An evaluation of the solubility of sulfur in silicate melts from experimental data obtained under a wide variety of experimental conditions has been undertaken. Under reducing conditions (fO below QFM), sulfur is present as sulfide (-II oxidation state), and precipitates as pyrrhotite or an FeSO sulfide phase when the sulfur concentration in the silicate melt is sufficiently high. The basic solubility reaction is: 1 2S2(g)+O2- (silicate melt)⇌ 1 2O2(g)+S2- ( silicate melt). For silicate melts with high FeO contents (> 10 wt.% FeO), sulfur dissolves as an Fe3SO2 (or 2FeO·FeS) species. When saturated with pyrrhotite: FeS(sulfide melt)+2FeO(silicate melt)⇌Fe3SO2(silicate melt)log K=log Xs - 21ogXFeO=-0 .8 (at 1200°C, 1 bar). Under saturated and unsaturated conditions: 1 2S2(g)+3FeO(silicate melt)⇌Fe3SO2(silicate melt)+> 1 2O2(g)log K=log Xs + 1 21og f{hook}O - 3 log XFeO- 1 2logf{hook}S=-4.1 (at 1200°C, 1 bar). log K also shows a positive pressure dependence (= +0.1 per kbar). For silicate melts with low FeO contents (< 1 wt.% FeO) that are saturated with H2O and pyrrhotite, sulfur shows an inverse correlation with FeO content: FeS(sulfide melt) + O2- (silicate melt)⇌FeO(silicate melt) + S2- (silicate melt)log K=log Xs + log XFeO = -5.8 (9007ndash;1000°C, 1-6 kbar). For silicate melts with intermediate FeO contents (1-10 wt% FeO), the behavior of sulfur is more complex, with sulfur probably present as an FeS species: FeS(sulfide melt)⇌FeS(silicate melt)log K=log Xs = -1.05-3000/T (K) (850-1200°C, 1 bar-2.2 kbar). The effects of temperature, pressure, melt composition and other experimental conditions have been assessed. The ability to predict the solubility of sulfur in silicate melts has implications for the formation of ore deposits associated with igneous systems. Application of the results to natural systems shows that some samples have a concentration of sulfur higher than that which could have been dissolved in the silicate melt, indicating that the magma carried excess sulfur, and/or sulfur has been added after crystallization of the magma. © 1990.
