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

合作研究：通过哥斯达黎加弧前的流体移动元素循环（卤素、硼、锂）

• 批准号: 1850699
• 负责人: Joost de Moor
• 金额: $ 7.33万
• 依托单位: University of New Mexico
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2019
• 资助国家: 美国
• 起止时间: 2019-05-01 至 2023-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1850699&HistoricalAwards=false
• 关键词: Collaborative; Research; Fluid; mobile; element

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.

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、洋壳和下地幔组成）沉入浮力较大的板块下方的地幔。 含水板块中的矿物在暴露于海底深处的高温下时会分解，释放出液体，最终导致岩浆和火山爆发。此外，这些流体被认为会影响俯冲带地震的发生。俯冲板片释放的流体含有流体移动的元素，如锂、氯和硼，流体的成分是俯冲板片（沉积物、洋壳、地幔）来源的特征。位于俯冲板块上方的冷温泉提供了一个进入俯冲带深处释放的流体的窗口。该项目将调查位于哥斯达黎加外海俯冲海沟和岩浆作用产生的火山之间的泉水的地球化学，以确定俯冲带内深层释放的板状流体的来源和数量。这些信息对于了解活动板块边缘沿着的火山和地震活动是必要的。这项工作还将为研究生和本科生提供科学培训，以及在美国和哥斯达黎加向公众介绍俯冲带和相关的危害。俯冲带对于了解地球的化学深层和浅层储层至关重要，但对弧前制度中的挥发行为知之甚少。在这里，我们建议量化的锂，B，卤素预算整个内部的陆上前弧的哥斯达黎加通过地球化学调查的前弧冷，温泉流体。锂、硼和卤素（Cl、F、Br、I）都是具有高度流体流动性的元素。它们在矿物中的不相容性限制了流体-岩石相互作用的改性，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，因为它是少数几个俯冲带之一，其中弧前泉水是陆上的，因为存在几个半岛，因此很容易取样。其中两个半岛（尼科亚和奥萨）直接位于孕震区上方，都是大型逆冲断层地震的发生地。在尼科亚半岛也观察到缓慢滑动事件。流体压力升高，通常认为是来自变质脱水反应，降低了有效应力，因此通常被援引为一种机制，为aseprocket滑动。弧前温泉的流体为我们提供了一个直接的窗口，在俯冲带的孕震深度的流体源。通过组合使用Li、B和Cl稳定同位素、元素浓度和元素比率（例如，Br/Cl、I/Cl、B/Cl、Li/B），将完成以下工作：1）确定沿着弧前和横跨弧前的板片源的变化，以及俯冲板片的俯冲变质作用如何控制弧前Li、B和卤素的浓度和同位素组成; 2）提供液体损失的综合视图-浅俯冲过程中的移动的元素有助于改进对俯冲带循环的评估;和3）评估相关性，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知识价值和更广泛的影响进行评估，被认为值得支持审查标准。