Potassium isotopic composition of low-temperature altered oceanic crust and its impact on the global K cycle
Potassium isotopic composition of low-temperature altered oceanic crust and its impact on the global K cycle
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低温蚀变洋壳钾同位素组成及其对全球钾循环的影响
DOI：
10.1016/j.gca.2021.08.001
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发表时间：
2021
影响因子：
5
通讯作者：
Kun Wang
中科院分区：
文献类型：
--
作者：
Haiyang Liu;Ying-Yu Xue;Guoliang Zhang;Weidong Sun;Zhen Tian;Brenna Tuller-Ross;Kun Wang
为了研究大洋地壳低温蚀变过程中钾同位素的变化规律，揭示其对全球钾循环的影响，对南太平洋综合大洋钻探（IODP）U1365（~100 Ma）和U1368（~13.5 Ma）的49个新鲜玄武岩和蚀变玄武岩进行了钾同位素测定。所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式丰度和元素迁移率上。这两个地点的沉积速率都非常低（分别为0.71和1.1米/百万年）。与新鲜玄武岩相比，两个地点的蚀变玄武岩均具有较高的K2 O含量和K/Nb比值，表明低温蚀变过程中有钾的加入。与未蚀变玄武岩相比，蚀变玄武岩的δ 41 K值变化范围较大（-0.76 ‰ ~-0.17 ‰），表明在洋壳低温蚀变过程中存在明显的K同位素分馏。我们认为，蚀变玄武岩的钾同位素特征是由海水和蚀变矿物之间的平衡和动力学同位素分馏的综合作用，在钾的掺入和吸附到蚀变矿物。U1365和U1368站低温蚀变洋壳（AOC）的加权平均δ 41 K值为−0.40 ± 0.33‰（2sd），与新鲜N-MORB值（−0.44 ± 0.17‰）没有区别，但明显低于其K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K同位素组成重的原因。此外，低温AOC的不均匀的K同位素组成，以及在各种地壳过程中的大规模分馏，进一步突出了K同位素系统跟踪俯冲地壳物质的实用性。
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 K 2 O contents and K/Nb ratios than fresh basalts, suggesting the addition of K during low-temperature alteration. The measured δ 41 K 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 δ 41 K 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 (δ 41 K = +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.