This page intentionally left blankCosmochemistryHowdidthesolar systems chemical compositionariseandevolve?This textbook provides the answers inthe rst interdisciplinary introductiontocosmo-chemistry. It makes this exciting and evolving eld accessible to undergraduate and graduatestudents from a range of backgrounds, including geology, chemistry, astronomy, andphysics.Theauthors twoestablishedresearchleaderswhohavehelpedpioneer developmentsintheeld provide a complete background to cosmochemical processes and discoveries,enablingstudents outside geochemistrytofullyunderstandandexplore the solar systemscomposition.Topics coveredinclude:* synthesis of nuclides instars* partitioningof elements betweensolids, liquids andgas inthesolar nebula* overviews of thechemistryof extraterrestrial materials* isotopic tools used to investigate processes such as planet accretion and elementfractionation* chronologyof theearlysolar system* geochemical explorationof planets.Boxes provide basic denitions and mini-courses in mineralogy, organic chemistry, andother essential background information for students. Review questions and additionalreadingfor eachchapter encouragestudents toexplorecosmochemistryfurther.HARRY (Hap) Y. McSWEEN Jr. is Chancellors Professor at the University of Tennessee.He has conducted research on cosmochemistry for more than three decades and wasone of the original proponents of the hypothesis that some meteorites are fromMars.He has been a co-investigator for four NASA spacecraft missions and serves onnumerous 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 ofthe Meteoritical Society, a Fellow of the American Academy of Arts and Sciences,recipient of the Leonard Medal, and has an asteroid named for him.GARYHUSS is a Research Professor and Director of the W. M. Keck CosmochemistryLaboratoryat theHawaii Institutefor Geophysics andPlanetology, Universityof Hawaiiat Mnoa. In more than three decades of research on cosmochemistry, he was among the rstto study presolar grains, the rawmaterials for the solar system. He presently studies thechronologyof the earlysolar system. He comes froma family of meteorite scientists: hisgrandfather, H. H. Nininger, hasbeencalledthefatherofmodernmeteoritics, andhisfather,Glenn Huss, and grandfather were responsible for recovering over 500 meteorites previ-ously unknown to science. Dr. Huss is a former president and Fellowof the MeteoriticalSocietyandalsohas anasteroidnamedfor him.CosmochemistryHarryY. McSween, Jr.Universityof Tennessee, KnoxvilleGaryR. HussUniversityof Hawaii at Ma noaCAMBRIDGE UNIVERSITY PRESSCambridge, New York, Melbourne, Madrid, Cape Town, Singapore,So Paulo, Delhi, Dubai, TokyoCambridge University PressThe Edinburgh Building, Cambridge CB2 8RU, UKFirst published in print formatISBN-13978-0-521-87862-3ISBN-13 978-0-511-72968-3 Harry Y. McSween, Jr. and Gary R. Huss 20102010Information on this title: www.cambridge.org/9780521878623This publication is in copyright. Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any partmay 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 Yorkwww.cambridge.orgeBook (NetLibrary)HardbackFor Sue and JackieContentsPreface pagexvii1 Introduction to cosmochemistry 1Overview 1What is cosmochemistry? 1Geochemistryversus cosmochemistry 3Beginnings of cosmochemistry(andgeochemistry) 6Philosophical foundations 6Meteorites andmicroscopy 6Spectroscopyandthecompositions of stars 9Solar systemelement abundances 10Isotopes andnuclear physics 11Spaceexplorationandsamples fromother worlds 14Newsources of extraterrestrial materials 18Organicmatter andextraterrestrial life? 20Thetools anddatasets of cosmochemistry 20Laboratoryandspacecraft analyses 21Cosmochemical theory 24Relationshipof cosmochemistrytoother disciplines 25Questions 26Suggestions for further reading 26References 272 Nuclides and elements: the building blocks of matter 29Overview 29Elementaryparticles, isotopes, andelements 29Chart of thenuclides: organizingelements bytheir nuclear properties 32Radioactiveelements andtheir modes of decay 35Theperiodictable: organizingelements bytheir chemistryproperties 38Chemical bonding 44Chemical andphysical processes relevant tocosmochemistry 46Isotopeeffects fromchemical andphysical processes 49Summary 51Questions 52Suggestions for further reading 52References 523 Origin of the elements 54Overview 54Inthebeginning 54TheBigBangmodel 55Observational evidence 56Nucleosynthesis instars 58Classication, masses, andlifetimes of stars 61Thelifecycles of stars 64Stellar nucleosynthesis processes 72Originof thegalaxyandgalacticchemical evolution 81Summary 82Questions 83Suggestions for further reading 84References 844 Solar system and cosmic abundances: elements and isotopes 85Overview 85Chemistryonagrandscale 85Historical perspective 85Howaresolar systemabundances determined? 87Determiningelemental abundances intheSun 88Spectroscopicobservations of theSun 88Collectingandanalyzingthesolar wind 96Determiningchemical abundances inmeteorites 99Importanceof CI chondrites 99MeasuringCI abundances 100Indirect methods of estimatingabundances 101Solar systemabundances of theelements 102Solar systemabundances of theisotopes 104Howdidsolar systemabundances arise? 110Differences betweensolar systemandcosmicabundances 111Howaresolar systemabundances usedincosmochemistry? 113Summary 116Questions 117Suggestions for further reading 117References 1185 Presolar grains: a record of stellar nucleosynthesis and processesin interstellar space 120Overview 120Grains that predatethesolar system 120Acosmochemical detectivestory 122Recognizingpresolar grains inmeteorites 125viii ContentsKnowntypes of presolar grains 127Identicationandcharacterizationof presolar grains 128Locatingandidentifyingpresolar grains 128Characterizationof presolar grains 129Identicationof stellar sources 132Grains fromAGBstars 132Supernovagrains 139Novagrains 139Other stellar sources 140Presolar grains as probes of stellar nucleosynthesis 140Input datafor stellar models 141Internal stellar structure 141Theneutronsource(s) for the s-process 142Constrainingsupernovamodels 143Galacticchemical evolution 144Presolar grains as tracers of circumstellar andinterstellar environments 146Siliconcarbide 146Graphitegrains fromAGBstars 146Graphitegrains fromsupernovae 148Interstellar grains 149Presolar grains as probes of theearlysolar system 149Summary 152Questions 153Suggestions for further reading 153References 1546 Meteorites: a record of nebular and planetary processes 157Overview 157Primitiveversus differentiated 157Components of chondrites 158Chondrules 158Refractoryinclusions 163Metals andsulde 164Matrix 164Chondriteclassication 165Primarycharacteristics: chemical compositions 166Secondarycharacteristics: petrologictypes 168Chondritetaxonomy 170Other classicationparameters: shockandweathering 170Oxygenisotopes inchondrites 171Classicationof nonchondriticmeteorites 173Primitiveachondrites 174Acapulcoites andlodranites 175ix ContentsUreilites 176Winonaites andIABsilicateinclusions 178Magmaticachondrites 178Aubrites 178Howarditeseucritesdiogenites 179Angrites 179Irons andstonyirons 180Classicationandcompositionof ironmeteorites 180Pallasites andmesosiderites 182Lunar samples 182Martianmeteorites 184Oxygenisotopes indifferentiatedmeteorites 185Summary 187Questions 188Suggestions for further reading 188References 1897 Cosmochemical and geochemical fractionations 192Overview 192What arechemical fractionations andwhyaretheyimportant? 192Condensationas afractionationprocess 195Condensationsequences 196Applicabilityof condensationcalculations totheearlysolar system 201Volatileelement depletions 205Gassolidinteractions 206Gasliquidinteractions 208Igneous fractionations 210Magmaticprocesses that leadtofractionation 210Element partitioning 211Physical fractionations 213Sortingof chondritecomponents 213Fractionations byimpacts or pyroclasticactivity 215Element fractionationresultingfromoxidation/reduction 217Element fractionationresultingfromplanetarydifferentiation 218Fractionationof isotopes 220Mass-dependent fractionation 220Fractionations producedbyionmoleculereactions 221Planetarymass-dependent fractionations 222Mass-independent fractionation 222Radiogenicisotopefractionationandplanetarydifferentiation 224Summary 225Questions 226Suggestions for further reading 226References 227x Contents8 Radioisotopes as chronometers 230Overview 230Methods of agedetermination 230Discussingradiometricages andtime 231Basicprinciples of radiometricagedating 231Long-livedradionuclides 237The40K40Ar system 238The87Rb87Sr system 242The147Sm143Ndsystem 252TheUThPbsystem 258The187Re187Os system 270The176Lu176Hf system 274Other long-livednuclides of potential cosmochemical signicance 276Short-livedradionuclides 278The129I129Xesystem 282The26Al26Mgsystem 284The41Ca41Ksystem 287The53Mn53Cr system 288The60Fe60Ni system 289The107Pd107Agsystem 291The146Sm142Ndsystem 293The182Hf182Wsystem 294The10Be10Bsystem 295Other short-livednuclides of potential cosmochemical signicance 297Summary 298Questions 299Suggestions for further reading 299References 3009 Chronology of the solar system from radioactive isotopes 308Overview 308Ageof theelements andenvironment inwhichtheSunformed 308Ageof thesolar system 315Earlysolar systemchronology 318Primitivecomponents inchondrites 319Accretionandhistoryof chondriticparent bodies 324Accretionanddifferentiationof achondriticparent bodies 327Accretion, differentiation, andigneous historyof planets andtheMoon 330Ageof theEarth 330Ageof theMoon 331Ageof Mars 332Shockages andimpact histories 336Shockages of meteorites 336xi ContentsShockages of lunar rocks 339Thelateheavybombardment 340Cosmogenicnuclides inmeteorites 340Cosmic-rayexposureages 340Terrestrial ages 345Summary 346Questions 347Suggestions for further reading 347References 34810 The most volatile elements and compounds: organic matter,noble gases, and ices 354Overview 354Volatility 354Organicmatter: occurrenceandcomplexity 355Extractableorganicmatter inchondrites 356Insolublemacromolecules inchondrites 362Stableisotopes inorganiccompounds 364Areorganiccompounds interstellar or nebular? 366Noblegases andhowtheyareanalyzed 370Noblegas components inextraterrestrial samples 371Nuclear components 371Thesolar components 372Planetarycomponents 373Planetaryatmospheres 375Condensationandaccretionof ices 377Summary 378Questions 379Suggestions for further reading 379References 38011 Chemistry of anhydrous planetesimals 382Overview 382Dryasteroids andmeteorites 382Asteroids: ageologiccontext for meteorites 383Appearanceandphysical properties 383Spectroscopyandclassication 385Orbits, distribution, anddelivery 389Chemical compositions of anhydrous asteroids andmeteorites 390Analyses of asteroids byspacecraft remotesensing 390Chondriticmeteorites 392Differentiatedmeteorites 396Thermal evolutionof anhydrous asteroids 398xii ContentsThermal structureof theasteroidbelt 403Collisions amongasteroids 406Summary 408Questions 409Suggestions for further reading 409References 41012 Chemistry of comets and other ice-bearing planetesimals 412Overview 412Icybodies inthesolar system 412Orbital andphysical characteristics 413Orbits 413Appearanceandphysical properties 414Chemistryof comets 418Comet ices 418Comet dust: spectroscopyandspacecraft analysis 419Interplanetarydust particles 422Returnedcomet samples 426Ice-bearingasteroids andalteredmeteorites 432Spectroscopyof asteroids formedbeyondthesnowline 432Aqueous alterationof chondrites 433Thermal evolutionof ice-bearingbodies 436Chemistryof hydratedcarbonaceous chondrites 436Variations amongice-bearingplanetesimals 439Summary 440Questions 441Suggestions for further reading 441References 44213 Geochemical exploration of planets: Moon and Marsas case studies 445Overview 445WhytheMoonandMars? 