Apollo 16 Landing Site: Geochemistry and Impact History

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Lunar impact glasses are droplets of melt produced by energetic cratering events that were quenched during ballistic flight and possess the unmodified refractory element ratios of the original fused target materials at the sites of impacts. These target materials are usually regolith. This study reports on the geochemistry of 866 impact glasses from 4 Apollo 16 regolith samples. A subset of these glasses has been dated by the 40Ar/39Ar method. Orbital geochemical data indicate that the Apollo 16 region is KREEP-poor and representative of typical highland basalt (HB), concordant with the observation that most of our impact glasses are KREEP-poor. Some of these glasses have a composition similar to that of the lunar meteorites and to that of the highland basalt glasses found in the Apollo 14 regolith, indicating that not all HB compositions are the same. Additionally, KREEP-rich and mare-derived glasses within our suite of glasses indicate that some of those exotic glasses have apparently been transported ˜250 km from the nearest exposure of mare materials. 40Ar/39Ar ages from 9 impact glasses show that the Moon experienced significant impacts at ˜800 Ma and at ˜3800 Ma, somewhere in the vicinity of the Apollo 16 landing site. We suggest that these glasses represent at least two distinct impact events at separate times on similar terrains, which must have included both mare and highlands compositions. Candidate craters include Theophilus and Cyrillus, both of which border the maria, ˜250 km east of the Apollo 16 landing site. Our analyses suggest that a combination of chemical composition and ages for impact glasses allows for a more substantive interpretation of the lunar impact history. Histograms alone are not sufficient.

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