¹Andrew S. Rivkin,²Cristina A. Thomas,^3,4Ian Wong,⁴Bryan Holler,⁵Helena C. Bates,⁶Ellen S. Howell,⁷Bethany L. Ehlmann,³Stefanie N. Milam,⁸Heidi B. Hammel
The Planetary Science Journal 6, 9 Link to Article [DOI 10.3847/PSJ/ad944c]
¹Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD, 20723, USA
²Northern Arizona University, Department of Astronomy and Planetary Science, PO Box 6010, Flagstaff, AZ 86011, USA
³NASA Goddard Space Flight Center, Astrochemistry Laboratory, Greenbelt, MD 20771, USA
⁴Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁵Planetary Materials Group, Natural History Museum, Cromwell Road, London SW7 5BD, UK
⁶Lunar & Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA
⁷Division of Geological & Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
⁸Association of Universities for Research in Astronomy, 1212 New York Avenue NW, Suite 450, Washington, DC 20005, USA

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¹Nicolas Dauphas,¹Zhe J. Zhang,¹Xi Chen,²Mélanie Barboni,^3,4Dawid Szymanowski,⁴Blair Schoene,⁵Ingo Leya,⁶Kevin D. McKeegan
Proceedings of the National Academy of Sciences (PNAS) 122, e2413802121 Link to Article [https://doi.org/10.1073/pnas.2413802121]
¹Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637
²School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85281
³Institute of Geochemistry and Petrology, ETH Zurich, Zurich 8092, Switzerland
⁴Department of Geosciences, Princeton University, Princeton, NJ 08544
⁵Space Sciences and Planetology, University of Bern, Bern 3012, Switzerland
⁶Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095

Crystallization of the lunar magma ocean yielded a chemically unique liquid residuum named KREEP. This component is expressed as a large patch on the near side of the Moon and a possible smaller patch in the northwest portion of the Moon’s South Pole-Aitken basin on the far side. Thermal models estimate that the crystallization of the lunar magma ocean (LMO) could have spanned from 10 and 200 My, while studies of radioactive decay systems have yielded inconsistent ages for the completion of LMO crystallization covering over 160 My. Here, we show that the Moon achieved >99% crystallization at 4,429 ± 76 Ma, indicating a lunar formation age of ~4,450 Ma or possibly older. Using the 176Lu–176Hf decay system (t1/2 = 37 Gy), we found that the initial 176Hf/177Hf ratios of lunar zircons with varied U–Pb ages are consistent with their crystallization from a KREEP-rich reservoir with a consistently low 176Lu/177Hf ratio of 0.0167 that emerged ~140 My after solar system formation. The previously proposed younger model age of ~4.33 Ga for the source of mare basalts (240 My after solar system formation) might reflect the timing of a large impact. Our results demonstrate that lunar magma ocean crystallization took place while the Moon was still battered by planetary embryos and planetesimals leftover from the main stage of planetary accretion. The study of Lu–Hf model ages for samples brought back from the South Pole-Aitken basin will help to assess the lateral continuity of KREEP and further understand its significance in the early history of the Moon.