Sulfur disproportionation in sub-arc COHS slab fluids drives mantle wedge oxidation

Sulfur degassed at volcanic arcs calls for dissolved S⁶⁺ released by subduction-zone fluids, oxidising (in association with C) the sub-arc mantle, but sulfur speciation in subduction fluids at subarc depths remains unclear. We apply electrolytic fluid thermodynamics to model the dissolution behaviour of pyrite in meta-carbonate sediments as a function of P, T and rock redox state up to 4.3 GPa and 730°C. At subarc depth and at the redox conditions of the fayalite-magnetite-quartz oxygen buffer, pyrite dissolution releases oxidised S in fluids by disproportionation into sulfate, bisulfite, and sulfide species. These findings indicate that oxidised, sulfur-rich COHS fluids form within subducting slabs at depths greater than 100 km independent from slab redox state and that sulfur can be more effective than the concomitantly dissolved carbon at oxidising the mantle wedge, especially when carbonates are stable in the mantle. Further open system modelling shows that such fluids are capable of oxidising the sub-arc mantle within a few million years.