Fe-Mg and Ni Partitioning between Olivine and Silicate Melt

Abstract

The mineral olivine is abundant in a wide range of mafic igneous rocks from around the solar system. The fractionation, or accumulation, of olivine often exerts a major control on the observed variations in magma and whole-rock compositions. We performed experiments on a synthetic Hawaiian picrite and examined the olivine (ol)-liquid (liq) partitioning of Mg and Fe2+. These experiments show that the exchange coefficient, KD, Fe2+-Mg=(FeO/MgO)ol/(FeO/MgO)liq , by weight, is 0.345±0.009 (1σ) and is independent of temperature and liquid composition. Using this result, we estimate that parental liquids for tholeiites from Kilauea, Mauna Loa, and Mauna Kea have approximately 19-21 wt. % MgO. Published experiments on model Martian compositions suggest that for the Fe-enriched and Al-depleted Martian basalts a slightly higher KD,Fe2+-Mg of 0.36 is more appropriate. Using this value we conclude that the olivine-phyric shergottites Y 980459, NWA 5789, 2990, and EETA 79001 are possible liquid compositions (others are not); if the canonical KD,Fe2+-Mg of 0.30 were used, we would have concluded that none of the bulk meteorites represent liquids. The behavior of Ni is nearly unique among most other major and trace elements: it is compatible in olivine. This compatibility is useful in constraining the evolution of lavas, as their Ni contents will be very sensitive to the fractionation or accumulation of olivine. We performed experiments investigating the partitioning of Ni between a liquid and olivine of approximately constant composition over a range of temperatures and pressures. These experiments successfully separate the effects of composition from those of temperature and pressure, showing that, for our liquid with ~ 18 wt. % MgO, the ol-liq Ni partition coefficient (by wt.) decreases from 5.0 to 3.8 as the temperature and pressure increase from 1400 to 1550°C and 1-atm to 3.0 GPa, respectively. We show that this temperature and pressure effect may contribute to the generation of high-NiO olivines observed in Hawaiian and other ocean-island basalts.