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

Abstract

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 δ44/42Ca values ranging from 0.36 to 0.41‰, 0.32 to 0.39‰ and 0.32 to 0.42‰, respectively. Overall, Dabie-Sulu meta-basaltic rocks have an average δ44/42Ca of 0.37 ± 0.06‰ (2SD, N = 17/19), comparable to terrestrial basalts previously reported (0.36 ± 0.05‰, 2SD; N = 34). Combined with no correlation of δ44/42Ca 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 δ44/42Ca of 0.46‰ and 0.49‰ also have high CO2 concentrations, indicating carbonation alteration that is evidenced by 0.04M HCl leaching experiments. The limited δ44/42Ca variation observed in the other metamorphic samples is most likely inherited from their igneous protoliths. Considering that terrestrial basalts have δ44/42Ca 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.