Cosmochemistry [1 ed.] 0521878624, 9780521878623

Table of contents :
Half-title......Page 3
Title......Page 5
Copyright......Page 6
Dedication......Page 7
Contents......Page 9
Preface......Page 19
What is cosmochemistry?......Page 21
Geochemistry versus cosmochemistry......Page 23
Meteorites and microscopy......Page 26
Spectroscopy and the compositions of stars......Page 29
Solar system element abundances......Page 30
Isotopes and nuclear physics......Page 31
Space exploration and samples from other worlds......Page 34
New sources of extraterrestrial materials......Page 38
The tools and datasets of cosmochemistry......Page 40
Mineral chemistry......Page 41
Trace element abundances......Page 42
Stable isotope compositions......Page 43
Cosmochemical theory......Page 44
Relationship of cosmochemistry to other disciplines......Page 45
Suggestions for further reading......Page 46
References......Page 47
Elementary particles, isotopes, and elements......Page 49
Chart of the nuclides: organizing elements by their nuclear properties......Page 52
Radioactive elements and their modes of decay......Page 55
The periodic table: organizing elements by their chemical properties......Page 58
Chemical bonding......Page 64
Chemical and physical processes relevant to cosmochemistry......Page 66
Evaporation and condensation......Page 68
Isotope effects from chemical and physical processes......Page 69
Summary......Page 71
References......Page 72
In the beginning......Page 74
The Big Bang model......Page 75
The Hubble Constant......Page 76
Big Bang nucleosynthesis......Page 77
Nucleosynthesis in stars......Page 78
Classification, masses, and lifetimes of stars......Page 81
The life cycles of stars......Page 84
CNO cycles......Page 92
Helium burning......Page 95
Synthesis of elements heavier than iron......Page 97
Explosive nucleosynthesis......Page 99
Nuclides produced by irradiation......Page 100
Origin of the galaxy and galactic chemical evolution......Page 101
Summary......Page 102
Questions......Page 103
References......Page 104
Historical perspective......Page 105
How are solar system abundances determined?......Page 107
Spectroscopic observations of the Sun......Page 108
Collecting and analyzing the solar wind......Page 116
Importance of CI chondrites......Page 119
Neutron activation analysis......Page 120
Helium......Page 121
Solar system abundances of the elements......Page 122
Solar system abundances of the isotopes......Page 124
How did solar system abundances arise?......Page 130
Differences between solar system and cosmic abundances......Page 131
How are solar system abundances used in cosmochemistry?......Page 133
Normalizing to solar system abundances......Page 135
Summary......Page 136
Suggestions for further reading......Page 137
References......Page 138
Grains that predate the solar system......Page 140
A cosmochemical detective story......Page 142
Recognizing presolar grains in meteorites......Page 145
Known types of presolar grains......Page 147
Locating and identifying presolar grains......Page 148
Characterization of presolar grains......Page 149
Nondestructive characterization......Page 150
Destructive techniques......Page 151
Grains from AGB stars......Page 152
Nova grains......Page 159
Presolar grains as probes of stellar nucleosynthesis......Page 160
Internal stellar structure......Page 161
The neutron source(s) for the s-process......Page 162
Constraining supernova models......Page 163
Galactic chemical evolution......Page 164
Graphite grains from AGB stars......Page 166
Graphite grains from supernovae......Page 168
Presolar grains as probes of the early solar system......Page 169
Summary......Page 172
Suggestions for further reading......Page 173
References......Page 174
Primitive versus differentiated......Page 177
Chondrules......Page 178
Refractory inclusions......Page 183
Matrix......Page 184
Chondrite classification......Page 185
Primary characteristics: chemical compositions......Page 186
Secondary characteristics: petrologic types......Page 188
Other classification parameters: shock and weathering......Page 190
Oxygen isotopes in chondrites......Page 191
Classification of nonchondritic meteorites......Page 193
Primitive achondrites......Page 194
Acapulcoites and lodranites......Page 195
Ureilites......Page 196
Aubrites......Page 198
Angrites......Page 199
Classification and composition of iron meteorites......Page 200
Lunar samples......Page 202
Martian meteorites......Page 204
Oxygen isotopes in differentiated meteorites......Page 205
Summary......Page 207
Suggestions for further reading......Page 208
References......Page 209
What are chemical fractionations and why are they important?......Page 212
Condensation as a fractionation process......Page 215
Condensation sequences......Page 216
Applicability of condensation calculations to the early solar system......Page 221
Volatile element depletions......Page 225
Gas–solid interactions......Page 226
CAIs......Page 228
Magmatic processes that lead to fractionation......Page 230
Element partitioning......Page 231
Sorting of chondrite components......Page 233
Fractionations by impacts or pyroclastic activity......Page 235
Element fractionation resulting from oxidation/reduction......Page 237
Element fractionation resulting from planetary differentiation......Page 238
Massependent fractionation......Page 240
Fractionations produced by ion–molecule reactions......Page 241
Massndependent fractionation......Page 242
Radiogenic isotope fractionation and planetary differentiation......Page 244
Summary......Page 245
Suggestions for further reading......Page 246
References......Page 247
Methods of age determination......Page 250
Basic principles of radiometric age dating......Page 251
Long-lived radionuclides......Page 257
40K–40Ar dating......Page 258
40Ar–39Ar dating......Page 259
The 87Rb–87Sr system......Page 262
History......Page 263
Technical details......Page 264
Applications......Page 267
Chronology with slope of an isochron......Page 268
Chronology using the initial 87Sr/86Sr ratio......Page 269
The 147Sm–143Nd system......Page 272
Technical details......Page 273
Reservoir evolution......Page 274
Chronology with 147Sm–143Nd isochrons......Page 277
