Cosmic ray exposure histories of Apollo 14, Apollo 15, and Apollo 16 rocks

Abstract

We investigate the exposure history of six lunar rocks using the abundances of cosmic‐ray produced noble gas isotopes and the 81 Kr‐Kr ages. Single‐stage exposure histories are inferred for basalt 14053, breccia 14305, and anorthosite 65315. We propose that basalt 14053 with a 81 Kr‐Kr age of 21.2±5.0 m.y. is a Cone crater ejecta and that anorthosite 65315 (1.3±0.7 m.y.) originates from South Ray crater. We present a comprehensive comparison of all cosmogenic noble gas ratios and abundances for rocks with single‐stage exposure and show that the agreement with values predicted by nuclear systematics and target element chemistry is good, in particular for isotope ratios. Basalt 14310 experienced a multistage exposure. The apparent 81 Kr‐Kr age, the depth sensitive isotopic ratios, and fission 136 Xe, mainly from 235 U induced by secondary cosmic‐ray neutrons, lead to the conclusion that this sample was preexposed 1.75 AE ago to cosmic rays for a duration of 350 m.y. Basalt 15058 and anorthosite 15415 also reveal multistage exposures. The abundance of the radionuclide 81 Kr in 15058 increases toward the surface of the rock, reflecting an enhanced production rate due to solar flare particles, whereas stable 83 Kr concentrations are constant with increasing depth within the rock. This indicates that basalt 15058 received most of its irradiation in a more shielded location than that effective during the last million years.