On the saturation surface and oxidation state of C-H-O-S-silicate melt systems

The equilibrium between a H2O–CO2–SO2–H2S gas phase and silicate melts is investigated by means of thermochemical calculations which join homogeneous reactions in the gas phase and heterogeneous gas–melt saturation modeling based on classical sub-regular multicomponent mixing and Toop- Samis polymeric approach. Sulfur in the melt phase is assumed to be present in two different oxidation states (sulfide and sulfate ions). The thermodynamic model is an extension of that presented in Moretti et al. [1] to account for iron speciation at high pressure with variable dissolved water contents. The consequences on the equilibrium conditions of different assumptions on the effective redox buffer in magma are examined for melts of basaltic and rhyolitic composition, determining the equilibrium conditions on the basis of i) constant FeII/FeIII, ii) constant fH2S/fSO2, and iii) constant relative fO2, expressed as difference in log-units to a solid buffer. The first two buffers are expected to be effective in basaltic and andesitic-rhyolitic magmas, respectively, according to the most abundant reservoir of redox couples. Furthermore, for each assumed redox buffer the pressure dependence of phase composition and oxidation state of the system shows strongly non-linear trends. The largest compositional differences are shown by sulfur species, however, the concentrations of H2O and CO2 in the two phases at equilibrium also show non-negligible dependence on the redox conditions. For each assumed redox buffer, sulfur dioxide in the gas phase, and sulfate ions in the liquid phase, are found to be present in appreciable quantities or represent the dominating sulfur species. The more reliable redox buffers represented by constant FeII/FeIII for basalt, and constant fH2S/fSO2 for rhyolite, show that oxygen fugacity paths during magma depressurization strongly deviate from those related to a solid buffer plus or minus a constant. Results here presented, although not yet accounting for the separation of S-bearing solid or liquid phases, may furnish insights on the composition of gases separated from magmas originated in various geodynamic settings, under different redox conditions. Reference [1] Moretti R., Papale P. and Ottonello G. (2003) In: Volcanic Degassing (Oppenheimer C., Pyle D. and Barclay J., eds.) Geol. Soc. London Spec. Publ., 213, 81-101