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。这一活动可表述为破坏性事件，如地震、海啸和火山爆发。导致这种灾难性行为的许多过程都是由流体驱动的。这项研究的重点是俯冲带浅层(15公里)的化学水文学，以揭示俯冲沉积物具有地震行为的过程。活动汇聚边缘近地表流体的低氯度和挥发分含量表明流体从深部向上倾斜迁移。这种奇异的流体成分被认为依赖于温度驱动的粘土矿物脱水。这一假设被观察到的远离流体通道的低氯度流体所阻碍，这表明了额外的流体清新机制。最重要的是，一套水-岩过程被假设发生在深处，以实现粘滑行为，但只在化石吸积棱柱中记录，而在活跃的棱柱中没有记录。为了解决这些问题，拟议的研究将分析从实验室热液压实试验中提取的流体和来自两个相反的活动边缘--增生的南开海槽(日本)和侵蚀的哥斯达黎加边缘--的自然流体。与其他水热系统相比，水热装置是一种独特的方法，因为它模拟了现场埋藏条件，增加了有效应力(P)，减少了孔隙率，并提高了温度(T高达150°C)。每个边缘的沉积物端元将在类似于地震孕育区上限的受控PT条件下进行测试，以利用主要和挥发性元素地球化学性质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被用来更好地区分天然流体中的地球化学信号。因此，将更好地了解水-岩相互作用和俯冲带中的流体流动。

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.