Calcium Isotope Fractionation In Earth's Mantle

Calcium is one of the major rock-forming elements in mantle and crust.It is also an essential element for life,which is the structural foundation for a variety of organisms.From 1990s,Ca isotopes began to attract lots of attention from geochemists.According to the Ca isotope data in previous studies,near 4‰ fractionation has been documented in terrestrial samples,which suggest the potential of Ca isotope as a proxy for a number of geological processes.Previous Ca isotopic studies have focused mostly on modern and ancient marine carbonates and sulphates,documenting large and systematic isotopic variations,which indicate a potential of Ca isotope as a tracer for crustal material recycling.However,the Ca isotope composition of deep earth is not clear and detailed work on igneous rocks is fairly limited.Many critical issues like the Ca isotopic composition of mantle and the behavior of Ca isotope during high temperature geological processes are still poorly understood.Currently,several questions are waiting to be answered:(1)the inter-mineral Ca isotopic fractionation mechanism among mantle minerals(clinopyroxene(cpx),orthopyroxene(opx),garnet(grt),olivine(ol))is not clear;(2)the Ca isotopic composition of bulk silicate earth(BSE)is poorly constrained;(3)the Ca isotopic composition of pyroxenite and eclogite,which also are important components of mantle lithology,have never been investigated;(4)it is also not clear whether mantle partial melting and mantle metasomatism would cause Ca isotope fractionation.Here,we selected a series of typical mantle xenoliths including peridotite,pyroxenite and eclogite from worldwide localities.Their Ca isotopic composition of bulk rock and mineral separates were analyzed in order to contribute new understanding on these problems.In order to better understand the inter-mineral Ca isotope fractionation mechanism,we analyzed the Ca isotope compositions of pyroxene pairs and olivine in peridotite xenoliths sampled from Hannuoba(Hebei province,north China),Lianshan(Jiangsu province,south China),and Penglai(Hainan province,south China).Our results suggest that the ?44/40Ca in these minerals follow the order of opx?ol>cpx and the opx from different samples show>1‰ variations in ?44/40Ca.The ?44/40Caopx-cpx(544/40Caopx-?44/40Cacpx)display a large variation from-0.01 ‰ to 1.11‰,which is correlated with both of the composition of cpx and opx(e.g.,Ca/(Ca+Mg+Fe),molar ratio).Based on the theoretical interpretation from first principle calculations,the equilibrium fractionation factor between opx and cpx should be in the range of 0.0‰ to 0.4‰ under the temperature of 800 ? to 1200 ?.And the compositional dependent inter-mineral fractionation should reflect kinetic effect.Mantle processes like rapid cooling,partial melting and mantle metasomatism may cause disequilibrium in mantle minerals.In order to achieve new equilibrium state,Ca will redistribute between cpx and opx.Since light isotopes travel faster than heavier isotopes,isotopic fractionation would occur and therefore show dependence with Ca content in both of cpx and opx.We also investigated the Ca isotopic composition of Siberian eclogite xenoliths as well as their mineral separates.Our results show that grt in eclogites are 0.2-0.3‰isotopic heavier than cpx.Such isotopic effect may reflect the equilibrium fractionation which controlled by mineral structure.In order to better constrain the Ca isotopic composition of bulk silicate earth,a suite of well-studied peridotite xenoliths from Tariat,Mongolia were studied.These samples have fertile major element compositions and nearly flat or slightly LREE-depleted patterns which suggest that they experienced no or little modifications by partial melting and mantle metasomatism and thus are appropriate to estimate the Ca isotope composition of BSE.We estimated the ?44/40Ca of BSE to be 0.94 ± 0.05‰(2SD)as a mean of average(3-4 duplicates)analyses of these Tariat lherzolites.Partial melting is one of the most important mantle processes.The effects of melt extraction on the Ca isotopic composition of melting residues are examined by comparing fertile peridotite with moderately to highly melt-depleted,unmetasomatized peridotites.The melt-depleted peridotites show?0.1‰ isotopically heavier than the fertile peridotite,which most likely reflect the effect of partial melting.Cpx(CaO?20.0 wt%)have lighter Ca isotopic composition than coexisting olivine(CaO<0.1 wt%),opx(CaO<1.5 wt%),and garnet(CaO?3-5 wt%).Because modal abundances of cpx decrease whereas those of olivine and opx increase in residues at higher melting degree,the residues may be progressively enriched in heavy Ca isotopes with the exhaustion of cpx(>25%melting degree).Mantle metasomatism is a complex phenomenon taking place in melt-depleted residues due to reaction with migrating melts or fluids.The highly metasomatized peridotite xenoliths from Siberian craton show significantly lighter Ca isotopic composition,with ?44/40Ca 0.7‰ lower than the BSE value.Their 844/40Ca also show correlations with metasomatism indicators like Ce/Eu,which sugget the effect of mantle metasomatism on Ca isotopic composition.We argue that such signature may reflect the low ?44/40Ca in metasomatic media(silicate or carbonate melt)or the kinetic effect of melt percolation in the mantle.Our results suggest that mantle metasomatism is the major reason for the Ca isotopic mantle heterogeneity.