In situ quantification of the nitrogen content of olivine-hosted melt inclusions from Klyuchevskoy volcano (Kamchatka): Implications for nitrogen recycling at subduction zones

Abstract

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°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). 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, bubblebearing) MIs have significantly lower N concentrations of ~1 ± 0.3 ppm. The N-CO 2 -Nb characteristics of completely homogenized MIs indicate that melts feeding Klyuchevskoy volcano have high absolute concentrations of both N and CO 2 , 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 CO 2 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.

The origin and dynamics of nitrogen in the Earth's mantle constrained by 15N15N in hydrothermal gases

“…Given the paucity of data, the influence of subduction on the N budget of mantle plumes remains unclear. Nitrogen in arc mantle sources is also important to study, as it is likely influenced by recycled surface-derived components (Fischer et al, 2002;Füri et al, 2021;Taran, 2009). Sub-arc nitrogen may be especially well suited for placing constraints on N isotope signatures that may be transferred to the deep Earth.…”

The origin of nitrogen in Earth's mantle: Constraints from basalts 15N/14N and N2/3He ratios

“…6 to fit the data. Taking [TiO2] = 0.4 wt% for recycled components (Kelley et al, 2003;Plank and Langmuir, 1998), it results that [N]slabs ~ 0.1 ppm N. A [N]slabs ~ 0.1 ppm N is not unreasonable: it is lower than N concentrations in the altered oceanic crust, consistent with ≥ 90% nitrogen losses from the slabs -likely occurring underneath sub-arc mantle sources (Füri et al, 2021;Labidi et al, 2021). The addition of 10 to 20% slabs (with [TiO2]slabs = 0.4 wt% and K2O/TiO2 = 0.5) to a depleted mantle (with [TiO2]depleted mantle = 0.13 wt% and K2O/TiO2 = 0.02) returns K2O/TiO2 ratios between 0.12 and 0.19, covering most of the enriched-MORB range (Fig.…”

“…Under higher temperatures relevant to hot subduction zones, laboratory experiments show a reversal of the nitrogen partition coefficient, such that slab-nitrogen would be quantitatively lost during devolatilization and mineral breakdown (Jackson et al, 2021). Füri et al, (2021) andLabidi et al, (2021) showed that the mantle wedges of two distinct subduction zones are enriched in N by ~ 1 to 2 orders of magnitude relative to the average MORB mantle. In the case of the Central American hot subduction zone, the N enrichment in the mantle wedge is indicative of near-quantitative nitrogen losses from the slab (Labidi et al, 2021).…”