Jabrane Labidi

E D Young

T P Fischer

P H Barry

C J Ballentine

J M De

J M De Moor

Recycling of nitrogen and light noble gases in the Central American

HAL is

Introduction

The origin of volatiles emitted from convergent margins provides fundamental constraints on how plate tectonics redistributes volatiles between terrestrial reservoirs [START_REF] Bekaert | Subduction-Driven Volatile Recycling: A Global Mass Balance[END_REF][START_REF] Hilton | Noble gases and volatile recycling at subduction zones[END_REF][START_REF] Plank | Why do mafic arc magmas contain∼ 4 wt% water on average? Earth Planet[END_REF]. The nitrogen cycle is underconstrained, partly because quantifying degassing N2 fluxes in arcs is challenging. For instance, basalt glasses are virtually absent in subduction zones, impeding quantification of nitrogen elemental abundances in the underlying mantle wedge. The systematic study of metamorphic rocks has suggested nitrogen is quantitatively retained within minerals in downgoing slabs [START_REF] Bebout | Devolatilization history and trace element mobility in deeply subducted sedimentary rocks: Evidence from Western Alps HP/UHP suites[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]. The latter is based on the composition of rocks from the European alps, where sediments underwent metamorphism in a cold subduction zone (630 °C at 100 km, [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]. In those rocks, nitrogen is hosted in the structures of clay minerals including micas and illites, as NH4 + substituting for potassium (Bebout et al., 1992[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]Nieder et al., 2011). Experimental work confirms that the fluid/rock nitrogen partition coefficient in a cold P-T pathway is in favor of N retention in minerals [START_REF] Jackson | Warm and oxidizing slabs limit ingassing efficiency of nitrogen to the mantle[END_REF]. However, under warmer conditions, fluid/rock partitioning favors N accumulation in fluids, potentially limiting nitrogen subduction to the mantle [START_REF] Jackson | Warm and oxidizing slabs limit ingassing efficiency of nitrogen to the mantle[END_REF]. Subduction temperature gradients were likely steeper in most of the Proterozoic and Archean [START_REF] Martin | Secular changes in tonalite-trondhjemite-granodiorite composition as markers of the progressive cooling of Earth[END_REF]. Therefore, it is unclear how modern fluxes determined in cold subduction zones should be extrapolated back into deep time.

The Central American Volcanic Arc (CAVA) is relatively well-characterized with evidence for sedimentary and igneous components from the slab variably contributing to mantle sources across the arc [START_REF] Patino | Local and regional variations in Central American arc lavas controlled by variations in subducted sediment input[END_REF]. It is a "warm" subduction zone, with predicted slab interface temperatures of ~ 800 °C at 100 km depth beneath Costa Rica [START_REF] Peacock | Thermal structure of the Costa Rica-Nicaragua subduction zone[END_REF], resulting in sporadic slab melting [START_REF] Hoernle | Arc-parallel flow in the mantle wedge beneath Costa Rica and Nicaragua[END_REF]. Thus, it may be considered an analog for subduction zones at a time when subduction temperature gradients were steeper. In Central America, metamorphic rocks cannot be used to constrain net subduction fluxes because no section of metasediments having undergone the subduction P-T pathway is known to occur in the geological record. Instead, net subduction fluxes of volatiles are estimated from comparing their concentrations and isotopic compositions in the offshore altered oceanic crust (Li andBebout, 2005, Busigny et al., 2019) with their concentrations and isotopic compositions in arc fumaroles that represent volcanic outgassing fluxes. This mass balance calculation can be hampered by ubiquitous infiltration of air into most fumaroles, as evidenced by noble gases systematics: although fumaroles typically show mantle-like helium isotope ratios, neon and argon budgets are overwhelmed by atmospheric components [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 129I evidence for crustal and magmatic volatile components[END_REF]. This is also a problem for nitrogen. Historically, SO2/N2 ratios from fumaroles and the overall SO2 outgassing flux has been used to determine a total N2 volcanic outgassing flux for the central American arc, yielding a value of 1.7 x 10 9 mol N2/yr, or 3.4 x 10 9 mol N/yr [START_REF] Hilton | Noble gases and volatile recycling at subduction zones[END_REF], Fischer et al., 2002). This is comparable to an estimate of the central American subduction nitrogen flux of ~1.1 x 10 9 mol N/yr (equivalent to ~5.7 x 10 8 mol N2/yr) from [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]. These flux estimates are given with no uncertainties, but they appear comparable within a factor of 2. Taken at face value, this would support inefficient N recycling from the surface to the deep mantle. However, fumaroles incorporate air-derived N2, after the infiltration of meteoric water within subsurface hydrothermal systems and/or because of sampling techniques [START_REF] Fischer | Subduction and recycling of nitrogen along the Central American margin[END_REF]. Thus, the outgassing N2 fluxes based on raw N2/SO2 ratios in fumaroles are overestimates of the outgassing N2 flux. As a remedy, the volcanic fraction of N2 in fumaroles was quantified on the basis of N2/Ar ratios: airsaturated waters have a known N2/Ar ratio of ~40 at STP but most hydrothermal gases in the CAVA have ratios > 80 [START_REF] Hilton | Noble gases and volatile recycling at subduction zones[END_REF][START_REF] Fischer | Subduction and recycling of nitrogen along the Central American margin[END_REF][START_REF] Elkins | Tracing nitrogen in volcanic and geothermal volatiles from the Nicaraguan volcanic front[END_REF][START_REF] Snyder | Sources of nitrogen and methane in Central American geothermal settings: Noble gas and 129I evidence for crustal and magmatic volatile components[END_REF][START_REF] Zimmer | Nitrogen systematics and gas fluxes of subduction zones: insights from Costa Rica arc volatiles[END_REF]. The N2 amount in excess of air-saturated water, termed N2*, was suggested to reflect the volcanic N2 fraction [START_REF] Hilton | Noble gases and volatile recycling at subduction zones[END_REF], Fischer et al., 2002). A volcanic nitrogen degassing flux of 2.9 x 10 8 mol N2/year was estimated for the central American arc [START_REF] Fischer | Subduction and recycling of nitrogen along the Central American margin[END_REF]. This degassing flux estimate is lower by a factor of 2 than the subduction nitrogen flux of ~5.7 x 10 8 mol N2/year (or ~1.1 x 10 9 mol N/yr), suggesting N2 sequestration into the deep mantle by subduction, even in a warm subduction zone [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] Li | Carbon and nitrogen geochemistry of sediments in the Central American convergent margin: Insights regarding subduction input fluxes, diagenesis, and paleoproductivity[END_REF].

Here, we take a new approach to constrain the origin of N2 as well as light noble gases in CAVA fumaroles and springs. We use the newly-developed 15 N 15 N tracer of atmospheric contamination [START_REF] Labidi | Hydrothermal 15 N 15 N abundances constrain the origins of mantle nitrogen[END_REF][START_REF] Yeung | Extreme enrichment in atmospheric 15N15[END_REF][START_REF] Young | A large-radius high-mass-resolution multiple-collector isotope ratio mass spectrometer for analysis of rare isotopologues of O2, N2, CH4 and other gases[END_REF]. Specifically, we use   , the 15 N 15 N concentration relative to a random distribution of 14 N and 15 N atoms among N2 molecules, as a tracer of surficial atmospheric contamination. The   tracer is defined as   = 30 R/( 15 R) 2 1 (‰), where 30 R= 15 N 15 N/ 14 N 14 N and 15 R= 15 N/ 14 N for the gas of interest. At relevant temperatures ranging from 200 to 1000 C, equilibrium among N2 isotopologues results in   values from 0.5 to 0.1‰, respectively [START_REF] Yeung | Extreme enrichment in atmospheric 15N15[END_REF]. This applies for any magmatic and crustal N2, whether it is mantle-derived, inherited from the slab, or shallow crustal reservoirs [START_REF] Labidi | Hydrothermal 15 N 15 N abundances constrain the origins of mantle nitrogen[END_REF]. In contrast, air has a pronounced disequilibrium 15 N 15 N enrichment, leading to an atmospheric   value of 19.1±0.3‰ (2σ) [START_REF] Yeung | Extreme enrichment in atmospheric 15N15[END_REF]. We make use of this disequilibrium as a tracer for air contributions in natural fluids and identify the compositions of mantle sources for N2 and light noble gases for 15 fumaroles and gases bubbling from springs in Costa Rica, Panama, Nicaragua, and El Salvador. Our 15 N 15 N data allows determination of the N2/ 3 He, 3 He/ 36 Ar and 3 He/ 22 Ne ratios for the CAVA mantle sources. Using an updated 3 He outgassing flux for the region, we provide a new range of estimates for the N2 outgassing flux along the Central American arc.

