Size Matters: Chemical Representativeness of Lunar Impact-Generated Glass Beads

Lunar impact glass beads are the products of meteorite impacts into lunar surface materials.While most beads exhibit compositions matching local regolith, some display significant geochemical anomalies, traditionally attributed to either ballistic transport of exotic materials or excavation of underlying rocks.However, such explanation neglects potential compositional biases introduced during the formation of impact glass beads.Incomplete homogenization of impact melt prior to ejection could induce larger deviations from bulk melt chemistry in smaller droplets, thereby leading to increased chemical variability in smaller beads.In this study, the major-element compositions and sizes of 635 homogeneous glass beads, along with trace-element compositions of 56 beads from the Chang'e-5 (CE-5) soil, were determined.The Kullback-Leibler divergence (KLD) was employed to quantify the dissimilarity between the compositional distributions of individual glass beads and the CE-5 regolith.KLD analysis of CE-5 basalt fragments and individual minerals relative to the CE-5 regolith reveal those materials with KLD > 0.1 exhibit significant chemical composition differences from CE-5 regolith.Glass beads with diameters > 100 m and KLD < 0.1 have the major-and rare earth-element compositions similar to the local CE-5 regolith.Of the 4 glass beads with diameters > 100 m and KLD > 0.1, three are interpreted as being exotic to the CE-5 region and one was derived from impact melting of underlying cumulates near the base of the local basaltic lava flows.In contrast, glass beads with diameters < 100 m exhibit a large range of chemical compositions.Among the 32 glass beads with diameters < 100 m and KLD > 0.1, 9 originated from the impact melting of single-mineralenriched components in local regolith, including 5 primarily composed of pyroxene, two of ilmenite, one of plagioclase, and one primarily of mesostasis assemblage.Only 14 were identified as exotic glass beads in these 32 glass beads.These results indicate that smaller impact glass beads exhibiting significant compositional deviations from local regolith are not necessarily from exotic sources.The sizes of impact glass beads should be considered when using their chemical compositions to infer local and regional characteristics of the Moon.