siliceous dolomites, although talc is also 

found as a hydrothermal mineral in submarine environments, 

in evaporites and in high pressure metamorphosed magnesianrich pelites. The origin of talc in some massive deposits is 

enigmatic. Certain chemical indicators may be used to infer a 

particular origin for talc. Hydrothermal talc may have high 

iron contents, talc derived from ultramafic rocks may contain 

significant nickel, and carbonate-derived talc may be recognized by a high fluorine content. Often, however, the chemical composition of talc is not diagnostic of a particular environment. Furthermore, lithological associations may indicate 

a particular origin, but late structural events may obscure the 

true origin of the deposit. Stable isotope compositions of talc 

may provide a less ambiguous means of determining the 

protolith or metasomatic fluid associated with talc formation. 

The 8180 and SD values of most magmatic fluids are approximately 6 to 10%o and -50 to -80%o, respectively. Connate 

fluids from carbonates evolved during metamorphism have SD 

and 8180 values that are significantly different from those of 

magmatic fluids: -20 to 25%o and as high as O%o, respectively. 

Taken together, the 5180 and BD values may uniquely define 

the origin of a given deposit. 

Isotopic analysis of talc samples were made from a number 

of different geological environments. Talc veins in dolomites 

from Campolungo, Swiss Alps and the Balmat talc mines 

(New York, U.S.A.) have a carbonate origin. The 8180 and 

BD values are 22 to 24%o and -25%o, respectively. Talc veins 

in ultramafic rocks from different locations have 5180 and BD 

values of 5 to 6%o and -75 to -80%o. Talc from the Pamour 

talc mine has 5180 values of 8 to 9%o, supporting an 

ultramafic origin. A talc halo around a diabase intruding 

carbonate (Western Mine, California, U.S.A.) has an intermediate 5180 value of 11.7%o indicating mixing between two 

isotopically distinct reservoirs. The 5180 and SD values from 

a talc mine and from the coesite-bearing whiteschists in the 

high pressure Dora Maira Massif, southern Alps (N. Italy) are 

5 to 6.5 %o and -25 to -32 %o, quite ,different from other talc 

deposits. Similar high-pressure rocks from the Adula nappe 

have the same isotopic compositions. The combined 8180 and 

SD values are similar to those of metamorphosed oceanic 

basalts and gabbros. These high pressure rocks may have 

undergone interaction with an metasomatic fluid derived from 

a subducting slab. 

288 

DISTURBANCE OF RB-SR ISOTOPE SYSTEM IN 

CALEDONIAN GRANITES OF NORTH KAZAKHSTAN 

BY HERCYNIAN THERMO-TECTONIC EVENT 

SHATAGIN Konstantine N., Laboratory of Isotope 

Geochemistry and Geochronology, IGEM RAS, 

Staromonetny 35, Moscow, 109017, Russia. 

Rb-Sr isotope system of Caledonian granites of North 

Kazakhstan bears a fingerprint of a rejuvenation event, 

previously dated at 240-300 Ma by K-Ar method. 

Results of detailed Rb-Sr isotope study of selected plutons are the following. 

Zerendinsky batholith has an age of 448±9 Ma and 

(

87Sr / 86Sr)0=0.7055±5 (isochrone on 11 whole-rock samples). Rb-Sr result on mineral separates from one isochrone sample suggest a reequilibration of strontium isotope composition between minerals some 290 Ma. Some 

scatter of mineral points around internal isochrone does 

not allow to imply that equilibrium was achieved, therefore, 290 Ma is considered to be maximum age of secondary event. 

Zolotonosha leucogranitic pluton has an age of 399±3 

Ma and (

87Sr / 86Sr)0=0.708± 1 (isochrone on 7 wholerock samples). Three points of alaskites lie under the isochrone. One of those samples was studied on the mineral 

level. Points of biotite, K-feldspar and albite do not feet a 

common isochrone. Date for biotite measures 381 Ma, 

and for albite and K-feldspar (combined) -- 285 Ma. The 

latter is consistent with homogenization of 87Sr / 86Sr between albite and K-feldspar as a result of the same event 

as for Zerendinsky batholith. 

Orlinogorsky alaskitic pluton presumably has the 

same age as Zoloton~sha pluton, as they belong to the 

common Orlinogorsky suite. Mineral separates (each divided to the dense fractions) was studied. K-feldspar --

albite isochrone (9 points) is consistent with 87Sr / 86Sr 

homogenization at 250±40 Ma, however, biotite date (3-

pont isochrone) is meaningless. 

Above facts confirm a presence of some Hercynian 

event in geological history of North Kazakhstan. A thermal nature of this event is deduced from fresh appearance of investigated samples and lack of traces of recristallization. 
BIOGEOCHEMICAL CONTROLS OF BENTHIC 

MICROBIAL MATS ON CH4 PRODUCTION AND FLUX 

FROM SALINE ALKALINE LAKES 

SHERWOOD LOLLAR. B., Dept. of Geology, U. of 

Toronto, M5S 3Bl Canada; GERITS, J., Dept. of Geography, 

U. of Toronto M5S 3Bl; FERRIS, F.G., LANSDOWN, J. 

and SCHULTZE-LAM, S., Dept. of Geology, U. of Toronto, 

M5S 3Bl. 

A biogeochemical and isotopic study was carried out to 

investigate benthic microbial mats and organic-rich sediments 

in a series of saline alkaline lakes on the Cariboo Plateau of 

central British Columbia. The lakes present a wide spectrum 

of hydrochemical regimes and range from relatively deep (> 

15m) perennial fresh and saline lakes to shallow hypersaline 

playa lakes. pH values vary significantly from 7.5 to > 10. 

Individual lakes support abundant microbial communities and 

associated authigenic mineral deposits of diverse and complex 

structure. Microbial mats are widely perceived to be modern 

analogues for the stromatolite-forming benthic microbial 

communities that constituted the most abundant life form of the 

Precambrian. They play a critical role in the formation of 

organic-rich sediments, the precipitation and diagenesis of 

carbonate and silicate minerals, and the biogeochemical 

transformation of carbon between organic, inorganic and 

atmospheric carbon pools. The primary objective of the 

research is to characterize and define the controls on these 

dynamic biogeochemical transformations. This presentation 

will focus on CH4 production in these lacustrine systems, 

carbon gas flux to the atmosphere, and the controlling 

influence of microbial mats in regulating that flux. 

Despite considerable geochemical diversity, 6 of the 7 lakes 

surveyed were supersaturated in CH4 with respect to the 

atmosphere. Of the 3 lakes studied in detail, CH4 

concentrations in Goodenough Lake are 50-400 times 

supersaturated with respect to the atmosphere while Deer and 

Probe Lake concentrations are up to 5000 times supersaturated. 

Flux measurements made elsewhere for lakes several hundred 

times supersaturated are on the order of 5 mg CH/m2/day (1). 

Profiles of dissolved gas and geochemical species collected 

within the lakes and across the sediment-water interface to 

depths of 30-40 em indicate that microbial mat development 

plays a major role in regulating the release of CH4 and other 

carbon gases from the sediments to the water column and the 

atmosphere. The degree of mat development appears to control 

interlake and seasonal variation in redox conditions at the 

sediment-water interface, and hence in the extent of CH4 

oxidation taking place within this zone. 

(1) Oremland et al. (1987) Geochim. Cosmo. Acta 51:2915-

2929. 

289 

NOBLE GAS SOLUBILITY IN SILICATE- AND 

METAL-MELTS AT HIGH PRESSURES, AND 

ITS APPLICATION TO THE ESTIMATION OF 

SURFACE TENSION 

SHffiATA, T., OZIMA, M., Dept. of Earth and 

Space Sci., Osaka University, Toyonaka-City, 

560,Japan, and TAKAHASHI, E., Faculty of Sci., 

Tokyo Inst. of Technology, Meguro-Ku, Tokyo 

152, Japana. 

We measured noble gas (He, Ne, Ar, Kr) solubility 

in basalt melts, olivine crystal, and metal (Au) melt 

at high pressures up to 2 Kbars. Basalt powder (ca. 

100 j.l) and olivine crystals (a few J.l) were contained 

in a platinum capsule (2 mm q> x 5 mm) and Au 

chips were put in a boron-nitride capsule (5 mm q> x 

5 mm.) A mixture of noble gases (volume ratio; 

He:Ne:Ar:Kr = 1:1:97:1) was used as a pressure 

medium. Samples were kept at 1250 oc, 1300°C, 

and 1600oC for 2, 5, and more than ten hours at 1, 2 

Kbars. 

Noble gas solubility of the gold melt at 2 Kbars 

was by five orders of magnitude smaller than those 

in the basalt melt, and there appeared little pressure 

effect on the solubility. Noble gas partition coefficients between silicate melt and gold melt at 2 Kbars 

are very small ( <10-5), being consistent with theresult by Matsuda et al.. Partition coefficients 

between basalt melt and olivine crystal are similar to 

those obtained by Hiyagon et al.at an atmospheric 

pressure. 

From noble gas solubility in basalt and metals, we 

estimated surface tension of these materials. To a 

fairly good approximation, surface tension (a) is related to noble gas solubility or to Henry's law constant (H) through the following relation, 

In (H) = ( -4xaR 2 + E)/KT , where R, K, T, and E 

stand for the radius of noble gas (we took a Vander 

Waals radius}, Boltzmann constant, absolute temperature, and solvent-solute interaction energy. In 

the case of noble gases which do not generally interact with solvent, E may be neglected. From the 

noble gas solubility data, we calculated the surface 

tension of basalt melts, which gave 180 dyne/em. 

The value agrees with those estimated by other 

methods. There was little pressure effect on the 

surface tension of basalt melt at least up to 2 Kbars. 

Matsuda et al., 1993, Science, v. 259, p.788-790. 

Hiyagon et al., 1986, Geochim. Cosmochim. Acta, 

v .50, p.2045-2057 0 
GEOCHEMISTRY OF CE AND ND ISOTOPES 

AND REB ABUNDANCES IN PRECAMBRIAN 

ORTHOGNEISSES FROM THE KAMIASO 

CONGLOMERATE, CENTRAL JAPAN: REB 

EVOLUTION HISTORY OF THE PRECAMBRIAN 

BASEMENT IN JAPAN 

SHIMIZU. H., Dept. Earth Sci., Kumamoto 

Univ., Kumamoto, 860, Japan, LEE, S.-G., 

KIGAM, Taejeon, 305-350, Korea, MASUDA, 

A., Univ. Electro-Communications, Chofu,182, 

Japan, and ADACHI, M., Dept. Earth Planet. 

Sci., Nagoya Univ., Nagoya, 464-01, Japan 

Cerium isotopic data, rare earth element 

abundances and major element compositions were 

determined for Precambrian orthogneisses and 

granites in the Jurassic Kamiaso conglomerate from 

central Japan; Sm-Nd age of 2.07 Ga with EN d(2.07 

Ga) = -1.9 was already reported by us for these 

samples. Three types of REB patterns are observed 

for the Kamiaso samples: (1) reverseS-shaped 

patterns with high enrichment of LREE, (2) patterns 

consisting of concave La-Nd span and reverseSshaped Nd-Lu span, and (3) patterns consisting of 

two or three rectilinear lines with inflection at Ho 

(and Nd). Major element data for the third REB 

pattern-type samples show low potassium content or 

high sodium content, implying that these samples 

retain the chemical features of their older protoliths. 

The other samples are characterized by high 

potassium content, which is typical of some 

Proterozoic potassic granites. Ece(O) values for the 

Kamiaso samples range from -1.1 to +2.8. 

On the basis of the Ce and Nd isotopic data and 

REB patterns, REB evolution of Precambrian 

basement of the Japanese Islands was deduced. (1) 

Proto lith of the Kamiaso orthogneisses was formed 

from LREE-depleted mantle in the late Archean as a 

part of the continental crust, and had the LREEenriched pattern that was observed for the third type 

of REB pattern. (2) Granitic rocks were fractionated 

from the protolith in an igneous event at 2.07 Ga; in 

which plagioclase and REB-bearing minerals such as 

zircon, monazite and allanite played a significant role 

in REB distribution. The 2.07 Ga processes are 

considered to have been fractionation among granitic 

rocks in the continental crust. (3) The granitic rocks 

were metamorphosed to gneisses at 1.8-1.6 Ga, 

during which process REB patterns for the whole 

rocks were essentially unchanged. 

290 

Xes-Xen ISOTOPE DATING 

SHUKOLYUKOV Yuri A., MESHlK. A. P., 

PRAVDIVTSEVA, 0. V., Vernadsky Institute of 

Geochemistry and Analytical Chemistry, Russian 

Academy of Sciences, 117975 Moscow, Russia 

Xes·Xen isotope dating method was suggested and 

intensively developed last years in Russia. The method 

based on the spontaneous fission of 238u yielded in the 

production of radiogenic xenon - Xes· During the irradiation of the mineral in nuclear reactor neutron-induced 

Xe of different isot~c composition - Xe0 is produced 

from the flSSion of 5u. If the mineral-monitor of wellknown age \\ill be irradiated together with the studied 

sample the ·age of the sample could be calculated as a 

function of Xes;Xen ratios measured in the sample and 

in the monitor, and age of the latter. So, the important 

advantage of Xes·Xen method is that for age calculation 

we do not need to measure any absolute concentrations, 

only Xe isotopic ratios is required.The second advantage 

of the method is the absence of trapped radiogenic xenon 

in aU of studied samples because of the low Xe contents 

in fluids in contrast to 40 Ar. Although some part of Xe 

may escape from the mineral during geological time, the 

activation energy of Xe migration of the rest is very high 

(up to 100-300 kcal/ mol for zircons). These values are 

30 kcal/ mol higher then for radiogenic Pb in the same 

samples. This ensures a good retention a high portion of 

Xes. By means of step wise annealing X~-Xen spectra 

could be obtained, with the high temperature plateau corresponding to the real age. Thus, the important advantage of the method is the ability of dating the opened isotopic systems in minerals. In contrast to the U-Pb method 

Xe5-Xen technique does not require the series of samples 

needed to plot isochrone, one sample is sufficient for the 

real age determination. 

Xes-Xen technique have been successfully applied for 

dating zircons from the BaikaJ region, Finland, KareUa, 

Ukraine, Antarctic. The obtained ages turns out to be in 

a perfect agreement with U-Pb method. 

Uranium deposits of Russia, Kazakhstan, Germany, 

France and South Africa, have been dated by Xe5-Xen 

method. It was found out that the hydrothermal ore deposition was a prolonged pulsate process with the definite spatial distribution. 
OXYGEN AND STRONTIUM ISOTOPES IN CONTRASTING :MET ALUMINOUS GRANITIC SUITES FROM TWO 

PRECMffiRIAN FOLDBELTS IN NE BRAZIL 

SIAL. AN., FERREIRA, V.P., Dept. of Geology., Stable 

Isotope Lab. (LABISE), UFPE, C.P. 7852, Recife, PE, 

50732-970, Brazil, LONG, L.E., Dept. of Geological Sciences, U. Texas, Austin, TX, 78712, USA 

Magmatic epidote (mEp)-bearing plutons in the Cachoeirinha-Salgueiro (CSF) and Serid6 (SFB) foldbelts, NE Brazil, 

form two contrasting geochemical groups that were emplaced 

respectively at 7-9 and 3-4 kbar ranges, as estimated from the 

Al-in-homblende geobarometer (Schmidt, 1992). 

In the CSF, granodiorite/tonalite plutons which typically intrude low-grade marine metaturbidites, have abundant microgranular hbl-bi diorite to tonalite enclaves. Amphibole-rich 

polycrystalline clots, up to 15 em long, are widespread and 

when occurring within the enclaves, they seem to be disaggregates from larger amphibolite fragments. They are composed 

of Ca-cpx and interstitial biotite, and zoned amphiboles (core 

and patchy zoning), whose grains commonly meet at - 120° 

triple junctions. They exhibit granoblastic texture in the interior of the clots, generally armored by an outer zone composed 

ofbi + hbl. In the SFB, mEp is found in qz diorites, tonalites, 

qz monzodiorites, qz monzonites and granites, which intruded 

gneisses and schists. 

Rb-Sr isochron data from CSF enclaves and host mEp-bearing granodiorite provide a highly linear regression (I = 

0.70598), indicating that enclave and host magma are cagenetic or had been isotopically equilibrated during cooling. 

In SFB mEp-bearing plutons, whole-rock 8180 is +7.8 to+ 

8.1%0SMOW> and qz diorite enclaves are- +7.8%o. 8180 in tonalites intruding basement rocks is about +6.4%osMow· CSF 

mEp-bearing plutons, although oxidized I -type granitoids, 

exhibit high 8180 (+ 11 to+ 13%osMow). Diopside-bearing varieties have very homogeneous 0-isotope compositions(+ 11.6 

to + 12%osMow), consistent with their restricted variability of 

major and REB-chemistry. Granodiorite/tonalite host and qz 

diorite enclaves display the same range of whole rock ol80, in 

accord with the indications of the isotope data. Values of ol&o 

in amphibole-rich clots in CSF plutons are + 10.1 to 

+ 11.6%0SMOW• an isotope range predictable for the source of 

their high 8180 host magmas. Therefore, 0 and Sr isotopes 

point to the amphibole-rich clots as probable fragments from a 

low-T hydrothermally-altered basaltic source for these magmas. Temperature estimates for them (670-690°C, according 

to Nabelek & Lindsley thermometer, 1985) are <than those 

for granodiorite hosts (700-850°C, according to Blundy & 

Holland thermometer, 1990), reinforcing the contention above. 

B1undy, J.D., and Holland, T.J.B., 1990, Calcic amphibole 

equilibria and a new amphibole-plagioclase geothermometer, 

Contrib. Mineral. Petrol., v. 104, p. 208-224. 

Nabelek, C.R., and Lindsley, T.H., 1985, Tetrahedral Al in 

hornblende: a potential thermometer for some mafic rocks., 

Geol. Soc. Amer, Annual Meet., Orlando, Fla. p. 673. 

Schmidt, M.W., 1992, Amphibole composition in tonalite as a 

function of pressure: an experimental calibration of the Alin-homblende barometer: Contrib. Mineral. Petrol., v. 110, 

p. 304-310. 

291 

THE POST-VARISCAN EVOLUTION OF THE 

CONTINENTAL MARGINS OF THE BAY OF BISCAY: 

A STUDY USING APATITE FISSION TRACK 

ANALYSIS 

SIDDALL R., Research School of Geological and 

Geophysical Sciences, Dept. of Geological Sciences, 

University College London, Gower Street, London, 

WClE 6BT, UK. 

The sensitivity of apatite fission tracks to cooling and 

unroofing episodes in Earth history is particularly suited to 

the tectonic processes of rifting and the long term 

evolution of continental margins. Apatite fission track 

analyses have been made on -70 samples of igneous, 

metamorphic and sedimentary rocks from coastal outcrops 

of Western France (the Armorican Massif) and northwestern Spain (the Hesperian Massif) the aim being to 

unravel the potential thermal overprint associated with the 

opening of the North Atlantic Ocean, specifically in the 

Bay of Biscay. 

The fission track ages range from -70 to 270 Ma, with 

the youngest ages towards the Atlantic coasts, and become 

progressively older eastwards. The data are interpreted in 

terms of the thermal histories resulting from the Mesozoic 

post-breakup exhumation of the rift flanks in the 

westernmost regions of Spain and France, whilst eastwards 

the earliest detectable cooling episode is Early Permian, 

reflecting the exhumation of the Variscan mountain chain. 

The data from the Armorican Massif are consistent 

with 2 previously unrecognised periods of burial and 

subsequent exhumation during the Mesozoic. The most 

recent (Campanian) burial episode is inferred to represent 

the deposition of chalk. The data from north-western Spain 

do not appear to reflect this second burial, probably 

because at this time this region was outside the chalk 

depocentre. 
AUSTRALIS: A MICROBEAM AMS SYSTEM FOR 

GEOCHRONOLOGY 

S.H. Sie and G.F. Suter, Heavy Ion Analytical Facility 

CSIRO Division of Exploration and Mining 

P.O. Box 136, North Ryde, NSW 2113, Australia 

The development of Accelerator Mass Spectrometry (AMS) 

represents significant progress in the resolution of problems 

of molecular and isobaric interferences, by virtue of 

acceleration to MeV energies and multiple charging in a 

tandem accelerator. The use of negative ions from the source, 

and availability of variety of molecular ions themselves can 

also be exploited to reduce or eliminate interference. The 

potential of AMS in isotope geology, both for geochronology 

and tracer applications has long been recognised. 

The CSIRO Heavy Ion Analytical Facility (HIAF) was 

established as an analytical laboratory based on the 3 MY 

Tandetron , dedicated to applications in the geosciences, and 

research relevant to the minerals industry. A basic AMS 

system that enables 14c and 10ae measurements has been 

developed [ 1]. This system is now being upgraded to enable 

in-situ measurements of ultra-traces and isotopic-ratios in 

mineralogical applications. The upgraded system will include 

a microbeam Cs ion source which is designed to produce as 

low as 1 f.1tn diameter Cs beam to enable analyses of 

monomineralic grains and microscopic features. The Cs 

primary beam will be mass analysed in order to minimize 

contamination of the sample. The detection system will be 

upgraded to enable analyses of elements up to U, at 2.5 MY 

terminal voltage for charge state 6+. The system will be 

known as AUSTRALIS: A.M.S. for Ultra Sensitive TRAce 

eLement and Isotopic Studies. 

AUSTRALIS will enable applications of a few 

geochronometers or tracers previously either tedious and 

limited to bulk applications, or completely inaccessible. 

Potentially useful applications include the common Pbi U/Pb 

(207Pb'2uopb) 87Rb/'7sr {T =4 8x101oy) 187Re/ 870s 1 io 1!2 . , (T 112=4.3x10 y) systems. 

Overviews of the system and of the anticipated 

applicationwill be presented. 

Sie, S.H, Ryan, C.G. and Suter, G.F.,1990. An AMS facility 

for minerals exploration research, Nucl. lnstr. Meth. B52, 

p.294-297 

292 

210p],_22%_230Jb_238u RADIOACTIVE DISEQUILIBRIA 

IN VOLCANICS FROM REUNION ISLAND AND 

VES1MANNAEYJAR (ICELAND) 

Sigmarsson, 0.*, Condomines, M.*, Bachelery, P.** and 

Jakobsson, S.*** *CRY and CNRS, Universit~ Blaise 

Pascal, Clennont-Ferrand, France. **Universit~ de la 

R~union, France. ***Museum of Natural History, Iceland. 

In general oceanic island basalts have a radioactive 

disequilibrium pattern of (226Ra)>(230Th)>(238U) but very 

few data exist on 210p],_226Ra and 228Ra_232Th systematics. 

Several recent basalts from Piton de Ia Foumaise (R~union 

island) and alkali basalt, hawaiite and mugearite from 

Vestmannaeyjar in Iceland (Surtsey and Heimaey) have been 

analyzed for 210pb_226Ra-230Th-238u radioactive 

disequilibria Basalts from both localities have (238ut230Th) 

around 1.2, (226Rat230Th) in the range of 1.3 to 1.4, and 

(210Pbt226Ra) at the time of eruption in the range 0.6-0.7. If 

these fractionation are only due to partial melting of a mantle 

source in radioactive equilibrium with identical partition 

coefficients, these results would imply a similar degree of 

melting in both islands. The existence of a 21 Opb deficit, 

despite of 228Ra_232Th equilibrium would imply a separation 

of the magma from the mantle between 30 and 100 years before 

eruption. However, although 230Th-238u fractionation is 

probably negligible during magma transfer and differentiation, 

it is not always the case for Ra-Th and Pb-Ra fractionations. 

In Piton de Ia Fournaise, the Ra enrichment cannot have 

taken place above the magma chamber. Samples from both the 

1986 summit and the contemporaneous lateral eruption display 

identical (226Rat230Th) values. On the other hand, picritic 

magma which has incorporated crystal mush from the magma 

chamber, has (226Raf230Jb) as low as 1.18. 

In Vestmannaeyjar, (226Rat230Th) vary from 1.4 in the 

1963 alkali basalt to 1.19 in the 1973 mugearite, while Th 

abundances increased from 1.49 to 3.21 Jlg/g. This decrease of 

(226Rat230Th) is probably due to plagioclase fractionation 

(with a bulk KDRa-0.3). The mugearite can be derived from 

an alkali basalt similar to the Surtsey magma by -50% 

fractional crystallization on a timescale ~ 10 years. 

Moreover, Pb-Ra fractionation can be due to several 

processes during magma differentiation or eruption: Pb loss as 

a volatile compound, Pb fractionation by sulfide 

crystallization. 

The results from these volcanic islands clearly show that 

magma chamber processes can affect U-series disequilibria and 

must be taken into account before assigning them to partial 

melting only. 
BASALT PETROGENESIS: A COMBINED 

APPROACH USING U-Th DISEQUILmRIA 

AND Sm-Nd ISOTOPIC SYSTEMATICS 

Sims K.W,W)&2; DePaolo, DJ.l; Murrell, M. T.2; 

Baldridge, w.s.2; and Goldstein, SJ.2. 

1) Berkeley Center for Isotope Geochemistry, Dept. of 

Geology and Geophysics, U.C. Berkeley. Berkeley CA 

94720 

2) LANL, Los Alamos, NM 87545 

Measurements of 238u-230Jb disequilibria in young oceanic 

basalts indicate that Th is enriched in the melt relative to U. 

particularly in MORBs, which have (238UJ230rb) as low as 

0.65. While this observation demonstrates that magmatic 

processes are capable of fractionating even highly 

incompatible element ratios, large U/l'h fractionations have 

been difficult to interpret because U and Th are thought to be 

too highly incompatible to allow such fractionations to occur 

by crystal-liquid partitioning alone, especially in MORBs 

where the chemistry suggests that total melt fractions are large. 

Although the mineral responsible for the U/l'h fractionation 

may be identified (garnet), the magnitude of this fractionation 

is still difficult to reconcile without calling on other processes, 

e.g., volatile exsolution, fluid/melt interaction, etc. 

To further investigate the controls on U-Th fractionation, 

we have measured Th and Nd isotopes (TIMS) and [U], [Th], 

[Sm] and [Nd] (ID-MS) in a series of young Hawaiian basalts 

and axial-MORBs which show a wide range of composition. 

The Hawaiian basalts range from tholeiites to basanites; the 

MORBs (EPR) range from N-MORB to E-MORB. We then 

compare inferred U/fh fractionation (238uf230Th) with 

inferred Sm/Nd fractionation to evaluate the relative 

partitioning behavior of U and Th (where the Sm/Nd source 

ratio is inferred by using the 1. 7 Ga source model of DePaolo 

(1988) and the measured 143NdJ144Nd of the sample). 

The Hawaiian lavas show a good correlation (r2 = 0.93) 

between Ulfh and Sm/Nd fractionation. The tholeiites are the 

least fractionated, the basanites are the most fractionated. In 

all samples Ufl'h fractionation is less than Sm/Nd 

fractionation. Correlation of Sm/Nd fractionation with U/lb 

fractionation allows the Hawaiian data to be inverted in terms 

of batch melting models. Assuming DNd = 0.020 and Dsm = 

0.050, this inversion yields: Du = 0.00143, Drh= 0.0006, with 

melt fractions varying from 5% (tholeiites) to <1% (basanites). 

The coherence of Sm-Nd and U-Th behavior is strong 

evidence that U/fh fractionation is a result of crystal-liquid 

fractionation associated with partial melting. In contrast, the 

MORB data show a systematic but significantly different 

relationship between U/lb and Sm/Nd fractionation. For both 

U-Th and Sm-Nd, N-MORBs are less fractionated than EMORBs. However. in all MORB samples U/fh fractionation 

roughly equals Sm/Nd fractionation. In fact, for most 

MORBs, U/lb is more fractionated than in Hawaiian basanites 

while Sm/Nd is less fractionated. 

Our data indicate there are fundamental differences in U-Th 

behavior between Hawaii and MORs. While Hawaiian lavas 

might be expected to be more fractionated than MORBs 

because of smaller melt fractions and deeper origin, they are 

instead less fractionated. If processes other than crystal-liquid 

partitioning are operating, they appear to be more important at 

MORs than in Hawaii. Differences in the melt generation 

geometry exist, but it is not obvious how these can generate 

the relationship between UII'h and Sm/Nd fractionation 

observed in MORBs. 

293 

K-Ar and 40Arf'Ar AGE OF BRUNHES-MATUYAMA 

BOUNDARY FROM LAVAS IN THE CHILEAN ANDES 

SINGER, Bradley S., Dept. de MincSralogie, Universite de 

Geneve, 1211 Geneve 4, Switzerland; EMAIL: Singer@sc2a. 

unige.ch. and NELSON, Stephen T., Woodward-Clyde 

Federal Services, 101 Convention Center Dr., Suite PliO 

MS 423, Las Vegas, NV 89109, USA. 

In obtaining K-Ar and 40 ArP9 Ar ages for >30 Pleistocene 

sap:tples from the frontal arc center of Volcan Tatara-San Pedro 

(36°S, 70.5°W), we have dated two samples from a sequence of 

10 lava flows that record transitional virtual geomagnetic pole 

(VGP) positions corresponding to the Brunhes-Matuyama 

Boundary (BMB). These 10 flows have VGPs centered on 

northern Australia. The uppermost flow is truncated by an 

unconformity overlain by lavas with normal polarity <330 ka. 

Conventional K-Ar (USGS, Menlo Park) and laser fusion 40 ArP9 Ar analyses (UCLA) were collected from groundmass, 

whole-rock. and plagioclase aliquots from two samples with 

transitional VGPs. Using the laser, heating steps removed Ar 

with a high atmospheric component from groundmass aliquots 

which were subsequently fused to yield Ar from which ages 

were calculated. Five 40 ArP9 Ar fusion analyses of groundmass 

from the stratigraphically lower andesite flow (QTWl0-3) give 

a mean age of 760 ± 8 ka, whereas four fusions of the 

stratigraphically younger basaltic andesite flow (QTW10-10) 

give 756 ± 37 ka. Eight of the nine laser fusion analyses have 

overlapping 1cr errors and these eight are within lcr error of KAr ages from the goundmass and whole-rock aliquots. The 

mean age (weighted by inverse of cr2

) for 8 analyses from 

QTWI0-3 is. 763 ± S ka and the 7 analyses from QTWI0-10 

give 762 ± 8 ka (Fig.). Four laser fusion analyses of 

plagioclase from QTWI0-3 give 786 ± 20 ka and two from 

QTWI0-10 give 754 ± 22 ka. Four other plagioclase aliquots 

give ages from 13 to 92 Ma, suggesting presence of a 

xenocrystic contaminant in these two lavas. ~0~-----------.----------~ 

weighted mean I weighted mean • 40139 1 

~= I;~;Slk:ll !76;±jkil_ :~== 800 I I wbole-nd 

Lava Flow QTWJ0-3 Lava Flow QTWI0-19 

As most plagioclase, whole-rock, and groundmass ages 

overlap within error, the groundmass plus whole-rock ages 

(Fig.) are interpreted as the time of extrusion. The only other 

direct date of BMB lavas is from Hawaii, where the 40 ArP9 Ar 

incremental heating age of three whole rocks is 783 ± 11 ka1

• 

Although younger than the Hawaiian age by 2.6%, our ages 

agree with new 40 ArP9 Ar laser fusion ages from sanidines that 

bracket the BMB between 790 and 760 ka2• The VGP for 

Hawaiian samples centers on eastern South America, thus two 

different "long-lived" dipolar orientations of the geomagnetic 

field3 may have been occupied at slightly different times during 

this polarity transition. 1

Baksi A.K., et al. (1992) Science. 256:356-357. 2

1zett G.A., and O'Bradovich J.D., J. Geophys. Res. (in press). 3

Hoffman A.K. (1992) Nature. 359:789-794. 
OBDUCTED MARGINS OF A CARIBBEAN 

PLATEAU: EVIDENCE FROM 40Ar-39Ar AGES 

AND TRACE ELEMENT CHEMISTRY FOR AN 

OCEANIC FLOOD BASALT PROVINCE 

SINTON. C.W. and DUNCAN, R.A., COAS, Oregon 

State University, Corvallis, OR 97331; 

csinton@ oce.orst.edu 

The anomalously thick oceanic crust beneath the 

Caribbean Sea appears to be the remnant of an oceanic plateau 

wedged between the North and South American plates. 

Tectonically uplifted margins of this feature provide an 

opportunity to investigate the volcanic stratigraphy, structure, 

and composition of this province. Here we present evidence 

that mafic rock suites from the Dumisseau Formation of Haiti, 

Curacao, the Western Cordillera of Colombia, Gorgona Island, 

and the Nicoya Peninsula of Costa Rica are cogenetic, using 

40 Ar-39 Ar ages and new and published trace element data sets. 

40 Ar-39 Ar ages from Curacao, Haiti, Gorgona Island, and 

Costa Rica range from 88-90 Ma. These ages are in accord 

with the Coniacian to Early Campanian fossil ages of the 

sediments that overlie the Caribbean crust. The radiometric 

ages further indicate that eruption occurred simultaneously over 

a wide area and during a relatively short time period, similar to 

continental flood basalt volcanism. 

Trace element patterns indicate at least three magma types 

within the Caribbean province. The lavas of the Western 

Cordillera of Colombia, the Nicoya Peninsula, and Curacao are 

uniform in trace element composition, having slightly enriched 

signatures and flat to almost flat REE patterns (La!Yb -1-1.5). 

Some tholeiitic basalts from Gorgona Island and a single 

sample from the Venezuela Basin are also included in this lava 

type. Lavas from the Nauru Basin and the Ontong Java 

Plateau are almost identical to this lava type in terms of trace 

elements. A LREE enriched lava type is typical of the upper 

stratigraphy of the Dumisseau Formation of Haiti and of the 

Beata Ridge (DSDP site 151). Komatiites and associated 

tholeiitic basalts from Gorgona Island have LREE-depleted 

patterns, similar to a single sample from the Colombian 

Basin. A lack of data from the Caribbean crust inhibits a more 

definite comparison between the submarine and the on-land 

portions of the plateau. 

Together, the ages and the trace element data support the 

existence of an oceanic plateau forming the core of the 

Caribbean plate. The formation of the plateau was 

cataclysmic, occurring over a relatively short time period. 

Lavas bear the signatures of both an enriched and a depleted 

.mantle source. Plate reconstructions indicate that the plateau 

could have formed over the present position of the Galapagos 

hotspot, providing a possible source for an enriched 

component. 

294 

Nd-ISOTOPE SYSTEMATICS OF TilE DUN MOUNTAIN 

OPHIOLITE: REASSESSMENT OF ITS ORIGIN AND 

IMPLICATIONS FOR MANTLE WEDGE PROCESSES 

DURING A BACK-ARC TO NASCENT ARC 

lRANSmON 

SIVELL WJ,. Chemistry Department, University of 

Western Sydney, Nepean, Kingswood 2747, Australia, and 

McCULLOCH, M. T., Research School of Earth Sciences, 

The Australian National University, Canberra 2601, 

Australia 

JnitiaJ143Nd/144Nd ratios are reported for early Permian and 

latest Carboniferous igneous rocks which comprise three 

discrete, sequentially emplaced magmatic suites within the 

northern Dun Mountain Ophiolite belt (DMOB), New Zealand. 

Stage 1 pillow basalts (£Nd(1j=+6.3 to +7.5) and Stage 2 

basalt flows (£Nd(l')=+8.4) possess the chemical characteristics 

of incompatible element enriched and depleted back-arc basin 

basalts (BABB) respectively. Stage 3 basic to silicic intrusives 

have refractory mineralogies and high uniform initial 

143Nd/144Nd ratios (£Nd(1)=+9.3±0.2) over a wide range of 

147Smf144Nd (0.29-0.17) including values which exceed those 

of MORB. They possess pronounced subduction-related 

geochemical signatures like island arc tholeiites from 

immature arcs but are isotopically more depleted than the 

preceding back-arc eruptives which contain a substantial 

enriched asthenospheric component similar to that in ocean 

island basalts (OIB). 

Regression analysis of the Sm-Nd isotopic data only for the 

Stage 3 ophiolitic rocks yields a precise age of 278 ± 9 Myr. 

(MSWD = 0.15), in agreement with the generally inferred early 

Permian age of the DMOB. Sm-Nd isotopic measurements for 

exclusively Stage 2 basalts suggest an older, albeit less 

precise, latest Carboniferous age of 295 ± 48 Myr. (MSWD = 0.48) for the strictly oceanic-crust portion of the ophiolite. 

The inclusion of data for a clinopyroxene-rich ferrogabbro 

cumulate from the Stage 2 suite extends the range of Sm/Nd 

values and provides an even older but precise age of 308 ± 12 

Myr. (MSWD = 0.12). The back-arc seafloor and nascent arc 

rocks of the DMOB which we recognize should be considered 

as temporally discrete ophiolite components. 

Island arc basalts typically have lower initiaJ143Nd/144Nd 

ratios than either MORB or proximal BABB because slabmodification of supra-subduction zone (SSZ) mantle wedge 

regions often masks Nd-isotope evidence of previous 

significant wedge depletion, otherwise indicated by ratios 

between non-slab derived incompatible elements. However, 

subduction of young and relatively sediment-free ocean crust 

caused a minimal decrease in Nd-isotope values of the 

ophiolitic Stage 3 nascent arc magmas, enabling a more 

accurate assessment of intrinsic mantle wedge isotopic 

depletion. 

The ophiolite implies a model for the general depletion of 

sub-arc SSZ mantle wherein preferential extraction of lowmelting OIB-like mantle material at back-arc spreading centres 

prevents replenishment of the wedge apex in these upwelling 

enriched asthenospheric components, at the same time 

generating an increasing volume of highly depleted mantle 

(HOM) residual after BABB removal. In the sub-arc region 

continuous melting of this HOM is promoted by influx of 

slab-derived fluids. 
GEOCHRONOLOGY AND CRUSTAL EVOLUTION 

OF THE ARCHAEAN BELOMORIAN (WHITE SEA) 

AND NORTH-SWEDISH HIGH-GRADE TERRAINS 

OF THE FENNOSCANDIAN (BALTIC) SHIELD 

SKI0LD. T., Museum of Natural History, S-104 05 

Stockholm, Sweden, BIBIKOVA, E., Vernadsky Inst 

of Geochem. and Analytical Chemistry, 117334 

Moscow, Russia, and BOGDANOV A, S., Lund 

University, S-22362 Lund, Sweden 

The granite-greenstone Karelian Province of the 

Fennoscandian (Baltic) Shield is bordered to the 

northeast, north and northwest by high-grade 

metamorphic terrains of the Belomorian (White Sea) and 

North-Swedish regions. Here we combine new U-Pb 

zircon data with petrogenetic Nd-isotope systematics 

which permit us to model the crustal evolution of these 

terrains. 

According to Nd-isotope data, both the Belomorian 

and the North-Swedish Archaean crust were formed no 

earlier than 3.0 Ga ago. This age limit is based on the 

calculation of maximum crustal residence ages involving 

a depleted mantle source. 

The earliest metamorphic event recognized in these 

areas occurred 2.85 ± .05 Ga ago (low pressure 

amphibolitic and granulitic conditions) and was followed 

by the emplacement of tonalites at 2.80-2.75 Ga. 

However, the main phases of igneous and metamorphic 

activity took place 2.7-2.6 Ga ago. Within that time 

interval the intrusion of gabbros (2.69 Ga), diorites (2.67 

Ga) and granites (2.58 Ga) took place. In the Belomorian, 

the 7-9 kbar metamorphism was accompanied by 

temperatures decreasing gradually from granulitic to 

amphibolitic conditions. The PT -regimes for the NorthSwedish Archaean rocks are less well known. 

The high-pressure metamorphism and its relatively 

short duration together with certain trace element 

distributions in the ortho-gneisses are taken to imply 

collisional conditions. 

Our results indicate that the early 3.0-2.8 Ga stages of 

crust formation in the Belomorian and the North-Swedish 

areas coincide in time with those of the Karelian province. 

However, when it comes to the time after 2.7 Ga, the 

metamorphic and magmatic records are different. Thus it 

is inferred that the Karelian granite-greenstone province 

represented a stable craton during the latest Archaean. 

Along its northeastern, northern and northwestern 

margins, however, high-grade metamorphic belts were 

formed. 

295 

GEOCHEMISTRY AND Sr, Nd, Ar ISOTOPE 

SYSTEMATICS OF HAITIAN K/T TEKTITES AND 

CHICXULUB IMPACT MELT. 

SMIT, J., BEETSMA. J.J., VONHOF, H.B., PRINGLE, 

M. J. and WIJBRANS, J.R., Dept. of Earth Sciences,' 

Vrije Universiteit, 1081 HV, Amsterdam, the Netherlands, 

M. GRAJALES N., Inst.Mex.Petr.Apart. 14-805, 07730 

Mexico d.f. 

(Trace) elements and Sr, Nd isotope ratios were measured 

from Cretaceous!fertiary (KIT) tektites from Beloc (Haiti) and 

impact melt from the Chicxulub crater. In addition 40 Ar;39 Ar 

r ages from black glass tektites from Beloc were determined. 

Some black glass tektites contain schlieren with high P 

content, derived from Ca-phosphate grains partially absorbed in 

the otherwise homogeneous tektite glass. This suggests, in 

combination with earlier determined high P20 5 contents in 

augite from KIT microkrystites of Shatsky Rise (Pacific), the 

presence of phosphorite in the carbonate cap of the Chicxulub 

target rocks in Yucatan. 

The Sr and Nd isotopic ratios of Haitian black glass tektites 

and a sample of hybrid impact basement melt (Y6-N17) are 

relatively unradiogenic. Both samples are further characterized 

by elevated Sr and intermediate Nd contents ( -500ppm and 

-20ppm respectively). The observed compositions have been 

modelled using binary mixing models involving estimates for 

the isotopic signatures of lithologies occurring in the 

Chicxulub area, including a 3 km thick(Late) Cretaceous 

carbonate platform underlain by Paleozoic siliciclastics. The 

unradiogenic component was thought to be represented by 

carbonates(phosphorites) rather than by mafic volcanics, 

because the latter are not described from and are not expected in 

the Chicxulub target rocks.The REE patterns of carbonate 

under- and overlying the tektites at Beloc, show a strong Ce 

depletion, indicative for precipitation from Pacific Ocean water. 

Assuming the carbonates(phosphorites) at Yucatan were also 

precipitated from Pacific water, we expect an unradiogenic 

signature in the Yucatan carbonates. The modelling suggests 

that in both samples (tektites and melt) a relative high 

proportion of unradiogenic carbonates (60-90%) have been 

mixed with the siliciclastics of the Yucatan basement. 

87sr;86sr(65Ma] 143Nd;l44Nd(65Ma] 

Haitian black glass 0.708446 0.512425 

Y6N17 impact melt 0.708612 0.512307 

Cretaceous carbonate 0.7075 (1000ppm) 0.51255 (8.8ppm) 

Paleozoic schists 0.7429 (102ppm) 0.511984 (38ppm) 

Replicate step heating 40 Ar/39 Ar experiments (8 single 

tektite grains) yielded a weighted mean age of 65.28 +/- 0.41 

Ma (1o, 83 steps used out of 91). This error includes a 0.3% 

error in J. Taylor Creek Rhyolite sanidine 85G003(age 27.92 

Ma) used as a flux monitor. Fish Canyon biotite irradiated in 

the same rabbit yielded an age of 28.09 +/- 0.12 Ma. Our 

results agree with previously reported ages for Beloc tektites 

and confirm the age of the KfT boundary. 
CONTINENTAL LITHOSPHERIC SOURCES FOR 

KIMBERLITE MAGMATISM 

SMITH, CRAIG B., Bernard Price Institute (Geophysics), 

University of the Witwatersrand, WITS 2050, South Mrica 

The mantle reservoir(s) that serve as sources for kimberlite 

magmatism are unlikely to be located in plumes, the transition 

zone or shallow continental lithosphere, given the complex 

age distribution of Mesozoic kimberlites and related rocks in 

southern Mrica, their bimodal isotopic character, petrologic 

constraints on melting depths, and the general restriction of 

kimberlites to continental areas. The Sr, Nd and Pb isotopic 

characteristics of kimberlite are of two main types, one being 

similar to slightly depleted OIB-like mantle (Group I) and a 

second characterized by ancient enrichment in LIL elements 

(Group II). Most kimberlites, however, also have a 

significant lithospheric (?) component incorporated during 

emplacement that to some degree masks the isotopic signature 

of the pristine source. 

The subcalcic, Cr-poor discrete nodule assemblages, 

ubiquitous in Group I kimberlites and now known from a few 

Group II localities, have isotopic characteristics closely akin to 

host kimberlites and must either be cognate (prior to 

modification and evolution of the melt) or representative of 

discrete melts derived from the same source. Hence, Sr, Nd, 

and Pb isotopes in the megacrysts reflect HIMU character for 

Group I kimberlite sources. The occurrence of this signature 

in megacryst suites throughout southern Mrica is not easily 

reconciled with hotspot origins for kimberlite, given the 

limited distribution of HIMU in South Atlantic plumes. The 

source is more likely resident in subcontinental 'tectosphere' 

that is substantially thicker than the circa 180-200 km 

currently accepted from petrologic studies of xenoliths but 

consistent with other lines of evidence. 

Group II kimberlites represent the enriched end of a 

spectrum of source isotopic compositions in the Mrican 

subcontinental lithosphere, with some off-craton southern 

Mrican kimberlites, Brazilian kimberlites and carbonatites 

and some components of east Mrican carbonatites having ' 

intermediate isotopic character. Hart et al. (1986) originally 

called such a continuum of compositions defined from oceanic 

rocks the LoNd array, and suggested residence within 

subcontinental lithosphere. In southern Mrica kimberlites and 

related rocks have tapped this reservoir in the continental 

setting. The occurrence of these compositions within deep 

subcontinental lithosphere provides support for the possibility 

that the DUP AL signature in some South Atlantic plumes may 

result from mixing of OIB sources with delaminated 

continental lithosphere stranded during Gondwana breakup. 

Hart, S.R., Gerlach, D.C. and White, W.M., 1986, A possible 

new Sr-Nd Pb mantle array and consequences for mantle 

mixing: Geochim. Cosmochim. Acta 50, 1551-1557 

296 

A COMBINED ISOTOPIC AND TRACE ELEMENT 

APPROACH TO DISCRIMINATING GROUNDWATER 

FLOW-PATHS 

Smith, David K,., Davisson, M.L., Volpe, A.M., Nuclear 

Chemistry Division, Lawrence Livermore National 

Laboratory, Livermore, CA 94550, Whittemore, D.O., 

Macfarlane, P.A., Kansas Geological Survey, Lawrence 

KS 66047, Niemeyer, S., Kenneally, J.M., Beiriger, J.M., 

Lawrence Livermore National Laboratory 

Regional studies of groundwater flow at the Nevada Test Site 

(NTS) and in the Dakota Aquifer in western and central 

Kansas emphasize the validity of multiple element and isotope 

applications in resolving disparate waters that have mixed, 

discriminating water-rock and redox reactions within a 

complex hydrostratigraphy and establishing reliable 

chronometers to trace groundwater flow. Isoto{'ic and trace 

element discrimination of unique end-members 1s required to 

interpret ~oundwater ages and the sources of recharge. 

Dissolved Srf86Sr, 36Cl/Cl, 14C, REE, transition metal, alkali 

and alkaline earths, halogens and actinides supplemented by 

analysis of major cations and anions are measured in concert 

from a single sample. The combined approach is important 

because hydrologic relations within these aquifers are complex 

and interpretation based exclusively on one tracer is likely to 

be ambtguous; as well, the mdependent tracers and 

chronometers serve as cross checks on one another. 

At NTS, older(> 40,000 year apparent age) groundwaters 

produced from the regional Paleozoic carbonate aquifer are 

enriched in 87Srf86Sr (0.7140-0.7150), Sr &600-800 Jlg!L) and 

Hco3· (400-500 mg/L) and depleted in 3 Cl/Cl (1-3 X 10"13) 

relative to younger (<20,000 year apparent age) waters derived 

from overlying Tertiary volcanic and volcaniclastic alluvial 

aquifers. Higher bicarbonate concentrations track proportional 

increases in elemental and radiogenic Sr apparently derived 

from dissolution of Paleozoic carbonates. Lower 36Cl/Cl ratios 

for the carbonate waters confirms the presence of a "dead" 

chloride source in the Paleozoic rocks. The dissolution of 

Paleozoic carbonates has a major influence on the resulting 14C 

atom abundance in groundwater and age correction models. 

REE analyses of NTS spring waters are controlled both by Eh 

and host rock signature. Spring waters discharging from 

silicic Tertiary aquifers yield evolved, fractionated chondrite 

normalized patterns in contrast to spring waters in possible 

connection with the regional Paleozoic aquifer which exhibit 

flat chondrite normalized REE patterns and negative Ce and no Eu anomalies. 

In Kansas, isotopic and elemental analysis of shallow 

groundwater collected along a 500km transect of the Dakota 

aquifer from southeastern Colorado to central Kansas indicates 

that a systematic modification of the groundwater chemistry is 

controlled by the flow system. Southwest of the Arkansas 

River, young (<15,000 year apparent age) oxidized water is 

associated wtth shallower de_pths and unconfined local flow 

systems where dissolved 87Srf50Sr (0.7094-0.7088), 36Cl/Cl (10-

15 x 10"13), Cr (0.01-0.3 Jlg/L) and U (7-25 Jlg/L) are 

relatively enriched. Northeast of the Arkansas River, older 

(15,000-40,000 year apparent age) and chemically reduced 

waters are associated with a confined, intermediate-scale flow 

systems within the Dakota aquifer; these waters are enriched 

in HC03- (300-550 mg/L), NH4 (0.5-1.8 mg/L) and cr (17-

1400 mg/L) and are d~?_Pleted in elemental and radiogenic Sr 

(Sr: 60-500 Jlg/L, 87Srf50Sr: 0.7080-0.7086) and 36Cl/Cl (0.01-

8.0 x 10"13). Ion exchange and carbonate equilibria are major 

controls on Sr and HC03- concentrations respectively. The 

confined flow system is laterally recharged by water from the 

southwest which flows beneath the Arkansas River Valley; 

lower 87Srf86Sr and higher Cl/Br ratios (1700-8500) are 

indicative of deeper Permian saltwaters that migrate upwards 

by diffusion and discharge into the base of the Dakota aquifer. 

Work performed under the auspices of the U.S. Department of 

Energy by the Lawrence Livermore National Laboratory under 

Contract W-7405-Eng-48. 
CRETACEOUS PARK: 40Ar-39Ar AGES FROM 

MESOZOIC LACUSTRINE BASINS, NORTHEAST 

CHINA 

SMITH. Patrick E., EVENSEN, Norman M., YORK, 

Derek., Dept. of Physics, University of Toronto, 

Toronto, Ontario, Canada, MSS 1A7 and RUSSELL, 

Dale, CUMBAA, Stephen, Canadian Museum of 

Nature, P.O. Box 3443, Station 'D', Ottawa, Ontario, 

Canada, K1P 6P4 

Freshwater sediments contained in small geographically 

restricted, fault-bound basins are often difficult to correlate 

globally. The middle Mesozoic Rehe Group, Liaoning, 

northeastern China is a notable example, being attributed 

to both the Jurassic and Cretaceous periods, based both on 

biochronology,and K-Ar and Rb-Sr geochronology. Precise 

dating of the Rehe sediments is particularly important 

because it provides a time-frame for the end of the tectonic 

isolation of Asia from the other continents. Thus, Mesozoic 

tectonics in northeast China influenced the balance between 

endemic and exotic taxa which represent important links for 

understanding the evolution of vertebrates, including 

dinosaurs, fish and birds, and the development of flowering 

plants. 

To precisely constrain the age of the Rehe sediments, 

we have used the laser 40 Ar -

39 Ar technique on single basaltic 

and andesitic whole-rock chips, complemented with analyses 

of minerals where possible, from volcanic horizons 

intercalated with lacustrine sediments, primarily from the 

Yixian Formation. Consecutive linear segments on plots of 39Ar/40Ar vs. 36Ar/40Ar, representative of gas fractions taken 

at intermediate temperatures, give well-defined isochron 

ages. A basalt from the Yixian Formation near the base of 

the Group gives an age of 125.4 ±0.2 Ma, whereas an 

andesite from the overlying Jiufotang Formation gives 121.7 

±0.4 Ma. Ages from intervening horizons are, within their 

uncertainties, in sequential agreement with these enclosing 

strata. Three dimensional plots of data from all gas 

fractions, where either 37Ar/40Ar or 38Ar/40Ar is considered 

in conjunction with the standard isochron plot, yield 

complex but often reproducible patterns. 

The age control provided by the dates on the volcanic 

intercalations allows the significance of apparent ages 

measured on the intervening lacustrine sediments 

themselves to be tested. Some of these samples yield very 

good isochrons yet give apparent ages which are 

significantly younger than the primary ages. These ages 

(110.3 ±0.2 Ma) may signify alteration events. 

These results show that the lower half of the Rehe 

Group, with its characteristic fauna of endemic fish, 

dinosaurs and other taxa, was deposited within a short time 

interval of 3-4 Ma during the lower Cretaceous and contains 

no strata of Jurassic age. 

297 

LEAD ISOTOPIC COMPOSmONS IN 

PARTICULATE AEROSOLS AND BWOD OF 

SAN FRANCISCO AREA RESIDENTS: 

IMPLICATIONS FOR SOURCES OF LEAD 

EXPOSURE 

SMITH, D.R. and OSTERLOH, J., Laboratory 

Medicine, University of California, San Francisco 

CA 94110, NIEMEYER, S., Nuclear Chemistry, 

LLNL, Livermore, CA 94550, and FLEGAL, 

A.R., Environmental Thxicology, University of 

California, Santa Cruz, CA 95060. 

Regulation of lead alkyl additives to gasoline over 

the past several decades has substantially reduced 

air lead levels, which has had a positive impact on 

reducing human lead exposures. However, 

suspended particulate aerosols may continue to 

pose an important source of lead exposure to 

humans due to the resuspension of fine urban dusts 

contaminated with lead, and continued emissions 

from non-regulated sources. Here we measured the 

stable lead isotopic compositions and lead 

concentrations in blood from eight adults, and in 

suspended particulate (>0.441-'m) aerosols and tap 

water from several locations in the San Francisco 

area to evaluate whether suspended particulate 

aerosol lead was measurably contributing to the 

blood lead levels of area residents. The isotopic 

compositions of lead in blood of the eight 

individuals were similar to one another (range in 

207Pbf206Pb = 0.8418- 0.8479 and 208Pbf206Pb = 

2.058 2.070, ± 0.07% 2RSE typical 

measurement error) and overlapped somewhat with 

the range in isotopic compositions of the 

particulate aerosols (2°7

PbP06Pb = 0.8340 - 0.8454 and 208PbP06Pb = 2.043-2.064, ± 0.07% 

2RSE). Thp water samples were similar but more 

variable in isotopic compositions, and were also 

uniformly low in lead concentrations ( < 1ng/mL). 

These preliminary data suggest that particulate 

aerosol lead, or isotopically similar lead from 

additional sources (e.g., soil dusts) continues to 

measurably contribute to the blood lead levels of 

environmentally lead-exposed humans in the San 

Francisco area. The differences in blood isotopic 

compositions likely reflects varying exposures to 

other lead sources with different isotopic compositions. 
THE USE OF Sr ISOTOPIC STRATIGRAPHY 

TO DATE ABYSSAL PERIDOTITE 

ALTERATION 

SNOW. I. Eu and FRANCE-LANORD, C. CNRS I 

Centre de Recherches Petrographiques et 

Geochimiques, 15 rue Notre Dame des Pauvres, BP 

20, 54501 Vandoeuvre Cedex, FRANCE. 

jesnow@crpg.cnrs-nancy .fr 

Abyssal peridotites are fragments of the oceanic 

mantle which have been transported to the sea floor by 

tectonic processes. They are typically serpentinized (60-

100%) and often heavily weathered. The weathering 

process in abyssal peridotites commonly deposits large 

amounts of seawater-derived Sr in the peridotite. In the 

sample suite studied here, the seawater Sr reservoir accounts for up to 400 times the amount of Sr derived 

from the mantle. This Sr is typically hosted in aragonite 

(Thompson, et al., 1972) deposited late during the process of weathering of the peridotite at low temperature. 

This study uses the high-precision record of the 

secular variation of 87Sr ! 86Sr in the oceans to estimate 

weathering ages of abyssal peridotites from 0-20 million 

years of age. Age estimation at this level (about ±1 rna) 

for the oceanic crust will be useful in cases where the 

magnetic record is difficult to interpret, or where the 

tectonic setting of an individual dredge haul is ambiguous. 

This approach entails several potential problems. First, the contribution of mantle-derived Sr from 

dinopyroxenes must be assessed. In the worst case scenario, this could account for a significant deviation in 

the true and apparent ages. In the typical case, however, the incorporation of all mantle-derived Sr into the 

carbonate fraction does not result in a significant change 

in the apparent age. Second, the contribution of Sr from 

entrained detrital sediments must be considered (Snow, 

et al., 1993). Once again, the worst-case scenario could 

cause a significant shift in apparent age, but in the typical case this contribution is insignificant. Third, the bottom water from which the peridotite aragonite is precipitated must be representative of ocean waters generally. Lastly, the aragonite must have formed relatively quickly after the exposure of the peridotite. If the 

carbonate is deposited over a long time, then the resulting "age" will be difficult to interpret. In regions where 

the crustal age is well-determined, such "too young" 

ages would constrain the interval of formation of the 

aragonite, and thus the duration of alteration of the 

peridotite. 

Thompson, G. (1970) A geochemical study of some 

lithified carbonate sediments from the deep sea. 

Geochimica et Cosmochimica Acta 36:1237-1253. 

Snow, J, S. Hart and H. Dick (1993) Orphan 87Sr in 

Abyssal Peridotites: Daddy was a Granite, Science 

December, 1993. 

298 

A TECHNIQUE FOR THE EXTRACTION OF 0-18 

FROM MICROLITER QUANTITIES OF WATER 

SOCKI, R.A., Romanek, C.S., and Gibson, 

E.K., Jr., LESC and SN-4, NASA Johnson 

Space Center, Houston, TX 77058, USA. 

As part of NASA's ongoing study to examine the 

0 and H isotopes from hydrous minerals associated 

with Antarctic meteorites, we are currently 

developing a technique to extract microliter 

quantities of water for 0-18 analysis. The need also 

exists for a routine 0-18 extraction method of 

analysis for water from fluid inclusions, structural 

water within hydrous minerals, and for use in doubly 

labeled water tracer studies in the medical sciences. 

Furthermore, eliminating the need for potentially 

dangerous and costly reagents (BrF 5 [I] and 

Guanadine-HCl [21) associated witli micro-analytical 

extraction techniques, is desirable. Our method is 

based on the traditional C02-H20 equilibration 

technique [3]. Others have attempted C02-H20 

equilibration on a microliter-size sample scale [4]. 

The method described here involves a reaction 

carried out exclusively in 6mm pyrex tubes which, 

after C02-H2o exchange is complete, are 

immediately attached to the inlet of the mass 

spectrometer for analysis. 

Pyrex tubes (6mm O.D.), stored in an oven at 

80°C, are attached to a small vacuum line consisting 

of both a C02 reservoir and a sample tube septum 

port. After pumping to high-vacuum 25111 of C02 is 

expanded into the sample tube and septum port. I 0 

to 20111 of H20 are then injected via the septa with 

an air tight 100111 syringe. H20 and C02 are frozen 

in the bottom of the tube for 3 min., sealed with a 

torch and placed in a furnace held at 180°C ±().5°C 

for 48 hrs. Sample tubes are then removed from the 

furnace and attached to a tube cracker on the inlet 

of an isotope ratio mass spectrometer (IRMS). After 

freezing the H20 with a dry ice alcohol mixture, 

C02 is admitted to the IRMS for 0-18 analysis. 

The method described here will allow for the 

routine measurement of microliter quantities of water 

without extensive extraction procedures. Routine 

analysis of a laboratory working water standard is 

reproducible to ±0.6 per mil using this technique. 

International standards V-SMOW, GISP, and SLAP 

will be analyzed to determine the accuracy of this 

method. 

References: 

[l]O'Neil, J.R. and S. Epstein, JGR 1966,71,4955, 

[2]Wong, W.W. et al., Anal Chem 1987,59,690, 

[3]Epstein, S. and T. Mayeda, GCA 1953,4,213, 

[4]Kishima, N. and H. Sakai, Anal Chem 1980,52,356 
PALAEOOCEANOGRAPHY WITH 10Be 

B.L.K.SOMAYAJULU,Physical Research 

Laboratory, Navrangpura,Ahmedabad 

380 009, lnd ia 

Cosmic Rays interact with the 

earth • s atmospheric constituents viz. 

N ,0 ,Ar etc.producing a variety of 

rldiJnucl ides with varying half I ives 

of which beryllium-lO(half life=1.5 

m.y) is an important one. Its congenial 

sea water chemistry, and the high 

sensitivity of its measurement in 

natural samples achieved through 

Accelerator Mass Spectrometry makes 

beryl I ium-1 0 a potential tracer to 

decipher meltwater flooding of the 

oceans during interglacial times. 

The basic philosophy behind this 

application is that the most concentrated solution of beryll ium-1 0 on 

earth is rain and snow/ ice and the 

latter accumulated for long periods 

during glacial times. During interglacial 

periods the ice/snow-meltwater 

containing high beryllium-10 content 

flooded the ocean and the beryll ium-1 0 

spike got removed by the detrital 

material, principalty clays (that are 

enriched AI) transiting through the 

water column to form the sediment 

in times shorter compared to th~ 

mixing time of the ocean (ie.x 10 

years). To be able to establish this 

method, one shou I d I ook for oceanic 

areas which were subjected to intense 

meltwater flooding in the past and 

measure beryll ium-1 0 and AI in their 

sediments as a function of delta 18-0 

which happens to be the prime 

indicator of meltwater inputs. 

This is done in the Gulf of Mexico 

sediments and a strong negative 

correlation (r=-0.95) between delta 

18-0 (range -1.5 °/oo to. 0.8 °/oo) 

and the beryllium-10/AI is obtained. 

Similarly beryll ium-1 0/ AI measurements 

were made in cores from the 

Mediterranean sea and the Arctic and 

are compared with delta 18-0 in nearby 

ones. The results are encouraging. 

299 

SHRIMP ISOTOPE STIJDIES OF GRANULITE ZIRCONS 

AND THEIR RELEVANCE TO EARLY PROTEROZOIC 

TECTONICS IN NORTHERN FENNOSCANDIA 

SORJONEN-WARD. Peter, Geological Survey of Finland, 

02150, Espoo, Finland, CLAQuE-LONG, Jon, Research 

School of Earth Sciences, Australian National University, 

Box 4, Canberra 2601, Australia, and HUHMA, Hannu, 

Geological Survey of Finland, 02150, Espoo, Finland. 