Our result has implications for the net subduction flux of nitrogen, suggesting that the net sequestration of nitrogen to the mantle is within error of zero.

Geological context and samples

The CAVA results from the eastward subduction of the Cocos Plate beneath the Caribbean Plate. Abundant literature describes the spectrum of sub-arc mantle sources, incorporating slab sedimentary components beneath Nicaragua, to volcanic seamounts derived from Galapagos underneath Costa Rica and Panama [START_REF] Carr | Volcanism and geochemistry in Central America: Progress and problems[END_REF][START_REF] Gazel | Galapagos-OI signature in southern Central America: Mantle refertilization by arc-hot spot interaction[END_REF][START_REF] Hoernle | Arc-parallel flow in the mantle wedge beneath Costa Rica and Nicaragua[END_REF][START_REF] Patino | Local and regional variations in Central American arc lavas controlled by variations in subducted sediment input[END_REF][START_REF] Ranero | Subduction erosion along the Middle America convergent margin[END_REF][START_REF] Schwarzenbach | Sulfur and carbon geochemistry of the Santa Elena peridotites: Comparing oceanic and continental processes during peridotite alteration[END_REF].

Eight gas samples from fumaroles were collected from oa s and Momotombo volcanoes during several field expeditions in the early to mid-2000's with gas chemistry and isotope data published in [START_REF] Zimmer | Nitrogen systematics and gas fluxes of subduction zones: insights from Costa Rica arc volatiles[END_REF], [START_REF] Elkins | Tracing nitrogen in volcanic and geothermal volatiles from the Nicaraguan volcanic front[END_REF][START_REF] De Leeuw | The He-CO2 isotope and relative abundance characteristics of geothermal fluids in el salvador and honduras: New constraints on volatile mass balance of the central american volcanic arc[END_REF] and [START_REF] Fischer | Temporal variations in fumarole gas chemistry at Poás volcano, Costa Rica[END_REF]. Fumaroles had outlet temperatures ranging from 98°C to 747°C. Samples were collected in pre-evacuated Giggenbach bottles filled with 5N NaOH solution (Giggenbach and Goguel, 1989). Gas splits were taken from the headspace of the bottles with sealed glass tubes shortly after sample collection and stored until analyzed for this work. Five gases from the oa s crater were collected between 2003 and 2006, from the fumarolic sites "Official" and "Naranja", at temperatures between 98°C and 158°C. For these, we report new N2 isotopologue and 40 Ar/ 36 Ar data. 3 He/ 4 He of these samples are 7.0±0.2 RA [START_REF] Hilton | Monitoring of temporal and spatial variations in fumarole helium and carbon dioxide characteristics at Poás and Turrialba volcanoes, Costa Rica (2001-2009)[END_REF] where RA is air 3 He/ 4 He or 1.384 x 10 -6 . Gas chemistry is available from previous work [START_REF] Fischer | Temporal variations in fumarole gas chemistry at Poás volcano, Costa Rica[END_REF]. One fumarole sample was collected at Santa Ana Volcano (Salvador), with a vent temperature of 400°C. It has a helium isotope ratio of 7.5±0.1 RA [START_REF] De Leeuw | The He-CO2 isotope and relative abundance characteristics of geothermal fluids in el salvador and honduras: New constraints on volatile mass balance of the central american volcanic arc[END_REF]. The He-Ne-Ar abundances are known [START_REF] De Leeuw | The He-CO2 isotope and relative abundance characteristics of geothermal fluids in el salvador and honduras: New constraints on volatile mass balance of the central american volcanic arc[END_REF]. No 40 Ar/ 36 Ar is available but other gases from El Salvador all have near-atmospheric 40 Ar/ 36 Ar ratios [START_REF] Snyder | Sources of nitrogen and methane in Central American geothermal settings: Noble gas and 129I evidence for crustal and magmatic volatile components[END_REF]. We additionally discuss two fumarole gas samples collected at Momotombo in Nicaragua, at an outlet temperature of ~750°C. For these two samples, we report new 40 Ar/ 36 Ar isotope measurements. N2 isotopologues, helium isotopes, He-Ne-Ar and major gas chemistry data for these samples are available from previous work [START_REF] Elkins | Tracing nitrogen in volcanic and geothermal volatiles from the Nicaraguan volcanic front[END_REF][START_REF] Yeung | Extreme enrichment in atmospheric 15N15[END_REF]. Hot springs found in the flanks of volcanic arcs are the most accessible samples, and hence are routinely used to characterize volcanic endmembers (e.g. [START_REF] Snyder | Sources of nitrogen and methane in Central American geothermal settings: Noble gas and 129I evidence for crustal and magmatic volatile components[END_REF]. As a means to compare these to our fumaroles, we also report data from seven gases from bubbling springs in Costa Rica and Panama, collected in 2018 as part of the Biology Meets Subduction initiative.

Sample collection sites are shown on figure 1. Samples were collected in 15 cm 3 copper tubes following standard procedures. Waters in the springs had temperatures between 29°C and 55°C. For these, we report major element gas compositions, N2 isotopologues and He-Ne-Ar systematic data.

Methods

Gas aliquots were split up to three ways and processed for (1) gas chemistry, (2) N2, and (3) noble gases systematics. Inorganic gas components (such as N2, He, Ar and O2) and methane were separated and quantified on a TCD gas chromatograph coupled with a quadrupole mass spectrometer as in earlier work [START_REF] Fischer | Temporal variations in fumarole gas chemistry at Poás volcano, Costa Rica[END_REF]. This allowed determining the N2/He and N2/Ar ratios of each gas splits. Nitrogen isotopologues were determined via high-resolution mass spectrometry at the University of California, Los Angeles [START_REF] Young | A large-radius high-mass-resolution multiple-collector isotope ratio mass spectrometer for analysis of rare isotopologues of O2, N2, CH4 and other gases[END_REF]. Noble gas analyses were conducted at the University of Oxford (UK) and at the University of New Mexico using standard methods [START_REF] Barry | Noble gases solubility models of hydrocarbon charge mechanism in the Sleipner Vest gas field[END_REF][START_REF] Lee | Incipient rifting accompanied by the release of subcontinental lithospheric mantle volatiles in the Magadi and Natron basin, East Africa[END_REF]. Details may be found in the supplementary online file.

Results

We present an integrated dataset that includes concentrations of He, Ne, Ar, N2, O2, and CH4, elemental ratios of those gases, noble gas isotope compositions, and N2 isotopologue ratios. Data for the fumaroles (n=8) are in table 1 [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] Snyder | Sources of nitrogen and methane in Central American geothermal settings: Noble gas and 129I evidence for crustal and magmatic volatile components[END_REF][START_REF] Zimmer | Nitrogen systematics and gas fluxes of subduction zones: insights from Costa Rica arc volatiles[END_REF]. The  15 N values exhibit a negative correlation with 30 values (Fig. 2).

Noble gas isotopes

Concentrations of He, Ne and Ar vary over 4 orders of magnitude and are correlated (Table 1 and2). We observe 40 Ar/ 36 Ar ratios between 297±5 and 342±5 for our samples (Table 1 and2), far from the upper mantle value of ~ 25,000 [START_REF] Moreira | Rare gas systematics in popping rock: isotopic and elemental compositions in the upper mantle[END_REF]) but close to air at 298.5 [START_REF] Lee | A redetermination of the isotopic abundances of atmospheric Ar[END_REF]. This is comparable to the known range of values for Central American rocks and gases of between 292±7 and 310±5 [START_REF] Fischer | Nitrogen isotopes of the mantle: Insights from mineral separates[END_REF][START_REF] Kennedy | Noble gases from Honduras geothermal sites[END_REF][START_REF] Snyder | Sources of nitrogen and methane in Central American geothermal settings: Noble gas and 129I evidence for crustal and magmatic volatile components[END_REF][START_REF] Staudacher | Recycling of oceanic crust and sediments: the noble gas subduction barrier[END_REF]. The 20 Ne/ 22 Ne and 21 Ne/ 22 Ne ratios are known only for the hot springs, and data appear indistinguishable from air (Table 2).

