Cécile Mironov

Andrey Deligny

Evelyn Füri

Maxim Portnyagin

Nikita Mironov

Cécile Deligny

Andrey Gurenko

Roman Botcharnikov

François Holtz

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

Keywords: Nitrogen, SIMS, melt inclusions, subduction, volatile recycling, Klyuchevskoy volcano

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

Introduction

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. Biological uptake of N from the atmosphere/hydrosphere, followed by the gradual transfer of N -in organic matter or as lattice-bound ammonium (NH 4 + ) in potassium-bearing minerals within seafloor sediments and altered oceanic crust -to sub-arc depths and beyond, depletes the surficial N reservoir (e.g., [START_REF] Zerkle | The geobiological nitrogen cycle: From microbes to the mantle[END_REF], whereas mantle degassing (at volcanic arcs, mid-ocean-ridges, and oceanic islands) releases molecular N 2 from Earth's interior to the atmosphere [START_REF] Sano | Volcanic flux of nitrogen from the Earth[END_REF]. However, the fate of N at convergent plate margins is still debated because various factors, such as the pressure-temperature path, lithologies, and structure of the subducting slab as well as the oxygen fugacity [START_REF] Jackson | Warm and oxidizing slabs limit ingassing efficiency of nitrogen to the mantle[END_REF][START_REF] Lee | Nitrogen recycling at the Costa Rican subduction zone: The role of incoming plate structure[END_REF][START_REF] Li | Nitrogen speciation in mantle and crustal fluids[END_REF][START_REF] Mallik | Nitrogen evolution within the Earth's atmosphere-mantle system assessed by recycling in subduction zones[END_REF][START_REF] Mikhail | The relationship between mantle pH and the deep nitrogen cycle[END_REF][START_REF] Mysen | Nitrogen in the Earth: abundance and transport[END_REF], influence the efficiency with which subducted N is returned to Earth's surface via arc volcanism or entrained into the mantle beyond the zone of arc magma generation (e.g., [START_REF] Bebout | Pathways for nitrogen cycling in Earth's crust and upper mantle: A review and new results for microporous beryl and cordierite[END_REF][START_REF] Bebout | Nitrogen-isotope compositions of metasedimentary rocks in the Catalina Schist, California: Implications for metamorphic devolatilization history[END_REF][START_REF] Busigny | A re-assessment of the nitrogen geochemical behavior in upper oceanic crust from Hole 504B: Implications for subduction budget in Central America[END_REF][START_REF] Busigny | Massive recycling of nitrogen and other fluid-mobile elements (K, Rb, Cs, H) in a cold slab environment: evidence from HP to UHP oceanic metasediments of the Schistes Lustrés nappe (western Alps, Europe)[END_REF][START_REF] Halama | Nitrogen recycling in subducted mantle rocks and implications for the global nitrogen cycle[END_REF][START_REF] Halama | Nitrogen recycling in subducted oceanic lithosphere: The record in high-and ultrahigh-pressure metabasaltic rocks[END_REF][START_REF] Li | Nitrogen concentration and δ 15 N of altered oceanic crust obtained on ODP Legs 129 and 185: Insights into alteration-related nitrogen enrichment and the nitrogen subduction budget[END_REF][START_REF] Li | Carbon and nitrogen geochemistry of sediments in the Central American convergent margin: Insights regarding subduction input fluxes, diagenesis, and paleoproductivity[END_REF][START_REF] Mallik | Nitrogen evolution within the Earth's atmosphere-mantle system assessed by recycling in subduction zones[END_REF][START_REF] Mitchell | Nitrogen sources and recycling at subduction zones: Insights from the Izu-Bonin-Mariana arc[END_REF][START_REF] Zimmer | Nitrogen systematics and gas fluxes of subduction zones: Insights from Costa Rica arc volatiles[END_REF]. As a consequence, different estimates of degassed versus subducted N in arc settings range from near complete fore-arc or sub-arc slab devolatilization (e.g., [START_REF] Elkins | Tracing nitrogen in volcanic and geothermal volatiles from the Nicaraguan volcanic front[END_REF]Fischer et al., 2002;[START_REF] Hilton | Noble gases and volatile recycling at subduction zones[END_REF][START_REF] Snyder | Sources of nitrogen and methane in Central American geothermal settings: Noble gas and 129 I evidence for crustal and magmatic volatile components[END_REF] to up to 80-92% transport of N beyond sub-arc depths into the (deep) mantle [START_REF] Barry | Release of subducted sedimentary nitrogen throughout Earth's mantle[END_REF][START_REF] Busigny | Nitrogen isotopes in ophiolitic metagabbros: A re-evaluation of modern nitrogen fluxes in subduction zones and implication for the early Earth atmosphere[END_REF].

Most previous studies of the N source(s) and degassing flux at convergent margins relied on analyses of gas and/or water samples from arc volcanoes and associated hydrothermal systems (e.g., [START_REF] Clor | Volatile and N isotope chemistry of the Molucca Sea collision zone: Tracing source components along the Sangihe Arc, Indonesia[END_REF][START_REF] Elkins | Tracing nitrogen in volcanic and geothermal volatiles from the Nicaraguan volcanic front[END_REF][START_REF] Fischer | Temporal variations in fumarole gas chemistry at Poás volcano, Costa Rica[END_REF][START_REF] Fischer | Subduction and Recycling of Nitrogen Along the Central American Margin[END_REF][START_REF] Halldórsson | Resolving volatile sources along the western Sunda arc, Indonesia[END_REF][START_REF] Lee | Nitrogen recycling at the Costa Rican subduction zone: The role of incoming plate structure[END_REF][START_REF] Mitchell | Nitrogen sources and recycling at subduction zones: Insights from the Izu-Bonin-Mariana arc[END_REF][START_REF] Sano | Volcanic flux of nitrogen from the Earth[END_REF][START_REF] Sano | Nitrogen recycling in subduction zones[END_REF][START_REF] Taran | Geochemistry of volcanic and hydrothermal fluids and volatile budget of the Kamchatka-Kuril subduction zone[END_REF][START_REF] Zimmer | Nitrogen systematics and gas fluxes of subduction zones: Insights from Costa Rica arc volatiles[END_REF]. A major concern for identifying and quantifying various contributory sources for the volcanic N output is that the N abundance and isotope ratio of these samples may have been affected by the addition of an atmospheric component (either air or air-saturated water) prior to or during sampling (e.g., Hilton et al., 2002). Although the three-component model of [START_REF] Sano | Volcanic flux of nitrogen from the Earth[END_REF], for example, provides a means to resolve the provenance of N, i.e., to estimate the relative contributions from air, sediments, and the mantle to the total N content, this approach requires knowledge of the isotopic composition ( 15 N/ 14 N ratio) and elemental abundance ratios (e.g, N 2 /Ar) of the endmembers. Furthermore, it relies on the assumptions that there is no additional N input from the arc crust, and that isotopic and elemental ratios in the volcanic products are representative of the N source. Nitrogen is assumed to predominantly exist as N 2 in arc melts and fluids [START_REF] Li | Nitrogen speciation in mantle and crustal fluids[END_REF][START_REF] Mikhail | The relationship between mantle pH and the deep nitrogen cycle[END_REF][START_REF] Mikhail | Nitrogen speciation in upper mantle fluids and the origin of Earth's nitrogen-rich atmosphere[END_REF][START_REF] Mysen | Nitrogen in the Earth: abundance and transport[END_REF] because of the relatively oxidizing conditions of the mantle wedge beneath arc systems (i.e., logfO 2 = QFM 0 to +2, where QFM refers to the quartz-fayalitemagnetite solid oxygen buffer; e.g., [START_REF] Parkinson | The redox state of subduction zones: insights from arc-peridotites[END_REF]. Since N 2 solubility in silicate melts has been shown to be comparable to that of argon [START_REF] Boulliung | Oxygen fugacity and melt composition controls on nitrogen solubility in silicate melts[END_REF][START_REF] Bernadou | Nitrogen solubility in basaltic silicate melt -Implications for degassing processes[END_REF][START_REF] Cartigny | Volatile (C, N, Ar ) variability in MORB and the respective roles of mantle source heterogeneity and degassing: the case of the Southwest Indian Ridge[END_REF][START_REF] Gaillard | The diverse planetary ingassing/outgassing paths produced over billions of years of magmatic activity[END_REF][START_REF] Libourel | Nitrogen solubility in basaltic melt. Part I. Effect of oxygen fugacity[END_REF], magmatic degassing is expected to result in limited fractionation of the N 2 /Ar elemental ratio. Degassing-induced changes of the N isotope ratio are also usually considered to be negligible (e.g. [START_REF] Cartigny | Volatile (C, N, Ar ) variability in MORB and the respective roles of mantle source heterogeneity and degassing: the case of the Southwest Indian Ridge[END_REF]Fischer et al., 2005). However, [START_REF] Li | Kinetic nitrogen isotope fractionation associated with thermal decomposition of NH 3 : Experimental results and potential applications to trace the origin of N 2 in natural gas and hydrothermal systems[END_REF] experimentally demonstrated that 15 N-depleted N 2 can be generated by the partial thermal decomposition of NH 3 . This process can modify the isotope ratio of the 'sedimentary' N component during slab devolatilization. Thus, these findings imply that the isotopic composition of volcanic N may not be representative of its source signature in all cases.

