Impact-related chemical modifications of the Chang’E-5 lunar regolith
Impact-related chemical modifications of the Chang’E-5 lunar regolith
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DOI：
10.1016/j.gca.2023.10.031
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发表时间：
2023-10
影响因子：
5
通讯作者：
Yunhua Wu;Shiyong Liao;Pan Yan;Zhiyong Xiao;Zongjun Yin;Wei Yang;Hao Wang;Hengci Tian;Hejiu Hui;Lili Pan;Hongxia Ma;Shitou Wu;Weibiao Hsu
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文献类型：
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作者：
Yunhua Wu;Shiyong Liao;Pan Yan;Zhiyong Xiao;Zongjun Yin;Wei Yang;Hao Wang;Hengci Tian;Hejiu Hui;Lili Pan;Hongxia Ma;Shitou Wu;Weibiao Hsu
月球上的撞击事件被认为是通过冲击变质作用和与外来成分混合等过程改变表面材料的组成。此前的研究表明，中国嫦娥五号任务采集的风化层主要是由当地的母质玄武岩演化而来，可能源于一次喷发的火山活动。嫦娥五号风化层相对简单和单调的原岩为研究撞击事件引起的化学修饰提供了一个独特的机会。在这项研究中，我们对一组不同的月球风化样品进行了详细的岩石学和矿物学分析，其中包括33个小型撞击玻璃颗粒(39-227mμm)，一个大凝聚体(∼1.6m m)和16个玄武岩碎屑(0.1m-1.6m m)。通过数值模拟，定量评价了玄武岩碎屑在不同冲击条件下的熔融行为。我们的主要目标是评估撞击玻璃与石屑碎屑之间的化学变化。虽然大多数均匀的冲击玻璃小球和较大的玄武岩碎屑(≥1 mm)显示出与当地风化岩相似的主体成分(例如，Al_2O_3、CaO和FeO)，但我们认识到冲击过程的额外影响，包括冲击粉碎、冲击熔化和结晶、差异挥发和潜在的选择性熔化。这些过程改变了月球风化层成分的结构和地球化学。嫦娥五号风化层中小碎屑的化学特征表明，细粒组分(如直径为300≤~300μm的碎屑)主要由岩屑和单矿物碎屑组成，其中很大一部分是中稳定碎屑。这些亚毫米级组分的熔化可能导致撞击玻璃的形成，其化学成分偏离当地平均风化层。这些结果对了解嫦娥五号风化层的成分演化具有重要意义。值得注意的是，我们的研究表明，具有不同主元素(如二氧化钛、氧化镁)和次要元素(如稀土、锆、钍和铀)的撞击玻璃球体可能来自当地母质玄武岩的同一原岩，而不是完全归因于外来的撞击喷射物。在这种情况下，小范围的局部撞击可能在嫦娥五号着陆点的撞击史上起到了至关重要的作用。
Impact events on the Moon have been recognized as modifying the composition of surface materials through processes such as shock metamorphism and mixing with exotic components. Previous studies have indicated that the regolith sampled by the Chinese Chang’E-5 mission was primarily evolved from local mare basalts, likely originating from a single episode of effusive volcanism. The relatively simple and monotonic protolith of the Chang’E-5 regolith presents a unique opportunity to investigate the chemical modifications induced by impact events. In this study, we conducted detailed petrographic and mineralogical analyses on a diverse set of lunar regolith samples, including thirty-three small impact glass particles (39–227 μm), one large agglutinate (∼1.6 mm), and sixteen basaltic clasts (0.1–1.6 mm). Numerical modeling was also conducted to quantitatively assess the melting behavior of basaltic clasts under different impact conditions. Our primary objective was to evaluate the chemical variations of impact glass in relation to lithic clasts. While the majority of homogeneous impact glass spherules and larger basaltic clasts (≥1 mm) exhibit similar bulk compositions (e.g., Al2O3, CaO and FeO) to the local regolith, we recognize additional effects of impact processes, including impact comminution, impact melting and crystallization, differential volatilization, and potentially selective melting. These processes have modified the texture and geochemistry of lunar regolith components. The chemical signatures of small clasts in the Chang’E-5 regolith indicate that the fine size fractions (e.g., those with diameters of ≤ 300 μm) are predominantly composed of lithic and monomineralic fragments with a substantial proportion being dominated by mesostasis. Melting of these sub-millimeter fractions may lead to the formation of impact glass with chemical compositions deviating from the average local regolith. These results have implications for understanding the compositional evolution of the Chang’E-5 regolith. Notably, our study suggests that impact glasses spherules with different major (e.g., TiO2, MgO) and minor (e.g., REEs, Zr, Th and U) element compositions could be derived from the same protolith of local mare basalts, instead of being exclusively attributed to exotic impact ejecta. In this case, small-scale local impacts may have played a crucial role in the impact history of the Chang'E-5 landing site.