445Global geologiccontext for lunar geochemistry 446Geochemical tools for lunar exploration 448Instruments onorbitingspacecraft 448Laboratoryanalysis of returnedlunar samples andlunarmeteorites 450Measuredcompositionof thelunar crust 451Samplegeochemistry 451Geochemical mappingbyspacecraft 452Compositions of thelunar mantleandcore 456Geochemical evolutionof theMoon 459Global geologiccontext for Mars geochemistry 462xiii ContentsGeochemical tools for Mars exploration 464Instruments onorbitingspacecraft 464Instruments onlanders androvers 465Laboratoryanalyses of Martianmeteorites 466Measuredcompositionof theMartiancrust 469Compositionof thecrust 470Water, chemical weathering, andevaporites 472Compositions of theMartianmantleandcore 475Geochemical evolutionof Mars 477Summary 477Questions 478Suggestions for further reading 478References 47914 Cosmochemical models for the formation of the solar system 484Overview 484Constraints onthenebula 484Fromgas anddust toSunandaccretiondisk 484Temperatures intheaccretiondisk 489Localizedheating: nebular shocks andtheX-windmodel 492Accretionandbulkcompositions of planets 495Agglomerationof planetesimals andplanets 495Constraints onplanet bulkcompositions 495Models for estimatingbulkchemistry 498Formationof theterrestrial planets 499Planetesimal buildingblocks 499Deliveryof volatiles totheterrestrial planets 503Planetarydifferentiation 504Formationof thegiant planets 507Orbital andcollisional evolutionof themodernsolar system 511Summary 512Questions 513Suggestions for further reading 514References 514Appendix: Some analytical techniques commonly usedin cosmochemistry 518Chemical compositions of bulksamples 518Wet chemical analysis 518X-ray uorescence(XRF) 519Neutronactivationanalysis 519Petrology, mineralogy, mineral chemistry, andmineral structure 520Optical microscopy 520Electron-beamtechniques 520xiv ContentsOther techniques for determiningchemical compositionandmineral structure 525Proton-inducedX-rayemission(PIXE) 525Inductivelycoupled-plasmaatomic-emissionspectroscopy(ICP-AES) 525X-raydiffraction(XRD) 525Synchrotrontechniques 526Mass spectrometry 527Ionsources 527Mass analyzers 528Detectors 530Mass spectrometer systems usedincosmochemistry 531Ramanspectroscopy 534Flight instruments 535Gamma-rayandneutronspectrometers 535Alpha-particleX-rayspectrometer 536Mssbauer spectrometer 536Samplepreparation 536Thin-sectionpreparation 536Samplepreparationfor EBSD 537Samplepreparationfor theTEM 537Preparingaerogel keystones 538Preparationof samples for TIMSandICPMS 538Details of radiometricdatingsystems usingneutronactivation 53940Ar39Ar dating 539129I129Xedating 540Suggestions for further reading 541Index 543xv ContentsPrefaceCosmochemistry provides critical insights into the workings of our local star and itscompanions throughout the galaxy, the origin and timing of our solar systems birth, andthecomplexreactions insideplanetesimals andplanets (includingour own) astheyevolve.Muchof thedatabaseof cosmochemistrycomes fromlaboratoryanalyses of elements andisotopes in our modest collections of extraterrestrial samples. A growing part of thecosmochemistry database is gleaned fromremote sensing andinsitu measurements byspacecraft instruments, which provide chemical analyses and geologic context for otherplanets, their moons, asteroids, and comets. Because the samples analyzed by cosmo-chemists aretypicallysosmall andvaluable, or must beanalyzedonbodies manymillionsof miles distant, this disciplineleads inthedevelopment of newanalytical technologies foruse in the laboratory or own on spacecraft missions. These technologies then spread togeochemistryandother elds wherepreciseanalyses of small samples areimportant.Despite its cutting-edge qualities and newsworthy discoveries, cosmochemistry is anorphan. It does not fall within the purviewof chemistry, geology, astronomy, physics, orbiology, but is rather an amalgamof these disciplines. Because it has nonatural home orconstituency, cosmochemistry is usually taught (if it is taught at all) directly fromitsscientic literature (admittedly difcult reading) or fromspecialized books on meteoritesand related topics. In crafting this textbook, we attempt to remedy that shortcoming. Wehavetriedtomakethissubject accessibletoadvancedundergraduateandgraduatestudentswithdiverseacademicbackgrounds, althoughwedopresumesomeprior exposuretobasicchemistry. Thisgoal mayleadtouneventreatment of somesubjects, andour readersshouldunderstandthat our intendedaudienceis broad.Cosmochemistryis advancingsorapidlythat wecanonlyhopetoprovideasnapshot ofthe discipline as it is currentlyunderstoodandpracticed. We have foundeventhat tobe achallenge, becausewecouldnot hopetopossess expertiseinall thesubjects encompassedby this discipline. We have drawn heavily on the contributions of many colleagues,especially those who educate by writing thoughtful reviews. That assistance is gratefullyacknowledged through our annotated suggestions for further reading at the end of eachchapter.Thetopics coveredinthechapters of this bookincludethefollowing, inthis order:* An introduction to how cosmochemistry developed, and to how it differs fromgeochemistry* Areviewof thecharacteristics andbehaviors of elements andnuclides* Adiscussionof howelementsaresynthesizedwithinstars, andhowthechemistryof thegalaxyhas evolvedover time* Anassessment of theabundances of elements andisotopes inthesolar system* Adescription of presolar grains, and howthey constrain stellar nucleosynthesis andprocesses ininterstellar space* Anintroductiontometeorites andlunar samples* An evaluation of processes that have fractionated elements and isotopes in interstellarspace, inthesolar nebula, andwithinplanetarybodies* Anexplanationof howradioactiveisotopes areusedtoquantifysolar systemhistory* Asynthesis of theradiometricageof thesolar systemandtheages of its constituents* Anassessment of themost volatilematerials organicmatter, noblegases, andices* Asurveyof thechemistryof anhydrous planetesimals andthesamples wehaveof them* Asurvey of the chemistry of ice-bearing cometsand asteroids and the samples we have ofthem* Examples of moderngeochemical explorationof planetarybodies theMoonandMars* Areviewof theformationof thesolar system, fromtheperspectiveof cosmochemistry* AnAppendixdescribingsomeimportant analytical methods usedincosmochemistryMore-established disciplines are taught using tried-and-true methods and examples, theresults of generations of pedagogical experimentation. Cosmochemistrydoes not yet offerthat. Most of those whodare toteachcosmochemistry, includingthe authors of this book,haveneveractuallybeenstudentsinacosmochemistrycourse. Intheauthorscase, wehavelearnedfromahandful of scientistswhohaveguidedour introductiontothe eld, includingCalvin Alexander, Bob Pepin, Ed Anders, JimHays, Dick Holland, Ian Hutcheon, KlausKeil, RoyLewis, Dimitri Papanastassiou, JerryWasserburg, andJohnWood. Wehopethatthis bookon cosmochemistry will guide other students and their teachers as they exploretogether this emerging, interdisciplinarysubject, andthat theywill enjoytheexperienceasmuchas wehave.xviii Preface1IntroductiontocosmochemistryOverviewCosmochemistryis dened, andits relationshiptogeochemistryis explained. Wedescribethe historical beginningsof cosmochemistry, and the lines of research that coalesced into theeldof cosmochemistryare discussed. Wethenbrieyintroduce the tools of cosmochem-istry and the datasets that have been produced by these tools. The relationships betweencosmochemistry and geochemistry, on the one hand, and astronomy, astrophysics, andgeology, ontheother, areconsidered.What is cosmochemistry?Asignicant portion of the universe is comprised of elements, ions, and the compoundsformedbytheir combinationsineffect, chemistryonthe grandest scale possible. Thesechemical components canoccur asgases or superheatedplasmas, less commonlyassolids,andveryrarelyas liquids.Cosmochemistry is thestudyof thechemical compositionof theuniverse andtheprocessesthat produced those compositions. This is a tall order, to be sure. Understandably, cosmo-chemistryfocusesprimarilyontheobjectsinour ownsolar system, becausethat iswherewehave direct access to the most chemical information. That part of cosmochemistry encom-passes the compositions of the Sun, its retinue of planets and their satellites, the almostinnumerable asteroids and comets, and the smaller samples (meteorites, interplanetarydustparticles or IDPs, returned lunar samples) derived from them. From their chemistry,determinedbylaboratorymeasurementsof samplesor byvariousremote-sensingtechniques,cosmochemists try to unravel the processes that formed or affected themand tox thechronology of these events. Meteorites offer a unique windowon thesolarnebula thedisk-shapedcocoonof gas anddust that envelopedthe earlySunsome ~4.57billionyearsago, andfromwhichplanetesimals andplanets accreted(Fig. 1.1).Within some meteorites are also found minuscule presolar grains, providing an oppor-tunitytoanalyzedirectlythechemistryof interstellar matter. Someof thesetinygrains arepure samples of the matter ejectedfromdyingstars andprovide constraints onour under-standing of howelements were forged inside stars before the Suns birth. Once formed, thesegrainswerereleasedtothe interstellar medium(ISM), thespacebetweenthestars. TheISMis lledprimarilybydiffusegases, mostlyhydrogenandhelium, but withoxygen, carbon,and nitrogen contributing about 1% by mass and all the other elements mostly inmicrometer-size dust motes. Much of the chemistry in the ISMoccurs within relativelydense molecular clouds, where gas densities can reach 103to 106particles per cm3, highby interstellar standards (but not by our everyday experience Earths atmospherehas ~ 31019atoms per cm3at sea level). These clouds are verycold, withtemperaturesrangingfrom10to100 K, sointerstellar grainsbecomecoatedwithices. Reactionsbetweenice mantles and gas molecules produced organic compounds that can be extracted frommeteorites and identied by their bizarre isotopic compositions. Many dust grains wereundoubtedly destroyed in the ISM, but some hardy survivors were incorporated into thenebulawhenthemolecular cloudcollapsed, andthencewereaccretedintometeorites.Processes that occur inside stars, in interstellar space, and within the solar nebulahave no counterparts in our terrestrial experience. They can be studied or inferred fromastronomical observations and astrophysical theory, but cosmochemical analyses ofmaterials actuallyformedor affectedbythese processes provide constraints andinsightsthat remote sensing and theory cannot. Our terrestrial experience places us onrmergroundindecipheringthegeologicprocesses occurringontheEarths Moon. Instudyinglunar rocksandsoils, wecanusefamiliar geochemical toolsdevelopedfor understandingthe Earth. We have also measured the chemical compositions of some other planetarybodies or their smaller cousins, geologicallyprocessedplanetesimals, usingtelescopes orinstrumentsonspacecraft. Insomecases, weevenhavemeteoritesejectedduringimpactsontothesebodies. Chemical measurements(whether fromlaboratoryanalysesof samplesor insitu analyses of rocks and soils by orbiting or landed spacecraft) add quantitativeFig. 