To circumvent the issues of atmospheric and shallow crustal contamination, the N characteristics of primary undegassed arc magmas may be investigated through analyses of melt inclusions (MIs) trapped in phenocrysts of volcanic rocks. These inclusions, which are small pockets of silicate melt entrapped during crystal growth, provide direct insight into pre-eruptive concentrations and isotope ratios of N (and other volatiles) in the parent melt [START_REF] Wallace | Volatiles in subduction zone magmas: concentrations and fluxes based on melt inclusion and volcanic gas data[END_REF]. Fischer et al. (2005) reported the first N isotope data for olivine separates from subduction zones, determined by in-vacuo crushing and static mass spectrometry analysis. This method is commonly used to extract and analyze noble gases from melt and/or fluid inclusions in mineral grains (e.g., [START_REF] Hilton | Noble gases and volatile recycling at subduction zones[END_REF], and it yields precise isotope ratios for arc magmas and their mantle source; nonetheless, since the proportion of 'melt' (i.e., the number and volume of MIs) in mineral separates is usually unknown, it allows determining N concentrations "per gram of olivine" only, and it does not provide a quantitative estimate of the N content of the trapped melt. Nitrogen concentrations in individual MIs can only be determined by in situ secondary ion mass spectrometry (SIMS) [START_REF] Füri | Nitrogen abundance and isotope analysis of silicate glasses by secondary ionization mass spectrometry[END_REF][START_REF] Regier | Analyzing nitrogen in natural and synthetic silicate glasses by secondary ion mass spectrometry[END_REF].

Although the N content of terrestrial MIs is too low to obtain 15 N/ 14 N ratios by this method, it permits to couple N analyses with measurements of other volatiles (e.g., H 2 O, CO 2 ) and/or major and trace elements (e.g., Nb). In this way, the N/CO 2 and N/Nb ratios of primary mantle melts can be assessed.

However, given that SIMS measurements are carried out in the (residual) glass phase, post-entrapment modification processes in the MIs (e.g., crystallization, diffusive volatile loss through the host crystal, and/or bubble nucleation during cooling and decompression; e.g., [START_REF] Danyushevsky | Experimental and petrological studies of melt inclusions in phenocrysts from mantle-derived magmas: an overview of techniques, advantages and complications[END_REF][START_REF] Roedder | Origin and significance of magmatic inclusions[END_REF][START_REF] Schiano | Primitive mantle magmas recorded as silicate melt inclusions in igneous minerals[END_REF][START_REF] Wallace | Volatiles in subduction zone magmas: concentrations and fluxes based on melt inclusion and volcanic gas data[END_REF] must be taken into account or reversed (e.g., by experimental homogenization at high temperature) to determine the volatile budget of the melt that was trapped in the host mineral prior to decompression [START_REF] Mironov | Quantification of the CO 2 budget and H 2 O-CO 2 systematics in subduction-zone magmas through the experimental hydration of melt inclusions in olivine at high H 2 O pressure[END_REF][START_REF] Moore | Bubbles matter: An assessment of the contribution of vapor bubbles to melt inclusion volatile budgets[END_REF]. This is of particular importance for CO 2 and N because the formation of a fluid bubble can substantially deplete the coexisting melt in dissolved CO 2 and N due to their low solubilities in silicate melt (e.g., [START_REF] Boulliung | Oxygen fugacity and melt composition controls on nitrogen solubility in silicate melts[END_REF][START_REF] Dixon | Degassing of alkalic basalts[END_REF][START_REF] Dixon | An Experimental Study of Water and Carbon Dioxide Solubilities in Mid-Ocean Ridge Basaltic Liquids. Part I : Calibration and Solubility Models[END_REF][START_REF] Libourel | Nitrogen solubility in basaltic melt. Part I. Effect of oxygen fugacity[END_REF][START_REF] Moore | Bubbles matter: An assessment of the contribution of vapor bubbles to melt inclusion volatile budgets[END_REF][START_REF] Wallace | Volatiles in subduction zone magmas: concentrations and fluxes based on melt inclusion and volcanic gas data[END_REF]. Therefore, the CO 2 and N contents measured in the glass phase of bubblebearing inclusions only provide minimum estimates of the volatile content of the parent melt. The volatile concentration in homogenized inclusions, in contrast, is expected to closely reflect that of the undegassed melt.

The goal of this study was to determine the pre-eruptive N content of primitive mafic melts from Klyuchevskoy volcano based on in situ analyses of olivine-hosted MIs by SIMS. To this end, we targeted MIs that were homogenized by heating in a pressure vessel under P-T-fO 2 conditions similar to those presumed to have existed when the MIs were trapped. The H 2 O-CO 2 systematics of these MIs were previously investigated by [START_REF] Mironov | Quantification of the CO 2 budget and H 2 O-CO 2 systematics in subduction-zone magmas through the experimental hydration of melt inclusions in olivine at high H 2 O pressure[END_REF]. The experimental treatment resulted in H + diffusion through the olivine structure and re-hydration of the melt to its original H 2 O content [START_REF] Portnyagin | Dehydration of melt inclusions in olivine and implications for the origin of silica-undersaturated island-arc melts[END_REF], as well as dissolution of CO 2 and other volatiles (e.g., N) from the fluid bubble back into the melt. This approach allows us to compare the N-CO 2 -Nb characteristics of the most primitive Klyuchevskoy melts with those of other mantle-derived melts, and, ultimately, to improve our understanding of the distribution of N in Earth's upper mantle.

Samples and methods

Samples from Klyuchevskoy volcano

Klyuchevskoy volcano, one of the largest and most active volcanoes in the world (with a summit elevation fluctuating between ~4750 and 4800 m above sea level due to frequent eruptions; e.g., [START_REF] Fedotov | The magmatic system of the Klyuchevskaya group of volcanoes inferred from data on its eruptions, earthquakes, deformation, and deep Structure[END_REF][START_REF] Portnyagin | Kliuchevskoi volcano diary[END_REF], is located on the Kamchatka Peninsula of eastern Russia where the Pacific plate is subducting beneath the Eurasian plate. It belongs to the Klyuchevskaya Group, a group of 12 volcanoes situated within the so-called Central Kamchatka Depression near the junction between the Kamchatka-Kuril and Aleutian island arcs in a complex geodynamic setting (e.g., [START_REF] Yogodzinski | Geochemical evidence for the melting of subducting oceanic lithosphere at plate edges[END_REF][START_REF] Levin | Seismic evidence for catastrophic slab loss beneath Kamchatka[END_REF][START_REF] Portnyagin | Transition from arc to oceanic magmatism at the Kamchatka-Aleutian junction[END_REF][START_REF] Portnyagin | Mantle temperature control on composition of arc magmas along the Central Kamchatka Depression[END_REF]. Based on the unusually heavy oxygen isotope composition of olivine and pyroxene phenocrysts from Klyuchevskoy, [START_REF] Dorendorf | Hydrated sub-arc mantle: a source for the Kluchevskoy volcano, Kamchatka/ Russia[END_REF] argued that the exceptional magma productivity is related to the addition of fluids derived from the subducting oceanic crust and the Hawaii-Emperor seamount chain to the mantle wedge. Furthermore, [START_REF] Portnyagin | Transition from arc to oceanic magmatism at the Kamchatka-Aleutian junction[END_REF] suggested that influx of fertile Pacific mantle, resulting from the detachment of a fragment of the subducting slab, leads to enhanced melt production beneath Klyuchevskoy. Indeed, the 3-D seismic velocity structure of the mantle beneath Kamchatka, as derived from teleseismic tomography, can be explained by a gap in the Pacific Plate and lateral asthenospheric mantle flow around the slab edge [START_REF] Javoy | The volatiles record of a "popping" rock from the Mid-Atlantic Ridge at 14°N: chemical and isotopic composition of gas trapped in the vesicles[END_REF]. High-resolution tomography also revealed that beneath Klyuchevskoy volcano, a near-vertical pipe-shaped conduit extends from 25 to 30 km depth to the summit which allows mantlederived mafic melts to ascend to the surface [START_REF] Koulakov | Three different types of plumbing system beneath the neighboring active volcanoes of Tolbachik, Bezymianny, and Klyuchevskoy in Kamchatka[END_REF].

