Ca-carbonate in the Orgueil (CI) carbonaceous chondrite: Mineralogy, microstructure and implications for parent body history
Ca-carbonate in the Orgueil (CI) carbonaceous chondrite: Mineralogy, microstructure and implications for parent body history
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DOI：
10.1016/j.epsl.2009.01.038
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发表时间：
2009-04-15
影响因子：
5.3
通讯作者：
Nicholson, Kirsty
中科院分区：
文献类型：
--
作者：
Lee, Martin R.;Nicholson, Kirsty
Orgueil CI碳质球粒陨石是一种角砾岩，它的一些组成碎屑含有碳酸钙颗粒，其矿物学尚未确定，但可能为其小行星或彗星母体的历史提供新的信息。其中一个碎屑的特征表明，除了碳酸钙外，它还含有磁铁矿、橄榄石和正辉石颗粒，这些颗粒被支撑在一个细晶皂石-蛇纹石-铁水合岩体中。碳酸钙颗粒通常是圆形的，直径小于10 μ m，后向散射菊池模式识别出所有被分析的方解石。电子后向散射衍射取向对比图显示，大多数方解石具有亚微米大小的e孪晶，其中一些是由腐蚀的晶粒边缘切割而成。这些微观结构表明，碎屑在含水蚀变过程中经历了非常轻微的冲击，可能伴随着母体风化层的冲击“园艺”。所研究的样品含有其他几个毫米大小的碎屑和细粒碎屑基质，这些碎屑基质不含碳酸钙、橄榄石和正辉石，但可能含有磷灰石、白云石和磁黄铁矿。在其中一个碎屑中有一个层状硅酸盐-磁铁矿脉网，据推测，在母体含水蚀变过程中，这些脉网起到了流体流动的管道作用。同一碎屑内的富硫反应环可能是在陆相前硫酸盐的落后再活化结晶过程中形成的。该研究为动态母体风化层中CI碳质球粒陨石的蚀变提供了很好的证据。Crown Copyright (C) 2009
The Orgueil CI carbonaceous chondrite is a breccia and some of its constituent clasts contain Ca-carbonate grains whose mineralogy is undetermined, but may yield new information on the history of its asteroidal or cometary parent body. Characterisation of one such clast shows that in addition to Ca-carbonate it contains magnetite, olivine and orthopyroxene grains that are supported within a finely crystalline saponite-serpentine-ferrihydrite groundmass. The Ca-carbonate grains are typically rounded, less than 10 mu m in diameter, and backscatter Kikuchi patterns identify all those analysed as calcite. Electron backscatter diffraction orientation contrast maps show that most calcites have sub-pm sized e-twins, some of which are cut by corroded grain margins. These microstructures demonstrate that the clast experienced very mild shock during aqueous alteration and probably accompanying impact 'gardening' of the parent body regolith. The sample studied hosts several other millimeter sized clasts and a fine grained clastic matrix that are free of Ca-carbonate, olivine and orthopyroxene although can contain apatite, dolomite and pyrrhotite. Within one of these clasts is a network of phyllosilicate-magnetite veins that are inferred to have acted as fluid flow conduits during parent body aqueous alteration. Sulphur-rich reaction rims within the same clast are likely to have formed during post-fall remobilization and crystallization of pre-terrestrial sulphates. This study provides good evidence for alteration of the CI carbonaceous chondrites in a dynamic parent body regolith Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved