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，密度较大的板块（俯冲板块，由海底沉积物、海洋地壳和下面的地幔组成）下沉到浮力较大的板块下面的地幔中。当暴露在海底深处的高温下时，含有水的板块中的矿物质会分解，释放出液体，最终导致岩浆和爆炸性火山活动的产生。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中含有锂、氯和硼等流体可动元素，其组成可诊断俯冲板块（沉积物、海洋地壳、地幔）的来源。位于俯冲板块上方的冷泉和温泉为了解俯冲带深处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外海俯冲沟和岩浆活动产生的火山之间的泉水流体的地球化学，以确定在俯冲带深处释放的板块衍生流体的来源和数量。这些信息对于了解沿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，并就美国和哥斯达黎加的俯冲带及其对公众的相关危害进行外联介绍。俯冲带对于了解地球深部和浅层储层的化学性质至关重要，但对弧前地区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温泉流体的地球化学调查，量化哥斯达黎加整个内陆陆地前弧的Li、B和卤素收支。锂、硼和卤素（Cl、F、Br、I）都是高度流动的元素。它们在矿物中的不相容性限制了流体-岩石相互作用的修饰，从而使它们成为流体来源的可靠示踪剂。哥斯达黎加是一个理想的研究地点，因为它是为数不多的俯冲带之一，其中由于几个半岛的存在，弧前泉处于水下，因此很容易取样。其中两个半岛（尼科亚半岛和奥萨半岛）正位于发震带上方，都是大型逆冲地震的发生地。在尼科亚半岛也观察到慢滑事件。流体压力升高，通常被认为是由变质脱水反应引起的，降低了有效应力，因此通常被认为是地震滑动的机制。来自弧前弹簧的流体为我们提供了一个了解俯冲带发震深度流体来源的直接窗口。通过对Li、B和Cl稳定同位素、元素浓度和元素比值（如Br/Cl、I/Cl、B/Cl、Li/B）的综合利用，将完成以下工作：1)确定沿弧前和弧前的陆块源变化，以及俯冲陆块的进阶变质作用如何控制弧前的Li、B和卤素浓度和同位素组成；2)提供了浅层俯冲过程中流体可动元素损失的综合观点，有助于改进对俯冲带旋回的评价；3)评估不同流体源与地震行为之间的相关性或相关性缺失。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.