Study on Formational Environment of Anomalous Carbonaceous Chondrites

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Three anomalous Antarctic meteorites, Yamato-82162, Yamato-86720, and Belgica-7904 have characteristics of both CI and CM chondrite groups, and were classified as CY group. They include abundant phyllosilicates, chemical compositions of which differ from those of CI and CM groups. Oxygen isotopic compositions of the CY bulk meteorites are also different from those of CI and CM chondrites. Metallic phases in the CY group coexist with wustite and magnetite, suggesting that the components of CY group condensated from an oxidized nebular gas which had depleted in hydrogen and helium in comparison to the solar gas.Pyroxene, plagioclase, and groundmass glass in chondrules of CY chondrites reacted with the oxidized nebular gas to form talc-rich phyllosilicates at temperatures lower than 600K, and later serpentine was produced from olivine at temperatures of about 300K.The phyllosilicates of CY group seems to have formed under an environment condition between those of CI and CM chondrites; Phyllosilicates in CM chondrites were produced by the parent body process without remarkable reaction with the nebular gas, those of CY group were formed by nebular process with remarkable reaction to produce the hydrous minerals, and those of CI group by both nebular and parent body processes. In addition, the CY group chondrites have experienced dehydration by shock heating on their parent body.