Formation and decomposition of vacancy-rich clinopyroxene in a shocked eucrite: New insights for multiple impact events
Formation and decomposition of vacancy-rich clinopyroxene in a shocked eucrite: New insights for multiple impact events
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DOI：
10.1016/j.gca.2022.05.017
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发表时间：
2022-05
影响因子：
5
通讯作者：
A. Zhang;Jie Li;Jia‐Ni Chen;Yuanyun Wen;Yan‐Jun Guo;Yang Li;N. Sakamoto;H. Yurimoto
中科院分区：
文献类型：
--
作者：
A. Zhang;Jie Li;Jia‐Ni Chen;Yuanyun Wen;Yan‐Jun Guo;Yang Li;N. Sakamoto;H. Yurimoto
撞击是决定行星和小行星形成和演化的基本过程。行星和小行星表面的一些物质受到多次撞击是不可避免的。然而，对于多次地层后撞击事件的明确的岩石学记录却很少被描述。在这里，我们报告了西北非洲8647的震荡长长岩的薄震荡熔体脉主要由细粒粒间或蛭状鸽岩和钙长岩组合组成，而不是致密的富空斜辉石。相反，脉体中富空斜辉石以不规则形状的残余颗粒普遍存在，被粒间或蛭状鸽石和钙长石组合所包围。激波熔体脉中夹带的二氧化硅碎片包含一个硅质岩心和一个石英边缘。富空斜辉石和钴矿的出现可以用两次撞击事件来最好地解释。第一次撞击事件产生了激波熔体脉，并导致富空斜辉石和钴矿的形成。第二次撞击事件加热了细粒熔融矿脉，导致富空斜辉石大面积部分分解和钴矿部分回变。本文首次报道了地外物质中激波诱导富空斜辉石的分解反应。我们认为，冲击陨石中高压矿物的广泛分解和/或反转化可以被认为是多次撞击事件的重要记录。
Impact is a fundamental process shaping the formation and evolution of planets and asteroids. It is inevitable that some materials on the surface of planets and asteroids have been impacted for many times. However, unambiguous petrological records for multiple post-formation impact events are rarely described. Here, we report that the thin shock melt veins of the shocked eucrite Northwest Africa 8647 are dominated by a fine-grained intergranular or vermicular pigeonite and anorthite assemblage, rather than compact vacancy-rich clinopyroxene. Vacancy-rich clinopyroxene in the veins instead is ubiquitous as irregularly-shaped, relict grains surrounded by intergranular or vermicular pigeonite and anorthite assemblage. The silica fragments entrained in shock melt veins contain a coesite core and a quartz rim. The occurrences of vacancy-rich clinopyroxene and coesite can be best explained by two impact events. The first impact event produced the shock melt veins and lead to the formation of vacancy-rich clinopyroxene and coesite. The second impact event heated the fine-grained melt veins and lead to the widespread partial decomposition of vacancy-rich clinopyroxene and the partial back-transformation of coesite. This paper is the first report of the decomposition reaction of shock-induced vacancy-rich clinopyroxene in extraterrestrial materials. We propose that widespread decomposition and/or back-transformation of high-pressure minerals in shocked meteorites can be considered as important records of multiple impact events.