The effect of water on the sulfur concentration at sulfide saturation (SCSS) in natural melts

We experimentally investigated the effect of water on the sulfur concentration at sulfide saturation (SCSS) in basaltic, andesitic, and rhyolitic melt compositions at 1250 degrees C, 1 GPa, and oxygen fugacities buffered at, or near, the graphite-carbon dioxide (CCO) buffer in a piston-cylinder apparatus. Water concentrations were measured in the sulfide-saturated melts using Raman spectroscopy. Experiments varied from anhydrous to 7.3 wt.% total dissolved water. The experiments demonstrate an increase in the SCSS of approximately 100 ppm per wt.% added water in all melt compositions. Combining these data with previous studies, two models for calculating the SCSS as a function of temperature, pressure, melt composition, and total dissolved water were created. The first model, Model A, incorporates the effect of oxygen fugacity in the melt composition by discriminating between ferric and ferrous iron and uses a generalized composition parameter, the MFM value, to describe melt composition. The second model, Model B, uses total iron as FeO and the mole fractions of major oxides per 100 g of melt to describe the melt composition. We find that Model B reproduces the SCSS of our calibrating data set, as well as an independent test set and a natural, high-Ni data set, to within about 5% relative variation, significantly better than Model A and in almost all cases within measurement uncertainties. Model B also reproduces a test set of sulfide-saturated silicic melts with sulfur concentrations near the limit of detection by electron microprobe to within 20% relative. We thus favor the empirical mole fractions of major oxides approach, and present the following equation to calculate the SCSS: ln(S, ppm)(SCSS) = -34.784 - 5772.3/T - 346.54 P/T - 20.393XH(2)O - 25.499XSiO(2) - 18.344XTiO(2) - 27.381XAl(2)O(3) - 17.275XFeO - 22.398XMgO - 20.378XCaO - 18.954XNa(2)O - 32.194XK(2)O where T is the temperature in Kelvin, P is the pressure in GPa, andXare the mole fractions of oxides per 100 g of melt. The model is applicable at pressures from 1 atm to 5 GPa, temperatures from 1050 to 1800 degrees C, and total melt water concentrations from anhydrous to 7.3 wt.% in terrestrial melts from komatiitic to rhyolitic in composition. Model B is also applicable to Martian basaltic melts (which are included in the calibration data set) and to many lunar compositions, but not high-Ti mare basalts. (C) 2015 Elsevier Ltd. All rights reserved.