Earth processes: reading the isotopic code

Earth Processes:

Reading the Isotopic Code

Geophysical Monograph Series

Including

IUGG Volumes

Maurice Ewing Volumes

Mineral Physics Volumes

GEOPHYSICAL MONOGRAPH SERIES

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1 Antarctica in the International Geophysical Year A. P. Crary, L. M. Gould, E. 0. Hulburt, Hugh Odishaw, and Waldo E. Smith (Eds.)

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5 Physics of Precipitation Helmut Weickmann (Ed.) 6 The Crust of the Pacific Basin

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13 The Earth's Crust and Upper Mantle Pembroke ]. Hart (Ed.)

14 The Structure and Physical Properties of the Earth's Crust John G. Heacock (Ed.)

15 The Use of Artificial Satellites for Geodesy Soren W. Henricksen, Armando Mancini, and Bernard H. Chovitz (Eds.)

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19 The Geophysics of the Pacific Ocean Basin and Its Margin: A Volume in Honor of George P. Woollard George H. Sutton, Murli H. Manghnani, and Ralph Moberly (Eds.)

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21 Quantitative Modeling of Magnetospheric Processes W. P. Olson (Ed.)

22 Derivation, Meaning, and Use of Geomagnetic Indices P. N. Mayaud

23 The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands Dennis E. Hayes (Ed.)

24 Mechanical Behavior of Crustal Rocks: The Handin

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25 Physics of Auroral Arc Formation S.-I. Akasofu and J. R. Kan (Eds.)

26 Heterogeneous Atmospheric Chemistry David R. Schryer (Ed.)

27 The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands: Part 2 Dennis E. Hayes (Ed.)

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(Maurice Ewing Volume 5) ]ames E. Hansen and Taro Takahashi (Eds.)

30 Magnetic Reconnection in Space and Laboratory Plasmas Edward W. Hones, Jr. (Ed.)

31 Point Defects in Minerals (Mineral Physics Volume 1) Robert N. Schock (Ed.)

32 The Carbon C•-cle and Atmospheric CO2: Natural Variations Archean to Present E.T. Sundquist and W. S . B roecker ( Eds . )

33 Greenland Ice Core: Geophysics, Geochemistry, and the Environment C. C. Langway, Jr., H. Oeschger, and W. Dansgaard (Eds.)

34 Collisionless Shocks in the Heliosphere: A Tutorial Review Robert G. Stone and Bruce T. Tsurutani (Eds.)

35 Collisionless Shocks in the Heliosphere: Reviews of Current Research Bruce T. Tsurutani and

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36 Mineral and Rock Deformation: Laboratory Studies --The Paterson Volume B. E. Hobbs and

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37 Earthquake Source Mechanics (Maurice Ewing Volume 6) Shamita Das, John Boatwright, and Christopher H. Scholz (Eds.)

38 Ion Acceleration in the Magnetosphere and Ionosphere Tom Chang (Ed.)

39 High Pressure Research in Mineral Physics (Mineral Physics Volume 2) Murli H. Manghnani and Yasuhiko Syono (Eds.)

40 Gondwana Six: Structure, Tectonics, and Geophysics Gary D. McKenzie (Ed.)

41 Gondwana Six: Stratigraphy, Sedimentology, and Paleontology Garry D. McKenzie (Ed.)

42 Flow and Transport Through Unsaturated Fractured Rock Daniel D. Evans and Thomas J. Nicholson (Eds.)

43 Seamounts, Islands, and Atolls Barbara H. Keating, Patricia Fryer, Rodey Batiza, and George W. Boehlert (Eds.)

44 Modeling Magnetospheric Plasma T.E. Moore and J. H. Waite, Jr. (Eds.)

45 Perovskite: A Structure of Great Interest to

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46 Structure and Dynamics of Earth's Deep Interior (IUGG Volume 1) D. E. Smylie and Raymond Hide (Eds.)

47 Hydrological Regimes and Their Subsurface Thermal Effects (IUGG Volume 2) Alan E. Beck,

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49 Slow Deformation and Transmission of Stress

in the Earth (IUGG Volume 4) Steven C. Cohen and Petr Vanϥek (Eds. )

50 Deep Structure and Past Kinematics of Accreted Terranes (IUGG Volume 5) John W. Hillhouse (Ed.)

51 Properties and Processes of Earth's Lower Crust (IUGG Volume 6) Robert F. Mereu, Stephan Mudler, and David M. Fountain (Eds.)

52 Understanding Climate Change (IUGG Volume 7) Andre L. Berger, Robert E. Dickinson, and J. Kidson (Eds.)

53 Plasma Waves and Instabilities at Comets and in

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54 Solar System Plasma Physics J.H. Waite, Jr., ]. L. B urch, and R. L. Moore (Eds.)

55 Aspects of Climate Variability in the Pacific and Western Americas David H. Peterson (Ed.)

56 The Brittle-Ductile Transition in Rocks A.G. Duba, W. B. Durham, J. W. Handin, and H. F. Wang (Eds.)

57 Evolution of Mid Ocean Ridges (IUGG Volume 8) John M. Sinton (Ed.)

58 Physics of Magnetic Flux Ropes C.T. Russall, E. R. Priest, and L. C. Lee (Eds.)

59 Variations in Earth Rotation (IUGG Volume 9)

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61 Cometary Plasma Processes Alan D. Johnstone (Ed.) 62 Modeling Magnetospheric Plasma Processes

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63 Marine Particles: Analysis and Characterization David C. Hurd and Derek W. Spencer (Eds.)

