Orbital data indicate that the youngest volcanic units on the Moon are basalt lavas in Oceanus Procellarum, a region with high levels of the heat-producing elements potassium, thorium, and uranium. The Chang'e-5 mission collected samples of these young lunar basalts and returned them to Earth for laboratory analysis. We measure an age of 1963 ± 57 Ma for these lavas and determine their chemical and mineralogical compositions. This age constrains the lunar impact chronology of the inner Solar System and the thermal evolution of the Moon. There is no evidence for high concentrations of heat-producing elements in the deep mantle of the Moon that generated these lavas, so alternate explanations are required for the longevity of lunar magmatism.

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Abstract-Dar al Gani (DaG) 400, Meteorite Hills (MET) 01210, Pecora Escarpment (PCA) 02007, and MacAlpine Hills (MAC) 88104 ⁄ 88105 are lunar regolith breccia meteorites that provide sampling of the lunar surface from regions of the Moon that were not visited by the US Apollo or Soviet Luna sample return missions. They contain a heterogeneous clast population from a range of typical lunar lithologies. DaG 400, PCA 02007, and MAC 88104 ⁄ 88105 are primarily feldspathic in nature, and MET 01210 is composed of mare basalt material mixed with a lesser amount of feldspathic material. Here we present a compositional study of the impact melt and impact melt breccia clast population (i.e., clasts that were generated in impact cratering melting processes) within these meteorites using in situ electron microprobe and LA-ICP-MS techniques. Results show that all of the meteorites are dominated by impact lithologies that are relatively ferroan (Mg# <70 ), have high Sc ⁄ Sm ratios (typically >10), and have low incompatible trace element (ITE) concentrations (i.e., typically <3.2 ppm Sm, <1.5 ppm Th). Feldspathic impact melt in DaG 400, PCA 02007, and MAC 88104 ⁄ 05 are similar in composition to that estimated composition for upper feldspathic lunar crust (Korotev et al. 2003). However, these melt types are more mafic (i.e., less Eu, less Sr, more Sc) than feldspathic impact melts returned by the Apollo 16 mission (e.g., the group 3 and 4 varieties). Mafic impact melt clasts are common in MET 01210 and less common in PCA 02007 and MAC 88104 ⁄ 05. We show that unlike the Apollo mafic impact melt groups (Jolliff 1998), these meteorite impact melts were not formed from melting large amounts of KREEP-rich (typically >10 ppm Sm), High Magnesium Suite (typically >70 Mg#) or High Alkali Suite (high ITEs, Sc ⁄ Sm ratios <2) target rocks. Instead the meteorite mafic melts are more ferroan, KREEP-poor and Scrich, and represent mixing between feldspathic lithologies and low-Ti or very low-Ti (VLT) basalts. As PCA 02007 and MAC 88104 ⁄ 05 were likely sourced from the Outer-Feldspathic Highlands Terrane our findings suggest that these predominantly feldspathic regions commonly contain a VLT to low-Ti basalt contribution.