Sulfur recycling in subduction zones and the oxygen fugacity of mafic arc magmas

Abstract

Sulfur is important to a range of subduction zone processes, but the factors controlling the sulfur content of primitive arc magmas are poorly understood. In particular, uncertainties about the oxidation state of primary melts in the sub-arc mantle hinder efforts to understand the behavior of sulfur in arc magmas. Here, we use olivine-hosted melt inclusion data from 32 arc segments globally to characterize sulfur contents of magmas from a variety of subduction zones and identify the key processes that control the sulfur content of arc magmas. Average primary magma sulfur contents estimated from these data range from 466±220ppm (Mariana Forearc) to 4,264±819ppm (Ecuador). Primitive and more evolved magmas in both hot- and cold-slab subduction zones commonly have higher sulfur contents than mid-ocean ridge basalt (MORB) magmas. In most cases, these elevated sulfur contents require a subduction-modified mantle source with more sulfur than MORB-source mantle. Correlations between magma S and Cl concentrations and S/Dy and Th/Yb ratios in the global data set confirm that sulfur is partially sourced from the subducting slab. By comparing melt inclusion sulfur contents to sulfur solubility limits imposed by sulfide saturation, we find that 88% of arc magmas in our compilation require conditions more oxidizing than the quartz-fayalite-magnetite buffer (QFM). The relationship between S/Dy and calculated magmatic oxygen fugacity (fO2) suggests that slab-derived sulfate is responsible for the higher oxidation state of arc magmas compared to MORB. Model calculations of redox equilibrium between S and Fe species in basaltic melts when sulfate is added to the mantle wedge demonstrate the feasibility of this causal link, which can explain the common range of arc magma fO2 values observed for subduction zones globally. Correlations between magma sulfur contents, fO2, and the proportion of added slab-derived material inferred from trace elements can drive associations between high Sr/Y magmas and fertile porphyry ore deposits, as well as the MORB-like Cu contents of arc magmas.