C-O-H-S fluids released by oceanic serpentinite in subduction zones: Implications for arc-magma oxidation

Subducted oceanic serpentinite carries H2O, ferric iron, carbon and sulfur into the subduction zone, where they are stepwise released during dehydration. These C-O-H-S fluids are intimately linked to magma oxidation and ore formation within magmatic arcs. However, the mechanism of transfer of carbon, sulfur and ferric iron into fluids is poorly known. It is also controversial whether the C-O-H-S fluids can oxidize arc magmas. We present new thermodynamic models for serpentinite to predict the species produced in C-O-H-S fluids during subduction. Closed system modeling provides molar concentrations of species at different P-T conditions, and shows that reducing species generally have high solubilities at low P-T conditions. Oxidizing species mainly partition into the fluid at higher P-T conditions. The concentration of iron in fluids is much lower than that of other major elements. Open-system fluid fractionation shows that only 5-14% of the carbon is lost to the mantle wedge, while the loss of sulfur is as high as 55-100% at sub-arc depths. Almost all carbon and sulfur lost are released as oxidizing species. The redox properties of the fluids are controlled by sulfur, resulting in an oxygen fugacity of the fluids that is 0.6-0.95 log units higher than the HM buffer. Sulfur loss is a function of the whole-rock composition (e.g., Mg/Si, Al, Ca, and especially Fe3+/Fetot ratio) and the geothermal gradient of the subduction zone, with elevated sulfur fluxes predicted for serpentinite in cold subduction zones. The maximum amount of sulfur released in a cold subduction regime is 5.5 times higher than in a warm subduction regime. 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 may contribute considerably to the composition of arc magmas.