The Fennoscandian Shield of northeastern Europe can be 

divided into three broad tectonic domains. The Svecofennian 

domain in the southwest of the shield consists essentially 

of juvenile crust formed rapidly at 1.9-1.8 Ga, while the 

Kola domain in the northeast represents a mosaic of Archean 

and Proterozoic elements amalgamated at some time between 

2.0 and 1.8 Ga. These two domains are separated from one 

another by the intervening 3.1-2.7 Ga Karelian craton, which 

formed a stable platform for terrestrial to shallow marine 

sedimentation and predominantly mafic to ultramafic 

volcanism throughout much of the early Proterozoic. 

Through much of northern Finland, the boundary between 

the Karelian craton and the Kola domain is represented by a 

thick low-angle thrust system that has emplaced a highly 

strained, predominantly felsic granulite complex over a 

lower to middle greenschist facies mafic volcanic sequence. 

An U-Pb zircon age of 1.93 Ga for plutonic rocks intruded 

prior to deformation constrains the maximum age of thrusting 

of the granulite complex, while a nearly concordant 1.91 Ga 

monazite age, along with the abrupt decrease in metamorphic 

grade away from the thrust zone, is thought to imply that 

emplacement, uplift and erosion of the granulites was a 

relatively rapid process. Whole-rock Sm-Nd data and some 

conventional zircon data also suggest that the sedimentary 

protoliths to the granulites were formed during the early 

Proterozoic, instead of being derived from an Archeandominated provenance. 

In order to obtain more precise information on provenance 

ages, as well as metamorphic and uplift history, a sample 

having a monazite age of 1.91 Ga, conventional U-Pb 

ages based on a poorly defined chord extrapolating to an 

intercept between 2.02 and 2.14 Ga, and whole-rock Sm-Nd 

T<DM> = 2.24 Ga with £<1900> = -2.4, was chosen for analysis 

using the SHRIMP in Canberra. The age spectrum for 

48 zircons has a mode close to the conventional age estimate, 

around 2.0 Ga. In some cases these ages were recorded from 

cores, with distinct overgrowths having ages nearer to 1.9 Ga. 

Whether or not these rims are metamorphic in origin is a 

problem requiring careful assessment and interpretation of 

zircon morphology, since SEM data also suggest rounded, 

relict detrital surfaces. Nevertheless, the data clearly 

indicate that the Lapland granulite protoliths were deposited, 

metamorphosed and exhumed relatively rapidly, rather than 

representing older lower crust uplifted and exposed during 

a significantly younger orogenic event. 
THERMOCHRONOLOGY OF A METAMORPHIC CORE 

COMPLEX AS SOCIA TED WITH THE BREAKUP OF 

GONDWANA: THE PAPAROA RANGE, SOUTH ISLAND, 

NEW ZEALAND 

SPELL. T. L., MCDOUGALL, I. (Research School of 

Earth Sciences, The Australian National University, 

Canberra, ACT 0200, Australia), and TULLOCH, A. J. 

(Institute of Geological and Nuclear Sciences, PO Box 

30368, Lower Hutt, New Zealand) 

Continental extension during the breakup of Gondwana in 

the late Cretaceous produced a metamorphic core complex 

preserved in the Paparoa Ran~e on South Island, New Zealand. 

Most features typical of Cordilleran core complexes are present 

including high metamorphic grade lower plate, or core, rocks 

(amphibolite facies gneisses), low grade upper plate rocks 

(lower greenschist facies metasediments), syntectonic granitic 

intrusions and volcanism, low angle mylonite zones (detachments) indicating extensional deformation which 

separate lower from upper plate rocks, and thick sequences of 

subaerial breccias and conglomerates. 

K/Ar ages for muscovite, biotite and K-feldspar (recording 

cooling through the -400-180 °C interval) from lower plate 

rocks ( ortho- and paragneisses of the Charleston Metamorphic 

Group, Buckland and Foulwind Granites) are generally restticted 

to the range -95-105 Ma. In contrast to results from core rocks, Paleozoic granites (Barrytown and Meybille) which 

intrude the upper plate metasedimentary rocks record extended 

cooling histories (>200 Ma) beginning in the early 

Carboniferous to Permian and ending in the Cretaceous. 

40Arf39Ar age spectra from hornblende, muscovite, biotite and 

K-feldspar from lower plate rocks (recording cooling from 

-500-130 °C) are relatively flat and nearly concordant with ages 

of -90-110 Ma. 40Arf3gAr spectra from upper plate granites are discordant both internally and between minerals with 

differing closure temperatures. Multidomain modelling of 

argon diffusion from K-feldspars indicates that granites from 

the lower plate cooled fairly rapid (-30 °C/Ma) from -100 to 

90 Ma followed by slower cooling (-5 °C/Ma) to -75 Ma. A 

K-feldspar age spectrum from an upper plate granite (Barrytown 

Granite) indicates closure of different diffusion domains over an -200 Ma interval with cooling rates of -<>.1 OC/Ma from -350 

to 140 Ma followed by cooling at -5 °C/Ma to -125 Ma. All 

data thus indicate rapid cooling of core rocks relative to upper 

plate rocks. 

The Pororari Group, subaerial breccias and conglomerates 

shed from the unroofing granites in the core complex over an -15 Ma interval beginning at -113 Ma, preserve information on the cooling history of the core complex for a substantial 

interval of time prior to that obtainable from presently exposed 

lower plate rocks. K/Ar and 40Arf39Ar dating of samples 

from the Pororari Group indicate rapid cooling rates similar to 

those of presently exposed lower plate rocks, thus suggesting 

that rapid unroofing of the core complex was continuous over at least a 5 Ma interval. 

Combined with published U/Pb and fission-track data these 

results yield calculated cooling rates (over the temperature 

interval-700-100 °C) for lower plate rocks ranging from 15 to 

75 °C Ma-l (averaging -35 °C Ma-l) whereas those for upper 

plate rocks are 5 to 0.5 °C Ma-l. Lower plate granites with 

intrusive ages both in the Paleozoic and Cretaceous yield 

identical rapid cooling curves around 100-90 Ma and estimated 

unroofing rates of -1.5 mm/a. The change to slower cooling 

rates around 90 Ma as seen in multidomain K-feldspar models 

is also seen in the combined K/Ar and fission-track data and 

may record cessation of continental extension and the 

beginning of seafloor spreading in the Tasman Sea. These data 

establish a temporal and spatial link between continental 

extensional tectonics on the Pacific margin of Gondwana, 

inception of seafloor spreading in the Tasman Sea (oldest crust 

-85 Ma), and subsequent rifting of New Zealand from Australia 

and Antarctica. 

300 

PALEOCLIMATE INFORMATION FROM SALT (HALITE) 

DEPOSITS 

SPENCER, Ron, YANG, W., ROBERTS,S., 

Dept. Geology and Geophysics, KROUSE, 

H.R., Dept. of Physics, The Univ. of 

Calgary, Calgary, Alberta T2N 1N4, 

Canada; LOWENSTEIN, T., Li, J., Dept. 

Geol. Sciences, S.U.N.Y. at Binghamton, 

Binghamton, New York 13901 

Many halite crystals contain fluid 

inclusions that outline primary . growth 

textures and allow interpretations of the 

depositional environment of the salt to be 

made. Analyses of these fluid inclusions can also provide data on the temperature of 

formation of the salts, the major dissolved 

ion concentrations and stable isotope 

composition of the surface brines from 

which the salts precipitated. Some halite 

crystals that grow at the air brine 

interface also trap small amounts of air in 

the fluid inclusions. The gas composition 

of these fluid inclusions can also be 

analyzed to provide a record of the 

composition of the atmosphere in the past. 

Salt deposits can also be dated by a variety of radiogenic techniques, allowing 

temporal control of the climate record. 

We have obtained climate data from a 50 metre core through the salts of the 

Qaidam Basin, Qinghai-Tibet Plateau, China, 

covering about 50 ka of record. These data 

indicate warm, but less arid conditions on the plateau from 50 until about 20 kaBP, 

with C02 /N2 in the atmosphere lower than at 

present. The low atmospheric C02 /N2 

continues until about 15 kaBP, and the 

period from 20 to 15 kaBP is characterized 

by colder, but drier conditions on the 

plateau. The Holocene record indicates 

warm, dry conditions and higher C02 /N2 in 

the atmosphere. 

Work is now in progress on a 180 

metre core from the Badwater Salt Pan, 

Death Valley, CA. It appears that this core will provide a high quality record of 

surface temperature, aridity, and stable 

isotope composition of meteoric inflow for 

this area. 
EFFECf OF [C{haq] ON THE CARBON AND OXYGEN 

ISOTOPE COMPOSITION OF FORAMINIFERAL CALCITE 

SPERO. H 1., BEMIS, B.E., Dept. of Geology, University 

of California, Davis CA. 95616, U.S.A., BUMA, J., AlfredWegener-Institut ftir Polar-und Meeresforschung, P.O. Box 

120161,27515 Bremerhaven, Gennany, and LEA, D.W., 

Department of Geological Sciences, University of California, 

Santa Barbara CA 93106. 

The oxygen and carbon isotopic compositions of 

foraminiferal calcite are commonly used in paleoceanographic 

reconstructions of oceanic temperatures, salinities, continental 

ice volumes, aspects of the ocean's biological pump and shifts in 

terrestrial and ocean carbon reservoirs. These interpretations are 

based on empirically derived relationships that relate the isotopic 

composition of calcite to the parameters in question. A number 

of environmental (e.g. temperature, salinity, atmosphere-ocean 

equilibration times, etc.) and physiological (e.g. symbiosis, 

respiration, calcification rates, etc.) parameters must be 

considered for accurate reconstructions. Here we describe the 

results of laboratory experiments which show that the carbon and 

oxygen isotopic composition of foraminiferal calcite is also a 

function of the carbonate chemistry of the sea water in which it 

grew. 

During the summer of 1993, the symbiont-bearing 

planktonic foraminifera, Orbulina universa, was grown in the 

laboratory under high light intensities (maximum 

photosynthesis) at 22°C in seawater with different [C02aql and 

[C032-].'The experimental results show that when alkalinity is 

kept constant the 513c of the test~ increases linearly from 

approximately 0 to 3.8 %o as [C02aql increases from 2 to 20 

J.UDOVl (ambient = 11 f.LM}. Above 20 J.LM, shell 513c remains 

constant. Across the same [CD2aql range, 5180 values increase 

from -2.8 to -1.9 %o and fmally stabilize at -1.7 %oat a [CD2aql 

of 45 J.LM (o18o calcite equilibrium = -1.82 %o). A clear 

covariance between ol3c and o18o values is observed. In 

contrast, under conditions of constant pH, the o13c of shell 

calcite decreases when [C<>2aql increases above 20 f.LM. In both 

sets of experiments, the ol3c of shell calcite decreases linearly 

as a function of increasing [C032-]. We hypothesize that these 

relationships are a function of both kinetic fractionation and 

phyisological factors such as symbiont photosynthesis. 

. . . . _ ................. ... = ... a -2 • ;.--t· • · I ! ·;~: . ::= :.

0 -2.5 : 

cc ~ 

~ 

-a 0 20 40 60 80 100 

Moleeular001 ~M) 

301 

SULFUR ISOTOPES EVIDENCE FOR SULFATE 

REMOVAL PROCESSES FROM SUBSURFACE BRINES, 

HELETZ FORMATION (LOWER CRETACEOUS), 

ISRAEL 

ST ARINSKY, A., Dept. of Geology, The Hebrew 

University, Jerusalem, 91904, Israel, SPIRO, B., NERC 

Isotope Geosciences Laboratory, Keyworth, NG 12 5GG, 

U .K, GA VRIELI, I., Geological Survey of Israel, 

Jerusalem, 95501, Israel, NIELSEN, H., Dept. of 

Geochemistry, Gottingen University, Germany and 

AIZENSHTAT, Z., Department of Chemistry, The 

Hebrew University, 91904, Jerusalem. 

Subsurface brines associated with oil ("oil brines") from 

four oil fields, Heletz, Brur, Kokhav and Negba, in the 

southern coastal plain of Israel show a deficiency in S04 

concentrations compared to brines from dry wells ("dry 

brines") in the same fields only a few hundred meters away. 

Both types of brines have SOiCl ratios lower than seawater 

or evaporated seawater with the same salinity, the lowest 

ratio being 1.9·10-4 (wt. ratio). Previous studies suggested 

that the brines have evolved from evaporated Miocene 

seawater, which precipitated the sulfates in the Mavqiim Fm. 

(834S=21.7-22.7%o). The present study shows that the 

residual brine in the Mavqiim Fm. has sulfate with 

834S=26.4%o. The 834S804 of the oil brines from the oil 

producing Heletz Fm. is 30-59%o. The 34S of S04 and H2S 

sampled together (two pairs), were nearly identical at about 

30%o. The oil has sulfur concentrations of 2-3% with 

relatively heavy 34S values of + 13 to + 15%o. 

The following overall evolutionary path for these brines 

was reconstructed on the basis of their sulfate concentration, 

SOiCl ratio and 34S in the S04, H2S and the oil. 

(1) Evaporation of a Messinian seawater which led to the 

precipitation of gypsum. 

(2) Precipitation of gypsum following dolomitization. 

(3) Bacterial sulfate reduction. 

Stages (1) through (3) removed some 50% of the initial 

sulfate content of the brines, changing the sulfate isotopic 

composition from marine (~34S~20%o) to ~26%o. 

(4) Migration of the residual brines eastward into the Lower 

Cretaceous Heletz Fm. and mixing with oil. 

(5) Bacterial sulfate reduction in the oil reservoir resulting in 

further decrease in SOiCl ratio and increase of 34S in the 

remaining sulfate. 

(6) Possible secondary enrichment of the oil with sulfur 

through reaction with the increasingly 34S enriched H2S and 

so4• 

The similar sulfur isotopic composition of the coexisting 

sulfide and sulfate still merits a detailed study. 
ARC INPUT PARAMETERS: LARGE SCALE SR, ND 

AND 0 ISOTOPIC ANATOMY AND ABUNDANCES OF 

REE AND U, K, AND RB OF ALTERED OCEANIC 

CRUST AT DSDP/ODP SITES 4171418 

STAUDIGEL, H., DAVIES, G. R., Centrum voor Isotopen 

Geologisch Onderzoek, Faculteit der Aardwetenschappen, 

Vrije Universiteit, De Boelelaan 1085, 1081 HV 

Amsterdam, HART, S.R. Woods Hole Oceanographic 

Institution, Woods Hole MA; K. MARCHANT Scripps 

Institution of Oceanography, San Diego, CA. 

To provide quantiative constraints on one of the major 

input parameters into magmatic arcs and the chemical transfer 

between hydrosphere and the mantle, we analysed the 0, Sr, 

and Nd isotope ratios and the abundances of REE, U, K, Rb, 

and Sr for large scale compositional domains at DSDP/ODP 

drill Sites 417A, 417D, and 418A. The three sites combined 

give the large scale composition of the upper 500 m oceanic 

crust over a distance of about 8 kilometers. We also 

contrasted the bulk composition of several "average" 10-30 m 

depth intervals at each site and the composition of different 

lithologies, including submarine extrusives ("FLO") and 

volcaniclastics ("VCL"). 

The o180SMOW and 87srf86srmeas of differently 

altered compositional domains in the upper ocean crust 

correlate well with a range of 7.7 - 19.2 and from 0.70364-

0.70744, and a mean of 9.96 and 0.70475, respectively. 

Seafloor alteration increases the Rb inventory of the upper 

crust by about an order of magnitude and the Sr contents only 

slightly. U abundances increase moderately under oxidizing 

alteration conditions and more dramatically during the (more 

common) reducing conditions. VCLs and 417A samples are 

the most altered and nearly double the observed range in 

isotope ratios. The consistency of composite analyses with 

individual samples from the same site and agreement between 

composites from two independent sites of similar alteration 

behavior confirm their fidelity, and the reproducibility. Even 

though rare earths are relatively little influenced by alteration, 

K correlates with 143Nd/144Nd and La/Sm, negatively and 

positively respectively, suggesting consistent addition of 

seawater REE's with increasing alteration. The exchange of 

REE is minor for the budget of the oceanic crust but important 

for the oceans. 

Even though VCLs and 417A composites are 

volumetrically a relatively minor component of "normal" 

oceanic crust, their high volatile and alkali inventory gives 

them a more important role during dehydration and partial 

melting reactions. Progressive subduction of oceanic crust is 

accompanied by chemical modification and dispersion of some 

of the seafloor-alteration imposed heterogeneity, but much of it 

will remain, in particular for the subduction of old oceanic 

crust, at high convergence velocities. Therefore, recycled 

oceanic crust is expected to be similar to fresh MORB with 

respect to 143:Ndt144Nd but with elevated and positively 

correlating o18osMOW and 87srfo6sr possibly resembling 

the EM II component. Nearly horizontal arrays in 

143Nd/144Nd- 87srt86sr correlation diagrams of island arc 

magmatic systems, may indicate involvement of partial melts 

from recycled altered volcaniclastics. 

302 

COMPARISON OF ANALYTICAL TECHNIQUES USED 

IN U-Pb DATING OF ZIRCON 

STEIGER R H , MEIER M., BICKEL R.A., Isotope 

geochemistry ETH, Zurich, CH-8092, Switzerland and 

PIDGEON R.T., School of Applied Geology, Curtin 

University of Technology, Perth, 6001, Western Australia 

Zircon dating has developed into the most accurate and 

versatile method of geochronology. During the last 25 years a 

number of analytical improvements have contributed to a reduction of sample requirement by several orders of magnitude. 

This has led to progress in sample selection and sample preparation allowing the dating of small zircon populations, single 

grains or of single phases thereof which exhibit specific properties and which can be attributed to growth during a particular 

geologic event. A major advance has also been made in the recognition and interpretation of interior and exterior morphology of zircon grains. Geochemical characterization and Hf isotopic signatures help determine the environment in which the 

zircon grains crystallized. The combined application of these 

methods has resulted in making zircon investigation one of the 

most powerful research tools available for provenance, petrogenetic as well as time scale studies. 

We are currently examining some previously analyzed zircon suites in order to determine to what degree modem analytical techniques can extract additional information from this 

valuable material. To this purpose we selected single grains 

from the original size-fractions of a reversely discordant zircon 

population of the pre-kinematic Caledonian Vagastie Bridge 

granite of NW Scotland. These fractions from sample RC662 

were originally studied by PIDGEON & AFTALION (1978) 

and found to plot along a 405±11 to 1540±42 Ma discordia line. 

Among the various techniques we plan to apply are high 

resolution-high precision conventional isotope dilution (ID) 

analysis on single-fragments or abraded single-grains, the Kober 

evaporation technique and spot analysis with the new ion probe 

SHRIMP II at Perth. 

Recently, the first data for single-fragment and abraded 

single-grain conventional ID analysis were obtained. They plot 

above and outside of error of the original size-fraction data 

points: 1) sub-microgram fragments of the zircon rim representing latest growth yield concordant ages near 430 Ma, 2) rim, 

zoned mantle and pop-out core from one single-grain align 

within limits of error on a discordia line intersecting concordia 

at 425 and 1540 Ma, 3) visibly core-bearing grains abraded to a 

fraction of their original size also fall on this line, except for one 

data point which plots markedly above this line and thus 

indicates the presence of a second inherited component. In 

agreement with our previous experience, the new data confirm 

that intrusive ages based on lower intersections extrapolated 

from zircon size-fractions may be too young. This is probably 

due to the presence of highly susceptible (and thus more 

discordant) U-Pb (zircon?) phases in the multi-grain fractions. 

We conclude that single-fragment dating considerably 

increases the resolving power of conventional ID zircon dating, 

particularly for the late growth phases. The ion probe analyses 

currently under way are expected to provide better resolution for 

the inherited components involved and thus help in evaluating 

the meaning of the extrapolated upper intersections obtained by 

classical techniques. 

Pidgeon, R.T., and Aftalion, M., 1978, Cogenetic and inherited 

zircon U-Pb systems in granites: Paleozoic granites of Scotland 

and England. Geol. Journal, Special issue No. 10, 183-220. 
MANTLE OVERTURNS AND 

MAJOR OROGENIES (MOMO) 

STEIN. M. Institute of Earth Sciences, The 

Hebrew University, Jerusalem, Israel, and 

HOFMANN, A.W. Max-Planck-Institut fur 

Chemie, Postfach 3060, 55020 Mainz, Germany. 

During Earth history, periods with areally 

extensive crust-formation rates appear to have 

alternated with periods of lower rates. We propose 

that these fluctuations are related to alternating styles 

of mantle convection. High crustal growth rates 

occur during periods of strongly penetrative twolayer convection or whole-mantle convection, and we 

call these "MOMO" periods (Mantle Overturn and 

Major Orogenies). They alternate with "Wilsonian" 

periods of two-layer convection, seafloor spreading, 

and continental drift coupled with low crustal growth 

rates. During Wilsonian periods, the upper mantle is 

relatively isolated from the lower mantle and is 

progressively depleted in incompatible trace 

elements. During MOMO periods, the upper mantle 

is partly replenished in these elements. 

This model is consistent with recent theoretical 

work on mantle convection and with several 

geochemical observations, the meaning of which has 

been controversial in the past. They all require 

significant but limited input of lower-mantle material 

into the upper mantle: (1) Initial isotopic 

compositions of Nd and Sr for several areally 

extensive major "orogenies" such as the Pan-African 

are relatively uniform and intermediate between 

primitive mantle and incompatible-element depleted, 

"MORB-type" mantle. (2) The trace element budget 

of continental crust requires extraction of trace 

elements from a mantle reservoir that is larger than 

the upper mantle but much smaller than the whole 

mantle. (3) The noble gas budget requires the 

continued presence of primordial 3 He and large 

amounts of 40 Ar in the lower mantle, but also 

significant gas transfer to the upper mantle. ( 4} The 

isotopic composition of lead is not consistent with 

Th/ U ratios in the upper mantle, if the continental 

crust and upper mantle evolved as a closed system. 

Other critical trace element ratios in the upper mantle 

and continental crust also do not add up to. possible 

primitive-mantle ratios. 

303 

SEDIMENTARY K-AR SIGNATURES IN CLAY 

FRACTIONS FROM MESOZOIC MARINE 

SHELF ENVIRONMENTS 

STEINITZ, G., KAPUSTA, Y., SANDLER, A. 

KOTLARSKY, P., Geological Survey, 

Jerusalem, Israel. 

K -Ar age determinations have been performed on 

silicate fractions (clay minerals, feldspar) separated 

from some 20 sedimentary rocks from Jurassic to 

Late Senonian formations in Israel, deposited in 

shallow marine carbonate shelf environments of the 

southern Tethys ocean. The results show that: 

1. In several cases (IR, > 10#-t) the K-Ar ages are 

significantly higher than the stratigraphic age, 

reflecting the age of detrital mica and feldspar, the 

only K-bearing minerals in the clay fractions. 

2. Some of the K-bearing clay fractions have K-Ar 

ages synchronous with the time of deposition. The 

K-Ar syn-sedimentary signal is recorded from 

different environments of deposition, the range of 

lithologies spanning rocks with IR < 10% (dolomite, 

chalk), through marls to shales (IR = 70-85%) and 

sandstones (IR > 90%). 

3. Among the clay minerals the syn-sedimentary 

signature was isolated in illite/ smectite of the < 2#-' 

fraction, assumed to be authigenic. The K-Ar ages 

have since been retained in these fractions, including 

in <0.2#-t fractions. 

4. A prominent syn-sedimentary signature is found 

in Early Cenomanian K-feldspars (97 rna), from 

shaly and calcareous rocks, confirmed by their 

authigenic origin based on idiomorphic crystal 

morphology and a limited size (4-10#-t) distribution. 

5. Post depositional closure of the K-Ar system in 

mineral fractions is indicated in the ages of < 2#-t 

fractions circa 15 rna lower than the stratigraphic 

age (90 rna) separated from dolomite, shale and 

sandstone. A distinct post-depositional event (60-65 

rna) is also recorded in the formation of authigenic 

K-feldspar within Senonian (85 rna) chalk and shale. 

The overall pattern of the results differs 

considerably from the K-Ar age patterns as observed 

in < 2#-t fractions from deep sea sediments. The 

difference may be connected with the occurrence of 

brines in these shelf environments. · 
CARBON ISOTOPE VALUES IN FOSSa RATITE 

EGGSHELLS AND ASSOCIA1ED MATERIALS: 

FURTHER EVIDENCE FOR HABITAT EXCLUSION OF 

SOUTH ASIAN SIVAPITHECUS SP. DURING THE LA1E 

NEOGENE 

STERN. LIBBY A , Johnson, Gary D., and Chamberlain, C. 

Page, Department of Earth Sciences, 6105 Sherman 

Fairchild Science Center, Dartmouth College, Hanover, 

New Hampshire 03755-3571, U.S.A. 

A 10+ million year sequence of ratite (e.g., ostrich and 

related paleognathids) eggshells from Siwalik Group sediments 

of northern Pakistan and India, shows a dramatic (-8%o) 

increase in the s13c in biomineralogic calcite by -4 Million 

years (Ma). s13c values of pre-7 Ma ratite eggshell carbonate 

are about the same as those found in carbonate from fossil 

mammal tooth enamel and paleosols in contemporary Siwalik 

facies. However, after 4 Ma, the ratite eggshell carbonate 

s13c values are -5%o less than this coexisting material. 

These data provide collective evidence of the development 

and enhancement of a C3fC4 vegetative mosaic which we 

interpret to mirror the sedimentologic/edaphic mosaic of the 

aggrading alluvial plain of an increasingly topographically 

differentiated Siwalik depositional system. 

Although this s13c shift has been previously recognized as 

coincident with a major faunal turnover occurring -7.4 Ma in 

the Potwar region of Pakistan, and roughly concurrent with the 

local LAD of Sivapithecus sp. in Pakistan (recorded also in the 

Potwar region at -8 Ma), the s13c values determined from the 

ratite eggshell material demonstrate the persistence of a C3 

vegetation signature well into the upper Pliocene. This has not 

been recognized in previous carbon isotope studies of paleosol 

and fossil tooth enamel in the South Asian Neogene. 

This relict C3 vegetation, most likely associated with the 

riparion aspect of the Siwalik depositional system, is 

interpreted to be of insufficience size or stability in the post-7 

record of the Potwar region of Pakistan to provide suitable 

habitat for the South Asian sivapithecines. The exclusion of 

this taxa from this portion of the South Asian monsoonal belt 

by -7 Ma and a subsequence eastward shift in the necessary 

habitat as seen from faunal indicators at Haritalyangar (India) 

and carbon isotope indicators Bakiya Khola (Nepal) suggests 

that interpretations of climate change and monsoonal 

development during the late Neogene in south Asia must be 

constrained by certain geographical and temporal qualifiers. 

We are currently refming this argument with additional 

material collected in a 500 km W-E transect originating in the 

Trans-Indus Ranges of northwestern Pakistan. 

304 

PROGRESSIVE CRATONIZATION IN THE 

SUPERIOR PROVINCE 

Stevenson. Ross K., GEOTOP/Dept. des Sciences 

de la Terre, Universite du Quebec a Montreal, 

P.O. Box 8888, St. C.V. Montreal, Que. H3C 3P8, 

Canada. 

Sm-Nd data are reported for several 

greenstone belts in the Sachigo Subprovince of the 

Superior Craton in NW Ontario. These greenstone 

belts are particularly noteworthy for their numerous 

granite-greenstone cycles during 300 Ma of late 

Archean history from 3.0-2.7 Ga. Thus, these belts 

offer an opportunity to monitor the evolution of the 

mantle and continental crust over a period of prolific 

crustal formation in the history of the Earth's 

continental crust. Recent tectonic analysis of the 

Sachigo Subprovince according to Phanerozoic terrane analysis concepts has subdivided the greenstone 

belts into different lithotectonic assemblages and 

grouped the belts into various tectonic terranes. 

When the Sm-Nd data is applied on this basis it can 

be used to compare the isotopic evolution of volcanic 

arc, platformal and oceanic tectonic settings. Isotopic 

data from each belt are consistent in that the earliest 

strata/assemblages show the smallest variation in £Nd 

values, usually ranging from chondritic (0) to +2. 

Successive assemblages show progressively greater 

ranges of £Nd values to more positive values for 

mafic samples and to more negative £Nd values for 

felsic and sedimentary samples. Thus a progressive 

evolution in both the depletion of the mantle and the 

recycling of crustal material from one assemblage to 

the next can be traced in each of these greenstone 

belts. Granitoid samples from the granitoid basement 

enveloping the greenstone belts mimic isotopic 

trends within the belts, indicating that granitoid 

development is concomitant with greenstone belt 

development and that the plutonism can often be 

attributed to volcanic events within the associated 

greenstone belt. The data are consistent with the 

formation of the North Caribou Terrane from an 

embryo of amalgamated island arc and oceanic 

plateaus, which developed through further accretion 

and intracrustal differentiation over 300 Ma 

(progressive cratonization) into a stable microcontinental platform onto which successive terranes 

and subprovinces accreted. 
PALEOHYDROLOGY OF THE LAtE PLIOCENE BASIN 

AND RANGE PROVINCE USING STRONTIUM ISOTOPES 

STEWART Brian W., HSIEH, Jean C.C., and MURRAY, 

Bruce C., Division of Geological and Planetary Sciences 

170-25, California Institute of Technology, Pasadena, CA 

91125, U.S.A. 

Lake sediments exposed in the Confidence Hills of Death 

Valley, California, preserve a record of climate change in the 

western United States Basin and Range Province during the 

late Pliocene and early Pleistocene. Alternating cycles of 

lacustrine evaporites and subaerial deposits in the well-exposed 

Confidence Hills section record variations in shoreline facies 

of a large Plio-Pleistocene lake; these changes in lake level 

may be correlated with climate variations inferred from the 

marine stable isotope record [1,2]. Determining the source of 

waters feeding Death Valley during lake highstands can 

provide important information about continental glaciation, 

climate, and weathering patterns. The two most likely sources 

of water to Death Valley during the late Pliocene are the paleoOwens River system, with water ultimately derived from the 

Sierra Nevada to the west, and the paleo-Amargosa River 

system feeding in from the east and north. 

We measured 87 Srf86Sr ratios in evaporite units (gypsum 

and anhydrite) spanning the length of the exposed Confidence 

Hills section in order to determine the source(s) of water 

entering Death Valley during the late Pliocene. Our samples 

span an age range of -350 Ka, from the base of the Reunion 

Event (2.15 Ma) to just above the top of the Olduvai event 

(1.79 Ma). All 87Srj86sr values fall within a range of 

0.7156±0.0003, with typical uncertainties on individual 

measurements of 0.00002. Although there are clear variations 

in 87srf86sr values outside of analytical uncertainty during this 

time period, the total range in values is small in comparison to 

possible sources of Sr to the Death Valley hydrologic system. 

The 87Sr/86sr values of Sierra Nevada granites generally fall 

in the range of0.705 to 0.709, with stream waters from the 

Yosemite area yielding values from 0.7078 to 0.7089 [3]. In 

contrast, exposures of Precambrian basement to the east of the 

Sierra Nevada can have values in excess of 0. 72. The absolute 

value and range of 87 Srj86sr ratios measured in Confidence 

Hills evaporites suggests that the source of waters into Death 

Valley during this time was relatively invariant, and that the Sr 

dissolved in the waters feeding the late Pliocene lake was 

derived primarily from Precambrian sources. Importantly, 

these data argue against the Owens River system as the 

dominant supplier of water to Death Valley during the late 

Pliocene. The most likely source of dissolved cations to the 

lake was either weathering of local Precambrian units within 

Death Valley itself or weathering of Proterozoic basement 

along the Amargosa River drainage system. Input of water 

from either of these sources would be enhanced if the Sierra 

Nevada were not sufficiently uplifted to create a strong rain 

shadow effect at the time of deposition of the Confidence Hills 

section. 

[1] Hsieh, J.C.C. and Murray, B.C. (1993) Geol. Soc. Am. Abstr. 

~25,A-456. 

[2] Beratan, K., Hsieh, J.C.C., Murray, B.C., and Gomez, S. 

(submitted) J. Sed. Pet. 

[3] Blum, J.D., Erel, Y. and Brown, K. (1993) Geocbim Cosmocbim 

Acta 58, 5019-5025. 

305 

STRONTIUM ISOTOPE SnJDY OF CATION FLUXES IN 

A SOJL-VEGETATION-A1MOSPHERE SYSTEM 

STEW ART, Brian W., Division of Geological & Planetary 

Sciences 170-25, California Institute of Technology, 

Pasadena, CA 91125, U.S.A., CHADWICK. O.A., JPLCaltech, Pasadena, CA 91109, U.S.A., and GRAHAM, 

R.C •• Department of Soil and Environmental Sciences, 

University of California, Riverside, CA 92521, U.S.A. 

Quantification of changes in soil weathering and nutrient 

cycling on a time scale of 101-102 years is essential in order to 

determine the effects of human-induced changes in 

atmospheric chemistry on soil-vegetation systems. As part of a 

U.S. Forest Service project in the San Dimas Experimental 

Forest in southern California, four unlined pits (5.3 x 5.3 x 2.1 

m) were filled in 1937 with a locally derived, homogenized 

sandy loam, and were planted with monocultures of native 

plant species after a nine year stabilization period. These 

experimental soil-vegetation systems offer an unparalleled 

opportunity to study soil development on a decade time scale. 

Studies of soil morphology [1] and c. N, and exchangeable 

cations [2] indicate that some changes have taken place in the 

lysimeter soils over their -50 year evolution, dependant 

primarily on vegetation type and associated faunal activity. 

We canied out mass balance calculations on bulk soil samples 

using the equations of Chadwick et al. [3] and Brimhall et al. 

[4]; the total element changes are sufficiently subtle on this 

time scale to be undetectable with these techniques, although 

further work on specific fractions may yield measurable 

differences. Here we present isotopic measurements of Sr 

extracted from the so~ exchange complex, and show that 

87srf86sr ratios do in fact provide a sensitive method for 

detennining patterns of cation release by weathering and input 

from atmospheric sources in soil-vegetation systems. Sr is an 

excellent proxy and serves as a tracer for divalent cations 

(especially Ca) in such systems. and Sr isotopic fractionation 

from natural processes is negligible. 

The two most important fluxes of strontium to soil 

exchange sites are dissolved Sr from atmospheric deposition, 

and Sr released by weathering of soil minerals. In the San 

Dimas study site, the 87srf86sr ratio of Sr in rainwater is 

...0.7091, and bulk soil Sr has a ratio of ...0.7099. We measured 

isotopic ratios of exchangeable Sr (using an NH4Cl-MeOH 

leach) in soils collected in 1987 from the pine-planted 

lysimeter. Samples range from the A-horizon (0-I em) down 

to a depth of 100 em, and all 87Sr/86sr values fall within 

0. 70953±0.00014, nearly in the middle of the two 

endmembers. This suggests that neither source dominates the 

budget of exchangeable cations on this time scale. In detail, 

the data exhibit a clear trend of decreasing 87 Srf86sr with 

depth, with the upper horizons having values slightly more 

radiogenic than the exchangeable Sr in the original soil, and 

the lower horizons trending below the original value, toward 

the atmospheric endmem ber. Our preliminary interpretation is 

that most weathering takes place in the A-horizon, resulting in 

release of cations, including Sr, to local exchange sites. In 

contrast, Sr from the atmosphere is delivered to exchange sites 

throughout the profile by soil water transport. 

[1] Graham, R.C. and Wood, H.B. (1987) Soil Sci. Soc. Am. J. 55, 

1638-1646. 

[2] Ulery, A.L., Graham, R.C., Chadwick, O.A. and Wood, H.B. (in 

press) Geoderma. 

[3] Chadwick, O.A., Brimhall, G.H. and Hendricks, D.M. (1990) 

Geomowbolo~y 3, 369-390. 

[4] Brimhall, G.H., Chadwick, O.A. et al. (1992) Science 255,695-

702. 
AGE ESTIMATES FROM SEMI-ARID AND ARID 

ARCHEOLOGICAL CONTEXTS: THE SUCCESSFUL 

APPLICATION OF A MULTIPLE TECHNIQUE DATING 

STRATEGY 

STOKES S., Research Laboratory for Archaeology and 

the History of Art, and School of Geography, University 

of Oxford, Oxford OX13QJ, UK, and VAN KLINKEN, 

OJ. Radiocarbon Accelerator Unit, University of Oxford, 

Oxford0X13QJ, UK. 

Chronological constraints on paleoenvironmentally- or 

archeologically-significant stratified sequences from arid and 

semi-arid areas have frequently been limited by difficulties in 

the application of routine radiocarbon dating methods and a 

lack of alternative dating techniques. In this paper we describe 

the results of a combined dating strategy, incorporating AMS 

radiocarbon dating of bone protein extract and optical dating 

of quartzose sediments, as applied to a significant early 

(possibly Paleoindian) indian burial site from New Mexico. 

We outline a series of bone protein separation and isolation 

techniques which we used and compare the AMS results on 

the fractions to demonstrate high degree of accordance which 

we have obtained. 

Two optical dating techniques are described; comprising the 

multiple and single aliquot, additive dose dating methods. 

Close agreement between these techniques, coupled with their 

broad agreement with the AMS radiocarbon assays and 

uranium equilibrium at the site, given us confidence in the 

results which we have obtained. 

Implications for the widespread application of such multiple 

dating strategies to similar arid and semi-arid region deposits 

will be discussed. 

306 

COSMOGENIC CHLORINE-36 PRODUCTION BY 

SPALLATION AND MUON-CAPTURE 

STONE. J . Research School of Earth Sciences, FIFIELD, 

L.K., EVANS, J.M., ALLAN, G.L. and CRESSWELL, R.G., 

Dept of Nuclear Physics, Research School of Physical 

Sciences and Engineering, Australian National University, 

Canberra, A.C.T., 0200, Australia 

Cosmogenic isotope measurements by AMS have progressed 

so far in recent years that the accuracy of exposure dating 

methods for terrestrial surfaces is now limited by knowledge of 

isotope production rates. A case in point is 36(:1, which can be 

measured with exceptional sensitivity (we have made 

measurements to better than ±10% at 3x1o4 atom/g), but 

whose production is complicated by numerous nucleon- and 

muon-induced reactions with Ca, K and 35a. We have 

calibrated rates for several of these from natural surface and 

sub-surface samples, aiming to isolate the effects of specific 

reactions by choice of target minerals and sample depths. 

(1) Ca Spallation was calibrated using calcite (CalK > UP, 

Ca/Cl > 1o4) from -10.3ka glacial pavements at high altitude 

in the Sierra Nevada, California. In these samples, 93-97% of 

36a production is due to Ca Spallation, 2% to negative muon 

capture by 40ea, 1-5% to capture of cosmogenic secondary 

neutrons and < 0.1% to capture of U- and Th-produced 

neutrons. Scaled to sea-level and latitudes> 60° (as per Lal; 

[1]) the 36a production rate from Ca spallation derived from 

the data is 0.733 atom/g/yr/%Ca. We assign an uncertainty of 

±10% to this value, largely to account for uncertainty in the 

exposure history. The result is in excellent agreement with the 

estimate of Zreda et al [2], of 0.67-0. n atom/g/yr/%Ca. 

(2) 36cl production in calcite by muon-induced reactions 

was calibrated from a 2-20m depth profile in marble. We 

estimate a surface rate of 0.043 atom/g/yr/%Ca for the negative 

muon capture reaction 40Caij1· ,a)36ct. at sea-level and high 

latitude, equivalent to -5.5% of total production from Ca. We 

calculate an overall neutron yield of -o. 75 n/stopped muon in 

calcite, hence a production rate of 130 ± 40 n/g/yr at the 

surface at sea level and high latitudes. The resulting rate for 

35a(n,y)36a from mu-genic neutrons under these conditions 

is therefore ....0.026 atom/g/yr/ppm Cl. Currently, uncertainties 

in the production parameters derived for muon reactions are - ±30% due to simplifications in the data inversion and 

disturbance of the profile surface. Better estimates (hopefully 

±10%) for the production parameters may be obtained with an 

improved sub-surface production model incorporating fastmuon-produced neutrons. 

(3) We have calibrated 36cl production from cosmic ray 

reactions with K using K-feldSpar (K/Ca > 250, K/Cl > 5x1o4) 

in Antarctic moraine boulders. Independent age control is not 

available for these samples, necessitating calibration against 

26 AJ110Be ages (13-17ka) on quartz from the same samples. 

The estimated rate is 1.77 atom/g/yr/%K, substantially higher 

than the published value [2], but in good agreement with a 

recent revision of this rate to 1.93 atom/g/yr/%K [3]. 

We hope to have further results on ( 4) production of 1 Ose 

from C and 0 in calcite, (5) calibration of K -> 36cl reactions 

from independently-dated surfaces, and (6) cross-checks on 

1 OBe and "26 AI production in quartz, by the time of the 

meeting. Applications will be discussed if time permits. 

[1] Lal, D., 1991, Earth Planet. Sci. Lett. 104,424-439. 

[2] Zreda, M.G., Phillips, F.M., Elmore, D., Kubik, P.W., 

Sharma, P. and Dom, R.I., 1991, Earth Planet. Sci. Lett. 105, 

94-109. 

[3] Phillips, F.M., pers. comm. and presentation at GSA 

Boston meeting, 1993 
ASSESSMENT OF CRUST-MANTLE CONTRffiUTIONS 

TO THE YOUNG (9Ma) SERIFOS AND OTHER AEGEAN 

GRANITES WITH MIXED S-/1-TYPE CHARACTERISTICS 

STQURAITI Christina, TARNEY, J. & NORRY, MJ., 

Dept. Geology, U. Leicester, UK, and BARREIRO, B.A., 

NERC Isotope Geoscience Laboratory, Keyworth, 

Nottingham, UK 

The Serifos pluton is one of the youngest granite complexes 

in the Aegean, and lies in the back-arc region, just to the north 

of the active Aegean Arc. Other granites in the Aegean tend to 

age progressively with distance northwards from the Arc. 

Serifos illustrates many of the problems of granitoid 

petrogenesis. The composition ranges from quartz-hornblende 

diorite (enclaves) through hornblende-biotite granodiorite and 

biotite granite to leuco- pegmatites and aplites near the margins 

of the complex. There are spectacular mineralised skarns and 

karstic Fe-Mn deposits. The fact that most rocks are generally 

not per-aluminous but are hornblendic signifies an I-type 

granite. On the other hand having 87Srf86Sr ratios for the 

complex that range from 0.709-0.711 and 143NcJ1144Nd ratios 

that range from 0.51220-0.51231 is clearly a strong S-type 

granite characteristic. How can these features be reconciled? 

A synthesis of available Sr and Nd isotopic data on Aegean 

and other E. Mediterranean granitoids and volcanics shows that 

most lie very close to a mixing curve between a Sr-enriched 

(relative to Nd) "Aegean mantle" component represented by 

Santorini basalts and a Nd-rich "Hellenic crustal" component 

represented by the metasedimentary basement rocks of Serifos 

and adjacent islands of the Cyclades. Whereas AFC 

(assimilation - fractional crystallisation) and similar models 

involving these end-members could theoretically reproduce the 

isotope data for Serifos, there is a problem in that Serifos 

occupies a very restricted field on the general isotopic mixing 

curve; other complexes also have a limited, but different, 

range. Moreover, the trace element data do not favour 

extensive AFC processes. 

Our preferred solution relies on the fact that continental 

development of the Hellenic region during the Mesozoic and 

early Tertiary was dominated by subduction-accretion 

processes, as testified by the abundant blueschist and eclogitic 

remnants scattered amongst the metasedimentary basement 

rocks throughout the region. The latter represent accreted 

continental greywacke-shale sediments that have been 

deformed and dragged-down during subduction-accretion, and 

some must have been uplifted and exhumed from mantle 

depths. We suggest that such sediments heavily, but variably, 

contaminate the underlying lithosphere. Magmas that are 

generated later from the lithosphere as a result of exhumation, 

extension or orogenic collapse will therefore inherit a "crustal" 

component dependent on the extent of the earlier subductionaccretion. If subduction-accretion is important in crustal 

development, particularly in the Phanerozoic and late 

Precambrian, then this may account for some of the crustal 

signatures of mantle-derived magmas as much as magmaassimilation processes. 

307 

EPISODIC METASOMATISM IN PELITIC SCHISTS 

FROM CONTACT METAMORPHIC AUREOLES NEAR 

JUNEAU, ALASKA 

STQWELL H H, and MENARD, T., Department of 

Geology, University of Alabama, Tuscaloosa, AL 35487 

Calcic pelitic schists from contact metamorphic aureoles 

around the Grand Island diorite complex near Juneau, Alaska 

experienced multiple fluid infiltration and metasomatic events. 

These events are recorded in the compositional zoning of 

garnet and in the metamorphic reaction history. 

Metamorphic aureoles around plutons of the Grand Island 

diorite complex range in width from 20 m to 200 m with 

peak metamorphism at staurolite grade. The rocks have a Ms + Bt schistosity and a weak crenulation cleavage. Calcic 

pelitic schists have the medium-grade assemblage Grt + Bt + 

PI + Ep + Qtz + Ms + Sph. 

Garnet grains are small (0.2 - 0.5 mm), contain inclusions 

of Qtz and Ep, and are partially resorbed to chlorite. Garnets 

2 mm from a quartz vein have cyclic zoning in XGrs (0.10 to 

0.35) and sympathetic zoning in XAlm (Fig. 1). With some 

exceptions, Xsps decreases and Fe/ {Fe+ Mg) increases towards 

the rim. Na zoning in garnet, with concentrations in the 

100's of ppm, has opposite trends as Ca. If the Na zoning in 

garnet reflects plagioclase compositions, XGrs and XA n 

increased (and decreased) together. Ti in garnet decreases from 

1800 to 100 ppm at about the same position as the first large 

increase of XGrs· The zoning data may reflect Cametasomatism, which would cause growth of plagioclase and 

sphene, reducing the availability of Na and Ti. Garnets 2-m 

away from veins are similar, but do not have the initial high 

XGrs· Zones with high XGrs may reflect successive fluid 

infiltration and Ca-metasomatism events. Inclusion of tiny 

mineral grains within garnet in these zones suggests rapid 

growth. Garnet growth with decreasing XGrs after each of 

these events may reflect hiatuses in fluid infiltration. 

The rocks were extensively retrograded to Czo + Ab + Ms 

+ sulfide, and in places, subsequently retrograded to minor 

chlorite and sericite. Epidote has subhedral cores of allanitic 

epidote zoned in REB's that correlate with garnet, surrounded 

by unzoned rims of retrograde clinozoisite. Plagioclase has 

equilibrium textures with the retrograde assemblage, and hence 

postdates the garnet. Retrograde metamorphism was 

associated with emplacement the foliated tonalite sill 10 km 

to the east, and possibly with the low-grade alteration and 

mineralization in the Juneau gold belt. 

Thus, fluid flow varied during metamorphism. Infiltration 

started near veins, and then became more pervasive. Pulses of 

fluid flow and intervening hiatuses may be related to 

emplacement of multiple plutons in the complex, causing 

repeated changes in temperature or fluid flow. The fluid 

character and the resulting metasomatism changed from event 

to event. 

Figure 1 

0.6 c 

.5! 

-~ 0.4 ... 

-

CD 0.2 

0 

E 

0.0 ~--~----~----_.----~----~ 

0.0 0.1 0.2 0.3 0.4 0.5 

distance (mm) 
COUPLED HE-S ISOTOPE VARIATION IN M.O.R. 

HYDROTHERMAL MINERALS 

STI JARI Finlay F ALLICK, A.E., Isotope Geology Unit, 

S.U.R.R.C., East Kilbride G75 OQO, UK , and TIJRNER G., 

Dept. Geology, Manchester University, Manchester M13 

9PL, UK. 

Helium isotopes have been measured in fluid inclusions 

hosted in sulphide and sulphate minerals from active mid-ocean 

ridge hydrothermal systems at bare rock (21 °N and 13<N, 

E.P.R., TAG and Snake Pit, M.A.R.) and sediment-covered 

sites (Middle Valley and Gorda Ridge, J D.F.). Sulphidehosted fluid 3Het4He from bare rock sites (7.1-9.5 RA) are 

indistinguishable from those of the venting hydrothermal fluids 

and are close to those of the local basalts. Fluid inclusion 

3Het4He from Middle Valley and the Gorda Ridge sulphides 

(4.5-6.8 RA) contain variable amounts of radiogenic helium 

along with the basalt-derived helium. The radiogenic He cannot 

be derived by protracted hydrothermal circulation in the 

basaltic basement. 

aJ4s values of EPR and MAR sulphides (0.5-7%o) are above 

values of basaltic sulphur (-O%o) and are consistent with the 

presence of reduced seawater sulphate. He isotope ratios are 

independent of aJ4s confmning that addition of seawater to 

hydrothermal fluids has no effect on He isotope composition. 

He and S isotopes in sulphides from sediment-covered systems 

are coupled, displaying a broad negative 3Hef4He-o34s trend. 

This is consistent with a sedimentary origin for the radiogenic 

He and either results from a mixture of hydrothermal fluid with 

sediment pore fluid enriched in radiogenic He, or direct 

interaction between hydrothermal fluids and sediments. 

Combined He-S isotope studies of continental hydrothermal 

mineralisation is a potentially powerful tracer of fluid origins. 

Pb isotope measurements are being performed to provide 

constraints on the degree of sediment-fluid interaction. 

308 

NEW AGE STANDARD MINERALS FOR KAr AND 40 Ar/39 Ar DATING; PRELIMINARY 

RESULTS OF THREE BIOTITE SEPARATES. 

SUDO M 1 UTO, K., 2 T AGAMI, T .,1 

MATSUMOTO, A.,2 HASEBE, N) UCHIUMI, S.,2 

ANNO, K.3 and NISHIMURA, S.l 

1 Dept. of Geology and Mineralogy, Fac. of Science, 

Kyoto Univ ., Kyoto 606, Japan; 2Dept. of 

Geochemistry, Geological Survey of Japan, Tsukuba 

305, Japan; 3Graduate School of Human and 

Environmental Studies, Kyoto Univ. Kyoto 606, Japan 

Interlaboratory age standards are of primary importance 

for reliable radiometric dating. To set up new K-Ar and 

40 Art39 Ar dating systems using VG 3600 mass 

spectrometers, installed in Kyoto University and 

Geological Survey of Japan (GSJ), we prepared and 

analyzed three biotite standard candidates of PlioceneCretaceous ages separated from Japanese igneous rocks. 

The biotite Sori-93 is a recollection of GSJ JG-1 

biotite standard from the same locality of Sori 

granodiorite. The contents of argon and potassium and 

the argon isotopic abundances were reported previously, 

yielding K-Ar age -91 Ma (e.g., Matsumoto et al., 

1989). At first we measured argon and potassium at 

GSJ. The contents and isotope ratios of argon were 

determined using two systems: the VG 1200 mass 

spectrometer by the peak comparison method (Matsumoto 

et al., 1989) and the Micromass 6 by the isotope 

dilution. The contents of potassium were determined with 

the Kotaki FIP-3D flame emission spectrometer. 

Subsequently, the measurement of argon was repeated 

with the VG3600 mass spectrometer installed in Kyoto 

University for interlaboratory calibration. 

We prepared the biotite NST from a welded tuff of 

the Nisatai Dacite. The K-Ar biotite and FT zircon ages 

are reported as -21 Ma (Tagami et al., submitted). To 

evaluate whether the age of the biotite is the 

concordance of the different grain sizes, the biotite was 

sieved to several fractions of different grain size. The 

measurements of argon and potassium were carried out 

on four and six fractions, respectively. The obtained 

results of K20 content varied from 7.34 wt% to 8.38 

wt% in the six fractions of 16 to 140 mesh sizes. 

However, the ages were approximately concordant (from 

20.9 Ma to 21.8 Ma) among 30 to 100 mesh sizes. 

This may indicate that the biotite is useful as the K-Ar 

age standard. To investigate the cause of potassium 

content variation among the grain sizes, potassium 

distribution analysis in single biotites with EPMA is 

currently planned. 

We also report the analysis of biotite from the 

Utaosa Rhyolite, which K-Ar ages are concordant at -2.5 

Ma (Uto et al., submitted). Further analytical results 

will be presented to assess its suitability to standard. 

Matsumoto A., Uto K., and Shibata K., 1989, K-Ar 

dating by peak comparison method --New technique 

applicable to rocks younger than 0.5 Ma--: Bull. 

Geol. Surv. Japan, v 40, p. 565-579. 
NITROGEN ISOTOPE SYSTEMATICS OF 

PRIMITIVE ORDINARY CHONDRITES 

SUGIURA. N~, KIYOTA, K., Dept. of 

Earth and Planetary Physics, Univ. of 

Tokyo, Tokyo, Japan. HASHIZUME,K., 

Dept. of Earth and Space Science, 

Osaka Univ.,Osaka, Japan. 

Searching for signature of presolar 

grains in ordinary chondrites, we have 

measured nitrogen isotopic composition 

of about 20 primitive ordinary · chondrites. The nitrogen was extracted 

by a stepped combustion method, and 

measured with a static mass 

spectrometer. 

The primitive chondrites can be 

classified into 3 major groups according 

to the nitrogen isotopic signature. In 

addition, there are a couple of 

ungrouped chondrites. 

Group-1: This is the largest group with 

7 members. This group is characterized 

by abundant but only slightly anomalous 

(-60 permil) nitrogen released at llOOC. 

Group-2: This group has four members 

and characterized by somewhat heavy (170 

permil) nitrogen isotopic composition. 

At least three members are gas-rich 

chondrites as inferred from the neon 

isotopic composition. It is quite likely 

that the heavy nitrogen is of solar wind 

origin. 

Group-3: This group has three members 

and characterized by isotopically light 

(-220 permil) nitrogen which shows 

bimodal release pattern (700C and 1100C) 

by stepped combustion. A large amount of 

primordial Ar is also released at these 

temperatures but the Ar and nitrogen are 

not necessarily in the same carrier 

according to the results of chemical 

treatment. 

Ungrouped chondrites are Y-74191 and 

Mezo·Madaras. The former has a very 

heavy (1000 permil) nitrogen, and the 

latter has a slightly heavy (200 permil) 

nitrogen. 

Five chondrites with a metamorphic 

grade higher than 3. 7 release nearly 

normal nitrogen although deviation of 

delta 15N values during stepped 

combustion suggests that anomalous 

nitrogen may be present. 

309 

U-Pb AGES FROM THE HIGH-GRADE BARRO ALTO 

MAFIC-ULTRAMAFIC COMPLEX (GOlAS, CENTRAL 

BRAZll.): MIDDLE PROTEROZOIC CONTINENTAL 

MAFIC MAGMATISM AND UPPER PROTEROZOIC 

CONTINENTAL COLLISION. 

SUITA1,2,4, M.T.F.; KAM03, S.L; KROGH3, T.E.; 

FYFE2, W.S. & HARTMANN4, LA. 1. Depto. 

Minera~ao, UFOP, Ouro Preto, 35400, MG, Brazil; 2. 

Dept. of Earth Sciences, UWO, London, Ont., Canada, 

N6A 5B7; 3. Dept. of Geology, Royal Ontario Museum, 

Toronto, Ont., Canada, M5S 2C6; 4. Instituto de 

Geocienciasl CPGq, UFRGS, Porto Alegre, 91500, RS, 

Brazil 

The Barro Alto Complex (BAC) is the largest maficultramafic layered intrusion (156 km x 8-25 km wide x 5-6 

km thick) in Gohis, Central Brazil, and among the largest 

in the world. The BAC, with the Niquelindia ~d Cana 

Brava Complexes, forms a 350 km-long belt of large 

layered mafic-ultramafic intrusions between the Archean 

Amazonian and Sao Francisco cratons. The BAC 

underwent granulite to amphibolite' facies metamorphism 

with local ductile deformation and associated 

retrogressions to amphibolite and greenschist facies. The 

first U-Pb data from the BAC are presented. Zircons were 

analyzed from granulite and amphibolite facies noritic 

rocks from the basal part of the BAC (the Serra de Santa 

Barbara Sequence, SSSB); a garnet-amphibolite facies 

quartz-diorite intrusion in the SSSB; a late-stage 

differentiate of pegmatitic hornblende-gabbro from the 

amphibolite facies BAC upper zone, the Serra da 

Malacacheta Sequence (SSM); and a garnet-sillimanitegneiss from the Juscelandia Sequence (!), tectonically 

interleaved with SSSB rocks, from which monazite was 

also analyzed. The upper intercept age for the quartzdiorite is ca. 1. 72-1.73 Ga and is interpreted to be a 

minimum primary age for the intrusion. The lower 

intercept age of ca. 780 Ma is suggested to be the time of 

syntectonic high-grade metamorphism. This metamorphic 

age is confirmed by those obtained for the mylonitized 

norites at amphibolite and granulite facies and from 

monazite from the gamet-sillimanitic-gneiss. The SSM 

pegmatitic gabbro has upper and lower intercept ages of 

ca. 1.29-1.35 Ga and 0. 77-0.82 Ga, respectively, that are 

interpreted as either a time of mafic magmatism related to 

the Urua~uano (Middle Proterozoic) Cycle or possibly local 

post-BAC magmatism. These U-Pb ages together with 

other studies provide constraints on the geotectonic setting 

and evolution of these large mafic-ultramafic complexes in 

Goias, suggest an important event involving underplating 

continental mafic magmatism during the transition of 

Lower to Middle Proterozoic, and confirm the syntectonic 

high-grade metamorphism associated with the continental 

collision of the Amazonian and the Sao Francisco Cratons 

during the Upper Proterozoic (Brasiliano/Pan-African 

Cycle) in Central Brazil. 
CONSTRAINTS ON DERIVING 'UPLIFT' HISTORIES 

FROM THERMOCHRONOLOGIC DATA 

SUMMBRFJEJ Q, ~. Department of Geography, 

University of Edinburgh, Edinburgh EH8 9XP, United 

Kingdom, and BROWN, Roderick, W., Department of 

Geological Sciences, University College, London, WClE 

6BT, Uni~Kingdom 

The inability of thermochronologic data directly to identify 

changes in surface elevation, as opposed to the movement of 

rocks relative to the earth's surface, is now becoming widely 

recognized, although the misleading use of the term 'uplift' 

continues to appear in research reports. We attempt to clarify 

the relevant terminology by clearly distinguishing between 

absolute and relative frames of reference in discussing 

displacements of the earth's surface and of movement of rocks 

in the crust. 

Although the consistent use of clearly defined terms is 

important in 'uplift' studies, our primary aim is to demonstrate 

that since patterns and rates of denudation are key factors in 

determining crustal cooling histories, an understanding of the 

factors controlling denudation rates is a prerequisite for the 

correct interpretation of thermochronologic data. In 

particular, we question the assumption implicit in many 'uplift' 

studies employing such data that denudational episodes only 

occur in response to specific 'uplift' events and that, 

consequently, thermochronologic data can be used to provide 

an unequivocal record of tectonic activity. The idea that an 

increase in elevation (arising from tectonically-driven uplift) 

will inevitably lead to an increase in denudation rates ignores 

the fact that the latter are controlled by local relief (through 

average slope), not elevation, and these two variables are 

unrelated in some morphotectonic settings. Moreover, there 

may be a significant time lag while tectonic3lly-engendered 

base level changes propagate across the landsurface, especially 

in passive margin and cratonic settings. Another problem is 

that denudation rates can change at a regional scale in 

response to changes in non-topographic variables such as 

climate and lithology. A further difficulty arises where 

significant changes in drainage boundaries could potentially 

lead to an episode of accelerated denudation (and crustal 

cooling); this is most likely to be important where external 

systems capture the internal drainage of high, intra-orogenic 

plateaus. 

Although thermochronologic data clearly have an important 

role to play in tectonic studies we argue that those interpreting 

such data need to consider carefully all the factors that might 

have contributed to the cooling history observed. 

310 

ISOTOPIC AND GEOCHEMICAL CONS1RAINTS ON 

THE DEVELOPMENT{)F A LARGE RHYOLIDC 

MAGMA SYS1EM: TAUPO VOLCANO, NEW ZEALAND 

SJIDON AN , BLAKE S., Dept. of Earth Sciences, Open 

University, Milton Keynes, MK7 6AA: U.K. and WILSON 

C.I .N., Inst. of Geological and Nuclear Sciences, Taupo, NZ. 

Taupo, a potentially active caldera volcano in central North 

Island, New Zealand, is one of the most productive rhyolitic 

systems in the world. This study attempts to relate the 

isotOpic and geochemical evolution of the volcano to recent 

work on its physical development We aim to address 

important questions in the study of large silicic magma 

systems: Are eruptions tapping a single magma chamber, or 

many? Do chambers have long residence times, or are they 

transient phenomena? On what timescales do replenishments 

of magma occur? 

Eruptive products from throughout Taupo's history have 

been 'imgetprinted' using Sr and Nd isotopes, in conjunction 

with major and trace element and petrographic data. The ranges 

of 87/86Sr (0.70489-0.70725) and ENd (-3.1 to +2.0), preclude 

a common origin for the rhyolites. Instead, analyses of 

rhyolites cluster on variation diagrams into compositional 

groups ('magma types') with distinct isotopic signatures, and 

limited chemical variability. Eruptions of a particular magma 

type are related in time and/or space, so that compositional data 

combined with a knowledge of the ages and vent sites, produce 

a picture of what magma was available, when, and where. 

Rather than repeated tapping of a single chamber, the evolution 

of Taupo has been punctuated by eruption of distinct types of 

magma with stepwise rather than gradual changes between 

them. 

The caldera-forming, ~800km3 Oruanui eruption at 26.5ka 

marks a major change in the magmatic system. Prior to this 

event the dominant control on composition was spatial. The 

period 320ka to 64ka was dominated by extrusion of lava 

domes, often in small groups that share the same isotopic 

signature. These are isotopically and chemically diverse (e.g. 

87/86sr= 0.70489-0.70725, eNd= -3.1 to +2.0, Si02 69-

77%,), although spatially related domes often have geochemical 

affinities that suggest discrete magma batches. Domes and 

pyroclastics from -64-27ka record the growth of an isotopically 

homogeneous (87186Sr =0.70555) magma chamber prior to the 

climactic Oruanui eruption, though even in this period, 

distinct, spatially-separated magma types continued to erupt 

The post-26.5ka sequence represents a new type, deimed by 

higher 87/86Sr (0.70598-0.70622), trace element contents, and 

higher temperature. There is no spatial control on chemistry, 

but some compositional breaks are seen with time. The itrst 

three, small volume, eruptions at 20.5-17ka were dacitic and 

have 87/86Sr ratios (0.70600) similar to those of post-26.5ka 

rhyolites, rather than Oruanui magma. The 25 eruptions from 

12ka-1800a were rhyolitic, with very similar major elements, 

although there appear to be two subtly distinct inputs of new 

magma, into the high level system beneath the same vent site 

region, on timescales of only a few thousand years. 

Taupo shows markedly different behaviour to many other 

rhyolite volcanoes of comparable productivity. The unususally 

high freqency of eruptions at Taupo, compared to systems like 

Long Valley, is paralleled by rapid changes in the type of 

magma available for eruption, with distinct chambers erupting 

magma for only -to3-to4 years. It is hoped to constrain 

magma residence times by comparing phenocryst formation 

ages from U-Th disequilibria studies with known eruption ages. 
STUDY ON EFFECT OF ALTERATION ON APPARENT 

RE-OS AGE FOR MOLYBDENITE THROUGH NEARINFRARED MICROSCOPY. 

