Calcium Isotope Fractionation in Multiple High-pressure Metamorphic Veins from the Dabie Orogen, China

To explore the behavior of Ca isotope fractionation in subduction-zone fluids, we report Ca isotopic compositions of multiple high-pressure (HP) metamorphic veins and their host ultrahigh-pressure (UHP) eclogites from Hualiangting in the Dabie orogen, China. The CaO contents of wall-rock eclogites gradually decrease whereas the δ^44/40Ca values increase toward the vein, implying that light Ca isotopes were preferentially released from the eclogites into the vein-forming fluid. The HP veins can be classified as omphacite-epidote, epidote-quartz, and kyanite-epidote-quartz veins from the vein-eclogite boundary to the vein interior, representing products of a single fluid that evolved at different stages in a relatively closed system. The precipitation processes of multiple veins were accompanied by decrease in the abundances of major Ca-bearing minerals (i.e. epidote and omphacite) and increase in the δ^44/40Ca values of the HP veins. These observations suggest significant Ca isotope fractionation during metamorphic fluid activity, with light Ca isotopes preferentially entered into epidote/omphacite relative to fluid during vein formation. Hence, Ca isotope could be a potentially useful tracer of fluid-rock interactions at subduction zones.