The U–Th–Pb system......Page 278
History......Page 281
The uranium–lead concordia diagram......Page 282
207Pb–206Pb dating......Page 286
The common-lead method......Page 288
The 187Re–187Os system......Page 290
Applications......Page 291
Chronology with Re–Os isochrons......Page 292
Reservoir evolution inferred from the 187Re–187Os system......Page 293
History......Page 294
Applications......Page 295
U–Th–He......Page 296
138La–138Ba and 138La–138Ce......Page 297
Shortived radionuclides......Page 298
History......Page 302
Technical details......Page 303
The 26Al–26Mg system......Page 304
History......Page 305
Applications......Page 306
Applications......Page 307
History......Page 308
The 60Fe–60Ni system......Page 309
History......Page 310
The 107Pd–107Ag system......Page 311
Applications......Page 312
Technical details......Page 313
Technical details......Page 314
The 10Be–10B system......Page 315
Applications......Page 316
244Pu–Xenon......Page 317
Summary......Page 318
Suggestions for further reading......Page 319
References......Page 320
Age of the elements and environment in which the Sun formed......Page 328
Age of the solar system......Page 335
Early solar system chronology......Page 338
Primitive components in chondrites......Page 339
CAIs......Page 341
Chondrules......Page 343
Accretion and history of chondritic parent bodies......Page 344
Ordinary chondrites......Page 345
CV and CO chondrites......Page 346
Accretion and differentiation of achondritic parent bodies......Page 347
HED meteorites......Page 348
Iron meteorites......Page 349
Age of the Earth......Page 350
Age of the Moon......Page 351
Age of Mars......Page 352
Shock ages of meteorites......Page 356
Shock ages of lunar rocks......Page 359
Cosmic-ray exposure ages......Page 360
Terrestrial ages......Page 365
Summary......Page 366
Suggestions for further reading......Page 367
References......Page 368
Volatility......Page 374
Organic matter: occurrence and complexity......Page 375
Extractable organic matter in chondrites......Page 376
Insoluble macromolecules in chondrites......Page 382
Stable isotopes in organic compounds......Page 384
Are organic compounds interstellar or nebular?......Page 386
Noble gases and how they are analyzed......Page 390
Nuclear components......Page 391
The solar components......Page 392
Planetary components......Page 393
Planetary atmospheres......Page 395
Condensation and accretion of ices......Page 397
Summary......Page 398
Suggestions for further reading......Page 399
References......Page 400
Dry asteroids and meteorites......Page 402
Appearance and physical properties......Page 403
Spectroscopy and classification......Page 405
Orbits, distribution, and delivery......Page 409
Analyses of asteroids by spacecraft remote sensing......Page 410
Chondritic meteorites......Page 412
Differentiated meteorites......Page 416
Thermal evolution of anhydrous asteroids......Page 418
Thermal structure of the asteroid belt......Page 423
Collisions among asteroids......Page 426
Summary......Page 428
Suggestions for further reading......Page 429
References......Page 430
Icy bodies in the solar system......Page 432
Orbits......Page 433
Appearance and physical properties......Page 434
Comet ices......Page 438
Comet dust: spectroscopy and spacecraft analysis......Page 439
Interplanetary dust particles......Page 442
Returned comet samples......Page 446
Spectroscopy of asteroids formed beyond the snowline......Page 452
Aqueous alteration of chondrites......Page 453
Chemistry of hydrated carbonaceous chondrites......Page 456
Variations among ice-bearing planetesimals......Page 459
Summary......Page 460
Suggestions for further reading......Page 461
References......Page 462
Why the Moon and Mars?......Page 465
Global geologic context for lunar geochemistry......Page 466
Instruments on orbiting spacecraft......Page 468
Laboratory analysis of returned lunar samples and lunar meteorites......Page 470
Sample geochemistry......Page 471
Geochemical mapping by spacecraft......Page 472
Compositions of the lunar mantle and core......Page 476
Geochemical evolution of the Moon......Page 479
Global geologic context for Mars geochemistry......Page 482
Instruments on orbiting spacecraft......Page 484
Instruments on landers and rovers......Page 485
Laboratory analyses of Martian meteorites......Page 486
Measured composition of the Martian crust......Page 489
Composition of the crust......Page 490
Water, chemical weathering, and evaporites......Page 492
Compositions of the Martian mantle and core......Page 495
Summary......Page 497
Suggestions for further reading......Page 498
References......Page 499
From gas and dust to Sun and accretion disk......Page 504
Temperatures in the accretion disk......Page 509
Localized heating: nebular shocks and the X-wind model......Page 512
Constraints on planet bulk compositions......Page 515
Models for estimating bulk chemistry......Page 518
Planetesimal building blocks......Page 519
Delivery of volatiles to the terrestrial planets......Page 523
Planetary differentiation......Page 524
Formation of the giant planets......Page 527
Orbital and collisional evolution of the modern solar system......Page 531
Summary......Page 532
Questions......Page 533
References......Page 534
Wet chemical analysis......Page 538
Neutron activation analysis......Page 539
Electron-beam techniques......Page 540
Scanning electron microscope......Page 542
Transmission electron microscope......Page 543
Auger nanoprobe......Page 544
X-ray diffraction (XRD)......Page 545
X-ray absorption near-edge spectroscopy......Page 546
Inductively coupled plasma......Page 547
Mass analyzers......Page 548
Quadrupole ion traps......Page 549
Electron multiplier......Page 550
Stable-isotope mass spectrometers......Page 551
Inductively coupled-plasma mass spectrometers......Page 552
Magnetic-sector ion microprobes......Page 553
Raman spectroscopy......Page 554
Gamma-ray and neutron spectrometers......Page 555
Thin-section preparation......Page 556
Sample preparation for the TEM......Page 557
Preparation of samples for TIMS and ICPMS......Page 558
40Ar–39Ar dating......Page 559
129I–129Xe dating......Page 560
Suggestions for further reading......Page 561
Index......Page 563

