26Al-depletions in anomalous and solar PLAC-like CAIs suggest high degrees of processing in the early solar nebula

Abstract

Due to their large nucleosynthetic anomalies [1], platy hibonite crystals (PLACs) are thought to be among the oldest solids that formed in the solar system. Most of them lack evidence for incorporation of 26Al, suggesting formation prior to injection and/or homogenization of 26Al in the solar system [1]. Our combined study of O, Ca and Ti isotopes in a large number of PLACs and related CAIs (hereafter PLAC-like CAIs) revealed a correlation between Δ17O, i.e., the mass-independent variation in O isotopes, and the range of large-scale anomalies in 48Ca and 50Ti (>100‰; [2]). Such a distribution is expected if the primordial (protosolar molecular cloud) dust was depleted in 16O relative to the inferred solar value [2,3]. In addition, we can now use this relationship to distinguish between solar (solar O, δ48Ca and δ50Ti ？ 0‰) and anomalous PLAC-like CAIs (16O-depleted relative to solar, range of δ48Ca and δ50Ti). We have studied the Al-Mg systematics in the same CAIs with the WiscSIMS Cameca IMS 1280 to investigate whether both solar and anomalous PLAC-like CAIs belong to the 26Aldepleted population or whether these CAIs represent multiple generations with respect to their incorporation of 26Al, in which case we would expect to find relationships between the degree of nucleosynthetic heterogeneity, O isotopes and 26Al incorporation.