铷-锶定年方法

铷－锶定年方法(Rb-Srdating)同位素地质年代学Thisdecaysystemwasoneofthefirsttobewidelyusedingeochronologyandremainsoneofthemostusefulgeochemicaltracers.Campbell&Wood(1906)foundnaturalradioactivityofrubidium(Rb)Hahnetal.(1937)&Mattauch(1937)identified87RbasthenaturallyoccurringradioactiveisotopeHahn&Walling(1938)pointedoutthefeasibilityofdatingRb-bearingmineralsbythedecayof87Rbto87SrHahnetal.(1943)reportedthefirstagedeterminationbytheRb-SrmethodWideusebegansince1950’saftertheavailabilityofmassspectrometeroftheNier’sdesignGast(1960)firstproposedSrisotopesforgeochemicaltracerNicolaysen(1961)proposedtheisochronmethodfortheRb-Srdating84Sr86Sr88Sr87Sr87Rb85Rb464837384950NumberofneutronsNumberofprotonsRb–Srisotopesystem87Rb87Sr+b-T1/2=4.88x1010(year);l=1.42x10-11(year-1)DecayconstantThehalflifeof87Rbwasdifficulttodetermineasthelowmaximumenergyoftheb-particlesitemitsandasslowrateofitsdecayPriorto1977,twodifferentvaluesofdecayconstantof87Rbwereinuse:l=1.39x10-11y-1andl=1.47x10-11y-1In1977,theSubcommissiononGeochronologyoftheInternationalUnionofGeologicalSciencesrecommendedavalueof1.42x10-11y-1(correspondingtohalflifetimeof4.88x109y)forthedecayconstantof87Rb,basedonthemeasurementsbyNeumann&Huster(1974),Davisetal.(1977)andSteiger&Jaeger(1977)85Rb87Rb72.17%27.83%铷(Rubidium)Group1alkalimetal,traceelementintheEarthAtomicabundanceAtomicabundanceratio:85Rb/87Rb=2.59265aconstantthroughouttheEarth,moonandmostmeteoritesduetoisotopichomogenizationinthesolarnebula.锶(Strontium)Group2alkalineearthmetal,traceelementintheEarthStableisotopes:84Sr,86Srand88SrRadiogenicisotope:87SrAtomicabundanceratio:88Sr/86Sr=8.375287Sr/86Srratiochangeswithtime.86Sr88Sr9.87%82.53%Atomicabundance87Sr84Sr7.04%0.56%AverageAnimportantadvantageoftheRb-SrisotopicsystemisrelativelylargevariationsoftheRb/SrratioinrocksAsdifferenceingeochemicalpropertiesofRbandSr,Rb/SrratiocanvarybyseveralordersofmagnitudeindifferentrocktypesordifferentmineralsAccuracyofanagedeterminationdependsheavilyonthespreadofmeasuredRb/Srratios,thismakesthissystemausefulgeochronologicaltoolContinentalcrust:32-78ppm

Rb260-333ppm

SrDepletedMantle:

0.6ppm

Rb

19.9ppm

SrAveragecontents(ppm)ofRb,K,Sr,CainigneousandsedimentaryrocksRbisahighlysoluble,highlyincompatibleelement.Srisalsorelativelysolubleandisfairlyincompatibleinmaficand,particularly,ultramaficigneoussystemBothRbandSraretraceelementsintheEarthGeochemicalfeatureofRbandSrSrisrelativelycompatibleinsilica-richigneoussystems,partitioningpreferentiallyintoplagioclaseThemantlehasarelativelyuniformandlow87Sr/86Srratio,whilethecontinentalcrusthasamuchmorevariable,and,onaverage,higherratio.Rbhasanionicradiusof1.48ÅThislargeionicradiusmeansitisexcludedfrommanymineralsand,henceitisoneofthemostincompatibleelementsItsradiusissufficientlysimilartothatofpotassium(1.33Å),therefore,RbsubstitutesreadilyforKinK-bearingminerals,suchasmicaandK-feldsparTheionicradiusofSris1.13ÅItisstillsufficientlylargeanditisalsoexcludedfrommanymineralsItisanincompatibleelement,butnotahighlyincompatibleoneSrsubstitutesforCa(ionicradius0.99Å)tovaryingdegrees.ItisquitecomfortableintheCasiteinplagioclaseSr2+canalsosubstitutesforK+,butrequiringthereplacementofSi4+byAl3+topreserveelectricalneutralityInigneousandhigh-grademetamorphicrocks,mostSrwillbeinplagioclase.SrcanalsosubstituteforCainotherminerals,suchascalcite,apatite,gypsum,sphene,etc.Srisalsoconcentratedinthecrustrelativetothemantle,butnottothedegreethatRbis.Parent-DaughterSystemD=D0+N(elt-1)87Sr=87Sr0+87Rb(elt-1)87Sr/86Sr=(87Sr/86Sr)0+87Rb/86Sr(elt–1)Isochron

