nucleation crystal growth and the thermal regime of cooling magmas crystallization at the margin of a quiet cooling magma has been studied numerically taking into account the kinetics of crystallization the variables are the latent heat value the growth and nucleation functions the initial magma temperature and the thermal contrast between magma and country rock we have investigated a wide range of values for these parameters corresponding to natural conditions we show that after a highly transient stage crystallization tends toward an equilibrium between heat production latent heat release and heat loss given the small diffusivity of country rocks latent heat release is the main factor controlling the temperature evolution in order to minimize the latent heat release crystallization occurs at a temperature where nucleation is small this can be close to either the liquidus or the solidus depending on the initial conditions the main process controlling crystallization is nucleation and not crystal growth nucleation occurs as a series of sharp pulses followed by longer periods of crystal growth the nucleation pulses give birth to thermal oscillations these oscillations can be sustained if the interior magma temperature is above the liquidus independently of the heat loss mechanism we show that the phenomenon occurs on the scale of a few centimeters which corresponds to the inch scale layering of many ultrabasic complexes the model allows us to calculate crystal sizes which are in good agreement with geological observations the crucial parameters which determine crystal size variations near the margins of igneous bodies are the initial thermal conditions as well as the nucleation and growth functions in the main cooling regime close to the liquidus significant size variations can be created by small thermal disturbances

controlling mechanisms for molybdenum isotope fractionation in porphyry deposits the qulong example molybdenite bearing porphyry deposits are the predominant supplier of molybdenum to industrialized society and one of the main hosts of mo in the upper continental crust the mo isotope compositions 98 95mo normalized to nist3134 equals 0 of molybdenite show considerable variation 1 62 to 2 27 but the factors controlling this variability remain poorly constrained this information is critical for underpinning genetic models of porphyry deposits understanding elemental cycling and utilizing the 98 95mo of marine sediments as a paleoredox proxy using the well characterized qulong porphyry cu mo deposit tibet as an example here we discuss how rapid cooling facilitated by mixing hot magmatic fluid with cold meteoric water can be a controlling factor on efficient mineralization and then tackle how fluid evolution regulates molybdenum isotope fractionation molybdenites which preferentially partition isotopically light mo rayleigh fractionation precipitated from a single fluid will develop a heavier 98 95mo composition over time and this also creates heterogeneous 98 95mo between molybdenite grains whereas a fluid undergoing multiple episodes of intensive boiling will gradually lose its isotopically heavy mo to the vapor phase molybdenites crystallizing successively from the residual liquid will then have lighter 98 95mo over time however when mineralization efficiency becomes too low a negligible variation in 98 95mo of molybdenite is observed given that the mineralization efficiency i e the amount of mo crystallized as molybdenite from the fluid rarely reaches 100 and molybdenite favors isotopically light mo the presence of a residual fluid with isotopically heavy mo is inevitable this residual fluid may then become trapped in alteration halos hence 98 95mo has the potential to aid in locating the mineralization center e g lighter 98 95mo toward the orebody the residual fluid may also feed surface hydrological systems and eventually impact mo cycling our study highlights that understanding the controls of isotope fractionation is critical to bridge the gap between ore formation and elemental cycling and that other transition metals e g cu fe and zn may follow similar trajectories

an experimental study of the kinetics of decompression induced crystallization in silicic melt 1 experiments were conducted to study the temporal evolution of feldspar crystallization kinetics during isothermal decompression pinatubo dacite was held at 780 c 220 mpa fo2 nno 2 h2o saturated conditions for an equilibration period decompressed to final pressures pf ranging from 175 to 5 mpa and then held for 0 3 931 hours according to the plagioclase liquidus curve in ph2o t space for the relevant melt composition these decompressions impose effective undercoolings teff of 34 266 c growth of preexisting phenocrysts and newly formed sparse microlites dominate crystallization at 75 pf 150 mpa teff 34 93 c and equilibrium crystal modes are achieved in 168 hours microlite nucleation is the dominant transformation process for 10 pf 50 mpa teff 125 241 c and chemical equilibrium is not attained by 168 hours under these conditions slow steady decompressions typically produced normally zoned euhedral and planar faceted feldspar crystals although anhedral morphologies were produced at very low pf contrary to expectation slowly decompressed samples were usually further from chemical equilibrium than rapidly decompressed samples after similar durations below the initial pressure although counterintuitive these trends are consistent with new constraints on the relative rates of feldspar nucleation and growth controlled by teff and melt viscosity experienced during each decompression path analysis of liquid to solid transformation kinetics using ttt style diagrams shows that crystallization occurs most rapidly at 100 mpa by a crystal growth mechanism the next most efficient crystallization conditions are at 25 mpa in a crystal nucleation dominated regime

fe isotopes and the contrasting petrogenesis of a i and s type granite abstract we present new fe isotope data of 42 s i and a type ferroan granites from the cambrian delamerian orogen in south australia the palaeozoic lachlan fold belt and western usa interpretation of these data together with modelling suggests that magmatic processes do result in quite complex fe isotopic differentiation trends and can lead to granites with isotopically heavy iron with 57 fe 0 35 by comparison mid ocean ridge basalts morbs have 57 fe 0 15 teng et al 2013 these variations are similar to those previously reported poitrasson and freydier 2005 heimann et al 2008 telus et al 2012 but contrary to some interpretations beard and johnson 2006 heimann et al 2008 heavy values are not necessarily the product of late stage hydrothermal fluid loss though this process is undoubtedly also an important factor in some circumstances a type ferroan granites reach very heavy 57 fe values 0 4 0 5 whereas i types are systematically lighter 57 fe 0 2 s type granites show a range of intermediate values but also tend to be isotopically heavy 57 fe 0 2 0 4 our results show that the iron isotopic values and trends are signatures that reflect granite generation processes a modelling using the rhyolite melts software suggests that contrasting trajectories and end points in fe isotope evolution towards granite depend on oxidation state of the evolving magma and whether or not the system is oxygen buffered iron isotopic evolution supports an origin of ferroan a type granite from protracted closed magma chamber fractionation of moderately reduced mafic magmas in these systems magnetite saturation is delayed and the ferric iron budget is finite i type systems originate with the supply of relatively oxidised hydrous subduction related magmas from the mantle wedge to the upper plate crust these then experience oxygen buffered open system afc processes in lower crustal hot zones s type magmas are crustal melts that crystallise under reduced conditions initially imposed at source by sulphidic or graphitic sedimentary protoliths the composition of the resulting melts reflects the domination of partial melting where conditions are hence buffered open system followed by subsequent late stage closed system fractionation of these extracted reduced magmas

iron isotope constraints on fe cycling and mass balance in oxygenated earth oceans the fe isotope composition of proterozoic to modern clastic sedimentary rocks and aerosols defines a range ind 56 fe values that is only slightly more variable than the range of fe isotope com positions measured in terrestrial igneous rocks indicating that chemical weathering sedimentary transport and diagenesis play only a minor role in producing fe isotope variations in environ ments where fe redox conditions have been controlled by current levels of atmospheric oxygen in contrast the fe isotope composi tions of hot fluids 300 8c from mid ocean ridge mor spread ing centers define a narrow range that is shifted to lower d 56 fe values by 0 2 0 5 as compared to igneous rocks these new data allow a conceptual model for the fe isotope composition of the oxic oceans that predicts large ranges in fe isotope composition under conditions of changing aerosol and mor fe fluxes such as during periods of major worldwide glaciation

a deep sea benthic community exposed to strong near bottom currents on the scotian rise western atlantic abstract this paper gives an overview of the structure of a benthic community at 4626 m depth on the nova scotian continental rise here abundances of polychaetes bivalves isopods and tanaids are conspicuously high compared to those reported from comparable depths bacterial numbers and atp concentrations are also high we suspect that these anomalous abundances result from enhanced food availability caused by the strong near bottom currents that flow through the area the polychaete and bivalve faunas have few adults and high species dominance suggesting that currents create large scale sediment transporting disturbances the composition of the crustacean fauna supports this inference in that it is dominated by forms that can enter the seabed and thereby escape adverse conditions on the sediment surface

iron isotopes in hot springs along the juan de fuca ridge we present fe isotope data for high temperature hydrothermal fluids sulfides and basalts from the juan de fuca ridge usgs basalt standard bcr 2 and the canyon diablo iron meteorite the fe isotopic compositions of all samples fall within a range of 1 however there is a small but statistically significant isotopic variation between fe dissolved in hydrothermal fluids and fe in canyon diablo specifically the hydrothermal fluids are lighter compared to canyon diablo yielding 56fe 56fe 54fe sample 56fe 54fe canyon diablo 1 1000 values from 0 30 to 0 77 in contrast the bcr 2 basalt and canyon diablo are isotopically indistinguishable measurements of 56fe in juan de fuca ridge basalts and sulfides suggest that fe isotopes are fractionated in hydrothermal systems however the data do not exclude the possibility of small 56fe differences between the rocks from which fluid fe is derived and other igneous fe in which case fractionation in igneous systems is indicated regardless these results are evidence of non biological mass fractionation of fe isotopes in nature further venting of hydrothermal fluids into the overlying water column could provide a source of isotopically light fe to the deep oceans contributing to fe isotopic variations observed in deep sea sediments

groundmass crystallization of mount st helens dacite 1980 1986 a tool for interpreting shallow magmatic processes the 1980 1986 eruption of mount st helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale groundmass plagioclase size measurements are coupled with measured changes in matrix glass plagioclase and fe ti oxide chemistry to document increasing groundmass crystallinity and thus to better constrain proposed physical models of the post may 18 1980 magmatic reservoir measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth approximately 10 9 cm s of groundmass plagioclase occurred immediately subsequent to may 18 relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10 11 cm s plagioclase growth rates then decreased to 1x10 11 cm s through 1986 changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst rim chemistry although the matrix glass composition appears to have remained approximately constant from 1981 1986 after a rapid compositional change from may 18 until the end of 1980 plagioclase microphenocrysts show increasingly more complex zoning patterns with time microphenocryst core compositions are commonly positively correlated with crystal size both of these observations indicate continuous groundmass plagioclase growth through 1986 magmatic temperatures estimated from fe ti oxide pairs are approximately constant through 1981 at eruption temperatures of 930 c and at log fo2 of 10 8 by 1985 1986 oxide temperatures decreased to 870 c chemical and textural changes can be explained by 1 rapid degassing and crystallization in response to the intrusion of magma into a shallow 4 5 km reservoir toward the end of the may 18 1980 eruption 2 continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir growth rates and consequent chemical changes appear to decrease at the end of 1980 this is coincident with the change in eruption style from explosive eruptions sometimes followed by dome growth to solely extrusive dome building events and can be explained by the expected viscosity increase of both degassing and increasing crystallinity the model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies casadevall et al 1983 gerlach and casadevall 1986 a b patterns of crater deformation chadwkck et al 1988 and post 1980 seismicity endo et al 1990 although it does not explain the experimental data of hill and rutherford 1989 on the growth rate of amphibole reaction rims textural measurements on mount st helens dacite can also be used to evaluate crystallization kinetics in silicic magmas systems for which experimental data is almost non existent plagioclase growth rates are 5 10 times slower than estimated plagioclase growth rates in basaltic systems a result consistent with the higher viscosity of a more silicic melt furthermore patterns of textural change both average crystal size and number density are similar to those observed during the 1984 mauna loa eruption by lipman and banks 1987 suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates such that the rate of crystallization scales with the melt viscosity

magmatic model for the mount st helens blast of may 18 1980 analytical and numerical solutions to the hydrodynamic equations of motion constrained by physical properties of juvenile ejecta in the mount st helens blast deposit were used to investigate magmatic conditions required to produce the initial devastating blast phase of the eruption of may 18 1980 evidence that the blast was magmatic includes equivalence in volume of juvenile blast ejecta to preemption inflation of the cone substantial vesicularity of this ejecta and continued vesiculation of large juvenile clasts after eruption observed or inferred ejecta velocities of 100 to 250 m s are shown to require 0 2 to 0 7 wt water vapor preexisting in magma unloaded by a landslide 200 to 900 m thick these conditions imply total magmatic water contents of 0 7 to 1 7 wt respectively such low required water content suggests that volcanic blasts may be regarded as a normal consequence of magma intrusion into an unstable edifice