SUZUKI,K., D~pt.Che~ ,Arts and Sci, Univ. 

Tokyo, Tokyo 153, Japan and KAGI, H., lnst. 

Mat. Sci. , Un i v. Tsukuba, I barak i 305, Japan 

Re-Os chronometry is a promising technique 

to determine the ages of sulfide minerals 

directly. We applied this method to molybdenite (MoS2) and obtai ned Re-Os ages, 

which show good agreement with related 

wall-rock ages (an indirect indicator of 

the real age). (Fig. 1) 

Luck and All~gre (1982) pointed out the 

possibility of Re loss from molybdenite 

through alteration. McCandless et al (1993) 

having investigated molybdenite through 

near infrared (NIR) microscopy, a 1 so reported that high NIR transparency of molybdenite would indicate alteration and Re 

loss. If their hypothesis were true, it 

means that Re-Os chronometry can give an 

apparent Re-Os age older than the real one. 

In our study, molybdenite from Matasvaara, 

Finland, whose Re-Os age agrees with the 

wall-rock age (Fig. 1), was observed through 

NIR. After the hypothesis by McCandless, 

this molybdenite should give low NIR transparency because it is considered to have 

experienced no such alteration as causes 

Re loss. However, thin section (0.2mm) of 

Matasvaara molybdenite was transparent to 

NIR and its inner texture can be easily 

seen. Therefore, high NIR transparency does 

not seem to reflect Re loss resulting in 

the significant deviation of Re-Os age. 

In order to clarify the effect of alteration on Re-Os age as well as NIR transparency, 1 a bora tory experiments were done. Molybdenite, in NaCl, A1Cl 3, was exposed to a 

heat of 180 °C for 40 days. The result was 

a remarkable increase in NIR transparency 

and intriguing changes were found in inner 

structures. For further understanding of 

the effect, experiments were continued 

through microprobe, NIR, and FT I R. 

5,000 

-&a 

~ 1,000 CD 500 -

cu 

0 - 100 I 

CD 50 a: 

50 100 500 1 ,000 

w a I I r o c k age (M a) 

Fig.l Re-Os ages and related wall rock ages. 

311 

PRECAMBRIAN DETRITAL MONAZITES AND 

ZIRCONS FROM JURASSIC SANDSTONES IN 

THE JAPANESE ISLANDS, REVEALED BY 

"CIUME" GEOCHRONOLOGY 

SUZUKI, K. and ADACHI, M., Dept. of Earth 

and Planetary Sciences, Nagoya University, 

Nagoya 464-01, Japan 

A total of 3769 monazite and zircon grains from 56 

Jurassic sandstones in the Mino and Hida terranes, 

central Japan were dated by the CHIME (CHemical 

Th- u-total Pb Isochron MEthod; Suzuki et al., 

1991; Suzuki and Adachi, 1991). Most monazite 

grains are chronologically unzoned and give CHIME 

ages of ca. 1700, 1450, 1250 or 240 Ma; several 

grains are zoned and show distinct core-overgrowth relations. The ca. 1700 Ma, most commonly 

observed, is considered to show the time of main 

metamorphism of the monazite-bearing gneisses that 

were exposed in the provenance of the Jurassic 

sandstone. The younger ages appear to represent 

some significant thermal events. Of particiJlar interest are several Archean cores of 3000 and 2700 

Ma within the ca. 1700 Ma monazites. Zircon ages 

range from ca. 3100 to 180 Ma, with conspicuous 

concentrations at ca. 2000 and 240-180 Ma. The 

ca. 2000 Ma age is interpreted to represent the 

emplacement time of origmal granites that provided 

large amounts of detritus to the protolith of the 

ca. 1700 Ma gneisses. The age spectra of monazites 

and zircons are nearly ideniicaf for sandstones 

from the Hida and Mino terranes. This is inconsistent with the current view that the Hida and 

Mino terranes· were totally independent until 

their accretion in middle Cretaceous time. 

The CHIME method, characterized by a high 

spatial resolution ( 2-3 JL m), is a powerful new 

tool for the study of clastic rocks in recycled 

orogens and for the terrane analysis. 

Suzuki, K., Adachi, M. and Tanaka, T., 1991, Sediment. 

Geol., 15, 141-147. Suzuki, K. and Adachi, M., 1991, 

Oeochem. J., 25, 357-376. 

PbO (wt.%) 

0.8 Detrital monazite grain 

from a Jurassic 

sandstone of the 

Hida terrane 

0.4 

o~---L----L----L----L---~----L---

0 4 8 Th02* (wt .%) 

240 

160 

80 

0 

0 

Frequency distribution of CHIME ages of 

detrital monazites 

and zircons from 

2 

Jurassic sandstones 

of the Hida and 

Mino terranes 

3Ga 
ROBLB GABBS IR TIIB SRC (MARTIAlf) 

ORTBOPYROXBRITB MBTBORITB ALB84001: 

COMPARISOR M7TB TBB RAKBLITBS 

SWINDLE, T. D., BURKLAND, M. K., and 

GRIER, J. A., Lunar and Planetary 

Laboratory, University of Arizona, 

Tucson, AZ 85721 USA 

ALB84001 is an Antarctic meteorite 

which was recently found to be a relative 

of the SNC (Shergottites, Nakhlites and 

Chassigny) meteorites, and hence probably 

comes from Mars. We have measured noble 

gases in two samples of ALB84001: all five 

gases in a 18.1 mg sample, and Kr and Xe 

in a stepwise heating of a 142 mg sample. 

We find that ALB84001 bears some 

remarkable resemblances to the three 

nakhlites, but it has some intriguing 

differences as well. 

The heavy noble gases in most 

shergottites can be plausibly fit as a 

mixture of the gas in EETA79001, 

( 129Xe/ 132Xe=2. 4, 13'Xe/ 132Xe=O. 36, 

uKr/ 132Xe=25), thought to represent the 

martian atmosphere, and Chassigny 

( 129Xe/ 132Xe=1. 0, 13'Xe/ 132Xe=O. 30, 84Kr/ 132Xe=1), perhaps representin~ the 

mantle. Nakhlites tend to have 29Xe/ 132Xe 

ratios of 1. 5 to 2. 0, but uKr/ 132Xe ratios 

of 5 or less, so they don't fall on the 

shergottite mixing line, but could 

represent a mixture of Chassigny-like gas 

and elementally fractionated atmosphere 

(Ott, 1988). 

The 129Xe/ 132Xe ratio in ALB84001 is as 

high as 2.05 in some extractions. Among 

the SNC's, only Nakhla (up to 2.2) and 

EETA79001 have higher values. The 

uKr / 132Xe is rather low (about 4) , again 

similar to Nakhla. This would be 

consistent with a scenario in which the 

(isotopically) atmospheric-like noble gas 

in nakhlites is carried by weathering 

products (Drake et al., 1993), since 

ALB84001 contains what are probably 

martian weathering products, like the 

nakhlites. However, unlike the nakhlites, 

most of the temperature extractions do not 

fall on the Chassigny-EETA79001 mixing 

line in Xe three-isotope clots such as 12'Xe/ 132Xe vs. 13'Xe/ 132Xe ( 1

r'xe/ 132Xe is too 

low). 

We calculate a preliminary cosmic-ray 

exposure age, based on Ne and Ar, of 15-20 

Ma, considerably longer than the 10 Ma 

exposure age of the nakhlites and 

Chassigny or the ~3 Ma of the 

shergottites. Thus, at least its most 

recent parent body was distinct from any 

of the previously known SNCs. 

Ott, u., 1988, Noble gases in SNC 

meteorites: Shergotty, Nakhla, 

Chassigny, GCA 52, 1937-1948. 

Drake, M.J., owen, T., swindle, T., and 

Musselwhite, D., 1993, Noble gas 

evidence of an aqueous reservoir near 

the surface of Mars more recently than 

1.3 Ga. LPSC XXIV, 431-432. 

312 

AGE OF FISH CANYON TUFF SANIDINE (FCTS): 

A SINGLE CRYSTAL 40 Ar /39 Ar DATING STANDARD 

SWISHER IILC.C. (1), DEPAOLO, DJ. (2), OWENS, T. 

(2,1): (1) Institute of Human Origins Geochronology 

Center, 2453 Ridge Rd., Berkeley, CA, 94709, USA; (2) 

Department of Geology and Geophysics, University of 

California, Berkeley, CA, 94720, USA. 

The Fish Canyon Tuff Sanidine (FCTS) is currently one of 

the most widely used 40 Ar/39 Ar neutron fluence monitors 

(dating standard) applicable for the calibration of late 

Cretaceous and Cenozoic rocks. Replicate analyses of 

individual crystals of the FCTS yield SE's on the order of 

0.1% or less, making it potentially an excellent single crystal 

40 Ar/39 Ar neutron fluence monitor. However, because of 

difficulties in assuring complete argon extraction of sanidines 

by K-Ar dating, the age of the FCTS is obtained largely from 

40 Arf39 Ar intercalibration with first principal biotite K-Ar 

standards. At IHO-GC, an age of 27.84 Ma2 is used based on 

40 Arf39 Ar intercalibration with MMhb-1 hornblende using a 

published age of 520.4 ± 1.6 Mal. However, the age of 

MMhb-I itself is disputed with younger age estimates of 513.9 

Ma that result in a FCTS age of 27.55 Ma3. Consequently, · 

despite the precision capable using the FCTS as a 40 Ar/39 Ar 

fluence monitor, its accuracy is limited by uncertainties as to 

the ages of the K-Ar standards used for intercalibration. 

To provide a more accurate age for the FCTS, we present 

age intercalibration with two K-Ar biotite standards University 

of California, Berkeley (UCB), GHC305 and Australian 

National University (ANU) GA 1550. The ages of these biotite 

standards are based on published Ar abundances of 1.428xi0-

11 for GHC-305 and 2.245xlo-11 for GA-1550 and potassium 

abundances as determined here by isotope dilution mass 

spectrometry. Review of potassium measurements of 

GHC305 by flame photometry at UCB accumulated over a 20 

year period, had shown short-term reproducibility on the order 

of 0.5% or less while long term variation ranged over 2%. 

Isotope dilution mass spectrometry results of this study 

yielded a mean potassium value of 7.58 ± 0.03 Ia GHC305·(7 

splits) and 7.63 ± 0.05 Ia for GAI550 (8 splits). Using these 

potassium values, the K-Ar age for GHC305 would be 105.49 

Ma and 98.77 Ma for GAI550. 

For 40 Art39 Ar intercalibration, the FCTS was irradiated 

along with GHC305 and GAI550 for 14 hours in the Los 

Alamos Reactor. The irradiation fluence or J was calculated 

separately for both GHC305 and GA1550 based upon 30 

analyses each (a single analysis was obtained by laser fusion 

of 1-3 biotite crystals). The J's were then used to calculate 

ages for 27 laser total fusion analyses of single crystals of the 

FCTS. Using the GHC305 J, we calculate a weighted mean 

age for FCTS of 28.06 ± 0.02 Ma and using GAI550 an age of 

28.04 ± 0.02 Ma. Combined GHC305 and GAI550 indicate 

an age of 28.05 ± 0.02 Ma for the FCTS. 

lsamson, S.D. and E.C. Alexander, Jr. 1987. Chern. Geol. 

Isot. Geosci. 66: 27. 

2Deino, A. and R. Potts. 1990. J. Geophys. Res. 95: 8453. 

3Lanphere, M.A. 1990. EOS 71:1658. 
U-SERIES DATING OF SPELEOTHEMS AND 

TRAVERTINES FROM ESINO RIVER VALLEY 

(CENTRAL ITALY) AND THEIR PALEOCLIMATIC 

AND GEOMORPHIC SIGNIFICANCE. 

TADDEUCCI, A., Dipartimento di Scienze 

Geologiche, Ill Universita di Roma, Roma, 

00146, ltalia, TUCCIMEI. P., Dipartimento di 

Scienze della Terra, Universita "La Sapienza", 

Roma, 00185, ltalia, e VOLTAGGIO M., Centro di 

Studio per il Quatemario e I'Evoluzione 

Ambientale, CNR, Roma, 00185, ltalia. 

U-series ages of speleothems from "Fiume-Vento" 

cave system and of travertines associated to the 

Esino River terraces have been obtained. The cave is 

located in the Esino River drainage basin (Central 

Italy) and has been developed at seven progressively 

deeper levels in a limestone formation due to 

lowering of local base level. 

The cave floors can be correlated with the Esino 

River terraces capped with strata of travertines 

formed when the river started to downcut the 

previously deposited sediment. Dating a large 

number of speleothems from each cave floor and the 

bottom layers of travertines makes possible to 

estimate when the base level dropped down. This can 

be due to a climatic change and/or to an uplift of the 

Appennini mountain range. 

Speleothems from the first and second level 

formed less than 10,000 years BP and samples from 

the fifth level date back to 130,000 and 200,000 years 

respectively. The travertine layers covering the river 

terraces of third order started to deposit around the 

end of the Last Glaciacion and can be correlated to 

the second cave level. 

Valley floor maximum downcutting rates over the 

last 15,000 years were calculated from the age of 

speleothems and travertines. They equal to about 

0,20-0,21 m/1 ooo y. 

Speleothems and travertines have also provided 

valuable paleoclimatic information. The periods of 

most abundant growth are clearly correlated to 

interglacials and periods of no deposition correspond 

to the glaciations. 

Dating different portions of speleothems made 

possible the measurements of their radial and vertical 

accretion rate which show an increasing trend during 

the interglacial periods. Over the last glaciation the 

deposition rate remained stationary, whereas their 

decrease could be expected. Probably the lower 

amount of circulating water and the consequent lower 

concentration of dissolved carbonate were balanced 

by a longer residence time of seepage water thro"'gh 

the wall-rock due to lowering of the base level. 

313 

TOWARDS ZIRCON FISSION-TRACK 

THERMOCHRONOLOGY: THE NATURAL 

LONG-TERM ANNEALING AND FOSSIL 

PARTIAL ANNEALING ZONE AROUND A 

GRANITIC PLUTON 

TAQAMI. I, SHIMADA, C. AND NISHIMURA, S., 

Dept. of Geology and Mineralogy, Kyoto Univ ., Kyoto, 

606, Japan. 

Thermal annealing characteristics of fission tracks in zircon 

have been better understood and precisely quantified through a 

series of laboratory experiments in recent years, establishing a 

reliable basis of zircon fission-track (ZFI) thermochronology 

(e.g. Tagami et al., 1990; Yamada et al., 1993, 1994; Hasebe 

et al., 1993). To reconstruct thermal histories of rocks using 

laboratory data, however, it should be elucidated that the track 

annealing process for geologic periods of time at low 

temperature is essentially the same as that for shorter 

laboratory heating duration at higher temperature. Here we 

report the analysis of ZFT annealing patterns in and around a 

granitic pluton to investigate the natural long-term annealing 

behavior. 

The studied area is the Shimanto accretionary prism, 

Shikoku, Japan, in which several Miocene granitic bodies are 

exposed. Most of them show clear intrusion relationships, 

accompanied by the contact metamorphic aureole. We 

collected sandstones from two coherent units of Cretaceous age 

around Takatsukiyama Granite of -15 Ma, the largest body 

(-5km in diameter) in the region. Sampling was carried out 

along an eastward traverse, with variable distances of -1 to 

-5x1o4 m from the contact. 

ZFT ages and confmed track lengths were measured on 

zircon separates, focusing particularly on the transient zone of 

track retaintivity, i.e., fossil partial annealing zone (FPAZ). 

As approaching to the contact, original ZFT ages of -100 Ma 

show a drastic, continuous reduction down to -15 Ma at -3 

km distance. Their age spectra accordingly change from 

multimodal, broad distributions reflecting provenance ages to 

unimodal, peaky distributions characterizing the heating event. 

Mean track lengths and their distributions also show 

remarkable variations in FP AZ, as a consequence of mixing 

both shortened tracks to various degree by the Miocene heating 

and unannealed ones with original lengths formed thereafter. 

This track shortening pattern in nature (heating duration of 

-10Sy, estimated from thermal modeling) is comparable to 

that in laboratory. The 'gap zones' of annealed tracks are 

observed in the advanced stage of laboratory annealing, 

indicative of track segmentation process subsequent to the 

initial shrinking stage (Yamada et al., 1994). The same 

features were found in the studied samples having greater 

reduction in age within FP AZ, and this further supports the 

same annealing process. Consequently, the measured 

variations of those ZFT parameters within FP AZ, in 

conjunction with laboratory annealing data, throw lights on 

the proper thermochronologic interpretation of ZFT data of 

detrital grains in a sedimentary rock. 

Hasebe et al., 1993, Chern. Geol., llO (in press). 

Tagami et al., 1990, Chern. Geol. (lsot. Geosci. Sec.), 80: 

159-169. 

Yamada et al., 1993, Chern. Geol. (lsot. Geosci. Sec.), 104: 

251-259. 

Yamada et al., 1994, this issue. 
AGE AND PALEOMAGNETIC STUDIES FOR 

ROCK SAMPLES FROM MAHANADI AND 

GODAVARI GRABEN, EAST INDIA 

TAKlGAMI, Y.,Kanto Gakuen University, Ohta-shi, 

Gumma, 373, Japan, HIROOKA, K., Toyama University, 

Toyama-shi, Toyama, 930, Japan, SAKAI, H., Toyama 

University, Toyama-shi, Toyama, 930, Japan, Funaki, M., 

National lnsitute of Polar Reserch, and Sakai, H., Kyushu 

University, Fukuoka-shi, Fukuoka, 810, Japan 

Age and Paleomagnetic studies were performed on 

rock samples collected from Mahanadi and Godavari 

graben, East India, in order to reconstruct the position 

of Indian continent that was a part of Gondwana land 

before Paleozoic. 

40Ar-39Ar plateau ages of about 510-520,560 and 680 

Ma were obtained for biotite samples separated from 

gneiss rocks. A pyroxene sample of a char noclote rock 

and dacite dyke rock samples had excess Ar and 

isochron ages were 500 - 570 Ma. There have been no 

40Ar-39Ar age data in this area before and plateau 

ages of 500 -550 Ma are consistent with previous 

40Ar-39Ar results obtained for rock samples of East 

Antarctica. The age of about 680 Ma has been not 

reported by the 40Ar-39Ar study for the Antarctic 

samples. 

Paleomagnetic studies ( thermal demagnetization ) 

were performed for same samples. From one sample, 

good magnetic direction ( D=- 53.1. , I= 11.4° , 

a 95=9.75. ) was obtained and this data is consistent 

with previous data for other samples from Gondwana 

land at the same age. 

314 

OXYOBN ISOTOPE S1UDIBS IN HUDSON BAY AND 

HUDSON STRAIT, CANADA 

TAN. fRANCIS C .. Marine Olemistry Division, Bedfmf 

Institute of Oceanography, Dartmouth, Nova Scoda, B2Y 

4A2, Canada, and STRAIN, Peter M., Marine Oaemistry 

Division, Bedfmf Institute of Oceanography t Dartmouth, 

Nova Scotia, B2Y 4A2, Canada. 

The oxygen isotopic composition of surface water in 

arctic regions is influenced by freshwater derived from bodl 

the melting of sea ice and the input of meteoric waters (land 

runoff, precipitation and glaciai melt). A full understandina 

of the chemistry and physics of such regions requires the 

knowledge of the spatial and temporal distributions of these 

low-salinity inputs. Oxygen isotope measurements can be 

used to distinguish between sea ice meltwater and the 

combined input of_ meteoric water from the other three 

sources. 

The oxygen isotopic composition of water in Hudson Bay, 

Hudson Strait, Foxe Basin and the Labrador Shelf bas been 

determined. The following types of oxygen isoWpe-salinity 

relationships have been observed: straight lines with diffenmt 

slopes and apparent freshwater components (81'0.,..), 

horizontal lines, bilinear or two line segments, and single 

points. The oxygen isotopic composition of water in Foxe 

Basin (-2.0 ppt) is nesly constant over the whole salinity 

range (30 to 40 psu). The apparent freshwater ccmponent 

(81'0..,) for three stations bas a mean value of 0 pJt 

indicating that sea ice meltwater is the dominant freshwater 

component. Similar oxygen isotope-salinity relationships are 

found in northwestern Hudson Bay-strait. Surface waters in 

northern Hudson Bay exhibit the most negative 11'0 value (-

4.0 ppt) and lowest salinity (29 psu) for the entire studied 

area suggesting the large contribution of river nmoft'. The 

apparent freshwater component (81'0...> ranges from 0 to -26 

ppt from Hudson Bay to the Labrador Shelf. 'l'he 

significance of this variability will be discussed. 
DESERT IS A POTENTIAL SINK OF C02 : 

ISOTOPIC EVIDENCE 

TANAKA, T., INAYAMA, E. & ASAHARA, Y. 

Dept. of Earth and Planetary Sciences, 

School of Science, Nagoya University, Chikusa, Nagoya 464-01, JAPAN 

Carbonate carbon in desert sand was 

examined in respect of its abundance, 87 Sr/ 86 Sr ratio and 14C concentration. 

18 samples of desert sand were collected 

from various deserts and semi-desert areas 

in the world . Samples were mostly collected from the edge of the deserts. 

More than 90% of the sand passed through 

an 80 mesh (160 11m) sieve and grains finer than the mesh were used for 

analysis. Samples were decomposed with 

concentrated phosphoric acid in a vacuum 

system, and volume of C02 gas recovered 

was measured after separation of H2 0. 

The C02 content in the sands was found to 

be 0.0"' 17.0 wt.% as carbonate. C0 2 tends 

to be concentrated more in sand of older 

deserts than in sand of younger ones. 

This indicates that the carbonate 

accumulated with age in desert sand beca~s~ of.little weathering caused by few 

precipitation. 87 Sr/ 86 Sr ratio was measured for the 

carbonate and its host sand. The ratios 

of the carbonate vary from 0.708 to 0.712, which are somewhat higher than those of 

marine carbonate in any geologic time. Low 

Rb/Sr ratios of the desert carbonate do 

not allow to accumulate the large amount 

of radiogenic 87 Sr in geologic time. 

Significant parts of the carbonate in 

desert are considered to originated in 

source different from limestone which was 

formed in ancient marine environments. 14C/ 13 C ratios of the carbonate were 

measured using a tandetron accelerator 

mass spectrometer at Nagoya University. The obtained ratios are distributed 

between 0. 055± 0. 002 and 0. 024± 0. 002 

relative to those of AD 1950. The ratios 

correspond to ages 24000± 300 and 

31000:£500 yBP, respectively. This 

indicates that all of the carbonate is not 

derived from detrital limestone formed in 

geologic time but some parts of the 

carbonate have been formed during historic 

time in desert. 

Average C02 content in the sand was 6.8 

wt% as carbonate(CaC03 ). When we assume 

that the area of desert, thickness of 

desert sand, density of desert sand are 

8000,000km2 , 10m ana 2g/~m \ respe~tively, 

the total amount of CO In aesert IS 

calculated to be 0.5x ,019 grams. The 

amount exceeds the carbon abundance in the 

worldwide biomass which is estimated to be 

0.55x 1018 grams as carbon. 

Desert stocks a large amount of C02 , 

because of its unique features such as few 

precipitation and quick evaporation. Desert is not a useless place as generally considered but is the potential sink of 

C02 • 

Samples are donated by Naoyuki Ando, Takemasa Ishii and Kiyohide Mizuno of 

Geological Survey of Japan and Liu Peijin of Academia Sinica and Shimizu 

Corporation. 14C/ 13C ratios were 

determined under the leading of Toshio 

Nakamura of Nagoya University. 

315 

VERTICAL AND LATERAL CONTRffiUTIONS TO 

CRUSTAL GROWTH: GEOCHEMICAL AND TECTONIC 

ASPECTS 

TARNEY John and JONES, C.E., Dept. Geology, 

University of Leicester, Leicester LEI 7RH, UK 

The composition of the Earth's continental crust differs 

from that of other planetary crusts, and is consistent both with 

relatively small degrees of melting and with the influence of 

hydrous fluids. Variations in the chemistry of crustal 

components with time primarily reflect differences in the 

thermal regime associated with subduction processes, but there 

are second order effects. Recent advances in modelling the 

thermal structure of subduction zones have helped constrain the 

conditions under which the slab melts, and have generally 

enhanced the role of the mantle wedge in magmagenesis and 

the importance of induced convection in replenishing the zone 

of magma generation. Mantle plumes may also have an 

important influence on crust generation, both directly and 

indirectly. 

Extensive and continued melting of the subducted slab to 

produce voluminous sodic tonalites and trondhjemites is only 

thought possible under Archaean thermal conditions when 

warm hydrous crust subsides into hot mantle and melting is 

controlled by hornblende and garnet equilibria. Arguably, 

similar conditions pertain - at least for a short time - at the 

present day wherever there is ridge subduction. However, such 

melts share trace element and isotopic characteristics with 

wedge-derived melts initiated by pressure-controlled 

breakdown of hornblende where there is induced convection in 

the mantle wedge. Where thermal gradients are cooler, with 

thicker lithosphere, more mature arcs, cooler slab, etc., melting 

compositions will be controlled by phlogopite. A sUIVey of the 

compositions of orogenic granitoids and volcanics shows that 

there are some consistent (but complementary) trace element 

differences which may reflect this mineralogical control of 

parental magmas at source. Extra thermal input from mantle 

plumes may change the thermal structure of the lithosphere and 

hence change the mineralogical controls on magmagenesis. 

As continental growth progresses through vertical 

magmatic accretion, there is increasing crustal development 

through lateral subduction-accretion, particularly where major 

river systems have discharged at active margins. Here oceanic 

components, ocean islands, arcs, plateaus, abyssal oozes, etc., 

are tectonically intermixed with continent-derived upper crustal 

sediment, often gneissified, and suffer melting during 

exhumation, thermal relaxation or active extension. The subcontinental lithosphere may become heavily contaminated with 

subducted sediment. Such regions provide a mixture of S- and 

1-type granitoids with varying proportions of "mantle" and 

"crust" components; this type of crustal development is 

particularly common in the Phanerozoic and can be recognised 

both chemically and isotopically. However, where continental 

breakup produces new cordilleran margins, as in the Andes, the 

thermal structure of the leading edge changes to favour mantle 

wedge melting, mostly with hornblende control on magma 

chemistry and a reducing sediment/crustal involvement. 

These models favour continual growth of continental crust, 

but with changing mechanisms of crust generation with time. 
SR AND PB ISOTOPE EVIDENCE FOR THE ORIGIN OF 

SKARN, SULFIDES AND FLUOR MINERALIZATIONS 

OF THE ITAOCA GRANITOID,SOUTHEASTERN BRAZIL. 

TASSINARI. C. C. G. Centro de Pesquisas Geocronologicas, 

Universidade de Sao Paulo,Sao Paulo, Po.Box 20899, 

Brazil; MELLO, I. S.C., Institute de Pesquisas Tecnol6gicas 

do Estado de Sao Paulo, Brazil; GOMES, D.P., CNPq, 

Brazil. 

Several mineralizations are associated with the post -tectonic 

Itaoca granitoid which is intruded into low-grade late 

Proterozoic metassedimentary sequences, in Ribeira region 

southeastern Brazil. 

The granitoid comprises monzogranites, quartz-syenites and 

granites corresponding to a high-potassium peraluminous 

calc-alkaline trend. The sedimentation of the host 

metamorphic sequences comprising terrigenous and carbonatic 

sediments developed during the time period 1.8 to 1.5 Ga. ago. 

The skarn type mineralizations are associated with 

metassedimentary roof- pendant inside the granitoid., which 

includ schists, pellitic hornfels and marbles. They are 

characterized by endoskarn, garnet-pyroxene-skarn and 

garnet-wollastonite-skarn. Scheelite-Powellite and 

wollastonite are the ore minerals Fluorite and sulfide 

mineralizations associated to the quartz-veins in fractures or 

shear zones in the granitoid also occur. 

Sr and Pb isotopic analyses were carried out on ore minerals 

and the host rocks in order to understand the petrogenesis of 

the mineralizations. 87Sr/86Sr isotopic ratios of the calcite in 

the skarns (0. 7103 to 0. 71 09) display values very similar with 

those obtained for calcites from (host) marbles (0.7100). 

However these values are slightly higher in comparison with 

the wollastonites(0.7092 to 0.7109). 

The Itaoca granitoid yielded a Rb-Sr whole rock isochron 

age of676+/-25 Ma, with Sr initial ratio of0.7097 +/- 0.0001 

which is in close agreement with the Sr isotopic initial 

composition of the skarn mineralizations. This fact suggests 

that the Sr was derived from the granitoid intrusion 

responsible for the origin of the skarn. Thus we infer a 

magmatic origin for the mineralization. 

Tardi and/or post-magmatic processes, such as albitization 

and greisenization probably related to the skarn genesis, took 

place in the central part of the granitic body, around 620 Ma 

ago, as supported by a Rb-Sr preliminary isochron. Therefore 

we assume these processes as contemporaneous with the skarn 

mineralization. 

The Fluorite veins have 87Sr/86Sr isotopic composition of 

0.7114, so higher than that of the skarn fluids. In addition, 

considering that the host metassedimentary sequences include 

abundant fluorite mineralizations, we consider these 

supracrustals as the source for the Fluorite. 

The galenas from the sulfide- bearing quartz veins have 

homogeneous Pb isotopic compositions values, and yielded 

Stacey-Krammers model ages of 1.4 Ga. This age and the 

corresponding isotopic compositions are in agreement with 

previous data on Pb-Ag mineralizations in the same host 

metassedimentary sequence. Thus the Pb of the galena inside 

the granitoid was leached from this sequence by exotic fluids 

of metamorphic origin and/or meteoric water. 

316 

ORIGIN OF HAWAIIAN BASALTS INFERRED FROM 

ISOTOPES . TATSUMOTO, M., U.S. Geological Survey, Box 25046, 

MS 963, Denver, CO 80225 

Our group at USGS, Denver has undertaken studies of Nd, 

Sr, and Pb isotopic compositions of basalts from EmperorHawaiian volcanoes e.g. (1,2,3) and of xenoliths in the basalts 

(4) in order to obtain isotopic constraints on their origin. 

Hawaiian volcanoes evolve similarly and can be subdivided into 

three main stages: shield building tholeiitic stage, caldera-filling 

alkalic stage, and posterosional (PE) nepherinitic stage. Our 

isotopic studies lead us to propose a three component mixing 

model for the sources: a mantle plume (Koolau end member), 

the lithosphere (Kea end member), and a depleted asthenosphere 

(posterosional end member). 

Hawaiian tholeiites can be explained by mixing of a plume 

with the sea water-altered lithosphere. The isotopic 

compositions of PE alkaline basalts are distinct from those of 

Koolau shield tholeiites and MORB, and are considered to have 

originated from the depleted asthP"'"~phere. High trace element 

concentrations in PE basalts are thought to be derived from 

metasomatic fluids from plumes added just prior to the 

eruption. 

Isotopic compositions of xenoliths in the Honolulu series 

are, however, similar to those of the host PE magma, 

indicating that they are genetically related and PE basalts 

originated from the deep lithosphere, probably the lithosphereasthenosphere boundary. The isotopic relationships of Nd and 

Pb may be generated -80 Ma which corresponds to the age of 

the lithosphere at Hawaii. The initial ENd = -8.5 for the 

xenoliths, which is slightly lower than that for the East Pacific 

MORB, suggests a possibility that the xenoliths and the host 

magma originated from a body that had intermediate 

characteristics and intruded into the lithosphere at or near the 

East Pacific Rise. Such basalts with intermediate characters are 

observed at "off-ridge" seamounts. 

We propose that the lithosphere, containing intermediatecomposition bodies, moves and approaches the Hawaiian hot 

spot resulting in the eruption of PE basalts. If such bodies do 

not occur close to a volcano, then PE basalts do not erupt. 

(1) Tatsumoto, M. 1978, Isotopic composition of lead and its 

implication to mantle evolution: Earth. Planet Sci. Lett 38, 

63-87. 

(2) Stille, P., Unruh, D. M. and Tatsumoto, M., 1986, Pb, Sr, 

Nd, and Hf isotopic constraints on the origin of Hawaiian 

basalts and evidence for a unique mantle source: Geochim. 

Cosmochim. Acta 50,2302-2319. 

(3) Hegner, E., Tatsumoto, M., and Unruh, D. M., 1986, Sr, 

Nd, and Pb isotopic evidence for origin of the West Maui 

volcanic suit, Hawaii: Nature 319,478-480. 

(4) Okano, 0., Tatsumoto, M., and Leemen, W. P., 1987, Sr, 

Nd, and Pb isotopes in Hawaiian basalt xenoliths: inferences 

for the origin of Hawaiian basalts: EOS, 68, 1521. 
ISOTOPE GEOCHEMISTRY OF AXIAL VOLCANIC 

CONSTRUCTS ALONG THE REYKJANES RIDGE 

TAYLOR. Rex N., Department of Geology, Royal 

Holloway and Bedford New College, University of 

London, Egham, Surrey, TW20 OEX I Department of 

Geology, University of Southampton, Highfield 

Southampton, S09 5NH, UK., THIRLWALL, Matthew 

F., Department ofGeology, Royal Holloway and Bedford 

New College, University of London, Egham, 

Surrey,TW~O OEX, UK., and MURTON, Bramley J. 

Institute of Oceanographic Sciences Deacon Laboratory, 

Brook Road, Wormley, Godalming, Surrey, UK. 

Recent bathymetric surveys of the Reykjanes Ridge 

between 57°N and 63°N have shown that the spreading axis 

is composed of a series of en echelon axial volcanic ridges 

(AVR's). Using this bathymetry, a detailed sampling 

program was undertaken to investigate the geochemical 

variation within and between these constructs. Sampling was 

performed using dredge and rock-chipper deployed from the 

RRS Charles Darwin. To increase the precision of sample 

location, the bottom-time for each dredge was kept to a 

minimum ( <20mins). As a result samples could be located 

within a =300m radius of the station. 

Major and trace element data and Nd, Sr and Ph isotope 

ratios are presented from AVR's within the southern section 

of the Reykjanes ridge (57°N to 59°N). Data are organised 

relative to location within each A VR (tip, centre or flank) 

and with respect to the apparent age of the volcanic 

construct. Significant variation in La/Sm exists within each 

AVR at an essentially constant isotopic composition. This 

probably reflects a variation in melting parameters within the 

mantle diapir, instead of representing variable mixing 

proportions between M ORB asthenosphere and Iceland 

plume. Ph isotope data do reflect such mixing, but the 

~b~Pb and ~b~Pb systematics indicate that mixing is 

not a simple north-south increase in the magnitude of the 

asthenospheric component. Both the plume and the 

asthenospheric component are required to change in Ph 

isotopic composition along the ridge. 

317 

SR AND NO-ISOTOPES OF THE DECCAN MANTLE 

PLUME: EVIDENCE FROM THE EARLY ALKALI 

IGNEOUS COMPLEXES OF MUNDWARA AND 

SARNU, RAJASTHAN. 

TEICHMANN, P.R., AND BASU, A.R., Dept. of Earth 

and Environmental Sciences, University of Rochester, 

Rochester, New York 14627, USA. 

We have recently documented (Basu et al., 1993) a high3He plume origin for the 68.5 Ma-old alkali igneous 

complexes of Sarnu and Mundwara, Rajasthan, which 

preceded the bulk of the Deccan volcanism to the south that 

erupted precisely at 65 Ma, the K-T boundary. Here, we 

report the Sr and Nd-isotopic characteristics and the trace 

element geochemistry of the different lithologic units of these 

igneous complexes. 

Alkalic ultramafic, mafic and syenitic lithologies, which 

comprise the major portions of the different hypabyssal 

intrusives, show tREE-enrichment from 20 to 1000 times 

chondrite in Ce. The relatively undifferentiated lithologies, 

such as picrites and gabbros, show also less tREEenrichment. However, the latter group show a very 

restricted ENd and Sr-isotopic ratios, mostly varying between 

E d = + 1.3 to +4.0 and 87Sr/86Sr = 0.70445 to 0.70490. 

~ese isotopic characteristics are remarkably similar to the 

Reunion mantle plume and to the least-differentiated 

Ambeali tholeiitic formations of the Deccan Traps. These 

Nd and Sr-isotopic signatures of the early alkali igneous 

pulses of the Deccan plume thus can be considered to 

represent the lower mantle source before its further 

modification by interactions with the Indian continental 

lithosphere. The high 3

He/4He ratios of 14 RAIR in these 

complexes also strongly support this Nd, Sr-isotopic 

characterization of the Deccan mantle plume. 

Basu, A.R. et al., 1993, Early and Late Alkali Igneous 

Pulses and a High-3He Plume Origin for the Deccan Flood 

Basalts, Science, v. 261, p. 902-906. 
40AR-39AR AND RB-SR GEOCHRONOLOGY OF THE 

PROTEROZOIC URUGUAY AN DIKE SWARM 

(URUGUAY), SOUTH AMERICA: TECTONIC 

SIGNIFICANCE. 

TEIXEIRA. W., CPGeo- Instituto de Geociencias, 

University of Sao Paulo, Sao Paulo, 05508-900, Brazil, 

RENNE, Paul, R., Institute of Human Origins, CA 94709, 

USA, BOSSI, J. and CAMPAL, N., Faculdad de 

Agronomia, Universidad de La Republica, Uruguay. 

The Uruguayan dike swarm in the Rio de La Plata Craton 

has been studied by means of 40Ar-39Ar determinations (one 

hornblende separate and 6 single crystals of biotite and 

muscovite) and Rb-Sr geochronology (7 internal isochrons on 

mineral concentrates and whole rocks). 40Ar-39Ar plateau 

ages (up to 101 steps in one case) on a coarse grained enclave 

(micropegmatite) in the mafic dikes, and felsic veins (located 

at the borders of the dikes) are concentrated in the range 

1,728-1,713 Ma, although sligthly younger pseudo-plateaus 

ages (defined by a fraction of the 39Ar released in the 

spectrum) were also obtained (1,700 - 1,687 Ma). 

Based on concordance of the hornblende and biotite plateau 

ages from the micropegmatite investigated (respectively 1,728 

± 2 Ma and 1,725 ± 2 Ma) it is proposed that it cooled rapidly 

and that its crystallization age is therefore estimated by the 

hornblende date. This age is considered as representing the 

emplacement time for the dike itself, and by extrapolation the 

best estimative for the age of the Uruguayan dike swarm. One 

Rb-Sr internal errorchron age of 1,779 ± 87 Ma (87Sr/86Sr = 

0.705) obtained for another dike is also consistent (considering 

the error) with the range of the 40Ar-39Ar ages obtained. 

Correlation of these data with dated Precambrian mafic 

dike swarms, contemporaneous granitoids, and characteristic 

geological features of correlative crustal blocks (with similar 

tectonic history) in southernmost South America and South 

Africa, suggest that the Uruguayan diking event was coeval 

and genetically associated with the origin of a continental rift 

which took place in Western Gondwanaland, after the stability 

of the Transamazonian cycle (2.2- 2.0 Ga). This igneous 

event is considered as anorogenic in character, which is 

consistent with the crustal history of the country rocks of the 

Craton and geological features characteristic of the dikes 

studied (e.g., igneous fabric, net contacts with host rocks}, 

demonstrating the emplacement of the swarm into a previously 

cooled, rigid crust. 

Detailed examination of the structure of the Ar spectra 

obtained (firsts steps of the plateaus; e.g. up to 4% of 39Ar 

released) plus interpretation of isotope correlation diagrams, 

when compared with most of the new Rb-Sr internal isochron 

ages (1.4 - 1.0 Ga and- 0.56 Ga) on the dikes and its felsic 

veins, together with the published K-Ar apparent ages on the 

dikes (1.6 - 1.4 Ga) indicate that the whole swarm was 

overprinted by differential, low temperature episodes long 

after its formation. This strong variation in the apparent ages 

obtained for both the Rb-Sr and K-Ar systems are interpreted 

as tectonically influenced by the tectonomagmatic activity 

characteristic of the Namaqualand (1.4-1.0 Ga), in South 

Africa, and further on by the Brasiliano events (Upper 

Proterozoic). Such events are well recognized in the tectonic 

evolution of the Gondwanaland. 

318 

THE POL YGENETIC NATURE OF THE BASEMENT 

ROCKS OF MALAITA AND SANTA ISABEL, SOL6MON 

ISLANDS (S. MARGIN OF ONTONG-JAVA PLATEAU) 

TEJADA. M.L.G .. MAHONEY, J.J., SPENCER, K.J., 

SOEST, Honolulu, HI, 96822, USA, and HAWKINS, 

M.P., British Geological Survey, Vanuatu 

Isotopic and chemical studies show that the igneous 

basements ofMalaita and Santa Isabel consist of 

petrogenetically unrelated rocks: a) Older Series Tholeiites 

and Sigana Tholeiites; b) Malaita and Sigana Alkalic Suites; 

and c) San Jorge Volcanics. The Early Cretaceous Older 

Series (Malaita) and Sigana Tholeiites (Santa Isabel) define a 

limited range of isotopic compositions despite the wide (>200 

km) spatial distribution of samples: sNd(T) = +3.4 to +6.6; 

(87Srf86Srh = 0.70369 to 0.70423; 206Pbf2°4Pb =18.245 to 

18.644. These values are remarkably similar to those of the 

contemporaneous Ontong-Java Plateau (OJP) lavas at ODP 

and DSDP Sites 807, 803, and 289 1 , located 1200-1700 km 

to the north; the combined results indicate a rather uniform 

hotspot-like mantle source for the world's largest oceanic 

plateau. As with the ODP lavas, rare-earth-element inversion 

modeling supports a plume-like source and suggests that the 

rocks are products of high degrees of partial melting (-30%) 

at relatively shallow average depths. 

Isotopic data also reveal that the two younger alkalic suites 

present on Malaita (the Younger Series and the North 

Malaita Alkalic Basalts) are distinct from each other, and 

differ from those of Santa Isabel (Sigana Alkalic Suite), 

indicating derivation from three different sources. The 

Younger Series (-45 Ma; R.A. Duncan, unpubl. data) have 

compositions overlapping the field for the primitive(?) mantle 

end-member postulated by Farley et al. 2 for Samoan lavas, 

consistent with the passage of the OJP over this hotspot 

during migration to its present location 3. In contrast, the 

North Malaita Alkalic Basalts have isotopic compositions 

near the OJP fields (e.g. sNd(T) = +4.5-4.7, (87Srf86Srh 

=0.70367-0.70370, and 206Pbf2°4Pb =18.754-18.826}, 

suggesting a possible petrogenetic link. The Sigana Alkalic 

Suite have isotopic compositions similar to "HIMU-type" 

Tubuaii and Mangaia lavas 4 , but plate tectonic 

reconstructions 3 do not show the OJP to have passed 

anywhere near present "HIMU-type" hotspots at the inferred 

time of eruption for these basalts (-65 to 45 Ma). 

The San Jorge Volcanics, which crop out to the south and 

west of the Sigana Tholeiites, are composed ofMORB-like 

lavas and lavas with arc-like isotopic and chemical 

compositions, indicating that the San Jorge Suite is not part 

of the OJP. Some San Jorge Volcanics could represent 

abyssal sea-floor abducted onto the leading edge of the OJP 

during its emplacement against the old Solomons Trench, and 

some could represent crust formed at an arc or back-arc 

spreading system that was trapped during the collision. 

1Maboney et al., AGU Monogr., 77, 233, 1993. 

2Farley et al. EPSL, ill,183, 1992. 

3Yan and Kroenke, Proc. ODP Sci. Res., 130, 697, 1993. 

4 Vidal, et al., Nature, 307, 536, 1984; Palacz and Saunders, 

EPSL, 79,270, 1986. 
ISOTOPIC ZONATION IN A PLAGIOCLASE 

XENOCRYST: PROGRESS TOWARDS CONSTRAINING 

TIME SCALES OF MAGMATIC PROCESSES. 

Te.pley. F J .. Davidson, J.P. and Holden, P., Dept. of Earth 

and Space Sciences, UCLA, Los Angeles, CA 90024, USA 

Micro drilling of a ~lagioclase xenocryst has revealed a 

significant range in 8 Sr/86sr ratios across the crystal and a 

large difference from its basaltic andesite. The xenocryst 

analyzed is believed to be derived from a local ignimbrite, and 

is currently hosted in a basaltic andesite magmatic inclusion 

from the dacite dome of Cerro Chascon, north Chile. These 

rocks show typical magma mingling textures. 

Micro drill sampling has enabled us to examine the scale of 

mixing and chemical (isotopic and elemental) disequilibrium. 

Whereas previous results from this crystal gave a single 

87srt86sr ratio of 0.708915 ± 37 (Davidson, et al, 1990), the 

results of this study demonstrate that across-crystal variations 

exist and are measurable, ranging from 0.709206 ± 13 near the 

core to 0.708502 ± 14 near the rim (see figure). A chemistry 

Sr blank of 100 pg was measured, amounting to less than 

0.5% of the Sr analyzed in the micro samples which weighed 

from 0.1 to 0.5 mg. Sr concentrations vary from c.500 to 

c.100 ppm, seemingly unsystematic with respect to location 

and isotope ratio. An contents are rather constant across the 

entire crystal (An57 to An64). The variations in isotope ratio 

are interpreted to reflect an approach to equilibrium through 

tracer diffusion at magmatic temperatures. The lack of Anx 

zonation reflects the slower diffusion rate controlling coupled 

CaAl-NaSi exchange. Diffusion was effectively arrested when 

the lava was extruded and chilled. Knowledge of appropriate 

isotope diffusion coefficients for Sr will enable us to estimate 

the time between incorporation of the xenocryst and eruption. 

Plagioclase Xenocryst 

Davidson, J.P., de Silva, S.L., Holden. P., & Halliday, A.N. 

1990, Small-scale disequilibrium in a magmatic inclusion 

and its more silicic host: J. Geophys. Res. 95, 17,661-

17,674. 

319 

WOOD IS NOT A LEAF: BIOLOGICAL 

CONSIDERATIONS WHEN INTERPRETING 

CLIMATE FROM 8D OF CELLULOSE 

NITRATE 

TERWILLIGER, V. J., Depts of 

Geography and Botany, University 

of Kansas, Lawrence, KS, 66045, 

USA and M. J. DeNiro, Dept of 

Geological Sciences, University 

of California, Santa Barbara, CA, 

93106, USA. 

The 8D of cellulose nitrate in 

tree rings should carry the 

climatic signature of water used in 

its synthesis. Poor understanding 

of hydrogen isotope effects that 

occur in the synthesis of cellulose 

prevents knowing if groundwater or 

leaf water produces the hydrogen 

isotope signature in wood 

cellulose. An isotopic signature 

from groundwater would reflect 

temperature whereas both 

temperature and humidity inferences 

could be derived from a leaf water 

signature. 

To elucidate the source of the 

hydrogen isotopic signal in wood 

cellulose, we grew avocados in 

isotopically constant water but at 

different humidities. The 8D of 

leaf cellulose nitrate was very 

significantly related to that of 

leaf water whereas the 8D of wood 

cellulose was related to that of 

stem water. There was some mixing 

of leaf and growth waters at the 

site of wood formation but humidity 

signals were virtually absent. The 

quantitative natures of the 

relationships between the 8D values 

of celluloses and waters suggested 

that the substrata for cellulose 

synthesis were partly from stored 

post-photosynthetic compounds. The 

contributions of these compounds to 

the 8D of celluloses were different 

from the effects of the immediate 

products of photosynthesis. 

Furthermore, proportionally 

different contributions of 

immediate and recycled substrata 

may occur in leaves than in wood 

cellulose. Conditions that evoke 

use of stored substrata are varied 

for trees and would contaminate 

climatic inferences from stable 

hydrogen analyses. 
A MONAZITE U-PB AGE PROFILE THROUGH THE 

LOWER AND MIDDLE CRUST: A CROSS SECTION 

THROUGH THE IVREA ZONE, NORTHERN ITALY 

TEUFEL S., GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, FRG, FRANZ, L., GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, FRG, 

Ahrendt, H .• Institut filr Geologie und Dynamik der LithosphHre, Goldschmidtstr. 3. D-37077 G6ttingen, FRG and 

HANSEN, B.T., Institut fUr Geologie und Dynamik der 

LithosphHre, Goldschmidtstr. 3, D-37077 G6ttingen, FRG 

We present preliminary results of a study about the behaviour 

of the U-Pb system in monazite during high and medium grade 

metamorphism. We report geochronological and thermobarometric data of samples collected along the Val Strona across the 

boundary between the Ivrea and Strona Ceneri Zone. Along this 

proftle metamorphic conditions change eastward from granulite 

facies (near the Insubric Line) to amphibolite facies (near 

Omegna). Monazites were extracted from ten metasediments 

and one augengneiss from both metamorphic units. 

So far, only two monazite ages of 295 Main the upper part 

and 275 Ma in the transition zone between amphibolite facies 

and granulite facies rocks are known (KOppel 1914). Age determinations on monazite samples of about 2 mg weight have 

been carried out at the Zentrallabor flir Geochronologie in 

MUnster. FRG. 

Within analytical error all analyses yield concordant hercynian 

U-Pb ages except two data points of a granulite facies sample. 

These points plot above the concordia, indicating a complex 

disturbance of the U-Pb system during post-hercynian times. 

U-Pb ages of the other monazites from granulite facies rocks 

lie in the range of 275- 285 Ma while monazite analyses from 

amphibolite facies rocks give ages of 287 - 295 Ma. For one of 

these rocks we obtained a concordant U-Pb monazite age of 265 

Ma showing a rejuvenation. Regarding the whole profile it is 

obvious that U-Pb ages correlate well with the degree of regional metamorphism. They decrease continuously from 295 Ma 

in amphibolite facies rocks near Omegna to 275 Main granulite facies samples near the Insubric Line. From these data no 

discontinuity between Ivrea and Strona Ceneri Zone can be 

detected. Our thermobarometric investigations revealed peak 

metamorphic conditions of 8.3 ±1 kb and 810 ±25 °C for granulite facies rocks continuously decreasing to amphibolite 

facies. All samples plot along a geothermal gradient of 26-30 

°C/km. Clockwise PT-t paths mainly reveal cooling in the granulite facies whereas the prograde metamorphic history is often 

documented within the amphibolite facies samples. 

The strong correlation between increase in metamorphic grade 

and decrease of the U-Pb ages leads to the following interpretation. During hercynian orogeny closing temperature of the UPb system in monazites were first understepped within the 

amphibolite facies rocks and distinctly later within the high 

grade series. Therefore the timing of regional metamorphism in 

the Ivrea Zone is best substantiated by concordant U-Pb ages of 

295 Ma within medium grade rocks. 

K6ppel, V •• 1974, Isotopic U-Pb Ages of Monazites and Zircons from the Crust-Mantle Transition and Adjacent Units of 

the Ivrea and Ceneri Zones (Southern Alps, Italy): Contr. 

Mineral. and Petrol., v.43, p.55-70. 

320 

Nd ISOTOPIC CONSTRAINTS ON THE EVOLUTION 

OF PALEOPRO'IEROZOIC SUPRACRUSTAL 

SEQUENCES AT THE NAIN-RAE PROVINCE 

BOUNDARY, EASTERN CANADA 

THERIAULT. R.I.. and HAMILTON, M.A,Continental 

Geoscience Division, Geological Survey of Canada, 

Ottawa, Ontario, Canada, KIA OES. 

The Nain /Rae Province boundaly zone is represented by 

the north-trending c:a. 1.8S Ga Torngat orogen of 

Labrador, eastern Canada. Paleoproterozoic supracrustal 

rocks spatially associated with the Torngat orogen consist 

of: 1) Mugford Group: relatively undeformed mafic 

continental volcanic rocks and lesser terrigenous clastic 

sediments unconformably overlying western Nain Province 

gneisses, 2) Ramah Group: siliciclastic platformal 

sediments at the western margin of the Nain Province, 3) 

Tasiuyak gneiss: migmatitic garnetiferous 

quartzofeldspathic paragneiss at the core of the Tomgat 

orogen, and 4) Lac Lomier paragneisses: heterogeneous 

shallow- and deep-water metasediments which, with 

Tasiuyak metasediments, have been suggested as potential 

correlative continental shelf facies units on the Rae 

margin. 

Weakly alkalic and tholeiitic basalts of the ca. 2.0 Ga 

Mugford Group have ~(1.8S Ga) from -K>.S to -6.6, 

indicating finite contnbutions from Nain Province 

continental lithosphere. The Ramah Group shows ~(1.8S 

Ga) ranging from -8.4 to -12.7. These values overlap and 

are slightly more positive than those of central Nain 

Province gneisses (~d(1.8S Ga) ranging from -9 to -20; 

Hamilton, 1993) and more negative than Rae Province 

orthogneisses west of the Torngat orogen (~d(1.8S Ga) 

from -4.6 to -9). Ramah Group data are consistent with 

mixing of crust similar to central Nain Province 

orthogneisses and more juvenile LREE-enriched crust such 

as Mugford Group basalts and tuft's. Lac Lomier 

metapelites yield ~d(1.8S Ga) values ranging from -S.l to 

-7.8, overlapping with those of Rae Province crust. The 

Lac Lomier data display a linear trend on an fsm/Nd vs ~d 

(1.8S Ga) diagram, consistent with derivation from a Late 

Archean LREE-enriched source such as Rae crust. The 

Tasiuyak gneiss gives ~d(l.8S Ga) values clustering in the 

-2.1 to -3.1 range. These values are distinctly more positive 

than those of Ramah Group and Lac Lomier paragneisses, 

as well as those of Rae and Nain Province crust. This 

implies that a juvenile component not identified elsewhere 

in the study area is present in the Tasiuyak gneiss. The 

isotopic homogeneity displayed by Tasiuyak materials may 

echo homogeneity of source material or thorough mixing 

of heterogeneous detritus prior to deposition. The soUrce 

terrane(s) providing the Tasiuyak gneiss with material of 

relatively juvenile character remains enigmatic. 

Hamilton, M.A., 1993, Ph.D. thesis, University of 

Massachussetts. 
CHEMICALLY PRODUCED MASS 

INDEPENDENT ISOTOPIC FRACTIONATIONS 

IN THE EARLY SOLAR SYSTEM NEW 

OBSERVATIONS 

miEMENS. M H, Dept. of Chemistry, University 

of California, San Diego, La Jolla, CA. 92093-0356, 

USA. 

Thiemens and Heidenreich Ill (1983) were the ftrst to 

experimentally demonstrate a chemically produced mass 

independent isotopic process. Perhaps the most striking 

feature was that the observed a170=a18o fractionation 

pattern was essentially identical to that observed by 

Clayton et al (1973) in Allende inclusions. It thus is 

possible that the meteoritic oxygen isotopic anomalies 

arise from chemical rather than nuclear processes. Since 

oxygen is the major element in stony planetary material 

this must reflect a major process in the formation of the 

solar system. 

The mechanism by which the mass independent 

fractionation occurs is now known to derive from 

molecular symmetry factors which occur during a 

reaction; thus the process is rather general. More 

recently, it has been demonstrated by this laboratory that 

several of the earth's most important molecules possess 

large mass independent isotoptic compositions. In fact, 

as is the case for the meteorites, these anomalous 

isotopic compositions have provided new insights into 

several atmospheric processes in the stratosphere and 

troposphere. It has also been demonstrated that there are 

several types of physical chemical processes which 

produce mass independent isotoptic fractionations, and 

in at least two different elements. 

Most recently it has been shown that in a gas phase 

reaction process a solid silicon oxide condensate with a 

16o rich mass independent isotopic composition arises. 

This further increases the plausibility that chemical 

processes may be the source of the observed meteoritic 

oxygen isotopic anomalies. There are several other 

recent advances in the study of chemically produced 

mass independent isotopic fractionation's as they relate 

to the early solar system. These observations will be 

discussed. 

Clayton, R.N. et al., 1973 Science, v. 182, p. 485. 

Thiemens, M.H. and Heidenreich, III, J.E. 1983, 

Science, 219, v. 1073. 

321 

CRUSTAL INTERACTION DURING CONSTRUCTION OF 

OCEAN ISLANDS Pb-Sr-Nd-0 ISOTOPE 

STRATIGRAPHY OF GRAN CANARIA SHIELD BASALTS 

M.P. THIRLWALL, C. JENKINS, P.Z. VROON & 

D.P. MATTEY, Geology, Royal Holloway Univ. 

London, Egham TW20 OEX, UK. 

The largest eruption rates and greatest eruptive volumes in the 

oceanic island of Gran Canaria took place at the beginning of 

the Miocene volcanic cycle, when at least lOOOm of mildly 

alkalic basalts were erupted, apparently over an interval as short 

as 1.5Ma. Five vertical sequences of basalts have been sampled, 

four ca. 1 Okm apart in the NW of the island (Tazartico, San 

Nicolas, El Risco and Agaete from S to N), and one 30km away 

in theSE of the island (Angosturas). The basalts show coherent 

major and trace element variation trends reflecting fractional 

crystallisation of olivine, augite, and, in samples with <6% 

MgO, plagioclase. Basalts with >8% MgO are restricted to the 

lower parts of 3 sequences, indicating that residence time in 

magma chambers increased as the basaltic shield(s) grew. Zr/Nb 

increases upward in all sequences, probably implying that the 

degree of melting also increased as the shield(s) developed. 

Zr/Nb ranges are different in each section, suggesting that the El 

Risco section may be equivalent to the upper parts of the Agaete 

and Tazartico sections, while the San Nicolas section may be 

equivalent to the lower parts. 

Radiogenic isotope ratios show limited variation (initial 

87srt86sr = 0.70320-0.70390, ENd= +6.0 to +4.3, 

206pbf204pb = 19.35-19.79), but are strongly controlled by 

stratigraphy. Almost all of the variation is displayed by the 

lower parts of the Agaete, Tazartico and San Nicolas sections, 

each of which shows tight ( <0.1 ENd-unit vertical scatter) 

parallel Sr-Nd correlations. In each sequence, younger rocks 

show less extensive Sr-Nd arrays, parallel and offset to higher 

87 Srt86sr and higher ENd than the older rocks. The upper part 

of the Agaete section is identical to the whole El Risco section, 

with 87srt86sr = 0.70333-0.70340 and ENd= 6.0 to 5.6, 

confirming the stratigraphic correlations suggested by Zr/Nb 

ratios. The section in the SE of the island has similar, or 

slightly lower, 87srt86sr, but much lower ENd (4.9-5.2). 

Pb isotoP.eS in each section show strong negative correlations 

in 206pb- 207pb space, with the lowest-ENd samples having 

highest .!\207pb ( +8), and the highest-ENd samples lying below 

the Northern Hemis~here Reference Line (NHRL). Ce/Pb is 

correlated with .!\20 Pb, reducing from 41±2 in samples near 

the NHRL to 26 at high .!\207pb. These data strongly suggest 

incorporation of a crustal component into the magmas: Pb-Pb 

correlations vector towards the field of local seafloor sediments, 

and indicate that ca. 50% sediment Pb has been incorporated into 

the most contaminated lava. Laser fluorination o18o analysis 

of phenocryst augite confirms that sediment incorporation took 

place within the ocean island crust, rather than by recycling of 

ancient crustal material into the mantle, because o18ocpx 

increases from 5.56 to 6.81 with increasing .!\207pb, 

Pb isotope compositions of Miocene shield basalts in Gran 

Canaria are thus significantly controlled by high-level sediment 

assimilation. Proximity to the NHRL, or supposedly mantlelike Ce/Pb ratios, can not be used as evidence that basalt 

compositions reflect their mantle sources. Nor can high MgO 

be used, as sediment effects are observable in rocks with up to 

10% MgO. The up-sequence progression to more fractionated 

compositions with more radio~enic Sr and Nd is not 

accompanied by change in ol 0, perhaps suggesting that, with 

greater magma input, magma chambers developed at higher 

crustal levels and assimilated pre-existing volcanic crust. 
EVALUATION OF THE LATE OROGENIC 

EXHUMATIONAL HISTORY OF THE CALABRIAN 

ARC, SOUTHERN ITALY USING FISSION TRACK 

ANALYSIS 

THOMSQN Stuart. N., Institut fiir Geologie, 

Ruhr-UniversWit Bochum, Universitatsstr. 150, 

P.O. Box 102148, Bochum 44721, GERMANY 

Fission track analysis has been used to provide 

important new low temperature and time 

constraints on the late-orogenic cooling and 

exhumational history of the crystalline basement 

rocks of the Calabrian Arc, southern Italy. 65 

samples yielded 57 apatite fission track ages, 54 

zircon fission track ages and 25 apatite confined 

fission track length distributions. Interpretation of 

this data indicates that a phase of increased cooling 

rates related to exhumation occurred between 

approximately 35 Ma, the mid-Oligocene and 15 Ma 

the middle Miocene (Thomson, in press). This period 

of exhumation was the result of both erosional and 

extensional tectonic processes. 

Additional chronological constraints using fission 

track analysis applied to localised late-orogenic 

extensional tectonism, recently identified within the 

basement rocks of the Calabrian Arc, confirms an 

Oligo-Miocene age for this tectonism. 

In conjunction, a sedimentary provenance study, 

also using fission track analysis, was carried out on 

Oligo-Miocene conglomerates and sandstones that 

are known to be derived by erosion from the 

crystalline basement rocks of the Calabrian Arc. The 

fission track data confirms a local source for the 

sedimentary rocks, but more importantly reveals 

that significant erosion of the basement rocks must 

have occurred during the Oligo-Miocene (Thomson, 

1994a). 

This information is combined to produce a new 

model of the Oligo-Miocene tectonic evolution of the 

Calabrian Arc. It proposes that the crystalline 

basement rocks were part of a critical orogenic 

wedge between the mid-Oligocene and the middle 

Miocene. 

Thomson, S. N., 1994a, Fission track analysis and 

provenance studies in Calabrian Arc sedimentary 

rocks, southern Italy: J. Geol. Soc. Lond., v. 151. 

Thomson, S. N., in press, Fission track analysis of the 

crystalline basement rocks of the Calabrian Arc, 

southern Italy: evidence of Oligo-Miocene lateorogenic extension and erosion: Tectonophysics. 

322 

RARE NOBLE GAS ISOTOPE MEASUREMENTS USING 

LASER-BASED RESONACE IONIZATION 

THONNARD. N. Institute for Rare Isotope Measurements, 

The University of Tennessee, Knoxville, 1N, 37996, USA 

The rare noble gases, especially 81Kr, 85Kr, and 39 Ar, 

having isotopic abundances in the 10·11 to IQ·lS range, and 

10-21 to 10-22 concentration in modem water, challenge 

present analytical capabilities. These radionuclei can 

contribute in many ways to a better understanding of processes 

in the environment, including dating of polar ice and very old 

ground water, deep ocean circulation, and modem water flow 

patterns, to name just a few. Atom Sciences, Inc. developed 

laser-based analytical methods for krypton, and was the only 

laboratory world-wide to have measured 81Kr from terrestrial 

samples, while 85Kr measurements were possible from 

samples 10 times smaller than used by the best decay counting 

laboratories. As the company decided to withdraw from this 

field, a group of researchers and non-profit organizations was 

organized to maintain and advance the capabilities. With 

funding from Oak Ridge National Laboratory, the Scripps 

Institution of Oceanography, and the University of Tennessee, 

Kno>.ville, the Institute for Rare Isotope Measurements (IRIM) 

was founded. By January 1994, laboratory space was made 

available to IRIM, building modifications initiated to 

accommodate the equipment, and a lease-purchase agreement 

signed for the equipment. It is anticipated that by the summer 

of 1994, the facility will be operational at IRIM. 