Citation preview

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Cosmochemistry

How did the solar system’s chemical composition arise and evolve? This textbook provides the answers in the first interdisciplinary introduction to cosmochemistry. It makes this exciting and evolving field accessible to undergraduate and graduate students from a range of backgrounds, including geology, chemistry, astronomy, and physics. The authors – two established research leaders who have helped pioneer developments in the field – provide a complete background to cosmochemical processes and discoveries, enabling students outside geochemistry to fully understand and explore the solar system’s composition. Topics covered include: * * * *

* *

synthesis of nuclides in stars partitioning of elements between solids, liquids and gas in the solar nebula overviews of the chemistry of extraterrestrial materials isotopic tools used to investigate processes such as planet accretion and element fractionation chronology of the early solar system geochemical exploration of planets.

Boxes provide basic definitions and mini-courses in mineralogy, organic chemistry, and other essential background information for students. Review questions and additional reading for each chapter encourage students to explore cosmochemistry further. HARRY (Hap) Y. McSWEEN Jr. is Chancellor’s Professor at the University of Tennessee.

He has conducted research on cosmochemistry for more than three decades and was one of the original proponents of the hypothesis that some meteorites are from Mars. He has been a co-investigator for four NASA spacecraft missions and serves on numerous advisory committees for NASA and the US National Research Council. Dr. McSween has written or edited four books on meteorites and planetary science, and coauthored a textbook in geochemistry. He is a former president and Fellow of the Meteoritical Society, a Fellow of the American Academy of Arts and Sciences, recipient of the Leonard Medal, and has an asteroid named for him. GARY HUSS is a Research Professor and Director of the W. M. Keck Cosmochemistry Laboratory at the Hawai‘i Institute for Geophysics and Planetology, University of Hawai‘i at Mānoa. In more than three decades of research on cosmochemistry, he was among the first

to study presolar grains, the raw materials for the solar system. He presently studies the chronology of the early solar system. He comes from a family of meteorite scientists: his grandfather, H. H. Nininger, has been called the father of modern meteoritics, and his father, Glenn Huss, and grandfather were responsible for recovering over 500 meteorites previously unknown to science. Dr. Huss is a former president and Fellow of the Meteoritical Society and also has an asteroid named for him.

Cosmochemistry Harry Y. McSween, Jr. University of Tennessee, Knoxville

Gary R. Huss University of Hawai‘i at Ma¯noa

CAMBRIDGE UNIVERSITY PRESS

Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo, Delhi, Dubai, Tokyo Cambridge University Press The Edinburgh Building, Cambridge CB2 8RU, UK Published in the United States of America by Cambridge University Press, New York www.cambridge.org Information on this title: www.cambridge.org/9780521878623 © Harry Y. McSween, Jr. and Gary R. Huss 2010 This publication is in copyright. Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published in print format 2010 ISBN-13

978-0-511-72968-3

eBook (NetLibrary)

ISBN-13

978-0-521-87862-3

Hardback

Cambridge University Press has no responsibility for the persistence or accuracy of urls for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate.

