Collaborative Research: Fluid-mobile element cycling (halogens, boron, lithium) through the forearc of Costa Rica

俯冲带是两个构造板块碰撞的区域，密度较高的板块(俯冲板块，由海底沉积物、洋壳和下伏地幔组成)下沉到浮力较大的板块下方的地幔中。含有水的板块中的矿物质暴露在海底深处遇到的高温下时会分解，释放出流体，最终导致岩浆的产生和火山爆发。此外，这些流体被认为影响了俯冲带地震的发生。俯冲板块释放的流体含有锂、氯和硼等流体活动元素，流体的组成对它们所来自的俯冲板块(沉积物、洋壳、地幔)的来源具有诊断作用。位于俯冲板上方的冷泉和温泉为俯冲带深部释放的流体提供了一个窗口。该项目将调查位于哥斯达黎加附近俯冲海沟和火山之间的由岩浆作用产生的泉水流体的地球化学，以确定俯冲带内深部板岩衍生流体的来源和数量。这些信息对于了解活动板块边缘的火山和地震行为是必要的。这项工作还将为研究生和本科生提供科学培训，并向美国和哥斯达黎加的公众介绍俯冲带及其相关危害。俯冲带对于了解地球深层和浅层储层的化学成分至关重要，但对弧前制度中的挥发性行为知之甚少。在这里，我们建议通过对弧前冷泉和热泉流体的地球化学调查来量化整个哥斯达黎加内亚航空前弧的Li、B和卤素收支。锂、硼和卤素(氯、氟、溴、碘)都是流动性很强的元素。它们在矿物中的不相容限制了流体-岩石相互作用的改造，从而使它们成为可靠的流体来源示踪剂。哥斯达黎加是一个理想的研究地点，因为它是少数几个俯冲带之一，由于存在几个半岛，弧前泉水是地下的，因此很容易进行采样。其中两个半岛(尼科亚和奥萨)位于地震孕育区的正上方，都是大型逆冲地震的发源地。尼科亚半岛也观察到了慢滑事件。流体压力升高通常被认为是由变质脱水反应引起的，它降低了有效应力，因此通常被认为是无地震滑动的一种机制。来自弧前泉水的流体为我们提供了一个直接了解俯冲带地震孕育深度的流体来源的窗口。通过Li、B和Cl稳定同位素、元素浓度和元素比值(如：溴/氯、碘/氯、硼/氯、锂/硼)的组合使用，将完成以下工作：1)确定沿和跨越前弧板片源的变化(S)，以及俯冲板片的递进变质作用如何控制整个弧前的Li、B和卤素浓度和同位素组成；2)提供浅俯冲过程中流体活动元素损失的综合观点，有助于改进对俯冲带旋回的评价；以及3)评估不同流体来源与地震行为之间的相关性或缺乏相关性。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

A subduction zone is an area where two tectonic plates collide and the denser plate (the subducting slab, consisting of seafloor sediments, oceanic crust, and underlying mantle) sinks into the mantle beneath the more buoyant plate. Minerals in the slab containing water break down when exposed to the high temperatures encountered at depth beneath the seafloor, releasing fluids which ultimately lead to the production of magmas and explosive volcanism. In addition, these fluids are thought to influence the occurrence of earthquakes in the subduction zone. Fluids released by the subducting slab contain fluid mobile elements, such as lithium, chlorine and boron, and the composition of the fluids is diagnostic of the source in the subducting slab (sediments, oceanic crust, mantle) from which they derived. Cold and thermal springs located above the subducting slab provide a window into the fluids released at depth in a subduction zone. This project will investigate the geochemistry of spring fluids located off of Costa Rica between the subduction trench and the volcanoes produced by magmatism to identify the sources and amount of slab-derived fluids released at depth within a subduction zone. This information is necessary to understand volcano and earthquake behavior along active plate margins. This work will also provide scientific training for graduate and undergraduate students, as well as outreach presentations on subduction zones and associated hazards to the public in both the United States and Costa Rica.Subduction zones are crucial for understanding the chemistry deep and shallow reservoirs of Earth, yet very little is known about volatile behavior in forearc regimes. Here we propose to quantify the Li, B, and halogen budget throughout the inner subaerial forearc of Costa Rica through a geochemical investigation of forearc cold and thermal spring fluids. Lithium, boron, and the halogens (Cl, F, Br, I) are all highly fluid-mobile elements. Their incompatibility in minerals limits modification by fluid-rock interaction, thereby making them reliable tracers of fluid source. Costa Rica is an ideal locality for study because it is one of the few subduction zones in which forearc springs are subaerial due to the presence of several peninsulas and therefore easily accessible to sampling. Two of the peninsulas (Nicoya and Osa) are directly over the seismogenic zone and both are sites of large megathrust earthquakes. Slow slip events have also been observed in the Nicoya Peninsula. Elevated fluid pressures, commonly thought to be derived from metamorphic dehydration reactions, lower the effective stress and are therefore commonly invoked as a mechanism for aseismic slip. Fluids from forearc springs provide us with a direct window to fluid sources at seismogenic depths in subduction zones. Through the combined use of Li, B, and Cl stable isotopes, elemental concentrations, and elemental ratios (e.g., Br/Cl, I/Cl, B/Cl, Li/B), the following will be completed: 1) Identify the change(s) in slab-derived source along and across the forearc and how prograde metamorphism of the subducting slab controls Li, B, and halogen concentrations and isotope compositions across the forearc; 2) Provide an integrated view of loss of fluid-mobile elements during shallow subduction processes contributing to an improved evaluation of cycling through subduction zones; and 3) Evaluate correlation, or lack thereof, between varying fluid sources and seismic behavior.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.