1.1An artists conception of the solar nebula, surrounding the violent young Sun. Figure courtesyof NASA.2 Cosmochemistrydimensionstoourunderstandingofplanetaryscience. All extraterrestrial materialsare fairgamefor cosmochemistry.Geochemistryversus cosmochemistryTraditionally, cosmochemistry has been treated as a branch of geochemistry usuallydened as the study of the chemical composition of the Earth. Geochemistry focuses onthe chemical analysis of terrestrial materials, as implied by the prexgeo, and geo-chemistry textbooks commonly devote only a single chapter to cosmochemistry, if thesubject is introducedat all. However, the line betweengeochemistryandcosmochemistryhas always beensomewhat fuzzy. The most prominent technical journal inthis discipline,Geochimica et Cosmochimica Acta, hascarriedbothnamessinceitsinceptionin1950. Theburgeoning eldof planetarygeochemistryappropriates thegeo prex, eventhoughitssubject is not Earth. Abroader and moreappropriate denition of geochemistry might be thestudy of element and isotope behavior during geologic processes, such as occur on andwithintheEarthandother planets, moons, andplanetesimals. Usingthisdenition, wewillinclude planetary geochemistryas anessential part of our treatment of cosmochemistry.It is worth noting, though, that the geochemical and cosmochemical behaviors of ele-ments do showsome signicant differences. Ageochemical perspective of the periodic tableis illustrated in Figure 1.2 (adapted fromRailsback, 2003). As depicted, this diagramisdecidedly Earth-centric, but the controls on element behavior during geologic processesapply toother bodies as well. Determining relative elemental abundances is an importantpart of geochemistry, andtherelativeabundancesof elementsintheEarthscrust varyovermanyorders of magnitude. Crustal abundances are illustratedinFigure 1.2, because mostgeochemical dataarebasedonreadilyaccessiblesamplesof thecrust. Geochemistryisalsoconcernedwithdeterminingthecompositionof theEarths interior its mantleandcore andamorecomprehensive gurewouldincludethoseabundancesaswell. Veryfewnativeelements (pure elements not chemicallyboundtoanyothers) occur naturallyinthe Earth,soFigure 1.2distinguishes elements that occur commonlyas cations or anions (positivelyand negatively charged particles, respectively), which allows themto combine into com-pounds (minerals), tobe dissolvedinnatural uids, or tooccur inmelts (magmas) at hightemperatures. The elements inFigure 1.2are alsogroupedbytheir so-called geochemicalafnities: lithophile(rock-loving) elementstendtoformsilicatesor oxides(theconstituentsof most rocks), siderophile (iron-loving) elements combine with iron into metal alloys,chalcophile (sulfur-loving) elements react with sulfur to formsuldes, and atmophile elementstend to formgases and reside in the atmosphere. Many elements exhibit several afnities,depending on conditions, so the assignments illustrated in Figure 1.2 offer only a roughapproximationof thecomplexityof element geochemical behavior. Finally, animportant partof geochemistry takesadvantageof thefact that most elements exist in more than oneisotopicform. Measuring isotopic abundances has great value as a geochemical tool, and the mostcommonlyusedisotopesystemsareillustratedbyheavyboxesinFigure1.2. Stableisotopesof some light elements provide information on sources of elements, the conditions under3 Introductionto cosmochemistrywhich minerals form, and the processes that separate isotopes fromeach other. Unstable(radioactive) nuclides and their decay products (radiogenic nuclides) similarly constrainelement sources and geologic processes, as well as permit the ages of rocks and events tobedetermined. Theisotopiccompositions of manyother elements interrestrial materials arenowbeinganalyzed, andafutureFigure1.2will certainlyexpandthelist of commonlyusedisotopicsystems.By way of contrast, Figure 1.3 illustrates a cosmochemical perspective of the periodictable. The element abundances shown in this gure are atomic concentrations in theSun (relative to the abundance of silicon), as best we can determine them. The Suncomprises >99.8%of the mass of solar systemmatter, so solar composition is approx-imatelyequivalent tothe average solar system(oftenincorrectlycalledcosmic) compo-sition. The behavior of elements inspace is governedlargelybytheir volatility, whichwequantifybyspecifyingthetemperatureinterval whereelementschangestatefromagastoasolidoncooling. (Theliquidstateis not generallyencounteredat theverylowpressures ofspace; liquids tend to be more common in geochemistry than cosmochemistry.) All elements50 51Sn Sb30333416ZnAsSeS48 49 52Cd In Te8081 82 83 84HgTl Pb Bi PoChalcophile47Ag75TaSiRbCrHf10 most abundant elements1120th most abundant elements2140th most abundant elementselements in trace abundanceAbundance in the Earths crust42Mo73 74W Re11Na12Mg3Li4Be19K20Ca21Sc22Ti23V24Cr25Mn37Rb38Sr39Y40Zr41Nb55Cs56Ba57La*72Hf87Fr88Ra89Ac*Lithophile29Cu26 27FeCo28Ni44 45 46Ru Rh Pd76 77 78 79Os Ir Pt Au15P31 32Ga Ge513 14BAl SiNo Ions1H35 3629 1017 18Br KrHeFNeClAr53 54I Xe85 86At Rn6 7CN8O+Actinide series58 59 60 62 63 64 65 66 67 68 69 70 71Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu90 91 92Th Pa U*Lanthanide seriesCations AnionsSiderophileAtmophile Cations AnionsAnionsGeochemicalPeriodic TablestableradioactiveradiogenicElements with mostcommonly used isotopes Fig. 1.2A geochemical periodic table, illustrating controls on element behavior during geologic processes.Abundances of elements in the Earths crust are indicated by symbol sizes. Cations and anionsare usually combined into minerals. Elements having afnities for silicate or oxide minerals(lithophile), metal (siderophile), sulde minerals (chalcophile), and non-rock (atmophile) phasesare identied. Elements having stable isotopes that are commonly used in geochemistry are shownas boxes with bold gray outlines. Radioactive and radiogenic isotopes used for chronology areshown by boxes with bold black outlines and arrows showing decay relationships.4 Cosmochemistryoccur asgasesat highenoughtemperatures, andtheyeither condenseat lower temperaturestoformsolidminerals or ices, or react withalreadycondensedphases toformother solidphases. Some elements condense at suchlowtemperatures that theyeffectivelyremainasgases. Thermodynamic data canbe usedtopredict the temperatures at whichsolidphasesbecome more stable than their components in a gas of solar composition. Assignment ofelements tothevarious refractoryandvolatilegroups inFigure1.3is basedonthetemper-ature at which 50%of each element has condensed into solid phases. It is convenient incosmochemistry to identify elements according to the kinds of minerals into which theycondense lithophile, siderophile, andchalcophile. Somevolatileelements onlycondenseat very lowtemperatures to formices, or do not condense at all. Also illustrated in Figure1.3are the most commonly used isotope systems in cosmochemistry; the complete list isconsiderablylonger thanfor geochemistry, andwouldinclude stable isotopes measuredin14Si24Cr312LiMgSr38Pd46H Highest abundance, >109Cosmic abundances(atoms relative to 106 Si)C Next 14 most abundant elements, >104Sc Next 14 most abundant elements, >1WElements in trace abundances, 30000K WeakH(Balmer) lines,He+features, strongUVcontinuum2060 915 90000800000101MyrB(blue-white) 1000030000K MediumHlines, neutralHeabsorption318 3.08.4 9552000 40011MyrA(white) 740010000K StrongHlines, somefeatures of heavyelements1.83.0 1.72.7 855 3Gyr400MyrF(yellow-white) 60007400K MediumHlines 1.11.8 1.21.6 2.06.5 73GyrG(yellow) 52506000K WeakHlines, Ca+H&Klines, NaDlines0.81.1 0.851.1 0.661.5 715GyrK(orange) 40005250K VeryweakHlines, Ca+,Fe, strongmoleculessuchas CH, CN0.40.8 0.650.80 0.100.42 17GyrM(red) 26003850K VeryweakHlines,molecules (e.g. TiO2),veryredcontinuum0.080.4 0.170.63 0.0010.08 56GyrL 10solarmasses (M). The earlypart of this evolutionis similar tothat for low- andintermediate-mass stars. But in stars with masses >10 M, the nuclear burning does not stop withhydrogen, helium, and carbon. In these stars, once the carbon in the core is exhausted,gravitycausesthecoretocontract andheat upsufcientlytoinitiateneonburning, followedinturnbyoxygenburning, magnesiumburning, andsiliconburning(Fig. 3.8). ThesestagesStages of stellar evolution> 10 solar-mass starsH-burning corestable envelopeMain sequenceHeH-burning shellHeCHHeCHNeHeCHNeOHeCHNeOSiSupernovaSupergiantSupergiant Red giantFig. 3.8 Stages of stellar evolution for stars of >10 M. The initial stages of burning in a massive star areanalogous to those in low- and intermediate-mass stars (Fig. 3.6). But after helium is exhausted inthe core, the star burns a series of newfuels, predominantly carbon, neon, oxygen, and silicon in thecore. As each fuel is exhausted in the core, it begins to burn in a shell around the core, creating anonion-shell structure with each layer containing products of different nucleosynthetic burning.When silicon has been consumed in the core, there is no more nuclear fuel to support the staragainst gravity and it collapses catastrophically. The result is a type II supernova explosion thatejects the outer and intermediate layers of the star, returning newly synthesized elements to thegalaxy. The interior zones collapse back on the remnant, which can be either a neutron star or, if theprogenitor was >25 M, a black hole.70 Cosmochemistryareincreasinglyrapidbecause less andless energyis beingreleased, until acoreof ironisproduced. Fusion reactions cannot proceed further because nuclear reactions no longersupply energy; instead they require energy to proceed (cf. Fig. 2.1). The iron core of thestar buildsupveryquicklytothe1.4 MChandrasekhar limit, themaximummass that canbe supported by electron degeneracy. This happens in about a day in a 25 Mstar. Thetemperatureinthecorereaches 10billiondegrees. Theironnuclei disintegrateintoprotonsand electrons. This requires energy, sothe core cools. The very high pressure in the corecauses theelectrons andprotonstocombineintoneutrons, aprocess that releases ablast ofneutrinos. Theneutrinos carryawayatremendous amount of energyfromthecoreandtheneutrons collapse to nuclear density (>1017g cm3), shrinking the neutron core to a tinyfraction of itsprevious sizealmost instantaneously. Theneutrons nowforma degenerategas(neutrondegeneracyisbroadlyanalogoustoelectrondegeneracy, theneutronscannot moveinspiteoftheirhighenergy andthey behavelikematter at