For Sue and Jackie

Contents

Preface

page xvii

1

Introduction to cosmochemistry Overview What is cosmochemistry? Geochemistry versus cosmochemistry Beginnings of cosmochemistry (and geochemistry) Philosophical foundations Meteorites and microscopy Spectroscopy and the compositions of stars Solar system element abundances Isotopes and nuclear physics Space exploration and samples from other worlds New sources of extraterrestrial materials Organic matter and extraterrestrial life? The tools and datasets of cosmochemistry Laboratory and spacecraft analyses Cosmochemical theory Relationship of cosmochemistry to other disciplines Questions Suggestions for further reading References

1 1 1 3 6 6 6 9 10 11 14 18 20 20 21 24 25 26 26 27

2

Nuclides and elements: the building blocks of matter Overview Elementary particles, isotopes, and elements Chart of the nuclides: organizing elements by their nuclear properties Radioactive elements and their modes of decay The periodic table: organizing elements by their chemistry properties Chemical bonding Chemical and physical processes relevant to cosmochemistry Isotope effects from chemical and physical processes Summary Questions Suggestions for further reading References

29 29 29 32 35 38 44 46 49 51 52 52 52

viii

Contents

3

Origin of the elements Overview In the beginning The Big Bang model Observational evidence Nucleosynthesis in stars Classification, masses, and lifetimes of stars The life cycles of stars Stellar nucleosynthesis processes Origin of the galaxy and galactic chemical evolution Summary Questions Suggestions for further reading References

4

Solar system and cosmic abundances: elements and isotopes Overview Chemistry on a grand scale Historical perspective How are solar system abundances determined? Determining elemental abundances in the Sun Spectroscopic observations of the Sun Collecting and analyzing the solar wind Determining chemical abundances in meteorites Importance of CI chondrites Measuring CI abundances Indirect methods of estimating abundances Solar system abundances of the elements Solar system abundances of the isotopes How did solar system abundances arise? Differences between solar system and cosmic abundances How are solar system abundances used in cosmochemistry? Summary Questions Suggestions for further reading References

85 85 85 85 87 88 88 96 99 99 100 101 102 104 110 111 113 116 117 117 118

5

Presolar grains: a record of stellar nucleosynthesis and processes in interstellar space Overview Grains that predate the solar system A cosmochemical detective story Recognizing presolar grains in meteorites

120 120 120 122 125

54 54 54 55 56 58 61 64 72 81 82 83 84 84

ix

Contents

6

Known types of presolar grains Identification and characterization of presolar grains Locating and identifying presolar grains Characterization of presolar grains Identification of stellar sources Grains from AGB stars Supernova grains Nova grains Other stellar sources Presolar grains as probes of stellar nucleosynthesis Input data for stellar models Internal stellar structure The neutron source(s) for the s-process Constraining supernova models Galactic chemical evolution Presolar grains as tracers of circumstellar and interstellar environments Silicon carbide Graphite grains from AGB stars Graphite grains from supernovae Interstellar grains Presolar grains as probes of the early solar system Summary Questions Suggestions for further reading References

127 128 128 129 132 132 139 139 140 140 141 141 142 143 144 146 146 146 148 149 149 152 153 153 154

Meteorites: a record of nebular and planetary processes Overview Primitive versus differentiated Components of chondrites Chondrules Refractory inclusions Metals and sulfide Matrix Chondrite classification Primary characteristics: chemical compositions Secondary characteristics: petrologic types Chondrite taxonomy Other classification parameters: shock and weathering Oxygen isotopes in chondrites Classification of nonchondritic meteorites Primitive achondrites Acapulcoites and lodranites

157 157 157 158 158 163 164 164 165 166 168 170 170 171 173 174 175

x

Contents

7

Ureilites Winonaites and IAB silicate inclusions Magmatic achondrites Aubrites Howardites–eucrites–diogenites Angrites Irons and stony irons Classification and composition of iron meteorites Pallasites and mesosiderites Lunar samples Martian meteorites Oxygen isotopes in differentiated meteorites Summary Questions Suggestions for further reading References

176 178 178 178 179 179 180 180 182 182 184 185 187 188 188 189

Cosmochemical and geochemical fractionations Overview What are chemical fractionations and why are they important? Condensation as a fractionation process Condensation sequences Applicability of condensation calculations to the early solar system Volatile element depletions Gas–solid interactions Gas–liquid interactions Igneous fractionations Magmatic processes that lead to fractionation Element partitioning Physical fractionations Sorting of chondrite components Fractionations by impacts or pyroclastic activity Element fractionation resulting from oxidation/reduction Element fractionation resulting from planetary differentiation Fractionation of isotopes Mass-dependent fractionation Fractionations produced by ion–molecule reactions Planetary mass-dependent fractionations Mass-independent fractionation Radiogenic isotope fractionation and planetary differentiation Summary Questions Suggestions for further reading References

192 192 192 195 196 201 205 206 208 210 210 211 213 213 215 217 218 220 220 221 222 222 224 225 226 226 227

xi

Contents

8

Radioisotopes as chronometers Overview Methods of age determination Discussing radiometric ages and time Basic principles of radiometric age dating Long-lived radionuclides The 40K–40Ar system The 87Rb–87Sr system The 147Sm–143Nd system The U–Th–Pb system The 187Re–187Os system The 176Lu–176Hf system Other long-lived nuclides of potential cosmochemical significance Short-lived radionuclides The 129I–129Xe system The 26Al–26Mg system The 41Ca–41K system The 53Mn–53Cr system The 60Fe–60Ni system The 107Pd–107Ag system The 146Sm–142Nd system The 182Hf–182W system The 10Be–10B system Other short-lived nuclides of potential cosmochemical significance Summary Questions Suggestions for further reading References

