Sulfur concentration at sulfide saturation (SCSS) in magmatic silicate melts

The sulfur concentration in silicate melts at sulfide saturation (SCSS) was experimentally investigated in a temperature range from 1150 to 1450 degrees C and a pressure range from 500 MPa to 1 GPa in a piston-cylinder apparatus. The investigated melt compositions varied from rhyolitic to basaltic and water concentrations varied from 0 to similar to 9 wt%. All experiments were saturated with FeS melt or pyrrhotite crystals. Temperature was confirmed to have a positive effect on the SCSS. Experimental oxygen fugacities were either near the carbon-carbon monoxide buffer or one log unit above the nickel nickel oxide buffer, and found to positively affect the SCSS. Combining our results with data from the literature we constructed a model to predict the SCSS in melts ranging in composition from komatiitic to rhyolitic, with water concentrations from 0 to 9 wt%, at pressures from 1 bar to 9 GPa and oxygen fugacities between similar to 2 log units below the fayalite-magnetite-quartz buffer to similar to 2 log units above it. The coefficients were obtained by multiple linear regression of experimental data and the best model found for the prediction of the SCSS is: ln(S in ppm)(SCSS) = 11.35251 - (4454.6)/(T) - 0.03190 (P)/(T) + 0.71006 ln(MFM) - 1.98063[(MFM)(X-H2Omelt)] + 0.21867 ln(X-H2Omelt) + 0.36192 ln X-FeOmelt where P is in bar, T is in K, MFM is a compositional parameter describing the melt based upon cation mole fractions: MFM = (Na + K + 2(Ca + Mg + Fe2+))/(Si X (Al + Fe3+)) X-H2Omelt is the mole fraction of water in the melt, andX(FEOmelt) is the mole fraction of FeO in the melt. This model was independently tested against experiments performed on anhydrous and hydrous melts in the temperature range from 800 to 1800 degrees C and 1-9 GPa. The model typically predicts the measured values of the natural log of the SCSS (in ppm) for komatiitic to rhyolitic (similar to 42 to similar to 74 wt% SiO2) melts to within 5% relative, but is less accurate for high-silica (> 76 wt% SiO2) rhyolites, especially those with molar ratios of iron to sulfur below 2. We demonstrate how this model can be used with the MELTS software to calculate the saturation of natural magmas with a sulfide phase and evolution of the SCSS during magmatic differentiation. (c) 2007 Elsevier Ltd. All rights reserved.