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

• Published in: The Astrophysical journal 2008-01, Vol.672 (1), p.713-721
• Main Authors: Krot, Alexander N, Nagashima, Kazuhide, Bizzarro, Martin, Huss, Gary R, Davis, Andrew M, Meyer, Bradley S, Ulyanov, Alexander A
• Format: Article
• Language: English
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/521973

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 super(26)Mg( super(26)Mg*), a decay product of the short-lived radionucllde super(26)Al (T sub(1/2) similar to 730.000 yr), that correspond to an initial super(26)Al/ super(27)Al ratio of similar to (4-7) x 10 super(-7). It is suggested that super(26)Al 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 super(26)Al 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 super(26)Mg*. Most CAIs composed of grossite (CaAl sub(4)O sub(7)), hibonite (CaAl sub(12)O sub(19)), Al-rich pyroxene, perovsklte (CaTiO sub(3)), and gehlenitic melilite (Ca sub(2)Al sub(2)SiO sub(7)-Ca sub(2)MgSl sub(2)O sub(7)) show either unresolvable or small super(26)Mg* corresponding to an initial super(26)Al/ super(27)Al ratio of similar to 4 x 10 super(-7). Some of the grossite-rich CAIs and the less refractory inclusions composed of melilite, spinel (MgAl sub(2)O sub(4)), Al, Ti-pyroxene, and anorthite (CaAl sub(2)Si sub(2)O sub(8)) have large super(26)Mg* corresponding to the initial super(26)Al/ super(27)Al ratio of similar to 5 x 10 super(-5). The super(26)Al-poor and super(26)Al-rich CAIs are characterized by super(16)O-rich ( Delta super(17)O < -20 %o) compositions typical of CAIs. We suggest that the super(26)Al-poor and super(26)Al-rich CAIs represent samples of at least two generations of CAIs formed before and after injection of super(26)Al into the solar system, respectively. Model yields of super(16)O, super(17)O, and super(18)O for SN wind prior to explosion, during explosion, and in total, combined with the observations that both super(26)Al-poor and super(26)Al-rich CAIs plot on a three-isotope oxygen diagram ( delta super(17)O vs. delta super(18)O) along a single line with a slope of similar to 1 are consistent with injection of super(26)Al with the SN wind into the protosolar molecular cloud rather with the SN explosion into the disk.