230 230 230 231 231 237 238 242 252 258 270 274 276 278 282 284 287 288 289 291 293 294 295 297 298 299 299 300

9

Chronology of the solar system from radioactive isotopes Overview Age of the elements and environment in which the Sun formed Age of the solar system Early solar system chronology Primitive components in chondrites Accretion and history of chondritic parent bodies Accretion and differentiation of achondritic parent bodies Accretion, differentiation, and igneous history of planets and the Moon Age of the Earth Age of the Moon Age of Mars Shock ages and impact histories Shock ages of meteorites

308 308 308 315 318 319 324 327 330 330 331 332 336 336

xii

Contents

10

11

Shock ages of lunar rocks The late heavy bombardment Cosmogenic nuclides in meteorites Cosmic-ray exposure ages Terrestrial ages Summary Questions Suggestions for further reading References

339 340 340 340 345 346 347 347 348

The most volatile elements and compounds: organic matter, noble gases, and ices Overview Volatility Organic matter: occurrence and complexity Extractable organic matter in chondrites Insoluble macromolecules in chondrites Stable isotopes in organic compounds Are organic compounds interstellar or nebular? Noble gases and how they are analyzed Noble gas components in extraterrestrial samples Nuclear components The solar components Planetary components Planetary atmospheres Condensation and accretion of ices Summary Questions Suggestions for further reading References

354 354 354 355 356 362 364 366 370 371 371 372 373 375 377 378 379 379 380

Chemistry of anhydrous planetesimals Overview Dry asteroids and meteorites Asteroids: a geologic context for meteorites Appearance and physical properties Spectroscopy and classification Orbits, distribution, and delivery Chemical compositions of anhydrous asteroids and meteorites Analyses of asteroids by spacecraft remote sensing Chondritic meteorites Differentiated meteorites Thermal evolution of anhydrous asteroids

382 382 382 383 383 385 389 390 390 392 396 398

xiii

Contents

Thermal structure of the asteroid belt Collisions among asteroids Summary Questions Suggestions for further reading References

403 406 408 409 409 410

12

Chemistry of comets and other ice-bearing planetesimals Overview Icy bodies in the solar system Orbital and physical characteristics Orbits Appearance and physical properties Chemistry of comets Comet ices Comet dust: spectroscopy and spacecraft analysis Interplanetary dust particles Returned comet samples Ice-bearing asteroids and altered meteorites Spectroscopy of asteroids formed beyond the snowline Aqueous alteration of chondrites Thermal evolution of ice-bearing bodies Chemistry of hydrated carbonaceous chondrites Variations among ice-bearing planetesimals Summary Questions Suggestions for further reading References

412 412 412 413 413 414 418 418 419 422 426 432 432 433 436 436 439 440 441 441 442

13

Geochemical exploration of planets: Moon and Mars as case studies Overview Why the Moon and Mars? Global geologic context for lunar geochemistry Geochemical tools for lunar exploration Instruments on orbiting spacecraft Laboratory analysis of returned lunar samples and lunar meteorites Measured composition of the lunar crust Sample geochemistry Geochemical mapping by spacecraft Compositions of the lunar mantle and core Geochemical evolution of the Moon Global geologic context for Mars geochemistry

445 445 445 446 448 448 450 451 451 452 456 459 462

xiv

Contents

14

Geochemical tools for Mars exploration Instruments on orbiting spacecraft Instruments on landers and rovers Laboratory analyses of Martian meteorites Measured composition of the Martian crust Composition of the crust Water, chemical weathering, and evaporites Compositions of the Martian mantle and core Geochemical evolution of Mars Summary Questions Suggestions for further reading References

464 464 465 466 469 470 472 475 477 477 478 478 479

Cosmochemical models for the formation of the solar system Overview Constraints on the nebula From gas and dust to Sun and accretion disk Temperatures in the accretion disk Localized heating: nebular shocks and the X-wind model Accretion and bulk compositions of planets Agglomeration of planetesimals and planets Constraints on planet bulk compositions Models for estimating bulk chemistry Formation of the terrestrial planets Planetesimal building blocks Delivery of volatiles to the terrestrial planets Planetary differentiation Formation of the giant planets Orbital and collisional evolution of the modern solar system Summary Questions Suggestions for further reading References

484 484 484 484 489 492 495 495 495 498 499 499 503 504 507 511 512 513 514 514

Appendix:

Some analytical techniques commonly used in cosmochemistry Chemical compositions of bulk samples Wet chemical analysis X-ray fluorescence (XRF) Neutron activation analysis Petrology, mineralogy, mineral chemistry, and mineral structure Optical microscopy Electron-beam techniques

