The partitioning of Fe, Ni and Co between olivine, metal, and basaltic liquid: An experimental and thermodynamic investigation, with application to the composition of the lunar core

The partitioning of Fe, Ni and Co between Mg-rich olivine, lunar basaltic liquid and metal has been measured over a range of temperatures and pressures. The results of the olivine/metal partitioning may be compared to those predicted from thermodynamic calculation; in general, the agreement is good, although the calculated distribution coefficients are slightly greater then those experimentally determined. This discrepancy increases with increasing temperature and pressure, but it is not possible to ascertain unambiguously which thermodynamic data might be responsible. At lower temperatures, inversion of the equilibria to yield "cosmothermometers" for measuring the temperature of equilibration between olivine and metal gives results for the pallasite meteorites in excellent agreement with independent estimates. The results have been applied to Apollo 15 Green Glass, presumed to approximate a primitive melt from the lunar mantle, to deduce the composition of Fe-Ni-Co metal in equilibrium with the lunar mantle. This composition is approximately (by weight) 54-60% Fe, 38-45% Ni and 1% Co; consequently, if metal has separated from the lunar mantle under equilibrium conditions to form a small lunar core, this core will be nickel rich, with a Ni/Fe ratio of ~0.7 ±0.15.