The 3 He/ 4 He ratios vary substantially, between 0.3 and 7.6±0.1 RA. Unlike Ne and Ar, the He budget is not significantly affected by atmospheric components and instead reflect appreciable deep contributions. The 3 He/ 4 He ratios are broadly correlated with 4 He/ 20 Ne ratios (Fig. 3). Most of the data are explained by two-component mixing between air

( 3 He/ 4 He =1 RA by definition, 4 He/ 20 Ne =0.3188) and a magmatic endmember. The magmatic component has a 3 He/ 4 He of ~7 (Fig. 3). The 4 He/ 20 Ne ratio of the magmatic component must be at least as high as the highest value from our dataset, ~270. Three

Costa Rican springs show a clear offset from the two-component mixing line, showing much lower 3 He/ 4 He ratios at a given 4 He/ 20 Ne value. Similar features in other Central American gases were interpreted to result from mixing with gases from crustal fluids [START_REF] De Leeuw | The He-CO2 isotope and relative abundance characteristics of geothermal fluids in el salvador and honduras: New constraints on volatile mass balance of the central american volcanic arc[END_REF].

Nitrogen, oxygen and methane concentrations

Nitrogen concentrations vary from 0.2 vol% to 93 vol% in the springs, and from 0.6 vol% to 1.9 vol% in the fumaroles (Table 1 and2). Oxygen concentrations are variable, resulting in O2/N2 ratios ranging over two orders of magnitude, between 3.8 x 10 -4 to 1.1 x 10 -1 (Supplementary Fig. 1). The O2/N2 ratios are not directly correlated with 30 values (supplementary Fig. 1), suggesting that O2/N2 is not a reliable indicator of the presence of nitrogen from air. Methane concentrations are below 0.1 vol % for most samples, with the exception of two of the Costa Rica hot springs, where CH4 concentrations are as high as ~80

vol %, suggesting contributions from shallow crustal gases [START_REF] Snyder | Sources of nitrogen and methane in Central American geothermal settings: Noble gas and 129I evidence for crustal and magmatic volatile components[END_REF]. N2/CH4

varies over 5 orders of magnitude, between ~1.2 x 10 -1 to ~2.1 x 10 4 , but remains uncorrelated with 30 values (Supplementary Fig. 1).

Nitrogen -noble gas ratios

We compute N2/Ar, N2/ 36 Ar, N2/He and N2/ 3 He ratios, with a ±20% relative uncertainty on individual samples. N2/Ar ratios are between 21 and 120 for most samples, with one sample from Momotombo at 507 (Fig. 4). For comparison, the air value is ~84, air-saturated water (ASW) at STP is ~41, and MORB gases have N2/Ar ratios of 55±5 (Javoy and Pineau, 1991), although a higher estimate of ~120±40 (Marty and Dauphas, 2003) was derived from MORB samples with unusually radiogenic 40 Ar/ 36 Ar ratios (data from Marty et al., 1999). For samples where 40 Ar/ 36 Ar was measured, N2/ 36 Ar ratios vary between and . Assuming the 40 Ar/ 36 Ar is atmospheric as for other Central American gas samples [START_REF] Snyder | Sources of nitrogen and methane in Central American geothermal settings: Noble gas and 129I evidence for crustal and magmatic volatile components[END_REF], the N2/ 36 Ar range extends to between and (Fig. 4). This includes values below and above both air ( ) and air-saturated water at STP ( ). The highest N2/ 36 Ar value remains well below estimates of the convective mantle of ~ [START_REF] Labidi | Hydrothermal 15 N 15 N abundances constrain the origins of mantle nitrogen[END_REF][START_REF] Marty | Nitrogen and argon isotopes in oceanic basalts[END_REF] even for samples where 30 is low.

The N2/He ratios vary between and , below the value of air of or air-saturated water ~ (Fig. 5). Similarly, N2/ 3 He ratios vary between and (Fig. 5), mostly below the value of air and ASW of and ~ respectively (Ballentine et al., 2002). The N2/He (and N2/ 3 He ratios) are correlated with 30 values and are described by two-component mixing hyperbolae between air (or air-saturated waters) and high-temperature components with elevated N2/He (and N2/ 3 He ratios) relative to MORB gases (Fig. 5).

Noble gas ratios

The 3 He/ 22 Ne ratios range over 4 orders of magnitude, between and (Fig. 6). These values are between air, at , and upper-mantle gases at ~ [START_REF] Moreira | Rare gas systematics in popping rock: isotopic and elemental compositions in the upper mantle[END_REF]. The 4 He/ 20 Ne ratios are also ranging between air and the upper-mantle value (Supplementary Figure 2). 3 He/ 22 Ne (and 4 He/ 20 Ne) ratios are correlated with 30 values (Fig. 6, Supp Fig. 2). At 30 = 0 the high-temperature component appears to have He/Ne ratios lower than MORB gases by about four orders of magnitude (Fig. 6, Supp Fig. 2). The 3 He/ 36 Ar ratio ranges between and (Fig. 6). This is higher than the air value of , but considerably lower than the uppermantle value of ~ [START_REF] Moreira | Rare gas systematics in popping rock: isotopic and elemental compositions in the upper mantle[END_REF][START_REF] Raquin | He, Ne and Ar systematics in single vesicles: mantle isotopic ratios and origin of the air component in basaltic glasses[END_REF]. A similar observation can be made of the 4 He/ 40 Ar ratio (Supp Fig. 2). The variations in 3 He/ 36 Ar (and 4 He/ 40 Ar) ratios are correlated with 30 values and at 30 =0, the high-T component shows 3 He/ 36 Ar ratios lower than MORB gases by about three orders of magnitude (Fig. 6, Supp Fig. 2). Most 22 Ne/ 36 Ar ratios in fumaroles and springs range between and

(with one outlier at ~ , the oa s gas with the 30 value closest to air).

The higher values are similar to the air and MORB values of and , respectively [START_REF] Mukhopadhyay | Early differentiation and volatile accretion recorded in deep-mantle neon and xenon[END_REF] and are observed for samples with air-like 30 values.

At 30 = 0, much lower 22 Ne/ 36 Ar values are observed, lower than both MORB and air by one to two orders of magnitude (Table 1 and2).

Discussion

Our rationale is that correlations between 30 and various isotope and element ratios can be used to identify and correct for air components and reveal pristine hightemperature endmembers. Air is identified with a 30 of 19.1±0.3‰, while any N2 produced by a geologic process such as magmatic degassing has 30 ~ 0 ‰ [START_REF] Labidi | Hydrothermal 15 N 15 N abundances constrain the origins of mantle nitrogen[END_REF]. Arguments against re-ordering and crustal contamination are given in the supplementary discussion.

Atmospheric N2 may easily go unnoticed

CAVA gas 30 values are variable, indicating commensurately variable contributions of air to the N2 budgets in hydrothermal discharges. Based on 30 data, air accounts for 40 to 100% of the N2 in the springs. In fumaroles, air accounts for 8 to 90% of the N2.

Atmospheric N2 is therefore ubiquitous. Systematics involving  15 N, [O2], O2/N2, N2/He and N2/Ar ratios may disentangle magmatic N2 from air-derived N2 in gas discharges (Sano et al., 2001, Fischer et al. 2002[START_REF] Elkins | Tracing nitrogen in volcanic and geothermal volatiles from the Nicaraguan volcanic front[END_REF]. The rationale for all of those approaches is that air has known  15 N, O2/N2, N2/He and N2/Ar ratios, that are different from magmatic components. Our 30 data present a series of challenges to these previous approaches. For example, samples with high 4 He/ 20 Ne ratios were suggested to illustrate a volatile budget largely uncontaminated by air, especially when a number of other criteria are met, e.g., O2/N2 ratios < 10 -3 , high outlet temperatures, or non-atmospheric N2/Ar ratios [START_REF] Elkins | Tracing nitrogen in volcanic and geothermal volatiles from the Nicaraguan volcanic front[END_REF]. The Santa Ana volcano fumarole is vented at 400°C, a 4 He/ 20 Ne ~ 200 times higher than air and a O2/N2 ratio of ~10 -4 (Table 1, Fig. 3), which would suggest minimal air contamination. However, the near-air 30 of 15.5±0.3‰ requires that ~80% of N2 is from air in this sample, showing that high 4 He/ 20 Ne, even in conjunction with high vent temperature and low O2/N2, is not necessarily sufficient evidence that N2 is dominantly magmatic (See supplementary discussion).

Atmospheric N2 undergoes isotopic fractionation in hydrothermal systems

Nitrogen isotope ratios could be a direct tracer of atmospheric N2, since air and magma-derived nitrogen are thought to have distinct  15 N values [START_REF] Fischer | Subduction and recycling of nitrogen along the Central American margin[END_REF], Sano et al., 2001). The 30 data confirm the veracity of this approach for most, but not all, samples. Gases (except those from oa s) fall on a single two-component mixing trend between air and a high-temperature component with near-zero 30 and  15 N of ~ 5‰ (Fig. 2). For those, the air component appears unfractionated with respect to  15 N. When mixed with high-temperature volatiles,  15 N increases and 30 values decrease (Fig. 2).

