Identifying primitive noble gas components in lunar ferroan anorthosites

Abstract

Lunar ferroan anorthosites are the ideal samples for investigating primitive volatile systematics. Not only are these lithologies thought to be direct crystallization products of the Lunar Magma Ocean (LMO), but many samples display short (T38 < 5 Myr) cosmic ray exposure (CRE) ages, minimizing the effects of cosmic ray spallation reactions. Here we report noble gas (He, Ne, Ar, Kr, Xe) abundances and isotope systematics for nine ferroan anorthosites (FAN) collected during the Apollo 16 mission and one anorthosite sample collected during the Apollo 15 mission. The CRE ages calculated for these samples range from T38 ~ 0.13 to ~226 Myr, indicating that not all anorthosites were emplaced at the lunar surface at the same time.In general, He-Ne-Ar-Kr-Xe isotope systematics can be accounted for by variable contributions from cosmogenic spallation reactions and solar-wind implantation. The Xe isotope systematics of lunar anorthosites offer our best chance of resolving primitive Xe components on the Moon. Three of the samples investigated here (60515, 65325, 60025) display a Xe isotope signature within error of terrestrial air. These samples have likely been comprised by anomalously adsorbed terrestrial air, as was also recognized by early Xe isotope studies of lunar anorthosites (e.g., Niemeyer and Leich, 1976). The three samples that have the shortest CRE ages (69955, 60135, 60015) display ratios of heavy Xe isotopes (134Xe and 136Xe) over lighter isotopes (130Xe and 132Xe) that are lower than air and solar wind. Mixing modeling for these three samples suggests that such signatures can be accounted for by the addition of up to ~30% cometary Xe (based on the reported Xe isotope composition of comet 67P/Churyumov-Gerasimenko; Marty et al., 2017) to mixtures of adsorbed terrestrial air and Solar Wind. One sample (60135) displays lower than solar 136Xe/132Xe from gases extracted in an intermediate temperature heating step, indicating that such a component may have only been superficially implanted. However, two other samples (69955, 60015) display heavy Xe isotope ratios deficits only in the highest temperature gas extraction steps, indicating that this component is hosted within the plagioclase crystal structure. It is not clear how a cometary component was introduced into the lunar crust. In one scenario, cometary Xe was mixed directly into the LMO during periods of high impact bombardment (such as the Late Veneer) prior to the formation of the lunar crust before ~4.2 Ga. Alternatively, cometary Xe may have been directly implanted into plagioclase crystals via diffusion as a result of micrometeorite impacts over geological time in the near surface lunar environment.