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

Abstract

Through freeze-thaw disaggregation of the Murchison (CM) carbonaceous chondrite, we have recovered a ~90 x 75 μm refractory inclusion that consists of corundum and hibonite with minor perovskite. Corundum occurs as small (~10 μm), rounded grains enclosed in hibonite laths (~10 μm wide and 30-40 μm 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 ≤ 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 (<1000x) in CI dust relative to gas at low pressures (below 10^(-4) atm). Both hibonite and corundum have δ17O ≈ δ18O ≈ -50‰, indicating formation from an 16O-rich source. The inclusion does not contain radiogenic 26Mg and apparently did not contain live 26Al when it formed. If the short-lived radionuclides were formed in a supernova and injected into the early solar nebula, models of this process suggest that 26Al-free refractory inclusions such as this one formed within the first ~6 x 10^5 years of nebular collapse.