Titanium isotope systematics of refractory inclusions: Echoes of molecular cloud heterogeneity

Shollenberger Q., Render J., Jordan M. K., McCain K. A., Ebert S., Bischoff A., Kleine T., Young E. D.,

Abstract

Calcium-aluminum-rich inclusions (CAIs) are highly refractory objects found in different chondrite groups and representsome of the oldest known solids of the Solar System. As such, CAIs provide key information regarding the conditions prevailingin the solar protoplanetary disk as well as subsequent mixing and transport processes. Many studies have investigatedCAIs for their isotopic compositions and reported nucleosynthetic isotope anomalies in numerous elements, which are typicallyexplained by the variable incorporation of isotopically highly anomalous presolar phases. However, with the exceptionof 54Cr-enriched nanospinels, the exact presolar phases responsible for the isotopic heterogeneities are yet to be identified. Toaddress this issue, we here present in-situ Ti isotopic analyses obtained on a diverse set of CAIs from various CV3 chondrites.The in-situ measurements were performed by targeting individual mineral phases of 15 CAIs with laser-ablation mass spectrometryand indicate significant inter- and intra-CAI isotopic heterogeneity in the neutron-rich isotope 50Ti. This is particularlypronounced for primitive fine-grained CAIs, whereas coarse-grained CAIs, which have been subject to melting, exhibitsmaller degrees of Ti isotopic heterogeneity.To further investigate this Ti isotopic heterogeneity, we additionally obtained Ti isotopic compositions of sequential acidleachates from two fine-grained and two coarse-grained CAIs derived from CV3 chondrites. In contrast to potential expectationsfrom the first part of the study, we do not observe any significant intra-CAI Ti isotopic heterogeneity between thedifferent leaching steps. The lack of intra-CAI Ti isotopic heterogeneity in the acid leachate samples of this study likely reflectsthat the leaching procedure is unable to efficiently separate the carriers of isotopically anomalous Ti in CAIs. By comparingthe bulk CAI Ti isotope compositions with Ti isotope data for hibonite-rich objects from the literature, we find that the rangeof Ti isotope compositions recorded by CAIs from various chondrite groups can be accounted for by the averaging of hibonitegrains. In turn, the variable Ti isotope compositions of hibonite grains can be explained by the averaging of isotopicallydiverse presolar grains present in the Sun's parental molecular cloud. This effect of averaging is statistically supported bythe central limit theorem, and the concept has the potential to be useful for other isotopic systems. 2022 Elsevier Ltd. All rights reserved.