64 Magnetospheric Substorms Joseph R. Kan, Thomas A. Potemra, Susumu Kokubun, and Takesi Iijima (Eds.)

65 Explosion Source Phenomenology Steven R. Taylor, Howard J. Patton, and Paul G. Richards (Eds.)

66 Venus and Mars: Atmospheres, Ionospheres, and Solar Wind Interactions Janet G. Luhmann, Mariella Tatrallyay, and Robert O. Pepin (Eds.)

67 High-Pressure Research: Application to Earth and Planetary Sciences (Mineral Physics Volume 3) Yasuhiko Syono and Murli H. Manghnani (Eds.)

68 Microwave Remote Sensing of Sea Ice Frank Carsey, Roger Barry, Josefino Comiso, D. Andrew Rothrock, Robert Shuchman, W. Terry Tucker, Wilford Weeks, and Dale Winebrenner

69 Sea Level Changes: Determination and Effects (IUGG Volume 11) P. L. Woodworth, D. T. Pugh, ]. G. DeRonde, R. G. Warrick, and J. Hannah

70 Synthesis of Results from Scientific Drilling in the Indian Ocean Robert A. Duncan, David K. Rea,

Robert B. Kidd, Ulrich von Rad, and Jeffrey K. Weissel (Eds.)

71 Mantle Flow and Melt Generation at Mid-Ocean

Ridges Jason Phipps Morgan, Donna K. Blackman, and John M. Sinton (Eds.)

72 Dynamics of Earth's Deep Interior and Earth Rotation (IUGG Volume 12) Jean-Louis Le Mou•l, D.E. Smylie, and Thomas Herring (Eds.)

73 Environmental Effects on Spacecraft Positioning and Trajectories (IUGG Volume 13) A. Vallance Jones (Ed.)

74 Evolution of the Earth and Planets (IUGG

Volume 14) E. Takahashi, Raymond Jeanloz, and David Rubie (Eds.)

75 Interactions Between Global Climate Subsystems: The Legacy of Hann (IUGG Volume 15) G. A. McBean and M. Hantel (Eds.)

76 Relating Geophysical Structures and Processes: The Jeffreys Volume (IUGG Volume 16) K. Aki and R. Dmowska (Eds.)

77 The Mesozoic Pacific: Geology, Tectonics, and VolcanismmA Volume in Memory of Sy Schlanger Malcolm S. Pringle, William W. Sager, William V. Sliter, and Seth Stein (Eds.)

78 Climate Change in Continental Isotopic Records P. K. Swart, K. C. Lohmann, J. McKenzie, and S. Savin (Eds.)

79 The Tornado: Its Structure, Dynamics, Prediction, and Hazards C. Church, D. Burgess, C. Doswell, R. Davies-Jones (Eds.)

80 Auroral Plasma Dynamics R.L. Lysak (Ed.) 81 Solar Wind Sources of Magnetospheric Ultra-Low

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82 Gravimetry and Space Techniques Applied to Geodynamics and Ocean Dynamics (IUGG Volume 17) Bob E. Schutz, Allen Anderson, Claude Froidevaux, and Michael Parke (Eds.)

83 Nonlinear Dynamics and Predictability of Geophysical Phenomena (IUGG Volume 18) William I. Neunnan, Andrei Gabrielov, and Donald L. Turcotte (Eds.)

84 Solar System Plasmas in Space and Time J. Burch, J. H. Waite, Jr. (Eds.)

85 The Polar Oceans and Their Role in Shaping the Global Environment O. M. Johannessen, R. D. Muench, and J. E. Overland (Eds.)

86 Space Plasmas: Coupling Between Small and Medium Scale Processes Maha Ashour-Abdalla, Tom

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A Review of Experiment and Theory R.M. Johnson and T. L. Killeen (Eds.)

88 Active Margins and Marginal Basins of the Western Pacific Brian Taylor and James Natland (Eds.)

89 Natural and Anthropogenic Influences in Fluvial Geomorphology John E. Costa, Andrew J. Miller, Kenneth W. Potter, and Peter R. Wilcock (Eds.)

90 Physics of the Magnetopause Paul Song, B.U.•. Sonnerup, and M.F. Thomsen (Eds.)

91 Seafloor Hydrothermal Systems: Physical, Chemical, Biological, and Geological Interactions Susan E. Humphris, Robert A. Zierenberg, Lauren S. Mullineaux, and Richard E. Thomson (Eds.)

92 Mauna Loa Revealed: Structure, Composition, History, and Hazards J. M. Rhodes and John P. Lockwood (Eds. )

93 Cross-Scale Coupling in Space Plasmas James L. Horwitz, Nagendra Singh, and James L. Burch (Eds.)

94 Double-Diffusive Convection Alan Brandt and

H.J.S. Fernando (Eds.)

Maurice Ewing Volumes

1 Island Arcs, Deep Sea Trenches, and Back-Arc Basins Manik Talwani and Walter C. Pitman III (Eds.)

2 Deep Drilling Results in the Atlantic Ocean: Ocean Crust Manik Talwani, Christopher G. Harrison, and Dennis E. Hayes (Eds.)

3 Deep Drilling Results in the Atlantic Ocean: Continental Margins and Paleoenvironment Manik Talwani, William Hay, and William B. F. Ryan (Eds.)

4 Earthquake PredictioniAn International Review David W. Simpson and Paul G. Richards (Eds.)

5 Climate Processes and Climate Sensitivity James E. Hansen and Taro Takahashi (Eds.)

6 Earthquake Source Mechanics Shamita Das, John Boatwright, and Christopher H. Scholz (Eds.)

IUGG Volumes

1 Structure and Dynamics of Earth's Deep Interior D. E. Smylie and Raymond Hide (Eds.)

2 Hydrological Regimes and Their Subsurface Thermal Effects Alan E. Beck, Grant Garven, and

Lajos Stegena (Eds. ) 3 Origin and Evolution of Sedimentary Basins and

Their Energy and Mineral Resources Raymond A. Price (Ed.)

