Controls on the solubility of rhenium in silicate melt: Implications for the osmium isotopic composition of Earth’s mantle

The solubility of Re and Au in haplobasaltic melt has been investigated at 1673–2573 K, 0.1 MPa–2 GPa and IW 1 to þ2.5, in both carbon-saturated and carbon-free systems. Results extend the existing, low pressure and temperature, dataset to more accurately predict the results of metal-silicate equilibrium at the base of a terrestrial magma ocean. Solubilities in run-product glasses were measured by laser Re and Au content of demonstrably contaminant-free glasses increases with temperature, and shows variation with oxygen fugacity (fO2) similar to previous results, although lower valence states for Re (1þ , 2þ) are suggested by the data. At 2 GPa, and DIW of þ1.75 to þ2, the metal-silicate partition coefficient for Re (DMet/Sil) is defined by the relation LogD Met=Sil 1⁄4 0:50 70:022 ð Þ 10 =TðKÞþ3:73 70:095 ð Þ For metal-silicate equilibrium to endow Earth’s mantle with the observed time-integrated chondritic Re/Os, (and hence Os/Os), DMet/Sil for both elements must converge to a common value. Combined with previously measured DMet/Sil for Os, the estimated temperature at which this convergence occurs is 4500 (7900) K. At this temperature, however, the Re and Os content of the equilibrated silicate is 100-fold too low to explain mantle abundances. In the same experiments, much lower Dmet/sil values have been determined for Au, and require the metal-silicate equilibration temperature to be o3200 K, as hotter conditions result in an excess of Au in the mantle. Thus, the large disparity in partitioning between Re or Os, and Au at core-forming temperatures argues against their mantle concentrations set solely by metal-silicate equilibrium at the base of a terrestrial magma ocean. & 2012 Elsevier B.V. All rights reserved.