^1,2Alan E. Rubin,³Brent D. Turrin
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14088]
¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California, USA
²Maine Mineral & Gem Museum, Bethel, Maine, USA
³Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey, USA
Published by arrangement with John Wiley & Sons

About 17% of L6 chondrites (15/87) show significant reduction features in BSE images in thin section. Because some thin sections of these meteorites do not show reduction features, this percentage is a lower limit. Reduction features include: (1) 4–5-μm-thick BSE-dark reduction rims on olivine and orthopyroxene grains and along fracture boundaries in these grains, (2) 4–12-μm-thick dark bands (probably poorly crystalline pyrrhotite) at the margins and along fractures in troilite grains, and (3) 2–5-μm-thick dark rinds of kamacite around some taenite grains. Only one of 70 L-group chondrites (1.4%) of lower petrologic type exhibits minor reduction. The L6 chondrites showing major reduction have 40Ar/39Ar plateau ages ranging from 156 ± 1 Ma for Guangnan to 4543 ± 3 Ma for Thamaniyat Ajras. Reduction occurred after silicate, sulfide, and metal grains had attained their present sizes during parent-body thermal metamorphism (and had been fractured by parent-body collisions). The precise plateau age of Thamaniyat Ajras probably marks the timing of the L6 reduction event. It seems likely the reductant was a low-viscosity fluid, plausibly CO, derived from oxidation of poorly graphitized and amorphous carbon within fine-grained matrix. Water-ice that had accreted to the L-chondrite asteroid was heated and mobilized during metamorphism, causing oxidation. After peak metamorphism, ~75% of the water had been used up or lost; the remaining water facilitated continuing graphite oxidation so that, after this point, overall reduction effects exceeded those of oxidation. L chondrites of lower petrologic type were less affected by reduction due to their lower metamorphic temperatures.

¹Yunhua Wu et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.10.031]
¹Planetary Environmental and Astrobiological Research Laboratory, School of Atmospheric Sciences, Sun Yat-sen University, Zhuhai 519082, China
Copyright Elsevier

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.