crystal size distribution csd in rocks and the kinetics and dynamics of crystallization crystal size distributions csds measured in metamorphic rocks yield quantitative information about crystal nucleation and growth rates growth times and the degree of overstepping t of reactions during metamorphism csds are described through use of a population density function n dn dl where n is the cumulative number of crystals per unit volume and l is a linear crystal size plots of ln n vs l for olivine pyroxene and magnetite in high temperature 1000 c basalt hornfelses from the isle of skye define linear arrays indicating continuous nucleation and growth of crystals during metamorphism using the slope and intercept of these linear plots in conjunction with growth rate estimates we infer minimum mineral growth times of less than 100 years at t 10 c and nucleation rates between 10 4 and 10 1 cm3 s garnet and magnetite in regionally metamorphosed pelitic schists from south central maine have csds which are bell shaped we interpret this form to be the result of two processes 1 initial continuous nucleation and growth of crystals and 2 later loss of small crystals due to annealing the large crystals in regional metamorphic rocks retain the original size frequency distribution and may be used to obtain quantitative information on the original conditions of crystal nucleation and growth the extent of annealing increases with increasing metamorphic grade and could be used to estimate the duration of annealing conditions if the value of a rate constant were known finally the different forms of crystal size distributions directly reflect differences in the thermal histories of regional vs contact metamorphosed rocks because contact metamorphism involves high temperatures for short durations resulting csds are linear and unaffected by annealing similar to those produced by crystallization from a melt because regional metamorphism involves prolonged cooling from high temperatures primary linear csds are later modified by annealing to bell shapes

zn fe and s isotope fractionation in a large hydrothermal system abstract the genesis of hydrothermal ore deposits is of crucial economic importance this study investigates the extent causes and consequences of zinc and iron isotope fractionation in a large hydrothermal system at the world class navan zn pb orebody ireland large variations in zn fe and s isotope compositions have been measured in microdrilled sphalerite zns at the millimetre scale 66 zn and 56 fe display a well defined positive correlation and both also correlate with 34 s these relationships represent the combined effects of kinetic zn and fe isotope fractionation during sphalerite precipitation and s isotope variation through mixing of hot metal rich hydrothermal fluids and cool bacteriogenic sulfide bearing brines combined with s isotope data 56 fe and 66 zn data on mine concentrates confirm that hydrothermal sulfide is a minor component of the overall deposit signature our data suggest that incoming pulses of metal rich hydrothermal fluid triggered sulfide mineralisation and that rapid precipitation of sphalerite from hydrothermal fluids will lead to strong kinetic fractionation of zn and fe isotopes at very short time and length scales thereby limiting the use of fe and zn isotopes as exploration tools within deposits but revealing the possibility of detecting new deposits from isotopically heavy zn fe geochemical halos

rb sr sm nd and pb isotope systematics of pyrite implications for the age and genesis of lode gold deposits lode gold deposits commonly consist of pyrite and lesser amounts of lead and zinc sulfides accompanied by quartz and calcite their exact origin remains controversial mainly because of the scarcity of reliable geochronological data here we present direct rb sr dating of pyrite from a lode gold deposit and propose the method as a useful geochronological technique for gold mineralization for which age data are poor a positive correlation between present day 87 sr 86 sr and 87 rb 86 sr ratios of pyrite fes 2 subsamples from the linglong gold mine of china corresponds to an age of 122 123 ma which dates the age of gold mineralization the sr nd and pb isotopic compositions of pyrites and the associated rocks suggest that the hydrothermal fluids responsible for the pyrite and gold mineralization were probably derived from a mixed source i e degassing of mafic magmas and meteoric water that had leached the country rocks

nature age and tectonic setting of granitoid hosted orogenic gold deposits of the jiaodong peninsula eastern north china craton china the jiaodong gold province along the southeastern margin of the north china craton has combined production and resources of 900 tonnes of gold more than 95 of the gold in this province is hosted in mesozoic granitoids which intruded late archean metamorphic basement rocks episodically at ca 160 150 and 130 126 ma the deposits of the jiaodong peninsula form one of the largest provinces of granitoid hosted lode gold deposits in the world the jiaodong gold deposits occur as massive auriferous quartz veins hosted in subsidiary second or third order faults and as disseminated and stockwork style mineralization hosted in alteration zones along major regional scale faults mineralization took place during brittle reactivation d3 of pre existing d2 ductile shear zones wall rock alteration is dominated by k feldspar and sericite with also sulfide minerals and extensive silicification closest to the orebodies the alteration was accompanied by the introduction of volatiles fe mg k rb ag as au bi and sb enrichment of base metals in the gold lodes is generally low although locally significant gold deposition took place at temperatures of 250 to 350 c for most gold deposits and the ore fluids are co2 bearing and have calculated salinities of 6 13 wt nacl equiv deposits from the zhao ye xixia and muping rushan belts have similar if not identical structural controls alteration patterns ore mineral assemblages and stable isotope signatures lead isotope data indicate that the ore stage leads were buffered by their host rocks the granitoid hosted gold deposits in the zhao ye belt are most radiogenic the greenstone hosted deposits in the xixia belt is the least radiogenic and the granitoid hosted deposits in the muping rushan belt have intermediate compositions gold mineralization in the zhao ye xixia and muping rushan belts in the jiaodong gold province appears to be broadly contemporaneously at ca 130 120 ma as constrained by shrimp u pb zircon dating on host rocks and a post gold dike as well as by ar ar rb sr and k ar dating of hydrothermal alteration minerals and ores this early cretaceous age is about 100 million years younger than the peak of ultra high pressure metamorphism in the qinling dabie sulu orogen between the north china and yangtze cratons and about 500 km south of the gold province however it overlaps with the extension and exhumation stage of the collisional orogeny and the subduction of the pacific plate beneath the eurasian continental crust whereas the majority of the jiaodong gold deposits occur on the margin of the north china craton some also occur in the suture zone between the two cratons they are significantly landward relative to the subduction zone of the pacific plate compared with typical orogenic lode gold deposits globally lode gold deposits of ca 130 120 ma occurred on both sides of the tan lu fault zone available data favor a temporal link of gold mineralization with pacific plate subduction

geochronology and origin of the sarekoubu gold deposit southern altaides china ar ar dating of vein quartz and cu zn fe isotopes abstract the sarekoubu gold deposit located north to altay city china has an unusual feature for abundant carbonic co2 ch4 n2 bearing fluid inclusions for many years the ages and origin of the sarekoubu deposit have been still controversial the gold deposit occurs in the fractured zone parallel to the strata of lower devonian kangbutiebao formation d1k22 and has three hydrothermal mineralizing stages i disseminated pyrite and white quartz vein stage ii fine quartz and pyrite veinlet stage and iii polymetallic sulfides grey quartz stage stages ii and iii are the main gold mineralization stages two gold bearing chalcopyrite quartz vein samples from stage iii were selected for 40ar 39ar age study the result shows that fluid inclusions in quartz of mineralizing stage have isochronal ages of 216 9 6 7 ma and 218 0 6 3 ma and inverse isochronal ages of 218 3 6 1 ma and 216 7 6 4 ma the fe isotope analysis shows that the sarekoubu gold deposit is different from vms sedex deposits the 56fe values of pyrite from main mineralization stages in the sarekoubu gold deposit have a large variation of fe isotope range from 0 498 to 0 759 which can be regarded as a closed hydrothermal system for fe isotopes the 65cu values of chalcopyrite both from the sarekoubu gold deposit and overprint gold mineralization of adjacent tiemurte deposit have a narrow range 0 28 0 922 which are not the same as vms deposits with a wide range of 65cu values the gold mineralization in the sarekoubu deposit is younger than the gold deposits near regional irtysh tectonic belt this may be related with the evolution of the thrust of the irtysh tectonic belt in the southern altaides

iron isotopic fractionation during continental weathering abstract the effect of continental weathering on the iron isotope compositions of natural materials is investigated unweathered igneous rocks pelagic clay and dust fall within the range 56 fe 0 0 3 rivers with large suspended loads also have 56 fe values near zero dilute streams have 56 fe values that trend towards lower 56 fe 1 suggesting that dissolved riverine iron is isotopically light relative to igneous rocks bulk soil and soil leaches display systematically different 56 fe profiles indicating that isotopically distinct fe pools are generated during pedogenesis nannofossil ooze which contains fe scavenged from the ocean water column has 56 fe 0 but is consistent with seawater dissolved fe having negative 56 fe it is inferred that continental weathering under modern oxidizing earth surface conditions preferentially releases dissolved fe with negative 56 fe which is transported in rivers to the ocean a preliminary analysis of the marine fe budget suggests that riverine fe has a substantial role in determining the 56 fe of both the modern and ancient oceans but other inputs particularly that from diagenesis of marine sediments may also be important since the chemical pathways of fe processing during weathering are dependent on oxidation state and biological activity fe isotopes may prove useful for detecting changes in these parameters in the geologic past

geology and alteration geochemistry of the porphyry cu au deposit at bajo de la alumbrera argentina this paper describes the igneous geology alteration mineralogy and geochemistry as well as the ore mineralization history of bajo de la alumbrera a world class porphyry copper gold deposit in northwestern argentina the deposit occurs in the k rich calc alkaline farallon negro volcanic complex located 200 km east of the main andean porphyry copper belt of chile the deposit consists of a composite stock of dacitic porphyry intrusions and extends into the surrounding andesitic volcanic rocks two of the earliest intrusions now occupying the center of the intrusive complex show intense hydrothermal alteration and mineralization these two porphyries host the highest ore grades and were subsequently intruded by several weakly mineralized or barren porphyries intense quartz magnetite k feldspar alteration pervades the first mineralized porphyry whereas similar but lower intensity alteration affected the second laterally this alteration grades into potassic alteration secondary k feldspar and biotite which affects all porphyries except the latest dikes andesitic wall rocks within a hundred meters of the intrusions are dominated by dark hydrothermal biotite an outer halo of propylitic alteration epidote chlorite albite calcite extends up to 1 km into the andesites feldspar destructive alteration sericite pyrite clay minerals gypsum overprints the potassic and propylitic alteration in the volcanic rocks and all porphyries the alteration is locally controlled by faults and late fractures and developed most pervasively upon the outer part of the potassic zone and toward the transition to propylitic alteration in the upper part of the deposit mass balance calculations show small positive changes in net volume for the potassic alteration and minor volume decreases for the chlorite epidote and feldspar destructive alteration the intensely quartz magnetite veined sample of quartz magnetite alteration indicates a total volume increase of 350 percent copper and gold are intimately associated at all scales they occur as composite gold sulfide grains chalcopyrite rarely bornite show a close correlation of ore grades in sample assays and have a closely overlapping distribution on the mine scale a well developed ore shell coincides with potassic alteration and partly with the distribution of intense quartz magnetite veins even though the bulk of the ore minerals now occupy texturally late positions in vugs and fractures a barren zone with intense quartz magnetite potassic alteration and veins but little or no sulfides is interpreted as the focus of upflow of high temperature ore fluids prior to ore mineral saturation and coprecipitation of copper sulfides and gold in the ore shell

crystallization of mount st helens 1980 1986 dacite a quantitative textural approach quantitative measurements of crystal size distributions csds have been used to obtain kinetic information on crystallization of industrial compounds randolph and larson 1971 and more recently on hawaiian basalts cashman and marsh 1988 the technique is based on a population balance resulting in a differential equation relating the population densityn of crystals to crystal sizel i e at steady staten no exp l itg whereno is nucleation density g is the average crystal growth rate is the average growth time and the nucleation ratej nog csd inn vsl plots of plagioclase phenocrysts in 12 samples of mount st helens blast dacite and 14 samples of dacite from the 1980 1986 mount st helens dome are similar and averageg 9 6 1 1 10 3 cm andno 1 2 106 cm 4 reproducibility of the measurements was tested by measuring csds of 12 sections cut from a single sample in three mutually perpendicular directions precision of the size distributions is good in terms of relative but not necessarily absolute values 10 growth and nucleation rates for plagioclase have been calculated from these measurements using time brackets of 30 150 years growth ratesg are 3 10 10 12cm s and nucleation ratesj are 5 21 10 6 cm3 s g andj for fe ti oxides calculated from csd data areg 2 13 10 13 cm sec andj 7 33 10 5 cm3 s respectively the higher nucleation rate and lower growth rate of oxides resulted in a smaller average crystal size than for plagioclase sizes of plagioclase microlites 0 01 mm in the blast dacite groundmass have been measured from backscatter sem photographs nucleation of these microlites was probably triggered by intrusion of material into the cone of mount st helens in spring 1980 this residence time of 52 days gives minimum crystallization estimates ofg 1 3 10 11 cm s andj 9 16 1o3 cm3 s the skeletal form of the microlites provides evidence for nucleation and growth at high values of undercooling t relative to the phenocrysts a comparison of nucleation and growth rates for the two crystal populations phenocrysts vs microlites suggests that while growth rate seems to be only slightly affected by changes in t nucleation rate is a very strong function of undercooling a comparison of plagioclase nucleation and growth rates measured in the mount st helens dacite and in basalt from makaopuhi lava lake in hawaii suggests that plagioclase nucleation rates are not as dependent on composition groundmass textures suggest that plagioclase microphenocrysts crystallized at depth rather than in the conduit in the dome or after extrusion onto the surface most of this crystallization appears to be in the form of crystal growth coarsening of groundmass microphenocrysts at small degrees of undercooling rather than extensive nucleation of new crystals this continuous crystallization in a shallow magmatic reservoir may provide the overpressurization needed to drive the continuing periodic domebuilding extrusions which have been the pattern of activity at mount st helens since december 1980

