The sulfate capacities of silicate melts

Abstract

The solubility of sulfur as sulfate (SO42− or S6+) in silicate melts has been measured experimentally under oxidizing conditions at 1200 to 1500 °C, using SO2-O2 and SO2-CO2 as the input gas mixtures, with S and major elements determined on the quenched glasses by electron microprobe. The silicate melts investigated range from simple systems to natural magmatic compositions. Eighteen compositions in CMAS (CaO-MgO-Al2O3-SiO2) were studied at 1400 °C and varying sulfur trioxide fugacity, fSO3, to demonstrate that [S6+]/fSO3 remains constant within the limits of fSO3 accessed, where [S6+] is the concentration of S6+. This observation validates the concept of the “Sulfate Capacity” defined as CS6+ = [S6+]/(fSO3). Fitting CS6+ to a model derived from reciprocal solution theory as a function of temperature and melt composition from 232 experiments gives:lnCS6+=-8.02+(21100+44000XNa+18700XMg+4300XAl+44200XK+35600XCa+12600XMn+16500XFe2+)/THere XM are the mole fractions of the cation oxide components on the single-cation basis, [S6+] is in parts per million by weight, and fSO3 is in bars referenced to the conventional standard state. As this equation shows, CS6+ is so sensitive to melt composition that the analytical uncertainties in the major-element compositions of the glasses dominate fitting the data to the model.Combining CS6+ with the analogous Sulfide Capacity, CS2-, from previously published work allows the ratio of S oxidation states, S6+/S2−, to be calculated in silicate melts as a function of oxygen fugacity (fO2), temperature and melt composition. These calculations are in reasonable agreement with spectroscopic measurements on experimental glasses at known fO2, and on natural volcanic glasses where measured Fe3+/∑Fe can be used to calculate fO2, despite the presence of H2O in both kinds of glasses. This implies that other S possible melt species like S4+, HS−, etcetera are insignificant. The fugacity of SO2, the main S species degassing from magma on eruption, in equilibrium with a melt of a given total S content varies markedly with fO2, with a maximum at S6+/S2− = 3 (S6+/∑S = 0.75), which corresponds approximately to Fe3+/∑Fe ≈ 0.2 for typical basaltic compositions. Sulfur degassing from magma as SO2 causes oxidation or reduction depending on whether S6+/∑S is greater or less than 0.75, with strong negative feedback on the magnitude of fSO2 in both cases.