Noble gases in lunar anorthositic rocks 60018 and 65315: Acquisition of terrestrial krypton and xenon indicating an irreversible adsorption process

Abstract

Lunar anorthositic breccias 60018 and 65315 contain a trapped Xe component with isotope ratios similar to those in terrestrial atmospheric Xe. We analyzed all five noble gases in eight grain size fractions of 60018 and in several bulk samples of 65315 in order to characterize the nature of the trapped components. Trapped gas abundances in 60018 are 100 to 1000 times lower than in a typical regolith breccia; they are grain size anticorrelated for grain sizes < 30 μm. In 65315, trapped He, Ne, and Ar concentrations are among the lowest ever observed for lunar material. Except for Xe, the isotopic abundances of the trapped components in both rocks are consistent with solar wind composition. Because of the uncertain results as to the origin of the terrestrial-like Xe component, we crushed samples of 60018 and 65315 in atmospheres spiked with 86Kr and 129Xe. These experiments showed unequivocally that anorthositic samples can be contaminated by Kr and Xe, and maybe also by Ar, from the ambient atmosphere. At least 75% of the acquired terrestrial and spike gases are released only at temperatures above 600°C. Krypton and xenon concentrations are grain-size anticorrelated, indicating surface adherence of the gas atoms. Furthermore, the amount of gas in a sample is approximately proportional to the partial pressure of the gas to which the sample was exposed, as described by Henry's law. Xenon uptake is about one order of magnitude more effective than Kr uptake, and activation energies, measured at room temperature, are in the range of 10–20 kJ/mol. Both characteristics are typical for ordinary physical adsorption. However, the high extraction temperatures show that somehow the gas atoms get fixed, either by strong chemisorptive bonding with activation energies around 200 kJ/mol, or by getting trapped beneath the surface. Therefore, we use the term “irreversible adsorption” for the phenomenon. Fixation is likely to take place during crushing, when mechanical, thermal, and maybe electromagnetic energy is supplied, although the air contamination of an uncrushed 65315 sample cannot be explained in this way. Irreversible adsorption of terrestrial Kr and Xe has been observed for several lunar anorthosites with low solar wind content, but also for some meteoritic samples, in particular diogenites, which are composed mainly of pyroxene. A similar process may have been responsible for the incorporation of nonterrestrial noble gas components into extraterrestrial material, e.g., the planetary trapped gases in meteorites or the surface-correlated components of 129Xe and fission-type Xe in some lunar breccias. Furthermore, other volatile elements might be affected by irreversible adsorption.