Large Impacts at 4 . 2 Ga from Uranium - Lead Dating of Lunar Melt Breccias

Introduction: Crystallization ages of lunar impact melt rocks provide the primary evidence for a spike in the impact flux at ~3.9 Ga. Here we report U-Pb isotopic ages of accessory phases (apatite, zirconalite) in lunar melt breccia 67955 that confirm a crystallization age of ~4.2 Ga and reveal a younger overprint possibly related to entrainment of the breccia by one or more younger basins such as Imbrium. Petrography: 67955 is a crystalline anorthositic norite breccia collected at North Ray crater. Its poikilitic texture suggested a plutonic igneous origin in early petrologic studies [1,2], but the abundance of FeNi metal, the Fe-Ni-Co compositions of the metal, and the high and chondritic relative abundances of siderophile trace elements in the metal indicates an origin as an impact melt rock rather than as an endogenous magmatic cumulate. The sample has been partially recrystallized, brecciated, and injected with glass veins, but areas with well-preserved, near-primary crystalline textures remain (Fig. 1) and were sampled for this study. Our isotopic and trace element studies of 67955 yielded a Sm-Nd mineral isochron age of 4.20 + 0.07 Ga, which was interpreted as the time of a major impact event on the Moon, and a geochemical signature of KREEP or Mg-suite plagioclase in the trace element compositions [3]. Major element compositions of the silicate phases fall within the Mg-suite field on a plot of An (plagioclase) vs. En (opx) [3].