Petrography and Chemistry of Impact-melt Clasts in Apollo 16 Breccias

Introduction: The preponderance of 3.9 Ga lunar impact-melt rock ages and the dearth of older samples of unequivocal impact origin, from either the Apollo collection or lunar meteorites, suggest an extraordinary bombardment event in the Earth-Moon system at ~3.9 Ga. We have begun to study another set of samples, impact melts and crystalline lunar spherules (CLS) within demonstrably old Apollo breccias, to search this untapped reservoir for evidence of pre-3.9 Ga lunar impacts. The impact-melt rocks that form the basis of the cataclysm hypothesis are predominantly mafic, KREEP-rich samples, affording enough radiogenic elements to be feasibly dated, but possibly dominated by the large volume of melt created in the stratigraphically young, nearside basins. In contrast, the feldspathic lunar meteorite breccias have low (!1 ppb) Th contents; consistent with an origin in the feldspathic lunar highlands, far from the incompatible element-rich Procellarum KREEP terrain (PKT) or mafic South Pole-Aitken basin [1]. Alternatively, the components of these breccias could have formed from the nearside feldspathic crust prior to the widespread distribution of material from these areas. Our previous work showing the relatively young 40 Ar39 Ar ages of impact-melt clasts in feldspathic lunar meteorite breccias supports the former interpretation that the breccias formed in the feldspathic lunar highlands [2]. However, discrepancies in the age distribution between the lunar meteorite impact-melt clasts and Apollo impact-melt rocks [e.g. 3, 4] may reveal a bias in selection techniques that merits further investigation. In contrast to the apparent youth of the meteorite breccias, high amounts of trapped 40 Ar and excess fission Xe present in many Apollo 16 feldspathic breccias indicates that they acquired their noble gases, including solar wind exposure, very early in lunar history [5]. These breccias lack mature regolith components such as agglutinates, indicating that following exposure, the breccia components were shielded for some time. Despite their recovery location near several large, nearside basins, the breccias lack the ubiquitous KREEPy impact-melt clasts found throughout the Apollo collection, indicating that these rocks were lithified and closed to new input before the exposure and dissemination of KREEP material. These characteristics make the Apollo 16 breccias attractive targets for a) searching for evidence of impact events that may pre-date formation of the large, nearside basins, and b) repeating identification and analysis techniques used for the lunar meteorite dataset on well-studied rocks from the lunar near side. Methods : 66075 is a feldspathic fragmental breccia and 60016 and 66035 are feldspathic regolith breccias. All are subcompact with low shock and rare occurrences of agglutinates and glass spheres [5]. We obtained 100-μm thick sections of each breccia to conduct pet rologic , geochemical , and geochronological studies on crystalline lunar spherules (CLS) [6] and impact-melt fragments. We identified and characterized impact-melt clasts in these rocks using previously-described