Lunar Impact History from Meteorite Impact Melt Clasts and Lessons Learned for Lunar Surface Sampling

Introduction: One of the important outstanding goals of lunar science is understanding the bombardment history of the Moon and calibrating the impact flux curve for extrapolation to the Earth and other terrestrial planets. Obtaining a sample from a carefully-characterized interior melt sheet or a ring massif is a reliable way to tell a single crater's age. A different but complementary approach is to use extensive laboratory characterization (microscopic, geochemical, isotopic, geochronological) of float samples to understand the integrated impact history of a region. Both approaches have their merits and limitations. In essence, the latter is the approach we have used to understand the impact history of the Feldspathic Highland Terrain (FHT) as told by lunar feldspathic meteorites. Here, we summarize data on impact-melt clast composition and ages in the feldspathic lunar meteorites, and use this work as an example of how this approach is valid for understanding regional lunar bombardment history of areas such as the South Pole-Aitken Basin (SPA). Impact-melt clasts in lunar meteorites: The feld-spathic lunar meteorites are regolith and fragmental breccias with high Al 2 O 3 / low Th content relative to the KREEPy, mafic impact-melt rocks of the Apollo collection. The stochastic nature of lunar meteorite launch events implies that these meteorites are more representative of the feldspathic lunar highlands than the Apollo and Luna samples [e.g., 1-3]. More than 100 impact melt clasts from 12 feldspathic lunar meteorites (and two possible nearside lunar meteorites, Cal-calong Creek and SaU 169) have been studied [4-10]. The clasts have textures similar to well-known rocks of impact origin, establishing their origins as impact-melt samples. The majority of clasts in the feldspathic meteorites and in Calcalong Creek are themselves highly feldspathic and lack a KREEP component. Based on their composition, we may infer that these clasts were formed by impacts into the feldspathic lunar crust, indicating their origin either before the formation of the Procellarum KREEP terrain (PKT) or far from the Apollo sampling sites. The meteorite clast ages range from ~4.0 Ga to younger than 2.0 Ga, with a statistical peak around 3.5 Ga (Fig. 1). It appears that impact melt rocks created in post-basin bombardment dominates the very surface of the lunar regolith and is readily incorporated into rego-lith breccias until the breccia lithification event. No samples are >1.1 σ older than 4.0 Ga, the older limit of the predominant age range among Apollo impact melt