AnExperimentalStudyonKinetics-ControlledCa-CarbonateAqueousReductionintoCH4(1and2GPa,550◦C):ImplicationsforCMobilityinSubductionZones

 Abioticmethane(CH4)generationundersubductionzoneconditionshasbeenexperimentallyinvestigatedthroughaqueousreductionofpureC-bearingmaterials(e.g.carbonatemineralsandorganicmatter).However,quantitativeassessmentsofCH4productionintheseexperiments,aswellasthepotentialeffectsofothercomponentssuchasSiO2onthereductionprocesses,havenotyetbeenwellestablished.Here,weperformedexperimentstoquantitativelyevaluatethetime-resolvedCa-carbonateaqueousreductionintoCH4atP=1and2GPaandT=550◦CintheCaO+COH,CaO+SiO2+COH,andCaO+SiO2+MgO+COHsystems,employingcalcite+water±quartz±serpentine(syntheticchlorine(Cl)-bearingchrysotileandnaturalFe−Al-bearingantigorite)asstartingmaterials.Redoxconditionsoftheexperimentswerebufferedbyiron−wüstite(IW)usingadoublecapsulesetting,correspondingtooxygenfugacity(fO2)values(expressedaslogunitsrelativetothefayalite–magnetite–quartzbuffer,FMQ)intheinnercapsuleofFMQ≈−5.5at1GPaandFMQ≈−6.0at2GPa.Thesolidproductsaremainlycomposedofportlandite±larnite±wollastonite±brucite,whileCa-carbonateand/orsilicatereactantscommonlyoccurasrelicts.QuadrupolemassspectrometricanalysisshowsthatCH4andH2OarethemajorCOHmolecularspeciesinthefluidproducts,withmolarratiosbetweenCH4andstartingcalciterepresentingthereactionprogressrangingfrom∼0.13to∼1.00.Comparisonsofexperimentalrunproductswiththermodynamicallypredictedphaseassemblages,togetherwithtime-seriesexperiments,indicatethatthereductionprocessesareprimarilycontrolledbyreactionkinetics.At1GPaand550◦C,rateconstantsof4.0×10−6s−1,7.4×10−6s−1,and2.6×10−6s−1wereretrievedforreactionsstartingwithcalcite+quartz+water,calcite+syntheticCl-bearingchrysotile+water,andcalcite+naturalFe−Al-bearingantigorite+water,respectively,significantlyhigherthantheconstantof0.8×10−6s−1forthesilicate-absentreaction.Besides,anincreaseinpressurescanalsoenhancethereductionefficiencyofCa-carbonatesuntilreachingequilibriumwiththefluids.Ourdataprovideexperimentalevidenceforkinetics-controlledCa-carbonateaqueousreductionintoCH4insubductionzones,indicatingthatsilicateinvolvementand/orpressureincreasecanacceleratethereactionratesthroughshort-livedfluid–rockinteractions,whichmayhaveimportantimplicationsfordeepCmobility.