Eruptive products of Klyuchevskoy mainly consist of lavas and pyroclastics ranging from high-Mg, low-Al basalts to low-Mg, high-Al basaltic andesites (e.g., [START_REF] Ariskin | Genesis of high-alumina basalts from Klyuchevskoi volcano[END_REF][START_REF] Bergal-Kuvikas | A petrological and geochemical study on time-series samples from Klyuchevskoy volcano, Kamchatka arc[END_REF][START_REF] Kersting | Klyuchevskoy Volcano, Kamchatka, Russia: The Role of High-Flux Recharged, Tapped, and Fractionated Magma Chamber(s) in the Genesis of High-Al 2 O 3 from High-MgO Basalt[END_REF][START_REF] Khubunaya | Mineralogical features of magnesian basalts as a reflection of fractionation in the magmatic chamber of Klyuchevskoi volcano[END_REF]. For this study, olivine phenocrysts (Fo > 85) from a high-MgO lava flow and tephra layer associated with the ~3 ky old Bulochka cinder cone on the NE flank of Klyuchevskoy volcano were selected. Olivine-hosted MIs are glassy in tephra samples and partially crystallized (i.e., with the presence of daughter pyroxene) in lava samples, and they all contain a fluid bubble and, occasionally, entrapped Cr-spinel (Figure 1a; Table 1; [START_REF] Mironov | Quantification of the CO 2 budget and H 2 O-CO 2 systematics in subduction-zone magmas through the experimental hydration of melt inclusions in olivine at high H 2 O pressure[END_REF][START_REF] Moore | Volatile contents of primitive bubblebearing melt inclusions from Klyuchevskoy volcano, Kamchatka: Comparison of volatile contents determined by mass-balance versus experimental homogenization[END_REF]. The fluid bubbles are rich in CO 2 and contain S-bearing phases (native S, sulfides or sulfates) and/or carbonates, as determined by Raman spectroscopy [START_REF] Moore | Volatile contents of primitive bubblebearing melt inclusions from Klyuchevskoy volcano, Kamchatka: Comparison of volatile contents determined by mass-balance versus experimental homogenization[END_REF].

Fluid bubbles occupy, on average, 4 ± 2 vol.% within both glassy inclusions from tephra and inclusions heated dry from lavas (e.g. [START_REF] Moore | Volatile contents of primitive bubblebearing melt inclusions from Klyuchevskoy volcano, Kamchatka: Comparison of volatile contents determined by mass-balance versus experimental homogenization[END_REF]; these rough estimates are based on 2D measurements of MIs (length, width, and thickness = width) and their bubbles (diameter). Importantly, previously studied MIs heated dry from Bulochka lavas and those used in re-hydration experiments show an excellent linear correlation between the volumes of MIs and their bubbles (with a narrow range of 3.9-6.4% for the relative bubble volume) (e.g., [START_REF] Mironov | H 2 O and CO 2 in parental magmas of Kliuchevskoi volcano inferred from study of melt and fluid inclusions in olivine[END_REF]. This implies that, in the studied samples, homogeneous melts were trapped as MIs without the addition of any extra fluid (e.g. [START_REF] Mironov | Estimation of CO 2 content in the gas phase of melt inclusions using Raman spectroscopy: case study of inclusions in olivine from the Karymsky volcano (Kamchatka)[END_REF]. The inclusions were presumably entrapped in olivine at T = 1150-1230°C, logfO 2 = QFM+0.8 to +1.8, and P ≥ 500 MPa (see Mironov et al., 2015, for details). The trapping pressure (600-800 MPa) was estimated from the H 2 O-CO 2 concentrations in homogenized MIs and by extrapolating the experimental H 2 O-CO 2 solubility data for tholeiitic melts of [START_REF] Shishkina | Solubility of H 2 O and CO 2 -bearing fluids in tholeiitic basalts at pressures up to 500 MPa[END_REF]. [START_REF] Mironov | Quantification of the CO 2 budget and H 2 O-CO 2 systematics in subduction-zone magmas through the experimental hydration of melt inclusions in olivine at high H 2 O pressure[END_REF] argued that these values agree well with independent estimates (P > 500 MPa) based on the density of CO 2 -rich fluid inclusions (~0.8 g cm -3 ) in high-Mg Klyuchevskoy olivine [START_REF] Mironov | H 2 O and CO 2 in parental magmas of Kliuchevskoi volcano inferred from study of melt and fluid inclusions in olivine[END_REF], as well as with petrological and modeling constraints for the crystallization conditions of magnesian Klyuchevskoy basalts (500-900 MPa in [START_REF] Kersting | Klyuchevskoy Volcano, Kamchatka, Russia: The Role of High-Flux Recharged, Tapped, and Fractionated Magma Chamber(s) in the Genesis of High-Al 2 O 3 from High-MgO Basalt[END_REF] and >700 MPa in [START_REF] Ariskin | Genesis of high-alumina basalts from Klyuchevskoi volcano[END_REF]).

Experimental methods

Homogenization experiments of olivine-hosted MIs were carried out at the Institute of Mineralogy, Leibniz University (Hanover, Germany), in vertically-oriented, internally-heated pressure vessels (IHPVs) equipped with a rapid quench device (see Mironov et al., 2015, for details). For the majority of the experiments, Au 80 Pd 20 capsules (20 × 3 mm), which were welded shut on one end, were loaded sequentially with powdered synthetic matrix glass (similar in major element composition to the Bulochka lavas and tephras; supplementary Table A.1.), 7-8 olivine grains (0.5-1.25 mm in size), distilled H 2 O, and powdered glass on top. The amount of H 2 O added varied between 5 and 11.3 wt.% relative to the glass. A 20% NaCl aqueous solution was added to the experiment #C4-4. The use of different H 2 O-bearing matrices permitted to study their effect on the diffusion rate of hydrogen in olivine [START_REF] Mironov | Quantification of the CO 2 budget and H 2 O-CO 2 systematics in subduction-zone magmas through the experimental hydration of melt inclusions in olivine at high H 2 O pressure[END_REF][START_REF] Portnyagin | Dehydration of melt inclusions in olivine and implications for the origin of silica-undersaturated island-arc melts[END_REF]. Once the top of the capsules was welded shut, they were weighed, heated to 110°C for 1 h, and weighed again to check for leaks (i.e., to ensure that there had been no loss of fluid during heating). Two capsules (experiments #C8-5 and #C9) were charged with loose olivine grains only, without any matrix glass or fluid. For experiment #C8-5, the top of the Au 80 Pd 20 capsule remained open. For experiment #C9, the olivine grains were placed into a small graphite container, which was encapsulated in Pt. The Pt capsule was welded shut on both ends.

The Au 80 Pd 20 capsules were kept in IHPVs at temperatures of 1150-1200°C and pressures of 300-500 MPa for 24-28 h, and then they were rapidly quenched (~150°C/s). For the different experiments, the nominal oxygen fugacity varied between the intrinsic fO 2 of the IHPV (logfO 2 = QFM+3.3 at H 2 O-saturated conditions) and more reducing conditions (QFM+0.6) when the IHPV was pressurized with a Ar-H 2 mixture. The experiment in the Pt capsule (#C9) was carried out at an oxygen fugacity equal to CCO (where CCO is the graphite-CO-CO 2 oxygen buffer) at atmospheric pressure and a temperature of 1400°C for 1 h.

Hereafter, MIs targeted for nitrogen abundance analyses are divided into three groups: 1)

'unheated' (natural) MIs in olivines that were extracted from tephra samples, 2) MIs in olivines from a lava flow that were 'heated dry' (without any matrix glass or fluid), and 3) MIs from the same lava flow that were experimentally 'heated with H 2 O' (with hydrous matrix and H 2 O or a 20% NaCl aqueous solution) at high pressure (Figure 1).

Analytical methods

Individual olivine grains containing natural or experimentally treated MIs were mounted in epoxy. The grains were grinded and polished to expose MIs on the surface using sand papers and diamond pastes of various particle sizes, and finally polished with a 0.25 µm grain size Al 2 O 3 suspension in water. The samples were then cleaned ultrasonically, removed from the epoxy mounts, and remounted in indium metal for further analyses.

For the study by [START_REF] Mironov | Quantification of the CO 2 budget and H 2 O-CO 2 systematics in subduction-zone magmas through the experimental hydration of melt inclusions in olivine at high H 2 O pressure[END_REF] 1.

