Impact-related chemical modifications of the Chang’E-5 lunar regolith

Abstract

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.