等时线t=(D-D0)Nlnl1+1Howtoobtainvalueoftheinitialvalue(D0)?elt=1+lt++(lt)22!(lt)33!+...elt1+lt

~~whenlt<<1,D=D0+N(elt-1)

=D0+Nlt87Sr/87Sr=(87Sr/86Sr)0+(87Rb/86Sr)ltRb-Sr

isochrondiagram(Nicolaysen,1961)Y＝c+mXSlopeofthislinem=elt-1c:initialisotopicratioincreasingratesof87Sr/86Srvalueswithdifferent87Rb/86SrDifferentsamples(whole-rocksandminerals)canhavevariantvaluesof87Sr/86Srratio,andhow?One87Rbatomdecaysandproducesone87Sratom5100.710.720.7387Rb/86Sr87Sr/86SrstartlaterRb-Sr

isochrondating5100.710.720.7387Rb/86Sr87Sr/86Srm=(elt-1)Initial87Sr/87SrratioTypeofisochronInternalisochron(内部等时线)Allanalyzedfractionscomefromonesample,e.g.,wholerockpowderanditsmineralsExternalisochron(外部等时线)Allanalyzedfractionscomefromdifferentsamples,whicharecollectedfromdifferentpartsofagenetic-relatedgeologicalbody,e.g.,agraniteplutonExternalisochron

Whole-rocks(WR)Internalisochron

WR&minerals87Sr/86SrWR1WR2WR3WR4WR5WR687Rb/86Sr87Sr/86SrWRBiotiteK-feldsparRb-freemineralTheisotopesystemofinterestwasat

isotopicequilibrium

attimet=0

Isotopicequilibriuminthiscasemeansthesystemhad

ahomogeneous,uniformvalueofD0Theisotopesystemasawholeandeachanalyzedpartofitwas

closed

betweent=0andtimet(usuallythepresenttime)

By“closedsystem”

wemeantherehasbeennotransferoftheparentorthedaughterelementintooroutofthesystemTherequirementofaclosedsystemandaninitiallyhomogeneoussystemabove,suggestsameaningforthenatureoftheevent,whichisdatedbyradiogenicisotopegeochemistry,andameaningfortime.Ingeneral,theeventisthelasttimethesystemwasopentocompleteexchangeoftheparentanddaughterelementsbetweenthevarioussubsystemsthatweanalyze.Thatisthelastpointintimethatthesystemhadahomogeneous,uniformvalueofradiogenicisotope.Thiseventisgenerallyathermalone,inwhichtimethesystemwashotenoughforisotopeexchangebetweensubsystemstooccurClosureTemperatureofIsotopeSystemClosuretemperature(TC)isdefinedas“thetemperaturethemineralexperiencedatthetimegivenbyitsage.(Dodson,1973)TC=Ea/Rln((A*R*TC2*D0/a2)/(Ea*Cr))whereEa=activationenergy;R=gasconstant;A=geometryfactor;D0=frequencyfactor;a=effectiveradius;Cr=coolingrateClosureTemperatureofIsotopeSystemRb-Srsystem:Muscovite450-500OCBiotite350OCK-Arsystem:Hornblende450-500OCBiotite300OCMuscovite350-400OCMicrocline150-250OCU-Pbsystem:Zircon>900OCGarnet>800OCMonazite650-740OCSphene(titanite)500-670OCRutile420-380OCApatite~350OCIntheRb-Srsystem,boththeinitialratioandtheagearevirtuallyalwaysunknown,meaningwemustsolveforbothsimultaneouslythroughtheisochronmethodToobtainameaningfulisochronthefollowingconditionsmustbemet:Rb-Srdating1.87Rb/86Srratioshouldbelarge

Ifyes,variationsinSrisotopiccompositionwillbelargerelativetoourabilitytomeasurethem.Underthebestofcircumstances,isotoperatioscanbemeasuredwithprecisionof10ppmorsoIfthetotalamountofradioactivelyproduced87Srissmallrelativetotheamountpresentinitially,thereislittlehopeofusingthesystemtoproducereasonable“ages”2.87Rb/86Srratiosshouldhavealargerange

Alargerangein87Rb/86SrratioleadstoalargerangeinisotoperatiosinthedaughterSr.Theerrorontheregressionslopeisafunctionoftherangeofvaluesusedinthecomputation.Sogivensimilaranalyticalprecisions,wewillobtainamoreprecisedatewithadecaysystemwherethevariationsinparent-daughterratioarelargethanwithonewherethesevariationsaresmall.3.minimal

deviationsfromclosedsystemRbtendtobemoremobilethanSrandsomemineralsarelessreactivethanothersMetamorphismwilldisturbasystemonthescaleofmineralgrainsAnatomcreatedbyradioactivedecaywillgenerallybeamisfitinthelatticesiteitoccupies.Thelatticesitemaybedamagedbythedecayprocess4.Srisotopiccompositionmusthavebeenhomogeneousatthetimeoftheevent