4 Slow Deformation and Transmission of Stress

in the Earth Steven C. Cohen and Petr Vani•ek (Eds.)

5 Deep Structure and Past Kinematics of Accreted Terrances John W. Hillhouse (Ed.)

6 Properties and Processes of Earth's Lower Crust Robert F. Mereu, Stephan Mudler, and David M. Fountain (Eds.)

7 Understanding Climate Change Andre L. Berger, Robert E. Dickinson, and J. Kidson (Eds.)

8 Evolution of Mid Ocean Ridges John M. Sinton (Ed.) 9 Variations in Earth Rotation Dennis D. McCarthy

and William E. Carter (Eds.)

10 Quo Vadimus Geophysics for the Next Generation George D. Garland and John R. Apel (Eds.)

11 Sea Level Changes: Determinations and Effects Philip L. Woodworth, David T. Pugh, John G. DeRonde, Richard G. Warrick, and John Hannah (Eds.)

12 Dynamics of Earth's Deep Interior and Earth Rotation

Jean-Louis Le Mou•l, D.E. Smylie, and Thomas Herring (Eds. )

13 Environmental Effects on Spacecraft Positioning and Trajectories A. Vallance Jones (Ed.)

14 Evolution of the Earth and Planets E. Takahashi,

Raymond Jeanloz, and David Rubie (Eds.) 15 Interactions Between Global Climate Subsystems:

The Legacy of Hann G.A. McBean and M. Hantel (Eds.)

16 Relating Geophysical Structures and Processes: The Jeffreys Volume K. Aki and R. Dmowska (Eds.)

17 Gravimetry and Space Techniques Applied to Geodynamics and Ocean Dynamics Bob E. Schutz, Allen Anderson, Claude Froidevaux, and Michael Parke (Eds.)

18 Nonlinear Dynamics and Predictability of Geophysical Phenomena William I. Newman, Andrei Gabrielov, and Donald L. Turcotte (Eds.)

Mineral Physics Volumes

1 Point Defects in Minerals Robert N. Schock (Ed.)

2 High Pressure Research in Mineral Physics Murli H. Manghnani and Yasuhiko Syona (Eds.)

3 High Pressure Research: Application to Earth and Planetary Sciences Yasuhiko Syono and Murli H. Manghnani (Eds.)

Geophysical Monograph 95

Earth Processes:

Reading the Isotopic Code Asish Basu

Stan Hart

Editors

American Geophysical Union

Published under the aegis of the AGU Books Board.

Library of Congress Cataloging-in-Publication Data

Earth processes: reading the isotopic code / Asish Basu and Stan Hart, editors.

P. cm.- (Geopysical monograph :95) Includes bibliographical references. ISBN 0-87590-077-1 (alk. paper) 1. Isotope geology. 2. Radioactive dating. I. Basu, Asish,

1944- . II. Hart, Stanley R. (Stanley Robert), 1935- III. Series.

QE501.4.N9E27 1996 551.7'01--dc20

96-1381

CIP

ISBN 0-87590-077-1

ISSN OO65-8448

Copyright 1996 by the American Geophysical Union 2000 Florida Avenue, N.W. Washington, DC 20009

Figures, tables, and short excerpts may be reprinted in scientific books and journals if the source is properly cited.

Authorization to photocopy items for internal or personal use, or the internal or personal use of specific clients, is granted by the American Geophysical Union for libraries and other users registered with the Copyright Clearance Center (CCC) Transactional Reporting Service, pro- vided that the base fee of $1.50 per copy plus $0.35 per page is paid directly to CCC, 222 Rosewood Dr., Danvers, MA 01923. 0065-8448/96/$01.50+0.35.

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Printed in the United States of America.

CONTENTS

Preface Stan Hart and Asish Basu xi

THE EARLY EARTH AND ITS ENVIRONS

Live •ø7pd in the Early Solar System and Implications for Planetary Evolution J. H. Chen and G. J. Wasserburg 1

4øAr-Z•Ar and Noble Gas Analyses of a H-type C!ast Included in a Shocked L-Chondrite From Antarctica

Ichiro Kaneoka, Nobuo Takaoka, and Keizo Yanai 21

Geochemical Constraints on the Origin of the Moon Stuart Ross Taylor and Tezer M. Esat 33

Accretion and Early Differentiation History of the Earth Based on Extinct Radionuclides Stein B. Jacobsen and Charles L. Harper, Jr. 47

Influence of Accretion on Lead in the Earth

Stephen J. G. Galer and Steven L. Goldstein 75

Three Time-Scales for the Mantle

Claude J. All•gre, Bernard Dupr•, and Eric Lewin 99

THE MANTLE AND ITS MELTING PRODUCTS

Partial Melting of Fertile Mantle Peridotite at High Pressures: An Experimental Study Using Aggregates of Diamond Ikuo Kushiro 109

Os Partitioning Between Phases in Lherzolite and Basalt Stanley R. Hart and Gregory E. Ravizza 123

Petrogenesis of U!tramafic Xenoliths From Hawaii Inferred From Sr, Nd, and Pb Isotopes Osamu Okano and Mitsunobu Tatsumoto 135

Temporal Isotopic Variations in the Hawaiian Mantle Plume: The Lanai Anomaly, the Molokai Fracture Zone and a Seawater-Altered Lithospheric Component in Hawaiian Volcanism Asish R. Basu and Billy E. Faggart, Jr. 149

Temporal Geochemical Evolution of Kilauea Volcano: Comparison of Hilina and Puna Basalt C.-Y. Chen, F. A. Frey, J. M. Rhodes, and R. M. Easton 161

Geochemistry and Geochronology of the Society Islands: New Evidence for Deep Mantle Recycling William M. White and Robert A. Duncan 183

