Iron and magnesium isotopic compositions of subduction-zone fluids and implications for arc volcanism

High-pressure (HP) metamorphic veins in eclogites provide insights into the composition and evolution of fluids in subduction zones. We here present the first Fe-Mg isotope data for three types of HP veins, eclogites and their mineral separates from the Dabie Orogen to constrain the Fe-Mg isotopic compositions of subduction-zone fluids and the Fe-Mg isotope behaviors during fluid-rock interaction and fluid evolution. The HP veins include omphacite-epidote (Omp-Ep), epidote-quartz (Ep-Qtz), and kyanite-epidote-quartz (Ky-Ep-Qtz) veins. The Omp-Ep veins first crystallized from eclogite-derived, solute-rich vein-forming fluids with the Ep-Qtz and Ky-Ep-Qtz veins successively crystallizing from the residual fluids after the formation of the Omp-Ep veins. The early Omp-Ep veins have much heavier Fe-Mg isotopic compositions compared to the host eclogites, indicating Fe-Mg isotope fractionation during fluid-rock interaction due to preferential dissolution of isotopically heavy Omp and Ep from the eclogites into the vein-forming fluids. The δ56Fe and δ26Mg values of the Omp-Ep, Ep-Qtz, and Ky-Ep-Qtz veins gradually decrease and positively correlate with the Eu/Eu* values of whole rock and epidote. This indicates that Fe-Mg isotope fractionation during fluid evolution results from continuous crystallization of isotopically heavy Omp and Ep from the vein-forming fluids. Our results thus demonstrate that Fe-Mg isotopes can significantly fractionate during the dissolution-precipitation processes of minerals in subduction zones.