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.