Temperature dependence of Pt and Rh solubilities in a haplobasaltic melt

Abstract

The temperature dependence of the solubilities of Pt and Rh in a haplobasaltic (anorthite-diopside 1-bar eutectic composition) melt has been investigated at 1 bar and 1300 to 1550°C using the mechanically assisted equilibration technique (Dingwell et al., 1994). The experiments were performed at almost constant oxygen fugacity (log fO 2 = −2.5 ± 0.3) over the entire temperature range. Major element concentrations in the quenched glass samples were determined using an electron microprobe. Pt and Rh concentrations were obtained by laser ablation inductive coupled plasma mass spectrometry. From our data, we obtain the following expressions for the solubilities of pure Pt and pure Rh in anorthite-diopside eutectic melt at 1 bar and log fO 2 = −2.5: log[Pt](ppm)=− 3320(340) T( K) +2.0(0.2) r 2=0.96, log[Rh](ppm)=− 5440(450) T( K) +3.9(0.3) r 2=0.97. Metal-silicate partition coefficients estimated from these results at likely conditions of core formation (3000 K and an oxygen fugacity 2 orders of magnitude below the iron-wüstite oxygen buffer; e.g., Righter and Drake, 1997) are 6.1 × 10 8 and 3.9 × 10 6 for Pt and Rh, respectively. On the basis of these results, high temperature is not sufficient to explain the abundances of these highly siderophile elements in the Earth’s mantle as a consequence of metal-silicate equilibrium during core formation.