Study on Formational Environment of Anomalous Carbonaceous Chondrites

三颗异常南极陨石Yamato-82162、Yamato-86720和Belgica-7904同时具有CI和CM球粒陨石群的特征，被归为CY群。它们包括丰富的层状硅酸盐，其化学组成不同于Cl和CM族。CY块体陨石的氧同位素组成也不同于CI和CM陨石。CY组中的金属相与方铁矿和磁铁矿共存，表明CY组的组分是从氧化的星云气体中凝聚而成的，与太阳气体相比，这种气体中的氢和氦已经减少。CY组陨石球粒中的辉石、斜长石和基质玻璃与氧化的星云气体在低于600 K的温度下反应，形成富滑石的层状硅酸盐，后来在300 K左右由橄榄石生成蛇纹石。CY组层状硅酸盐的形成环境介于CI和CM组之间; CM榴辉岩中的层状硅酸盐是由母体过程产生的，与星云气体没有明显的反应，CY组为星云过程形成，并有明显的反应生成含水矿物，CI组为星云过程和母体过程共同作用形成。此外，CY组的幼鱼经历了母体休克加热引起的脱水。

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.