Nebular history of an ultrarefractory phase bearing CAI from a reduced type CV chondrite

Abstract

Ultrarefractory (UR) phases in calcium-aluminum-rich inclusions (CAIs) could have formed at higher temperature compared to common CAI minerals and thus they potentially provide constraints on very high temperature processes in the solar nebula. We report a detailed characterization of the mineralogy, petrology and oxygen isotopic composition of an UR phase davisite (CaScAlSiO6) bearing CAI from a reduced type CV chondrite. The CAI is an irregular-shaped, compound inclusion that consists of five units that are composed of melilite + spinel + Al,Tirich pyroxene ± perovskite with various modal abundances of minerals and lithologies, and surrounded by the Wark-Lovering (WL) rim. Absence of secondary iron- and/or alkali-rich phases and occurrence of low-iron, manganese-enriched (LIME) olivine indicate that primitive chemical and isotopic compositions are preserved in the inclusion.Davisite occur only in one lithological unit that consists of three chemically and isotopically distinct parts: i) 16O-poor (？20‰ ≤ δ18O ≤ 0‰) regions with reversely-zoned melilite and davisite; ii) 16O-rich (？50‰ ≤ δ18O ≤ ？40‰) regions consisting of unzoned, gehlenitic melilite,diopside and spinel; and iii) spinel framboids composed of 16O-rich spinel and 16O-poor melilite. Random distribution of chemical and isotopic heterogeneities with sharp boundaries in the CAI indicates its formation by an aggregation of mineral assemblages formed and processed separately at different time and/or space in the solar nebula. Although isotope exchange between 16O-rich solids and 16O-poor gases prior to the final agglomeration of the CAI cannot be ruled out, we suggest that modification of chemical and isotopic composition of porous CAI precursors or aggregation of isotopically distinct mineral assemblages are alternative scenarios for the origin of oxygen isotopic heterogeneity in CAIs. In either case, coexistence of spatially and/or temporally distinct 16O-rich and 16O-poor gaseous reservoirs at the earliest stage of the solar system formation is required. The grain-scale oxygen isotopic disequilibrium in the CAI indicate that post-formation heating of the inclusion (i.e., the WL rim formation event) was short (e.g., ？ 10^3 hours at 1400 K; ？ 10^5 hours at 1100 K), which can be achieved by rapid outward transport of the CAI. High Ti3+/Titot ratios of pyroxene from CAI interior and the rim and LIME composition of the rim olivine document that the entire CAI formation process took place under highly reducing conditions.