Mineralogy of phyllosilicate-rich micrometeorites and comparison with Tagish Lake and Sayama meteorites
Mineralogy of phyllosilicate-rich micrometeorites and comparison with Tagish Lake and Sayama meteorites
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DOI：
10.1016/s0012-821x(02)00777-x
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发表时间：
2002-09
影响因子：
5.3
通讯作者：
T. Noguchi;Tomoki Nakamura;W. Nozaki
中科院分区：
文献类型：
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作者：
T. Noguchi;Tomoki Nakamura;W. Nozaki
利用同步辐射X射线衍射技术和透射电子显微镜研究了4块富页硅酸盐微陨石的结构特征。三个是富含皂石的砂岩，一个是富含蛇纹石的砂岩。在富含皂石的火山岩中，我们不能找到蛇纹石，反之亦然，在富含蛇纹石的MM中。在富含皂石的火山岩中，主要组成矿物是皂石、含Fe和Ni的硫化物和磁铁矿。两个富含皂石的砂岩中含有细粒的富含镁硅铁矿的聚集体。聚集体由<50 nm的多边形镁钨铁矿与少量的Fe硫化物颗粒共存组成。它们的结构，化学成分，以及对塔吉什湖碳质球粒陨石的基质碎片加热实验的结果强烈表明，这些聚集体是由富镁和富铁的碳酸盐颗粒进入地球大气层时分解形成的。在进入地球大气层之前，富含皂石的陨石的估计主要矿物组合与富含碳酸盐的塔吉什湖岩性非常相似，我们认为陨石和陨石来自类似的小行星。富含蛇纹石的MM基质的主要矿物组合和结构与经历过严重水蚀变的狭山CM 2球粒陨石的基质相似。MM中蛇纹石的化学成分表明，MM的水蚀变程度弱于狭山。在MM中，cronstedtite不与tochilinite共存，这与经历了弱到中度水蚀变的CM2钙钛矿不同。然而，不能排除富含蛇纹石的MM来自CM球粒陨石小行星的可能性，因为托奇利尼特可以在进入大气加热过程中优先分解，因为它的分解温度低于cronstedtite。
Four phyllosilicate-rich micrometeorites (MMs) were investigated by a synchrotron radiation X-ray diffraction technique and transmission electron microscopy. Three are saponite-rich MMs and one is a serpentine-rich one. In the saponite-rich MMs, we could not find serpentine, and vice versa in the serpentine-rich MM. In the saponite-rich MMs, major constituent minerals are saponite, Fe- and Ni-bearing sulfides, and magnetite. Two saponite-rich MMs contain fine-grained magnesiowüstite-rich aggregates. The aggregates consist of <50 nm polygonal magnesiowüstite coexisting with minor Fe sulfide grains. Their texture, chemical composition, and the result of heating experiments on matrix fragments of the Tagish Lake carbonaceous chondrite strongly suggest that these aggregates were formed by the breakdown of Mg- and Fe-rich carbonate grains when the MMs entered the Earth’s atmosphere. The estimated major mineral assemblage of the saponite-rich MMs before entering the Earth’s atmosphere is very similar to that of the Tagish Lake carbonate-rich lithology, and we suggest that the MMs and the meteorite were derived from similar asteroids. The major mineral assemblage and texture of the matrix of serpentine-rich MM are similar to the matrix of the Sayama CM2 chondrite that experienced heavy aqueous alteration. Chemical compositions of serpentine in the MM suggest that the degree of aqueous alteration of the MM is weaker than that of Sayama. In the MM, cronstedtite does not coexist with tochilinite, which is different from CM2 chondrites that experienced weak to moderate aqueous alteration. However, the possibility that the serpentine-rich MM was derived from the CM chondrite asteroid cannot be ruled out, because tochilinite can be preferentially decomposed during atmospheric entry heating due to its lower decomposition temperature than that of cronstedtite.