2008ApJ...672..713K

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 (T_1/2∼730,000 yr), that correspond to an initial ²⁶ Al/²⁷Al ratio of ~(4-7)x10^–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 (Ca₂Al₂SiO₇Ca₂MgSi₂O₇) show either unresolvable or small ²⁶Mg* corresponding to an initial ²⁶Al/²⁷Al ratio of ∼4x10^–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 ^ 26^Mg* corresponding to the initial ²⁶Al/²⁷Al ratio of ∼5x10^–5. The ²⁶Al-poor and ²⁶Al-rich CAIs are characterized by ¹⁶O-rich (Δ¹⁷O←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 ^ 18^O for SN wind prior to explosion, during explosion, and in total, combined with the observations that both ²⁶Al-poor and ²⁶ Al-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.