Abstract The naturally occurring radium (Ra) isotope quartet was measured for the first time at an intraoceanic arc hydrothermal setting along the Kermadec Island Arc (SW Pacific). 224Ra (3.7 d half life) and 223Ra (11.4 d half life) are assumed to be tracers solely originating from hydrothermal discharge and have been used to resolve near-field hydrothermal plume dynamics by applying both advection and eddy diffusion-based dispersion models. Assuming dominant advective processes, we identified different horizontal plume age structures at Brothers volcano, with a slow moving plume inside the caldera (0.060 cm/s) and a faster moving plume outside the caldera (0.323 cm/s), indicating a longer plume residence time within the caldera structure. The decreasing trend in Ra activity also allows the calculation of horizontal eddy diffusion coefficients inside and outside of the caldera, with 0.164 m2/s and 3.545 m2/s for 224Ra, respectively. These trends are in accordance with the radial horizontal eddy diffusion model. Our results reveal that net advection might not necessarily play a major role at Brothers, as radial horizontal eddy diffusion reproduces the observed Ra activity distribution to the same degree. Additional model simulations based on a combination of advection and diffusion, favor slow net advection to explain the Ra activity distribution. Within the rising non-buoyant plume, Ra isotopes suggested constant dilution along the first 170 m of the vertical water column and a vertical transport velocity on the order of 0.156 cm/s for altitudes up to 150 m above the vent and around 0.004 cm/s for altitudes above 150 m. When a common source of diffuse and focused venting is assumed and only small amounts of Ra are added to the ascending plume, due to recoil originating from high water to rock ratios, fluids from diffuse vents show a slower migration rate through the subsurface than fluids from focused vents. Our data imply that the fluids from diffuse vents needed approximately five days longer to percolate through the underlying subsurface to reach the water column plume, compared to the fluids from direct vents which emanate through chimney structures. This study demonstrates the potential of short-lived Ra isotopes as tracers of near-field plume dynamics, which will improve our understanding of hydrothermal vents as sources for various trace elements to the global ocean.

中文翻译：

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兄弟火山 (Kermadec Arc) 的近场热液羽流动力学：短期镭同位素研究

摘要 在沿克马德克岛弧（西南太平洋）的洋内弧热液环境中首次测量了天然存在的镭 (Ra) 同位素四重体。224Ra（3.7 天半衰期）和 223Ra（11.4 天半衰期）被假定为仅源自热液排放的示踪剂，并已通过应用基于平流和涡流扩散的弥散模型来解决近场热液羽流动力学问题。假设占主导地位的平流过程，我们在兄弟火山确定了不同的水平羽流年龄结构，火山口内缓慢移动的羽流 (0.060 cm/s) 和火山口外移动速度更快的羽流 (0.323 cm/s)，表明羽流驻留时间更长火山口结构内的时间。Ra 活性的下降趋势也允许计算火山口内外的水平涡扩散系数，224Ra 分别为 0.164 m2/s 和 3.545 m2/s。这些趋势符合径向水平涡扩散模型。我们的结果表明，净平流可能不一定在 Brothers 中发挥主要作用，因为径向水平涡流扩散以相同的程度再现了观察到的 Ra 活动分布。基于对流和扩散的组合的附加模型模拟有利于缓慢的净对流来解释 Ra 活动分布。在上升的非浮力羽流中，Ra 同位素表明沿垂直水柱的前 170 m 不断稀释，垂直传输速度约为 0。通风口上方 150 m 以下的高度为 156 cm/s，150 m 以上的高度约为 0.004 cm/s。当假设有一个共同的扩散和集中排放源并且只有少量的 Ra 被添加到上升的羽流中时，由于高水岩比引起的反冲，来自扩散排放口的流体显示出比来自地表的流体更慢的迁移速度。集中通风口。我们的数据表明，与通过烟囱结构喷出的直接通风口的流体相比，来自扩散通风口的流体需要大约五天的时间才能渗透通过下伏的地下到达水柱羽流。这项研究证明了短寿命 Ra 同位素作为近场羽流动力学示踪剂的潜力，