Journal Article

Deep Nitrogen Fluxes and Sources Constrained by Arc Lava Phenocrysts

Shaw, Alison M., Cahoon, Emily, Curtice, Joshua M., Li, Kan, Kurz, Mark D., Bekaert, David V., Tyne, Rebecca L., Turner, Stephen J., Hudak, Michael R., Barry, Peter H., Wallace, Paul, Walowski, Kristina, Broadley, Michael W., Nielsen, Sune G., Muth, Michelle

2024

Overview

Nitrogen (N) dominates Earth's atmosphere (78% N2) but occurs in trace abundances in silicate minerals, making it a sensitive tracer of recycled surface materials into the mantle. The mechanisms controlling N transfer between terrestrial reservoirs remain uncertain because low N abundances in mineral‐hosted fluid inclusions (FIs) are difficult to measure. Using new techniques, we analyzed N and He isotope compositions and abundances in olivine‐ and pyroxene‐hosted FIs from arc volcanoes in Southern Chile, Cascadia, Central America, and the Southern Marianas. These measurements enable an estimate of the global flux of N outgassing from arcs (4.0 × 1010 mol/yr). This suggests that Earth is currently in a state of net N ingassing, with roughly half of subducted N returned to the mantle. Additionally, the N outgassing flux of individual arcs correlates with the thickness of subducting pelagic sediment, suggesting that N cycling in the modern solid Earth is largely controlled by sediment subduction. Plain Language Summary Nitrogen (N) largely behaves like an inert gas, and so it is substantially more concentrated at Earth's surface than in Earth's deep interior. Over geologic time, N can be transported between the solid Earth and the surface, and its concentration can change in both of these settings. Volcanic gases transport N from the interior to the surface, while some surface N returns into the solid Earth via plate subduction. Here, we present measurements of N and helium (He) gas trapped within crystals in volcanic rocks to determine how much N is transported to the surface through volcanism associated with plate subduction. We find that the amount of N returning to the surface through volcanism is less than estimates of how much N is transported into the solid Earth, suggesting that, overall, N is being returned to the planet's deep interior. Additionally, we observe that the amount of oceanic sediment that is subducted correlates with the amount of N that comes out of volcanoes, making it the primary carrier of N into the solid Earth. Key Points Arc lavas yield fluxes of 4.0 × 1010 mol N/yr, similar to estimates from volcanic arc gases, likely resulting in net mantle ingassing of N Nitrogen isotopes and N‐He mixing models highlight that small contributions of sediment dominate volcanic arc N budgets Subducted sediment thickness correlates with N2/3He ratios, and likely controls arc N fluxes rather than slab parameters or thermal state

• Publisher: John Wiley & Sons, Inc,American Geophysical Union,Wiley
• Subject: Abundance / Crystals / Earth / Earth mantle / Earth Sciences / Earth surface / Fluid inclusions / fluxes / Geological time / Helium / Helium isotopes / Isotope composition / Nitrogen / Olivine / Outgassing / Pelagic sediments / Plates (tectonics) / Rare gases / Recycled materials / Sciences of the Universe / Sediment / Sediments / Silicate minerals / Silicates / Subduction / Subduction (geology) / Tracers / volatile cycling / Volcanic activity / volcanic arcs / Volcanic gases / Volcanic rocks / Volcanism / Volcanoes