Condensate refractory inclusions from the CO3.00 chondrite Dominion Range 08006: Petrography, mineral chemistry, and...

We have found two refractory inclusions in the CO3.00 carbonaceous chondrite Dominion Range (DOM) 08006 that appear to be primary condensates from the early solar nebula. One, inclusion 56-1, contains the first four phases predicted to form by equilibrium gas-solid condensation: corundum; hibonite; grossite; and perovskite. The other, 31-2, contains nine predicted condensate phases: hibonite; grossite; perovskite; melilite; spinel; FeNi metal; diopside; forsterite; and enstatite. Except for melilite/spinel, the phases occur in the predicted sequence from core to rim of the inclusion, which has an irregular shape inconsistent with a molten stage. This inclusion preserves the most complete record of condensation in the early solar nebula that has yet been found. The physical evidence reported here supports equilibrium condensation calculations that predict the observed sequence as well as the assumptions upon which they are based, such as total pressure (~10^-3 atm), bulk system composition (solar), and C-O-H proportions. All phases in both inclusions and the associated ferromagnesian silicates are 16O-rich, with ∆¹⁷O between -25 and -20‰, implying that this is the original composition of the vast majority of primary condensates and that ¹⁶O-poor compositions observed in many isotopically heterogeneous inclusions are largely due to subsequent isotopic exchange. While the nebula was well-mixed with respect to oxygen isotopic composition, clearly resolved anomalies in Ca and Ti isotopic compositions indicate that some isotopic heterogeneity existed early and was preserved during condensation. Inclusion 31-2 did not incorporate live ²⁶Al and has nucleosynthetic anomalies in the heavy Ca and Ti isotopes (i.e., δ⁴⁸Ca = 4.3 ± 1.9‰; δ⁵⁰Ti = 8.8 ± 2.0‰). In contrast, inclusion 56-1 has radiogenic ²⁶Mg excesses yielding a (²⁶Al/²⁷Al)₀ ratio of (1.0 ± 0.1) × 10^-5 and negative nucleosynthetic isotopic anomalies in Ca (δ⁴⁸Ca = -10.3 ± 4.2‰) and Ti (δ⁵⁰Ti = -4.3 ± 2.9‰). Thus, it represents a deviation from the mutual exclusivity relationship between ²⁶Al incorporation and large nucleosynthetic anomalies. The reservoirs in which these inclusions formed had similar O-isotopic and different Al-, Ca- and Ti-isotopic compositions, suggesting that while the CAI-forming region was well-mixed with respect to oxygen isotopic composition, clearly resolved anomalies in Ca and Ti isotopic compositions indicate that some isotopic heterogeneity existed and was preserved during condensation.