Recycling of nitrogen and light noble gases in the Central American subduction zone: Constraints from 15N15N

Abstract

How much nitrogen and light noble gases are recycled in modern subduction zones is unclear. Fumaroles act as a means for passive degassing in arcs. They receive variable contributions of volatiles from arc magmas, themselves sourced from the mantle wedge. The gas compositions reflect the extent of volatile enrichment in sub-arc mantle sources and constrain slab dehydration. However, contributions from atmospheric components in fumaroles are unavoidable. For N2, neon and argon, the atmospheric components are challenging to discern from slab-derived components. Here, we report 15N15N measurements from eight fumaroles and seven bubbling springs, along the Central American arc. Our new 15N15N data are coupled with noble gases measurements and show that air-derived components in volcanic gas discharges can easily be underestimated, in both fumaroles and springs, using conventional stable isotope or noble gases methods. We show that, in the absence of 15N15N data, previously used tracers for air (e.g., δ15N, N2/Ar, N2/He, among others) may lead to erroneous conclusions regarding the origin of volatiles in mixed gases. In contrast, 15N15N data provide quantitative constraints on the nature and contributions of both atmospheric and magmatic components. Most springs are heavily dominated by air-derived N2, while fumaroles show substantial contributions of volcanic endmembers. Based on the fumarole data, we show that magma sources beneath the central American arc are enriched in all volatiles relative to 3He, by two to three orders of magnitude compared to the MORB source. We use new 15N15N data to obtain source N2/3He, 3He/36Ar and 3He/22Ne ratios which we then use to compute volcanic N2, Ar and Ne degassing fluxes. Using this approach, we show that outgassing fluxes appear to match subduction fluxes in the Central America subduction zone. We determine an N2 outgassing flux of between 4.0×108 and 1.0×109 mol N2/y, comparable to the subduction flux of 5.7×108 mol N2/yr determined previously. We obtain a similar conclusion for 22Ne and 36Ar. Overall, the volatile fluxes in the central American subduction zone no longer seem to require net transfer of N2, Ar, and Ne, to the deep mantle.