Abstract The Moon is depleted in water and other volatiles compared to Earth and the bulk solar composition. Such depletion of volatile elements and the stable isotope fractionations of these elements can be used to better understand the origin and early differentiation history of the Moon. In this study, we focus on the moderately volatile element, potassium, and we report the K elemental abundances and isotopic compositions (δ41K relative to NIST SRM 3141a) for nineteen Apollo lunar rocks and lunar meteorites (twenty-two subsamples), spanning all major geochemical and petrologic types of lunar materials. The K isotopic compositions of low-Ti and high-Ti basalts are indistinguishable, providing a lunar basalt average δ41K of –0.07 ± 0.09‰ (2SD), which we also consider to be the best estimate of the lunar mantle and the bulk silicate Moon. The significant enrichment of K in its heavier isotopes in the bulk silicate Moon, compared with the bulk silicate Earth (δ41K = –0.48 ± 0.03‰), is consistent with previous analyses of K isotopes and other moderately volatile elements (e.g., Cl, Cu, Zn, Ga, and Rb). We also report analyses of K isotopes for lunar nonmare samples, which show large variations of K isotopic ratios compared to lunar basalts. We interpret this large K isotopic fractionation as the result of late-stage magma ocean degassing during urKREEP formation, which is also coupled with Cl isotope fractionation. Degassing of urKREEP likely triggered redistribution of K isotopes in the Moon, enriching the urKREEP reservoir in heavy K isotopes while implanting the light K isotopic signatures onto the lunar surface. This scenario suggests a heterogeneous distribution of K isotopes in the Moon as a consequence of its magmatic evolution.

中文翻译：

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月球钾同位素组成

摘要 与地球和大部分太阳组成相比，月球缺乏水和其他挥发物。这种挥发性元素的消耗和这些元素的稳定同位素分馏可用于更好地了解月球的起源和早期分化历史。在这项研究中，我们专注于中等挥发性元素钾，并报告了十九个阿波罗月球岩石和月球陨石（二十二个子样本）的 K 元素丰度和同位素组成（δ41K 相对于 NIST SRM 3141a），涵盖所有主要月球材料的地球化学和岩石学类型。低钛和高钛玄武岩的 K 同位素组成无法区分，提供的月球玄武岩平均 δ41K 为 –0.07 ± 0.09‰ (2SD)，我们也认为这是对月球地幔和大块硅酸盐月球的最佳估计. 与大块硅酸盐地球（δ41K = –0.48 ± 0.03‰）相比，大块硅酸盐月球中重同位素 K 的显着富集与先前对 K 同位素和其他中等挥发性元素（例如，Cl、Cu）的分析一致、Zn、Ga 和 Rb)。我们还报告了月球非海样本的 K 同位素分析，与月球玄武岩相比，K 同位素比率的变化很大。我们将这种大的 K 同位素分馏解释为 urKREEP 形成期间后期岩浆海洋脱气的结果，这也与 Cl 同位素分馏有关。urKREEP 的脱气可能触发了月球中 K 同位素的重新分布，在将轻 K 同位素特征植入月球表面的同时，丰富了 urKREEP 库中的重 K 同位素特征。