Impact History and Regolith Evolution on the Moon: Geochemistry and Ages of Glasses from the Apollo 16 Site

Abstract

Lunar impact glasses are quenched droplets of melt that carry geochemical records of their target compositions, formation ages, and time‐integrated exposure in the upper layers of the lunar regolith. Here we present the first study to obtain major element, trace element, and Ar isotopic data for impact glasses from the Apollo 16 regolith sample 66031. Thirty particles were analysed with 27 of them yielding useable age information. The glasses have a wide range of major and trace element compositions, similar to that observed in lunar meteorites. Half of these glasses have compositions similar to Apollo 16 soils and are considered to be “locally derived”, whereas the others represent diverse source regions and are considered to be “exotic” particles that were delivered from a considerable distance to the landing site. Almost 40% of the samples analysed for this study have formation ages younger than 500 Ma. Duplicate particles produced in single impact events contribute minimally to the age distribution, and diurnal or transient heating of the regolith does not appear to have had a significant effect on the 40Ar/39Ar ages. Rather, the ages reflect primarily the formation of these glasses by impact melting, with the distribution modified to some degree by preservation bias. As most of these glasses are likely formed by relatively small impactors, their age distribution cannot be compared directly with the crystalline lunar melt rocks to constrain the impact mass flux through time.

Constraining the formation and transport of lunar impact glasses using the ages and chemical compositions of Chang’e-5 glass beads

“…The small number of glasses identified as exotic to the landing site based on their chemical compositions ( n = 26) makes it difficult to evaluate their age and crater size distributions relative to the locally derived glasses. Nevertheless, together with previous observations ( 17 , 40 ), they support the hypothesis that small populations of glasses in lunar soils were transported over much greater distances (in excess of 150 km) although the impact conditions necessary to produce such populations remain poorly defined.…”

“…Previous studies have demonstrated the predominantly local provenance and relatively young ages of the majority of lunar impact glasses at a given site and their formation in modest size events have been suggested ( 3 , 5 , 10 , 27 , 40 ). However, neither the precision on the isotopic ages nor the constraints on local geology and CSFD model ages of potential parent craters have been adequately defined to allow a detailed comparison in those previous studies.…”

“…Previous studies of impact glasses ( 1 , 7 , 8 , 13 , 27 , 40 ) identified a set of physical aspects that have impeded a deeper understanding of the chemistry and ages of impact glasses in the lunar regolith. These aspects include, but are not limited to, typical sizes of craters responsible for the major contribution of glasses to lunar soils, limits on transport distances, and influence of regolith thickness and the nature of underlying bedrock on impact melt production on the Moon.…”

“…Glasses produced by impacts can be distinguished from those formed by volcanic eruptions using chemical and textural characteristics (1)(2)(3)(4). Previous studies of the 39 Ar- 40 Ar and U-Pb chronometric systems in lunar regolith glasses provide a foundation for investigating the volcanic and impact history of the Moon (3,(5)(6)(7)(8)(9)(10)(11)(12)(13). Volcanic glasses provide unique information about the lunar mantle (14,15), whereas impact glasses reflect the compositions of crustal target materials (16)(17)(18) and collisional dynamics of the inner solar system (4,9,19,20).…”

Formation of Small Craters in the Lunar Regolith: How Do They Influence the Preservation of Ancient Melt at the Surface?

“…Some impact glasses, however, may not have a local origin but still represent relatively small craters. For example, Norman et al (2019) suggested that relatively small impact events thousands of kilometers away from the Apollo 16 landing site might have contributed exotic glasses to the regolith at this landing site within the last 500 My. However, in our model, the most distant small crater that contributed material to the target region is only ∼5 km away (Crater 4; see Figure 2d), and most of the craters whose ejecta were transported to the central target region after their formations are located no farther than 1 km away.…”