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The origin of sulfite (S 4+ ) species in silicate glasses was evaluated using XANES at the S K-edge. Systematic investigations show that the presence of S 4+ species in silicate glasses is an analytical artifact related to changes in the sulfur species caused by irradiation with an electron beam during EMPA or by irradiation with an intense focused X-ray beam during synchrotron analysis. The data shown here indicate that S 2and S 6+ are the only significant sulfur species occurring in silicate glasses synthesized under geologically relevant conditions.

/ P S with extent of S degassing at Erta Ale, indicating negligible effect on fO 2 , and further suggesting that H 2 S is the dominant gas species exsolved from the S 2À -rich melt (i.e., no redistribution of electrons). High SO 2 /H 2 S observed in Erta Ale gas emissions is due to gas re-equilibration at low pressure and fixed fO 2. Sulfur budget considerations indicate that the majority of S injected into the systems is emitted as gas, which is therefore representative of the magmatic S isotope composition. The composition of the Masaya gas plume (þ4.8%) cannot be explained by fractionation effects but rather reflects recycling of high 34 S oxidized sulfur through the subduction zone.

Subduction zone magmas are more oxidised on eruption than those at mid-ocean ridges. This is attributed either to oxidising components, derived from subducted lithosphere (slab) and added to the mantle wedge, or to oxidation processes occurring during magma ascent via differentiation. Here we provide direct evidence for contributions of oxidising slab agents to melts trapped in the sub-arc mantle. Measurements of sulfur (S) valence state in sub-arc mantle peridotites identify sulfate, both as crystalline anhydrite (CaSO4) and dissolved SO42− in spinel-hosted glass (formerly melt) inclusions. Copper-rich sulfide precipitates in the inclusions and increased Fe3+/∑Fe in spinel record a S6+–Fe2+ redox coupling during melt percolation through the sub-arc mantle. Sulfate-rich glass inclusions exhibit high U/Th, Pb/Ce, Sr/Nd and δ34S (+ 7 to + 11‰), indicating the involvement of dehydration products of serpentinised slab rocks in their parental melt sources. These observations provide a link between liberated slab components and oxidised arc magmas.