For krypton, the technique presently consists of a multi-step 

process starting with (1) degassing of the sample, (2) 

separating Kr from the remainder of the gas, (3) going 

through a first isotopic enrichment reducing interfering 

isotopes by 105, ( 4) proceeding with a second isotopic 

enrichment of -103, and finally (5) detecting the rare krypton 

isotope in a time-of-flight mass spectrometer utilizing 

resonance ionization spectroscopy. A detection limit of -100 

85Kr atoms has been demonstrated in the final mass 

spectrometer. Argon measurements will require charge 

exchange to a metastable state of a fast Ar+ beam, followed by 

collinear laser excitation and field ionization. 

The objectives ofiRIM are: (I) to define shortcomings of the 

present system, (2) to design and implement modifications 

that will increase reproducibility and accuracy, increase 

throughput by reducing analysis time, and reduce the 

minimum sample size, (3) to conduct the necessary research 

to implement 39 Ar measurements, and (4) to initiate research 

in the geosciences utilizing the new facility both with 

collaborators at the University of Tennessee and from other 

institutions. We anticipate few-percent 39 Ar and 81Kr 

measurements from I 0 liters of water and 85Kr measurements 

from 1 liter of water, throughput reaching 500 samples per 

year, and IRIM becoming a user facility for the geosciences 

community. 

We will describe the technology developed to date and the 

improvements planned for the next few years. Data will be 

presented for 81 Kr and 85Kr measurements from polar ice, 

groundwater, and tracer studies, together with a summary of 

some of the unique research opportunities made possible with 

this technology. 
POSSffiLE IGNEOUS SOURCE FOR OLYMPIC DAM1YPE COPPER OCCURRENCES IN PROTEROZOIC 

HEMATITIC BRECCIAS, YUKON, CANADA 

ffiORKELSON. D.J., and WALLACE, C.A., 

Canada/Yukon Geoscience Office, Box 2703 (F-3), 

Whitehorse, Yukon, VIA 2C6, Canada. 

Numerous hematitic breccia zones of Middle Proterozoic 

age in northern Yukon host local enrichments of Copper, 

Cobalt, Uranium and Gold. Collectively known as 

Wernecke breccia, these zones are exposed in inliers of the 

Wernecke Supergroup, a deformed, thick miogeoclinal 

succession deposited in Middle Proterozoic time. Wernecke 

breccia crops out as individual zones up to S km2

, distributed 

in curvilinear arrays over thousands of km2

• The origin, 

timing, and significance of breccia development are being 

addressed by combined field and geochemical studies. 

Existing data implies that brecciation and mineralizaton 

resulted from explosive hydrothermal activity related to latestage magmatic processes. 

W emecke breccia consists of rounded to angular clasts in 

a hydrothermally precipitated matrix. The clasts were 

derived mainly from the Wernecke Supergroup and to a 

lesser degree from mafic to intermediate dykes which 

intruded the Wernecke Supergroup prior to brecciation. 

Metasomatism affecting the breccia zones and surrounding 

host rocks produced ubiquituous growth of specularite and 

carbonate, and commonly associated magnetite, quartz, 

chlorite, iron and copper sulphides, and uranium minerals. 

The metasomatism began before and continued after 

brecciation. Both metasomatism and brecciation were 

apparently caused by voluminous upsurge of hydrothermal 

solutions. 

Previous studies have depicted Wernecke breccia as 

diatremes, phreatomagmatic breccias, modified evaporite 

diapirs, debris flows, and mud diapirs. In these models, 

except for the diatreme hypothesis, brecciation is considered 

to have occurred prior to lithification of the Wernecke 

Supergroup. However, the recent discovery of kinked 

phyllite clasts of the Wernecke Supergroup in breccia 

lacking secondary petrofabrics indicates that an event of 

contractional deformation preceded brecciation; the models 

involving unlithified strata are untenable. A diatreme origin 

is plausible, although the breccias do not contain fragments 

of crustal basement, mantle, or chilled vesiculated magma. 

The most likely cause of brecciation is explosive 

expansion of volatile-rich fluids. An igneous influence is 

implied by the spatial coincidence of intrusions and breccia 

zones, and common enrichments of Fe, Cu, and Co. The 

fluids may have been derived from volatile-rich residual 

liquids which accumulated at the top of fmctionating 

tholeiitic magma chambers beneath the Wernecke 

Supergroup. In this depiction, breccia generation would 

represent a late- to post-magmatic stage, when the magma 

chambers were largely solidified; fluids enriched in Fe and 

volatiles escaped toward the surface and boiled explosively. 

The link between mineralizing fluids and tholeiitic 

magmatism is being tested by chemical and isotopic 

analysis. 

323 

EFFECTS OF CRYSTAL STRUCTURE ON Th-U-Pb 

EMISSIONS FROM ZIRCONS AS DETERMINED ON THE 

CURTIN SHRIMP II. 

TODT,W.A., Max Planck Institute for Chemistry, Mainz, 

55020, Germany, PIDGEON. R.T., KENNEDY, A.K. and 

VAN BRONSWJK,W., Curtin University of Technology, 

Kent Street, Bentley, 6012, Western Australia. 

SHRIMP measurements provide a unique means for determining 

Th-U-Pb ages on small areas within individual zircon crystals. 

Such measurements are made by comparing the Th-U-Pb ion 

emissions of selected areas with those of a concordant, 

crystalline standard zircon. However, in any one zircon 

population the structural state of zircons can vary from 

completely crystallised to strongly metamict. Our aim was to 

examine the effects of the degree of metamictness of zircons on 

Th-U-Pb ion emissions during ion microprobe analysis. For this 

study we have investigated 5 optically homogeneous Sri Lankan 

zircon grains which varied in colour from translucent yellow to 

opaque black and had a range of uranium contents from 400 ppm 

to over 21 OOppm and showed a corresponding range in metamict 

state. The state of metamictisation was determined using XRD 

analysis and Raman spectroscopy. Conventional ages and 

uranium and thorium contents of the zircons were measured by 

isotope dilution and filament Pb emission characteristics were 

determined by the Kober method. Our results at the time of 

preparation of the this abstract suggest that the extent of 

radiation damage of the zircon is not a major factor in disturbing 

Pb/U ratios during SHRIMP analyses. 
BEHAVIOR OF GOLD DURING MAGMATIC 

EVOLUTION OF ISLAND ARC THOLEIITES 

TOGASHI. S. and TERASHIMA , S., 

Geol. Surv. Japan, Tsukuba, Ibaraki, 305,Japan. 

High gold concentrations ( 10-25 or more ppb) in 

titanomagnetite concentrates from volcanic rocks of 

the Izu-Oshima and Fuji volcanic areas in the East 

Japan arc imply that gold is strongly partitioned into 

titanomagnetite. The concentration of gold in volcanic 

rocks increases during the early stage of fractional 

crystallization, dominated by olivine, pyroxenes and 

plagioclase, but decreases with subsequent fractional 

crystallization involving titanomagnetite. The delay 

of titanomagnetite formation during fractional 

crystallization results in high concentrations of gold in 

island arc tholeiites. 

A wide variation of gold concentration in rocks of 

primary magmas is observed across a single volcanic 

area(Fig.), and is positively correlated with the 

concentrations of incompatible elements such as Rb 

and Zr. These features are inheritated from a mantle 

source, since variable degrees of partial melting do 

not significantly affect the gold concentrations of the 

primary magmas. 

100 -.....--------------. 

Izu Oshima 

0.1 

0 2 4 6 8 10 

Rb ppm 

324 

U, Th, K, Li, and He and Ar ISOTOPES IN ROCKS, 

MINERALS, AND UNDERGROUND WATERS 

TOLSTIKHIN Igor, Department of Earth's Sciences, 

Cambridge University, Cambridge UK; LEHMANN 

Bernhard, LOOSLI Hugo, Physics Institute, University of 

Bern, 3012 Bern, and GAUTSCID Andreas, Swiss National 

Co-operative for the Disposal of Radioactive Waste 

(NAGRA), Wettingen, Switzerland. 

He and Ar isotope and U, Th, K, and Li concentrations 

were measured in sediment and metamorphic rocks from 

Quaternary-Carboniferous Molasses basin, Switzerland, with 

the aim to clear up processes controlled noble gas abundances 

in the minerals, rocks and related groundwaters. 

Concentrations of parent elements [ppm] vary within : 1 < 

U < 12, 1 < Th < 30, 1 < Li < 350. A comparison of 

calculated, assuming the water-rock system has been closed 

since the sedimentation a~e of given aquifer, and measured 

concentrations of 4He and He shows almost complete loss of 

the both nuclides from all but one sample. This one, Permian 

sandstone, contains excess trapped He with concentrations of 

4He and 3He up to 200x1o-6 and 15x1o-12 cm3/g, 

respectively, which are -10 and -30 times greater than the 

calculated values. 

[U] and [Th] in rocks and their porosity enable us to 

estimate [He] in the water filled pore volumes. The estimated 

values are a factor of 10-1000 higher than those measured in 

groundwaters, so a small addition of He from stagnant zones 

of the basin via mixing and/or diffusion into moveable waters 

flowing within high-permeability channels could provide its 

observed concentrations. 

Distribution of He isotopes in rocks and, more important, 

in mineral separates, definitely shows that trapped He can be 

a principal component of He in rocks even if measured 

3Het4He ratio is similar to or lower than the production ratio. 

Rocks (minerals) contain negligible amount of radiogenic in 

situ produced He, which is almost completely lost, while He 

isotope inventory in the rocks is provided by trapped He as a 

minor component of trapped fluid. 

In evaporites measured 3Hef4He ratios greatly exceed 

calculated values, up to (3Hef4He)meas = 16·10-8 

-100·(3Het4He)caic· Both mantle and atmospheric sources for 

the excess 3He can be eliminated. Because of 3H is a 

precursor of radiogenic 3He, specific H/He fractionation in 

the course of diagenesis and loss of He could increase 

3Het4He ratio. However other processes (e.g., spallation) can 

not be excluded at that stage of our knowledge. Trapped 

excess 3He in sandstone formed due to erosion of granites is 

inte~reted as a result of a small contribution of mantle He. 

( Het4He) ratios in groundwaters are similar to those in 

water bearing rocks or in related aquitards, also implying 

local sources for He isotopes in underground waters. 

Accordin~ to available data, i.e6 the total mineralisation, 

[Cl], [He], Hei4He and 40 Art3 Ar ratios, signatures of 

Permian and overlying aquifers can not be provided by 

advection of groundwaters from the basement. 

Water-sediment interactions and redistribution of species 

between stagnant and moveable waters appears to be a 

processes responsible for observed parameters of 

groundwaters in the Molasses basin. 
Sr, Nd, AND Pb ISOTOPE DATA OF CRUSTAL MELTS 

FROM TilE ISLAND OF SERAM, INDONESIA: 

EVIDENCE FOR DISEQUILIBRIUM MELTING DURING 

ANATEXIS 

TOMMASINI Simone <

1

•

2

>, DAVIES, G. R., STAUDIGEL, 

H., HELMERS, H., (1) Faculteit der Aardwetenschappen, 

Vrije Universiteit, 1081 HV Amsterdam, The Netherlands, 

MANETTI, P., Dipartimento di Scienze della Terra, 50121 

Firenze, Italy, and POLl, G., (2) Dipartimento di Scienze 

della Terra, 06100 Perugia, Italy 

Problems associated with granitoid petrogenesis (e.g. 

magma mixing, AFC, source composition) have been 

addressed with varying degrees of success using both 

radiogenic isotope and trace element ratios. The assumption 

that crustal reservoirs attain chemical and isotopic equilibrium 

during high grade metamorphism and anatexis is central to 

these approaches. However, parameters such as the rate of 

elemental diffusion, the time-scale of the heating event, and 

the rate of melt extraction are critical in controlling the extent 

of chemical and isotopic equilibrium in crustal environments. 

Recently published experiments on the diffusion of certain 

trace elements in rock-forming minerals (e.g. U, Pb, REE, Sr 

in garnet, feldspars, micas) suggest that isotope disequilibrium 

may be particularly important at crustal anatexis temperatures 

(750-800°C) during short thermal events (1-10 Ma). Despite 

this possibility, little assessment has yet been made of how 

minerals control the trace element and isotopic composition of 

crustal melts by remaining closed systems during melting. 

On the island of Seram, Eastern Indonesia, the Neogene 

obduction of hot oceanic lithosphere onto Palaeozoic 

metasedimentary material produced rapid crustal anatexis ( < 1 

Ma). Samples were obtained during the Snellius II expedition. 

Partial melting of metapelites produced cordierite-bearing 

S-type granitoids. Nd and Pb isotope ratios are the same in 

granitoids and metapelites (

143NdJI44Nd; = 0.51212±5, 206Pbf04Pb; = 18.738±5), whereas Sr isotope ratios are variable 

in granitoids (

87Srr6

Sr; = 0.7166-0.7268) and significantly 

lower than in metapelites (

87Srr6

Sr; = 0.7321). The less 

radiogenic Sr isotope signature of granitoids is consistent with 

feldspars not having undergone equilibration with the 

bulk-rock during anatexis, and contributing a large proportion 

of the Sr budget in the melt. 

To further evaluate the nature of melting, detailed Sr 

isotope analyses of single minerals of both granitoids and 

metapelites are being carried out with microdrilling 

techniques. The overall data set will permit an accurate 

assessment of the extent of chemical and isotopic equilibrium 

attained by crustal reservoirs during anatexis. 

325 

GEOCHEMICAL AND ISOTOPIC MONITORING OF MT. 

ETNA 1989-93 ERUPTIVE ACTIVITY: BEARING ON 

THE SHALLOW FEEDING SYSTEM:. 

TONARINI S., 1st. Geocronologia e Geochimica 

Isotopica, CNR, 56127 Pisa, Italy~ ARMIENTI, P.~ 

D'ORAZIO, M. and INNOCENTI, F., Dip. Scienze della 

Terra, Univ. Pisa, 56126 Pisa, Italy~ PETRINI, R., 1st. 

Min. Petr .• Univ. di Trieste, 34100 Trieste, Italy. 

An unusually long eruptive sequence characterized Mt Etna 

during 1989-1993. In this period, the persistent activity at the 

summit craters was accompanied by two eruptions wich took 

place in 1989-90 and 1991-93. The former eruption produced 

lavas among the most primitive of this century while the 

latter was the largest in the last three centuries. Geochemical 

and isotopic data collected during this time span reveal the 

occurrence of complex petrogenetic processes in the feeding 

system of Mt Etna. The early alkali basalts emitted from 

Sud Est Crater during 1989 eruption displaied enrichments of 

Rb and K accompanied by higher levels of radiogenic-Sr and 

isotopic disequilibrium between host-rock and clinopyroxene 

(0.70364 and 0.70356 respectively). In the following K. Rb 

and 87Sr/86Sr ratio decreased within one week, reaching the 

typical value measured in the alkali basalts outpured in the 

Valle del Leone (0.70355). Nd isotopic ratio did not show 

significant variations. These features can be explained by a 

selective contamination process involving essentially Rb, K 

and radiogenic-Sr. 

During the 1991-93 eruption the largest volume of lavas 

was emitted between January and May 1992 and was 

characterized by uniform geochemical and isotopic 

compositions and Sr-isotopic equilibrium between whole 

rock and pyroxene (0.70355). On the basis of their constant 

composition and huge volume, these lavas are considered as a 

distinct magma batch supplied to the feeding system of the 

volcano (JML batch). In other periods (December 1991 and 

June 1992) we observed lower and variable Sr isotopic 

compositions coupled with disequilibria between lavas and 

their clinopyroxenes (0.70351 whole rock and 0.70345 cpx 

for the sample 15 Dec. 1991). Nd isotope ratios are 0.51287-

0.51289. Our data do not allow to discriminate between the 

hypothesis of mixing or selective contamination, however 

lavas emitted during December 1991 evolved towards the 

isotope composition of JML that was never exceeded~ the 

regular behaviour of Rb, K and radiogenic-Sr of 1989 lavas 

was not observed during the 1991-93 eruption. Thus we 

prefer to interpret these variations as mainly due to mixing 

processes between an end-menber of JML composition and 

another one having the same, or lower, isotopic composition 

of the cpx found in the sample of December 1991. 

Starting from June 1992, when the output rate 

significatively decreased, the products seem to reveal 

fractionation effects as suggested by the continuous rise in 

concentrations of incompatible elements. 

In conclusion, several petrogenetic processes are 

contemporaneously active at Mt Etna~ they encompass 

magma mixing, selective contamination and fractional 

crystallization. Their relative importance varies according to 

the original volume of each magma batch and to the status of 

the shallow feeding system. The complexity of these 

processes, acting in the short time span of a few months, 

claims caution about the geochemical representativity of 

single lava sample for each eruption. 
ATMOSPHERIC 813C VARIATIONS, 1850-1990 A.D., AS 

RECORDED IN C4 GRASSES. 

TOOLIN. L. J., NSF Accelerator Facility, University of 

Arizona, Tucson, Arizona 85721, USA, and EASTOE, C.J., 

Department of Geosciences, University of Arizona, 

Tucson, Arizona 85721, USA. 

We previously reported1 813C values obtained on native 

species of a grass genus (Setaria}, which utilize the fourcarbon (C4) photosynthetic pathway. We were able to show 

that the 813C values of the grasses tracked changes in 

atmospheric 813C through time. Our mean Holocene value(-

6.5 °/ oJ agreed with the pre-industrial value of C02 in ice 

cores. Modem (1990 AD) Setarias from southeastern Arizona 

yielded a mean value of -8.2 °/00, in agreement with an 

atmospheric mean measured in the same region during 1983-

1984 AD. 

Here, we present 813C values measured on historically-dated 

herbarium specimens of Setarias, from field collections made 

in the southwestern USA from 1850-1990 AD. The data show 

a long-term decrease in 13C relative to 12C, from the 1800's to 

the present. Short-term variations ("wiggles") are also 

revealed. For example, there appears to be a peak of morenegative values in the early 1930's, followed by a trend to 

heavier values until 1945, with a subsequent return to lighter 

values from 1946 to about 1950. Such wiggles are compared 

to short-term trends seen in ice-core C02, C3 tree rings and 

C4 com. 

1 Toolin, L. J. and Eastoe, C. J., 1993, Late PleistoceneRecent Atmospheric 813C in C4 grasses: Radiocarbon, v. 35, 

p. 263-269. 

326 

CONTRASTING MORPHOLOOY AND U-Th-Pb-SYSTEMATICS OF ZIRCONS IN A SEf OF PERALUMINOUS, 

METALUMINOUS AND PERALKALINE BRASILIANOGRANITOIDS 

TOPFNER, C.P., Institut fiir Allgemeine und Angewandte 

Geologie, Ludwig-Maximilians-Universitllt, 

80333Munchen,Luisenstr.37,Gennany 

Since in 1980, PUPIN published a petrogenetic granitoid 

classification scheme based on the accessory mineral zircon, 

there is general agreement about the fact that tectonically, 

genetically and petrographically comparable granitoids show 

morphologically comparable zircons. Nevertheless, there is 

only little known about the factors governing the zircons' 

morphology during magmatic crystallization. 

In the Brazilian Federal States of Sao Paulo and 

southeastern Minas Gerais, acid to intermediate plutonic rocks 

cover more than 60% of the exposed crystalline basement 

belonging to the Ribeira Mobile Belt. These granitoids are 

directly related to the geodynamic evolution during the 

Brasiliano Cycle (680-450 Ma). Six different syn- to 

postkinematic granitoid massifs covering the genetic maintypes 

of PUPIN's classification were sampled: 

- a synkinematic, two mica-, garnet-, sillimanite-, and 

tourmaline-bearing, peraluminous leucogranite with a zircon 

population according PUPIN's stock 1 (orogenic aluminous 

leucogranites of crustal origin) 

- three synkinematic, metaluminous monzogranite-, 

granodiorite-, and quartz monzodiorite-batholiths with Kfeldspar megacrysts up to several em and zircon populations 

according PUPIN's stocks 4a, 4b and 4c respectively (orogenic 

hybrid granitoids belonging to the calc-alkaline low- (4a), 

medium- (4b), and high-temperature (4c) series) 

- a postkinematic, pink, subalkaline rapakivi granite complex 

with a zircon population according PUPIN's stock 5 (orogenic 

hybrid granitoids of the subalkaline series) 

- a peralkaline larvikite/nordmarkite complex with zircons 

according PUPIN's stock 6 (anorogenic alkaline series 

granitoids of mantle origin). 

The table below summarizes selected preliminary results by 

isotope dilution mass spectrometry on morphologically 

characterized zircon fractions. Further analysis and 

accompanying cathodoluminescence analysis are in progress. 

Preva111DCJ s1J:OOD type She u Pbrad Pbooa [U/'l'hlto 

@@~ 

[pal [w-l 

150-180 641.6 

100-125 417.8 

63-80 462.1 

[ppa] [ppa] [1101] 

40.26 34.42 1.236 

29.63 11.25 2.105 

30.49 14.16 2.019 

(i)® 125-180 384.3 

•• 100-125 347.6 

63-80 291.0 

37.90 2.47 1.360 

34.26 2.60 1.396 

~® 150-180 557.9 

.b 100-125 633.1 

63-80 710.3 

~® 150-180 375.6 

•a 100-125 384.8 

63-80 487.7 

39.41 2.67 1.177 

40.30 1.17 1.248 

51.61 5.15 1.112 

®® 150-180 227.4 

100-125 281.1 

63-80 335.3 

22.22 1.81 0.940 

26.83 0.62 0.973 

31.46 0.46 0.903 

®® 150-180 73.9 

100-125 81.2 

63-80 84.8 

8. 70 0.26 0.613 

9.48 0.17 0.579 

9.84 0.18 0.668 
U-Pb ZIRCON AGES AND Pb ISOTOPIC 

COMPOSITIONS OF MIDDLE PROTEROZOIC 

CRYSTALLINE BASEMENT IN THE CENTRAL 

ANDES, WESTERN BOLIVIA AND NORTHERN 

cmLE 

TOSDAL, R.M., U.S. Geological Survey, 345 

Middlefield Road, Menlo Park, CA 94025 U.S.A. 

Middle Proterozoic crystalline rocks outcrop in isolated 

fault-blocks in the western Andes inN. Chile and W. Bolivia, 

east of the Early Proterozoic (-2.0 Ga) Arequipa massif in 

southern Peru. Those exposures plus indirect evidence from 

clasts in Tertiary rocks and Pb isotopic compositions of 

Neogene volcanic rocks indicate that a Middle Proterozoic 

basement terrane extends as far south as 21°S and east to 

-67°W. Near Berenguela in western Bolivia (-17.3°S), 

clasts in the Oligocene and Miocene Mauri Formation 

include boulders of sparse granites and more common gneiss 

characterized by amphibolite- or granulite-facies metamorphic assemblages (biotite-green amphibole-plagioclase in 

mafic gneiss and gamet-(up to 1 em in diameter)-biotiteperthite-plagioclase-quartz in quartzofeldspathic gneiss). 

Protoliths are mostly igneous, ranging in composition from 

basaltic to granitic. Two augen gneisses have U-Pb 

crystallization ages of 1171±20 Ma and 1158±12 Ma. Other 

gneisses with uncertain protoliths have similar ages. A 

quartz-perthite-plagioclase gneiss has an U-Pb age of 

1100±17 Ma. U-Pb data for a migmatitic gametiferous 

quartzofeldspathic gneiss scatters about a chord with an 

upper intercept age of 1098±48 Ma, which may approximate 

the age of metamorphism. No inherited zircons are 

recognized. These U-Pb ages agree with published U-Pb 

ages and with Rb-Sr and Sm-Nd model ages for other 

basement occurrences north of 21 °S and for inherited zircons 

in Mesozoic granites as far south as 26.3°S. 

Present-day Pb isotopic compositions for the Middle 

Proterozoic rocks (19 samples: 108pbj204Pb=37.68-41.05; 

207PbJ204Pb=15.54-15.67; 206PbJ204Pb=17.12-18.24; five 

are more radiogenic) indicate time-averaged Th/U and Jl 

values higher than those of average crust. An orthogneiss 

from the Middle Proterozoic Belen Schist (18.3° S) in N. 

Chile has a similar Pb isotopic composition, but associated 

pelitic schists extend to less radiogenic values and overlap 

the field for the -2.0-Ga Arequipa massif. As a group, the 

Middle Proterozoic rocks have higher present-day 

206PbJ204Pb than those of the Arequipa massif, which is also 

characterized by elevated time-averaged ThiU and Jl values. 

They, furthermore, have higher present-day 208pbJ204Pb and 

207pbJ204Pb than Middle Proterozoic rocks in western 

Argentina, which are characterized by crustal average ThiU 

and distinctly lower Jl values. Age-corrected Pb isotopic 

compositions are also distinct for the crystalline terranes. 

The Pb isotopic data clearly indicate long-term involvement 

of old radiogenic continental crust, presumably of Archean 

age, in the Proterozoic rocks in S. Peru, N. Chile and W. 

Bolivia. This implies that these Proterozoic rocks were 

originally part of, or adjacent to, such an Archean craton. 

Furthermore, any fragment rifted away from this part of 

western South America in the Late Proterozoic or Early 

Paleozoic can be identified by these Pb isotopic and U-Pb 

geochronologic characteristics. 

327 

Sm-Nd, Rb-Sr AND Pb-Pb DATING OF ARCHAEAN 

SEDIMENTS: CONTRASTING RESULTS FROM WHOLEROCK AND CLAY MINERAL SEPARATES 

TOULKE&IDIS. T.l.2,3, GOLDSTEIN, S.L.l, CLAUER, 

N.2, STILLE, P.2, TODT, W.l and KRONER, A.3 

1 MPI fUr Chemie, 55020 Mainz, Germany, 2cGS-CNRS, 

67084 Strasbourg, France and 3Institut fUr Geowissenschaften, University of Mainz, 55099 Mainz, Germany 

Diagenetically-formed clay minerals in sediments can often 

be used to determine the time elapsed since closure of the 

mineral isotopic systems following the last thermal evenL We 

report preliminary results of an isotopic study of one shale and 

two carbonate occurences as well as clay minerals separated 

from carbonates. 

Samples are from the early Archaean Barberton Greenstone 

Belt, South Africa, which underwent regional thermal events in 

the Kaapvaal craton at -3.1, 2.7, 2.4, and 2.1 Ga ago. One 

carbonate sample comes from the -3.4 Ga old Onverwacht 

Group (OG), while the other two samples are from the 

overlying -3260-3227 Ma old Fig Tree Group (FTG). All 

sampled sedimentary sequences contain primary sedimentary 

structures, textures and, partly, mineralogies. 

Pb-isotopic data for 45 individual chips of one OG wholerock sample form a linear array corresponding to a Pb-Pb age of 

2628±131 Ma. Sm-Nd and Rb-Sr isotopes were analyzed in 

WR samples of the FTG carbonates. The Sm-Nd data give an 

age of 2612±84 Ma with initial ENd(t)=-8.1±0.5, and the RbSr data give an age of 2745±148 Ma with initial 87sr,S6sr= 

0.7014±0.002. Rb-Sr and Pb isotopes were analyzed in shales, 

and form a Rb-Sr array of 2761±52 Ma with initial 

S7sr,S6sr=0.7147±0.002, and a Pb-Pb age of 2601±29 Ma. 

All three whole-rock isotopic systems of the carbonates and 

the shales indicate ages of -2.7 Ga, much younger than the 

depositional age of the successions. This agreement indicates 

that a thermal event at 2.7 Ga has reset all three whole-rock 

isotopic systems. Clay fractions (mainly illites) separated for isotopic analyses 

from the FTG carbonates were additionally treated with 1N 

HCI. The Sm-Nd data of leached, residual and untreated clay 

fractions (n=8) form a linear array corresponding to an age of 

3102±64 Ma with initial eNd(t)=-1.0±0.6. This age is younger 

than the depositional age but older than the authigenic clays 

and is interpreted to reflect a thermal overprint related to 

widespread granitoid magmatism around the Barberton 

Greenstone Belt area. The highest temperature reached by the 

samples since the time of clay mineral formation was <3oo·c. 

The unusual feature, that the clay mineral isochron is older than 

the whole-rock isochron, can be explained by the fact that the 

primary minerals of the whole-rocks were altered, recrystallized 

and isotopically reset at -2.7 Ga at a temperature of <25o·c, 

while the newly fonned clay minerals swvived. 
ESR DATING OF VOLCANIC QUARTZ: APPLICATIONS 

TO THERMAL HISTORY 

Toyoda S. and Ikeya, M., Department of Earth and 

Space Science, Faculty of Science, Osaka University, Toyonaka, Osaka, 560, Japan. 

Unpaired electrons created by natural ionizing radiation 

are accumulated in minerals. Accumulated dose (D~ is 

obtained from concentration of unpaired electrons measured 

by ESR (electron spin resonance) spectroscopy. ESR ages 

are obtained by dividing ~ by annual radiation dose (D). 

This method has been successfully applied to speleothems, shells, corals, tooth enamel, gypsum in sediments, quartz in 

tephra and so on1>. 

Dating methods using isotopes and fiSsion tracks have 

been used in investigations of thermochronology. However, 

possibility of ESR dating method for this field has never 

been discussed. In this paper, results of basic research on 

thermal stability of paramagnetic centers (unpaired electrons) 

in quartz are presented as well as discussion on possibility of ESR method for applications to thermochronology. As an 

example, thermal history around an intrusive dike is 

investigated. 

Quartz grains were extracted from a portion of Mannari 

granite in Japan and from some members of Valles Rhyolite in New Mexico, U.S.A. ESR signals were observed by ESR 

measurement due to AI center (an electronic hole trapped at 

an aluminum ion replacing silicon), Ti center (an electron 

trapped at a titanium ion replacing silicon), and E' 1 center 

(an electron at an oxygen vacancy). 

Thermal stabilities of the centers were investigated with 

isochronal (stepwise) and isothermal heating experiments. As 

results of isothermal heating experiments, the centers were 

found to . decay with second order kinetics. Activation 

energies were obtained from an Arrhenius diagram where 

decay coefficients were plotted The thermal stability of 

oxygen vacancies in quartz was also investiaated where a 

method was used according to our proposal> in order to 

convert oxygen vacancies to their paramagnetic states (E' 1 centers). 

Closure temperatures were obtained for each center and 

oxygen vacancy by a calculation simulating cooling of 

granite3

> as well as dike intrusion. As for quartz in granite 

with a cooling rate of 10 K/Myr, the temperatures were 

obtained to be 91 oc for E\ center, 78°C for AI center, and 

31 °C for Ti center. 

The concentrations of oxygen vacancies were reported to 

be correlated with radiometric age of host rocks in Ma-Ga 

range4> as well as E' 1 centersSl. It is found in this study that 

oxygen vacancies are created by artificial 'Y ray irradiation 

simulating external ~ and 'Y rays. Their amount created is 

consistent with that in natural quartz where external ~ and 

'Y rays were assumed to create oxygen vacancies. The results 

support our theory4

> that oxygen vacancies are created by external ~ and 'Y ray in natural quartz. A thermal history 

around intrusive andesite will be discussed using oxygen 

vacancies in quartz extracted from host rocks of granite where external ~ and 'Y rays are assumed to create oxygen vacancies in quartz. 

1) Ikeya, M., 1988, Dating and radiation dosimetry with electron 

spin resonance: Magn. Reson. Rev., v.l3, p.91-134. 

2) Dodson, M.H., 1973, Closure temperature in cooling 

geochronological and petrological systems: Contr. Mineral. 

Petrol., v.40, p.259-274. 

3) Toyoda, S., and Ikeya, M., 1991, Thermal stabilities of 

paramagnetic defect and impurity centers in quartz: Basis for 

ESR dating of thermal history: Geochem. J., v.25, p.437-445. 

4) Toyoda, S., Ikeya, M., Morikawa, J., and Nagatomo, T., 

Enhancement of oxygen vacancies in quartz by natural external 

~ and y ray dose: a possible Geochronometer of Ma-Ga 

range: Geochem. J., v.26, p.111-115. 

5) Odom, A.L., and Rink, W.J., 1989, Natural accumulation of 

Shottky-Frenkel defects: Implication for a quartz 

geochronometer: Geology, v.17, p.55-58. 

328 

CHARACfERIZATION OF DEFECfS INDUCED 

BY NEUTRON IRRADIATION: XRD RIETVELD 

REFINEMENT RESULTS 

Hsin-Yi TsenK, Peter Heaney, and T.C. Onstott 

Dept. of Geological and Geophysical Sciences, 

Princeton University, Princeton, NJ 08544, 

USA 

To investigate the relationship between the 

microstructures resulting from neutron irradiation 

and argon diffusion in vacuum, techniques capable 

of quantifying these structures are required. Powder 

X-ray diffraction (XRD) analyses followed by 

Rietveld refinement were applied to irradiated and 

unirradiated K-feldspars. Benson Mines orthoclase 

(Ko.915Nao.067Al1.06Si2.960s, corresponding to 

Or94) was chosen because of its purity, lack of 

naturally formed microstructures, and because its 

argon diffusion behaviour is extremely well 

determined. XRD powder samples were irradiated 

by total (thermal +fast) neutron doses of 6.3x1Q18 

n/cm2 and 1.26x1Q19 n/cm2. We observed that clear 

powdered orthoclase takes on a distinct brownish 

coloration after irradiation. We utilized the fiber 

filter mounting method for sample preparation to 

reduce preferred orientation problems and obtained 

reproducible X-ray diffraction patterns. XRD 

analyses revealed peak broadening in irradiated 

orthoclase compared to the unirradiated orthoclase. 

Line shapes were modeled using a pseudo-Voigt 

profile function with the GSAS Rietveld refinement 

program. The anisotropic strain percentage 

calculated from the Lorentzian profile parameters 

reveals a positive correlation with the neutron dose 

(see table). By contrast, the calculated particle sizes 

are between 0.5 J.Lm and 5 J.Lm, so the contribution of 

particle size broadening can be approximately 

neglected. This strain-induced broadening probably 

results from vacancies created by collision cascades 

of recoiling atoms during neutron bombardment. 

Since 39 Ar is initially concentrated within these 

cascade zones compared to 40Ar, enhanced 

diffusivity of 39 Ar with respect to 40 Ar is not 

surprising. 

Rietveld Refinement Result 

Neutron Dose unirradiated 6.3x1018 1.26x1019 

nlcm2 n/cm2 

"12 3.111 3.026 2.906 

Strain 1/fOOtl 0.2425% 0.3153% 0.4065% 

Strain .lfOOtl 0.2052% 0.2697% 0.3206% 

Particlemootl 1.822x104 A 5.076x103 A 4.108x104 A 

Particle 1.roon 4.813x103A 4.813x103 A 4.813x103 A 
RELATING HYDROTHERMAL FLUID FLOW 

TO THERMAL MATURITY FOR A TRIASSIC 

BASIN: RESULTS FROM FLUID INCLUSIONS 

AND FISSION TRACK LENG1H 

DISTRIBUTIONS 

Hsin-Yi Tsen&, T. C .. Onstott, Dept. of 

Geological and Geophysical Sciences, Princeton 

University, Princeton, NJ 08544, USA; 

R. C. Burruss, U.S. Geological Survey, Denver, 

CO 80225, USA; and 

D. S. Miller, Dept. of Geology, Rensselaer 

Polytechnic Institute, Troy, NY 12180, USA. 

The Late Triassic Taylorsville basin yields 

detrital fission track apatite and zircon dates that are 

younger than the age of deposition. The apatite 

fission track ages range from 135 to 200 Ma with 

the younger ages located near the center of the 

basin. The sparcity of intrusive igneous rocks in the 

basin and the spatial position of the fission track 

samples suggest that either the Taylorsville basin 

was more deeply buried than today and was 

exhumed prior to the Early Cretaceous. 

Maximum homogenization temperatures of 

secondary aqueous fluid inclusions in calcite veins 

in shale and sandstone cuttings from a borehole 

near the center of the basin range from 150° to 

200°C between 6650 and 10070 ft. depth, with 

highest temperatures in the deepest samples. A 

calcite vein at 8376 ft contains both two-phase 

aqueous and one-phase Cf4 -rich inclusions in 

healed microcracks. The CI4-rich inclusions 

homogenize to vapor at about -82.1 °C. If both 

types of inclusions were trapped simultaneously, 

the density of the Cl4-rich inclusions implies an 

entrapment pressure about 400 bars at the 

homogenization temperature (165°C) of the 

aqueous inclusions. This pressure falls between the 

hydrostatic and lithostatic pressures at the present 

depth of burial. If we assume that the pressure 

regime was hydrostatic at the time of trapping then 

the inclusions were trapped at 4 km in a thermal 

gradient of about 40°C/km. If the pressure gradient 

was close to lithostatic, then the thermal gradient 

would have been about 90°C/km. 

The high temperature recorded by the secondary 

aqueous inclusions are consistent with the pervasive 

resetting of zircon and apatite fission track dates. In 

order to fit the fission track length distributions of 

the apatite data, however, a cooling rate of 1 - 2 0

C/Ma following the thermal maximum is required. 

To match the integrated dates, the thermal 

maximum would have occurred at -200 Ma. The 

timing of the maximum temperature is consistent 

with rapid burial of the Taylorsville basin to twice 

its present day depth and thermal reequilibration 

with a 40°C/km geothermal gradient, followed by 

slow exhumation. 

329 

SIMS U-Pb AGE OF ZIRCONS IN ARCHEAN GRANULITES FROM THE LIMPOPO BELT, SOUTHERN 

AFRICA. 

TSUNOGAE. T .. Fac. Education, Shimane Univ., Matsue 

690, Japan, and YURIMOTO, H., Inst. Geosci., Univ. 

Tsukuba, Ibaraki 305, Japan. 

New U-Pb age of single zircons in Archean granulites was 

examined by the secoodary ion mass spectrometry (SIMS). A 

Cameca IMS-3f ion microprobe at the University of Tsukuba 

was employed f<X" the present zircon analysis. Mass resolution 

of secondary ions f<X" the SIMS analysis was set up to 6500 

(M/.o.M, 10% valley) which is sufficient to resolve all major 

interferences from U, Pb, md Th isotopes. Although the Pb 

intensity analyzed by the 3f (about 1 to 2 cps/ppm) is lower 

than that by SHRIMP, it is sufficient to apply to the U-Pb 

method of zircon with high radiogenic Pb concentration. The 

relationship between 206pb +;DV md uo+;u+ ratios was used 

f<X" the 3f analysis to obtain unknown ~b./238U ratio in terms 

of standard zircon (SL13 provided by the ANU) as mentioned 

by Compston et d. (1984). 207Pb.fD6Pb. ratio was estimated as 

measured. because Pb isotopic fractionation on the SIMS 

analysis was negligibly small. 

The analytical procedure was applied to zircons in granulites 

from the Limpopo ~elt in southern Africa. The belt can be 

lithologically subdivided into the Central Zone (CZ; T=750 

-8500C, P> 12 kbar) and the Northern and Southern Marginal 

Zones (NMZ and SMZ; T=700-7500C, P=7 .0-7 .5 kbar). 

Preliminary analyses of zircons in the tonalitic Sand River 

Gneiss (CZ) display ages of 3200-3350 Ma, which show good 

agreement with previous Sm-Nd age (3200-3300 Ma; Harris et 

al., 1987) and U-Pb age (<3290 Ma; Retief et al., 1990). 

The CZ shows SIMS U-Pb ages of 3100-3350 Ma, while 

the NMZ shows younger ages of 2650-2800 Ma. Rim of the 

2780 Ma zircon from the NMZ suggests a later thermal event 

at about 1800 Ma. There are two possible explanations for the 

differences in U-Pb ages between the CZ and NMZ; (1) timing 

of peak metamorphism of the CZ was older than that of the 

NMZ, which is consistent with the differences in P-T path. (2) 

older relict zircon from the CZ did not suffer significant reset of 

U-Th-Pb system during peak metamorphism, because of rapid 

uplifting and cooling history of the CZ. 

Compston,W., Williams,I.S., and Meyer,C., 1984, JGR, 8 9, 

(suppl.) B525-B534. 

Harris,N.B.W., Hawkesworth,CJ., Van Calsteren,P .• and 

McDermott,F., 1987, EPSL. 8 3, 85-93. 

Retief,E.A., Compston,W., Armstrong,A .• and Williams,I.A .• 

1990, In J.M.Barton)r., Eds .• Extended Abstracts, The 

Limpopo Belt. RAU, Johannesburg, 95-99. 
GAS GEOCHEMISTRY OF DEEP-SEA 

VENTING HYDROTHERMAL FLUIDS 

RELATED WITH ARC VOLCANISM: 

EVIDENCE FOR SUBDUCTING SLAB 

CONTRIBUTION IN AN ARC VOLCANISM 

TS1JNOGAI U , ISHIBASHI, J., WAKITA, H., 

Lab. for Earthq. Chern., Fac. of Science, Univ. of Tokyo, 

Hongo Bunkyo-ku, Tokyo 113, JAPAN, 

GAMO, T., ORI, Univ. of Tokyo, Minami-dai, Nakanoku, Tokyo 164, JAPAN 

Chemical and isotopic compositions of 

dissolved gases are analyzed for hydrothermal 

fluids from the Suiyo seamount (located in the 

southern part of the Izu-Bonin arc; depth: 1,370 

m) and the Peak B seamount (located in the 

southern end of the Mariana arc; depth: 1,4 70 

m). Gas geochemistry of the fluids are compared 

with that of MOR hydrothermal fluids and 

subaerial arc volcanic gases. The origin of 

gases is discussed in relation to arc volcanism. 

This is the first gas composition study of deepsea venting hydrothermal fluids in relation to 

arc volcanism. 

Using Japanese submersibles SHINKAI 2000 

and SH/NKAI 6500, we collected twelve fluid 

samples (up to 310 °C) from the Suiyo 

'seamount and eleven fluid samples (up to 127 

OC) from the Peak B seamount. Gas compositions of these hydrothermal fluids have several 

common characteristic features: (1) The C02 

concentration is substantially higher than that 

of the MOR fluids, and the C02/3He ratio 

(11-12x109) is close to that found in the 

subaerial arc volcanic gases. (2) The 3He/4He 

ratio of 8.1 and 7.5 (R/Ratm) are the highest 

value for an island-arc and are almost identical 

to the MOR ratio. (3) The 813C values of 

dissolved gases are unusually as high as 

813C(C02) = -1 %o and 813C(CH4) = -8.5 %o at 

the Suiyo seamount and 813C(C02) = -1 %o at 

Peak B seamount. These are differ from those of 

the ordinary island-arc and MOR values. 

The observed high 3 He/4 He ratios and high 

813C values are difficult to explain by either 

contamination or fractionation processes during 

hydrothermal circulation. We conclude that they 

reflect the gas compositions of source magma. 

A modeled calculation demonstrates that a 

simple contribution of the subducting slab of 

about 3 % to the MORB mantle will produce a 

magma with the gas composition of these 

seamounts and this is the only case to generate 

the observed composition. 

These characteristic gas compositions of the 

seamounts are not common to subaerial arc 

volcanoes, but may represent "original" features 

of arc magmatism. Formation of such magma 

may be due to its particularly different 

environment from subaerial volcanoes: 

extraordinary thin and young arc crust, lack of 

old sedimentary layer in crust and insignificance 

of biogenic materials. 

330 

AGES OF MESOZOIC PLUTONS IN THE 

DAMYANG-GEOCHANG AREA, RYONGNAM 

MASSIF, KOREA 

TUREK. A., Dept. of Geology, University of 

Windsor, Windsor, ON. Canada N9B 3P4, and 

KIM,C.B., Dept. of Geology, Chonnam National 

University, Puk-k:u, Kwangju, Korea 500-757. 

The Damyang-Geochang area of Korea is in the 

southwest part of the Ryongnam massif and is madeup of Precambrian gneisses that have been intruded by 

Triassic to Jurassic, felsic to mafic, batholiths and 

stocks. All have been involved in several orogenies 

from Precambrian to Mesozoic. A total of eight KAr, Ar-Ar, and Rb-Sr ages has been reported for 

some of the rocks in the area and the ages range from 

159-228 Ma. 

This study reports the first U-Pb zircon ages for 

the Korean peninsula. In the Damyang-Geochang 

area plutonism appears to span from 219 Ma to 176 

Ma. The first period of Mesozoic plutonism was the 

emplacement of: the foliated granite, the foliated 

gabbro and the Daegang foliated granite, at 219-212 

Ma. A second plutonic event occurred at 187-183 

Ma in which a foliated granodiorite and the Sunchang 

foliated granite were emplaced. The Namweon 

granitic batholith and an associated diorite stock, both 

non-foliated and post-tectonic, were emplaced at 177-

176 Ma. The concordia plots for the above zircon 

data indicate derivation from a Precambrian protolith, 

1.9-2.6 Gain age. Pb isotopic composition measured 

on feldspars has an upper crustal signature. 

Moreover, the younger plutons are derived from a 

more radiogenic source than the older ones. The 

Jirisan gneiss complex constitutes part of the 

Ryongnam massif and yields a zircon age of 1940 

Ma. 
EXOTIC AND EXOGENIC OSMIUM 

TUREKIAN, KARL K., Dept. of Geology and 

Geophysics, Yale University, P.O. Box 208109, 

New Haven, CT 06520-8109 

The characteristic 187Qsf186Qs signatures of 

meteoritic, mantle and crustal materials imprint 

the oceans and leave a record of terrestrial and 

cosmic events in marine deposits. The record over 

the past 100 million years shows the results of 

meteorite impacts and mountain building. 

331 

THE DEVELOPMENT OF IDEAS ABOUT THE 

GROWTH OF 40Ar IN THE ATMOSPHERE 

TUREKIAN, K.K., PORCELU, D., 

SEIDEMANN, D.E., Dept. of Geology and 

Geophysics, Yale University, P.O. Box 

208109, New Haven, CT 06520-8109 

The origin of the third most abundant gas in 

the atmosphere, argon-40, is clearly the decay of 

40K in the solid Earth with subsequent degassing. 

The crust and the mantle are the two obvious loci 

of production. The amounts of potassium in 

these two reservoirs are of the same order of 

magnitude. The mechanisms of escape from the 

two reservoirs are not obviously coupled. A 

simple first order degassing law has been 

modified to accommodate the decreasing 

efficiency of degassing geared to the total heat 

production and escape from the Earth. 

Estimates of contemporary degassing rates 

from the mantle (at ocean spreading centers) and 

the crust (based on resetting of 40K-40 Ar mineral 

chronometers) provide the entry into the 

problem. 
PROPOSAL FOR A QUATERNARY 40ArJ39Ar DATING 

STANDARD 

TURRIN. B.D),2, DONNELLY-NOLAN, J.M.l, FLECK, 

R.JI, CHAMPION, D.E.I, BECKER, T.A.2 CURTIS, G.H2 

DEINO, A.L.2, RENNE, P.R.2, SHARP, W.D.2, and 

SWISHER, C.C.III2: 1) U.S. Geological Survey, 345 

Middlefield Rd., Menlo Park, CA, 94025, USA. 2) Institute of 

Human Origins, Geochronology Center, 2453 Ridge Rd., 

Berkeley, CA, 94709, USA. 

Accuracy of the 40 Ar/39 Ar dating technique, a relative dating 

method which compares the age of an unknown sample to the 

age of a known standard, is limited by the accuracy of the 

''known age" of the standard and by how well the interfering 

neutron reactions from 36 Area. 39 Area. and 40 ArK are known 

and corrected. The 36 Area and 39 Area corrections are reproducible between separate irradiations ~ 1 %, Dalrymple et al., 

1981 ). The 40 ArK value varies by as much as an order of 

magnitude between measurements (Dalrymple et al., 1981). 

This correction, which becomes significant in young samples 

where the 40Arradiogenic (40Ar*)/K ratio is typically low, can 

be minimized in two ways: 1) reduce the amount of neutron 

flux, and 2) use Cd shielding. However, if the standard is 

significantly older than the unknowns, this correction becomes 

large relative to the measured 40 Ar*/39 Ar ratio of the 

unknowns. For example, using the youngest available standard 

[the Fish Canyon sanidine (FCS) 27.84 Ma], the youngest 

material that can be dated while maintaining optimum 

40 Ar* ;39 Ar ratios, and relatively small corrections for 

interfering neutron reactions, is 100 to 300 ka (Dalrymple et 

al., 1981 ). An ideal 40 Ar/39 Ar standard for dating rocks in the 

range 1 o4 to 10 7 yrs would be -1 Ma. 

For this standard, we propose the sanidine from the rhyolite 

of Alder Creek, Ca., which is the type section of the Cobb 

Mountain Normal-Polarity Subchron (CMNPS) of the Matuyama Reversed-Polarity Chron of the Geomagnetic Polarity 

Time Scale. Because the CMNPS is a unique world-wide 

stratigraphic event of short duration ~1 0 ka), sanidine from the 

rhyolite of Alder Creek is an ideal 40 Ar/39 Ar standard. 

Sanidine from this unit has been dated at 1.186 ±0.006 Ma 

using the FCS (Turrin et al., 1994). This age is concordant 

with the age of 1.19 Ma for the CMNPS as suggested by the 

Astronomical Polarity Time Scale (Shackleton et al., 1990). 

The use of the CMNPS standard will optimize irradiation 

times of Quaternary age samples, thus providing accurate and 

precise age measurements relative to the age of the CMNPS for 

Quaternary stratigraphic and climatic dating studies. 

Dalrymple, G.B., Alexander, C.E., Lanphere, M.A., and Kraker, 

P.G., 1981, Irradiation of Samples for 40 Ar/39 Ar Dating Using 

the Geological Survey 1RIGA Reactor: U.S. Geological Survey 

Professional Paper 1176, 55p. 

Shackleton, N.J., Berger, A., and Peltier, W.R., 1990, An alternative astronomical calibration of the Lower Pleistocene time 

scale based on ODP Site 677: Royal Society of Edinburgh Trans., 

v. 81, p. 251-261. 

Turrin, B.D, Donnelly-Nolan, J.M., and Hearn, B.C., 1994, 40 Ar/39 Ar ages from the rhyolite of Alder Creek, California: Age 

of the Cobb Mountain Normal-Polarity Subchron revisited, 

Geology, v. 22, p. 251-254. 

332 

PAN-AFRICANREJUVENATION IN 

SUPRACRUSTALS OF KERALA 

KHONDALITE BELT (KKB) AND THEIR 

ISOTOPIC SIGNATURES 

UNNIKRISHNAN-WARRIER. C., Dept. of 

Geosciences, Osaka City University, Sugimoto, 

Sumiyoshi-Ku, Osaka-558, Japan., YOSHIDA. 

M., Dept. of Geosciences, Osaka City University, 

Sugimoto, Sumiyoshi-Ku, Osaka-558, Japan and 

SANTOSH, M., Centre for Earth Science Studies, 

P.B. 7250, Akkulam, Thuruvikkal P.O, 

Trivandrum-695 031, India. 

The southern tip of the high-grade terrain of Peninsular 

India is composed of a vast supracrustal sequance known as 

Kerala Khondalite Belt (KKB). Lithological units of this 

supracrustal sequance are composed of dominantly by 

charnockite- khondalite-leptynite group of rocks. They also 

host the incipient charnockites. Minor lithounits are the 

interlayered basic granulites, calc-silicates and quartzites. 

Radio isotope investigations (Sm-Nd) on mineral separates 

from the garnet-biotite gneiss and the sillimanite-cordierite 

bearing gneiss from KKB yield- 500 Ma simillar to the age 

of incipient charnockites from this terrain, where as the RbSr ages are slightly younger. The widespread incipient 

charnockite development in the KKB gneisses adjacent to 

Pan-African alkalic granite-syenite complex links the 

charnockitic alteration of the gneisses with the C02 derieved 

from deep crustal origin. Fluid inclusion studies as well as 

Carbon isotope studies of graphites from these lithologies 

also suggest that deep crustal fluids are instrumental for the 

genesis of incipient charnockites. This Pan-African tectonics 

can be well correlated with other parts of East Gondwana. 
WIDE-AREA LINE ANALYSIS OF ISOTOPIC ABUNDANCES ON 

INHOMOGENEOUS INSULATORS USING AN ION MICROPROBE 

UYEDA,C.,TSUCHIYAMA,A,and YAMANAKA,T.,Inst. 

Earth and Planetary Sci.,Col.General Education 

Osaka University Toyonaka Osaka 560 Japan 

In general, the isotopic analysis using an ion 

microprobe is accompanied by various types of instrumental mass fractionation deriving from the 

ionization process at the sample surface 11 • The 

variation of the charge potential due to the inhomogeniety of the sample conductivity often interfere with the in-situ analysis for various inhomogeneous geological sample. For this reason, 

isotopic in-situ analysis is not popularly done 

for inhomogeneous insulator samples. 

We have developed a new method which imporves 

the condutivity of the sample surface, and homogenizes the surface potential 21 • A metal channel 

is located along the line of analysis. Point to 

point analysis is performed along the boundary 

of the metal channel and the sample. Both the 

sample and the metal surface are bonbarded with 

a same ion beam with the diameter of 40-80 JL m. 

Using this method, the line analysis of Mg isotopes is done with the spatial resolution of 4 JL m, 

the error of less than 3%o. 

A sample is attached between two laboratory 

standards, and the analysis line is located across 

the sections of the three samples. Isotopic variation of the sample is measured precisely by comparing the sample data with that of the two standards. The length of the analysis line is 6111D wide. 

An isotopically enriched standard is placed 

between the laboratory standards from time to time 

to check whether the system reproduces the spatial 

isotopical variation correctly. In general, the 

amount of instn.unental mass fractionation changes 

considerably with the position of the primary ion 

at the sample surface, and it is necessary to 

evaluate this effect day-by-day especially in 

the case of wide-area line analysis. 

It is noted that the search of a new isotopic 

anomaly is done far more effectively by performing an in-situ line analysis than by separating 

the individual grain from the bulk samples as it 

is commonly done 31 • Also the systematic survey 

of isotope data can be done for various components 

in the meteorite. The intrinsic mass fractionation of refractory elements, for example, would 

be a subject of large significance41 • Every components in the meteorites may have experienced 

mass fractionation by evaporation or condensation. 

Therefore it is desiable to carry out a systematic 

survey on all the components of the meteorites, 

in order to obtain the whole view of the process. 

l)NISHIMURA.H & OKANO.J(1974)J.Appl.Phys.Suppl.2 

Pt.l 339. 2)UYEDA,C.,et al,Proc.SIMSLK conf. (in 

print) 3)e.g. McKEEGAN,J.& ZINNER,E. (1987)GCA 51 

1468, 4)UYEDA.C, TSUCHIYAMA,A.& OKANO,J. (1989byEarth Planet Sci.Lett. 107 138. 

333 

RADIOMETRIC DATING OF THE TORTONIAN/ 

MESSINIAN BOUNDARY (N. APPENNINES, ITALY) 

V AI G.B., Dip. Scienze Geologiche, 1-40127, Bologna, 

Italy., LAURENZI, M.A., Istituto Geocronologia e 

Geochimica Isotopica. CNR, .56127 Pisa, Italy, 

A series of new AriAr datings on biotites have been made 

from the Monte del Casino (MdC) section in the Northern 

Appennines, Italy. The aim was to check the reliability of the 

previous radiometric datings obtained from closely related 

sections, which have provided mean dates of 7.72±0.15 close 

to the Gr. suterae FAD, 7.35±.0.08 Ma (biotite) and 

7.28±0.10 Ma(plg) for the interval between Gr. saheliana 

(near M5 datum) and Gr. conomiozea FADs, and an 

interpolated age of 7.26±0.10 Ma for the boundary taken at 

the conomiozea FAD (Vai et al., 1993). 

The new section sampled is continuous, 

biostratigraphically well subdivided, and contains a series of 

superposed biotite-rich volcanoclastic horizons (BRH) 

spanning the TortonianiMessinian (TIM) boundary over a 

total thickness of about 18 m.The MgOIFeOITi20 and 

FeOIMgOIK20 tests made on biotites show an acceptable to 

excellent degree of homogeneity suggesting that the BHR can 

be interpreted as original fallout deposits. 

Seven out of the nine BRH have been sampled and five of 

them have been dated till now. The two first horizons give a 

mean date of 7.30±.0.02 Ma which differs from the mean of 

three previous Kl Ar dates from the same horizons in a 

different section nearby (s. above). The following two 

horizons have yielded dates of 7.40 Ma and 7.28 Ma, with a 

mean of 7.34±0.02 Ma, quite consistent with previous data. 

The date of 6.71±0.04 Ma obtained from the uppermost 

horizon is the first one bracketing the TIM boundary (about 4 

m from above). It differs from a preliminary ~ Ar age of 

7.15 Ma (and 7.2 Ma by Odin, pers. com.) obtained from a 

previous sample of the same horizon. 

Considering the end members of the new dates from the 

last three horizons a mean sedimentation rate of 10±,0.5 

miMa is obtained to be compared with the figure of 27±3 

m/ma by Vai et al. (1993). By linear interpolation between 

the two facing dates, the TIM is about 7.07±0.05 Ma old, 

which is fairly consistent with the previous calibration of 

7.26±.0.10 by Vai et al. (1993) and also with the 

astronomical calibration at 7.10 Ma (Krijgsman et al., 1993). 

Two further dates of about7.1 (biotite) and 6.85 (sanidine) 

Ma, reported by A. Deino at the MICOP Ancona 1992, are 

also consistent, although more precision on their 

stratigraphic calibration is needed. At present , the TIM 

(Conomiozea) boundary can be assumed at 7.1±0.2 Ma.All 

ages presented here are referred to an age of 27.55 Ma for the 

FCT biotite (lzett et al., 1991). 

Izett G.A., Dalrymple G.B. and Snee L.W., 1991, 

40 Ar/39 Ar age of Cretaceous-Tertiary boundary tektites from 

Haiti: Science, 252, 1539-1542. 

Krijgsman W., Hilgen F.J., Langereis C.G. & Zachariasse 

W.J., 1993, The age of the Tortonian/Messinian boundary; 

Earth Planet. Sci. Letter, in press. 

Vai G.B., Villa I.M. & Colalongo M.L., 1993, First direct 

dating of the Tortonian/Messinian boundary; C.R. Acad. Sci. 

Paris, 316, II, 1407-1414. 
He-Ne AND Sr-Nd-Pb ISOTOPE SYSTEMATICS IN 

HAWAIIAN BASALTS: EVIDENCE FOR THE 

MIGRATION OF GASEOUS AHEAD OF LITHOPHILE 

PLUME COMPONENTS? 

P J Yalbracht Inst. de Physique du Globe, 75252 Paris, 

France, M. Honda, I. McDougall, The Australian National 

University, ACT0200 Canberra, Australia, H. Staudigel, 

and G.R. Davies, Vrije Universiteit, 1081HV Amsterdam, 

The Netherlands 

Hawaiian volcanoes, the type expression of plume-related 

oceanic volcanism, were studied for their geochemical and 

chronological relationships between the isotopic ratios of 

gaseous (He, Ne, Ar, Kr, Xe) and lithophile (Sr, Nd, Pb) 

elements. An apparent decoupling of these isotopic parameters 

between different volcanic stages suggests that gaseous plume 

components migrate ahead of the lithophile components. 

A wealth of scientific information is available from all four 

major stages of Hawaiian volcanic activity, beginning with the 

earliest seamount stage at Loihi Seamount (Stage 1), through 

the massive shields (Stage II), and late alkalic (Stage Ill), to 

the rejuvenated volcanism (Stage IV). Previous studies suggest 

that the lithophile fraction of the volcanic products (i.e. their 

Sr-Nd-Pb isotopic signature) appears to be decoupled from the 

gaseous fraction. Stage I has a strong "Enriched Pacific 

MORB" lithophile component, whereas Stage II lavas, 

forming about 95% of these volcanoes, are dominated by a 

relatively "enriched" plume lithophile signature (especially on 

Oahu (the Koolau series), Lanai, and Kahoolawe) similar to 

the Bulk Silicate Earth. Stage III and IV lithophiles show more 

depleted compositions. With respect to 3Het4He, Stage I 

appears to be dominated by a plume component, while later 

stages become increasingly similar to MORB. Hence the 

plume component appears in Stage I (Loihi Seamount), based 

on helium isotope systematics, but in Stage II ba~ on Sr-NdPb isotope systematics. This apparent decoupling of noble gas 

and lithophile tracers either requires another component in the 

Hawaiian ba~alts, or a separation of gaseous from lithophile 

components. 

We have mea'iured the isotopic composition of all five 

noble gases and Sr-Nd-Pb on a suite of young samples from 

the deep submarine rift zones from Loihi Seamount and 

Mauna Loa, and from the deep flanks of Hualalai. Our data 

corroborate previously published results: the combined 

isotopic data indicate an offset ahead in time of the helium 

(and neon) relative to the Sr-Nd-Pb isotopic record. This is 

most clearly demonstrated by Stage I, where helium is 

dominated by the plume component, which later becomes 

apparent in the Sr-Nd-Pb record during Stage II. During the 

latter stage, helium and neon already show a shift towards a 

MORB-like component, which later becomes apparent in the 

Sr-Nd-Pb record during Stages III and IV. Hence helium (and 

neon) signatures may be seen roughly one stage ahead of the 

equivalent Sr-Nd-Pb record. We envisage this to be caused by 

more rapid migration of the noble ga'ies in a separate (C02-

rich?) gas-phase in the melt extraction column. This is 

followed in time by the magmatic phase carrying the solid 

lithophile elements; helium and neon can be correlated with 

the solid lithophile elements on the scale of an evolving 

Hawaiian volcano, but are shifted ahead in time on the scale of 

individual samples. If this model is correct, a less depleted, 

and less degassed reservoir, relative to the upper MORB 

mantle, characterizes the Hawaiian plume component, and 

may originate in the lower mantle. 

334 

COMPARISON OF CUMAtE DATA WITII AN AMSDATED RECORD OF SALT MARSH ELEVATION, 

HAMMOCK RIVER MARSH, CONNECTICUT DURING 

THE LAST 1800 YEARS 

VAN DE PLASSCHE, 0., Institute .of Earth Sciences,Vrije 

Universiteit, 1081 HV Amsterdam, the Netherlands, VAN 

DER BORG, K. and DEJONG, AF.M., RJ. van dec Graaff 

Laboratory, Rijks Universiteit Utrecht, 3508 TA UTRECHT, 

the Netherlands 

The surface of mature salt marshes occurs from slightly below 

local mean high water (MHW) to somewhat above local mean 

high water spring (MHWS) tide. Salt marshes are able to keep 

up with high average rates of sea-level rise, provided sediment 

supply is sufficient and conditions for marsh plant growth are 

favorable. Salt marsh deposits thus contain a record of sea-level 

change, sediment supply and growth conditions. The deposits of 

New England type salt marshes are rich in organics, composed 

largely of the subsurface remains of plants growing at the marsh 

swface. Dominant peat forming plant species are Spartina 

alterniflora stunted, Spartina patens. and Distichlis &PiCata in the 

lower parts, and Juncus mwdii, Scit:pus robustus. ~ 

americanus, and PhtapU.tes australis on the higer parts. 

