| 其他摘要 | Chondrites are directly formed from the solar nebular, when various types of dust and small grains accreted to form primitive asteroids. Premolar grains in chondrites open a window for understanding the origins of the Solar system and evolutions of stellars. The studies of shock metamorphism from chondrites provide important clues for learning about geological histories and impact processes of other asteroid bodies. Presolar grainsl, also called stardust grains, formed in stellar outflows from late-type stars and supernova ejecta, and survived their interstellar journey into the solar system. Presolar grains have been identified in meteorites, interplanetary dust particles, Antarctic micrometeorites and cometary matter. They are C-rich phases (nanodiamonds, silicon carbide, graphite and silicon nitride), and O-rich phases (oxides and silicates). The laboratory study of presolar grains can provide new information on stellar nucleosynthesis, galactic chemical evolution and grain formation in circumstellar environments. Isotopic studies and elemental analyses of these presolar grains can provide detailed information about the source of the stars. Silicon carbides, the best-studied presolar grains, are divided into several groups based on C-, N-, and Si-isotopes: mainstream, AB, X, Y, Z, possible nova grains, and U/C grains. The abundances of rare types are much higher in smaller grains. Here we report the results of C, Si, N, S, Mg-Al and Ca-Ti isotopic measurements of selected presolar SiC grains from KJE fraction (0.5-0.8 ?m) of the Murchison meteorite. 1113 SiC were identified based on their C and Si isotopic ratios. 25 grains were selected for N, S, Mg-Al and Ca-Ti isotopic analysis. A C-type grain, with high heavy Si (?29Si/28Si = 1345 ± 19 ‰, ?30Si/28Si = 1272 ± 19 ‰), has a huge 32S excess (?33S/32S = –944 ± 33 ‰, ?34S/32S = –941 ± 14 ‰). High-inferred 44Ti/48Ti provide definitive proof of a SN origin. The 32S excess in the C grain, larger than that predicted for the Si/S zone in supernova, is evidence for a radiogenic origin of short-lived 32Si. 32S excesses in X-SiC and Si3N4 are much smaller than expected, and contamination must be invoked to explain the data. Bote the 12M? and 15r SN models give their own better fit to the isotopic ratios of the grains. The extremely large 30Si excesses in the Y and Z grains imply an origin in low-mass AGB stars with metallicities between 0.001 and 0.002. Graphite has different isotopes signatures from those observed in SiC. The correlation among morphology, Raman spectra and isotopic compositions would imply the stellar origin. 103 graphites, size larger than 3 μm, were selected from different fractions of KFA1, KFB1 and KFC1. We would get (1) graphites in low-density show more “cauliflower” morphology, whereas the high-density fraction is dominated by grains with onion morphology; (2) on average low-density grains have higher Raman D/G ratios than medium- and high-density grains, tending to have smaller in-plane crystallite sized of sp2-bonded carbon; (3) 14 out of 24 graphites in KFA1 showing 15N or 18O excess differ from grains from KFB1 and KFC1 with normal N and O isotopes. Therefore, we can come to the conclusion that most of the graphites in KFA1 have an origin of supernova, while grains in KFB1 and KFC1 comes from AGB stars. 6 out of 8 graphites, with kerogen-type Raman spectra, show constant abnormal C isotopic ratios, implying its form of organic carbon. Since N and O isotopes are normal in these grains, we could not infer their origins. Chelyabinsk meteorite is a LL5 type ordinary chondrite, showing different impact characteristics between fractions. The studies of petrology and mineralogy can provide more information on shock metamorphism. Melt veins and melt pocket with maskelynite imply this meteorite in its parent body has experienced strong shock metamorphism, at least S4 level. Olivine grains in or near the melt vein show chemical heterogeneity, but no high-pressur |
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