Diffusion and solubilities of Rh, Ru and Ir in olivine and spinel

The true solubilities of highly siderophile elements in silicate and oxide minerals may be difficult to measure because of their low abundances and the tendency of these elements to occur as minute inclusions or “micronuggets”. In order to mitigate this latter problem, we present a new approach that makes use of the solid-state diffusion of such elements into pre-existing crystals. The diffusion of Rh, Ru and Ir was measured in San Carlos olivine, and synthetic monocrystalline forsterite and MgAl₂O₄-spinel, at atmospheric pressure, 1300 to 1400 °C and oxygen fugacities from air to that of the quartz-fayalite-magnetite (FMQ) oxygen buffer. The activities of all main components were controlled in each experiment by solid-state buffer assemblages: SiOR₂R and MgO in olivine experiments were controlled by forsterite (olivine) + periclase (magnesiowustite) (fo + per or sco + mw), or forsterite (olivine) + protoenstatite (fo + en or sco + opx). Activities of AlR₂ROR₃R and MgO in the spinel experiments were buffered by spinel + periclase (sp + per) or spinel + corundum (sp + cor) mineral assemblages. The diffusion profiles were measured by LA-ICP-MS in both traverse and drilling modes, and by NanoSIMS. Good agreement was obtained between the methods with spatial resolution significantly higher in LA-ICP-MS drilling mode and NanoSIMS at 0.2 μm and 0.1 μm, respectively, compared to 5 μm by LA-ICP-MS in the traverse mode. Diffusion coefficients were more than five times slower, and solubilities five times lower, in the low MgO activity experiments compared with those carried out at high MgO activity at the same temperature and fO₂. Our results show that at 1400P^oPC and FMQ + 1.5 the maximum solubility of Rh in olivine in equilibrium with metal is 0.7 ppm in high SiOR₂R activity mineral associations (those with orthopyroxene) and <0.3 ppm in low SiOR₂R activity associations (those with magnesiowustite). Solubility of Rh in forsterite obtained at the same conditions was 0.2 ppm for the enstatite association and 0.17 ppm for the periclase association. Concentrations of Ir in olivine buffered with magnesiowustite were close to those buffered with orthopyroxene: 0.11 ppm and 0.12 ppm, respectively, at 1300 °C and FMQ + 0.1 The Ru concentration in olivine was 0.21 ppm in the orthopyroxene association at FMQ + 6.6 (in air). Solubilities of platinum group elements (PGEs) in spinel show a strong dependence on both fO₂ and a_SiO2: 150 ppm of Ir at FMQ + 1.5 compared to 1 ppm at FMQ + 6.6 for the cordierite assemblage, and 7 ppm for the periclase assemblage. Conclusions from this study show that olivine is unlikely to play a role in fractionating any of the compatible platinum group elements during partial melting of the mantle.