FeO‐rich rims and veins in Allende forsterite: Evidence for high temperature condensation at oxidizing conditions

Abstract

Abstract— Forsteritic olivine grains in the Allende meteorite are commonly rimmed by FeO‐rich olivine. New evidence is presented in this paper that the fayalitic rims formed by condensation from a gas and not by thermal equilibration of forsterite with FeO‐rich metal or FeO‐rich olivine in the interior of a parent body. A similar origin is inferred for fayalitic veins within forsterite crystals.A good correlation of FeO with MnO was observed along profiles from forsteritic cores to fayalite‐rich rims, excluding oxidation of metal as a source of rim FeO. The fayalite content of the rim is generally lower than that of the adjacent matrix olivines excluding formation of FeO‐rich rims by equilibration with present matrix. This is also supported by systematic differences in Cr between rim and matrix olivines.Chromites, associated with FeO‐rich olivine, were found at the boundary between forsterite and fayalite‐rich rim, within the rim itself and in fayalitic veins. Condensation of chromite from a gas phase is likely for the latter two occurrences since no other suitable source of Cr is available. Chromite at the forsterite‐fayalite interface was formed by diffusion of Cr from forsterite. Increasingly oxidizing conditions, apparently enabled formation of chromite.The steep compositional gradient between forsterite and fayalite provides severe constraints on the thermal history of forsterite grains. The width of oxidized zones (halos) around metal inclusions in the interior of forsterite grains are not compatible with this steep gradient. Their development requires longer diffusion times than would be allowed by the rims, indicating that these forsterite grains must have had an independent history of oxidation prior to formation of the rim.Condensation calculations indicate that FeO‐rich olivine can be formed by condensation from a gas with enhanced oxygen fugacity. Increasing the oxygen fugacity would provide conditions allowing the thermodynamic stability of chromite.High temperatures and oxidizing conditions must have prevailed for some time in that part of the solar nebula where the Allende meteorite formed.