Sulfur speciation in silicate melts at high pressure

We have determined the pressure dependence of the ratio S 6+ /(S 2- +S 6+ ) in silicate melts by measuring the effects of pressure on the concentrations of sulfide (S 2- ) and sulfate (SO 4 2- ) species at known fugacities of sulfur and oxygen. For S 2- we controlled f (S 2 ) using mixtures of Ag and Ag 2 S with oxygen fugacity held at the CCO buffer. For S 6+ we measured molten CaSO 4 solubility as a function of pressure. We define sulfide capacity C S 2 - and sulfate capacity C S 6 + from the S 2- and S 6+ contents of the melt as (Fincham and Richardson, 1954): log C S 2 - = log S melt 2 - + 1 2 l o g fO 2 fS 2 and log C S 6 + = log S melt 6 + - 3 2 l o g ( fO 2 ) - 1 2 l o g ( f S 2 ) . The dependences of C S 2 - and C S 6 + on pressure were found, with P in bars and T in K, to be: log C S 2 - P , T = l o g C S 2 - 1 , T - P ( 0.056 ) T and log C S 6 + P , T = l o g C S 6 + 1 , T - P ( 0.165 ) T . The negative pressure dependences are due to the differences in partial molar volumes between sulfide S 2- and oxide O 2– species V S 2 - - V O 2 - and sulfate SO 4 2 - and oxide O 2– V S 6 + - V O 2 - which we calculate to be ∼ 10.7 cm 3 /mol and ∼ 31.6 cm 3 /mol respectively. These are similar to the differences in volumes between CaS and CaO (10.96 cm 3 /mol) and CaSO 4 and CaO (29.2 cm 3 /mol). We used these logC S 2 - and log C S 6 + equations to calculate the pressure dependence of the “crossover” oxygen fugacity at which S 2- transforms to S 6+ in silicate melts of different composition. The crossover is shifted in absolute f (O 2 ) by + 0.25 log units or, relative to FMQ, by −0.41 log units as pressure is increased from 1 bar to 1 GPa at 1400°C. This demonstrates that the effect of pressure on sulfur oxidation state is small and may be neglected for many purposes. The pressure dependence of the S 2— S 6+ crossover means that there would be some electron exchange between Fe 2+ and S 6+ during decompression in a closed system (S 2- +8Fe 3+ = S 6+ +8Fe 2+ ). The effect is small, but largest in melts which start at oxygen fugacities close to and above FMQ and would, for a basalt containing 1500 ppm S lead to an increase in oxygen fugacity of 0.4 to 0.5 log f (O 2 ) units during decompression from 1.5 GPa to 1 bar.