518 518 518 519 519 520 520 520

xv

Contents

Other techniques for determining chemical composition and mineral structure Proton-induced X-ray emission (PIXE) Inductively coupled-plasma atomic-emission spectroscopy (ICP-AES) X-ray diffraction (XRD) Synchrotron techniques Mass spectrometry Ion sources Mass analyzers Detectors Mass spectrometer systems used in cosmochemistry Raman spectroscopy Flight instruments Gamma-ray and neutron spectrometers Alpha-particle X-ray spectrometer Mössbauer spectrometer Sample preparation Thin-section preparation Sample preparation for EBSD Sample preparation for the TEM Preparing aerogel “keystones” Preparation of samples for TIMS and ICPMS Details of radiometric dating systems using neutron activation 40 Ar–39Ar dating 129 129 I– Xe dating Suggestions for further reading Index

525 525 525 525 526 527 527 528 530 531 534 535 535 536 536 536 536 537 537 538 538 539 539 540 541 543

Preface

Cosmochemistry provides critical insights into the workings of our local star and its companions throughout the galaxy, the origin and timing of our solar system’s birth, and the complex reactions inside planetesimals and planets (including our own) as they evolve. Much of the database of cosmochemistry comes from laboratory analyses of elements and isotopes in our modest collections of extraterrestrial samples. A growing part of the cosmochemistry database is gleaned from remote sensing and in situ measurements by spacecraft instruments, which provide chemical analyses and geologic context for other planets, their moons, asteroids, and comets. Because the samples analyzed by cosmochemists are typically so small and valuable, or must be analyzed on bodies many millions of miles distant, this discipline leads in the development of new analytical technologies for use in the laboratory or flown on spacecraft missions. These technologies then spread to geochemistry and other fields where precise analyses of small samples are important. Despite its cutting-edge qualities and newsworthy discoveries, cosmochemistry is an orphan. It does not fall within the purview of chemistry, geology, astronomy, physics, or biology, but is rather an amalgam of these disciplines. Because it has no natural home or constituency, cosmochemistry is usually taught (if it is taught at all) directly from its scientific literature (admittedly difficult reading) or from specialized books on meteorites and related topics. In crafting this textbook, we attempt to remedy that shortcoming. We have tried to make this subject accessible to advanced undergraduate and graduate students with diverse academic backgrounds, although we do presume some prior exposure to basic chemistry. This goal may lead to uneven treatment of some subjects, and our readers should understand that our intended audience is broad. Cosmochemistry is advancing so rapidly that we can only hope to provide a snapshot of the discipline as it is currently understood and practiced. We have found even that to be a challenge, because we could not hope to possess expertise in all the subjects encompassed by this discipline. We have drawn heavily on the contributions of many colleagues, especially those who educate by writing thoughtful reviews. That assistance is gratefully acknowledged through our annotated suggestions for further reading at the end of each chapter. The topics covered in the chapters of this book include the following, in this order: *

* *

An introduction to how cosmochemistry developed, and to how it differs from geochemistry A review of the characteristics and behaviors of elements and nuclides A discussion of how elements are synthesized within stars, and how the chemistry of the galaxy has evolved over time

xviii

Preface

* *

* *

* * * * *

* * *

An assessment of the abundances of elements and isotopes in the solar system A description of presolar grains, and how they constrain stellar nucleosynthesis and processes in interstellar space An introduction to meteorites and lunar samples An evaluation of processes that have fractionated elements and isotopes in interstellar space, in the solar nebula, and within planetary bodies An explanation of how radioactive isotopes are used to quantify solar system history A synthesis of the radiometric age of the solar system and the ages of its constituents An assessment of the most volatile materials – organic matter, noble gases, and ices A survey of the chemistry of anhydrous planetesimals and the samples we have of them A survey of the chemistry of ice-bearing comets and asteroids and the samples we have of them Examples of modern geochemical exploration of planetary bodies – the Moon and Mars A review of the formation of the solar system, from the perspective of cosmochemistry An Appendix describing some important analytical methods used in cosmochemistry

More-established disciplines are taught using tried-and-true methods and examples, the results of generations of pedagogical experimentation. Cosmochemistry does not yet offer that. Most of those who dare to teach cosmochemistry, including the authors of this book, have never actually been students in a cosmochemistry course. In the authors’ case, we have learned from a handful of scientists who have guided our introduction to the field, including Calvin Alexander, Bob Pepin, Ed Anders, Jim Hays, Dick Holland, Ian Hutcheon, Klaus Keil, Roy Lewis, Dimitri Papanastassiou, Jerry Wasserburg, and John Wood. We hope that this book on cosmochemistry will guide other students and their teachers as they explore together this emerging, interdisciplinary subject, and that they will enjoy the experience as much as we have.

1

Introduction to cosmochemistry

Overview Cosmochemistry is defined, and its relationship to geochemistry is explained. We describe the historical beginnings of cosmochemistry, and the lines of research that coalesced into the field of cosmochemistry are discussed. We then briefly introduce the tools of cosmochemistry and the datasets that have been produced by these tools. The relationships between cosmochemistry and geochemistry, on the one hand, and astronomy, astrophysics, and geology, on the other, are considered.

