车里雅宾斯克(Chelyabinsk)陨石的岩石矿物特征与冲击变质作用

2013年2月15日,俄罗斯车里雅宾斯克(Chelyabinsk)发生了伴随罕见的空中爆炸的大规模陨石雨事件。本文对3块代表不同冲击变质程度的车里雅宾斯克陨石碎块进行了研究。它们都具有部分熔壳,其中1块仅出现碎裂,1块含有冲击熔融细脉,1块基本由冲击熔融囊和冲击熔脉组成。冲击变质程度最低的样品,代表了该陨石母体小行星的原始岩石矿物学特征:即具有粗粒的岩石结构和均一的矿物化学组成,但仍保留一些残余球粒,表明受到了明显的热变质作用,其岩石类型可划分为5型。铁镁质硅酸盐高的Fe O含量(橄榄石Fa:27.9mol%~28.2mol%,辉石Fs值:23.3mol%~23.7mol%)、以及较低的Fe-Ni金属含量,表明其化学群属于低铁低金属的LL群。我们所分析的样品与前人报导的结果相似,未发现不同岩性的岩屑,表明车里雅宾斯克陨石的原始岩矿特征较为均一。3块陨石碎块中,随着冲击程度的增强,其冲击变质特征依次表现为硅酸盐矿物的破碎、熔长石化更为普遍、陨硫铁与铁镍合金共熔、硅酸盐熔脉的形成、铬铁矿与长石共熔、以及大量熔融囊的发育等。但是,在冲击熔融囊和熔脉中,以及相邻围岩中均未发现高压矿物相。熔脉中的橄榄石晶屑和相邻围岩的橄榄石颗粒表现为化学成分的不均一,在背散射电子图像中呈不同灰度的结构。这与其他强烈冲击变质陨石中橄榄石的林伍德石或瓦茨利石相变相似。该陨石中林伍德石或瓦茨利石的缺失很可能是由于强烈撞击后高温产生的退变质。这也表明车里雅宾斯克陨石的母体小行星可能遭受了非常强烈的撞击事件。

Petrology, mineralogy and shock metamorphism of the Chelyabinsk meteorite

On Feb 15, 2013, a meteorite exploded extremely intense above the city of Chelyabinsk, Russia. We studied three representative fragments of the Chelyabinsk meteorite, which preserved at least part of fusion crust and show different degree of shock metamorphism. One of the fragments is almost intact with only fractures, another fragment contains thin shock-induced melt veins, and the third consists mainly of shock-induced melt pockets and thick veins. The little shocked fragment preserved the initial petrographic and mineral chemical features of the parent asteroid of Chelyabinsk, including coarse-grained texture, homogeneous compositions of olivine and pyroxene, and blurred chondrule outlines but can be recognized, which indicate a high degree of thermal metamorphism and a petrographic type of 5 of chondrites. The high and homogeneous FeO contents of Mg-Fe-silicates (Fa: 27.9mol%~28.2mol%; Fs: 23.3mol%~23.7mol%) and the low abundance of Fe-Ni metal classify Chelyabinsk into LL group (low FeO and low metal). The classification of Chelyabinsk as a LL5 ordinary chondrite confirms previous work, and there was no other type of lithic clasts. Fractures in silicate minerals, more abundant maskelynite, eutectic texture of troilite and metal, presence of silicate shock melt veins, chromite-plagioclase intergrowth, and melt pockets were observed in order of increasing shock degree. However, no high-pressure polymorphs of silicates have been found in the shock melt pockets or veins. Olivine fragments within the melt pockets and veins and those at their walls show significant chemical heterogeneity, similar to the typical texture of olivine transforming to ringwoodite or wadsleyite in other heavily shocked meteorites. The absence of ringwoodite and wadsleyite is likely due to retrograde metamorphism at high post-shock temperature. This suggests a very severe impact event on the parent asteroid of Chelyabinsk.