near0K). Thesurroundinglayersarenolonger supported andcollapse onto theneutronball at supersonicspeed. Theinfallingmaterial bounces off of theneutroncoreandashockwaveis created, whichalongwiththeneutrinos released fromthe core, eject the outer layers of the star at speeds of 10 000 to20 000 kms1. Thestar explodesasatypeII core-collapsesupernova, withaluminosityof108times the solar luminosity. Most supernovae leave behind a neutron star, with a radius ofafewkilometersandamasssimilar tothat of theSun, rotatingasfast as30timesper second(theseareobservedaspulsars). If themass left behindbytheexplosionis highenough, theremainingmaterial maycollapseintoablackhole. Themasscut that seemstoseparatethesetwooutcomes is ~25 M.ThestarsdestinedtobecometypeII supernovaearethesupergiantsthat occupythetopofthe HRdiagram(Fig. 3.7). Supergiants have very high luminosities and appear in all spectralclasses. These are all very massive stars that have two or more shells of nuclear burning. Thesestars are often losing mass at a tremendous rate. Stars of >~33Mmay lose their entireenvelopesthroughmassivestellar windsandbecomeWolf Rayettestars. Theseunusual starshavenewlysynthesizedmaterial at their surfaces. Theyalsoeventuallybecomecore-collapsesupernovae. Supernovae are classied spectroscopically. Those without hydrogen in theirspectra are type I supernovae and those with hydrogen are type II supernovae. Because stars of108K, the proton reaction rates on13C,15O,17F, and18F begin to compete effectively with the (,+) reactions. Isotopes such as13C and15Nare bypassedand a different equilibrium is established. If this equilibrium is quenched, such as in a novaexplosion, the unstable nuclei -decay to their respective stable daughters, resulting in low 12C/13Cand 14N/15N, and 12C/16O can be greater than one, very different from the outcome of normal CNOburning. After Champaign and Wiescher (1992).76 CosmochemistryQuiescent burning of heavy isotopesQuiescent burning of heavier isotopes takes place in the cores of stars and in shellssurrounding the core (Fig. 3.8). Carbonburning requires temperatures of ~500 milliondegrees anddensities of about 3106gcm-3. Suchtemperatures are reachedonlyinstarsmore massive than ~810 M. Carbon burning consists of several reactions, the mostimportant of which are12C(12C,2)16O,12C(12C,)20Ne,12C(12C,)24Mg, and12C(12C,p)23Na. Light particles (n, p,-particle) released during carbon burning can be capturedbyother speciestosynthesizeminor amountsof manyelements. Inthecoreof a25 Mstar,carbonburninglastsabout 600years. Near theendof thecarbon-burningphase, carboncanbegintointeract withoxygenandneonas well.Thenext reactiontoproceedefcientlyis20Ne(,)24Mg. The neon-burningphaselastsonly ~1 year in a 25 Mstar, and at the end of this stage, the composition of the stellar core isnowprimarily16Owithsome24Mg. Astemperatureanddensityrise, oxygen burningtakesplaceasoxygenreacts with itself to producesilicon andsulfur, alongwithisotopesof argon,calcium, chlorine, potassium, and other elements up to approximately scandium. Thereactionnetworkis quitecomplex. This stagelasts about 6months ina25 Mstar.Theultimatestageof quiescent nuclear burningis silicon burning. However, rather than28Si reacting withitself, silicon burns withthe aidof photodisintegration. The number ofphotons present increases as the fourth power of the temperature, so by the time oxygenburning ends, the silicon nuclei are sitting in a sea of high-energy -ray photons.Photodisintegration plays a major role in all subsequent nucleosynthesis. During siliconburning, -particles, protons, and neutrons are ejected fromstable nuclei to be used infurther synthesis. Nucleii uptoandslightlybeyondtheironpeakareproduced. Productionof nuclei beyond the iron peak requires more energy than is released by adding moreparticles to the nuclei, so for the most part, nucleosynthesis terminates, leaving an ironcore. Thesilicon-burningstagelasts for about adayina25 Mstar.Synthesis of elements heavier than ironThe mainmechanismbywhichnuclides beyondthe ironpeakare producedis byneutroncapture. The basic processes involved in neutron capture were laid out by Burbidge,Burbidge, Hoyle, and Fowler (1957) (this classic paper is commonly known as B2FH).The common ingredient in these processes is the capture of a neutron by a nucleus,increasing the atomic mass by one unit. If the resulting nucleus is stable, it remains anisotope of the original element. If not, the atom -decays (a neutronemits anelectronandbecomes a proton) and becomes an isotope of the next heavier element. Any isotope,whether stable or unstable, can capture another neutron. The rate of capture compared totherateof decayleadstotwobasicend-member processes, the s-processandthe r-process.The s-process is capture of neutrons on a time scale that is slowcompared to the rate of-decay. The r-processisneutroncaptureonsucharapidtimescalethat manyneutronscanbecapturedbefore -decayoccurs.Thes-processoccursduringtheAGBstageinlowandintermediatemassstars, whenthehydrogen shell is burning outward fromthe core and the heliumshell repeatedly ignites,77 Originof the elementsgeneratingthirddredge-upmixingevents. It canalsooccur inmassive stars duringanalo-gous stages. Therearetwomainreactions that provide theneutrons todrive the s-process,13C(,n)16Oand22Ne(,n)25Mg. Bothreactionsrequire -particles(4Henuclei). The13C(,n)16Oreactionoperates primarilyintheintershell regionbetweenthehydrogenandheliumshells while the helium shell is quiescent. The22Ne(,n)25Mg reaction is marginallyactivated during the helium-shellash. Because the neutrons fromthe two reactions arereleased at different temperatures, the details of the s-process nucleosynthesis that results aredifferent andprovideaprobeof nucleosynthesis instars.Thes-process is responsible for approximately half of the isotopes heavier than iron.Figure3.12isaportionoftheChart oftheNuclideswiththe s-processpathhighlighted. Thes-process path denes the center of the region of the stable isotopes on the Chart of theNuclides (valley of -stability). Whenthe s-process is activated, thestableisotopes acquireneutrons and move to the right (constant Z) until the resulting nuclide is unstable and-decays tobecome anisotope of the next heavier element. As the process continues, theisotopes that do not capture neutrons very efciently (said to have lowneutron-capturecross-sections) buildup, whilethosewithhighneutron-capturecross-sections aredepleted.If the neutron uence (total number of neutrons available) is high enough, the abundances oftheisotopes approachasteadystate, wheretheabundances of isotopes of similar mass areproportional totheinverseof theneutron-capturecross-sections.Cd106 Cd108Sn112pXe124Atomic number (Z)5452504858 60 62 64 66 68 70Neutron number (N)Cd110 Cd111 Cd112 Cd113 Cd114 Cd116In113 In115Sn114 Sn115 Sn116 Sn118 Sn119Sb121Te123 Te122 Te120p s s,r s,r s,r s,r rp s,rp p p s s,r s,r s,r s,rSb123Te124I127Xe129 Xe128 Xe130 Xe13174 76Sn122 Sn124Te128 Te126 Te125Xe126r72rs,r rXe132Te130r r s,r s,r s s s ps,rp p s s s,r s,r s,rI127Ba130 Ba132 Ba134s,rp p sXe134 Xe136r rBa135 Ba136 Ba137 Ba138s,r s,r s,r s,rr-process5678 80 82(,-,)(n,)Sn117 Sn120Fig. 3.12 PortionoftheChartof theNuclidesshowings-processandr-processpathways. Thes-processpathway,shownbythedarklineinthecenterofthevalleyof-stability,showshowanuclidethatsuccessivelycapturesindividual neutronswouldevolve. Eachaddedneutronmovesthenuclidetotherightonthediagram, until itreachesanunstablenuclide, inwhichcaseitwill -decaytothestablenuclidewithahigherZ. Incontrast, insituationswherenuclidescaptureneutronsveryrapidly(r-process), theywill bedrivenfartotherightofthevalleyof-stabilityuntil thetimescaleforneutroncapturematchesthatfor-decay. Theywill thenmovetohigherZandcapturemoreneutronsuntil theyeitherreachasizethatcausesthemtossion(break)intosmallernuclei(whichcanthencapturemoreneutrons)oruntiltheneutronsdisappear, inwhichcasetheywill -decaybacktotherststableisotopealongpathsofconstantA(arrows).78 CosmochemistryExplosive nucleosynthesisExplosive nucleosynthesis occurs under conditions where temperature and density arechanging rapidly with time, either due to the passage of a shock wave or because of arunawayexplosion. Wehighlight someof themoreimportant processes here.Explosive hydrogen burningoccurs viathehot CNOcycleinnovaexplosions whenthedensity of hydrogen accreted to a white dwarf or neutron star froma binary companionreaches acritical densityof ~103gcm3. Theresultingthermonuclear runawaypowers thenovaexplosion. Explosivehydrogenburningalsooccursincore-collapsesupernovaeastheshockwavepasses throughthehydrogen-richenvelope.Explosiveheliumburning occurs in supernova explosions when the shock wave passesthroughhelium-richlayers. Ifthelayerhasasufcientlyhigh22Neabundance, thepassageoftheshockwavetriggersthe22Ne(,n)25Mgreaction, whichreleasesaburst of neutrons. Thisneutronburst drivesthestableseednuclei totheneutron-richsideof thevalleyof -stability.Explosive oxygen, carbon, and neon burningoccur intheoxygen-richlayers of massivestars duringthepassageof thesupernovashockwave. Thepassageof theshockgeneratestemperaturesnear3billiondegrees. Akeyproduct is28Si, but someiron-groupelementsarealsoproduced. Inlayers that are heatedtoless than3billiondegrees, photodisintegrationreactions[(,p), (,n), (,)] onstablenuclei producenuclei ontheneutron-decient sideofthe valley of -stability. Outside the layer that experiences explosive oxygen burning,conditions areappropriatefor explosivecarbonandneonburning. Explosivecarbonburningvia the reactions,12C(12C,2)16O,12C(12C,)20Ne,12C(12C,)24Mg, and12C(12C,p)23Na, isalsotheunderlyingenergysourcefor typeIasupernova.Ther-process is theprocess of rapidneutron-capturenucleosynthesis andis responsiblefor about half of theisotopes heavier thaniron. It occurs whenrapidlyexpandingmaterialcontainsheavyseednuclei inthepresenceof ahighdensityof neutrons. Thetimescaleforneutroncaptureinthe r-processissofast that nuclidesaredrivenfar totheneutron-richsideof the valley of -stability (Fig. 3.12). When the neutron source is removed, the isotopes-decayalong paths of constant Aback towardthe valley of -stability, following the arrowson Figure 3.12, until they reach a stable conguration. An analogous p-process may alsooperate in which a nucleus captures protons and moves up and tothe left on Figure 3.12before decaying back toward the valley of -stability. The low-abundance isotopes to the leftof thevalleyof -stabilitythat arebypassedbythe s-processmaybeproducedeither bythesupposed p-process or byphotodisintegrationreactions suchas thosedescribedabove.The exact mechanismandsite of the r-process are not well understood, although core-collapse supernovae are likely sites. There are two peaks in the solar systemr-processabundances, one at A= ~130 and one at A= ~195. Asingler-process cannot producethis distribution, so multipler-process sites are indicated. Spectroscopic studies of verymetal-poor stars, which formed during therst billion years after the Big Bang, alsoshowevidence of multipler-process sites. These observations suggest that the elementsfrom~90 6wt. %), low-titanium(1.56%), andverylow-titanium(50%of