PARTITIONING OF PALLADIUM, IRIDIUM, AND PLATINUM BETWEEN SULFIDE LIQUID AND BASALT MELT - EFFECTS OF MELT COMPOSITION, CONCENTRATION, AND OXYGEN FUGACITY

The partitioning of Pd, Ir, and Pt, between immiscible (Fe, Ni)-monosulfide liquid and basalt melt has been investigated at 1300-degrees-C and at low pressure over the concentration range 40 to 20,000 ppm platinum-group element(s) (PGE) in the sulfide liquid and at oxygen fugacities from the C-CO-CO2 to the wustite-magnetite buffers. The experiments used sealed silica-glass tubes with internal oxygen buffers: PGE in glass were determined by radiochemical neutron activation analysis (RNAA). Partition coefficients (D) vary markedly with compositions of the sulfide liquid and silicate melt, increasing with decrease in oxygen fugacity, S, Fe, and possibly Ni, and with increase in total concentration of PGE. For 5 ppb PGE in the silicate melt and the iron-silica phase-fayalite (IQF) buffer, D(Pd) and D(Pt) are about 2 x 10(3), and D(Ir) is about 3 x 10(3); whereas, at the maximum concentration of PGE investigated, D(Pd) and D(Pt) are about 2 x 10(4), and D(Ir) is about 3 x 10(4). A single experiment confirms the marked fractionation of Pt from Pd predicted for partitioning with alloy in S-bearing and S-saturated silicate melts. The experimental D(PGE) values for low concentration of PGE are similar to D(PGE) calculated for many sulfide ore deposits, but are several orders of magnitude lower than calculated values for concordant sulfide PGE deposits in layered complexes.