| Experiments were conducted to investigate the effects of sulfur fugacity, oxygen fugacity, temperature, melt composition, and dilution with Os and Ir on the solubility of Ru in sulfide liquid and the two-phase field of laurite and Ru-Os-Ir alloy. Regardless of the intensive parameter investigated, Ru solubilities are far too high in molten sulfide to permit crystallization of the platinum-group minerals laurite and Ru-Os-Ir alloy in natural sulfide-saturated magmas. Natural laurite-alloy assemblages require sulfur fugacities on the order of 10−0.37 to 10−0.05 atm to crystallize with chromite at 1200–1250°C. Calculations indicate that moderately oxidized (logfO2 > FMQ-1) silicate magmas with moderate FeO contents can crystallize these phases without saturation in an immiscible sulfide liquid. Hence, natural laurite and IPGE alloy pairs may be representative of primary magmatic phases and could contribute to the fractionation of the platinum-group elements in natural basaltic magmas. |
