4.3.1. Nitrogen Flux
 The gas systematics determined here can be used to calculate releases of N2 and He from the Central American arc and to estimate global fluxes associated with subduction processes. By assuming characteristic N2/3He ratios, the flux of nitrogen can indirectly be determined, either from a single volcano, from an arc system, or on a global scale. Ratios of 3He/4He arc volcanoes average 6.5 Rair, but are generally lower than the ratios of the upper mantle (8 Rair) [Poreda and Craig, 1989]. The introduction of 4He from subducted sediments provides the most plausible explanation for these lowered ratios; otherwise, one would have to assume a fairly uniform and ubiquitous crustal noble gas component throughout all island arc systems, while concurrently contributing essentially no crustal carbon or nitrogen. Recent work by Basu et al.  with fluid inclusions of xenoliths of the Kamchatka arc system confirms that helium and other volatiles are effectively subducted to mantle depths, and are involved in the metasomatism within the mantle wedge. This finding contradicts the existence of a noble gas barrier in subduction zones, i.e., that subduction processes remove all noble gases at depths of less than 100 km producing arc gases with no remnant subducted component [e.g., Staudacher and Allègre, 1989; Torgersen, 1989; Tolstikhin and Marty, 1998]. Snyder et al.  show that the island arc output may be produced through a 100% release of 4He and a 4–46% release of CO2 in subarc conditions.
 If indeed this noble gas barrier in subduction zones is absent, we are able to calculate the subducted nitrogen flux relative to helium, and then assess the flux of nitrogen in arc volcanoes from N2/3He and N2/4He measurements. This approach allows discussion of forearc devolatilization and the estimation of upper and lower limits for total fluxes. This method differs from that used by Sano et al.  who derive a global flux of 1.9 × 108 mol N2/yr for island arcs, and a combined arc + back arc flux of 3 × 108 mol N2/yr, based on an adjusted arc N2/3He ratio of 5.6 × 106 and the widely cited assumption that the 3He flux from arc volcanics is 20% of the mid-ocean ridge flux (976 mol MOR 3He/yr) [Torgersen, 1989]. This approach also differs from the approach of Hilton et al.  who determined a flux of excess nitrogen of 197.9 × 108 mol/yr based on gas ratios and COSPEC determinations of the total SO2 flux.
 We use the N2excess/3He and 4He/3He ratios of volcanic gases to determine the ratio of N2/4He (Figure 15), applying a similar approach to that developed for determining the CO2 flux from the Central American geothermal fields [Snyder et al., 2001] The majority of the samples, with the exception of those from Nicaragua, have N2/4He ratios between 1 × 104 and 2 × 104. The global flux of subducted sediment is 1.3 × 109 t/yr [Plank and Langmuir, 1998]. Assuming that the sediments are 0.02 wt.% nitrogen [Waples and Sloan, 1980] and contain 4 × 10−8 mol/kg helium (9 × 10−7 cm3/g) [Hunt, 2000; Mukhopadhyay et al., 2001], the corresponding subducted fluxes are ϕN2,ss = 92 × 108 mol N2/yr and ϕ4He,ss = 5.2 × 104 mol 4He/yr.
Figure 15. Excess nitrogen versus helium, showing the high-nitrogen input from the Momotombo geothermal wells. Ratios of N2/3He are generally greater than those of island arcs, also shown in Figure 13 and are less than those of subducted sediments.
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 The total flux of island arc helium (ϕ4He,arc-tot) is the sum of the helium contributed from the mantle (ϕ4He,arc-um) and from subducted sediments:
Similarly, the subducted component of the arc 4He flux may then be expressed as:
We assume that all of the helium from subducted sediments is released beneath the arc (ϕ4He,arc-ss ≈ ϕ4He,ss) and that the subducted 3He flux is negligible (ϕ3He,arc-ss ≪ ϕ3He,arc-tot), since sediment 3He/4He ratios (Rss) are generally on the order of 0.1 Rair. In this case, the total 3He flux is nearly the same as the arc 3He flux contributed by the upper mantle (ϕ3He,arc-tot ≈ ϕ3He,arc-um). The upper mantle has 3He/4He ratios of Rum = 8.0 Rair and island arc systems have average 3He/4He ratios of Rarc = 6.5 Rair. We can derive the following relation for the island arc flux of 3He(ϕ3He,arc-tot):
The total arc 3He flux determined from (7) is only 2.5 mol/yr, which is 2 orders of magnitude less than previous estimates [Torgersen, 1989]. It is also noteworthy that this is a maximum estimate, since a greater flux would require either a larger flux of subducted 4He, or a lower mantle ratio. Even if slightly different values are applied to (7), such as those of Hilton et al.  (ϕ4He,ss = 8.43 × 104, Rarc = 5.37), one arrives at a similar 3He flux (1.9 mol/yr). Our model implies a that the global 3He flux from island arc systems is much lower than previous calculated values which range from 92 to 230 mol 3He/yr [Hilton et al., 2002; Torgersen, 1989; Marty and Tolstikhin, 1998; Allard, 1992], since an elevated 3He flux would produce 3He/4He ratios indistinguishable from the mantle value of 8 Rair. The implications of this lower flux will be discussed further.
