The irradiation history of lunar samples

Abstract

From new data on Apollo 12 samples illustrated in this paper and data available in the literature, the galactic cosmic ray and solar wind exposure of lunar samples has been investigated by means of rare gas and Gd isotopic measurements. Neutron exposures obtained from Gd isotopic measurements on Apollo 11 and Apollo 12 soil samples are the same to within 10% and Xe/(Ba + 1.65 Ce) to within 40%. These data permit regolith mixing depths of 10-20 meters to be calculated for the Apollo 11 and Apollo 12 sites. The Apollo 12 double core shows only a small, but significant, gradient in neutron exposure (~10% larger at the bottom). Consequently, the core could not have been stratified in its present configuration for more than 50 x 10⁶ yr. Apollo 12 rocks have neutron exposures which are similar to the Apollo 11 rocks. No variations in neutron exposure are observed for samples from different depths in 12002. Strong evidence for a range in irradiation depths for lunar rocks is obtained from the relative yields of rare gas spallation products and neutron dosages calculated from the measured Gd isotopic variations. However, it appears that the average irradiation depths were not large (probably ~60 cm). Almost all rocks must have resided at depths greater than ~4 meters most of the time since their original crystallization. An internally consistent set of spallation product exposure ages has been calculated for 14 Tranquillity Base rocks. Six of seven "low-K" rocks appear to be grouped with an exposure age of around 100 m.y. Thermal release data show large variations (~ΔM/M) in the composition of SUCOR (surface correlated) Kr and Xe indicating that SUCOR may not be well defined. SUCOR Xe appears to be complex and may contain, in addition to solar wind, additional components from the lunar atmosphere (for example, fission Xe) which have been implanted by solar wind action.