For this study, nitrogen ( 14 N) abundances were determined with the CAMECA IMS 1280 HR at CRPG. Spot-analyses of 14 N 16 O -secondary molecular ions were carried out in mono-collection mode on an electron multiplier at a nominal mass resolution m/Δm ≈ 14,000 using a 10 kV Cs + primary ion beam with a current of 10 nA and a spot diameter of ~20 µm, following the analytical protocol of [START_REF] Füri | Nitrogen abundance and isotope analysis of silicate glasses by secondary ionization mass spectrometry[END_REF]. The 30 Si -and 16 O 2 -count rates were determined on a Faraday cup (FC2)

during each cycle to monitor the stability of secondary ion intensities. In addition, the 27 Al -signal was monitored to distinguish MIs from their host olivine. Following pre-sputtering for 180 s with a beam raster of 10 × 10 µm to minimize any surface contamination prior to signal acquisition, twenty cycles were collected for each analysis through the mass sequence given the high water content of the inclusions, they were likely affected by the tail of the neighboring 29 SiH 2 -peak; therefore, 15 N 16 O -count rates are not reported, and nitrogen isotope ratios ( 15 N/ 14 N)

could not be determined. We also note that host olivines (e.g., C4-2-1, C4-3-1, C10-2-1, C9-3, and K6), which can clearly be distinguished from glass inclusions based on their low 27 Al -signals, contained no detectable nitrogen, i.e., 14 N 16 O -count rates (2-3 cps) were equal to or lower than those of the calibrant containing 0.022 ± 0.034 ppm N (3-4 cps, n = 12).

Results

Previous analyses revealed that the glass phase of unheated, glassy, bubble-bearing MIs from Bulochka tephra contains between 2.9 and 3.4 wt.% H 2 O and 880 to 1200 ppm CO 2 (Figure 2a,b; Overall, the hydrous high-pressure experiments resulted in gradual shrinking of the fluid bubble up to its complete dissolution, and a concomitant increase in H 2 O and CO 2 concentrations in the melt [START_REF] Mironov | Quantification of the CO 2 budget and H 2 O-CO 2 systematics in subduction-zone magmas through the experimental hydration of melt inclusions in olivine at high H 2 O pressure[END_REF]. These observations are consistent with the notion that a large amount of volatiles partitioned into the fluid phase of bubble-bearing inclusions (and/or, in the case of H 2 O, was lost by diffusion through the host crystal). Inclusions heated dry, in contrast, contain significantly less The glass phase of unheated MIs from Bulochka tephra contains a small amount of nitrogen, i.e., 0.8 ± 0.3 to 1.2 ± 0.3 ppm N (Figure 2; Table 1). The N concentration in MIs heated dry (at logfO 2 = QFM+0.6 or CCO) varies between 1.7 ± 0.3 and 9.6 ± 1.4 ppm. Notably, these MIs were not completely homogenized (i.e., they still contain a bubble with a relative volume of 1.5 to 3 % after the experiments; Figure 2c); consequently, a significant fraction of N and CO 2 must have remained in the fluid phase. It is noteworthy that their dissolved N contents are higher than those of unheated inclusions with large fluid bubbles, and Figure 2d shows that the glasses of these inclusions are characterized by exceptionally high N/CO 2 ratios (as high as 0.05 ± 0.01).

Inclusions that were experimentally re-hydrated at P = 300 MPa and logfO 2 = QFM+3.3 contain 0.8 ± 0.3 to 14.9 ± 0.5 ppm N in the quenched glass. The highest N abundances of up to 25.7 ± 0.5 ppm were observed in the most hydrated MIs that were completely homogenized at 500 MPa.

Nitrogen concentrations of MIs heated with H 2 O clearly correlate with CO 2 and H 2 O abundances in the glass phase (Figure 2a andb). In parallel, the N content is related to the volume of the residual fluid bubble (Figure 2c), i.e., a reduction of the bubble volume results in increased dissolved N concentrations. Finally, Figure 2d shows that the N/CO 2 ratio progressively increases from a minimum value of ~0.0005 towards 0.005 ± 0.001 as a result of homogenization.

Discussion

Effects of the experimental treatment on N concentrations in melt inclusions

Previous experimental studies of olivine-hosted MIs from Klyuchevskoy volcano [START_REF] Mironov | Quantification of the CO 2 budget and H 2 O-CO 2 systematics in subduction-zone magmas through the experimental hydration of melt inclusions in olivine at high H 2 O pressure[END_REF][START_REF] Portnyagin | Dehydration of melt inclusions in olivine and implications for the origin of silica-undersaturated island-arc melts[END_REF] revealed that homogenization at high P and high T in the presence of H 2 O results in H + diffusion through the olivine structure and re-hydration of the melt. The rapid diffusion of hydrogen in Klyuchevskoy olivines is primarily controlled by metal (Mg, Fe) vacancies, as confirmed by FTIR spectroscopy, with a H diffusivity (D H ) on the order of 10 -11 m 2 s -1 at 1200 o C.

As a consequence of H + diffusion through olivine and an increasing H 2 O content in the trapped melt, the pressure inside MIs increases, thereby causing dissolution of CO 2 from the fluid bubble back into melt. Since the diffusivity of C in olivine has been shown to be very low (i.e., <10 -16 m 2 s -1 at P ≤ 3

GPa and T = 1200-1450 o C; [START_REF] Tingle | Experiments and observations bearing on the solubility and diffusivity of carbon in olivine[END_REF], the C content of the melt is not expected to be modified by inward or outward diffusion through the host crystal during the experiments.

The results of this study reveal that the experimental treatment results in increasing N concentrations, as well as high N/CO 2 ratios, in the glass phase of MIs that were heated with a fluid and hydrous matrix in closed capsules. In contrast, glasses of the dry runs contain a significantly lower amount of H 2 O and CO 2 than those of unheated (natural) MIs, and they record anomalously high N/CO 2 ratios (Figure 2d). These observations indicate that the experimental treatment can have a variable effect on the volatile element systematics of MIs. In either case, the data may imply that 1)

MIs are enriched in N as a result of inward N diffusion through the host olivine, or, alternatively, 2) a significant amount of N was stored in the fluid bubble together with CO 2 prior to the experiments, and is re-dissolved into the melt at high P and high T. These two hypotheses are discussed below.

Inward diffusion of N

The open capsule of the dry experiment #C8-5 was exposed to a Ar-H 2 gas mixture in the IHPV; for this run series, an external source for N detected in the olivine-hosted MIs can be ruled out.

In contrast, for the hydrous experiments in closed capsules, olivine grains were in contact with synthetic matrix glass and a fluid (H 2 O or a 20% NaCl aqueous solution), whose N content is unknown. In addition, a small amount of air was likely trapped in the capsules of these run series and the dry run series #C9 during loading and welding. Since the olivines were in contact with sources variably enriched in N (gas, matrix, fluid) at high temperatures, the possibility of inward diffusion of N through the olivine structure must be explored.

Nitrogen (in-)diffusion in San Carlos olivine was recently studied by [START_REF] Watson | Nitrogen diffusion in silicate minerals, with implications for nitrogen transport and cycling in the lithosphere[END_REF] through experiments at T = 650-1400°C and P = 0.001-1 GPa. The results revealed that the N diffusivity (D N ) in olivine varies between 10 -19.2 m 2 s -1 and 10 -18.4 m 2 s -1 at temperatures of 1150°C and 1400°C, respectively, and that the diffusing species is either molecular N 2 or monatomic N. It is noteworthy that these experiments were carried out by N + ion implantation (~900 ppm) followed by heating to mobilize N, and they resulted in high near-surface 15 N concentrations between ~50 and 270 ppm in olivine, as determined by nuclear reaction analysis. In contrast, SIMS analysis shows that the N content of Klyuchevskoy olivines is below the detection limit (i.e., 14 N 16 O -count rates were comparable to those of the reference glass containing 0.022 ± 0.034 ppm N) following the homogenization experiments. Most importantly, since nitrogen diffusion is eight orders of magnitude slower than H + diffusion, the N contents of the studied MIs are unlikely to be affected by diffusion on timescales of 1-28 h.

Similar amounts of N (1.7-9.6 ppm; Figure 2; Table 1) are detected in the glass phase of MIs heated dry in the presence of trapped air (#C9) or in a N-poor Ar-H 2 atmosphere (#C8-5). Most importantly, the N contents of inclusions within olivine grains that were heated in contact with Nbearing matrix glass and a fluid range from very low to the highest values (0.8 to 25.7 ppm N), and they are clearly correlated with H 2 O and CO 2 concentrations. These observations demonstrate that there is no direct relationship between the N concentrations in MI glasses and any N that may be present in the different matrices the host olivines interacted with (gas, matrix, fluid). Based on the available data, we therefore conclude that the elevated N abundances in MIs are unlikely to be the result of contamination by inward migration of N from the surrounding gas, matrix or fluid through olivine by either diffusion or transport along dislocations in the crystal structure.