What is cosmochemistry? A significant portion of the universe is comprised of elements, ions, and the compounds formed by their combinations – in effect, chemistry on the grandest scale possible. These chemical components can occur as gases or superheated plasmas, less commonly as solids, and very rarely as liquids. Cosmochemistry is the study of the chemical composition of the universe and the processes that produced those compositions. This is a tall order, to be sure. Understandably, cosmochemistry focuses primarily on the objects in our own solar system, because that is where we have direct access to the most chemical information. That part of cosmochemistry encompasses the compositions of the Sun, its retinue of planets and their satellites, the almost innumerable asteroids and comets, and the smaller samples (meteorites, interplanetary dust particles or “IDPs,” returned lunar samples) derived from them. From their chemistry, determined by laboratory measurements of samples or by various remote-sensing techniques, cosmochemists try to unravel the processes that formed or affected them and to fix the chronology of these events. Meteorites offer a unique window on the solar nebula – the disk-shaped cocoon of gas and dust that enveloped the early Sun some ~ 4.57 billion years ago, and from which planetesimals and planets accreted (Fig. 1.1). Within some meteorites are also found minuscule presolar grains, providing an opportunity to analyze directly the chemistry of interstellar matter. Some of these tiny grains are pure samples of the matter ejected from dying stars and provide constraints on our understanding of how elements were forged inside stars before the Sun’s birth. Once formed, these

2

Fig. 1.1

Cosmochemistry

An artist’s conception of the solar nebula, surrounding the violent young Sun. Figure courtesy of NASA.

grains were released to the interstellar medium (ISM), the space between the stars. The ISM is filled primarily by diffuse gases, mostly hydrogen and helium, but with oxygen, carbon, and nitrogen contributing about 1% by mass and all the other elements mostly in micrometer-size dust motes. Much of the chemistry in the ISM occurs within relatively dense molecular clouds, where gas densities can reach 103 to 106 particles per cm3, high by interstellar standards (but not by our everyday experience – Earth’s atmosphere has ~ 3 × 1019 atoms per cm3 at sea level). These clouds are very cold, with temperatures ranging from 10 to 100 K, so interstellar grains become coated with ices. Reactions between ice mantles and gas molecules produced organic compounds that can be extracted from meteorites and identified by their bizarre isotopic compositions. Many dust grains were undoubtedly destroyed in the ISM, but some hardy survivors were incorporated into the nebula when the molecular cloud collapsed, and thence were accreted into meteorites. Processes that occur inside stars, in interstellar space, and within the solar nebula have no counterparts in our terrestrial experience. They can be studied or inferred from astronomical observations and astrophysical theory, but cosmochemical analyses of materials actually formed or affected by these processes provide constraints and insights that remote sensing and theory cannot. Our terrestrial experience places us on firmer ground in deciphering the geologic processes occurring on the Earth’s Moon. In studying lunar rocks and soils, we can use familiar geochemical tools developed for understanding the Earth. We have also measured the chemical compositions of some other planetary bodies or their smaller cousins, geologically processed planetesimals, using telescopes or instruments on spacecraft. In some cases, we even have meteorites ejected during impacts onto these bodies. Chemical measurements (whether from laboratory analyses of samples or in situ analyses of rocks and soils by orbiting or landed spacecraft) add quantitative

3

Introduction to cosmochemistry

dimensions to our understanding of planetary science. All extraterrestrial materials are fair game for cosmochemistry.

Geochemistry versus cosmochemistry Traditionally, cosmochemistry has been treated as a branch of geochemistry – usually defined as the study of the chemical composition of the Earth. Geochemistry focuses on the chemical analysis of terrestrial materials, as implied by the prefix “geo,” and geochemistry textbooks commonly devote only a single chapter to cosmochemistry, if the subject is introduced at all. However, the line between geochemistry and cosmochemistry has always been somewhat fuzzy. The most prominent technical journal in this discipline, Geochimica et Cosmochimica Acta, has carried both names since its inception in 1950. The burgeoning field of planetary geochemistry appropriates the “geo” prefix, even though its subject is not Earth. A broader and more appropriate definition of geochemistry might be the study of element and isotope behavior during geologic processes, such as occur on and within the Earth and other planets, moons, and planetesimals. Using this definition, we will include planetary geochemistry as an essential part of our treatment of cosmochemistry. It is worth noting, though, that the geochemical and cosmochemical behaviors of elements do show some significant differences. A geochemical perspective of the periodic table is illustrated in Figure 1.2 (adapted from Railsback, 2003). As depicted, this diagram is decidedly Earth-centric, but the controls on element behavior during geologic processes apply to other bodies as well. Determining relative elemental abundances is an important part of geochemistry, and the relative abundances of elements in the Earth’s crust vary over many orders of magnitude. Crustal abundances are illustrated in Figure 1.2, because most geochemical data are based on readily accessible samples of the crust. Geochemistry is also concerned with determining the composition of the Earth’s interior – its mantle and core – and a more comprehensive figure would include those abundances as well. Very few native elements (pure elements not chemically bound to any others) occur naturally in the Earth, so Figure 1.2 distinguishes elements that occur commonly as cations or anions (positively and negatively charged particles, respectively), which allows them to combine into compounds (minerals), to be dissolved in natural fluids, or to occur in melts (magmas) at high temperatures. The elements in Figure 1.2 are also grouped by their so-called geochemical affinities: lithophile (rock-loving) elements tend to form silicates or oxides (the constituents of most rocks), siderophile (iron-loving) elements combine with iron into metal alloys, chalcophile (sulfur-loving) elements react with sulfur to form sulfides, and atmophile elements tend to form gases and reside in the atmosphere. Many elements exhibit several affinities, depending on conditions, so the assignments illustrated in Figure 1.2 offer only a rough approximation of the complexity of element geochemical behavior. Finally, an important part of geochemistry takes advantage of the fact that most elements exist in more than one isotopic form. Measuring isotopic abundances has great value as a geochemical tool, and the most commonly used isotope systems are illustrated by heavy boxes in Figure 1.2. Stable isotopes of some light elements provide information on sources of elements, the conditions under

