Abstract
Determining the impact chronology of the Moon is an important yet challenging problem in planetary science even after decades of lunar samples and other analyses. In addition to crater counting statistics, orbital data, and dynamical models, well-constrained lunar sample ages are critical for proper interpretation of the Moon’s impact chronology. To understand which properties of lunar impact glasses yield well-constrained ages, we evaluated the compositions and sizes of 119 Apollo 14, 15, 16, and 17 impact glass samples whose compositions and 40Ar/39Ar ages have already been published, and we present new data on 43 others. These additional data support previous findings that the composition and size of the glass are good indicators of the quality of the age plateau derived for each sample. We have further constrained those findings: Glasses of ≥200 μm with a fraction of non-bridging oxygens (X(NBO)) of ≥0.23 and a K2O (wt%) of ≥0.07 are prime candidates for argon analyses and more likely to yield well-constrained 40Ar/39Ar ages. As a result, science resulting from impact glass analyses is maximized while analytical costs per glass are minimized. This has direct implications for future analyses of glass samples for both those in the current lunar collection and those that have yet to be collected.
Highlights
Lunar samples continue to be invaluable to the study of the Moon’s impact history
Impact glasses, which form from droplets of melt that have been ballistically transported from energetic impact events and are deposited into surrounding regolith, are one important source of data
The lithophile element compositions (e.g., Al2 O3, MgO, and TiO2 ) for the new set of 43 lunar impact glasses were determined by electron microprobe analysis, which allows for the determination of chemical composition in small samples and is well-suited for impact glasses that are typically hundreds of microns in size [5]
Summary
Lunar samples continue to be invaluable to the study of the Moon’s impact history. The Moon’s well-preserved surface and its close proximity allow for a better understanding of Earth’s own impact history and can help inform models of the solar system’s dynamical evolution. Despite the wealth of data provided by these sources, the impact history of the Moon has proven to be complex and difficult to decipher [1,2]. Correct interpretation of the impact sample data is crucial for reconstructing an accurate impact history. Impact glasses, which form from droplets of melt that have been ballistically transported from energetic impact events and are deposited into surrounding regolith, are one important source of data. These glasses retain the chemical composition of their target material and age of impact, making them powerful probes of regional geology and impact history [3]. The volcanic glasses can be characterized by a high MgO/Al2 O3 ratio, chemical homogeneity, and Mg-correlated abundances of Ni, among other criteria
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