Trapping of Atmospheric Gases During Crushing of Lunar Samples

Introduction. The presence of terrestrial atmospheric noble gases in lunar samples has been known since the early analyses of lunar samples delivered by the Apollo program [1-4]. Some lunar samples contain trapped gases with clear terrestrial atmosphere Xe isotopic signatures [3, 4]. As a part of the sample preparation procedure some of them have been crushed before analyses. Therefore, it was thought that trapping of the atmospheric gases happened at this stage. However, the terrestrial atmospheric gases have been found also in the samples that have not been crushed prior to analysis [3]. Nevertheless, even in these cases the origin of the trapped gases has been attributed to terrestrial contamination, though the mechanism by which they are trapped remains unclear. In the present study, we investigated this issue by taking into consideration nitrogen, which is the most abundant atmospheric constituent, and is also more chemically active than noble gases. Along with analyses of N in lunar samples which have an excess of atmospheric noble gases [5], we performed several crushing experiments to see in what proportion noble gases and nitrogen can be trapped and compared this with their abundance in the lunar samples. Experimental. For the analyses of noble gases, nitrogen and carbon we used the ‘Finesse’ instrumentation [6], which allows simultaneous measurements of abundances and isotopic compositions of different species present. First, we analyzed each sample as it is, using stepped combustion and vacuum crushing. More than 10000 strokes cumulatively were applied during crushing, resulting in production of a fine-grained powder. The crushed samples were subsequently removed from the crusher and an aliquot of the sample was then analyzed by stepped combustion. The transfer from the crusher to the combustion furnace includes brief exposure of the crushed material to the atmosphere. In a separate experiment, we crushed (3000 strokes) one of the samples (69921) under atmospheric conditions i.e. without evacuation of air from the crusher, and then analyzed the crushed material using stepped combustion. Altogether, three lunar soils (12070, 14141 and 69921) were used in this study. The nitrogen, carbon and noble gas step-combustion data for these samples have been published previously [7]. Results and discussion. Based on our initial analysis of nitrogen in the lunar rock, 12064, which showed a clear excess of atmosphere-like Ne, we found [5] that the (N2/Ne)tr ratio is lower than the terrestrial atmospheric ratio by a factor of 6x10. This indicates that atmospheric nitrogen is much less efficiently trapped by crushing than Ne, contradicting the general idea that chemically active atmospheric species should be trapped during crushing of silicate materials in air with higher efficiency than the inert ones. This result can be considered as evidence for a non-atmospheric origin of the trapped Ne in this sample.