Crystallization and impact history of a meteoritic sample of early lunar crust (NWA 3163) refined by atom probe geochronology

Abstract

Abstract Granulitic lunar meteorites offer rare insights into the timing and nature of igneous, metamorphic and impact processes in the lunar crust. Accurately dating the different events recorded by these materials is very challenging, however, due to low trace element abundances (e.g. Sm, Nd, Lu, Hf), rare micrometer-scale U-Th-bearing accessory minerals, and disturbed Ar-Ar systematics following a multi-stage history of shock and thermal metamorphism. Here we report on micro-baddeleyite grains in granulitic mafic breccia NWA 3163 for the first time and show that targeted microstructural analysis (electron backscatter diffraction) and nanoscale geochronology (atom probe tomography) can overcome these barriers to lunar chronology. A twinned (∼90°/ ) baddeleyite domain yields a 232Th/208Pb age of 4328\xa0±\xa0309\xa0Ma, which overlaps with a robust secondary ion mass spectrometry (SIMS) 207Pb/206Pb age of 4308\xa0±\xa018.6\xa0Ma and is interpreted here as the crystallization age for the igneous protolith of NWA 3163. A second microstructural domain,