Stable potassium (K) isotope characteristics at mid-ocean ridge hydrothermal vents and its implications for the global K cycle
Stable potassium (K) isotope characteristics at mid-ocean ridge hydrothermal vents and its implications for the global K cycle
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DOI：
10.1016/j.epsl.2022.117653
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发表时间：
2022-09
影响因子：
5.3
通讯作者：
Xin-Yuan Zheng;B. Beard;M. Neuman;M. Fahnestock;J. Bryce;C. Johnson
中科院分区：
文献类型：
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作者：
Xin-Yuan Zheng;B. Beard;M. Neuman;M. Fahnestock;J. Bryce;C. Johnson
最近发现的各种陆地样品中稳定钾同位素比值（41 K/39 K或δ 41 K）的显著变化表明，钾同位素可以成为全球钾循环的新示踪剂，但海水δ 41 K比块状硅酸盐地球高出约0.6‰的关键观察结果仍然无法解释。一个不受约束的组成部分，这一难题的关键是热液系统，代表了一个主要的钾源和汇在海洋中。本文报道了戈达海脊和东太平洋海隆（EPR）109 ° N洋底中脊（莫尔）热液流体的δ 41 K结果，包括记录了局部火山爆发引起的流体化学主要扰动的时间序列样品。流体δ 41 K值介于新鲜玄武岩和海水之间，为-0.46 ‰ ~-0.15 ‰。在火山爆发后不久收集的“零时”流体的δ 41 K值向海水值偏移，随后在2002年内恢复到爆发前的值。流体δ 41 K的变化很大程度上受水-岩相互作用的影响，但不能仅仅用海水和高温下从玄武岩中浸出的钾的简单混合来解释。相反，这些数据意味着小，但显着的同位素分馏，富集重钾同位素的玄武岩，可能造成的低温蚀变在热液循环的补给阶段。我们的研究结果排除了莫尔热液体系是海水δ 41 K值偏高的原因。利用流体δ 41 K数据和这里限制的热液系统的K同位素分馏，K质量平衡模型暗示了海洋沉积汇（可能是自生粘土形成）在全球K循环中的关键作用。此外，将本文限制的K同位素分馏应用于已发表的蛇绿岩δ 41 K数据显示，奥陶纪期间海水δ 41 K显著降低的可能性，这可以解释为当时不同气候和构造的反风化作用增强。
Recent discoveries of significant variations in stable K isotope ratios (41 K/39 K or δ 41 K) among various terrestrial samples indicate that K isotopes can be a novel tracer for the global K cycle, but a key observation that seawater δ 41 K is∼ 0.6‰ higher than the bulk silicate Earth remains unexplained. An unconstrained component critical to this puzzle is hydrothermal systems that represent both a major K source and sink in the ocean. Here we report δ 41 K results on mid-ocean ridge (MOR) hydrothermal fluids from the Gorda Ridge and∼ 9° N East Pacific Rise (EPR), including time-series samples that recorded major perturbations in fluid chemistry induced by a local volcanic eruption. Fluid δ 41 K values range from-0.46‰ to-0.15‰, falling between those of fresh basalts and seawater. δ 41 K values of “time-zero” fluids collected shortly after the volcanic eruption are shifted towards the seawater value, followed by a return to pre-eruption values within∼ 2 years. Fluid δ 41 K variations are largely influenced by water–rock interactions, but they cannot be solely explained by simple mixing of seawater and K leached from basalts at high temperatures. Instead, these data imply small but significant isotope fractionation that enriches heavy K isotopes in basalts, likely caused by low-temperature alteration during the recharge stage of hydrothermal circulation. Our results preclude MOR hydrothermal systems as the cause for the heavy δ 41 K value of seawater. Using fluid δ 41 K data and K isotope fractionation constrained here for hydrothermal systems, a K mass-balance model implies a critical role for a marine sedimentary sink, possibly authigenic clay formation, in the global K cycle. Also, applying the K isotope fractionation constrained here to the published δ 41 K data from ophiolites shows the possibility for significantly lower seawater δ 41 K during the Ordovician, which can be explained by enhanced reverse weathering in response to distinct climate and tectonics at that time.