:IRC. 956 

Geophysics and Petrology of the 

Deep Crust and Upper MantleA Workshop Sponsored by the 

U.S. Geological Survey and 

Stanford University 

U.S. GEOLOGICAL SURVEY CIRCULAR 956 

Geophysics and Petrology of the 

Deep Crust and Upper Mantl...,._eA Workshop Sponsored by the 

U.S. Geological Survey and 

Stanford University 

Jay 5. Noller, Stephen H. Kirby, and Jane E. Nielson-Pike, Editors 

U.S. GEOLOGICAL SURVEY CIRCULAR 956 

7987 
Department of the Interior 

DONALD PAUL HODEL, Secretary 

U.S. Geological Survey 

Dallas L. Peck, Director 

Library of Congress Cataloging-in-Publication Data 

Geophysics and petrology of the deep crust and upper mantle. 

(U.S. Geological Survey Circular 956) 

Includes bibliographies. 

Supt. of Docs. No.: I 19.4/2:956 

1. Earth-Crust-Congresses. 2. Earth-Mantle-Congresses. I. Noller, 

Jay S. II. Kirby, Stephen H. III. Nielson-Pike, Jane E. IV. U.S. Geological 

Survey. V. Stanford University. VI. Series: U.S. Geological Survey Circular 

956. 

QE75.C5 No. 956 

[QE511] 

557.3 s 

[551.1'3] 

87-600255 

Free on application to the Books and Open-File Reports Section, 

U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225 
CONTENTS 

Page 

Introduction and Overviews 

Workshop Overview and Summary 

Stephen H. Kirby, Jane E. Nielson-Pike, Jay s. Noller, 

and George A. Thompson-----------------------------------------

Character of the Lower Lithosphere: Lithology 

Howard G. Wilshire------------------------------------------------ 6 

Character of the Lower Lithosphere: Composition 

Jane E. Nielson-Pike---------------------------------------------- 9 

The Physical State and Properties of the Continental Lithosphere Based on 

Xenolith Studies: An Overview 

Stephen H. Kirby-------------------------------------------------- 13 

Microstructures in Mantle Peridotites: Asthenospheric/Lithospheric 

Flow 

Adolphe Nicolas--------------------------------------------------- 14 

Composition and Structure of the Continental Crust and the Upper Mantle 

Petrologic Variability of the Oceanic Uppermost Mantle 

Henry J.B. Dick-------------------------------------------------- 17 

The Xenolith Magnetic Record - Toward a Better Understanding of the 

Magnetic Structure of the Lithosphere 

Peter Wasilewski------------------------------------------------- 21 

The Interpretation of Seismic Velocities in the Continental Crust Based on 

Laboratory Seismology 

Nikolas I. Christensen------------------------------------------- 23 

Geological and Geophysical Nature of the Lower Continental Crust as 

Revealed by Exposed Cross Sections of the Continental Crust 

David M. Fountain------------------------------------------------ 25 

Thermal, Seismic, and Petrologic Characteristics of the Lower Crust and 

Upper Mantle--A Case History Based on Xenoliths from Eastern 

Australia 

Suzanne Y. O'Reilly and William L. Griffin----------------------- 27 

Correlation of Xenolith Petrology and Seismic Data: An Example from EastCentral Queensland 

William L. Griffin, F.L. Sutherland, and J.D. Hollis------------- 30 

Crust and Upper Mantle Beneath the Northern Plains: Evidence from Montana 

Xenoliths 

B. Carter Hearn, Jr., and Elaine S. McGee------------------------ 32 

The Mantle Beneath the Red Sea Rift: Xenoliths from Western Saudi Arabia 

Anne W. Vaughan-------------------------------------------------- 35 

A Mechanism for the Incorporation of Upper Mantle Material into the Middle 

and Upper Crust: Evidence from Southern Alaska 

Walter D. Mooney, Gary s. Fuis, Elizabeth L. Ambos, Robert A. Page, 

George Plafker, Warren J. Nokleberg, and David L. Campbell------- 37 

Ill 
Page 

The Lower Crust and Upper Mantle Beneath the Sierra Nevada Batholith: 

Evidence from Xenoliths in Late Cenozoic Volcanic Rocks 

Franklin c.w. Dodge---------------------------------------------- 38 

The Ground Truth from Crustal Xenoliths: A Multifaceted Approach 

Elaine R. Padovani----------------------------------------------- 40 

New View of the Continental Crust from Seismic Reflection Profiling 

George A. Thompson--------T-------------------------------------- 44 

Evidence for Moho Evolution in Orogenic Belts Based on COCORP Profiles 

Christopher J. Potter, Richard w. Allmendinger, Ernest c. Hauser, 

and K. Douglas Nelson----------------------------------------- 46 

Constraints on the Physical Nature of Deep Crustal Structures in 

Southeastern California from the CALCRUST Seismic Reflection 

Experiment 

John N. Louie and Robert w. Clayton------------------------------- 48 

Deformation, Preferred Orientation and Property Anisotropy in the Continental 

Lithosphere 

Causes and Effects of Anisotropy in Deformed Rocks 

H.-R. Wenk and George c. Johnson---------------------------------- 51 

Asthenosphere Structure and Anisotropy Beneath Rifts 

Adolphe Nicolas--------------------------------------------------- 53 

Crustal Evolution of the Rhinegraben Area: Exploring the Lower Crust in the 

Rhinegraben Rift by Unified Geophysical Experiments 

Karl Fuchs, K.-P. Bonjer, D. Gajewski, E. Luschen, c. Prodehl, 

K.-J. Sandmeier, F. Wenzel, and H. Wilhelm--------------------- 55 

Crustal Structure and Tectonics in Southern California 

Thomas A. Hearn--------------------------------------------------- 56 

Upper Mantle Structure Beneath Southern California: Observation Using 

Seismic Tomography and Interpretation of Resulting Flow and Stress 

Bradford H. Hager, Eugene Humphreys, and Robert w. Clayton-------- 58 

Synthetic Seismogram Modeling of Deep Reflections in the Basin and Range 

Province 

Erik B. Goodwin--------------------------------------------------- 60 

Physical State and Processes of the Continental Lithosphere 

Geophysical Implications of Mantle Xenoliths: Evidence for Fault Zones in 

the Deep Lithosphere of Eastern China 

Stephen H. Kirby, B. Carter Hearn, Jr., He Yongnian, 

and Lin Chuanyong---------------------------------------------- 63 

Polyphase History of Ultramafic Rocks in the Marble Mountains, Northern 

California Klamath Mountains 

Mary M. Donato---------------------------------------------------- 66 

The Effects of Weak Crust Upon the Strength of the Continental Lithosphere 

Robert F. DeRito-------------------------------------------------- 69 

Thermobarometry of Ultramafic Xenoliths: State of the Art and Global 

Applications 

Anthony A. Finnerty ---------------------------------------------- 71 

IV 
Page 

Crustal Growth and the Lower Crust of Magmatic Arcs 

Robert w. Kay and S. Mahlburg Kay--------------------------------- 74 

Chemical and Physical State of the Mantle Related to the Transport and 

Reactions of Fluids and Melts 

Dissolution of Volatiles in Mantle Olivine: Rheological Implications 

Harry w. Green II------------------------------------------------ 77 

Mantle Degassing, Mantle Convection, and Volcanism 

Ivan Barnes------------------------------------------------------ 79 

Solid and Fluid Inclusions in Mantle Xenoliths: Their Origins and 

Implications for the Properties of the Mantle 

Jay S. Noller---------------------------------------------------- 83 

Fluid-Wallrock Interactions in the Mantle: Evidence from Composite 

Lherzolite-Hornblendite Xenoliths 

Jane E. Nielson-Pike, James R. Budahn, and Howard G. Wilshire---- 86 

v 

Introduction and Overviews 

WORKSHOP OVERVIEW AND SUMMARY 

STEPHEN H. KIRBY, JANE E. NIELSON-PIKE, U.S. Geological Survey, Menlo Park, 

California 94025, JAY S. NOLLER, u.s. Geological Survey, Denver, Colorado 

80225, and GEORGE A. THOMPSON, Stanford University, Stanford, California 

94305 

The extended abstracts collected here are the result of a workshop on 

the nature of the lower crust and uppermost mantle that was convened at 

Stanford University in January 1986 under the sponsorship of the u.s. 

Geological Survey (Deep Crustal Studies Program) and Stanford University 

(Department of Geophysics). The purpose of the workshop was to bring 

together for the first time leading scientists from some of the diverse 

disciplines that are represented in studies of the lower crust and 

uppermost mantle. Large and expensive consortia devoted to geophysical 

soundings, such as COCORP, 1 CALCRUST, 2 TACT, 3 and PACE 4 have revolutionized 

crustal research in the United States in the past 10 years. However, the 

structural, thermal, and magnetic models derived from these projects 

usually do not produce unique solutions, and they require more data about 

the chemical, ,mineralogical, and textural character of samples that 

represent the lower crust and upper mantle. 

1 Consortium for Continental Reflection Profiling 2 Southern California Consortium for Crustal Studies (southern 

California) 3 Trans-Alaska Crustal Transect (central Alaska) 4 Pacific-Arizona Crustal Experiment (southern Basin and Range and 

southern California) 
The earth scientists who participated in the workshop are engaged in 

_research of the structure, state, petrology, composition, and evolution of 

the deep continental lithosphere. Each of the disciplines represented 

contributes significant data about the nature of the deep lithosphere, but 

each approach has limitations. For example, geophysical observations 

provide clues about the structure and state of the deep continental 

lithosphere, from large-scale seismic refraction and reflection surveys, 

heat-flow observations, and measurements of the Earth's magnetic and 

gravity fields. However, structural interpretations of these observations 

require assumptions about the petrology, composition, and physical 

properties of deep lithospheric materials. Petrological and geochemical 

studies of xenoliths and deep terranes, our most direct samples of the deep 

lithosphere, yield information about lithologies, mineral compositions, and 

in situ physical conditions of the deep lithosphere. However, xenoliths 

are modified compositionally by magma-forming processes, and also by 

decompression and alteration during their rapid ascent to the surface. 

Deep terranes also undergo reequilibration and alteration during slow 

emplacement within the crust. Field geological observations of exposed 

deep crustal and mantle terranes in regions of Neogene crust building can 

provide information about textures caused by deformation and reveal 

structural relationships among rock types at depth. However, the tectonic 

emplacement produces structural complexities that can obscure the evidence 

of crust-forming processes. Laboratory measurements of physical 

properties, such as elastic wave velocity (VP, Vs), density, the stability 

of mineral assemblages, and rock rheology, all are crucial to modeling the 

constitution and thermomechanical evolution of the continental 

lithosphere. However, measurements are made on samples of limited size and 

over relatively short times, compared to the conditions of natural physical 

phenomena in the deep crust and uppermost mantle. Theoretical 

investigations of large-scale tectonic, thermal, and mass-transport 

processes, such as continental rifting and collision, flexure, plate-scale 

faulting, and large-scale plutonism and volcanism, offers the potential for 

understanding the evolution of the continental lithosphere. However, such 

models are based on assumptions about process and initial conditions; they 

can be improved by use of information derived from the other disciplines 

listed above. Given these limitations, it is clear that, once brought 

together, scientists from a variety of disciplines might help each other 

both to better constrain interpretations of the deep continental 

lithosphere and to focus research on areas and materials of greatest 

interest. 

In this summary we want to highlight the main themes of the 

presentations and discussions, which focused on the processes that may 

govern the growth and subsequent modification of the continental crust, as 

deduced from geophysical and geological studies of the uppermost mantle and 

lower crust. 

Mechanisms of crustal growth--The mechanisms by which the crust grows 

must be deduced from the structure and petrology of crustal rocks. At 

best, large-scale geophysical profiles provide defocused images of 

structure in the deep continental lithosphere. The most tightly 

constrained interpretations are from areas where detailed mapping allows 

the extrapolation to depth of structures that are exposed at the surface. 

2 
Presentations and discussions emphasized interpretations of regions where 

crustal growth has occurred since the Mesozoic or is active today, such as 

areas above subduction zones and regions of large-scale intraplate 

extension. 

The diverse nature of the crust-mantle boundary--Many presentations 

and discussions focused both on the nature of the crust-mantle boundary and 

on the definition of the Moho in the context of variable boundary structure 

and composition. It is clear that the idealized Moho, a stepwise change in 

composition or mineralogy, represented by a change in P-wave velocity from 

6-7 km/s to greater than 8 km/s, is an inadequate description of the crustmantle boundary in areas of active crustal formation. Examples of a 

laminated or gradational crust-mantle boundary are suggested by the 

petrology and geochemistry of xenoliths from Australia and the southwestern 

United States, and from structural and petrological studies of deep 

terranes in Alaska, Wyoming, and the Ivrea Zone of southern Europe. P-wave 

velocities in some of these areas may reflect magmatic processes, the 

related effects of a steep geothermal gradient, and (or) metamorphic 

reactions. The crust-mantle boundary in regions of recent crustal 

extension may be a young feature, with depth influenced by magmatic or 

metamorphic processes. In discussions seismologists emphasized that the 

term "Moho" (Mohorovicic discontinuity) should be reserved for the 

description of the Earth's velocity distribution based on seismic 

observations, and not used as a synonym for a crust-mantle boundary based 

on petrological or other criteria. 

The origin of deep flat-lying crustal reflectors--Deep subhorizontal 

reflectors are found in many large-scale seismic reflection profiles. 

These reflections commonly are abundant in the lower crust and absent below 

the Moho. The generation of these structures may be closely related to 

crustal growth and formation of the crust-mantle boundary. In discussions, 

proposed explanations for these structures included zones of planar mafic 

intrusions, metamorphic segregation, tectonic juxtaposition of unrelated 

terranes, and preferred orientation anisotropy developed in deep shear 

zones. Data from many disciplines support a zone of multiple mafic 

intrusions as the cause of many of the reflecting horizons, especially in 

regions of crustal extension, where dike injection probably is part of 

crustal growth processes. The positive buoyancy of mafic magmas in the 

mantle should provide a strong driving force for their segregation into the 

lower crust. 

Seismic anisotropy and its implications for the flow and stress state 

of the crust and upper mantle--The physical properties of most rock-forming 

minerals vary with crystallographic direction, and this anisotropy extends 

to rocks if the mineral components have crystallographic preferred 

orientations. Metamorphic rocks may acquire preferred mineral orientation 

during plastic deformation at depth. A consequence of anisotropy can be 

large-scale azimuthal variations in elastic wave velocity, observed in the 

oceanic lithosphere and in the uppermost mantle under continents. These 

observations offer the exciting possibility of mapping the flow fields in 

the crust and mantle from seismic observations. Discussion addressed 

theoretical relations between the state of stress, the flow field, the 

resulting preferred orientations and anisotropies, and also the tectonic 

interpretations of Pn anisotropies in the uppermost mantle of the 

continents. 

3 
Fluids in the deep lithosphere--From determinations of their stable 

. isotopes, the waters released in volcanic eruptions are meteoric or of 

metamorphic origin from relatively shallow depths. Except for co2, dry 

volcanic gases contain species produced by single-stage or multistage 

reactions between crustal or surface waters and silicic magmas. 

Estimations of the Earth's co2 budget also point to a significant deep 

crustal and (or) mantle reservoir, and studies of microstructures in mantle 

xenoliths demonstrate the presence of co2 in the deep lithosphere. Work on 

volatiles in xenoliths and rocks of the deep lithosphere is difficult 

because overprinted effects of near-surface alterations must be identified 

and deleted. Fuller knowledge of the abundance and composition of 

volatiles is crucial to understanding the physical properties of the deep 

continental lithosphere. 

RECOMMENDATIONS 

• Large-scale geophysical profiling consortia should call upon as much 

geophysical, geological, and petrological data as possible. Profile and 

deep drill-hole site selections should be guided by nearness to exposures 

of deep lithospheric terranes or to xenolith localities. Geologic mapping 

and petrological, geochemical, and physical properties studies of relevant 

materials should be part of the geophysical programs. Petrological and 

geochemical studies should focus on materials and sites that have the 

greatest geophysical significance. 

· Xenoliths and rocks of exposed deep terranes should be the objects of 

multidisciplinary, collaborative studies. Xenolith studies should include 

characterizations of localities by statistical population counts. 

Laboratory studies should include mineral compositions, proportions, 

textures, and preferred orientat.ions, whole-rock major- and trace-element 

compositions, mineral isotopic compositions, and bulk physical properties 

such as density, magnetic properties, and elastic and inelastic mechanical 

properties. Complete data of the sorts listed are most valuable when all 

are acquired on the same samples; however, sample size is a limiting 

factor. 

· Systematic studies are needed of the effects of compositional 

variations (including major and minor elements and fluid proportions) on a I I 

range of physical properties and on composition-based geothermometers and 

barometers. Also, determinations of cation diffusion rates in pyroxenes 

are needed to evaluate the extent to which minerals will reequilibrate in 

response to changes in pressure, temperature, and composition. These, and 

allied microstructural studies, can aid in defining the effect of 

alteration in near-surface environments. 

· Seismic techniques aimed at determining the fine structure of the lower 

crust and upper mantle are needed. These techniques include highresolution reflection profiles that use closer seismometer and shot 

spacings, and interpretations of reflected arrivals using the spectral 

response of model structures, as exemplified by the CALCRUST experiments 

reported in this workshop. 

· Experiments should be mounted to test the possibility that velocity 

anisotropies due to mineral preferred orientations cause enhanced 

4 
reflectivity in flat-lying shear zones. In a simple shear setting, mineral 

preferred orientations typically have a symmetry related to the shear 

direction and shear plane. If anisotropy at depth causes flat-lying 

reflections, the reflectivity should vary with the azimuth of the 

profile. This should not be the case if the velocity contrasts and 

reflectivity of laminated structures are isotropic. 

· Geophysicists and geophysical consortia should maintain contact with 

those investigating chemical, petrological, laboratory, and theoretical 

aspects of deep continental materials. Future meetings, such as this 

workshop, can facilitate interdisciplinary communications. 

5 
CHARACTER OF THE LOWER LITHOSPHERE: LITHOLOGY 

HOWARD G. WILSHIRE, U.S. Geological Survey, Menlo Park, California 94025 

The lower lithosphere comprises lower crust and upper mantle between 

about 20 and 100 km depth under continents, and shallower depths under 

oceans. Direct evidence of the lithologic character of the lower 

lithosphere is derived from study of ophiolites, which represent oceanic 

crust and mantle, exposed lower continental crustal sections and associated 

peridotite massifs, and xenoliths in volcanic rocks, which represent both 

oceanic and continental lithosphere. All of these occurrences are 

fragmented, and represent varied igneous, metamorphic, and tectonic 

histories. 

The basis for describing the lithology of these rocks is not uniform, 

and commonly reflects the special focus of authors on one or another of 

these occurrences. This in turn obscures the similarities in range of 

lithologies, and, more importantly, the structural relationships among 

them. Table 1 lists the common rock types in a way that emphasizes the 

overlap--and differences--among the different modes of occurrence; that is, 

by omitting special place names that are not in common use and do not 

convey the mineralogy of the rocks, and omitting names that are acronyms. 

Peridotite is the dominant mantle component of all occurrences. 

Important differences among the peridotites are mainly reflected in the 

nature of the aluminous phase (plagioclase, spinel, or garnet) and in the 

proportion of clinopyroxene present (<3-5 pet in harzburgite, >3-5 pet in 

lherzolite). Occurrences of peridotites with different aluminous phases in 

xenolith suites have been calibrated by experimental work. This has led to 

the generally accepted hypothesis that the upper mantle is mineralogically 

stratified. Plagioclase peridotite represents the shallowest (<0.9-1 .1 

GPa) and garnet peridotite is the deepest (>2.0-2.5 GPa) accessible mantle 

level. Variations in amount of modal clinopy~oxene are thought to be 

related to varying degrees of partial melting, which in turn may reflect 

geodynamic environment of formation (Nicolas, in press). However, 

reenrichment of depleted peridotite may occur by melt infiltration in a 

variety of tectonic settings (Hamilton, 1981; Nicolas, in press) and by 

metasomatic processes (Wilshire, in press). Minor mafic lithologies 

commonly occur as inclusions in volcanic rocks and interleaved within the 

peridotites of massifs. These include spinel, garnet, and plagioclase 

pyroxenites, rocks rich in hydrous phases, gabbro, and their metamorphic 

equivalents; eclogite is found rarely in peridotite inclusions in 

kimberlite. 

The lower crustal component of these occurrences is very diverse. In 

ophiolites it is dominantly gabbroic, and in continental settings it varies 

from dominantly silicic granulite of possible supracrustal paragenesis to 

mafic granulite formed by metamorphism of mafic intrusions from the mantle 

(Fountain and Salisbury, 1981; Hamilton, 1981); inclusion assemblages in 

volcanic rocks commonly contain gabbro or metagabbro or both. Less 

abundant lithologies include dunite, pyroxenite, and wehrlite at the base 

6 
of feldspathic rock sections in ophiolites and eclogite, charnockite, and 

anorthosite in continental magmatic arc settings. 

An important observation regarding the minor lithologies that occur in 

peridotite is that they duplicate the lower crustal section in ophiolites 

and alpine massifs. The origin of these minor rock types is controversial, 

but it is of vital importance in using inclusion assemblages in volcanic 

rocks as guides to the lithology and thermal structure of the 

lithosphere. The interpretation of these minor rock types varies according 

to their mode of occurrence. For example, gabbro bodies in massifs are 

clearly dikes, but as xenoliths they are interpreted either as cognate 

precipitates from the host lavas or as unrelated fragments of the crust. 

Although gabbro dikes in massifs are commonly recrystallized, granulite 

xenoliths are almost universally ascribed to lower crustal strata. Similar 

dichotomies apply to pyroxenite, eclogite, and wehrlite, and in certain 

instances, even to peridotite. 

Integration of observations on the structural relationships of minor 

lithologies and peridotite in massif and xenolith occurrences (see 

Wilshire, in press, for review) indicates that the minor lithologies in 

both occurrences are dikes, and that they were emplaced in peridotite in a 

chronological sequence trending from higher to lower pressure. That is, 

partial melting and dike emplacement occurred sequentially in a rising 

peridotite diapir. The dike rocks so formed do not represent laterally 

continuous "strata" in the mantle or crust, but may be mineralogically (and 

texturally) identical to such "strata." Paleogeotherms reconstructed from 

such assemblages (to the extent that the currently available geobarometers 

are truly accurate) may represent the trajectory of a diapir and not a 

regional thermal structure of the lithosphere. 

REFERENCES CITED 

Fountain, D.M., and Salisbury, M.H., 1981, Exposed cross-sections through 

the continental crust: Implications for crustal structure, petrology, 

and evolution: Earth and Planetary Science Letters, v. 56, p. 263-277. 

Hamilton, Warren, 1981, Crustal evolution by arc magmatism: Philosophical 

Transactions of the Royal Society of London, A, v. 301, p. 279-291. 

Nicolas, A., in press, Structure and petrology of peridotites: Clues to 

their geodynamic environment: Reviews of Geophysics. 

Wilshire, H.G., in press, A model of mantle metasomatism, in Mullen, E.D., 

and Pasteris, J.D., eds., Mantle metasomatism and alkaline magmatism: 

Boulder, Colo., Geological Society of America Special Paper 215. 

7 
TABLE 1. Comparison of lower lithospheric lithologies in xenolith and massif occurrences 

[Group classification from Wilshire (in press). Overall relative abundances indicated by: all capital letters, 

abundant; lower cases letters, less abundant] 

Group 

Cr-diopsi<je 

Al-augite 

Gabbro/metagabbro 

Garneti ferous 

ultramafic 

Cr-diopside 

Al-augite 

Gabbro/metagabbro 

Garnetiferous 

ultramafic 

Cr-diopside 

Al-augite 

Feldspathic 

ultramafic 

Gabbro/metagabbro 

Garnet i ferous 

ultramafic 

Cr-diopside 

Feldspathic 

ultramafic 

G'ibbro/metagabbro 

Ultramafic lithologies 

HARZBURGITE 

Dunite 

Wehr lite 

Garnet lherzolite 

SPINEL LHERZOLITE 

SPINEL HARZBURGITE 

Dunite 

Spinel lherzolite 

Garnet lherzolite 

Garnet-spinel lherzolite 

SPINEL LHERZOLITE 

Spinel har zburgi te 

Spinel dunite 

Spinel lherzolite 

Spinel wehr lite 

Feldspathic lherzolite 

Anhydrous mafic/silicic 

lithologies 

OPHIOLITES 

Clinopyroxeni te 

GABBRO 

Plagiogranite 

ALPINE MASSIFS 

Garnet websterite 

Spinel clinopyroxenite 

Spinel websterite 

Spinel orthopyroxenite 

Garnet-spinel clinopyroxenite 

Spinel clinopyroxenite 

Spinel websterite 

2 Px gabbro 

2 Px metagabbro Hornblendite 

Garnet-sillimanite granulite 

Charnockite 

Anorthosite 

Plagiogranite 

XENOLITHS IN BASALTS 

Garnet clinopyroxenite 

Spinel clinopyroxenite 

Spinel websterite 

Spinel orthopyroxenite 

Garnet-spinel pyroxenite 

Spinel clinopyroxenite 

Spinel websterite 

Feldspathic garnet 

clinopyroxenite 

Feldspathic 

clinopyroxenite 

Feldspathic websterite 

2 PX GABBRO 

Olivine gabbro 

2 PX METAGABBRO 

2 PX GARNET GRANULITE 

Garnet-spinel granuli~e 

Charnocki te 

Anorthosite 

XENOLITHS IN KIMBERLITE$ 

GARNET HARZBURGITE 

Garnet clinopyroxenite 

Garnet-splnel lherzolite Garnet websterite, eclogite 

Garnet-spinel harzburgite 

Spinel lherzolite 

Feldspathic garnet 

eli nopyroxeni te 

2 Px garnet granulite 

2 Px granulite 

8 

Hydrous lithologies 

Hornblende wehrlite 

Hornblende gabbro 

Phlogopite peridotite 

Phlogopite pyroxenite 

Kaersutite peridotite 

Kaersutite pyroxenite 

Kaersutite hornblendite 

Fe-Ti glimmerite 

2 Px hornblende metagabbro 

Phlogopite lherzolite 

Phlogopite pyroxenite 

Mg-Cr glimmerite 

Phlogopite pyroxenite 

Phlogopite lherzolite 

Kaersutite pyroxenite 

Fe-Ti glimmerite 

Kaersutite hornblendite 

2 Px hornblende gabbro 

Hornblende metagabbro 

Amphibolite 

Phlogopite harzburgite 

Mg-Cr glimmerite 

Richterite peridotite 

Richterite hornblendite 
CHARACTER OF THE LOWER LITHOSPHERE: COMPOSITION 

JANE E. NIELSON-PIKE, U.S. Geological Survey, Menlo Park, California 94025 

Direct evidence of the composition of lower lithosphere is derived 

from the study of xenolith suites, ophiolites, and tectonic exposures of 

deep terranes that include alpine peridotite massifs and granulite-facies 

metamorphic rocks (Wyllie, 1967). Indirect evidence comes from the 

compositions of lavas derived from mantle and deep-crustal areas. 