copper isotopic zonation in the northparkes porphyry cu au deposit se australia abstract significant systematic cu isotopic variations have been found in the northparkes porphyry cu au deposit nsw australia which is an orthomagmatic porphyry cu deposit copper isotope ratios have been measured in sulfide minerals chalcopyrite and bornite by both solution and laser ablation multi collector inductively coupled plasma mass spectrometry mc icp ms the results from both methods show a variation in 65cu of hypogene sulfide minerals of greater than 1 relative to nist976 significantly the results from four drill holes through two separate ore bodies show strikingly similar patterns of cu isotope variation the patterns are characterized by a sharp down hole decrease from up to 0 8 0 29 0 56 1 n 20 in the low grade peripheral alteration zones phyllic propylitic alteration zone to a low of 0 4 0 25 0 36 1 n 30 at the margins of the most mineralized zones cu grade 1 wt in the high grade cores of the systems the compositions are more consistent at around 0 2 0 19 0 14 1 n 40 the cu isotopic zonation may be explained by isotope fractionation of cu between vapor solution and sulfides at high temperature during boiling and sulfide precipitation processes sulfur isotopes also show an isotopically light shell at the margins of the high grade ore zones but these are displaced from the low 65cu shells such that there is no correlation between the cu and s isotope signatures fe isotope data do not show any discernable variation along the drill core this work demonstrates that cu isotopes show a large response to high temperature porphyry mineralizing processes and that they may act as a vector to buried mineralization

integrated fe and s isotope study of seafloor hydrothermal vents at east pacific rise 9 10 n abstract in this study we report on coupled fe and s isotope systematics of hydrothermal fluids and sulfide deposits from the east pacific rise at 9 10 n to better constrain processes affecting fe isotope fractionation in hydrothermal environments we aim to address three fundamental questions 1 is there significant fe isotope fractionation during sulfide precipitation 2 is there significant variability of fe isotope composition of the hydrothermal fluids reflecting sulfide precipitation in subsurface environments 3 are there any systematics between fe and s isotopes in sulfide minerals the results show that chalcopyrite precipitating in the interior wall of a hydrothermal chimney displays a limited range of 56fe values and 34s values between 0 11 to 0 33 and 2 2 to 2 6 respectively the 56fe values are on average slightly higher by 0 14 relative to coeval vent fluid composition while 34s values suggest significant s isotope fractionation 0 6 0 2 during chalcopyrite precipitation in contrast systematically lower 56fe and 34s values relative to hydrothermal fluids by up to 0 91 and 2 0 respectively are observed in pyrite and marcasite precipitating in the interior of active chimneys these results suggest isotope disequilibrium in both fe and s isotopes due to s isotopic exchange between hydrothermal h2s and seawater so42 followed by rapid formation of pyrite from fes precursors thus preserving the effects of a strong kinetic fe isotope fractionation during fes precipitation in contrast 56fe and 34s values of pyrite from inactive massive sulfides which show evidence of extensive late stage reworking are essentially similar to the hydrothermal fluids multiple stages of remineralization of ancient chimney deposits at the seafloor appear to produce minimal fe isotope fractionation similar affects are indicated during subsurface sulfide precipitation as demonstrated by the lack of systematic differences between 56fe values in both high temperature fe rich black smokers and lower temperature fe depleted vents

iron isotopic evolution during fractional crystallization of the uppermost bushveld complex layered mafic intrusion we present 56fe 56fe 54fe relative to standard irmm 014 data from whole rock and magnetite of the upper and upper main zones uumz of the bushveld complex with it we assess the role of fractional crystallization in controlling the fe isotopic evolution of a mafic magma the uumz evolved by fractional crystallization of a dry tholeiitic magma to produce gabbros and diorites with cumulus magnetite and fayalitic olivine despite previous experimental work indicating a potential for magnetite crystallization to drastically change magma 56fe we observe no change in whole rock 56fe above and below magnetite saturation we also observe no systematic change in whole rock 56fe with increasing stratigraphic height and only a small variation in 56fe in magnetite separates above magnetite saturation whole rock 56fe errors twice standard deviation 2 throughout the uumz ranges from 0 01 0 03 to 0 21 0 09 56feaveragewr 0 10 0 09 n 21 isotopically light outlier 56fewr 0 15 and magnetites range from 0 28 0 04 to 0 86 0 07 56feaveragemgt 0 50 0 15 n 20 similar to values previously reported for other layered intrusions we compare our measured 56fewr to a model that incorporates the changing normative mineralogy calculated temperatures and published fractionation factors of fe bearing phases throughout the uumz and produces 56fewr values that evolve only in response to fractional crystallization our results show that the fe isotopic composition of a multiply saturated multiple phases on the liquidus magma is unlikely to change significantly during fractional crystallization of magnetite due to the competing fractionation of other fe bearing cumulus phases this article is protected by copyright all rights reserved

collision between the north and south china blocks a crustal detachment model for suturing in the region east of the tanlu fault a crustal detachment model based on the interpretation of linear aeromagnetic anomalies surface geological observations and deep seismic profiles is proposed for the continent continent collision between the north and south china blocks east of the tanlu fault the model suggests that during the mid mesozoic collision between the two continental blocks the upper crust of the south china block in the subei yellow sea region was detached from the lower crust and thrust over the north china block for 400 km whereas the lower part of the lithosphere was subducted under the north china block along a subsurface suture running east from nanjing the sinistral offset of the qinling suture by the tanlu fault is only 110 120 km in the deep crust much less than previously suggested

syneruptive mixing degassing and crystallization at redoubt volcano eruption of december 1989 to may 1990 abstract the 1989 1990 eruption of redoubt volcano included initial energetic vertical venting of pumiceous tephra followed by shedding of dense pyroclastic flows from successive unstable domes the suite of quenched magmatic materials thus produced coupled with detailed seismic observations provides a record of establishment and subsequent evolution of an active magma column in the upper crust the initial eruptive event involved a dacite magma with rhyolite melt and an andesite magma with dacite melt the dacite magma contains reaction rim free hornblende high cl concentrations 0 12 0 02 wt of melt and is microlite free all evidence for rapid ascent from depth in contrast the andesite component is microlite rich though this characteristic may have arisen by mixing between the two magmas rather than by slow ascent abundant dense juvenile lava like clasts with low h2o 0 1 0 3 wt were ejected in the initial eruption event implying that the magma column had reached the near surface and its upper portion had undergone degassing and bubble collapse prior to explosive disruption later eruptive events contain evidence of progressive blending of the two magmatic components as well as mixing between fresh reservoir derived and old conduit resident magma magma apparently ascended to the surface from the 6 10 km deep source chamber in a series of pulses rather than by steady flow or gradual emptying of a shallowly intruded body increased shallow long period seismicity which may indicate subsurface degassing from the flowing magma column occurred 5 days before each major eruptive event petrologic evidence indicates that each eruption contained some magma that ascended quickly from the chamber with time the proportion of new magma in eruptive batches may have decreased however even the final dome contains magma that left the chamber no more than a few days before extrusion together the pulses apparently record passage of an andesite plume through redoubt s dacite magma reservoir

magma fragmentation by rapid decompression the processes leading to magma fragmentation and the generation of pyroclastic debris during explosive volcanic eruptions are of fundamental importance in volcanology observations of explosive eruptions1 3 as well as theoretical analyses of the underlying processes4 have raised the question of whether the rapid decompression of highly viscous vesicular magma that results from the collapse of a lava dome or volcanic edifice can in itself produce explosive magma fragmentation and pyroclast formation here we report the results of a laboratory investigation of pyroclast formation following rapid decompression samples of magma from the 1980 eruption of mount st helens when rapidly depressurized from initial pressures of up to 12 mpa and at temperatures in the range 750 825 c fragmented to form pyroclastic products that are in many respects similar to those formed in real eruptions moreover we observe explosive fragmentation at temperatures well below those normally associated with magmatic processes suggesting that even relatively cool magma bodies can be very hazardous when subjected to rapid unloading events

biodiversity of suprabenthic peracarid assemblages from the blanes canyon region nw mediterranean sea in relation to natural disturbance and trawling pressure abstract blanes canyon and its adjacent margin are important fishery areas mainly by bottom trawling located in a highly energetic oceanographic setting in the nw mediterranean sea here we assess the spatial and temporal variability in abundance diversity and community structure of the suprabenthic peracarid assemblages in this region and examine this variability in relation to the natural and anthropogenic trawling fisheries disturbance regimes the sampling was conducted between march 2003 and may 2004 in three main fishing grounds the canyon head average depth 490 m the canyon wall average depth 550 m and the eastern adjacent slope average depth 820 m as well as in two non exploited areas in the western at 900 m depth and eastern at 1500 m depth slope near the canyon mouth a total of 138 species were identified with amphipods being the most speciose and abundant group followed by mysids in terms of abundance our results show high spatial and temporal variability in suprabenthic assemblages densities were higher in the canyon head and western slope which appear to be the preferential routes for water masses and particle fluxes in months of flood events and other energetic processes in the canyon head where periodic erosion processes are more active low diversity high dominance and higher turnover diversity were observed apparently coupled with significant temporal fluctuations in the densities of the highly motile component of suprabenthos mysids predatory and scavenging amphipods in the sedimentary more stable eastern slope high diversity values were observed accompanied by a higher relative contribution of the less motile groups i e amphipods most isopods cumaceans these groups have a closer interaction with the sediment where they exploit different food sources and are more susceptible to physical disturbance temporal variability in their diversity may be related to changes in food quality rather than quantity in the canyon wall temporal fluctuations in diversity indices were only revealed in relation to the overall higher and more continued fishing pressure observed in the canyon wall fishing ground cara norte sot site here species richness and abundance declined with increasing fishing pressure but the lowest trophic and taxonomic diversities were observed under intermediate levels of disturbance these findings underline i the differences between relatively low and highly motile taxa in terms of response to disturbance events ii the differences between assemblages subjected to different levels of natural disturbance and trawling pressure which modify the common bathymetric patterns of abundance and diversity often described from continental margins

contrasting origins of late mesozoic adakitic granitoids from the northwestern jiaodong peninsula east china implications for crustal thickening to delamination two suites of granitoids the late jurassic 158 3 ma linglong suite and the early cretaceous 130 126 ma guojialing suite crop out in the northwestern jiaodong peninsula eastern china the linglong suite is a monzogranite comprising alkali feldspar plagioclase quartz and fe rich biotite the guojialing suite includes at least five plutonic bodies of both granodiorite and monzo granite the rocks are composed of plagioclase alkali feldspar quartz mg rich amphibole and mg rich biotite both the linglong and guojialing suites have adakitic affinity they are enriched in lree with high la yb ratios and show positive eu anomalies the rocks are also enriched in lile and depleted in hfse with high sr y ratios the linglong granite shows very uniform sr nd isotopic compositions with initial 87 sr 86 sr ratios of 0 7119 0 7126 and e nd t values of 21 3 to 21 6 which are similar to those of the local neoarchaean basement the guojialing suite has variable initial 87 sr 86 sr ratios 0 7108 0 7120 and e nd t values 10 8 to 17 2 which are distinct both from those of the neoarchaean basement and from those of the local enriched lithospheric mantle inferred from the coeval mafic dykes in the studied area detailed petrological and geochemical data indicate that the linglong suite was derived by partial melting of neoarchaean metamorphic lower crustal rocks at depth of 50 km with a eclogite residue whereas the guojialing suite was formed by the reaction of delaminated eclogitic crust derived melt with the upwelling asthenospheric mantle the petrogenesis of these two contrasting adakitic granitoids suggests intensive lower crustal delamination during early cretaceous times following a crustal thickening process from the late stage of the early jurassic to early stage of the late jurassic with crustal thickness of 50 km respectively

