Zinc isotopic behavior of mafic rocks during continental deep subduction

Abstract

Zinc isotopes may act as a new tool of tracking recycling of crustal materials that causes compositional heterogeneity of the mantle. This application relies on an investigation of Zn isotopic behaviors during slab subduction. In this study, we report Zn isotopic compositions for a suite of metabasalts (greenschists, amphibolites, and coesite-bearing eclogites) from the Dabie Orogen (China), which were formed via the subduction of mafic rocks into different depths and up to > 200 km. Three out of eight greenschists are characterized by lighter δ66ZnJMC-Lyon (0.10‰–0.16‰) than those of global basalts (0.28‰ ± 0.05‰), which may be caused by crustal assimilation of the protoliths by sedimentary rocks due to their extremely high 87Sr/86Sr (up to 0.7130) and low εNd values (down to −12.3). The remaining greenschists have relatively low 87Sr/86Sr and their δ66Zn values (0.21‰–0.38‰) overlap the ranges of amphibolites (0.18‰–0.32‰) and coesite-bearing eclogites (0.18‰–0.36‰). There is no correlation between δ66Zn and sensitive indicators of dehydration (Rb/TiO2, Ba/Yb, and H2O+), suggesting that no detectable Zn isotope fractionation has occurred during the deep subduction of mafic rocks even into > 200 km, which is attributed to the limited loss of Zn during prograde metamorphism and dehydration. Thus, Zn isotopic compositions of the deeply subducted mafic rocks are inherited from their protoliths. Considering that these metamorphosed rocks have higher δ66Zn than that of the mantle value by up to 0.2‰, the recycled/subducted mafic crust can incorporate isotopically heavy Zn into the mantle. The subducted slabs may partially melt and generate a metasomatized mantle, resulting in changes of Zn isotopic composition of the hybridized mantle as have been observed in some mantle xenoliths and basaltic lavas.