A NEW METHOD TO COMPUTE FLUIDS SATURATION IN C-H-O-S-SILICATE MELT SYSTEMS

We developed a method to calculate equilibrium between a C-O-H-S fluid phase and a silicate melt based on a previous model for the saturation of H2O-CO2 fluids (Papale, 1999) and on a thermochemical approach for calculating sulfide and sulfate solubilities of simple and complex melts. In particular, this second approach combines the Toop-Samis polymeric model with the Flood - Grjotheim theoretical treatment of silicate melts (Ottonello et al., 2001; Moretti, 2002). Moreover, fugacities in the gaseous phase are computed through the SUPERFLUID code (Belonoshko et al., 1992). The C-H-O-S saturation model allows determining the partition of H2O, CO2, and S between silicate melt and coexisting fluid, and the composition of the fluid phase in terms of H2O, CO2, SO2, and H2S, as a function of pressure, temperature, volatile-free liquid composition, oxygen fugacity, and total amount of volatile components in the system. For the sake of simplicity, we assumed that no reduced or oxidized sulfur-saturated solid or liquid phases nucleate or separate from the liquid-gas system. Minima in sulfur solubility as a function of oxygen fugacity are depicted, in good agreement with theory and experiments. Applications are given for rhyolitic and basaltic melts with various oxygen fugacities in the range NNO±2, and pressure from a few hundred MPa to atmospheric. The developed model accounts for the reciprocal effects of volatiles on their saturation contents, and the complex relationships between the saturation surface of a multicomponent fluid and the liquid composition, volatile abundance, P-T conditions and oxidation state. Belonoshko A, Shi PF & Saxena S, Comp. Geosci, 18, 1267- 1269, (1992). Moretti R, PhD Thesis, University of Pisa Ottonello G, Moretti R, Marini L& Vetuschi Zuccolini M, Chem. Geol, 174, 157-179, (2001). Papale P, Amer. Mineral, 84, 477-492, (1999).