Behavior of calcium isotopes during continental subduction recorded in meta-basaltic rocks

In order to investigate the behavior of Ca isotopes during subduction, we report high precision Ca isotopic data of a suit of well-characterized meta-basaltic rocks from the Dabie-Sulu orogen, including six greenschists, six amphibolites and seven eclogites. Except two samples that may have been affected by carbonation alteration, greenschists, amphibolites and eclogites yield comparable delta Ca-44/42 values ranging from 0.36 to 0.41 parts per thousand, 0.32 to 0.39 parts per thousand and 0.32 to 0.42 parts per thousand, respectively. Overall, Dabie-Sulu meta-basaltic rocks have an average delta Ca-44/42 of 0.37 +/- 0.06 parts per thousand (2SD, N = 17/19), comparable to terrestrial basalts previously reported (0.36 +/- 0.05 parts per thousand, 2SD; N = 34). Combined with no correlation of delta Ca-44/42 with H2O and Rb/TiO2, this study suggests that prograde metamorphism dehydration cannot significantly fractionate Ca isotopic compositions of meta-basaltic rocks. Given the comparable CaO contents in meta-basaltic rocks with increasing metamorphism grade, isotope fractionation may be limited by insignificant Ca loss during dehydration. Two heavy greenschists with delta Ca-44/42 of 0.46 parts per thousand and 0.49 parts per thousand also have high CO2 concentrations, indicating carbonation alteration that is evidenced by 0.04M HCl leaching experiments. The limited delta Ca-44/42 variation observed in the other metamorphic samples is most likely inherited from their igneous protoliths. Considering that terrestrial basalts have delta Ca-44/42 systematically lower than the upper mantle, subduction of mafic lithologies (e.g., oceanic slabs) thus could reflux light Ca isotopes back into the mantle and may create mantle heterogeneity. (C) 2019 Elsevier Ltd. All rights reserved.