Multiple Generations of Refractory Inclusions in the Metal‐Rich Carbonaceous Chondrites Acfer 182/214 and Isheyevo

Abstract

Ca,Al-rich inclusions (CAIs) are believed to have formed by evaporation, condensation, and melting of the pre-existing solids during the earliest stages of the solar system evolution. Most CAIs in unmetamorphosed chondrites contain detectable excesses of 26Mg(26Mg*), a decay product of the short-lived radionuclide 26Al ( -->T1/2 ~ 730,000 yr), that correspond to an initial 26Al/27Al ratio of ~ -->(4–7) × 10−5. It is suggested that 26Al was injected into the protosolar molecular cloud or protoplanetary disk by a nearby core-collapse supernova (SN Type II) and uniformly distributed in the solar system; CAI formation started shortly after injection of 26Al and lasted less than 20,000 yr . Here we show that CAIs from the metal-rich carbonaceous chondrites Acfer 214 (CH) and Isheyevo (CH/CB-like) have a bimodal distribution of 26Mg*. Most CAIs composed of grossite (CaAl4O7), hibonite (CaAl12O19), Al-rich pyroxene, perovskite (CaTiO3), and gehlenitic melilite (Ca2Al2SiO7-Ca2MgSi2O7) show either unresolvable or small 26Mg* corresponding to an initial 26Al/27Al ratio of ~ -->4 × 10−7. Some of the grossite-rich CAIs and the less refractory inclusions composed of melilite, spinel (MgAl2O4), Al,Ti-pyroxene, and anorthite (CaAl2Si2O8) have large 26Mg* corresponding to the initial 26Al/27Al ratio of ~ -->5 × 10−5. The 26Al-poor and 26Al-rich CAIs are characterized by 16O-rich (Δ -->17O < − 20‰) compositions typical of CAIs. We suggest that the 26Al-poor and 26Al-rich CAIs represent samples of at least two generations of CAIs formed before and after injection of 26Al into the solar system, respectively. Model yields of 16O,17O, and 18O for SN wind prior to explosion, during explosion, and in total, combined with the observations that both 26Al-poor and 26Al-rich CAIs plot on a three-isotope oxygen diagram (δ17O vs. δ18O) along a single line with a slope of ~1 are consistent with injection of 26Al with the SN wind into the protosolar molecular cloud rather with the SN explosion into the disk.