同位素地质学.pdf

ISOTOPE GEOLOGY Radiogenic and stable isotopes are used widely in the Earth sciences to determine the ages of rocks, meteorites, and archeological objects, and as tracers to understand geological and environmental processes. Isotope s determine the age of the Earth, help reconstruct the climate of the past, and explain the ation of the chemical elements in the Universe. This textbook provides a comprehensive introduction to both radiogenic and stable isotope techniques. An understanding of the basic principles of isotope geology is important in a wide range of the sciences geology, astronomy, paleontology, geophysics, climatology, archeology, and others. Claude Alle gre is one of the world’s most respected and best-known geochemists, and this textbook has been developed from his many years of teaching and research experience. Isotope Geology is tailored for all undergraduate and graduate courses on the topic, and is also an excellent reference text for all Earth scientists. claude alle gre is extremely well known globally in the Earth science research and teaching community. He is currently an Emeritus Professor at the Institut Universitaire de France, Universite Denis Diderot, and the Institut de Physique du Globe de Paris, and has had a long and illustrious career in science. He is a er Director of the Department of Earth Sciences, Universite Paris VII, er Director of the Institut de Physique du Globe de Paris, past President of the French Bureau of Geological and Mining Research BRGM, and er National Education Minister for Research and Technology for the French government. In his career he has won most of the available honours and awards in the geosciences, including the Crafoord Prize from the Swedish Royal Academy of Sciences, the Goldschmidt Medal from the Geochemical Society of America, the Wollaston Medal from the Geological Society of London, the Arthur Day Gold Medal from the Geological Society of America, the Me daille d’Or du CNRS, the Holmes Medal from the European Union Geosciences, and the Bowie Medal from the American Geophysical Union. He is member of several academics Foreign Associate of the National Academy of Sciences USA,Foreign Member oftheAcademyofArtand Science, ForeignMember ofthe Philosophical Society, Foreign Member of the Royal Society, Foreign Member of the National Academy of India, and Membre de l’Acade mie des Sciences de Paris. He is also a Commandeur de la Le gion d’Honneur, a past President of the European Union of Geosciences, past President of the NATO Earth Sciences Committee, and er editor of the journals Physics of the Earth and Planetary Interiors and Chemical Geology. He has written hundreds of research articles, and 25 books in French. Isotope Geology CLAUDE J. ALLE GRE Institut de Physique du Globe de Paris and Universite Denis Diderot CAMBRIDGE UNIVERSITY PRESS Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, So Paulo Cambridge University Press The Edinburgh Building, Cambridge CB2 8RU, UK First published in print at ISBN-13 978-0-521-86228-8 ISBN-13978-0-511-45524-7 Claude J. Allegre 2008 2008 Ination on this title www.cambridge.org/9780521862288 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. 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. Published in the United States of America by Cambridge University Press, New York www.cambridge.org eBook EBL hardback Dedication I dedicate this book to all those who have helped me to take part in the extraordinary adventureofdevelopingisotopegeology. Tomyfamily,whohaveprobablysueredfrommyscientichyperactivity. TothosewhowereparagonseandhavebecomeverydearfriendsJerryWasserburg, PaulGast,GeorgeWetherill,AlNier,JohnReynolds,MitsunobuTatsumoto,ClairPatterson, GeorgeTilton, Harmon Craig, Samuel Epstein, Karl Turekian, Paul Damon, Pat Hurley, EdgarPicciotto,WallyBroecker,andDevendraLal.Ihavetriedtostandontheirshoulders. To my colleagues and friends withwhom I have shared the intense joyof international scienticcompetitionStanHart,KeithO’Nions,AlHofmann,MarcJavoy,DonDePaolo, CharlesLangmuir,JeanGuySchilling,ChrisHawkesworth,andmanyothers. Tomyundergraduateandgraduatestudents,andpostdoctoralfellows,tomylaboratory staand rst and foremosttothosewhohave participated in almost allofthis adventure Jean-LouisBirck,Ge rardManhe s,Franc oiseCapmas,LydiaZerbib,andthesorelymissed Dominique Rousseau.Without them, none of this would have been possible, because modernresearchisprimarilyteamworkinthefullsenseoftheword. CONTENTS Prefaceix Acknowledgmentsxi 1 Isotopes and radioactivity1 1.1 Reminders about the atomic nucleus1 1.2 The mass spectrometer2 1.3 Isotopy11 1.4 Radioactivity18 2 The principles of radioactive dating29 2.1 Dating by parent isotopes29 2.2 Dating by parent–daughter isotopes30 2.3 