Mantle lithosphere (peridotite) is refractory and rich in Mg, Fe, and 

Si, with lesser abundances of Ca, Cr, and Ni. Compared to averages of 

crustal rocks, dominant mantle is poor in "incompatible" elements (those 

that preferentially enter a melt), which include K, Rb, Ba, U, Th, and the 

light rare-earth elements (LREE) (Jagoutz and others, 1979; BVSP, 1981). 

The LREE are depleted relative to heavy rare-earth elements (HREE) and 

relative to chondrite abundances. Also, compared to crustal values, the 

isotopic ratios of Sr and Pb, daughter products of the incompatible 

elements K, U, and Th, are low in abundance, and those of Nd, daughter of 

the compatible element Sm, are high (Menzies, 1983). Lavas with analogous 

compositional characteristics are judged to be of mantle origin. In 

general, peridotite richer in clinopyroxene also is relatively richer in 

the incompatible elements (Frey, 1984). These riompositional 

characteristics are taken to indicate that variable amounts of melting and 

melt extraction removed clinopyroxene from mantle peridotite during and 

after early crust formation (Morgan and others, 1980; 1981). 

The compositions of Mg-Cr-rich spinel-peridotite xenoliths from 

basalt, found worldwide, have greater chemical variability than do 

peridotites of the other occurrences (Frey, 1984). In some xenolith 

suites, clinopyroxene-poor peridotite is enriched in incompatible elements 

and in LREE relative to HREE (LREE/HREE). Isotopic ratios of Sr and Nd are 

widely variable (Menzies, 1983) and many of these samples lie nearer "bulk 

Earth" (and crustal) values than does depleted peridotite from massifs 

(Frey and others, 1985). Clinopyroxenes of some clinopyroxene-poor 

peridotite xenoliths also show the paradoxical depletion of incompatible 

major elements and relative enrichment of incompatible minor and trace 

elements (Kempton and others, 1984). Most explanations propose that these 

rocks underwent two events: depletion by melting, and a later metasomatic 

reenrichment (Frey, 1984; Dawson, 1984; Roden and Murthy, 1985). 

Controversy about mantle metasomatism centers around how, where, and 

when metasomatism occurred. A favored hypothesis is that metasomatism 

pervades mantle regions worldwide, and is a necessary precursor to the 

generation of alkali basalt magmas (Boettcher, 1984). The metasomatic 

agent is a tenuous fluid or a melt derived from a few percent of melting in 

deeper (perhaps primitive) mantle (Frey, 1984). A competing idea is that 

metasomatism is consequent to production of basaltic melts (Wilshire, in 

press), and occurs in magmatic belts; for example, beneath rift zones. 

Young (less than 200 Ma) metasomatism of peridotite wallrocks by mafic 

_mantle intrusions has been demonstrated in field and petrologic studies of 

9 
both xenoliths and massifs (Wilshire and others, 1980; Quick, 1981). 

Recent interpretations of Nd-Sm model ages from samples identified as 

pervasively metasomatized suggest that the event was ancient (Archean) 

(Hawkesworth and others, 1983). The presence and composition of highpressure megacrysts, particularly diamond, show other mantle chemical 

heterogeneities that apparently are ancient, but of poorly understood 

origins (Richardson and others, 1984). 

The minor mantle lithologies are dominantly mafic (gabbroic or 

basaltic) compositions, with variable mantle isotopic ratios. Hydrous rock 

types of undisputed mantle origin (Varne and Graham, 1971) demonstrate that 

hydrous fluids exist in the mantle, although the rocks are differentiated 

and no longer have liquid compositions (Irving, 1980; Frey, 1984). These 

rocks may contain or be wholly composed of amphibole and (or) mica, with 

various accessory minerals, including apatite (Griffin and others, 1984). 

They have more K2o and other incompatible trace elements, and enriched 

LREE/HREE, compared to anhydrous pyroxenites. C02-rich fluid inclusions in 

xenolith minerals are evidence for co2-bearing mantle fluids (Andersen and 

others, 1984). Controversy centers on whether mantle fluids are dominantly 

H20-rich or rich in H2o and co2 . Current research indicates that co2-rich 

fluids probably occur only in restricted portions of the upper mantle 

(Schneider and Eggler, 1984). Still unknown is the compositional range of 

mantle fluids, how mantle melts and fluids are produced and segregated, and 

how melts or fluids migrate through large volumes of solid rock. 

Geochemical studies of crustal xenoliths were rare until recently. 

Crustal rock types are dominantly mafic rocks metamorphosed at granulite 

and amhibolite facies, but also found are more felsic types, such as 

pelitic gneiss and schist, quartzofeldspathic granulite, charnockite, and 

minor anorthosite. Mantle eclogite can be distinguished from much crustal 

eclogite by high contents of pyrope in garnet (Coleman and others, 1965). 

Trace-element and isotopic compositions of crustal xenoliths range from 

values like normal crust to the relatively depleted values characteristic 

of mantle rocks. Ages deduced from the isotopic systems are dominantly 

Precambrian, and are commonly interpreted as the ages of regional crust 

formation (Padovani and others, 1986). In some assemblages, compositional 

parameters are paradoxical and apparently indicate multistage depletion and 

enrichment during crustal formation (Esperanca, 1986). 

The volume of deep lithosphere samples available for study is small, 

and the geochemical data set is even more restricted. For the suite of 

crustal xenoliths, interpretations of geochemistry are thought to be 

constrained by direct comparisons with exposed deep crustal terranes. In 

contrast, compositional variations of mantle xenolith suites are commonly 

considered to indicate that xenoliths are derived from mantle regions 

unlike those represented by exposed mantle terranes, such as ophiolites and 

peridotite massifs. New geochemical work on massifs may change this 

situation, because currently there are few data that can really be compared 

between peridotite of xenoliths and exposed terranes. 

Present ideas of mantle composition and structure from xenoliths and 

basalt compositions suggest that "primitive" mantle exists at depth and is 

a chemical reservoir that has produced metasomatic agents since the 

Proterozoic. Primitive mantle is thought to have a major-element 

composition like "pyrolite" and REE abundances twice those of chondrites. 

Unfortunately, no known mantle rocks have primitive compositions. Isotopic 

10 
variations of lavas are taken to indicate that a number of apparently 

discrete reservoirs have developed since formation of the primitive 

mantle. It is clear that chemical and physical inhomogeneities exist and 

may be related, although their scale and age are still unknown. Zindler 

and others (1984) have attempted to show how mantle heterogeneities may be 

preserved in a dynamic mantle. 

REFERENCES CITED 

Andersen, T., O'Reilly, S.Y., and Griffin, W.L., 1984, The trapped fluid 

phase in upper mantle xenoliths from Victoria, Australia: Implications 

for mantle metasomatism: Contributions to Mineralogy and Petrology, v. 

88' p. 72-85. 

Boettcher, A.L., 1984, The source regions of alkaline volcanoes, in 

Explosive volcanism: Inception, evolution, and hazards: Washington, 

D.C., National Academy of Sciences, p. 13-22. 

BVSP (Basaltic Volcanism Study Project), 1980, Ultramafic xenoliths in 

terrestrial volcanics and mantle magmatic processes, in Basaltic 

volcanism on the terrestrial planets: New York, Pergamon, p. 282-310. 

Coleman, R.G., Lee, D.E., Beatty, L.B., and Brannock, W.W., 1965, Eclogites 

and eclogites: Their differences and similarities: Geological Society 

of America Bulletin, v. 76, p. 483-508. 

Dawson, J.B., 1984, Contrasting types of upper-mantle metasomatism?, in 

Kornprobst,-J., ed., Kimberlites II. The mantle and crust-mantle 

relationships: Amsterdam, Elsevier, p. 289-294. 

Esperanca, S., 1986, Lower crustal xenoliths from the Camp Creek locality, 

central Arizona [abs.]: Geological Society of America Abstracts with 

Programs, v . 1 8, p. 1 05 . 

Frey, F.A., 1984, Rare earth element abundance in upper mantle rocks, in 

Henderson, P., ed., Rare earth element geochemistry: Developments in 

geochemistry 2: Amsterdam, Elsevier, p. 153-203. 

Frey, F.A., Suen, C.J., and Stockman, H.W., 1985, The Ronda high 

temperature peridotite: Geochemistry and petrogenesis: Geochimica et 

Cosmochimica Acta, v. 49, p. 2469-2491. 

Griffin, W.L., Wass, S.Y., and Hollis, J.D., 1984, Ultramafic xenoliths 

from Bullenmerri and Gnotuk maars, Victoria, Australia: Petrology of a 

sub-continental crust-mantle transition: Journal of Petrology, v. 25, 

p. 53-87. 

Hawkesworth, C.J., Erlank, A.J., Marsh, J.S., Menzies, M.A., and van 

Calsteren, P., 1983, Evolution of the continental lithosphere: Evidence 

from volcanics and xenoliths in southern Africa, in Hawkesworth, C.J., 

and Norry, M.J., eds., Continental basalts and mantle xenoliths: 

Cheshire, Eng., Shiva Publishing, p. 111-138. 

Irving, A.J., 1980, Petrology and geochemistry of composite ultramafic 

xenoliths in alkalic basalts and implications for magmatic processes 

within the mantle: American Journal of Science, Jackson Volume, v. 280-

A, p. 389-426. 

Jagoutz, E., Palme, H., Baddenhausen, H., Blum, K., Cendales, M., Dreibus, 

G., Spettel, B., Lorenz, v., and Wanke, H., 1979, The abundances of 

major, minor, and trace elements in the earth's mantle as derived from 

primitive ultramafic nodules: Proceedings of the 10th Lunar and 

Planetary Science Conference, p. 2031-2050. 

11 
Kempton, P.D., Menzies, M.A., and Dungan, M.A., 1984, Petrography, 

petrology, and geochemistry of xenoliths and megacrysts from the 

Geronimo volcanic field, southeastern Arizona, in Kornprobst, J., ed., 

Kimberlites I: Kimberlites and related rocks: Amsterdam, Elsevier, p. 

71-83. 

Menzies, M., 1983, Mantle xenoliths in alkaline magmas: Evidence for 

mantle heterogeneity modified by magmatic activity, in Hawkesworth, 

C.J., and Norry, M.J., eds., Continental basalts and mantle xenoliths: 

Cheshire, Eng., Shiva Publishing, p. 92-110. 

Morgan, J.W., Wandless, G.A., Petrie, R.K., and Irving, A.J., 1980, 

Composition of the earth's upper mantle - II: Volatile trace elements 

in ultramafic xenoliths: Proceedings of the 11th Lunar and Planetary 

Science Conference, p. 213-233. 

1981, Composition of the earth's upper mantle - I: Siderophile trace 

elements in ultramafic nodules: Tectonophysics, v. 75, p. 233. 

Padovani, E.R., Wandless, G.A., Reid, M., and Hart, S.R., 1986, 

Characterization of the deep crust in an active intracontinental rift: 

Evidence from xenoliths at Kilbourne Hole Maar [abs.]: Geological 

Society of America Abstracts with Programs, v. 18, p. 168. 

Quick, J.E., 1981, Petrology and petrogenesis of the Trinity Peridotite, an 

upper mantle diapir in the eastern Klamath Mountains, northern 

California: Journal of Geophysical Research, v. 86, p. 11,837-11,863. 

Richardson, S.H., Gurney, J.J., Erlank, A.J., and Harris, J.W., 1984, 

Origin of diamonds in old enriched mantle: Nature, v. 310, p. 198-202. 

Roden, M.F., and Murthy, V.R., 1985, Mantle metasomatism: Annual Reviews 

of Earth and Planetary Sciences, v. 13, p. 269-296. 

Schneider, M.E., and Eggler, D.H., 1984, Compositions of fluids in 

equilibrium with peridotite: Implications for alkaline magmatismmetasomatism, in Kornprobst, J., ed., Kimberlites I: Kimberlites and 

related rocks:---Amsterdam, Elsevier, p. 383-394. 

Varne, R., and Graham, A.L., 1971, Rare earth abundances in hornblende and 

clinopyroxene of a hornblende lherzolite xenolith: Implications for 

upper mantle fractionation processes: Earth and Planetary Science 

Letters, v. 13, 11-18. 

Wilshire, H.G., in press, A model of mantle metasomatism, in Mullen, E.D., 

and Pasteris, J.D., eds., Mantle metasomatism and alkaline magmatism: 

Boulder, Colo., Geological Society of America Special Paper 215. 

Wilshire, H.G., Pike, J.E.N., Meyer, C.E., and Schwarzman, E.C., 1980, 

Amphibole-rich veins in lherzolite xenoliths, Dish Hill and Deadman 

Lake, California: American Journal of Science, Jackson Volume, v. 280-

A, p. 576-593. 

Wyllie, P.J., ed., 1967, Ultramafic and related rocks: New York, Wiley, 464 

p. 

Zindler, A., Staudigel, H., and Batiza, R., 1984, Isotope and trace element 

geochemistry of young Pacific seamounts: Implications for the scale of 

upper mantle heterogeneity: Earth and Planetary Science Letters, v. 70, 

p. 175-195. 

12 
THE PHYSICAL STATE AND PROPERTIES OF THE CONTINENTAL LITHOSPHERE BASED ON 

XENOLITH STUDIES: AN OVERVIEW 

STEPHEN H. KIRBY, U.S. Geological Survey, Menlo Park, California 94025 

The deep-seated origin of the forces responsible for large-scale 

deformation of the continental lithosphere and the difficulty in extrapolating surface geological and geophysical observations to significant 

depth have motivated efforts to probe the deep continental lithosphere by 

various methods. The first is continental scientific drilling, both 

proposed and in progress. Second is large-scale geophysical profiling, 

such as seismic reflection and refraction surveys, aeromagnetic surveys, 

and gravity and electrical profiling. These efforts, which include integrated geological observations with sophisticated modeling studies, have 

revolutionized our understanding of the lower continental crust and 

uppermost mantle. Third is xenolith and deep terrane studies. Rocks of 

deep origin, brought up by volcanic eruptions or by uplift and erosion, 

presently represent our only samples of the lower crust and upper mantle. 

They present a challenge of interpretation because of the various ways they 

have been physically and chemically altered during their ascent to the 

surface and because their geologic context is poorly known, especially in 

the case of xenoliths. Nevertheless, as far as direct sampling below 

15 km, they represent the only game in town. 

Research on deep rocks can illuminate the deep lithosphere in a number 

of important ways: (1) They provide petrological and mineralogical 

information on rock types and distributions. Combined with direct 

laboratory measurements of their physical properties, their study provides 

powerful constraints on the models used to interpret the geophysical 

observations. (2) They give insights into the physical processes that 

operate in the deep lithosphere and that may have caused it to evolve into 

its present state. These insights stem from the structures and textures 

that are unique to the physical processes that are responsible for them. 

(3) There can be quantitative connections between the chemistries, 

textures, and structures on the one hand and the physical state of the 

lower lithosphere, such as the state of stress, temperature, and 

pressure. These connections come from laboratory studies of these features 

under controlled conditions. (4) Laboratory measurements of physical 

properties of deep rocks contribute not only in the first and third areas 

above but also provide basic information on the mechanical behavior of the 

continental lithosphere. For example, the flexure of the continental 

lithosphere depends on the depth distribution of plastic strength, and 

recent work suggests that the deep continental Moho represents a 

discontinuous increase in creep strength. 

13 
MICROSTRUCTURES IN MANTLE PERIDOTITES: ASTHENOSPHERIC/LITHOSPHERIC FLOW 

ADOLPHE NICOLAS, Laboratoire de Tectonophysique, University of Nantes, 

44072 Nantes, France 

Microstructures in mantle peridotite (kimberlite and basalt xenoliths 

and various types of massifs) reflect different degrees of plastic 

deformation produced in different pressure-temperature conditions. These 

microstructures have been classified, and using petrofabric analysis and 

dislocation microstructure studies, it has been possible to show that they 

correspond in the majority of cases to a shear flow regime produced by 

dislocation creep. 

A first-order problem is to distinguish among these microstructures 

those corresponding to an asthenospheric flow and those corresponding to a 

lithospheric one. An answer to this problem would have consequences on the 

general modeling of asthenosphere dynamics and on the interpretation of 

local mantle structures (asthenosphere diapirs or oceanic ridge structure, 

for example). An answer now seems possible provided that (1) one accepts 

that a signature of asthenospheric deformation is the presence of a 

basaltic melt within the deformed peridotite and (2) one is able to 

identify this situation in the now frozen peridotite structure. I present 

here evidence of the presence of such melts in various peridotite 

occurrences. 

Harzburgite in ophiolitic massifs--This is the simplest situation in 

which the presence of melt during plastic flow of the upper harzburgite and 

dunite of the mantle section of ophiolites can be demonstrated (Nicolas and 

Prinzhofer, 1983). It can be seen in the field that gabbro dikes terminate 

into diffuse plagioclase-clinopyroxene impregnations in the peridotite. In 

the same area, such impregnations either are foliated along with the 

peridotite, proving that they penetrated the rock before or during the 

flow, or they are not deformed as much as the enclosing rocks, in which 

case the impregnation postdates the flowage. This proves the presence of 

melt during the plastic flow. Interestingly, the microstructure of these 

peridotites is a coarse mosaic. In dunite it mimics a magmatic accumulate 

structure. At this scale, the plastic deformation is only recorded by the 

olivine and enstatite fabrics which are, mainly the olivine one, remarkably 

strong. It is suggested that, due to the presence of a melt, grain 

boundary migration is greatly enhanc~d, resulting in a rapid consumption of 

crystals improperly oriented for slip, thus explaining the strong fabric as 

well as the development of a coarse-mosaic structure. 

Plagioclase lherzolite from ophiolitic massif--In such massifs, 

exemplified by Lanzo (western Alps) and Trinity (Klamath Mountains) 

peridotites, the evidence of melt during plastic flow is given by field 

analysis. There, a complete sequence is followed from lherzolite in which 

the plagioclase is evenly distributed as dispersed aggregates parallel to 

the foliation, to small plagioclase-rich lenses formed by melt 

concentration parallel to the shear flow plane, and finally to gabbro dikes 

in a tension fracture orientation (Boudier and Nicolas, 1977). Outside the 

areas where this sequence is observed, it is difficult to ascertain whether 

14 
the plagioclase aggregates parallel to the foliation in homogeneous 

plagioclase lherzolite represent melt pockets. 

Spinel lherzolite xenoliths in basalt--It is proposed that the 

orthopyroxene-clinopyroxene spinel clusters, which are characteristic of 

the undeformed protogranular $tructure, crystallized from melt pockets in 

these lherzolites. It is now widely accepted that these clusters were 

derived from the garnets of garnet lherzolite, but the transformation is 

usually regarded as a subsolidus reaction. The evidence used to claim that 

the transformation proceeds by melting of the garnet and crystallization of 

the clusters from this melt is mainly indirect. In any case, the classical 

sequence of deformation described in these xenoliths affects these 

clusters. Thus, this deformation is at best contemporaneous with 

crystallization, and more probably subsequent to it, recording then 

lithospheric flow. 

REFERENCES CITED 

Boudier, F., and Nicolas, A., 1977, Structural controls on partial melting 

in the Lanzo peridotites, in Dick, H.J.B., ed., Magma genesis: Bulletin 

Oregon Department of Geology and Mineral Industries, v. 96, p. 63-78. 

Nicolas, A., and Prinzhofer, A., 1983, Cumulative or residual origin for 

the transition zone in ophiOlites: Structural evidence: Journal of 

Petrology, v. 24, p. 188-206. 

15 

Composition and Structure of the Continental Crust and 

the Upper Mantle 

PETROLOGIC VARIABILITY OF THE OCEANIC UPPERMOST MANTLE 

HENRY J.B. DICK, Department of Geology and Geophysics, Woods Hole 

Oceanographic Institution, Woods Hole, Massachusetts 02543 

Asthenosphere ascending between diverging lithospheric plates beneath 

an ocean ridge undergoes decompression melting. The melt is squeezed to 

the surface and erupts to form new crust over the ascending mantle. 

Peridotite dredged from the sea floor is likely to be the most depleted 

residue of this process. Because the degree of melting is linked to the 

depth of mantle upwelling, initial temperature, and chemistry, knowledge of 

the lateral chemical and mineralogic variability of residual mantle 

peridotite from the ocean ridges provides direct information on the process 

of mantle convection, as well as direct constraints on the generation of 

the spatially associated basalts. 

EXPOSURE AND EMPLACEMENT 

The large majority of abyssal peridotite has been recovered from the 

great oceanic fracture zones; smaller amounts have been dredged and drilled 

from rift mountains. At some fracture zones, areas exist where only 

altered peridotite and minor basalt have been dredged for thousands of 

square kilometers (see Bonatti and Honnorez, 1976). This is prima facie 

evidence that the flow of magma out of the mantle is not uniform, but is 

channeled away from fracture zones, erupting preferentially near the 

midpoints of adjacent ridge segments (Whitehead and others, 1984). 

17 
The exposure of abyssal peridotite is related to spreading rate. At 

the very slow spreading SW Indian and American-Antarctic Ridges (<1 cm/yr 

half rate) close to 67 percent of rocks dredged from fracture zones are 

altered mantle peridotite. At the slightly faster spreading Mid-Atlantic 

Ridge, fracture-zone dredge hauls contain 10-30 percent peridotite. At 

fast spreading ridges, like the East Pacific Rise, peridotite is rare. 

Emplacement clearly involves the diapiric ascent in the solid or partially 

molten state either to the base of the crust or, at some fracture zones, 

directly to the sea floor. Subsequent tectonism and faulting allow 

percolation of water into the peridotite, producing extensive hydrothermal 

alteration, principally serpentinization. In areas where the crust is thin 

(fracture zones) or the faults are large (median valley walls at slow 

spreading ridges) continued faulting allows the lower density serpentinite 

to work its way up through faults in the crust to the surface (Aumento and 

Loubat, 1971; Bonatti and Honnorez, 1976). 

MINERALOGY 

Modal analyses of 266 peridotite samples (570,740 points, 1 mm 

spacing) from 42 dredge hauls at 21 localities on 6 ocean ridges give an 

average of 74.8±5.3 percent olivine, 20.6±3.7 percent enstatite, 3.6±2.0 

percent diopside, 0.5±0.2 percent chromian spinel, and 0.5±1 .2 percent 

plagioclase. This is deceptive (note the standard deviation of 

plagioclase) as only 39 percent of these rocks contain plagioclase. 

Peridotite in most dredge hauls contains little or no plagioclase. 

Locally, there are areas where almost all dredged peridotite contains 

abundant plagioclase (up to 18 vol. pet.): at the Argo Fracture Zone, for 

example, peridotite in one dredge haul contained 3.1 percent plagioclase, 

while in another 9 km away there was none. This localized distribution of 

plagioclase is the same as in alpine peridotite bodies, where adjacent 

massifs in the same mountain belt contain either abundant or no 

plagioclase. 

The proportions of minerals are not directly related to spreading 

rate. There is a relationship with proximity to mantle hot-spot regions: 

diopside-poor harzburgite, reflecting the highest degree of mantle melting 

beneath the ridge, occurs in greatest abundance in the vicinity of 

postulated mantle plumes (Dick and others, 1984, Michael and Bonatti, 

1985). Along the American-Antarctic and SW Indian Ridges (spreading rates 

at 0.9 cm/yr), the average composition of dredged peridotite varies from 

lherzolite with 7.26 percent diopside at the Vulcan Fracture Zone far from 

the Bouvet hot spot, to harzburgite with 1 percent diopside at the Bouvet 

Fracture Zone. 

Mineral compositions are very uniform, consistent with a residual 

or1g1n. Enstatite is always close to saturation with respect to diopside, 

indicating that melting was restricted to the four-phase, two-pyroxene 

field (Dick and Fisher, 1984); this places an upper temperature limit on 

the mantle beneath ocean ridges. Exceptions are the alumina content of 

pyroxene and the Cr/(Cr+Al) ratio of spinel, which have large systematic 

variations with mineral proportions along the ocean ridges. Enstatite 

alumina varies from 6.0 to 2.0 weight percent, spinel Cr/(Cr+Al) from 0.160 

(o 0.525, and olivine ranges from Fo90 .

0to Fo 91 •4, as the average modal 

proportion of olivine in the peridotite in the dredge hauls increases from 

18 
67 to 80 volume percent and diopside decreases from 5.9 to 0.2 volume 

percent. This is not expected in residues of fractional crystallization, 

where proportions are controlled by complex physical processes in magma 

chambers, whereas progressive changes in mineral compositions and 

proportions are anticipated in the residues of varying degrees of 

melting. Dick and others (1984) found that these variations in abyssal 

peridotite indicate a range from 10 to 30 percent melting. 

Mineral compositions in plagioclase peridotite have more Ti-rich 

pyroxenes and more titanian and ferric spinels than does plagioclase-free 

peridotite. The spinels are notably similar to spinels in ocean-ridge 

basalts. Plagioclase (An68 to An98 ) occurs in interstitial lenses, often 

in reaction relation with spinel, frequently with reverse zoning, with the 

most sodic plagioclase in the most plagioclase-rich rock. 

