Dark inclusions in CO3 chondrites: new indicators of parent-body processes
Dark inclusions in CO3 chondrites: new indicators of parent-body processes
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CO3 球粒陨石中的深色包裹体：母体过程的新指标
DOI：
10.1016/s0016-7037(02)01045-1
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发表时间：
2003
影响因子：
5
通讯作者：
K. Tomeoka
中科院分区：
文献类型：
--
作者：
D. Itoh;K. Tomeoka
通过对Kainsaz、Ornans、Lancé和Warrenton四个CO_3球粒陨石的岩相学和扫描电子显微镜研究，首次发现所有陨石中都存在暗色包裹体(DI)。DIS的大小大多比CV3球粒陨石报告的要小。它们的证据表明，它们是由球粒陨石前体的水蚀变和随后的脱水形成的，因此可能具有类似于CV3球粒陨石中的dis的形成历史。CO3球粒陨石中的DIS主要由细粒的富铁橄榄石组成，根据结构可分为两种类型。第I类岩石含有细粒基质中的圆形、多孔性细粒聚集体，其结构表明它们是球粒假象的碎片。充填富铁橄榄石的矿脉在I型砂岩中很常见，这为它们在母体上经历了水蚀变提供了证据。II型DIS缺乏圆形的多孔团聚体，具有基质样的无特征质地。花岗岩的整体化学成分和花岗岩中橄榄石颗粒的矿物学特征表明，这两类花岗岩具有密切的成因关系。这些碎屑很可能是在不同于陨石当前位置的位置经历了水蚀变和随后的脱水作用的碎屑。其主要元素组成、金属相矿物学特征以及广泛的分散性表明其前驱物质为CO球粒陨石物质。CO母体通常被认为是干燥的、均匀的和未经处理的。但研究表明，CO母体是一个由含水区和无水区组成的非均质砾岩，在小行星加热过程中，含水区发生了水化并随后脱水。角砾化也可能在母体中活跃。DIS和基质在各自的寄主CO3球粒陨石(岩石学3.1~3.6亚型)中同样受到热变质作用的影响，但DIS的二次加工程度(水蚀变和随后的脱水)与寄主球粒陨石的岩石学品位没有明显的相关性。这些观察结果表明，在热变质作用发生之前，DIS已融入到寄主球粒陨石中，影响DIS的次生过程主要发生在热变质之前。
A petrographic and scanning electron microscopic study of the four CO3 chondrites Kainsaz, Ornans, Lancé, and Warrenton reveals for the first time that dark inclusions (DIs) occur in all the meteorites. DIs are mostly smaller in size than those reported from CV3 chondrites. They show evidence suggesting that they were formed by aqueous alteration and subsequent dehydration of a chondritic precursor and so probably have a formation history similar to that of DIs in CV3 chondrites. DIs in the CO3 chondrites consist mostly of fine-grained, Fe-rich olivine and can be divided into two types on the basis of texture. Type I DIs contain rounded, porous aggregates of fine grains in a fine-grained matrix and have textures suggesting that they are fragments of chondrule pseudomorphs. Veins filled with Fe-rich olivine are common in type I DIs, providing evidence that they experienced aqueous alteration on the parent body. Type II DIs lack rounded porous aggregates and have a matrix-like, featureless texture. Bulk chemical compositions of DIs and mineralogical characteristics of olivine grains in DIs suggest that these two types of DIs have a close genetic relationship. The DIs are probably clasts that have undergone aqueous alteration and subsequent dehydration at a location different from the present location in the meteorites. The major element compositions, the mineralogy of metallic phases, and the widely dispersed nature of the DIs suggest that their precursor was CO chondrite material. The CO parent body has been commonly regarded to have been dry, homogeneous, and unprocessed. However, the DIs suggest that the CO parent body was a heterogeneous conglomerate consisting of water-bearing regions and water-free regions and that during asteroidal heating, the water-bearing regions were aqueously altered and subsequently dehydrated. Brecciation may also have been active in the parent body. The DIs and the matrices are similarly affected by thermal metamorphism in their own host CO3 chondrites (petrologic subtypes 3.1 to 3.6), but the degree of the secondary processing (aqueous alteration and subsequent dehydration) of the DIs has no apparent correlation with the petrologic grades of the host chondrites. These observations suggest that the DIs had been incorporated into the host chondrites before the thermal metamorphism took place and that the secondary processes that affected the DIs largely occurred before the thermal metamorphism.