4

Cosmochemistry

Elements with most commonly used isotopes

Abundance in the Earth’s crust Si 10 most abundant elements Rb 11–20th most abundant elements Cr 21–40th most abundant elements Hf elements in trace abundance

Geochemical Periodic Table

stable radioactive radiogenic

Cations Cations 3 Li

4 Be

11 Na

12 Mg

19

5 B

Lithophile

K

Ca

21 Sc

22 Ti

23 V

37 Rb

38 Sr

39 Y

40 Zr

41 Nb

55

56 Ba

57

72

75

Cs

La*

Hf

Ta

87

88

89

Fr

Ra

Ac*

20

58

*Lanthanide series Ce +

Actinide series

Fig. 1.2

Anions

90 Th

24 Cr

Chalcophile 30 Zn

8

O

Anions S

14

47

48

49

52

Si

Ag

Cd

In

Te

Tl

81

82 Pb

31 Ga

32 Ge

As

50

51

Sn

Sb

25 Mn

83

84

Bi

Po

Siderophile

59 Pr

91 Pa

60 Nd

28 Ni

Fe

27 Co

42

44

45

46

P

29 Cu

Mo

Ru

Rh

Pd

73

74

76

77

78

79

80

Re

W

Os

Ir

Pt

Au

Hg

62 Sm

33

63

64

65

66

67

68

69

70

71

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

No Ions

H

Se

13

26

1

34

Al

15

Atmophile

16

Anions 6

C

7 N

2 He

9

10

F

Ne

17

18

Cl

Ar

35

36

Br

Kr

53

54

I

Xe

85

86

At

Rn

92 U

A geochemical periodic table, illustrating controls on element behavior during geologic processes. Abundances of elements in the Earth’s crust are indicated by symbol sizes. Cations and anions are usually combined into minerals. Elements having affinities for silicate or oxide minerals (lithophile), metal (siderophile), sulfide minerals (chalcophile), and non-rock (atmophile) phases are identified. Elements having stable isotopes that are commonly used in geochemistry are shown as boxes with bold gray outlines. Radioactive and radiogenic isotopes used for chronology are shown by boxes with bold black outlines and arrows showing decay relationships.

which minerals form, and the processes that separate isotopes from each other. Unstable (radioactive) nuclides and their decay products (radiogenic nuclides) similarly constrain element sources and geologic processes, as well as permit the ages of rocks and events to be determined. The isotopic compositions of many other elements in terrestrial materials are now being analyzed, and a future Figure 1.2 will certainly expand the list of commonly used isotopic systems. By way of contrast, Figure 1.3 illustrates a cosmochemical perspective of the periodic table. The element abundances shown in this figure are atomic concentrations in the Sun (relative to the abundance of silicon), as best we can determine them. The Sun comprises > 99.8% of the mass of solar system matter, so solar composition is approximately equivalent to the average solar system (often incorrectly called “cosmic”) composition. The behavior of elements in space is governed largely by their volatility, which we quantify by specifying the temperature interval where elements change state from a gas to a solid on cooling. (The liquid state is not generally encountered at the very low pressures of space; liquids tend to be more common in geochemistry than cosmochemistry.) All elements

5

Introduction to cosmochemistry

Elements with most commonly used isotopes

Cosmic abundances (atoms relative to 106 Si)

cosmogenic

Cosmochemical Periodic Table

radioactive (short-lived) radiogenic

Moderately Volatile Main Component 1230–640 K 1350–1250 K 16 Siderophile + Chalcophile S

Refractory 1850–1400 K

42 Mo 74 W

75 Re

44 Ru

29

45 Rh

76 Os

H Highest abundance, >109 C Next 14 most abundant elements, >104 Ti Next 11 most abundant elements, >102 Sc Next 14 most abundant elements, >1 W Elements in trace abundances,