Samples from oa s are different. They form a unique linear mixing trend where both air-derived and high-temperature N2 are distinct from elsewhere in the arc system, with  15 N endmember values of 4.0±0.3‰ and +1.0±0.3‰ for air and the high-T component, respectively (Fig. 2). This represents a ~4‰ shift for both endmembers compared to values observed elsewhere on the arc. Our observation creates ambiguities at oa s, since there, and there only: (1) high-temperature  15 N is only marginally higher than air; and (2) air-derived N2 has a  15 N very similar to the MORB value of 5±2‰ [START_REF] Javoy | he volatiles record of a "popping" rock from the id-Atlantic Ridge at 14°N: chemical and isotopic composition of gas trapped in the vesicles[END_REF]Pineau, 1991, Marty et al., 1999).

The data from oa s require that atmospheric N2 experienced a ~4‰ mass-dependent 15 N/ 14 N fractionation in the sub-surface. This may result from the degassing of once airsaturated waters. The isotopic fractionation associated with N2 dissolved in water was experimentally documented to be +0.9 to -0.4 ‰ from 6 to 60 °C [START_REF] Lee | Kinetic nitrogen isotope fractionation between air and dissolved N2 in water: Implications for hydrothermal systems[END_REF]. The dissolved-gas isotope fractionation was shown to cross over at about 40 °C, from a positive to a negative sign [START_REF] Lee | Kinetic nitrogen isotope fractionation between air and dissolved N2 in water: Implications for hydrothermal systems[END_REF]. Above 40 °C, the isotopic difference between dissolved and gaseous N2 increases: isotopically light N2 is increasingly partitioned in the dissolved fraction as temperature increases. This behavior indicates that a kinetic isotope effect attends N2 dissolution/degassing in geothermal waters [START_REF] Lee | Kinetic nitrogen isotope fractionation between air and dissolved N2 in water: Implications for hydrothermal systems[END_REF]. Using the experimental results of [START_REF] Lee | Kinetic nitrogen isotope fractionation between air and dissolved N2 in water: Implications for hydrothermal systems[END_REF], we calculate the consequences of N isotope fractionation during open-system degassing using Rayleigh fractionation. At 60 °C, with a gas/dissolved-gas 15 N/ 14 N fractionation factor of 1.0004, the lowest  15 N values of ~ 3.5

‰ with air-like 30 require that the N2 sampled by the fumaroles is the residuum left over after degassing of about 99.98 % of the dissolved nitrogen. Extrapolating the temperature dependence of the fractionation factor to 100 °C, 97% degassing of N2 is required to account for a  15 N of ~ 3.5 ‰.

We envision the hydrothermal degassing to be the natural consequence of hydrothermal systems experiencing continuous degassing in the absence of continuous meteoric (air-saturated) water recharge. It is puzzling however that among the sites we sampled, air nitrogen undergoes isotopic fractionation only at oa s. In order to explain this observation, we suggest a restricted set of conditions is met in the oa s subsurface, but not elsewhere. Gases from Momotombo and Santa Ana are vented at temperatures between 747 °C and 400 °C, respectively. No liquid water remains stable at those temperatures.

Thus, isotope partitioning between water and gas is not occurring, and N2 remains unfractionated. Spring gases are vented at temperatures between 29 °C and 59 °C (Table 2), at temperatures where degassing is probably limited. In contrast, at oa s, the fumaroles are vented at ~100-150 °C [START_REF] Fischer | Temporal variations in fumarole gas chemistry at Poás volcano, Costa Rica[END_REF]. At these temperatures, liquid and gaseous water are stable and water/gas partitioning can occur. These conditions are comparable to those observed for hydrothermal gases from Iceland or Yellowstone, where negative  15 N values were previously observed in conjunction with air like 30 values [START_REF] Labidi | Hydrothermal 15 N 15 N abundances constrain the origins of mantle nitrogen[END_REF].

Our conclusion is also consistent with the observation of sporadically negative  15 N (with no available 30 data) in oa s fumaroles sampled in 2001. Values as low as ~ -3.0‰ were observed in fumarole vented at temperature between 76 and 108 °C [START_REF] Fischer | Subduction and recycling of nitrogen along the Central American margin[END_REF].

In other fumaroles (89-101 °C) sampled between 1998 and 2001, while unfractionated airlike  15 N and N2/Ar values were reported [START_REF] Vaselli | Fumarole migration and fluid geochemistry at Poás volcano (Costa Rica) from 1998 to 2001[END_REF]. More work is warranted to follow the potential 30- 15 N evolution of a given fumarole through time, but in the interim, we suggest caution in interpreting  15 N data at oa s in the absence of 30.

Is high-temperature nitrogen also fractionated?

Whether high-temperature components with 30 ~0‰ also experience  15 N isotope fractionation in hydrothermal systems is an open question. At Momotombo, fumaroles have a high-temperature endmember with  15 N ~+5‰ (Fig. 2.). There, vent temperatures are > 700 °C. This precludes liquid water to exist even at depths, ruling out the possibility of any water/gas isotope exchange. This is consistent with a previous observation of unfractionated  15 N for fumaroles when they are vented at > 300 °C [START_REF] Fischer | Nitrogen isotopes of the mantle: Insights from mineral separates[END_REF].

t oa s however, the interpretation is not as straightforward, where the high-temperature endmember has a  15 N ~+1‰ (Fig. 2), which is lower than at omotombo by ~4‰, implying that a fractionation similar to that for the air component could have occurred. It is conceivable that high-T volatiles have been delivered to the water table prior to hydrothermal degassing, where they would experience 15 N/ 14 N fractionation together with the air-derived gases. In this interpretation, the high-T  15 N value of ~+1‰ obtained by extrapolation to 30 = 0 would not record the actual magmatic  15 N value at oa s, and the oa s trend would be parallel to the main array as the result of a fractionation equally affecting both the atmospheric and high-T volatiles. However, with our dataset alone, we cannot exclude that high-T volatiles had been delivered to the fumaroles after hydrothermal degassing. If so, the oa s high-T endmember could be truly unique in  15 N, and the oa s trend being parallel to the main array (Fig. 2) would be fortuitous. This latter scenario relies on a coincidence, and thus appears unlikely, although it cannot be fully ruled out.

N2/Ar ratios place independent constraints on hydrothermal degassing

The solubilities of argon and nitrogen are distinct by about a factor of 2 in geothermal waters at temperatures between 20 and 100 °C (Ballentine et al, 2002). Thus, water/gas processes may be tracked with N2/Ar ratios, independently of nitrogen isotopes. Low N2/Ar ratios for air-like 30 values are observed at oa s, while most other samples have near-air values for N2/Ar at air-like 30 (Fig. 4), confirming that at oa s liquid-gas partitioning occurred in the sub-surface. We can again use a Rayleigh fractionation calculation to infer the amount of degassing implied by the N2/Ar ratio data, where N2/Ar = (N2/Ar)0 f  -1 (1) and

α = (K N2 /K Ar ), ( 2
)
where f is the fraction of remaining Ar in the water, (N2/Ar)0 the known starting composition of ASW, and α is the fractionation coefficient given for a gas/liquid system given by Equation ( 2), and Ki is the Henry's-law constant for species i, as compiled in Ballentine et al., (2002). Taking (N2/Ar)0 to be 41, we find that the lowest N2/Ar value of ~ 10 implied by our data occurs for about 94% degassing of N2 (75% degassing of Ar) prior to sampling (Fig. 4). To first order, this degree of degassing is consistent with estimates made on the basis of 15 N/ 14 N values (section 5.2).