The first step in reconstructing former local MHW(S) 

positions from salt marsh deposits is to establish and date paleomarsh surface levels. We determined the level and age of 45 

former marsh swfaces within the upper 1.7 m of marsh deposit 

in Hammock River marsh, Connecticut by means of AMS-dated 

subsuface stalk parts of Sci.J;:pus robustus and~ 

alterniflora and rhizomes of Spartina patens and Distichlis 

~preserved in the marsh peat. The relation of subsurface 

plant remains to marsh surface is based on observations of the 

depth of modem root systems and of the vertical length of plant 

remains observed in hundreds of cores. Of each sample, two 

targets were measured to obtain radiocarbon ages with 1 sigma 

values less than 50 years. 

The reliability of the 45 dates is assessed by reference to stratigraphic position and internal consistency and an attempt is made 

to explain deviations in terms of sample contamination, sample 

preparation, instrument performance and statistical errors. 

The paleo-marsh surface curve. which is a reasonable 

approximation of the local MHW curve, is compared with paleoclimate data for the past 1800 years. 
RECONSTRUCTION OF PALAEOCLIMATIC 

REGIONAL VARIATIONS USING 13C DATA FROM 

RADIOCARBON DATABASES 

VAN KLINKEN G.J., Radiocarbon Accelerator Unit, 

University of Oxford, Oxford OXl 3QJ, UK, and VAN 

DER PLICIH, J., Centre for Isotope Research, Groningen 

University, 9747 AG Groningen, The Netherlands. 

Stable isotopes of hydrogen, oxygen, and to a lesser 

extent carbon are used, to study palaeoclimatic change. The 

sensitivity of carbon to climatic influences is far less 

pronounced than in the other two isotopes. Carbon isotopic 

ratios in ecosystems are established during the 

photosynthetic process in plants, and are then passed on in 

the food\veb without much modification. In a previous study 

(Van Klinken et al. in press), we have analysed a very large 

data set derived from 14C databases, and have shown that 13Cf12

C ratios in plant and animal tissues have close 

relationships with environmental factors such as temperature 

and relative humidity, giving an especially clear response in 

animal bone. These small-scale variations in 13C/12C ratios in 

\Vood, charcoal and bone result in a regular pattern of 

regional variation across Europe. The close association of 

carbon with the biosphere makes the use of carbon isotopic 

ratios especially valuable. 

We have extended our database with infotmation from 

areas with more extreme climatic conditions. and a 

discussion of potential applications of this type of analysis in 

palaeoclimatic reconstruction will be presented. 

Van Klinken, G . .T., Vander Plicht, .T., and Hedges, R.E.M., 

1994. Bone 13C/12C ratios reflect (palaeo-)climatic 

variations. Geophysical Research Letters in press. 

335 

DEFINITION OF CONTINENTAL HISTORY IN WEST 

GONDWANA USING SM-ND AND U-PB 

GEOCHRONOLOGY. 

VAN SCHMUS. W.R., Dept. of Geol., Univ. of Kansas, 

Lawrence, KS, 66045, BRITO NEVES, B.B., and 

BABINSKI, M., Inst. Geociencias, Univ. Sao Paulo, Sao 

Paulo-SP-Brasil, HACKSPACHER, P., Inst Geociencias 

et Ciencias Exatas, UNESP, Rio Claro-SP-Brasil, 

TOTEU, S.F., C.R.G.M., B.P. 333, Garoua, Cameroon. 

We report Srn!Nd and U/Pb results for the Borborema 

Province of NE Brasil and the Central African fold belt in 

Cameroon; we use these data to define the internal architecture 

of the region, with emphasis on defining the northen margin 

of the Sao Francisco-Congo craton (SF-CC), its fragmentation 

history, and continental domains assembled between it and the 

West African craton during the 600 Ma Brasiliano-Pan African 

orogeny associated with formation of West Gondwana. 

Archean cores of the SF-CC formed about 3 Ga as juvenile 

crust. 2.1 Ga granulites present between these cores represent 

fusion of them into the SF-CC. 2.1 Ga gneisses also occur 

north of the craton as defineable blocks in the 600 Ma fold 

belt. Most 2.1 Ga rocks in this region are reworked Archean, 

since Srn!Nd data typically yield Archean T(DM) ages. 

Several Mesoproterozoic graywacke (turbidite)-bimodal 

volcanic assemblages occur between older crustal blocks north 

of the SF-CC. U/Pb analyses from metavolcanic and granitic 

rocks of these domains in NE Brazil show that these 

supracrustal rocks formed about 1.0 Ga. Srn!Nd results for 

them and post-tectonic 600 Ma granites hosted by them do not 

indicate incorporation of large amounts of Paleoproterozoic or 

Archean material. We interpret these assemblages as formed in 

1.0 Ga rifts or small oceans in or along a large, pre-Gondwana 

continent (Rodinia supercontinent?). This 1.0 Ga rifting was a 

major tectonic and crust-forming event north of the SF-CC; it 

is apprently is also common in southern parts of West 

Gondwana. The 1.0 Ga rifting event may have created several 

continental fragments that were later incorporated into 

Brasiliano-Pan African collisional orogens at 600 Ma. There 

are also several other remnants of older crust in NE Brazil and 

central Africa that may represent exotic crustal blocks not 

originally part of the SF-CC; data are not sufficient as yet to 

answer this question. Syn- to post-tectonic granitoid plutons 

associated with the Brasiliano-Pan African collisional orogen 

all yield Nd isotopic signatures corresponding to the ages of 

their host rocks, typically giving T(DM) ages of ca. 1.2-1.5 

Ga (rift domains) or 2.6-3.0 Ga (cratonic fragments), 

consistent with magmas generated by partial melting of 

continental crust during or immediately following assembly of 

West Gondwana. We have not yet found evidence for juvenile 

600 Ma crust in this collisional orogen or significant juvenile 

2100 Ma crust within older cratonic fragments. 

In summary, combined use of U/Pb and Srn!Nd isotopic 

systems allows precise definition of the pre-collisional history 

of continental crust in this major 600 Ma continental collision 

belt and will allow us to define the pre-collisional history of 

the diverse crustal domains involved in the assembly of West 

Gondwana. 
STABLE CHLORINE ISOTOPE MEASUREMENTS IN 

GROUNDWATER PLUMES CONTAINING 

CHLORINATED HYDROCARBONS 

VAN WARMERDAM. E.M .• FRAPE, S.K., 

DRIMMffi, R.J., and CHERRY, J.A., University of 

Waterloo, Waterloo, Ontario, Canada, N2L 3G1 

Chlorinated hydrocarbons such as perchloroethylene 

(PCB), trichloroethylene (TCE) and 1, 1,1-trichloroethane 

(TCA), are common groundwater contaminants used in a 

wide variety of industries. Concentrations in these 

groundwater plumes vary from ppb to as high as the 

solubility limits of the products. For isotopic analysis of 

chlorine in these solvents, the chlorinated hydrocarbon is 

incinerated, converting the organo-chlorine to 

hydrochloric acid. The hydrochloric is then converted to 

methyl chloride and the Cl 37/35 value is measured using 

a mass spectrometer. The present known isotopic range 

of inorganic chloride is 3.5°/00 as identified by Long et al. 

Ongoing research examining stable chlorine isotopes in 

PCB, TCE and TCA shows that the range of 

measurement in pure phase organic products using the 

incineration method is 6.25°/00• The stable isotopes of 

chlorine undergo fractionation as a result of the 

manufacturing process. Each different manufacturing 

process of PCB, TCE and TCA results in a particular Cl 

37/35 content. A "fingerprint" value for PCB, TCE and 

TCA produced by different manufacturers can be 

identified based on the unique chlorine ratio associated 

with that particular product and manufacturer. Analytical 

techniques are being refined for Cl 37/35 analysis of 

chlorinated organic solvents found in groundwater 

plumes. At present, plumes with dissolved 

concentrations as low as 10 ppm can be analyzed for the 

Cl ratios. Various groundwater plumes will be studied to 

identify whether the fingerprints vary between the 

plumes. If the ratios differ, this technique may improve 

our ability to associate plumes or portions of plumes with 

particular sources or source areas. 

Long, A., Eastoe, C.J., Kaufmann, R.S., Martin, J.G., 

Wirt, L., and Finley, J.B., 1993, High-precision 

measurement of chlorine stable isotope ratios: 

Geochimica et Cosmochimica Acta, v. 57, p. 2907-

2912. 

336 

GEOCHRONOLOGICAL CONSTRAINTS ON 

THE MECHANISMS OF EXHUMATION OF 

REGIONAL METAMORPHIC TERRAINS 

YANCE. D .. Dept. of Earth Sciences, The Open 

University, Milton Keynes, MK7 6AA, UK. 

GALLAGHER, K., School of Geological 

Sciences, Kingston University, Kingston-uponThames, KTl 2EE. KELLEY, S. and AYERS, 

M., Dept. Earth Sciences, The Open University, 

Milton Keynes, MK7 6AA, UK. 

One-dimensional numerical modelling of regional 

metamorphism involving overthrusting and 

subsequent exhumation with constant erosion rates 

predicts cooling at increasing rates as the rock 

approaches the surface. This is in contrast with many 

regions of the Alps and the Himalaya, where thermal 

histories derived from a combination of garnet 

chronometry and Ar-Ar thermochronology require 

initially very fast cooling (up to 100°C Ma-l) 

followed by successively slower cooling. This shape 

of cooling path can be generated by either crustal 

extension or an exponentially decreasing erosion 

rate. In theory, the chronological data might be able 

to distinguish between the two mechanisms or at 

least yield a maximum time constant if the case of 

exponentially decreasing erosion rates was 

important 

We have modelled cooling histories for both 

extension (two dimensions) and a decreasing erosion 

rate (one dimension). The results indicate that in 

some cases both mechanisms can reproduce the 

observed thermal histories while in others the 

erosional time constant required to reproduce the 

observed cooling paths may be too small. In the case 

of the Tauem window, for example, the data require 

that the time constant for erosion is about 5 Ma with 

initial erosion rates of 5mm a-1. In the south-central 

Alps cooling is so fast early on that the time constant 

has to be unfeasibly small - < 1 Ma. An additional 

result of the modelling is that the isothermal 

decompression often associated by metamorphic 

petrologists with extension can equally well be 

produced by the onset of rapid erosion immediately 

after the metamorphic peak. 

The above models will be further tested using new 

laser Ar-Ar data from the Zanskar region of the 

Himalaya where a uniquely well-exposed 

extensional shear zone cuts through the top of the 

metamorphic pile. 
MECHANISMS AND KINETICS OF ATMOSPHERIC, 

RADIOGENIC, AND NUCLEOGENIC ARGON RELEASE 

FROM CRYPTOMELANE DURING 40ARf39AR ANALYSIS 

VASCONCELOS. P.M., Department of Earth Sciences, 

University of Queensland, Brisbane Qld 4072, Australia, 

RENNE, P.R., BECKER, T.A., Institute of Human 

Origins, Geochronology Center, 2453 Ridge Road, 

Berkeley, California 94709, U.S.A., and WENK, H.R., 

Department of Geology and Geophysics, University of 

California, Berkeley, California 94720, U.S.A. 

Resistance furnace (RF) 40 Arf39 Ar analyses of two 

compositionally distinct cryptomelane samples indicate that 

atmospheric 40Ar, 38Ar and 36Ar, radiogenic 40Af*, and 

nucleogenic 39 Ar and 38Af isotopes are released from the 

mineral structure at discrete temperature steps ranging from 

300 to 900 OC. The RF analyses permit us to identify the 

relative contributions of Ar from different reservoirs (adsorbed 

gases, intercrystalline space, and intracrystalline tunnel sites) 

in cryptomelane. The surface sites and intercrystalline spaces 

host primarily modem atmospheric Ar, which is released at 

temperatures ranging from 300-500 °C; the tunnel sites host 

radiogenic, nucleogenic, and paleoatmospheric Ar, which are 

liberated between 650-900 OC. 

Correlation between the resistance furnace results, laserheating release spectra, and in situ observation of the thermal 

behavior of cryptomelane in the high voltage electron 

microscope {HVEM) suggests that degassing of specific sites 

in the mineral are associated with discrete temperature 

intervals. The lower degassing temperature (peak at - 400 OC) 

is associated with release of atmospheric Ar from intergranular 

spaces. Only minor 40 Ar* and 39 Ar are released at this stage. 

Minor amounts of Ar released at intermediate steps {T = 500 

°C and 600 OC) are associated with the break down of poorly 

crystallized phases or Ar displacement by volume diffusion 

from the cryptomelane tunnel sites. The most significant 

degassing temperature (peak at T - 800 °C) is associated with 

the phase transformations of cryptomelane into hausmannite 

and manganosite. Most {>94% in sample PEG-01 and >60% 

in F115-139 .8m) of the 40 Ar* and 39 Ar are released at this 

temperature range, which makes the gas fraction liberated in 

the 700-900 OC interval the most significant for cryptomelane 

dating by the 40 Arf39 Ar method. 

The degassing behavior of cryptomelane in the RF 

experiment confirms that this mineral is stable during the 

sample preparation procedures preceding 40 Arf39 Ar analysis. 

The samples release no radiogenic or nucleogenic Ar at 

temperatures below 300 OC (Co-cryptomelane) and 400 °C (!(-

cryptomelane), and the high temperature release of 40 Ar*, 

39 Ar, and 38 Ar (700-900 OC) can be directly correlated to the 

break down of specific crystal sites in which these isotopes are 

hosted. This coherent behavior is drastically different from the 

continuous low temperature leakage of Ar observed in poorly 

crystalline clays and cryptocrystalline minerals, suggesting 

that cryptomelane is a much more competent, stable, and Ar 

retentive mineral than previously studied low temperature 

phases. The coherent behavior of cryptomelane further' 

suggests that this mineral is suitable for dating 

paleoweathering processes by the K-Ar and the 40 Arf39 Ar 

methods. 

337 

UNRAVELLING mE GROWfH "STRATIGRAPHY" IN 

GRANULITE ZIRCONS FOR SHRIMP DATING OF mE 

LOWER CRUST: A CASE STUDY FROM mE IVREA 

ZONE (SOUTHERN ALPS) 

VAVRA Gerhard, GEBAUER, D., SCHMID, R, Department of Earth Sciences, ETH ZUrich, CH-8092 ZUrich, 

Switzerland 

The timing of metamorphic and magmatic events in the 

lower crust is largely derived from conventional U/Pb zircon 

data, which do not consider in detail the processes affecting 

the U/Pb system in zircon. The cathodoluminescence of 

granulite zircons reveals a variety of growth and alteration 

domains which can be U/Pb-dated by ionprobe and assigned to 

petrogenetic processes. 

In a restitic metasedimentary granulite from the Ivrea 

zone, zircons were separated from adjacent metapelitic and 

quartzitic layers. In both layers, isometric overgrowths surround Precambrian detrital cores. The first period of growth, 

characterized by radial growth sectors, is only obseiVed in the 

metapelitic layer, whereas the second and final period of 

growth, characterized by rounded shells, is present in both 

layers. The 206Pb/238u ages of the first period scatter between 295 and 262 Ma with a mean value of 277 ± 7 Ma (95 

% c. 1.). Several growth pulses indicate to a long lasting 

growth period. In contrast, the second growth period defines a 

rather short event at 262 ± 3 ~ The data imply that anatectic 

melting started in the metapelitic layer at 295 ± 8 Ma. The Zr 

was probably provided by reacting biotite (Bi+Sil+Qtz-

>Gt+Ksp+H20). Several anatectic pulses may have been triggered by gabbroic intrusions. In the quartzitic layer, the detrital cores were corroded by anatectic melt Thus, the start of 

anatexis in this layer cannot be dated The final growth episode at 262 ± 3 Ma is likely to be related to a phase of rapid 

uplift and isothermal decompression. As a result, additional 

anatectic melt may have formed and crystallized within a short 

period. The typical rounded shape of shells grown during this 

period may be due to rotation of grains at tectonic shearing. 

In a metagranitoid from the same complex, zircon overgrowths can be correlated with those in the metasedimentary 

layers. In addition, the large protolithic cores of this sample 

allow detailed UIPb-dating of alteration processes. Unaltered 

Ctystal domains yield an age of 345 ± 5 Ma for the intrusion. 

Recrystallization causing strong loss of U, Th and radiogenic 

Pb from some domains occurred at 275 ± 3 Ma. The recrystallization is related to the overstepping of a critical temperature, 

but does not mark the temperature maximum. A second type 

of alteration is present as diffusion-controlled flow structures 

which digested recrystallized and non-recrystallized domains. 

The structures indicate volume diffusion which is most likely 

related to the thermal peak of granulite metamorphism. Since 

diffusion did not efficiently expel the inherited radiogenic Pb, 

an U/Pb age for the thermal peak cannot be determined However, this age is closely bracketed by the prograde recrystallization event at 275 ± 3 Ma and late zircon overgrowth from a 

crystallizing anatectic melt at 271 ± 3 Ma. 

The ionprobe dating of zircon resolves the granulite facies 

metamorphism into a pre-climax temperature increase of long 

duration, with anatectic melt forming in several steps since 

295 ± 8 Ma, and a final event of rapid anatectic melt 

formation and crystallization at 262 ± 3 Ma, shortly following 

the temperature maximum. A separate thermal pulse is 

recorded by partial recrystallization at 225 ± 5 Ma. 
BORON ISOTOPES IN GROUNDWATER: 

IDENTIFICATION OF SOURCES OF 

CONTAMINATION 

YENGOSH A. Hydrological Service, P.O. Box 

6381, Jerusalem 91063, Israel, HEUMANN, K.G, 

JURASKE, S Institute of Inorganic Chemistry, 

University of Regensburg, Universitatsstrabe 31, 

Regensburg D-93053, Germany, and BARTH, S. 

CRPG-ENSG, 15 rue Norte Dame des Pauvres, 

BP 20, F-54501 Vandoeuvre-les Nancy Cedex, 

France. 

One of the main goals of environmental 

protection is to prevent deterioration of water 

quality through early recognition of contamination 

sources. The relatively large mass difference 

between the two stable isotopes of boron resulted in 

a wide range of ollB* values in natural waters, 

ranging from -15%o to +60%o. The different 

isotopic signatures have been used for tracing the 

sources of dissolved boron, and in particular to 

distinguish marine from non-marine sources. In 

addition to the natural load, the application of 

synthetic boron compounds, especially sodium 

perborate as a bleaching agent in detergents, leads 

to an enrichment of boron in waste waters, and 

consequently in natural aquatic systems. The 

anthropogenic annual discharge flux (10llg B /yr in 

Western Europe alone) is significant and even 

comparable to the natural global boron influx to the 

oceans (3x1ollg B /yr). 

Boron isotope composition of anthropogenic 

boron has been examined in raw and treated 

sewage effluents as well as contaminated 

groundwater from the the Dan Region Project and 

Coastal Plain aquifer in Israel. It is shown that the 

olb values of sewage effluents (5.3%o to 12.9%o) 

overlap with those of natural borate minerals 

(-0.9%o to 10.2%o) but are significantly different 

from those of regional groundwater ( -30%o) and 

sea water (39%o). Groundwater contaminated by 

artificial recharge of treated sewage yields a high 

B/Cl ratio with a distinctive anthropogenic isotopic 

signature (6.8%o to 25%o) which is different from 

those of natural marine-derived saline groundwater 

from the Coastal Plain aquifer (35%o to 60%o). 

This enables utilization of the boron isotope 

composition of groundwater as a tracer for sources 

of contamination of groundwater, particularly 

detergents compounds in aquatic systems. 

* ollB=(llB/10B)sample/(11B/lOB)NBS 951-1] X 

1000 

338 

Ar-Ar CHRONOLOGY OF THE LADAKH 

COLLISION ZONE, NORTHWEST HIMALAYA 

_ VENKA1ESAN, T. R, Physical Research 

Laboratory, Ahmedabad 380009, India. PANDE, K, 

Physical Research Laboratory Ahmedabad 380009, 

India, SHARMA,K.K and CHAWALA, H.S, Wadia 

Institute of Himalayan Geology, Dehradun 248001, 

India. 

In the Ladakh sector a more or less complete record 

of magmatism, sedimentation and deformation is 

preserved to enable a study of crustal growth and 

collision tectonics between the Indian and Eurasian 

plates. The Indus Suture zone has long been 

recognised as a zone of Tertiary collision between the 

two plates. The presence of a second Suture - the 

Shyok-Nubra - (Northern Megashear in Kohistan 

sector) - has also been recognised. However, the 

sequence and timing of the formation of these two 

Sutures are still debated Some believe that Shyok-Nubra 

Suture is older than the Indus Suture while others 

prefer a younger age for the former. 

To resolve this controversy detailed Ar-Ar 

chronology of the various igneous constituents of the 

two Sutures has been undertaken to supplement field 

studies. Here we report the data for volcanics from 

the Shyok-Nubra zone. Of the six whole rock samples 

analysed only one yielded a plateau age of 58 Ma. 

Other samples exhibit age spectra indicating a slow 

cooling and a loss of about 90-95% argon. From these 

spectra we can infer a thermal event between 14-28 

Ma. Interestingly such a range of ages for Tertiary 

Himalayan granites (Dietrich and Gansser, 1981) and 

biotite Rb-Sr isochrons of 28-29 Ma from Ladakh 

(Trivedi et al., 1982) have been reported. 

Dietrich,V and Gansser,A., 1981, The Luecogranites 

of the Bhutan Himalaya : Schweiz. mineral. petrogr. 

Mitt. v61, p177-202. 

Trivedi) .R., Gopalan,K., Sharma,K.K., Gupta,K.R, and 

Choubbey,V .N, 1982, Rb-Sr age of Gaik granite, 

Ladakh batholith, Northwest Himalaya: Proc. Indian 

Acad. Sci.(Earth Planet Sci), v91, p65-73. 
LOW TEMPERATURE (S300°C) HYDROGEN 

ISOTOPE EXCHANGE BETWEEN HYDROUS 

MINERALS AND MOLECULAR HYDROGEN 

VENNEMANN. Torsten. W. and O'NETI..., J.R., 

Department of Geological Sciences, University of 

Michigan, Ann Arbor, MI 48109, USA. 

In order to establish a consistent set of mineral-water 

D/H fractionation factors, we have used H2 as an exchange 

medium for hydrous minerals. Mineral-water fractionation 

factors may then be calculated from known water-}\ 

fractionations. The small size of }\ as opposed to }\0 

allows for good penetration to the interiors of the crystal, 

enhancing D/H exchange with the OH-sites at temperatmes 

below those of previous calibrations based on mineral-water 

exchange. Preliminary results obtained on the basis of a 

method where the gas provides an "infinite" reservoir of}\ 

and approach to equilibrium is estimated by a change in the 

~D values of biotite, muscovite and homblend~ at 

temperatures ~300"C (Vennemann and O'Neil, 1993), 

together with recent ion-microprobe analyses of individual 

crystals (mm's to em's in size) exposed to gases enriched 

in HD for short periods of time (5-7 days), confirm 

penetration of and exchange with }\ well below the surface 

of the crystals. 

For temperatures below 3000C a method is chosen where 

the mineral provides an "infinite" reservoir of hydrogen and 

the approach to equilibrium is estimated by a change in the 

~D values of two isotopically distinct gases which 

approach an equilibrium distribution of isotopes from 

compositions relatively enriched and depleted in deuterium. 

To remove interfering gases such as C02, N2, H20 which 

remain within the crystal even after prolonged evacuation 

at temperatures as high as 3000C, and to avoid problems of 

preferential adsorption of HD, the minerals are subjected to 

"He-flushing" prior to exchange with }\, and the gases 

used for exchange are expanded onto and removed from the 

minerals at temperatures of exchange. The infinite-gas 

method employed at temperatures above 300"C results in 

about 90% exchange of D/H between epidote and hydrogen 

after 20 days at 3000C. 100% exchange is obtained in 

periods of less than 2 to 3 days for epidote grains of 60 to 

125 J.Lm. in size with the infmite-mineral method. This 

method also provides epidote-}\ fractionation factors that 

are within error of the infinite-gas method (1000lna = 442 

and 435, respectively). 

It is concluded that laboratory calibrations of the 

hydrogen isotope exchange between molecular }\ and 

hydrous minerals provides a suitable method for the 

indirect evaluation of the geologically important low 

temperature fractionation factors between hydrous minerals 

and water. Minerals under investigation at present include 

epidote, muscovite, kaolinite, serpentine and chlorite. 

Vennemann T.W., and O'Neil, J.R, 1993, EOS, V.74, p. 

330. 

339 

STABLE ISOTOPE AND 40 Ar 139 Ar CONSTRAINTS 

ON PREALPINE RECONSTRUCTIONS AND P-T-t 

PATHS WITHIN THE CENTRAL SESIA-LANZO 

ZONE, WESTERN ALPS, NORTHERN ITALY 

VENTURINI. G., COSCA, M.A., SHARP, Z.D., and 

HUNZIKER, J.C. lnstitut de Mineralogie, Universite 

de Lausanne, BFSH-2 1015, Switzerland, 

The Sesia-Lanzo zone is composed of a belt of high 

pressure (HP) (granulite to eclogite) and greenschist 

(GS) facies units in the Western Alps of Northern Italy 

(Venturini et al., 1994). A new geological map of the 

Sesia-Lanzo zone based on detailed mapping and 

compilations by the authors has resulted in a new 

geotectonic model of this area. Stable isotope and 

40 Ar I 39 Ar analyses have been made on mineral 

separates from the Sesia-Lanzo zone to determine the 

relationship between the Prealpine basement, the 

Permo-Mesozoic cover sequences, and the effects of 

Alpine metamorphism. 

Stable isotope thermometry applied to eclogite 

mineral separates (o18o Qz = 21.5%o, o18o Gt = 17.9%o, 

o18o Omph = 17.0%o, o18o Ru = 16.9%o) yield 

temperatures of 570°C, consistent with cation exchange 

thermometers. 

Over 30 whole rock oxygen isotope values have_ been 

determined from orthogneiss and paragneiss. The o18o 

values of orthogneiss and 1-Type metagranites are 

similar (o18o = 8.2 - 8.8 %o) and distinct from S-type 

metagranites (o18o = 11.2%o), while paragneiss give 

values >10%o (o 180quartzites = 11.9 - 12.4 %o, 

o180leucocratic gneiss = 11.2). The consistent and 

distinct isotopic signature between rock types is too 

small to unequivocally determine protolith lithologies 

in strongly sheared rocks. 

Incrementally heated 40 Ar I 39 Ar age spectra have 

been collected from 29 phengite both the internal HP 

unit and the external GS unit. Cooling ages from the 

well-preserved HP unit are 60-85 Ma and 45-60 Ma for 

the GS unit. Our field and isotopic data, combined with 

published data (Hurford and Hunziker, 1991), support 

unique cooling histories prior to 35 Ma for the HP and 

GS units. The two units follow a common P-T-t path 

subsequent to 35 Ma. 

Hurford,A.J. and Hunziker J.C., 1991, Constraints on the 

late thermotectonic evolution of the western Alps: 

evidence for episodic rapid uplift: Tectonics, 10-4, p. 

758-769. 

Venturini, G., Martinotti, G., Armando, G., Barbero, M., 

Hunziker, J.C., 1994, The Sesia-Lanzo Zone (Western 

Italian Alps): new field observations and 

lithostratigraphic subdivisions: Schweiz. mineral. 

petrogr. Mitt., in press. 
Sr And Nd Isotopic Constraints On The Generation Of 

Large-Scale Compositional Zoning In Epizonall\{agma 

Bodies 

P L Verplanck and G L Farmer (Dept. of Geological 

Sciences and CIRES, University of Colorado, 

Boulder, CO 80309-0216; 303-492-1276) 

M McCurry (Dept. of Geology, Idaho State 

University, Pocatello, ID 83209) 

A detailed isotopic and chemical study of Oligocene-age 

Organ Needle pluton (ONP) in south-central New Mexico and 

its Precambrian granitic wall-rock is underway to test models 

for the origin of compositional zoning in large volume silicic 

magma bodies. The dominant volume of the ONP is syenitic 

in composition, but the pluton is capped by an alkali feldspar 

granite {AFG), with a narrow transition between the two 

lithologies (1 to 10 meters). The coarse-grained syenite has 

Si02=60%-70%, £Nd=-2.3 to -2.8, and 87sr,t86sr -0.7055-

0.7065 with systematic major and trace element variations 

that correlate with mineralogical variations. The chemical 

data, combined with the narrow range in isotopic 

composition, suggest that the bulk of the pluton evolved by 

closed-system fractional crystallization. The AFG 

(Si02=74%-77%) has £Nd=-5 to -5.5 and 87sr/86sr -0.710-

0.715, values not only distinct from the underlying syenite, 

but also from the wall-rock, dominantly Precambrian granite 

(eNd(36Ma)= -10 to -14 and 87sr/86sr(36 Ma)=0.750-

0.800). The isotopic constraints preclude the possibility that 

the AFG formed by protracted closed-system fractionation of 

the underlying magma body or by roof-rock melting. 

Although the interior of the pluton has a relatively uniform 

isotopic composition and systematic chemical variations, the 

margins of the pluton are more chemically and texturally 

diverse with isotopic compositions that overlap those of the 

AFG. At the easternmost (and structurally deepest) exposed 

portions of the ONP, the margin of the pluton is coarsegrained but inequigranular with Si02=57% to 63%, £Nd=-4 

to -5.5, 87sr/86sr-0.708-0.711, and displays a wide range of 

trace element compositions over a geographically small 

region. Within 400m of the contact the border lithology 

grades into the more homogeneous interior portion of the 

pluton. Samples of the marginal portions of the pluton along 

other structurally higher transects also have relatively low 

£Nd (-4.5 to -5.5) but higher 87srf36sr (up to 0.719) and 

higher Si02 (63%-68% ). Overall the side-wall material has 

isotopic compositions similar to the AFG, suggesting that a 

genetic link may exist between these lithologies. The most 

likely model involves assimilation of wall-rock at the base or 

sides of a magma body in conjunction with side-wall 

differentiation and melt migration upward, culminating in the 

ponding of silicic magma over the main magma body at the 

top of the chamber. 

340 

EVOLUTION OF THE DEPLETED MANTLE: THE 

HAFNIUM PERSPECTIVE 

VERVOORT, JEFFREY D. and PATCHETT, P. 

JONATHAN, Department of Geosciences, 

University of Arizona, Tucson, AZ, 85721, USA. 

The Nd isotopic data set for mantle-derived rocks is 

characterized by two major temporal trends. First, 

there is a near linear increase in initial epsilon values 

of mantle-derived rocks from approximately 2.7 Ga to 

the present. This trend has been widely accepted as 

reflecting a widespread depleted mantle reservoir 

formed by the extraction of crust from the mantle. The 

existence of such a reservoir is easy to reconcile with 

the abundance of preserved Archean and younger crust. 

Second, the initial epsilon values of mantle-derived 

rocks appear to be static at approximately + 1 to +3 

from 3.8 to 2.7 Ga. These high, positive values require 

the existence of highly depleted mantle reservoirs in 

the early Archean which would appear to necessitate 

widespread differentiation of the mantle within the 

first few hundred million years of Earth's formation 

(e.g., Bennett et al., 1993). However, the Nd isotopic 

data for altered or metamorphosed early Archean 

rocks are potentially problematic in a number of ways 

and the validity of these data need to be evaluated. 

Part of the problem derives from the fact that the SmNd system does not need to be disturbed very much in 

order to modify the apparent initial Nd isotopic 

compositions of these rocks. Various authors have 

suggested that the highly positive epsilon values of 

many of the earliest Archean rocks are spurious and are 

a product of later disturbance of the Sm-Nd system. 

Because of this concern for the validity of early 

Archean Nd isotopic data, it is important to test 

models for early mantle depletion with other isotopic 

systems. Because zircons are robust with respect to 

alteration, their Lu-Hf isotopic systems are not as 

susceptible to later perturbations as either Sm-Nd or 

Lu-Hf systems in whole-rock samples. Therefore, our 

approach has been to obtain zircons from carefully 

selected suites of mantle-derived rocks throughout 

Earth's history, examine them for concordance, then 

analyze the concordant populations for Hf composition. 

If a zircon population is concordant, or nearly so, this 

should be sufficient evidence that the Lu-Hf system in 

the zircons has not been disturbed and their initial 

isotopic compositions are accurate. In order to relate 

the Hf and Nd isotopic data we have also analyzed 

the whole-rock portions of these rocks for Sm-Nd. 

Preliminary results from this study will be 

presented for Hf isotopic compositions of previously 

dated, concordant, or near-concordant, zircons from 

Amitsoq Gneisses in the Nuuk and lsukasia regions in 

Greenland as well as for zircons from the early 

Proterozoic juvenile terranes of the Flin Flon belt of 

Canada. 

Bennett, V.C., Nutman, A.P., and McCulloch, M.T., 

1993, Nd isotopic evidence for transient, highly 

depleted mantle reservoirs in the early history of 

the Earth: Earth Planet. Sci. Letters, v. 119, p. 299-

317. 
GEOCHEMICAL TYPOLOGY OF PLUME·LITHOSPHERE 

INTERACTIONS IN THE MARQUESAS AND SOCIETY 

ARCIDPELAGOS (FRENCH POLYNESIA) 

VIDAL. Ph .. CNRS and Univ. Blaise Pascal, Clermont· 

Ferrand, DUPUY, C., BARCZUS,. H. G., CNRS, 

Montpellier, CAROFF, M., MAURY, R., CNRS and 

Univ. Brest, France. 

Isotope (Sr, Nd, Pb) and trace element data on basalts from 

Marquesas archipelago can be interpreted as a mixture of EM II, 

IDMU and DM components. They also support the existence of 

three geographic groups, each of these groups having its own 

specificity reflecting the variable proportions of these 

components as well as the processes involved in the basalts 

generation. In addition, there is a correlation between magmatic 

types and isotopic compositions. For example, in one group 

(Ua Pou, Tahuata, Fatu Hiva), the magmatic activity started 

4.5 Ma ago with tholeiites having a HIMU source, and ended 

up 2 Ma ago with alkali basalts having an EM II source. 

These regularities in the magmatic evolutions are 

accompanied by two strikingly contrasting situations : 

• every time a HIMU source is involved in the genesis of 

shield·building basalts, the post shield-building lavas exhibit a 

EM signature. This is the ease in two Marquesas groups. 

- conversely, when an EM source is involved in the genesis of 

the shield·building basalts, the post shield-building lavas 

exhibit a DM-type. This is the case in the third Marquesas 

group and in Society. 

These observations are discussed in the frame of the current 

models on plume-oceanic lithosphere interactions. 

341 

8SI(r-133Xe ZIRCON DATING 

1M Villa 1, J Eikenberg2, E Lehmann2 

lUniv Bern, Isotopengeologie, Erlachstr. 9a, 3012 

Bern, CH - 2Paul Scherrer Inst, 5232 Villigen, CH 

Xer-Xen dating by neutron irradiation was 

established 20 years ago [1]. Our modification 

includes Kr and involves the following procedure: 

1. Separation of =90% pure zircons. While U/Pb 

always requires handpicking, U-Kr-Xe only needs to 

avoid secondary U minerals. 

2. Computer simulation, via cross-sections, of all 

induced activities as a function of fluence & cool-off 

time; of main interest: short-lived U-proxies 239Np, 

133Xe; health hazards 132Te/l, 140JlaJI.,a. 

3. Thermal neutron irradiation of unknown samples 

together with age monitors. 

4. Hot-lab 'Y spectroscopy of 239Np to calculate U (& 

further trace elements from other radioisotopes). 

5. Cooling period minimising health hazards but 

limiting 133Xe decay. 

6. Stepwise heating analysis of Kr and Xe. 

We established the feasibility on magmatic zircons 

from the Limpopo belt, Zimbabwe, whose 

concordant U/Pb age is 2591±1 Ma. We observed: 

.u.. U contents via Kr, Xe agree with prel. U from Np. 

]2. Degassing artefacts: the ratio of artificial 85Kr to 

133Xe should follow Rayleigh fractionation as T 

grows, as both derive from the same target, U, but it 

jumps irregularly, correlating with release bursts. 

~. N aturalloss mechanisms are complex: in 14 unirradiated overprinted U-minerals, 86Krfl36Xe is 

compatible with volume diffusion in only 1 case. 

Conclusions. What are the main advantages? 

I. Complementarity to U/Pb, possible screening of 

most suited samples for handpicking. 

n. Usual advantage of stepwise heating: constraining 

disturbances and/or formation ages. 

ill. Checking deviations from ideality of neutroninduced 85KrJ133Xe and natural fission 86KrJ136Xe. 

IV. Assessment of kinetic processes: using two 

noble gases is one step further than 40 Arf39 Ar, 

where non-diffusive transport requires a much more 

elaborate proof and is therefore still denied by some. 

[1] Shukolyukov et al, Dokl. Akad. Nauk 219 (1974) 952 
GEOCHRONOLOGY OF SUPRACRUSTAL 

SEQUENCES IN 1HB SLAVE PROVINCE, NWT, 

CANADA: IMPLICATIONS FOR AGE OF BASEMENT. 

Vll.J..ENEUVE. M.E., Geological Survey of Canada, 

Ottawa, Ont., K1A OE8, RELF, C., Canada-NWT 

Mineral Initiatives Office, Yellowknife, NWT, Canada, 

HRABI, B., Queen's University, Dept. of Geological 

Sciences, Kingston, Ont., Canada and JACKSON, V., 

DIAND Geology, Yellowknife, NWT, Canada. 

The Slave Province of northwest Canada comprises 

abundant 2.7-2.65 Ga volcanic-tubidite successions 

(Yellowknife Supergroup) and numerous 2.7-2.55 Ga 

plutons. Significantly older basement is restricted to the 

western Slave Province, as evidenced by the reconnaissance 

Sm-Nd study of Davis and Hegner (1992). U-Pb 

geochronology has been carried out on a transect along the 

eastern margin of this inferred basement terrane. 

At two locations along this zone, ca. 3.1 Ga rhyolite 

volcanic rocks have been discovered. In the central Slave 

Province, a massive felsic volcanic unit, dated at 

3118+ 11/-8 Ma, has been mapped at the base of the Winter 

Lake supracrustal belt. Similar rocks are found in the 

north-west Slave Province. In the Napaktulik Lake map 

area, two samples of felsic volcanic rocks, from a sequence 

of interlayered mafic volcanics, felsic volcanics and 

meta-sediment, give ages of3142±1 Ma and 3134±1 Ma. 

These rocks provide the first evidence for widespread 

volcanic sequences significantly older than the Yellowknife 

Supergroup. 

Cross-bedded quartz arenites overlie the Winter Lake 

rhyolite. A sample of this unit yields detrital zircons that 

fall within a tightly restricted age range of3.13-3.16 Ga. 

About 60 km north of the Napaktulik rhyolites, upper 

amphibolite grade quartz arenites form competent layers in 

gneisses, which are in fault contact the 2.7 Ga Anialik 

River volcanic belt. Detrital zircons from this unit range in 

age from 3.15-3.30 Ga. A conglomerate in the gneisses has 

zircons with a broader age range; from 2.88 Ga to 3.4 Ga. 

Late orogenic (<2.6 Ga) polymictic conglomerates are 

dominated by syn-Yellowknife Supergroup (ca. 2.7 Ga) 

detritus, but also contain 2.96 Ga and 2.72-2.76 zircons. 

The latter ages are coeval with a rhyolite porphyry dated at 

2729+8/-7 Ma and a 2720+8/-6 Ma deformed granite from 

the central Slave Province. 

This data indicates at least four periods of basement 

crust formation in the western Slave Province. The absence 

of 3.6-3.96 Ga ages suggests that the Acasta gneiss block is 

of limited extent. The preponderance of3.1-3.3 Ga age 

detritus and rhyolites suggests that western basement 

terrane(s) are dominantly crust of this age, although a ca. 

2.85-2.96 event is also documented. Finally, a magmatic 

episode began at 2. 75 Ga and CQntinued through onset of 

Yellowknife Supergroup deposition. 

Davis, W.I. and Hegner, E., 1992, Neodymium isotopic 

evidence for the tectonic assembly of Late Archean crust 

in the Slave Province, northwest Canada. Contrib. 

Mineral. Petrol, v. 111, p. 493-504. 

342 

MAGMATIC PROVIRCBS IR -:rBB ZIIIBABNB CRM!OII: 

IMPLICATIORS FOR IIAiftLE BETEROGEREITIBS 

ARD SOURCE RBGIOR DBPLBTIOR DURIRG THE 

LA.TB ARCBAEAR ERA I 

M.L.Vinyuill 

Dept of Geology, University of 

Zimbabwe, P.O. Box MP 167, Mt 

Pleasant, Barare,Zimbabwe. 

Basement-cover relationships and new age data provide evidence that the aafic/ 

ultramafic volcanics that are an integral 

part of the circa 2. 7 Ga event in the 

Zimbabwe craton were deposited onto preexisting crust in zones of ensialic 

rifting. 

Geochemical data bases on these 

late Archaean volcanics from two 

greenstone belts (with siailar structural 

styles and overlapping ages) located in 

the southern and northern parts of the 

Ziababwe craton are coapared with a view 

to model their source composition and 

evolutionary histories. 

The lavas from the two greenstone belts are easily distinguished, and subdivided 

into two distinct geochemical provinces, on the basis of their major and trace 

element geochemistry. Higher Al203, Ti02, 

Pe203, P205, Zr, Y, San and lower Mgt 

contents are found in the northern segment vo~canics while the lavas from the south 

show a distinct enrichment in S~02, total 

alkalis and Mn02 in addition to their 

higher Mgt. 

Major elemen't geotectonic discrimination 

diagrams favour a geodynamic eruptive 

environment that was transitional between 

an oceanic and continental marginal 

setting for the formation of both suites 

of volcanics. Trace element plots favour 

derivation of the lavas in both provinces 

from plume related activity, with the 

lavas from the northern part being derived 

from a more depleted mantle domain 

(Fig.l). The implications of the existance 

of distinct magmatic provinces in one superevent are discussed with reference to 

the geotectonic environment and the 

various processes governing mantle melt 

formation in the late Archaean era. 

20 -=~--------------------~ 

10 

Pig.l. Y/Rb vs zr/Rb 

plot 
CHLORINE ISOTOPIC COMPOSITION OF 

MARINE AEROSOlS 

VOLPE. C. , Scripps Inst. of Ocean., San Diego, La 

Jolla, CA 92093 and SPIVACK, A. J ., Dept. of 

Earth Sciences, University of North Carolina at 

Wilmington, Wilmington, NC 98405 

Chlorine is involved in many important 

atmospheric chemical reactions, in particular, the 

homogeneous and heterogeneous catalytic destruction 

of stratospheric ozone. Methyl chloride and 

anthropogenic halocarbons are the principle sources 

of reactive chlorine in the stratosphere, however, 

their relative contributions to the inorganic chlorine 

budget of the stratosphere are not well known. 

Chlorine may also play a role in tropospheric 

chemistry. 

It has been suggested that chlorine liberated from 

synthetic halocarbons may be isotopically distinct 

from natural sources, yet, the isotopic composition of 

the various atmospheric chlorine species is still 

unknown. If the major chemical reservoirs and 

sources of atmospheric chlorine (anthropogenic 

halocarbons, methyl chloride, volcanic HC~ and 

tropospheric and stratospheric inorganic chlorine) 

prove to have unique chlorine isotopic compositions, 

it may then be possible to utilize the isotopic 

compositions of these species to constrain chlorine 

fluxes between the earth's surface, the troposphere 

and the stratosphere. In addition, if isotopic 

fractionation is associated with particular chemical 

reactions, it should also be possible to utilize isotopic 

variations to constrain atmospheric reaction 

mechanisms. 

Here we report the frrst isotopic composition 

determinations of an atmospheric chlorine specie, 

marine aerosols. These data are part of a larger 

project aimed at determining the isotopic systematics 

of atmospheric chlorine. Isotopic compositions were 

determined by the mass spectrometric analysis of 

dicesium chloride with an external reproducibility of 

0.08 per mil. All of the aerosol samples analyzed so 

far are isotopically enriched relative to sea water, up 

to + 2.5 per mil. The data fall on a trend that is 

consistent with the "fractional distillation" of chlorine 

with a best fit fractionation factor of 0.9969. 

Laboratory experiments confrrm that HCl volatilized 

from marine sea salt particles is isotopically light 

relative to sea water with a fractionation factor of 

0.9969. 

343 

lOBe/)Be RATIO AS A SEAWATER MASS TRACER. A 

WORLDWIDE STUDY OF FERROMANGANESE CRUSTS 

VON BLANCKENBURG, F., BELSHAW, N.S., O'NIONS, R.K. 

(Earth Sciences, University of Cambridge, UK), and HEIN, 

J.R., (U.S. Geological Survey, Menlo Park, CA 94025) 

lOBe /)Be ratios for samples taken from the surface layers 

of oceanic ferromanganese crusts distributed worldwide have 

. been determined using the new SIMS-based (ISOLAB) 

technique (Belshaw et al. 1994). Cosmogenic 10Be 

(T112=1.5Ma) is mainly produced by interaction of cosmic rays 

with the atmosphere, from where it is precipitated by rain into 

the oceans. Stable 9Be is incorporated into the deep oceans 

primarily by wind-blown dust. Accelerator Mass Spectrometry 

(AMS) results of modem sea water suggest higher 10Be/)Be 

ratios for the Pacific ( -1 * I0-7) than for the Atlantic ( -6* IQ-8, 

Kusakabe et al. 1990). 

In an attempt to investigate whether this pattern has 

prevailed in the past, we have separated Beryllium from the 

outermost 1 millimetre of oceanic ferromanganese crusts using 

dissolution and a carrier-free all chromatographic technique. 

These crusts, sampled from seamounts, have formed 

authigenically; their isotope ratios should therefore have been 

those of sea water at time of formation. At typical growth rates 

of 3-lOmm/Ma the measurements reflect the 1 0Be/)Be ratio 

integrated over the ca. 100-300ka time interval represented by 

the samples. The results (see figure below) show a similar 

difference between the Pacific and Atlantic as does modem sea 

water. This includes both hydrogenous and hydrothermal 

crusts. Some scatter in the data could be due to radioactive 

decay of lOBe during growth of the layer. 

The prevalence of the difference between Atlantic and 

Pacific lOBe/)Be ratios over the last few ka indicates that the 

present oceanic circulation pattern has been maintained 

through this period. The isotopic composition of Be, of which 

the residence time in the deep oceans must be less than the 

mixing time of the oceans (1000a), is sufficiently sensitive to 

detect global patterns while averaging regional variations. It 

may therefore serve as a sea water mass tracer suitable to study 

aleo-oceanic circulation attems. 

10 e/9 • F C B Be m erromanganese rusts 

)> 

1.2 •u 

Belshaw, N.S., O'Nions, R.K .. and von Blanckenburg, F., 

1993, A new SIMS technique for 1Dse,BBe ratio 

measurement: J. Mass Spec. Ion Phys., submitted. 

Kusakabe, M., Ku, T., Southon, J.R., and ~1easures, C.I., 

1990, Beryllium isotopes in the ocean: Geochem. J., v.24, 

p. 263-272. 
GBOCBBMICAL (RBB) AND ISOTOPIC (Rd, 0) 

IRBOMOGBRBITY OF TIIB LATB ARCBABAR UPPBR 

KARTLB 01' T11B BALTIC SBIBLD (BVIDBHCB 

PROM TIIB PBRIDOTITIC KOMATIITBS) 

VREVSKY,A.B., SVETOV,S.A.,Inst.of Precambrian Geology & Geochronology, Russian 

Acad.Scien.,St.Petersburg,199034, Russia) 

The peridotitic komatiites (PK) from the 

Archaean greenstone belts of the Eastern 

Baltic Shield can be divided based on their 

REE abundances in four geochemically 

distinct types with: 1) flat chondritenormalized REE patterns (0.4-1.5, 1.5-3.0, 

3.0-5.0 x chondrite abundances); 2) LREE 

depleted and flat HREE (La/Sm)N = 0.6-0.7; 

3) light LREE enrichment (La/Sm) N 

1.4-1.6; 4) strong negative Eu anomaly 

Eu/Eu * = 0. 04-0. 1. 

This geochemical types of PK can be 

interpreted by different petrological 

models: 1) single-stage 50-60% partial 

melting at P-35 kb of heterogenic in REE 

mantle source; 2) secondary 25-30\ partial 

melting at P- 20-30 kb of residue after 

segregation of first stage 10-15% partial 

melting liquid; 3) contamination of 

komatiite liquid of felsic crustal 

material; 4) secondary melting of mantle 

residue after segregation of anorthositic 

liquid. 

This geochemical and petrological data 

suggests the existence of lateral 

inhomogeneity of the upper mantle in AR2 • 

The most depleted mantle in REE ( 0. 4-1. 5 x 

chondr.) is supposed for Kola peninsula and 

N.Karelia, chondrite type mantle (1.5-3.0) 

- for East and Central Karelia and slightly 

enriched (3.0-5.0) -for E.Finland. 

Supposed lateral inhomogeneity of upper 

mantle can be related with isotopic age 

of volcanic rock of greenstone belts: Kola 

and N.Karelia 2830-2790 m.y., East and 

Centr.Karelia - 2920-2995 m.y., E.Finland 

- 2790-2754 m. y. 

Komatiites have a wide range of 6 o18 

values(+4 to +7 °/00 )and initial eNd 

values (+1. 5 to +4. 5). The 6 o 18 for the less 

a'ltered whole-rock samples are +4.0-5.5°/oo 

and for carbonatized komatiites + 6.5-

+7. 5° I oo· Different initial Nd isotopic 

composition of the PK are in agreement with 

the geochemical types of PK and there 

pertogenesis. 

344 

LARGE SCALE ASSIMILATION AND MIXING 

PROCESSES IN E1NA VOLCANO (ITALY): EVIDENCE 

FROM SR-ND-PB ISOTOPES AND TRACE-ELEMENTS 

YROON. P Z., Department of Geology, Royal Holloway 

University of London, Egham, Surrey 1W20 OEX, England, 

DOWNES, H., Department of Geology, Birkbeck College, 

University of London, Malet Street, London WC1E 7HX, 

England, and MARRINER, G.F., Department of Geology, 

Royal Holloway University of London, Egham, Surrey 

1W20 OEX, England. 

We analysed 105 samples from five stratigraphic sections at 

the southern wall of the Valle del Bove for major, trace 

elements and Sr-Nd-Pb isotopes for selected samples. All 

samples are quite evolved (Si02=50.37-62.55, Mg0=1.36-

5.66) and sodium rich (K20/Na20=0.35-0.54). Isotopic ranges 

are fairly small for Sr and Nd isotopes: (87Sr/86Sr=0.70334-

0.70360, 143Nd/144Nd= 0.51291-0.51285, but slightly larger 

for Pb isotopes (e.g. 206Pb/204Pb= 19.7-20.0). Different 

magmatic cycles exist in each stratigraphic section, which can 

be divided on the basis of the Si02, Zr/Nb, La!Y and K/Nb. In 

general the Si02, Zr/Nb and KINb decrease with stratigraphic 

height and scatter is found in the top part of the sampled 

sections. 

Positive correlations exist between Si02 and 87Sr/86Sr, 

206Pb/204Pb, Zr/Nb, Pb/Ce and K/Nb. These are readily 

explained by assimilation of up to 20% average Calabrian crust 

into the most mafic magma. The effects of assimilation on Nd 

and Sr isotopes are small due to the large concentrations of Sr 

(=1600 ppm) and Nd (=70 ppm), whereas Pb isotopes are 

easily affected (Pb=6 ppm). 

However, not all within suite variations of isotopes and 

trace elements can be explained by assimilation and/or 

fractional crystallisation. The uncontaminated lavas display a 

mixing array between two end-members. The ftrst end-member 

has low Si02 (=50%), high LILE, LREE, HFSE contents and 

higher 87Sr/86Sr. The second end-member has higher Si02 

(=54%), lower LILE, LREE and HFSE contents and slightly 

lower 87Sr/86Sr. These end-members are probably a function 

of the complex Etna source region, which could contain 

subducted, lithospheric and asthenospheric components due to 

its peculiar tectonic position. We speculate that the low Si02 

group is related to an asthenospheric source, whereas the high 

Si02 group is related to lithospheric mantle which has been 

previously modified by recent subduction processes. 
Obsidian from Japan as a 

'putative' glass age standard 

"JAS-G 1" 

Kiyoshi WADATSUMI*l, Naoko KITADA*l, 

Gullo BIGAZZI*2, Alan DEIN0*3, Marin ella 

LAURENZI*2, Bart KOWALLIS*4, Keisuke 

NAGAO*S, Harue MASUDA*l <*1; Osaka City 

Univ. *2; lnstituto di Geochronologia e 

Geochimica lsotopica *3; Geochronology 

Center of the Institute of Human Origin 

*4;Brigham Young Univ. *5; Earth's Interior 

Inst.) 

The JAS-G 1 obsidian glass is produced 

as a chilled margin and no hydration 

from the analysis of index. On account 

of K-content and obtain spontaneous 

fission-tracks enough to counting easily. 

we introduce following: 

1) Geological setting of JAS-G 1 glass 

2) result of chemical analysis 

3) data of age determination of K-Ar, 

Ar-Ar and FT(using plateau method) 

age 

At the FT dating, Moldavite glass is 

recommended as a glass age standard. 

However, Moldavite is not abundant in 

and then it difficult to keep supplying. 

This JAS-G 1 sample be able to keep 

supplying well. Therefore we consider 

this sample as a 'putative' age standard 

sample of glass fission-track. 

345 

PLIO-PLEISTOCENE VOLCANIC ROCKS AND 

INCISION OF THE NORTH FORK FEATHER RIVER, 

CALIFORNIA: TECTONIC IMPLICATIONS 

WAKABAYASHI. John (1), PAGE, W. D. (2), RENNE, 

P.R. (3), Sharp, W. D. (3), and Becker, T. A. (3): (1) 

Consultant, 1329 Sheridan Ln., Hayward, CA 94544; (2) 

Geosciences Department, F22A, Pacific Gas and Electric 

Company, P.O. Box 77()()()(), San Francisco, CA 94177; 

(3) Institute of Human Origins Geochronology Center, 

2453 Ridge Road, Berkeley CA 94709. 

Three major sequences of Quaternary basalt have flowed 

from the Modoc Plateau into the Almanor basin. These flows 

have yielded Ar-Ar dates of approximately 1.1 Ma, 0.6 Ma 

and 0.4 Ma respectively. The oldest two of these flow 

sequences flowed down the North Fork Feather River canyon; 

the youngest flow ponded in the Almanor basin. Continued 

incision of the North Fork Feather River left the basalt flows 

preserved as terrace-like remnants with the highest terrace set 

being the oldest flow. Two older volcanic units were deposited 

in the ancestral canyon, but are now preserved only as limited 

remnants. These units are a 2.4 Ma basalt flow and a 2.8 Ma 

andesite related to the Y ana volcanic center (Tuscan 

Formation). The various volcanic units record post-Pliocene 

deformation in the northeastern Sierra Nevada. Several faults 

have been identified and vertical separation rates range up to 

0.15 mm/yr on individual faults. The aggregate east-down 

vertical separation rate between Lake Almanor and the Sierra 

Nevada crest (Sierra Nevada Frontal fault system) is 

approximately 0.4 mm/yr. The aggregate west-down vertical 

separation northeast of Lake Almanor (east side of the TahoeMedicine Lake trough) is approximately 0.2 mm/yr. 

Relative elevations of volcanic remnants above the 

bottom of the North Fork Feather River canyon place 

constraints on the long term incision rate of the North Fork 

Feather River canyon. The incision rate of the North Fork 

Feather River ranges from 0.2 to 0.4 mm/yr depending on the 

fault block and the time interval. Incision rates appear to have 

accelerated since 2.8 Ma. The apparent acceleration of 

incision rates may reflect uplift of the Sierra Nevada that 

exceeds erosion rates resulting in the geomorphic expression 

of this uplift as a mountain range. The volcanic flows are inset 

below remnants of a Tertiary erosion surface. Extrapolation of 

incision rates for different fault blocks suggest that canyon 

incision, and by inference, Sierra Nevada uplift in this region, 

began 3.5 to 4.8 million years ago (preferred value of 4.6 to 

4.8 Ma). The insight into late Quaternary faulting and the 

timing of Sierra Nevada uplift from this investigation has 

proved extremely valuable in assessing the seismiC hazards for 

hydroelectric facilities in the region. 
CHRONOSTRATIGRAPHY OF THE TRANSVAAL AND 

GRIQUALAND WEST BASINS, SOUIH AFRICA 

Walraven, F., Isotope Laboratory, Geological Survey, 

Private Bag X112, Pretoria 0001, South Africa. 

The Transvaal Supergroup is a major Precambrian succession or chemical and clastic sediments with intercalated basic 

volcanics occupying two large intracratonic basins on the 

Kaapvaal Craton of Southern Africa, viz. the Transvaal and 

Griqualand West basins. In the Transvaal basin a promi-nent 

group of intermediate to acid volcanics forms the upper 

portion of the supergroup. Limited age data available for the 

Transvaal Supergroup in the past confined its chrono-logical 

time span between the age of the Ventersdorp Super-group 

(close to 2. 71 Ga) which forms part of the basin floor and 

that of the Bushveld Complex (ca. 2.06 Ga) which in-trudes 

the uppermost strata of the supergroup. 

In recent years high-resolution geochronological investigations of the Transvaal Supergroup have been stimulated by 

chronostratigraphic studies and comparisons with other late 

Archean to Proterozoic successions in various parts of the 

world (in particular the Minas Supergroup, Quadrilatero Ferrifero and the Carajas basin of South America and the 

Hamersley Supergroup of Australia). 

The high-resolution age data obtained so far show that 

basin-wide clastic sedimentation in the Transvaal Supergroup 

commenced at ca. 2.64 Ga (the Black Reef and Vryburg 

Forma-tions). Prior to this a number of protobasins had been 

formed in both the eastern and western parts of the Transvaal basin had been formed. This brings the time gap separating the Ventersdorp and Transvaal Supergroups down to 

less than 60 Ma. 

Chemical sedimentation in both the Transvaal and Griqualand West basins followed and continued up to approximately 2.43 Ga ago, after which time clastic sedimentation 

(mainly shale and sandstone) took over. Several episodes of 

non-deposition, locally coupled with erosion, occurred 

throughout the filling of the basins, a prominent unconformity being that separating the chemical and clastic sediments. Ages in the order of 2.24 Ga have been found for 

basic lava within the clastic sediments while basic vol-canics 

at the top of the succession are less than 2.1 Ga in age. Direct 

ages are lacking for the intermediate to acid volcanics 

forming the upper part of the succession in the Transvaal 

basin. However, age data on related units together with 

geological considerations suggest that these volcanics formed 

part of the Bush veld Complex magmatic event. Thus their 

age is considered to be close to 2.06 Ga. 

The Transvaal Supergroup therefore appears to have had an 

extremely long life span of almost 600 Ma. The new age data 

also show better agreement with the ages reported from the 

Carajas basin and Minas and Hamersley Supergroups, 

thereby providing additional evidence for the coeval nature 

of these major precambrian successions in Southern Africa, 

South America and Australia. 

346 

TEMPORAL FRAMEWOR~ FOR THE EVOLUTION 

OF AUSTRALOPITHECUS AF ARENSIS 

WALTER. Robert C .. Institute of Human Origins, 2453 

Ridge Rd., Berkeley, CA 94709, WOLDEGABRIEL, 

Giday, Los Alamos National Laboratory, Box 1633, 

EE5-1 /0462, Los Alamos, NM 87545, HART, William 

K., Department of Geology, Miami University, 

Oxford, OH 45056, and ARONSON, James L., 

Department of Geology, Case Western Reserve 

University, Cleveland, OH 44106. 

From 1973 to 1981 the west-central Afar region of 

Ethiopia yielded important Pliocene hominid fossils 

from the Hadar and Middle Awash deposits. These 

sediments produced an array of hominid specimens that 

represent the presently known time range of 

Australopithecus afarensis, the oldest hominid species. 

Renewed investigations at these sites beginning in 1990 

has produced much additional hominid material that 

extends our knowledge of the range of afarensis. 

The Hadar Formation and the Middle Awash 

deposits are part of the Miocene to Pleistocene Awash 

Group. This basin developed in the Afar Rift, adjacent 

to the Ethiopian escarpment, and from time to time 

included paleolakes as well as a thoroughgoing 

meandering river system fed by braided streams. 

Numerous widespread rhyolitic tephras from volcanic 

sources within the Afar, the Ethiopian Highland, and 

Main Ethiopian Rift occur in the sedimentary sections. 

These tephras permit correlation and help to establish 

temporal relationships within and between the 

fossiliferous sedimentary basins. 

Single-crystal 40 Ar I 39 Ar dates on these tephras 

enable us to calibrate faunal evolution with great 

precision, and to overcome problems of contamination 

and paucity of datable components that plagued earlier 

dating attempts using bulk methods. Roughly fifteen 

new 40 Ar ;39 Ar dates on tephras from the Hadar and 

Middle Awash establish a refined chronology for 

biostratigraphic zones, including that of afarensis. The 

presence of the Sidi Hakoma Tuff (3.4 ± 0.03 Ma; 

equivalent to the Tulu Bor Tuff of the Turkana Basin) 

links the stratigraphies of the Hadar and Middle 

Awash. 

The oldest specimen of A. afarensis derives from the 

Middle Awash deposits between the Cindery Tuff (3.86 

± 0.20 Ma) and Vitric Tuff 1 (3.89 ± 0.02 Ma; equivalent 

to the Moite Tuff of the Turkana Basin). The youngest 

afarensis derives from the Hadar Formation between 

Bouroukie Tuff-2 (2.95 ± 0.05 Ma) and Bouroukie Tuff-1 

(3.10 ± 0.1 Ma). Therefore, the currently known time 

range of A. afarensis is from about 3.9 Ma to 3.0 Ma, or 

roughly 900,000 years. Combined, however, these 

deposits date to greater than 4.2 Ma and to less than 2.3 

Ma, which offers the potential for future discovery of 

first and last appearances of afarensis, and pre and 

post-afarensis specimens. 
A TECHNIQUE FOR STEPWISE POTASSIUM·ARGON 

ANALYSIS OF ROCK SAMPLES BY LEACHING WITH 

HOT HYDROCHLORIC ACID 

WAMPLER J. M., School ofEarth and Atmospheric 

Sciences, Georgia Institute of Technology, Atlanta, 

Georgia, 30332-0340, U. S. A. 

A simple reflux reactor in which a rock sample may be 

continuously leached by a hot hydrochloric acid solution has 

been constructed with provision for stepwise analysis of the 

argon and potassium ex1racted. The reactor operates with an 

internal atmosphere of carbon dioxide, whose pressure deter· 

mines the extraction temperature by controlling the boiling 

of the acid solution. At the end of each extraction step, the 

accumulated argon may be removed for isotopic analysis, 

while the water, HCl, and C02 are retained by cold trapping 

for use in the next extraction step. The potassium and other 

solids left by evaporation of the acid solution may be 

removed for analysis by sealing off the bottom end of a glass 

tube where the non·volatile products of the extraction had 

accumulated. The pressure of the internal gas must be a 

little less than one atmosphere when the glass tube is sealed 

off with a torch. 

