  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.

Earth and Planetary Astrophysics