There is a caveat to this interpretation. Data for  15 N and N2/Ar appear consistent with fractionation within hydrothermal systems, but no direct correlation is observed between  15 N and N2/Ar (Table 1). This is perhaps because degassing may yield variable 15 N/ 14 N and N2/Ar fractionations depending on the exact temperature and pressure conditions [START_REF] Lee | Kinetic nitrogen isotope fractionation between air and dissolved N2 in water: Implications for hydrothermal systems[END_REF][START_REF] Warr | Determining noble gas partitioning within a CO2-H2O system at elevated temperatures and pressures[END_REF]. Under conditions making the gas behavior non-ideal, degassing may change the N2-Ar solubility relationship, as suggested for heavier noble gases [START_REF] Labidi | Hydrothermal 15 N 15 N abundances constrain the origins of mantle nitrogen[END_REF][START_REF] Warr | Determining noble gas partitioning within a CO2-H2O system at elevated temperatures and pressures[END_REF]. Additionally, small contributions of hightemperature volatiles are likely to cause shifts in  15 N with no obvious increases in N2/Ar ratios (Fig. 4). This is because high-temperature endmembers with 30 = 0‰ appear to have N2/Ar and N2/ 36 Ar ratios of ~ 100 and ~ 10 4 , respectively (Fig. 4). These values are essentially similar to air, consistent with pioneer work on the CAVA [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 129I evidence for crustal and magmatic volatile components[END_REF], but remain lower than volcanic endmembers at other arcs [START_REF] Taran | Geochemistry of volcanic and hydrothermal fluids and volatile budget of the Kamchatka-Kuril subduction zone[END_REF][START_REF] Zelenski | Geochemistry of volcanic and hydrothermal gases of Mutnovsky volcano, Kamchatka: evidence for mantle, slab and atmosphere contributions to fluids of a typical arc volcano[END_REF]. We note that one of the Momotombo fumaroles has a high N2/Ar of ~500, higher than other high-temperature gases by a factor of 5. The significance of variable N2/Ar ratios in Central American high-temperature endmembers must be systematically investigated in future work, with 30 data.

 15 N-N2/He relationships constrained by 30 data

In contrast to argon, the solubility of helium in geothermal waters is nearly indistinguishable from that of nitrogen in the relevant temperature range (Ballentine et al., 2002). Data can be accounted for by mixing between air and an endmember with N2/ 3 He ratios ~ (Fig. 5). This is higher than MORB gases that are characterized by N2/ 3 He ratios of about 10 6 [START_REF] Javoy | he volatiles record of a "popping" rock from the id-Atlantic Ridge at 14°N: chemical and isotopic composition of gas trapped in the vesicles[END_REF]Pineau, 1991, Marty andHumbert, 1999). The oa s fumaroles involve a mixing scenario that is similar to the one for Momotombo, involving high-T endmembers with a N2/ 3 He of ~10 8 (Fig. 5). This observation argues against a significant contribution of MORB derived volatiles to Poa s volcanic gas discharges. Overall light  15 N values from oa s were previously suggested to reflect a contribution from MORB-derived volatiles, contrary to the majority of other CAVA gas discharges that showed predominantly slab-derived N [START_REF] Fischer | Subduction and recycling of nitrogen along the Central American margin[END_REF][START_REF] Elkins | Tracing nitrogen in volcanic and geothermal volatiles from the Nicaraguan volcanic front[END_REF]. Our new 30 data suggest a revision to the interpretation of 15 N depletions and show that they rather reflect contributions of fractionated air (Fig. 2) mixed with a predominantly non-MORB magmatic component with elevated N2/He ratios (Fig. 5).

Magmatic endmembers throughout the arc are enriched in N2

In an attempt to characterize subduction volatiles, we constrain the high-temperature endmembers for nitrogen isotopes and N2/Ar, N2/He, He/Ne and He/Ar ratios.

Endmember compositions are calculated assuming two-component mixing with air constrained by 30 values, as described above. We focus on fumarole data to constrain endmember compositions, since they show the lowest 30 and no evidence of crustal contamination (see supplementary discussion).

The derived N2/He ratios are higher than MORB by two orders of magnitude. In contrast, He/Ne, He/Ar and N2/ 36 Ar are all lower than MORB by two to three orders of magnitude (Fig. 456). The simplest explanation is that slab-derived N2, Ne and Ar were added to a depleted mantle source with an unmodified helium signature (Fig. 7). A straightforward quantification of slab-derived N2, Ar and Ne to the mantle wedge is possible using N2/ 3 He, 3 He/ 22 Ne and 3 He/ 36 Ar ratios. This approach is viable if 3 He subduction is negligible (Staudacher and Allegre, 1988), notwithstanding that slab components are required to contribute minute 4 He ingrowth from subducted uranium [START_REF] Hilton | Noble gases and volatile recycling at subduction zones[END_REF].

he high-, arc magma endmembers at oa s and omotombo have N2/ 3 He ratios two orders of magnitude higher than MORB, requiring nitrogen addition to mantle sources in these arc systems. Sedimentary nitrogen may be a straightforward contributor to the Momotombo source, explaining the high  15 N of + 5‰ there (Fig. 7). Sediments on the modern oceanic crust directly offshore of Central America are likely candidates, since they have an average  15 N of ~5‰ (Li and ebout, 2005). oa s volcano is more complicated, because the lower  15 N (Fig. 2) may not be a genuine representation of the Poás mantle source. We note that because N2/ 3 He are indistinguishable between oa s and omotombo, they must have received a comparable amount of slab-derived N.

Sub-arc magmatic endmembers have lower 3 He/ 22 Ne and 3 He/ 36 Ar ratios by 2 to 3 orders of magnitude compared to those of MORBs. Like nitrogen, this likely requires substantial addition of slab-derived neon and argon to a mantle source. Samples with nearzero 30 have 22 Ne/ 36 Ar lower than both MORB and air by one to two orders of magnitude (Table 1), with values trending toward ~ 0.

. This is similar to the median ratio of an entire section of altered oceanic crust and sediments of ~ [START_REF] Chavrit | The contribution of hydrothermally altered ocean crust to the mantle halogen and noble gas cycles[END_REF].

The similarity in 22 Ne/ 36 Ar between the magmatic endmembers at CAVA and the slab is consistent with a mantle wedge being overwhelmed by the addition of slab-derived neon and argon. Samples with near-zero 30 also have atmospheric 40 Ar/ 36 Ar values (Table 1), indicating that Ar from the mantle wedge has retained the noble gas isotopic signature of its subducted atmospheric source. This is likely explained by the contribution of surfacederived argon with atmospheric 40 Ar/ 36 Ar ratios to the mantle wedge. This sheds new light on atmospheric 40 Ar/ 36 Ar observed systematically in all known Central American fumaroles [START_REF] Snyder | Sources of nitrogen and methane in Central American geothermal settings: Noble gas and 129I evidence for crustal and magmatic volatile components[END_REF] and essentially all gas discharges sampled to date from subduction zones (Hilton et al., 2002, Sano andFischer, 2013): A likely origin for the atmospheric Ar in high-T endmembers is devolatilizing subducted protoliths that are otherwise known to have atmospheric Ar isotopic signatures [START_REF] Chavrit | The contribution of hydrothermally altered ocean crust to the mantle halogen and noble gas cycles[END_REF][START_REF] Holland | Seawater subduction controls the heavy noble gas composition of the mantle[END_REF][START_REF] Staudacher | Recycling of oceanic crust and sediments: the noble gas subduction barrier[END_REF]. Our high-T data shows that not all of the subducted air-derived Ar is transferred into the deeper mantle and that some of it is released via arc volcanism. Our data also dispels the notion that atmospheric Ar-isotope signatures of arc-gases are exclusively the result of shallow atmospheric contamination in the hydrothermal system.

Revisiting volatile fluxes in the central American subduction zone.

The perspective that volatiles from the mantle wedge incorporate recycled argon is problematic, as it limits the use of nitrogen excesses, N2 * , calculated on the basis of N2/Ar ratios (Fischer et al. 1998,2002, Hilton et al., 2002). We show here that magmatic volatiles deliver measurable amounts of argon (with atmospheric 40 Ar/ 36 Ar) to the fumaroles.

Consequently, assumptions on N2 * may naturally underestimates the outgassing flux of N2.

Estimates for the fluxes of N2, Ar, and Ne can be obtained from 3 He degassing fluxes. Using a combination of S/CO2, He/CO2 and 3 He/ 4 He ratios, and an SO2 flux of 21. We derive N2/ 3 He ratios between 8 x 10 7 and 2 x 10 8 for magmatic volatiles in Central America (Fig. 5), from samples with 30 < 5‰. This results in a N2 flux between 4.0 x 10 8 and 1.0 x10 9 mol N2/y (or 1.1 x 10 10 and 2.8 x 10 10 g N/ y). This range for the nitrogen mass flux is higher than previous estimates of 8.2 x 10 9 g N/ y [START_REF] Fischer | Subduction and recycling of nitrogen along the Central American margin[END_REF], Hilton et al., 2002). Our derived range of nitrogen fluxes overlaps the estimated flux of subducting nitrogen of 1.6 x 10 10 g N/ y (1.1 x 10 9 mol N/y, or 5.7 x 10 8 mol N2/yr), given with no uncertainty in [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] and [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]. This suggests that in the Central American subduction zone, the N2 cycle is not required to be out of equilibrium.

