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

• Main Authors: L.F. White, D.E. Moser, K.T. Tait, B. Langelier, I. Barker, J.R. Darling
• Format: Article
• Language: English
• Published: Elsevier 2019-09-01
• Series: Geoscience Frontiers
• Online Access: http://www.sciencedirect.com/science/article/pii/S1674987118302457

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°/<401>) baddeleyite domain yields a 232Th/208Pb age of 4328 ± 309 Ma, which overlaps with a robust secondary ion mass spectrometry (SIMS) 207Pb/206Pb age of 4308 ± 18.6 Ma and is interpreted here as the crystallization age for the igneous protolith of NWA 3163. A second microstructural domain, < 2 μm in width, contains patchy overprinting baddeleyite and yields a Th-Pb age of 2175 ± 143 Ma, interpreted as dating the last substantial impact event to affect the sample. This finding demonstrates the potential of combining microstructural characterization with nanoscale geochronology when resolving complex P-T-t histories in planetary materials, here yielding the oldest measured crystallization age for components of lunar granulite NWA 3163 and placing further constraints on the formation and evolution of lunar crust. Keywords: Baddeleyite, U-Th-Pb isotopes, EBSD, Atom probe tomography, Geochronology, Northwest Africa 3163