| Significant Ca isotopic fractionation(?44/42 Ca ~ 2‰)has been found among natural samples on Earth.Thus Ca isotope has been widely used in tracing various kinds of geological progresses.However,it's difficult to obtain high-precision Ca isotopic data due to the large abundance difference,obvious relative mass range and large isotopic fractionation during analysis.Small bias on standard materials have been noticed in published data from different labs.Furthermore,most of the studies about Ca isotope focused on low-temperature processes.Limited studies were related to high-temperature processes,and most of them focused on mafic-ultramafic rocks.Few work were conducted on the fractionation mechanism of intermediate to felsic rocks during magmatic differentiation and crustal melting.A high-precision analysis method has been established in this study.A set of well-characterized granitoids were analyzed to further constrain the Ca isotope fractionation behavior during magmatic differentiation and crustal melting.Regarding current issues in Ca isotope analysis such as calibration of chemical separation and heterogeneous evaporation,determination of the double spike isotopic composition,and data deduction,we build an analysis method of Ca isotopic composition on a Triton Plus thermal ionization mass spectrometry(TIMS).Normal Ca isotopic composition and stable Ca isotopic composition measured with 43Ca–48Ca double-spike of thirteen reference materials,including carbonates(SRM915a and915b),Atlantic seawater and ten kinds of rocks ranging from peridotite to sandstone,are reported in this study.Analytical error of ?44/42 Ca and ?40/44 Ca is lower than ±0.03‰ and ± 1.0 with eight times repeat measurement,respectively.All the reference materials yield identical ?43/44 Ca and ?48/44 Ca within ± 0.6,supporting the hypothesis that terrestrial samples were fractionated from a single Ca isotopic composition.The variation of ?40/44 Ca may reflect different enrichment of radiogenic 40 Ca.The measurements of SRM 915 b and Atlantic seawater yield ?44/42 Ca values of 0.357 ±0.011‰(2SE,n = 41)and 0.907 ± 0.007‰(2SE,n = 147)relative to SRM 915 a,respectively.Ca isotopic compositions of ten geological reference materials are consistent with published data within analysis error.?44/42 Ca of nine igneous rocks ranges from 0.265 ± 0.014‰(2SE)to 0.536 ± 0.009‰(2SE),indicating obvious Ca isotopic fractionation during high-temperature processes.Eight non-adakitic granitoids and nine low-Mg adakitic granitoids from the Dabie Orogen are analyzed to constrain Ca isotopic fractionation behavior during intermediate to felsic magmatic processes.With a variable content of SiO2 and CaO,non-adakitic granitoids yield a homogeneous Ca isotopic composition(0.37 ± 0.03‰,2SD,n = 8).This can be explained by the opposite Ca isotopic effect of hornblende and plagioclase of melt during crystallization differentiation.The low-Mg adakitic granitoids have a large Ca isotopic composition range(0.15‰),but their limited CaO content suggest they experienced insignificant crystallization differentiation.The low-Mg adakitic granitoids are derived from partial melting of a thickened lower continental crust with substantial garnet and clinopyroxene in their residue.Caisotopic composition of garnet is considered to be heavier than clinopyroxene due to its relatively short Ca–O bond length.The good correlation between ?44/42 Ca and(Dy/Yb)N indicates that the variation of Ca isotopic composition among low-Mg adakitic granitoids is possibly owning to the effect of garnet in residual phase during crustal melting process.In addition,since granitoids are important parts of upper crust and Ca isotopic fractionation of non-adakitic granitoids is insignificant during magmatic differentiation and crustal melting processes,Ca isotopic composition of upper crust can be estimated to be 0.36 ± 0.01(2SE,n = 19),a weighted mean value of non-adakitic granitoids in this study and published intermediate to felsic geological reference materials. |
PDF Full Text Request