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Measured and modeled Ca and Ti isotopic fractionation effects in a diverse suite of refractory inclusions are used to understand processes of condensation in the solar protoplanetary disk where they and their precursor materials formed. This coordinated approach reveals largely decoupled isotopic signatures and implies that few, if any, of the studied inclusions can be considered primary condensates. All studied inclusions are enriched in light Ca isotopes (∼−0.2 to − 2.8 ‰ / amu ), but only two show correspondingly light Ti isotopes. Studied inclusions exhibit both heavy and light Ti isotope enrichments (∼0.3 to − 0.4 ‰ / amu ). These refractory element isotopic signatures, therefore, suggest admixture and reprocessing of earlier formed materials with distinct condensation histories. Along with coordinated measurements of 50 Ti isotopic anomalies, which span a range from ∼0 to ∼40 epsilon-unit excesses, the comparison of measured and modeled fractionation of Ca and Ti isotopes provides a powerful approach to understanding primitive nebular processes and environments in the protoplanetary disk. Remarkable evidence for Ca isotopic zoning within a typical Type B1 inclusion exemplifies the potential record of the earliest solar nebula that is likely lost and/or overprinted in the isotopic compositions of more volatile elements (e.g., Mg, Si, and O) by later modification processes. [ABSTRACT FROM AUTHOR]

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 26 Al/ 27 Al ratios and could have formed prior to injection or widespread distribution of 26 Al in the solar nebula. In this study, we report 26 Al– 26 Mg 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 26 Al– 26 Mg, calcium and titanium). Our results confirm (1) the large range of nucleosynthetic anomalies in 50 Ti and 48 Ca (our data range from −70‰ to +170‰ and −60‰ to +80‰, respectively), (2) the substantial range of Δ 17 O values (−28‰ to −17‰, with Δ 17 O = δ 17 O − 0.52 × δ 18 O), and (3) general 26 Al-depletion in PLAC-like CAIs. The multielement approach reveals a relationship between Δ 17 O and the degree of variability in 50 Ti and 48 Ca: PLAC-like CAIs with the highest Δ 17 O (∼−17‰) show large positive and negative 50 Ti and 48 Ca anomalies, while those with the lowest Δ 17 O (∼−28‰) have small to no anomalies in 50 Ti and 48 Ca. These observations could suggest a physical link between anomalous 48 Ca and 50 Ti carriers and an 16 O-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 16 O-poor (Δ 17 O ⩾ −17‰) relative to the primordial gaseous (CO + H 2 O) reservoir (Δ 17 O < −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. [ABSTRACT FROM AUTHOR]