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 <1200 °C) than previously calculated. A major consequence is that the crossover from S 2− at low f O 2 to S 6+ at high f O 2 shifts upwards by ∼0.6 log f O 2 units relative to the FMQ buffer as temperature declines from 1200 °C to 1050 °C. The large temperature effect on sulfur speciation also means that there is electron exchange between Fe 2+ and S 6+ during magma cooling and ascent leading to high measured Fe 3+ /Fe 2+ in quenched melts, high calculated f O 2 and relatively reduced sulfur with high S 2− /S 6+ even at +2-3 log f O 2 units above the FMQ buffer. This self-oxidation mechanism at low temperatures is a major contribution to the observation that hydrous S-bearing arc magmas are more oxidised than MORB, which are generated at low f O 2 and which erupt at higher temperatures. Furthermore, these oxidised hydrous melts are sulfide saturated and should precipitate an Fe-rich sulfide throughout their path of ascent and differentiation in lower, mid and upper -crustal levels. Given that such sulfides would scavenge Cu and other chalcophile metals we suggest that the occurrence of Cu-(±Au) porphyry deposits is governed less by the capacity of magmas to remain Cu-rich, and more by the architecture and evolution of the cumulate pile, the timing and depth of volatile saturation, and the efficiency of Cl-rich fluids in later mobilisation of metals.