Sulfur oxidation state and solubility in silicate melts

Abstract

We have determined the solubility of sulfur (S) as sulfide (S2–) for 13 different natural melt compositions at temperatures of 1473–1773 K under controlled conditions of oxygen and sulfur fugacities (fO2 and fS2, respectively). The S and major element contents of the quenched glasses were determined by electron microprobe. The sulfide capacity parameter (CS2–) was used to express S2– solubility as a function of the oxygen and sulfur fugacities according to the equation: $$\log C_{{S^{2 - } }} = \log S_{melt} \left( {wt\% } \right) + 0.5\log \left( {\frac{{fO_{2} }}{{fS_{2} }}} \right)$$ log C S 2 - = log S melt w t % + 0.5 log f O 2 f S 2 . Sulfide capacities of silicate melts were found to increase with temperature and the FeO content of the melt. We combined our sulfide data at 1473–1773 K with (O’Neill and Mavrogenes, J Petrol 43:1049–1087, 2002) results at 1673 K, and obtained by stepwise linear regression the following equation for sulfide capacity $$\log C_{{S^{2 - } }} = 0.225 + \left( {25237X_{FeO} + 5214X_{CaO} + 12705X_{MnO} + 19829X_{{K_{2} O}} - 1109X_{{Si_{0.5} O}} - 8879} \right)/T{ }$$ log C S 2 - = 0.225 + 25237 X FeO + 5214 X CaO + 12705 X MnO + 19829 X K 2 O - 1109 X S i 0.5 O - 8879 / T . XMO is the mole fraction of the oxide of M on a single-oxygen basis, and T is in Kelvin. The sulfide capacity equation was combined with sulfate capacity (CS6+) data for similar compositions and at the same temperatures (Boulliung and Wood, Geochim Cosmochim Acta 336:150–164, 2022), to estimate the S redox state (S6+/S2– ratio) as a function of melt composition, temperature and oxygen fugacity. Results obtained are in good agreement with earlier measurements of S6+/S2– for basaltic and andesitic compositions. We observe a significant increase, however, relative to FMQ of the oxygen fugacity of the S2– to S6+ transition as temperature is lowered from 1773 to 1473 K. We used our results to simulate sulfur-degassing paths for basaltic compositions under various redox conditions (FMQ –2 log fO2 units to FMQ + 2). The calculations indicate that, given an initial concentration of 0.12 wt% S in an ascending melt at 250 MPa, most of the S (> 80%) will be degassed before the magma reaches 100 MPa pressure.

Mantle Oxidation Induced by Recycled Carbonate: Insights From Mg‐Zn‐Fe‐Cu Isotopic Systematics of Intraplate Basalts

“…Insignificant Cu isotope fractionation with decreasing pressure is observed during basaltic melt ascent through the lithosphere and syn-eruptive degassing in Kilauea Iki lava (Savage et al, 2015;Wieser et al, 2020). Alternatively, Boulliung and Wood (2023) found that the effect of pressure on the S 2− to S 6+ transition is much smaller than that in Matjuschkin et al (2016), and only ∼1.7 log fO 2 units (ΔFMQ) are enough to affect SCSS of melts when pressure increases from 0 to 5 GPa at 1700 K (Figure 7b). A simplified model demonstrates that the quantity of dissolved sulfides relies on the fO 2 and the Cu isotopic compositions of melts become lighter with increasing dissolution of sulfides (Figures 7a and 7b).…”

“…However, free sulfate-bearing fluids may be only stable in crustal pressures (<1 GPa) under oxidized conditions (Matjuschkin et al, 2016). Sulfides can dissolve into silicate melts with increased sulfur content at sulfide saturation (SCSS), which is a function of P-T-fO 2 and chemical compositions (Boulliung & Wood, 2023;Chowdhury & Dasgupta, 2020;Jugo et al, 2010). If sulfides dissolve into oxidized basaltic melts during mantle melting, such melts are expected to acquire light Cu isotopic signatures.…”