Magnesium isotopic systematics of mafic rocks during continental subduction

Magnesium isotopic compositions of a set of prograde metamorphosed rocks (greenschists, amphibolites, and eclogites) from East China were measured in order to understand the behavior of Mg isotopes during continental subduction. The delta Mg-26 values of the greenschists vary from -0.269 +/- 0.057 parts per thousand to -0.133 +/- 0.042 parts per thousand (2SD), and correlate negatively with the MgO contents and Nb/U ratios, possibly due to the assimilation of isotopicallly heavy felsic schists (delta Mg-26 up to -0.099 parts per thousand) during the genesis of MORB-like protoliths. The two subgroups of amphibolites have slightly different Mg isotopic compositions. The group I amphibolites with cumulate origin have delta Mg-26 values ranging from -0.243 +/- 0.061 parts per thousand to -0.192 +/- 0.050 parts per thousand, which reflects clinopyroxene accumulation nature of the protolith. The group II amphibolites have relatively light delta Mg-26 values varying from -0.358 +/- 0.061 parts per thousand to -0.224 +/- 0.056 parts per thousand, which may result from either source contamination or crustal assimilation of isotopically light components (e.g., carbonates). Eclogitic garnet and clinopyroxene display large inter-mineral Mg isotope fractionations (Delta Mg-26(Cpx-Grt) = 1.097 +/- 0.056 parts per thousand to 1.645 +/- 0.081 parts per thousand) equilibrated at peak metamorphic temperatures. The delta Mg-26 values of the eclogites vary from -0.348 +/- 0.041 parts per thousand to -0.137 +/- 0.063 parts per thousand, overlapping the ranges of greenschists and amphibolites. Such Mg isotopic variations are not related to unrepresentative sampling nor host-eclogite chemical interactions, instead, they reflect the protolith heterogeneities. Collectively, the similarity of Mg isotopic compositions among the greenschists (-0.196 +/- 0.044 parts per thousand), amphibolites (-0.271 +/- 0.042 parts per thousand), and eclogites (-0.226 +/- 0.044 parts per thousand), suggests that Mg isotope fractionation during continental subduction, if any, is limited. Published by Elsevier Ltd.