If the internal pressure is near one atmosphere, the reactor operates near 100°C, a temperature high enough that 

hornblende will dissolve at a rate sufficient for a practical 

schedule of steps. Biotite dissolves incongruently, and so it 

remains to be seen whether stepwise acid extraction will be 

useful for potassium-argon studies of biotite. The acidextraction technique should be particularly useful for studies 

of illitic clay samples, which often contain different populations of clay particles that cannot practically be separated 

from one another. Owing to the smallness of the clay particles, a sufficient rate of dissolution can be obtained at 100°C. 

The stepwise acid-ex1raction technique offers an alternative to stepwise 40 Arf39 Ar analysis, which should be particularly useful in cases where loss of 39 Ar by recoil causes problems (for example, in the study of clay samples). In other 

cases, age spectra obtained by acid extraction should provide 

information that complements 40 Arf39 Ar age spectra, si nee 

the pattern of susceptibility of common minerals to acid dissolution is quite different from the pattern of thermally 

induced argon release from these minerals. For example, a 

small amount of excess argon in a basaltic rock may be more 

clearly evident in an age spectrum obtained by acid extraction than in one obtained by stepwise heating, since lowpotassium minerals such as olivine and pyroxene should dissolve much more rapidly than feldspar. A drawback of the 

technique is that it will often not be practical to obtain a 

complete age spectrum by acid extraction, because of the 

very slow dissolution of minerals such as feldspar. 

347 

OXYGEN AND HYDROGEN ISOTOPIC 

COMPOSmONS IN GROUNDWATERS OF THE 

PENGHUISLANDS, TAIWAN 

WANG. Chun&-Ho and PENG, Tsung-Ren 

Institute of Earth Sciences, Academia Sinica, P.O. 

Box 1-53, Nankang, Taipei, Taiwan 11529, ROC 

Deep (>90 m) and shallow ( <90 m) ground waters 

together with several surface waters in the Penghu 

Islands were analyzed for their oxygen and hydrogen 

isotopic compositions. The following isotopic ranges 

were found for the groundwaters: o18o, -4.5%o to 

-7.6%o; oD, -31%oto -49%o. All o18o and oD data 

plot along the global meteoric-water line, and only 

those of surface waters shift towards more positive 

values during the period of high evaporation and low 

precipitation. Isotopic values of groundwaters from 

the Penghu Islands are generally heavier both in D 

and 18o relative to those of groundwaters from the 

western Taiwan. 

The preliminary results show that: (1) deep 

groundwaters of the Penghu Islands show no 

evidence of sea water contamination; (2) evaporation 

has a strong influence on the isotopic values of 

surface waters, but less influence on the shallow 

groundwaters and no effect on the deep 

groundwaters; (3) stable isotopic data of 

groundwaters suggest that the sources of deep and 

shallow groundwaters may have different origins. 

14c results confirm that deep groundwaters were 

formed during the last glacial period. 
MODELING OF ISOTOPIC FRACTIONATION DURING 

THE PHASE TRANSITION OF A DIFFUSIONCONTROLLED RESERVOIR 

WANG Jianhual, DAVIS, Andrew M.2, and CLAYTON, 

Robert N.l.2,3, lDepartment of the Geophysical Sciences, 

2Enrico Fermi Institute, 3Department of Chemistry, 

University of Chicago, Chicago, IL 60637, USA 

The Rayleigh fractionation law, R = Rofl/a-1, is commonly used to describe the isotopic composition of a reservoir 

phase transferring into another phase such as occurs during 

evaporation and fractional crystallization. Application of this 

law requires that the reservoir be completely mixed throughout 

the fractionation event add that the transformed phase be isolated from the reservoir. Diffusion limits mixing of the reservoir in many cases where isotopic fractionation occurs. We developed a theoretical model to study isotopic fractionation during the phase transition of a diffusion-controlled reservoir. This 

model quantitatively describes isotopic fractionation during 

evaporation of a solid reservoir and instrumental fractionation 

in thermal ionization mass spectrometry. 

The present model uses a one-dimensional approach that assumes that the isotopic fractionation occurs only at the planar 

phase-transition surface of a reservoir. The coordinate of length 

is always perpendicular to the surface. The f1rst step in solving 

this problem is to consider a semi-infinite reservoir. For this 

case, the origin (x = 0) is fixed at the phase-transition surface 

and the reservoir occupies the positive axis (x;;:: 0). The isotopes of an element are fractionated at the surface with a factor 

of a and diffusively transported in the reservoir with a diffusion 

coefficient of D. The rate of the phase trcmsition is v. The following set of equations describes this scenario with non-dimensionalized X (X = ;;) and t (t = ~~ for isotopic ratio R(X,t): 

aR a2R aR . aR 

at = ax2 + ax , 0 < X < 00 , With R = Ro when t=O, ax + 

a-1 a R = 0 at X = 0 and R = Ro at X = 00 for t > 0. Using the 

Laplace transform, the analytical solution can be obtained as 

follows: 

R(X, t) _ 1 ! ..& X+t (a-1) -X~ X-t 

R - - 2 euc -r + 2 e t;UC - r + 0 2-vt 2-v t 

(1 _g) 0-a)(aK+t) ..& ax+(2-a)t - 2 cr 2 euc _r 

a · 2a-vt 

The next step is to consider a reservoir with a finite thickness, w. By fixing the opposite end to the transition surface of 

the reservoir at X = 0 and nondimensionalizing w (W = ~). we 

have the following set of equations:~-:::~~~ , 0 < X < W-t, 

. aR aR a-1 w1th R = Ro when t = 0, ~ = 0 at X = 0 and~- - R = oX ciX a 

0 at X= W-t. This set of equations was solved numerically by 

substituting Y for X (Y = :--:2 and using Crank-Nicolson's 

implicit method. This model was used to accurately interpret 

instrumental isotopic fractionation in thermal ionization mass 

spectrometry and isotopic fractionation in residues of forsterite 

evaporated from the solid state. 

348 

A STABLE ISOTOPE STUDY ON DIET AND 

PALEOENVIRONMENT OF OLIGOCENE 

MAMMALS FROM BADLANDS NATIONAL 

PARK, S. DAKOTA 

WANG, Y., Department of ESPM, University 

of California, Berkeley, CA 94720, CERLING, 

T. E. and DECOURTEN, F., Department of 

Geology & Geophysics, University of Utah, Salt 

Lake City, Utah, 84112 

Stable isotopes of fossil mammal tissues from 

Badlands National Park, South Dakota demonstrate 

that diagenesis of biogenic carbonates in the tooth 

enamel hydroxyapatite over time is unexpectedly 

limited and· fossil tooth enamel of Oligocene 

mammals preserved their paleodietary signatures. 

However, the stable isotopic compositions of bone 

carbonates have been altered completely. Bone 

apatite has similar isotopic values as tooth dentine, 

while bone carbonate displays similar isotopic ratios 

as carbonate in the matrix, suggesting that bone 

apatite is more resistant to diagenetic modification 

than bone carbonate. The 813 C values of tooth 

enamel of titanotheres of Early-Oligocene age 

indicate that they had a C3-based diet. Hyracodon 

and oreodont specimens from early MiddleOligocene display 813c signatures of C3 diet which 

are consistent with their modem relatives. The 813c 

values of tooth enamel of Mesohippus specimens 

also indicate that these early horses are c3 eaters, 

implying that a graminivorous dietary specialization 

had not been developed during the Early and early 

Middle Oligocene time. The 813c values of 

Perchoerus from Late Oligocene suggest a pure 

dietary intake of c3 plants, which might indicate 

that there were no C4 plants at that time in the area 

since peccaries are omnivorous. Our results suggest 

that the Oligocene herbiroves studied here were 

browsers and lived in an environment probably 

composed mostly of trees and shrubs, which is in 

accordance with dental morphology. 
ABIOGENIC NATURAL GAS POOL IN 

PRODUCTION GASES FROM SONGLIAO 

BASIN, P.R. CHINA 

WANG, X. , Chen, J. , Li, C. , 

Yang, H. , Wen, Q. , Shao, B. , State 

Key Lab. of Gas Geochemistry , 

LIGCAS, Lanzhou 730000, P. R. 

China, and Guo, Z. , Daqing Institute of 

Petroleum Exploration &. Development, 

Daqing 163712, P.R. China 

Songliao Basin is famous for its 

petroleum and natural gas. This basin is an 

intracratonic rift basin, in which deep faults 

of basement, crust and super- crust are well 

developed and volcanic activity is very 

frequent from Cretaceous to Quaternary 

period. The o13C value of methane in out of 

13 drilled wells ranges from - 16. 7 ,.._, 

-24. 2%0 , which shows isotopic composition 

charactrisitics of abiogenic methane. 

Owing to the isotopic fractionation effect 

during the degradation of kerogen of source 

rocks, the o13C value of methane homologues 

( CH4, C2Hs, Cslia, C4H1o, etc. ) from 

biogenic natural gas, becomes heavier with 

the increase of carbon number, i. e. , o13C1 < 

o13C2<o13Cs<o13C.(i. e. -56. O%o<-37. O%o 

<-33. 0%0<-31. 0%0). As for the abiogenic 

natural gas, the o13C of methane homologues 

formed by inorganic synthetic reaction of 

primary methane ( FIT ) which shows a 

different feature from biogenic natural gas. 

Its o13C values put in oppsite order compared 

with the biogenic natural gas, i. e. , o13Ct > 

o13C2 > o13Cs > o13C4. The o13C values of 

methane homologues obtained from some wells 

in east Songliao Basin show the aforementioned feature (i. e. -18. 7%'0>-22. 4%0 

>-24. 1%o>-28. 2%o). 

349 

PRELIMINARY RESULTS OF FELDSPAR 40Arf39Ar 

THERMOCHRONOMETRY FROM THE KTB DEEP 

CRUSTAL BOREHOLE, GERMANY 

WARNOCK. A.C. and ZEITLER, P.K., Dept of Earth and 

Environmental Sciences, Lehigh University, Bethlehem, 

Pennsylvania, 18015-3188, USA. 

The KTB deep crustal borehole, located in the Oberpfalz 

region of Germany, will soon reach its fmal depth of 10 km. 

This great depth and the relatively steep(= 28°C/km) 

geothermal gradient measured in the hole provides the basis for 

our study of diffusive argon loss in feldspar during slow 

cooling. The temperature at 10 Ian is expected to be 

approximately 300°C, well into the closure interval of feldspar 

(=1500 to 350°C). Laboratory studies have successfully 

established the existence of discrete diffusion domains within 

single crystals of feldspar. These diffusion domains appear to 

be capable of recording individual segments of thermal 

histories. The KTB borehole offers a unique opportunity to 

explore the concept of diffusion domains outside of the 

laboratory. 

Samples from the upper portion of the borehole ( <1500C) 

record the post-orogenic thermal history of different levels in 

the Variscan crust. The vertical section sampled in the 

borehole, therefore, provides a third dimension to the geology 

seen at the surface. With such a cross-section, we will learn 

about the roles of thermal relaxation and denudation during the 

decay of a major metamorphic event, as well as improve our 

understanding of the regional tectonics. 

Of the 20 core samples collected so far, the results of six 

pilot samples (five K-feldspar and one plagioclase) spanning 

depths of 0 m to 6670 m (=200°C) will be presented. 
APPARENT DIFFUSIVE LOSS 40 Arf39 Ar AGE 

PROFILES IN AMPHIBOLES. 

WARTHO Jo-Anne, Department of Earth and Space 

Sciences, Geology Building 3806, University of California, 

Los Angeles, CA 90024-1567, USA. 

Phone: (310) 825-5505, email: jo@argon.ess.ucla.edu 

Turner et al (1966)1 and Turner (1968)2 presented 

theoretical incremental loss curves by which 40 Arf39 Ar age 

spectra of meteorite samples could be interpreted as revealing 

natural diffusion profiles, produced as a result of diffusive 

loss of argon from the sample. Harrison and McDougall 

(1980)3 used Turner's theoretical curves to interpret age 

spectra of metamorphic hornblende samples which had been 

partially degassed as a result of the thermal effects of a later 

granite intrusion. The age spectra of three hornblende 

samples were interpreted as revealing argon loss by volume 

diffusion from uniform spheres. However, studies by Gaber 

et al (1988}4, Wartho et al (1991}5, and Lee et al (1991)6 have 

shown that in vacuo argon release from amphiboles is 

controlled by phase changes and not by volume diffusion. 

These processes suggest that both naturally and imposed 

gradients of argon and inferences of kinetic parameters will be 

obscured. 

More recently, experiments by Rex et al (1993)7 showed 

how biotite contamination within amphibole samples could 

result in apparent diffusive loss 40 Arf39 Ar age gradients. 

However, some age spectra have been rePorted for 

amphiboles that appear to be characterised by 40 Ar gradients 

that cannot be easily explained by contamination problems 

(e.g. Harrison and McDougall, 19803; Berry and McDougall, 

19868, and Conrad and McKee, 19929). In addition, Kelley 

and Turner (199110) have found evidence of 40 Ar loss 

profiles in 40 Arf39 Ar laser spot analysed amphibole grains. 

Analysis of two amphibole samples yielding argon loss 

profiles (including one hornblende sample from the original 

Harrison and McDougall (19803) study) has revealed the 

presence of complex compositional zoning within the 

amphiboles. Comparison of the 37 Arcar9 ArK ratios (obtained 

from 40 Arf39 Arstep-heating and laser analyses) and CalK 

ratios from electron microprobe data in these zoned 

amphiboles indicates that the apparent diffusi~e loss 

40 Arf39 Ar age profiles in these two samples are due to partial 

chemical re-equilibration within the amphibole grains (during 

a later thermal event) and, therefore, is not solely caused by 

thermally assisted volume diffusive loss of argon from the 

sample. The reset ages obtained from the various amphibole 

compositions in these samples do not correlate to any know 

thermal events and appear to be due to partial argon loss 

during major element readjustment of the amphibole grains. 

1 - Earth Planet. Sci. Lett., 1, 155-7; 2 - Origin and 

Distribution of the Elements (L.H. Ahren, Ed.) pp 387-98; 

3 - Geochim.Cosmochim. Acta, 44, 2005-20; 4 - Geochim. 

Cosmochim. Acta, 52, 2457-65; 5- Am. Mineral., 76, 

1446-8; 6- Geology, 19, 872-6; 7- Isot. Geosci., 103, 271-81; 

8- Isot. Geosci., 59, 43-58; 9- Econ. Geol., 87, 185-8; 

10- Earth Planet. Sci. Lett., 107, 634-48. 

350 

GEOCHRONOLOGY OF THE AREQUIPA MASSIF, 

PERU: CORRElATION WITH lAURENTIA 

WASTBNBYS, Hardolph A., Jack Satterly 

Geochronology Laboratory, Royal Ontario Museum, 100 

Queen's Park, Toronto, M5S 2C6, 

Previous Rb-Sr and U-Pb geochronology of Proterozoic 

granulite-facies gneisses in the enigmatic Arequipa Massif 

concluded that high-grade metamorphism had occurred ca. 

1900 Ma and shortly after formation of the protoliths. 

However, new U-Pb zircon geochronology(Wasteneys et al, 

1993) has demonstrated, instead, the presence of two 

Grenvillian granulite-facies domains implying that the Rb-Sr 

isochrons were not reset and that the complex Pb-loss 

patterns in unabraded, multi-grain zircon fractions obscured 

the metamorphic zircon component More importantly, the 

ages for metamorphism (1198 Ma near Quilca and 970 Ma 

near Mollendo), orthogneiss protoliths (ca.1950 Main both 

domains) and detrital sources (minimum ca. 1600 Ma) from 

the two domains suggests correlations with the Grenville 

Province of Canada which support the proposed 

Neoproterozoic continental reconstruction of Dalziel (1994) 

in which the conjugate margin of Laurentia was the preAndean margin of South America. The data also lend 

support to equivocally Grenvillian Rb-Sr and U-Pb ages for 

other Andean basement complexes in northern Chile and 

Bolivia. The ca. 1950 Ma ages for the Arequipa orthogneiss 

protolithic zircons are found in zircons of the NW zone of 

the Makkovik Province, Labrador while younger, ca. 1650 

Ma detrital zircons may have been derived from the 

voluminous Trans-Labrador Batholith which cuts both 

Grenville and Makkovik Provinces. The 970 Ma age for 

granulitic metamorphism in the Mollendo domain correlates 

with metamorphic domains of the eastern Grenville Province 

which have partially overprinted the trans-Labrador 

Batholith. The ca. 1200 Ma Quilca domain metamorphism 

correlates with the main phase of high-grade metamorphism 

in the allochthonous Central Metasedimentary Belt of the 

Grenville Province in Ontario constrained by Lumbers et al. 

(1991) to the 1240-1160 Ma period. In the Frontenac 

domain of the CMB granulite-facies metamorphism 

commenced prior to 1180 Ma ca. based on new U-Pb ages 

for zircons in anatectic melts and terminated, probably by 

isothermal decompression, at 1166 Ma based on zircon 

coronas on baddeleyite from gabbros, 1160 Ma monazite 

and 1155 Ma sphene ages. Possibly subduction-related 

magmatism in the Elsevir and Adirondack terranes, which 

ended by 1240 Ma, heralded an imminent collision between 

elements of South America and Laurentia. Similar age 

magmatic rocks in northern Chile and Bolivia basement 

complexes may record the same event 

Dalzieli.W.D., 1994, Precambrian Scotland as a LaurentiaGondwana link: The origin and significance of cratonic 

promontories. Geology (submitted). 

Lumbers, S.B., Heaman, L.M., Vertolli, V.M., and Wu, T.W. 

1991, Nature and timing of Middle Proterozoic 

magmatism in the Central Metasedimentary Belt, 

Grenville Province, Ontario. in GAC Special Paper 38, 

p.243-276. 

Wasteneys, H.A, Clark, AH., Langridge, R.J. and Farrar, E, 

1993 Granulite-facies metamorphism in the Arequipa 

Massif, southern Peru: Grenvillian, not Early Proterozoic 

in age. GAC/MAC Progam with Abstracts p. 108 
EVOLUTION OF THE KERGUELEN PLUME WITH 

TIME AND ITS ROLE IN THE GENERATION OF 

INDIAN OCEAN CRUST. 

WEIS D, Petrol. Geodyn. Chimique, CP160-02, 

Universite Libre de Bruxelles, Av. F.D. Roosevelt, 50, 

1050 Brussels, Belgium and FREY, F.A., EAPS, MIT, 

Cambridge, MA 02139, USA. 

Indian Ocean (10) basalts are important because they 

are isotopically different from Atlantic and Pacific 

basalts. In addition the Kerguelen Plume (KP) as a main 

source for these differences is a definite possibility. 

In order to constrain both the time evolution of the KP 

and its extension in space, we have undertaken a study 

of different 10 basalts: Indian ocean crust (152 to 

39 Ma), Nineyteast Ridge basalts (90ER- 83 to 38 Ma) 

and different volcanic series from the Kerguelen 

Archipelago (KA), i.e. a 38 Ma stratigraphic sequence of 

transitional basalts from the Western KA, 26 Ma mildlyalkaline lavas from the eastern KA and the youngest 

expression of the KP on the KA, the <2 Ma highlyalkaline Ross volcano. 

All the 10 crust and 90ER basalts are tholeiitic and 

some of the basalts forming the 10 crust have 

incompatible element contents typical of depleted 

MOAB. However of the tholeiitic basalts of varying age 

from the 90ER and many from the 10 crust are mildly 

enriched in incompatible elements (e.g., La!Yb= 2 to 5). 

In contrast transitional and alkalic lavas from the KA are 

highly enriched in LILE (e.g., La/Yb up to 15). The lava 

compositions erupted in the KA differ from those 

erupted along the 90ER and spreading ridge axes. 

Isotopically, IOC basalts present Pupal signatures 

(207pbJ204Pb>15.55, 87Srf86Sr>0.704) starting around 

115 Ma, which corresponds to the first unambiguous 

evidence of activity of the KP in the 10. For 90ER, the 

tectonic setting seems to control the geochemical 

signature of the basalts, tholeiitic-relatively depleted, i.e. 

MOAB-type isotopic ratios when an active spreading 

ridge is involved in the process to very enriched 

(87Srf86Sr>0.705, ENd <-3), almost pure KP signature. 

The MOAB-type source is nevertheless always 

entrained by the KP as reflected by ME and TE 

concentrations in these 10 and 90ER lavas. 

With age, KA lavas change from transitional to alkalic 

with accompanying changes in isotopic ratios, with 

87Srf86Sr from 0.7038 to 0.7058 and correlative 

variations in ENd from +5 to -5. The youngest lavas have 

the lowest 206pbf204pb ratios and have the highest 

Dupal signature. They are like the plume (B7srt86Sr= 

0.7052-0.7053; tNd=-2, -3; 206pbf204Pb=18.02-18.12; 

207pbJ204Pb=15.53-15.56; 208pbJ204pb= 38.50-38.60). 

In conclusion, our data show the KP to be responsible 

for Dupal anomalies in the 10 basalts starting at 120 Ma. 

Although 10 and 90ER lavas are compositionally 

different from KA lavas, there is much isotopic overlap. 

Both in 90ER and KA lavas, isotopes require 3-different 

components (sources) and aging is not a major 

explanation for the variations observed. The 10 setting 

evolution (on/off ridge activity) seems to favor 

entrainment of surrounding mantle by the KP which may 

explain the existence of specific geochemical features. 

351 

THERMOCHRONOLOGIC IDSTORY OF THE McKINLEY 

PLUTON, DENALI NATIONAL PARK, ALASKA 

WEST. Andrew W, and LAYER, Paul W., Geophysical 

Institute, University of Alaska, Fairbanks, Alaska 99775, 

USA. 

The McKinley pluton is one of the northern-most plutons 

of the Alaska-Aleutian Range batholith. This biotite-granite 

intrusion makes up most of the 6,194 m high Mt. McKinley 

(Denali) massif. Geochemical studies indicate that the entire 

pluton is homogeneous and was intruded as a single unit 

Previous K-Ar and apatite fission-track studies suggest that 

the pluton has had a protracted cooling/uplift history since its 

emplacement at -56 Ma. The great local relief available for 

sampling makes the McKinley pluton ideal for a 

thermochronometric study through 40 Arf39 Ar dating of Kfeldspars and biotites. We have conducted detailed step 

heating experiments on samples collected from a range in 

elevation of 2,100 to 5,990 m. 

Six biotites and one primary white mica have relatively 

flat age spectra with plateau ages between 55.1 and 55.8 Ma. 

They show a very slight age/elevation dependence with the 

lower biotites being younger. These consistent mica ages 

most likely reflect uniform cooling of the pluton at this time. 

Seven K-feldspars samples exhibit saddle-shaped age 

spectra that vary significantly from each other. High Cl/K 

values are seen in the first -5% of gas release in some 

samples and appear to coincide with old ages (excess argon). 

Cycled heating experiments carried out on the K-feldspars 

indicate that they contain multiple diffusion domains that we 

have modeled using the computer programs ofLovera (1992). 

From this analysis, a detailed, internally consistent, 

thermochronologic history spanning -35 m.y. for the 

McKinley Pluton emerges. The pluton cooled slowly at rates 

of 100 to 40 C/m.y., with the lower samples cooling at the 

slower rates. The vertical variation of K-feldspar ages 

records an average uplift rate of 1.3 mm/Ma from 43 Ma to 

25 Ma. Thus, the McKinley Pluton reached high levels in the 

crust by 20 Ma such that even the smallest domains of the Kfeldspars do not record the rapid uplift of Mt. McKinley that 

began at 5 Ma indicated by apatite fission track dating. 

Selected Cooling Histories for K-spars 

400 

360 

0300 

.5 

0. 250 

E 

., 

... 200 

/.~==· 

.

......./3 ····· • AW-4c(5.150m) 

/ " AW·6c (6,960m) 

• AW-7(3.600m) 

150 o JB-8 (3,500 m) 

100 

10 " ~ ~ ~ ~ ~ u 50 " 50 

Age in Ma 

Lovera, 0. M., 1992, Computer programs to model 40Arf39Ar 

diffusion data from multidomain samples: Computers 

and Geosciences, v. 18, p. 789-813. 
AGE AND NATURE OF CONtiNENTAL CRUST IN 

YEMEN: NEW CONSTRAINTS ON PAN-AFRICAN 

CRUSTAL ACCRETION RATES 

WHITEHOUSE. M.J., Dept. of Earth Sciences, University 

of Oxford, Oxford OX1 3PR, UK, WINDLEY, B.F., Dept. 

of Geology, University of Leicester, Leicester LE1 7RH, 

UK, and BA-B'TTAT, M.A., Dept. of Geology, University 

of Sana'a, Sana'a, Yemen. 

The Arabian-Nubian shield is of considerable importance 

to our understanding of late Precambrian plate-tectonics and 

crust-mantle evolution. Currently there exists a major 

controversy regarding the crustal growth rate of the arc 

terranes of the shield, with estimates ranging from 20% to 

100% of the present world-wide rate (reviewed in Pallister et 

al., 1990). The key problem revolves around the questions (1) 

where is the eastern boundary of the arcs of the shield; (2) 

does Pan African age crust continue under the cover rocks of 

eastern Arabia; and (3) do the Pan African island arcs 

continue to the Moho? 

We present here isotopic data obtained on samples 

collected during our 1992/3 geotraverse across the previously 

poorly studied Precambrian of Yemen which provide 

additional constraints on the crustal growth-rate. In southcentral Yemen we have identified two major gneiss blocks, 

the western Abas terrane and the eastern Al Mahfid terrane 

which respectively have Nd model ages (depleted mantle 

source) of ca. 1.9-2.2 Ga and 2.6-2.8 Ga. Our field 

observations suggest that Pan African reworking of these 

terranes was relatively minor and we conclude that the model 

ages probably approximate true crustal accretion ages. These 

terranes are separated by a major island arc terrane which is 

intruded by late-Pan African granitoids whose isotope 

systematics yield information on the deep structure of the arc. 

Our data, together with earlier studies in Saudi Arabia 

(Stoeser et al., 1991) indicate that a much greater proportion 

of pre- Pan African crust exists to the east of the major arc 

terranes of the western Arabian peninsula than has previously 

been assumed. Thus, we are able to revise estimates for 

crustal growth rates in this major late-Precambrian orogen. 

Pallister, J.S. et al., 1990, Use and abuse of crustal accretion 

calculations, Geology, v. 18, p. 35-39. 

Stoeser, D.B. et al., 1991, Pan-African accretion and 

continental terranes of the Arabian Shield, Saudi Arabia 

and Yemen, EOS, v. 72, p. 299. 

352 

STRO~ISOTOPECHRONOSTRATIGRAPHY 

AND GEOCHEMISTRY OF THE DARAI LIMESTONE, 

PAPUA NEW GUINEA: JUHA IX WELL 

WHITFORD. D.J., ANDREW AS., HAMILTON, P.J., 

Australian Petroleum Cooperative·Research Centre, 

CSIRO Division of Petroleum Resources, North Ryde, 

NSW 2113, Australia, ALLAN T.L. University of 

Technology, Sydney, NSW 2007, Australia, VALENTI, 

G.L., South Pacific Chevron, Brisbane, QLD 7000, 

Australia. 

The Miocene Darai Limestone crops out extensively in the 

Papuan Foldbelt, an area of high current hydrocarbon 

exploration activity. Dense tropical jungle, severe karst, and 

thick (up to 1600 m) limestone prevent the acquisition of 

useable seismic data. Consequently, exploration in the area 

is based on photo geologic and surface geologic mapping. 

However, neither biostratigraphy nor lithostratigraphy 

provide a workable subdivision of the Darai. Strontium 

isotope methods have been undertaken to develop a Darai 

chronostratigraphy in order to refine structural and 

stratigraphic models. 

Drill cuttings have been sampled at approximately 30m 

intervals over a 1. 5 km thick section in the Juha IX well and 

analysed for Sr, C and 0 isotopic composition and major 

and trace element abundances. There is a generally 

consistent decrease in 87 Sr/86sr with depth that 

corresponds to an increase in age from approximately 8 - 30 

Ma. Comparison of 87 Sr/86sr in whole-rock and fossil 

isolates from other locations in the Darai Limestone shows 

excellent agreement between apparent ages, consistent with 

petrographic evidence for v~ early diagenesis. 

Carbon isotope values (81 C = + 1±1 %o PDB) for nondolomitized samples do not vary with depth at Juha IX. 

Oxygen isotope values vary more than 9 %o over the same 

dNth range. From the base to the top an overall increase in 

51 0 of 2 %o corresponds with known global cooling 

leading to late Miocene-Pliocene glaciation. A decrease in 

5l8o in limestones from within the age range 22 - 12 Ma 

may reflect the involvement of meteoric water during 

deposition. 

Major and trace element abundances define two zones: an 

upper zone above about 2000 ft (< ~17 Ma) where dolomite 

is sporadically developed associated with scattered 

enrichment in Fe, Mn, Si, Al, Na, P and Sr. Below 2000 ft (> 

~17 Ma) the limestone is geochemicallyvery uniform. The 

geochemical zonation appears to correspond to a change in 

depositional environment from older shallow water platform 

carbonates to more continuously open marine shelf 

conditions with a significant terrigenous input. 

The internal consistency of the Sr isotopic results froni 

Juha IX suggests that the technique provides a robust 

quantitative tool by which to define local and, possibly, 

regional stratigraphy and structure. Combined C and 0 

isotope studies together with major and trace element 

abundances can be used to resolve the effects of primary 

depositional variations from those due to diagenetic 

alteration. 
ZIRCON GEOCHRONOLOGY USING A SMALL 

ION MICROPROBE 

Wiedenbeck,M, and Goswami, J.N., Physical 

Research Laboratory, Navrangpura, Ahmedabad 

380009, India. 

We have made the first successful use of a 

sma1l geometry ion microp<f£be &,Cameca ims 4_f) 

for high precision Pb/2 Pb ages m 

individual Archean zircons with a spatial 

resolution of $20 J.l.m. Despite the low 

sensitivity provided by the ims 4f, resulting 

from operating at high mass resolu~ion _(M!~J:v1 

--4600 required to resolve all maJor. I~oba_nc 

molecular interferences), we can d1stmgmsh 

Archean components whose ages differ by as 

little as 50 Ma. A single measurement requires 

90 minutes of data acquisition, thus a 

comprehensive study of a zircon pop~lation c~n 

be accomplished in a few days. W1 th certam 

precautions, the ims 4f can accurately track 

all masses between 204 and 254~ however the 

determination of U-Pb ratios would greatly 

reduce sample throughput due to the persistent 

need to measure calibration standards. 

We have analyzed three zircon suites with 

previously established ages of 1.1, 2.6 and 

3.8 Ga; our results are within ±2cr of the 

'true' values. We conclude that no important 

systematic eiTors affect our data (e.g. no 

significant hydride ~nte1~erences, no 

significant mass fractwnation, accurate 

common Pb con-ection). 

A limitation of our technique is the 

relatively low count rates which the ims 4f 

provides when operating at high ma~s 

resolution ( -1 cnt/sec/ppm Pb); th1s 

impediment has a partic.ular impac~04 on 20~ur 

ability to measure prec1~~ly the • Pb/. Pb 

in samples with low 6

Pb concentrauon.s. 

Nonetheless, this is not a major hindrance m 

dating Archean samples. Another problem 

encountered was how to define which analyses 

belong to the least 'disturbed sub-population 

within an isotopically hetero- geneous 

population (e.g. samples containing xenocrysts 

only slightly older or Pb-loss. compone~ts only 

slightly young~r than the mam populatiOn). In 

such cases the minimum age calCulated for a 

sample can vary significantly d.epending on the 

criterion chosen for defimng the least 

disturbed sub-population. We have ~dentifi~d 

several plausible criteria for handling th1s 

situation. 

For early to mid-Precam.brian. sa~~P.les Jhe 

internal scatter of the radwgen1c Pb/ Pb 

ratio is often larger than the precision limit 

imposed by ion counting statis.ti~s. Our ~ge 

measurements yielded lcr precisions rangmg 

between ±14 Ma for the mid-Proterozoic to ±3 

Ma for the mid-Archean. 

353 

THERMAL EVOLUTION OF SOUTH-ALPINE CRUST: 

FROM VARISCAN COOLING TO ALPINE 

OVERPRINTING 

WIJBRANS. J.R., and BERTOTTI, G., Faculty of Earth 

Sciences, Vrije Universiteit, 1081 HV Amsterdam 

Netherlands. 

In the Central Alps, Lepontine metamorphism is 

characterised by young Miocene cooling ages. In contrast, 

much older Variscan to Mesozoic ages are found in the adjacent 

Southern Alps. The western Southern Alps (Ivrea and StronaCeneri zones) have been extensively studied, but in the central 

sector of the Southern Alps radiometric data are scarce and 

inconsistent We report a detailed investigation of the 

crystalline basement on the eastern shore of Lake Como, 

applying for the first time in the central Southern Alps the 

40Arf39Ar technique to single crystals of muscovite, biotite and 

hornblende. 

Following the Variscan orogeny, the Southern Alps 

underwent Permian magmatism and graben formation, Triassic 

to Early Jurassic extension leading to the formation of the 

passive margin of the Adria Microplate, and, beginning from 

the Late Cretaceous/Early Tertiary, Alpine shortening. In the 

Lake Como area, a Pre-Alpine crustal section is exposed from 

the sedimentary cover to a paleo-depth of ca. 15 km. The 

basement section is separated into two parts by a 100 m thick 

mylonite band, interpreted as a normal fault related to early 

Mesozoic extension. 

Our data allow us 'to quantify the poorly known thermal 

evolution of this crustal segment from the end of the Variscan 

orogeny until the onset of Alpine shortening. By comparing 

the radiometric signatures of clasts in the mylonite zone with 

the same minerals outside the fault zone, we also investigate 

the effects of deformation and frictional heating on the system. 

Our data confirm the Variscan age of the main metamorphic 

phase. They further reveal a strong thermal pulse in mid-crustal 

rocks causing muscovite plateau ages at 195 - 200 Ma, which 

is during Pre-Alpine rifting. The hanging wall and footwall 

blocks both experienced thermal overprinting, but the hanging 

wall block substantially less severely, resetting biotite to ca. 

195 Ma, compared to 140-145 Ma biotite cooling ages in the 

footwall block. We also report Late Cretaceous (ca. 90 Ma) 

steps, which we interpret as caused by overprinting associated 

with the first stages of Alpine shortening. 
THE EARLY SUN: PRECURSOR TO He and Ne 

ISOTOPES IN THE EARTH 

WITTENBERG, L.J ., Fusion Technology Institute, 

University of Wisconsin, Madison WI, 53706-

1687, USA 

Mantle gases from the Earth containing high 

3Hef4He ratios relative to the atmosphere but in 

combination with Ne isotopes of nearly atmospheric 

ratios have attracted much attention. This paper 

proposes that the formation of these gases occurred 

during the early Sun's pre-main-sequence phase and 

suggests a mechanism for their implacement deep 

within the Earth. 

Astrophysical models (1) propose that our solar 

nebula was created by a super-nova explosion. 

Small particles of SiC, containing nearly pure 22Ne, 

are remnants of that explosion. Dust particles and 

gases within the nebula, eventually, formed a solar 

nucleus. As in-fall continued, the deuteriumburning nuclear reaction, 2H(p;y)3He, was initiated. 

A wind, containing mostly H, but also the newly 

formed 3He and the enriched 22Ne isotopes, swept 

over the dust particles and small planetesimals 

which had formed a disk surrounding the Sun. 

Some of the planetesimals were chemically reduced 

forming the iron cores of the early terrestrial planets. 

Because the noble gases had limited solubility in the 

metallic core, the He and Ne dissolved mostly in the 

molten silicate phases 

The presence of the hydrogen wind suggests the 

possibility of H dissolution within the metallic core. 

Experimental solubility measurements (2) indicate 

that the H/Fe ratio could have been -3 x 10-5. 

Because of the large nuclear cross-section of 3He 

for neutrons, the nuclear reaction, 3He(n,p )3H, 

must be considered. A high flux of neutrons can be 

postulated because of the abundant, neutron-rich, 

fissionable heavy isotopes. The 3H would have 

dissolved, also, in the core; however, the 3H 

quickly decayed to 3He, which was insoluble in the 

iron and could only be released by the fonnation of 

bubbles. These bubbles would migrate slowly 

through the core and be retained for long periods of 

time in the D" layer. 

The 2HJH ratio was -2.5 x 10-5 in the early Sun. 

If this ratio were retained for the 3HJH, the Earth's 

core contained -1014 g of3He. A crude estimate 

indicates that the deep mantle/.lumes over the age of 

the Earth have released -101 g (3He). Hence, 99% 

of the entombed 3He still exists. 

Palla, F., and Stahler, S. W., 1991, The Evolution 

of Intermediate-Mass Protostars: 1. Basic 

Results, Astrophys. J., v. 375, p. 288-299. 

Fukai, Y.,1993, The Metal-Hydrogen System: 

Basic Bulk Properties, Springer-Verlag, Berlin, 

p. 101-119. 

354 

GRENVILLIAN PLUTONISM,.DEFORMATION, AND 

METAMORPIDSM IN Tiffi GRENVlllE PROVINCE, 

ONTARIO, CANADA 

WODICKA, N., PARRISH, R R, Geological Smvey of 

Canada, Ottawa, Ontario, K1A OE8, Canada; and 

JAMIESON, R A, Dept. of Earth Sciences, Ddlhousie 

University, Halifax, Nova Scotia, B3H 3J5, Canada 

Discrimination of individual thennotectonic events in the 

Grenville Province requires a comprehensive IDlderstanding of 

the timing of plutonism, defonnation, and metamorphism. 

We have applied a combination of U-Pb and 40 Arf39 Ar 

geochronology, petrology, and s1ructural geology to 

reconstruct the complex Grenvillian history of the Parry 

SolUld domain, southwest Grenville orogen, Ontario. 

The Parry SolUld domain is a far-travelled allochthon 

isolated from a proposed root zone to the southeast by 

parautochthonous and allochthonous rocks of the Central 

Gneiss Belt. Rocks of the Parry SoiDld domain lUlderwent 

two high-grade thermotectonic events prior to their final 

emplacement onto the Central Gneiss Belt. U-Pb ages of 

syntectonic pegmatites and metamorphic zircons in mafic 

gneisses indicate that the earliest event, which produced 

widespread granulite facies assemblages, took place at -1160 

Ma. We believe that collisional thickening and deep burial 

was the primmy cause for this early granulite facies 

metamorphism; however, we emphasize that it was, at least 

in part, spatially and temporally associated with emplacement 

of the Parry Island anorthosite and mafic dykes at -1163 Ma. 

Monazite and titanite ages from schists and gneisses record a 

later defonnation event at -1120 Ma, which coincided with 

significant exhumation and rapid cooling of Parry Sound 

domain rocks. Evidence for broadly contemporaneous 

sediment accumulation suggests that these data may be 

interpreted in tenns of a tectonic model involving rapid 

exhumation in response to thrusting and concomitant erosion. 

The thermochronological data provide evidence for yet 

another event at -1080-1075 Ma. This thermal event probably 

resulted from major reimbrication of a structurally higher, 

crustal-scale shear zone at -1080-1050 Ma, a consequence of 

continent-continent collision to the southeast of the exposed 

Grenville Province (McEachern and van Breemen, 1993). 

Finally, the juxtaposition of hanging wall rocks with 

· 

40 Arf39 Ar hornblende ages ranging from 1070-1020 Ma 

against footwall units with yo\Ulger cooling ages of 970-960 

Ma has recorded evidence of late stage normal displacement, 

possibly as a result of gravitational collapse of the Grenville 

orogen. 

McEachern, S. J. and van Breemen, 0., 1993, Age of 

defonnation within the Central Metasedimentaty Belt 

bolUldary thrust zone, southwest Grenville orogen: 

constraints on the collision of the Mid-Proterozoic Elzevir 

terrane: Can. J. Earth Sci., 30, p. 1155-1165. 
MINERALOGY AND TEMPORAL RELATIONS OF 

AUTHIGENIC MINERALS IN ALTERED TUFFS AND THE 

UTILITY OF ALKALI ZEOLITES AS POTENTIAL LOWTEMPERATURE DATEABLE MINERALS 

WOLDEGABRIELGiday. BROXTON David E., and BYERS 

Frank M., Jr., Los Alamos National Laboratory, Los Alamos, NM, 

87545 

Finely-crystalline authigenic silicate mineral assemblages from 

altered Miocene and Plio-Pleistocene tuffs in the Barstow and 

Spanish Canyon Formations and the Lake Tecopa Basin and 

adjacent areas in south-central California provide information 

about the history of diagenesis. Altered tuff samples were studied 

petrographically and the authigenic minerals were separated and 

characterized by X-ray powder diffraction. The distal tuffs in the 

upper part of the Barstow Formation are fine-grained and contain 

multiple non welded shard morphologies with no apparent postdepositional reworking. The authigenic minerals are dominated 

by clinoptilolite, opal-CT, analcime, K-feldspar, quartz, illite/ 

smectite {liS), phillipsite, barite, and calcite that formed in a saline, 

alkaline lacustrine environment. The mineral assemblages are 

typical of a low-temperature environment and show minimal postdiagenetic alteration. The bedded tuffs in the Spanish Canyon 

Formation are reworked and are dominated by smectite, 

clinoptilolite, opal-CT. and cristobalite with minor amounts of 

calcite, barite, and authigenic K-feldspar. The Spanish Canyon 

zeolites formed in an open hydrologic system. In the Lake Tecopa 

basin, some of the altered tuffs are replaced by phillipsite and Kfeldspar, whereas others are dominated by a mixture of 

clinoptilolite, erionite, phillipsite, liS, and calcite that formed in 

a saline, alkaline environment. Delicate shard morphology is 

preserved in the clinoptilolite- and K-feldspar-rich tuffs. 

Clinoptilolite-rich tuffs north of the Lake Tecopa basin are 

Miocene in age and were probably altered during ground water 

percolation. 

Representative authigenic mineral fractions (0.1 - 0.35, 0.35-

1, 1 - 3, and 3 - 20 J.Lm) separated from the altered tuffs were 

dated by the K/Ar method. Phillipsite and K-feldspar from the 

altered Plio-Pleistocene tuffs yielded concordant apparent ages 

of0.4 and 0.5 Ma, respectively. Clinoptilolites (6.7- 16.2 Ma), II 

S (10.7 Ma), and K-feldspars (13.8- 15 Ma) from the altered 

Miocene tuffs yielded similar apparent ages. Although 

clinoptilolite in the Miocene tuffs predated K-feldspar, the K/Ar 

results indicate that only one of the clinoptilolite fractions (13.5 

Ma) from an undisturbed unit in the Barstow Formation is similar 

to the K-feldspar ages. The other two clinoptilolites are from 

fractured samples of the same unit close to the axis of the Barstow 

Syncline and yielded younger dates (6.7- 8.7 Ma). The decrease 

in apparent age is due to Ar loss probably related to fracture 

permeability in those samples from the folded zone and to 

dehydration. Experimental results indicate minimal Ar loss from 

hydrated alkali zeolites. 

In most cases, eruption and alteration ages of the tuffs are 

similar to each other and suggest early diagenetic alteration. The 

results also indicate that zeolites can retain Ar and K and may be 

used with other minerals to constrain the time oflow-temperature 

diagenetic processes especially in saturated continental 

environments with possible application to zeolites in deep sea 

sediments .. 

355 

NOPAL 1 NATURAL ANALOGUE: A STUDY OF 

RADIONUCLIDE MIGRATION 

VIRGINA WONG, P.C. GOODELL, AND E.Y. 

ANTHONY, Univ. of Texas at El Paso, El Paso, 

Texas, 79968. 

This abstract reports on activities of naturallyoccurring radioisotopes for the Nopal 1 uranium deposit, 

located in the Peiia Blanca uranium district in northern Mexico. 

The Nopal 1 deposit has been identified as one of the most 

promising site for analogue studies to the proposed nuclear 

waste repository site at Yucca Mountain, Nevada. Activities of 

21 samples were measured using gamma-ray spectroscopy. The 

samples consist of an ore sample and alteration minerals of 

kaolinite, hematite, limonite, manganese oxide, and carbonate 

minerals. They were collected from the 0 m (meter), +10m, 

+ 20 m, and + 30 m levels of the mine. 

In general, samples in the 0 m level have the highest 

activities (in Bq/ g) followed by low activities in the upper levels 

of the mine. The ore sample from the 0 m level has the 

highest activity, at 78 Bq/g. The samples associated with 

kaolinite alteration show highest activity in the 0 m level with 

decreasing activity in the + 10 m, + 20 m, and + 30 m levels 

(activity range = 27Bq/g to 0.9Bq/g). Hematite samples 

collected from 0 m and + 10 m levels have an activity range 

from 27 Bq/g to 2 Bq/g. Limonite samples collected from 0 

m, + 10 m, and +20 m levels behave erratically in their 

activities - the sample from the 0 m level has an activity of 24 

Bq/g, two limonite samples from the + 10 m level have 

activities of 85 Bq/g and 2 Bq/g, and a sample from +20m 

level has 19 Bq/g. The manganese oxide sample from the 0 m 

level has an activity of 40 Bq/g, and carbonate samples from 

+ 10 m level show low activities, ranging from 0. 7 to 0.5 

Bq/g. 

The ore sample, the carbonate samples, and the 

manganese oxide sample are in secular equilibrium. Kaolinite 

samples collected from the four mine levels all show 

enrichment of 234r'fh relative to 226Ra except for the kaolinite 

sample from the 0 m level. This sample is deficient in 226Ra 

relative to 230Jb implying mobility within the past 8 ka. 

Hematite samples show various kinds of disequilibrium. The 

sample from the 0 m level is enriched in 234U relative to 234r'fh 

and 230Jb, implying that the nsu decay chain was broken in the 

past 250 ka. In addition, most hematite samples from the + 10 

m level show evidence of disequilibrium in the three ratios in 

the 238U decay chain f"'Tbf234U, 234U/2»fh, and 2»fhf226Ra), 

implying that the mobility for this alteration type is complex in 

nature. Limonite samples are observed to be in secular 

equilibrium in the nsu and 235U decay chains except for the 

sample from the 0 m level. This sample is enriched in 230Jb 

relative to 226Ra, implying mobility of radionuclides in the past 

8 ka. 
ISOTOPE AND HYDROLCXHCAL FEATURES 

OF THE TARIM BASIN, CHINA 

WUSHIKI, H., The Institute of Physical and 

Chemical Research.(RIKEN), Saitama 351-01, 

Japan, IAKAHAS.HL_KL, RIKEN, Saitama 

351-01, Japan, HUANG, Z., Xinjiang Institute 

of Biology, Pedology, and Desert Research, 

Academia Sinica, Urumuqi, China, XIONG, J., 

Xinjiang Institute of Biology, Pedology, and 

~sert Research, Academia Sinica, Urumuqi, 

China, :MASUDA, A., University of ElectroCommunications, Tokyo 182, Japan, and 

YABUKI, S., RIKEN, Saitama351-01, Japan. 

The purpose of this study is to investigate the 

hydrological environment (the origin of water and 

the hydrologic cycle) in Tarim Basin, based on the 

regional characteristics of 180 and D contents of 

natural water. During the three field work seasons 

of 1990, 1991 and 1992, water samples, river, 

precipitational and ground water, were collected for 

the isotope analyses and chemical analyses. 

Analytical results were discussed as follows. 

River water: Regional distribution range of the 

isotopic ratios was found at the highest level in 

southern region of Tarim basin, spanning -66. 8"'-

46. 7%o, in bD. North region fell on the second and 

west region on the lowest level, -79.1,...,-72. 7%o and 

-88. 7"'-78.3%o, respectively. Rivers on the 

northern slope of West Kunlun Mts., which is 

situated at the southern edge of Tarim basin, have 

the isotopic ratios nearly same to the summer 

precipitation at West Kunlun Mts. Therefore, they 

were considered to be replenished mainly by the 

summer precipitation at West Kunlun Mts. site. 

Precipitation: Precipitations, both rain and snow, 

were sampled in 1990/1991 and 199111992 at the 

northern slope of West Kunlun Mts. The isotope 

ratios varied greatly with season, ranging from 

-28.6 to +46%o and from -287 to +5.0%o in b18Q 

and bD, respectively. There is a distinct separation 

in isotopic value between winter and summer~ 

transitional period can be placed between April and 

11ay with an approximate value set of -15 %o and 

-100 %o. On the b180-bD plot, the summer 

precipitation holds larger d-index than the winter 

one, suggesting different origin of water vapor. 

Ground water: Shallow ground water of phreatic 

surface depth of 1m or less are widely distributed 

in the peripheral zone of Taklamakan sands. Under 

the ground surface with poor vegetation, the 

i~otope enrichment process in ground water may be 

dtfferent from that under the ground with much 

vegetation. Furthermore, we also performed a 

chemical and isotopic (Sr isotopes) studies and the 

results will be presented in our report. 

This work was partly supported by Special 

Coordination Funds of Science and Technology 

Agency of Japanese Government. 

356 

A STUDY OF COMBINED STABLE-ISOTOPE AND 

TRACE-ELEMENT COMPOSITION IN LACUSTRINE 

OSTRACODES AS PALEOCLIMATIC INDICATORS IN 

THE NORTHERN GREAT PLAINS, NORTH AMERICA 

Xl&...L., ITO, E. and ENGSTROM, D., all in Dept. of 

Geology and Geophysics, Univ. of Minnesota, 310 

Pillsbury Dr., Minneapolis, MN 55455 

The calcite shells of ostracodes are an important source of 

biogenic carbonate that can be analyzed for isotopic 

composition and trace element ratios to provide an account 

of chemistry and temperature changes in closed-basin lakes. 

The variations in lake chemistry and temperature can in turn 

provide high-resolution records of hydrologic and climatic 

change. 

Accurate interpretation of the geochemical record from 

fossil ostracodes requires a solid understanding of ostracode 

shell formation and life-history. To explore these issues, 

we cultured ostracodes in the laboratory and compared the 

results with those obtained from monthly field collection of 

live ostracodes from several lakes in the northern Great 

Plains. The oxygen-isotope fractionation factors of the 

ostracode species, Candona rawsoni, have been calculated 

as 1.0305 at 25°C and 1.0322 at l5°C based on laboratory 

cultures. From monthly field collections in Coldwater 

Lake, ND and Roslyn Lake, SD, large variations in ol8o, 

Mg/Ca and Sr/Ca among individual adults from the same 

collections warn strongly that analysis of just a few 

individuals per sampling interval will most certainly lead to 

erroneous stratigraghic interpretations. 

To study climatic changes in the northern Great Plains, 

sediment cores have been taken from several closed-basin 

lakes spanning an E-W moisture gradient across the region. 

The results from one of these sites, Coldwater Lake, North 

Dakota, will be discussed here. A preliminary study of the 

upper part of a short core covering the past 110 years shows 

strong coherence between instrumental records of aridity 

and variations in both o18o and Mg/Ca in ostracodes. 

Sr/Ca in ostracodes shows a strongly negative correlation to 

Mg/Ca that results from aragonite precipitation and the 

selective removal of Sr from solution with increasing 

salinity. The lower part of the short core represents the last 

several hundred years including the so-called "Little Ice 

Age" event. Results thus far imply that this period was" 

highly arid in the northern Great Plains and that climatic 

gradients with wetter regions to the east and south were 

very pronounced. 

The entire Holocene record, contained within an 18m long 

core, shows great potential for providing high-resolution 

records of regional climate. Preliminary results show a 

number of major excursions in o18o and Mg/Ca 

representing periods of wetness and aridity that are 

linked by abrupt transitions in hydrology and climate. 
THE STIJDY OF ISOTOPE GEOLOOE OF EARLY PRECAMBRIAN 

AND URANIUM MINERALIZATION IN lliE EASTERN PART OF 

NOR'lli-GHINA PLA'IFORM 

XIA YULIANG, ZHENG MAOGONG AND TAO QUAN 

Beijing Research Institute of Uranium Geology,China 

P.O.Box 764, Beijing 100029, China 

On the basis of obtained the U-Pb isotopic data 

from 132 samples of zircon, 32 whole-rock samples 

and 45 samples of uraninite(or pitchblende) ,we have 

put forward as following: 

(1) The crust movement of early Precambrian 

A number of isotopic ages showed that the crust 

movement of ear·ly Precambrian in the Eastern Part 

of the North-china Platform could be separated into 

four obvious cyclses which from the older to the 

younger are as follows: 

1). Qianxi Movement-------3000 ± 100 Ma 

2). Fuping Movement-------2800 ± 100 Ma 

3). Wutai Movement-------2500 ± 100 Ma 

4). Lul iang Movement-------1800 ± 100 Ma 

(2) Time division of early Precambrian 

mismatitic granitoids 

According to the data of obtianed isotopic ages 

of migmatitic granitoidS in North-china Platform, 

the migmatization in the area could be divided into 

four periods which corresponded to the importent 

tectonic movement cycles of earth's crust. The 

migmatization at Wutai Movement spreaded to the 

most widespread range. This showed that the Wutai 

Movement at the end of Archean was the strongest 

period of the crust movement and could have a 

significance of all the world. 

(3). Uranium mineralization 

The mineralized types of uranium on the early 

Precambrian in the Eastern Part of North-China 

Platform are known as follows: 1) migmatitic 

hydrothermal type;2) metamorphic hydrothermal type; 

and 3) magmatic type.No matter what the mineralized 

type, the time of all the uranium mineralizations 

was the products of Luliang Movement, the age of 

which is about 1800 ± 100 Ma. 

357 

HELIUM ISOTOPIC COMPOSITIONS IN NATURAL 

GASES IN CHINA 

~.NAKAI, S., WAKITA, H., Laboratory for 

Earthquake Chemistry, Faculty of Science, University of 

Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113, Japan. 

The isotopic composition of helium has been determined for 

more than 170 natural gases including fuel gases in 20 

commercial oiVgas fields from 16 sedimentary basins, and hot 

spring gases in recent volcanic and geothermal areas in China. 

The 3HefHe ratios range three orders of magnitude from 0.004 

times the atmospheric ratio (0.004Ra) in a pre-Cambrian 

sedimentary reservoir in Sichuan basin up to 5.2Ra in a recent 

volcanic geothermal area at Tengchong, and 4HeJl~e ratios 

vary from 0.3 to 22000. 

The crust of Chinese continent thickens gradually from 

shallower than 30km in east to deeper than 65km in west. 

Several zones of steep gravity gradient divide China into four 

main tectonic units. The distribution of 3Het4He ratio is 

consistent with the tectonic classification. The Eastern domain 

is typical areas undergoing continental thinning. Crustal 

extension linked with uplift of mantle material produces a 

series of basins in which R/Ra varies from 0.008 to 5.0. The 

extensive mantle-derived helium is introduced by melts whose 

generation is associated with crustal thinning. It is worthy to 

mention that the mantle-derived He occurs larger in geographic 

areas than do the distribution of surface volcanics. This may 

suggest that mantle-derived melts are emplaced in the deep parts 

of the lithosphere and that the migration of He has extensively 

occurred. In contrast, the tectonically stable area of the Central 

domain and Northwest segment, where the crust has been 

thickened, is characterized by typical crustal He isotopic 

composition and a negligible input of mantle component. In 

the case of Southwest segment of the Qinghai-Tibet plateau 

where crust has also been thickened, high values of R/R.a 

ranging from 0.21 to 5.2 were observed in Tengchong, a 

Quaternary volcanic geothermal area near the Indo-Eurasian 

collisional margin. Tengchong area is a fossil subduction zone 

where the subduction movement changed to continent-continent 

collision. The mantle-derived He is closedly related to the 

volcanics caused by the collision between the Indian and 

Eurasian plates. The highest 3Het4He ratio in Tengchong, 

5.2Ra, is the highest value reported to date for mainland China, 

however, the value is lower than that of present typical 

subduction volcanism. 

In general a good correlation is observed between the 

distribution of terrestrial heat flow data and 3Het4He ratios. 

The heat flow values in Central domain and Northwest segment 

are generally as low as -45mW/m2, while higher values from 

50 up to 120 m W /m2 are predominant in the Eastern domain 

and Qinghai-Tibet plateau. This suggests that uprising 

magmatic melts carry both the primordial 3He and heat in the 

crust of Eastern domain and Qinghai-Tibet plateau. 
FISSION~ TRACK ANNEALING IN ZIRCON; CONFINED 

TRACK LENGTH ANALYSIS AND THE ARRHENIUS 

PLOT 

YAMADA R, TAGAMI, T. and NISHIMURA, S., Dept. 

of Geology and Mineralogy, Fac. of Science, Kyoto Univ., 

Kyoto, 606, Japan. 

To establish the basis of zircon fission-track 

thennochronology, we carried out a series of laboratory 

annealing experiments. The variation of confined fission track 

lengths was determined on zircon grains from the Nisatai 

Dacite. We primarily performed the bias estimation of track 

length measurement in order to achieve higher accuracy. 

Consequently, it was found that etching temperature, track 

etching criteria, measurement criteria of horizontal confined 

tracks (HCTs) and the orientation ofHCTs significantly control 

observed track lengths where the samples were highly annealed 

(e.g., ~ °C for 1 hr annealing). The frrst three factors may 

be related to the presence of gap zones in annealed zircons; 

whilst the fourth is caused by anisotropic etching and annealing 

characteristics. As a result, the experimental and analytical 

criteria adopted are: etching temperature, 250 °C; track etching 

criterion, 2 J.1IIl width of the surface tracks perpendicular to caxis; measurement of HCT's having 1 ± 0.5 J.Lm width; and 

HCTs with the angles over 60° to c-axis. 

Based on those experimental criteria, the Arrhenius-plot 

experiment was subsequently performed at 350- 750 °C for 

10-1- 103 hr (i.e., 4.5 min.- -40 days). On the Arrhenius 

diagram, the fading contours of constant nonnalized mean track 

length (r) showed as a series of straight lines. To describe the 

decrease in r with increasing temperature or heating time, two 

types of model fitting were perfonned, called the parallel and 

fanning models. We defined the lowest temperature limit of 

zircon partial annealing zone (ZPAZ) by r ...().95, and the 

highest by total fading of surface tracks. The extrapolation of 

laboratory data to a geological time scale gives an estimate of 

the ZPAZ: -210- 280 °C by the parallel model and -200- 330 

°C by the fanning model for the heating time of ten m.y. For 

an order of longer duration, the temperatures of ZP AZ decrease 

-20 °C. The closure temperature (Tc) of zircon fission-track 

system shows a significant discrepancy among literature: 

previously estimated, -175 °C (e.g., Harrison et al., 1979) or 

-240 °C (e.g., Hurford, 1986). Because Tc corresponds to the 

middle of ZP AZ, the present results support the higher value. 

The variation of etching time to satisfy the criterion revealed 

that the removal of alpha-radiation damage occur drastically at r 

...().93. 

Harrison, T. M., Armstrong, R. L., Naeser, C. W. and 

Harakal, J. E., 1979, Geochronology and thennal history of 

the Coast Plutonic Complex, near Prince Rupert, British 

Columbia: Can. J. Earth Sci., 16, 400-410. 

Hurford, A. J ., 1986, Cooling and uplift patterns in the 

Lepontine Alps South Central Switzerland and an age of 

vertical movement on the Insubric fault line: Contrib. 

Mineral Petrol., 92, 413-427. 

358 

•'GENESIS OF CONTINENTAL CRUST THROUGH 

ARC MRGMRTI SM 

YRNRGI,T., Dept. Earth and Planetary Sc:l., 

Kyushu Unlu., Fukuoka 812, .Japan 

Geographical configurations of Island 

arcs Indicate the alternatlue growth of 

uolcanlc and frontal arcs. The 

emplacement of granitic plutons occurs 

alternatiuely In uolcanic and frontal arcs. 

The uolcanic arc Is recorded geologically as 

a belt of Intimate association of uolcanlc 

rocks and subsequent granitic plutons, and 

the frontal arc as a belt of granitic 

plutons In metamorphic or sedimentary 

formations with Imbricated structures 

(Yanagl, 1981 ). Geological comparison 

between mature and Immature Island arcs 

suggests that the arc crust euolues with 

time toward the crust with much 

continental characteristics. 

The following characteristics of arc 

uolcan!c rocks are Important for crustal 

growth: ( 1) Intimate association of 

tholeiitic and calc-alkaline uolcanic rocl<s; 

(2) repeated occurrence of basaltic and 

dacitic lauas in a uolcanlc succession; (3) 

gradational change of uolcanlc rocks from 

tholeiitic to calc-alkaline series; (4) 

crystallization differentiation which 

Included the crystallization of plagioclase 

with other mafic minerals; (5) marked · 

enrichment of K20 associated with slight 

depletion in compatible elements; (6) an 

euoluUon limit represented by, for 

instance, Rb/Sr ratios of 8.23-8.28 and 

Si02 contents of 68-66 wt.% and (7) 

repetitiue magma miHing. 

Batch fractionation In refilled magma 

chambers best accounts for these 

characteristics, transforming mantlederiued magmas through calc-alkaline 

magmas to magmas of composition uery 

close to the bulk upper crust. Coupled 

magma chambers proposed by Yanagl et al. 

( 1 99 1) is a system which keeps the batch 

fractionation. R Similar mechanism may 

operate in imbricated sedimentary 

formations to form the frontal arc 

Immediately behind the aseismic front. 
IDENTIFiCATION OF TWO VOLCANiC CHAINS IN TilE 

NORTH TAIWAN-LUZON ARC 

YANG. Tsan-yao. F., Dept. Chemisuy, WHOI, Woods 

Hole, MA 02543; LEE, T., Inst. Earth Sci., Acad. Sinica, 

Taipei, Taiwan, CHEN, C-H., Dept. Geology, Nat'l Taiwan 

Univ., Taipei, Taiwan, PUNONGBA Y AN, RS., RASDAS, 

A.S., and DAAG,A.S., PHIL VOLCS, Quezon, Philippines. 

Two volcanic chains of North Taiwan-Luzon Arc are 

recognized in this study. They are separated by 50 km in the 

south, however, they converge together northward intersecting 

- 20~. The volcanic islets belonging to the West Volcanic 

Chain are all Miocene in age and have the characteristics of 

low relief, lateritic platforms, wave-cut terraces, and covered 

with massive recrystallized limestone. In contrast, most of the 

islets of the East Volcanic Chain are Quaternary in age. Some 

are active volcanoes with perfect cone shapes, and have well 

preserved eruptic records of near vent facies. 

Before the eruption of East Volcanic Chain, regional 

uplifting occurred in this area. This might be due to th~ arccontinent collision in the northern part of the arc (Tmwan) 

and the aseismic ridge colliding with the Taiwan-Luzon Arc 

around 3-5 Ma ago. Subsequently, the arc magmatism 

shiefed eastward during Quaternary. We propose that this 

sudden eastward shifting of the arc volcanic activities can be 

interpreted as caused by an abrupt change of the dip angle of 

the subducted slab when the aseismic ridge began to sink. 

< w 

U) 

< ~ 

~ \ 0 

< 

w 

U) 

w 

z 

0.. 

0.. 