iron isotope fractionation during skarn type alteration implications for metal source in the han xing iron skarn deposit abstract the han xing iron mineralization in the central north china craton is a typical fe skarn deposit associated with altered diorites here we report the fe isotopic compositions of whole rocks and mineral separates from this deposit with a view to evaluate the fe isotope fractionation during the formation of fe skarn deposit and to constrain the metal source the fe isotopes show a large variation both in whole rocks and mineral separates altered diorites show a wide range in 56 fe values 0 07 to 0 21 relative to the fe isotope standard irmm 014 which positively correlate with their tfe 2 o 3 tio 2 ratios fe 2 o 3 and feo calculated as tfe 2 o 3 the positive correlation indicates that heavy fe isotopes were preferentially leached from diorites during the skarn type alteration among the metallic minerals pyrite and pyrrhotite are isotopically heavier 0 12 to 0 48 than the magnetite 0 07 to 0 21 fe isotope fractionation between mineral pairs demonstrates that magnetite did not attain fe isotopic equilibrium with pyrite and pyrrhotite whereas pyrite and pyrrhotite might have attained isotopic equilibrium petrological observations and major element data also suggest that iron was leached from the diorites during the skarn type alteration if the leached iron provides the main fe budget of the han xing fe skarn deposit magnetite in ores would be isotopically heavier than the unaltered diorite however our results are in contrast with the magnetite being isotopically lighter than the unaltered diorite this suggests that the major fe source of the han xing fe skarn deposit is not from the leaching of diorites and might be from magmatic fluid which is isotopically lighter than the silicate melt our data demonstrate that fe isotopes can be used as important tracers in deciphering the metal source of fe skarn deposits

slope transport of suspended particulate matter on the aquitanian margin of the bay of biscay abstract spatial and temporal characteristics of the water masses and the dispersion of the suspended particulate matter were investigated using current meter hydrographic and nephelometric observations gathered during the ecofer experiment 1989 1991 in the cap ferret canyon on the aquitanian margin of the bay of biscay while characteristics of the deep water masses were stable from one year to another large hydrographic change in the upper 500 m related to winter renewal induced by poleward advection of warm and saline water along the continental slope the slope circulation and seasonal eddy activity appear as important dynamical mechanisms that control the entrainment and the dispersion of the suspended particulate matter from the neritic domain to the deep ocean a predominantly northward along slope current with occasional reversal characterizes this circulation the nephelometric structures also showed seasonal changes in the overall suspended particulate matter content but recurrent features such as the presence of intermediate nepheloid layers at the shelf break depth and various depths along the slope 500 1000 and 2000 m were observed these nepheloid layers extended off the slope to about 10 30 km but especially laterally along the slope their presence indicated that suspended particulate matter exchanges between the shelf and the slope occurred mainly in the head of the canyon and along the southern open slope the intermediate nepheloid layers around 500 m depth detached from the slope particularly in regions where the bottom slope is close to critical for the m2 internal tide

experimental determination of fe isotope fractionation between aqueous fe ii siderite and green rust in abiotic systems abstract iron isotope fractionation between aqueous iron and siderite has been measured using abiotic synthesis experiments at 20 c applying a rayleigh distillation model to three experiments in which siderite feco3 was continuously precipitated from solutions over the course of tens of hours the derived fractionation factor is 103ln fe ii siderite 0 48 0 22 2 this experimentally determined fractionation factor is significantly smaller by 1 5 than that calculated from vibrational frequencies and mossbauer shifts geochim cosmochim acta 64 2000 849 865 geochim cosmochim acta 65 2001 2487 2497 and lies on the high end of that inferred from natural mineral assemblages where the fe ii aq siderite fractionation factor was inferred to lie between 1 7 and 0 3 contrib mineral petrol 144 2003 523 547 the measured fe ii aq siderite fractionation from this study suggests that siderite from archean and proterozoic rocks which have moderately negative 56fe values 1 5 to 0 5 may reflect precipitation from fe ii aq whose origin was hydrothermal in contrast natural siderites that have higher 56fe values apparently require non hydrothermal sources of fe and or different pathways of formation or may reflect the effects of cation substitution on the fe ii aq carbonate fractionation factor

fe isotope fractionation in magmatic hydrothermal mineral deposits a case study from the renison sn w deposit tasmania abstract we present 50 new iron isotopic analyses of source granite and mineral separates from the renison tin deposit in western tasmania the aim of the study is to characterise the composition of minerals within a tin deposit associated with a reduced s type magma we have analysed bulk samples of granite and separates of pyrrhotite pyrite arsenopyrite magnetite chalcopyrite and siderite by multi collector inductively coupled mass spectrometry the isotopic compositions of mineral separates are consistent with theoretical predictions of equilibrium fractionation based on mossbauer spectroscopy and other parametric calculations mineral mineral pairs yield temperatures of formation that are in agreement with prior detailed fluid inclusion studies but are spatially inconsistent with declining fluid temperatures with distance from the causative intrusion limiting the use of fe isotopes as a potential geothermometer at least in this case comparison of our data with published data from other deposits clearly demonstrates that pyrite magnetite and chalcopyrite from the hottest ore fluids 300 400 c at renison are isotopically heavier than minerals sampled from a deposit formed at similar temperatures but associated with a more oxidised and less differentiated intrusion

secular variation of iron isotopes in north atlantic deep water a high precision iron isotope time series for a ferromanganese crust demonstrates that the iron isotope composition in north atlantic deep water has changed substantially over the past 6 million years and that iron isotope variations in the crust are closely correlated to those of lead isotopes the close correlation between the two isotope series indicates that the observed iron isotope variations predominantly reflect those of iron input from terrigenous sources and provides no evidence for biologically induced mass fractionation within north atlantic deep water

grain size distribution of volcaniclastic rocks 1 a new technique based on functional stereology the power of explosive volcanic eruptions is reflected in the grain size distribution and dispersal of their pyroclastic deposits grain size also forms part of lithofacies characteristics that are necessary to determine transport and depositional mechanisms responsible for producing pyroclastic deposits however the common process of welding and rock lithification prevents quantification of grain size by traditional sieving methods for deposits in the rock record here we show that functional stereology can be used to obtain actual 3d volume fractions of clast populations from 2d cross sectional images tests made on artificially consolidated rocks demonstrate successful correlations with traditional sieving method we show that the true grain size distribution is finer grained than its representation on a random 2d section our method allows the original size of vesicular pumice clasts to be estimated from their compacted shapes we anticipate that the original grain size distribution of welded ignimbrites can also be characterized by this method our method using functional stereology can be universally applied to any type of consolidated weakly to non deformed clastic material regardless of grain size or age and therefore has a wide application in geology

kinetics of crystallization of igneous rocks the geochemistry of igneous rocks is discussed with the primary objectives of bringing together the theories underlying the kinetics of crystallization of igneous rocks and illustrating the use of these theories in understanding experimental and observational data the primary purpose of the chapter is to introduce current thinking about the kinetics of igneous rocks and to provide a basis for understanding other work a basic assumption made in the discussion is that the rate of any chemical reaction including the crystallization of igneous rocks is zero at equilibrium and proceeds at a finite rate only at a finite deviation from equilibrium as such an understanding of the processes operating in igneous rocks requires an understanding of how deviation from equilibrium affects the rates and mechanisms of the processes occurring during crystallization these processes are detailed with special emphasis given to nucleation and crystal growth jmt

sulfur isotopes in sediment hosted orogenic gold deposits evidence for an early timing and a seawater sulfur source we report sulfur isotopic compositions of sulfides of various paragenetic stages in the giant sukhoi log sediment hosted orogenic au deposit in russia the overall mean value and the significant variability in early pyrite indicate that the sulfur was from the reduction of seawater sulfate the later generations of sulfide have 34s values in successively smaller ranges coincident with the mode that is around the median value of the whole data set together with textural evidence sulfide trace element data and gold occurrence the data demonstrate that metamorphism has gradually homogenized the early sulfur accompanied by the segregation of quartz and the release of au from the lattice of early pyrite and its reprecipitation as inclusions in later pyrite the s isotopic compositions of sulfides in sukhoi log and many other major orogenic au deposits hosted in sedimentary rocks of various ages show a pattern generally parallel to the seawater sulfate curve through geologic time indicating that the sulfur in most sediment hosted orogenic au deposits was probably also originally from the reduction of seawater sulfate we conclude that sulfidation and gold mineralization in many sediment hosted orogenic gold deposits was early during basin evolution when seawater was the principal active fluid rather than later during or after basin inversion as proposed in current models

iron isotope fractionation during crystallization and sub solidus re equilibration constraints from the baima mafic layered intrusion sw china abstract to better understand fe isotope fractionation between mafic minerals and oxides during crystallization of mafic magma and sub solidus re equilibration the fe isotopes of whole rocks and separated minerals olivine clinopyroxene magnetite and ilmenite of the baima mafic layered intrusion sw china have been investigated the separated minerals show a systematical decrease in 57 fe values from magnetite 0 15 to 0 51 to olivine 0 11 to 0 15 and clinopyroxene 0 35 to 0 05 and then to ilmenite 0 82 to 0 10 demonstrating regular fractionation between these minerals except for a few of samples most of the olivine and clinopyroxene are similar to those of mantle xenoliths in fe isotopes indicating that fe isotope equilibrium reached during magma crystallization was well preserved by contrast the fe isotopes of the magnetite and ilmenite may be evidently modified by sub solidus re equilibration via the fe 3 versus ti 4 and fe 2 exchange between the oxides furthermore the sub solidus re equilibration in fe isotope is strongly controlled by the proportions of magnetite and ilmenite in rocks therefore although the 57 fe ilm of the lower zone rocks with magnetite ilmenite ratios as high as 6 10 was reduced evidently by the sub solidus re equilibration the magnetite preserved their original fe isotope compositions by contrast the fe isotopes of both most magnetite and ilmenite in the middle zone had been markedly modified by sub solidus re equilibration owing to the moderate magnetite ilmenite ratios 4 7 the decreases of both 57 fe mt and 57 fe ol upwards in the cyclic units of the lower zone reveal that extensive early fractional crystallization of the magnetite resulted in depletion of heavier fe isotopes in the magma on the other hand early crystallization of olivine and clinopyroxene gave rise to the slight elevation of 57 fe ol values upwards in the cyclic units of the middle and upper zones the stratigraphic reversals in the 57 fe mt and 57 fe ol values suggest multiple magma recharges additionally the 57 fe ol values in the base of the lower zone 0 10 to 0 15 indicate that the parental magma were heavy in fe isotope due to extensive silicate mineral fractionation at depth this study indicates fractionation in fe isotope between silicates and oxides during magma crystallization and sub solidus re equilibration

textural and chemical variation in plagioclase phenocrysts from the 1980 eruptions of mount st helens usa this study presents major and trace element chemistry of plagioclase phenocrysts from the 1980 eruptions of mount st helens volcano despite the considerable variation in textures and composition of plagioclase phenocrysts distinct segments have been cross correlated between crystals the variation of sr and ba concentration in the melt as calculated from the concentration in the phenocrysts using partition coefficients suggests the cores and rims crystallised from compositionally different melts offset by the plagioclase crystallisation vector in both of these melts sr and ba are correlated despite the abundance of plagioclase in the 1980 dacites we propose that rapid crystallisation of plagioclase upon magma ascent caused a shift in melt composition towards lower sr and higher ba as documented in the rims of the phenocrysts although the cores of the phenocrysts crystallised at relatively shallow depths they preserve the sr and ba of the deep seated melts as they ascended from a deeper region further magma ascent resulted in microlite nucleation which is responsible for a similar shift to even lower sr concentration as observed in the groundmass of post 18 may 1980 samples

fe isotope behaviours during sulfide dominated skarn type mineralisation abstract fe isotope compositions of mineral separates and bulk samples from dongguashan and fenghuangshan skarn type cu fe deposits in tongling district middle lower yangtze valley cu fe au mo polymetallic belt have been investigated differences in 57 fe values exist in co precipitated bornite chalcopyrite and pyrite chalcopyrite mineral pairs the bornite is enriched in light iron isotopes relative to coexisting chalcopyrite whereas pyrite samples show a tendency of heavy isotopes enrichment relative to chalcopyrite moreover the 57 fe values of endoskarn and the earliest formed fe mineral phase magnetite of the three deposits investigated in tongling district are consistently lower relative to ore related igneous rocks suggesting fe isotopic fractionation occurs during fluid exsolution and exsolved fluid is enriched in light fe isotopes relative to the stock furthermore spatial and temporal variations in 57 fe values within single mineral phases are observed suggesting iron isotopic fractionation occurs between mineral and fluid phases as a result of precipitation of fe bearing minerals during fluid evolution overall according to fe isotope compositions of three representative skarn deposits formed under the same geological setting in tongling district this study indicates that fe isotopes have the potential to unraveling ore forming processes as well as constraining the metal sources of skarn deposits combining with results of previous studies a protocol for using fe isotopes to trace skarn type metallogeny is proposed

