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

该奖项的全部或部分资金来自《2021年美国救援计划法案》(公法117-2)。该项目调查俯冲和火山活动导致的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发挥着关键作用，特别是对挥发性元素，如氢、碳、硫、氯和氮(N)，这些元素主要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，但它能在多大程度上通过俯冲返回地幔深处，以及控制深层氮封存的因素，目前尚不清楚。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)数十座火山的氮产出，以及2)确定弧岩浆中氮的来源。这项工作包括确定在N被脱气到地球表面之前被困在晶体中的流体的小包裹体中的N浓度和同位素比率。在他在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有关的教材和演示，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。他还将通过世界卫生组织的REU项目指导夏季本科生。通过俯冲带和火山弧进行的火山循环改变了地球的大气组成，有助于在地质时期保持稳定的气候和宜居的地球。然而，对深度N周期的全球视角才刚刚开始浮现。地幔是N的净来源(通过火山作用)还是净沉入(通过俯冲)仍然是一个争论的主题，并可能在很大程度上受俯冲板块的热演化控制。研究人员将利用手头的样品产生两个全面的沿弧形N和δ15N数据集，检查沿洞穴和SVZ从镁铁质弧状熔岩中分离出来的矿物中的流体包裹体。由于这些系统在热上相似，但具有不同的氮源和俯冲动力学，这些数据允许通过俯冲带和火山弧隔离对氮循环的地质控制。N_2/3He和N_2/Ar的比值将限制这些弧段的N通量，而δ15N将允许源识别。该项目利用了NU Noblesse HR惰性气体质谱仪的独特分析能力，该质谱仪连接到WHOI的专用样品粉碎和氮净化生产线，避免了现代大气中的样品污染，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。

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.