Xenon systematics of individual lunar zircons, a new window on the history of the lunar surface

We demonstrate a new way of investigating the processing of the lunar surface (and other planetary regoliths) that combines Xe-S-Xe-N ages (based on uranium fission) in individual zircons with their xenon isotopic record of solar wind and cosmic ray exposure. We report the first xenon isotopic analyses of individual lunar zircons (from Apollo 14 soil and breccias samples). Parallel analyses of a suite of zircons from the Vredefort impact structure in South Africa revealed Xe-S-Xe-N ages that agree well with U-Pb systematics, suggesting that the diffusion kinetics of xenon and lead in zircon are similar in the pressure-temperature environment of sub-basin floors. In contrast, all Apollo 14 zircons examined exhibit Xe-S-Xe-N ages markedly younger than the associated U-Pb and (207)pb-Pb-206 ages, and soil zircons with (207)pb-Pb-206 ages greater than 3900 Ma produced an abundance of Xe-S-Xe-N ages <1000 Ma. The young ages cannot be explained by thermal neutron irradiation on the lunar surface, and diurnal heating is unlikely to cause preferential loss of xenon. As such these young soil zircon ages likely record regolith gardening processes. The breccia zircons typically record older ages, >2400 Ma, suggesting that these samples may be useful for investigating ancient events and regolith processing at an earlier epoch. However, none of the zircons contain xenon from now-extinct Pu-244 implying that either the samples have completely degassed since similar to 3900 Ma or that the initial Pu/U ratio of the Moon is lower than that on Earth. We also describe a methodology for conducting component deconvolution that can be applied to multi-isotopes systems beyond xenon. We have also determined new xenon isotopic yields from rare earth element spallation in the lunar environment and high precision yields for neutron induced fission of U-235 in geologic samples. (C) 2020 Elsevier Ltd. All rights reserved.