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

Abstract

Ultrarefractory (UR) phases in CAIs could have formed at higher T compared to common CAI minerals and thus they potentially provide constraints on very high-T processes in the solar nebula. We report a detailed characterization of an UR phase davisite bearing CAI from a reduced type CV chondrite. Absence of secondary iron- and/or alkali-rich phases and occurrence of LIME olivine indicate that primitive chemical and isotopic compositions are preserved in the CAI. Davisite occur only in one lithological unit that consists of three chemically and isotopically distinct parts: i) $^{16}$O-poor regions with reversely-zoned melilite and davisite; ii) $^{16}$O-rich regions consisting of unzoned, gehlenitic melilite, diopside and spinel; and iii) spinel framboids composed of $^{16}$O-rich spinel and $^{16}$O-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. Although isotope exchange 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 O-isotopic heterogeneity in CAIs. In either case, coexistence of spatially and/or temporally distinct $^{16}$O-rich and -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 CAI was short, which can be achieved by rapid outward transport of the CAI. High Ti$^{3+}$/Ti$^{\mathrm{tot}}$ ratios of pyroxene and presence of LIME olivine document that the entire CAI formation process took place under highly reducing conditions.