Experimental study on the speciation and solubility of sulfur, and the behavior of highly siderophile elements in sulfide- and sulfate-saturated basaltic melts and 1300°C and 1 GPa

The speciation and solubility of sulfur, and the solubilities of Pd, Ir, Pt, and Au were investigated in sulfide- and sulfate-saturated basaltic melts between 1300 to 1355°C and 1.0 to 1.2 GPa. Sulfur solubilities (as wt. % S) of 0.14 ± 0.02 wt.% and of 1.5 ± 0.2 wt.% were determined for sulfide-saturated and sulfate-saturated experiments, respectively.; Sulfur speciation in the melt changed from S2−-dominated (X(S6+) = molar S6+/[S6+ + S 2−]) = 0.03 ± 0.04) in sulfide-saturated experiments to S6+-dominated (X(S6+) = 0.81 ± 0.06) in sulfate-saturated experiments. This change in speciation was combined with previous data on sulfur speciation in basalts to derive an expression for the change in sulfur speciation as a function of oxygen fugacity.; Models for the change in sulfur solubility as a function of oxygen fugacity were derived. These models predict a decrease in the degree of partial melting required to produce sulfur-undersaturated melts with increasing oxygen fugacity in supra-subduction zone settings. Thus, if the source region is oxidized, the magmas generated can be both sulfur-rich and sulfur-undersaturated at relatively low degrees of partial melting.; Concentrations of [Pd]_melt = 11.9 ± 1.2 ppm and [Au] _melt = 4.7 ± 0.9 ppm in glasses from sulfate-saturated experiments were much higher than in sulfide-saturated experiments ([Pd]_melt = 0.63 ± 0.31 ppm; [Au]_melt = 0.52 ± 0.08 ppm). In contrast, platinum concentrations were not affected by oxidation state and were similar in oxidized ([Pt]_melt = 1.6 ± 0.3 ppm) and reduced experiments ([Pt]_melt = 1.23 ± 0.46 ppm). Iridium concentrations decreased with increasing oxidation state (from [Ir]_melt = 0.24 ± 0.08 ppm in sulfide-saturated experiments to [Ir] _melt = 0.07 ± 0.02 ppm in sulfate-saturated experiments).; The results of this work support the hypothesis that oxidation of mantle sulfides in suprasubduction zone settings favors Au-enrichment in the generated magmas, which translates to potentially higher Au-content in associated porphyry and epithermal deposits. Reduction of a relatively oxidized magma by assimilation of carbon-bearing country rocks can trigger sulfide saturation and produce orthomagmatic ore deposits.; Because the solubility of sulfur in oxidized melts decreases significantly with decompression, oxidized, water- and sulfur-rich basaltic magmas are likely to exsolve water- and sulfur-rich volatile phases upon ascent. Therefore, the results are consistent with the contribution of a volatile phase exsolved from basaltic magmas at depth to explain the high-sulfur degassing in some arc-related volcanoes.