Instead, the revisited fluxes being identical within uncertainty, they allow N2 to be quantitatively recycled through the mantle wedge and returned to the surface by degassing rather than being delivered to the deep mantle. An additional source of outgassing underestimation, here, is that we ignore forearc devolatilization for N2, known to occur in the CAVA [START_REF] Inguaggiato | Nitrogen isotopes in thermal fluids of a forearc region (Jalisco Block, Mexico): Evidence for heavy nitrogen from continental crust[END_REF]. Future work will be necessary to constrain this fraction of the N2 flux that will inevitably increase the outgassing flux further. In any case, our newly defined N2 outgassing fluxes are high, and in the range of subducting fluxes. Because this subduction zone is relatively "warm" and therefore more similar to subduction zones in the past over geological timescales [START_REF] Keller | Plate tectonics and continental basaltic geochemistry throughout Earth history[END_REF][START_REF] Martin | Secular changes in tonalite-trondhjemite-granodiorite composition as markers of the progressive cooling of Earth[END_REF], the finding that nitrogen is recycled by subduction and arc magmatism in the Central American system suggests that nitrogen subduction could have been inefficient through geological time. This conclusion is also in agreement with recent experimental work suggesting warm subduction zones limit nitrogen recycling in the deep mantle [START_REF] Jackson | Warm and oxidizing slabs limit ingassing efficiency of nitrogen to the mantle[END_REF].

We estimate the 3 He/ 36 Ar of magmatic inputs to be ~ 10  4 (Fig. 6). A 3 He/ 36 Ar ratio of ~ 10  4 leads to a degassing 36 Ar flux of ~5.0 x 10 4 mol/y (1.8 x 10 6 g 36 Ar/y) in Central America. Using 1.4 x 10 16 g of subducted crust, depths of ~6 km of oceanic crust and ~500 m of sediments, and the noble gases abundances in the oceanic crust [START_REF] Chavrit | The contribution of hydrothermally altered ocean crust to the mantle halogen and noble gas cycles[END_REF], we derive a 36 Ar subduction flux of 1.1 x 10 8 cm 3 STP/y, or 4.5 x 10 4 mol 36 Ar/y. Because of the large uncertainties in 36 Ar concentrations in rocks [START_REF] Chavrit | The contribution of hydrothermally altered ocean crust to the mantle halogen and noble gas cycles[END_REF], this subduction flux estimate is probably associated with a ~50% uncertainty, and is presented here for illustration only. We note however that subduction and outgassing 36 Ar flux compare favorably. Like nitrogen, our flux analysis seems to allow quantitative release of slabderived 36 Ar to the mantle wedge, which would not require substantial 36 Ar recycling into the deep mantle in this subduction zone. The same conclusion holds for neon. Using a 3 He/ 22 Ne of ~2 x 10 -2 (Fig. 6), we obtain a 22 Ne degassing flux of ~2.5 x 10 2 mol/y (5.5 x 10 3 g 22 Ne/y). The 22 Ne/ 36 Ar of subducting components is largely variable, around a mode of ~10 -2 [START_REF] Chavrit | The contribution of hydrothermally altered ocean crust to the mantle halogen and noble gas cycles[END_REF]. This yields a 22 Ne subduction flux of 1.1 x 10 6 cm 3 STP/y, or 4.5

x 10 2 mol 22 Ne/y. Again, although large uncertainties must be taken into consideration, the neon subduction and degassing fluxes appear equivalent, arguing in favor of quantitative 22 Ne recycling in the central American subduction zone back to the surface.

The simplest interpretation of the fluxes derived here is that the Central American subduction zone acts as a subduction barrier for N, Ar, and Ne: volcanic fluxes for N2, Ar, and Ne, determined with the 15 N 15 N approach, no longer require subduction of these elements past the sub-arc melting region in the central American subduction zone.

Conclusion

Gas discharges in Central America are volatile mixtures involving contributions from at least three endmembers: air (±fractionated air), crust, and mantle-derived components.

Using 30 as a tracer of nitrogen from air in fumaroles and springs, we show that O2/N2 ratios are unreliable tracers of air in these systems. We also show that  15 N and N2/Ar ratios experienced fractionation during water degassing at depth, rendering these unreliable as signatures of air where liquid water is thermodynamically stable. This is problematic since the degassing fractionation may lead to  15 N values that insidiously mimic MORB gases. This does not seem to affect magmatic components, but more work is needed on oa s -where interaction with the hydrothermal system may be significant, before a firm conclusion can be reached. A preliminary conclusion is that without 30 data, the  15 N-N2-Ar systematics may be deceptive, and can lead to confusion with regard to the origin of N2 in mixed gases.

We derive estimates of N2/He, He/Ar and He/Ne ratios that reliably exclude -, and variably fractionated atmospheric components, including air, air-saturated water (ASW), and degassed water. The high-temperature endmembers with 30 = 0‰ de ined by the data from omotombo and oa s have similar N2/Ar and N2/ 36 Ar ratios of ~ 100 and ~ 10 4 , respectively. Here, we adopt values of ~100 and 3 x 10 4 for N2/Ar and N2/ 36 Ar ratios for the high-T endmember at Momotombo. Nitrogen and argon have indistinguishable solubilities in silicate melts and thus are not fractionated by magmatic degassing at ~1200 °C [START_REF] Libourel | Nitrogen solubility in basaltic melt. Part I. Effect of oxygen fugacity[END_REF]. Thus, the low N2/ 36 r at omotombo is not resulting from fractionations during magmatic degassing. Instead, it may be a feature of the omotombo mantle source. t oa s, fumarole samples are consistent with the same N2/Ar ratio for the high-temperature component as at Momotombo. If hydrothermal degassing affected the high-volatiles at oa s, the pristine N2/Ar and N2/ 36 r for high-gases at oa s could be higher than the observed values. Note that combining a 3 He/ 36 Ar ratio of ~ 10  4 (see Fig. 6B) to the observed N2/ 3 He of ~ 10 8 (Fig. 4) returns to a N2/ 36 Ar of ~ 10 4 , consistent with the conclusion of a low N2/Ar in our sampling of high-temperature endmembers (see section 5.4). 3 and4. They are shown for mixtures between air and high-temperature components: MORB, and a N 2 -36 Ar-22 Ne enriched endmember that fits the data. The mixing relationships are curved because of the logarithm scale, but also because of how distinct the N 2 /Ne of the mixing endmembers may be. Air has a N 2 / 20 Ne ratio of air is 4.5 x 10 4 . For MORB, we calculate a the N 2 / 20 Ne of 1.3 x 10 6 . This is based on a N 2 / 3 He of 3 x 10 6 for the MORB mantle (Javoy and Pineau, 1991). We combine this N 2 / 3 He estimate with 3 He/ 4 He and 4 He/ 20 Ne values for the MORB mantle from [START_REF] Moreira | Rare gas systematics in popping rock: isotopic and elemental compositions in the upper mantle[END_REF]. Using the 3 He/ 22 Ne ratio of the MORB mantle [START_REF] Moreira | Rare gas systematics in popping rock: isotopic and elemental compositions in the upper mantle[END_REF], we derive a N 2 / 22 Ne of 1.7 x 10 7 . Conversely, we here derive N 2 / 3 He, 3 He/ 22 Ne ratios of the central American high-temperature volatiles. Combining those with 3 He/ 4 He of ~7 from Fig. 3, we derive N 2 / 20 Ne and N 2 / 22 Ne ratios of 3.4 x 10 5 and 3.3 x 10 6 , respectively. On panel b, the curvatures are function the N 2 /Ar of the mixing endmembers, which are estimated on Fig. 4. Fig. 7 Cartoon representing various processes and reservoirs constrained in this study. Scales are grossly exaggerated, especially for melt conduits and magma chambers. A slab with sediments, basalts, and oceanic lithospheric mantle is shown to enter subduction. The slab composition is not constrained here but is likely enriched in all volatiles relative to 3 He [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] Chavrit | The contribution of hydrothermally altered ocean crust to the mantle halogen and noble gas cycles[END_REF]. In the slab, nitrogen is fixed as NH 4 + [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] references therein) so no  30 values are defined;  30 is only relevant for N 2 molecules. Both slab devolatilization and slab melting are shown for illustration. Any slab melting would be relevant underneath Costa Rica [START_REF] Hoernle | Arc-parallel flow in the mantle wedge beneath Costa Rica and Nicaragua[END_REF] while slab devolatilization and sediment-derived fluids would contribute to sources underneath Nicaragua and El Salvador [START_REF] Patino | Local and regional variations in Central American arc lavas controlled by variations in subducted sediment input[END_REF]. The melting region is characterized as the high-temperature endmembers constrained in this work (Fig. 7). Melts and super-critical fluids are suggested to host dissolved N 2 [START_REF] Libourel | Nitrogen solubility in basaltic melt. Part I. Effect of oxygen fugacity[END_REF][START_REF] Mikhail | Nitrogen speciation in upper mantle fluids and the origin of Earth's nitrogen-rich atmosphere[END_REF]. Upon partial melting of a NH 3 -bearing mantle source, magmatic N 2 would form with a  30 0‰. T -T N 2 is then contributed to hydrothermal systems in the sub-surface by near-quantitative magmatic degassing. Air circulation in the subsurface allows air-saturated waters to undergo degassing, and subsequent gas release with fractionated compositions.