Depleted and Enriched Upper Mantle Sources for Basaltic Rocks From Diverse Tectonic Environments in the Northeast Pacific Ocean: The Generation of Oceanic Alkaline vs. Tholeiitic

Basalts

Brian L. Cousens 207

The Character of the Subcontinental Mantle in Southeast Asia: Evidence From Isotopic and Elemental Compositions of Extension-Related Cenozoic Basalts in Thailand Samuel B. Mukasa, G. Matthew Fischer, and Sandra M. Barr 233

CONTENTS

Petrogenesis of Italian Alkaline Lavas Deduced From Pb-Sr-Nd Isotope Relationships Massimo D'Antonio, George R. Tilton, and Lucia Civetta 253

The Lithospheric Manfie Beneath Central Europe: Nd Isotopic Constraints for Its Late Proterozoic Enrichment and Implications for Early Crustal Evolution Peter Stille and Urs Schaltegger 269

Isotopic Studies of Processes in Mafic Magma Chambers: IH The Muskox Intrusion, Northwest Territories, Canada Brian W. Stewart and Donald J. DePaolo 277

Lead Isotope Mapping of Crustal Reservoirs Within the Grenville Superterrane: I. Central and Southern Appalachians A. K. Sinha, J.P. Hogan, and J. Parks 293

A P-T-t Path for an (Ultra?-) High-Pressure Ultramafic/Mafic Rock-Association and Its Felsic Country-Rocks Based on SHRIMP-Dating of Magmatic and Metamorphic Zircon Domains. Example: Alpe Arami (Central Swiss Alps) Dieter Gebauer 307

UNTANGLING CRUSTAL CONGERIES

Mineral Inclusions in Heavy Minerals of the Ultrahigh-Pressure Metamorphic Rocks of the Dora-Maira Massif and Their Bearing on the Relative Timing of the Petrological Events H.-P. Schertl and W. Schreyer 331

Shock Metamorphosed Zircons with Correlated U-Pb Discordance and Melt Rocks With Concordant Protolith Ages Indicate an Impact Origin for the Sudbury Structure T. E. Krogh, S. L. Kamo, and B. F. Bohor 343

U-Pb Reverse Discordance in Zircons: The Role of Fine-Scale Oscillatory Zoning and Sub-Micron Transport of Pb James M. Mattinson, Cinda M. Graubard, David L. Parkinson, and William C. McClelland 355

SOILS, SEAWATER AND STANDARDS

Recent Erosional History of a Soil Profile Based on Cosmogenic In-Situ Radionuclides •4C and •øBe D. Lal, M. Pavich, Z. Y. Gu, A. J. T. Jull, M. Caffee, R. Finkel, and J. Southon 371

Preferential Weathering of Potassium Feldspar in Mature Soils Todd K. Hinkley 377

Evaluation of the Sedimentary Manganese Deposits of Mexico and Morocco for Determining Lead and Strontium Isotopes in Ancient Seawater B. R. Doe and R. A. Ayuso, Kiyoto Futa, and Z. E. Peterman 391

Industrial Lead Contamination of Natural Ecosystems in Japan Masayo Murozumi, Se•/i Nakamura, and Clair Patterson 409

Evaluation of a •2Pb-ZøsPb Double Spike for High-Precision Lead Isotope Analysis W. Todt, R. A. Cliff, A. Hanser, and A. W. Hofmann 429

PREFACE

Publication of this monograph will coincide, to a preci- sion of a few per mil, with the centenary of Henri Becquer- el's discovery of "radiations actives" (C. R. Acad. Sci., Feb. 24, 1896). In 1896 the Earth was only 40 million years old according to Lord Kelvin. Eleven years later, Boltwood had pushed the Earth's age past 2000 million years, based on the first U/Pb chemical dating results. In exciting progression came discovery of isotopes by J. J. Thomson in 1912, invention of the mass spectrometer by Dempster (1918) and Aston (1919), the first measurement of the isotopic composition of Pb (Aston, 1927) and the final approach, using Pb-Pb isotopic dating, to the correct age of the Earth: close--2.9 Ga (Gerling, 1942), closer--3.0 Ga (Holmes, 1949) and closest--4.50 Ga (Patterson, Tilton and Inghram, 1953).

Almost no facet of planetary science has subsequently escaped isotopic scrutiny. The unifying theme of this mono- graph reflects part of this astonishing vista of geoscience problems which can be dissected with today's isotopic "tool box." From the early solar system to recent weathering profiles, from the classical isotopes of Pb to the extinct curiosities of 146Sm and lø7pd, from the convection of Sr isotopes by mantle plumes to the advection of anthropogenic Pb by humans, this monograph has vision in many dimen- sions. There is no adequate title for such a panoply.

The Early Earth and Its Environs

The constraints on the timing of condensation and aggregation in the early solar system can be derived from the excess •ø7Ag produced by decay of now-extinct •ø7pd. Chen and Wasserburg review the evidence from iron and stony-iron meteorites and show that these time differences cluster in a very narrow range of about 12 Ma. Events can also affect meteorites long after their birth. Using noble gas studies, Kaneoka, Takaoka and Yanai uncover complex shock degassing processes in an H-chondrite clast included in a shocked L-chondrite. Closer to home, early catastrophic events also affected the moon, and Taylor and Esat review the evidence for the giant impact hypothesis for lunar origin. They argue that some depletions of volatile elements (e.g., Rb) occurred in the solar nebula some 10-50 Ma prior to lunar origin, and that most of the strong depletions of very volatile elements in the moon were in fact inherited from the

