A hibonite-corundum inclusion from Murchison: A first-generation condensate from the solar nebula

Through freeze-thaw disaggregation of the Murchison (CM) carbonaceous chondrite, we have recovered a ~90 x 75 micrometers refractory inclusion that consists of corundum and hibonite with minor perovskite. Corundum occurs as small (~10 micrometers), rounded grains enclosed in hibonite laths (~10 micrometers wide and 30-40 micrometers long) throughout the inclusion. Perovskite predominantly occurs near the edge of the inclusion. The crystallization sequence inferred petrographically--corundum followed by hibonite followed by perovskite--is that predicted for the first phases to form by equilibrium condensation from a solar gas for Ptot is less than or equal to 5 x 10^(-3) atm. In addition, the texture of the inclusion, with angular voids between subhedral hibonite laths and plates, is also consistent with formation of the inclusion by condensation. Hibonite has heavy rare earth element (REE) abundances of ~40 x CI chondrites, light REE abundances ~20 x CI chondrites, and negative Eu anomalies. The chondrite-normalized abundance patterns, especially one for a hibonite-perovskite spot, are quite similar to the patterns of calculated solid/gas partition coefficients for hibonite and perovskite at 10^(-3) atm and are not consistent with formation of the inclusion by closed-system fractional crystallization. In contrast with the features that are consistent with a condensation origin, there are problems with any model for the formation of this inclusion that includes a molten stage, relic grains, or volatilization. If thermodynamic models of equilibrium condensation are correct, then this inclusion formed at pressures <5 x 10^(-3) atm, possibly with enrichments (