  3 x 10 9 mol/yr,[START_REF] Hilton | Noble gases and volatile recycling at subduction zones[END_REF] derived a 3 He flux of 5.4 mol/yr for the entire CAVA. Using the CAVA CO2 flux fromFischer et al., (2019) of 94 x 10 9 mol/yr, and a CO2/ 3 He ratio of 2 x 10 10 (Kagoshima et al., 2015), we obtain a 3 He flux 4.7 mol/yr. Here, we take a 3 He flux of 5.0 mol/yr for the sake of illustration. We combine our new elemental ratios estimates for the CAVA source (Fig.4-6) with 3 He fluxes to derive new estimates for N2, 36 Ar, and 22 Ne outgassing fluxes in the central American arc.

  atmospheric components. N2, Ar and Ne enrichments by two or three orders of magnitude compared to a MORB source are observed. They are attributed to mantle wedge sources incorporating slab-derived gases. Using known 3 He degassing fluxes, we calculated N2, Ne and Ar degassing fluxes for the Central American arc that could balance subduction fluxes, within uncertainties, no longer requiring the Central American arc to have heavily imbalanced volatile cycles.

Fig. 3 :

 3 Fig. 3: Measured 3 He/ 4 He ratios versus 4 He/ 20 Ne ratios of the samples, normalized to air by convention (De Leeuw et al, 2007). Most samples indicate simple two-component air-magma mixing. Three Costa Rica hot springs (HS) however show additions of crustal 4 He.

Fig. 4 :

 4 Fig. 4: Measured N 2 /Ar and N 2 / 36 Ar ratios versus  30 values.-, and variably fractionated atmospheric components, including air, air-saturated water (ASW), and degassed water. The high-temperature endmembers with 30 = 0‰ de ined by the data from omotombo and oa s have similar N2/Ar and N2/ 36 Ar ratios of ~ 100 and ~ 10 4 , respectively. Here, we adopt values of ~100 and 3 x 10 4 for N2/Ar and N2/ 36 Ar ratios for the high-T endmember at Momotombo. Nitrogen and argon have indistinguishable solubilities in silicate melts and thus are not fractionated by magmatic degassing at ~1200 °C[START_REF] Libourel | Nitrogen solubility in basaltic melt. Part I. Effect of oxygen fugacity[END_REF]. Thus, the low N2/ 36 r at omotombo is not resulting from fractionations during magmatic degassing. Instead, it may be a feature of the omotombo mantle source. t oa s, fumarole samples are consistent with the same N2/Ar ratio for the high-temperature component as at Momotombo. If hydrothermal degassing affected the high-volatiles at oa s, the pristine N2/Ar and N2/ 36 r for high-gases at oa s could be higher than the observed values. Note that combining a 3 He/ 36 Ar ratio of ~ 10  4 (see Fig.6B) to the observed N2/ 3 He of ~ 10 8 (Fig.4) returns to a N2/ 36 Ar of ~ 10 4 , consistent with the conclusion of a low N2/Ar in our sampling of high-temperature endmembers (see section 5.4).

Fig. 5 :

 5 Fig. 5: Measured N 2 /He and N 2 / 3 He ratios versus  30 values. Mixing lines are shown for mixtures between air and three high-temperature components: MORBs, and two variably N 2enriched endmembers required to fit the data. Springs with additions of crustal 4 He are shown in the smallest symbols. They are not used for fluxes calculations -see discussion in the supplementary discussion.

Fig. 6 :

 6 Fig. 6: Measured 3 He/ 22 Ne and 3 He/ 36 Ar ratios versus  30 values. Springs with additions of crustal 4 He are shown in the smallest symbols. They are not used for fluxes calculations -see discussion in the supplementary discussion. Mixing lines are the same as in figure3 and 4. They are shown for mixtures between air and high-temperature components: MORB, and a N 2 -36 Ar-22 Ne enriched endmember that fits the data. The mixing relationships are curved because of the logarithm scale, but also because of how distinct the N 2 /Ne of the mixing endmembers may be. Air has a N 2 / 20 Ne ratio of air is 4.5 x 10 4 . For MORB, we calculate a the N 2 / 20 Ne of 1.3 x 10 6 . This is based on a N 2 / 3 He of 3 x 10 6 for the MORB mantle(Javoy and Pineau, 1991). We combine this N 2 / 3 He estimate with 3 He/ 4 He and 4 He/ 20 Ne values for the MORB mantle from[START_REF] Moreira | Rare gas systematics in popping rock: isotopic and elemental compositions in the upper mantle[END_REF]. Using the 3 He/ 22 Ne ratio of the MORB mantle[START_REF] Moreira | Rare gas systematics in popping rock: isotopic and elemental compositions in the upper mantle[END_REF], we derive a N 2 / 22 Ne of 1.7 x 10 7 . Conversely, we here derive N 2 / 3 He, 3 He/22 Ne ratios of the central American high-temperature volatiles. Combining those with 3 He/ 4 He of ~7 from Fig.3, we derive N 2 / 20 Ne and N 2 / 22 Ne ratios of 3.4 x 10 5 and 3.3 x 10 6 , respectively. On panel b, the curvatures are function the N 2 /Ar of the mixing endmembers, which are estimated on Fig.4.

Table 1 . Nitrogen and light noble gases isotope and concentrations for central Americans fumaroles

 1

					PO 06-1-3	PO 03-2	Pnar 06-1	PO 06-1-1	PO 06-1-2	ES02 10	Nic-3	Nic-2
					Poas	Poas	Poas	Poas	Poas	Santa Ana MomotomboMomotombo
	dates of collection		11/5/06	3/31/03	2/24/06	2/24/06	2/24/06		1/5/02	1/5/02
	latitude		10.12	10.12	10.12	10.12	10.12	13.5111	12.2519	12.2519
	longitude		-84.1359	-84.1359	-84.1359	-84.1359	-84.1359	-89.3748	-86.3224	-86.3224
	temperature (Cº)		113	98	153	113	113	400	747	747
	quantity of processed N2	10 -6 mol	2.1	2.2	8.8	8.0	5.8	22.4
	N2 vol fraction (ucla)	x10 -2	9.21E-01	8.47E-01	3.10E-01	7.43E-01	6.01E-01	7.14E-01
	d 15 N	vs. air	-3.69	0.35	-1.38	-1.38	-1.7	0.2	5.41	3.89
	D30	vs. stochastic	17	3.43	10.92	10.25	11	15.5	1.5	3.9
	1 se d 1 se D30 15 N		0.032 0.496	0.018 0.527	0.02 0.288	0.021 0.309	0.022 0.307	0.008 0.138	0.1 0.3	0.1 0.2
	CO2		0.992	0.990	0.975	0.993	0.990	0.983	0.859	0.861
	He		4.56E-05	5.00E-06	5.59E-06	4.97E-06	8.53E-06	6.54E-06	7.61E-06	1.66E-05
	H2		1.17E-04	3.25E-04	1.40E-02	6.62E-05	7.05E-05	4.64E-03	1.24E-01	1.20E-01
	Ar		2.42E-04	1.12E-04	1.24E-04	3.10E-04	4.21E-04	9.98E-05	1.52E-04	3.70E-05
	O2		4.19E-04	1.25E-05	1.45E-04	5.28E-04	7.90E-04	4.54E-06	1.14E-05	8.93E-06
	N2		6.70E-03	1.00E-02	1.06E-02	6.56E-03	8.94E-03	1.20E-02	1.62E-02	1.88E-02
	CH4		6.44E-06	1.25E-06	1.20E-04	7.24E-06	9.48E-06	1.95E-05	8.88E-06	7.66E-06
	He/Ar		1.89E-01	4.44E-02	4.50E-02	1.60E-02	2.03E-02	6.55E-02	5.00E-02	4.48E-01
	N2/He		1.47E+02	2.00E+03	1.91E+03	1.32E+03	1.05E+03	1.83E+03	2.13E+03	1.13E+03
	N2/Ar		2.77E+01	8.89E+01	8.58E+01	2.11E+01	2.12E+01	1.20E+02	1.07E+02	5.07E+02
	O2/N2		6.25E-02	1.25E-03	1.36E-02	8.04E-02	8.83E-02	3.80E-04	7.03E-04	4.76E-04
	N2/CH4		1.04E+03	8.00E+03	8.87E+01	9.06E+02	9.43E+02	6.14E+02	1.83E+03	2.45E+03
	40 Ar/ 36 Ar					297	293		305
	3 He/ 4 He	(RA)	6.44	7.15	6.96	7.15	7.15	7.56	6.99	6.99
	N2/	3 He		1.63E+07	2.00E+08	1.96E+08	1.32E+08	1.05E+08	1.73E+08	2.18E+08	1.16E+08
	N2/	36 Ar		8.49E+03	2.72E+04	2.62E+04	6.47E+03	6.49E+03		3.26E+04	1.78E+05
	4 He/	20 Ne		8.93E+00	1.90E+02	3.61E+02	1.56E+02	1.56E+02	6.66E+01	2.75E+02	2.75E+02
	3 He/ 22 Ne		8.04E-04	1.90E-02	3.52E-02	1.56E-02	1.56E-02	7.05E-03	2.69E-02	2.69E-02
	X value		28	596	1133	488	488	209	864	864
	3 He/ 36 Ar		5.20E-04	1.36E-04	1.34E-04	4.90E-05	6.20E-05		1.50E-04
	4 He/ 40 Ar		1.89E-01	4.46E-02	4.51E-02	1.61E-02	2.03E-02		5.02E-02

