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

Levke Kööp, Daisuke Nakashima, Philipp R. Heck, Noriko T. Kita, Travis J. Tenner, Alexander N. Krot, Kazuhide Nagashima, Changkun Park, Andrew M. Davis

Geochimica et Cosmochimica Acta
In Press, Accepted Manuscript, Available online 26 April 2017


“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 isotopic 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."