Experimental data on the speciation of sulfur as a function of oxygen fugacity in basaltic melts

The speciation of sulfur as a function of oxygen fugacity was calculated in glasses of basaltic composition saturated experimentally with either sulfide or sulfate phases. The experiments were conducted on mixtures of synthetic and natural materials equilibrated at 1300 °C and 1 GPa in a piston-cylinder apparatus. Sulfur speciation was calculated by measuring the peak shift of the sulfur Kα radiation relative to a sulfide standard, whereas oxygen fugacity was calculated from the composition of olivine and spinel present in the assemblages. The results are consistent with sulfur being present as sulfite (S4+) in addition to sulfate (S6+) in oxidized melts. Therefore, sulfur speciation derived from SKα peak shifts should be seen as ”sulfate mole fraction equivalents“ (X(S6+)eq.). Using the data available, an empiric function: X(S6+)eq.=0.86/(1+exp(2.89−2.23ΔFMQ)) relating sulfate mole fraction equivalents and oxygen fugacity was derived. This function can be approximated by a linear function: ΔFMQ=2.28X(S6+)eq.+0.32 which could be used as an independent oxybarometer over the range in which most of the change of speciation occurs (+0.8 ≤ ΔFMQ ≤ +1.9).