Nature-Episodic-growth-of-the-Gondwana-supercontin

©2006NaturePublishingGroupEpisodicgrowthoftheGondwanasupercontinentfromhafniumandoxygenisotopesinzirconA.I.S.Kemp1,C.J.Hawkesworth1,B.A.Paterson1&P.D.Kinny2Itisthoughtthatcontinentalcrustexistedasearlyas150millionyearsafterplanetaryaccretion1,butassessingtheratesandprocessesofsubsequentcrustalgrowthrequireslinkingtheapparentlycontradictoryinformationfromtheigneousandsedi-mentaryrockrecords.Forexample,thestrikingglobalpeaksinjuvenileigneousactivity2.7,1.9and1.2Gyragoimplyrapidcrustalgenerationinresponsetotheemplacementofmantle‘super-plumes’,ratherthanbythecontinuousprocessofsubduc-tion2–4.Yetuncertaintiespersistoverwhethertheseagepeaksareartefactsofselectivepreservation5,andoverhowtoreconcileepisodiccrustformationwiththesmoothcrustalevolutioncurvesinferredfromneodymiumisotopevariationsofsedimentaryrocks6,7.Detritalzirconsencapsulateamorerepresentativerecordofigneouseventsthantheexposedgeology1,8,9andtheirhafniumisotoperatiosreflectthetimesincethesourceoftheparentalmagmasseparatedfromthemantle.These‘model’agesareonlymeaningfulifthehostmagmalackedamixedorsedimentarysourcecomponent10,butthelattercanbediagnosedbyoxygenisotopes,whicharestronglyfractionatedbyrock-hydrosphereinteractions.Herewereportthefirststudythatintegrateshafniumandoxygenisotopes,allmeasuredinsituonthesame,preciselydateddetritalzircongrains.ThedatarevealthatcrustgenerationinpartofGondwanawaslimitedtomajorpulsesat1.9and3.3Gyrago,andthatthezirconscrystallizedduringrepeatedreworkingofcrustformedatthesetimes.Theimplicationisthatthemechanismsofcrustformationdifferedfromthoseofcrustaldifferentiationinancientorogenicbelts.Palaeozoicsedimentaryrocksthataccumulatedalongthepalaeo-PacificmarginoftheformerGondwanasupercontinentareidealfortestingpunctuatedcrustalgrowthmodels.Theirdepositionpostdatesthemajorenvisagedperiodsofrapidglobalcrustalgrowth(2.7and1.9Gyrago;ref.2),andtheiraveragecrustalSm/Ndratios7anduniformityofdetritalzirconagepopulations11–13testifytotheefficientmixingofmaterialerodedfromlargeareasofcontinentalcrust.Detritalzirconsintheserockshaveagesthatrangeto3.5Gyr,spanningmostofEarth’shistory.ThetwoOrdoviciangreywackes(sandstonescontaining.15%clay)usedinthisstudyarefromtheeasternAustraliansegmentofGondwana,andweremetamorphosedinthegreenschistfaciesduringformationoftheOrdoviciantoDevonianLachlanfoldbelt.Wealsoanalysedtheinherited(pre-magmatic)zirconcoresoftwocordierite-bearinggraniteplutonsthatintruded430Myrago,andthoseofanequivalentvolcanicrock(SupplementaryTable1).Thegraniticmagmasweregeneratedfrombroadlymetasedimentaryprecursorsatmid-crustaldepths14,buttheirabundantinheritedzirconshavethesamerangeofages12,13,15andisotopecompositionsasdodetritalzirconsintheexposedmetasedimentarycountryrocks.Approximately100zirconsfromeachrockweremountedinresin,polishedtoexposetheircentresandimagedbycathodoluminescence.Oscillatoryorsectorzonationisconsistentwithamagmaticpara-genesisforthemajorityofthese,althoughfiveirregularlybandedgrainswithlowTh/Uratios(,0.10)mayrepresentmetamorphicgrowth.TheU–Pb,oxygenandLu–Hfisotopecompositionsofeachzirconwereacquiredbysequential,insituanalysisusingthreedifferentinstruments.ThecrystallizationageswerefirstdeterminedbySHRIMP(SensitiveHighResolutionIonMicroProbe)U–PbisotopeanalysisattheCurtinUniversityofTechnology,Perth,underroutineoperatingconditions1.Concordantgrains,orthosethatwere,10%discordantforU–Pbisotopes,werethenanalysedforoxygenisotopeswithaCamecaims1270ionmicroprobeattheUniversityofEdinburgh,theanalysissitebeingdirectlyadjacenttotheSHRIMPpit.Last,HfisotoperatiosweremeasuredfromthesamepartofthezirconattheUniversityofBristolwithaFinniganNeptunemulti-collectorinductivelycoupledplasma-massspectrometer(ICP-MS)and193-nmArFlaser(SupplementaryMethods).AlldataaregiveninSupplementaryTables2,3and4.Oxygenisotoperatiosareexpressedinthestandardd18Onotation,signifyingdeviationofthemeasured18O/16OvaluefromViennastandardmeanoceanwater(VSMOW)inpartsperthousand.Zirconsinequilibriumwithpristinemantle-derivedmeltshaved18Ovaluesof5.3^0.3‰(ref.16).Higherd18Ovaluesreflectacomponentof18O-enriched,supracrustalmaterialinthemagmafromwhichthezirconprecipitated,whichcouldincludesedimentaryrock(10–40‰)orhydrothermallyalteredoceaniccrust(10–20‰;ref.17).Oxygenisotoperatiosofigneousrocksaremostsensitivetosedimentaryinput,asshownbytheelevatedd18Oofmetasedimentary-derivedgranites(9–15‰;ref.18).Followingref.19,weinferthatzirconswithd18Ooflessthan6.5‰formedfrommeltsthatcontainedaminortonegligiblesedimentarycomponent.Zirconsofthiscompositionwouldcrystallizefrommaficmeltswithd18Ovaluesof,7‰,orsilicicmagmaswithd18Obelow8‰,assumingequilibriummelt/zirconfractionation20.Duringthisstudyweobservedabruptcontrastsind18Oofnearly6‰betweeninheritedzirconcoresandtheirthin(,50mm)magmaticover-growths.Thisaccordswiththeempiricallyestablishedlowdiffusionrateofoxygeninzirconunderhigh-temperatureconditions21,andsuggeststhatthemeasuredd18Oofinheritedzirconsapproximatestheprimaryvalue.The176Hf/177Hfdatafordetritalandinheritedzirconswithdifferentd18Ovaluesareplottedasafunctionofthei