The influence of undercooling and sector zoning on Clinopyroxene–Melt equilibrium and thermobarometry

Macdonald, Alice; Ubide, Teresa; Mollo, Silvio; Pontesilli, Alessio; Masotta, Matteo

doi:10.1093/petrology/egad074

Thermobarometryprovidesacriticalmeansofassessinglocationsofmagmastorageanddynamicsinthelead-uptovolcaniceruptionsandcrustalgrowth.Acommonapproachistoutilisemineralsthathavecompositionssensitivetochangesinpressureand/ortemperature,suchasclinopyroxene,whichisubiquitousinmafictointermediatemagmas.However,clinopyroxenethermobarometrymaycarrysignificantuncertaintyandrequireanappropriateequilibriummeltcomposition.Inaddition,thedegreeofmagmaundercooling(ΔT)affectsclinopyroxenecompositionandzoning,withcommonsectorzoningpotentiallyobfuscatingthermobarometryresults.Here,weuseasetofcrystallisationexperimentsonaprimitivetrachybasaltfromMt.Etna(Italy)atΔT=25–233◦C,P=400–800MPa,H2O=0–4wt%andfO2=NNO+2,withclinopyroxenecrystalsdefinedbyAl-richzones(prismsandskeletons)andAl-poorzones(hourglassandovergrowths)toassesscommonequilibriummodelsandthermobarometricapproaches.Underthestudiedconditions,ourdatasuggestthatthecommonlyappliedFe–Mgexchange(cpx-meltKdFe–Mg)isinsensitivetoincreasingΔTandmaynotbeareliableindicatorofequilibrium.ThecombineduseofDiHd(CaMgSi2O6+CaFeSi2O6)andEnFs(Mg2Si2O6+Fe2Si2O6)modelsindicatetheattainmentofequilibriuminbothAl-richandAl-poorzonesforalmostallinvestigatedΔT.Incontrast,CaTs(CaAl2SiO6)andCaTi(CaTiAl2O6)modelsrevealsubstantialdeviationsfromequilibriumwithincreasingΔT,particularlyinAl-richzones.WepostulatethatthisreflectsslowerdiffusionofAlandTiinthemeltcomparedwithCaandMgandrecommendtheconcurrentapplicationofthesefourmodelstoevaluateequilibriumbetweenclinopyroxeneandmelt,particularlyforsector-zonedcrystals. Thermobarometerscalibratedwithonlyisothermal–isobaricexperimentscloselyreproduceexperimentalP–TatlowΔT,equivalenttonaturalphenocrystscoresandsector-zonedmantles.ModelsthatalsoconsiderdecompressionexperimentsaremostaccurateathighΔTandarethereforesuitableforoutermostphenocrystrimsandgroundmassmicrolites.RecentmachinelearningapproachesreproduceP–TconditionsacrossallΔTconditions.Applyingourexperimentalconstraintstosector-zonedmicrophenocrystsandgroundmassmicroliteseruptedduringthe1974eccentriceruptionatMt.Etna,wehighlightthatbothhourglassandprismsectorsaresuitableforthermobarometry,giventhatequilibriumissufficientlytestedfor.ThecombinationofDiHd,EnFs,CaTsandCaTimodelsidentifiescompositionsclosesttoequilibriumwiththebulkmeltcomposition,andresultsinsmallerdifferencesinP–TcalculatedforhourglassandprismsectorscomparedwithapplyingonlyDiHdandEnFsequilibriummodels.Thisprovidesaframeworktoassesscrystallisationconditionsrecordedbysector-zonedclinopyroxenecrystalsinmaficalkalinesettings.

The influence of undercooling and sector zoning on Clinopyroxene–Melt equilibrium and thermobarometry / Macdonald, Alice; Ubide, Teresa; Mollo, Silvio; Pontesilli, Alessio; Masotta, Matteo. - In: JOURNAL OF PETROLOGY. - ISSN 0022-3530. - 64:10(2023), pp. 1-18. [10.1093/petrology/egad074]