359 

C-, 0-, AND Sr- ISOTOPE 

GEOCHEMISTRY OF ~COUTE FROM 

SOME SEAMOUNT PHOSPHORITE IN 

NORTH PACIFIC BASIN 

YEH, H.-W., Institute of Earth Sciences, 

Academia Sinlca, P.O.Box 1-55, Nankang, 

Taipei, Taiwan, HEIN~ J R , U.S. Geological 

Survey, Menlo Park, CA. 94025, CHAN, L.-

H., Department of Geology and Geophysics, 

Louisiana State ~i versity, Baton Rouge, LA. 

70808, and WII J HIRE, J C , Oceanogrophy 

Department, University of Hawaii, Honolulu, 

Hi. 96822. 

The occurrence of phosphorite on seamounts in 

Pacific Basin was first known in late 1950. It is 

now apparent that phosphorite is a common r~k 

type on seamounts in general. However, the ongm 

of the phosphorite is still an enigma; the "u.pwelling, diagenetic" model is inadequete to exph~m 

it. C-., 0-, and Sr-isotope methods are potential 

tools to gain understanding of its origin, provided 

we have good knowledge on the isotope 

geochemistry of the phosphorite. Her<:, ~e 

discuss the isotope geochemistry of francohte, tn 

the light of C-, 0-, and Sr-isotope ~ata on 

phosphorite samples collected from etghteen 

seamounts of North Pacific. Francolite, a 

phosphate mineral, is the phosphbrus carrying 

phase in the phosphorite. The C and 0 are from 

the structural carbonate. 

The isotopic results fait into two groups against 

the background of prevailing Cenozoic deep-sea 

environment. The majority of the data have the 

characteristics of Cenzoic benthic foraptifera. A 

few of them appear to be hydrothermal in 

character. a 13c, a 18o and 87srf86sr all are 

correlated according to the aformentioned 

groupings. The range of isotopic data are several 

ppt in a 13c and a 18o and about 1.5ppt in 

87 Srt86sr, regardless the ages of the seamounts 

and the foram precursor. 

The conclusions drawn from the data are: 

l.The phosphorite do not suffer sig~ifican.t. 

postformational modification of the tsotoptc 

composition. . . . 2. The phosporite appears to form tn tsotoptc 

equilibrium with ambient sea water. . 

3.The isotopic behavior of structural carbonate Ill 

francolite appears to be practically identical to that 

of calcite. 

4.The isotopic composition of francolite do not 

carry the isotopic signature of the precursor 

carbonate. 

5.The isotopic compositions of the francolite 

reffects the environmental condition of the 

phosphogenesis. 
EXHUMATION OF THE RED SEA & GULF OF ADEN 

RIFT-FLANKS: FISSION TRACK CONTRASTS 

WITHIN THE VOLCANIC & NON-VOLCANIC 

MARGINS OF THE YEMEN REPUBLIC. 

YELLAND. Andrew. J .. Department of Geology, 

Royal Holloway, University of London, Egham, 

Surrey, TW20 OEX, United Kingdom, MENZIES, 

M.A., Department of Geology, Royal Holloway, 

University of London, Egham, Surrey, TW20 OEX, 

United Kingdom, HURFORD, A.J., Department of 

Geological Sciences, University College, London, 

WClE 6BT, United Kingdom. 

Crustal exhumation along the proto-Red Sea margin, 

appears to have occured in the early Miocene; co-eval 

with extension, and post-dating major Oligo-Miocene 

flood volcanism (33-18 Ma). The sequence of break-up 

(magmatism-extension-uplift & denudation) fits 

neither active nor passive rifting models in the 

traditional sense. 

Regional synthesis of fission track data from ArcheanProterozoic basement and Mesozoic-Cenozoic sediments 

throughout the Red Sea and Gulf of Aden margins of 

Yemen provides important thermo-chronometric 

constraints on the relative stability of crustal domains 

both proximal and distal to active extension. 

FT data from highly extended and exhumed coastal 

domains contrast markedly with those from more stable 

internal crustal domains which appear to have 

maintained a relatively positive structural and 

topographic stance since the break-up of Gondwana. 

Conclusions which are supported by recent 

sedimentological and stratigraphic observations. 

Furthermore, recent reactivation of Mesozoic ijurassic & 

pre-Jurassic) structural elements parallel and oblique to 

the Gulf of Aden is thought to have an important 

influence on the exhumational signature presently found 

along the Gulf of Aden margin, and hence to the 

evolution of rift margin topography. 

360 

EXPERIMENTAL AND THEORETICAL STUDIES OF 

ARGON RECOIL 

YORK Derek, EVENSEN, Nonnan M. and SMITH, 

Patrick E., Department of Physics, University of Toronto, 

60 St. George St., Toronto, Ontario, MSS 1A7, Canada. 

The laser fusion technique has shifted the focus of 40 Ar39 Ar analysis toward smaller sample and domain sizes in 

which fast-neutron-induced recoil of daughter nuclei e7 Ar and 39 Ar) over distances on the order of O.l-0.21Jlll is significant. 

Recoil can lead to redistribution of daughter argon in the 

sample (affecting release patterns) or to partial loss of 

daughter argon from the sample (producing anomalously old 

ages). 

We use both theoretical modelling and experimental 

studies to investigate recoil behaviour. In the experimental 

work, samples are micro-encapsulated in evacuated quartz 

ampoules before irradiation. Then an in vacuo "crusher" 

releases trapped recoil gas from the ampoules, followed by 

laser heating of the solid sample to release occluded argon. 

The low irradiation temperatures of the McMaster University 

reactor allow clean separation of the recoil and thennal 

release components of the sample argon, even in samples 

with low closure temperatures such as glauconies. 

Recoil loss is a function of the geometry of the sample 

in relation to the distribution of parent nuclides e9y( and 4

°Ca). Analytical models of Ar loss for some elementary 

sample geometries have appeared in the literature, but we 

have derived a simple approach which allows quantitative 

treatment of complex geometries. Examples of some basic 

geometries (e.g. spherical, planar, cylindrical, cubic) and 

some occasionally counter-intuitive consequences will be 

presented. 

Surprisingly, vacuum irradiation, while significantly 

reducing recoil loss, does not result in complete reimplantation of recoil argon in adjacent sample or the 

container walls. This behaviour is quantitatively modelled by 

assuming (in accordance with experimental data) that reimplantation occurs only above a threshold energy of the 

recoiling argon nucleus. In glaucony grains, where recoiling 

nuclei pass through several thin laminae before halting, recoil 

losses exceed I 0% even in vacuo. Samples with less void 

space, such as the hornblende standard Hb3gr, show losses 

much less than I%, which may however still be significant in 

very precise geochronology using fine-grained standards. 

More extensive studies of recoil behaviour of this and other 

standards are in progress and will be reported. 37 Ar, recoiling from an a-particle in the reaction 4

°Ca(n, 

a)37 Ar, has more recoil energy than 39 Ar, produced by an (n, 

p) reaction, though rather than the -4x difference suggested 

by the ratio of a-particle to neutron mass, the energy is only 

about 2.Sx greater. Nevertheless, because of its greater range, 37 Ar would be expected to be more readily lost by recoil than 39Ar. Experiments on glauconies show, however, that the 37 Arf9 Ar ratio of recoil gas is equal to that of the solid. This 

is explicable by a theory based on the threshold energy of 

implantation discussed above, and should be true regardless 

of sample geometry. Experiments are underway on a variety 

of materials to test this hypothesis. 
231pA AND 230ni AS CHEMICAL TRACERS OF 

PARTICLE FLUX IN PALEOCEANOGRAPHY: 

APPliCATION IN TilE SOUTH INDIAN OCEAN 

Yu. E. -F, Dept of Marine Chemistry and Geochemistry, 

Woods Hole, Massachusetts, Ph. D candidate, Bacon 

P. M., Dept of Marine Chemistry and Geochemistry, Woods 

Hole, Massachusetts, Senior Scientist, U. S. A, and Francois 

R., Dept of Marine Chemistry and Geochemistry, Woods 

Hole, Massachusetts, Associate Scientist, U.S. A 

Holocene and Last Glacial Maximum (LGM) sections of 

cores forming a nodh-south transect across the present 

Subtropical Convergence (STC), Subantarctic Front (SAF), 

and Polar Front Zone (PFZ) in the South Indian Ocean were 

analyzed for 23<>rh, U isotopes and 23lpa to reconstruct 

changes in the flux of sediment components. Results from the 

Holocene show a maximum in 23Gfb-normalized opal flux in 

the region of the PFZ. North of the present PF, opal flux is 

lower than it is immediately south of the PF. There are also 

remarkably higher 231 Paf230Jb ratios, reflecting enhanced 

scavenging, at the PFZ; the latter observation is consistent with 

the higher opal rain rates at the PFZ. Data from the LGM 

sections show similar latitudinal features. The data also 

suggest a higher opal flux for sediments in SAF during the 

LGM; an approximately constant opal flux is found for 

sediments in PFZ. For carbonate flux, lower values are found 

for LGM sections of all cores in both PFZ and SAF; whereas 

the terrigenous flux shows a reverse pattern for change between 

LGM and the Holocene. Both 231 paJ230Jn ratios and 230Jn_ 

normalized total flux remain approximately uncbanges in the 

regions of both PFZ and SAF. The later feature is also 

confirmed by the downcore records from a core (MD88-773) in 

the south of the PF. However, the pronounced maximum in 

231 paJ230rb ratio and total flux are found during deglaciation 

in the core. We interpret these records as reflecting variations 

in particle rain rates associated with climate-induced latitudinal 

migration of the southern ocean frontal system. 

361 

THE O,C,Sr,Pb ISOTOPE COMPOSITIONS OF 

REEF-FACIES CARBONATES OF CORE NY-1 IN 

THE SOUTH CHINA SEA AND THEIR 

PALAEOENVIRONMENT IMPLICATIONS 

YU JIN-SHENG, CHEN YU-WEI, GUI XUNT ANG, YU FU-JI, ZHANG DE-KE, WEI 

GU ANG-JIAN, Guangzhou Branch, Institute of 

Geochemistry, Chinese Academy of Science, 

Wushan, Guangzhou, 51064 0, P.R. China. 

"Core NY-1" located in the Yongshu Reef, Nansha Islands of the South China Sea, ( 09°33'N , 112° 

53. 3'E), was drilled by The Mutildisciplinary Oceanographic Expedition T earn of Chinese Academy 

of Sciences to the Nansha Islands, during May and 

June, 1990. The depth of this core is 152. 07m, 

which consists of mainly reef-facies carbonates. 

4 stag&S can be divided for the 0 , C isotope compositions of reef-facies carbonates of core NY -1. The 

average values of 618 0 (%0 ) and 613C (%0 ) for the 4 

stages are: (I )-5. 0;-0. 5; (I )-7. 7;-5. 3; 

( II ) - 6. 4 ; - 2. 8; ( IV ) 1. 3; 2. 1 , respectively. The 

boundary age between I and II stages in depth of 

about 90m is 730ka, according to the palaeomagnetic 

dating. The characters of the I stage of 6180 fluctuating curves are the low frequencies , small amplitudes and corresponding correlation with the glacial 

and interglacial stages, during the Brunhes normal epoch. It has positive correlations between 618 0 and 

613C values in II and IV stages, which genetically related to dolomitization. 

The maximum, minimum values and all of the 

positive values of 6. 87Sr in the core of whole hole are 

only occured in the I stage. The characters of frequently changing fluctuation appeared in IV stage for 

both Pb and Sr isotope compositions. The leads with 

highly J.1. values were taken in during dolomitization. 

The history of palaeoclimatic and palaeoenvironmental changes in south China sea were recorded 

in the 0, C ,Sr, Pb isotope compositions of reef-facies 

carbonate, which is another significant information 

storage concerning global environmental. change during Quaternary Period. 
OXYGEN ISOTOPE EXCHANGE DURING 

CHONDRULE FORMATION: AN EXPERIMENTAL 

STUDY 

YU, Y., HEWINS, R. H., Dept. of Geological 

Sciences, Rutgers University, Piscataway, NJ 08855 

USA, and MAYEDA, T. K., CLAYTON. R. N., 

Enrico Fermi Institute, University of Chicago, 

Chicago, IL 60637 USA 

Oxygen isotopic compositions of chondrules from 

primitive meteorites fall along a slope-1 trend in the 

three-isotope diagram. This is a mixing line which has 

been interpreted to be the result of isotopic exchange 

between chondrule material and the surrounding 

nebular gas, perhaps during the heating event which 

melted the chondrule. We have conducted laboratory 

experiments to determine rates and other details of this 

exchange process by melting chondrule-sized droplets 

of meteoritic material in an atmosphere consisting of a 

controlled mixture of water vapor, hydrogen, and 

helium. Experiments were done in a vertical gasmixing furnace at 1400°C for periods of 5 minutes to 

12 hours. Reaction progress was deter.mined by 

measurement of 618o and 617 0 in the quenched 

droplets. The exchange trajectory is expected to be 

linear on the three-isotope graph, connecting the 

initial composition to the comp<:>sition in equilibrium 

with the gas reservoir. (At equilibrium, the melt which 

is of ultramafic composition is about 2%o lighter in 

6180 than the water vapor.) The expected linear 

behavior is observed for single-phase charges in two 

sets of experiments, one with solid olivine, the other 

with a pyroxene composition above the liquidus. Two 

additional sets of experiments were carried out with 

partially molten charges, as is often the case in natural 

chondrules. In these experiments, the trajectory is 

markedly nonlinear, reflecting different exchange 

rates for the solid and liquid parts of the charge, which 

had different initial isotopic compositions. 

The rate of isotopic exchange in these experiments 

is determined by the diffusion rate within crystals 

and/or melt. The slowest exchange occurred for solid 

olivine from the Eagle Station pallasite, which 

exchanged 35% in 12 hours. The fastest exchange 

occurred for the pyroxenic melt, which exchanged 

50% in the shortest practicable experiments of 5 

minutes. The experiments demonstrate that the 

diffusion rates within molten chondrules are sufficient 

for extensive isotopic exchange during the period of 

time during which a chondrule is hot. However, the 

experiments do not test the dependence of the 

exchange rate on the gas/melt surface processes, which 

are likely to be rate-determining in the case of 

chondrule formation due to the much lower gas 

pressure in the solar nebula compared to laboratory 

conditions. 

362 

U-Pb AND Sr ISOTOPIC STUDIES ON GRANITOIDS 

OF TAIWAN AND CHINMEN AND THEIR 

IMPLICATIONS FOR THE CRUSTAL EVOLUTION 

Tzen-Fu Yui. Institute of Earth Sciences, 

Academia Sinica, R.O.C., Larry Heaman, 

Dept. of Geology, Royal Ontario Museum, 

Canada, and Ching-Ying Lan, Institute of 

Earth Sciences, Academia Sinica, R.O.C. 

The time of granitoid emplacement in 

orogenic belts provides valuable information 

for deciphering the crustal evolution in 

complex metamorphic terrains. With these 

objectives in mind, several dating systems, 

such as Rb-Sr, K-Ar and 40Art39Ar, have 

been employed in the past to date the 

granitoids in Taiwan and Chinmen. However, 

since the studied areas have experienced 

complicated metamorphic histories, the dating 

results can be somewhat ambiguous due to 

partial resetting by various kinds of 

subsolidus reactions. Some critical problems, 

therefore, remain unsolved. On the other 

hand, the U-Pb zircon/monazite dating can be 

quite successful at unraveling the timing of 

granitoid emplacement in high grade 

metamorphic terrains because the blocking 

temperature for Pb in these minerals is 

rather high (>6000C). This paper presents UPb zircon/monazite dates as well as Sr isotope 

data for granitoids from Taiwan and Chinmen. 

The results show that the linear correlations 

between 8 7 Sr/8 6 Sr vs. 8 7 Rb/8 6 Sr are not true 

isochrons, but apparent isochrons. Two endmember mixing/ AFC model or late-stage 

alteration can account for the available data. 

The implications on the crustal evolution for 

the southeast China will be discussed. 
IN-SITU OXYGEN ISOTOPE ANALYSIS BY 

SIMS 

YURIMOTQ. H., Institute of Geoscience, University of 

Tsukuba, Tsukuba,lbaraki 305, Japan 

Secondary-ion mass spectrometry (SIMS) is a powerful 

technique used for elemental and isotopic analyses at a microscale surface region. Normally, positive ions, such as o2+ and 

Cs+, have been used for the primary beam in SIMS analysis. 

In the case of bombardment by positive ions, one of the 

difficulties that limits insulator analysis in SIMS is severe 

electrostatic charging of the sample surface. In contrast, charge 

compensation with negative-ion bombardment is superior to 

those using positive-ion bombardment. The use of an aprimary ion beam is widespread for insulator analysis by SIMS 

owing to ease of generation. One of the interesting 

applications of SIMS is in situ oxygen isotope analysis of 

intra-mineral grains in meteorites, which are natural insulator 

materials. In the case of an oxygen isotope analysis by SIMS, 

however, the use of an o- primary ion beam is obviously 

inadequate, since the primary species dilutes the oxygen in the 

sample. 

Recently, a new type of intense negative-ion source for 

SIMS based on plasma sputtering was developed [1]. The ion 

source combined with a SIMS instrument succeeded in 

generating negative metal ions, such as Au and Cu, with an 

·intense beam current and with excellent stability. Under the 

negative metal ion irradiation, no severe electrostatic charging 

occurred on the surface of polished thin section covered by gold 

thin film. The secondary-ion emission was stable during 

analysis of over several tens hours. 

Oxygen isotopic measurements were made with a 

focused negative primary ion beam of gold. Primary beam 

currents were adjusted for each run to obtain a 16o- count rate 

of -3 x 1 o5 counts s- 1. The beam size was 10-20 Jlm in 

diameter. A mass resolution power was set to -2,000 (1% 

valley), which was sufficient to resolve all significant 

interferences. Measurements were made by cycling through the 

mass sequences 16 and 18 in a magnetic peak jumping mode. 

After the magnetic peak jumping, a precise peak centering of 

each mass was made by electrostatic peak switching scan. 

Secondary ion signals were detected with electron multiplier 

operated in a pulse counting mode. Signals measured in the 

electron multiplier were corrected for the counting system dead 

time. The isotopic composition measured by the SIMS differs 

from true isotopic composition of the sample by the matrix 

dependent instrumental mass fractionation. In order to correct 

the instrumental mass fractionation, a terrestrial single crystals 

whose composition were same to unknown samples was 

prepared as standards. Oxygen isotope-ratios relative to SMOW 

for the standards were determined by conventional massspectrometry method. 

Under this analytical conditions, o 18o of spinel, 

pyroxene, plagioclase and melilite were determined with the 

accuracy of less than 5 o/oo (cr) from the region of 10 Jlm in 

diameter. 

1. H. Yurimoto, Y. Mori and H. Yamamoto, 1993, Rev. 

Sci. lnstrum. 64, 1146-1149. 

363 

VARIATIONS ON GEOCHEMISTRY 

AND Nd-Sr ISOTOPE GEOLOGY OF 

BIMODAL VOLCANIC ROCKS FROM 

TWO TECTONIC SETTINGS IN SOUTH 

-EAST CHINA. 

Gen -di ZHANG, Zhong FANG, Ying -fei XIA, 

Xiang-cong TAO and Hui-min LI, 

Department of Earth Sciences, Nanjing University, 

Nanjing 210008, China. 

A sets of Carboniferous bimodal volcanic rocks 

bedded in ~hi !u Qroup phyllites (SLG) are in a Later 

Palaeozoic rift , that is located at Hainan Island , where 

is belonged to the extensional margin of South China 

Plate. And a series of Permian -Triassic boundary basic 

-felsic subvolcanic rocks intruded in late Permian ~ong­

!an Qroup coal-bearing formation (LTG) are in an inner fault depression of Lower Yangtze Plate, eastern 

China. 

The variations between the two types of bimodal 

volcanics are mainly from geochemistry and isotope geology of basic volcanic rocks , which are one kind of 

members in the bimodal volcanics. (A) SLG tholeiites: 

SiOz 48.82%, MgO 9.21%, (KzO+Naz0)2.88%, 

LREE 77. 67 PPm, (LLREE/LHREE) 5. 9, (Eu/Eu" ) 

1.14, ESr(t)0---+149, ENd(t)+4. 23---+7. 56. 

(B) LTG basalts: SiOz 50.74%, MgO 4.18%, (KzO 

+Na20) 4. 93%, LREE 166. 95 PPm, (LLREE/ 

LHREE) 8. 96, (Eu/Eu * ) 0. 84, E Sr ( t) + 36 ~ + 

188, ENd(t)-8. 61~+3. 57. But the geochemical 

and isotopic difference between the felsic volcanic ends 

of SLG and LTG are also obvious, such as, (A) SLG 

rhyolites: SiOz 77. 69%, (NazO + KzO) 5. 03%, 

(NazO/KzO) 0. 029, LREE 115. 8 PPm, (LLREE/ 

LHREE)5. 44, (Eu/Eu*)O. 2, ENd(t)-6. 84;(B) 

LTG rhyodacites: SiOz 65. 4%, (NazO + KzO) 7. 

04%, (NazO/KzO) 0. 931, LREE 166. 3 PPm, 

(LLREE/LHREE)19, (Eu/Eu* )0. 81, ENd(t)-

15.73. 

Conclusions: The Carboniferous SLG bimodal volcanic rocks were formed in continental rifting. The evolution of the rifting terminated at the stage of trasition 

from intra -continental to inter -continental rift. The 

Permian -Triassic boundary LTG basic-felsic volcanic 

rocks are located in inner fault depression of continental 

plate. There are some old detrital zircons found from 

LTG volcanics, the U -Pb ages of the single zircons in 

the basic member are 2281 Ma and 7 46 Ma, which are 

determined by isotopic dilution mass spectrometer 

method. This is one of some important evidence about 

contamination between mantle and crustal rocks ;Others 

are negative values of ENd ( t) , higher (La/Yb) values, etc .. The contamination is one of common features 

about inner fault depressions in contin~ntal plate. 
THREE TYPES OF OXYGEN ISOTOPE PROSPECTING 

MODELS AND APPLICATIONS 

ZHANG LIGANG, CHEN ZHENSHENG and LIU JINGXIU 

Yichang Institute of geology and Mineral 

Resources, Yichang 443003, PR China 

A systematic 2- or 3-dimentional mapping of 

oxygen isotope compositions have been carried 

out for the altered wall-rocks of 18 different ore 

deposits in China. Accroding to the _6 180 

contours obstained, three types of oxygen-isotope 

prospecting models have been established as follows: 

1. low 6 180-center system: Mineralizing centers 

have the lower 6 180 values relative to the 

surrounding rocks. The mineralizations are hosted 

either in volcanic craters on sedimental strata or 

within fracture zones. Strong water-rock 

interactions took place on or near the surface. 

Cases in point are the Eren Ag deposit and the 

Bainaimiao Au deposit in the Inner Mongolia, the 

Xijiruoshan Au prospect in Heilongjiang Province. 

The 6 180 values of the altered wall-rocks from 

the Xijiruoshan Au prospect are as low as -8~ 

on the surface and decrease further downward to the 

position of Au-bearing ore-body. In the Eren Ag 

deposit, the 6 180 values of vein-quartzes are lower 

than those of the altered wall-rocks, indicating 

that they are formed at 2 separate stages. 

2. high 6 180-center system: Mineralizing centers 

have t he h i g he r 6 180 v a I u e s r e I a t i v e t o t h e 

surrounding rocks. The mineralizations occur within 

fracture zones in subvolcanic or intrusive rocks. 

Cases in point are the Lengshuikeng and Yingshan 

Ag deposits in Jiangxi Province, the Jiaojia-type 

Au deposit in Shandong Prov1nce, and the 

Tuanjiegou Au deposit in Heilongjiang Province. 

The 6 180 values of the altered rocks from the 

Tuanjiegou Au deposit are as high as +17~ 

on the surface and gradually ~ecrease downwards. 

3. high-low-high 6 180-center system: The 

high 6 180 zones (9-10~) are of prophyry Cu (Mo) 

mineralizations, outwards occur the low 6 180 

zones (0-8%o) with Au or Pb-Zn mineralizations, 

further outwards occur the highest 6 180 zones 

(>10%o) without mineralization. Mineralizations 

are frequently related to hypabyssal rocks. Cases 

in point are the Tongshan Cu deposit, the 

Twenty-one Station Cu(Mo) deposit and Malinxi 

Cu deposit in Heilongjiang Province. 

We have performed the prospecting experiments 

of oxygen isotope geochemistry based on the oxygen 

isotope prospecting models and the nature of 

water-rock interactions. As a result, a few 

economic ore deposits have been found. 

364 

GRAIN SIZE CHARACTERISTICS OF A MIDDLE 

ORDOVICIAN ERUPTION AND ITS SOURCE AREA 

ZHANG. Y -S, and HUFF, W.D., Dept. of 

Geology, University of Cincinnati. Cincinnati, 

OH 45221, USA 

Grain size data from 30 K-bentonite samples representing 

the Middle Ordovician Millbrig K-bentonite in North 

America and its counterpart in northern Europe, the 

Kinnekulle K-bentonite, were measured to locate their source 

area. The source material of the Millbrig and Kinnekulle Kbentonites was silicic magma from a great eruptive event. 

The K-bentonite consists of clay minerals and a small portion 

of phenocrysts. The clay minerals were formed by the 

alteration of volcanic glass and no longer represent the 

original volcanic ash. The phenocrysts, however, are mostly 

·unchanged. They were used as the major grain size 

'information source of the original volcanic ash. During the 

measurement, fine particles :s; 0.074 mm were sieved out and 

only the coarse phenocrysts > 0.074 mm were used for grain 

size analysis. Log normal normalization of the phenocryst 

raw data was conducted to obtain the grain size characteristics 

of the original volcanic ash. 

Based on the raw grain size data of phenocrysts, the coarse 

phenocryst percentage, moment mean, and the log normal 

normalized median were used to locate the vent and estimate 

the magnitude. The results are internally consistent, showing 

a high percentage of coarse phenocrysts in the southeastern 

region of North America and in the west of Baltoscandia, 

with the highest (59 %) in Alabama. It indicates that the 

coarse phenocrysts accumulated in these areas and are 

considered proximal to the source. The coarse phenocrysts 

content decreases from the southeast to the nQrthwest in north· 

America, 59% to 1.35%, and from the west to the east in 

Baltoscandia, 20 % to 0.58%. Moment mean and log normal 

normalized median calculations show the same trend. In 

North America, the moment mean ranges from 0.212 mm in 

southeast to 0.120 mm in the northwest, and the median 

from 0.166 mm to 0.107 mm. In Baltoscandia, moment 

mean varies from 0.128 mm in the west to 0.099 mm in the 

east, and the median from 0.075 mm to 0.053 mm 

accordingly. Combining the distribution patterns of coarse 

phenocryst percentage, moment mean, and median both in 

North America and in Europe, the vent position of the great 

eruption which produced the source materials for the Millbrig 

and Kinnekulle K-bentonites is on the western margin of 

Iapetus near the (present day) southeastern margin of 

Laurentia. 

ThrQugh this study, it appears that the grain size analysis 

of the phenocrysts inK-bentonite may be an important 

source of information about Paleozoic volcanoes, and that the 

grain size characteristics of phenocrysts in K-bentonite altered 

form volcanic ash may serve as an approximate representative 

of the original ash. 
REFERENCE MATERIALS. FOR ISOTOPE 

GEOLOGY FROM CHINA 

ZHANG ZICHAO Yichang Institute of Geology 

and Mineral Resources, CAGS, P.O. Box 602, 

Yichang, 443003, P.R. China 

Following ISO Guides, a series of reference materials for geochronology and stable isotope analysi~ 

were produced in China in the last five years.These 

reference materials were prepared and certified by 

multi-laboratories and formally approved and issued 

by the State Bureau of Technical Supervision. The 

property values of these reference materials were 

obtained based on comparison with NBS'SRM and other 

reference materials which are widely used in the 

isotope geology community. The property values of 

certified reference materials issued are listed in 

Table 1. 

Table 1 Information of isotope geological 

reference materials issued in China 

GBW"No I materIa I I property values 

04411 potassium Rb 249.47(v.glg) Sr 168. 9 2 ( v. gl g) 

feldspar 87Sri 88Sr 0. 75999± 0. 00020 

04418 ho·· 40Ar 109.06 oo-•ccSTPig) 

K 0. 729 %, t 2060±8 Ma 

04419 rock Sm 3. 0 3 2 ( ll gIg) Nd 1 0. 1 0 ( v. gIg) 148Ndi 144Nd 0. 512725± 0. 000007 

04412 U-series u 9. 31 ( "' gIg) u•u I saaau t. 86 

dating 280Thi 884U 0. 57 t 86 ± 4 Ma 

04413 U-series u 2. 20 (.,. g/ g) auu;sasu 1. 42 

dating 280Th/ 284U 0. 69 t 118± 6 Ma 

04401 water 6 DsMow -0. 4 %o 6 180sMOW 0. 32_%o 

04402 water 6 DsMow -64. 8%o 6 180sMOW -8. 79_%o 

04403 water 6 DsMow -189. 1%o 6 180sMOW -24. 52_%o 

04404 water 6 DsMow -428. 3%0 6 180sMOW ·55. 16%o 

04405 CaCOa 6 18CPDB 0. 57%o 6 180PDB -8, 49_%o 

04406 CaCOa 6 18CPDB -10. 85_%o 6 180PDB -}2, 40%/, 

04407 cb··· 6 18CPDB -22. 4 3%o 

04408 cb··· 6 18CPDB -36. 91%/, 

04409 quartz 6 180sMOW 11. 11 _%o 

04410 quartz 6 180sMOW -1. 7 5%o 

04414 Ag2S 6 88ScDT -0. 02%o 6 84 ScDT -0. 07_%o 

04415 Ag2S 6 88ScDT 11. 36%o 6 84 8cDT 22. 15%o 

04416 CaCOa 6 18CPDB 1.61%o 6 180PDB ':' 11. 59%o 

04417 CaCOa 6 18CPDB -6. 06%o 6 180PDB -24. 12%o 

Abbreviation of Chinese charaters for 

National Standard Reference Meterial. 

•• ho=hornb lende. • •• cb=carbon black. 

365 

ISOTOPE EVIDENCE OF MUL TIGENESIS 

URANIUM DEPOSITS IN DALCI'GSHANKUNSHAN IN ANHUI PROVENCE,CHINA 

ZHENG MAOGONG,.ZHIJ JIECHEN 

Beijing Research Institute of 

Uranium Geology,Ch ina 

The uranium ore bodies occure mainly in 

Jurassic sandstone as tiny scattered and vein. The 

quartz-syenite (137Ma)intruded the sedementery rock, 

and the mineralization is controlled by both 

stratigraphy and tectonic fractures. 

Sassed on the istope dating of whole rock of 

the ores and pitchblende, the uranium metallogenetic 

process can be divided into three stages: 

The earliest stage, 174Ma, is the same as forming 

age (Rb-Sr 177Ma) of the sandstone. It means the 

minerals are simultaneous with the host rock; The 

second and the third stages are 137Ma and 112Ma 

repectively, which are coincident with two magma 

activities (Rb-Sr 130Ma,115Ma). 

The results on U-Pb system studies indicate 25-

86% of uranium had lost from sandstOne, and the 

uranium in syenite is too low to providing ·more 

materials for ore~forming. 

6 180 of the uranium mineralization ranges 9.16 

%o-14.58%o;the 6 180 of syenite is 9.52%o;the 6 180 

of sandstone is 12.18%o. It suggest that the oreforming solution is composed four mixture water 

which inclu~ metamorphic water enriched in heayy 

0 from deep and precipitation with light 0. 

Sassed on the studies above mentioned, it has 

been ascer lained that the uranium deP08its are of 

multiple stages and polygenesis, the sandstone (Jt-2) 

is an importent uranium source and uranium 

preconcentration body; the ore-forming solution is 

derived from four sources; the therma 1 events 

originated from the magmatic-stractural activities 

caused multistage uranium mobilization , 

concentration and superimposation. So Dalongshan 

uranium deP08 its belong to comp 1 ex genes i c depOSits. 
BERYLLIUM ISOTOPIC INVESTIGATION OF 

SEDIMENTARY COLUMNS OUTBOARD OF 

SUBDUCTION ZONES 

ZH:ENG S *, Morris, 1 .**, Tera, F., Department of 

Terrestrial Magnetism, Carnegie Institution of 

Washington, Washington, D.C. 20015, USA 

Klein, J., and Middleton, R., Tandem Accelerator 

Laboratory, University of Pennsylvania, Philadelp~ 

PA 19104, USA 

The presence of 1 Ose in island-arc volcanic lavas 

strongly supports the conclusion that the oceanic sediments 

on the subducted plate are subducted in some convergent 

margins. In order to evaluate the mechanism of sediment 

subduction, an important quantity to consider is the 

inventory of lOse in the sediment column just outboard of 

the trench, i.e. the total number of atoms per unit area 

obtained by integrating over the thickness of the sediment 

column. 

The lOse inventories ofDSDP/ODP cores 495, 183 and 

777B Oocated just outboard of the Central American, 

Aleutian and Marianas trenches, respectively) are 12.9, 9.3 

and 1.2 xtol2atoms/cm2, respectively. The lowest value 

is equivalent to that predicted by globally uniform 

deposition of 1 Ose; the others require preferential 

deposition of lOBe in these near-continent regions of high 

particle flux. Large volumes (lOOm, <400ka) of young, 

rapidly deposited arc turbidites are present at the top of 

DSDP 499 in the Guatemala trench and increase the lOse 

inventory of the total sediment column by 5.8xtol2 

atoms/cm2. The underlying marine section at 499 shows 

the same lithology and lOse inventory as at 495, despite 

dewatering and -50% reduction in volume. Plots of lOse 

vs depth in the three cores show scatter about the best fit 

regressions, where the regressions correspond to 1 Ose 

half-lifes of 1.6-2.1 Ma. Both features indicate nonuniform 10se deposition rates in these sites of continuous 

accumulation. 

The 1 Ose concentrations in the Central America, 

Aleutian and Mariana arcs are 2-23,2.5-15 and 0.1-1 x to6 

atoms/g. respectively, and represent -40%, 40%and 0%, 

respectively, of the lOse flux into the trench. The data 

require nearly complete subduction of marine sediments at 

central America and the Aleutians, and indicate basal 

accreation of young sediments in the Marianes. 

* Current address: Dept of Geosciences, UC Irvine, 

Irvine, CA 92717 

· ** Current address: Dept of Earth & Planetary Sciences, 

Washington University, StLouis, MO 63130 

366 

MANTLE DEGASSING AND DIAMOND FORMATION: 

CARBON AND SULFUR ISOTOPE PERSPECTIVE 

ZHENG,Y.-F., Department of Earth and Space 

Sciences, University of Science and Technology 

of China, Hefei 230026, PR China 

The reservoir effects of carbon- and sulfur-bearing 

gaseous species degassing from the mantle on the 

isotope composition of diamond carbon and melt 

sulfur are quantitatively modeled in terms of the 

principles of Rayleigh distillation. The modeling is 

based on the observations that larger carbon and 

sulfur isotope fractionations can occur in the mantle 

during diamond formation at high temperatures. 

Assuming the 613C value of -5%o (PDB) for the 

mantle, C02 degassing can result in the large negative 

613C values of diamond ( < -30%o ), whereas CH4 

degassing can drive the diamond 613C values in the 

positive direction ( > +3%o ). The present results 

differ from the previous expectations based on a 

closed-system assumption and therefore can be used 

to explain the fall range of 613C values from -34.4 

to +5%o in the natural diamonds. In this context, 

a small range of 613C values from -9 to -2%o for 

"peridotitic" diamonds can be produced by relatively 

weaker degassing, whereas a large range of 613C values 

from -3·L4 to +5%o for "ecologitic" diamonds ran 

be attributed to involvement of relatively strongN 

degassing during diamond formation. 

Assuming· the h:Hs value of O%o (CDT) for 

th<' mantiP. the residual sulfur in t.llf' mantiP can 

significantly he either depleted in 34 S ( P·•s < -~<,!co) 

due to S02 degassing or cnricl1<'d in 34 S (P·•s > +~%o) 

clue to ll·.zS degassing. Furthermore, a large variation 

in thl:' kHS values of the mantle is accompanif'CI 

diagnostically by very low sulfur contents (distinctly 

!owN than the average content); a large variation in 

the sulfur contents but narrow P 4S values around O%o 

is attributable to the degassing under conditions of 

melt sulfide mole fraction close to the threshold value 

(0.90"'0.93 for H2S degassing and 0.30"'0.41 for S02 

degassing, depending on the degassing temperature). 

According to the present theoretical models. the 

large variations in the diamond 613C values and the 

mantle-derived material 634S values do not mean 

a mantle source with the inhomogeneous isotope 

compositions of carbon and sulfur. In other \'v'Ords, 

there is no need to postulate that the mantle carbon 

and sulfur are isotopically i_nhomogeneous due to the 

recycling of crustal sediments. given that the carbon 

and sulfur in diamond and its sulfide inclusion are 

residual ones. Diamond formation is probably related 

to th<' solid -gas reactions in the mantle and thus to 

the suhsequ<'nt removal of gaseous species and the 

fractional crystallization of the melt at depth. 
KINETICS OF SULFUR ISOTOPIC EXCHANGE DURING 

THE REPLACEMENT OF SULFIDES 

Zhiyu Jiang, P. K. Seccombe, Department of Geology, 

The University of Newcastle, NSW, 2308, Australia 

Sulfide replacement is a common process in many 

important sulfide deposits. The 834S relationships among 

sulfide minerals usually do not follow the sequence 834SPY > 

834Spo or 834Ssp > 834Sc::p >o34San which is established for sulfides 

formed under conditions of chemical and isotopic equilibrium. 

Such differences between observed and expected isotopic 

fractionation are usually attributed to chemical and isotopic 

disequilibrium during ore formation. 

Sulfur isotopic exchange in replacement may be 

controlled largely by surface reactions, such as dissolutiondeposition or transformation from one sulfide to another. The 

widespread occurrence of replacement textures among sulfides 

indicates that the sulfur content in ore fluids may often be too 

small to form new sulfide directly with metal ions even though 

the latter may be in high concentration in the fluid. Dissolution 

of the early formed sulfides would develop a high sulfur 

concentration in solution near these mineral surfaces. Metal 

ions then easily combine with the dissociated sulfur to form 

new sulfides so that the replacement textures and structures 

always appear near the surfaces. For the chemical reactions 

involved in this kind of replacement, it is believed that the 

reaction rate of deposition of the new sulfides is larger than the 

reaction rate of dissolution of the replaced sulfides, such that 

the overall rate of reaction is controlled by the rate of the 

dissolution reaction. This reaction process under chemical 

disequilibrium conditions thus may largely restrict the isotopic 

fractionation between the fluid and the deposited sulfides. The 

isotopic composition of sulfur in the replacing sulfides would 

be mainly controlled by the isotopic composition of sulfur in 

the replaced sulfide, as well as the isotopic kinetics of the 

dissolution process. Since the 32S species has a lower 

dissociation energy than the 34S species, more light sulfur would 

be transfered into the solution during the initial dissolution. At 

the same time, a surface complex would develop on the 

dissolving mineral, which is likely to possess a larger 34S/32S 

ratio than the replaced sulfide due to the retention of residual 

heavy sulfur. Further, the presence of this surface complex 

would tend to block dissociation of light sulfur beneath and an 

increase in the dissociation rate of the residual heavy sulfur 

from the complex can be expected, due to its relatively high 

concentration in the complex. Ultimately, a mass balance 

would reached, whereby the o34S value of sulfur in the solution 

would be kept similar to, or slightly larger than the isotopic 

composition of the sulfur in the replaced sulfide depending on 

the geometric shape of the dissolving mineral. 

The replacement textures and structures would tend to 

disappear and isotopic equilibrium would be approached due to 

a relatively large reaction rate of for sulfide dissolution as the 

reaction rate of new sulfide precipitation slowed. The presence 

of later sulfide overgrowths on the same sulfide phase which 

has undergone earlier replacement suggests that the precipitation 

of the new sulfide is mainly controlled by temperature. Under 

these conditions, isotopic equilibrium may attain among the 

newly formed sulfide minerals, but the isotopic compositions 

for the overgrowth and replaced sulfides may be different. 

367 

POLYCYCLIC GRANULilE FACIES METAMORPHISM 

DE1EC1ED BY SM-ND DATING OF GARNET: 

IMPLICATIONS FOR SM-ND CLOSURE 

lEMPERATURE 

ZHOU, Band HENSEN, B. J. Department of Applied 

Geology, University ofNSW, Kensington, 2033, Australia 

The granulite facies gneisses along the Prydz Bay coastline in 

eastern Antarctica, have long been regarded as part of an 

extensive Proterozoic (c. 1000 Ma) terrane; recent work, 

however, has revealed that parts of the region have experienced 

polycyclic granulite metamorphism. On S9)strene Island in the 

western part of Prydz Bay, garnet grains in the mafic granulite 

show two stage breakdown to a coarse-grained outer (early) 

Opx-Plg±Hb symplectite and a fine-grained inner (late) OpxPlg-Spl symplectite. Conditions of the early, garnet forming, 

metamorphic event are 9 ± 1 kbar and 850 ± 50 oc, and the 

late, isochemical, breakdown of the garnet occurred at 6 ± 1 

kbar and 700 ± 50 oc (Thost et al1991). Sm-Nd garnet-whole 

rock isotope analyses on a garnet core indicates an age of 988 

± 12 Ma for the fll'St granulite metamorphism; and the second 

thermal event is dated at 498 ± 12 Ma from a nearby pelitic 

sample. This later granulite facies event is also found in other 

parts of Prydz Bay with ages ranging from 488 to 517 Ma. In 

view of these results, it is believed that the garnet cores record 

the earlier high P-T event and retain a memory of the timing 

of this event. The subsequent reheating to c. 700 -750 oc at c. 

500 Ma did not reset the Sm-Nd isotope system within garnet 

cores. Therefore it is concluded that the closure temperature of 

Sm~Nd system for garnet in these rocks is in excess of 700 °C. 

Our investigation also demonstrates that the presence of 

microscopic to submicroscopic inclusions (in particular, zircon 

and monazite) in garnets may have significant implication in 

closure temperature studies of the Sm-Nd system of garnet. 

Most of the monazite inclusions were completely 

reequilibrated during garnet growth, thus the monazitecontaminated garnet analyses fall on the isochrons defmed by 

clean garnet-whole rock/matrix. However, inherited 

compositions were recorded in one case where the monazitecontaminated analyses defme various mixing lines with 

apparent older ages. Model calculations show that as little as 

0.001 wt% monazite contamination would result in Sm-Nd 

contents of the garnet concenuates dominated by the monazite. 

A similar amount of zircon contamination would have little 

influence on the Sm-Nd system. However, it would have very 

significant influence on the U-Pb garnet ages. 

In view of the high closure temperature at > 700 oc rather 

than 600 ·c for Sm-Nd diffusion in garnet, it is worth 

considering that small degree of contamination of garnet with 

inherited zircons could be responsible for the apparent age 

differences, and deduced differences in diffusivity between the 

Sm-Nd and U-Pb systems as reported recently. 

Thost, D. E., Motoyoshi, Y., & Hensen, B. J. 1991. Twostage decompression in garnet-bearing mafic granulites from 

S9)Strene Island, Prydz Bay, East Antarctica. Jomnal of 

Metamorphic Geology, v. 9, p. 245-256. 
LOMU-PM CONNECTION: A CONTINENTAL 

EMI DEBATE 

ZHOU. Xin-Hua. and ZHANG, Jian-Bo, Both at 

Institute of Geology, Academia Sinica, Beijing, 

100029, P.R. CHINA 

Based on Sr-Nd-Pb-0 isotope systematics of highpotassic volcanic rocks (dominated by leucitite and 

basanite) erupted in northeastern China, combined with an 

overview of globaJ data with typical EMI signature, such 

as WaJvis, Leucite Hill (Wyoming) and Smoky Butte 

(Montana), the LOMU nature of EMI component has 

been emphasized. However, it was suggested naming 

this end-member as LOMU to bypass ambiguity( Zhou 

and Zhu, 1989) and has been accepted by intemationaJ 

geochemical community(Zindler, 1993). The quantitative 

approaches, including major and trace elements modeling, 

multiple space (five to ten isotopes and trace element 

ratios) topographic anaJysis, and mixing caJculations, 

show that the observed geochemical data can be well 

interpreted by an old primitive mantle component 

modified by multiple metasomatic episodes at different 

times. Furthermore, the study reveaJs a close connection 

between LOMU and PM, as wen as the linear array 

formed by those data with a typical EMI signatures on 

Sr-Nd plot, as suggested here, the LOMU array'. This 

LOMU array has distinct evolution path with respect to 

the normaJ mantle array(i.e. NormaJ J.1 array, or called as 

NOMU anay) and LoNd array. The LoNd array, proposed 

by Hart(I986), reflects the connection between EMI and 

HIMU, and fits most oceanic EMI data, whereas the 

LOMU array is the locus of connection between EMI and 

PM, which indicates the nature of EMI component, and 

fits most continentaJ EMI data. The genesis of most EMI 

type rocks could be geochemically interpreted by this 

connection. In addition, affinity to old continental 

lithosphere of LOMU component is also stressed and 

tectonic implications are discussed. 

Zhou, X.-H. and Zhu, B.-Q., 1989, Geochemical mapping 

of subcontinentaJ mantle beneath eastern China, 28th 

International Geological Conference, Washington 

D.C., Abstr. Vo1.3, p.III-414. 

Zindler, A .• 1993, V .M. Goldschmidt medal and Lester 

W. Strock and family honorarium, Geochim. 

Coscbim. Acta. Vol. 57, p.1891-1892. 

368 

THE STALAGMITES RECORDS OF 

ISOTOPIC PALEOTEMPERATURE 

IN BEIJING REGION 

Z hu H ongshan,Dept. of Earth Sciences, 

Graduate School, Academia Sinica, 

Beijing 100039, P.R.China 

This paper deals with the isotopic paleotemperature and geochronology of the stalagmites 

of Quaternary limestone caves in Beijing region. 

The geochronology of spelean stalagmites has 

been studied by the U-sereis lisequilibrium 

method of dating. The results show that the 

stalagmites were formed during the time from 

440 to 78ka B.P .. 

The results of the oxygen isotope measureInents of the stalagmites indicate that the stalagmites were formed under isotopic equili briunl condition. Isotopic paleotemperature 

records of the stalagmites indicate that there 

are three apparent periodcal changes during 

the time 

from 440 to 78 ka B .P. in Beijing region. The 

period of reoccurance is about 105 years. The 

whole period can be divided into three spans 

rangion from 480 to 304 ka, 304 to 226 ka and 

220 to 115 ka B.P. respectively. Their corresponding change of paleotemperature i~ about 

10°C 3°C-12°C, 12°C-5°C-16°C and 16°C-5°C 

-12°C respectively. The highest mean annual 

temperature is 12°C-16°C and the lowest is 

3°C-5°C. They may represent the temperature 

of interglacial and glacial respectively. The difference of temperature between glacial and interglacial is about 7° C-9° C in Beinjing region. 

The moden mean annual temperature(12° C) 

in Beijing region is close to the highest temperature during the period from 440 to 78 ka 

B.P .. 

Compared with the paleotemperature curve 

and time scales of Emiliani and Shackleton 

(1974)*, the curve of isotope paleotemperature 

in Beijing region shows a 

sitnilar tendency of change. 

* Etniliani, C., and N.J. Shackleton(1974) The 

Brunhes epoch: Isotopic paleotemperatures 

and geochronology. Science, 1~3, 511-514. 
ION PROBE DATING OF MONAZITE: EXAMPLES FROM 

THE ISUA AND THE LEWISIAN TERRANES 

ZHU. Xiangkun, o•NIONS, R.K., BELSHAW, N.S., 

REED, S.J.B., Dept. of Earth Sci., Univ. of Cambridge, 

Cambridge, CB2 3EQ, UK. and. MOORBATH, S., Dept. 

of Earth Sci., Univ. of Oxford, Oxford, OX1 3PR, UK 

U-Pb geochronology of monazite is an established but 

under-utilized technique. Some studies have shown that 

monazite is rarely inherited in intermediate- to high-grade 

metamorphic rocks and usually yields concordant or near 

concordant age with closure temperature ~700°C for the U-Pb 

system. Here we present the results of micron-scale 

chronology obtained by secondary ion mass spectrometry 

(SIMS) analysis and related EMP A studies of monazites from 

gneisses of the classic Archean areas: Isua, Western 

Greenland and the Lewisian, Northwest Scotland. Individual 

monazite grains were located in polished thin sections by 

optical microscopy and SEM study and their Pb isotope 

composition analysed in situ using the ISOLAB-120 operated 

at -7000 resolving power. 

The sample studied from the Isua is a tourmaline gneiss. 

Three monazite grains were selected for SIMS analysis. All 

these grains are unzoned and have a similar chemical 

composition. Two larger (-100 J.lm) grains coexisting with 

allanite and apatite give 201J>bf206Pb ages of 3568 ± 19 Ma and 

3580 ± 20 Ma. The small (-10 J.lm) grain included in 

tourmaline yields a 201J>bf206Pb age of 3578 ± 13 Ma, 

indistinguishable from the ages obtained from two other 

grains in the same thin section. The consistency of the ages 

obtained from the different size grains of monazite suggests 

that these ages are concordant. 

Samples studied from the Lewisian are garnetiferous felsic 

gneisses. Monazite occurs as discrete grains and inclusions in 

garnet, biotite and feldspar, and usually shows zoning with 

higher Th abundance at the rims, but with REE distribution 

patterns that are similar between both the rims and cores. 201J>bf206Pb ages of 2715 ± 23 Ma and 2530 ± 8 Ma are 

obtained from the core and rim, respectively, of a large (-400 

J.lffi) complexly zoned discrete monazite grain. Thus, this 

single crystal preserves a high-grade metamorphic history 

extending over some 200 Ma. The central regions of two 

smaller (-200 J.lm) discrete monazite grains yield 201J>bf206Pb 

ages of 2745 ± 15 Ma and 2741 ± 12 Ma, respectively. A 

small (-20 J.lm) grain of monazite fully armoured by garnet 

gives a 2

01J>bf206Pb age of2761 ± 12 Ma. This is the oldest age 

reported for the Lewisian high-grade metamorphism so far. 

This study shows that monazite is a valuable phase for in 

situ SIMS chronology on intermediate- to high-grade 

metamorphic rocks. It illustrates that the host minerals, 

especially garnet, may play an important role in the behaviour 

of Pb in monazite during metamorphism. The combination of 

detailed petrographic study and spatially resolved chemical 

analysis with SIMS chronology offers ,a significant advance in 

the study of these materials. 

369 

Pb-Sr-Nd ISOTOPIC AND GEOCHEMICAL 

PROFILES OF THE CRUST AND MANTLE 

IN EASTERN JOINT PART BETWEEN 

YANGTZE AND SOUTH CHlNA BlOCKS 

Zhu B.-Q., Wang Y.-X. and Wang H.-F. 

Institute of Geochemistry, Chinese 

Academy of Sciences, Wushan, Guang 

Zhou, 510640, P.R. China 

Systematic studies on Pb-Sr-Nd isotopic and chemical compositions ,within 

crust and mantle along the geophysical 

sections through Yangtze(YB) and south 

China blocks (SCB) in Anhui and Zhejiang area, eatern China, have been 

done. Pb isotopic results evidently show 

sympathelical variation within the crust 

(feldspars in Mesozoic granites) and 

mantle (Cenozoic basalts), which the 

206pbj204pb and 208pbj204pb in SCB are 

higher than 18.40 and 38.40, and those 

in the YB are close to 18.10 and 38.20 

respectively. The boundary line between 

the YB and the SCB has been well established at 29.1-29.2"N through the detail 

Ph isotopic mapping, which is consistent 

with the evidence of the geophysical 

section. It's indicated from the Nd isotopic evidence of alkaline basalts and 

granitoids that the mantle beneath the 

boundary area was more depleted (e~d= 

+6.5-+8.5) and its crust possesses higher 

Nd model ages(ToM=1.8-2.5 Ga), w~ich is 

consistent with zircon U-Pb age(1.8 Ga), 

than those in the interior of the two 

blocks( mantle eNd< +6.5, crust TnM<l. 7 

Ga}. The Sr isotopic ratios indicated that 

the YB mantle preserved primitive feature from Proterozoic to present(. 7042-

.7045), whereas the SCB mantle was depleted at present(. 7035) and enriched 

during Mesozoic(. 7088). The trace element 

data show that the crust in SCB is ,evidently enriched in U, Th, Rb, Sn, W and 

etc. and depleted in Zr, Sr, Ba and etc. 

It's indicated that Pb and Sr isotopic 

systematics possess strong terrane features in YB and SCB, but ·sm-Nd systematic is mainly show vertical fractionation between crust and mantle, YB and 

SCB have . entirely different evolution 

histories, and their crust and mantle 

have been welded since Proterzoic. 
TEMPORAL ASSOCIATION OF TYRNYAUZ W-MO 

DEPOSIT WITH ELDJURTA GRANITE: EVIDENCE 

FROM RB-SR DATING 

ZHURAYLEV Andrew Z., Institute of mineralogy, 

geochemistry, and crystallic chemistry of rare elements (IMGRE), Moscow 121357 Russia, and 

NEGREY E.V., IGEM RAS, Staromonetny 35, 

Moscow. 109017, Russia 

Eldjurta granitic pluton is situated within Peredovoy 

range of North Caucasus. Tyrnyauz W -Mo deposit is spatially related· to this pluton and occupies its supra-intrusive zone. On the basis of geological background and K-Ar 

ages of igneous rocks, it was suggest that W-Mo ores of 

Tyrnyauz deposit are older than granites. However, there 

was no any isotope data on the age of ore formation. 

We determined ages of Eldjurta granite and ore-bearing metasomatite from Tyrnyauz deposit by Rb-Sr method. 

It is widely accepted that precision of isochrone dating 

depends heavily on an amount of sample points. The granite sample contains only 3 minerals (biotite, K-feldspar 

and plagioclase), datable by Rb-Sr method, and the metasomatite ·· only 2 (amphibole and biotite). In order to 

increase amount of isochrone points we separated dense 

fractions of the minerals using heavy liquids and hand 

picking under microscope. 

Age of Eldjurta granite was determined by 13-point 

isochrone and measures 1982±8 Ty, (

87Sr / 86Sr)0= 

0.70685±3. Age of ore-bearing metasomatite was determined by 7-pont isochrone and measures 1963±15 Ty, 

(

87Sr / 86Sr)0=0.70810± 15. 

The same age of granite and metasomatite, coupled 

with their spatial association, suggests a genetic relation 

between Eldjurta pluton and Tyrnyauz deposit. A higher 

(

87Sr / 86Sr)0 in metasomatite may be interpreted as a 

result of contamination by host hornfels. 

370 

SULFUR ISOTOPIC COMPOSITION OF MASSIVE 

SULFIDE FROM THE MIDDLE VALLEY SEDIMENTCOVERED SEAFLOOR SPREADING CENTER 

ZIERENBERG. ROBERT A., US Geological 

Survey, Menlo Park, CA 94025, USA; 

GOODFELLOW, Wayne D. and FRANKLIN, 

James M., Canadian Geological Survey, Ottawa, 

Ontario K1A OE8. 

Middle Valley is a sediment-buried part of the 

Juan de Fuca Ridge spreading center. Turbiditic 

sediment deposited during the Pleistocene fills the 

axial valley to depths ranging from 300 m to > 

1,200 m. An active hydrothermal vent field with 

temperatures as high as 27 4 ° C and a fossil 

geothermal system that deposited a large massive 

sulfide deposit have been sampled by the ALVIN 

submersible and by drilling during Leg 139 of the 

Ocean Drilling Program. Turbiditic sediment away 

from the hydrothermal systems contains 0. 1 to 0.3 

wt. percent sulfur in pyrite with o 34S values of 

-45%o to -20%o, typical for sulfide formed by 

biogenically mediated reduction of seawater sulfate. 

This early diagenetic pyrite is overprinted by 

hydrothermally-derived pyrite with o 34S values of 

6o/oo to 12%o. The extent of overprinting increases 

both with increasing depth in the sediment and with 

proximity to the hydrothermal upflow zones. 

Sediment cored below the active vent field contains 

disseminated anhydrite with o 34S ranging from 

21%o to 28%o, indicating partial reduction of 

seawater sulfate. 

Massive sulfide drilled at an extinct 

hydrothermal center 3 km SW of the active vent 

field is at least 95 m thick. A primary hightemperature mineral assemblage of pyrrhotite + 

isocubanite + chalcopyrite + sphalerite has been 

partially replaced during lower temperature 

hydrothermal alteration by pyrite ± magnetite ± 

chalcopyrite ± sphalerite. Massive sulfide from 

both the active and fossil hydrothermal systems has 

a narrow range of o 34s values mostly between 5%o 

to 9o/oo. Pyrite, pyrrhotite, sphalerite, and 

chalcopyrite are not in isotopic equilibrium and 

generally have similar values in adjacent 

subsamples. The isotopic data are consistent with 

petrographic evidence for extensive 

recrystallization of sulfide minerals and 

replacement of pyrrhotite by pyrite via sulfurconserving reactions. 

The deepest drill holes (936 m) in Middle Valley 

penetrated an interlayered sequence of basaltic sills 

and sediment altered to subgreenschist to lower 

greenschist assemblages. Disseminated pyrite and 

pyrite, pyrrhotite, sphalerite, and chalcopyrite in 

veins cutting the basalts have a narrow range of 

o 34S values generally between 7%o to 12%o. 

Primary i~neous sulfide (monosulfide solid 

solution, 0 4S approx. O%o) was leached from the 

basalt during hydrothermal alteration. 
DEBUNKING THE K CONUNDRUM 

ZINDLER. A., BOURDON, B., and ELLIOTI. 

T, Lamont-Doherty Earth Observatory of 

Columbia Univ., Palisades, NY 10964, USA. 

Galer and O'Nions (Nature 316,778-782, 1985) 

noted that K values measured in MORBs, or inferred 

from MORB U-Th systematics (KTb), are substantially lower than KPb values inferred from MORB 

Pb isotope systematics. They proposed that the 

solution to this "K conundrum" was a "steady-state" 

upper mantle with a K of 2.5, low U, Th and Pb 

concentrations, and short but different residence 

times for these elements. In this context, MORB Pb 

isotope ratios are inherited from a high-K (-4.0) 

lower mantle and are not perturbed by a brief period 

of residence in the upper mantle. Although appealing in some respects, this model suffers from 

being an ad hoc mathematical construct that is 

difficult to support by direct observation. 

A geologically more sound alternative solution to 

the K conundrum lies in a reinterpretation of MORB 

Pb isoto~ systematics. Because the slope of the 

MORB 06pbJ204Pb vs. 208pbJ204Pb array is similar 

to that predicted for evolution over 4.5 Ga in a 

constant K reservoir, KPb values, as they are used to 

define the conundrum, are calculated by assuming a 

single-stage evolution for K, in contrast to the multistage scenarios known to best describe Rb-Sr, SmNd and U-Pb evolution in the earth. We have found, 

however, that the growth of Pb isotope ratios in the 

MORB mantle (DMM) is consistent with a marked 

change in K evolution during the middle period of 

Earth history. If KoMM starts at -4.0, and remains 

constant until-2 to 3 Ga b.p., and then continuously 

declines to a present-day value of -2.5, observed 

MORB Pb isotopic systematics are reproduced in 

the context of a low-K, present-day DMM. This 

change in the evolution of K must occur in response 

to a substantial increase in the amount of U recycled 

from the continental crust to DMM via altered 

oceanic crust, and could herald the advent of oxidizing conditions at the surface and/or a change in the 

nature of subduction due to thermal evolution of the 

mantle (McCulloch, EPSL 115, 89-100, 1993). 

In contrast to the model of Galer and O'Nions, 

our Post-Archean U Recycling (or PURE) model is 

conceptually consistent with the quasi-continuous 

depletion of the MORB mantle in response to the 

growth of the continental crust. Artificially short 

and different residence times for U, Th and Pb in the 

upper mantle are not required. A Monte Carlo 

exploration of the quantitative predictions and 

consequences of the PURE model for the mantle and 

crust will be presented and discussed at the meeting. 

371 

CASPIAN SEA AND GULF OF MEXICO AS 

CLIMATE REGISTORS FOR CONTINENTS OF 

THE NORTH-ATLANTIC SECTOR 

ZUBAKOV V.A., Hydrological Inst., 

St .Petersburg, .. 199053, Russia. 

The Caspian hydrologic-chronomet- ric series of 48 cycles covers 7 Ma 

and includes 35 paleomagnetic bench 

marks, tens of datings by Th/U, TL, 

F.t. KAr methods and many hundreds 

of 14c. It is separated into 4 independent scales compatible with the 

series for Russian and Siberian 

plains. Many boundarieQ of the Caspian events are properly correlated 

with the peaks of isotope-oxygen 

curves according to foraminifers 

from ODP 625B section in the Gulf of 

Mexico (Joyce,Tjalsma,Prutzman,1990, 

1993). The boundaries of 5.1, 4.6, 3.6and 

3.3Ma fix supercryothems 24, 22, 18 

and 16 in the Ponte-Caspian which 

divide the Zankli/,an transgression into 4 stages. At ).OMa the uplift and the first glaciation of the 

Great Caucasus as well as the Caspian isolation.occurred. At 2.5-2.4 Ma 

(SKT 12) the Akchgylian transgression reached its maximum (+100m), and 

its water made the wey through a new 

Manych strait. At 1.9Ma (Termo-SKT9) 

Dreissena and ancestry of the Apsheron-:rauna penetrated into the Caspian, and at 0.9-0.8Ma - the ancestry of the Baku fauna did. 

At the Brunhes epoch the Caspian 

was an isolated basin but at glacial phases (Orto-KT 18,16,14,12,6; 5d, 

5b)some water from the Caspian discharged to Azov-Black Sea basin together with the predominant Didacna 

species. The Khvalynian hist~ 

the Caspian is reconstructed in details, compatible with the boundaries of GRIP and Holocene ( 15cycles). Thus, the Caspian is the global recorder of Pliocene glaciation of 

the Antarctica and of the moistening trend of continents of the Nortlr 

Atlantic sector during Pleistocene. 

Since continental climate-chronologie scale of Plio-Pleistocene is important for a development of climate 

theory and predicting future climate, 

the US scientists are invited to 

join their efforts for cooperation. 
Isotoping dating (Ar-Sr-Nd) and tracing (REE) 

of diagenetic and hydrothermal activities in 

Rotliegendes Sandstones (Central Europe) 

Zwingmann, H., Clauer, N., Stille, P. and Samuel, J. (CNRS) 

Centre de Geochimie de Ia Surface, 67084 Strasbourg, France 

An isotopic, geochemical and petrographic investigation on 

Late Permian (Rotliegendes) and Late Carboniferous rocks 

from gas wells permits the influence of burial and hydrothermal events on the formation of authigenic day mineral phases 

to be determined. The sequences occur at a 4.5-to-5.5 km 

burial depth in a series of NW -SE trending graben and horst 

structures, where Rotliegendes and Carboniferous units are 

juxtaposed along distinct faults. Acidic fluids from the Late 

Carboniferous units which where expulsed through these fault 

systems into the Rotliegendes units, cause feldspar dissolution 

and authigenic clay-mineral precipitation. The clay minerals 

can be quantified by REE-chemistry, chemical and petrophysical investigations. Petrographic and XRD investigations have 

identified several morphological types of authigenic clay minerals {illite, kaolinite, dickite, chlorite). Their REE patterns 

and Li concentrations are dependent on the distance from major fault systems: authigenic illites next to these faults yield 

REE patterns highly enriched in LREE and in Eu, as well as 

in Li. The decreasing positive Eu-anomaly and the Li content 

in fine fractions (-< l~m) can be used as a geochemical tracer 

for hydrothermal fluid pathways.Some illites show extremely 

high 147Smjl44Nd ratios up to 0.48, which are related to close 

proximity to coal bearing Late Carboniferous units. 

