Article ID Journal Published Year Pages File Type
4677184 Earth and Planetary Science Letters 2013 13 Pages PDF
Abstract

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.

Related Topics
Physical Sciences and Engineering Earth and Planetary Sciences Earth and Planetary Sciences (General)
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