Modeling the interplay of fO2 and fS2 along the FeS-silicate melt equilibrium

In this paper we will discuss a simplified thermodynamic description for the saturation of FeS, either liquid or
solid, in magmatic melts. The Conjugated-Toop–Samis–Flood–Grjotheim model [Moretti R. and Ottonello G.,
2005. Solubility and speciation of sulfur in silicate melts, the Conjugated-Toop–Samis–Flood–Grjotheim (CTSFG)
model. Geochimica et Cosmochimica Acta, 69, 801–823] has furnished the theoretical reference frame, since it
already accounts for the solubility of gaseous sulfur and the speciation and oxidation state of sulfur in silicate
melts. We provide a new model to predict the saturation of magmatic silicate melts with an FeS phase that is
internally consistent with these previous parameterizations. The derived model provides an effective sulfogeobarometer,
which is superior with respect to previous models. For magmas rising from depth to surface, our
appraisal of molar volumes of sulfur-bearing species in silicate melts allows us to model oxidation–reduction
processes at different pressures, and sulfur concentrations for saturationwith either liquid or solid phases. In this
respect, the nature of the oxygen fugacity buffer is critical. On the basis of model results on some typical
compositions of volcanological interest, the sulfur contents at sulfide saturation (SCSS) have been calculated and
the results duplicate the experimental observations that the SCSS is positively correlatedwith pressure forwatersaturated
acidic melts and negatively correlated with pressure for water-poor basaltic melts. This new model
provides fO2–fS2 pairs of FeS saturation of natural silicatemelts. In caseswhere the redox constraint is lacking, the
model can be used to investigate whether the dissolved sulfur content approaches SCSS or not, and if so, to
estimate at which fO2 value the silicate melt is saturated with a sulfide phase

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