 Using the subducted 3He flux of 2.5 mol/yr value, we may calculate the nitrogen flux, if N2/4He ratios are known for the arc:
The molar ratio of N2/4He for subducted sediments is 17 × 104, based on the nitrogen content of deep marine sediment cores [Waples and Sloan, 1980] and the helium content of fine-grained sediments [Hunt, 2000; Mukhopadhyay et al., 2001]. In the hypothetical case where subducted helium is lost at shallow depths and not incorporated into subarc magma, the measured N2/4He ratio should increase. The actual data (Figure 16) reveal that all but one of the measured samples have N2/4He ratios which are lower than the subducted end-member, as a result of the addition of mantle helium. The Momotombo samples have ratios closest to those expected for subducted sediments. This case will be discussed separately in section 4.3.3, because it is likely a product of crustal contamination based on CH4 content and 129I/I ratios. The magmatic component is predominant in the other geothermal localities, and has an N2/4He ratio that is generally between 1 × 104 and 2 × 104, significantly lower than that of subducted sediments. Applying (8) to this range of ratios, yields a flux of 27 × 108 to 54 × 108 mol N2/yr. This estimate is between that of Sano et al.  (6.4 × 108 mol N2/yr) and that of Hilton et al.  (197 × 108 mol N2/yr) and requires that 29–58% of the subducted nitrogen in sediments is released beneath arcs. Since this estimate does not take into account preferential loss of helium in the forearc it would be an upper limit and the actual subducted helium flux would be lower. Nor does it take into account crustal production of 4He, although the drop in N2/4He ratios would be offset by the drop in 3He/4He ratios in (8) (Figure 12). Our estimate of arc flux is comparable to the mid-ocean ridge flux (22 × 108 to 28 × 108 mol N2/yr) [Zhang and Zindler, 1993; Marty and Zimmermann, 1999]. If the present mantle-exospheric exchange of nitrogen is in steady state, then 20 × 108 to 43 × 108 mol/yr (22–47%) of the subducted nitrogen is unaccounted for, and perhaps is released in forearc regions.
Figure 16. Excess nitrogen versus carbon dioxide, showing an enrichment in nitrogen relative to CO2 in the Miravalles Field, which is nearly an order of magnitude greater than subducted sediment, and an enrichment in Momotombo nearly 2 orders of magnitude greater.
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 It is worth mentioning that Hilton et al.  also calculate a substantial subducted input associated with the oceanic crust (1.92 × 106 mol/yr He, 3.11 × 1010 mol/yr N2); however, it is not clear how the oceanic crust would devolatilize and preserve volatile signatures in island arc systems similar to those of the subducted sediment. We assume no discernible contribution from oceanic basalt crust, for a number of reasons. First, if the crustal helium were to be completely released and mixed with the helium in the sediments, the resulting 3He/4He would be essentially the same as the subducted crust. A much higher helium release would also require an unrealistically high release of nitrogen from individual island arc systems in order to preserve the observed N2/3He ratios. Second, the positive δ15N values observed in arc systems [Fischer et al., 2002; Sano et al., 2001], cannot be produced by bulk mixing of subducted sediment and oceanic crust. Assuming the given fluxes, a sediment δ15N value of +7‰, and an oceanic crust δ15N of −5‰, the bulk subducted input should be −1.3‰ while the d15N values observed in volcanic systems are generally positive. Finally, in a hydrological sense, it is easier to equilibrate helium, carbon dioxide, and nitrogen ∼200 m of porous sediment rather than in ∼6 km of fractured basaltic crust.