impactor, and not produced during the impact event. Unlike

the moon, the Earth preserves no direct rock record of its earliest history. However, inferences can be made by utilizing isotopic anomalies attributed to extinct radioactivi- ties. Jacobsen and Harper discuss new data for •82W which suggest an accretion and core formation interval of only 2- 15 Ma, whereas their 142Nd anomaly in one of the Earth's oldest surviving rocks argues for silicate mantle differentia- tion within 100 Ma of accretion. Because no primordial Earth rocks have yet been found, the "age of the Earth" is necessarily a "model" age. Galer and Goldstein revisit this venerable question; they derive limits to the time interval between initial condensation in the solar nebula and comple- tion of Pb fractionation into the core of 80_+40 Ma. They further argue that the also venerable "Pb paradox" is a natural consequence of this U/Pb fractionation interval. Also weighing in on the side of a younger Earth, All•gre, Dupr6, and Lewin suggest a core segregation age of 4.47 Ga. They further delineate two other time scales for Earth's mantle--a

crustal differentiation time (mean value 2.0-2.2 Ga) and a plate tectonic, or upper mantle, mixing time of about 1 Ga.

The Mantle and Its Melting Products

Using the novel new diamond aggregate technique, Kushiro revisits the thorny question of how melt composi- tion changes with pressure and temperature, based on experimental melting of a natural lherzolite. By monitoring the extent of melting (F), ranging from 5 % to 35 %, he is able to confirm the decrease of SiO2 and A1203 and the increase of MgO and FeO with pressure of melting. He also clearly delineates the P-T-F regimes from which tholeiites, alkali basalts and picrites may be derived (and all without invoking any isotopes!). From these kinds of studies, it has long been supposed that ocean ridge tholeiites (MORB) represent higher-degree melts than ocean island alkali basalts. Yet, paradoxically, Os contents are typically 10 times lower in the former than in the latter. Hart and

Ravizza report on the partitioning behavior of Os between various phases in a natural lherzolite and in several basalts. They document both a very high sulfide/melt fractionation (> 106) and the likelihood that almost all of the Os in typical lherzolites will be contained in the sulfide. This leads to a

solution for the paradox which invokes direct dissolution of mantle sulfide into upwardly percolating basaltic melts.

xi

While oceanic basalts are clearly the melting products of mantle peridotite, it is seldom possible to directly relate the two in a given area. Okano and Tatsumoto report several clear-cut examples from the Honolulu volcanic series, Hawaii, where one spinel lherzolite xenolith has a Sr, Nd and Pb isotopic signature similar to that of the older Koolau shield tholeiites, whereas a variety of other xenoliths are isotopically similar to the post-erosional basalts which entrained them. U/Pb and Sm/Nd mineral "ages" from these peridotites are, at 80-90 Ma, the same as the age of litho- sphere under Hawaii, confirming their isotopic identity with, and derivation from, the lithosphere as opposed to being from the Hawaiian plume itself. The unusual (non-litho- spheric) enriched isotopic signature of the Koolau tholeiites is shown to be also present on Lanai and Kahoolawe, in the study reported by Basu and Faggart. However, this signa- ture was not found in any of a large series of basalts dredged along at least 1900 km of the Hawaiian plume trail. Basu and Faggart suggest that a large 3-Ma pulse of enriched plume material was released when the Molokai Fracture Zone migrated across the plume. Temporal variations of the Hawaiian plume on a shorter time scale were also studied, for the oldest 100 Ka sequence on Kilauea volcano, by Chen, Frey, Rhodes and Easton. "Old" Kilauea not only looks similar to recent Kilauea in trace elements and in Sr, Nd, and Pb isotopes but also retains its distinctive difference from Mauna Loa, showing that the plumbing systems for the two volcanoes have not intermin- gled through time.

While Hawaii may be the archetype of mantle plumes, it apparently shares some attributes with its brethren. The Society plume shows a plume trail with age progression, and on the island of Tahaa, isotopically enriched shield-building basalts are followed, after a 1.2 Ma hiatus, by more depleted post-erosional basalts. In addition, White and Duncan's study of the Society plume shows that the enriched Tahaa basalts extend to a virtually pure EM2 mantle component, and that good correlations between isotopes and ratios such as Pb/Ce, Zr/Nb and Rb/Sr show that these trace element signatures can be attributed to the mantle source and are not merely artifacts of the melting process. This important finding strengthens the case for the presence of a sediment component in EM2 mantle.

Enriched basalts are not always restricted to plumes, as Cousens reports the common occurrence of LREE-enriched alkali basalts, along with typical LREE-depleted tholeiites, from the NE Pacific Seamount province, contiguous to the Juan de Fuca/Explorer ridge system. Tholeiitic and alkalic basalts are geographically intermingled, and all are generally N-MORB in Sr and Nd isotopic signature. Cousens appeals to a plum-pudding or marble-cake-mantle model to explain these results, with the plums being enriched in lithophile trace elements and of HIMU-like isotopic signature. The complexities of a seamount/spreading ridge environment

such as this are significantly exacerbated when the ex- tensional regime is intra-continental, as in the study of Cenozoic rift basalts from Thailand by Mukasa, Fischer and Barr. While crustal contamination appears to be minimal in these basalts, they do have a strong DUPAL Pb isotope signature (EM2). Mukasa et al. conclude that this signature was locally introduced into the mantle by subduction of marine sediments during crustal amalgamation or conver- gence events in the Mesozoic.

