SO2 solubility and degassing behavior in silicate melts

Abstract

We have determined the S contents of 14 different silicate melts equilibrated with Air/SO2 mixtures at 1 atm pressure and temperatures of 1473–1773 K in 50 K intervals. Under these oxidizing conditions sulfur dissolves in the melts predominantly as sulfate (S6+ or SO42–). The sulfur contents of the quenched products were determined by EPMA (Electron Probe Micro-Analysis) and by SIMS (Secondary Ion-Mass Spectrometry) with the latter being employed for low sulfur contents (i.e., ≤60 ppm). Equilibrium S contents were found to increase strongly with decreasing temperature and with some compositional parameters, especially CaO content. The data were used to calculate the sulfate capacity (log CS6+), for each melt composition at each temperature. Sulfate capacity is related to the equilibrium constant for sulfur dissolution and is defined as follows in terms of SO42– concentration and fugacities of oxygen and sulfur:logCS6+=logSO4melt2-(wt.%)-12logfS2-32logfO2By determining log CS6+ at different Air/SO2 ratios we showed that log CS6+ is independent of sulfur content and hence that Henry’s Law is obeyed in the experimental composition and temperature range. We used stepwise linear regression to fit sulfate capacity to an equation incorporating compositional and temperature terms. Parameters which did not pass the F-test at the 95% confidence level were excluded. This approach led to the following fit equation:logCS6+=-12.659+(3692XCaO-7592XSiO2-13736XTiO2+3762XAlO1.5+34483)/TThe standard error of the fit is 0.154 and R2 is 0.985.The data were used to simulate scenarios for SO2-degassing from ascending silicate melts of basaltic and andesitic compositions. The important observations are that SO2 partitions strongly into an H2O-rich fluid phase and that most S is lost at relatively high pressures in the crust. So, for example we estimate that ≥85% of the initial S should be lost before the ascending magma reaches 100 MPa pressure.