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First in situ analyses of nitrogen in primitive subduction-related melts

A quantitative understanding of nitrogen fluxes and pre-eruptive magmatic concentrations in subduction zones is key for constraining the N recycling efficiency between Earth's internal and external reservoirs. Although the three-component mixing model of Sano et al. [1], for example, provides a means to estimate the relative contributions from air, subducted sediments, and the mantle to the total N content of gas and/or water samples from arc volcanoes and associated hydrothermal systems, assessing the N content of primitive arc magmas and their mantle source remains a challenge. Given that melt inclusions (MIs) in Mg-rich olivine represent the best proxies for primary arc melts, we applied, for the first time, an in situ high-resolution secondary ion mass spectrometry (SIMS) method [2] to determine the N concentration in olivine-hosted MIs from Klyuchevskoy volcano in Kamchatka. To reverse the effects of post-entrapment modification processes, the MIs were partially to completely homogenized at high temperatures (1150–1400°C) and pressures ranging from 0.1 to 500 MPa under dry to H 2 O-saturated conditions at variable oxygen fugacities (CCO to QFM+3.3) [3]. After the experiments, N concentrations in water-rich MI glasses correlate positively with H 2 O and CO 2 contents as well as with N/CO 2 ratios, and negatively with the volume of the remaining fluid bubble. Glasses of completely homogenized (fluid bubble-free) MIs contain up to 25.7 ± 0.5 ppm N, whereas glasses of three unheated (natural, glassy) MIs have significantly lower N concentrations of ~1 ± 0.3 ppm. The N-CO 2 characteristics of completely homogenized MIs indicate that melts feeding Klyuchevskoy volcano are strongly enriched in both N and CO 2 compared to primary mid-ocean ridge melts, thus indicating subduction of large amounts of N and its (partial) return to the crust and atmosphere by arc-related magmatism.