4Constraining the duration of magmatic activity on the Moon is essential to understand how the lunar mantle evolved chemically through time. Determining age and initial isotopic compositions of mafic lunar meteorites is a critical step in defining the periods of magmatic activity that occurred during the history of the Moon and to constrain the chemical characteristics of mantle components involved in the sources of the magmas. We have used the in situ Pb-Pb SIMS technique to investigate eight lunar gabbros and basalts, including six meteorites from the Northwest Africa (NWA) 773 clan (NWA 2727, NWA 2700, NWA 3333, NWA 2977, NWA 773, and NWA 3170), NWA 4734, and Dhofar 287A. These samples have been selected as there is no clear agreement on their age and they are all from the dominant low titanium chemical group. We have obtained ages of 2981 ± 12 Ma for NWA 4734 and 3208 ± 22 Ma for Dhofar 287. For the NWA 773 clan, four samples (the fine-grained basalt NWA 2727 and the three gabbros NWA 773, NWA 2977, NWA 3170) out of six yielded isochron-calculated ages that are identical within uncertainties and yielding an average age of 3086 ± 5 Ma. The age obtained for the fine-grained basalt NWA 2700 is not precise enough for comparison with the other samples. The gabbroic sample NWA 3333 yielded an age of 3038 ± 20 Ma suggesting that two distinct magmatic events may be recorded in the meteorites of the NWA 773 clan. The present study aims to identify and assess all potential issues that are associated with different ways to date lunar rocks using U-Pb–based methods. To achieve this, we have compared the new ages with the previously published data set. The entire age data set from lunar mafic meteorites was also screened to identify data showing analytical issues and evidence of resetting and terrestrial contamination. The data set combining the ages of mafic lunar meteorites and Apollo rocks suggests pulses of magmatic activity with two distinct phases between 3950 and 3575 Ma and between 3375 and 3075 Ma with the two phases separated by a gap of approximately 200 Ma. The evolution of the Pb initial ratios of the low-Ti mare basalts between approximately 3400 and 3100 Ma suggests that these rocks were progressively contaminated by a KREEP-like component.

中文翻译：

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月球陨石的 Pb-Pb 年龄和初始 Pb 同位素组成：NWA 773 族、NWA 4734 和 Dhofar 287

4 限制月球上岩浆活动的持续时间对于了解月球地幔如何随时间进行化学演化至关重要。确定月球镁铁质陨石的年龄和初始同位素组成是确定月球历史上岩浆活动时期和限制岩浆来源中地幔成分化学特征的关键步骤。我们使用原位 Pb-Pb SIMS 技术调查了八块月球辉长岩和玄武岩，包括来自西北非 (NWA) 773 族群的六块陨石（NWA 2727、NWA 2700、NWA 3333、NWA 2977、NWA 773 和 NWA 317 )、NWA 4734 和佐法尔 287A。之所以选择这些样品，是因为它们的年龄没有明确的一致性，而且它们都来自占主导地位的低钛化学组。我们获得了 NWA 4734 的年龄为 2981 ± 12 Ma，Dhofar 287 的年龄为 3208 ± 22 Ma。对于 NWA 773 族，四个样品（细粒玄武岩 NWA 2727 和三个辉长岩 NWA 773、NWA 2977、NWA）在六个产生的等时计算年龄中，在不确定性内是相同的，平均年龄为 3086 ± 5 Ma。获得的细粒玄武岩 NWA 2700 的年龄不够精确，无法与其他样品进行比较。辉长岩样品 NWA 3333 的年龄为 3038 ± 20 Ma，表明 NWA 773 族的陨石中可能记录了两个不同的岩浆事件。本研究旨在识别和评估与使用基于 U-Pb 的方法对月球岩石进行测年的不同方法相关的所有潜在问题。为了实现这一点，我们将新时代与之前发布的数据集进行了比较。还筛选了来自月球基性陨石的整个年龄数据集，以确定显示分析问题的数据以及重置和地面污染的证据。结合镁铁质月球陨石和阿波罗岩石年龄的数据集表明，岩浆活动的脉冲在 3950 至 3575 Ma 和 3375 至 3075 Ma 之间有两个不同的阶段，两个阶段之间的间隔约为 200 Ma。大约 3400 到 3100 Ma 之间低钛玄武岩的 Pb 初始比率的演变表明这些岩石逐渐受到类 KREEP 成分的污染。结合镁铁质月球陨石和阿波罗岩石年龄的数据集表明，岩浆活动的脉冲在 3950 至 3575 Ma 和 3375 至 3075 Ma 之间有两个不同的阶段，两个阶段之间的间隔约为 200 Ma。大约 3400 到 3100 Ma 之间低钛玄武岩的 Pb 初始比率的演变表明这些岩石逐渐受到类 KREEP 成分的污染。结合镁铁质月球陨石和阿波罗岩石年龄的数据集表明，岩浆活动的脉冲在 3950 至 3575 Ma 和 3375 至 3075 Ma 之间有两个不同的阶段，两个阶段之间的间隔约为 200 Ma。大约 3400 到 3100 Ma 之间低钛玄武岩的 Pb 初始比率的演变表明这些岩石逐渐受到类 KREEP 成分的污染。