Phase equilibrium constraints on the pre-eruptive conditions of alkaline basalts of the Main Ethiopian Rift and their bearing on the production of peralkaline rhyolites

BimodalmagmatismischaracteristicofthegeodynamicevolutionoftheMainEthiopianRift(MER),whichisareferenceareaforthestudyoftheprocessesleadingtocontinentalbreak-upbeforeseafloorspreading.Thereareabundantemissionsofbasaltsandrhyolites,whichareinpossibleparent–daughterrelationships.However,theP–T–H2Oconditionsofproductionandstorageofthebasalticendmemberremainunclear.CrystallizationexperimentshavebeenconductedonanalkalibasaltfromtheMERtodefineitspre-eruptiveconditionsandshedlightonthecompositionalevolutionofderivativeliquidsandsourceconditions.Theexperimentswereperformedat100to200MPa,975◦Cto1080◦C,varyingH2O/CO2ratios,correspondingtomeltwatercontentsof1to5wt%,atfO2slightlylowerthantheFayalite-Magnetite-Quartz(FMQ)solidbuffer.Comparisonbetweenthepetrologicalattributesofthestartingrockandtheexperimentsshowsthatthebasalticmagmawasstoredat150to200MPa,1050±10◦C,with1to2wt%H2Oinmelt,withfO2nearFMQpriortoeruption.Geochemicalmodellingshowsthatthecorrespondingmantlesourcecontainedabout0.1wt%H2O,reflectingametasomatizedsource.ExtensivecrystallizationofsuchbasaltsproducesSiO2-richliquids,whicharenotyetperalkaline,however.Thisunderscoresthatextremefractionation(>90wt%)isrequiredinordertoproduceperalkalinederivativesfrommildlyalkalinebasalts.Thisextremefractionationandthewater-richnatureofthestartingbasaltreadilyexplaintheH2O-richconditionofperalkalinerhyolitesthathavefueledcalderaformingeruptionsintheRift.