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

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 isotopiccompositions are preserved in the CAI. Davisite occur only in one lithological unit that consists of three chemically and isotopicallydistinct parts: i) $^{16}$O-poor regions with reversely-zoned meliliteand davisite; ii) $^{16}$O-rich regions consisting of unzoned, gehlenitic melilite, diopsideand spinel; and iii) spinel framboidscomposed of $^{16}$O-rich spinel and $^{16}$O-poor melilite. Random distribution of chemical and isotopicheterogeneities 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 isotopeexchange prior to the final agglomeration of the CAI cannot be ruled out, we suggest that modification of chemical and isotopiccomposition of porous CAI precursors or aggregation of isotopicallydistinct mineral assemblages are alternative scenarios for the origin of O- isotopicheterogeneity 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 isotopicdisequilibrium 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 pyroxeneand presence of LIME olivine document that the entire CAI formation process took place under highly reducing conditions.