Constraining the origin of isotopic variations in chondritic materials through titanium and chromium isotopic analyses of refractory inclusions

Nucleosynthetic isotope anomalies in meteorites indicate a non-uniform distribution of stable isotopes in the protosolar disk. Understanding the origin of the isotopic dichotomy between carbonaceous chondrites (CCs) and non-carbonaceous (NCs) meteorites, especially in Ti (ε⁵⁰Ti) and Cr (ε⁵⁴Cr), is essential for exploring the early evolution of the Solar System and the planetesimal formation. Calcium-aluminum-rich inclusions (CAIs), the oldest Solar System condensates rich in CCs, control the isotopic compositions of refractory elements in CCs. However, the Ti-Cr isotopic compositions of CCs cannot be solely ascribed to commonly studied coarse-grained CAIs (CGs). Fine-grained CAIs (FGs), which have avoided melting after condensation, likely preserve the isotopic signature of CAI-forming gases, making them important for understanding the isotopic compositions of CCs and the origin of the NCCC isotopic dichotomy.

This study investigates the Ti-Cr isotopic compositions of ten FGs and four CGs from three CV chondrites. These CAIs exhibit ε⁵⁰Ti values consistent with previous studies, while their ε⁵⁴Cr values are more variable than were previously obtained. Notably, some FGs present higher ε⁵⁴Cr and ε⁵⁰Ti values than CGs, suggesting a distinct origin for FGs that cannot be attributed to the matrix–CG mixing. Moreover, our results indicate that the diversity in isotopic composition of CCs cannot be fully explained by differences in the abundance of refractory materials among individual CCs. The negative correlation between ε⁵⁰Ti and ε⁵⁴Cr values in CCs suggests that the isotopic variability arose from either metal grains with low ε⁵⁴Cr values, or differences in isotopic composition among each CC matrix.