 Our determination of the erupted N2 flux in arc systems diverges from previously cited values. Sano et al.  assume a N2/3He ratio that is 2 orders of magnitude lower, even though their sampled N2/3He and N2/Ar ratios are in the same range as ours. Consequently, their assumption of the 3He flux is overestimated and in order to lower the N2/3He ratios to produce a reasonable N2 flux, they assume fractionation during magmatic degassing, and carry out a correction based on end-member percentages derived from N2/Ar ratios and δ15N values. As was mentioned previously, we believe the δ15N values vary due to the possibility that nitrogen of different thermal maturity is contributed from the crust or subducted sediments [Zhu et al., 2000]. The low variability in N2/3He ratios observed in diverse volcanic systems [Sano et al., 2001] also precludes the importance of variable fractionation during magmatic degassing. Furthermore, if solubility controlled fractionation did occur, the corrected N2/3He ratios should be higher, rather than lower, since He is an order of magnitude more soluble in magma than either Ar or N2 [Tolstikhin and Marty, 1998; Lux, 1987]. On the other hand, our calculations of both the erupted N2 and 3He fluxes are less than those of Hilton et al.  due to the fact that their fluxes are derived by extrapolating the SO2 emissions from each arc system from the power law distribution of individual volcanoes. This method generally produces total fluxes which are greater than the sum of the individual measured fluxes of SO2 and the uncertainty of this flux is carried on to the other gases.
4.3.2. Helium and Carbon Contributions From Subducted Sediments
 The arc flux of 3He contributed from subducted sediments is insignificant when compared to the total erupted flux of 2.5 mol 3He/yr. Given that the subducted 4He flux is 5.2 × 104 mol/yr and assuming a sediment ratio of 0.1Rair, the subducted 3He flux is at most 7.3 mmol/yr, or only 0.3% of the total erupted flux. The amount of subducted 4He is significant, however. With an arc 3He/4He ratio of 6.5 Rair, the flux of arc 4He is 27 × 104 mol/yr, of which 20% is derived from subducted sediments. The total arc flux of helium is minor, however, compared to the mid-ocean ridge helium flux (Figure 16).
 Helium and carbon may be considered two extremes in terms of the proportion of subducted material devolatilized beneath the arc, and the proportion of gas released from magmatic degassing (Table 2). Even if a minor part of arc helium is derived from the subducted slab, helium devolatilization from subducted sediments must be nearly complete. On the other hand, while only a small portion of carbon is devolatilized from the sediments, it still supplies nearly all of the arc volcanic CO2 inventory [e.g., Snyder et al., 2001]. Both CO2 and N2 in arc volcanoes are derived from subducted sediments and, as with helium, a large proportion of nitrogen must be released beneath the arc. It is also likely, given mid-ocean ridge compositions, that the majority of the carbon is transported deeper into the mantle, rather than being released at shallow depths [Kerrick and Connolly, 2001a, 2001b], while a significant portion of the marine nitrogen sediment inventory is probably released in the arc and forearc areas.
Table 2. Global Fluxes
| ||Subducted Sediment Flux (mol/yr)||Flux From Arcs (mol/yr)||Flux From MORB (mol/yr)a,b,c||Percent Island Arc Sediment Devolatilization||Percent Arc Contribution from Subducted Sediment|
|CO2d||89 × 1010||4 × 1010 to 37 × 1010||150 × 1010||4.2–46%||86–98%|
|N2e||92 × 108||27 × 108 to 54 × 108||22 × 108 to 28 × 108||29–58%||∼100%|
|4Hee||5.2 × 104||27 × 104||8.7 × 107||∼100%||20%|
 Previous studies have estimated fluxes from arc volcanoes to be 3.1 × 1012 mol/yr for CO2 [Sano and Williams, 1996] and 3 × 108 mol/yr for N2 [Sano et al., 2001]. The resulting N2/CO2 ratio for island arcs should therefore be 0.1 × 10−3. This is lower than our estimate of N2/CO2 of 0.8 × 10−3 in subducted sediments (Figure 16) and would require either significant additions of mantle CO2 or the preferential release of slab CO2 relative to nitrogen beneath the volcanic front, neither of which are likely. We believe that our flux estimates provide consistency between the two systems. Assuming that the global CO2 flux is 0.04 × 1012 to 0.37 × 1012 mol/yr [Snyder et al., 2001] and the N2 flux is 27 × 108 to 54 × 108 mol/yr, requires N2/CO2 ratios for island arcs between 7 × 10−3 and 135 × 10−3. The observed ratios in Central America all fall within this range of between 5 × 10−3 and 50 × 10−3 (Figure 17). If we were to assume the CO2 flux of Sano and Williams , the Miravalles geothermal data alone would suggest that the global flux of N2 is 170 × 108 mol/yr, which is double our estimate of the total subducted flux of nitrogen and nearly 2 orders of magnitude greater than the arc flux actually suggested by Sano et al. .