Radioactive chains36 2.4 Dating by extinct radioactivity49 2.5 Determining geologically useful radioactive decay constants54 3 Radiometric dating s58 3.1 General questions58 3.2 Rich systems and solutions to the problem of the open system64 3.3 Poor systems and the radiometric isotopic correlation diagram78 3.4 Mixing and alternative interpretations94 3.5 Towards the geochronology of the futurein situanalysis99 4 Cosmogenic isotopes105 4.1 Nuclear reactions105 4.2 Carbon-14 dating112 4.3 Exposure ages126 4.4 Cosmic irradiation from nucleosynthesis to stellar and galactic radiation138 5 Uncertainties and results of radiometric dating153 5.1 Introduction153 5.2 Some statistical reminders relative to the calculation of uncertainties155 5.3 Sources of uncertainty in radiometric dating166 5.4 Geological interpretations180 5.5 The geological timescale188 5.6 The age of the Earth193 5.7 The cosmic timescale200 5.8 General remarks on geological and cosmic timescales209 5.9 Conclusion215 6 Radiogenic isotope geochemistry220 6.1 Strontium isotope geochemistry220 6.2 Strontium–neodymium isotopic coupling234 6.3 The continental crust–mantle system248 6.4 Isotope geochemistry of rare gases277 6.5 Isotope geology of lead294 6.6 Chemical geodynamics313 6.7 The early history of the Earth341 6.8 Conclusion353 7 Stable isotope geochemistry358 7.1 Identifying natural isotopic fractionation of light elements358 7.2 Modes of isotope fractionation364 7.3 The modalities of isotope fractionation373 7.4 The paleothermometer382 7.5 The isotope cycle of water393 7.6 Oxygen isotopes in igneous processes400 7.7 Paleothermometry and the water cycle paleoclimatology406 7.8 The combined use of stable isotopes and radiogenic isotopes and the construction of a global geodynamic system421 7.9 Sulfur, carbon, and nitrogen isotopes and biological fractionation428 7.10 The current state of stable isotope geochemistry and its future prospects432 8 Isotope geology and dynamic systems analysis436 8.1 Basic reservoir analysis steady states, residence time, and mean ages439 8.2 Assemblages of reservoirs having reached the steady state443 8.3 Non-steady states445 8.4 The laws of evolution of isotope systems459 References473 Appendix490 Further reading496 Solutions to problems497 Index of names508 Subject index510 The color plates are situated between pages 220 and 221. viii Contents PREFACE Isotopegeologyistheospringofgeologyononehandandoftheconceptsandsof nuclearphysicsontheother.Itwasinitiallyknownas‘‘nucleargeology’’andthenas‘‘isotope geochemistry’’beforeitscurrentnameofisotopegeologycametobepreferredbecauseitis basedonthemeasurementandinterpretationoftheisotopiccompositionsofchemicalele- ments making up the various natural systems.Variations in these isotope compositions yield useful ination for the geological sciences in the broad sense.The rst break- through for isotope geology was the age determination ofrocks and minerals, which at a stroketransedgeologyintoaquantitativescience.Nextcamethemeasurementofpast temperatures andthebirth ofpaleoclimatology.Then horizonsbroadenedwith the emer- genceoftheconceptofisotopictracerstoencompassnotonlyquestionsoftheEarth’sstruc- turesandinternaldynamics,oferosion,andofthetransportofmaterial,butalsoproblems ofcosmochemistry,includingthoserelatingtotheoriginsofthechemicalelements.Andso isotopegeologyhasnotonlyextendedacrosstheentiredomainoftheearthsciencesbuthas alsoexpandedthatdomain,openingupmanynewareas,fromastrophysicstoenvironmen- talstudies. Thisbook is designedtoprovidean introductiontothe s, techniques, andmain ndingsofisotopegeology.Thegeneralcharacterofthesubjectdenesitspotentialreader- shipnal-yearundergraduatesandpostgraduatesintheearthsciencesorenvironmental sciences,geologists,geophysicists,orclimatologistswantinganoverviewoftheeld. This is an educational textbook.To my mind, an educational textbook must set out its subject matter and explain it, but it must also involve readers in the various stages in the reasoning. One cannot understand the development and the spirit of a science passively. Thereader mustbeactive.Thisbookthereforemakes constantuseofquestions,rcises, andproblems. Ihavesoughttowriteabookon isotopegeologyinthevein ofTurcotteand Schubert’s Geodynamics Cambridge University Press or Arthur Beiser’s Concepts of ModernPhysicsMcGraw-Hill,whichtomymindaremplary. Asitisaneducationaltextbook,inationissometimesrepeatedindierentplaces.As modern research inthe neurosciences shows, learning isbasedon repetition, and so I have adopted this approach. This is why, for example, although numerical constants are often giveninthemaintext,manyofthemarelistedagainintablesattheend.Inothercases,Ihave deliberatelynotgivenvaluessothatreaderswillhavetolookthemupforthemselves,because