^1,2Bidong Zhang,³Yangting Lin,¹Desmond E.Moser,²Paul H.Warren,³Jialong Hao,¹Ivan R.Barker,¹Sean R.Shieh,¹Audrey Bouvierd
Earth and Planetary Science Letters 569, 117046 Link to Article [https://doi.org/10.1016/j.epsl.2021.117046]
¹The University of Western Ontario, Department of Earth Sciences, London, Ontario N6A 3K7, Canada
²Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095, USA
³Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
⁴Universität Bayreuth, Bayerisches Geoinstitut, Bayreuth 95447, Germany
Copyright Elsevier

The lunar Mg-suite magmatic rocks are commonly thought to represent mafic intrusions into the anorthositic flotation crust of the lunar magma ocean (LMO). Their geochronology is, therefore, important for constraining evolution models of the LMO. Petrogenetic models of the Mg-suite hold that their parent magmas were derived from primary LMO sources (Mg-cumulates, An-rich plagioclase, and melts enriched in KREEP—potassium, rare earth elements, and phosphorus). Previous radiogenic isotopic age interpretations of Mg-suite and putatively older, related ferroan anorthosites (FANs) overlap over a 200-million-year interval. The Apollo 78238 norite is an exemplary Mg-suite rock, with a relict coarse igneous texture modified by shock metamorphism. In-situ secondary ion mass spectrometry U-Pb analyses of zircon and baddeleyite in 78238 yield discordant arrays, attributed to recent impact metamorphism, with upper intercepts that constrain its crystallization age. The four oldest baddeleyite analyses give a weighted mean 207Pb/206Pb age of 4332 ± 18 Ma (2σ, MSWD = 0.06, P = 0.98), which is interpreted as the crystallization age of the norite. The overlap of the baddeleyite age with previously reported Sm-Nd and Pb-Pb mineral isochron ages for 78238 (Edmunson et al., 2009) supports a moderately fast cooling of the norite. Moreover, it is distinguishably younger than the most precisely dated sample of FAN (Apollo 60025), measured at 4360 ± 3 Ma by Sm-Nd and Pb-Pb mineral isochrons (Borg et al., 2011). Together with the baddeleyite 207Pb/206Pb age of Apollo Mg-suite troctolite 76535 at 4328 ± 8 Ma (White et al., 2020), the chronological record of the 78238 norite indicates a significant Mg-suite magmatic event at 4.33 Ga and a lower age limit on LMO differentiation.

¹Thomas Kenkmann
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13657]
¹Institute of Earth and Environmental Sciences, Geology, Albert-Ludwigs-Universität Freiburg, Albertstraße 23b, Freiburg im Breisgau, 79104 Germany
Published by arrangement with John Wiley & Sons

The number of newly discovered and confirmed impact structures on earth is growing continuously. In this review paper, the main attributes of 198 confirmed impact structures and 10 further structures, for which final confirmation based on the identification of shock features is not yet entirely satisfying, are presented. The impact craters are compared statistically, with regard to their morphology, structure, and status of erosion or burial. The size– and age–frequency distributions of terrestrial impact structures are presented. Additional aspects concern target petrography and shock effects found in the craters. Based on the discovery statistics of presently known crater structures, an estimate can be made of the number of craters that await discovery. The paper is complementary to the recently published atlas of terrestrial impact structures by Gottwald et al. (2020).