Figure 17. Range of global flux values for subducted sediments, island arcs, and the mid-ocean ridge. Nitrogen differs from carbon dioxide and helium in that the subducted flux exceeds the total island arc and MOR flux.
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4.3.3. Impact of Crustal Devolatilization in Nicaragua
 Two questions arise regarding the Nicaraguan segment of the arc system. The first is whether, despite methane and 129I data to the contrary, the excess nitrogen flux from Momotombo and the surrounding volcanic chain could conceivably be from subduction zone recycling. Subduction of sediments along the Middle America Trench makes up 6% of the global flux (1.3 × 109 t/yr) [Plank and Langmuir, 1998]. Ratios of N2/4He in Costa Rica and El Salvador suggest that the entire arc has a magmatic N2 flux of 1.6 × 108 to 3.2 × 108 mol/yr. Our calculations are in general agreement with those of Fischer et al.  and Hilton et al.  who determined the non-air nitrogen component to be 2.9 × 108 mol/yr along the Central American arc. The Nicaraguan portion of the arc is roughly 20% of the total Central American arc length and, if Momotombo is representative, the N2/4He ratio is 7 times that of the rest of the arc. The 3He/4He ratios in Momotombo are essentially identical to the rest of the arc, so the 3He flux may be assumed to be the same. That said, the N2 flux along the Nicaraguan portion of the arc is 2.2 × 108 mol/yr or approximately equal to that of the entire arc. For this to occur without a crustal source, all of the subducted nitrogen along the entire Middle American Trench would have to be devolatilized and redirected along the Nicaraguan segment of the volcanic chain, while only 0.1% of the subducted CO2 would be similarly focused [Snyder et al., 2001]. The implausibility of this scenario, as well as the association of N2excess with methane and low 129I/I ratios, point to an organic source in the crust, probably linked to the remobilization of organic matter in the Sandino Basin [Darce et al., 2000]. Fischer et al.  provide an alternative explanation, and assert that variations in both N2excess and δ15N in Central American volcanics are due primarily to the subducted input of nitrogen-rich hemipelagic material, at least along the Guatemalan portion of the arc. If this were true, however, it would also imply that other arc systems, particularly those in regions with abundant hemipelagic material would have unusually high N2/He ratios which are not generally observed (Figure 12).
 The second question is whether the Nicaraguan segment produces a nitrogen flux through periodic eruptions in which enough crustal nitrogen is released to have an impact on the global volcanic flux. The flux of CO2 from the Masaya Volcano, just to the southeast of Momotombo, during eruptive events from 1998 to 1999 was calculated to be 2.3 × 1010 mol/yr based on COSPEC measurements and gas ratios [Burton et al., 2000]. This flux is equivalent to 3% of the global flux of sedimentary carbon [Plank and Langmuir, 1998] or between 6% and 56% of the total island arc CO2 flux [Snyder et al., 2001] and points to the nonsteady state conditions in volcanic systems. Assuming Central American magmatic N2/CO2 ratios of 5.5 × 10−3 (Figure 17), the flux from this volcano alone represents 1.3 × 108 mol N2/yr, which is 40–80% of the total Central American flux. If we assume that Masaya has N2/CO2 ratios similar to Momotombo of (50 × 10−3) (Figure 17), then the nitrogen flux from this volcano alone during eruptive periods is 12 × 108 mol/yr (between 22% and 44% of the total global arc flux in this study). This may be contrasted to gas fluxes determined similarly from COSPEC measurements and gas determinations at Poás Volcano in Costa Rica [Zimmer et al., 2001]. They calculated the CO2 flux for Poás at 1.89 × 1010 mol/yr, which is similar to that of Masaya Volcano in Nicaragua [Burton et al., 2000]. Nonetheless, they determined the nitrogen flux to be only 0.67 × 108 mol/yr (1.2–2.5% of the global flux estimated in this study). The total crustal contributions of nitrogen through metamorphic reactions in areas similar to the Nicaraguan Depression could therefore be as large, if not larger, than the flux of nitrogen derived from magmatic island arc sources.