BULK COMPOSITION 

Primary bulk compositions of highly altered peridotite can be computed 

using the measured relict mineral compositions and modes. Such 

calculations indicate that, irrespective of degree of melting, both soda 

and titania amount to less than 0.02 weight percent in the plagioclase-free 

peridotite. Given the concentrations of these elements in basalt, this 

peridotite contains at best only 1 percent potential basalt, indicating 

that melt extraction was highly efficient in most areas of the abyssal 

upper mantle. 

When the bulk compositions of the plagioclase peridotites are 

computed, however, they contain substantial soda and titania, which 

increase linearly with the proportion of plagioclase in the rock. This 

compositional variation can be modeled as a linear addition of abyssal 

basalt to residual plagioclase-free peridotite. Since both soda and 

titania are thought to behave as incompatible elements during melting, they 

should occur in negligible amounts in any residue, as we have found for the 

plagioclase-free peridotite. This, as can also be seen by numerical 

modeling, unequivocally rules out a residual origin for the plagioclase. 

The plagioclase-peridotite is best explained as hybrid rocks consisting of 

residual peridotite impregnated with trapped melt from which the 

plagioclase crystallized. The nonuniform distribution of plagioclase in 

abyssal peridotite, then, is additional evidence for localized flow of melt 

in the mantle, perhaps channeled by some form of gravitational instability 

(Whitehead and others, 1984). 

On average, xenolith suites and subcontinental alpine peridotite (such 

as the Balmuccia and Baldissero peridotites) contain significant soda and 

titania in contrast to abyssal and ophiolitic alpine peridotites. Either 

they contain significant quantities of trapped melt (as does abyssal 

plagioclase peridotite), or they underwent little or no melting during 

their emplacement to the base of the crust. This may be the case for 

alpine peridotite emplaced from shallow lithosphere depths to the base of 

the crust during the initial stages of continental rifting. 

This work was supported by NSF Grant No. OCE-8416634, and is Woods 

Hole Oceanographic Institution Contribution No. 6164. 

19 
REFERENCES CITED 

Aumento, F., and Loubat, H., 1971, The Mid-Atlantic Ridge near 45°N. XVI. 

Serpentinized ultramafic intrusions: Canadian Journal of Earth 

Science, v .• 8, p. 631-663. 

Bonatti, E., and Honnorez, J., 1976, Sections of the earth's crust in the 

equatorial Atlantic: Journal of Geophysical Research, v. 81, p. 4101-

4116. 

Dick, H.J.B., and Fisher, R.L., 1984, Mineralogic studies of the residues 

of mantle melting: Abyssal and alpine-type peridotites, in Kornprobst, 

J., ed., Kimberlites II: The mantle and crust-mantle relationships: 

Amsterdam, Elsevier, p. 295-308 

Dick, H.J.B., Fisher, R.L., and Bryan, W.B., 1984, Mineralogic variability 

of the uppermost mantle along mid-ocean ridges: Earth and Planetary 

Science Letters, v. 69, p. 88-106. 

Michael, P.J., and Bonatti, E., 1985, Peridotite composition from the North 

Atlantic: regional and tectonic variations and implications for 

partial melting: Earth and Planetary Science Letters, v. 73, p. 91-

104. 

Whitehead, J., Dick, H.J.B., and Schouten, H., 1984, A mechanism for 

magmatic accretion under spreading centres: Nature, v. 312, p. 146-

148. 

20 
THE XENOLITH MAGNETIC RECORD-TOWARD A BETTER UNDERSTANDING OF THE 

MAGNETIC STRUCTURE OF THE LITHOSPHERE 

PETER WASILEWSKI, NASA Goddard Space Flight Center, Greenbelt, Maryland 

20771 

The continental lithosphere emerges as an important study region for 

the 1980's. In this regard Magsat, launched by NASA, was timely and 

pioneering in that it has provided the impetus for considering the 

continental lithosphere as a magnetic material. However, to handle the 

Magsat anomalies requires that we consider the development of magnetization 

contrast in the context of crustal evolution. Rock magnetization data 

appropriate to this task are poorly developed or nonexistent. At the 

Goddard rock magnetism laboratory this problem is being addressed. Study 

materials were specifically chosen in order to address (1) the distribution 

of magnetic minerals and associated Curie points in the lithosphere, (2) 

the distribution of possible lithologies and their magnetic properties, and 

(3) the role of prograde and retrograde metamorphism in magnetic contrast 

development. Xenoliths, tectonically exposed crustal sections, and high 

metamorphic grade Precambrian terranes constitute the principal study 

materials. 

Xenoliths are accidental fragments from any part of the upper mantle 

or crust traversed by the host volcanic rock and carried to the surface of 

the Earth. Xenoliths contain information about the magnetic. minerals, 

level of magnetization associated with the minerals, and fundamental 

magnetic domain structure characteristics of otherwise inaccessible parts 

of the continental crust and upper mantle. In the ideal case with no 

decompression melt, no hydrothermal alterations, and no crack filling, the 

xenolith has thermoremanent magnetization characteristics appropriate to 

the included magnetic mineralogy and vector directions similar to the 

host. The Curie point, initial susceptibility, and magnetic domain 

structure are then characteristic in situ magnetic properties. The natural 

remanence measured in the laboratory is a thermoremanence and not therefore 

an indication of the in situ remanence. The remanence would have to be 

modeled in laboratory experiments at elevated temperature. I will discuss 

granulite grade and upper mantle xenoliths from the northeast and southwest 

Honshu arc (Japan), the Aleutians, Eastern Australia, the Colorado plateau 

and Rio Grande rift, and South Africa. I will summarize the magnetic 

contrasts across the crust-mantle boundary, as evidenced in the xenolith 

record, which supports the concept of the Moho as a magnetic boundary. 

Magnetic domain structure of the continental lithosphere will be considered 

using three separate data bases recently completed at Goddard. The 

xenolith data base will be contrasted with a surface crustal data base 

which includes all types of igneous and metamorphic rocks and a high-grade 

metamorphic rock data base which includes the Ivrea zone, the Kapuskasing 

zone, and granulite and amphibolite rocks from Norway, Scotland, 

Antarctica, and Minnesota. The magnetic domain structure is the basis for 

the physical theory of rock magnetism, and the magnetic hysteresis loop 

results used to evaluate the domain structure suggests the amount, size, 

21 
and other characteristics of the magnetic material. This is critical 

information necessary in consideration of the temperature and time 

dependence of magnetic remanence, and the temperature dependence of the 

induced component of magnetization. 

22 
I HE INTERPRETATION OF SEISMIC VELOCITIES IN THE CONTINENTAL CRUST BASED ON 

LABORATORY SEISMOLOGY 

NIKOLAS I. CHRISTENSEN, Department of Geosciences, Purdue University, West 

Lafayette, Indiana 47907 

Much of our knowledge of the lower continental crust and upper mantle 

originates from seismology. Both reflection and refraction studies have 

provided useful information of crustal structure in many regions throughout 

the world. Seismic refraction wave velocities also provide additional 

information on the nature of the continental lithosphere. The 

interpretation of these velocities in terms of composition relies on 

laboratory studies of the seismic properties of rocks. 

Laboratory investigations of wave velocities in continental rocks are 

in many ways complicated compared to similar studies of ocean rocks. Deep 

continental pressures are much higher, so it is often necessary to obtain 

measurements at confining pressure to 1 GPa. Also, temperature becomes 

significant in reducing velocities in many lower continental crustal 

regions and this requires knowledge of temperature derivatives of 

velocities. Perhaps most important, mapping in high-grade metamorphic 

provinces has shown that compositional heterogeneity is a characteristic 

feature of the lower continental crust. In contrast with the oceanic 

crust, which based on deep sea drilling and studies of ophiolites appears 

generally basaltic in composition, the lower continental crust probably 

ranges significantly in composition from region to region. 

New measurements of velocities of compressional waves in high-grade 

metamorphic rocks at hydrostatic confining pressures and temperatures to 

500 °c show average temperature coefficients (aVQ/aT)Q of 

-0.5 x 10- 3 km s- 1 °C- 1 at pressures above 0.5 GPa. Critical thermal 

gradients (dT/dz)c for velocity reversals in the lower crust are between 8 

and 12 °C/km. Velocities in continental rocks correlate well with chemical 

composition. Also of major importance is mineralogy, which is a function 

of metamorphic grade. Velocity changes associated with progressive 

metamorphism of pelitic rocks, graywacke, carbonate, basalt, diorite, and 

granite show complex but well-defined patterns. In general, velocities 

increase with increasing metamorphic grade; however, for some compositions 

velocity reversals accompany a progressive isochemical sequence of 

advancing grade. A similar behavior is observed for shear-wave velocities. 

A phenomenon seldom considered in discussions of continental seismic 

properties but that may be of importance even at lower crustal depths is 

pore pressure. New measurements in continental rocks at high confining 

pressures show that both compressional and shear wave velocities are 

significantly lowered by the application of pore pressure. Within the deep 

crust, many regional metamorphic reactions involve water. It seems likely 

that changes in pore pressure accompanying these reactions have significant 

effects on seismic velocities. Laboratory measurements show high Poisson's 

ratios may be diagnostic of crustal regions with high pore pressure. 

One of the most distinctive features of the fabric of many continental 

rocks is the tendency for their minerals to assume parallel or partially 

23 
parallel crystallographic orientation. Preferred orientation of minerals 

in metamorphic rocks produces appreciable seismic anisotropy for both 

compressional and shear waves. Distinctive anisotropy patterns 

characterize different metamorphic mineral assemblages, which is perhaps 

best illustrated by velocity studies of mylonites from metamorphic core 

complexes of the southwestern United States. Extreme anisotropy of 

compressional and shear waves and associated shear-wave splitting are 

important seismic properties of phyllonites from major thrust zones such as 

the Brevard of the southern Appalachians. Fabric studies and velocity 

measurements of ultramafic xenoliths from kimberlites also demonstrate that 

seismic anisotropy is an important property of the continental upper mantle 

as well as the continental crust. 

24 
GEOLOGICAL AND GEOPHYSICAL NATURE OF THE LOWER CONTINENTAL CRUST AS 

REVEALED BY EXPOSED CROSS SECTIONS OF THE CONTINENTAL CRUST 

DAVID M. FOUNTAIN, Program for Crustal Studies, Department of Geology and 

Geophysics, University of Wyoming, Laramie, Wyoming 82071 

Nearly intact cross sections of the continental crust, exposed in 

orogenic belts around the world (Fountain and Salisbury, 1981), provide 

insights into the structure, evolution, and geophysical nature of the lower 

continental crust. Importantly, these terranes were formed at different 

times in a variety of tectonic environments, thus providing a spectrum of 

modes of crustal evolution and structure. Archean cross sections, such as 

the Pikwitonei and Kapuskasing bodies in the Superior province, represent 

the deep crust below Archean gneiss-greenstone terranes. The Musgrave and 

Fraser Ranges in Australia provide glimpses of the deep crust of 

Proterozoic mobile belts. Several Phanerozoic examples display the lower 

crust of magmatic arcs (Kohistan in Pakistan, Fiordland in New Zealand, and 

the Tehachapi Mountains in southern California) and extensional regimes 

(Ivrea zone, northern Italy). Most of these terranes underwent several 

tectonic and thermal events during their evolution, thereby providing 

examples of a variety of deep crustal processes. 

The composition of these lower crustal samples is variable with the 

average chemical composition ranging from tonalitic (for example, 

Pikwitonei) to mafic (for example, Ivrea, Kapuskasing). Each section is 

characterized by diverse upper amphibolite-granulite facies lithologies, 

which may include various proportions of metasedimentary and metavolcanic 

rocks, layered mafic complexes, anorthosite, tonalite gneiss, mafic gneiss 

and ultramafic bodies. The presence of isoclinal folds, boudinage 

structure and, in a few cases, mylonitic shear zones in these terranes 

attests to the dominance of ductile deformation mechanisms in the lower 

crust. These structures, coupled with igneous and metamorphic processes, 

in many cases result in a horizontally layered deep crust from centimeter 

to kilometer scale. 

Efforts are underway to study the physical properties of rocks from 

these cross sections in order to gain insight into the geophysical nature 

of the lower continental crust. Laboratory measurements at high confining 

pressures indicate that there is a wide range in P-wave velocities within 

any given section. Therefore, large reflection coefficients (>0.15) can be 

expected. These large reflection coefficients are the result of many 

factors, including high-grade metamorphic mineral assemblages, intrusion of 

mafic bodies, and tectonic juxtaposition of diverse rock packages. When 

coupled with geometrical effects, these reflection coefficients can produce 

abundant, reflective horizons in the deep crust such as those observed in 

some COCORP and BIRPS profiles. These effects can be demonstrated on 

synthetic reflection seismograms. 

Large variations in heat production have been measured in rocks from 

the Pikwitonei, Kapuskasing, and Ivrea lower crustal sections (>0.1 to 1.2 

~W/m ), and average heat production values (around 0.4 ~W/m ) are greater 

than anticipated for the deep crust. These variations may not be 

25 
resolvable in heat-flow studies because of their relatively small scale. 

These data indicate that the lower crust may provide a significant 

contribution to continental heat flow. 

Lower crustal rocks have high magnetic susceptibilities, with 

magnetite constituting the magnetic phase (Williams and others, 1985). 

Susceptibility is independent of temperature, suggesting that modern lower 

crust can be a magnetic source in stable regions. Finally, high 

susceptibility can occur in deep crust rocks of various compositions. 

Geophysical interpretations of lower continental crustal structure 

based on these cross sections must be tempered by considerations of 

complications introduced by tectonic emplacement of these terranes and the 

fact that the Moho has not been satisfactorily identified in any of these 

cross sections. Nevertheless, studies of this type will permit development 

of realistic geophysical models which can be used to interpret continental 

geophysical data. 

REFERENCES CITED 

Fountain, D.M., and Salisbury, M.H., 1981, Exposed cross sections through 

continental crust: Implications for crustal structure, petrology and 

evolution: Earth and Planetary Science Letters, v. 56, p. 263-277. 

Williams, M.C., Shive, P.N., Fountain, D.M., and Frost, B.R., 1985, 

Magnetic properties of exposed deep crustal rocks from the Superior 

Province of Manitoba: Earth and Planetary Science Letters, v. 76, p. 

176-184. 

26 
THERMAL, SEISMIC, AND PETROLOGIC CHARACTERISTICS OF THE LOWER CRUST AND 

UPPER MANTLE-A CASE HISTORY BASED ON XENOLITHS FROM EASTERN AUSTRALIA 

SUZANNE Y. O'REILLY, School of Earth Sciences, Macquarie University, North 

Hyde, New South Wales, 2113 Australia, and WILLIAM L. GRIFFIN, Geologisk 

Museum, Sarsgate 1, Oslo, Norway 

Upper mantle and lower crust xenoliths from eastern Australia provide 

samples of deep-seated rock types whose physical parameters can be used to 

constrain geophysical modeling. Selected xenoliths can be used for 

geothermobarometric calculations to determine the temperature and pressure 

(hence depth) of equilibration of these rocks and to construct a 

paleogeotherm which represents the ambient geothermal gradient at the time 

the xenoliths were entrained in the host basaltic magma. Xenoliths in 

young volcanics (<50 Ma) will reflect the current measurable thermal state 

of the underlying lithospheric column. 

A xenolith-derived geotherm was constructed for present-day 

southeastern Australia (O'Reilly and Griffin, 1985) using well-equilibrated 

xenoliths (with ideal assemblages of unzoned 

garnet+orthopyroxene+clinopyroxene) from two young (<3 Ma) maars in western 

Victoria. This geotherm gives a much higher temperature at any pressure 

than conventional continental or even oceanic geotherms. It has a strong 

curvature from about 1-3 GPa, indicating significant advective transfer of 

heat in the lower crust and upper mantle, consistent with intrusions of 

basaltic magmas in those regions. The locus of this geotherm contrasts 

markedly with that of the steady-state conductive geotherm, which has a 

shallower curvature at low pressures. This highlights the problems of 

extrapolating geothermal gradients to depth using surface heat-flow data. 

Pressure and temperature data for xenoliths from other Australian 

basaltic provinces of various ages (200 to 10 Ma) also plot on this 

geotherm and provide evidence of episodic thermal events over that time 

span which coincide with volcanism in discrete provinces. The eastern onethird of the Australian continent is relatively young (Paleozoic to 

Holocene) in contrast with the western two-thirds, which is Precambrian 

craton. It therefore appears that tectonically active continental regions, 

especially those with intraplate basaltic activity, are characterized by 

high geothermal gradients. This contrasts with the lower geothermal 

gradients of cratons, which correspond more closely with conductive 

geothermal profiles. 

These differences in thermal state of contrasting continental lithosphere types are crucial in the interpretation of seismic data, especially 

in determining the significance of the attainment of a "mantle" seismic 

velocity of approximately 8 km/s and higher. The other crucial factor is 

the real petrological nature of the lower crust and upper mantle. 

Using all available xenolith data for eastern Australia (Griffin and 

O'Reilly, in press; Bezant and others, unpub. data), it is possible to 

construct a lower crust-upper mantle stratigraphy that is consistent with 

petrologic, pressure-temperature, and geophysical data. In this model, the 

crust-mantle boundary lies at about 25 km, which is the depth where 

27 
ultramafic rocks (spinel lherzolite) become abundant and increase in 

proportion with depth. Layers of mafic rocks (which range in composition 

from pyroxenite through gabbro to anorthosite) decrease in proportion away 

from the crust-mantle boundary (both upward and downward). All lower 

crustal wallrocks are felsic to mafic granulite. 

Reversed seismic profiles for eastern Australia (Finlayson and others, 

1979) show that there is a gradient in V from about 25 km to 55 km, where 

Vp is about 8 km/s and thus represents tRe seismic Moho. This depth of 55 

km is the predicted depth where the southeastern Australian geotherm 

crosses the spinel- to garnet-lherzolite boundary (Griffin and others, 

1984). Therefore it is suggested that in continental regions of high 

geothermal gradient, the seismic Moho may represent the transition from 

spinel- to garnet-lherzolite, rather than the crust-mantle boundary. 

Calculated (O'Reilly and Griffin, 1985) and measured (Bezant and 

others, unpub. data) V 's for real mantle rock types are consistent with 

this interpretation. ¥hese VP values for spinel lherzolites are less than 

8 km/s for two main reasons: (1) the higher geothermal gradient lowers VP 

and (2) real mantle-derived spinel lherzolite generally contains 

significant (at least 40 percent) pyroxene, which gives a lower VP than the 

olivine-rich rock usually used for modeling the upper mantle. 

Xenolith data worldwide (Griffin and O'Reilly, in press) show that the 

lower crust is dominantly mafic. Therefore different thermal profiles are 

critical in determining whether or not the equilibrium mineral assemblage 

lies in the eclogite or the granulite facies, and hence in determining the 

Vp of these deep-seated regions. As cooling takes place from the high 

southeastern Australia geotherm to a lower cratonic geotherm, an increasing 

proportion of mafic rocks converts from granulite to eclogite assemblages, 

and hence V 's increase dramatically from about 6.9-7.6 km/s to 7.5-8.5 

km/s (Chris~ensen, 1982) for the mafic rocks. The calculated seismic 

profiles for these differing thermal regimes show that the seismic Moho 

will roughly coincide with the crust-mantle boundary in cratonic 

lithosphere. In lithospheric sections with high geothermal gradients, the 

seismic Moho will lie 20-30 km deeper than the crust-mantle boundary. 

These calculated VP-z profiles closely parallel observed reversed seismic 

profiles for western and eastern Australia. 

With successive cooling, mafic assemblages convert from granulite to 

eclogite at shallower depths. Because eclogite has significantly higher 

seismic velocity, the net effect is to decrease successively the depth to 

the seimic Moho until it coincides with the crust-mantle boundary in stable 

cratonic areas. 

Correlation of the geothermobarometry data with MAGSAT information can 

lead to mapping of the Curie-point isotherm within the lithosphere and 

thence to a regional surface heat-flow map. This method of contouring 

surface heat flow allows interpolation in regions where no direct heat-flow 

data are available. It also avoids anomalies due to perturbation of 

surface heat flow measurements by artesian water circulation. In addition, 

because pressure and temperature are constrained at depth, there is no 

necessity to assume arbitrarily a steady-state conductive heat-flow model, 

which may result in misleading extrapolation of heat-flow production to 

high pressure. 

28 
REFERENCES CITED 

Christensen, N.I., 1982, Seismic velocities, in Carmichael, R.S., ed., 

Handbook of physical properties of rocks,-v. 2: Boca Raton, Florida, 

CRC Press, p. 108-133. 

Finlayson, D.M., Prodehl, c., and Collins, C.D.N., 1979, Explosion seismic 

profiles, and implications for crustal evolution in southeastern 

Australia: BMR Journal of Geology and Geophysics, v. 4, p. 243-252. 

Griffin, W.L., and O'Reilly, S.Y., in press, The composition of lower crust 

and the nature of the continental Moho--Xenolith evidence, in Nixon, 

P.H., ed., Mantle xenoliths: London, Wiley. 

Griffin, W.L., Wass, S.Y., and Hollis, J.D., 1984, Ultramafic xenoliths 

from Bullenmerri and Gnotuk maars, Victoria, Australia: Petrology of a 

sub-continental crust-mantle transition: Journal of Petrology, v. 45, 

p. 53-87. 

O'Reilly, S.Y., and Griffin, W.L., 1985, A xenolith-derived geotherm for 

southeastern Australia and its geophysical implications: 

Tectonophysics, v. 111, p. 41-63. 

29 
CORRELATION OF XENOLITH PETROLOGY AND SEISMIC DATA: AN EXAMPLE FROM EASTCENTRAL QUEENSLAND 

WILLIAM L. GRIFFIN, F.L. SUTHERLAND and J.D. HOLLIS, Geologisk Museum, 

Oslo, Norway, and the Australian Museum, Sydney 

Some volcanoes in east-central Queensland (148°-153° E., 21°-27° S.) 

contain abundant crustal and mantle xenoliths. The surrounding basins are 

covered by high-quality seismic reflection and refraction data, so this is 

a good place to relate xenoliths to geophysical models. The volcanism in 

this region ranges in age from about 55 Ma to <1 Ma; in several provinces 

an episode of central volcanism, with tholeiite, trachyte and rhyolite 

occurred in the period 35-25 Ma. The later eruptions are commonly very 

undersaturated, and include most of the xenolith localities. 

Deep seismic reflection studies in the Bowen and Eromanga basins show 

a "transparent" middle crust and a "layered zone" with numerous subhorizontal reflectors between 22 and 36-40 km. Refraction data show 

VP .. 6.0-6.3 km/s in the transparent zone, with a sharp increase (0.35-0.6 

km/s) at the top of the layered zone. VP increases with the depth through 

the lower part of this zone, and jumps from -7.7 to -8.1 km/s at the bottom 

of this zone, interpreted as the Moho (Mathur, 1983; Finlayson and others, 

1984). 

Mantle xenoliths include dominant Cr-diopside spinel lherzolite, minor 

garnet websterite and Al-augite series pyroxenite. Crustal xenoliths 

include a variety of two-pyroxene granulite (2px+plag±apat) and garnet 

granulite (plag+gnt+cpx±opx±qtz). Corona structures show that some of 

these rocks formed by cooling and recrystallization of gabbroic mineral 

assemblages. Garnet commonly is broken down to very fine-grained 

aggregates of 2px+plag+spin; this is interpreted as due to contact 

metamorphism around crustal magma chambers. Some granulite xenoliths have 

compositions appropriate to basaltic liquids, but most appear on chemical 

grounds to be cumulates from basaltic melts. 

Pressure and temperature (P-T) calculations .for the garnet websterite 

and garnet granulite are complicated by oxidation of the pyroxenes, 

breakdown of garnet, and minor zoning of the minerals. Nevertheless, the 

garnet websterite data fall along the southeastern Australian geotherm of 

O'Reilly and Griffin (1985), and the garnet granulite data scatter about 

this line at P = 0.7-1.0 GPa. Temperatures for two-pyroxene granulite are 

700-850 °C, equivalent to P = 0.6-0.9 GPa, while oliv-opx-sp temperatures 

on spinel lherzolite are 875-1050 °c, equivalent toP= 0.9-1.6 GPa on this 

geotherm. The data suggest that the southeastern Australia geotherm of 

O'Reilly and Griffin (1985) is valid for the present area as well. Both 

areas lie within negative magnetic anomalies on maps of MAGSAT data (Mayhew 

and Johnson, unpub. data). 

The P-T estimates suggest that the granulite xenoliths are derived 

from depths of approximately 23-30 km, and the lherzolite xenoliths from 

about 28-55 km. Vp values for the granulite, calculated from density and 

chemical composition and corrected for T and P, average about 6.7 km/s at 

0.8 GPa, and are consistent with measured Vp's in the upper part of the 

30 
layered zone. Calculated Vp's for the spinel lherzolite are 8.1-8.2 

km/s. The Vp gradient from approximately 6.8 to 7.7 km/s with depth in the 

layered zone suggests a mixture of these two rock types, and the P-T 

estimates show that spinel lherzolite is the dominant rock type already at 

30 km depth. The crust-mantle boundary thus lies within the layered zone, 

while the seismically defined Moho simply represents the depth where the 

proportion of relatively low-VP mafic rocks drops below a critical level. 

The layered zone seen on the reflection profiles is therefore 

interpreted as consisting mainly of mafic granulite in the upper part, and 

of mixed mafic granulite, pyroxenite, and spinel lherzolite in the lower 

part. The chemistry and microstructures of the granulite xenoliths, and 

the apparent contact metamorphism of many of them, suggest that this 

layered zone formed as the result of repeated intrusions of basaltic melts 

into the lower crust and uppermost mantle. This conclusion is probably 

applicable to much of eastern Australia (Griffin and O'Reilly, in press). 

REFERENCES CITED 

Finlayson, D.M., Collins, C.D.N., and Lock, J., 1984, Wave velocity 

features of the lithosphere under the Eromanga basin, eastern 

Australia, including a prominent mid-crustal (Conrad ?) 

discontinuity: Tectonophysics, v. 101, p. 267-291. 

Griffin, W.L., and O'Reilly, S.Y., in press, The lower crust in eastern I Australia: Xenolith evidence: Journal of the Geological Society of 

London. 

Mathur, S.P., 1983, Deep reflection experiments in eastern Australia reveal 

differences in the nature of the crust: First Break, v. 3, p. 9-16. 

