Sulfide textures of a unique CO3-chondrite (Y-82094) and its petrogenesis

The unique CO3-chondrite Y-82094 contains abundant FeNi metals. FeNi metal grains are often surrounded by troilite. More than one hundred troilite rimmed FeNi metals have been found in three polished thin sections of Y-82094 (CO3). These structures are frequently found in all CO3s. The troilite rim was formed by the reaction of FeNi metal in the core with S-rich gas. However, there are some differences in the metal-sulfide association between Y-82094 and other CO3s. In Y-82094,rims include two features that have not been observed in other CO3s : fizzed troilite and deviation of a reaction rim. In addition, FeNi metal cores include abundant large phosphates (∿10μm). In Y-82094,there is a large discrepancy of subtype classification between 3.0 obtained from variation in olivine composition, and 3.5 determined by thermoluminescence (TL). These features could be explained by shock metamorphism accompanying shock melting and subsequent rapid cooling. The shock effects were particularly effective for opaque mineral assemblages but not effective for silicates. Y-82094 has other unique features which cannot be explained by shock metamorphism; (1) mean chondrule size of Y-82094 being larger than those of CO3s irrespective of low petrologic subtype from olivine random analyses, and (2) unique bulk chemical composition. Thus Y-82094 is a unique CO3. This could mean that the formation and thermal history of Y-82094 is different from other CO3,implying that Y-82094 does not come from a different parent body from that of the other CO3s