Distribution of gold and rhenium between nickel-iron and silicate melts: implications for the abundance of siderophile elements on the Earth and Moon

Abstract

Abstract The distribution equilibrium of Au and Re between nickel-iron and basaltic melts was studied at 1400–1600°C, using radioactive tracers. Metal/silicate distribution coefficients were 1–3 orders of magnitude higher than earlier estimates, as follows. Mauna Loa basalt—Fe10Ni90: DAu = 3.3 × 104, DRe = (2.4−89) × 104. Gorda Ridge basalt—Fe10Ni90: DAu = (18−75) × 104. Synthetic lunar basalt—Fe70 Ni30: DAu≥ 2 × 104, DRe ≥ 2 × 103. The experimental ΔG1800° for the distribution of Au between nickel-iron and Mauna Loa basalt is −40 kcal/mole, compared to a calculated value of about −110 kcal/mole for a reaction involving simple Au3+ ions. Presumably the difference represents stabilization of Au(III) by complex formation with ligands such as Cl−, H2O, etc. Gold abundances in lunar basalts are roughly consistent with the measured DAu, but those in terrestrial basalts are two orders of magnitude too high. This discrepancy may reflect complexing by volatiles in the Earth's upper lithosphere, as well as oxidative destruction of metal in the final stages of accretion. In the absence of a metal phase, siderophile trace elements would remain trapped in the upper mantle and crust.