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 +2.5, 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 ablation ICP-MS, which allows for the explicit assessment of contamination by metal inclusions. The 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 (1+, 2+) are suggested by the data. At 2 GPa, and ΔIW of +1.75 to +2, the metal-silicate partition coefficient for Re (DMet/Sil) is defined by the relationLogDMet/SilRe=0.50(±0.022)×104/T(K)+3.73(±0.095)For metal-silicate equilibrium to endow Earth's mantle with the observed time-integrated chondritic Re/Os, (and hence 187Os/188Os), 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 (±900) 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 <3200 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.

► Re solubility in silicate melt increases with temperature and oxygen fugacity.
► An unexpectedly low 2+ valence state for dissolved Re is suggested.
► The solubility of Re is ∼3 orders of magnitude greater than that of Au.
► Re abundances in the mantle cannot be explained by metal-silicate equilibrium.
► The Re/Os ratio of the mantle cannot be set by equilibrium core formation.