In Situ Quantification of the Nitrogen Content of Olivine-Hosted Melt Inclusions from Klyuchevskoy Volcano (kamchatka): Implications for Nitrogen Recycling at Subduction Zones

Assessing the N content of arc magmas and their mantle source remains a challenge because the volatile element composition of melts and gases can be modified during magma ascent, storage, and eruption. 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 to determine the N concentration in olivine-hosted MIs from Klyuchevskoy volcano in Kamchatka. To reverse the effects of post-entrapment modification processes (i.e., exsolution of volatiles into a fluid bubble), the MIs were partially to completely homogenized at high temperatures (1150-1400 degrees C) and pressures ranging from 0.1 to 500 MPa under dry to H2O-saturated conditions at variable oxygen fugacities (CCO to QFM + 3.3). After the experiments, N concentrations in water-rich MI glasses correlate positively with H2O and CO2 contents as well as with N/CO2 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, bubble-bearing) MIs have significantly lower N concentrations of similar to 1 +/- 0.3 ppm. The N-CO2-Nb characteristics of completely homogenized MIs indicate that melts feeding Klyuchevskoy volcano have high absolute concentrations of both N and CO2, as well as large excess of these volatiles relative to Nb, compared to primary mid-ocean ridge melts. This implies that large amounts of N and CO2 in Klyuchevskoy melts and their mantle source are derived from the subducting slab, and that these subducted volatiles are (partially) returned to the crust and atmosphere by arc-related magmatism.