Composition, structure, and age of the Apollo 16 subregolith basement as deduced from the chemistry of post‐imbrium melt bombs

Abstract

Irregularly shaped, slag‐like impact‐melt bombs with variolitic crystallization texture, mostly 1 to 10 cm in size, occur throughout the regolith of the Apollo 16 landing site. We have analyzed 16 samples from Stations 11 and 13 near North Ray Crater using the microscope, electron microscope, microprobe, neutron activation, and 40Ar‐39Ar age‐dating techniques. We interpret the melt rocks as rapidly quenched, high‐velocity ejecta (“glass bombs”) from impact craters less than 1.5 km in diameter located in the vicinity of the Apollo 16 landing site. A comparison between glass bombs and typical regolith samples from various sampling stations reveals differences in their bulk chemical compositions. More than 80% of the glass bombs cannot be derived from the melting of the regolith, but can be produced by melting the subregolith basement. The differences in chemical composition of metal particles in glass bombs and regolith samples are consistent with these findings. Mixing calculations show that the glass bombs were formed by melting of two or more of the most abundant lithologies (granulites, feldspathic melt breccias, anorthosites, and various mafic melt breccias of VHA and LKFM affinity) that constitute the subregolith basement. From these results, we conclude that the subregolith basement represents a polymict megabreccia target. The glass bombs are post‐Cayley in age. They were produced by several different impact events more than 370 Ma ago by which they were emplaced in proximity to the North Ray Crater area. They were buried under a younger regolith layer until the North Ray impact event 50 Ma ago brought them up to the lunar surface again, and since then they have been exposed to cosmic radiation.