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

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

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.