The effect of oxygen fugacity and temperature on solubilities of nickel, cobalt, and molybdenum in silicate melts

Abstract

Solubilities of Ni, Co and Mo in silicate melts of anorthite-diopside eutectic composition were determined at reducing conditions, with ƒ O 2 values ranging from 10 −8.6 to 10 −12.6 atm and at temperatures of around 1400°C. In a log (solubility) vs. log ( ƒ O 2 ) diagram, Ni and Co data plot along straight lines with slopes of 0.45 ± 0.01 for Ni and 0.52 ± 0.01 for Co, approximately corresponding to a valence state in the melt of +2 for both metals. No indications for the presence of metallic Ni and/or Co were found, even at the lowest ƒ O 2 (IW-3) reached with gas mixing. This result is in agreement with independent experimental data for Ni solubilities in silicate melts by Dingwell et al. (1994) but is in conflict with recent experimental results by two other groups ( Colson, 1992; Ehlers et al., 1992). The experimental data for Mo suggest a change in valence slightly below the IW-1-buffer, from +6 to +4. This is the first reliable determination of the valence of Mo in silicate melts. No effect of the composition of gas mixtures used for fixing the ƒ O 2 was found; i.e., CO 2-CO-N 2 and CO 2-H 2-N 2 mixtures defining the same ƒ O 2 gave the same results. Temperature dependences for Ni, Co, and Mo solubilities were derived from experiments at temperatures ranging from 1349 to 1438°C. Solubilities of all three metals decrease with increasing temperature at constant oxygen fugacities. Metal/silicate partition coefficients were calculated from solubilities. Although Ni and Co metal/ silicate partition coefficients decrease with increasing temperature at ƒ O 2 appropriate for core formation, temperature extrapolated Ni-metal/silicate partition coefficients are much higher than corresponding Co-metal/silicate partition coefficients. Thus, the nearly chondritic Ni Co ratio of the mantle is difficult to reconcile with a global mantle-core equilibrium. In addition, although the metal/silicate partition coefficient of Mo decreases with temperature at ƒ O 2 values appropriate for core formation, the decrease is less steep than that of Ni and Co leading to a Ni Mo concentration ratio in a hypothetical magma ocean ten times above the Ni Mo ratio characteristic of the Earth's mantle.