C-O-H-S fluids released by subducted serpentinite and the implications for arc-magma oxidation

Serpentinites are able to carry H2O, ferric iron, carbon and sulfur into subduction zones, where they stepwise released fluid during dehydration. These C-O-H-S fluids are intimately linked to magma oxidation and ore formation within magmatic arcs. However, the mechanism of the transfers of carbon, sulfur and ferric iron into fluids is poorly known. It is also controversially discussed if C-O-H-S fluids can oxidize arc magmas. Our new thermodynamic models focus on the species produced by C-O-H-S fluids during subduction. Closed system modeling provides molar concentrations of such species under different P-T conditions, showing that reducing species generally have high solubilities at low P-T conditions. Oxidizing species mainly remain in the fluid after increasing P-T conditions. Open system fluid fractionation shows that only 5-14% of carbon is transported to the mantle wedge, while the amount of sulfur loss is as high as 55-100% at sub-arc depths. Almost all lost carbon and sulfur are released as oxidizing species. The redox properties of the fluids are controlled by sulfur, resulting in an oxygen fugacity of the fluids being 0.6-0.95 log units higher than that of the HM buffer. Sulfur loss is a function of the whole-rock compositions and the subduction geothermal gradient, demonstrating elevated sulfur fluxes to be released from serpentinite in cold subduction zones compared to warm ones. Our modeling results are consistent with petrological observations and global-arc-basalt oxygen fugacity calculations and highlight that the C-O-H-S fluids released from the subducted slab can considerably contribute to the composition of arc magmas.