Origin of Nucleosynthetic Isotope Variability in the NC Reservoir: Evidence from Ti, Cr, and Mo Isotopes

Nucleosynthetic isotope anomalies allow distinguishing between non-carbonaceous (NC) and carbonaceous (CC) type meteorites, and have revealed correlated isotope variations especially among NC bodies. Understanding the origin of this NC trend is important for identifying the processes that produced the NC isotope heterogeneity, and for using these isotope anomalies to reconstruct the early evolution of the solar protoplanetary disk. We report mass-independent Ti, Cr, and Mo isotope compositions for a comprehensive set of previously not or only poorly investigated meteorites, as well as acid leachates obtained from the sequential digestion of primitive ordinary chondrites. Some of the samples investigated in this study fill previously identified apparent gaps in the NC trend, suggesting these gaps reflect unrepresentative sampling of a more continuous isotopic trend. Bulk meteorites and leachates exhibit distinct isotope systematics, indicating that the NC isotope variability does not reflect selective thermal processing of presolar carriers in the disk. The NC trend also cannot reflect the continuous addition of CC dust from the outer to the inner disk, because early- and late-formed NC meteorites display largely overlapping isotopic compositions. Instead, we find that the NC isotope heterogeneity is best accounted for by fractionation and mixing among chemically and isotopically distinct dust components, similar to the processes that produced the isotopic variability among carbonaceous chondrites. On this basis we argue for the presence of substructures in the inner disk, which facilitated fractionation and mixing among distinct dust components, and helped preserve a long-lived dust reservoir from which NC planetesimals accreted over an extended period of time.