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 s uch as spinel, perovskite, Al,Ti-diopside, melilite, hibonite, and anorthit ic plagioclase, which may be partially to completely surrounded by halos of Na-(+/-Cl)-rich minerals. Porous, aggregate, and compact textures of the r efractory cores in enstatite chondrite CAIs and rare Wark-Lovering rims are also similar to CAIs from other chondrite groups. However, the small size (<100 mu m), low abundance (<1% by mode in thin section), occurrence of onl y spinel or hibonite-rich types, and presence of primary Ti-(+/-V)-oxides, and secondary geikelite and Ti,Fe-sulfides distinguish the assemblage of en statite chondrite CAIs from other groups. The primary mineral assemblage in enstatite chondrite CAIs is devoid of ind icators (e.g., oldhamite, osbornite) of low O fugacities. Thus, high-temper ature processing of the CAIs did not occur under the reducing conditions ch aracteristic of enstatite chondrites, implying that either (1) the CAIs are foreign to enstatite-chondrite-forming regions or (2) O fugacities fluctua ted within the enstatite-chondrite-forming region. In contrast, secondary g eikelite and Ti-Fe-sulfide, which replace perovskite, indicate that alterat ion of perovskite occurred under reducing conditions distinct from CAIs in the other chondrite groups. We have not ascertained whether the reduced alt eration of enstatite chondrite CAIs occurred in a nebular or parent-body se tting. We conclude that each chondrite group is correlated with a unique as semblage of CAIs, indicating spatial or temporal variations in physical con ditions during production or dispersal of CAIs.