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Precise Pb-Pb ages derived from lunar impact breccias: the key to dating large lunar basins

Understanding the early lunar bombardment history hinges on reliable formation ages of the large lunar basins. Recent studies have shown that Zr minerals and Ca phosphates in petrographically, chemically, and microstructurally well-characterized lunar impactites can yield easily reproducible U-Pb ages that can be related to impacts. Geological, textural, and chemical arguments, plus impact melt distribution models, suggest that the widespread 3.92 Ga and 4.21 ages in breccias at the Apollo 14-17 landing sites likely reflect the Imbrium and Serenitatis impacts, respectively. However, the ages of other basins are more uncertain. Lunar granulites – impactites that were thermally metamorphosed by impact melt sheets – also should record basin-forming events. We have combined precise in situ Pb-Pb dates with Pb-Pb isochron dating to constrain the early history of the Apollo 17 granulite 77017. The rock contains annealed anorthositic gabbro clasts with relict igneous textures and a finer, thermally annealed matrix. High Ir abundances and the presence of metal indicate that the gabbro crystallized from impact melt. Baddeleyites define a homogeneous Pb-Pb age distribution (4175±3 Ma, n = 5), which is interpreted to date the crystallization of this impact melt. The phosphate Pb-Pb ages range between 4.18 and 4.13 Ga and are consistent with variable resetting by the metamorphic heating event. A Pb-Pb isochron yields a precise date for the granulite facies metamorphism (4143±1 Ma). The heating event may either relate to the impact that formed the Crisium basin or to another nearby basin-forming impact on the feldspathic highland terrane (Nectaris or Smythii).


Harry Becker1, Thomas Haber2, Dennis Vanderliek1, Wajiha Iqbal2, Tiantian Liu3, Erik Scherer2, Winfried Schwarz4, Martin Whitehouse5, Carolyn H. van der Bogert2, Harald Hiesinger2
1Freie Universität Berlin, Germany; 2Universität Münster; 3Museum für Naturkunde Berlin; 4Universität Heidelberg; 5Swedish Museum of Natural History
GeoBerlin 2023