Origin of Low-26Al/27Al Corundum/Hibonite Inclusions in Meteorites

Most meteoritic calcium-rich, aluminum-rich inclusions formed from a reservoir with ²⁶Al/²⁷Al ≈ 5 × 10⁻⁵, but some record lower (²⁶Al ²⁷Al)₀, demanding they sampled a reservoir without live ²⁶Al. This has been interpreted as evidence for “late injection” of supernova material into our protoplanetary disk. We instead interpret the heterogeneity as chemical, demonstrating that these inclusions are strongly associated with the refractory phases corundum or hibonite. We name them “low-²⁶Al/²⁷Al corundum/hibonite inclusions” (LAACHIs). We present a detailed astrophysical model for LAACHI formation in which they derive their Al from presolar corundum, spinel, or hibonite grains 0.5–2 μm in size with no live ²⁶Al; live ²⁶Al is carried on smaller (<50 nm) presolar chromium spinel grains from recent nearby Wolf–Rayet stars or supernovae. In hot (≈1350–1425 K) regions of the disk, these grains and perovskite grains would be the only survivors. These negatively charged grains would grow to sizes 1–10³ μm, even incorporating positively charged perovskite grains, but not the small, negatively charged ²⁶Al-bearing grains. Chemical and isotopic fractionations due to grain charging was a significant process in hot regions of the disk. Our model explains the sizes, compositions, oxygen isotopic signatures, and the large, correlated ⁴⁸Ca and ⁵⁰Ti anomalies (if carried by presolar perovskite) of LAACHIs, and especially how they incorporated no ²⁶Al in a solar nebula with uniform, canonical ²⁶Al/²⁷Al. A late injection of supernova material is obviated, although formation of the Sun in a high-mass star-forming region is demanded.