thecumulative39Ar releasefor all four sub-samples. Theageof eruptionandcrystallizationonthelunar surfaceinferredfor thisbasalt is3.860.02(2)Ga (=5.5431010yr1). Other basalts are not sowell behaved. For Apollo11basalt10072, the plateau for 5098%cumulative release givesan age of 3.5 Ga (Fig. 8.3), which isconsistent with the87Rb87Sr age(3.570.05Ga)andthe147Sm143Nd age (3.570.03Ga).The close agreement between the dates obtained by three independent techniques shows that40Ar39Ar ages are reliable when the systemhas remained closed and a good plateau isobtainedontheage- spectrumdiagram. Ontheother hand, Apollo11basalt 10017doesnotgiveaplateau(Fig. 8.3) andonlyaminimumageof 3.2Gacanbeinferred.The87Rb87Sr systemRubidium-87 -decays to87Sr with a half-life of ~4.88 1010years (=1.42 1011yr1).Rubidiumhas two naturally occurringisotopes,85Rb and87Rb (Table 4.2). Rubidiumis analkali metal andbelongs toGroup1Aof theperiodictable(Fig. 2.4). Other alkali metals arelithium, sodium, potassium, cesium, andfrancium. The ionic radius of rubidium(1.48) issimilar enough to that of potassium(1.33 ) to permit it to substitute for potassiumin allpotassium-bearingminerals. As a consequence, rubidiumdoes not formminerals of its own,but insteadoccurs ineasilydetectableamounts inmanyminerals.242 CosmochemistryStrontiumhasfour naturallyoccurringisotopes(Table4.2). It isamember of thealkalineearths (Group 2A) along with beryllium, magnesium, calcium, barium, and radium(Fig. 2.4). Strontiumsubstitutesfor calciumandis abundant in mineralssuchas plagioclase,apatite, andcalciumcarbonate.HistoryThenatural radioactivity of rubidiumwasdemonstratedby Campbell andWood (1906), and87Rb was identied as the radioactive isotope by Hemmendinger and Smythe (1937).Mattauch (1937) determined the isotopic composition of strontium separated from arubidium-rich mica and found that it was nearly pure87Sr. This led Hahn and Walling(1938) todiscuss usingthe87Rb87Sr systemtodetermineages of rocks. Theyenvisioneddeterminingabundances of rubidiumandstrontiumbychemical means, whichmeant thattheir methodwas limited tominerals suchas mica andpotassiumfeldspar that formwithhighrubidiumconcentrationsandessentiallynocommonstrontium. Althoughthechemicalstrontiummethod was used for several years, it was clear that isotopic analyses weredesirabletopermit detectionof andcorrectionfor commonstrontium. After WorldWar II,withtheimprovedmass spectrometers andthenewlydevelopedisotopedilutiontechniquefor measuring very lowelemental abundances (Inghram, 1954), the isotopic87Rb87Srmethodreplacedthechemical strontiummethod.The87Rb87Sr technique wasrst applied to meteorites in 1956. However, the decayconstant for rubidiumwas then poorly known. Measurements of one chondrite and oneachondrite by Schumacher (1956) gave very old ages, 5.46 and 5.80 Ga, using the then-recommendedhalf-life of 58Gyr. Schumacher proposeda geological valuefor thehalf-life that gave87Rb87Sr ages for the meteorites of 4.70.4 Ga, in reasonable agreementwiththe PbPbages determinedbyPatterson(1956).87Rb87Sr chronologywas plaguedbythe uncertaintyinthe half-life of87Rbthroughout the 1950s and1960s. Values rangedfrom47Ga as determinedbydirect countingtoa geological value of 50Ga. The currentuncertainty in the half-life is still on the order of 2%. Steiger and Jger (1977) recom-mended a value of 48.8 billion years, but cosmochemists often use values of 49.4 to50 Ga. One can easily recalculate ages to a different decay constant using the followingequation:t1:42 date 1:39 10111:42 1011(8:17)Inthis equation, t1.42is the date basedon = 1.421011yr1andthe original date wascalculatedusing = 1.391011yr1.The87Rb87Sr methodreachedits modernlevel of maturityduringthemidtolate1960sasscientistspreparedfor thereturnof lunar samples. Thetechnologythat enabledthisworkwas a new generation of automated mass spectrometers with digital data acquisition.Leaders indevelopingthis newtechnologywere theAustralianNational Universitygroupheaded by W. Compston and the Caltech group headed by G. J. Wasserburg. The otherdevelopment was the application of the mineral orinternal isochron technique to met-eorites (see below). The87Rb87Sr systemplayed an important role in unraveling the history243 Radioisotopes as chronometersof the Moonandinestablishingthe antiquityof meteorites. It is oftenusedinconjunctionwithother techniques toevaluatetheevolutionof differentiatedbodies.Technical detailsTherearetwobasicwaystoapplythe87Rb87Sr techniquetonatural samples. Theoriginalmethodis tosimply measure the isotopic compositionof strontiumandthe abundance ofrubidiumina rockandthencalculate a date. If the rockcontains nocommonstrontium, adatecanbecalculatedfrom:t 1 ln87Sr87Rb1_ _(8:18)Here,87Sr*denotes radiogenic87Sr from in situdecay. However, most rocks docontaincommonstrontium, whichmust besubtractedfromthemeasured87Sr todetermine87Sr*:t 1 ln87Srtotal87Sr087Rb1_ _(8:19)It is difcult froma single measurement to determine the amount of common Sr (87Sr0), soinpractice, measurements are generallymade onseveral cogenetic rocks or minerals, andtheresults areplottedona87Rb87Sr evolutiondiagram.Let us assume that a geological event causedthe strontiumisotopes tobe homogenizedwithin a rock. This meansthat at time zero, all of the minerals in the rock had the sameinitial87Sr/86Sr ratio[(87Sr/86Sr)0]. TheRb/Sr ratiodiffers amongtheminerals of arock, sowiththe passage of time, the87Sr/86Sr ratio in each mineral changes at a rate that is proportional totheRb/Sr ratio(Fig. 8.4). AlthoughFigure8.4illustrateswhat happenstothe87Sr/86Sr ratiowithtime inthe various minerals ina rock, it is not particularlyuseful indatingthe rock.ReturningtoEquation(8.18) above, wecanwrite:87Sr 87Sr0 87Rbet1_ _(8:20)1.31.21.11.00.90.80.787Sr/86Sr4.5 3.0 1.5 PresentAge (Ga)87 Rb/86 Sr~1087Rb/86Sr~587Rb/86Sr ~287Rb/86Sr ~1Fig. 8.4 CompstonJeffery diagram showing the growth of radiogenic87Sr as a function of time in mineralswith different87Rb/86Sr ratios.244 CosmochemistryDividing through by86Sr, which is stable and is not involved in radioactive decay, thefollowingequationis obtained:87Sr86Sr87Sr86Sr_ _087Rb86Sret1_ _(8:21)Thisistheequationfor astraight lineonadiagramof87Sr/86Sr vs.87Rb/86Sr, alsoknownasa87Rb87Sr evolutiondiagram. Thelineisan isochron, withtheslopeandintercept givenbyslope et1 intercept 87Sr86Sr_ _0Figure 8.5shows schematicallythe data fromFigure 8.4plottedona87Rb87Sr evolutiondiagram, alsoknownas anisochrondiagram. At time zero, the event that we hope todatehomogenizedthestrontiumisotopes throughout thesystem, soeachof theminerals startedwith the same87Sr/86Sr ratio, [(87Sr/86Sr)0]. With the passage of time, some of the87Rbdecays to87Sr, andthe minerals eachmove upandtothe left onthe diagram. Because theposition of each mineral at any given time is described by Equation (8.21), the mineralsalways fall onalineararray, the slopeof which isproportional to theamount oftime that haspassed. Thedateis thus givenbysolvingtheequationfor theslopefor time t:t 1 ln slope 1 (8:22)To apply this method, the scientist measures the87Sr/86Sr and87Rb/86Sr ratios in severalsamples and plotsthe resultson a87Rb87Sr evolution diagram. Each datapoint has ameasure-ment uncertainty associated with it, so the points will not fall exactly on a straight line even in the1.31.21.11.00.90.80.787Sr/86Sr1 2 3 4 5 6 7 8 9 1087Rb/86SrFig. 8.5 Rubidiumstrontium evolution diagram showing the isochron generated by the minerals inFigure 8.4. Note that the87Rb/87Sr ratio decreases slightly as87Rb decays to87Sr.245 Radioisotopes as chronometersbest-behavedsystem. Anerror-weightedlinear regressionof thedataprovidestheequationfortheisochron. Regressionmethodsthat takeintoaccount theuncertaintiesinboth87Sr/86Sr and87Rb/86Sr and provide as output the slope and intercept of the isochron along with theiruncertainties and a measure of the goodness oft to the data are available in the literature(York, 1966, 1969; Williamson, 1968; Ludwig, 2003) andareusedbymost laboratories.An isochron that is built frommeasurements of several mineral phases provides aninternal checkonwhether or not the systemhas remainedclosed, one of the requirementsfor avalidage. If thesystemhas not remainedclosedandhas beenaffectedbysubsequentevents, theisochronwill be disturbed.Onthe87Rb87Sr evolutiondiagram, thedatawillplot farther fromthe isochron than expected based on measurement errors. Such a line isknown as anerrorchron. Figure 8.6 shows schematically what might happen to theisochron in Figure 8.5 if the rubidiumand strontiumare partially or totally redistributed.Disturbed isochrons may also result if the minerals being measured are not cogenetic,i.e. they did not formtogether fromthe same isotopic reservoir. If the isochron is highlydisturbed, theproblemis easytorecognize, but if it is onlyslightlydisturbed, the t tothedatamaylookprettygood, but theerrorchronwill not giveanaccurateage.Ideally one would like to construct the isochron based on distinct mineral phases. Inpractice, it isnot alwayspossibletoget cleanmineral separatesfor analysis. However, if the1.31.21.11.00.90.80.71 2 3 4 5 6 7 8 9 10Rb loss Sr loss87Rb/86Sr87Sr/86SrFig. 8.6 Rubidiumstrontium evolution diagram illustrating what happens when an isochron is disturbed,perhaps by a later thermal event. If the thermal event happens in a closed system, the strontiumisotopes simply re-equilibrate. Minerals with high87Sr/86Sr ratios exchange with minerals with low87Sr/86Sr ratios and, if the process goes to completion, a newequilibriumcomposition shown by thedashed horizontal line results. When the system cools, radiogenic87Sr begins to accumulate againand a new, sloped isochron is generated (dashed line with solid points). If rubidiumis lost or gainedwhile the strontium isotopes remain undisturbed, the points on the original isochron (opensymbols) move either left or right on the diagram and the correlation line becomes steeper orshallower. Unless the gain or loss is directly proportional to the amount of rubidium present, thecorrelation will be destroyed. Both kinds of disturbance can happen simultaneously. The result, ifthe system is not completely reset, is a general correlation that cannot be interpreted as anisochron.246 Cosmochemistryrock meets the conditions necessary for a valid age, an isochron can be generated fromimpure mineral separates or even different fragments of the rock that have differentproportions ofthe constituent minerals. Thedifferent mixturesofmineralsproducedifferent87Rb/86Sr ratiosandprovidethespreadtogeneratetheisochron. Thereis adanger inusingthis type of sample. Suppose a rock consists of two phases that did not formin a singleevent, such as plagioclase rich in strontiumcut by a vein of a rubidium-rich orthoclase.Several samples of this rock containing different mixtures of the two phases could bemeasured, resultingina series of87Rb/86Sr and87Sr/86Sr ratios that formalinear arrayona87Rb87Sr evolutiondiagram. However, inthis case, thearrayis not anisochron, but is amixinglinebetweenthetwophases. Figure8.7shows twoexamples of sucharrays, alongwiththe hypothetical mineral phases (endmembers of the mixinglines) fromwhichtheywere generated. The data fall solidlyonthe linear arrays, sothere is nowaytodeterminefromthe t of thedatatothelinethat thesearenot isochrons. If theslopeor intercept of thearraywereunreasonable, suchasinthecaseof arrayCDonFigure8.7, onecouldtell, butarrayABcouldeasilybemistakenfor anisochron. Althoughsuchcasesarerare, onemustbeawareof this possibility.ApplicationsAwide variety of rock types can be dated by the87Rb87Sr system, provided that thesamples satisfy the assumptions that the systemwas initiallyisotopically homogeneous(hada uniform87Sr/86Sr ratio) and did not gain or lose rubidiumor strontiumafter if formed. Both1.31.21.11.00.90.80.71 2 3 4 5 6 7 8 9 10ABCD87Rb/86Sr87Sr/86SrFig. 