Dissolution of N from the fluid bubble back into the melt

Given that inward diffusion of N can be ruled out as the cause of high N concentrations in homogenized olivine-hosted MIs from Klyuchevskoy volcano, the results of this study imply that a significant amount of N partitioned into the fluid phase during decompression and bubble growth within the inclusions after entrapment in olivine, and that N dissolved back into the melt as a result of the experimental treatment. Importantly, the strong negative correlation between the N content of the melt and the fluid bubble volume for MIs heated with H 2 O (Figure 2c) indicates a closed-system behavior of the MIs, i.e., all N was initially stored inside the inclusions, within which it was partitioned between the melt and fluid bubble. According to this scenario, the observed significant increase of the N/CO 2 ratio in the melt with progressive MI re-hydration implies that the fluid-melt partition coefficient of N is higher than that of CO 2 .

The volatile compositions of MIs heated dry deviate from the trend defined by inclusions heated with H 2 O towards anomalously high N/CO 2 at a given N content (Figure 2d). These distinct N-CO 2 -H 2 O systematics can be explained, in part, by different fO 2 (and T, for #C9) conditions compared to the hydrous runs; lower fO 2 has an important effect on N solubility in silicate melts (Section 4.2; [START_REF] Boulliung | Oxygen fugacity and melt composition controls on nitrogen solubility in silicate melts[END_REF]Bernardou et al., 2021;[START_REF] Grewal | The speciation of carbon, nitrogen, and water in magma oceans and its effect on volatile partitioning between major reservoirs of the Solar System rocky bodies[END_REF][START_REF] Libourel | Nitrogen solubility in basaltic melt. Part I. Effect of oxygen fugacity[END_REF][START_REF] Roskosz | Dual speciation of nitrogen in silicate melts at high pressure and temperature: An experimental study[END_REF][START_REF] Li | Nitrogen distribution between aqueous fluids and silicate melts[END_REF]. The dry #C9 run series was carried out at 1 atm, nominally reducing conditions (logfO 2 = CCO), and at a higher temperature of 1400 o C than the hydrous experiments, whereas the fO 2 and T conditions during the dry high-pressure experiment #C8-5 (QFM+0.6 and 1200 o C) were comparable to those run with hydrous matrix and H 2 O (Table 1). However, the nominal oxygen fugacity logfO 2 = QFM+0.6 corresponds to the water-saturated system, and, hence, dehydration of MIs in this dry experiment may have shifted the actual redox conditions to more reducing. Depending on the amount of water remaining in the #C8-5 MIs, the logfO 2 within these inclusions may be lower than the nominal QFM+0.6 value by up to 2 logarithmic units. Furthermore, since the dry experiments in open Au 80 Pd 20 (#C8-5) and Pt capsules (#C9) resulted in melt dehydration (Figure 2b) due to the contrast in water activity between the interior and exterior of the heated MIs, the internal pressure in the MIs decreased, assuming that the volume of MIs remained constant.

Consequently, CO 2 partitioned into the expanding fluid bubble to compensate for the decreasing pressure and increasing CO 2 fugacity. This redistribution process reduced the CO 2 concentration in the melt phase. The process of CO 2 exsolution from the melt and growth of a large bubble containing a CO 2 -dominated fluid, accompanied by reducing fO 2 in the melt, may have modified N speciation in the fluid as well as N solubility and dissolution mechanism(s) in the melt.

Overall, this study reveals that the widely used technique of MI heating without re-hydration can produce complex N-CO 2 -H 2 O interactions because the loss of water from the MIs might affect the speciation and behavior of all other volatiles and prevent complete homogenization. Since N in the seven MIs heated dry shows a peculiar behavior, which leads to increased N/CO 2 ratios in melts that are in contact with large fluid bubbles, the N-CO 2 characteristics of the glasses of dehydrated MIs are not representative of either those of the primary melt or partially degassed melts that existed in nature.

In contrast, the highest N and CO 2 concentrations observed here in completely homogenized, rehydrated MIs represent a good proxy for the volatile characteristics of the most primitive Klyuchevskoy melt. Since it cannot be excluded that some volatiles were lost from the melt by degassing prior to the entrapment of inclusions by the host mineral phase, these measured values may represent minimum primary (i.e., mantle-derived) melt N and CO 2 contents. Similarly, if degassing occurred prior to melt entrapment, the observed N/CO 2 ratios would be minimum values, assuming that the fluid-melt partition coefficient of N is higher than that of CO 2 .

N partitioning between fluid and melt in Klyuchevskoy magmas

Experimental investigations revealed that N solubility in silicate melts is primarily controlled by the oxygen fugacity (fO 2 ); in addition, the chemical composition of the melt (i.e., the degree of melt polymerization), the hydrogen fugacity (fH 2 ), and the P-T conditions also affect N solubility [START_REF] Boulliung | Oxygen fugacity and melt composition controls on nitrogen solubility in silicate melts[END_REF][START_REF] Bernadou | Nitrogen solubility in basaltic silicate melt -Implications for degassing processes[END_REF]Dalou et al., 2019aDalou et al., , 2019b;;[START_REF] Fogel | Nitrogen solubility in aubrite and E chondrite melts[END_REF][START_REF] Grewal | The speciation of carbon, nitrogen, and water in magma oceans and its effect on volatile partitioning between major reservoirs of the Solar System rocky bodies[END_REF][START_REF] Grewal | The fate of nitrogen during coremantle separation on Earth[END_REF][START_REF] Kadik | Solubility of nitrogen, carbon, and hydrogen in FeO-Na 2 O-Al 2 O 3 -SiO 2 melt and liquid iron alloy: influence of oxygen fugacity[END_REF][START_REF] Kadik | Influence of oxygen fugacity on the solubility of nitrogen, carbon, and hydrogen in FeO-Na 2 O-SiO 2 -Al 2 O 3 melts in equilibrium with metallic iron at 1.5 GPa and 1400°C[END_REF][START_REF] Li | Nitrogen distribution between aqueous fluids and silicate melts[END_REF][START_REF] Libourel | Nitrogen solubility in basaltic melt. Part I. Effect of oxygen fugacity[END_REF][START_REF] Mallik | Nitrogen evolution within the Earth's atmosphere-mantle system assessed by recycling in subduction zones[END_REF][START_REF] Miyazaki | Solubilities of nitrogen and noble gases in silicate melts under various oxygen fugacities: Implications for the origin and degassing history of nitrogen and noble gases in the Earth[END_REF][START_REF] Mosenfelder | Nitrogen incorporation in silicates and metals: Results from SIMS, EPMA, FTIR, and laserextraction mass spectrometry[END_REF][START_REF] Mulfinger | Physical and Chemical Solubility of Nitrogen in Glass Melts[END_REF][START_REF] Mysen | Nitrogen and hydrogen isotope compositions and solubility in silicate melts in equilibrium with reduced (N+H)-bearing fluids at high pressure and temperature: Effects of melt structure[END_REF][START_REF] Roskosz | Nitrogen solubility in molten metal and silicate at high pressure and temperature[END_REF][START_REF] Roskosz | Dual speciation of nitrogen in silicate melts at high pressure and temperature: An experimental study[END_REF]. At atmospheric pressure and under reducing conditions, N chemically bonds into the silicate network in high concentration (up to ~3000 and 6000 ppm N in andesitic or basaltic melts, respectively, at IW -8 and 1425ºC; [START_REF] Boulliung | Oxygen fugacity and melt composition controls on nitrogen solubility in silicate melts[END_REF]. In contrast, under oxidizing conditions (IW ≥ -1), N is 'physically' incorporated in very low concentrations (≤1 ppm) in the form of N 2 molecules into cavities of the silicate melt network without the formation of covalent or ionic bonds [START_REF] Boulliung | Oxygen fugacity and melt composition controls on nitrogen solubility in silicate melts[END_REF][START_REF] Libourel | Nitrogen solubility in basaltic melt. Part I. Effect of oxygen fugacity[END_REF][START_REF] Miyazaki | Solubilities of nitrogen and noble gases in silicate melts under various oxygen fugacities: Implications for the origin and degassing history of nitrogen and noble gases in the Earth[END_REF]; this dissolution mechanism is comparable to that of the noble gases (e.g., [START_REF] Paonita | Noble gas solubility in silicate melts: a review of experimentation and theory, and implications regarding magma degassing processes[END_REF]. Under either reducing or oxidizing conditions, N solubility in silicate melts increases with increasing pressure, at least for pressures up to 4-5 GPa [START_REF] Roskosz | Nitrogen solubility in molten metal and silicate at high pressure and temperature[END_REF][START_REF] Roskosz | Dual speciation of nitrogen in silicate melts at high pressure and temperature: An experimental study[END_REF].

