The H2O content of granite embryos

Omar Bartoli (1) , Bernardo Cesare (1) , Laurent Remusat (2) , Antonio Acosta-Vigil (3) , Stefano Poli (4)

1 Dipartimento di Geoscienze [Padova]
2 LMCM - Laboratoire de Minéralogie et Cosmochimie du Muséum
3 IACT - Instituto Andaluz de Ciencias de la Tierra
4 Dipartimento di ScienzedellaTerra,Università di Milano

QuantificationofH2Ocontentsofnaturalgraniteshasbeenanon-goingchallengeowingtotheextremelyfugitivecharacterofH2Oduringcoolingandascentofmeltsandmagmas.Hereweapproachthisproblembystudyinggranitesintheirsourceregion(i.e.thepartiallymeltedcontinentalcrust)andwepresentthefirstNanoSIMSanalysesofanatecticmeltinclusions(MI)hostedinperitecticphasesofmigmatitesandgranulites.TheseMIwhichtotallycrystallizeduponslowcoolingrepresenttheembryosoftheupper-crustalgranites.TheapproachbasedonthecombinationofMIandNanoSIMShasbeenheretestedonamphibolite-faciesmigmatitesatRonda(SSpain)thatunderwentfluid-presenttofluid-absentmeltingat∼700◦Cand∼5kbar.Small( 5μm)crystallizedMItrappedingarnethavebeenremeltedusingapiston-cylinderapparatusandtheyshowleucograniticcompositions.WemeasurehighandvariableH2Ocontents(meanof6.5±1.4wt%)intheselow-temperature,low-pressuregraniticmelts.Wedemonstratethat,whentheentirepopulationfromthesamehostisconsidered,MIrevealtheH2Ocontentofmeltinthespecificvolumeofrockwherethehostgarnetgrew.MeanH2OvaluesfortheMIindifferenthostcrystalsrangefrom5.4to9.1wt%.ThisrangeisinrathergoodagreementwithexperimentalmodelsforgraniticmeltsattheinferredP-Tconditions.OurstudydocumentsforthefirsttimetheoccurrenceofH2Oheterogeneitiesinnaturalgraniticmeltsatthesourceregion.Theseheterogeneitiesareinterpretedtoreflectthebirthofgraniticmeltsunderconditionsof"mosaic"equilibrium,wherethedistinctfractionsofmeltexperiencedifferentbufferingassemblagesatthemicro-scale,withconcomitantdifferencesinmeltH2Ocontent.Theseresultsconfirmtheneedforsmall-scalegeochemicalstudiesonnaturalsamplestoimproveourquantitativeunderstandingofcrustalmeltingandgraniteformation.ThesameapproachadoptedherecanbeappliedtoMIhostedinhigher-temperature,granulite-faciesrocksthatrepresenttheparentsofmanyupper-crustalgranites.Thiswillresultinabetterunderstandingofformationandevolutionofgraniticmagmas.

Mots clés: meltinclusions NanoSIMS granite H2O content; crustal melting; peritectic garnet; granite embryos

Domaines: Minéralogie

Omar Bartoli, Bernardo Cesare, Laurent Remusat, Antonio Acosta-Vigil, Stefano Poli. The H2O content of granite embryos. Earth and Planetary Science Letters, 2014, 395, pp.281-290. ⟨10.1016/j.epsl.2014.03.031⟩. ⟨hal-01001959⟩