A multielement isotopic study of refractory FUN and F CAIs: Mass-dependent and mass-independent isotope effects

Calcium-aluminum-rich inclusions (CAIs) are the oldest dated objects that formed inside the Solar System. Among these are rare, enigmatic objects with large mass-dependent fractionation effects (F CAIs), which sometimes also have large nucleosynthetic anomalies and a low initial abundance of the short-lived radionuclide ²⁶Al (FUN CAIs). We have studied seven refractory hibonite-rich CAIs and one grossite-rich CAI from the Murchison (CM2) meteorite for their oxygen, calcium, and titanium isotopic compositions. The ²⁶Al-²⁶Mg system was also studied in seven of these CAIs. We found mass-dependent heavy isotope enrichment in all measured elements, but never simultaneously in the same CAI. The data are hard to reconcile with a single-stage melt evaporation origin and may require reintroduction or reequilibration for magnesium, oxygen and titanium after evaporation for some of the studied CAIs. The initial ²⁶Al/²⁷Al ratios inferred from model isochrons span a range from <1 × 10⁻⁶ to canonical (∼5 × 10⁻⁵). The CAIs show a mutual exclusivity relationship between inferred incorporation of live ²⁶Al and the presence of resolvable anomalies in ⁴⁸Ca and ⁵⁰Ti. Furthermore, a relationship exists between ²⁶Al incorporation and Δ¹⁷O in the hibonite-rich CAIs (i.e., ²⁶Al-free CAIs have resolved variations in Δ¹⁷O, while CAIs with resolved ²⁶Mg excesses have Δ¹⁷O values close to −23‰). Only the grossite-rich CAI has a relatively enhanced Δ¹⁷O value (∼−17‰) in spite of a near-canonical ²⁶Al/²⁷Al. We interpret these data as indicating that fractionated hibonite-rich CAIs formed over an extended time period and sampled multiple stages in the isotopic evolution of the solar nebula, including: (1) an ²⁶Al-poor nebula with large positive and negative anomalies in ⁴⁸Ca and ⁵⁰Ti and variable Δ¹⁷O; (2) a stage of ²⁶Al-admixture, during which anomalies in ⁴⁸Ca and ⁵⁰Ti had been largely diluted and a Δ¹⁷O value of ∼−23‰ had been achieved in the CAI formation region; and (3) a nebula with an approximately canonical level of ²⁶Al and a Δ¹⁷O value of ∼−23‰ in the CAI formation region.