ARE IPGE ALLOYS STABLE IN THE PRESENCE OF IMMISCIBLE SULFIDE LIQUID?

To more completely assess the primary origin of IPGE alloys, we conducted experiments to evaluate the effects of T, fO₂, fS₂ and melt composition on the solubility of Ru in molten Fe-Ni-sulfide. Fe-Ni-S melt + Ru were held in olivine crucibles, and experiments were done in a vertical-tube gas-mixing furnace at 1200-1400C for 1-5 days. At constant fO₂ and fS₂, Ru solubility is prograde with T, but if fO₂ is varied parallel to FMQ, solubility is retrograde. At log fS₂ of -1.8, Ru solubility decreases from >15 wt% at log fO₂ of -11, to ~0.3 wt% at log fO₂ of -8. At a log fO₂ of -9, a similar reduction in Ru solubility occurred as log fS₂ decreased from -1 to -2.7. Substitution of Ni for Fe in the sulfide results in an increase in Ru solubility, with values ranging from ~3 wt% at Fe/Ni of 36 to ~10 wt% at Fe/Ni of 6 (log fO₂, fS₂ of -9.5, -1.8). Dilution of Ru with a 1:1 mix of Os + Ir results in a 4 and 10-fold decrease in melt Ru content for alloys with 50 and 25 wt% Ru, respectively. For the fO₂-fS₂ conditions required for sulfide liquid saturation in natural basaltic magmas, pure Ru solubility is expected to exceed 10 wt%, and dilution by Os + Ir is still likely to require wt% levels of Ru for IPGE alloy saturation. Since Ru abundances of ore-grade massive sulfide is <50 ppm, our results would preclude IPGE alloy saturation in the presence of immiscible sulfide liquid. The negative fO₂ dependence of Ru solubility in sulfide liquid is opposite that for Ru (and other PGEs) in silicate melt, suggesting that a decrease in fO₂ will favor the partitioning of Ru into the sulfide liquid. This effect may be partially offset, however, by concurrent changes in sulfide melt Fe/Ni ratio.