This process of local injection and incubation of sedi- ments is garnering more widespread recognition, and D'Antonio, Tilton and Civetta appeal to a similar process to explain Sr, Nd and Pb isotope relationships in the Italian alkaline igneous province. They argue for selective tapping of injected crustal material, by partial melting or fluid processes, from a sunken lithospheric slab now situated under the Apennines. Direct study of exhumed enriched lithosphere is reported by Stille and Schaltegger, for mafic/ultramafic rocks from the Alpine realm ranging in age from 20 to 500 Ma. Utilizing Nd isotope tracers, they show evidence for a common evolutionary path for all of these rocks, diverging from a depleted mantle array about 700- 900 Ma. They argue that the sub-European mantle was enriched at this time by intrusion and entrapment of carbon- atite-like fluids or melts derived from the asthenosphere. For decades, geochemists have struggled with the problem of distinguishing between crustal contamination of magmas at shallow levels and derivation from deep subcontinental- lithosphere enriched during prior subduction or metasomatic events. In the case of the Italian alkaline province discussed above, this argument has raged back and forth many times. Study of the crystallization of a large magma chamber can aid in identifying compelling fingerprints for AFC (crustal contamination) processes. Stewart and DePaolo report such a study of the 1260 Ma Muskox Intrusion, utilizing Sr and Nd isotopic tracers. They show that while a zone of wall- rock-derived silicic magma existed at the roof of this intrusion, the assimilation of this into the injected pulses of new mafic magma proceeded at a low rate ( < 5 55) compared with the overall crystallization rate. While increased mixing did occur as the system lost vigor, silica-rich liquids were still produced by in situ differentiation with little input from the silicic "roof magma."

Untangling Crustal Congeries

The continental crust is exceedingly complex, and isotopic tools have much to contribute, both in a classic age- dating sense and in a taxonomic or finger-printing sense. Sinha, Hogan and Parks provide an example of the latter, by using Pb isotopes to map various crustal domains in Gren- ville age rocks from the Appalachians. Whereas inter- regional correlation of terrains is difficult by geologic mapping, they are able to assign a variety of basement rock

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units, stretching from Pennsylvania to Georgia, to three principal Pb isotope "groups"; they further argue that tectonic processes juxtaposed these groups into what we know as the Grenville Superterrane. Looking at a smaller region, with œmer age resolution, does not diminish the complexity but seemingly just adds new layers in fractal-like progression. Gebauer unravels four stages of magmat- ic/metamorphic history in and around one small mafic/ul- tramafic body in the central Swiss Alps, using a combination of ion-probe U-Pb dating and cathode-luminescence studies of zircons. In addition to inherited pre-Alpine zircon domains, he delineates events at 43 Ma, 35.4 Ma and 25.1 Ma and, from these, constructs a model for uplift and cooling rates in this region of the Alps. Focusing down even further, Schertl and Schreyer search for high-pressure mineral inclusions encapsulated in zircon, monazite and rutile from the intriguing ultra-high-pressure rocks of the Dora-Malta massif (western Alps). In general, inclusions are found which are chemically identical to the minerals in the groundmass which define the peak metamorphic assemblage. The inclusion-bearing minerals thus grew during the ultra- high-pressure event (> 100 km depth), and this leads the way to further understanding of this event by ion probe U/Pb age dating of these inclusion-bearing minerals.

Not all crustal complexities can be blamed on endogenous processes, and separating the results of these from results of meteorite impact events has frequently proven challenging. The Sudbury Igneous complex is one such case, with a contentious inside/outside controversy dating back over three decades. Following their elegant success with zircon dating of the K/T ejecta from Chicxulub crater, Krogh, Kamo and Bohor weigh-in on the side of an impact origin for Sudbury, utilizing the correlation of precise U-Pb zircon dating with planar deformation features in the zircon. One controversy retired! And while the U-Pb zircon technique becomes ever more useful and celebrated, nagging mysteries still lie dormant. So-called "reverse discordance" is one

such mystery, where the two U/Pb zircon ages are greater than the Concordia age. Variably attributed to U loss or radiogenic Pb gain, Mattinson, Graubard, Parkinson and McClelland give us the best, possibly final, illumination of this problem in a careful and derailed study of reversely discordant zircons. They show that the problem results from excess "parentless" Pb which is derived locally and redis- tributed within a finely zoned zircon structure by diffusion and alpha-recoil implantation.

Soils, Seawater and Standards

River input of elements to the sea is modulated both by overall erosion rate and by complex elemental fractionations which occur during weathering and soil formation. In developing a soil erosion measurement technique based on the in situ cosmogenic production of InC and løBe, Lal,

Pavich, Gu, Jull, Caffee, Finkel and Southon obtain erosion rates of >_30 /•m/year (inc) to _<3 /•m/year (løBe) for a quartz vein in Virginia. While much ground remains to be explored, this technique shows great promise for measuring erosion rates of quartz-bearing rocks. When eroded products are carried away in streams, this is rarely done as a "con- gruent" process, as emphasized by Hinkley in his study of soils and drainage in a sub-alpine watershed (Sierra Nevada) underlain by homogenous granitoid basement. Plagioclase and K-feldspar are concentrated in the sand and clay/silt fractions of the soils, respectively. During rapid run-off conditions, the K-feldspar fractions dominate, whereas Ca and Sr from plagioclase dominate the earliest stages of water/substrate interactions and the normal "baseline"

stream conditions. As a repository of the dissolved load from continents, seawater acts as a recorder of climatic and tectonic processes affecting the continents. Learning to hear the melody from the cacophony of this record is a chal- lenge. Doe, Ayuso, Futa and Peterman evaluate the use of ancient sedimentary manganese deposits for reconstructing the Sr and Pb isotopic melody of the oceans. While the Sr in these deposits is generally more radiogenic than open- ocean seawater, suggesting some commingling of older detritus, the Pb appears to behave in a more consistent man- ner. As we expect the oceans to be homogeneous in Sr but heterogeneous in Pb, these Mn deposits may provide a much needed new recorder of seawater Pb.