Considering the P-T-fO 2 conditions of the mantle wedge beneath convergent margins, nitrogen is expected to predominantly dissolve as N 2 in sub-arc mantle melts, although some nitrogen can exist as various N-H complexes [START_REF] Li | Nitrogen distribution between aqueous fluids and silicate melts[END_REF][START_REF] Mallik | Nitrogen evolution within the Earth's atmosphere-mantle system assessed by recycling in subduction zones[END_REF][START_REF] Mikhail | Nitrogen speciation in upper mantle fluids and the origin of Earth's nitrogen-rich atmosphere[END_REF][START_REF] Mysen | Nitrogen in the Earth: abundance and transport[END_REF][START_REF] Mysen | Solubility and solution mechanisms of NOH volatiles in silicate melts at high pressure and temperature -Amine groups and hydrogen fugacity[END_REF]. Accordingly, dissolved N 2 is a "passive" volatile element that strongly partitions into the fluid phase during magma decompression [START_REF] Watson | Diffusive fractionation of volatiles and their isotopes during bubble growth in magmas[END_REF]. During degassing, dissolved N 2 and CO 2 are then expected to be fractionated according to their relative solubilities,

where S N2 = 2.2 ± 0.5 × 10 -9 mol g -1 bar -1 [START_REF] Libourel | Nitrogen solubility in basaltic melt. Part I. Effect of oxygen fugacity[END_REF] and S CO2 = 9.3 × 10 -9 mol g -1 bar -1 [START_REF] Burnard | Correction for volatile fractionation in ascending magmas: Noble gas abundances in primary mantle melts[END_REF][START_REF] Dixon | Degassing of alkalic basalts[END_REF][START_REF] Dixon | An Experimental Study of Water and Carbon Dioxide Solubilities in Mid-Ocean Ridge Basaltic Liquids. Part I : Calibration and Solubility Models[END_REF] as determined for nominally anhydrous tholeiitic melts at 1425°C and 1200°C, respectively, by gas-melt equilibration at atmospheric pressure. (We note here that slightly variable solubility values -in units of cm 3 STP g -1 bar -1 , ppm atm -1 or mol g -1 hPa -1 -are reported in the literature; e.g., [START_REF] Boulliung | Oxygen fugacity and melt composition controls on nitrogen solubility in silicate melts[END_REF][START_REF] Cartigny | Volatile (C, N, Ar ) variability in MORB and the respective roles of mantle source heterogeneity and degassing: the case of the Southwest Indian Ridge[END_REF][START_REF] Marty | An evaluation of the C/N ratio of the mantle from natural CO 2 -rich gas analysis: Geochemical and cosmochemical implications[END_REF]. Consequently, the N 2 /CO 2 ratio in the residual melt is expected to decrease during (closed-or open-system) degassing en route to the surface (e.g., [START_REF] Burnard | Correction for volatile fractionation in ascending magmas: Noble gas abundances in primary mantle melts[END_REF][START_REF] Marty | Nitrogen content of the mantle inferred from N 2 -Ar correlation in oceanic basalts[END_REF][START_REF] Marty | Volatiles (He, C, N, Ar) in mid-ocean ridge basalts: assesment of shallowlevel fractionation and characterization of source composition[END_REF], whereas the N 2 /CO 2 ratio in the exsolving gas phase will be higher than that of the melt. In this respect, it is noteworthy that Taran (2009) measured a molar N 2 /CO 2 ratio of ~0.03 (i.e., CO 2 /N 2 = 32, corrected for atmospheric contamination) in volcanic gases at Klyuchevskoy volcano during eruption in 1994. This value is ~4 times higher than the ratio determined here for the primary melt based on MI data (i.e., molar N 2 /CO 2 ≈ 0.008, assuming that all N is dissolved as N 2 ), consistent with the notion that degassing of sub-arc melts will result in higher N 2 /CO 2 ratios in the exsolving gas phase. However, given that the N 2 /CO 2 ratio of volcanic and hydrothermal gas discharges is controlled by the P-T-fO 2 (-fH 2 ) conditions and the fractionation style and extent of degassing, as well as by additional processes such as the volatile input from the arc crust [START_REF] Hilton | Noble gases and volatile recycling at subduction zones[END_REF] and calcite precipitation [START_REF] Ray | The effects of volatile recycling, degassing and crustal contamination on the helium and carbon geochemistry of hydrothermal fluids from the Southern Volcanic[END_REF], relating the N 2 /CO 2 ratio of volcanic gases to that of homogenized melt inclusions is not straightforward. Additional results from in situ N measurements of MIs, analyses of gases discharging from volcanoes and hydrothermal systems, and high P and T experiments will be key for improving our understanding of the behavior and fate of N at convergent plate margins.

N and CO 2 contents of primary Klyuchevskoy melts

Quantifying the N and CO 2 contents of undegassed mantle-derived melts is challenging because the low solubility of these volatiles in silicate melts (Section 4.2) results in extensive volatile loss from almost all magmas, and, possibly, degassing-induced N/CO 2 fractionation in the crust or shallow mantle. In addition, atmospheric (and crustal) contamination can modify the volatile characteristics of magmas en route to the surface (e.g., [START_REF] Cartigny | Volatile (C, N, Ar ) variability in MORB and the respective roles of mantle source heterogeneity and degassing: the case of the Southwest Indian Ridge[END_REF].

Whereas SIMS analyses of MIs provide reliable constraints on the CO 2 contents of MORB magmas and their mantle source (i.e., CO 2,DMM = 137 ± 54 ppm; Le [START_REF] Voyer | Heterogeneity in mantle carbon content from CO 2undersaturated basalts[END_REF], N concentration estimates have so far relied on analyses of vesicles and glass by crushing and melting, respectively, of vesicular submarine basaltic glasses (e.g., [START_REF] Marty | Nitrogen content of the mantle inferred from N 2 -Ar correlation in oceanic basalts[END_REF]. [START_REF] Marty | Volatiles (He, C, N, Ar) in mid-ocean ridge basalts: assesment of shallowlevel fractionation and characterization of source composition[END_REF] measured the N concentrations in MORB glasses from different locations around the globe by static mass spectrometry and obtained values of ≤0.15 ppm N (Figure 3). [START_REF] Fischer | Volatile abundances in the sub-arc mantle: insights from volcanic and hydrothermal gas discharges[END_REF] then derived the N-CO 2 characteristics of primary undegassed MORB melts from the N 2 / 3 He and CO 2 / 3 He ratios of these MORB glasses, and by estimating the 3 He content of the upper mantle (1.79 × 10 -15 mol/g) and the degree of partial melting (10%); thus, MORB melts are inferred to initially contain ~0.04 to 1.7 ppm N (Figure 3). Assuming a partial melting degree of ~10% and strong partitioning of N into the melt, this concentration range implies that the MORB mantle contains <0.17

ppm N, which is roughly comparable with more recent estimates of the N content of the MORB mantle, i.e., 0.09 ± 0.05 to 0.27 ± 0.16 ppm [START_REF] Marty | The origins and concentrations of water, carbon, nitrogen and noble gases on Earth[END_REF], and references therein) or 0.21-0.69 ppm [START_REF] Bekaert | Subduction-Driven Volatile Recycling: A Global Mass Balance[END_REF]. It is noteworthy that [START_REF] Johnson | The nitrogen budget of Earth[END_REF] argued for a higher N abundance of 1.2 ± 0.8 ppm in the MORB source based on a compilation of literature data for various terrestrial samples. This higher value appears consistent with that of Javoy and Pineau (1991) (~1 ppm), derived from the N content (~12 ppm) of the famous "popping" rock 2πD43 (ABP0002-043-VG8482 in PetDB), which has been interpreted to be the best preserved sample of undegassed melt derived from the upper mantle (e.g., [START_REF] Cartigny | Towards a consistent mantle carbon flux estimate: Insights from volatile systematics (H 2 O/Ce, δD, CO 2 /Nb) in the North Atlantic mantle (14 °N and 34 °N)[END_REF][START_REF] Javoy | The volatiles record of a "popping" rock from the Mid-Atlantic Ridge at 14°N: chemical and isotopic composition of gas trapped in the vesicles[END_REF][START_REF] Moreira | Rare gas systematics in popping rock: isotopic and elemental composition in the upper mantle[END_REF][START_REF] Sarda | Mid-ocean ridge ppping rocks: Implications for degassing at ridge crests[END_REF]. However, 2πD43 is an E-MORB (i.e., with a high La/Sm N ratio of 1.9; Le [START_REF] Voyer | The effect of primary versus secondary processes on the volatile content of MORB glasses: An example from the equatorial Mid-Atlantic Ridge (5°N-3°S)[END_REF], and its high volatile content has been argued to be the result of bubble accumulation in a magma chamber (e.g., [START_REF] Chavrit | Mapping modern CO 2 fluxes and mantle carbon content all along the mid-ocean ridge system[END_REF][START_REF] Moreira | Noble gases as tracers of mantle processes and magmatic degassing[END_REF][START_REF] Jones | New constraints on mantle carbon from Mid-Atlantic Ridge popping rocks[END_REF]. Although the N content of the MORB mantle is still debated, the MORB source appears to predominantly generate melts containing <2 ppm N (Figure 3).

