| Abundant geochemical and petrological data exists to suggest that chromite may be an important phase in concentrating and fractionating the PGEs in mafic/ultramafic systems. To investigate this possibility, chromite/silicate- and sulphide-melt partitioning experiments have been conducted at conditions of pressure, temperature and fO2 appropriate for basalt genesis. Trace elements in run products were determined in situ using laser ablation ICP-MS, which allowed us to discern both phase homogeneity and detect the presence of sulphide or alloy contaminants. Experiments yielded absolute chromite/silicate melt partition coefficients (Ds) for Re (0.15) and maximum values for the PGEs (Ru < 10.1; Os < 4.3; Ir < 1; Pd < 0.23; Rh < 0.17). Results indicate that Re and some PGEs are incompatible in chromite, whereas compatibility of other PGEs is significantly less than previously considered. As an alternative to the PGEs being in solid solution in the chromite structure, textures produced in our experiments show entrapment of PGE-rich phases (sulphide/alloy) during chromite crystallisation, which may best explain the PGE-chromite association. |