8.7 Two mixing lines on a rubidiumstrontium evolution diagram. On any three-isotope diagram witha common denominator (or numerator), mixtures of two components will fall on straight linesconnecting the two components. Different mixtures of components A and B produce a line with apositive slope that could be mistaken for an isochron. Different mixtures of C and D produce a linewith a negative slope that is easy to recognize as a mixing line. If supposed internal isochrons arebased only on different whole-rock samples, one must be aware of the possibility that a correlationon the evolution diagram could be a mixing line.247 Radioisotopes as chronometerstheslopeandtheintercept of the87Rb87Sr isochrongivechronological information. If theconditionsfor radiometricdatingaremet, theslopeof thearraygivestheagebeforepresentwhenanobject formed. Differencesintheintercept valuesamongobjectsthat formedfromthesamematerial reservoir alsoprovideinformationabout relativetimedifferences.Chronologywithslopeof anisochronTwo types of isochrons can be constructed. An internal isochron is one constructed frommeasurementsof different cogeneticmineralsinthesamerock. Theinternal isochrongivesinformationabout theformationtimeofthat rock. Anisochroncanalsobeconstructedfrombulksamplesof different rocksthought tohaveformedfromthesamesourcematerial at thesame time. Such isochrons are known aswhole-rock isochrons. Early attempts to datechondritesbythe87Rb87Srmethodwerecarriedout byconstructingwhole-rockisochrons,rst for meteorites of all classes, and later for meteorites from single classes. Theseisochrons demonstratedthat chondrites are~4.5Gaold(e.g. Minster andAllgre, 1981).Internal87Rb87Sr isochrons were particularly useful in unraveling the history of theMoonandMars. Asanexample, considermarebasaltsfromtheSeaofTranquilitycollectedbyApollo11. Theselunar basalts consist primarilyof calcium-rich(anorthitic) plagioclaseand low-calciumpyroxene, along with ilmenite, crystobalite, and other minor phases.Strontiumis concentrated in the calciumsites in plagioclase and is excluded frommostother phases, resulting in a variation of a factor of several hundred in the strontium87Sr/86Sr0.69850.69900.69950.70000.70050.70100.70150.70200.70250.70300.70350.70400 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11PlagioclaseWhole-rockPyroxenePyroxeneIlmeniteCristobaliteCristobaliteT=4.6 Ga(b) Lunar rock 10044T=3.630.11 Ga87Rb/86Sr87Rb/86Sr87Sr/86Sr0.71500.71000.70500.70000 0.1 0.2 0.3PlagioclasePyroxeneWhole-rockFragmentIlmeniteT=3.510.05 GaT=4.6 Ga(a) Lunar rock 10047Fig. 8.8 Rubidiumstrontium evolution diagrams for two different Apollo 11 rocks. Rock 10047 has highpotassium and rubidium and the measured87Sr/86Sr ratios of the rubidium-rich phases, pyroxeneand ilmenite, are quite high, with the result that the isochron produced fromplagioclase, pyroxene,ilmenite, and two whole-rock fragments has a precisely determined slope. Rock 10044 is a low-potassium rock with lower rubidium content. The87Rb/86Sr ratios are lower as are the measured87Sr/86Sr ratios. However, because the data are very precise, the isochron is still well determined.After Papanastassiou et al. (1970).248 Cosmochemistryabundanceamongminerals. Ontheotherhand, rubidiumispresent inmost mineralsandtheabundancevariesbyonlyafactorofabout ten. Asaresult, the87Rb/86Srratiointhe samplesvaries bya factor of several hundredamongthe mineral phases. This large spreadandthehigh-precisionmeasurementsof the87Sr/86Sr ratiousingthermal ionizationmassspectrom-etry(TIMS) permit preciseagedeterminations. Figure8.8showsinternal isochronsfor twodifferent Apollo 11 rocks, one with high total potassiumcontent and one with lowtotalpotassium. Mineral separates were obtained by a combination of hand picking, densityseparation, andmagneticseparation. Notehowsmall themeasurement errors arecomparedtothespreadintheisotoperatiosamongthesamplesandtheprecisionoftheresultingdates.Chronologyusingtheinitial87Sr/86Sr ratioTheintercept of a87Rb87Sr isochrongives theinitial87Sr/86Sr ratioof thesamplewhenitcrystallized. The intercepts of isochrons will be different in objects that formed fromthesame reservoir at different times due to the continuing decay of87Rb to87Sr within thereservoir. As time passes, each newobject to formwill have a slightly higherinitial87Sr/86Srratio [(87Sr/86Sr)0]. In order to use the initial value for chronology, it is necessary todetermine it veryprecisely. The best samples for this workare those withverylowRb/Srratios, sothat the87Sr/86Sr ratiohas not evolvedverymuchsincetheobject formed. As anexample, let us consider the (87Sr/86Sr)0ratio for eucrites, basaltic achondrites that wethink come fromthe asteroid 4 Vesta. Eucrites have lowRb/Sr ratios and the resulting87Sr/86Sr ratios have a total spreadof only~0.2%. PapanastassiouandWasserburg(1969)measuredthe87Rb/86Sr ratiosandmadehighprecisionmeasurementsof the87Sr/86Sr ratios0.70100.70000 0.010 0.020 0.0250.70050.69950.69900.005 0.015StannernJonzacNuevo LaredoPasamonteSioux CountyJuvinasMoore CountyBasaltic achondritesT = 4.300.26 Ga(87Sr/86Sr)BABI= 0.6989904787Rb/86Sr87Sr/86SrFig. 8.9 Rubidiumstrontium evolution diagramfor basaltic achondrites. A least-squares regression throughthe data gives an isochron age. The intercept of the isochron gives the (87Sr/86Sr)0 ratio at the timethe parent planet of the basaltic achondrites differentiated. After Papanastassiou and Wasserburg(1969).249 Radioisotopes as chronometersin several of these meteorites. Their data are plotted on a87Rb87Sr evolution diagram(Fig. 8.9). The best-t line through the data has a slope corresponding to an age of(4.300.26) 109years (recalculatedto =1.421011yr1). Theinitial ratioindicatedby the isochron is 0.6989900.000047. The isochron age has an uncertainty of ~6%, buttheinitial ratiois determinedtobetter than0.01%. PapanastassiouandWasserburg(1969)also calculated the initial ratios for each individual eucrite fromthe measured87Rb/86Srand87Sr/86Sr ratios by assuming that the formation age for all meteorites was 4.5 Ga. Ifthe87Rb/86Sr ratiosarelowenough, the87Sr/86Sr ratioevolvedsolittleover 4.5Gathat theestimated initial ratio is not sensitive to the exact choice of the formation age of themeteorites. The two precisely measured meteorites with the lowest87Rb/86Sr ratios(Juvinas andSiouxCounty), andtherefore the ones withthe least radiogenic87Sr, hadthesmallest uncertainties. The average of the initial ratios for these two meteorites gave(87Sr/86Sr)0= 0.6989760.000055, which is essentially identical to the intercept of theisochron. Papanastassiou and Wasserburg (1969) dened the Basaltic Achondrite BestInitial value (BABI, Table 8.3) as an estimate of the (87Sr/86Sr)0ratio in the early solarsystem.It wassoondiscoveredthat therewereother solar systemobjectsthat wereolder andhadmoreprimitivestrontiumthanthebasalticachondrites. Table8.3compares the(87Sr/86Sr)0value determinedfor BABI withthose fromsome other important samples fromthe earlysolar system. If Allende, AngradosReis, andthebasalticachondrites formeddirectlyfromthe solar nebula, then the time intervals shown in the right-hand column of Table 8.3 arevalid. Note that the uncertainties on these time intervals, a fewmillion years, are muchsmaller than the uncertainty in the eucrite isochron shown in Figure 8.9 (~260 Myr).However, the validity of the time intervals determined fromthe initial ratios dependscompletelyonthe validityof the idea that theyall formeddirectlyfromthe bulkmaterialof thesolar nebula.The (87Sr/86Sr)0ratio of a systemdepends not only on time but also on the details ofthe history of the systemfromwhich the nal object formed. Figure 8.10 illustrates thisTable8.3 Initial strontium[(87Sr/86Sr)0] ratios for solar-systemsamplesSample (87Sr/86Sr)0(87Sr/86Sr)0adjust toNBS 9871Tif formedfromchondriticreservoirAllende(ALL)2 0.698770.000020.698880.000020Angrados Reis(ADOR)30.698840.00004 0.698950.00004 (4.52.8) 106yrsBasalticachondrites(BABI)40.698980.00003 0.699090.00003 (13.52.3) 106yrs1Thenumerical valueof theNBS987standardwas increased, soall numbers reportedrelativetothat standardmust alsobeincreasedtocomparewithmodernvalues.2Gray et al. (1973).3Papanastassiou et al. (1970).4PapanastassiouandWasserburg(1969).250 Cosmochemistrypoint. Suppose that a batch of material evolves as a closed systemfor some period oftime, but thenexperiences afractionationevent that produces anewmaterial. Anexamplemight be differentiation of an asteroid or planet into a core, mantle, and crust. In ourexample, we will assume that the mantle has a lower87Rb/86Sr ratio than the materialfromwhich it formed, so the87Sr/86Sr ratio evolves more slowly, along a line with ashallower slope inFigure 8.10. The (87Sr/86Sr)T1ratioof the mantle reects the time (T1)that it separated fromthe original material. Nowsuppose that at a later time (T2), themantle generates amagmathat crystallizes intoseveral minerals. Eachof the minerals hasa different87Rb/86Sr ratioandthus evolves along a line with its ownslope. If the systemremains closed until today, an isochron can be generated fromthese several minerals(inset in Fig. 8.10), and the time of crystallization can be determined fromthe isochron.Theinitial ratioobtainedfromthis isochronreects theevolutionof the87Rb87Sr systeminthemantle of the original body, but not theoriginal bodyas awhole. Theoriginal bodyas a whole continued to evolve along the original trajectory, shown in Figure 8.10 as adottedline. Withappropriateknowledgeof the87Rb/86Sr ratioof theoriginal body, atwo-stage model can be constructed that accurately reects the history of the system.However, if interpreted with a single-stage model, the history inferred fromthe initialratio of the isochron will not be correct. Amore complicated history that produces amagma with the (87Sr/86Sr)0shown schematically by (87Sr/86Sr)T2in Figure 8.10 could87Sr86Sr87Sr86Sr87Sr86Sr87Sr86Sr87Sr86Sr( )T0T1T2T3F1F2( )( )T2T2T10( )87Rb/86Sr( )Fig. 8.10 Schematic diagramshowing a hypothetical multi-stage fractionation of rubidiumand strontiumandthe subsequent isotopic evolution of strontium. At T0, a system with xed87Rb/86Sr and87Sr/86Srratios (black circle) begins to evolve. At time T1, when the system has reached F1, a fractionationevent occurs which extracts material with a lower87Rb/86Sr ratio. This newmaterial evolves towardpoint F2, while the original reservoir, which now has a slightly higher 87Rb/87Sr ratio, evolves