A link between oxygen, calcium and titanium isotopes in 26Al-poor hibonite-rich CAIs from Murchison and implications for the heterogeneity of dust reservoirs in the solar nebula

PLACs (platy hibonite crystals) and related hibonite-rich calcium-, aluminum-rich inclusions (CAIs; hereafter collectively referred to as PLAC-like CAIs) have the largest nucleosynthetic isotope anomalies of all materials believed to have formed in the solar system. Most PLAC-like CAIs have low inferred initial 26Al/27Al ratios and could have formed prior to injection or widespread distribution of 26Al in the solar nebula. In this study, we report 26Al-26Mg systematics combined with oxygen, calcium, and titanium isotopic compositions for a large number of newly separated PLAC-like CAIs from the Murchison CM2 chondrite (32 CAIs studied for oxygen, 26 of these also for 26Al-26Mg, calcium and titanium). Our results confirm (1) the large range of nucleosynthetic anomalies in 50Ti and 48Ca (our data range from −70‰ to +170‰ and −60‰ to +80‰, respectively), (2) the substantial range of Δ17O values (−28‰ to −17‰, with Δ17O = δ17O − 0.52 × δ18O), and (3) general 26Al-depletion in PLAC-like CAIs.The multielement approach reveals a relationship between Δ17O and the degree of variability in 50Ti and 48Ca: PLAC-like CAIs with the highest Δ17O (∼−17‰) show large positive and negative 50Ti and 48Ca anomalies, while those with the lowest Δ17O (∼−28‰) have small to no anomalies in 50Ti and 48Ca. These observations could suggest a physical link between anomalous 48Ca and 50Ti carriers and an 16O-poor reservoir. We suggest that the solar nebula was isotopically heterogeneous shortly after collapse of the protosolar molecular cloud, and that the primordial dust reservoir, in which anomalous carrier phases were heterogeneously distributed, was 16O-poor (Δ17O ⩾ −17‰) relative to the primordial gaseous (CO + H2O) reservoir (Δ17O < −35‰). However, other models such as CO self-shielding in the protoplanetary disk are also considered to explain the link between oxygen and calcium and titanium isotopes in PLAC-like CAIs.