Nitrogen sources and recycling at subduction zones: Insights from the Izu‐Bonin‐Mariana arc

Abstract

We report new chemical and nitrogen isotopic data from 29 volcanic and hydrothermal gas samples covering eight centers in the Izu‐Bonin‐Mariana (IBM) arc to investigate the sources, flux, and mass balance of nitrogen at a “cool” convergent margin. The majority of samples have high N2/He (1217–17,300) and low CO2/N2,exc. (78–937), implying addition of nitrogen from the subducting slab. This inference is supported by positive (i.e., sediment‐like) δ15N values (up to 5.5‰) in most samples. The exception to these trends is Agrigan in the Mariana arc, with low N2/He (∼200), high CO2/N2,exc. (∼1500), and negative δ15N. Mixing calculations suggest an average of 34% of the nitrogen in our samples is derived from subducted sediment, or 75% after correction for atmospheric contamination. Sediment‐derived N2 fluxes estimated by three different methods range from 0.25 × 108 to 1.11 × 108 mol yr−1 N2, representing 4%–17% of the total nitrogen input flux or 11%–51% of the sedimentary nitrogen input flux. The altered oceanic crust is identified as an important contributor to the arc nitrogen budget, and the δ15N of the residual nitrogen subducted into the mantle is estimated at approximately −1.9‰. Despite similarities in gas chemistry and δ15N values, our conclusions regarding nitrogen recycling for IBM are markedly different than those for the Nicaraguan segment of the Central American arc, and we suggest that thermal regime is the major control on nitrogen recycling within subduction zones. The global nitrogen cycle is estimated to be in steady state, suggesting either that subducted sediments are an unlikely source for heavy nitrogen in plume‐related rocks or secular variation in the isotopic composition of subducted sediments. Better constraints on nitrogen recycling at other arcs are required to test these conclusions.