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1、Chapter3VariationsofStableIsotopeRatiosinNature3.1ExtraterrestrialMaterialsIsotopevariationsfoundinextraterrestrialmaterialshavebeenclassifiedaccordingtodifferentprocessessuchaschemicalmassfractionation,nuclearreactions,nucle-osynthesis,and/ortodifferentsourcessuchasinterplan
2、etarydust,solarmaterials,andcometmaterial.Variousgeochemicalfingerprintspointtothereservoirfromwhichtheplanetarysamplewasderivedandtheenvironmentinwhichthesamplehasformed.Theycanbeattributedtoavarietyofprocesses,rangingfromhetero-geneitiesintheearlysolarnebulatotheevolutionof
3、aplanetarybody.FormoredetailsthereaderisreferredtoreviewsofThiemens(1988),Clayton(1993,2004),andMcKeeganandLeshin(2001).ExtraterrestrialmaterialsconsistofsamplesfromtheMoon,Mars,andavari-etyofsmallerbodiessuchasasteroidsandcomets.Theseplanetarysampleshavebeenusedtodeducethee
4、volutionofoursolarsystem.Amajordifferencebe-tweenextraterrestrialandterrestrialmaterialsistheexistenceofprimordialisotopicheterogeneitiesintheearlysolarsystem.TheseheterogeneitiesarenotobservedontheEarthorontheMoon,becausetheyhavebecomeobliteratedduringhigh-temperatureproces
5、sesovergeologictime.Inprimitivemeteorites,however,compo-nentsthatacquiredtheirisotopiccompositionsthroughinteractionwithconstituentsofthesolarnebulahaveremainedunchangedsincethattime.Heterogeneitiesinisotopecompositionindicateincompletemixingofdistinctpresolarmaterialsduring
6、formationofthesolarsystem.Suchisotopeanomaliesinlightelementshavebeendocumentedonallscales,frommicroscopiczoninginmeteoriticmineralstobulkasteroids.Themostextremeexamples,however,havebeendocumentedfromminutepresolargrainsextractedfromprimitivemeteoritesandmeasuredwiththeionm
7、icroprobe.Thesegrainsofsiliconcarbide,graphite,anddiamondshowisotopevariationsthatmayvarybyoneormoreordersofmagnitude.Theyhaveacquiredtheirisotopecharacteristicsbeforethesolarsystemhasbeenformed.TheimplicationsofthesevariationsformodelsofstellarformationhavebeensummarizedbyZ
8、inner(1998)andHoppeandZinner(2000).TheabundanceJ.Hoefs,StableIsotopeGeochem
