The Presolar Heritage of ⁵⁰Ti and ²⁶Al Heterogeneity in the Protoplanetary Disk

Nucleosynthetic isotope anomalies have been documented within the solar protoplanetary disk for several isotopes, including 48 Ca, 50 Ti, 54 Cr, 94 Mo, and 96 Zr. However, the existence of isotopic heterogeneities for short-lived radionuclides such as 26 Al remains debated. Here, we investigate potential heterogeneity in the 26 Al distribution by examining nucleosynthetic 50 Ti and 26 Al– 26 Mg isotope systematics in high-temperature early solar system solids—Ca-Al-rich inclusions (CAIs) and hibonites from CM chondrites—and comparing these with presolar grain data. We present high-precision titanium isotope measurements for 13 CAIs, comprising 11 26 Al-rich “normal”-type and two 26 Al-poor fractionation and unidentified nuclear effect (FUN)-type CAIs. Our data, combined with previously published results, reveal a fundamental decoupling between significant nucleosynthetic 50 Ti anomalies and high abundances of 26 Al, a pattern also present in presolar grains. We propose that this mutual exclusivity of 50 Ti and 26 Al observed in CAIs and CM hibonites is inherited directly from protosolar molecular cloud material. Specifically, this isotopic signature reflects the mixing of newly formed, 26 Al-rich molecular cloud material with older, Galactically inherited 26 Al-poor dust characterized by large nucleosynthetic 50 Ti variability. Consequently, solids exhibiting substantial 50 Ti anomalies ( 26 Al-poor FUN-type CAIs and CM hibonites) predominantly inherited older, 26 Al-poor presolar dust, whereas those with minimal 50 Ti anomalies and abundant 26 Al ( 26 Al-rich normal CAIs and CM hibonites) incorporated more of the younger, 26 Al-rich component. This inherited isotopic signature provides a consistent explanation for disk-scale nucleosynthetic heterogeneity in bulk meteorites, challenging the widely accepted notion that the canonical 26 Al/ 27 Al ratio of 5.25 × 10 −5 in 26 Al-rich CAIs represents the solar system average.