Non-nebular origin of dark mantles around chondrules and inclusions in CM chondrites
Non-nebular origin of dark mantles around chondrules and inclusions in CM chondrites
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CM 球粒陨石中球粒和包裹体周围深色地幔的非星云起源
DOI：
10.1016/j.gca.2005.11.009
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发表时间：
2006
影响因子：
5
通讯作者：
J. Wasson
中科院分区：
文献类型：
--
作者：
J. Trigo‐Rodríguez;A. Rubin;J. Wasson
我们对九个横跨水蚀变序列的CM球粒陨石的研究得出结论，CM球粒陨石中的球粒陨石和包裹体周围致密的黑色细地幔并不是Metzler等人所模拟的在太阳星云中获得的离散的细粒边缘。CM球粒陨石中的吸积尘埃：太阳星云过程的证据。地平线。宇宙人。Acta56,1992,2873-2897]。导致凝聚的星云过程产生的物质的孔隙率远远高于黑暗地幔中的物质。我们推测，地幔是由CM母行星上的多孔星云物质通过撞击压实(一种在球粒和包裹体附近产生最低孔隙度的过程)产生的。压实作用之后是水蚀变，在球粒间区的空隙中形成了千里石、蛇纹石、含镍硫化物和其他次生产物。Metzler等人的研究成果。报道了地幔厚度与封闭物体半径之间的关系。在大和791198中，当包括所有大小的中心物体和暗块时，我们发现没有相关性，但如果我们限制考虑半径为35μm的中心物体，则发现显著的相关性(R2=0.44)；在QUE 97990中也发现中等相关性。我们认为，李子布丁状前体的撞击诱导剪切产生了观察到的“地幔”；在撞击事件中，这些“地幔”被邻近较强的球粒和包裹体保护而不被粉碎。一些蚀变程度较低的CM球粒陨石中的地幔显示出明显的层，这可能在很大程度上反映了孔隙度的差异。通常，灰色、均匀的内层被外层包围，外层由较暗的硅酸盐组成，带有BSE-明亮的斑点。Metzler等人将CM球粒陨石描述为“原生增积岩”。不是在星云中形成的，而是在母体上形成的。这些碎屑中没有太阳耀斑粒子的踪迹和太阳风注入的稀有气体，反映了它们的岩化性质和低的表面积/体积比，当时它们居住在风化层中，受到太阳粒子的照射。这些碎屑类似于普通球粒陨石风化角砾岩中的浅色变质碎屑(也没有太阳耀斑粒子轨迹和太阳风气)。
Our examination of nine CM chondrites that span the aqueous alteration sequence leads us to conclude that compact dark fine mantles surrounding chondrules and inclusions in CM chondrites are not discrete fine-grained rims acquired in the solar nebula as modeled by Metzler et al. [Accretionary dust mantles in CM chondrites: evidence for solar nebula processes. Geochim. Cosmochim. Acta56, 1992, 2873–2897]. Nebular processes that lead to agglomeration produce materials with porosities far higher than those in the dark mantles. We infer that the mantles were produced from porous nebular materials on the CM parent asteroid by impact-compaction (a process that produces the lowest porosity adjacent to chondrules and inclusions). Compaction was followed by aqueous alteration that formed tochilinite, serpentine, Ni-bearing sulfide, and other secondary products in voids in the interchondrule regions. Metzler et al. reported a correlation between mantle thickness and the radius of the enclosed object. In Yamato 791198 we find no correlation when all sizes of central objects and dark lumps are included but a significant correlation (r2=0.44) if we limit consideration to central objects with radii >35μm; a moderate correlation is also found in QUE 97990. We suggest that impact-induced shear of a plum-pudding-like precursor produced the observed “mantles”; these were shielded from comminution during impact events by the adjacent stronger chondrules and inclusions. Some mantles in CM chondrites with low degrees of alteration show distinct layers that may largely reflect differences in porosity. Typically, a gray, uniform inner layer is surrounded by an outer layer consisting of darker silicates with BSE-bright speckles. The CM-chondrite objects characterized as “primary accretionary rocks” by Metzler et al. did not form in the nebula, but rather on the parent body. The absence of solar-flare particle tracks and solar-wind-implanted rare gases in these clasts reflect their lithified nature and low surface/volume ratios during the period when they resided in the regolith and were subject to irradiation by solar particles. The clasts are analogous to the light-colored metamorphosed clasts in ordinary-chondrite regolith breccias (which also lack solar-flare particle tracks and solar-wind gas).