O'Reilly, S.Y. and Griffin, W.L., 1985, A xenolith-derived geotherm for 

southeastern Australia and its geophysical implications: 

Tectonophysics, v. 111, p. 41-63. 

31 
CRUST AND UPPER MANTLE BENEATH THE NORTHERN PLAINS: EVIDENCE FROM MONTANA 

XENOLITHS 

B. CARTER HEARN, JR., and ELAINE S. McGEE, U.S. Geological Survey, Reston, 

Virginia 22092 

Xenoliths in Eocene intrusions and volcanic rocks in north-central 

Montana provide data on the chemistry and evolution of a mostly buried 

terrane of Archean ancestry, and its lower-crustal and upper-mantle 

roots. This terrane is a northern extension of the Wyoming block of 

Archean rocks 2.5 Ga and older, which has been overprinted by Proterozoic 

metamorphism and magma emplacement between 1.7 and 1.9 Ga (Peterman, 

1981). Aeromagnetic and gravity patterns indicate discontinuities in the 

buried Precambrian rocks in eastern Montana (Zietz and others, 1971). Some 

of these discontinuities have been interpreted as metamorphic and tectonic 

boundaries, and have had recurrent influence on Phanerozoic sedimentation 

and shallow tectonic activity. 

Seismic studies show crustal thicknesses of 45 to 55 km and present a 

variety of crustal models. The profile closest to the xenolith sites shows 

a three-layer model that has an upper layer 3 to 8 km thick with a P-wave 

velocity of 6.1 km/s, a middle layer 15 km thick with a velocity of 6.4 

km/s, a lower layer 25 to 30 km thick with a velocity of 6.6 km/s, and an 

upper mantle with a velocity of 8.2 km/s (McCamy and Meyer, 1964). In 

east-central Montana, a north-south profile shows a three-layer model that 

has a velocity of 6.1 km/s for the depth interval 2 to 20 km, 7.0 km/s for 

20 to 35 km, 7.6 km/s for 35 to 55 km, and 8.1 km/s for the upper mantle 

(McCamy and Meyer, 1964). Intersecting that profile, a profile trending N. 

70° E. in the Large Aperture Seismic Array (LASA) shows a two-layer model 

that has a velocity of 6.1 km/s for the upper crust, 6.7 km/s for the lower 

crust, and velocity of 8.3 km/s for the upper mantle (Borcherdt and Roller, 

1968). More recent studies have found significant lateral variations of 

velocities beneath LASA that have been interpreted in terms of crustal 

heterogeneity, or a dipping or offset Moho, or a vertical low-velocity zone 

extending from the shallow crust to a depth of 140 km in the upper mantle 

(Glover and Alexander, 1969; Warren and others, 1973; Aki and others, 

1976). 

The greatest depth range of xenoliths, from the upper mantle to the 

shallow basement, is found in the Williams kimberlites. The deepest uppermantle samples are garnet peridotite and megacrysts, which are found in the 

Williams kimberlites and in the Macdougal Springs diatreme. Spinel 

peridotite, from the shallow upper mantle, is found in the Williams 

kimberlites and in three other Missouri Breaks diatremes, and is sparse in 

a few shonkinite intrusions in the Eagle Buttes, and in the Bearpaw 

Mountains phonolites. Eclogite is absent from all the xenolith suites. 

Xenoliths of probable mid- to lower-crustal origin are mafic to felsic 

garnet granulite and mafic to felsic garnet amphibolite. Metamorphic 

textures range from equigranular massive to banded on a scale of 1-3 mm. 

These are found in the Williams kimberlites, in two other Missouri Breaks 

diatremes, and in the phonolitic volcanic rocks of the Bearpaw Mountains. 

32 
Mid- to shallow-crustal xenoliths of garnet-free amphibolite, pelitic 

schist and gneiss, gabbro, and granite are found in the above localities 

and are locally abundant in the Bearpaw Mountains, Eagle Buttes, and 

Highwood Mountains. Those rock types also make up the exposed Precambrian 

basement in the core of the Little Rocky and Little Belt Mountains. 

Biotite-pyroxene-carbonate xenoliths in Missouri Breaks diatremes are 

enigmatic; these xenoliths are believed to represent carbonate-rich magmas 

and their cumulates trapped in the lower crust. 

Thermobarometry for garnet peridotite xenoliths indicates a perturbed 

middle Eocene geotherm (Hearn and McGee, 1984). Textures in some xenoliths 

indicate partial to complete reaction of garnet to a garnet-spinel facies 

or spinel facies assemblage, with indications of either metasomatic 

introduction of K2o or redistribution of K2o already present in the form of 

phlogopite and (or) amphibole. Spinel peridotite in Williams kimberlites 

tends to bear amphibole and (or) phlogopite. 

Preliminary chemical data (J.J. Cramer, 1985, written commun.) show a 

wide compositional range for garnet granulite and garnet amphibolite 

xenoliths, from nepheline-normative to quartz-normative basalt-like 

compositions; garnet amphibolite is generally higher in total Fe and more 

alkaline compared with garnet granulite. Rare-earth patterns also range 

widely, from primitive (flat REE, 8X chondrite; quartz-normative garnet 

granulite) to evolved characteristics (LREE-enriched, La 80-140X chondrite, 

Lu 7-15X chondrite; nepheline-normative garnet amphibolite), and suggest 

that the lower crust here has had a complex evolutionary history. 

Xenolith lithologies can be correlated tentatively with the two-layer 

and three-layer seismic-velocity models. The upper crust (6.0-6.1 km/s) 

probably consists of felsic pelitic schist and gneiss, gabbroic and 

granitic intrusions, and felsic amphibolite. In the two-layer model, the 

lower crust (6.7 km/s) could consist of garnet amphibolite and garnet 

granulite, with interlayered felsic, intermediate, and mafic types (based 

on composite xenoliths). In the three-layer models, the middle crust (6.4 

or 7.0 km/s) could consist of felsic and intermediate garnet amphibolite 

and garnet granulite, and the lower crust (6.6 or 7.6 km/s) could consist 

of mainly mafic garnet-pyroxene-rich granulite. Other lithologic 

distributions are possible, such as garnet amphibolite being dominant in 

the middle crust, and garnet granulite being dominant in the lower crust. 

However, the densities of probable lower crustal xenoliths do not fit 

the lower crustal seismic velocity in the nearest profile. The measured 

densities for 14 felsic to mafic granulite and amphibolite xenoliths range 

from 2.67 to 3.44 g/cm3, and average 3.01 g/cm3. For a depth of 30 km, 

based on the data of Christensen and Fountain (1975), these densities imply 

P-wave velocities ranging from 6.5 to 8.0 km/s and averaging 7.3 km/s, much 

higher than the 6.6 km/s velocity for the lowermost crust in the nearest 

seismic profile. Possible explanations are that (1) the xenolith suite is 

biased by the diatremes or by the collector, or (2) the seismic profile is 

not representative of the crust beneath the diatremes because of limited 

resolution of the seismic experiment or lateral heterogeneities in the 

lower crust. The xenolith data suggest that a lower crustal layer having a 

velocity that averages 7.0 to 7.6 km/s actually is present. Further 

seismic modeling and research on geochemical and geophysical properties 

should provide greater resolution. 

33 I 
REFERENCES CITED 

Aki, K., Christoffersson, A., and Husebye, E.S., 1976, Three-dimensional 

seismic structure of the lithosphere under Montana LASA: Seismological 

Society of America Bulletin, v. 66, no. 2, p. 501-524. 

Borcherdt, C.A., and Roller, J.C., 1968, Preliminary interpretation of a 

seismic refraction profile across the large aperture seismic array, 

Montana: U.S. Geological Survey Open-File Report, 31 p. 

Christensen, N.I., and Fountain, D.M., 1975, Constitution of the lower 

continental crust based on experimental studies of seismic velocities 

in granulite: Geological Society of America Bulletin, v. 86, no. 2, 

p. 227-236. 

Glover, P., and Alexander, S.S., 1969, Lateral variations in crustal 

structure beneath the Montana LASA: Journal of Geophysical Research, 

v. 74, no. 2, p. 505-531. 

Hearn, B.C., Jr., and McGee, E.S., 1984, Garnet peridotites from Williams 

kimberlites, north-central Montana, U.S.A., in Kornprobst, J., ed., 

Kimberlites II: The mantle and crust-mantle relationships: Amsterdam, 

Elsevier, p. 57-70. 

McCamy, K., and Meyer, R.P., 1964, A correlation method of apparent 

velocity measurement: Journal of Geophysical Research, v. 69, no. 4, 

p. 691-699. 

Peterman, Z.E., 1981, Archaean gneisses in the Little Rocky Mountains, 

Montana: u.s. Geological Survey Professional Paper 1199-A, p. A1-A6. 

Warren, D.H., Healy, J.H., Bohn, J., and Marshall, P.A., 1973, Crustal 

structure under LASA from seismic refraction measurements: Journal of 

Geophysical Research, v. 78, no. 35, p. 8721-8734. 

Zietz, I., Hearn, B.C., Jr., Higgins, M .w., Robinson, G. D., and Swanson, 

D.A., 1971, Interpretation of an aeromagnetic strip across the 

northwestern United States: Geological Society of America Bulletin, v. 

82, no. 12, p. 3347-3372. 

34 
THE MANTLE BENEATH THE RED SEA RIFT: XENOLITHS FROM WESTERN SAUDI ARABIA 

ANNE w. VAUGHAN, Department of Geology, Stanford University, Stanford, 

California 94305 

Formation of the Red Sea was accompanied by eruption of large volumes 

of alkaline basalt on the Red Sea margin of the Arabian continental 

shield. At least nine Saudi Arabian basalt fields contain xenoliths of 

mantle and crustal material. The mantle nodules provide a unique 

opportunity to study the mantle beneath the Saudi Arabian Red Sea margin. 

Two Saudi Arabian mantle xenolith localities have been described 

previously: Al Birk, by Ghent and others (1980), and Harrat Hutaymah, by 

Thornber and Pallister (1985). This study examines the mantle xenolith 

suite from Harrat al Kishb in·western Saudi Arabia. Harrat al Kishb is a 

large (6,700 km2) volcanic plateau of Pliocene-Pleistocene to Holocene 

age. The alkaline olivine basalts of Harrat al Kishb contain abundant 

xenoliths of mantle and crustal rocks that have been collected by R.G. 

Coleman during his work with the U.S. Geological Survey in Saudi Arabia. 

The al Kishb mantle suite consists of a variety of Type I and Type II 

spinel-bearing peridotite and pyroxenite nodules, several garnet-spinelamphibole-bearing pyroxenites, and one lherzolite transitional between 

Types I and II. Several rare composite nodules show relations between some 

of these mantle lithologies. 

The al Kishb Type I nodules exhibit far greater variation in 

lithology, mineral composition, and textures than Type II. Type I nodules 

are dominantly coarse-grained allotriomorphic-granular harzburgite and 

lherzolite. Pyroxenes have exsolved spinel and many are zoned, apparently 

due to re-equilibration to lower temperature conditions. Type I nodules 

are Mg-Cr rich, containing Cr-diopside, olivine with Fo90_91 •

5• and Mg-Cr 

rich spinel. Type I websterite crystallized at high temperature as 

subcalcic diopside and has exsolved and recrystallized with cooling. One 

composite nodule contains a 6-cm-wide vein of Type I websterite within Type 

I peridotite. 

Al Kishb Type II xenoliths are dominantly websterite and 

clinopyroxenite. Textures range from mosaic-equigranular to 

porphyroclastic, to allotriomorphic-granular with consertal grain 

boundaries suggestive of an igneous origin. Type II nodules are rich in 

Al, Fe, and Ti, and poor in Mg and Cr, relative to Type I xenoliths. One 

composite nodule contains a 5-6-em-wide vein of Type II pyroxenite within 

Type I peridotite. Type II pyroxenite apparently formed by crystallization 

of aluminous augite and spinel from basaltic liquids within veins in the 

mantle. Subsequent exsolution and recrystallization occurred in response 

to decreasing temperature. 

Intrusion of basaltic veins into the mantle may alter the peridotite 

surrounding the veins. One nodule of transitional lherzolite from Harrat 

al Kishb may represent peridotite from a reaction zone bordering a vein. 

The nodule mineral chemistry is transitional between Type I and II, and is 

compositionally zoned across the xenolith. One edge of the nodule is 

enriched in Fe and Al, and depleted in Mg and Cr. As one traverses the 

35 
nodule, away from this one edge, Fe and Al decrease steadily and Mg and Cr 

increase. Apparently the xenolith edge was at or near a contact with a 

vein. 

Several unusual garnet-spinel-amphibole-bearing pyroxenite xenoliths 

have been found at Harrat al Kishb. These nodules contain anhedral garnet 

forming "coronas" around rounded anhedral spinel. Ti-rich pargasite is 

always present in these nodules, and is not found in any garnet-free 

nodules from al Kishb. The compositions of pyroxene and spinel from these 

nodules are similar to other Type II spinel pyroxenites, except that the 

garnet-bearing pyroxenites are more Fe, Ti, and Na rich. 

In summary, the variety of lithologies, compositions, and textures 

within the Harrat al Kishb mantle xenolith suite is indicative of a 

heterogeneous upper mantle beneath western Saudi Arabia. The estimated 

range of temperatures and pressures of reequilibration of Harrat al Kishb 

mantle xenoliths is about 900-1150 °c, at about 1.5-2.2 GPa. Compared to a 

stable continental geotherm, this pressure-temperature range suggests 

elevated temperatures in the mantle, presumably due to mantle upwelling 

beneath the Red Sea rift. Exsolution and recrystallization textures of al 

Kishb mantle nodules suggest reequilibration to decreasing temperatures, 

and possibly pressure, within the upwelling mantle beneath the Saudi 

Arabian Red Sea margin. 

REFERENCES CITED 

Ghent, E.D., Coleman, R.G., and Hadley, D.G., 1980, Ultramafic inclusions 

and host alkali olivine basalts of the southern coastal plain of the 

Red Sea, Saudi Arabia: American Journal of Science, v. 280-A, p. 499-

527. 

Thornber, C.R., and Pallister, J.S., 1985, Mantle xenoliths from Northern 

Saudi Arabia [abs.]: Eos, Transactions, American Geophysical Union, v. 

66' p. 393. 

36 
A MECHANISM FOR THE INCORPORATION OF UPPER MANTLE MATERIAL INTO THE MIDDLE 

AND UPPER CRUST: EVIDENCE FROM SOUTHERN ALASKA 

WALTER D. MOONEY, GARY S. FUIS, ELIZABETH L. AMBOS, ROBERT A. PAGE, GEORGE 

PLAFKER, ·wARREN J. NOKLEBERG, U.S. Geological Survey, Menlo Park, 

California 94025, and DAVID L. CAMPBELL, U.S. Geological Survey, Denver, 

Colorado 80225 

Ideas regarding the possible composition of the lower crust are 

necessarily limited by what we view as "reasonable" given known geologic 

processes. Recent seismic refraction measurements in southern Alaska 

provide evidence for the incorporation of large amounts of mafic and 

ultramafic oceanic rocks into the qontinental crust. Underplating of the 

continent by paleosubduction zone complexes is the mechanism responsible. 

Southern Alaska is an area that is generally agreed to consist of a 

mosaic of oceanic and continental fragments that have been accreted to the 

continental margin of North America in the past 200 m.y. Paleomagnetic 

studies indicate that these terranes have moved north thousands of 

kilometers relative to the North American craton since the early 

Mesozoic. The seismic refraction profiles described here provide the first 

information on the deep crustal structure of the various terranes. 

Seven hundred kilometers of seismic refraction profiles were recorded 

in southern Alaska in 1984 and 1985. The profiles were shot as part of the 

Trans-Alaska Crustal Transect (TACT) program--a combined geological and 

geophysical study of the structure, composition, and evolution of the 

Alaskan crust from the Pacific Ocean to the Arctic continental margin. 

Our interpretation of the seismic refraction data is that the 

accretionary Chugach terrane and the composite Peninsular/Wrangellia 

terrane are underlain by an extensive sequence of continuous, north-dipping 

alternating low- and high-velocity layers. Velocities within the highvelocity layers are appropriate for ultramafic rocks. Each layer could in 

fact be considered "Moho." We interpret these layers as a stack of oceanic 

plates that have been subducted, the deepest plate being the youngest and 

currently subducting plate, while the upper plates have already been 

incorporated into the continental crust. This interpretation implies that 

continental growth in southern Alaska is in part accomplished by the 

underplating of paleosubduction zones at the active continental margin. On 

the basis of similar results obtained on Vancouver Island to the south, we 

believe that underplating may be a generally important mechanism for the 

incorporation of mafic and ultramafic rocks into the continental crust. 

37 
THE LOWER CRUST AND UPPER MANTLE BENEATH THE SIERRA NEVADA BATHOLITH: 

EVIDENCE FROM XENOLITHS IN LATE CENOZOIC VOLCANIC ROCKS 

FRANKLIN C.W. DODGE, U.S. Geological Survey, Menlo Park, California 94025 

Xenoliths from several small late Cenozoic volcanic plugs, pipes, and 

flow remnants provide a sampling of the lower crust and upper mantle 

underlying the central Sierra Nevada batholith. The nature of the 

xenoliths varies, apparently systematically inward from the margins to the 

core of the batholith. 

Localities of xenolith-bearing rocks at the edges of the batholith 

include the Jackson Butte and Golden Gate Hill dacite domes on the west 

side and the Oak Creek and Waucoba basalt flows on the east side. 

Fragments of near-surface country rock are the most common xenolith type, 

although magnesian peridotite xenoliths, generally of hornblende- or 

spinel-bearing lherzolite, are also present at these sites. The magnesian 

peridotite (Cr-diopside ultramafic group of Wilshire and others, 1985) is 

representative of mantle materials under the batholith's margins. 

Contrasting xenolith suites occur at three major and several 

associated minor localities concentrated in the core of the batholith. 

Peridotite and olivine-bearing pyroxenite as well as partially fused 

gabbroid and granitoid are common inclusions in the Blue Knob alkali basalt 

plug. A variety of high-grade metamorphic, commonly garnet-bearing 

xenoliths, including abundant eclogite and granulite, and batholithic rock 

fragments, are present in an andesite pipe near the town of Big Creek. 

Olivine-free pyroxenite dominates the xenolith assemblage, that also 

contains abundant plagioclase-rich granulite and sparse Fe-rich peridotite 

in a trachybasalt flow remnant capping Chinese Peak. 

These suites are believed to represent different source levels beneath 

the batholith. Blue Knob ultramafic xenoliths may have been derived from 

upper mantle materials. Ultrapotassic basalt related to the Blue Knob 

intrusion was generated at pressures greater than 2.0 GPa (Van Kooten, 

1980; Dodge and Moore, 1981). The garnet-bearing Big Creek inclusions were 

largely derived from ancient mafic oceanic floor materials; preliminary 

geobarometric estimates indicate equilibration at pressures of 1.5 to 1.7 

GPa. Chinese Peak xenoliths represent a continental mafic-ultramafic 

complex intruded into a granulite terrane. Crystallization pressure of 

approximately 1 .5 GPa has been determined on Chinese Peak xenoliths. 

Coupling of the xenolith and other surface data with a seismic~density 

profile across the central Sierra Nevada (Oliver, 1977) permits 

construction of a meaningful model of the crust and mantle beneath the 

batholith. The felsic granitoid and associated greenschist- to 

amphibolite-grade metamorphic rocks of the batholith make up a surface 

seismic-density layer (V = 6.0 km/s, p = 2.67) that extends only to a 

depth of 10 to 15 km. srudies in the southermost Sierra Nevada, where 

deeper crustal levels are exposed (Ross, 1985), indicate that a largely 

meta-igneous assemblage of hornblende-rich gneissic amphibolite- to 

granulite-grade rocks underlies the batholith and indeed may form its 

roots. These rocks form a roughly 10 km thick lens of the upper crust that 

38 
thins toward the margins of the batholith. The lower portion of the crust 

(VP = 6.9 km/s, p = 3.03) constitutes about half the entire crust. Its 

upper part is made up·of a series of deformed mafic-ultramafic intrusions 

that cut granulite-grade metasedimentary rocks. Mafic eclogite occurs at 

greater depths and may, perhaps with the disappearance of a minor phase, 

such as hornblende, span the crust-mantle seismic boundary. Olivine-rich 

ultramafic igneous rocks occur exclusively at greater mantle depths and in 

the mantle beneath the margins of the batholith. 

REFERENCES CITED 

Dodge, F.C.W., and Moore, J.G., 1981, Late Cenozoic volcanic rocks of the 

southern Sierra Nevada, California: II. Geochemistry: Geological 

Society of America Bulletin, Part I, v. 92, p. 912-914. 

Oliver, H.W., 1977, Gravity and magnetic investigations of the Sierra 

Nevada batholith, California: Geological Society of America Bulletin, 

v. 88, p. 445-461. 

Ross, D.C., 1985, Mafic gneissic complex (batholithic root?) in the 

southernmost Sierra Nevada, California: Geology, v. 13, p. 288-291. 

VanKooten, G.K., 1980, Mineralogy, petrology, and geochemistry of an 

ultrapotassic basaltic suite, central Sierra Nevada, California, 

U.S.A.: Journal of Petrology, v. 21, p. 631-684. 

Wilshire, H.G., and others, 1985, Mafic and ultramafic xenoliths from 

volcanic rocks of the western United States: u.s. Geological Survey 

Open-File Report 85-139, 49 p. 

39 
THE GROUND TRUTH FROM CRUSTAL XENOLITHS: A MULTIFACETED APPROACH 

ELAINE R. PADOVANI, U.S. Geological Survey, Reston, Virginia 22092 

Direct sampling of rock types that characterize the Earth's 

lithosphere is limited to outcrops, drill cores, and fragments (xenoliths) 

brought to the Earth's surface by erupting magmas. Xenoliths can provide 

important information about crustal evolution at depth and can place 

constraints on properties of the lower crust and crust-mantle interface 

from geophysical measurements. Such calibration is critical for refining 

models of rheological structure and transport phenomena in the continental 

lithosphere. Because xenolith suites are nonrenewable, a multifaceted 

analytical approach should be undertaken wherever samples of sufficient 

size and abundance are encountered, and after establishing that in situ 

properties have not been overprinted by the emplacement process. 

Projects that have integrated petrology and geochemistry of xenolith 

suites with geophysical studies are few. In the United States, two 

xenolith occurrences have received intensive scrutiny from both geophysical 

and petrological perspectives. Studies of samples from Kilbourne Hole and 

Potrillo maars in the southern Rio Grande rift, and from Moses Rock dike 

and the Mule Ear diatreme on the Colorado Plateau, are good examples of 

what can be done to improve our understanding of the chemical and physical 

properties of crustal depths below our present drilling capabilities (see 

McGetchin and Silver, 1972; Padovani and Carter, 1977; Hart and others, 

1979; Padovani and Hart, 1981; Wasilewski and Padovani, 1981; Padovani and 

others, 1982; Reid and others, 1985; Wandless and Padovani, 1985). At the 

same time, these studies reveal pitfalls that must be avoided if xenolith 

suites are to be utilized to their fullest potential. The following brief 

discussion touches on models of the crust and upper mantle, and on 

techniques used to ensure the best possible use of material available for 

both geophysical and geochemical purposes. 

RIO GRANDE RIFT 

Techniques.--The crustal xenolith suites at Kilbourne Hole and 

Potrillo maars are e~ptional because of the variety and abundance of rock 

types sampled from middle to lower crustal depths. In addition, their 

large size (10-40 em maximum diameter) has permitted a spectrum of 

observations and analyses to be made on individual samples. Preliminary 

petrographic and electron microprobe analyses revealed that most samples 

are pristine, and, except for minor amounts of decompression melting, have 

anisotropies and mineralogies that are representative of in situ conditions 

at depth. Techniques used to maximize information from each sample were: 

(1) photographic documentation before and after sawing, coring, and 

numbering; (2) acquisition of oriented cores where possible for geophysical 

measurements; (3) preservation of a reference slab for archival purposes; 

4) preparation of sufficient material from an adjacent representative slab 

to accommodate the requirements of a variety of geochemical analyses; and 

(5) preparation of a sufficient number of polished thin sections to allow 

40 
complete characterization of major- and minor-element geochemistry of 

mineral phases across the slab used for other types of geochemical 

analysis. Approximately 20-25 xenoliths have been characterized with 

respect to mineralogy, seismic velocity, density, magnetic properties, 

major-, minor-, and trace-element compositions, and (or), isotopic 

signatures (Rb/Sr, Sm/Nd, U/Th/Pb, 0). 

Pitfalls.--Major pitfalls to be avoided in studying this type of 

xenolith suite are associated with sample preparation and with 

nonrecognition of decompression melt and (or) its melt crystallization 

products, all of which can lead to gross misinterpretations of physical 

properties and petrogenesis. 

Model.--The lower crust beneath the southern Rio Grande rift is 

dominated by metasedimentary and meta-igneous granulites and appears to be 

of intermediate composition. Rb/Sr and 207/204-206/204 Pb isochrons give 

apparent ages of 1.6-1.1 Ga for the crust in this region, but both Rb/Sr 

and Sm/Nd mineral isochrons within the xenolith suite give ages of 0-37 

Ma. Oxygen isotopic compositions are typical crustal values and do not 

have a recognizable mantle component. These data suggest that the lower 

crust cooled to a stable shield geothermal gradient after cratonization and 

then was reheated to 750-900 °c during magmatic activity related to opening 

of the Rio Grande rift until the crust was sampled by maar eruptions 0.125 

m .y. ago. 

Laboratory measurements yield intermediate velocities for lower 

crus~al xenoliths of felsic granulite, which have densities of 2.9-3.1 

g/cm • Ilmeni te-dom.inated mineralogies and elevated temperatures in the 

lower crust are responsible for the shallow Curie isotherm at 10-15 km, 

thus regional magnetic anomalies are due to both the topology of the Curie 

isotherm and petrologic variations within the crust. These results agree 

with observed properties of the lower crust from geophysical measurements 

(Jiracek and others, 1979; Seager and Morgan, 1979; Baldridge and others, 

1984) and suggest that crustal granulite may have a major influence on 

observed properties of the crust in reactivated intracontinental rifts. 

COLORADO PLATEAU 

Techniques.--Crustal xenolith suites from Moses Rock dike and Mule Ear 

diatreme were sampled in order to compare observed seismic velocity 

structure beneath the Colorado Plateau with velocities measured on 

representative xenoliths. In contrast to the rift samples, most Colorado 

Plateau xenoliths have suffered extensive hydrothermal alteration during 

and (or) after emplacement, an effect not recognized by McGetchin and 

Silver (1972). However, systematic collection from the dikes resulted in 

specimens that exhibit a spectrum of alteration in the same rock type from 

"pristine" to "altered." Due to the small size of the "pristine" samples 

(8-15 em maximum diameter), drilling of cores was given highest priority, 

and unused portions of cores and remnents were reserved for electron 

microprobe, scanning electron microscope (SEM) and petrographic analyses. 

Pitfalls.--The major pitfall in the study of this type of xenolith 

suite is nonrecognition of hydrothermal overprinting of lower crustal 

amphibolite and granulite. Petrography and SEM observations reveal a 

direct correlation between progressive alteration and presence of 

_microcracks extending into the xenoliths from the kimberlitic host. In 

41 
---- ---··--------- ---- ---------- T--------- -----------

addition, seismic velocities measured in altered samples are too low for 

such rocks to be characteristic of deep crustal levels. 

Model.--Seismic velocities measured on or calculated for pristine 

samples agree with crustal velocity models, but not with the crustal model 

of McGetchin and Silver (1972). It is concluded that the crust below 15 km 

is not hydrated on a majot scale and that the "greenschist facies event" 

postulated for the lower crust by these authors is more likely "localized" 

in each xenolith as a result of the emplacement process. The crustal 

reconstruction of Padovani and others (1982) results in a profile that has 

similarities to the Ivrea Zone (Fountain, 1976) and is characterized by 

rhyolite, granite, and low-grade metamorphic rocks in the upper crust, and 

by mafic, garnet-bearing amphibolite and intermediate to mafic granulites 

in the lower crust. 

SUMMARY 

To achieve the most benefit from available xenolith material, it is 

clear that a multifaceted approach is needed. The examples cited reflect 

some of the information to be gained and pitfalls to be encountered in the 

study of xenolith suites from contrasting crustal environments. A 

coordinated approach from the petrologic, geochemistry, and geophysical 

communities is clearly needed if we are to profit from the potential 

knowledge to be gained fron the xenolith suites. 

REFERENCES CITED 

Baldridge, w.s., Olsen, K.H., and Callender, J.F., 1984, Rio Grande rift: 

Problems and perspectives, in Guidebook, 35th Field Conference, Rio 

Grande Rift: Northern New Mexico: Santa Fe, New Mexico Geological 

Society, p. 1-12. 

Fountain, D.M., 1976, The Ivrea-Verbano and Strona-Ceneri zones, northern 

Italy: A cross-section of the continental crust--New evidence from 

seismic velocities of rock samples: Tectonophysics, v. 33, p. 313-327. 

Hart, S.R., Padovani; E.R., and Roden, M.K., 1979, Sr-isotopic relationships in lower crustal nodules from Kilbourne Hole, New Mexico 

[abs.]: Geological Society of America Abstracts with Programs, v. 7, 

no. 7, p. 4 39. 

Jiracek, G.R., Ander, M.E., and Holcombe, H.T., 1979, Magnetotelluric 

soundings of crustal conductive zones in major continental rifts, in 

Riecker, R.E., ed., Rio Grande rift: Tectonics and magmatism: 

Washington, D.C., American Geophysical Union, p. 208-222. 

McGetchin, R.R., and Silver, L.T., 1972, A crustal-Upper-mantle model for 

the Colorado Plateau based on observations of crystalline rock fragments in Moses Rock Dike: Journal of Geophysical Research, v. 77, p. 

7022-7037. 

Padovani, E.R., and Carter, J.L., 1977, Aspects of deep crustal evolution 

beneath-south central New Mexico, in Heacock, J.G., ed., The Earth's 

crust, its nature and physical properties: Washington, D.C., American 

Geophysical Union, p. 19-55. 

Padovani, E.R., Hall, J.; and Simmons, G., 1982, Constraints on crustal 

hydration below the Colorado Plateau from v

0 measurements on crustal 

xenoliths: Tectonophysics, v. 84, p. 313-32~. 

42 
Padovani, E.R., and Hart, S.R., 1981, Geochemical constraints on the 

evolution of the lower crust beneath the Rio Grande Rift, in Conference 

on the processes of planetary rifting: Napa, California, Lunar 

Planetary Institute Contribution No. 457, p. 149-152. 

Reid, M., Hart, S.R., and Padovani, E.R., 1985, Importance of sedimentary 

protoliths to the lower crust exemplified by Kilbourne Hole 

paragneisses - Sr, Nd, Pb, isotope geochemistry: Eos, Transactions, 

American Geophysical Union, v. 66, no. 46, p. 1110. 

Seager, W.R., and Morgan, P., 1979, Rio Grande Rift in southern New Mexico, 

west Texas, and northern Chihuahua, in Reicker, R.E., ed., Rio Grande 

Rift: Tectonics and Magmatism: Washington, D.C., American Geophysical 

Union, p. 87-106. 

Wandless, G.A., and Padovani, E.R., 1985, Trace element geochemistry of 

lower crustal xenoliths from Kilbourne Hole maar, New Mexico: Eos, 

Transactions, American Geophysical Union, v. 66, no. 46, p. 1110. 

Wasilewski, P.J., and Padovani, E.R., 1981, Crustal magnetization beneath 

the Rio Grande rift based on xenoliths from Kilbourne Hole and Potrillo 

maar: Conference on the processes of planetary rifting, Napa, Calif., 

Lunar Planetary Institute Contribution no. 457, p. 153-155. 

43 
NEW VIEW OF THE CONTINENTAL CRUST FROM SEISMIC REFLECTION PROFILING 

GEORGE A. THOMPSON, Department of Geophysics, Stanford University, 

Stanford, California 94305 

Below the sedimentary section, the crystalline crust of the continents 

has long been defined by seismic refraction P-wave velocities of about 5-7 

km/s and the mantle below by velocities greater than about 7.8 km/s. The 

transition so defined is the M discontinuity. The observed crustal 

velocities are appropriate for silicic to mafic igneous and metamorphic 

rocks, such as those exposed in deeply eroded mountain belts, including 

granulite and serpentinized ulramafic rocks. The upper mantle velocities 

are appropriate for peridotite and eclogite. Multichannel seismic 

reflection profiling has increased the resolving power in the deep crust 

from kilometers to about 100 m and revealed that the crust in many regions 

is full of reflectors in contrast to a comparatively transparent mantle. 

Thus the "reflection Moho" is generally picked as the base of the 

reflective section; it is usually marked also by a zone of discontinuous 

higher amplitude reflectors; this zone in the basal part of the crust is 

one-half to several kilometers thick. 

Perhaps the most surprising revelation of the reflection sections is 

the subhorizontal layered fabric seen in large parts of the crystalline 

crust. Deep-seated zones of thrust and normal faulting (mylonites) are 

also well imaged, and these inclined reflectors add assurance that the 

subhorizontal relectors are not an artifact of the method. 

A representative reflection section in a tectonicallY active region 

with granitic plutons at the surface might consist, from the top down, of 

(1) a nearly transparent section (few reflectors) to a depth of 10 km or 

so, (2) several kilometers of discontinuous subhorizontal reflectors that 

suggest high and low velocities, (3) a few kilometers of fewer reflectors 

or nearly transparent section, and (4) a kilometer or more of subhorizontal 

reflectors at the base of the crust. The available interpretations for (2) 

range from igneous through metamorphic to tectonic. An igneous 

interpretation may suggest tabular mafic intrusions underplating the 

granitic terrane; the mafic intrusions might be interlayered with gneiss 

and might contain cumulate layers. A tectonic interpretation necessarily 

emphasizes subhorizontal zones of ductile deformation below the brittle, 

seismogenic upper crust; contrasting lithologies sheared out horizontally 

into a roughly layered mass would fit the reflection data. Micaceous 

minerals have a much lower velocity perpendicular than parallel to their 

cleavage and can strongly influence the reflectivity of layered mylonites. 

The interpretation of the lowest, Moho reflectors must also consider 

tectonic and igneous possiblities. In active regions such as the Basin and 

Range, the Moho seems to cut and be superimposed upon all the older 

structures. It may be a zone of shear and flow in the thermally softened 

rocks just above the stronger mantle peridotite. The laminated appearance, 

however, requires interlayering of contrasting rocks on a scale of tens to 

hundreds of meters; the Ivrea zone is one possible model. A combination of 

partial melting and metamorphism of lower crust, mafic intrusions from the 

44 
mantle, and concentrated deformation within the soft zone near the base of 

the crust seems likely. 

45 
EVIDENCE FOR MOHO EVOLUTION IN OROGENIC BELTS BASED ON COCORP PROFILES 

CHRISTOPHER J. POTTER, RICHARD W. ALLMENDINGER, ERNEST C. HAUSER, and 

K. DOUGLAS NELSON, Institute for Study of the Continents, Cornell 

University, Ithaca, New York 14853 

COCORP deep seismic reflection profiling along four transects of 

Phanerozoic orogens in the United States (two Appalachian transects and two 

Cordilleran transects) has shown that high-amplitude, relatively flat and 

continuous Moho reflections occur beneath orogenic interiors (characterized 

by thick-skinned thrusting and post-thrusting crustal extension), while the 

reflection Moho becomes much less distinct as seismic profiles cross from 

orogenic interiors into thin-skinned thrust belts. Examples of these 

transitions occur in the eastern Great Basin (Allmendinger and others, in 

press), eastern Washington-northwestern Montana (Potter and others, in 

press), the Georgia Piedmont (Cook and others, 1981; Nelson and others, 

1985), and the New England Appalachians (Ando and others, 1984). Beneath 

thin-skinned thrust belts and cratonic areas, the reflection signature of 

the Moho is a downward disappearance of reflections, rather than a 

prominent set of reflections. The following characteristics suggest that, 

in regions where overthickened crust developed during compressional 

orogenesis, the position and geometry of the Moho have evolved after 

thrusting: (1) the occurrence of Moho reflections as very prominent, 

relatively flat features confined to orogenic interiors, (2) the fairly 

constant 30-35 km depth to Moho in these regions, and (3) angular 

relationships preserved (and observed on seismic profiles) between the 

reflection Moho and dipping lower_crustal structure. 

Cordilleran examples of the well-defined "reflection Moho" occur under 

the Basin and Range in Nevada along a COCORP traverse at about 40 °N 

latitude (Klemperer and others, 1987), the crystalline complexes of 

northeastern Washington and northern Idaho (Potter and others, in press), 

as well as Death Valley and the Mojave Desert (Cheadle and others, 1986). 

In the Appalachians, apparently similar Moho reflections occur beneath the 

Georgia coastal plain and in New Hampshire. In the Appalachian examples 

the subhorizontal Moho reflections underlie dipping crustal reflections 

that are interpreted as Paleozoic thrusts (Ando and others, 1983; Nelson 

and others, 1985). In all of these places, the Moho reflections are at 

two-way travel times of 10-11.5 s (about 30-35 km). 

The Moho varies smoothly·and continuously beneath a complexly deformed 

crust in these interior regions, and there are several examples of angular 

relationships between dipping deep crustal structures and the relatively 

flat Moho. Beneath central Georgia, dipping reflections which mark the 

Alleghenian suture can be traced nearly to the Moho; preliminary migration 

studies indicate that the deepest of the dipping suture-related reflections 

in the lower crust are truncated by a horizontally laminated reflection 

sequence at two-way travel times appropriate for the Moho (Nelson and 

others, 1985). In the Cordilleran interior of the northwestern United 

States, gently east-dipping reflections, interpreted as broad ductile 

deformation zones related to Eocene extension, appear to sole into the 

46 
Moho; west-dipping reflections from thrust structures can also be traced 

__ into the deep crust (Potter and others, in press). Beneath eastern Nevada, 

a crust-penetrating normal fault can be traced to, but not deeper than, the 

Moho (Hauser and others, in press). These examples strongly suggest that 

the geometry and position of the Moho evolves dynamically beneath orogenic 

belts, with significant Moho adjustments occurring after major episodes of 

crustal shortening. The well-defined reflection Moho appears confined to 

regions where post-thrusting magmatism and extension was important; thus, 

its development may be related to magmatic processes in the lowermost crust 

(both underplating from mantle-derived magmas and partial melting of the 

deep crust). In addition, examples listed above suggest that high strain 

zones along the Moho may have accommodated large displacements during postthrusting extension. 

REFERENCES CITED 

Allmendinger, R.W., Hauge, T.A., Hauser, E.C., Potter, C.J., and Oliver, 

J.E., in press, Tectonic heredity and the layered lower crust in the 

Basin and Range Province, western United States: Geological Society of 

London Special Volume on Extensional Tectonics. 

Ando, C.J., Czuchra, B.L., Klemperer, S.L., Brown, L.D., Cheadle, M.J., 

Cook, F.A., Oliver, J.E., Kaufman, S., Walsh, T., Thompson, J.B., 

Lyons, J.B., and Rosefeld, J.L., 1984, Crustal profile of mountain 

belt: COCORP deep seismic reflection profiling in New England 

Appalachians and implications for architecture of convergent mountain 

chains: American Association of Petroleum Geologists Bulletin, v. 68, 

p. 819-837. 

Cheadle, M.J., Czuchra, B.L., Ando, C.J., Byrne, T., Brown, L.D., Oliver, 

J.E. and Kaufman, S., in press, Geometries of deep crustal faults: 

Evidence from the COCORP Mojave survey, in Barazangi, M., and Brown, 

L.D., eds., Reflection seismology: The continental crust: Washington, 

D.C., American Geophysical Union, Geodynamics Series, v. 14. 

Cook, F.A., Brown, L.D., Kaufman, s., Oliver, J.E., and Petersen, T.A., 

1981, COCORP seismic profiling of the Appalachian orogen beneath the 

coastal plain of Georgia: Geological Society of America Bulletin, Pt. 

1, v. 92, p. 738-748. 

Hauser, E.C., Burgess, S., Burtch, S., Mutschler, J., Potter, C.J., Hauge, 

T.A., Allmendinger, R.W., Brown, L.D., Kaufman, S., and Oliver, J.E., 

in press, Crustal structure of eastern Nevada from COCORP deep seismicreflection data: Geological Society of America Bulletin. 

Klemperer, S.L., Hauge, T.A., Hauser, E.C., Oliver, J.E., and Potter, C.J., 

1986, The Moho in the northern Basin and Range, Nevada, along the 

COCORP 40 °N. seismic-reflection transect: Geological Society of 

America Bulletin, v. 97, p. 603-618. 

Nelson, K.D., Arnow, J.A., McBride, J.H., Willemin, J.H., Huang, J., Zheng, 

L., Oliver, J.E., Brown, L.D., and Kaufman, S., 1985, New COCORP 

profiling in the southeastern United States, Part 1: Late Paleozoic 

suture and Mesozoic rift basin: Geology, v. 13, p. 714-717. 

Potter, C.J., Sanford, W., Yoos, T., Prussen, E., Keach, R.W., Oliver, 

J.E., Kaufman, S., and Brown, L.D., in press, Deep seismic profiles 

across the interior of the north American Cordillera, Washington and 

Idaho: Implications for orogenic evolution: Tectonics. 

47 
CONSTRAINTS ON THE PHYSICAL NATURE OF DEEP CRUSTAL STRUCTURES IN 

SOUTHEASTERN CALIFORNIA FROM THE CALCRUST SEISMIC REFLECTION EXPERIMENT 

JOHN N. LOUIE and ROBERT W. CLAYTON, Seismological Laboratory, California 

Institute of Technology 252-21, Pasadena, California 91125 

Seismic reflection studies of deep-crustal structures have, to this 

date, concentrated on imaging the geometry of the reflectors. While 

reflector geometry can be useful in the geologic interpretation of these 

structures, many ambiguities regarding the physical nature of deep 

reflectors remain. Reflections from deep crustal structures beneath the 

Mojave Desert of California were first recognized by Dix in 1965. The 

multiplicity and strength of the reflections he recorded led him to believe 

that the velocity of the crust could oscillate with depth, rather than 

increase monotonically. The more extensive COCORP survey across the 

western Mojave succeeded in imaging the geometry of several deep-crustal 

reflectors over a wide area (Cheadle and others, 1986). With the exception 

of one reflection which they believe can be traced to surface exposures, 

Cheadle and others' (1986) geologic interpretation of the mid- and deepcrustal reflections was guided by the geometry in stacked sections. They 

consider the major reflectors to be detachment fault structures. This 

interpretation ignores the possibility, proposed by Kosminskaya (1964), 

that crustal velocity discontinuities may be due to the effect of physical 

conditions such as pressure or temperature. It is this possibility that 

motivated the CALCRUST consortium to add a secondary experiment to record 

reflections from the deep crust during its May-June 1985 seismic reflection 

survey in the eastern Mojave Desert. 

Both of the previous surveys had used only the arrival time and 

"character" of the recorded reflections in their interpretations. The 

character of the reflections they evaluated was the average character over 

the entire source-to-receiver offset range of the experiment such as on a 

stacked seismic section. However, much more information is available in 

the multioffset seismograms which can be used to constrain the physical 

properties of the reflector, such as changes in compressional and shear 

velocity and density. We separate the problem of inverting the seismograms 

for such properties into two parts. The first-order problem is determining 

whether the reflector constitutes an abrupt change in these properties (a 

step discontinuity), is a more gradual variation over some depth range (a 

gradient), or is composed of layers having thicknesses on the same scale as 

the seismic wavelength (the simplest case of which is an isolated thin 

layer). The ability to classify observe~ reflections into one of these 

three categories is crucial for deciding whether the structures represent 

major changes in composition or physical conditions, or are the result of 

fault movement. The second-order problem is to use the variation of 

reflection amplitude with offset to yield information on the relative 

changes in the velocities and density. Such information is needed to 

decide whether the structures represent changes in mineralogy or are due to 

_variations in physical properties such as porosity or anisotropy. 

48 
The aim of our initial effort is to distinguish whether step 

discontinuity, gradient, or thin-layer models best fit the character of the 

deep-crustal reflections observed by the CALCRUST survey. We investigated 

the reflection spectral response of the three conceptual models through 

elastic finite-difference modeling in two dimensions. The synthetics show 

that a thin layer produces a distinctive increase in the peak frequency of 

the reflection with offset. This increase is adequately described by 

simple acoustic interference relations which allow the thickness and 

velocity of the thin layer to be derived from the dispersion. 

The spectra of normal-incidence reflections were used to support the 

concept of a layered Moho and derive layer thicknesses on the order of 100 

m (Clowes and Kanasewich, 1970; Meissner, 1973). Later workers (Hale and 

Thompson, 1982; Jones and Nur, 1984; Jones~ 1985) combined these ideas with 

laboratory measurements of candidate rock types for deep-crustal reflectors 

and the Moho to support the thin-layer model as the cause of high-amplitude 

normal-incidence reflections observed on deep seismic reflection sections. 

A total of 108 km of seismic reflection profiling was collected in May 

and June 1985 by the CALCRUST consortium. The survey was located along 

five lines in the Ward, Rice, and Vidal Valleys of the eastern Mojave 

Desert in southeastern California. While the main objective of the survey 

was to collect high-resolution seismic reflection data from the shallow 

part of the crust, the consortium was able to augment the main survey with 

a secondary experiment that resulted in reversed, long-offset, high fold 

common-midpoint records over a substantial portion of three lines. One of 

these lines, in the southern Ward Valley, yielded 8 km of reversed commonmidpoint gathers spaced at 75 m. Many gathers exceed 100 fold and include 

offsets beyond 15 km. These data show abundant reflections from the middle 

and deep crust, and from the Moho, at times from 5.5 to 10 s. Particularly 

strong reflections can be seen between 5.5 and 6 s, between 7 and 7.5 s, 

and from the Moho from 8.5 to 10 s. These reflections are easily 

recognized in a large number of the common-midpoint gathers. They can 

commonly be traced from the shortest to the longest offsets~ even on 

gathers that have not been enhanced by processing beyond demultiplexing, 

correlation, and sorting. 

Hand-picked examples of deep-crustal reflections were examined for 

spectral dispersion. Reflections with and without dispersion were found 

from several areas providing the highest quality data. A strong reflection 

at 6 s, about 15 km depth, can be identified as a thin layer at many 

midpoints. The amount of dispersion shows that the layer, a few hundred 

meters in thickness, must have a velocity at least 10 percent higher than 

the velocity of the overlying medium. This is also true of many of the 

other very strong thin-layer reflectors observed. Such a velocity increase 

in a thin layer is most likely due to the presence of higher-grade 

metamorphic mineral phases or to a more mafic composition. On the other 

hand, many strong reflections between 6.8 and 7.6 s do not show dispersion 

and may result from broad-scale vertical velocity changes at step 

discontinuities. This suggests that the lower crust, at perhaps 18 km 

depth, undergoes a change in composition or physical state. 

High-quality Moho-depth reflections could be picked on virtually every 

trace of the Ward Valley data set. These events are grouped into two 

zones, arriving at about 8.4 and 9.4 s. The earlier reflection does not 

_show dispersion, which is strong on the later arrival. This suggests, along 

49 
with correlations with regional refraction profiles, that the earlier 

reflection is due to a step increase in velocity at the top of a highvelocity subcrustal zone. The later arrival is then from the Moho, which 

is shown to include a layer a few hundred meters thick with a velocity 

exceeding that of the underlying mantle. This interpretation leads to 

speculation that such a layer could be the residual from partial melting 

and extrusion of mantle material or be a zone of highly anisotropic mantle 

oriented by tectonic mobilization of the Moho. 

REFERENCES CITED 

Cheadle, M.J., Czuchra, B.L., Ando, C.J., Byrne, T., Brown, L.D., Oliver, 

J.E., Kaufman, S., 1986; Geometries of deep crustal faults: Evidence 

from the COCORP Mojave survey, in Barazangi, M., and Brown, L., eds., 

Reflection seismology: The continental crust: Washington, D.C., 

American Geophysical Union, Geodynamics Series, v. 14. 

Clowes, R.M., and Kanasewich, E.R., 1970, Seismic attenuation and the 

nature df reflecting horizons within the crust: Journal of Geophysical 

Research, v. 75, p. 6693-6705. 

Dix, C.H., 1965, Reflection seismic crustal studies: Geophysics, v. 30, p. 

1068-1084. 

Hale, L.D., and Thompson, G.A., 1982, The seismic reflection character of 

the continental Mohorvicic discontinuity: Journal Geophysical 

Research, v. 87, p. 4625-4635. 

Jones, T.D., 1985, Nature of seismic reflections from the crystalline 

basement: COCORP Wind River line, Wyoming: Journal of Geophysical 

Research, v. 90, p. 6783-6791. 

Jones, T.D., and Nur, A., 1984, The nature of seismic reflections from deep 

crustal fault zones: Journal of Geophysical Research, v. 89, p. 3153-

3171. 

Kosminskaya, I.P., 1964, On layering of the earth's crust: Journal of 

Geophysical Research, v. 69, p. 802-805. 

Meissner, R., 1973, The "Moho" as a transition zone: Geophysical Surveys, 

v. 1 ' p. 1 95. 

50 
Deformation, Preferred Orientation and Property 

Anisotropy in the Continental Lithosphere 

CAUSES AND EFFECTS OF ANISOTROPY IN DEFORMED ROCKS 

H.-R. WENK, Department of Geology and Geophysics, University 6f California, 

Berkeley, California 94702 and GEORGE c. JOHNSON, Department of Mechanical 

Engineering, University of California, Berkeley, California 94720 

Plastic deformation of rocks in the crust and in the mantle has been 

modeled with Arrhenius-type flow laws which relate stress and strain, 

treating them essentially as scalars and not accounting for the deformation 

history. This assumption is valid for a limited range of mechanisms, for 

example, superplastic flow, dislocation climb, and some mechanisms of 

recrystallization. Wherever dislocation glide is involved in the 

deformation process, preferred orientation develops which depends on the 

strain mode, and preferred orientation is present in mantle xenoliths as 

well as metamorphic tectonites. The development of texture has a profound 

influence on the stress-strain behavior: (1) It imposes a threedimensional geometric factor due to crystal orientation which is expressed 

by the Taylor factor, (2) Strain hardening occurs due to interaction of 

dislocations in active slip systems, and (3) Latent hardening is expected 

because of microstructural difficulties in activating inactive slip systems 

at assumed critical shear stresses. The influence of geometric factors on 

the plasticity of polycrystals can be approached with the full-constraint 

Taylor theory (Wenk and others, in press). We observed that for calcite, 

the effects of strain mode are even stronger than in metals, where the 

anisotropy of plastic flow was first documented. For example, at low 

temperature in axial deformation the specimen hardens rapidly, whereas in 

51 
plane strain it softens during straining. The differences can be 

attributed to activation of different slip systems (number and relative 

importance) in each case, particularly the role of mechanical twinning. 

Wherever dislocation glide is significant and wherever preferred 

orientation is present, anisotropic flow laws should be considered to 

explain deformation in geological materials. Particularly emphasized is 

the importance of deformation experiments in geometries different from 

axisymmetric compression. 

Rocks which have preferred crystallographic orientation, such as 

mantle xenoliths and many crustal tectonites, do display anisotropy of 

physical properties. Of particular importance to geophysicists are the 

anisotropy in thermal conductivity and of acoustic properties. If the 

texture, given in terms of the three-dimensional orientation distribution 

function, and the single-crystal properties are known, the properties of 

the rock can be quantitatively calculated. We have developed analytical 

expressions for the elastic constants of marble in terms of the elastic 

constants of the calcite single crystals and the orientation distribution 

(Johnson and Wenk, in press). The texture is expressed by expanding the 

distribution function in a series of spherical harmonics (Bunge, 1982; 

Wenk, 1985). In the case of orthorhombic sample symmetry, only six 

coefficients of this expansion are needed to fully characterize the elastic 

properties of the polycrystal when either the Voigt or the Reuss averaging 

techniques are used. 