mixed magma pyroclastic surge deposits associated with debris avalanche deposits at colima volcanoes mexico the transition between the terminal cones and the ancestral edifices of nevado de colima and fuego de colima volcanoes is marked by the deposits of gigantic volcanic debris avalanches of the mount st helens msh or bezymianny type unusual mafic juvenile fragments and cauliflower bombs as well as juvenile fragments of mixed and more evolved composition are abundant in dune bedded pyroclastic surge deposits directly associated with these catastrophic events at both volcanoes at nevado these mafic juvenile fragments represent the most primitive magma ever erupted by the volcano sio2 52 50 the lavas directly preceding and following the debris avalanche event are silicic andesites sio2 59 at fuego these juvenile fregments have 56 sio2 the lavas from the upper parts of the caldera wall are dacites 65 sio2 whereas the terminal cone is composed of andesites 57 to 62 sio2 at nevado petrologic evidence for interaction of mafic magma with andesitic or dacitic magma in a high level magma chamber just before the eruption of pyroclastic surge deposits consists of 1 banded juvenile bombs of intermediate composition 2 the range of composition of these bombs from sio2 52 to 58 3 the presence of highly magnesian olivine with reaction rims 4 inverse zoning in clinopyroxene with strong mg enrichment towards the rim 5 resorption of plagioclase and 6 significant compositional heterogeneity in the vitric phase volcanic debris avalanche events at nevado and fuego de colima may thus correspond with major breaks in the petrological evolution of the volcanoes and the start of a new magmatic cycle injection of mafic magma into the presently perched viscous surface dome of the active fuego cone as occurred in 1818 and 1913 could enhance the likelihood of southward collapse of the flank of an already unstable edifice and it must be considered in future hazard assessment of this active volcano risk to life and property for the entire colima region associated with such catastrophic phenomena would be immeasurably greater in comparison with hazards related to the last explosive outburst in 1913 which resulted in emplacement of pyroclastic flows over uninhabited areas of the upper flanks of the volcano

geochronology and geochemistry of metamorphic rocks in the jiaobei terrane constraints on its tectonic affinity in the sulu orogen abstract tectonic affinity of tectono lithological units close to ultrahigh pressure metamorphic belt is a key issue for understanding the geodynamics of continental collision this is particularly so for the jiaobei terrane northeast of the dabie sulu orogenic belt in china new data from la icpms zircon u pb dating whole rock elements and nd sr isotopes and mineral o isotopes are presented for metamorphic rocks from this terrane the results place geochronological and geochemical constraints on their protolith nature and metamorphic timing and thus on its tectonic affinity to one of the two triassic collided continents the north and the south china blocks protolith ages for ttg gneiss amphibolite and mafic granulite are 2 7 2 5 and 2 4 ga respectively regional metamorphism took place extensively at 1 76 ga protolith of the ttg gneiss was generated by partial melting of mantle derived rocks at the root of a thickened crust protolith of the amphibolite was probably a product of arc like magmatism protolith of the mafic granulite was derived from a depleted mantle source both of them were locally contaminated by supracrustal materials protoliths of paragneiss and schist in the fenzishan group were mostly derived from supracrustal sources but protolith of amphibolite in the fenzishan group is of mantle derived signature unlike the uhp metaigneous rocks in the dabie sulu orogenic belt that show unusual 18 o depletion the jiaobei metamorphic rocks have basically preserved their original mantle like o isotope compositions in general the nature and timing of geological events recorded in the metamorphic rocks from the jiaobei terrane are comparable with those from the north china block rather than the south china block thus the jiaobei terrane is concluded to have tectonic affinity to the former but behave like a micro continent during the triassic continental collision the 1 76 ga regional metamorphism in the jiaobei terrane is likely related to reworking of the arc continent collisional orogen in the periphery of the north china block rather than the 1 85 ga collision event between the eastern and western north china blocks the present study lends support to the common assumption that the suture boundary between the north and south china blocks in the sulu orogen is located along the wulian yantai fault tectonic mingling along the wulian yantai fault is probably related to subduction erosion during the continental collision

monitoring the 1980 1982 eruptions of mount st helens compositions and abundances of glass the mount st helens eruptive sequence of 1980 through 1982 reflects the tapping of successively less water rich more highly crystallized and more viscous highly phyric dacitic magmas these changes reflect both syn and preeruption processes the decreasing water content points to a continued decline in the volume and intensity of explosive pyroclastic activity this decreasing water content appears to be composed of a long term trend established during a long period of repose about 130 years imposed on short term trends established during short periods about 7 to 100 days of repose between eruptions in the present eruptive cycle the last two eruptive cycles of this volcano the t a d 1800 and w cycles about a d 1500 exhibited similar trends these changes are inferred from a combination of petrographic bulk chemical and electron and ion microprobe analyses of matrix and melt inclusion glasses

iron and lithium isotope systematics of the hekla volcano iceland evidence for fe isotope fractionation during magma differentiation abstract in this study potential iron isotope fractionation by magmatic processes in the earth s crust was systematically investigated high precision iron isotope analyses by mc icp ms were performed on a suite of rock samples representative for the volcanic evolution of the hekla volcano iceland the whole series of hekla s rocks results from several processes i basaltic magmas rise and induce partial melting of meta basalts in the lower part of the icelandic crust the resulting dacitic magma evolves to rhyolitic composition through crystal fractionation during this differentiation the 56 54feirmm 014 values increase successively from 0 051 0 021 for the primitive dacites to 0 168 0 021 for the rhyolites this increase can be described by a rayleigh fractionation model using a constant bulk fractionation factor between all mineral phases m and the silicate liquid l of 56 54fem l 0 1 ii the basaltic magma itself differentiates by crystal fractionation to basaltic andesite composition no fe isotope fractionation was found in this series all basalts and basaltic andesites have an average 56 54feirmm 014 value of 0 062 0 042 2sd n 9 identical to mean terrestrial basaltic values reported in previous studies this observation is consistent with the limited removal of iron from the remaining silicate melt through crystal fractionation and small mineral melt fe isotope fractionation factors expected at temperatures in excess of 1050 c iii andesites are produced by mixing of basaltic andesite with dacitic melts the iron isotope composition of the andesites is matching that of the basaltic andesites and the less evolved dacites in agreement with a mixing process in the hekla volcanic suite li concentrations are positively correlated with indicators of magma differentiation all hekla rocks have 7li values typical for the upper mantle and demonstrate the absence of resolvable li isotope fractionation during crystal fractionation as a fluid mobile trace element li concentrations and isotopes are a potential tracer of magma fluid interaction at hekla li concentrations and isotope compositions do not indicate any extensive fluid exsolution hence the heavy fe isotope composition of the dacites and rhyolites can be predominately attributed to fractional crystallisation iron isotope analyses on single samples from other icelandic volcanoes torfajokull vestmannaeyjar confirm heavy fe isotope enrichment in evolving magmas our results suggest that the iron isotope composition of highly evolved crust can be slightly modified by magmatic processes

sedimentology of box cores from the cap ferret canyon area bay of biscay abstract sedimentological and geochemical investigations of 45 box cores collected in various morphological settings of the cap ferret canyon bay of biscay are presented to document accurately present day sedimentary processes on the eastern atlantic continental slope the magnitude and variations through time and space of the canyon s channelling or sinking effect on fine grained particles behaviour in comparison with sediment flux across the continental margin was particularly considered and discussed 1 all the parameters grain size carbonate and water content major and trace elements measured both in surface sediment and downcore demonstrate that the characteristics at the sediment interface vary with water depth and with the morphological setting 2 surface sediment is generally coarser grained more terrigenous and deposited at higher rate in the canyon than outside the terrigenous particle supply must be preferentially directed and trapped within the canyon s depression due to present day dynamic conditions 3 the downcore gradients reflected in grain size variations yield information on settling processes the coarse grained population has the characteristics of a winnowed sediment similar to those on the outer shelf while the fine grained population has grain size spectra very similar to the present day fine grained suspensions 4 the carbonate particles are partly derived from direct pelagic production distinct grain size distribution and like terrigenous grains are partly reworked similar downslope decrease in the coarse grained fraction the relatively low caco 3 content observed in the canyon and its downward increase up to values observed at shallower depths may result from a channelling of terrigenous suspensions within the canyon 5 at the present high sea level stand the canyon should become a trap for sediments without much gravity remobilisation as indicated by a lack of sedimentary structures in box cores however a simple increase in sediment trapping can hardly account for the downcore gradients observed in the box cores these trends which are observed on other continental margins monaco et al 1993 journees specialisees de la soc geol france geosciences marines 16 17 december 1994 abstract p 83 indicate a probable increase in terrigenous supplies and or in settling energy

iron isotope behavior during fluid rock interaction in k feldspar alteration zone a model for pyrite in gold deposits from the jiaodong peninsula east china abstract mechanisms for fe isotope fractionation in hydrothermal mineral deposits and in zones of associated k feldspar alteration remain poorly constrained we have analyzed a suite of bulk samples consisting of granite displaying k feldspar alteration precambrian metamorphic rocks and pyrite from gold deposits of the jiaodong peninsula east china by multi collector inductively coupled plasma mass spectrometry pyrites from disseminated j type ores show a 56fe variation from 0 01 to 0 64 overlapping with the signature of the host granites 0 08 to 0 39 in contrast pyrites from quartz veins l type ores show a wide range of fe isotopic composition from 0 78 to 0 79 negative values are never seen in the j type pyrites the fe isotope signature of the host granite with k feldspar alteration is significantly heavier than that of the bulk silicate earth the fe isotopic compositions of precambrian metamorphic rocks across the district display a narrow range between 0 16 and 0 19 which is similar to most terrestrial rocks concentrations of major and trace elements in bulk samples were also determined so as to evaluate any correlation between fe isotope composition and degree of alteration we note that during progressive k feldspar alteration to rocks containing 70 wt sio2 75 ppm rb and this work demonstrates that the fe isotope composition of pyrite displays a significant response to the process of pyrite precipitation in hydrothermal systems and that systematic fractionation of iron isotopes occurs during fluid rock reaction in the k feldspar alteration zone of the linglong granite the implications of the results are that processes of mineralization and associated fluid rock interaction which are ubiquitously observed in porphyry style cu au mo and other hydrothermal deposits may be readily traceable using fe isotopes

40ar 39ar geochronological constraints on the formation of the dayingezhuang gold deposit new implications for timing and duration of hydrothermal activity in the jiaodong gold province china abstract china s largest gold resource is located in the highly endowed northwestern part of the jiaodong gold province most gold deposits in this area are associated with the ne to nne trending shear zones on the margins of the 130 126 ma guojialing granite these deposits collectively formed at ca 120 5 ma during rapid uplift of the granite the dayingezhuang deposit is a large 120 t au orogenic gold deposit in the same area but located along the eastern margin of the late jurassic linglong metamorphic core complex new 40ar 39ar geochronology on hydrothermal sericite and muscovite from the dayingezhuang deposit indicate the gold event is related to evolution of the core complex at 130 4 ma and is the earliest important gold event that is well documented in the province the dayingezhuang deposit occurs along the linglong detachment fault which defines the eastern edge of the ca 160 150 ma linglong granite granodiorite massif the anatectic rocks of the massif were rapidly uplifted at rates of at least 1 km m y from depths of 25 30 km to form the metamorphic core complex the detachment fault with precambrian metamorphic basement rocks in the hangingwall and the linglong granitoids and migmatites in the footwall is characterized by early mylonitization and a local brittle overprinting in the footwall gold is associated with quartz sericite pyrite k feldspar altered footwall cataclasites at the southernmost area of the brittle deformation along the detachment fault our results indicate that there were two successive yet distinct gold forming tectonic episodes in northwestern jiaodong one event first reactivated the detachment fault along the edge of the linglong massif between 134 and 126 ma and then a second reactivated the shears along the margins of the guojialing granite both events may relate to a component of northwest compression after a middle early cretaceous shift from regional nw se extension to a ne sw extensional regime

