To constrain the behavior of K isotopes during the low-temperature alteration of oceanic crust and reveal its impact on the global K cycle, we measured the K isotopic compositions of 49 fresh and altered basalts recovered from the Integrated Ocean Drilling Program (IODP) Sites U1365 (~100 Ma) and U1368 (~13.5 Ma) in the South Pacific Ocean. The measured basalts representing the uppermost oceanic crust have been subjected to low-temperature (<150 °C) alteration of different types and intensities as reflected by different secondary mineral modal abundances and element mobility. Both Sites have remarkably low sedimentation rates (0.71 and 1.1 m/Myr, respectively). The altered basalts from both Sites show higher K₂O contents and K/Nb ratios than fresh basalts, suggesting the addition of K during low-temperature alteration. The measured δ⁴¹K values of the altered basalts vary over a large range (−0.76‰ to −0.17‰) compared to unaltered basalts, indicating significant K isotopic fractionation during low-temperature alteration of the oceanic crust. We propose that the K isotopic characteristics of the altered basalts were caused by the combined effects of equilibrium and kinetic isotopic fractionations between seawater and alteration minerals during the incorporation and adsorption of K into alteration minerals. A weighted average δ⁴¹K value of −0.40 ± 0.33‰ (2sd) is given for the low-temperature altered oceanic crust (AOC) at Sites U1365 and U1368, which is indistinguishable from the fresh N-MORB value (−0.44 ± 0.17‰), but significantly lower than its K source, i.e., seawater (δ⁴¹K = +0.12 ± 0.07‰, 2sd). Therefore, low-temperature AOC serves as an important sink for K and is a cause for the heavy K isotopic composition of seawater. In addition, the heterogeneous K isotopic compositions of the low-temperature AOC, together with large scale fractionation during various crustal processes, further highlights the utility of the K isotope system to trace subducted crustal materials.

为了限制大洋地壳低温蚀变过程中 K 同位素的行为并揭示其对全球 K 循环的影响，我们测量了从综合大洋钻探计划 (IODP) 站点 U1365 回收的 49 块新鲜和蚀变玄武岩的 K 同位素组成(~100 Ma) 和 U1368 (~13.5 Ma) 在南太平洋。测量的代表最上层洋壳的玄武岩经历了不同类型和强度的低温（<150°C）蚀变，这反映在不同的次生矿物模态丰度和元素迁移率上。两个站点的沉降速率都非常低（分别为 0.71 和 1.1 m/Myr）。两个站点的蚀变玄武岩显示出更高的 K ₂O 含量和 K/Nb 比值高于新鲜玄武岩，表明在低温蚀变过程中添加了 K。与未蚀变玄武岩相比，蚀变玄武岩的测量 δ ⁴¹ K 值变化范围很大（-0.76‰至-0.17‰），表明在洋壳低温蚀变过程中存在显着的 K 同位素分馏。我们认为蚀变玄武岩的 K 同位素特征是由海水和蚀变矿物在 K 并入和吸附到蚀变矿物过程中的平衡和动力学同位素分馏的综合作用引起的。加权平均 δ ⁴¹U1365和U1368站点的低温蚀变洋壳（AOC）的K值为-0.40±0.33‰（2sd），与新鲜的N-MORB值（-0.44±0.17‰）没有区别，但显着低于其钾源，即海水（δ ⁴¹ K = +0.12 ± 0.07‰，2sd）。因此，低温 AOC 是 K 的重要汇，是海水重 K 同位素组成的原因。此外，低温 AOC 的非均质 K 同位素组成，以及各种地壳过程中的大规模分馏，进一步突出了 K 同位素系统在追踪俯冲地壳物质方面的效用。