Nitrogen isotopes in lunar soils as a measure of cosmic-ray exposure and regolith history

Previous studies have shown that the bulk of the nitrogen in soils has been implanted by the solar wind, that the N-15/N-14 ratio of the implanted nitrogen has apparently increased with time, and that both soils and rocks contain a nitrogen component greatly enriched in N-15, relative to natural abundance ratios, due to cosmic-ray spallation. An investigation was, therefore, conducted to test the feasibility of using spallation-produced N-15 for determining cosmic-ray exposure ages. Samples from several depths in the Apollo 15 deep drill core were analyzed. The data obtained have also been evaluated with regard to possible models for the prior history of the material now in the Apollo 15 core. In addition, analyses have been performed of several other soil samples considered for various reasons to be of particular interest, such as orange soil 74220 and its companion 74240, Apollo 16 drill core sample 60002, and the apparently peculiar fillet soil 67461. Wherever possible, analyses were carried out by step-wise heating rather than by total pyrolysis. On the basis of the results of the investigation it is concluded that nitrogen isotope ratio determinations provide a usable method for determining cosmic-ray exposures for soils.