Since the Apollo 14 mission delivered samples of the Fra Mauro formation, interpreted as ejecta of the Imbrium impact, defining the age of this impact has emerged as one of the critical tasks required for the complete understanding of the asteroid bombardment history of the Moon and, by extension, the inner Solar System. Significant effort dedicated to this task has resulted in a substantial set of ages centered around 3.9 Ga and obtained for the samples from most Apollo landing sites using a variety of chronological methods. However, the available age data are scattered over a range of a few tens of millions of years, which hinders the ability to distinguish between the samples that are truly representative of the Imbrium impact and those formed/reset by other, broadly contemporaneous impact events. This study presents a new set of U-Pb ages obtained for the VHK (very high K) basalt clasts found in the Apollo 14 breccia sample 14305 and phosphates from (i) several fragments of impact-melt breccia extracted from Apollo 14 soil sample 14161, and (ii) two Apollo 15 breccias 15455 and 15445. The new data obtained for the Apollo 14 samples increase the number of independently dated samples from this landing site to ten. These Apollo 14 samples represent the Fra Mauro formation, which is traditionally viewed as Imbrium ejecta, and therefore should record the age of the Imbrium impact. Using the variance of ten ages, we propose an age of 3922 ± 12 Ma for this event. Samples that yield ages within these limits can be considered as possible products of the Imbrium impact, while those that fall significantly outside this range should be treated as representing different impact events. Comparison of this age for Imbrium (determined from Apollo 14 samples) with the ages of another eleven impact-melt breccia samples collected at four other landing sites and a related lunar meteorite suggests that they can be viewed as part of Imbrium ejecta. Comprehensive review of ⁴⁰Ar/³⁹Ar ages available for impact melt samples from different landing sites and obtained using the step-heating technique, suggests that the majority of the samples that gave robust plateau ages are indistinguishable within uncertainties and altogether yield a weighted average age of 3916 ± 7 Ma (95 % conf., MSWD = 1.1; P = 0.13) and a median average age of 3919 + 14/-12 Ma, both of which agree with the confidence interval obtained using the U-Pb system. These samples, dated by ⁴⁰Ar/³⁹Ar method, can be also viewed as representing the Imbrium impact. In total 36 out of 41 breccia samples from five landing sites can be interpreted to represent formation of the Imbrium basin, supporting the conclusion that Imbrium material was distributed widely across the near side of the Moon. Establishing temporal limits for the Imbrium impact allows discrimination of ten samples with Rb-Sr and ⁴⁰Ar/³⁹Ar ages about 50 Ma younger than 3922 ± 12 Ma. This group may represent a separate single impact on the Moon and needs to be investigated further to improve our understanding of lunar impact history.

自阿波罗（Apollo）14任务提供了Fra Mauro编队的样本（被解释为Imbrium撞击的弹射体）以来，确定这种撞击的年龄已成为全面了解月球小行星轰炸历史所需的关键任务之一，并且通过扩展，内部的太阳系。致力于此任务的大量工作导致了大约3.9 Ga左右的大量年龄，并使用各种年代学方法从大多数阿波罗登陆点获得了样品。但是，可用的年龄数据分散在数千万年的范围内，这妨碍了区分真正代表Imbrium撞击的样本和由其他广泛同时发生的撞击事件形成/重置的样本的能力。这项研究提供了一组新的U-Pb年龄，这些年龄是从Apollo 14角砾岩样品14305中发现的VHK（非常高的K）玄武岩碎屑和来自（i）从Apollo 14土壤样品14161中提取的若干冲击熔融角砾岩的碎片中获得的，以及（ii）两个阿波罗15角砾岩15455和15445。从阿波罗14样本获得的新数据将来自该着陆点的独立日期的样本数量增加到十个。这些Apollo 14样品代表了Fra Mauro地层，该地层在传统上被视为Imbriumjecta，因此应记录Imbrium撞击的年龄。使用十个年龄的方差，我们建议此事件的年龄为3922±12 Ma。屈服年龄在这些限制范围内的样品可以被认为是Imbrium冲击的可能产物，而那些明显超出此范围的事件应被视为代表不同的影响事件。将此年龄的年龄（由Apollo 14样品确定）与在其他四个着陆点以及相关的月球陨石上收集的另外11个冲击熔融角砾岩样品的年龄进行比较，表明它们可以被视为Imbrium射出物的一部分。全面审查来自不同着陆点的冲击熔体样品的⁴⁰ Ar / ³⁹ Ar年龄可用阶梯加热技术获得，这表明大多数具有稳固高原年龄的样品在不确定性内是无法区分的，加权平均年龄为3916± 7 Ma（95％conf。，MSWD = 1.1； P = 0.13），中位平均年龄为3919 + 14 / -12 Ma，两者均与使用U-Pb系统获得的置信区间相符。这些样品，日期为⁴⁰ Ar / ³⁹Ar方法，也可以视为代表Imbrium的影响。在来自五个着陆点的41个角砾岩样品中，总共36个可以解释为代表了Imbrium盆地的形成，支持了Imbrium材料广泛分布在月球近侧的结论。建立对Imbrium撞击的时间限制，可以区分10个Rb-Sr和⁴⁰ Ar / ³⁹ Ar年龄小于3922±12 Ma的样品，其年龄为50 Ma。该组可能代表对月球的单独撞击，需要进一步调查以增进我们对月球撞击历史的理解。