→Sulphur speciation and the oxidation state of the mantle wedge

Recycling oceanic crust in subduction zones via fluids or melts through the overlying mantle and to the volcanoes on top of the lithosphere involves successive oxidation. The mantle wedge, which is the source region of all arc magmas, is known to be more oxidised relative to normal mantle or oceanic crust by one to two orders of magnitude (Brandon & Draper, 1996). The oxidation of the mantle wedge was previously attributed to an interaction with water released from the subducting slab, but a simple water addition does not provide the full oxidation mechanism, because water can only oxidise rocks if hydrogen is released as a reaction product. Other possible volatile agents, presumably affecting redox conditions, should be considered. For instance, the flux of sulphur from subducted slab can be responsible for the oxidation of the mantle wedge, because the speciation and solubility of S in magmas largely depend on its oxidation state. We have performed well constrained high-pressure and high-temperature experiments on iron-free melts with controlled oxygen fugacities (f O2)and measured the oxidation state of S in the quenched glasses using X-ray absorption near edge structure (XANES). Glass compositions include soda-lime-glass, K2Si4O9, albite and a trondhjemite-glass representing a residual liquid during partial melting of subducted oceanic crust. Experiments were performed in CSPV and IHPV at 850 and 1000◦C, 200 MPa and at f O2’s ranging from QFM-2.5 to QFM+4. The S6+/ΣS ratio in the glass is a function of f O2. At f O2> QFM+1.5 sulphur is dissolved as S6+. At f O2< QFM-1.