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Ultrarefractory (UR) phases in CAIs could have formed at higher T compared tocommon CAI minerals and thus they potentially provide constraints on veryhigh-T processes in the solar nebula. We report a detailed characterization ofan UR phase davisite bearing CAI from a reduced type CV chondrite. Absence ofsecondary iron- and/or alkali-rich phases and occurrence of LIME olivineindicate that primitive chemical and isotopic compositions are preserved in theCAI. Davisite occur only in one lithological unit that consists of threechemically and isotopically distinct parts: i) $^{16}$O-poor regions withreversely-zoned melilite and davisite; ii) $^{16}$O-rich regions consisting ofunzoned, gehlenitic melilite, diopside and spinel; and iii) spinel framboidscomposed of $^{16}$O-rich spinel and $^{16}$O-poor melilite. Randomdistribution of chemical and isotopic heterogeneities with sharp boundaries inthe CAI indicates its formation by an aggregation of mineral assemblages formedand processed separately at different time and/or space. Although isotopeexchange prior to the final agglomeration of the CAI cannot be ruled out, wesuggest that modification of chemical and isotopic composition of porous CAIprecursors or aggregation of isotopically distinct mineral assemblages arealternative scenarios for the origin of O-isotopic heterogeneity in CAIs. Ineither case, coexistence of spatially and/or temporally distinct $^{16}$O-richand -poor gaseous reservoirs at the earliest stage of the solar systemformation is required. The grain-scale oxygen isotopic disequilibrium in theCAI indicate that post-formation heating of the CAI was short, which can beachieved by rapid outward transport of the CAI. HighTi$^{3+}$/Ti$^{\mathrm{tot}}$ ratios of pyroxene and presence of LIME olivinedocument that the entire CAI formation process took place under highly reducingconditions.

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