K-Ar dating of the authigenic illite phase from 40 samples (fractions from -<0.2 to>-63~m) yield ages between 220 

and 160 Ma. K-Ar ages of 220-200 Ma predominate in the 

horst areas and decrease to 180.160 Ma in the graben area. 

With increasing distance to the major faults system the K-Ar 

ages become progressively younger. The dependance of age 

on distance from the Carboniferous units reflects the decreasing inftuence of hydrothermal fluid infiltration from the faults 

into the graben sequences. The geochemical and K-Ar data 

allow to reconstruct the timing and the migration-pathways of 

illite-forming hydrothermal ftuids. The extent of hydrothermal 

alteration is therefore mainly controlled by tectonic position in 

the system. 

Rb-Sr data of authigenic illite and chlorite phases support 

these results. The Rb-Sr isochrons defined by leachates of clay 

fractions (L), residues from leaching (R) and untreated aliquot 

(U) yield ages simular to the K-Ar ones. Sr-isotope data point 

to an increasing water-rock interaction from the horst area, 

where the hydrothermal fluids originated, into the graben area. 

Sm-Nd isochrons (L-R-U) yield ages of350 to 260 Ma which 

correspond approximately to the sedimentation ages of the 

Carboniferous and Permian units, respectively. The Sm-Nd 

ages seem to reflect early diagenetic processes in these stratigraphic units in contrast to the K-Ar and Rb-Sr results. The 

formation of the hydrothermal illite can be dated by the K-Ar 

and Rb-Sr systems and is correlated to a period of tectonic 

activity which is the opening of the central Atlantic Ocean 

during an early-to-mid Kimmerian event. 

372 

()C AND ()0 SIGNALS Of= PEDOGENIC CARBONATES 

FROM PLEISTOCENE CONTI~'ENT AL DEPOSITS, AS 

PALAEO-ENVIRONMENTAL INDICATORS. 

KEPPENS E., ALAM S., HAN J., NIELSEN P., VAN 

RIET A. and PAEPE R.: Vrije Universiteit Brussel, 

pleinlaan 2, 1050 Brussels, Belgium. 

bC and ()0 values of pedogenic carbonate concretions 

from two Pleistocene continental sequences (one in NW 

Bangladesh with 11 palaeosols and one in Central China 

with 18 palaeosols) are compared and discussed. The major 

objectives of this study were to test: (1) the reproducibility 

of the bC and bO signals within lithostratigraphic or 

petrological units, and (::!) their variation from one unit to 

the other. All together more than 2000 bC and 60 analyses 

were carried out one more than 1000 samples, leading to the 

following observations. 

(1) Lithostratigraphic units that are expected to have 

formed under different palaeo-environmental or 

palae()Climatic conditions, provide isotopic signatures (()C 

and 00) that can be cleariy distinguished. In other words the 

differences between analytical results from a same 

lithostratigraphic unit are much smaller than between 

samples from different units. This would mean that climatic 

or environmental changes are recorded as significant 

differences in isotopic signature. 

(2) Carbon isotopic compositions vary between -8%o and 

-2.5o/oo in China, and between -11.5o/oo and + 1.5o/oo in 

Bangladesh. The relatively large 6C variations may be 

explained by climatically induced changes in floral 

compositions in terms of C3-/C4-plants. Changes in 

humidity are translated into variations of C3- and C4- plant 

proportions, resulting in changes in soil C<J:! 6C values. 

The extreme 6C values found in the sequence in Bangladesh 

\vould correspond to nearly single "phase" C3 (6C: -11.5%o) 

or C4 (6C: -11.5o/oo) florae. 

(3) Variations in the oxygen isotopic composition are 

much smaller: between -10%o and -8%o in China and 

between -6o/oo and -3.5o/oo in Bangladesh. These smaller 

differences can be induced by temperature changes. Contrary 

to the mechanism in the marine environment, here the 

"direct" temperature-effect, causing the change of the 

fractionation factor between (soil-)water and (soil-) 

carbonate, is opposite to the "indirect" temperature-effect, 

controlling the isotopic composition of the (meteoric and 

thus soil-) water. A temperature rise of e.g. + l0°C causes 

shifts in the 60 of pedogenic carbonates between Oo/oo (in the 

wanner climates) and + 7.5%c (in the colder climates). 
40Arf39Ar GEOCHRONOLOGY OF AIR-FALL ASHES 

FROM THE MIOCENE PELAGIC SEQUENCE OF THE 

UMBRIA-MARCHE APENNINES (ITALY) 

DEINO, A. (Geochronology Center, Institute of Human 

Origins, 2453 Ridge Road, Berkeley, CA 94709), and 

MONTANARI. A. (Osservatorio Geologico di Coldigioco, 

62020 Frontale di Apiro -MC-); COCCIONI, R. (Ist. di 

Geologia, 61029 Urbino), and ODIN, G.S. (Dep. Geol. 

Sedimentaire, Univ. P. et M. Curie, 75252 Paris, France) 

We have dated with 40Arf39Ar laser total-fusion and 

incremental heating techniques several crystal separates of 

biotite, sanidine, and plagioclase from distal air-fall ash layers 

from the Miocene pelagic sequence of the northeastern 

Apennines (Umbria and Marche regions of Italy). The results 

of this work pennit a detailed geochronologic calibration of 

biostratigraphic boundaries, magnetic polarity reversals, and 

chemo-stratigraphic characteristics of this pelagic sequence from 

the mid-upper Aquitanian to the uppennost Messinian. We 

have obtained the following boundary age estimates: 

Burdigalian/Aquitanian, 20.1 Ma; Burdigalian/Langhian, 15.8 

Ma; Serravallian{fortonian, 10.9 Ma; Tortonian/Messinian 

(7 .2-6.8 Ma); Messinian/Zanclean (5.3 Ma). 

U·M STRATIGRAPHIC SECTION "' ~ 

• 

• 

• 

• 

• 

373 

"EXCESS" ARGON AND HALOGENS IN GRANITEDERIVED FLUIDS 

McCONVILLE Paul 1, STIJART, f.1 and TIJRNER, G.2 1 SURRC, East Kilbride, Glasgow, G75 OQU, UK 

2 Dept. of Geology, Manchester University, M13 9PL, UK 

The 40 Ar-39 Ar dating scheme is complicated by the release of 

'excess' argon (40 ArE) along with 40 Ara from in-situ decay of 4°K (i.e. radiogenic 40Ar* is mixture of 40Ara and 40ArE). 

Recent experiments have found that 40 ArE correlates with Clderived 38Ar [1] raising the possibility of a Cl-based 40ArE 

correction [2]. Noble gas and chemical (K, Cl, Br, I) 

information obtained by 40 Ar-39 Ar studies show that 40 ArE in 

granitic feldspars is trapped in fluid inclusions and that it is 

possible to determine the origin of these fluids [3,4]. 

To test this suggestion, we firstly re-examined 10 year-old 

unpublished 40Ar-39Ar data on the Hamlet Bjerg granite from 

the East Greenland Caledonides. We expected consistent Rb-Sr 

and K-Ar cooling histories because Rb-Sr systematics and 

petrographic evidence suggest that the granite cooled slowly 

(4-ll°C/Ma) and bas not been affected by later hydrothermal 

events. 

Conventional K-Ar, 40Ar-39Ar stepped heating and laser 

ages of biotite grains agree well with the Rb-Sr mineral-WR 

isochron of 415 ± 23 Ma [5]. However laser 40Ar_39Ar dates 

of adjacent feldspar grains lie on an isocbron of 497 ± 7 Ma (n 

= 9, MSWD = 0.9). 40Ar*/K correlates with CVK in the 

feldspar. The age determined from the 40Ar*/K intercept (Cl = 

0) agrees with the Rb-Sr isochron age. The 40 Ar*/Cl ratio -4 

x 1Qr4 given by the slope is somewhat higher than found in 

nomlal crustal rocks (lQ-6 to 10"5). The high initial 87Srf86Sr 

ratio (0.7218 ± 14) is consistent with a mantle source 

contminated by Late Proterozoic country rocks. About -10"10 

mol g·1 40ArE would have to be incorporated into the feldspar 

(oligoclase) to elevate its age by 100 Ma. This is less than 

observed in other feldspars [3,4] and yet is clearly discernible 

in the isotope systematics . 

Assuming 40 ArE and halogens dissolved in circulating 

hydrothermal fluids are trapped in fluid inclusions, what do 

their concentrations and composition tell us about granite 

fonn.ation? Are the fluids magmatic or meteoric in origin? Is 

40 ArE locally derived? What is the significance of the 40 Ar*/Cl 

ratio? How reliable are corrections to K-Ar ages based on 40Ar"'/Cl? 

To answer these questions, we have analysed new samples 

from Hamlet Bjerg and the Strontian granite (Scottish 

Caledonides) using the 40 Ar-39 Ar method. We shall present 

results from in vacuo crushing and laser stepped heating of 

feldspar, quartz and micas and discuss the sources of fluids in 

the c;ontext of magma sources and fluid-rock interactions 

during cooling. 

[1] Kelley, S. P., et al., Earth Planet. Sci. Lett. 79, 303-318, 

1986. [2] Turner, G. and Bannon, M. P., Geochim. 

Cosmochim. Acta 56, 227-243, 1992. [3] Burgess, R., et 

al., Earth Planet. Sci. Lett. 109, 127-167, 1992. [4] 

Burgess, R. and Parsons, 1., Contrib. Mineral. Petrol. 115, 

345-355, 1994. [5] Rex, D. C. and Gledhill, A. R., Rapp. 

Gr0nlands geol. Unders. 104,47-72, 1981. 
Abraham, J.D., 49 

Achmedov, A.M., 257 

Ackert, R., 184 

Adachi, M., 290, 311 

Adams, C., 151 

Aeschbach-Hertig, W ., 172 

Aggarwal, J.K., 1 

Agrinier, P., 2 

Ahrendt, H., 320 

Aizenshtat, Z., 301 

Akers, W., 128 

Alam, S., 372 

Al-Kadasi, M., 216 

Aleinikoff, J.N., 2 

Alexander, C.M. O'D., 3 

Allan, G.L., 58, 169, 306 

Allan, J.F., 3, 69 

Allan, T.L., 352 

Allegre, C.J ., 280 

Alpers, C.N., 4 

Amari, S., 4, 28 

Amato, J.M., 5 

Amelin, Y., 5, 68 

Ames, L., 6 

Amouric, M., 97, 273 

Amundson, R.G., 6, 157 

Anderson, R.S., 7, 265 

Anderson, S.P., 7 

Anderson, T.B., 44 

Andersson, U.B., 8 

Andre, L., 8 

Andrew, A.S., 352 

Andriessen, P.A.M., 9, 25, 270, 

277 

Anno, K., 308 

Anthes, G., 9 

Anthony, E.Y., 10, 355 

Aoki,M., 90 

Aquilina, L., 82 

Arakawa, Y., 10 

Archibald, D.A., 18 

Arculus, R.J., 168 

Arehart, G.B., 90 

Armienti, P., 325 

Armstrong, R.A., 11, 186 

Armstrong, R.L., 217 

Arnaud, N.O., 11, 166 

Arne, D.C., 12 

Arnold, J.R., 234, 262 

Arnold, M., 273 

Arnould, M., 12 

AUTHOR INDEX 

Aronson, J.L., 13, 57, 346 

Arth, J.G., 13 

Asahara, Y., 14,315 

Ash, RD., 14 

Aspden, J., 234 

Austin, P., 168 

Awwiller, D.N., 15 

Ayalon, A., 15 

Ayers, M., 336 

Ayliffe, L.K., 15, 16 

Ayuso, R.A., 13 

Ba-B'ttat, M.A., 352 

Babinski, M., 16, 335 

Bachelery, P., 292 

Bacon, C.R., 17 

Bacon, P.M., 361 

Baker, J., 17 

Baker, J.A., 18, 216 

Bakke, A., 210 

Baksi, A.K., 18 

Baldridge, W.S., 293 

Baldwin, S.L., 19, 34 

Ball, J.D., 19 

Ballevre, M., 97, 273 

Banks, D.A., 218 

Barczus, H.G., 341 

Barker, A.P., 20 

Barling, J., 20, 137 

Barovicb, K.M., 219 

Barreiro, B.A., 21, 160, 248, 

307 

Barth, S., 21, 338 

Basei, M.A.S., 22 

Baskakov, A. V., 114 

Baskaran, M., 22 

Basu, A.R., 23,317 

Baumgartner, P.O., 30, 226 

Baur, H., 42, 172 

Beaumont, V., 23 

Bebout, G.E., 24 

Beck, J.W., 219 

Becker, T.A., 24, 265, 332, 

337,345 

Beer, J., 153 

Beets, C.J., 25 

Beetsma, J.J., 25, 295 

Begemann, F., 238 

Beiriger, J.M., 296 

Beliatsky, B.V., 217 

Bell, K., 30 

374 

Belshaw, N.S., 26, 343, 369 

Belyatsky, B.V., 26 

Bemis, B.E., 301 

Bennett, V.C., 27 

Benoit, P.H., 27 

Berger, G.W., 28 

Bernatowicz, T., 28 

Bernius, M.T., 242 

Bertotti, G., 353 

Beverding, A.M., 29 

Bhattacharya, S.K., 29 

Bibikova, E., 60, 295 

Bickel, RA., 302 

Bickford, M.E., 222 

Bickle, M.J., 52 

Bideau, D., 2 

Bigazzi, G., 173, 345 

Biino, G.G., 237 

Bijma, J., 301 

Bikerman, M., 30 

Bill, M., 30 

Bird, J.R., 58 

Black, P.M., 196 

Blake, S., 310 

Blakey, S., 216 

Blenkinsop, J., 31 

Bloch, J.D., 272 

Blomqvist, R., 105 

Blum, J.D., 31, 50 

Blusztajn, J.S., 32, 130 

Bodvarsson, G.S., 93 

Bogaard, P. V.D., 32 

Bogdanova, S., 60, 295 

Bogdanovski, O.G., 33 

Boghossian, N.D., 33 

Bohlke, J.K., 77 

Bohor, B.F., 141 

Bohrson, W.A., 34 

Bonini, J.A., 34 

Bonotto, D.M., 35 

Booij, E., 35 

Borchardt, G., 36 

Boriani, A., 36 

Borole, D. V., 37 

Bossi, J., 318 

Bottrell, S.H., 20 

Bougault, H., 85 

Boulegue, J., 82 

Bourdon, B., 37, 276, 371 

Bourles, D.L., 41 

Boven, A., 8 
Bowring,.S.A., 38, 131, 138 

Boyce, A.J., 38 

Brack;P., 229 

Bradshaw, J.D., 151 

Brandon, A.D., 39, 71 

Breaks, F.W., 67 

Brenninkmeijer, C.A.M., 138 

Brewer, T.S., 74 

Bridges, J.C., 14, 104 

Brimhall, G.H., 39 

Briot, D., 40 

Brito Neves, B.B., 335 

Broecker, W.S., 40, 195, 279, 

286 

Brook, E.J., 41, 184 

Brown, E.T., 41 

Brown, K., 31 

Brown, R.W., 42, 188,310 

Broxton, D.E., 355 

Bruno, L.A., 42, 153, 282 

Bryant, G., 105 

Buchardt, B., 43, 152 

Bukshpan, S., 190 

Burbank, D.W., 190 

Burgess, R., 15, 43 

Burkland, M.K., 312 

Burnard, P.G., 16, 44 

Burnham, K., 44 

Burruss, RC., 329 

Burton, K. W., 45, 204 

Byers, F.M., Jr., 355 

Caffee, M.A., 127 

Caffee, M.W., 45, 93, 100, 190, 

234,285 

Caldwell, E.A., 46, 287 

Camargo, P.B.De, 46 

Cameron, K.L., 47, 233 

Campajola, L., 77 

Campal, N., 318 

Campbell, J.D., 171 

Campbell, K.R., 71, 76 

Capo, R.C., 47 

Carignan, J., 48, 108 

Caroff, M., 341 

Carr, G.R., 48, 240 

Carr, S.D., 243 

Carroll,J.,49 

Carroll, M.R., 166, 193 

Carter, A., 148 

Castaneda, C., 29 

Castrec, M., 82 

Cavazza, W., 31 

Cerling, T.E., 49, 348 

Chabaux, F., 50 

Chadwick, O.A., 47, 144, 305 

Chakravarti, A., 29 

Chakravarty, N., 29 

Chamberlain, C.P., 50, 304 

Chamberlain, K.R., 51, 106 

Champion, D.E., 51, 33:2 

Chan, L.-H., 52, 359 

Chapman, H.J., 52, 269 

Chase, R.L., 69,217 

Chaudhuri, S., 61 

Chaussidon, M., 53 

Chawala, H.S., 338 

Chazot, G., 216 

Cheilletz, A., 53 

Chen, C-H., 54, 359 

Chen, H-W., 189 

Chen, J., 54, 158, 349 

Chen, J.C., 242 

Chen, J.H., 25, 55 

Chen, S-M., 57, 120 

Chen, Y-G., 56 

Chen, Y-W., 57,361 

Chen, Y.D., 55, 56 

Chen, Z-S., 364 

Chenery, C., 140 

Cheng, Z., 54 

Cheng, Z.-Y., 195 

Chernet, T., 57 

Cherry, J.A., 336 

Chesley, J.T., 208 

Childe, F.C., 58 

Chivas, A.R., 58, 74, 107, 169, 

247 

Christensen, J.N~. 122 

Christie, D.M., 256 

Chung, S-L., 59 

Churikova, T.G., 59 

Cisar, D.J., 49 

Civetta, L., 66 

Claesson, S., 60 

Claoue-Long, J., 299 

Clark, A.H., 55 

Class, C., 60 

Clauer, N., 61, 327, 372 

Clayton, R.N., 76, 247, 278, 

348,362 

Clemens-Knott, D., 61 

Clement, S.W.J., 62 

Cliff, B., 149 

Cliff, R.A., 62 

Cliff, S.S., 63 

Coccioni, R, 223, 373 

Cohen, A.S., 50,63 

375 

Cole, D.R., 143, 223 

Coleman, D.S., 38 

Coleman, M.L., 64,227 

Coler, D.G., 64 

Colin, F., 41 

Colley, H.C., 245 

Colodner, D.C., 260 

Compston, D.M., 65 

Compston, W., 62,65 

Condomines, M., 292 

Conrad, I.E., 84 

Conrad, M.E., 66, 150, 215 

Constantine, A., 98 

Conticelli, S., 66 

Copeland, P., 67 

Cordani, U.G., 220 

Corfu, F., 67, 281 

Cornett, J., 83 

Cornett, R.I., 178 

Corriveau, L., 5, 68 

Cosca, M.A., 68, 266, 339 

Coulon, C., 80 

Cousens, B.L., 69 

Cox, S, 243 

Cox, S.J.D., 112 

Coyle, D.A., 69 

Craig, H., "'0 

Cramer, J., 83 

Creaser, R.A., 71 

Cresswell, R.G., 169, 306 

Criss, R.E., 71, 76 

Crowe, B.M., 133, 248, 255 

Crowhurst, P.V., 72 

Crowley, S.F., 19 

Cui, Y., 106 

Cunibaa, S., 297 

Curtis, C.D., 64 

Curtis, D.B., 83 

Curtis, G.H., 77, 265, 332 

D'Antonio, M., 66 

D'Orazio, M., 325 

Daag, A.S., 359 

Dahl, P.S., 72 

Dai, T., 73 

Dalrymple, G.B., 73, 183 

Daly, J.S., 74, 214 

Dammer, D., 74 

Danhara, T., 154 

Danti, K., 90 

Darbyshire, D.P.F., 75 

Darling, G., 118 

Davidson, E.A., 46 

Davidson, J.P., 259,319 
Davie, R., 58 

Davies, G.R., 75, 135, 177, 

225,302,325,334 

Davis, A.M., 76,247,348 

Davis, A.S., 117 

Davis, M., 45 

Davis, R., 262 

Davis, S., 45 

Davisson, M.L., 71, 76, 296 

De Jong, A.F.M., 334 

Deak, J., 77 

Dean, J.A., 48 

Deck, B., 286 

Decourten, F., 348 

Defant, M.J., 168 

Deino, A.L., 77, 173, 265, 332, 

345,373 

DeJong, K., 97 

DeLaeter, J.R., 78, 166 

DeLange, G.J., 25 

Deloule~ E., 78, 268 

Demaiffe, D., 20, 79 

Demeny, A., 79 

Deniel, C., 80 

DeNiro, M.J., 319 

Dep, L., 80, 91 

DePaolo, D.J., 81, 111, 159, 

293,312 

Derry, L.A., 81, 104 

Descarceaux, S., 41 

Deseo, E., 77 

Desmons, J., 147 

Devey,_C.W., 134, 142 

DeWit, M.J., 186 

Dia,A.N., 82 

Diamond, L.W., 249 

Dick, G.S., 265 

Dick, H., 32 

Dickey, N., 165 

Dickinson, W.R, 109, 112 

Dietrich, W .E., 7, 285 

Ding, T-P., 82, 158 

Dingwell, D.B., 193 

Dixon, P.R., 83 

Doe, B.R., 83 

Dolenec, T., 94 

Donahue, D.J., 29 

Dong, H., 84 

Donnelly-Nolan, J.M., 84, 332 

Donovan, J., 3 9 

Dorn, R.I., 195 

Dosso, L., 85 

Downes, H., 205, 271, 344 

Draper, D.S., 166 

Drimmie, R.J., 85, 336 

Drummond, M.S., 168 

Duddy, I.R., 221 

Duffield, W.S., 274 

Dunai, T.J., 86 

Duncan, R.A., 86, 91,256,294 

Dunkl, 1., 87 

Dunkley, P., 21 

Dunlap, C.E., 87 

Dunlap, W .J., 88 

Dunphy, J.M., 88 

Dupuy, C., 341 

Durakiewicz, T., 89 

Dymek, R.F., 287 

Eastoe, C.J., 326 

Ebibara, M., 136, 280 

Edwards, B.R., 106 

Edwards, RL., 108, 219 

Eggenkamp, H., 64 

Eichelberger, J.C., 209 

Eikenberg, J., 341 

Ekwueme, B.N., 89 

Eldridge, C.S., 90 

Elliot, D.H., 101 

Elliott, C., 13 

Elliott, T.E., 90, 202, 276, 371 

Elmore, D., 80, 91, 176, 250, 

262 

Emslie, R.F., 124 

Englert, P.A.J., 29, 127, 262 

Engstrom, D., 356 

Epstein, S., 97, 206 

Erba, E., 91 

Erel, Y., 31 

Esser, B.K., 92 

Esser, R.P., 92 

Evans, J.A., 93 

Evans, J.M., 306 

Evensen, N.M., 297, 360 

Evenson, E.B., 114 

Ewing, R.C., 238 

Eyal, M., 96 

Fabryka-Martin, J., 80, 83, 93 

Faganeli, J., 94 

Faggart, B.E., 23 

Fahey, A.J., 94 

Fallick, A.E., 38, 215, 308 

Fang, Z .• 95, 363 

Fanning, C.M., 171 

Farmer, G.L., 340 

Farquhar, R.M., 95 

Farrar, E., 18, 55 

376 

Farrell, J.W., 121 

Fehn, U., 96, 224 

Feinstein, S., 96 

Feng, X., 97 

Feraud, G., 53, 97, 273 

Ferguson, K.M., 98 

Ferrara, G., 98, 201 

Ferreira, V.P., 99, 291 

Ferris, F.G., 289 

Fifield, L.K., 58, 169, 306 

Filippi, M.L., 99 

Finkel, R.C., 45, 100, 127, 190, 

234,265,285 

Fitton, J.G., 188 

Fitzgerald, J.F., 128 

Fitzgerald, P.G., 100, 262 

Fitzgerald, RC., 74 

Fleck, R.J., 101, 332 

Flegal, A.R., 268, 279, 297 

Fleming, T.H., 101 

Foden, J.D., 102 

Fogel, RA., 149 

Foland, K.A., 101, 102, 185 

Forde, E.J., 103 

Foster, D.A., 42, 72, 103, 221, 

235 

France-Lanord, C., 81, 104, 

298 

Franchi, I.A., 104 

Francis, D., 48 

Francois, R, 361 

Franklin, J.M., 370 

Franz, L., 320 

Frape, S.K., 85, 105, 336 

Freedman, P.A., 122 

Frei, R., 105, 194, 230 

Freifeld, B., 66 

Frey, F.A., 351 

Friedman, R.M., 106 

Frost, C.D., 51, 106, 284 

Fu, H-Q., 259 

Fuck, R.A., 252 

Funaki, M., 314 

Furfaro, D., 251 

Fyfe, W.S., 309 

Gabel, M., 157 

Gagan, M.K., 107 

Galer, S.J.G., 263 

Gallagher, K., 107, 336 

Gallahan, W.E., 35 

Gallup, C.D., 108, 219 

Garno, T., 330 

Gans, C., 29 
Gans, P.B., 188, 231 

GArtner, M., 263 

Gariepy, C., 108 

Gautschi, A., 324 

Gavrieli, 1., 301 

Gebauer, D., 109, 337 

Gehrels: G.E., 109 

Geissman, J., 10 

Gelman, M.L., 110 

Gerstenberger, H., 110 

Gertis, J., 289 

Getty, S.R., Ill 

Ghaleb, B., 164 

Giacobbe, A .• 116 

Gibb, A., 26 

Gibson, E.K., Jr., 271, 298 

Giggenbach, W.F., 200 

Gill, J.B., 87, 241 

Gilmour, J.D., 14, Ill 

Giobbi Origoni, E., 36 

Giraudeau, J., 186 

Giret, A., 206 

Giuliani, G., 53 

Gleadow, A.J.W., 42, 112, 176, 

235,236,261 

Gleason, J.D., 112 

Goldfarb, R., 210 

Goldman, 1., 127 

Goldstein, S.J., 293 

Goldstein, S.L., 60, 263, 327 

Gomes, D.P., 316 

Goodell, P.C., 355 

Goodfellow, W.O., 370 

Goodfriend, G.A., 113 

Gopalan, K., 113 

Gorokhov, I.M., 114 

Gorokhovsky, B.M., 191 

Goslar, T., 273 

Gosse, J.C., 114 

Goswami, J.N., 115, 353 

Gotih, RP., 257 

Graf, Th., 115 

Graham, D. W., 116, 125 

Graham, R.C., 305 

Granger, D.E., 116 

Graubard, C.M., 207 

Graves, G., 50 

Gtay, L.B., 84, 117 

Gregoire, M., 206 

Gregory, R.T., 98, 117 

Grey, K., 275 

Grier, J.A., 312 

Griesshaber, E., 118 

Griffin, T.J., 231 

Grimm, R.E., 262 

Grinenko, L.N., 118 

Grinenko, V.A., 119 

Grove, M., 119, 128 

Gui, X-T., 120, 361 

Guise, P.G., 266 

Gunn, S.H., 17, 120 

Guo, Y., 158 

Guo, Z., 349 

Gust, D.A., 168 

Haapala, I., 259 

Hacker, B.R., 121 

Hackspacher, P., 335 

Haggerty, S.E., 200 

Hajdas, I., 195 

Halicz, L., 284 

Hall, A.J., 38 

Hall, C.M., 84, 121, 122, 188 

Hall, R, 103 

Hallett, R.B., 122 

Halliday, A.N., 84, 122, 188 

Hames, W.E., 123, 138 

Hamilton, M.A., 124, 320 

Hamilton, P.J., 352 

Hamlin, S.N., 4 

Han, J., 124, 372 

Han, Y., 153 

Hanan, B.B., 125 

Hancock, R.G.V., 95 

Hansen, K., 125 

Hansen, B.T., 320 

Hanson, G.N., 133, 279 

Hanyu, T., 126 

Harangi, Sz., 79 

Harper, C.L., Jr., 126, 156 

Harris, C., 127, 186 

Harris, J.W., 43, 198 

Harris, L., 285 

Harris, L.J., 127 

Harrison, T.M., 119, 128 

Harrop, P.J., 129 

Hart, C.J.R., 129 

Hart, S.R., 32, 130, 302 

Hart, W.K., 57,346 

Hartmann, J., 130 

Hartmann, L.A., 309 

Hasebe, N., 131, 308 

Hashizume, K., 309 

Hawkesworth, C.J., 22, 145, 

211,245 

Hawkins, D.P., 131 

Hawkins, M.P., 318 

377 

Heaman, L.M., 74, 132, 187, 

228,246,258,362 

Heaney, P., 328 

Hebeda, E.H., 258 

Hedenquist, J.W., 90 

Heijnis, H., 132 

Heimann, A., 101 

Heimann, M., 139 

Hein, J.R., 45, 50, 343, 359 

Heisinger, B., 130 

Heithersay, P.S., 48 

Heizler, M.T., 92, 128, 133, 

212 

Hekinian, R., 2 

Helfer, M., 282 

Helmers, H., 325 

Hemming, N.G., 279 

Hemming, S.R., 133 

Hemond, C., 134 

Henderson, C.M.B., 185 

Henderson, G.M., 204 

Hendricks, D.M., 47 

Henry, C.D., 134 

Hensen, B.J., 367 

Hertogenk, J., 142 

Herzog, G.F., 175 

Hess, J.C., 135 

Heumann, A., 135 

Heumann, K.G., 338 

Hewins, R.H., 362 

Hidaka, H., 136 

Hildebrand, A.R., 161 

Hill, K.C., 72, 221, 261 

Hilton, D.R, 86, 146, 137 147 

Hirooka, K., 314 

Hiyagon, H., 137, 221, 280 

Hodder, P.S., 138 

Hodg~s. K. V., 138 

Hoehndorf, A., 139 

Hofer, M., 172 

Hoffmann, G., 139 

Hofmann, A. W., 249, 303 

Hogan, L.G., 86 

Hohmann, R., 172 

Hoke, L., 140 

Holden, P., 319 

Holk, G.J., 140 

Holliger, P., 136 

Holm, P.M., 141, 161,246 

Holmes, C.W., 141 

Holmes, R, 50 

Honda, M., 136, 142, 211, 334 

Hoogewerff, J.A., 142 

Hoover, J.D., 213 
Horita, J., 143 

Housh, T.B., 38 

Howard, K.A., 103, 143 

Howell, D.G., 109 

Hrabi, B., 342 

Hsieh, J.C.C., 144, 305 

Hu, A.; 144 

Huang, B., 228 

Huang, M., 145 

Hu~g, Y.-M., 145 

Huang, Z., 356 

Hudson, G.B., 285 

Huff, W.O., 364 

Huhma, H., 146, 299 

Hulston, J.R., 146, 147 

Hunziker, J.C., 30, 68, 99, 147, 

226,288,339 

Hurford, A.J., 147, 148, 360 

Huss, G.R., 94, 148 

Hutcheon, 1., 181 

Hutcheon, I.D., 149 

Hutchison, R., 14, 104 

Ikeya, M., 328 

Iliffe, T.M., 22 

Imboden, D.M., 172 

Inayama, E., 315 

Inger, S., 149 

Ingle, J.C., 150 

Ingraham, N.L., 46, 287 

Ingram, B.L., 150 

Innocenti, F., 325 

Ireland, T.R., 151, 196 

Irwin, J.J., 151 

Isachsen, C.E., 38 

Isdale, P.J., 107 

Ishibashi, J., 330 

Israelson, C., 43, 152 

Itaya, T., 152, 154 

Ito, E., 153, 356 

Ivanenko, V.V., 163 

Ivanov, V., 8 

Ivy, S.D., 42, 153, 282 

Iwano, H., 154 

Iwasaki, 1., 154 

Iwata, N., 155 

Iyer, S.S., 155 

Jackson, C., 127 

Jackson, V., 342 

Jacobsen, S.B., 126, 156 

Jaeckel, P., 181 

Jagoutz, E., 33, 156 

Jahren, A.H., 157 

Jakobsson, S., 292 

Jamieson, R.A., 354 

Javoy, M., 2, 23, 157 

Jefferey, A., 146, 147 

Jemielita, R., 234 

Jenkins, C., 321 

Jenkins, R.J.F., 65 

Jessberger, E.K., 216 

Jiang, S-Y., 158 

Jiang, S., 82 

Jiang, W., 124 

Jiang, Z., 158 

John, B.E., 103, 143 

Johnson, C.M., 219 

Johnson, G.D., 304 

Johnson, K., 159 

Johnson, R.G., 108 

Johnson, T.M., 159 

Johnston, J.C., 160 

Jones, C., 160 

Jones, C.E., 122, 315 

Jones, P.W., 75 

Jergensen, N.O., 161 

Joron, J-L., 85 

Jull, A.J.T., 194, 250, 262 

Jull, T., 29 

Juraske, S., 338 

Kagi, H., 3ll 

Kamensky, I.L., 249 

Kamioka, H., 14 

Kamo, S.L., 161, 309 

Kampata, M., 79 

Kaneoka, 1., 126, 155, 162 

Kaplan, I.R., 146, 147 

Kapusta, Y., 162, 190, 303 

Karasaki, K., 66 

Karhu, J.A., 163 

Karpenko, M.I., 163 

Karpenko, S.F., 33 

Kashgarian, M., 164 

Kastner, M., 245 

Kato, K., 276 

Kaufman, A., 164 

Kaufmann, R.S., 165 

Kawashima, A., 165 

Kawashita, K., 220 

Keeling, R.F., 286 

Kelley, S.P., 11, 43, 166, 261, 

336 

Kelly, E.F., 144 

Kelts, K., 153 

Kenneally, J.M., 296 

Kennedy,A.K., 166,251,323 

378 

Kennedy, M.J., 167 

Kent, A.J.R., 167 

Kepezhinskas, P.K., 168 

Keppens, E., 372 

Kersting, A.B., 168 

Ketcham, D.H., 169 

Keywood, M.D., 58, 169 

Khai, N.Vu., 170 

Kigam, T., 290 

Kilius, L., 83 

Kim, C.B., 330 

Kim, J.S., 115 

Kim, K., 29 

Kim, S-J., 170 

Kimbrough, D.L., 171 

Kinny, P.O., 171 

Kipfer, R., 172 

Kirchner, J. W., ll6 

Kirschner, D.L., 172 

Kiseleva, V. Yu., 254 

Kiss, E., 58 

Kistler, R.W., 101, 173 

Kitada, N., 173, 345 

Kitchen, N.E., 174 

Kiyota, K., 174, 309 

Klein, J., 114, 175, 234, 262, 

366 

Klotzli, U.S., 175 

Knies, D.L., 176 

Koehler, A.M., 29 

Kohl, C.P., 100,234 

Kohn, B.P., 96, 112, 176 

Kohn, M.J., 177 

Kolodny, Y., 203 

Konnerup-Madsen, J., 246 

Koppers, A.A.P., 177,256,282 

Korsch, RJ., 261 

Korschinek, G., 130 

Kostitsyn, Yu.A., 59 

Kotarba, M., 178 

Kotlarsky, P., 162, 190, 303 

Kotov, A.B., 191 

Kotzer, T.G., 178 

Kowallis, B., 173, 345 

Kramers, J.D., 179 

Krishnamurthy, R.V., 179 

Krogh, T.E., 56, 161, 180, 309 

Krol, M.A., 180 

Kroner, A., 89, 181, 327 

Krouse, H.R., 118, 119, 155, 

181,207,272,300 

Kruger, F.J., 182 

Ku, T., 49 

Ku, T-L., 182, 189, 199 
Kubik, P.W., 153, 262 

Kuc, T., 273 

Kulkarni, K.M., 183 

Kunk, M.J., 134, 183 

Kurz, M.D., 63, 1~4 

Kusakabe, M., 182 

Kutyavin, E.P., 114 

Kuzmin, D.S., 254 

Kyle, P.R., 92, 122, 130, 184, 

241 

Lal, D., 262 

Lambert, R.St.J., 39 

Lan, C-Y., 362 

Landoll, J.D., 185 

Langmuir, C.H., 37 

Lanphere, M.A., 17, 185 

Lansdown, J., 289 

Larimer, R.M., 127 

Laurenzi, M.A., 173, 186, 333, 

345 

Lavelle, M., 186 

Lawn, B.R., 114 

Layer, P.W., 187, 210, 351 

Lea, D.W., 301 

LeCheminant, A.N., 187 

Lee, D.-C., 122, 188 

Lee, E.H., 242 

Lee, J., 188 

Lee, J.K.W., 189 

Lee, K.S., 242 

Lee, S.-G., 290 

Lee, T., 189, 359 

Leeman, W.P., 98 

Lehman, T., 197 

Lehmann, B.; 324 

Lehmann, E., 341 

Leland, J.F., 19o 

Lempert, G.D., 190 

Lerche, 1., 49 

~sko, K.T., 127 

Levchenkov, O.A., 191 

Levkovskaya, G.M., 191 

Levsky, L.K., 26 

Lewis, A.J., ·192 

Lewis, R.S., 4, 28,-192, 247 

Leyboume, M.I., 69 

Li, C., 349 

Li, H-m., 95, 363 

Li, H., 193, 198 

Li, J., 300 

Li, X-H., 120 

Li, Y., 82 

Libby, W.G., 78 

Libourel, G., 193 

Liebetrau, V., 194, 253 

Liegeois, J-P., 8 

Lienkaemper, J.J., 36 

Lifton, N.A., 194, 250 

Lin, J.C., 195 

Lin, R-F., 195 

Lindsay, J.M., 196 

Lippolt, H.J., 135 

Lipschutz, M.E., 91, 176 

Lister, G.S., 19 

Litherland, M., 234 

Liu, C., 228 

Liu, J., 193, 198, 364 

Liu, T -K., 56 

Liu, Y., 196 

Lloyd, J.W., 20 

Loague, K., 7 

Lokhov, K.I., 191 

Long, L.E., 99, 169, 197, 291 

Longstaffe, F.J., 15 

Loosli, H., 324 

Lorand, J.-P., 264 

Loske, W.P., 197 

Lovera, O.M., 128 

Lowenstein, T., 300 

Lowry, D., 38, 198 

Lu, H-Z., 193, 198 

Luchitta, 1., 45 

Ludden, J.N., 48, 88 

LudWig. K.R., 199 

Luo, S., 182, 199 

Luz, B., 203 

Lyon, G.L., 200 

Lyon, I.C., 281 

M. Grajales, N., 295 

Ma, Z., 247 

Maas, R., 171 

Macdougall, J.D., 200 

Macfarlane, P.A., 76, 296 

Machado, N., 252 

Machavaram, M., 179 

MacPherson, C. G:, 18, 198 

MacRae, R.A., 229 

Magenheim, A.J., 201 

Maggetti, M., 253 

Magro, G., 201 

Mahoney, J.B., 202 

Mahoney, J.J., 318 

Mancktelow, N.S., 286 

Manetti, P., 325 

Mansfeld, J., 60 

Marcantonio, F., 202, 239 

379 

Marchant, K., 302 

Markel, D., 203 

Markewich, H. W., 244 

Marriner, G.F., 344 

Marshall, B.D., 203 

Marshall, J.D., 19 

Marshintsev, V.K., 149 

Martel, D.J., 45, 204 

Marti, K., 115 

Martinelli, L.A., 46 

Masarik,J.,80,204 

Mason, P.RD., 205 

Masson, H., 172 

Masterman, S., 210 

Masuda, A., 145, 165, 205, 

257,275,290,356 

Masuda, H., 173, 345 

Mathez, E.A., 149 

Matsumoto, A., 308 

Mattey, D.P., 18, 198, 211, 321 

Matthai, S.K., 65 

Matthews, A., 17, 206 

Mattielli, N., 206 

Mattinson, J.M., 171, 207, 209 

Matukov, D.I., 191 

Maury, R.C., 168, 341 

Mayeda, T.K., 362 

Mayer, B., 207 

Mazor, E., 208 

McCandless;T.E., 208, 274 

McClelland, W.C., 207,209 

McConnell, V.S., 209 

McConville, P., 373 

McCoy, D., 210 

McCulloch, M.T., 27, 210, 294 

McCurry, M., 340 

McDermott, F., 211 

McDougall, 1., 74, 142, 167, 

211, 300, 334 

McDowell, F.W., 47,212 

McFadden,L.D.,248 

Mcintosh, W.C., 10, 92, 133, 

184, 212, 241 

McKeegan, K.D., 128 

McKibben, M.A., 90, 213 

McKinley, J.P., 213 

McLelland, J., 214 

McLennan, S.M., 133 

McMillan, N.J., 47 

McNaughton, N.J., 171 

McNicoll, V.I., 258 

McWilliams, M., 214 

Meier, M., 21, 229, 237, 302 

Melezhik, V.A., 215 
Mello, I.S.C., 316 

Melnikov, N.N., 114 

Menard, T., 307 

Menchaca, L.B., 215 

Menuge, J.P., 74 

Menzies, M.A., 18, 216, 360 

Meshik, A.P., 216, 290 

Mezger, K., 133 

Michael, P.J., 125, 201, 217 

Middleton, R, 114, 175, 262, 

366 

Mikhalsky, E.V., 217 

Millar, I.L., 218 

Millard, H.T., 244 

Miller, B.R., 286 

Miller, D.S., 329 

Miller, E.L., 188 

Miller, M., 238 

Miller, M.P., 218 

Milling, M.E. Jr., 219 

Min, G.R., 219 

Mineev, S.D., 119 

Mironov, A.G., 220 

Misuzaki, A.M.P., 220 

Mitchell, M.M., 221 

Mitrofanov, F.P., 56 

Miyazaki, A., 137, 221 

Mock, T.D., 222 

Monaghan, M.C., 176,222 

Moncaster, S.J., 20 

Monger, H.C., 223 

Montanari, A., 223, 224, 373 

Montgomery, D.R., 7 

Moorbath, S., 275, 369 

Moore, W.S., 245 

Moran, J.E., 96,224 

Morante, R., 225 

Moree, M., 225 

Morettini, E., 226 

Morin, D., 5 

Morishita, Y., 226 

Morozova, I.P., 254 

Morris, J., 366 

Mortensen, J.K., 227 

Mortimer, G., 210 

Mortimer, R.J.G., 227 

Morton, A.C., 74 

Moser, D.E., 228 

Moureau, J., 79 

Mu,Z., 228 

Muecke, G.K., 229 

Muhs, D.R., 2 

Muir, R., 151 

Mukasa, S.B., 122, 184, 241 

Mundil, R., 229 

Murphy, W., 93 

Murray, B.C., 305 

Murrell, M.T., 251, 293 

Murton, B.J., 317 

Nagao, K., 173, 345 

Nagasawa, H., 230 

NAgler, Th.F., 230, 253 

Nair, R.R., 37 

Nakai, S., 357 

Napier, B., 127 

Nauert, J.L., 231 

Navada, S.V., 183 

Negrey, E. V., 370 

Nelson, D.R., 231 

Nelson, S.T., 293 

Nemchin, A.A., 232,251 

Nepstad, D.C., 46 

Nesje, A., 41 

Neumaier, S., 130 

Newberry, R.J., 210 

Neymark, L.A., 8, 232 

Niedermann, S., 115 

Nielsen, H., 301 

Nielsen, P., 372 

Niemeyer, S., 296, 297 

Nier, A.O., 233 

Nimz, G.J., 93, 233 

Nishiizumi, K., 100, 234, 262 

Nishimura, A., 14 

Nishimura, S., 131, 308, 313, 

358 

Nishri, A., 203 

Noack, M., 178 

Noble, S.R., 234 

Noble, W.P., 235 

Nolte, E., 130 

Norman, E.B., 127 

Norry, M.J., 75, 307 

Nunez-Betelu, L.K., 229 

Nutman, A.P., 27 

O'Brien, A., 235 

O'Neil, J.R., 270, 339 

O'Nions, R.K., 26, 45, 50, 63, 

204,236,343,369 

O'Sullivan, A.J., 236 

O'Sullivan, P.B., 221, 236, 237, 

261 

Oberli, F., 21, 229, 237 

Oberthuer, T., 139 

Odin, G.S., 223, 373 

Okada, T., 152 

380 

Olinger, C.T., 234 

Onstott, T.C., 238, 328, 329 

Orsi, G., 77 

Orstom, T., 53 

Osterloh, J., 297 

Otomo, I., 276 

Ott, U., 238 

Ovchinnikova, G.V., 26, 191 

Owens, T., 312 

Oxburgh, R., 239 

Ozima, M., 239, 289 

Paces, J.B., 232 

Paepe, R., 372 

Page, R. W., 240 

Page, W.D., 24, 240, 345 

Palacz, Z., 241 

Palin, J.M., 206 

_Palmer, M.R. 1, 158, 192 

Pande, K., 338 

Pankhurst, R.J., 184, 241 

Panter, K.S., 184, 241 

Pantulu, G. V., 113 

Papanastassiou, D.A., 242, 272, 

287 

Park, B.K., 242 

Parkinson, D.L., 207, 243 

Parrish, R.R., 175, 243, 354 

Patchett, P.J., 33, 112, 244, 340 

Patterson, D.P., 211 

Patterson, R.T., 31 

Pattou, L., 264 

Paul, R.L., 127 

Pavich, M.J., 244 

Paytan, A., 245 

Peacor, D.R., 84 

Pearce, J.A., 245 

Pearson, P.N., ·152 

Peate, D. W., 245 

Peccerillo, A., 66 

Pedersen, L.E., 246 

Pedersen, S., 246 

Pell, S.D., 247 

Pellin, M.J., 24 7 

Peng, T-H., 40 

Pe.ng, T-R., 347 

Pennisi, M., 98 

Perry, F. V., 133, 248, 255 

Pessenda, L.C.R., 46 

Peterman, Z.E., 203 

Peters, M. T., 52 

Petford, N., 248 

Petit-Jean, C., 130 

Petrini, R., 325 
Pettke, Th., 249 

Petty, G., 176 

Peucker-Ehrenbrink, B., 249 

Pewe, T.L., 28 

Pezdi, J., 94 

Pezzotta, f., 252 

Phillips, F.M., 91, 195, 250 

Phillips, W.M., 250 

Pickett, D.A., 251 

Pidgeon, R T., 232, 251, 302, 

323 

Pik, R., 80 

Pillinger, C.T., 104, 218 

Pimentel, M.M., 252 

Pin, C., 99 

Pinarelli, L., 36, 252 

Pisotsky, B.I., 257 

Plank, T., 90 

Pokrovsky, B. G., 253 

Poller, U., 194, 253 

Ponomarchuk, V.A., 254 

Porat, N., 254 

Porcelli, D., 255, 331 

Poreda, RJ., 49 

Poths, J., 10, 234, 255 

Poulson, S., 50 

Praegel, N.-0., 141 

Pravdivtseva, 0. V., 256, 290 

Prellwitz, H.S., 30 

Pringle~ M.S., 91, 177, 256, 

282 

Pringle, M.J., 295 

Prinos, S., 165 

Pu, Z., 73 

Puli, G., 325 

Punoilgbayan, R.S., 359 

Pushkarev, Y.D., 257 

QI-LU., 257 

Quade, J., 250 

Radhakrishna, B.P., 113 

Rae, J.E., 227 

Ragettli, R., 258 

Ragnarsdottir, K.V., 1 

Raiilbird, R.H., 258 

Raisbeck, G.M., 41 

Ralska-Jasiewiczowa, M., 273 

RAmo, O.T., 259 

Ramos, F.C., 259, 263 

Rao, S.M., 183 

Rao, U., 96 

Rasdas, A.S., 359 

Rasskasov, S., 8 

Ravenhurst, C.E., 260 

Ravizza,G.E.,92,249,260 

Raza, A., 261 

Recy, J., 219 

Reddy, S.M., 261 

Redfield, T.F., 262 

Reed, S.J.B., 369 

Reedy,R.C.,29,80,204,262 

Rehldimper, M., 263 

Reid, M.R, 34, 190, 259, 263 

Reinfrank, R., 60 

Reinhard, E.A., 270 

Reinhardt, E., 31 

Reisberg, L., 264 

Reischmann, T., 9, 264 

Relf, C., 342 

Rengarajan, R., 113 

Renne, P.R., 24, 171, 240, 265, 

318,332,337,345 

Repka, J.L., 265 

Restrepo, P., 266 

Revesz, K., 77 

Rex, D.C., 266 

Reynolds, P.H., 229 

Richardson, S.H., 267 

Rink, W.J., 267 

Ritson, P.R., 268 

Robert, F., 23, 53, 78, 268 

Roberts, S., 300 

Robinson, A.C., 269 

Roddick, J.C., 124, 269 

Roden, M.K., 260 

Roe, L.J., 270 

Roedder, E., 151 

Rohrman, M., 270 

Rohrman, P., 9 

Rokop, D.J., 83 

Roldan-Quintana, J., 212 

Romanek, C.S., 271, 298 

Ronen, A., 254 

Rosenbaum, J.M., 271 

Ross, G.M., 33, 109,272 

Roy, A.B., 115 

Roy-Barman, M., 272 

Rozanski, K., 273 

Rubenstone, J.L., 195 

Ruffet,G.,53,97,273 

Ruiz, J., 112, 208, 266, 274 

Russell, D., 297 

Russell, J., 275 

Rye, R.O., 4 

Ryerson, F.J., 128 

Sahoo, S.K., 275 

381 

Saito, K., 276 

Sakai, H., 314 

Salters, V.J.M., 276 

Saltus, R.W., 176 

Samson, S.D., 64, 277 

Samuel, J., 372 

Sanders, C.A.E., 9, 277 

Sandler, A., 303 

Sanfo, Z., 41 

Sano, Y., 278 

Santos, R. V., 278 

Santosh, M., 332 

Sanyal, A., 279 

Safiudo-Wilhelmy, S.A., 279 

Sarda, P ., 280 

Sarin, M.M., 113 

Sasada, T., 280 

Sass, E., 284 

Savin, S.M., 144 

Saxton, J.M., 281 

Schaltegger, U., 281 

Scheveers, R, 282 . 

Schirnick, _C., 32 

Schltichter, C., 42, 153, 282 

Schmid, R., 337 

Schmidt, M.P., 283 

SchmidtMumm, A., 139 

Schnell, U.A.L., 283 

Schultze-Lam, S., 289 

Schwarcz, H.P., 254, 267 

Scoates, J.S., 51, 106, 284 

Scrivener, RC., 218 

Sears, D.W.G., 27 

Seccombe, P .K., 367 

Segev, A., 284 

Seghedi, 1., 205 

Seidemann, D.E., 331 

Seidl, M.A., 285 

Semikhatov, M.A., 114 

Sergeev, S.A., 194, 285 

Setsuya, N., 54 

Severinghaus, J.P., 286 

Seward, D., 286 

Shadel, C., 287 

Shao, B., 349 

Sharma, K.K., 338 

Sharma, M., 287 

Sharma, P., 83, 91, 93, 176, 

250 

Sharp, W.O., 24, 265, 332, 345 

Sharp, Z.D., 30, 68, 172, 288, 

339 

Shatagin, K.N., 288 
Shepherd, T.J., 75 

Sherry, T., 50 

Shenwood,L.B.,289 

Shi, Y., 153 

Shibata, T., 289 

Shieh, Y-N., 170 

Shimada, C., 313 

Shimizu, H., 290 

Shimizu, N., 206 

Shukolyukov, Yu.A., 216,256, 

290 

Sial, A.N., 99, 169, 291 

Siddall, R., 291 

Sie, S.H., 292 

Siegel, D.I., 165 

Sigmarsson, 0., 292 

Signer, P., 42, 258 

Silantjev, S.A., 33 

Simonetti, A., 30 

Sims, K.W.W., 293 

Sinelnikova, I.M., 191 

Singer, B.S., 293 

Singh, B., 39 

Sinigoi, S., 61 

Sinton, C.W.; 294 

Sisterson, J., 29 

Sivell, W.J., 294 

Skiold, T., 295 

Smellie, J.L., 184, 241 

Smit, J., 295 

Smith, B.M., 215 

Smith, C.B., 296 

Smith, O.K., 296 

Smith, D.R., 297 

Smith, G.I., 195 

Smith, I.E.M., 196 

Smith, M., 21 

Smith, P.E., 95, 297, 360 

Smith, R.D., 47 

Smithies, H., 231 

Snee, L.W., 183 

Snow, J.E., 298 

Socki, R.A., 271, 298 

Somayajulu, B.L.K., 113, 299 

Sorjonen-Ward, P., 299 

Southon, J.R., 45, 77, 93, 100, 

127,182,234,265 

Speer, J.A., 64 

Spell, T.L., 67, 300 

Spencer, K.J., 318 

Spencer, R., 300 

Spera, F., 116 

Spero, H.J., 301 

Spicuzza, M.J., 209 

Spiro, B., 140, 301 

Spivack, A.J., 201, 343 

Starinsky, A., 284, 301 

Staudacher, T., 280 

Staudigel, H., 35, 135, 177, 

202, 225, 256, 302, 

325,334 

Steckler, M.S., 96 

Steiger, R.H., 194, 285, 302 

Stein, M., 303 

Steinitz, G., 162, 190, 303 

Stephens, W.E., 38 

Stem, L.A., 304 

Stevenson, R.K., 304 

Stewart, B. W., 305 

Stewart, J.H., 109 

Stille, P., 327 

Stoffers, P., 134 

Stokes, S., 306 

Stolper, E.M., 206 

Stone, J., 306 

Stopforth, S., 155 

Stott, G.M., 67 

Stouraiti, C., 307 

Stowell, H.H., 307 

Strack, E., 130 

Strain, P.M., 314 

Stuart, F., 308, 373 

Stuart, F.M., 44, 129 

Stuckless, J.S., 203 

Stump, E., 262 

Sudo, M., 308 

Sugi, S., 276 

Sugiura, N., 137, 174, 221, 309 

Suita, M.T.F., 309 

Summerfield, M.A., 310 

Sun, S-s., 59, 240 

Suppel, D.W., 48 

Sur, B., 127 

Suter, G.F., 292 

Sutherland, S., 40 

Sutton, A.N., 310 

Suzuki, K., 311 

Svetov, S.A., 344 

Swabey, S.E.J., 211 

Swager, C.P., 231 

Swindle, T.D., 312 

Swisher, C.C., III, 224, 265, 

312,332 

Szakacs, A., 205 

Taddeucci, A., 313 

T~gami, T., 131, 308, 313, 358 

Takahashi, E., 289 

382 

Takahashi, K., 136, 165, 356 

Takigami, Y., 314 

Tan, F.C., 314 

Tanaka, T., 14, 315 

Tao, Q., 357 

Tao, X-c., 95, 363 

Tamey, J., 160, 307, 315 

Tassinari, C.C.G., 316 

Tatsumoto, M., 316 

Taylor, F.W., 219 

Taylor, H.P., Jr., 61, 140 

Taylor, R.N., 317 

Teagle, D., 122 

Teichmann, F.R., 317 

Teixeira, W., 318 

Tejada, M.L.G., 318 

Tellam, J.H., 20 

Teng, R.T.D., 96 

Tepley, F.J., 319 

Tera, F., 366 

Terashima, S., 324 

Terrasi, F., 77 

Terwilliger, V.J., 319 

Teufel, S., 320 

Theriault, R.J., 320 

Thiemens, M.H., 63, 138, 160, 

321 

Thirlwall, M.F., 18, 103, 205, 

317,321 

Thomas, R.L., 85 

Thomaz-Filho, 220 

Thompson, RN., 247 

Thomson, J., 260 

Thomson, S.N., 322 

Thonnard, N., 322 

Thorkelson, D.J., 323 

Tieszen, L., 157 

Titterington, D.M., 199 

Todt, W.A., 251, 323, 327 

Togashi, S., 324 

Tolstikhin, LN., 179, 236, 249, 

324 

Tommasini, S., 325 

Tonarini, S., 98, 325 

Toolin, L.J., 326 

Topfner, C.P., 326 

Toplis, M.J., 193 

Torres, R., 7 

Tosdal, R.M., 327 

Toteu, S.F., 335 

Toulkeridis, T., 327 

Toyoda, S., 328 

Travin, A.V., 254 

Tiumbore, S.E., 46 
Tseng, H-Y., 328, 329 

Tsuchiyama, A., 333 

Tsunogae, T., 329 

Tsunogai, U., 330 

Tuccimei, P., 313 

Tulloch, A.J., 300 

Turchenko, T.L., 114 

Turek, A., 330 

Turekian, K.K., 331 

Turner, G., 14, 15, 16, 43, 44, 

111, 129, 281, 308 

Turner, P.J., 274 

Turrin, B.D., 24, 84, 265, 332 

Uchiumi, S., 308 

Unnikrishnan-Warrier, C., 332 

Uto, K., 308 

Uyeda, C., 333 

Vaasjoki, M., 259 

Vai, G.B., 333 

Valbracht, P.J., 334 

Valenti, G.L., 352 

Valley, J.W., 174, 177, 209 

Van Bergen, M.J., 142 

Van Bronswjk, W., 251,323 

Van Calsteren, P., 145 

Van De Plassche, 0., 334 

VanDerBeek, P.A., 9, 270 

VanDer Borg, K., 334 

VanDer Plicht, J., 335 

Van Donkelaar, C., 181 

Van Klinken, G.J., 306, 335 

Van Riet, A., 372 

Van Schmus, W.R., 16, 335 

Van Wagoner, N.A., 69 

Van Warmerdam, E.M., 336 

Vance; D., 336 

Vasconcelos, P.M., 337 

Vaughan, A.P.M., 218 

Vavra, G.~ 337 

Velsko, C.A., 76 

Vengosh, A., 338 

Venkatesan, T.R.-, 338 

Vennemann, T.W., 339 

Venturini, G., 339 

Verplanck, P.L., 340 

Vervoort, i.D., 340 

Vetrin, V.R., 56 

Vetter, U., 139 

Viana, M.G., 252 

Vidal, Ph., 80, 341 

Villa, I.M., 249, 341 

Villeneuve, M.E., 342 

Vincent, J., 29 

Vinyu, M.L., 342 

Visona, D., 186 

Vogt, S., 91, 176 

Volpe, A.M., 296 

Volpe, C., 343 

Voltaggio, M., 313 

von Blanckenburg, F., 26, 343 

Vonhof, H.B., 295 

Vrevsky, A.B., 344 

Vroon, P.Z., 103, 142, 321, 344 

Wada, H., 280 

Wadatsumi, K., 170, 173, 345 

Wagner, G.A., 69 

Wakabayashi, J., 24, 345 

Wakita, H., 330, 357 

Walder, A.J., 122 

Wallace, C.A., 323 

Wallace, M.W., 167 

Walraven, F., 346 

Walter, M., 2 

Walter, R.C., 265, 346 

Wampler, J.M., 347 

Wan, D., 82 

Wang, C-H., 347 

Wang, H.-F., 369 

Wang,J., 76,348 

Wang, ·L., 182 

Wang, S., 153 

Wang, X., 349 

Wang, Y., 6, 348 

Wang, Y.-X., 369 

Warnock, A.C., 349 

Wartho, J-A., 350 

Wasserburg, G.J., 25, 55, 242, 

255,272,287 

Wasteneys, H.A., 350 

Watanabe, T., 154 

Weaver, S., 151 

Wehmiller, J., 164 

Wei, G-J., 361 

Wei, K.-Q., 195 

Weis, D., 20, 79, 206, 351 

Weise, S., 118 

Weiss, R.F., 286 

Wells, S. G., 248 

Wen, Q., 349 

Wenk, H.R., 337 

Wesolowski, D.J., 143 

West, A. W., 351 

Wheeler, J., 261 

Wheller, G.E., 137 

383 

Whitehouse, MJ., 252, 275, 

352 

Whitelaw, M., 10 

Whitford, D.J., 352 

Whittemore, D.O., 296 

Wickham, M., 93 

Wickham, S.M., 52 

Wiedenbeck, M., 115, 353 

Wiedman, C., 164 

Wieler, R., 42, 172, 258 

Wieser, M., 155 

Wightman, S., 93 

Wijbrans, J.R., 91, 256, 282, 

295,353 

Williams, A.E., 213 

Williams, I.S., 196, 247 

Williams, S.N., 278 

Williams, W.J.W., 10 

Wilson, C.J.N., 310 

Wilson, M., 271 

Wiltshire, J.C., 359 

Windley, B.F., 352 

Witt, W.K., 231 

Wittenberg, L.J., 354 

Wodicka, N., 354 

WoldeGabriel, G., 346, 355 

Wong, V., 355 

Wooden, J.L., 17, 173 

Woodhead, J.D., 142 

Wright, J.E., 5 

Wushiki, H., 356 

Wyche, S., 231 

Xia, J., 356 

Xia, Y., 357 

Xia, Y-f, 363 

Xie, J., 73 

Xiong, J., 356 

Xu, S., 357 

Xue, S., 175 

Yabuki, S., 356 

Yakovleva, S.Z., 191 

Yamada, R., 358 

Yamanaka, T., 333 

Yamazaki, M., 154 

Yanagi, T., J58 

Yang, H., 349 

Yang, J-D., 95 

Yang, T-y. F., 359 

Yang, W., 300 

Yeh, H.-W., 359 

Yelland, A.J., 216, 360 

Yiou, F., 41 
York, D., 297, 360 

Yoshida, M., 332 

Young, J., 29 

Yu, E.-F., 361 

Yu,.F-J., 361 

Yu, J-H., 259 

Yu, J-S., 120, 361 

Yu, Y-W., 54 

Yu, Y., 362 

Yui, T-F, 362 

Yurimoto, H., 329, 363 

Zakariadze, G.S., 33 

Zantedeschi, C., 186 

Zartman, R.E., 232 

Zeitler, P.K., 180, 349 

Zhang, D-K., 361 

Zhang, G-d., 95, 363 

Zhang, J-B., 368 

Zhang, L., 364 

Zhang, Y -S, 364 

Zhang, Z., 365 

Zhatnuev, N.S., 220 

Zheng, M., 357, 365 

Zheng, S., 366 

Zheng, Y.-F., 366 

Zhiyu, J., 367 

Zhou, B., 367 

Zhou, T., 54 

Zhou, X-H., 368 

Zhu, B.-Q., 369 

Zhu, H., 368 

fhu, J., 365 

Zhu, X., 369 

Zhuravlev, A.Z., 370 

Zierenberg, R.A., 370 

Zindler, A.; 37, 90, 202, 239, 

371 

Zinger, T.F., 191 

Zinner, E., 4 

Zreda, M.G., 91,250 

Zubakov, V.A., 371 

2uravlev, D.Z., 257 

Zwingmann, H., 372 

'fl U.S. CDVElRENr PRlN'.l'IR; OFFICE: 1994-583-048/80018 384 


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