Nitrogen Transiting the Subduction and Redox Barrier

地球的宜居性取决于保持大气中氮的高丰度。氮气在火山爆发时被添加到大气中，当它与地壳结合时从大气中消失。地壳在地质时期俯冲到地球内部。在俯冲过程中，一些氮被保留在地壳中，一些被释放到流体和岩浆中，最终将氮释放到大气中。氮在地壳中保留和从地壳中释放的比例预计取决于地壳的氧化还原状态。地球化学的一个重要观察结果是，在地球的外部（地壳+大气）和内部（地幔）中，氮与钾的比例是恒定的。在还原条件下，氮和钾的地球化学耦合是有预期的。因此，地球外部和内部氮/钾比率的恒定表明，地壳内部的还原环境主要负责将氮输送到地球内部。然而，氮和钾在地球化学上耦合的具体氧化还原边界仍然不明确。定义这个边界是这个项目的目标。通过定义这一边界，将推导出俯冲物质氧化还原状态的约束条件，从而进一步了解地球上富氮大气是如何维持的，以及俯冲带中发生的氧化还原敏感过程，包括火山放气、大陆地壳形成和矿石成因。该项目通过为本科生提供研究机会和为年轻科学家提供培训来推进STEM领域。为了确定氮钾耦合的氧化还原边界，将进行一系列的实验，模拟氮钾在俯冲过程中所受的条件。每个实验的氧化还原状态将由金属氧化物缓冲液控制，并将由XANES和/或Pt-in-Fe氧气压计独立验证。氮的形态将在每次实验的玻璃杯和被困流体中确定。实验还将进行化学分析，以确定氮和钾对矿物和岩浆/流体的相对亲和力如何随着压力、温度和氧化还原电位的变化而变化。然后，这些数据将应用于板坯融化/脱水的地球化学模型，以约束地壳在保持氮钾偶联的同时可能承受的压力、温度和氧化还原条件。

Earth's habitability depends on maintaining a high abundance of nitrogen in the atmosphere. Nitrogen is added to the atmosphere during volcanic eruptions and is removed from the atmosphere when it is incorporated into the crust. Crust is subducted into Earth's interior over geological time. During the subduction process some nitrogen is retained in crust and some is liberated into fluids and magmas that ultimately return nitrogen to the atmosphere. The proportions of nitrogen retained in and liberated from crust are expected to depend on the redox state of crust. A critical observation in geochemistry is that the ratio of nitrogen to potassium is constant between Earth's exterior (crust+atmosphere) and interior (mantle). Geochemical coupling of nitrogen and potassium is expected under reducing conditions. Therefore, the constancy of nitrogen/potassium ratios between Earth's exterior and interior suggests that reduced environments within crust are largely responsible for transporting nitrogen into Earth's interior. The specific redox boundary where nitrogen and potassium become geochemically coupled, however, remains poorly defined. Defining this boundary is the goal of this project. By defining this boundary, a constraint on the redox state of subducted materials will be derived that will yield further insight into how a nitrogen-rich atmosphere has been maintained on Earth and into redox-sensitive processes occurring in subduction zones, including volcanic outgassing, continental crust formation, and ore genesis. This project advances STEM fields by providing research opportunities for undergraduate students and by providing training to a young scientist.  A series of experiments will be conducted that simulate the conditions that nitrogen and potassium are subjected to during subduction to define the redox boundary of nitrogen-potassium coupling. The redox state of each experiment will be controlled by metal-oxide buffers and will be independently verified by XANES and/or Pt-in-Fe oxybarometers. Nitrogen speciation will be determined within the glasses and trapped fluids of each experiment. Experiments will also be chemically analyzed to determine how the relative affinity of nitrogen and potassium for minerals and magma/fluids changes as a function of pressure, temperature, and redox potential. These data will then be applied to geochemical models of slab melting/dehydration to constrain the pressure, temperature, and redox conditions that crust can be subjected to while maintaining nitrogen-potassium coupling.