THE IMPLICATIONS OF SULFUR SPECIATION IN SILICATE MELTS FOR SULFUR SOLUBILITIES
Seminars
Semester 1
Prof. Hugh O’Neill is an earth scientist whose career has focused on experimental petrology and geochemistry. His work has aimed at advancing our understanding of how rocky planets form, using experimental and theoretical studies of the thermodynamic properties of minerals and melts to solve large-scale geological problems. He has contributed to the understanding of several key questions relating to our planet, including: the chemical composition of Earth and how it differs from other planets; the origin of the Moon; the geochemical evolution of the Earth’s mantle; and the origin of basalts. His current research interests include the composition of the Earth compared to other rocky planetary bodies, the solubilities of volatile species in magmas, diffusion and other transport properties of minerals, the trace-element geochemistry of basalts, and X-ray Absorption Spectroscopy of geomaterials. As well as being a Fellow of the Royal Society and of the Australian Academy of Sciences, he is a Fellow of the American Geophysical Union, the Mineralogical Society of America and the Geochemical Society.
Sulfur solubilities in silicate melts in equilibrium with a gas phase have been experimentally measured as a function of temperature at ambient pressure under both reduced (sulfur as S2-) and oxidized (sulfur as S6+ or SO42-) conditions for a wide range of melt compositions. The results can be systematized using thermodynamic models for the sulfide and sulfate capacities of the silicate melts (CS2- and CS6+), based on a two-sublattice treatment of the melts (i.e., cation and anion sublattices). Combining the two models gives the range of oxygen fugacities over which the speciation of S in the melts changes from predominantly S2- to predominantly S6+, at ambient pressure. The results show that the midpoint of the transition, where S2-/S6+ = 1, occurs at Fe3+/ΣFe= 0.16 for a synthetic parental mid-ocean ridge basalt composition at 1230ºC, with Fe3+/ΣFeincreasing slightly with decreasing temperature. The models may also be combined with experimental data from the literature at super-ambient pressures, to determine the maximum solubilities of sulfur in silicate melts coexisting with FeS-rich sulfide (the Sulfide Content at Sulfide Saturation or SCSS) or with CaSO4 (the Sulfate Content at Anhydrite Saturation or StCAS). The resulting models are not well constrained as regards the effects of pressure and H O contents. To address these issues, new experiments have been done in the piston-cylinder apparatus, in which haplobasaltic melt compositions were brought into equilibrium with both FeS-rich matte and anhydrite simultaneously, at 1200 to 1500ºC and 0.5 to 4 GPa. The quenched glasses were analysedby electron microprobe, and both S6+/ΣS and Fe3+/ΣFe measured by XANES spectroscopy. The results show a limited range of S6+/ΣS (0.6 to 0.8) at near constant Fe3+/ΣFe (0.17 ± 0.02), but with very large total S contents that are difficult to reconcile with extrapolations from ambient-pressure models. The discrepancy suggests additional S species other than just S2- and S6+ may be present at intermediate redox states at high pressure, although this is not evident from the XANES spectra.