alongthe dashed line. The dotted line shows the evolution of the body as a whole. When the materialreaches F2 at time T2, a second fractionation event occurs that generates four new materials(minerals perhaps) with different 87Rb/86Sr ratios. Each mineral evolves along its own trend, and ifthe system remains closed, the four new minerals will generate an isochron (inset).251 Radioisotopes as chronometersalso be constructed. Reservoir evolution models typically are not unique and additionalinformation is often required to choose the correct model. We will discuss reservoirevolution in more detail below.The147Sm143Nd systemSamariumhas seven naturally occurring isotopes, two of which,147Smand148Smareradioactive (Table 4.2). The-decay of long-lived147Smto143Nd (t1/2=1.06 1011years; =6.54 1012yr1) is a widely used chronometer. In contrast, the half-life of148Sm, which -decays to144Nd with a half-life ~7 1015years, is too long for it to beuseful for chronology. Thenow-extinct nuclide146Sm, whichhas ahalf-lifeof 1.03108years (=6.71 109yr1), is potentially useful as a chronometer for the rst fewhundredmillionyearsof solarsystemhistory(seebelow). It alsois the daughterof short-lived150Gd,which -decays to146Smwithahalf-lifeof 1.8106years.All three of the radioactive samarium isotopes decay to isotopes of neodymium.Neodymium has seven isotopes, one of which,144Nd, is slightly radioactive with ahalf-life of 2.4 1015years, too long to be of use for chronology. Both samariumandneodymiumare rare earth elements (REE) and are not fractionated fromeach other verymuch by melting and crystallization. They are both also refractory lithophile elements,and so are not fractionated much by evaporation and condensation processes or metal-silicate fractionationinthe solar system. Inigneous systems, neodymiumis preferentiallyconcentrated in the melt during partial melting because the slightly larger ionic radius ofNd3+makes it more incompatible than Sm3+. Thus, basaltic magmas have lower Sm/Ndratios than the source rocks fromwhich they form, and on Earth, crustal rocks such asgranite have lower Sm/Nd ratios than basalts. But overall, the range in Sm/Nd ratios inigneous systems is only a factor of a few.The147Sm143Nddecayschemehas beenuseful inbothcosmochemistryandgeochem-istryintwodifferent ways. The147Sm143Ndsystemcangiveanisochron, fromwhichbotha date andthe initial143Nd/144Ndratiocanbe obtained. Because most of the materials ofinterest tocosmochemistryareold, theslowdecayrateandrelativelyminimal fractionationof samariumfromneodymiumare overcome by long decay times. Very precise isotopicmeasurements alsofacilitatedatingbythe147Sm143Ndmethod. The147Sm143Ndsystemisalsogoodfor datingmajor fractionationevents, suchasformationof thecrust andmantle.In this application, the143Nd/144Nd ratio of the bulk systemcan be used to extract the time ofdifferentiation. Thegeochemical behavior of the147Sm143Ndsystemisoppositethat ofthe87Rb/87Srsystemduringpartial melting. Inthe147Sm143Ndsystem, theradioactiveisotopeisleft behindintheresidual solids, whereasinthe87Rb/87Sr system, theradioactiveisotopeis concentrated in the melt phase. This difference means that the combination of the147Sm143Ndsystemandthe87Rb/87Sr systemsis verypowerful ininvestigatingthepetro-genesis of igneous rocks. Because it is difcult tofractionate samariumfromneodymiumunder avarietyof conditions, the147Sm143Ndsystemcanprovidechronological informa-tion in situations where the87Rb87Sr or UThPb systems have been compromised by laterevents.252 CosmochemistryHistoryThe147Sm143Ndsystemwas rst usedincosmochemistrybyLugmair et al. (1975a), whopublishedapreciseisochronagefor anApollo17basalt (Fig. 8.11). Thesystemdevelopedrapidlythereafter. Notethat thespreadin147Sm/144Ndinthemineral phasesof thisbasalt islessthan50%, not factorsof ten, several hundred, or evenafewthousand, ascanbeseeninother systems. Thefact that preciseisochronages canstill beobtainedis atestament tothecaretakeninchemical separationof samariumandneodymiumandtheprecisionof modernmass spectrometricanalysis.Lugmair et al. (1975a) alsoshowedthat theinitial143Nd/144Ndratioinferredfromtheirisochronwasmeasurablydifferent fromthat expectedfromtheclosed-systemevolutionofachondritic composition. DePaolo and Wasserburg (1976a, 1976b) expanded on thisobservationtocreate a methodologyandreportingnotationinwhichthe timingof eventsthat fractionatedsamariumfromneodymiumcanbeestimatedfromdifferencesbetweenthemeasured143Nd/144Nd ratios and the ratio expected fromthe evolution of achondriticuniformreservoir,or CHUR. Usingthissystem, theratiosmeasuredat thepresent timearecompared to the inferred composition of the CHURtoday, in contrast to the87Rb87Srsystem, wheretheinitial ratiofromtheisochroniscomparedwithaninitial ratioinferredfortheearlysolar system(e.g. BABI or ALL).Technical detailsSeveral samariumand neodymiumisotopes are isobars (Table 4.2) and cannot be separ-ated by mass spectrometry. Thus, samarium and neodymium must be completelyseparated by chemical procedures prior to measuring themin a mass spectrometer. Theion-exchange chemistry necessary to separate these elements is nowwell developed (seeAppendix). Isotope measurements can be done either by TIMS or ICPMS (inductively0.5150.5140.5130.20 0.22 0.24 0.260.28 0.30147Sm/144Nd143Nd/144NdApollo 17Rock 75075T=3.700.07 GaI =0.5082512PyroxeneWhole rockIlmenitePlagioclase202Y147Sm/144Nd0.20 0.25 0.30Fig. 8.11 Samariumneodymium evolution diagram for lunar mare basalt 75075. Data points for the totalrock, plagioclase, ilmenite, and pyroxene form a precise linear array, the slope of which gives acrystallization age of 3.700.07 Ga for this rock (= 6.54 1012yr1). The insert shows thedeviations from the best-t line in parts in 104. After Lugmair et al. (1975a).253 Radioisotopes as chronometerscoupled plasma mass spectrometry). The elemental ratios can be determined by isotopedilution using either TIMSor ICPMS(see Appendix).IsochronsIn suitable systems, it is possible to obtain relatively precise internal isochrons fromthe147Sm143Nd system. In an igneous system, the REE partition differently among thecrystallizingminerals. Inolivineandorthopyroxene, thelight REEaredepletedrelativetothe heavy REE, resulting in an enrichment of the parent samariumover the daughterneodymium. In plagioclase, the light REE are enriched. This partitioning provides thefractionationof parent anddaughter necessarytoproduce anisochron(Fig. 8.11). As withother isochronsystems, thedateis calculatedfromtheslopeof theisochron.Reservoir evolutionFigure 8.12 is a schematic diagramof the evolution of the143Nd/144Nd ratio in the bulksolar systemand in two planetary objects. The143Nd/144Nd ratio of the solar systemincreases with time due to the decay of147Sm, and the increase can be numericallymodeledif oneknows the147Sm/144Ndratiointhesolar system. Intheoriginal theoreticalframework, the rst solids begantocondensefromthehot solar nebulaat 4.57Ga. But thestartingtimecanalsorepresent meltingor local evaporationandcondensationof themixedgas and dust inherited fromthe molecular cloud. The exact time of formation of the rstsolids in the solar system is assumed because it cannot be determined from the147Sm143Nd system. Due to variations in the relative abundances of samariumand neo-dymiumcausedbythermal processingof nebular material, planetarybodies couldaccretewithdifferent Sm/Ndratios. Theevolutionof twosuchbodiesisillustratedinFigure8.12a.PlanetA formedat time TAwitha lower Sm/Ndratiothanthe bulksolar system, soitsbulk143Nd/144Ndratioevolvedmore slowly with time than the bulk solar system. PlanetB formedat time TBwitha higher Sm/Ndratio, andthus its143Nd/144Ndratioevolvedmore quickly.Inthe earlysolar system, most planetarybodies mayhaveformedinonlyafewmillionyears. The147Sm143Ndsystemisnot particularlysensitivetoshort timeintervalsat 4.6Gabecause of the slow decay ratio of147Sm. In addition, because the fractionation ofsamariumfromneodymiumtends to be relatively small, the slopes of the lines on plotslikeFigure8.12tendtobesimilar. As aresult, theprecisionrequiredtoresolvedifferencesinformationtime basedonthe intercepts of the isochrons is beyondcurrent experimentalcapabilities. Also, in contrast to the situations for the87Rb/87Sr and UThPb systems,for which large fractionations of parent and daughter elements have provided samplesof strontiumand lead with essentially primordial compositions, there are no examplesof objects without signicant samarium that can give a direct measurement of the(143Nd/144Nd)0.DePaulo and Wasserburg (1976a, 1976b) showed howto get around these problems.Rather thanattemptingtodeterminetheintercept of anisochronat extremelyhighprecisionandtocompareit toapoorlyknowninitial valuefor thesolar system, theyinsteadchoseto254 Cosmochemistrycompare the present bulk143Nd/144Nd ratiowiththat expected for a uniformreservoir ofchondriticcomposition(the chondriticuniformreservoiror CHUR). Inother words, theyworkedwiththevalues that plot ontheright axis of Figure8.12, wherethedifferences arethe greatest. With sufciently precise measurements of143Nd/144Nd and147Sm/144Nd forthe sample, andwell-knownparameters for CHUR, one cancalculate the evolutionof the143Nd/144Ndratioas a functionof time for bothand ndout whenthe sample andCHURhadthesamecomposition. Withcurrent levelsof analytical precision, the143Nd/144Ndratiocanbemeasuredwithanaccuracyof ~50ppmandthe147Sm/144Ndratiocanbemeasuredto ~0.05%. This translates into an uncertainty of ~10 ppmin the143Nd/144Nd ratio at 4.6 Ga.CHURCHUR4.57 Ga Present Time143Nd/144Nd143Nd/144Nd4.57 Ga Present TimeTATATBTB(a)(b)Fig. 8.12 (a) Schematic diagramof143Nd/144Nd versus time for the bulk solar system(CHUR, dark heavy line)and for two hypothetical planets formed from solar system material. When planet A formed attime TA, it acquired a lower147Sm/144Nd ratio than the parent reservoir, so fromthat point onward,the143Nd/144Nd ratio evolved more slowly than the bulk solar system. Planet B formed slightlylater with a higher147Sm/144Nd ratio and its143Nd/144Nd ratio evolved faster than the bulk solarsystem. (b) Schematic diagram of143Nd/144Nd versus time for two-stage evolution of materialseparated from the CHUR reservoir. At time TA, a planet forms with a higher 147Sm/144Nd ratio thanthe bulk solar system and subsequently evolves along a steeper trend. Some time later, at time TB,the planet differentiates into two layers with different 147Sm/144Nd ratios. Each layer evolves fromthat point on with a different trend. It is generally not possible to determine from the samariumneodymium data alone whether a sample is the result of a single or multi-stage history. But in theexample in panel (b), the planetary layer with the lowest147Nd/144Nd ratio cannot be the result of asingle-state evolution because the evolution line intersects the CHUR line in the future.255 Radioisotopes as chronometersIn contrast, the uncertainty in the intercept of an isochron obtained with data of similarprecisionis ontheorder of 200ppm.Acritical input valuenecessarytousethebulk143Nd/144NdratiotodeterminethetimeofSm/Nd fractionation is the present day composition of