Homogenizationthattakesplacethroughdiffusion,ishighlytemperaturedependent.Thehigherthetemperaturesobtainedduringthe'event',themorehomogenizedthesystemwillbe.Rb-Srdatingofigneousrocks(火成岩)NumerouscaseshavealreadyshownthattheRb-Sr

isochrondatingmethodisasuitabletoolfordatingofcogeneticigneousrocksWhole-rockRb-Sr

isochronofbiotitegranite,Liruei

pluton,centralNigeriaBreemenetal.(1975)Rb-Srdatingofmeteorite(陨石)Pinsonetal.(1965)usedwhole-rockspecimensofchondritesandachondritestoconstructRb-Sr

isochronsGopalan&Wetherill(1968,1969,1970)usedwhole-rockspecimensofchondritesandachondritesbelongingtoselectedclassesweredatedseparatelyKempeandMueller(1969)usedindividualspecimensofchondritesandachondritesfordatingbymeansofinternalisochronsMeteoritesarecomposedofsilicateandoxidemineralssimilartothoseofmaficigneousrocksontheEarth(e.g.,olivine,plagioclase,magnetite,ilmenite)TheyalsocontainvaryingamountsofmetallicironandnickelintheformofdispersedgrainsorinnearlypureformSubdivisionofmeteoritesbasedontheircomposition:Stony,stonyiron,andmetallicironStonymeteorite~95%Ironmeteorite~4%Stonyironmeteorite~1%Stonymeteoritesaresubdividedintochondrites(球粒陨石）andachondrites(非球粒陨石),onthebasisoftextural,mineralogicalandchemicalcriteriaChondritesarestonymeteoritesthatcontainsmallsphericalpellets(小球,~1mm)knownaschondrulesStonymeteoriteslackingchondrulesareachondritesChemicalcompositionsandtexturesofachondritesaresimilartothoseofmaficorultramaficigneousrocksontheEarthandreferredtoasbasalticachondrites球粒陨石非球粒陨石WRRb-Sr

isochronof10chondritesoftheLLclassMinster&Allegre(1981)Theseresultsdemonstratesthatthechondriteshadthesame87Sr/86Srratioat4.493GaTheSrisotopecompositionatthetimeofformationofthesolarsystemappearstohavebeenhomogeneousTheSrbecomingpartoftheEarthalsohadthesameisotopiccompositionastheSrinasteroidsandmeteoroidsatthetimeoftheirformationInitialvalueofSrisotopiccompositionoftheEarthAchondritemeteoriteshavelowerRb/Srratiosthanthechondrites,andtheSrinachondritescontainslessradiogenic87Sr,hence,amoreprecisevalueofinitial87Sr/86SrratiocanbederivedfromtheachondritesbytheisochronmethodPapanastassiouandWasserburg(1969)reportedavalueof0.698990±0.000047forinitial87Sr/86Srratiofrom7whole-rockachondritemeteoritesThisisknownasBABI(basalticachondritebestinitial)LowervaluesthantheBABIhavealsoreportedforachondriteslater,e.g.,0.69884±0.00003knownasADOR(Wasserburgetal.,1977;Lugmair&Galer,1992)87Sr/87Sr=(87Sr/86Sr)0+(87Rb/86Sr)ltbasalticachondrite

bestinitialRb-SrdatingoflunarrocksRb-Sr

isochronoftheconstituentmineralsofolivinebasalt(Apollo12),OceanofStorms,MoonWhole-rockRb-Sr

isochronofbasaltsamplesfromtheOceanofStorms,MoonPapanastassiou&Wasserburg(1970,1971)Rb-SrdatingofmetamorphicrocksSrisotopichomogenizationandevolutionofrocksduringthermalmetamorphicprocessesEvolutionofSrisotopeofwhole-rocksandmineralsinanepisodeofthermalmetamorphismofshortdurationSlopeofthewhole-rockisochroncorrespondstoti,thetimeelapsesincecrystallizationoftheserocksSlopeofmineralisochroncorrespondstotm,thetimeelapsesincetheendofthethermalmetamorphismWhole-rockandmineralisochronsofgraniticrocksfromtheCarn

ChuinneagcomplexintheHighlandsofnorthernScotlandLong(1964)andPidgeon&Johnson(1974)TheAmitsoqgneissisoneoftheoldestterrestrialrocksknownWhole-rockisochronsfortheAmitsoqgneissfromsouthwesternGreenlandyielded3660MaMoorbathetal.(1972)Sr

同位素地层年代学Time(Ma)87Sr/86SrSrisotopeevolutiondiagramoftheEarthMarinegeochemistry,illustratingtherangeofpossiblesourcesandsinks,aswellasinternalprocessing,ofdissolvedmaterialinseawaterTaylorandMcLennan(1985)Logofseawate