Constraining nitrogen sources and recycling efficiency in hot subduction zones using fluid inclusions in mineral separates from mafic lavas

该奖项全部或部分由《2021年美国救援计划法案》（公法117-2）资助。这个项目通过俯冲和火山作用研究地球表面和地幔之间的氮循环。研究和公共服务将在伍兹霍尔海洋研究所（WHOI）进行。俯冲带在全球地球化学循环中起着至关重要的作用，特别是对于挥发性元素，如氢、碳、硫、氯和氮(N)，它们大部分通过火山脱气返回地表。虽然氮是地球大气中最丰富的气体，但它可以通过俯冲回到深部地幔的程度以及控制深部氮封存的因素仍然未知。在这个项目中，研究人员将1)量化中美洲火山弧（CAVA）和智利安第斯山脉南部火山带（SVZ）数十座火山的氮输出，2)确定弧岩浆中氮的来源。这项工作包括在氮被脱气到地球表面之前，确定被困在晶体中的小流体包裹体中的氮浓度和同位素比率。在WHOI任职期间，PI将与马萨诸塞州布罗克顿的新高地特许学校（6-12年级）合作，开发与俯冲和火山作用相关的教材和演示，这些教材和演示可以直接纳入马萨诸塞州的物理科学标准。他还将通过WHOI的REU项目指导暑期本科生。通过俯冲带和火山弧的挥发性循环改变了地球的大气成分，有助于在地质时期维持稳定的气候和宜居的星球。然而，关于深N周期的全球视角才刚刚开始出现。地幔是氮的净来源（通过火山作用）还是净汇（通过俯冲作用）仍然是一个有争议的话题，可能在很大程度上取决于俯冲板块的热演化。研究人员将利用手头的样品产生两个全面的沿弧N和δ15N数据集，检查沿CAVA和SVZ的基性弧熔岩矿物分离物中的流体包裹体。由于这些系统在热上相似，但具有不同的N来源和俯冲动力学，因此这些数据可以隔离地质控制通过俯冲带和火山弧的N循环。N2/ 3he和N2/Ar的比值将限制沿这些弧段的N通量，而δ15N将允许源判别。该项目利用Nu Noblesse HR稀有气体质谱仪的独特分析能力，该质谱仪连接到WHOI的专用样品破碎和N净化线，避免了现代大气对样品的污染，并提供了目前可用的最高精度的俯冲带N数据。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。

This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2).This project investigates nitrogen recycling between Earth’s surface and the mantle through subduction and volcanism. The research and public service will be conducted at the Woods Hole Oceanographic Institution (WHOI). Subduction zones play a critical role in global geochemical cycles, especially for volatile elements such as hydrogen, carbon, sulfur, chlorine, and nitrogen (N) that largely return to the surface through volcanic degassing. While nitrogen is the most abundant gas in Earth’s atmosphere, the extent to which it can return to the deep mantle through subduction and, the factors that control deep N sequestration, remain unknown. For this project, the researcher will 1) quantify nitrogen outputs from dozens of volcanoes in the Central America Volcanic Arc (CAVA) and the Southern Volcanic Zone (SVZ) of the Chilean Andes, and 2) identify the sources of N in arc magmas. This work involves determining N concentrations and isotope ratios in small inclusions of fluids that are trapped in crystals before N has been degassed to Earth’s surface. During his tenure at WHOI, the PI will partner with New Heights Charter School (Grades 6-12) in Brockton, MA to develop teaching materials and demonstrations related to subduction and volcanism that can be directly incorporated into the Massachusetts state standards for physical sciences. He will also mentor summer undergraduate students through the REU program at WHOI.Volatile recycling through subduction zones and volcanic arcs modifies Earth’s atmospheric composition and helps to maintain a stable climate and a habitable planet over geologic time. However, a global perspective on the deep N cycle is only beginning to emerge. Whether the mantle is a net source (via volcanism) or a net sink (via subduction) of N remains a subject of debate and may be largely governed by the thermal evolution of the subducting slab. The researcher will use samples that in hand to produce two comprehensive, along-arc N and δ15N data sets, examining fluid inclusions in mineral separates from mafic arc lavas along the CAVA and the SVZ. Because these systems are thermally similar but have distinct N sources and subduction dynamics, these data allow for the isolation of geologic controls on N recycling through subduction zones and volcanic arcs. Ratios of N2/3He and N2/Ar will constrain N fluxes along these arc segments while δ15N will allow for source discrimination. This project leverages the unique analytical capacity of a Nu Noblesse HR noble gas mass spectrometer connected to a dedicated sample crushing and N purification line at WHOI, avoiding sample contamination from the modern atmosphere, and providing the highest precision subduction zone N data currently available.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.