We have investigated a sample of deformed marble from the Santa Rosa 

mylonite zone (southern California). The texture is strong (c axes 

maximum = 6 times random distribution) and the calculated anisotropy of the 

P-wave velocities is 17 percent. The estimated response agrees with the 

data, which is closer to the Reuss average than to the Voigt average. 

Experimental factors influence the resolution. While we have used calcite 

as an example to demonstrate the method, the same technique can be applied 

to peridotites that dominate the elastic and plastic behavior of the upper 

mantle. 

REFERENCES CITED 

Bunge, H.J., 1982, Texture analysis in materials science: London, 

Butterworths. 

Johnson, G.C., and Wenk, H.R., in press, Elastic properties of calcite 

polycrystals with preferred orientations (trigonal crystal and 

orthorhombic specimen symmetry): Journal of Applied Physics. 

Wenk, H.-R., 1985, Preferred orientation in deformed metals and rocks: An 

introduction to modern texture analysis: Orlando, Florida, Academic 

Press. 

Wenk, H.-R., Takashita, T., Van Houtte, P., and Wagner, F., in press, Yield 

strength and texture development of calcite polycrystals: Journal of 

Geophysical Research, v. 91, B2. 

52 
ASTHENOSPHERE STRUCTURE AND ANISOTROPY BENEATH RIFTS 

ADOLPHE NICOLAS, Laboratoire de Tectonophysique, University of Nantes, 

44072 Nantes, France 

From structural studies conducted in ultramafic massifs and in 

peridotite xenoliths, some unifying concepts are now emerging on the 

relation between spreading and rifting environments and the nature and 

structure of the underlying mantle (Boudier and Nicolas, in press; Nicolas, 

in press). With the exception of some garnet peridotites, all the 

peridotites reflect in their structure and mineralogy a similar history of 

ascent in asthenosphere diapirs following paths, specific for each group, 

through the lithosphere up toward the Earth's surface. The main 

petrological facies--spinel-lherzolite, plagioclase-lherzolite and 

harzburgite--define specific groups, within the corresponding massifs, with 

specific internal organizations, associations, and environments. It is 

proposed that this diversity arises from a single critical parameter, which 

is the ascent rate in the asthenosphere diapir. For ascent rates (equated 

with spreading rates) larger than 1-2 cm/yr, the asthenosphere diapir rises 

to the depth of the Moho, yielding enough melt to generate a normal oceanic 

crust with a highly residual harzburgitic mantle. For smaller rates but 

possibly still larger than 0.5 cm/yr, the asthenosphere diapir meets with 

the overlying lithosphere at depths <30 km (in the plagioclase lherzolite 

field) where the melting is stopped. This results in the creation of an 

oceanic rift with a thinner oceanic crust, possibly replaced by a 

metasedimentary crust and a plagioclase lherzolite residual mantle. 

Finally for still smaller ascent rates, the asthenosphere diapir meets with 

the lithosphere at depths >30 km, resulting in the creation of a 

continental graben, a limited volcanism, and spinel lherzolite as the 

residual mantle. 

Thus, the mantle signature of a rifting situation with opening rates 

between 0.5 and around 1 cm/yr is constituted by plagioclase lherzolite 

like that of Zabargad Island in the Red Sea. The study of solidus plastic 

deformation in such massifs makes it possible to predict the asthenospheric 

flow pattern characterizing a rifting environment, provided that the 

massifs can be reoriented in the framework of rifting. This is possible in 

an ophiolitic massif liKe Trinity in the Klamath Mountains (Le Sueur and 

Boudier, in press), where the limit with the mafic section, equated with 

the paleo-Moho, provides the horizontal and the strike of the diabase dike 

swarm provides the rift direction. This also seems possible in Lanzo 

(western Alps) and Zabargad because the geophysical data suggest in both 

cases that they represent the head, presumably little tilted, of a much 

larger mantle intrusion rooted in the mantle. 

In these massifs, as well as in a few others where there are no 

constraints on their primitive attitude (Ojen in southern Spain, Collo in 

Algeria, Cap Corse in Corsica), the foliation is steep and the flow 

lineation moderately inclined; this lineation is horizontal in Trinity. 

The foliation strike is parallel to the Red Sea trend in Zabargad and to 

the diabase dike swarm attitude in Trinity. 

53 
We interpret these data as reflecting a wedge-shaped asthenospheric 

intrusion below the rift. The steep thermal boundaries between lithosphere 

and asthenosphere on each side of the rift are due to the small spreading 

rate. The moderately inclined to horizontal flow lineations indicate that 

the asthenospheric flow direction is itself inclined. This suggests that 

the asthenospheric flow diverges from discrete deeper diapirs and that at 

what are considered shallow depths (5-10 km) it has an important horizontal 

component parallel to the rift. 

The seismic anisotropy below the Rhine graben, which is characterized 

by the fast velocity being parallel to the graben direction (Fuchs, 1983), 

is accordingly interpreted as a consequence of the asthenospheric flow 

being dominantly parallel to the graben direction. 

REFERENCES CITED 

Boudier, F., and Nicolas, A., 1986, Harzburgite and lherzolite subtypes in 

ophiolitic and oceanic environments: Earth and Planetary Science 

Letters, v. 76, p. 84-92. 

Fuchs, K., 1983, Recently formed elastic anisotropy and petrological models 

for the continental subcrustal lithosphere in southern Germany: 

Physics of Earth and Planetary Interiors, v. 31, p. 93-118. 

LeSueur, E., and Boudier, F., in press, Structure du complexe basique et 

ultrabasique de Trinity, Californie: Genese d'une ophiolite atypique: 

Bulletin de la Societe Geologique de France. 

Nicolas, A., 1986, A melt extraction model based on structural studies in 

mantle peridotites: Journal of Petrology, v. 27, p. 999-1022. 
CRUSTAL EVOLUTION OF THE RHINEGRABEN AREA: EXPLORING THE LOWER CRUST IN THE 

RHINEGRABEN RIFT BY UNIFIED GEOPHYSICAL EXPERIMENTS 

KARL FUCHS, K.-P. BONJER, D. GAJEWSKI, E. LUSCHEN, C. PRODEHL, K.-J. 

SANDMEIER, F. WENZEL, AND H. WILHELM, Geophysikalisches Institut, 

Universitaet Karlsruhe, Federal Republic of Germany 

Unified geophysical investigations of the lithosphere in the 

Rhinegraben rift system have revealed new details of the lower crust and 

its role in the rifting process. The new findings allow to compare four 

different geophysical notions and properties of the lower crust for their 

compatibility: (1) The lower crust as the layer beneath the CONRAD 

discontinuity with a P-wave velocity of about 6.5 km/s or greater 

(refraction seismics); (2) the laminated band of reflections, as seen in 

the near-vertical reflection seismic experiments in many parts of the 

continent; (3) the ductile part of the crust below the brittle-ductile 

transition, void of earthquakes in seismic active regions; and (4) the 

electrical conductivity of the lower crust characterizing dry or wet 

conditions or still unknown conduction phenomena. 

In the Rhinegraben area, the lamination of the lower crust serves as 

an outstanding marker of deep tectonic movements during the rifting 

process, in which the crust of the Rhinegraben rift system has been 

subjected to three different natural dynamic processes: (1) Uplift of 2 km 

to 3 km with subsequent erosion of the Rhinegraben shoulders--Black Forest 

and Vosges Mountains--caused decompression, possibly leading to the 

formation of a low-velocity, high-electrical-conductivity zone right on top 

of the laminated lower crust beneath the elevated shoulders of the Black 

Forest; (2) the brittle crystalline wedge of the graben proper subsided 

nearly undeformed into the lower crust, which became about 5 km to 7 km 

thinner below the graben than below the shoulders; and (3) the deepest 

hypocenters in the Black Forest (Dinkelberg area), if projected into the 

neighboring reflection profile, are located 7 km to 8 km within the 

laminated lower crust beneath the southern Black Forest, indicating a 

discrepancy between the top of the lower crust as defined by the brittleductile transition as seen by the deepest earthquakes, and as defined by 

the top of the laminated reflection band. The Rhinegraben rift system 

reveals properties and behavior of the lower crust under a wide variety of 

tectonic situations. 

55 
CRUSTAL STRUCTURE AND TECTONICS IN SOUTHERN CALIFORNIA 

THOMAS A. HEARN, Institute for the Study of the Continents, Cornell 

University, Ithaca, New York 14853 

Arrivals from local earthquakes in southern California have given us a 

look into the seismic structure of the crust. Tomographic and leastsquares methods applied to the traveltimes enable us to find velocity and 

structural variations. Substantial velocity variations exist in both the 

middle crust and on the Moho discontinuity. The Moho shows almost 10 km of 

topography and has a substantial anisotropic velocity component to it. 

Together the results have considerable implications about the mode of 

tectonic deformation in southern California. 

In the middle crust we find that the San Andreas, Garlock, and San 

Jacinto faults are clearly outlined. The Peninsular Ranges region has 

correspondingly high velocities that are sharply bounded by the southern 

San Jacinto fault, underlain by a patch of anomalously slow mantle near San 

Diego. Comparison with gravity data indicates that these batholiths are 

very dense. The Mojave Desert, especially the Antelope Valley region, is 

composed of midcrustal low-velocity material that shows low gravity 

values. Again, sharp boundaries corresponding to the San Andreas and 

Garlock faults are indicated. Offshore, the crust thins and higher Pn 

velocities are found. 

The San Jacinto block shows Mojave-type velocities and not Peninsular 

Ranges-type velocities. San Jacinto Peak may have overridden the Mojave 

plate on an upper-crustal detachment surface. This accounts for the 

tremendous height and escarpment of San Jacinto Peak. Also, this 

overthrusting is helping to develop that portion of the big bend of the San 

Andreas fault in southern California. In response to this overthrusting, 

the San Jacinto and Elsinore faults may have developed to alleviate the 

forces of overthrusting. These faults have little slip relative to the 

southern San Andreas, but greater seismicity. They do not necessarily 

extend to Moho depths but may only go to midcrustal detachment surfaces. 

This would allow the San Andreas at depth to act as the major plate 

boundary, but at the surface the plate movement could be distributed over 

several fault zones. 

The Transverse-Ranges, similar to the San Jacinto Mountains, have no 

midcrustal expression. They are essentially allochthonous, and the 

batholiths do not extend into the midcrust. They have been caught in the 

convergent zone of the San Andreas. The Moho discontinuity shows a local 

root underneath the eastern Transverse Ranges and a broad root that 

counterbalances the combined load of the western Transverse Ranges and the 

topographically high Antelope Valley. 

The Salton trough, a region of crustal divergence, shows the high 

midcrustal velocities that have previously been found by Fuis and others 

(1982). This high velocity reflects intruded igneous rocks that also 

underlie the narrow high gravity anomaly in the central trough. The Moho 

underneath the trough shows a transition from 27-km crust under the 

Peninsular Ranges eastward to thin 22-km crust under the Colorado River. 

56 
The Salton trough has formed by passive pulling apart of the plates and 

mantle material upwelling to fill the gap. 

The thin crust of the Colorado River region corresponds to an area of 

surface metamorphic core complexes and detachment surfaces. The crust 

there has undergone extension with the upper crust reacting by brittle 

fracturing on normal faults and the lower crust reacting by ductile 

stretching. This separation must occur on detachment surfaces. 

A dramatic difference exists between the Pn velocities and the 

midcrustal Pg velocities; similarly, the topography on the Pn and Pg 

refractors contrasts. While the Pg velocities show a dramatic velocity 

contrast across the plate boundary with only a few kilometers of topographic change, the Moho discontinuity shows not only a gross velocity 

difference between the plates but also substantial topography. The upper 

crust has reacted brittlely to plate collision. In the lower crust, plate 

collision forces are diminished by the ductile nature of the rocks. At 

that depth, the forces of isostasy and mantle convection determine the 

structure. These two regimes, again, must be separated by detachment 

surfaces. 

The anisotropy found on the Moho is a 0.15 km/s velocity variation 

with the fast direction at N. 75° w. Anothe~ study of Pn anisotropy in 

southern California (Vetter and Minster, 1981) found that the fast 

direction near Pasadena parallels the plate boundary while in the Mojave 

region no significant anisotropy exists. The gross anisotropy that is 

found over the array represents an average. It roughly parallels the plate 

boundary, indicating that it may be stress induced. Pn anisotropy is 

difficult to interpret since it determines only two of three velocity 

directions. The vertical dimension generally corresponds to the smallest 

velocity direction. If sub-Moho material has rotated about any nonvertical 

axis, this could cause anisotropic variations. Pn anisotropy is found in 

the oceanic Moho with its fastest horizontal axis perpendicular to the 

ridge crest (Raitt and others, 1969; Raitt and others, 1971) and its 

slowest axis vertical. The mantle in California could also have such a 

stress-induced anisotropy. 

REFERENCES CITED 

Fuis, G.S., Mooney, W.D., Healy, J.H., Meehan, G.A., and Lutter, W.J., 

1982, Crustal structure of the Imperial Valley region, in The · 

October 15, 1979 Imperial Valley earthquake: U.S. Geological Survey 

Professional Paper 1254, p. 25-50. 

Raitt, R.W., Shor, G.G., Francis, T.J.G., and Morris, G.B., 1969, 

Anisotropy of the Pacific upper mantle: Journal of Geophysical 

Research, v. 74, p. 3095-3109. 

Raitt, R.W., Shor, G.G., and Kirk, H.K., 1971, Mantle anisotropy in the 

Pacific Ocean: Tectonophysics, v. 12, p. 173-186. 

Vetter, U., and Minster, J.B., 1981, Pn velocity anisotropy in southern 

CalifOrnia: Bulletin of·the Seismological Society of America, v. 71, 

p. 1511-1530. 

57 
UPPER MANTLE STRUCTURE BENEATH SOUTHERN CALIFORNIA: OBSERVATION USING 

SEISMIC TOMOGRAPHY AND INTERPRETATION OF RESULTING FLOW AND STRESS 

BRADFORD H. HAGER, EUGENE HUMPHREYS, and ROBERT W. CLAYTON, Seismological 

Laboratory, California Institute of Technology, Pasadena, California 

91125; HUMPHREYS also affiliated with Department of Geology, University of 

Oregon, Eugene, Oregon 97403 

We use a tomographic method of inversion of teleseismic P-wave delays 

recorded using the Southern California Array to determine the threedimensional structure of the upper mantle beneath southern California 

(Humphreys and others, 1984). The algorithm employed is a modified form of 

an algebraic reconstruction technique used in medical X-ray imaging. 

Deconvolution with an empirically estimated point spread function helps in 

focusing the image. The inversion assumes an isotropic medium. 

The inversion reveals two prominent features. A thin, essentially 

vertical high-velocity curtain, 2-3 percent faster than the surrounding 

mantle, lies directly beneath the Transverse Ranges. This feature is 

wedge-shaped in east-west section, attaining a maximum depth of about 

250 km beneath the San Bernardino Mountains. The second feature is a lens 

of low velocity material, 2-4 percent slow, extending to a depth of about 

125 km beneath the Salton Trough. 

The seismic rays used in the tomographic inversion are from teleseisms 

and all traverse the upper mantle beneath southern California at a fairly 

steep angle. To test the possibility that some of the anomalies may be 

explained by variations in vertically oriented anisotropy, we use arrivals 

from two regional sources whose rays intersect the Transverse Range anomaly 

at moderately shallow angles. The results of this test suggest that 

anisotropy does not cause the variation in arrival times. 

We interpret the features in the upper mantle beneath southern 

California as being the result of transient small-scale convection. In 

this model, the high-velocity curtain beneath the Transverse Ranges is a 

cold blob falling off the base of the lithosphere, and the slow region 

beneath the Salton trough is the result of penetration of the asthenosphere 

into the lithosphere. We calculate the flow and stress in the mantle for 

the (three-dimensional) distribution of density contrasts inferred from 

seismic tomography by using a simple analytic model with a constantviscosity mantle underlying a rigid lithosphere. These velocities and 

stresses might be used to predict a three-dimensional distribution of 

seismic anisotropy caused by preferred orientation of olivine crystals. 

We hypothesize that this convective instability may have been 

triggered by the rifting that resulted in the opening of the Gulf of 

California about 5 m.y. ago. we test the validity of this hypothesis by 

calculating a suite of two-dimensional convection models with temperaturedependent rheologies that use a rifted lithosphere as an initial 

condition. For certain ranges of parameters, a thin curtain of cold 

material penetrates to ~250 km depth in about 5 m.y. We plan to compute 

finite strain fields in this model to determine the anisotropy expected 

from alignment of crystals by the flow. 

58 
REFERENCE CITED 

Humphreys, E., Clayton, R.W., and Hager, B.H., 1984, A tomographic image of 

mantle structure beneath southern California: Geophysical Research 

Letters, v. 11, p. 625-627. 

59 
SYNTHETIC SEISMOGRAM MODELING OF DEEP REFLECTIONS IN THE BASIN AND RANGE 

PROVINCE 

ERIK B. GOODWIN, Geophysics Department, Stanford University, Stanford, 

Califo~nia 94305 

An unexpected result of deep seismic reflection profiling in the Basin 

and Range province has been the recognition of a deep zone of subhorizontal 

discontinuous reflections beneath the entire province (Klemperer and 

others, 1986; Allmendinger and others, 1987). This reflective zone is 

always confined to the middle and lower crust except in highly extended 

areas or "metamorphic core complexes," where it appears to reach shallow 

levels (Gans and others, 1985; Hurich and others, 1985; McCarthy, 1986). 

In these areas, uplift and tectonic denudation have been great enough that 

the reflectivity can be correlated with rocks sampled in drill holes or 

exposed at the surface. 

Reprocessed seismic reflection records from the Picacho Mountains, 

Arizona, illustrate three features of this unusual reflectivity. (1) The 

reflections begin at 3.7 km (below a low-angle detachment) and maintain 

their strength and unusual character to the Moho at a depth of 28 km. The 

reflection record is seismically transparent beneath the Moho, as it is 

elsewhere in the Basin and Range. (2) There is a smooth transition from 

the reflections in the upper and middle crust to those at the Moho. The 

"reflection" Moho appears to be a Tertiary feature, implying that the 

shallow reflections are also a by-product of extension. (3) The 

reflections have similar amplitudes throughout the seismic record, perhaps 

meaning that the velocity differences between adjacent layers in the 

shallow and deep crust are similar. 

From one- and two-dimensional seismic modeling it is clear that the 

shallow reflections are coming from many thin layers with alternating high 

and low velocities. We know this because a 5.5-km-deep drill hole on the 

flank of the Picacho Mountains provides unparalleled velocity control for 

the shallow crust. One-dimensional synthetic seismograms based on the 

velocity log from the well show that the shallow reflections result from 

subtle velocity differences (approximately 5 percent or less) between 

adjacent layers. Thus, most reflections are the result of complex 

interference patterns and are sensitive to velocity changes of individual 

layers. There are, however, several zones within the well which are 

strongly reflective and show velocity changes as high as 10 percent. The 

two-dimensional seismic modeling requires a lateral change in the velocity 

along a layer to make the reflections discontinuous. Laterally the 

velocity must vary by 5-10 percent over short distances, but this is 

consistent with outcrop relationships in the adjacent Santa Catalina 

Mountains. 

The similarity of the shallow reflections with the deeper ones 

suggests that the source of the deep reflections is also a series of thinly 

layered high- and low-velocity plates. One of the important implications 

of this modeling is the presence of relatively isolated zones which are 

strongly reflective amidst a less reflective matrix. The source of these 

60 
zones, whose reflection coefficients might approach 0.05, is puzzling. In 

the vicinity of the drill hole, a plausible explanation for the larger 

reflections is that they are coming from shear zones that have 

preferentially concentrated phyllosilicate minerals; that is, they are the 

result of textural and compositional differences due to extension. The 

crust in this area could be interpreted as having slipped apart on many 

surfaces with limited lateral extent. There is evidence for this in 

wireline logs from the drill hole, which show a strong correlation between 

low-velocity zones and high gamma ray. (The gamma-ray log essentially 

reacts to potassium-rich minerals, such as biotite, which might accumulate 

in slip zones.) Nevertheless, it is unclear at what depth, if any, this 

source of reflectivity might be replaced by another. As an example, 

perhaps the deeper part of the seismic record is in fact the result of 

compositional differences caused by intrusion into the lower crust. 

Certainly the presence of reflections beneath the Picacho Mountains from 

1.5 s to the Moho is an exciting observation indicating that these highly 

e~tended areas can serve as windows into the middle crust. 

REFERENCES CITED 

Allmendinger, R.W., Hauge, T.A., Hauser, E.C., Potter, C.J., Klemperer, 

S.L., Nelson, K.D., Knuepfer, P., and Oliver, J.E., l987, Overview of 

the COCORP 40 °N transect, western United States: The fabric of an 

orogenic belt Geological Society of America Bulletin, v. 98, p. 308-

319. 

Gans, P~B., Miller, E.L., McCarthy, J., and Ouldcott, M.L., 1985, Tertiary 

extensional faulting and evolving ductile-brittle transition zones in 

the northern Snake Range and vicinity: New insights from seismic 

data: Geology, v. 13, p. 189-193. 

Klemperer, S.L., Hauge, T.A., Hauser, E.A., Oliver, J.E., and Potter, C.J., 

1986, The Moho in the northern Basin and Range province, Nevada, along 

the COCORP 40 °N. seismic reflection transect: Geological Society of 

America Bulletin, v. 97, p.603-618. 

Hurich, C.A., Smithson, S.B., Fountain, D.M., and Humphreys, M.C., 1985, 

Seismic evidence of mylonite reflectivity and deep structure in the 

Kettle dome metamorphic core complexc Washington: Geology, v. 13, p. 

577-580. 

McCarthy, J., 1986, Reflection profiles from the Snake Range metamorphic 

core complex: A window into the mid-crust, in Barazangi, M., and 

Brown, L., eds., Reflection seismology: The-continental crust, 

Washington, D. c., American Geophysical Union, Geodynamics Series, v. 

14, p. 281-292. 

61 

Physical State and Processes of the Continental 

Lithosphere 

GEOPHYSICAL IMPLICATIONS OF MANTLE XENOLITHS: EVIDENCE FOR FAULT ZONES IN THE 

DEEP LITHOSPHERE OF EASTERN CHINA 

STEPHEN H. KIRBY, U.S. Geological Survey, Menlo Park, California 94025, 

B. CARTER.HEARN, JR., U.S. Geological Survey, Reston, Virginia 22092, 

HE YONGNIAN and LIN CHUANYONG, Institute of Geology, State Seismological 

Bureau, Beijing, China 

We are studying the peridotite xenoliths brought up from the mantle by 

alkalic basalts that erupted within or near the Tancheng-Lujiang Fault Zone 

(TLFZ), a major plate-scale rift zone over 2,000 km in length in eastern 

China. Our objective is to gain some insight into the deformation 

conditions in the middle and lower continental lithosphere cut by such a 

fault zone. 

TECTONIC SETTING OF THE TANCHENG-LUJIANG FAULT ZONE 

Although evidence exists for earlier deformation, the latest 

deformation phase commenced in the late Mesozoic. Microseismicity 

continues to the present over much of the TLFZ but moderate to major 

earthquakes have occurred mainly along its southern half. Alkalic-basaltic 

volcanism peaked in the latest Tertiary; Pleistocene to Holocene activity 

suggests that it has not yet ceased. Lastly, abundant geodetic data and 

geologic observations of deformed Holocene deposits in Shandong Province 

indicate that tectonic activity continues to the present, with rightlateral fault motion becoming increasingly important in the southern 

63 
segment. As it is roughly parallel to and contemporaneous with the opening 

of the Sea of Japan, the TLFZ can be interpreted as a failed marginal basin 

back-arc from the Japanese subduction zone. The strike-slip motion in the 

south may reflect the accommodation within the Eurasian plate of the 

collision with the Indian-Australian plate. 

PETROLOGY 

Most of the xenoliths are spinel lherzolite, with clinopyroxene 

typically greater than 10 percent by volume. The compositions of the 

pyroxenes indicate that these rocks probably originated at depths of 

30-80 km where in situ temperatures were 850-1050 °c. 

TEXTURES 

Most of the xenoliths display porphyroclastic or granular textures and 

some are markedly foliated and lineated by the distribution of dark 

minerals; these are clearly metamorphic tectonites deformed in the 

mantle. Nodules collected in the vicinity of the TLFZ are significantly 

finer grained than the ones found well outside the zone; typical 

recrystallized grain sizes are less than 1 mm. In one volcanic field near 

Yitong in Jilin Province, we found abundant foliated peridotite 

ultramylonites with recrystallized grain sizes less than 50 ~· The 

textures resemble those in crustal shear zones in which shear strains 

exceed several hundred percent. Straightforward application of the 

relation between recrystallized grain size in olivine and the applied 

differential stress suggests that the Yitong mylonites were deformed 

locally at differential stresses of about 200 MPa. 

DEFORMATION STRUCTURES 

Preliminary oxidation decoration of some of the fault-zone xenoliths 

indicates that most of them display dislocation configurations 

characteristic of diffusion-controlled recovery (a process by which defect 

damage introduced by deformation is healed). These configurations include 

cell structure (subgrains) and helical dislocations. These features are 

commonly found in ultramafic xenoliths from many locations worldwide. In 

one locality at Nushan Cinder Cone near Jiashan in Anhui Province, we 

discovered {110} and {100} slip bands, known to occur in great abundance 

only in environments of shock metamorphism. 

CONCLUSIONS 

The high strain rates implied by the shock metamorphic features of the 

Nushan xenoliths suggest that they may have developed as a plastic 

accommodation of the movements associated with great earthquakes that have 

occurred along the southern half of the TLFZ. Likewise, we believe that 

the Yitong mylonite xenoliths are fragments of the TLFZ deep within the 

continental lithosphere where deformation is accommodated in a ductile 

fault zone. Since fine grain size and slip bands are readily annealed at 

basalt magma temperatures, these xenoliths must have been incorporated, 

transported, and cooled over a short time interval, implying a close 

64 
·temporal connection between the eruptions and the fault movements producing 

the observed textures and structures. In summary, studies of mantle 

xenoliths from fault zones offer unique opportunities to investigate the 

mechanics of faulting in the deep continental lithosphere. 

