Chemical hydrology of subduction zones: Processes, signals and fluid flow

俯冲带在地球的地质活动中起着中心作用。这种活动可以表现为地震、海啸和火山爆发等破坏性事件。导致这种灾难性行为的许多过程都是由流体驱动的。本研究的重点是俯冲带浅部（<15公里）的化学水文学，以阐明使俯冲沉积物能够产生地震的过程。低氯度和挥发性含量的近地表流体在活跃的会聚边缘表明，流体从深源向上倾迁移。外来的流体成分，这已被记录为几个活跃的会聚边缘，被认为是取决于温度驱动的粘土矿物脱水。这一假设是阻碍了观察到的低含氯量流体远离流体通道，这表明额外的流体清新机制。最重要的是，一套水岩过程已被假设发生在深度，使粘滑行为，但只记录在化石增生棱镜，而不是在积极的。为了解决这些问题，拟议的研究将分析流体提取实验室热液压实试验和天然流体从两个相反的活动边缘，增生南海海槽（日本）和侵蚀哥斯达黎加边缘。与其他热液系统相比，热液装置是一种独特的方法，因为它模拟了有效应力（P）增加、孔隙率降低和温度升高（T高达150°C）的原位埋藏条件。将在与孕震区上限相似的受控PT条件下对每个边缘的沉积物端部成员进行测试，以使用主要和挥发性元素地球化学以及硼和锂同位素研究粘土脱水和水-岩相互作用。热液测试的地球化学指纹将用于更好地区分天然流体中的地球化学信号。从而更好地了解俯冲带中的水-岩相互作用和流体流动。

Subduction zones play a central role in the geological activity of the earth. This activity may be expressed as devastating events such as earthquakes, tsunamis and explosive volcanism. Many processes that lead to such catastrophic behavior are driven by fluids. This study focuses on the chemical hydrology of the shallow portion (<15 km) of subduction zones to shed light on processes that enable subducting sediments to behave seismogenically. Low-chlorinity and volatile content of near surface fluids in active convergent margins suggest that fluids migrate updip from deep sources. The exotic fluid composition, which has been documented for several active convergent margins, is thought to depend on temperature driven clay mineral dehydration. This assumption is hampered by observed low-chlorinity fluids away from fluid pathways that suggest additional fluid freshening mechanisms. Most importantly, a suite of water-rock processes has been hypothesized to occur at depth to enable stick-slip behavior but are only documented in fossil accretionary prisms but not in active ones. To tackle these problems the proposed study will analyze fluids extracted from laboratory hydrothermal compaction tests and natural fluids from two contrary active margins, the accretionary Nankai Trough (Japan) and the erosional Costa Rica margin. The hydrothermal device is a unique approach compared to other hydrothermal systems because it simulates in-situ burial conditions with increasing effective stress (P), decreasing porosity and increasing temperatures (T up to 150°C). Sediment end members of each margin will be tested under controlled PT conditions similar to the upper limit of the seismogenic zone to study clay dehydration and water-rock interaction using major and volatile element geochemistry and boron and lithium isotopes. Geochemical fingerprints from the hydrothermal tests will be used to better distinguish geochemical signals in natural fluids. Thus, a better insight into water-rock interaction and fluid flow in the subduction zones will be achieved.