The N and CO 2 abundances in sub-arc mantle melts are expected to be higher than in magmas derived from the MORB mantle, as a result of devolatilization of the subducting slab, and addition of N and CO 2 from oceanic sediments and/or altered oceanic crust into the mantle wedge [START_REF] Fischer | Volatile abundances in the sub-arc mantle: insights from volcanic and hydrothermal gas discharges[END_REF]. However, CO 2 concentrations in the glass phase of most MIs from subduction-related settings are significantly lower (≤500 ppm) than those predicted for primary arc magmas (>3000 ppm; e.g., [START_REF] Wallace | Volatiles in subduction zone magmas: concentrations and fluxes based on melt inclusion and volcanic gas data[END_REF] because melts are partially degassed prior to trapping within minerals of relatively evolved composition and/or because most inclusions contain a fluid bubble, which can host a substantial fraction of the total CO 2 initially dissolved in the melt (e.g., [START_REF] Mironov | Quantification of the CO 2 budget and H 2 O-CO 2 systematics in subduction-zone magmas through the experimental hydration of melt inclusions in olivine at high H 2 O pressure[END_REF][START_REF] Moore | Bubbles matter: An assessment of the contribution of vapor bubbles to melt inclusion volatile budgets[END_REF][START_REF] Moore | Volatile contents of primitive bubblebearing melt inclusions from Klyuchevskoy volcano, Kamchatka: Comparison of volatile contents determined by mass-balance versus experimental homogenization[END_REF]. Homogenized MIs from Klyuchevskoy volcano are, therefore, unique in that they contain up to 4844 ± 823 ppm CO 2 (this study; [START_REF] Mironov | Quantification of the CO 2 budget and H 2 O-CO 2 systematics in subduction-zone magmas through the experimental hydration of melt inclusions in olivine at high H 2 O pressure[END_REF]. These homogenized MIs also have exceptionally high CO 2 /Nb ratios of ~3300 to 5500, significantly higher than the CO 2 /Nb ratio of the MORB mantle (CO 2 /Nb = 505 ± 168 to 810 ± 220; [START_REF] Cartigny | Towards a consistent mantle carbon flux estimate: Insights from volatile systematics (H 2 O/Ce, δD, CO 2 /Nb) in the North Atlantic mantle (14 °N and 34 °N)[END_REF][START_REF] Hirschmann | Comparative deep Earth volatile cycles: The case for C recycling from exosphere/ mantle fractionation of major (H 2 O, C, N) volatiles and from H 2 O/Ce, CO 2 /Ba, and CO 2 /Nb exosphere ratios[END_REF][START_REF] Voyer | Heterogeneity in mantle carbon content from CO 2undersaturated basalts[END_REF][START_REF] Mysen | Nitrogen in the Earth: abundance and transport[END_REF][START_REF] Rosenthal | Experimental determination of C, F, and H partitioning between mantle minerals and carbonated basalt, CO 2 /Ba and CO 2 /Nb systematics of partial melting, and the CO 2 contents of basaltic source regions[END_REF] and of popping rock 2πD43 (CO 2 /Nb = 556; [START_REF] Cartigny | Towards a consistent mantle carbon flux estimate: Insights from volatile systematics (H 2 O/Ce, δD, CO 2 /Nb) in the North Atlantic mantle (14 °N and 34 °N)[END_REF]. These high CO 2 /Nb ratios imply that >80% of the CO 2 in primary Klyuchevskoy melts and their mantle source is derived from the subducting slab [START_REF] Mironov | Quantification of the CO 2 budget and H 2 O-CO 2 systematics in subduction-zone magmas through the experimental hydration of melt inclusions in olivine at high H 2 O pressure[END_REF]. In addition, the N/Nb ratio of completely homogenized inclusions -which is on the order of 20-25 (#C10-2;

supplementary Table A.1) -differs substantially from that of the MORB mantle (N/Nb ≈ 0.24-9.5, assuming N = 0.05-2 ppm [START_REF] Bekaert | Subduction-Driven Volatile Recycling: A Global Mass Balance[END_REF][START_REF] Johnson | The nitrogen budget of Earth[END_REF][START_REF] Marty | The origins and concentrations of water, carbon, nitrogen and noble gases on Earth[END_REF] and Nb = 0.21 ppm [START_REF] Salters | Composition of the depleted mantle[END_REF] for the MORB mantle) and popping rock (N/Nb ≈ 0.5; [START_REF] Cartigny | Towards a consistent mantle carbon flux estimate: Insights from volatile systematics (H 2 O/Ce, δD, CO 2 /Nb) in the North Atlantic mantle (14 °N and 34 °N)[END_REF][START_REF] Javoy | The volatiles record of a "popping" rock from the Mid-Atlantic Ridge at 14°N: chemical and isotopic composition of gas trapped in the vesicles[END_REF]. Using these N/Nb estimates, we calculate that 52 to 99% of N in the Klyuchevskoy melts may be derived from the subducting slab. [START_REF] Marty | Volatiles (He, C, N, Ar) in mid-ocean ridge basalts: assesment of shallowlevel fractionation and characterization of source composition[END_REF] and popping rock 2πD43 [START_REF] Cartigny | Towards a consistent mantle carbon flux estimate: Insights from volatile systematics (H 2 O/Ce, δD, CO 2 /Nb) in the North Atlantic mantle (14 °N and 34 °N)[END_REF][START_REF] Javoy | The volatiles record of a "popping" rock from the Mid-Atlantic Ridge at 14°N: chemical and isotopic composition of gas trapped in the vesicles[END_REF]. Also shown are calculated N and CO 2 abundances in undegassed MORB mantle melts and sub-arc melts from [START_REF] Fischer | Volatile abundances in the sub-arc mantle: insights from volcanic and hydrothermal gas discharges[END_REF]. The N-CO 2 characteristics of undegassed MORB and sub-arc mantle melts were derived from the N 2 / 3 He and CO 2 / 3 He ratios of MORB glasses and volcanic or hydrothermal gas discharges, respectively, and by assuming a similar 3 He content of the crystalline mantle below mid-ocean ridges and volcanic arcs (see [START_REF] Fischer | Volatile abundances in the sub-arc mantle: insights from volcanic and hydrothermal gas discharges[END_REF] for details). The grey rectangle indicates the CO 2 (137 ± 54 ppm; Le [START_REF] Voyer | Heterogeneity in mantle carbon content from CO 2undersaturated basalts[END_REF] and N content (0.09 ± 0.05 to 1.2 ± 0.8 ppm; [START_REF] Marty | The origins and concentrations of water, carbon, nitrogen and noble gases on Earth[END_REF][START_REF] Johnson | The nitrogen budget of Earth[END_REF][START_REF] Bekaert | Subduction-Driven Volatile Recycling: A Global Mass Balance[END_REF] of the MORB source.

Overall, the data from this study demonstrate that melts generated beneath Klyuchevskoy volcano have high absolute concentrations of N and CO 2 compared to most melts generated at midocean ridges. In addition, Klyuchevskoy melts have highly elevated N/Nb and CO 2 /Nb ratios compared to all MORB magmas and their mantle sources. These findings demonstrate that N is subducted (at least) to mantle depths relevant for magma genesis at the Kamchatka-Kuril arc. It is, however, noteworthy that Klyuchevskoy MIs, as well as the parent melt composition calculated for the Kuril-Kamchatka arc based on measured volatile fluxes (Fisher and [START_REF] Fischer | Volatile abundances in the sub-arc mantle: insights from volcanic and hydrothermal gas discharges[END_REF], plot at the lower

Figure 1 :

 1 Figure 1: a) Unheated melt inclusion K16-mi, b) C8-5-3 heated dry at 300 MPa and 1200°C, and c) C10-2-1 heated with H 2 O at 500 MPa and 1150°C.

H 2 O

 2 and CO 2 in the quenched glass, and the volume of the fluid bubble is larger than that of the MIs treated in the presence of H 2 O. These observations suggest that MI heating without re-hydration has a distinct effect on the volatile characteristics of the melt.

Figure 2 :

 2 Figure 2: Nitrogen concentration in the glass phase of Klyuchevskoy MIs versus a) the CO 2 content of the glass phase, b) the H 2 O content of the glass phase, c) the (residual) fluid bubble volume (relative to the total volume of the inclusion), and d) the N/CO 2 (ppm/ppm) ratio. Uncertainties of SIMS nitrogen concentration measurements are 2σ and are, in most cases, smaller than symbol sizes.