Migration and dispersal of Pb throughout Earth's eco- sphere is readily studied, courtesy of the massive anthropo- genic Pb tracer spike injected over the past century. Muro- zumi, Nakamura and Patterson show that this industrial Pb

can be isotopically characterized and is present in soils, waters, plants and animals in three of the most remote ecosystems in Japan. Cadmium, copper and zinc pollution are evident in these remote areas as well, all atmospherically transported along with the Pb.

And here we end, almost 70 years after the first crude isotopic measurements of Pb. Todt, Cliff, Hanser and Hof- mann give a detailed evaluation of the 2ø2pb-2øspb double- spike technique and show that isotopic measurement preci- sion of better than 0.01% can be achieved with this tech-

nique. After 100 years of remarkable progress since Becque- rel's discovery of radioactivity, what will we do for an encore?

Stan Hart

Woods Hole Oceanographic Institution Massachusetts

Asish Basu

University of Rochester New York

Editors

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AGU Meeting, circa 1970. George apparently preferred flashier lies, Tats apparently is left-handed. Did Tatsjust give George a Pb brick?

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A Tribute to Mitsunobu Tatsumoto and George Tilton, Our Preeminent Heroes of Pb Isotope Geochemistry

Mitsunobu Tatsumoto, popularly known as Tats among his colleagues, was educated in Japan at the Tokyo Higher Normal School, studying physics and chemistry, followed by three years of study at the Tokyo University of Educa- tion (Bunrika). At the Bunrika, he continued nine more years of studies and research in geochemistry for which he received a Doctor of Science degree in 1957. Tatsumoto held three post-doctoral research positions in the United States, first at Scripps Institution of Oceanography, 1957-58, then at Texas A & M College, 1959, followed by a three year stay at the California Institute of Technology (1959- 1962). In September 1962, he joined the U.S. Geological Survey and has been associated with the Branch of Isotope Geology in Denver since that time. In July, 1995, after 33 years of uninterrupted service, Tats took official retirement from the USGS. In recognition of his outstanding contribu- tions to the development of multi-isotope geochemistry to investigate fundamental problems of crust-mantle evolution, the understanding of lunar history and the origin of meteor- ites, he received the Distinguished Service Award of the United States Department of the Interior in 1988. Previous- ly, in 1976, he was awarded the degree of Doctor honoris causa from the University of Paris for lunar and meteorite research. Tats became a Fellow of the Meteoritical Society in that same year, and, in 1987, the American Geophysical Union recognized his numerous contributions in isotope geochemistry and elected him a Fellow. Although he was pleased with these honors, he derived his most enduring satisfaction from working with the pool of young, enthusias- tic scientists from the U.S. and abroad, whom he trained in his laboratory and who now hold many academic and governmental positions. Tats is an internationally known isotope geochemist who has done exceptional research in solar system chronology, and earth history and evolution, using the uranium-thorium-lead (U-Th-Pb), samarium- neodymium (Sm-Nd) and lutetium-hafnium (Lu-Hf) isotope systems. His research career began in the early 1950's in Japan, where he worked with H. Hamaguchi on projects involving chemical investigations of deep sea deposits. After arriving in the United States in 1957, he continued his research on the marine geochemistry of uranium and lead, first at Scripps, then followed by several years of fruitful collaboration with Clair Patterson at Cal Tech dealing mostly with lead-isotopes in detrital minerals and their

significance for crust-mantle evolution. It was here that he was inoculated with the "Chicago bug." After joining the U.S.G.S. in 1962, he was engaged in research on the U-Th- Pb systematics of terrestrial volcanic rocks and meteorites. He believed in the science of measurements, omnia in mensura et numero, that everything rests on measurements and numbers. With his pioneering work on accurate isotopic analyses of lead, neodymium and hafnium in terrestrial, lunar and meteorite samples, he has continued to contribute outstanding research results for which he is widely ac- claimed in the scientific community.

In 1973, Tats and co-workers analyzed very precisely the Pb-isotopic ratios in troilite from the iron meteorite Canyon Diablo, establishing the primeval Pb-isotopic composition in the solar system. He and his colleagues were the first investigators to compare the effects of thermal metamor- phism on the U-Th-Pb, Rb-Sr and Sm-Nd systems using the brecciated achondrite meteorite Pasamonte. This work has

served as a foundation for evaluating apparent disturbances to the isotopic chronometers, when these three isotopic systems are applied to metamorphosed samples. The following year, 1978, was marked by another outstanding contribution by Tatsumoto to the Pb-isotopic composition of oceanic basalts and its implications for mantle evolution. This now classic review paper, representing more than a decade's worth of efforts to understand chemical heterogene- ity in the mantle using Pb-isotopes, subsequently inspired new lines of ideas and data interpretation concerning heterogeneous mantle reservoirs. In the next few years, Tats and colleagues pioneered the first comprehensive studies of lunar basalts, oceanic basalts and granitoids using the new Lu-Hf isotopic system. This system was developed in his laboratory, and the results obtained in the first several studies served as the basis for most subsequent investiga- tions in the field. From 1978 to 1987, Tats directed and co- authored several studies of oceanic basalts with an emphasis on Hawaiian volcanism; the combined Pb, Sr, Nd and Hf isotopic relationships indicated that three isotopically distinct mantle sources were contributing to the generation of Hawaiian basalts. These studies provided a working model for the origin of the Hawaiian basalts and pointed out the power and necessity of a multi-isotopic approach in the evaluation of sources for oceanic island volcanism. The

efforts by Tats and colleagues in using the Lu-Hf isotopic

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system for investigating the evolution of lunar basalts were recognized by NASA as one of the highlights of lunar and planetary research in 1983. During 1984-1986, Tats was involved in the exciting consortium studies of two "lunar" meteorites. The Pb-isotopic data from his laboratory provided strong and independent evidence that these meteor- ites did, in fact, have a lunar origin.

