A Distinct Nucleosynthetic Heritage for Early Solar System Solids Recorded by Ni Isotope Signatures

As the earliest dated solids, calcium-aluminum-rich inclusions (CAIs) provide a unique window into the early solar system. However, for many elements, CAIs have been shown to exhibit a very different nucleosynthetic isotope signature from that of later-formed bulk meteorites. To explore this critical difference between solar system materials, we investigate a broad set of CAI samples for both mass-dependent and non-mass-dependent (nucleosynthetic) isotope variations in the siderophile element nickel (Ni). We find that fine-grained CAIs show little if any mass-dependent Ni isotopic fractionation, whereas coarse-grained inclusions exhibit a broad range of isotopically heavy signatures. Because mass-dependent variations appear to be coupled with nucleosynthetic anomalies in CAIs, a part of this Ni isotope variability could be due to thermal processing that acted on these samples. Nucleosynthetic Ni isotopic signatures show that CAIs share a genetic heritage with carbonaceous meteorites and provide a clear distinction from the isotopic reservoirs occupied by terrestrial Ni and non-carbonaceous meteorites. However, whereas nucleosynthetic Ni isotope heterogeneity in previously investigated bulk meteorites was ascribed to variation in the neutron-poor isotope ⁵⁸Ni, we here find that CAI signatures require variability in other, more neutron-rich Ni isotopes. Taken in aggregate with previous work, this highlights a change in the nucleosynthetic character from CAIs to later-formed solids that cannot be explained by variable admixture of a single presolar phase or material from a specific supernova shell. Instead, these data reveal the complex evolution of the solar system, including blending and reprocessing of matter from several generations and types of stars.