Table 2 Nitrogen and light noble gases isotope and concentrations for central American springs

	sample ID		LH180406	CL180409	CH180410	PX180416	PS180405	BS180407	HA180403	CW180415	XF180416
	spring name		Los Pozos	Colobre	Chiguiri Abajo	San Juan PraxAir	Playa Sandalo	Bajo Mended	Hatillo	Cauhita	San Juan PraxAir
	latitude		8.870954	8.404478	8.705078	10.488755	8.575544	8.66645	9.360224	9.735746	10.485523
	longitude		-82.689901	-80.803745	-80.269193	-84.113598	-83.36416	-82.349098	-83.916639	-82.825737	-84.113229
	temperature (Cº)		55	51	31	29	31	41	33	35	29
	quantity of processed N2	10 -6 mol	18.0	540.0	7.2	38.0	28.0	588.0	556.0	39.2	2.8
	N2 vol fraction (ucla)	x10 -2	2.89E-02	8.67E-01	1.16E-02	6.10E-02	4.49E-02	9.44E-01	8.92E-01	6.29E-02	4.49E-03
	d 15 N	vs. air	-1.38	0.55	1.60	0.20	0.71	4.19	0.22	0.34	1.30
	D30	vs. stochastic	18.39	15.27	12.52	19.35	18.42	8.73	7.29	18.47	15.09
	1 se d 15 N		0.029	0.004	0.028	0.007	0.008	0.004	0.004	0.004	0.031
	1 se D30		0.825	0.099	0.563	0.22	0.24	0.168	0.077	0.097	0.617
	CO2		0.997	0.018	0.985	0.944	0.082		0.031	0.054
	He		1.47E-07	4.98E-04	1.54E-05	1.97E-06	1.90E-06		4.19E-04	1.04E-05
	H2		9.07E-07	3.92E-06	1.77E-07	9.81E-08	1.51E-05		7.48E-05	1.62E-05
	Ar		4.79E-05	1.12E-02	3.76E-04	3.52E-04	1.07E-03		6.90E-03	1.71E-03
	O2		5.75E-04	4.78E-03	1.87E-03	1.19E-02	2.77E-03		3.12E-02	1.10E-02
	N2		2.04E-03	9.63E-01	1.32E-02	4.41E-02	5.03E-02		9.30E-01	9.92E-02
	CH4		9.62E-08	2.52E-03	4.53E-05	2.66E-06	8.64E-01		3.09E-04	8.34E-01
	He/Ar		3.07E-03	4.46E-02	4.09E-02	5.60E-03	1.77E-03	2.57E-03	6.07E-02	6.09E-03	3.85E-02
	N2/He		1.39E+04	1.93E+03	8.54E+02	2.23E+04	2.65E+04	4.23E+04	2.22E+03	9.56E+03	1.21E+03
	N2/Ar		4.27E+01	8.62E+01	3.50E+01	1.25E+02	4.69E+01	1.09E+02	1.35E+02	4.64E+01	4.64E+01
	O2/N2		2.81E-01	4.96E-03	1.42E-01	2.69E-01	5.50E-02		3.36E-02	1.11E-01
	N2/CH4		2.12E+04	3.83E+02	2.91E+02	1.65E+04	5.82E-02		3.01E+03	1.19E-01
	4 He	x10 -9 cc/ccSTP	3.21E+01	4.39E+05	1.28E+04	1.12E+03	1.65E+02	2.23E+04	3.98E+05	4.40E+03	3.72E+03
	20 Ne	x10 -9 cc/ccSTP	2.24E+00	1.37E+04	1.23E+02	6.81E+02	3.98E+02	1.58E+04	7.07E+03	6.15E+02	5.85E+01
	40 Ar	x10 -6 cc/ccSTP	3.85E+00	1.15E+04	3.59E+02	3.95E+02	4.83E+02	8.67E+03	6.35E+03	7.49E+02	9.65E+01
	36 Ar	x10 -9 cc/ccSTP	1.28E+01	3.74E+04	1.05E+03	1.29E+03	1.59E+03	2.67E+04	2.06E+04	2.43E+03	3.16E+02
	40 Ar/ 36 Ar		301	308	342	305	304	325	308	309	306
	3 He/ 4 He	(RA)	7.48	8.78	6.37	6.52	1.55	0.27	1.82	2.05	7.59
	20 Ne/ 22 Ne		10.00	9.83	9.89	9.82	9.84		9.85	9.87	9.91
	21 Ne/ 22 Ne		0.023	0.029	0.029	0.029	0.029		0.029	0.029	0.029
	N2/ 3 He		1.33E+09	1.57E+08	9.59E+07	2.45E+09	1.22E+10	1.12E+11	8.72E+08	3.33E+09	1.14E+08
	N2/ 36 Ar		1.29E+04	2.66E+04	1.20E+04	3.83E+04	1.43E+04	3.53E+04	4.17E+04	1.80E+04	1.42E+04
	4 He/ 20 Ne		1.43E+01	3.21E+01	1.04E+02	1.65E+00	4.15E-01	1.41E+00	5.63E+01	7.15E+00	6.37E+01
	3 He/ 22 Ne		1.50E-03	3.87E-03	9.19E-03	1.48E-04	8.85E-06	5.34E-06	1.41E-03	2.02E-04	6.70E-03
	X value		45	101	327	5	1	4	177	22	200
	3 He/ 36 Ar		9.70E-06	1.69E-04	1.25E-04	1.57E-05	1.17E-06	3.16E-07	4.78E-05	5.41E-06	1.25E-04
	4 He/ 40 Ar		3.08E-03	4.47E-02	4.10E-02	5.62E-03	1.77E-03	2.50E-03	6.09E-02	6.11E-03

Fig.

1

: location of all the samples studied here. Green, red and blue symbols are fumarole locations from Santa Ana, Momotombo and oa s (Data in table 1). Yellow symbols are springs (Data in table 2).

Acknowledgement

This study was supported by the Deep Carbon Observatory through Sloan Foundation grant numbers G-2018-11346 to EDY. This work was also supported by grant G-2016-7206 from the Alfred P. Sloan Foundation and the Deep Carbon Observatory to PHB. Karen Lloyd and Donato Giovannelli are thanked for co-organizing the Subduction meets Biology initiative.

Further, we acknowledge the National Science Foundation (NSF) award MGG-2015789 to PHB.

We thank Yuri Taran and an anonymous reviewer for constructive and helpful comments.

Rosemary Hickey-Vargas is thanked for editorial handling.