crystal size distribution csd in rocks and the kinetics and dynamics of crystallization ii makaopuhi lava lake crystal size distribution csd theory has been applied to drill core samples from makaopuhi lava lake kilauea volcano hawaii plagioclase and fe ti oxide size distribution spectra were measured and population densities n were calculated and analyzed using a steady state crystal population balance equation n n0 exp l g slopes on ln n versus crystal size l plots determine the parameter g a product of average crystal growth rate g and average crystal growth time the intercept is j g where j is nucleation rate known temperature depth distributions for the lava lake provide an estimate of effective growth time allowing nucleation and growth rates to be determined that are independent of any kinetic model plagioclase growth rates decrease with increasing crystallinity 9 9 5 4 10 11 cm s as do plagioclase nucleation rates 33 9 1 6 10 3 cm3 s ilmenite growth and nucleation rates also decrease with increasing crystallinity 4 9 3 4 10 10 cm s and 15 2 2 10 3 cm3 s respectively magnetite growth and nucleation rates are also estimated from the one sample collected below the magnetite liquidus g 2 9 10 10 cm s j 7 6 10 2 cm3 s moments of the population density function were used to examine the change in crystallization rates with time preliminary results suggest that total crystal volume increases approximately linearly with time after 50 crystallization a more complete set of samples is needed for material with 50 crystals to define the entire crystallization history comparisons of calculated crystallization rates with experimental data suggests that crystallization in the lava lake occurred at very small values of undercooling this interpretation is consistent with proposed thermal models of magmatic cooling where heat loss is balanced by latent heat production to maintain equilibrium cooling

the experimental calibration of the iron isotope fractionation factor between pyrrhotite and peralkaline rhyolitic melt abstract a first experimental study was conducted to determine the equilibrium iron isotope fractionation between pyrrhotite and silicate melt at magmatic conditions experiments were performed in an internally heated gas pressure vessel at 500 mpa and temperatures between 840 and 1000 c for 120 168 h three different types of experiments were conducted and after phase separation the iron isotope composition of the run products was measured by mc icp ms i kinetic experiments using 57 fe enriched glass and natural pyrrhotite revealed that a close approach to equilibrium is attained already after 48 h ii isotope exchange experiments using mixtures of hydrous peralkaline rhyolitic glass powder 4 wt h 2 o and natural pyrrhotites fe 1 x s as starting materials and iii crystallisation experiments in which pyrrhotite was formed by reaction between elemental sulphur and rhyolitic melt consistently showed that pyrrhotite preferentially incorporates light iron no temperature dependence of the fractionation factor was found between 840 and 1000 c within experimental and analytical precision an average fractionation factor of 56 fe 54 fe pyrrhotite melt 0 35 0 04 2se n 13 was determined for this temperature range predictions of fe isotope fractionation between fes and ferric iron dominated silicate minerals are consistent with our experimental results indicating that the marked contrast in both ligand and redox state of iron control the isotope fractionation between pyrrhotite and silicate melt consequently the fractionation factor determined in this study is representative for the specific fe 2 fe ratio of our peralkaline rhyolitic melt of 0 38 0 02 at higher fe 2 fe ratios a smaller fractionation factor is expected further investigation on fe isotope fractionation between other mineral phases and silicate melts is needed but the presented experimental results already suggest that even at high temperatures resolvable variations in the fe isotope composition can be generated by equilibrium isotope fractionation in natural magmatic systems

chronology and pyroclastic stratigraphy of the may 18 1980 eruption of mount st helens washington many timed observations make it possible to subdivide the 9 hour plinian eruption of mount st helens on may 18 1980 into six phases defined by eruption style the phases are correlated with stratigraphic subunits of ashfall tephra and pyroclastic flow deposits the suite of pyroclastic deposits indicates that the eruption became more pumice rich and compositionally diverse with time perhaps owing to concurrent eruption of less evolved gas poor parts of the magma body with the more evolved gas rich parts the paroxysmal phase i 0832 0900 consisted of landslides lithic pyroclastic flows of a lateral blast and other explosions and a weak pre plinian column phase i pyroclastic deposits include lithic ash flow deposits intercalated with and overlying the voluminous debris avalanche deposit and basal pumice lapilli tephra that underlies a pisolitic ash layer the early plinian phase ii 0900 1215 consisted of vertical ejection of tephra with an early pulse of small pyroclastic flows on the upper flanks 1010 1035 a brief period of lithic ash ejection 1035 1100 and a pumice rich pulse that accompanied growth in height of the eruption column 1100 1215 deposits include minor pyroclastic flows on the crater rim and a reversely graded sequence of proximal tephra that include the lower pumice lapilli layer the lower lithic ash layer and the middle pumice lapilli layer all of which consist of evolved white dacitic pumice 63 64 sio2 during the early ash flow phase iii 1215 1500 the height of the eruption column decreased vertical ejection of tephra ceased and pyroclastic flows were fed from intermittent fountains phase iii deposits consist of a poorly exposed sequence 12 m of ash flow tuff that consist of many thin flow units 2 m each containing pumiceous white dacite 63 64 sio2 and denser gray silicic andesite 61 62 sio2 and fine grained ash cloud deposits interbedded with a nongraded middle pumice ash layer the climactic phase iv 1500 1715 developed in two stages fountain fed pyroclastic flows followed by a short pulse 1625 1715 of vigorous vertical ejection of tephra these stages were accompanied by the peak seismic energy release and peak eruption column height respectively climactic deposits consist of a thick 35 m sequence of thick lapilli rich ash flow sheets 4 12 m each with white and gray pumice and streaky scoria bands 60 sio2 in pumice breccia clasts and the reversely graded upper pumice lapilli layer that is interbedded with fine grained ash cloud deposits during the late ash flow phase v 1715 1815 eruption intensity waned but included a brief episode of small pyroclastic flows 1745 1815 phase v deposits consist of small distributary lobes of ash flow tuff containing white and gray pumice and minor fine ash deposits phase vi activity 1815 to may 19 1980 consisted of a low energy ash plume with transient increases in intensity while seismicity continued at depth sustained vertical discharge of phase ii prodeced evolved dacitewith high s cl ratios ash flow activity of phase iii is attributed to decreases in gas content indicated by reduced s cl ratios and increased clast density of the less evolved gray pumice climactic events are attributed to vent clearing and exhaustion of the evolved dacite

origin and classification of the late triassic huaishuping gold deposit in the eastern part of the qinling dabie orogen china implications for gold metallogeny the huaishuping gold deposit is located in the xiong ershan mountains of the qinling dabie orogen of central china the mineralisation is structurally controlled and hosted by faulted rhyolite quartz andesite volcaniclastic rocks and volcanic breccia assigned to the jidanping formation towards the top the palaeoproterozoic xiong er group the deposit has a resource of around 32 t with an average grade of 5 5 g t au alteration at the deposit progressed from an early k feldspar quartz pyrite assemblage through quartz pyrite gold quartz base metal sulfides to a late stage assemblage of quartz carbonate the 34s v cdt values for pyrite in the ore range from 13 3 to 1 6 the calcite has c isotopes ranging from 6 1 to 2 5 v pdb and o isotopes from 10 6 to 15 8 v smow the 18o quartz ranges from 10 5 to 15 1 and the d values for fluid inclusions in quartz range from 93 to 76 the 56fe value for the mineralisation varies between 0 1 and 0 5 with corresponding 57fe values between 0 2 and 0 7 the isotope systematics indicates that the hydrothermal fluids were derived from metamorphic fluid but the source of gold remains uncertain re os dating of molybdenite yields a date of 202 8 ma interpreted as the age of the gold mineralisation this age is consistent with the triassic onset of extensional tectonics following the collision between north and south china

dynamics of the bathyal benthic boundary layer in the northwestern mediterranean depth and temporal variations in macrofaunal megafaunal communities and their possible connections within deep sea trophic webs the distribution patterns of benthopelagic fauna and the macrofauna megafauna trophic relationships in the benthic boundary layer bbl were studied the study is based on data collected during 6 sampling cruises off the catalan coast western mediterranean during 1991 1995 at depths ranging from 389 1355 m crustaceans were the dominant benthopelagic macrofauna in the bbl level closest to the sea bed 0 1 5 m above bottom on the catalan sea slope copepods and peracarid crustaceans mysids amphipods isopods and cumaceans were dominant whereas euphausiids and natantian decapods some taxa of gelatinous plankton siphonophores medusae and chaetognaths and benthopelagic fishes were also well represented groups seasonal changes in megafaunal decapod crustaceans abundance seem to be linked to changes in the density and the biological cycle of bbl macrofauna which constitute an important part of the available food exploited by megafauna both the advective and the vertical flow of organic matter in the north western mediterranean should simultaneously influence peaks of available food bbl macrofauna for bathyal megafaunal decapods recruitment of macrofaunal suprabenthos and infauna species at the level of canyons and neighbouring slope zones mainly occurred between late autumn late winter and would probably be mainly induced by an advective component however the macrofaunal sizes consumed by megafaunal decapods are found more abundantly represented in spring and summer populations in parallel the vertical fluxes seem to determine maxima in the abundance of planktonic organisms especially copepods which also occur in late spring summer size natatory capability and energetic value are important factors in the selection of food resources by

spatial evolution of zn fe pb isotopes of sphalerite within a single ore body a case study from the dongshengmiao ore deposit inner mongolia china analyses of sphalerite minerals from the characteristic brecciated zn pb ores of the main ore body in the giant dongshengmiao deposit have revealed variations in 66zn from 0 17 to 0 40 and in 56fe from 1 78 to 0 35 further the investigated pyrrhotite samples have iron that is isotopically similar to that of associated sphalerite minerals the most distinctive pattern revealed by the zinc and iron isotope data is the lateral trend of increasing 66zn and 56fe values from southwest to northeast within the main ore body the lead isotopic homogeneity of ore sulfides from the main ore body suggests that there is only one significant source for metal thus precluding the mixing of multiple metal sources as the key factor controlling spatial variations of zinc and iron isotopes the most likely control on spatial variations is rayleigh fractionation during hydrothermal fluid flow with lighter zn and fe isotopes preferentially incorporated into the earliest sulfides to precipitate from fluids precipitations of sphalerite and pyrrhotite have played vital roles in the zn and fe isotopic variations respectively of the ore forming system accordingly the larger isotopic variability for fe than zn within the same hydrothermal system perhaps resulted from a larger proportion of precipitation for pyrrhotite than for sphalerite the lateral trend pattern revealed by the zinc and iron isotope data is consistent with the occurrence of a cystic shaped breccia zone which is characterized by marked elevation in cu the results further confirm that zn and fe isotopes can be used as a vectoring tool for mineral prospecting

deep sea macrofaunal assemblages within the benthic boundary layer of the cap ferret canyon bay of biscay ne atlantic abstract the benthic boundary layer bbl assemblages from the cap ferret canyon bay of biscay were quantitatively sampled at two sites located within its main channel near mooring deployments mooring sites ms 1 ca 2400 m ms 2 ca 3000 m with a suprabenthic sled equipped with four nets fishing at different heights above the bottom the macrofaunal abundance above the sea floor was mainly represented by isopoda 42 2 amphipoda 19 0 euphausiacea 17 3 cumacea 13 5 mysidacea 2 8 and tanaidacea 2 6 at both sampling sites the highest total densities were generally recorded in the immediate vicinity of the sea floor 10 40 cm water layer and a drastic decrease occurred higher in the bbl community the bbl assemblages from the two sampling sites were similar in their faunal composition major taxa and their mean density estimates were not statistically different ms 1 525 3 ind 100 m 2 ms 2 283 3 ind m 2 although the recorded values during each cruise were always lower at the deeper site the bbl macrofauna abundance showed obvious temporal fluctuations at both sites probably linked with a seasonal organic input from the euphotic zone vertical flux via phytodetritus deposition on the sea bottom

ancient geochemical cycling in the earth as inferred from fe isotope studies of banded iron formations from the transvaal craton variations in the isotopic composition of fe in late archean to early proterozoic banded iron formations bifs from the transvaal supergroup south africa span nearly the entire range yet measured on earth from 2 5 to 1 0 in 56fe 54fe ratios relative to the bulk earth with a current state of the art precision of 0 05 for the 56fe 54fe ratio this range is 70 times analytical error demonstrating that significant fe isotope variations can be preserved in ancient rocks significant variation in fe isotope compositions of rocks and minerals appears to be restricted to chemically precipitated sediments and the range measured for bifs stands in marked contrast to the isotopic homogeneity of igneous rocks which have 56fe 0 00 0 05 as well as the majority of modern loess aerosols riverine loads marine sediments and proterozoic shales the fe isotope compositions of hematite magnetite fe carbonate and pyrite measured in bifs appears to reflect a combination of 1 mineral specific equilibrium isotope fractionation 2 variations in the isotope compositions of the fluids from which they were precipitated and 3 the effects of metabolic processing of fe by bacteria for minerals that may have been in isotopic equilibrium during initial precipitation or early diagenesis the relative order of 56fe values appears to decrease in the order magnetite siderite ankerite similar to that estimated from spectroscopic data although the measured isotopic differences are much smaller than those predicted at low temperature in combination with on going experimental determinations of equilibrium fe isotope fractionation factors the data for bif minerals place additional constraints on the equilibrium fe isotope fractionation factors for the system fe iii fe ii hematite magnetite fe carbonate 56fe values for pyrite are the lowest yet measured for natural minerals and stand in marked contrast to the high 56fe values that are predicted from spectroscopic data some samples contain hematite and magnetite and have positive 56fe values these seem best explained through production of high 56fe 54fe reservoirs by photosynthetic fe oxidation it is not yet clear if the low 56fe values measured for some oxides as well as fe carbonates reflect biologic processes or inorganic precipitation from low 56fe ferrous fe rich fluids however the present results demonstrate the great potential for fe isotopes in tracing the geochemical cycling of fe and highlight the need for an extensive experimental program for determining equilibrium fe isotope fractionation factors for minerals and fluids that are pertinent to sedimentary environments

