Subduction zone rocks oxidized by supercritical fluid: Constraints from an ultrahigh-pressure eclogite-vein system in the Dabie orogen, China

The role of subducting slab-derived supercritical fluids in transferring oxidized components in subduction zones remains poorly constrained. We investigated the oxidation state and effect of supercritical fluids by a combined study of petrology and geochemistry on an ultrahigh-pressure eclogite-vein system from the Dabie orogen, China. These samples record the composition and evolution of supercritical fluids at subarc depths. Multiphase crystal inclusions trapped within minerals in the vein encompass calcite, anhydrite, and magnetite, suggesting that the supercritical fluids contain such cations as S6+, C4+, and Fe3+. Whole-rock and mineral compositions of the eclogites vary systematically with distance from the vein, suggesting that the eclogites adjacent to the vein were metasomatized by vein-forming supercritical fluids. The garnet-clinopyroxene oxybarometric results indicate that the metasomatism of supercritical fluids resulted in the elevation of oxygen fugacity in the eclogites. The metasomatized eclogites exhibit decreases in Fe and S contents, Fe3+/Fetotal ratios, and modes of omphacite and pyrite. This observation suggests that the supercritical fluids dissolving omphacite and pyrite were accompanied by a Fe-S redox coupling process during fluid-rock reactions. This process further releases Fe3+ and S6+ from the eclogites to supercritical fluids. In combination with the mineral composition of the vein, it is inferred that sulfur was likely transported farther by the supercritical fluids. Therefore, some supercritical fluids were oxidizing at the subducting slab-mantle wedge interface and thus can oxidize the metasomatized rocks. Therefore, supercritical fluids are efficient carriers of oxidized components from the subducting slab to the mantle wedge, eventually contributing to the oxidization of arc magma sources.