SULFUR ABUNDANCE AND ITS SPECIATION IN OXIDIZED ALKALINE MELTS

The sulfur concentrations and the relative proportions of S2- and S6were measured by electron microprobe in a series of melt inclusions tr apped in phenocrysts from different subduction-related and within-plat e volcanoes. The melt inclusions correspond to potassic and shoshoniti c primary melts to tholeiitic and hawaiitic primitive melts. In the th oleiitic and the transitional basaltic melt inclusions, sulfur is main ly present as S2- (S6+/S-total = 0.03 to 0.08), and varies from 0.13 t o 0.18 wt%. The occurrence of immiscible sulfides attests to their sat uration. In shoshonitic and potassic primary melts, sulfur (S = 0.12 t o 0.32 wt%) is dissolved as both S2- and S6+ (S6+/S-total = 0.3 to 0.7 ). Their oxygen fugacity, estimated from the S6+/S-total ratios, range s 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 t o NiNiO, as illustrated by samples from Mt. Etna, Italy. Variations of sulfur, at constant S6+/S-total ratio and temperature in both the pot assic and shoshonitic primary melts, indicate that these melts are und ersaturated, with respect to a S-rich condensed phase, in agreement wi th their relatively high oxidation state. It strongly suggests that su lfur behavior in relatively oxidized primary alkaline melts is control led by the mantle source melting conditions.