65 
POLYPHASE HISTORY OF ULTRAMAFIC ROCKS IN THE MARBLE MOUNTAINS, NORTHERN 

CALIFORNIA KLAMATH MOUNTAINS 

MARY M. DONATO, U.S. Geological Survey, Menlo Park, California 94025 

The Marble Mountain terrane, a metamorphosed tectonic melange of preMiddle Jurassic age, consists of middle-amphibolite-facies metasedimentary, 

metavolcanic, and metamorphosed ultramafic rocks. The latter form numerous 

fault-bounded bodies ranging from massifs 10 km or more across (such as the 

Tom Martin and the Seiad massifs) to small, abundant, concordant slivers 

only a few meters wide. The larger bodies are composed largely of dunite 

and lesser amounts of harzburgite. At least parts of these bodies are 

believed to be remnants of mantle peridotite. The smaller bodies are 

commonly schistose and consist mainly of olivine, talc, antigorite, 

tremolite, and magnetite, and are considered to be metamorphosed 

serpentinite (Medaris and others, 1980; Donato, 1985). Petrologic and 

petrofabric evidence from various textural and compositional types suggests 

a composite three-phase history of the ultramafic rocks. The earliest 

phase, ductile deformation of peridotite, is reflected in olivine 

directional fabrics which indicate high-temperature, low-strain-rate 

plastic shear flow in the upper mantle (Nicolas and Poirier, 1976). These 

fabrics are rare or absent in smaller massifs, but are locally common in 

several of the larger ultramafic bodies in the terrane (Hanks, 1981; 

Lundquist, 1982; Cannat, 1985; Donato, 1985). The fabrics are commonly 

overprinted by later recrystallization and deformation features, which are 

attributed to both mantle and crustal processes. Petrologic data provide 

evidence of the next phase of evolution and suggest a complex thermal and 

tectonic history. Metamorphosed rodingites, Ca-metasomatic rocks formed 

within or adjacertt to ultramafic rocks during serpentinization, occur in 

association with many of the ultramafic rocks in the terrane (Rawson, 1984) 

and necessitate a low-temperature, hydrous environment early in this 

phase. Serpentinite and associated rodingite subsequently developed hightemperature, high-pressure metamorphic assemblages indicating conditions 

near 725 °C and 0.7 GPa (Rawson, 1984). These high-grade assemblages are 

now observed in metaserpentinite in the lower structural levels of the 

melange (Rawson, 1984; Lieberman and Rice, 1983), but occur only locally in 

the higher structural levels (Donato, 1985). Representative high-grade 

assemblages in metaperidotite bodies are: olivine+enstatite±anthophyllite, 

olivine+enstatite+talc±tremolite, and olivine+tremolite+anthophyllite. 

These assemblages are not isofacial with assemblages in contiguous 

calcareous, pelitic, mafic, and ultramafic rocks in the higher structural 

levels of the melange, which contain low-pressure, middle-amphibolitefacies assemblages. Furthermore, many of the high-grade ultramafic 

assemblages show textural evidence for retrograde metamorphism to middle 

amphibolite facies. These facts suggest that the high-grade assemblages 

are products of an early, high-temperature, high-pressure metamorphism 

which were subsequently overprinted by a later, lower-grade event. 

Low-pressure amphibolite-facies metamorphism and contemporaneous 

isoclinal folding of the assembled melange occurred during the most recent 

66 
phase of tectonism recognized in the peridotite. The effects of this phase 

were pervasive in the higher structural levels of the melange, and probably 

obscured most evidence of earlier events in smaller ultramafic bodies. 

Continued (renewed?) serpentinization is indicated by probable secondgeneration metarodingite. Temperatures of metamorphism are estimated to be 

500-550 °C based on garnet-biotite geothermometry and on comparison of 

assemblages in contemporaneously metamorphosed metapelite, metacarbonate, 

metabasite, and ultramafic rocks with experimentally determined reaction 

boundaries. Andalusite in metapelite constrains pressures at 0.3 to 0.5 

GPa. Ultramafic assemblages formed at this time are olivine+talc±carbonate, olivine+talc+antigorite, and olivine+talc+tremolite±magnetite. 

In summary, three distinct phases of tectonism can be recognized in 

the ultramafic rocks: (1) High-temperature plastic deformation and 

recrytallization in the upper mantle, (2) serpentinization in a lowtemperature, hydrous regime and subsequent upper-amphibolite-facies 

metamorphism in the lower crust, and (3) renewed serpentinization followed 

by low-pressure metamorphism in the upper crust. The first phase of 

deformation probably reflects plastic flow of upper mantle beneath a preJurassic mid-ocean or marginal basin. The second phase may represent 

transport of the ultramafic rock from the upper mantle along intra-oceanic 

thrust faults during the imbrication of the melange. During this phase, 

ocean-floor sediments and volcanic rocks were tectonically interleaved with 

partia~ly serpentinized peridotite and subsequently metamorphosed in the 

lower crust. The third phase of tectonism may have taken place in the 

region between a subduction zone and the magmatic axis of a Lower to Middle 

Jurassic volcanic arc, and probably represents the final accretion of the 

ultramafic rocks to the continental margin. 

REFERENCES CITED 

Cannat, M., 1985, Tectonics of the Seiad Massif, northern Klamath 

Mountains, California: Geological Society of America Bulletin, v. 96, 

p. 15-26. 

Donato, M.M., 1985, Metamorphic and structural evolution of an ophiolitic 

tectonic melange, Marble Mountains, northern California: Stanford, 

Calif., Stanford University, Ph.D. thesis, 258 p. 

Hanks, C.L., 1981, The emplacement history of the Tom Martin Ultramafic 

Complex and associated metamorphic rocks, north-central Klamath 

Mountains, California: Seattle, University of Washington, M.S. thesis, 

112 p. 

Lieberman, J.L., and Rice, J.M., 1983, Prograde metamophism of marble and 

peridotite in the Seiad Ultramafic Complex, California [abs.]: 

Geological Society of America Abstracts with Programs, v. 15, no. 5, p. 

436. 

Lundquist, S.M., 1982, Deformation history of the ultramafic and associated 

rocks of the Seiad Complex, Seiad Valley, California: Seattle, 

University of Washington, M.S. thesis, 167 p. 

Medaris, L.G., Jr., Welsh, J.L., Ferns, M.L., and Kays, M.A., 1980, 

Prograde metamorphism in the western Paleozoic and Triassic belt, 

Klamath Mountains province [abs.]: Geological Society of America 

Abstracts with Programs, v. 12, no. 3, p. 120. 

67 
Nicolas, Adolphe, and Poirier, J.P., 1976, Crystalline plasticity and solid 

state flow in metamorphic rocks: London, Wiley, 444 p. 

Rawson, S.A., 1984, Regional metamorphism of rodingites and related rocks 

from the north-central Klamath Mountains, California: Eugene, 

University of Oregon, Ph.D. thesis, 252 p. 

68 
THE EFFECTS OF WEAK CRUST UPON THE STRENGTH OF THE CONTINENTAL LITHOSPHERE 

ROBERT F. DeRITO, U.S. Geological Survey, Menlo Park, California 94025 

Lithospheric strength depends not only upon the presence of intralithospheric zones of weakness, but also upon the distribution of these 

zones and the manner in which intervening competent zones interact. 

Variability in thickness and lithology of the continental crust therefore 

provides for a wide range of weakening scenarios. 

Since crustal material is generally weaker than upper mantle material 

at a given temperature, a discontinuity in mechanical properties probably 

exists across the Moho (Kirby, in press). A general picture emerges 

therefore of a three-component lithosphere, composed of a strong zone each 

in the upper crust and upper mantle, separated by a weak lower crust. 

During flexure, long-term zones of weakness within the crust may be 

delineated with the aid of a strength index, I , which, for a Maxwell 

material, is defined 

1 -

where E is the total axial strain present in a fiber, and where Ee and Ev 

are the elastic and viscous components of strain, respectively. Strong, 

predominantly elastic zones exist where I is in the neighborhood of 1. 

Numerical experimentation (DeRito, 1983; DeRito and others, unpubl. data) 

shows that mechanically weak zones are characterized by strength index 

values less than about 0.3 (greater than 70 percent viscous strain). The 

position of rheologic zone boundaries depends, in general, upon time since 

loading as well as upon temperature and lithologic distribution. 

The overall thickness of the lithosphere, ht, may be defined by the 

depth to the I = 0.3 isopleth in the upper mantle, or by the depth to a 

critical isotherm, Tern· The effective thickness, he, is the thickness of 

the elastic plate that best fits a given flexural profile. A weak crust 

works to weaken the lithosphere as a whole, so that, in general, he < ht. 

Certain modes of interaction may strongly amplify this weakening effect, 

depending upon the degree of shear coupling between competent layers. Two 

extreme modes of interaction are: (1) Simple asthenospheric weakening--

relative slip between the competent upper crust and upper mantle is 

negligible, and (2) detachment weakening--negligible shear stress is 

transmitted across the weak lower crust. 

The net weakening effect depends critically upon the position of the 

weak crustal zone within the lithospheric column as defined by ht (DeRito 

and others, 1983). For example, detachment weakening is most effective, 

reducing he by half or more, when the weak zone is centered at depths of 

about ht12 , that is, when the base of the continental crust occurs at midlithospheric depths; at these same depths, simple asthenospheric weakening, 

69 
when operative, may have a negligible effect on he· The latter mechanism 

is most effective when the base of the crust is at depths approaching ht. 

Complete flexural delamination may occur if the isotherm T0 m rises to 

depths shallower than the Moho, or perhaps if the weak zone in the lower 

crust attains a great enough thickness. In such cases, the upper mantle 

would not be significantly involved in flexure and the lower crust might 

behave as a true, buoyant asthenosphere; this is true even in the latter 

case where the upper mantle remains mechanically competent. Narrow crustal 

loads may produce unexpectedly broad downwarps due to the small density 

contrast between the upper and lower crust compared to that between the 

upper crust and the mantle asthenosphere. The fractional change in 

flexural wavelength relative to a "normal," mantle-compensated lithosphere, 

IJ., is 

where pi is the density of basin infill, Pa is the density of the mantle 

asthenosphere, and p0 is the density of the crustal asthenosphere. Note 

that this effect may be appreciable since the density contrast between 

well-consolidated sedimentary rocks and the less-mafic (weak) constituents 

of the continental crust may be exceedingly small (Hinze and others, 1978). 

Lithospheric flexure generally produces a characteristic gravity 

signature when the Moho is fully involved (Watts and Cochran, 1974; Karner 

and Watts, 1983). Flexure in the absence of such a signature may indicate 

mechanical delamination. 

REFERENCES CITED 

DeRito, R.F., 1983, The physical meaning and thermal dependence of the 

thickness of the mechanical lithosphere during flexure: Buffalo, State 

University of New York, M.S. thesis, 180 p. 

DeRito, R.F., Cozzarelli, F.A., and Hodge, D.S., 1983, Mechanism of 

subsidence of ancient cratonic rift basins: Tectonophysics, v. 94, 

p. 141-168. 

Hinze, W., Bradley, J.W., and Brown, A.R., 1978, Gravimeter survey in the 

Michigan deep borehole: Journal of Geophysical Research, v. 83, p. 

5864-5868. 

Karner, G. D., and Watts, A. B., 1983, Gravity anomalies and flexure of the 

lithosphere at mountain ranges: Journal of Geophysical Research, 

v. 88, p. 10, 449-50, 472. 

Kirby, S. H., in press, Rock mechanic observations pertinent to the 

rheology of the continental lithosphere and the localization of strain 

along shear zones: Tectonophysics. 

Watts, A.B., and Cochran, J.R., 1974, Gravity anomalies and flexure of the 

lithosphere along the Hawaiian-Emperor seamount chain: Geophysical 

Journal of the Royal Astronomical Society, v. 36, p. 57-90. 

70 
THERMOBAROMETRY OF ULTRAMAFIC XENOLITHS: STATE OF THE ART AND GLOBAL 

APPLICATIONS 

ANTHONY A. FINNERTY, Department of Geology, University of California, 

Davis, California 95616 

Thermobarometry of garnet peridotite xenoliths from kimberlites has 

become an important means of investigating mantle processes. It is 

difficult to estimate the accuracy of any thermometer or barometer for 

application to natural assemblages, however, because of the paucity of 

absolute references for pressure and temperature in typical garnet 

lherzolite. The difficulty of testing, and large disagreements in 

pressure-temperature (P-T) estimates between some thermobarometers, has 

fostered skepticism of the technique. One requirement for application of 

thermobarometry to any rock is preservation of mineral compositions that 

were in chemical equilibrium at some pressure and temperature. It is not 

possible to prove that any rock has retained such equilibrated 

compositions, but a necessary condition for equilibration is mineral 

homogeneity. Xenoliths from kimberlites and other magmas, because they are 

transported to the surface quickly relative to chemical diffusion times 

within minerals, are most likely to preserve evidence of the P-T conditions 

from which they were derived, but even these may be altered or have 

suffered complex transport histories that in some cases may induce chemical 

changes that affect P-T estimates. 

Unfortunately, the minerals of real rocks are chemically complex, and 

the results of laboratory experiments cannot be applied as thermobarometers 

without first accounting somehow for the differences between experimental 

and natural mineral compositions. Many thermodynamic or empirical 

corrections schemes have been proposed, so that the potential user of 

thermobarometers for mantle rocks is faced-with making a choice from more 

than 50 proposed thermometers or barometers! 

A suite of chemically homogeneous garnet lherzolite xenoliths from 

northern Lesotho has been used to test accuracy and precision of a large 

number of combinations of thermometers and barometers for garnet 

lherzolite, by comparison of P-T estimates for a diamond-bearing and a 

graphite-bearing xenolith with the experimentally determined diamondgraphite univariant curve and by comparison of phlogopite-bearing xenoliths 

to the high-T stability limit of phlogopite. Precision was evaluated by 

measuring the scatter of P-T estimates for each of four xenoliths from a 

wide range of P and T when many point analyses of the constituent minerals 

were used for P-T estimation. 

The petrologic criteria for accuracy were not strongly selective for 

thermometers, but an empirical equation using only reversed data for the 

enstatite-diopside miscibility gap and incorporating a pressure effect 

gives minimal scatter and good agreement with petrologic constraints. Of 

the many formulations of the aluminous enstatite garnet-field barometer, 

only the original isopleths of MacGregor (1974) yield results in accord 

with the petrologic constraints, despite more recent, reversed 

determinations that show the MacGregor isopleths to be slightly 

71 
erroneous. Evidently, thermodynamic treatments of experimental data, for 

correction of effects of components present in natural rocks but not in 

experiments, at their present stages of development unbalance effects that 

fortuitously compensate in the empirical thermometer and barometer. 

Equilibration conditions for suites of garnet lherzolite xenoliths 

from kimberlites and related rocks from worldwide localities have been 

determined using the preferred combination of thermometer and barometer. 

Low-T suites that define paleogeotherms close to the 40 mW/m2 conductive 

continental geotherm are characteristic, as are high-T suites of deeper 

origin that record temperatures higher than the conductive geotherm. Both 

suites are not present at all localities. The low-T suite in some areas 

defines a geotherm that is displaced to higher temperatures signifying a 

higher heat-flow regime, and reflecting the local tectonic environment. 

The high-T suites characteristically show an origin from a restricted depth 

range 20 to 40 km wide that may be a reflection of convective or diapiric 

heat transport and (or) proximity to a magmatic focus. 

A gap in P-T space commonly separates low-T from high-T suites at a 

given locality. This may reflect the presence at depth of materials other 

than lherzolite, possibly low-Ca garnet harzburgite and (or) magma. The 

P-T location of intersection of the low-T and high-T suites usually cannot 

be located accurately because of the gap, but falls between 1,000 and 1,200 

°C. This may correlate with the location of the solidus curve for 

peridotite in the presence of H2o and co2 • Both P-T estimates and chemical 

data are consistent with the interpretation that the transition is the 

lithosphere-asthenosphere boundary. 

High-T peridotite from mobile belts surrounding the Kaapvaal craton in 

southern Africa originated from depths of 130-170 km, compared to 

170-220 km from within the craton, leading to the interpretation that the 

Kaapvaal craton has a relatively cool root and that the base of the 

lithosphere shelves from beneath the craton toward the Atlantic and Indian 

oceans. An analogous relationship derived from fewer localities in North 

America is interpreted as evidence that the lithosphere-asthenosphere 

boundary beneath this continent may shelve to the north and south. 

A barometer based upon solubility of Ca in olivine coexisting with two 

pyroxenes, which does not require the presence of an aluminous phase, has 

been experimentally calibrated (Finnerty and Higden, 1981). Inasmuch as 

the inflections observed in geotherms persist when different, independent 

reactions are employed for thermometers and barometers, they cannot be 

artifacts of the methods of P-T estimation techniques. 

When the olivine barometer is applied to homogeneous spinel lherzolite 

xenoliths from alkali basalt of San Carlos, Arizona, P-T estimates range 

from 940 °C, 1.2 GPa to 1070 °c, 2.6 GPa, with a slope of ~8 °C/100 MPa. 

The lower P limit nearly coincides·with the 45-km crustal thickness estimated from seismic waves for the Colorado Plateau, and the high-T, lowslope geotherm is suggestive of anomalous heating of the underlying mantle, 

consistent with the low shear wave velocity of ~4.5 km/s. 

Spinel lherzolite xenoliths from 12 localities in eastern China 

equilibrated at similar temperatures, but higher pressures than those of 

San Carlos, defining a P-T slope of ~20 °C/100 MPa. An upper temperature 

limit of 1,075 °c coincides with that of the San Carlos suite, and with the 

inflection point observed in garnet lherzolite suites from many 

localities. More spinel lherzolite geotherms must be estimated to test 

72 
whether this observation bears significance; for example, could it be 

related to a temperature-induced change in mechanical properties of mantle 

rocks? 

73 
CRUSTAL GROWTH AND THE LOWER CRUST OF MAGMATIC ARCS 

ROBERT W. KAY and S. MAHLBURG KAY, Department of Geological Sciences, Snee 

Hall, Cornell University, Ithaca, New York 14853 

In magmatic arcs, the common basalt and andesite bodies (and intrusive 

equivalents) that reach the shallowest arc levels do not originate directly 

from the mantle (e.g., they are not primary compositions). In the Aleutian 

arc, we have identified olivine tholeiite as a primary arc lava and have 

proposed that the early fractionated phases (olivine and clinopyroxene) 

have accumulated at Moho depth and represent newly formed upper mantle (Kay 

and Kay, 1985). We have recovered xenolithic fragments that represent 

direct evidence of these processes (Conrad and Kay, 1984; DeBari and 

others, in press). The magma composition that results from this 

fractionation is high-Al basalt, which represents the composition added to 

the crustal section of the Aleutian arc. More generally, for arcs with 

even the smallest proportion of high-Al ·basalt, and especially for arcs 

with olivine tholeiitic basalts (representing the little-fractionated, 

near-primary basalt), we find little support for models that assume that 

crustal additions are andesitic. Shallow-level silicic volcanic and 

plutonic rocks can be derived by a combination of crystal fractionation 

from a high-Al basalt and assimilation of a low-melting fraction from the 

arc crust. The actual crustal section of a particular arc depends on the 

preexisting crust (oceanic and cratonic are end-members) at the arc 

magmatic axis, and on the structural response of the arc. But in all 

cases, the composition of new crustal addition, and in particular the newly 

added lower crust, is basaltic. 

This creates a problem (Kay and Kay, in press), for if the bulk 

composition of the Earth's continental crust is andesitic, the crustal 

formation process is not duplicated in present-day arcs. We are left with 

two choices. First, the mean composition of new crust (non-recycled 

additions from the mantle to the continental crust) was more andesitic in 

the past, in particular at the time around 2.5 Ga. when large amounts of 

crust formed. Second, basaltic continental crust~ once formed, is 

transitory, and returns to the mantle by crustal delamination. A third 

process, deep subduction of siliceous sediment, might basify crustal 

composition, but that would only contribute to the problem. 

REFERENCES CITED 

Conrad, w.s., and Kay, R.W., 1984, Ultramafic and mafic intrusions from 

Adak Island: Crystallization history, and implications for the nature 

of primary magmas and crustal evolution in the Aleutian Arc: Journal 

of Petrology, v. 25, p. 88-125. 

DeBari, Sc, Kay, S.M., and Kay, R.W., in press, Xenolithic evidence for the 

nature of the crust-mantle boundary and primitive magmas in the 

Aleutian arcs: Journal of Geology. 

Kay, S.M., and Kay, R.W., 1985, Role of crystal cumulates and the oceanic 

74 
crust in the formation of the lower crust of the Aleutian arc: Geology, 

v. 13, p. 461-464. 

Kay, R.W., and Kay, S.M., in press, Petrology and geochemistry of the lower 

continental crust: Journal of the Geological Society of London. 

75 

Chemical and Physical State of the Mantle Related to the 

Transport and Reactions of Fluids and Melts 

DISSOLUTION OF VOLATILES IN MANTLE OLIVINE: RHEOLOGICAL IMPLICATIONS 

HARRY w. GREEN II, Department of Geology, University of California, Davis, 

California 95616 

The evidence is overwhelming that volatiles remain in the Earth's 

mantle. Xenoliths of mantle rocks accidentally incorporated in volcanic 

eruptions ubiquitously display fluid bubbles on healed cracks within 

olivine and pyroxene crystals (see Roedder, 1965). Analyses of these 

bubbles repeatedly have shown that they are virtually pure co2, with trace 

amounts of noble gases. Similarly, volatile emanations accompanying 

volcanic eruptions are rich in co2 • Although H2o also is a major component 

of such gases, isotopic studies argue strongly that the water is a crustal 

contaminant (see I. Barnes, this volume). Xenoliths from kimberlite pipes 

always have their grain boundaries serpentinized, but isotopic studies have 

shown that this water also is crustal contamination. Many of these 

xenoliths, however, contain phlogopite that clearly is part of the mantle 

paragenesis and has a mantle isotopic signature. On the other hand, 

analyses of "hydrous" minerals in xenoliths frorn oceanic and tectonic areas 

usually show them to be oxyamphiboles and fluorphlogopites. Thus, a 

variety of independent lines of evidence demonstrates the existence of co2 in the mantle, but, except for the subcontinental lithosphere, the evidence 

for H2o is weak. 

For 15 yea~s, I have been collecting evidence that some volatile 

species dissolve in mantle olivine and precipitate as submicroscopic fluid 

bubbles during solid diapirism preceding incorporation of xenoliths in 

77 
alkalic basalt (Green and Radcliffe, 1975) or during transportation of 

xenoliths to the surface in kimberlite (Green and Gueguen, 1983). Direct 

evidence for the contents of the fluid precipitates has been lacking except 

that they commonly display an amorphous phase on their surfaces that 

contains carbon. Precipitated bubbles in fresh olivine do not have 

serpentine on their surfaces, nor do rare recrystallized olivine grains 

contained within olivine porphyroclasts and thereby isolated from the latestage (crustal) serpentinization of the grain-boundary network. Whatever 

is the nature of the fluid, bubble densities indicate a solubility up to 

several hundred parts per million at depths in excess of 100 km. 

Recent experimental evidence from my laboratory indicates the 

following: (1) Olivine equilibrated with co2 dissolves no carbon at low 

pressure (0.1 GPa, 1200 °C), but dissolves of the order of 100 wt ppm at 

3.0 GPa, 1200 °C (Tingle and others, 1985); (2) experimental deformation of 

a dry synthetic harzburgite in the range 1.0-3.0 GPa, 1,100-1,400 °C 

displays a marked pressure dependence (activation volume -30 cm3/mole). 

Extrapolation of these data to conditions approximating the base of the 

olivine-bearing mantle (13 GPa, 1,500 °c, T = 0.1-1 MPa) predicts a strain 

rate of 10-21 s-1 (Green and Hobbs, 1984); (3) a single experiment 

conducted at 1200 °C, 3.0 GPa, in which olivine was equilibrated with 

carbonate, yielded a strength about half of the volatile-free value (R. 

Borch and H.W. Green, unpub. data). 

Experimental data from several laboratories over 20 years have 

demonstrated that dissolution of small amounts of H2o weakens olivine by a 

factor of 2 or more (see Mackwell and others, 1985). These results, 

coupled with the new data summarized above, suggest that dry olivine is too 

strong to be consistent with plate tectonics; dissolution of H2o or co2 in 

olivine probably is responsible for flow of peridotite at stress levels 

present in the mantle. The evidence for volatiles in the mantle suggests 

that co2 is the more likely choice, but more experimental data are needed 

to confirm carbon-induced weakening. 

REFERENCES CITED 

Green, H.W., and Gueguen, Y., 1983, Deformation of peridotite in the mantle 

and extraction by kimberlite: A case history documented by fluid and 

solid precipitates in olivine: Tectonophysics, v. 92, p. 71-92. 

Green, H.W., and Hobbs, B.E., 1984, Pressure dependence of creep in dry 

polycrystalline olivine: Eos, Transactions, American Geophysical 

Union, v. 65, p. 1107. 

Green, H.W., and Radcliffe, S.V., 1975, Fluid precipitates in rocks from 

the earth's mantle: Geological Society of America Bulletin, v. 86, p. 

846-852. 

Mackwell, S.J., Kohlstedt, D.L., and Paterson, M.S., 1985: The role of 

water in the deformation of olivine single crystals: Journal of 

Geophysical Research, v. 90, p. 11,319-11,334. 

Roedder, E., 1965, Liquid co2 inclusions in olivine-bearing nodules and 

phenocrysts from basalts: American Mineralogist, v. 50, p. 1746-1782. 

Tingle, T.N., Green, H.W., and Finnerty, A.A., 1985: The solubility and 

diffusivity of carbon in olivine: Eos, Transactions, American 

Geophysical Union, v. 66, p. 1135. 

78 
MANTLE DEGASSING, MANTLE CONVECTION, AND VOLCANISM 

IVAN BARNES, U.S. Geological Survey, Menlo Park, California 94025 

Carbon dioxide (C02) discharges throughout the world correspond to 

zones of seismic activity. Volcanoes characteristically occur in the 

seismically active zones. Isotopic compositions show that the 13co2 is 

from the mantle in some places. co2 may be a cause for mantle 

convection. Discharge of co2 on spreading centers leads to the suggestion 

that co2 may be a causative factor in seafloor spreading. 

