Fayalite in the Kaba and Mokoia carbonaceous chondrites

The olivines in the Kaba and Mokoia CV3 carbonaceous chondrites range from almost pure forsterite (Fo_99.6) to almost pure fayalite (Fa_99.9)). Individual grains of fayalite of such high purity have not previously been reported from meteorites; they can reach 100 μm in diameter and occur in the matrix, chondrules, forsterite-enstatite aggregates, and rims around Ca, Al-rich inclusions, chondrules, and aggregates. The fayalite is three to nine times richer in Mn than in bulk CI chondrites. Many grains are associated with magnetite, troilite, and pentlandite, an assemblage that suggests relatively oxidizing conditions. We propose that the fayalite formed through reaction of SiO(g), released by decomposition of enstatite, with magnetite and sulfide. The lower thermal limit for fayalite formation is determined by the above reaction, whereas the upper thermal limit is controlled by magnetite stability. Thermodynamic calculations indicate that fayalite coexisting with magnetite and sulfides could have formed in a nebular environment that had a H₂O H₂ ratio substantially greater than the canonical solar nebula. Coexisting olivines having endmember compositions indicate that they experienced little or no thermal metamorphism.