the CHUR. Samariumand neo-dymiumare both refractory and are not expected to have fractionated very much in theaccretion disk, so bulk chondrites are expected to give similar147Sm/144Nd and143Nd/144Ndratios. Jacobsen and Wasserburg (1980) found that the147Sm/144Nd ratios had a rangeof ~4%andthe143Nd/144Ndratios exhibitedarangeof 5.3parts in104amongchondrites.TheCHURvalues(Table8.4)havebeenrenedonlyslightlysincetheoriginal set of valuesproposedbyJacobsenandWasserburg(1980).The143Nd/144Ndratioof asampleis typicallyreportedas:0j 143Nd_144Nd_ _0meas143Nd_144Nd_ _0CHUR1____104(8:23)This epsilonnotation gives the difference betweenthe measuredratiointhe sample andtheCHURvaluefor thepresent timeinpartsper 10000. Thedifferenceinthe147Sm/144NdratiorelativetotheCHUR(theelemental fractionation) is givenby:f Sm=Ndj147Sm_144Nd_ _j147Sm_144Nd_ _0CHUR1 (8:24)To interpret these numbers, consider again the line for the bulk solar system(CHUR) inFigure8.12. The143Nd/144Ndratioat anytime, T, inthepast canbecalculatedas follows:143Nd144Nd_ _TCHUR143Nd144Nd_ _0CHUR147Sm144Nd_ _0CHURexpT 1 (8:25)wherethetime, 0, referstothepresent time. Theratiofor4.56GaisshowninTable8.4. Thesame equationis usedtocalculate the evolutionof a sample, withthe values143Nd/144Ndand147Sm/144Ndobtainedfromthesampleinsertedinplaceof theCHURvalues. Thevalueof epsilon can be calculated for any specied time fromEquation (8.23) by inserting theratios for the sample and CHURcalculated for that time using Equation (8.25), althoughcaremust betakentospecifythetimeif it is other thanthepresent.Table8.4 Acceptedvalues for CHURPresent 4.56Ga143Nd/144Nd 0.512638 0.5066866147Sm/144Nd 0.1966 0.20255JacobsenandWasserburg(1980), Bouvier et al. (2008).256 CosmochemistryOnecandeterminethetimeat whichtheparent reservoir for asamplewasseparatedfromtheCHURbysettingEquation(8.25) for theCHURequal toEquation(8.25) for thesampleandsolvingfor T:Tmodel 11 143Nd144Nd_ _0Sample143Nd144Nd_ _0CHUR147Sm144Nd_ _0Sample147Sm144Nd_ _0CHUR____(8:26)Thetime, Tmodel, is amodel agethat assumes asingle-stateevolutionof thesampleonceitwas separatedfromtheCHUR. OnFigure8.12a, thetimeTmodelis thetime, TA, for planetAandthetimeTBfor planet B.More-complicatedhistories canbe envisionedandmodeled. Figure 8.12bshows atwo-stage history. In this example, a planet separates fromthe CHUR at time TAand itscompositionevolves alonga steeper trend. Some time later, this bodydifferentiates intoamantle and a crust, with the crust having a lower Sm/Nd ratio and the mantle a higher Sm/Ndratiothanthebulkplanet. Thecrust andthemantletheneachevolvealongtheir owntrends.Using the present day147Sm/144Nd and143Nd/144Nd ratios for crust and mantle, one cancalculate the time whenall layers hadthe same143Nd/144Ndratio. This is the time of thedifferentiation. If the bulk147Sm/144Nd ratio of the planet can be reconstructedfromthe dataon crust and mantle, the evolution of the bulk planet (therst stage of the two-stageevolution) canbecalculated. But if asingle-stagemodel is usedtoevaluatemeasurementsof either the crust or the mantle, the model time of separation fromthe CHURwill be incorrect.Multi-stagemodelsarenot uniqueandareoftendifcult toconstrainwith147Sm143Nddataalone. However, incombinationwithother information, the147Sm143Ndsystemisapower-ful tool for unravelingthedifferentiationhistoryof aplanetarybody.ApplicationsThe147Sm143Nd systemis particularly well suited for dating basaltic and ultramacigneous rocks, whichcannot bedatedwiththe87Rb/87Sr or UThPbsystems. Ingeneral,the more mac the rock, the lower the REEconcentrationandthe higher the147Sm/144Ndratio. TheREEs arenot easilyaffectedbyweatheringor bymetamorphism, socrystalliza-tionages canbe obtainedevenfromrocks that have beenmetamorphosed. These featuresmakethe147Sm143Ndsystemparticularlywell suitedtodateterrestrial Archeanrocks.Chronologywith147Sm143NdisochronsThe147Sm143Nd systemhas been widely used to datesamplesof differentiatedbodies. Theinitial application of the147Sm143Nd system in cosmochemistry was to produce anisochronage for a lunar mare basalt (Fig. 8.11). The147Sm143Ndsystemis still usedforlunar samples, although care must be taken to avoid the effects of secondary, impact-produced melting. Precision has improved over the years, as shown in Figure 8.13 for amagnesium-suite lunar norite. The internal isochron gives an age of 4.3340.037 Ga(Edmunson et al., 2009). Thesame authors alsodeterminedaconcordant207Pb206Pbage257 Radioisotopes as chronometersof 4.3330.059 Ga for this rock, conrming that the147Sm143Nd systemcan producepreciseandaccurateagesfor undisturbedrocks. Thissystemhasalsoplayedamajor roleindatingof Martianmeteorites. Reliable147Sm143Ndagesof ~1.3Gahavebeenobtainedforthe nakhlites and for Chassigny. These dates are concordant with dates obtained from40Ar39Ar,87Rb87Sr, and UThPb systems (Nyquist et al., 2001b; Misawa et al.,2006). Dates for theMartianshergottites aremorecontroversial (seeChapter 9).Reservoir evolutioninferredfromthe147Sm143NdsystemThe147Sm143Nd systemis actively used to investigate the differentiation and magmatichistoryof theMoon, Mars, andtheEarth. This systemis usedtogether withthe87Rb87Sr,176Lu176Hf, UThPb, and187Re187Os systems toextract informationonthetimingandnatureof meltinganddifferentiationinplanetarybodies. Theseresults will bediscussedinChapters 9and13.The UThPb systemUranium has two long-lived isotopes,235U and238U, that decay to207Pb and206Pb,respectively. Thoriumhas onelong-livedisotope,232Th, that decays to208Pb. Theisotopicabundancesof uraniumandthoriumaresummarizedinTable8.5. Theisotopicabundancesof terrestrial leadaregiveninTable4.2.The principal decaymode of238Uis -decay, but a small fractionof the decays are byspontaneous ssion. Theemissionof an -particleinitiates aseries of decays knownas theuranium series (Fig. 8.14), whichends at206Pb. Theuraniumseries canbesummarizedas238U !206Pb 84He 6 (8:27)0.5220.5200.5180.5160.5140.5120.5100.5080 0.1 0.2 0.3 0.4 0.5 0.6147Sm/144Nd143Nd/144NdWhole rockPlagioclasePlagioclasePlagioclaseGlassMg-pyroxeneFe-pyroxeneFe-pyroxene+10120.1 0.2 0.3 0.4 0.5Nd147Sm/144NdT = 4.3340.037 GaI =0.270.74Lunar norite 78238Intermed-pyroxeneFig. 8.13147Sm143Nd evolution diagram for nine mineral fractions and a whole-rock sample of lunar norite78238. The inset shows the deviations from the best-t line in parts in 104. In spite of a smallamount of scatter attributed to the inclusion of tiny fragments of other rock types into the measuredsample, the isochron is very well dened. After Edmunson et al. (2009).258 CosmochemistryThe decaypathsplits at several points where decaycanbe either by - or -decay, but allpaths endat206Pb(Fig. 8.14).The decayof235Uis alsoprimarilyby -decaythroughthe actinium series (Fig. 8.15).Theactiniumseries canbesummarizedas235U !207Pb 74He 4 (8:28)Similarly, thedecayseries initiatedbythe -decayof232Th(Fig. 8.16) canbesummarizedas232Th !208Pb 64He 4 (8:29)Although these decay series consist of 43 isotopes of 12 elements, none is a member ofmorethanoneseries, soeachdecaychainalways leads toaspecicisotopeof lead:Table8.5 Decayconstants andisotopic abundances of uraniumandthoriumIsotope Abundance Decaymode Half-life Decayconstant232Th 100 alpha 14.010109years =4.94751011yr1235U 0.7200 alpha 0.7038109years =9.84851010yr1238U 99.2743 alpha 4.468109years =1.551251010yr1Steiger andJger (1977).206Pb206Tl206Hg210Po210Bi210Pb210Tl214Po214Bi214Pb218Rn222Rn218Po218At226Ra230Th234U238U234Pa234ThAtomic number (Z)92908886848280124 126 128 130 132 134 136 138 140 142 144 146Neutron number (N)Fig. 8.14 Portion of the chart of the nuclides illustrating the decay of238U to206Pb. The decay occurs by aseries of -decays, which cause the nuclide to move down and to the left, and -decays, which causethe nuclide to move up and to the right.259 Radioisotopes as chronometers238U !206Pb235U !207Pb232Th !208PbThe half-lives of238U,235U, and232Th are all very much longer than those of theradioactivedaughter isotopesintheir decaychains. Therefore, aconditionknownassecularequilibriumis quicklyestablishedinwhichthe decayrates of the daughter isotopes inthedecaychainequal that of theparent isotope. Inaclosedsystem, oncesecular equilibriumisAtomic number (Z)92908886848280124 126 128 130 132 134 136 138 140 142 144Neutron number (N)207Pb207Tl211Po211Bi211Pb215Po215Bi215At219Rn223Fr219At223Ra235U231Th231Pa227Th227AcFig. 8.15 Portion of the chart of the nuclides illustrating the decay of235U to207Pb. This decay series is oftencalled the actinium series.Atomic number (Z)908886848280126 128 130 132 134 136 138 140 142Neutron number (N)208Pb208Tl212Pb212Bi212Po216Po220Rn224Ra228Th228Ac228Ra232ThFig. 8.16 Portion of the chart of the nuclides illustrating the decay of232Th to208Pb.260 Cosmochemistryestablished, the rate of productionof the stabledaughter at the endof the chainequals therateof decayof theparent isotopeat theheadof thechain.Uraniumandthoriumareactinideelements. Their chemical behavior is similar under mostconditions. Both are refractory elements, both occur in nature in the +4 oxidation state, and theirionicradii areverysimilar (U+4=1.05, Th+4=1.10). However, uraniumcanalsoexist inthe +6stateastheuranyl ion(UO2+2), whichformscompounds that are solubleinwater. Thus,under oxidizing conditions, uraniumcan be separated fromthoriumthrough the action of water.In contrast to refractory uraniumand thorium, lead is a moderately volatile element.Uraniumand thorium are lithophile, while lead can exhibit lithophile, siderophile, orchalcophile behavior. This means that inmanycosmochemical situations, it is possible tostrongly fractionate the daughter lead from parent uranium and thorium, a favorablesituationfor radiochronology. Onthe other hand, leadtends tobe mobile at relativelylowtemperatures andcanbe either lost fromasystemor introducedat alater time. As alreadymentioned, uranium can also become mobile under oxidizing conditions. This meansthat the UThPb systemis more susceptible toopen-systembehavior than several othercommonly used dating techniques. However, as we discuss below, there are ways torecognizeandaccount for theopen-systembehavior inmanycases.HistoryThe rst attempts to determine ages of natural samples using uraniumwere chemical methods,becausethenatureof isotopeswasnot yet understood. Twomethodsbasedonuraniumwereinitiallyused, the chemical leadmethodandthe heliummethod.Bothmethods assumedthat thedaughter element was initiallyabsent fromtherockor mineral analyzedandthat thecontribution of thoriumto the daughter elements could be neglected. Thus, initially, thesemethods were applied only to uranium-rich, nearly lead-free materials. It was soon determinedthat most rocks andminerals slowlyleakhelium, sothat dates determinedbymeasuringtheuraniumcontent andtheheliumcontent of arockweretypicallytooyoung. It alsoturnedoutthat thecontributionfromthoriumwas not negligibleinmanycases.Bythelate1920s, isotopicmeasurements hadshownthat uraniumandthoriumdecayedtodifferent isotopes of lead. Athirdleadisotope,207Pb, wasalsoidentiedandwasshowntoberadiogenic. It appearedtobetheproduct ofthedecayofactinium, but it waspostulatedthat the ultimate source might