determination of the fe ii aq magnetite equilibrium iron isotope fractionation factor using the three isotope method and a multi direction approach to equilibrium abstract magnetite is ubiquitous in the earth s crust and its presence in modern marine sediments has been taken as an indicator of biogeochemical fe cycling magnetite is also the most abundant fe oxide in banded iron formations bifs that have not been subjected to ore forming alteration magnetite is therefore an important target of stable fe isotope studies and yet interpretations are currently difficult because of large uncertainties in the equilibrium stable fe isotope fractionation factors for magnetite relative to fluids and other minerals in this study we utilized the three isotope method 57fe 56fe 54fe to explore isotopic exchange via an enriched 57fe tracer and natural mass dependent fractionation using 56fe 54fe variations during reaction of aqueous fe ii fe ii aq with magnetite importantly we employed a multi direction approach to equilibrium by reacting four 57fe enriched fe ii solutions that had distinct 56fe 54fe ratios which identifies changes in the instantaneous fe isotope fractionation factor and hence identifies kinetic isotope effects we find that isotopic exchange can be described by two 56fe 54fe fractionations where an initial rapid exchange 66 isotopic mixing within 1 day involved a relatively small fe ii aq magnetite 56fe 54fe fractionation followed by slower exchange 25 isotopic mixing over 50 days that was associated with a larger fe ii aq magnetite 56fe 54fe fractionation this later fractionation is interpreted to approach isotopic equilibrium between fe ii aq and the total magnetite all four fe ii solutions extrapolate to the same final equilibrium 56fe 54fe fractionation for fe ii aq magnetite of 1 56 0 20 2 at 22 c additional experiments that synthesized magnetite via conversion of ferrihydrite by reaction with aqueous fe ii yield final 56fe 54fe fractionations that are identical to those of the exchange experiments our experimental results agree well with calculated fractionation factors using the reduced partition function ratios for fe h 2 o 6 2 from rustad et al 2010 and stoichiometric magnetite from mineev et al 2007 and these relations may be combined with the experimental constraints to determine the temperature dependence of the fe ii aq magnetite fractionation factor 10 3 ln f e i i aq magnetite 0 145 0 002 10 6 t 2 0 10 0 02 where t is in k part of the reason for large discrepancies in calculated fe isotope fractionation factors for magnetite likely lies in the stoichiometry of the mineral in specific studies given the significant effect of octahedral versus tetrahedral fe isotope fractionation that has been calculated when our results are applied to bif genesis our experimentally determined fe ii aq magnetite fractionation factor indicates that magnetite siderite mineral pairs in 2 5 ga bifs did not form in fe isotope equilibrium with each other or with ancient seawater iron oxides in such bifs are therefore more likely to have formed through processes that were isolated from equilibrium with the oceans indicating that such bif minerals may not be suitable proxies for ancient paleoenvironments

sediment budget and transfer of suspended sediment from the gironde estuary to cap ferret canyon abstract a study of the particulate organic carbon content the grain size of suspended sediment and turbidity in a transect between the gironde estuary and cap ferret canyon indicates that particles from the gironde river are transferred to near the bottom of the canyon the separation of turbid estuarine water into bottom and surface nepheloid layers takes place in the gironde itself and in the nearshore zone the development of these two separate turbid layers is due to vertical a turbid surface plume and horizontal bottom layer salinity gradients the bottom nepheloid layer is caused primary by a horizontal supply of suspended sediment from the gironde rather than from resuspension or vertical sedimentation as suggested by other workers on the median and outer shelf the distribution of particles in the water column is influenced by thermal stratification the thickness of the bottom nepheloid layer increases from the estuary towards cap ferret canyon as a function of the thickness of the bottom waters detached nepheloid layers are observed at the shelf break coinciding with isopycnal surfaces due to the presence of cold waters in the cap ferret canyon superimposed advective inputs from the northern part of the shelf and resuspension of particles from mud patches contribute to the supply of particles towards the canyon suspended sediment in transit over the shelf break represents 2 3 of the total discharge from the gironde estuary this is a considerable amount as 50 of the suspended sediment is trapped in the west gironde mud patch and another undefined portion of the gironde sediment load is transported along the coast by tidal currents

bathyal suprabenthic assemblages from the southern margin of the capbreton canyon kostarrenkala area se bay of biscay abstract the bathyal suprabenthic fauna of the kostarrenkala area capbreton canyon se bay of biscay was sampled during daytime at eight stations located on a bathymetric transect between 175 and 1000 m depth using a multinet suprabenthic sled two hundred and five suprabenthic taxa were recorded in this area mainly amphipods cumaceans isopods and mysids total abundances ranged from 752 to 2640 ind 100 m 2 showing a decreasing trend with depth diversity values h ranged between 3 83 and 5 72 increasing significantly with depth multivariate analysis of abundance data discriminated three assemblages according to depth shelf break 72 sp 1924 ind 100 m 2 upper slope 93 sp 1485 ind 100 m 2 and mid slope 135 sp 857 ind 100 m 2 assemblages each assemblage was characterised by a distinct dominant species the shelf amphipod westwoodilla caecula at the shelf break the isopod munnopsurus atlanticus on muddy sand bottoms of the upper bathyal and the amphipod rhachotropis gracilis on mid slope muddy bottoms below the mud line such a structure of bathyal assemblages seems to be generalised for the whole margin of the bay of biscay

coupled fe and s isotope evidence for archean microbial fe iii and sulfate reduction direct fossil evidence for early microbial life on earth is rare microbiological data indicate that sulfate and iron reduction are both among the earliest forms of microbial respiration and direct evidence for the early origin of sulfate reduction comes from sulfur isotopic anomalies in ancient sediments fe isotope geochemistry potentially provides a new way of identifying microbial iron reduction early in earth9s history we present fe isotopic data for sedimentary pyrite from the 2 7 ga belingwe sedimentary basin in zimbabwe isotopically light fe and a remarkable covariation between fe and s isotopes provide strong evidence for coexisting fe and s reduction our results are consistent with an early origin for sulfate reduction and provide direct geochemical evidence for the antiquity of bacterial fe reduction the covariation of fe and s isotopes may provide a useful new tracer of microbial evolution early in earth history

iron isotope fractionation between aqueous ferrous iron and goethite abstract the equilibrium fe isotope fractionation factor between aqueous fe ii and goethite has been experimentally measured to be 1 05 0 08 in 56 fe 54 fe 2 at 22 c using the three isotope method experiments were done using two sizes of goethite 81 11 nm and 590 42 nm and the experimental products were subjected to serial extraction using acid partial dissolution techniques to determine if surface fe iii atoms have different isotopic properties than the bulk goethite these experiments indicate that the interaction of fe ii aq and goethite is dynamic and results in complete or near complete fe isotope exchange over 30 days involving at least four components fe ii aq goethite sorbed fe ii and fe iii surface the equilibrium fractionation factor between fe ii aq and fe ii sorb is the same for both sizes of goethite at 56 fe fe ii aq fe ii sorb 1 24 0 14 this fractionation factor is significantly different than the results of previous studies on fe ii sorption to goethite the proportion of the fe iii surface component is greatest in the experiments that used the smallest goethite and the fe iii surface fe ii aq fractionation is estimated to be at least 2 1 the high fe iii surface fe ii aq fractionation may exert a significant influence on the fe isotope compositions of aqueous fe ii in natural systems that contain nanoparticulate goethite including those involving bacterial iron reduction these results demonstrate that the isotopic properties of nano scale minerals may be distinct from micron scale or larger minerals as is the case for other thermodynamic properties of nanoparticles

deformation monitoring at mount st helens in 1981 and 1982 for several weeks before each eruption of mount st helens in 1981 and 1982 viscous magma rising in the feeder conduit inflated the lava dome and shoved the crater floor laterally against the immobile crater walls producing ground cracks and thrust faults the rates of deformation accelerated before eruptions and thus it was possible to predict eruptions 3 to 19 days in advance lack of deformation outside the crater showed that intrusion of magma during 1981 and 1982 was not voluminous

fe isotope fractionation on fes formation in ambient aqueous solution iron sulfide phases are the ultimate repository of iron and reduced sulfur in sediments the sulfur isotope geochemistry of pyrite has had much to tell us about modern and ancient earth environments and it is likely that fe isotopes will too once fractionations for key processes are known we report the results of an experimental study of fe isotope fractionation on precipitation of fes synthetic mackinawite from excess aqueous fe ii solutions by addition of sodium sulfide solution at 2 40 c the results show a significant kinetic isotope effect in the absence of a redox process no detectable effect of temperature on the fractionation factor was observed the fe isotope fractionation for zero age fes is fe ii fes 0 85 0 30 across the temperature range studied giving a kinetic isotope fractionation factor of fe ii fes 1 0009 0 0003 on ageing the fes in contact with aqueous fe ii becomes progressively isotopically heavier indicating that the initial fractionations are kinetic rather than equilibrium from published reaction mechanisms the opportunity for fe isotope fractionation appears to occur during inner sphere ligand exchange between hexaqua fe ii and aqueous sulfide complexes fe isotope fractionation on mackinawite formation is expected to be most significant under early diagenetic situations where a readily available reactive fe source is available since fes aq is a key reactive component in natural pyrite formation kinetic fe isotope fractionations will contribute to the fe isotope signatures sequestered by pyrite subject to the relative rate of fes2 formation versus fes fe ii aq isotopic equilibration

iron isotope fractionation during magmatic differentiation in kilauea iki lava lake magmatic differentiation helps produce the chemical and petrographic diversity of terrestrial rocks the extent to which magmatic differentiation fractionates nonradiogenic isotopes is uncertain for some elements we report analyses of iron isotopes in basalts from kilauea iki lava lake hawaii the iron isotopic compositions 56fe 54fe of late stagemeltveins are 0 2 permil per thousand greater than values for olivine cumulates olivine phenocrysts are up to 1 2 per thousand lighter than those of whole rocks these results demonstrate that iron isotopes fractionate during magmatic differentiation at both whole rock and crystal scales this characteristic of iron relative to the characteristics of magnesium and lithium for which no fractionation has been found may be related to its complex redox chemistry in magmatic systems and makes iron a potential tool for studying planetary differentiation

iron isotope fractionation during skarn type metallogeny a case study of xinqiao cu s fe au deposit in the middle lower yangtze valley abstract fe isotope compositions of mineral separates and bulk samples from xinqiao cu s fe au skarn type deposit were investigated an overall variation in 57 fe values from 1 22 to 0 73 has been observed which shows some regularity the 57 fe values of endoskarn and the earliest formed fe mineral phase magnetite are ca 1 2 and ca 0 3 lower respectively relative to the quartz monzodiorite stock indicating that fluid exsolved from the stock is enriched in light fe isotopes moreover spatial and temporal variations in 57 fe values are observed which suggest iron isotope fractionation during fluid evolution precipitation of fe bearing minerals results in the fe isotope composition of residual fluids evolving with time precipitation of fe iii minerals incorporating heavy iron isotopes preferentially leaves the remaining fluid enriched in light isotopes while precipitation of fe ii minerals preferentially taking up light iron isotopes and makes the fe isotopic composition of the fluid progressively heavier the regularity of fe isotope variations occurred during fluid exsolution and evolution indicates that the dominant fe source of xinqiao deposit is magmatic overall this study demonstrates that fe isotope composition has great potential in unraveling ore forming processes as well as constraining the metal sources of ore deposits

