Calcium-aluminum-rich inclusions in enstatite chondrites

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Scientific paper

Like Ca,Al-rich inclusions (CAIs) from carbonaceous and ordinary chondrites, enstatite chondrite (EC) 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 EC CAIs and rare Wark-Lovering rims are also similar to CAIs from other chondrite groups. However, the small size (<100 ?m), low abundance (<1% by mode in thin section), occurrence of only spinel or hibonite-rich types, and presence of primary Ti(+/-V)-oxides, and secondary geikelite and Ti,Fe-sulfides distinguish the assemblage of EC CAIs from other groups. The primary mineral assemblage in EC CAIs is devoid of indicators (e.g. oldhamite, osbornite) of low oxygen fugacities. Thus, high-temperature processing of the CAIs did not occur under the reducing conditions characteristic of ECs, implying that either: (1) the CAIs are foreign to EC-forming regions, or (2) oxygen fugacities fluctuated within the EC-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 EC 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 and/or dispersal of CAIs.

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