Rare gas studies of the galactic cosmic ray irradiation history of lunar rocks

Abstract

Eight analyses of all five noble gases in whole rock samples and mineral separates from lunar rocks 10017, 10044 and 10069, in conjunction with available literature data, permit qualitative conclusions concerning average irradiation depths and enable internally consistent exposure ages for Tranquillity Base rocks to be calculated. Correlated variations in He³Ne²¹ and Ar³⁸Ne²¹ reflect shielding differences. Diffusion losses cannot explain this correlation, even though evidence for gas loss is obtained from mineral separate data. A qualitative shielding sequence can be constructed which agrees reasonably well with the sequence based on Gd isotopic data. There are no systematic differences between meteoritic and lunar He, Ne and Ar spallation spectra, with the possible exception of Ar³⁸Ne²¹. Observed variations in meteoritic and lunar Xe spallation spectra, except at mass 131, are due primarily to differences in chemical composition. The relative Xe¹²⁶ production rates from Ba and rare earths can be derived. Resonance neutron capture on Ba¹³⁰ is a plausible source for the anomalous Xe¹³¹ observed in lunar rocks; however, the required fluxes are quite large. Surface production rates from measurements of radioactive spallation products in lunar samples give more concordant exposure ages than those obtained from meteoritic production rates. This concordance implies that the average irradiation depths for these rocks were small (≲150 g/cm²). Surface production rates for Xe¹²⁶ and Ne²¹ yielded exposure ages for fourteen Tranquillity Base rocks which are in reasonable agreement for most rocks. Five low-K rocks have exposure ages around 100 m.y. in what may be a significant grouping. At least four major impacts are required to produce the fourteen rocks.