Multiple generations of refractory inclusions in the metal-rich carbonaceous chondrites Acfer 182/214 and Isheyevo

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 Mg-26(Mg-26*), a decay product of the short-lived radionuclide Al-26(T-1/2 similar to 730, 000 yr), that correspond to an initial Al-26/Al-27 ratio of similar to(4-7) x 10(-5). It is suggested that Al-26 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 Al-26 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 Mg-26*. Most CAIs composed of grossite (CaAl4O7), hibonite (CaAl12O19), Al-rich pyroxene, perovskite (CaTiO3), and gehlenitic melilite (Ca2Al2SiO7-Ca2MgSi2O7) show either unresolvable or small Mg-26* corresponding to an initial Al-26/Al-27 ratio of similar to 4 x 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 Mg-26* corresponding to the initial Al-26/Al-27 ratio of similar to 5 x 10(-5). The Al-26-poor and Al-26-rich CAIs are characterized by O-16-rich (Delta O-17 < - 20 parts per thousand) compositions typical of CAIs. We suggest that the Al-26-poor and Al-26-rich CAIs represent samples of at least two generations of CAIs formed before and after injection of Al-26 into the solar system, respectively. Model yields of O-16, O-17, and O-18 for SN wind prior to explosion, during explosion, and in total, combined with the observations that both Al-26-poor and Al-26-rich CAIs plot on a three-isotope oxygen diagram (delta O-17 vs. delta O-18) along a single line with a slope of similar to 1 are consistent with injection of 26Al with the SN wind into the protosolar molecular cloud rather with the SN explosion into the disk.