Sulfur abundance and its speciation in oxidized alkaline melts

Abstract

The sulfur concentrations and the relative proportions of S2− and S6+ were measured by electron microprobe in a series of melt inclusions trapped in phenocrysts from different subduction-related and within-plate volcanoes. The melt inclusions correspond to potassic and shoshonitic primary melts to tholeiitic and hawaiitic primitive melts. In the tholeiitic and the transitional basaltic melt inclusions, sulfur is mainly present as S2− (S6+Stotal = 0.03 to 0.08), and varies from 0.13 to 0.18 wt%. The occurrence of immiscible sulfides attests to their saturation. In shoshonitic and potassic primary melts, sulfur (S = 0.12 to 0.32 wt%) is dissolved as both S2− and S6+ (S6+Stotal = 0.3 to 0.7). Their oxygen fugacity, estimated from the S6+Stotal ratios, ranges from NiNiO to NiNiO + 1 log unit. Hawaiitic melts may also dissolve up to 0.3 wt% sulfur possibly because of their oxidation state close to NiNiO, as illustrated by samples from Mt. Etna, Italy.Variations of sulfur, at constant S6+Stotal ratio and temperature in both the potassic and shoshonitic primary melts, indicate that these melts are undersaturated, with respect to a S-rich condensed phase, in agreement with their relatively high oxidation state. It strongly suggests that sulfur behavior in relatively oxidized primary alkaline melts is controlled by the mantle source melting conditions.