Calcium-aluminum-rich inclusions in enstatite chondrites (I): Mineralogy and textures

Like calcium-aluminum-rich inclusions (CAIs) from carbonaceous and ordinary chondrites, enstatite chondrite CAIs are composed of refractory minerals such as spinel, perovskite, Al, Ti-diopside, melilite, hibonite, and anorthitic plagioclase, which may be partially to completely surrounded by halos of Na-(±Cl)-rich minerals. Porous, aggregate, and compact textures of the refractory cores in enstatite chondrite CAIs and rare Wark-Lovering rims are also similar to CAIs from other chondrite groups. However, the small size (<100 μm), low abundance (<1% by mode in thin section), occurrence of only spinel or hibonite-rich types, and presence of primary Ti-(±V)-oxides, and secondry geikelite and Ti, Fe-sulfides distinguish the assemblage of enstatite chondrite CAIs from other groups. The primary mineral assemblage in enstatite chondrite CAIs is devoid of indicators (e.g., oldhamite, osbornite) of low O fugacities. Thus, high-temperature processing of the CAIs did not occur under the reducing conditions characteristic of enstatite chondrites, implying that either (1) the CAIs are foreign to enstatite-chondrite-forming regions or (2) O fugacities fluctuated within the enstatite-chondrite-forming region. In contrast, secondary geikelite and Ti-Fe-sulfide, which replace perovskite, indicate that alteration of perovskite occurred under reducing conditions distinct from CAIs in the other chondrite groups. We have not ascertained whether the reduced alteration of enstatite chondrite CAIs occurred in a nebular or parent-body setting. We conclude that each chondrite group is correlated with a unique assemblage of CAIs, indicating spatial or temporal variations in physical conditions during production or dispersal of CAIs.