Calcium Isotope Fractionation During Magmatic Processes

Calcium is the fifth most abundant element and the most abundant alkaline earth metal in the Earth's crust,which is widely distributed in all the Earth Spheres.At present,researches on Ca isotopes in Earth sciences are mainly focused on the low-temperature processes on the Earth's surface.Limited by the smaller isotope fractionation,Ca isotope fractionation during many high-temperature processes remains to be investigated.In this study,typical samples were investigated to constrain a serious of fundamental issues of Ca isotope systematics of high temperature Earth,including Ca isotope fractionation between Ca-rich minerals(i.e.,garnet and clinopyroxene),the mean Ca isotopic composition of the upper mantle,fractionation during mantle partial melting and mantle heterogeneity.Garnet and clinopyroxene separates of eclogites and garnet peridotites from the Dabie-Sulu orogen were analyzed for high-precision Ca isotopic compositions.Garnet has Ca isotopic compositions heavier than its co-existing clinopyroxene,with?44/42Cagrt-cpx varying from 0.06±0.03‰ to 0.38±0.04‰.Combined with previous Fe-Mg-O isotopic data and EMPA results on these minerals,?44/42Cagrt-cpx could have recorded equilibrium fractionation during crystallization of these two minerals during prograde metamorphism.The coordination number of Ca2+in garnet is the same as in clinopyroxene.Garnet enriches heavy Ca isotopes due to its shorter Ca-O bond length.?44/12Cagrt-cpx is negatively correlated with the jadeite proportion in clinopyroxene,reflecting a mineral compositional effect of clinopyroxene.Combined with previous work on the fractionation factor between Ca-rich clinopyroxene and melt,the Ca isotope fractionation between clinopyroxene and melt under low pressure is likely close to 0.As the pressure rise,both the garnet proportion in the residue and jadeite proportion in residual clinopyroxene increase,and accordingly,fractionation between the residue and melt is expected to increases significantly.Ca isotopes thus may be used as a potential tool to trace the derivation depth of magmas.A collection of non-metasomatized mantle peridotites from different regions around the world reveals that Ca isotopic compositions of peridotite residue become heavier with increasing extent of melt extraction.Significant Ca isotopic difference has not been identified between garnet-peridotites and spinel-peridotites.Based on the correlations of Ca isotopic compositions of these peridotite samples with indices reflecting the extent of melt extraction(CaO,MgO and Al2O3),mean ?44/40Ca and?44/42Ca of the upper mantle are estimated to be 0.90±0.03‰ and 0.42±0.02‰(2SE),respectively.?40/44Ca of the upper mantle is-0.41±0.45 ?,indicating that the upper mantle is slightly depleted in radiogenic 40Ca relative to NIST SRM 915a.Mantle peridotite xenoliths metasomatized by various components reveal significant decrease in ?48/22Ca by both silicate melt and carbonate melt metasomatism,yielding?44/42Ca variations of 0.14±0.05‰?0.44±0.06‰ and 0.16±0.05‰?0.322±0.03‰,respectively.Silicate melts and carbonate melts have different effects on the major element composition of metasomatized samples,which provides a potential identification for these two types of metasomatism.In order to further constrain Ca isotope fractionation during mantle partial melting,six MORB samples from Pacific and Atlantic mid-oceanic ridges were investigated.Theses samples have homogenous Ca isotopic compositions with a mean ?44/42Ca of 0.42±0.03‰(2SD,N=6),similar to that of the upper mantle.This observation indicates that Ca isotope fractionation between the melt and mantle source is very limited during mantle melting in the ridges,likely due to Ca-rich clinopyroxene as the dominant Ca-carrier in the residue.Measurements on oceanic island basalts from four hotspots,including the Pitcairn island,Pacific as typical EM1 end-member samples,the Canary island basalts in Atlantic representing a typical HIMU end-member,the La Reunion island basalts in Indian Ocean and the Hawaii island basalts,reveal a significant ?44/42Ca variation(0.34±0.03‰?0.42±0.03‰).Their ?44/42Ca are systematically lower than the upper mantle and MORB,likely indicating recycling of crustal materials.The mean ?44/42Ca of Pitcairn OIBs is 0.35±0.03‰(2SD,N=6),which is lower than the other samples.Limited by the current analysis precision,it is,however,hard to distinguish whether the metasomatizing component was subducted oceanic crust or recycled carbonates.Therefore,using Ca isotopes to trace deep carbonate recycling requires development of ultra-high-precision analysis methods as well as more researches to identify the Ca isotopic effects of recycled carbonate and non-carbonate components.The Ca isotope behavior of key minerals is important to explain the Ca isotope fractionation during magmatic processes.At high melting pressures,heavy Ca isotopes are enriched in garnet and jadeite-rich clinopyroxene relative to the melt,and the melt thus has a light Ca isotopic composition.The Ca isotopic composition of the mantle peridotite partial melts is homogenous due to the very limited fractionation between Ca-rich clinopyroxene and melts at low pressures.The Ca isotopic compositions of the mantle peridotites and different types of OIBs are systematically lower than the upper mantle,with a significant variation range,reflecting the heterogeneity of the mantle on Ca isotopes.When using Ca isotopes to trace the crust-mantle interaction,the influence of magma process on the Ca isotopic composition of recycled materials should be carefully considered.