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

Abstract

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 approximately zero 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. Further-more, a relationship exists between 26Al incorporation and ？17O (i.e., 26Al-free CAIs have re-solved variations in ？17O, while all CAIs with resolved 26Mg excesses have ？17O values close to ？23‰). 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 ？17O, (2) a stage of 26Al-admixture, during which anomalies in 48Ca and 50Ti had been largely diluted and a ？17O value of ~ ？23‰ had been achieved in the CAI formation region, and (3) a nebula with an approximately canonical level of 26Al and a ？17O value of ~ ？23‰ in the CAI formation region.