  , the MIs were analyzed for their CO 2 and H 2 O content by relative to the total MI volume, and the CO 2 and H 2 O concentrations in the glasses, is given in Table

	SIMS (CAMECA IMS 1280 HR) at CRPG (Nancy, France). Subsequently, the inclusions were
	analyzed for major elements, S, and Cl by electron microprobe (JEOL JXA 8200) at GEOMAR (Kiel,
	Germany) and for trace elements (Ti, Nb, Ba, La, Th) by SIMS (CAMECA IMS 4f) at the Yaroslavl
	Branch of the Institute of Physics and Technology (Yaroslavl, Russia). Details on the analytical
	techniques and the compositions of MIs, matrix glass, and host rocks are provided by Mironov et al.

(2015)

. The results are summarized in supplementary Table A.1., together with unpublished data for four additional MIs. Information directly relevant for this study, such as the size of MIs, the volume of the bubble

Table 1 ;

 1 [START_REF] Mironov | Quantification of the CO 2 budget and H 2 O-CO 2 systematics in subduction-zone magmas through the experimental hydration of melt inclusions in olivine at high H 2 O pressure[END_REF]. Heating of initially re-crystallized and predominantly dry MIs from lavas in the presence of H 2 O at a pressure of 300 MPa results in higher H 2 O and CO 2 abundances (i.e., the most hydrated inclusions at a pressure of 500 MPa, resulting in the highest observed H 2 O and CO 2 contents of 4.6-5.6 wt.% and 3800-4800 ppm, respectively, in the quenched glass.

up to 5.2 wt.% and 3700 ppm CO 2 ) in the glass phase. Although the volume of the fluid bubble decreased as a result of the experimental treatment, most of these MIs still contain a residual bubble of variable size, suggesting that volatiles initially present in the fluid bubble were not completely redissolved into the melt. Complete homogenization (i.e., disappearance of the fluid bubble) was achieved for

Table 1 :

 1 Experimental conditions and volatile (N, H 2 O, CO 2 ) contents of olivine-hosted melt inclusions from Klyuchevskoy volcano.

	Sample ID Conditions	Phases in MI	D (µm)	FB (vol.%)	N (ppm)	CO 2 (ppm)	H 2 O (wt.%)
	C4-1-1 C4-1-2 C4-1-3	300 MPa, 1150°C, 24 h, QFM+3.3 matrix glass + 9.3 % H 2 O	g, f, sp g, f g, f	100 107 100	<0.1 0.7 0.1	14.9 ± 0.5 5.4 ± 0.3 13.1 ± 0.5	3202 2869 3713	4.12 3.68 4.78
	C4-2-1a		g, f, sp, cpx	125	0.5	3.4 ± 0.3	2980	3.27
		300 MPa, 1150°C, 24 h, QFM+3.3				3.4 ± 0.3
	C4-2-1b	matrix glass + 5 % H 2 O	g, f, sp	90	0.3	5.8 ± 0.3	3269	3.69
	C4-2-2		g, f, sp	79	0.4	4.5 ± 0.3	2781	3.93
	C4-3-1	300 MPa, 1150°C, 24 h, QFM+3.3	g, f, sp	138	0.3	9.5 ± 0.4	2914	3.43
	C4-3-2	matrix glass + 11.3 % H 2 O	g, f	57	0.3	5.2 ± 0.3	3669	5.21
	C4-4-2 C4-4-3 C4-4-5	300 MPa, 1150°C, 24 h, QFM+3.3 matrix glass + 20 % NaCl	g, f, sp g, f g, f	112 153 80	1.3 1.2 0.8	3.8 ± 0.3 0.8 ± 0.3 2.2 ± 0.3	2425 1510 2670	2.87 2.23 3.15
	C8-5-1*		g, f, s	80	2.7	2.4 ± 0.5	47	0.18
	C8-5-3	300 MPa, 1200°C, 28 h, QFM+0.6	g, f, s	113	1.5	2.4 ± 0.3	90	0.25
	C8-5-4a	no matrix	g, f, s	89	2.2	3.5 ± 0.4	211	0.30
	C8-5-4b*		g, f, s	84	3.0	9.6 ± 1.4	192	0.28
	C9-1 C9-2 C9-3	1 atm, 1400°C, 1 h, CCO no matrix	g, f, s g, f, s, sp g, f, s	105 61 75	2.8 2.6 2.9	1.7 ± 0.3 8.3 ± 0.4 2.7 ± 0.3	621 193 456	0.14 0.05 0.07
	C10-2-1		g, s, sp	106	0	20.8 ± 0.5	3822	4.64
		500 MPa, 1150°C, 24 h, QFM+0.6				22.1 ± 0.4
	C10-2-3*	matrix glass + 5.6 % H 2 O	g, s, sp	70	<0.1	20.5 ± 1.4	4310	5.65
	C10-2-5*		g, s, sp	88	<0.1	25.7 ± 0.5	4844	5.44
	K6-mi1		g, f, sp	88	2.3	0.8 ± 0.3	879	2.87
	K6-mi2	Natural	g, f, sp	288	1.8	0.7 ± 0.3	911	3.49
	K16-mi		g, f, sp	178	1.1	1.2 ± 0.3	1198	3.42
	Notes: g -glass; f -fluid; sp -Cr-spinel; cpx -clinopyroxene; s -sulfide; D -average two-dimensional
	diameter of the melt inclusion; FB -volume of the residual fluid bubble relative to the total volume of the
	inclusion.

N concentrations were determined in this study, whereas the size of MIs and their H 2 O and CO 2 contents are from

[START_REF] Mironov | Quantification of the CO 2 budget and H 2 O-CO 2 systematics in subduction-zone magmas through the experimental hydration of melt inclusions in olivine at high H 2 O pressure[END_REF]

(with the exception of the four MIs marked with asterisks). Uncertainties (2σ) of CO 2 and H 2 O concentrations, determined by SIMS, are estimated to be 17% and 9%, respectively (supplementary Table A.1).

  Measured N and CO 2 contents of Klyuchevskoy melt inclusions (heated with H 2 O or unheated) from this study in comparison with measured N and CO 2 concentrations in MORB glasses from

	1000	MIs Heated with H 2 O (high fO 2 )
			MIs Unheated
		100	MORB glasses Popping rock 2πD43 MORB mantle melts (calculated)	Kuril Kamchatka-
		10	Sub-arc melts (calculated)
	N (ppm)	1		MORB source
		0.1
	0.01
	0.001
		1	10	100	1000	10000
				CO 2 (ppm)
	Figure 3:

Acknowledgements

We thank Nordine Bouden for his help during the SIMS analyses. Comments from Long Li, Pierre Cartigny, and Tobias Fischer helped to improve the manuscript. EF and CD were supported by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement no. 715028). MP, NM, and RB acknowledge support from the joint DFG-

end of the estimated N and CO 2 concentrations for arc melts globally in Figure 3. The relatively low N and CO 2 concentrations are consistent with a low influx of biogenic sediments at this convergent margin [START_REF] Aiuppa | CO 2 flux emissions from the Earth's most actively degassing volcanoes, 2005-2015[END_REF][START_REF] Plank | The chemical composition of subducting sediment and its consequences for the crust and mantle[END_REF]. This implies that the N abundance in (homogenized) olivine-hosted MIs from other convergent margins, with a higher influx of organic N(-C)-bearing sediments, may significantly exceed that of Klyuchevskoy MIs.

Conclusions

SIMS is a powerful technique for probing the N content of the glass phase of individual MIs in mantle-derived samples. However, given the low N abundance in Earth's mantle, application of this method requires the availability of N-poor, homogeneous reference glasses. Moreover, MIs also commonly contain fluid bubbles that formed during cooling and decompression, and such bubbles may host a substantial portion of N that was initially dissolved in the melt. This post-entrapment modification process must be accounted for or reversed when determining the pre-degassed N budget based on analyses of MI glasses. Experimental homogenization and re-hydration of olivine-hosted MIs from a high-MgO lava flow of Klyuchevskoy volcano, coupled with a new high-resolution SIMS method, allowed us, for the first time, to determine the pre-eruptive concentration of N in an arc magma. The results demonstrate that primary Klyuchevskoy melts have high N concentrations as well as high N/Nb (and CO 2 /Nb) ratios compared to MORB mantle melts, thus indicating that a significant amount of N is subducted (at least) to sub-arc mantle depths beneath Kamchatka and recycled back to the crust and atmosphere by subduction-related magmatism. It is noteworthy that the MIs studied here were entrapped at high pressure in primitive olivine (Fo > 85). Since more commonly studied arc melts may be partially degassed prior to entrapment within more evolved minerals, such MIs may record lower N (and CO 2 ) abundances -unless the influx of organic N(-C)-bearing sediments significantly exceeds that of the Kamchatka-Kuril arc. Therefore, additional in situ N measurements of primitive MIs in other arc-related magmas, together with analyses of gases discharging from volcanoes and hydrothermal systems and high P and T experiments, are crucially needed for completing our understanding of the behavior and fate of N at subduction zones.

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.