Dauphas Nicolas, Zhang Zhe J, Chen Xi, Barboni Mélanie, Szymanowski Dawid, Schoene Blair, Leya Ingo, McKeegan Kevin D
Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637.
School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85281.
Proc Natl Acad Sci U S A. 2025 Jan 14;122(2):e2413802121. doi: 10.1073/pnas.2413802121. Epub 2025 Jan 6.
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 Lu-Hf decay system ( = 37 Gy), we found that the initial Hf/Hf ratios of lunar zircons with varied U-Pb ages are consistent with their crystallization from a KREEP-rich reservoir with a consistently low Lu/Hf 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.
月球岩浆海洋的结晶产生了一种化学性质独特的残余液体，称为KREEP。这一成分在月球近侧表现为一个大片区，在月球背面南极-艾特肯盆地的西北部可能有一个较小的片区。热模型估计，月球岩浆海洋（LMO）的结晶过程可能持续了10到200百万年，而对放射性衰变系统的研究得出的LMO结晶完成时间的年龄并不一致，跨度超过160百万年。在此，我们表明月球在4429±76百万年前实现了>99%的结晶，这表明月球的形成年龄约为4450百万年或可能更老。利用镥-铪衰变系统（ = 37 Gy），我们发现不同铀-铅年龄的月球锆石的初始铪/铪比值与它们从一个富含KREEP的储库中结晶的情况一致，该储库在太阳系形成后约140百万年出现，其镥/铪比值始终较低，为0.0167。先前提出的月海玄武岩源区约4.33 Ga的较年轻模型年龄（太阳系形成后240百万年）可能反映了一次大型撞击的时间。我们的结果表明，月球岩浆海洋的结晶发生在月球仍受到行星胚胎和行星吸积主阶段遗留的小行星撞击的时候。对从南极-艾特肯盆地带回的样本进行镥-铪模型年龄研究，将有助于评估KREEP的横向连续性，并进一步了解其在月球早期历史中的重要性。