inationonehastoseekoutisrememberedbetterthaninationserveduponaplate. Readers must therefore work through the rcises, failing which they may not fully understandhowtheideasfollowonfromoneanother.Ihavegivensolutionsaswegoalong, sometimes in detail, sometimes more summarily. Atthe end ofeach chapter, I have set a numberofproblemswhosesolutionscanbefoundattheendofthebook. Another messageIwanttogetacrosstostudentsofisotopegeologyisthatthisisnotan isolated discipline. It is immersed both in the physical sciences and in the earth sciences. Hence the deliberate use here and there of concepts from physics, from chemistry Boltzmanndistribution,Arrheniusequation,etc.,orfromgeologyplatetectonics,petro- graphy, etc. to encourage studyofthese essential disciplines and, whereneedbe, to make readers look up ination in basic textbooks. Isotope geology is the outcome of an encounter between nuclear physics and geology; this multidisciplinary outlook must be maintained. Thisbookdoesnotsetouttoreviewalltheresultsofisotopegeologybuttobringreadersto apointwheretheycanconsulttheoriginalliteraturedirectlyandwithoutdi⁄culty.Among current literature on the same topics, this book could be placed in the same category as GunterFaure’sIsotopeGeologyWiley,tobereadinpreparation forAlanDickin’s excel- lentRadiogenicIsotopeGeologyCambridgeUniversityPress. TheguidelineIhaveoptedtofollowhasbeentoleaveasideaxiomatic expositionandto take instead adidactic, stepwise approach.The nal chapteralone takes a more synthetic perspective,whilegivingpointersforfuturedevelopments. I have to give a warning about the references. Since this is a book primarily directed towardsteachingIhavenotgivenafullsetofreferencesforeachtopic.Ihaveendeavoredto giveduecredittothesignicantcontributorswiththeproperorderofprioritywhichisnot alwaysthecasein modernscienticjournals.BecauseitiswhatIammostfamiliarwith,I havemadeextensiveuseofworkdoneinmylaboratory.Thisleadstoexcessiveemphasison myown laboratory’s contributions in some chapters. Ifeelsuremycolleagueswill forgive meforthis.Thereferencesattheendofeachchapteraresupplementedbyalistofsuggestions forfurtherreadingattheendofthebook. x Preface ACKNOWLEDGMENTS Iwouldliketothankallthosewhohavehelpedmeinwritingthisbook. MycolleaguesBernardDupre ,BrunoHamelin,E ricLewin,Ge rardManhe s,andLaure Meynadier made many suggestions and remarks right from the outset. Didier Bourles, Serge Fourcade, Claude Jaupart, and Manuel Moreira actively reread parts of the manuscript. IamgratefultootothosewhohelpedinproducingthebookSandraJeunet,whoword- processedadi⁄cultmanuscript,LesE ditionsBelin,andaboveallJoe lDyon,whodidthe graphics.ChristopherSutcliehasbeenamostco-operativetranslator. Myverysincerethankstoall. CHAPTER ONE Isotopes and radioactivity 1.1 Reminders about the atomic nucleus InthemodelrstdevelopedbyNielsBohrandErnestRutherfordandextendedbyArnold Sommerfeld, the atom is composed oftwo entities a central nucleus, containing mostof themass,andanarrayoforbitingelectrons.1Thenucleuscarriesapositive chargeofZe, which isbalancedby the electron cloud’snegative chargeof Ze.The numberofprotons, Z,ismatchedinanelectricallyneutralatombythenumberofelectrons.Eachoftheseparti- clescarriesanegativeelectricchargee. As a rough description, the nucleus ofany element is made up oftwo types ofparticle, neutrons and protons. A neutron is slightly heavier than a proton with a mass of mn1.67495 1027kg compared with mp1.67265 1027kg for the proton.While of similar masses, then, the two particles dier above all in their charges.The proton has a positive chargeewhiletheneutroniselectricallyneutral.ThenumberofprotonsZis theatomic number.ThesumANZofthenumberofneutronsNplusthenumberof protons Z gives the mass number.This provides a measure ofthe mass ofthe nuclide in question ifwe take as our unit the approximate mass ofthe neutron or proton.Thomson 1914andAston1919showedthat,foragivenatomicnumberZ,thatis,foragivenposition inMendeleyev’speriodictable,thereareatomswithdierentmassnumbersA,andthere- forenucleiwhich dier inthenumberofneutronstheycontain seePlate1.Such nuclides areknownastheisotopesofanelement. Thusthereisoneofhydrogenwhosenucleusiscomposedofjustasingleprotonand another ofhydrogen deuteriumwhosenucleus comprisesbothaprotonandaneu- tron; these are the two stable isotopes of hydrogen. Most elements have several naturally occurring isotopes. However, some, including sodium Na, aluminum Al, manganese Mn,andniobiumNb,havejustonenatural,stableisotope. Theexistenceofisotopeshasgivenrisetoaspecialofnotationfornuclides.Thesym- boloftheelementH,He,Li,etc.iscompletedbytheatomicnumberandthemassnumber 1 1H; 2 1H; 6 3Li; 7 3Li,etc.Thisnotationleavestheright-handsideofthesymbolfreeforchemi- cal notationsusedfor molecularorcrystalline compoundssuchas 2 1H2