epithermal gold mineralization acupan baguio district philippines geology mineralization alteration and the thermochemical environment of ore deposition a large gold rich adularia sericite style epithermal vein system formed at 0 65 ma at acupan baguio district philippines during a period of high k magmatism low salinity asymp 0 5 wt nacl equiv gassy up to 0 41 m co 2 meteoric fluids with temperatures of up to 300 degrees c or more were focused into a series of steeply dipping interconnected faults and joints in the virac granodiorite balatoc diatreme lucbuban gabbro and zig zag formation prolonged and varied geothermal activity resulted in the formation of a series of composite banded epithermal veins with an average width of 1 m that extend to depths of at least 1 km relative to the present day surface the epithermal event was characteried by periods of hydrothermal brecciation interspersed with more quiescent periods when delicate to coarsely banded vein material was precipitated five distinct mineral assemblages have been defined in the epithermal veins type a characterized by fine grained chalcedony type b fine grained gray quartz type c coarse grained white quartz type d coarse grained calcite and type e coarse grained anhydrite electrum and au ag tellurides have been recognized in type b c and d bands interaction of ascending chloride brines with the wall rocks over the lifetime of the system produced intense symmetrical meterwide halos of altered rock characterized by sericitic silicic and sericite chlorite mineral assemblages that grade out to a weak diffuse widespread propylitically altered zone minor adularia alteration is restricted to the deeper mine levels sulfide oxide silicate carbonate phase relationships in the acupan vein system constrain the mineralizing fluids to values of log f sub o 2 between 28 and 33 log f sub h 2 between 1 5 and 3 7 and log log f sub h 2 s between 0 6 and 1 8 at 300 degrees c abundant quartz k mica pyrite alteration and vein mineralization are consistent with silica saturated reduced weakly acidic to weakly alkaline fluid compositions boiling caused the fluids to evolve to more alkaline conditions leading to the sporadic precipitation of adularia in the veins and altered wall rocks dissolved co 2 concentrations were sufficiently high up to 0 41 m to stabilize calcite at the expense of calc silicate minerals mg super 2 concentrations were low in the mineralizing fluids but increased during dissolution of primary hornblende and biotite in the country rocks resulting in the development of a sericitechlorite alteration assemblage in the deep mine levels h 2 s and h 2 co 3 were the predominant sulfur and carbon bearing species and the low salinity gassy nature of the fluids favored gold and silver transport as bisulfide complexes h 2 te and hte super were the predominant aqueous tellurium species entrainment of minor amounts of magmatic volatiles during type c and d mineralization is responsible for te enrichment in the dominantly meteoric fluids and log f sub te 2 ranged between 7 8 and 8 6 at 300 degrees c during periods of hessite deposition magmatic contributions also account for the co sub 2 super and h 2 s rich nature of the fluids and for magmatic helium isotope signatures

variation in copper isotope ratios and controls on fractionation in hypogene skarn mineralization at coroccohuayco and tintaya peru hypogene chalcopyrite and bornite in copper skarn and related porphyry ore from coroccohuayco and tintaya peru have been analyzed for copper isotope ratios fifty six samples have a range in 65 cu values of 1 29 to 2 98 per mil relative to nist srm 976 using the sample standard bracketing technique to correct for machine drift and machine isotope fractionation effects the range of 65 cu values of vein hosted mineralization at coroccohuayco overlaps with disseminated mineralization in skarn ore but also extends to lower values the overlap of these ranges indicates that different mineralizing events in these deposits cannot be identified solely by their copper isotope compositions geologic and isotopic considerations of the samples analyzed in this investigation indicate that copper mineralization proximal to fluid sources including porphyries and principal fluid pathways tends to be isotopically lighter than more distal samples this suggests that important copper isotope fractionation occurred during mineralization and may have been the result of equilibrium fractionation and or indirect fractionation mechanisms related to the chemical characteristics of the hydrothermal fluid these fractionation mechanisms have produced copper isotope zonation over tens of meters in hypogene ore during the process of ore precipitation at coroccohuayco

new data on equilibrium iron isotope fractionation among sulfides constraints on mechanisms of sulfide formation in hydrothermal and igneous systems abstract fe s and cu reduced partition function ratios factors allow calculation of equilibrium isotope fractionation factors factors for chalcopyrite are calculated from experimental and theoretical partial phonon densities of state states kobayashi et al 2007 the fe factors for mackinawite are calculated from mossbauer spectroscopy data bertaut et al 1965 excellent agreement exists between fe factors for chalcopyrite calculated from the experimental and theoretical 57fe phonon densities of states supporting the reliability of the fe factors for chalcopyrite the 34s factor for chalcopyrite is consistent with experimental data on equilibrium sulfur isotope fractionation factors among sulfides and theoretical 34s factors except those recently calculated by a dft approach up to date experimental isotope exchange data on equilibrium fe isotope fractionation factors between minerals and aqueous fe were critically reevaluated in conjunction with fe factors for minerals and the following expressions for factors for aqueous fe2 and fe3 were obtained 10 3 ln fe aq 3 1 0063 10 6 t 2 t 273 k 10 3 ln fe aq 2 0 6537 10 6 t 2 t 273 k application of these new factors to fe and s isotope fractionation at seafloor hydrothermal conditions shows that chalcopyrite may precipitate in equilibrium with fe in hydrothermal fluids in contrast fe and s isotope compositions of pyrite are may not be controlled by isotope equilibration with fe in the hydrothermal fluid previously reported data on fe isotope fractionation between pyrite and chalcopyrite in intrusions in the grasberg igneous complex seem to be reasonable and are in general agreement with previous non isotopic estimates fe isotope fractionation between pyrite and chalcopyrite appear to reflect equilibrium in the intrusions of the grasberg igneous complex and fe isotopes may therefore be considered as a potential geothermometer the pyrite chalcopyrite fe isotope geothermometer however cannot be applied to the skarn facies of the grasberg complex where the initial fe isotope compositions may have been disturbed by subsequent processes related to skarn formation

macrofaunal density and biomass in the campeche canyon southwestern gulf of mexico abstract the composition density and community structure of the benthic macrofauna were investigated in sediments of the campeche canyon in the sw gulf of mexico total macrofaunal density ranged from 9466 2736 ind m 2 at the continental shelf station to 1550 195 ind m 2 in the canyon density values significantly diminished with distance from the coast and depth only a few stations in the center of the canyon displayed larger density values e 37 with 4666 1530 ind m 2 e 36 with 5791 642 ind m 2 and e 26 with 6925 2258 ind m 2 densities were positively correlated to organic nitrogen in the sediment r 0 82 and coarse silt r 0 43 and negatively with depth r 0 74 and distance from the coast r 0 68 at all stations the polychaete worms contributed most to the multi species community structure the nematodes and foraminifera displayed their highest densities in the center of the canyon the biomass values declined significantly with depth we conclude that the macrofauna density and biomass changed in response to organic matter contents in the sediment both with distance from the coast and with depth

the relationship of mantle derived fluids to gold metallogenesis in the jiaodong peninsula evidence from d o c s isotope systematics abstract the largest gold district in china is the jiaodong peninsula where three types of gold deposits are recognized quartz vein fracture altered and breccia types the first two developed along a group of ne trending faults and are hosted by granitic intrusions dated at 160 to 150 ma biotite granite and 130 to 126 ma granodiorite and by metamorphic rocks of the precambrian crystalline basement the breccia type gold system is mainly located around the northern margin of the jiaolai cretaceous basin where mineralisation is controlled by both detachment fractures and ne trending faults this study is based on stable isotope determinations from ten gold deposits including linglong jiaojia sanshandao cangshang wang ershan dayigezhuang denggezhuang pengjiakuang fayunkuang and dazhuangzi as well as the linglong jurassic biotite granite guojialing cretaceous granodiorite and archean gneiss the stable isotope systematics reflect the style of the three types of gold deposits but also show that they belong to the same metallogenic system in which the hydrothermal fluids were derived from a mantle fluid reservoir and mixed with crustal fluids the ore forming age is later than both the jurassic biotite granite and cretaceous granodiotite but overlaps with the 121 to 114 ma ages of lamprophyre and diabase dykes the hydrothermal fluids that were responsible for both gold mineralisation and the retrograde alteration of the diabase and lamprophyre dykes are similar and represent a co 2 and potassium rich system this fluid system is interpreted to be the consequence of cretaceous lithospheric thinning asthenospheric upwelling and mantle degassing in eastern china

the pre eruption conditions for explosive eruptions at merapi volcano as revealed by crystal texture and mineralogy abstract we analyze the textures and mineralogy of merapi tephra generated during explosive vei 3 4 eruptions over the past 2000 years and compare these data with those observed for merapi dome and flow lavas we find that the merapi pumiceous tephra and lava textures differ significantly with respect to small size crystal populations but that phenocryst textures are generally similar a similar initial phase of crystallization is indicated for tephras and lavas in mid crustal 10 km depth reservoirs subsequent textural differences are mainly affected by ascent rate and degassing during ascent and for dome lavas with temporary storage in shallower reservoirs these differences also correspond to different eruptive styles our analyses include study of pumices lava and breadcrust bomb samples including samples from some of the most recent explosive episodes prior to the 2010 eruption textural analyses of youthful breadcrust bomb samples yield insights on one type of transition between effusive and explosive eruptive styles involving pressure build up under a degassed crystalline shallow conduit plug in general comparison of the crystal size distributions and calculated residence times among the effusive and explosive eruptive styles suggests that the two main magma product types resided for similar lengths of time in a mid crustal reservoir before ascending toward the surface and either erupting explosively tephra or stagnating in a shallow magma chamber prior to extrusion lava the interpretation is supported by the occurrences of amphibole in pristine condition in tephra and in altered state in lava finally the 1872 and 2010 explosive eruptions are examined and compared with others over the past three millennia the 2010 bulk rock compositions overlap with products of other major explosive merapi eruptions such as the ngrangkah tegalsruni temusari and kepuharjo tephras the 2010 products show a gradational late stage mafic enrichment dissimilar to 1872 which showed little

ore forming fluids associated with granite hosted gold mineralization at the sanshandao deposit jiaodong gold province china the sanshandao gold deposit with total resources of more than 60 t of gold is located in the jiaodong gold province the most important gold province of china the deposit is a typical highly fractured and altered disseminated gold system with high grade quartz sulphide vein veinlet stockworks that cut mesozoic granodiorite there are four stages of veins that developed in the following sequence 1 quartz k feldspar sericite 2 quartz pyrite arsenopyrite 3 quartz base metal sulfide and 4 quartz carbonate fluid inclusions in quartz and calcite in vein veinlet stockworks contain c o h fluids of three main types the first type consists of dilute co2 h2o fluids coeval with the early vein stage molar volumes of these co2 h2o fluid inclusions ranging from 50 60 cm3 mol yield estimated minimum trapping pressures of 3 kbar homogenization temperatures obtained mainly from co2 h2o inclusions with lower co2 concentration range from 267 375 c the second inclusion type with a co2 h2o ch4 composition was trapped during the main mineralizing stages these fluids may reflect the co2 h2o fluids that were modified by fluid rock reactions with altered wallrocks isochores for co2 h2o ch4 inclusions with homogenization temperatures ranging from 204 325 c and molar volumes from 55 to 70 cm3 mol provide an estimated minimum trapping pressure of 1 2 kbar the third inclusion type aqueous inclusions trapped in cross cutting microfractures in quartz and randomly in calcite are post mineralization and have homogenization temperatures between 143 228 c and salinities from 0 71 7 86 wt nacl equiv stable isotope data show that the metamorphic fluid contribution is minimal and that ore fluids are of magmatic origin most likely sourced from 120 126 ma mafic to intermediate dikes this is consistent with the carbonic nature of the fluid and the cross cutting nature of those deposits relative to the host mesozoic granitoid

ultrahigh ϵnd eclogites from an ultrahigh pressure metamorphic terrane of china the su lu ultrahigh pressure uhp metamorphic terrane is part of the qinling dabie suture zone formed by the collision between the sino korean and yangtze cratonic blocks geochemical and isotopic investigation of the uhp metamorphic rocks has revealed a spectacular history for some eclogitic rocks retrograded coesite bearing eclogites from the weihai area in the eastern end of the su lu terrane have recorded an extraordinary nd isotopic growth with a world record high nd value of 264 range 170 to 264 such isotopic characteristics are consistent with the observed extreme lree fractionation and very high sm nd ratios the sm nd isotopic data allow us to calculate the minimum age of protolith formation at 1 7 ga assuming their evolution from a depleted or a chondritic mantle geochemical data indicate that the eclogite protoliths were of basaltic compositions originally but underwent metasomatism water rock interaction at high pressures and temperatures leading to the present apparent andesitic composition and extreme lree depletion this process took place in the proterozoic 1 7 ga and was not related to the mesozoic continental collision and ultrahigh pressure metamorphism at 210 220 ma ago because of a protracted retrograde metamorphism garnet sm nd systems were not closed until 160 ma ago 50 ma after the peak eclogite metamorphism this implies that the exhumation history is significantly different in different parts of the dabie and su lu uhp metamorphic terranes from geochemical analyses and isotopic age determinations we argue that at least two ancient tectonic events in the middle and late proterozoic can be identified prior to the last mesozoic continental collision and accompanying uhp metamorphism in the su lu and dabie terrane