Solubility and speciation of sulfur in silicate melts under crustal conditions

• Sulfide and sulfate capacities experimentally constrained in silicate melts at 1050 to 1250 °C. • The change in S 6+ /S 2− of a silicate melt with log f O 2 is strongly temperature dependent < 1200 °C. • Fe-S redox exchange may oxidise arc magmas by several log units in f O 2 as they crystallise. • Sulfide saturation is widespread in oxidised (> ΔFMQ +2) calc-alkaline magmas. • Cu-rich fluids should form by remobilising cumulate sulfides not sulfide-undersaturated melts. We have determined the solubility of sulfur as either sulfide (S 2− ) or sulfate (S 6+ ) in a wide range of silicate melts at 1 atm pressure and temperatures of 1050 to 1250 °C. The method involved suspension of the melt in either a mixture of CO 2 -CO-SO 2 (sulfide solubility) or SO 2 and air (sulfate solubility) for periods of up to 120 hours. Sulfur concentrations, measured by electron microprobe were converted into sulfide capacity C S 2 − and sulfate capacity C S 6 + using (Fincham and Richardson; 1954): log C S 2 − = log [ S 2 − ] + 1 2 log ( f S 2 f O 2 ) log C S 6 + = log [ SO 4 2 − ] − 1 2 log f S 2 − 3 2 log f O 2 [ S 2 − ] and [ SO 4 2 − ] refer to weight % sulfur dissolved in the melt as sulfate and sulfide respectively. Our new results demonstrate that extrapolation of earlier data to temperatures below 1200 °C yields good agreement for sulfide capacity but overestimates sulfate capacity. This means that sulfide is appreciably more stable relative to sulfate in crustal magmas (temperatures