The distribution of H2O between silicate melt and nominally anhydrousperidotiteandtheonsetofhydrousmelting in thedeepuppermantle - CNRS - Centre national de la recherche scientifique Accéder directement au contenu
Article Dans Une Revue Earth and Planetary Science Letters Année : 2014

The distribution of H2O between silicate melt and nominally anhydrousperidotiteandtheonsetofhydrousmelting in thedeepuppermantle

Résumé

The partitioningofH2O betweenamantleperidotiteassemblageandlowdegreehydrousmelthas beeninvestigatedat6GPa(correspondingto ∼180kmdepth)atatemperatureof1400 ◦C. Peridotite mineralphaseswereanalysedfrom6meltingexperimentsperformedinanaturalchemicalsystem.The experimentscontained ∼80wt%ofalowdegreehydrousmeltthatwasobtainedthroughaseriesof experimentswherethemeltcompositionwasiterativelyadjusteduntilsaturationwiththeappropriate peridotiteassemblagewasachieved.Themeltisfluid-undersaturatedattheconditionsoftheexperiment. Ion microprobemeasurementsofthemineralphasesindicateolivineH2O concentrationsof434 ± 61 ppmwtandaverageclinopyroxene(cpx)concentrationsof1268 ± 173 ppmwtH2O. Orthopyroxene (opx) andgarnetcontain700 ± 46 ppmwtand347 ± 83 ppmwtH2O, respectively.TheH2O contentof the hydrousmeltswasdeterminedbymassbalancetobe11 ± 0.5 wt%H2O. H2O partitioncoefficients betweenmineralsandmelt(Dmin/melt H2O = Xmin H2O/Xmelt H2O ) are0.0040±0.0006 forolivine,0.0064±0.0004 for opx, 0.0115±0.0016 forcpxand0.0032±0.0008 forgarnet. Using thedeterminedH2O partitioncoefficientstheonsetandextentofmeltingatconditionsequivalent to180kmbelowmid-oceanridgeswasdeterminedasafunctionofmantleH2O content.Current estimatesfortheH2O contentofthedepletedmantle(50-200ppmwtH2O) areinsufficienttoinduce mantle meltingatthisdepth,whichrequires ∼700 ppmwtH2O toproduce0.1%meltingand1600 ppmwtH2O for1%melting,alonganadiabatwithapotentialtemperatureof1327 ◦C. Meltingcanoccur at theseconditionswithinthemantlesourceofoceanislandbasalts,whichareestimatedtocontainup to900ppmwtH2O. Ifadiabatictemperaturesare200 ◦C higherwithinsuchplumerelatedsources,then melt fractionsofover1%canbereachedat180kmdepth.Inaddition,amodelforthedistributionof H2O betweenperidotitemineralphasesasafunctionofdepthandatH2O-undersaturatedconditions is constructed.Themodelindicatesthatforafixedmantlecompositioncontaining150ppmwtH2O, the olivineH2O contentwillincreasewithdepthsolelyduetochangesininter-phasepartitioningand modal proportionsofminerals.ThechangeintheolivineH2O concentrationwithdepthcorrespondsto proposedchangesinthedominantolivineslipsystemfordeformationbydislocationcreep,thatmight provideanexplanationforthereductioninseismicanisotropyobservedatdepths >200 km.
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Dates et versions

hal-00999398 , version 1 (03-06-2014)

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Davide Novella, Daniel J.Frost, Erik H.Hauri, Helene Bureau, Caroline Raepsaet, et al.. The distribution of H2O between silicate melt and nominally anhydrousperidotiteandtheonsetofhydrousmelting in thedeepuppermantle. Earth and Planetary Science Letters, 2014, 400, pp.1-13. ⟨10.1016/j.epsl.2014.05.006⟩. ⟨hal-00999398⟩
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