Yamato-86720: A CM carbonaceous chondrite having experienced extensive aqueous alteration and thermal metamorphism
Yamato-86720: A CM carbonaceous chondrite having experienced extensive aqueous alteration and thermal metamorphism
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Yamato-86720：经历了广泛的水蚀变和热变质作用的 CM 碳质球粒陨石
DOI：
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发表时间：
1989
期刊：
影响因子：
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通讯作者：
K. Yanai
中科院分区：
文献类型：
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作者：
K. Tomeoka;H. Kojima;K. Yanai
对 Yamato-86720 (Y-86720) 的岩相学和矿物学研究表明，它可能是一种 CM 碳质球粒陨石，经历了与大多数 CM 球粒陨石截然不同的蚀变历史。 Y-86720 含有异常高丰度的硫铁矿（9 vol%）；大多数陨石以亚微米到微米的颗粒形式存在，分散在整个陨石中。存在 Ca-Mg 碳酸盐和少量 Fe-Ni 金属（主要是镍纹石），但不存在磁铁矿和五氯苯酚。这块陨石含有球粒和聚集体，它们完全被光学半透明材料（可能是页硅酸盐）所取代。因此，Y-86720 可能广泛受到水蚀变的影响；蚀变程度可能是 CM 球粒陨石中最高的。基体主要由细晶粒的镁铁橄榄石、近非晶态的富硅镁铁材料和富铁材料组成；后者可以是水铁矿。页硅酸盐很少见。结构表明橄榄石和富含 Si-Mg-Pe 的材料是通过加热改变层状硅酸盐而产生的。球粒和聚集体的替代产物显示出比蛇纹石更低的 (Mg+Fe)/(Si+Al) 比率和相对较高的 Na 含量（以 Na2O 计高达 2.0 wt%），表明不仅存在蛇纹石，而且还存在另一种类型的页硅酸盐。然而，替代产品始终显示出较高的分析总数；因此它们可能已经脱水并部分转变为无水相。这些结果表明Y-86720在水蚀变后经历了轻度的热变质作用；它似乎在还原条件下被加热到 500°C 以上。大多数细粒陨石可能在变质过程中从基质中分离出来。许多陨石被类似水铁矿的材料所取代；因此，Y-86720可能受到热变质作用后额外的温和水相变化的影响。
A petrographic and mineralogical study of Yamato-86720 (Y-86720) shows that it may be a CM carbonaceous chondrite that has experienced a con siderably different alteration history from most CM chondrites. Y-86720 has an unusually high abundance of troilite ( 9 vol%); most troilite occurs in submicron to micron grains dispersed throughout the meteorite. Ca-Mg carbonates and minor Fe-Ni metal, mostly taenite, are present, but magnetite and PCP are absent. This meteorite contains chondrules and aggregates that were completely replaced by optically translucent materials, presumably phyllosilicates. Thus Y-86720 was probably extensively affected by aqueous alteration; the degree of alteration may be the highest of the CM chondrites. Matrix consists largely of fine grains of Mg-Fe olivine, a nearly amorphous Si Mg-Fe-rich material, and an Fe-rich material; the latter may be ferrihydrite. Phyllosilicates are rare. The textures suggest that the olivine and the Si-Mg-Pe rich material were produced by alteration of phyllosilicates by heating. The replacement products of chondrules and aggregates show lower (Mg+Fe)/(Si+ Al) ratios than serpentine and relatively high Na contents (up to 2.0 wt% as Na20), suggesting the presence of not only serpentine but another type of phyllosilicate. However, the replacement products show consistently high analytical totals; thus they may have been dehydrated and partially altered to anhydrous phases. These results suggest that Y-86720 has experienced mild thermal metamorphism after the aqueous alteration; it appears to have been heated above 500C in a reduced condi tion. Most fine-grained troilite probably segregated from matrix during the metamorphic process. Many of the troilites are replaced by a ferrihydrite-like material; thus, Y-86720 may have been affected by additional mild aqueous altera tion after the thermal metamorphism.