INTRODUCTION 

A compilation of carbon dioxide (C02 ) discharges throughout the world 

shows that co2 discharges occur in zones of seismicity (Barnes and others, 

1984). Active volcanism occurs in these same seismically active zones. 

This fact led to an examination of the sources of volatiles in volcanic 

terranes to determine which if any of the volatiles are of magmatic origin 

in the strictest sense. 

There is a great deal of confusion about the relative importances of 

amounts and sources of volatiles in volcanic eruptions. Much of the 

confusion may stem from the occurrence of clouds during volcanic eruptions 

that reach the atmosphere. Because most atmospheric clouds are composed of 

water droplets, there is an intuitive identification of the clouds above 

erupting volcanoes as "steam" clouds even in the absence of any data 

(Christiansen and Peterson, 1981). The intuitive belief in "steam" clouds 

is reinforced by the large number of studies of fumaroles that show steam 

to be the most abundant component of fumarolic gases (Gerlach, 1980). In 

the most detailed and careful analyses of fumarolic gases to date, it was 

shown that fumarolic gases are in chemical equilibrium (Giggenbach, 

1980). The demonstration that volcanic gases are in internal chemical 

equilibrium is of great importance but does not show the origins of any of 

the volatile components. 

VOLATILES FROM RECENT ERUPTIONS 

Recent eruptions in Alaska and Washington (Christiansen and Peterson, 

1981) have offered opportunities to study the chemical and isotopic 

compositions of volcanic volatiles and to identify the sources of the 

volatiles. The similarity in major component compositions of fumarolic 

gases from Mount St. Helens and Erta'Ale and the continuum they form with 

other fumarolic gases (Gerlach, 1980) show that an explanation for one will 

probably serve as an explanation for all. The gases are approximately 80 

percent steam, 10 percent co2 and equilibrium amounts of H2S, so2, H2 , CO, 

HCl and less abundant gases such as COS. Isotopic evidence is essential to 

determine the sources of the H2o and co2; chemical evidence is sufficient 

to show the source of chloride that provides the HCl. Metamorphic brines 

occur both at the site of two explosive volcanic eruptions in Alaska and in 

the metamorphosed volcanic rocks near Mount St. Helens. The major analyses 

79 
are repeated here in table 2. The term "metamorphic brine" refers to the 

waters, rich in chloride, that are present during the reactions that 

produced the metamorphic rocks. The brine near Mount St. Helens, for 

example, is supersaturated with respect to the minerals (albite, potassium 

feldspar, calcite, quartz, and chlorite) that constitute the ~etam~~phosed 

volcanic host rock of the Eocene Ohanapecosh Formation. The 7sr/ Sr data 

show that the brine (0.7033), calcite (0.7032), and silicates (0.7036) are 

genetically related. The metamorphic brines may explain the common reports 

of chloride in fumarolic discharges because, at temperatures in excess of 

300 °C, hydrochloric acid is very slightly ionized and the undissociated 

hydrogen chloride is sufficiently volatile to make condensates quite 

acid. Condensate from fumarolic gases in the crater of Mount St. Helens 

had a pH of 0.8 and was 0.2N solution of HCl. If the metamorphic brines 

contribute appreciably to volcanic eruptions~ very acid rain rich in 

chloride would result. No such rain was found after the,eruption of Mount 

St. Helens, so steam from the brine was not a major volatile during the 

eruption, although such steam constitutes 80 to 87 percent of the gas from 

the fumaroles. Seawater with 19,000 mg/L chloride would behave very much 

the same as the metamorphic brines. 

SOURCE OF C02 

Of the anhydrous gases at Mount St. Helens and the locality of the two 

recent volcanic explosions in Alaska, co2 is the major component. 

Anhydrous carbon dioxide is the gas found in fluid inclusions in deep-sea 

basalts both in the Atlantic (Pineau and others, 1976) and Pacific (Moore 

and others, 1977) Oceans. The isotopic compositions of the co2 in the 

fluid inclusions, -4.7 to -8.1 per mil (PDB), gives a guide to the isotopic 

compositions of co2 from the mantle. The isotopic composition of the co2 from the explosion pits in Alaska is -6.36 per mil, midrange for mantle 

co2 , and the co2 from Mount St. Helens (-10.7 per mil) is close to the 

range fixed by deep-sea fluid inclusions. Dissolved co2 (-3.0 per mil) and 

gaseous co2 (-4.2 per mfl) from San Miguel, Azores may also be from the 

mantle as is the co2 (15.7 per mil) discharging at Chaves, Portugal (de 

Almeida, 1982). co2 (rom the mantle (-5.0 to -8.4 per mil) also discharges 

from the active spreading center of Iceland. co2 of mantle origin (-4.0 to 

-5.1 per mil, Taylor /and others, 1967) also discharges in the Eifel 

District, West Germany, the type locality of maars and "phreatic" eruptions 

(Steininger, 1819; SChmincke, 1977). co2 of mantle origin (-6.5±0.5 per 

mil) also discharges from Erta'Ale volcano in Ethiopia (Allard and others, 

1977), Asal, -4.8 to -5.9 per mil (Allard, 1980), Usu volcano, Japan, -4.4 

per mil (Allard, 1981b), Krakatoa, Indonesia, -7.0 per mil (Allard and 

others, 1981) and Soufriere, Antilles, -5.5 per mil (Allard, 1981a). Other 

volcanoes also disc~arge co2 , although the isotopic compositions are not 

yet known. The Canary Islands (Ramon-Vasallo, written commun.) and 

Philippine Islands (feliciano, 1926) are examples. Carbon dioxide also 

discharges from submarine spreading centers as at ·the Galapagos (Corliss 

and others, 1979), but, of course, is much more difficult to detect than 

co2 from subaerial volcanoes because of the solubility of co2 in seawater. 

Although co2 discharges seem ubiquitous from volcanoes and spreading 

centers, the co2 is not always accompanied by silicate melts; indeed, co2 

Jdischarges last for millenia unaccompanied by melts (Barnes and others, 

80 
1984). If co2 discharges in the absence of lava, then the co2 pressure 

(not fugacity) must equal or exceed the total pressure at the source of the 

co2• From isotopic and tectonic evidence, the source in some places must 

be the mantle. Thus it must be concluded that the mantle contains a free 

co2 gas phase~ The co2 is buoyant, because at no temperature or pressure 

does the density (Kennedy and Helser, 1966) of co2 equal that of a silicate 

melt. If the gas is buoyant, so must be gas-melt mixtures in the mantle, 

leading to convective movement without the requirement of addition of heat 

to the convective system. The convective upwelling provides a mechanism 

for seafloor spreading. 

REFERENCES CITED 

Allard, P., 1980, Caracteristiques geochimiques des volatiles emis par 

l'eruption volcanique de Novembre 1978, dans le Rift d'Asal: Bulletin 

Societe Geologique France, v. 22, p. 825-830. 

Allard, P., 1981a, Composition chimique et isotopique des gaz emis par le 

nouveau dome andesitique de la Soufriere de Saint-Vincent (Antilles): 

Comptes Rendus Academie Sciences Paris, v. 293, ser. 2, p. 721-724. 

-----1981b, Composition isotopique du carbone dans les gaz d'un ma~~a 

dacitique (Volcan Usu, Japon)t· Relation entre le rapport 13c; C des 

volatils et le rapport tl7sr/80sr de la phase silicatee dans le 

volcanisme d'arc: Comptes Rendus Academie Sciences Paris, v. 293, ser. 

2, p. 583-586. 

Allard, P., Jehanno, c., and Sabroux, J.C., 1981, Composition chimique et 

isotopique des produit gazeux et solides de l'activite eruptive du 

Krakatoa (Indonesie) pendant la periode 1978-1980: Comptes Rendus 

Academie Sciences Paris, v. 293, ser. 2, p. 1095-1098. 

Allard, P., LeGuern, F., and Sabroux, J.C., 1977, Thermodynamic and 

Isotopic Studies in Eruptive Gases: Geothermics, v. 5, p. 37-40. 

Barnes, I., Irwin, W.P., and White, D.E., 1984, World distribUtion of 

carbon dioxide springs and major zones of seismicity: u.s. Geological 

Survey Miscellaneous Investigations Series Map I-1528, 10 p. 

Christiansen, R.L., and Peterson, D.W., 1981, Chronology of the 1980 

eruptive activity, in Lipman, P.W., and Mullineaux, D.R., eds., The 

1980 eruptions of Mount St. Helens, Washington: u.s. Geological Survey 

Professional Paper 1250, p. 17-30. 

Corliss, J.B., Dymond, J., Gordon, L.I., Emond, J.M., VonHerzen, R.P., 

Ballard, R.D., Green; K., Williams, D., Bainbridge, A., Crane, K. and 

Van Andel, T., 1979, The submarine thermal springs on the Galapagos 

Rift: Science, v. 203, p. 1073-1083. 

de Almeida, F.M., 1982, Novos dados geothermometicos sobre aguas de Chaves 

e de s. Pedro do Sul [New geothermometric data on the waters of Chaves 

and S. Pedro do Sul): Comunicacoes dos Services Geologicos de 

Portugal, v. 68, p. 179-190. 

Feliciano, J.M., 1926, A study of the thermal springs in the Philippines: 

Proceedings Third Pan Pacific Science Congress, Tokyo, v. 1, p. 804-

811 . 

Gerlach, T.M., 1980, Interpretation of volcanic gas data from tholeiitic 

and alkaline lavas, preliminary study and interpretation of magmatic 

gas, in La Guern, F., ed., Programme interdisciplinaire de recherche 

81 
sur la preveion et la surveillance des eruptions volcaniques, Bulletin 

31: Gif-sur-Yvett, France, p. 69-92. 

Giggenbach, W.F., 1980, Geothermal gas equilibria: Geochimica et 

Cosmochimica Acta, v. 44, p. 2021-2032. 

Kennedy, G.C., and Holser, W.T., 1966, Pressure-volume-temperature and 

phase relations of water and carbon dioxide, in Clark, S.P., Jr., ed., 

Handbook of physical constants, revised edition: Boulder, Colo., 

Geological Society of America, Memoir 97, p. 371-383. 

Moore, J.G., Batchelder, J.N., and Cunningham, C.G., 1977, C0 2-filled 

vesicles in mid-ocean basalt: Journal of Volcanology and Geothermal 

Research, v. 2, p. 309-327. 

Pineau, F., Javoy, M., and Bottinga, Y., 1976, 13c;12c ratios of rocks and 

inclusions in popping rocks of the Mid-Atlantic Ridge and their bearing 

on the problem of isotopic composition of deep-seated carbon: Earth 

and Planetary Science Letters, v. 29, p. 413-421. 

Schmincke, H-U, 1977, Phreato magmatsche Phasen in Quartaren Vulkanen der 

Osteifel [Phreatomagmatic phases in Quaternary volcanoes of East 

Fifel]: Geologisches Jahrbuch Series A, v. 39, p. 3-45. 

Steininger, D.J., 1819, Geognostiche Studien am Mitteirheine [Geologic 

Studies in the Middle Rhine]: Mainz, F. Kupferberg, 223 p. 

Taylor, H.P., Jr., Frechen, J., and Degens, E.T., 1967, Oxygen and carbon 

isotope studies of carbonatites from the Laacher See district, West 

Germany and the Alno district, Sweden: Geochimica et Cosmochimica 

Acta, v. 31, p. 407-430. 

TABLE 2. Results of analyses of brines from the 

Alaska Peninsula and Mount St. Helens 

Alaska Mount 

Peninsula St. Helens 

pH (units) 5.9 8.34 

T (°C) 52.8 8 

Species (mg/L) 

Si02 120 9.2 

Ca 1 ,500 7' 100 

Mg 460 5.5 

Na 17,700 6' 100 

K 450 6.9 

Hco3 1 '240 18.0 

so4 140 280.0 

Cl 32,000 22,000 

B 360 3.0 

Isotope ratio (per mil SMOW) 

50 -37.9 -43.4 

o180 +.42 -4.29 

82 
SOLID AND FLUID INCLUSIONS IN MANTLE XENOLITHS: THEIR ORIGINS AND IMPLICATIONS 

FOR THE PROPERTIES OF THE MANTLE 

JAY S. NOLLER, U.S. Geological Survey, Denver, Colorado 80225 

Solid and fluid inclusions in mantle xenoliths aid in distinguishing 

between the chemical and physical effects of processes taking place within 

the mantle and those resulting from decompression events and alteration 

during ascent of the xenoliths to the earth surface. Studies of ultramafic 

massifs and composite xenoliths show that magmas may react with Mg-rich 

wallrocks in the upper mantle, altering the wallrock to more Fe-rich 

compositions. This alteration process is commonly called "mantle 

metasomatism." Previous investigations suggest that mantle metasomatism is 

carried out by fluids that evolve from magmas and react with the peridotite 

wallrocks to produce compositional variations in minerals as a function of 

distance from the dikes (Wilshire and others, 1980; Andersen and others, 

1984; Menzies and others, 1985; Nielson and Noller, in press). Criteria on 

separating mantle dike--wallrock reactions from xenolith--host basaltic 

magma interaction during ascent have been gathered from the study of grain 

coatings, intragranular inclusions, and fluid inclusions in (composite) 

mantle xenoliths (Rocchia, 1982; Green and Gueguen, 1983; Noller, 1986). 

Composite ultramafic xenoliths rimmed by kaersutite selvages (dikes) 

from southwestern United States and France contain four types of solid and 

fluid inclusions. Three types of solid inclusions are spinel inclusions, 

amphibole glass, and basalt glass. Spinel inclusions consist of trails of 

dark spinel octahedra (up to 20 ~), aligned along parallel lines 

approximately 20 ~ to 50 ~m apart, and their abundance gradually decreases 

away from kaersutite selvages. Amphibole glass consists of clear films, 

containing fluid inclusions, with dendritic and honeycomb morphologies 

along grain boundaries and fractures. These films branch from reaction 

rims around amphiboles in both kaersutite selvages and lherzolite. Basalt 

glass typically consists of brown and black films that line cleavages and 

fractures and, to a lesser degree, grain boundaries in the xenoliths. 

Basalt-glass films are ubiquitous throughout most mantle xenoliths, 

indicating that basaltic magmas interact with xenolith interiors to a great 

extent. 

These three types of solid inclusions can be used to discriminate 

between actual mantle processes and processes acting upon a fragment of 

mantle as it is entrained and carried to the Earth's surface by the 

basaltic magma. The spinel inclusions are interpreted to have formed by a 

process associated with the emplacement of kaersutite dikes in the mantle; 

thus, these inclusions are important indicators of fluid compositions in 

the mantle. Amphibole glass probably was derived from "flash" melting of 

amphibole during decompression in the host basaltic magma during rapid 

ascent (Switzer and Melson, 1969; Francis, 1976). This hypothesis is 

supported by the observations that (1) amphibole glass is present in 

fractures unoccupied by spinel inclusions, and (2) amphibole grains have 

honeycomb-textured boundaries infilled with amphibole glass. The fractures 

apparently formed from expansion and fracturing of the xenoliths as 

83 
decompression occurred. Continuous decompression expansion of the 

xenoliths opened the way for injection of both decompression melts and host 

basalt without total disaggregation of the xenoliths. 

Fluid is present as inclusions in amphibole glass and as planes of 

secondary inclusions in the xenoliths that locally branch from basalt 

injections. Thermometric analyses of fluid inclusions in xenoliths from 

Dish Hill, California, indicate that they are composed of essentially pure 

co2, which is consistent with other fluid inclusion studies (Roedder, 1965; 

Pasteris, in press). These inclusions have densities of 0.93 to 0.40 

g/cm3, which indicate filling pressures of 0.2-0.7 GPa at crustal depths of 

9-26 km. However, the fluid in these inclusions ·may not be representative 

of the fluid present at the time of their entrapment (Mathez and Delaney, 

1981; Pasteris, in press). The range in the densities of these co2 inclusions is interpreted·to represent continuous entrapment of 

fluid during ascent. 

Solid and fluid inclusions are important sites of incompatible-element 

concentrations in mantle xenoliths. Thus, scientists who study mantle 

rocks are confronted with an analytical dilemma: laboratory leaching of 

xenolith samples before analysis may destroy inclusions that are carrying 

intrinsic mantle components, whereas failure to leach the samples probably 

leaves a host-rock contaminant. It is apparent from detailed maps of 

xenoliths containing basalt glass that it may be difficult to uniquely 

identify mantle trace-element compositions with whole-rock analysis. In 

addition, measurements of physical properties also are affected. There 

are, for example, problems in determining magnetic properties of the lower 

lithosphere by using xenoliths, and careful petrographic work must be 

performed prior to analysis. The precipitation of Fe-oxides from 

decompression melts and exsolution of metals (Fe and Ni) during ascent can 

produce a post-entrainment magnetization that does not represent lower 

lithospheric conditions (Garcia-Spatz and Wasilewski, 1985). 

REFERENCES CITED 

Andersen, T., O'Reilly, S.Y., and Griffin, W.L., 1984, The trapped fluid 

phase in upper mantle xenoliths from Victoria, Australia: Implications 

for mantle metasomatism: Contributions to Mineralogy and Petrology, v. 

88, p. 72-85. 

Francis, D.M., 1976, The origin of amphibole in lherzolite xenoliths from 

Nunivak Island, Alaska: Journal of Petrology, v. 17, p. 357-378. 

Garcia-Spatz, R., and Wasilewski, P., 1985, Magnetic·properties of mantle 

xenoliths and ilmenite nodules and the magnetic character of the upper 

mantle [abs.]: Eos, Transactions, American Geophysical Union, v. 66, 

no. 18, p. 380. 

Green, H.W., and Gueguen, Y., 1983, Deformation of peridotite in the mantle 

and extraction by kimberlite: A case history documented by fluid and 

solid precipitates in olivine: Tectonophysics, v. 92, p. 71-92. 

Mathez, E.A., and Delaney, J.R., 1981, The nature and distribution of 

carbon in submarine basalts and peridotite nodules: Earth and 

Planetary Science Letters, v. 56, p. 217-232. 

Menzies, M.A., Kempton, P.D., and Dungan, M., 1985, Interaction of 

continental lithosphere·and asthenospheric melts below the Geronimo 

84 
volcanic field, Arizona, u.s.A.: Journal of Petrology, v. 26, pt. 3, 

p. 663-693. 

Nielson, J.E., and Noller, J.S., in press, Processes of mantle 

metasomatism: Constraints from observations of composite peridotite 

xenoliths, in Mullen, E.D., and Pasteris, J.D., eds., Mantle 

metasomatism and alkaline magmatism: Boulder, Colo., Geological 

Society of America Special Paper 215. 

Noller, J.S., 1986, Solid and fluid inclusions in mantle xenoliths: An 

analytical dilemma?: Geology, v. 14, p. 437-440. 

Pasteris, J.D., in press, Fluid inclusions in mantle xenoliths, in Nixon, 

P., ed., Mantle xenoliths: London, Wiley. 

Recchia, J., 1982, Interactions entre nodules de peridotites et basalteshotes: Exemples de l'Ardeche et des Causses: Paris University, 

doctoral thesis 3d cycle, 127 p. 

Roedder, E., 1965, Liquid co2 inclusions in olivine-bearing nodules and 

phenocrysts from basalts: American Mineralogist, v. 50, p. 1746-1782. 

Switzer, G., and Melson, w. G., 1969, Partially melted kyanite eclogite 

from the Roberts Victor Mine, South Africa: Smithsonian Contributions 

to the Earth Sciences, no. 1, 9 p. 

Wilshire, H.G., Nielson-Pike, J.E., Meyer, C.E., and Schwarzman, E.L., 

1980, Amphibole-rich veins in lherzolite xenoliths, Dish Hill and 

Deadman Lake, California: American Journal of Science, Jackson 

Volume, v. 280-A, p. 576-593. 

85 
FLUID-WALLROCK INTERACTIONS IN THE MANTLE: EVIDENCE FROM COMPOSITE 

LHERZOLITE-HORNBLENDITE XENOLITHS 

JANE E. NIELSON-PIKE, JAMES R. BUDAHN, and HOWARD G. WILSHIRE, U.S. 

Geological Survey, Menlo Park, California 94025 

Metasomatic reactions between magmatic hornblendite veins and their 

peridotite wallrocks have been well documented by systematic major- and 

minor- element analyses of minerals in the peridotite (Wilshire and 

others, 1980). Additional whole-rock major-, minor-, and trace-element 

data on the nature of the reactions are reported here. Sample Ba-2-1, 

from the Dish Hill, California basanite cone, is a large (17 em in largest 

dimension) composite xenolith of spinel lherzolite with a hornblendite 

selvage 1 to 9 mm thick at one end. The selvage, which contains about 5 

percent biotite and 5 percent apatite in addition to Ti-rich pargasite, 

has been shown to be a dike emplaced in the mantle (Wilshire and Trask, 

1971 ) . 

Systematic data derived from Ba-2-1 include major element 

compositions of minerals (Wilshire and others, 1980) and whole-rock 

volumes (reported here) sampled at increasing distance from the selvage, 

chondrite-normalized rare earth element (REEcn) contents of the same 

whole-rock volumes (reported here), and REE and Nd-Sr isotopic 

compositions of both clinopyroxene separates from the lherzolite (location 

unknown) and amphibole from the dike selvage (Menzies and others, 1985). 

The systematic analyses all show that lherzolite bulk and mineral 

compositions are richer in Fe, Na, K, Ti, and Mn in a 4-cm-wide zone 

adjacent to the hornblendite dike than are minerals and volumes of 

lherzolite farther from the dike. In the contact zone the bulk rock also 

has higher H2o+ and co2 than the rock volumes farther from the dikes, and 

light REEcn (LREE) are enriched relative to heavy REEcn (HREEcn) so that 

the ratio LREE/HREEcn is greater than 1 .0. This REE pattern is like that 

of the selvage amphibole, but the REEcn abundances are much less in the 

bulk rock. The systematic data show that compositional variations of the 

minerals were caused by reaction between the lherzolite and fluids from 

which the hornblendite crystallized. Bulk chemical variations also are 

due in part to introduced amphibole and solid inclusions (Nielson and 

Noller, in press); the abundances of both systematically decrease with 

distance from the hornblendite dike. 

Published compositions of unlocated clinopyroxene separates from Ba2-1 showed LREE/HREEcn less than 1 .0 and Nd-Sr isotopic compositions that 

are highly depleted compared to amphiboles from the dike (Menzies and 

others, 1985). The whole rock compositional variations reported here 

raise questions about these data, because they indicate that clinopyroxene 

REE and isotopic compositions likely vary with proximity to the 

hornblendite dike. Clinopyroxene separates (unlocated) from Ba-1-72, 

another Dish Hill composite sample, have LREE/HREEcn greater than 1 .0, and 

isotopic compositions similar to amphibole of the dikes. 'The strong 

similarities between Ba-2-1 and Ba-1-72 further suggest that data on 

unlocated separates may be misleading. For example, in both Ba-2-1 and 

86 
Ba-1-72, major-element mineral compositions vary with proximity to the 

hornblendite, and the REE composition of the hornblendite dikes are 

identical. The only physical difference between the two samples is that 

Ba-1-72 is a thin (7 em) slab of lherzolite sandwiched by two hornblendite 

dikes and Ba-2-1 is a large mass of lherzolite with a dike at one end. 

Thus, Ba-1-72 lherzolite is likely to have been completely soaked by dike 

fluids, and the minerals more extensively metasomatized (Nielson and 

Noller, in press). However, the apparent differences in REE and isotopic 

compositions led Menzies and others (1985) to separately classify the 

samples and to infer an entirely different history for each sample. 

The systematic whole rock REE compositions reveal two important 

facts: (1) The pattern of LREEcn-enrichment is directly related to 

proximity to the hornblendite, and thus is due to a process of mantle 

metasomatism, regardless of whether the LREE occupy clinopyroxene 

structural sites or occur in inclusions within and around other 

minerals. (2) The REEcn pattern of the volume of rock adjacent to the 

dike is like that characteristic of metasomatized peridotite xenoliths 

from many worldwide localities (Frey and Green, 1974). Like the Dish Hill 

composite xenoliths, these other metasomatized peridotites show enrichment 

of Fe and associated elements, LREE enrichment relative to HREE, and some 

contain hydrous seconoary minerals. These features commonly are ascribed 

to processes acting over large regions of the mantle. These processes are 

believed to be necessary precursors to alkali basalt magmatism (see 

Wilshire, in press). Evidence from the Dish Hill xenoliths indicates that 

the processes of local metasomatism consequent upon intrusion of volatilerich melts, and the postulated regional metasomatic processes of 

unspecified origin must mimic one another, or the origin of samples that 

lack structural context has been misinterpreted. 

REFERENCES CITED 

Frey, F.A., and Green, D.H., 1974, The mineralogy, geochemistry, and 

origin of lherzolite inclusions in Victoria basanites: Geochimica et 

Cosmochimica Acta, v. 38, p. 1023-1059. 

Menzies, M.A., Kempton, P.D., and Dungan, M.A., 1985, Interaction of 

continental lithosphe~e and asthenospheric melts below the Geronimo 

volcanic field, Arizona, U.S.A.: Journal of Petrology, v. 26, p. 663-

93. 

Nielson, J.E. and Noller, J.S., in press, Processes of mantle 

metasomatism: Constraints on interpretations from composite 

peridotite xenoliths, in Mullen, E.D., and Pasteris, J.D., eds., 

Mantle metasomatism and alkaline magmatism: Boulder, Colo., 

Geological Society of America Special Paper 215. 

Wilshire, H. G., in press, A model of mantle metasomatism, in Mullen, 

E.D., and Pasteris, J.D., eds., Mantle metasomatism and alkaline 

magmatism: Boulder, Colo., Geological Society of America Special 

Paper 215. 

Wilshire, H.G., Pike, J.E.N., Meyer, C.E., and Schwarzman, E.C., 1980, 

Amphibole-rich veins in lherzolite ·xenoliths, Dish Hill and Deadman 

Lake, California: American Journal of Science, v. 280-A, Jackson 

Volume, Part 2, p. 576-593. 

87 
Wilshire, H.G. and Trask, N.J., 1971, Structural and textural 

relationships of amphibole and phlogopite in peridotite inclusions, 

Dish Hill, California: American Mineralogist, v. 56, p. 240-255. 

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