High-temperature condensation of iron-rich olivine in the solar nebula

Abstract

Olivine is the most abundant mineral in many chondritic meteorites. It is also the first major Mg-silicate to condense from a solar gas upon cooling. The composition of olivine provides important information concerning its origin. Results of calculations are presented for the Fe, Mn and Ni contents of olivine condensing from a gas with solar composition but variable oxygen fugacity. A basic computational method is described that takes into account the depletion of the gas in Mg and Si during condensation. The first olivine that condenses from a solar gas has less than 10 −2 mol% FeO, independent of the oxygen fugacity of the otherwise solar gas. At lower temperatures when most of the Mg is condensed more FeO can be incorporated in forsterite by condensation if the oxygen fugacity is sufficiently high. The FeO/MnO ratio of the first condensing olivine is above 1000, decreasing with decreasing temperature and increasing oxygen fugacity to values of around 100 which are actually observed in fayalitic olivine rims in Allende. The Ni-content of condensed olivine is below 10 −7 mol%, even at high oxygen fugacities, indicating that condensation of NiO in olivine is a very unlikely process. Any Ni in olivine in the ppm range is, therefore, indicative of metal-olivine equilibration. Under increasingly oxidizing conditions (H 2O/H 2 ratios of ∼ 0.01–0.1) chromite is an early phase to condense, forming before forsterite. Isolated olivine grains in Allende and olivine on the outside of chondrules have FeO-rich rims. Texture, composition and morphology of these rims indicate that they formed by condensation from a gas. Our calculations suggest that the gas was enriched by a factor of 100 in oxygen relative to solar-composition gas and that Mg was almost completely condensed when rim formation occurred. The sharp boundary between forsteritic and fayalitic olivine suggests an increase in oxygen fugacity and rapid formation of fayalitic olivine by condensation. Cr-enrichment at the rim interface with the underlying forsterite probably reflects stability of chromite under oxidizing conditions. Conventional models of FeO-bearing silicate formation require oxidation of metal + silicate assemblages at temperatures as low as 400 K. It is shown here that diffusion at this temperature is so slow that the time needed for formation of the observed FeO-bearing phases would exceed the lifetime of the solar nebula. Other oxygen-fugacity indicators reflect similar conditions. For example, the hercynite component of spinel in fine-grained Al-rich inclusions indicates equilibration with a gas of similar composition as that responsible for condensation of fayalitic olivine, suggesting formation of FeO-rich spinel by gas-solid reactions. Mechanisms for producing oxidizing conditions in the solar nebula are briefly discussed.