Sulfur oxidation state and solubility in silicate melts

We have determined the solubility of sulfur (S) as sulfide (S 2– ) for 13 different natural melt compositions at temperatures of 1473–1773 K under controlled conditions of oxygen and sulfur fugacities ( f O 2 and f S 2 , respectively). The S and major element contents of the quenched glasses were determined by electron microprobe. The sulfide capacity parameter (C S2– ) was used to express S 2– solubility as a function of the oxygen and sulfur fugacities according to the equation: log C_S2 - = log Sₘₑₗₜ ( wt% ) + 0.5log ( fO₂ fS₂ ) 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_S2 - = 0.225 + ( 25237XFeO + 5214XCaO + 12705XMnO + 19829X_K₂ O - 1109X_Si0.5 O - 8879 )/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 . X MO 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 (C S6+ ) 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 (S 6+ /S 2– ratio) as a function of melt composition, temperature and oxygen fugacity. Results obtained are in good agreement with earlier measurements of S 6+ /S 2– for basaltic and andesitic compositions. We observe a significant increase, however, relative to FMQ of the oxygen fugacity of the S 2– to S 6+ 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 f O 2 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.