PROTON MICROPROBE RESULTS FOR THE PARTITIONING OF PLATINUM-GROUP ELEMENTS BETWEEN MONOSULPHIDE SOLID SOLUTION AND SULPHIDE LIQUID

Abstract

Partition coefficients (D) for Ni, Cu, and platinum-group elements (PGE) between monosulphide solid solution (mss) and Fe-sulphide liquid (liq) have been determined experimentally using an electron microprobe (EMP) to analyze experimental run products. The EMP detection limit is approximately 0.05 weight per cent for the PGE, consequently few results were obtained for Pt and Ir and the precision for Pd and Rh at low concentrations was poor. These run products have been reanalyzed using a proton microprobe (PMP), which has a detection limit between 10 and 50ppm for these elements. It is now clear that Dmss/liquid for all the elements show a strong dependence on the S content of the run in S-undersaturated and S-saturated runs. However, in S-oversaturated runs the S content of the run does not appear to influence Dmss/liq. The greater precision of the PMP data establishes that in S-oversaturated runs Dmss/liq at 1000°C are consistently higher than those at 1100°C. In contrast, Dmss/liq in the S-undersaturated and S-saturated runs are similar at both temperatures. This difference in behaviour is thought to arise because in the S-undersaturated and S-saturated runs the amount of S in the mss is controlled by the S content of the run. As the S content in the mss increases, the number of vacancies in the structure of the mss also increases, and Dmss/liq rises. In contrast, in S-oversaturated runs the mss has absorbed the maximum amount of S and thus the S content of the run no longer influences the structure of the mss and hence does not control Dmss/liq. Thus, the effect of temperature on Dmss/liqonly becomes apparent in the S-oversaturated runs. The tendency for Os, Ir, Ru and Rh to partition into mss and the exclusion of Cu, Pt and Pd from mss maybe used to explain a number of phenomena; the zonation of massive sulphide bodies, the tendency for Os, Ir, Ru and to a lesser extent Rh to be enriched in cumulates with minor sulphides, and the presence of two types of sulphides in mantle nodules (an Os-Ir-rich mss and Cu-Pd rich pentlandite). The tendency of sulphide liquids to crystallize RuOsIr and Pt-Fe minerals at low f S2 may explain the enrichment of RuOsIr in ultramafic mafic cumulate rocks in the following manner. Sulphide solubility increases as pressures falls. Thus, sulphide droplets in rising basalt magma could be partly resorbed. The PGM could crystallize from this liquid. If these PGM survive long enough, then they could be incorporated into the early cumulate phases such as olivine and chromite. This would explain both the presence of PGM in many olivine and chromite cumulates and the tendency of more evolved magmas to have high Pd/Ir ratios.