At the time of his retirement in July 1995, Tats had authored and co-authored 133 scientific papers, and 128 abstracts were presented by him and co-workers at meet- ings. Although officially retired, he plans to continue to work in his laboratory at the USGS as an emeritus research- er. He is known among colleagues throughout tile world as a man with a great sense of humor, boundless energy and enthusiasm for new research and for his youthful, vigorous and yet courteous manner.

George Tilton, rarely known as Tilt among his col- leagues, was educated at Blackburn College and the Univer- sity of Illinois (B.S. in chemistry, 1947). He then took part in the incredible isotope geochemistry "bloom" at the University of Chicago, doing a thesis under Harrison Brown on the U-Pb geochemistry of Piedmont granites. His advisor suggested that the study would be "duck soup," because of the high lead content of granites. The good news was that George showed that granites had ten times less lead than commonly supposed; the bad news was that this made for a difficult and technically demanding thesis! In 1951, Ph.D. in hand, George moved to the Carnegie Institution of Washington to make a foursome with the emerging isotope dynasty there (Aldrich, Davis, Wetherill). Geochronology was to "come of age" globally during Tiltoh's 14-year stay at CIW, emerging from the realm of heroic analytical efforts and poorly understood "dates" to semi-routine analyses and interpretable "ages." Migrating west in 1965, following the earlier tracks left by Wetherill, George settled at the University of California, Santa Barbara. Not to be dislodged by occasional clashes with the mores of Southern California, George did spend a few sabbatical years away (ETH, Zurich 1971-1972; CIW, 1978-1979) before nominally retiring in 1991. But then, George just published his 89th paper last year, so perhaps UCSB just thinks he's retired!

George Tilton has been formally honored in many ways. He is a fellow of the Geological Society of America, the American Geophysical Union and the American Association for the Advancement of Science. He was elected to the

National Academy of Sciences in 1977 and was awarded an honorary "Doktor der Naturwissenschaften" from the Swiss Federal Institute of Technology, Zurich, in 1984. And, from respect for his quiet statesmanship, George served as president of the Geochemical Society in 1980-1981. More importantly, perhaps, George is honored day in and day out by all who know him, for his spirit is gentle, generous and nourishing to all.

George Tilton will undoubtedly go down in scientific history as Mr. Zircon. While contributing importantly to Patterson's early work on the age of meteorites and the Earth in 1953-1955, George was also developing the technology for isotope dilution of U, Th and Pb of granitic rocks. In a classic 1955 paper, he "took apart" a granite from Ontario and derived precise U-Th-Pb data on six different phases, and reported the first zircon U/Pb age. This was followed in 1957 by a second classic paper reporting zircon U/Pb ages from a wide variety of granitoid rocks, and thus was born the whole field of U-Th-Pb geochronology as we know it today. The techniques have evolved radically since then, thanks largely to the torch which was passed on to Tom Krogh before George left Carnegie for the wild west; no one misses the operose borax fusions, dithizone extractions and lead sulfide precipitations!

While George continued to refine and expand the under- standing and utility of U/Pb geochronology throughout his career (continuous diffusion models, sphene and monazite dating, ages of ophiolites and ultra-high-pressure metamor- phic complexes), he also turned his talents to the use of Pb isotopes as the "DNA testing" of Earth science. In 1958, George demonstrated that tektites were not ETs (extra- terrestrials) based on their Pb isotopic composition; not George's fault that this work didn't put a stop to the tektite controversy! In 1964, the momentous Gast, Tilton and Hedge paper demonstrated the chemical heterogeneity of Earth's mantle, based on Sr and Pb isotopes in volcanic rocks from two oceanic islands. This started something that has endured 30 years of scrutiny without satisfactory closure, and George and his students have contributed forcefully all along the way. Not George's fault that we haven't figured this one out yet! There's more!--Pb in Precambrian galenas, and the age of the Earth (with Steiger, 1969; and Sinha, 1973); origin of volcanic arc magmas (Cascades, with Church, 1973; Andes, with Barriero, 1980; Mukasa, 1984; Bonin Islands, with Dobson, 1989; Sierra Nevada, with Chen, 1991); and the geochemistry of mantle rocks (heat production, 1963; subcontinental lithosphere, with BenOthman and Menzies, 1990; partitioning in lherzo- lites, with Meiier and Kwon, 1990). All investigations focused and targeted with pin-point accuracy, and what a splendid range of vision.

While George's activities on extraterrestrial matter were dormant from 1955 to 1973, they then became activated with gusto. He revisited the question of the age of meteor- ites in 1973 and 1976 (with Chen), applying vastly im- proved techniques. In a tribute to Patterson's early work, the "ages" only changed by some 1 •h percent, but with uncertainties now 10 times smaller. Interestingly, 1973 and 1976 also saw publication of Tatsumoto's meteorite Pb-Pb age work (with }Cmight and All•gre, 1973; with Unmh and Desborough, 1976). The close agreement between these two

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groups left little doubt about the age of meteorites! There followed a 10-year period when George and his students pursued a series of important lunar and meteoritic studies, all utilizing careful Pb isotope technology. While ETs appear since 1988 to be hibernating in George's lab, they are probably not retired either.

We applaud Tats and George; they have made our science richer, they have made our lives richer. They have shown us the way, and they have done it with a sense of human- ness which we can all follow proudly. We salute also the many contributors who have gifted this monograph with their talent and their time, and made it a fitting tribute to our two heroes.

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•---Pump

C

G

i ^

The first mass spectrograph, constructed by A. J. Dempster at the University of Chicago (Dempster, A. J., Physical Review 11, 316, 1918). The source G, collector E, and analyzing chamber A were housed in a brass vacuum chamber partitioned by the slits S 1 and S2.

ooo

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