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

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 N-2/He (1217-17,300) and low CO2/N-2,N-exc. (78-937), implying addition of nitrogen from the subducting slab. This inference is supported by positive (i.e., sediment-like) delta N-15 values (up to 5.5%) in most samples. The exception to these trends is Agrigan in the Mariana arc, with low N-2/He (similar to 200), high CO2/N-2,N-exc. (similar to 1500), and negative delta N-15. 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 N-2 fluxes estimated by three different methods range from 0.25 x 10(8) to 1.11 x 10(8) mol yr(-1) N-2, 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 delta N-15 of the residual nitrogen subducted into the mantle is estimated at approximately -1.9 parts per thousand. Despite similarities in gas chemistry and delta N-15 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.