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Keywords:

  • Central America;
  • nitrogen;
  • methane;
  • noble gases;
  • 129I;
  • geothermal

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Field Sampling and Analytical Techniques
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[1] Gases and fluids from four geothermal fields of Central America were analyzed for nitrogen, methane, and helium concentrations, isotopic composition, and 129I/I ratios in order to determine the sources of volatiles in these systems. Results for gas ratios and isotopic compositions for three of the fields are consistent with observations from other subduction zones. Ratios of N2/3He are only slightly higher than average arc values of 1 × 108 and the volcanic flux of N2 for the Central American systems is estimated to be between 1.6 × 108 and 3.2 × 108 mol/yr. Analysis of 129I/I ratios indicates the presence of a subducted organic component (25–30 Ma) as well as of a much older crustal component (40–65 Ma) throughout the study area. The magmatic flux of nitrogen and noble gases in Central America was then extrapolated to determine the degree of nitrogen recycling in island arc systems. Global N2 flux is estimated at 2.7 × 109 to 5.4 × 109 mol/yr, which is comparable to the global mid-ocean ridge flux, and represents 29–58% of the subducted sediment flux. This flux estimate is consistent across the N2–CO2–He systems and suggests that nearly all of the nitrogen supplied to the mantle wedge is devolatilized beneath the volcanic front. The Momotombo geothermal field of Nicaragua is characterized by exceptionally high excess nitrogen and methane values, and the close correlation of these two gases indicates a common source. While it is not uncommon for sedimentary basins with high heat flow to have excess nitrogen, the Momotombo geothermal field is unique in that the high N2/3He gases have essentially magmatic 3He/4He ratios. The high excess nitrogen component of Nicaragua is related to the older iodine end-member, pointing to a crustal origin. The crustal nitrogen contributions along the Nicaraguan portion of the arc are on the order of 2.2 × 108 mol N2/yr or roughly equal to the magmatic contribution along the entire Central American volcanic arc. The results for Momotombo indicate that the release of nitrogen during reorganization of island arc systems may have a significant impact on the global flux of volcanic nitrogen.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Field Sampling and Analytical Techniques
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[2] Excess molecular nitrogen in island arc systems is frequently cited as compelling evidence for the recycling of subducted marine organic matter. Nonetheless, the origin of free nitrogen in arc gases is ambiguous in certain cases where it is associated with other gases such as methane, which are derived from shallow crustal organic matter. The purpose of this investigation is to determine whether nitrogen found in gases from deep geothermal production wells in Central America is primarily of subducted magmatic origin, or from other sources such as metamorphic reactions in the shallow crust, and to determine what implications the observed fluxes have on the global nitrogen cycle.

[3] Little is known about the geochemical behavior of the major constituents of marine organic matter, as it is recycled through subduction systems. Nitrogen can potentially provide an important tracer since the background concentration in the mantle is quite low [Bebout, 1995]. While multiple lines of evidence suggest that subducted carbonates and altered oceanic slab provide the majority of the carbon dioxide in island arc volcanic systems [e.g., Marty and Jambon, 1987; Kerrick and Connolly, 2001a, 2001b; Sano and Williams, 1996; Snyder et al., 2001; van Soest et al., 1998; Basu et al., 2001], the fate of organic matter at convergent plate margins remains open to speculation. It has been suggested that isotopically light carbon observed in some arc settings is derived from subducted organic matter [Sano and Marty, 1995], yet secondary carbonates in altered oceanic basalt can also carry an isotopically light signature [Furnes et al., 2001; Burns et al., 1990; Alt and Teagle, 1999]. Because nitrogen is an important constituent of marine organic matter, yet is not found in appreciable concentrations in marine carbonates, the nitrogen system provides a stronger indicator of the migration of organic constituents at convergent margins than the carbon system.

[4] Nitrogen is a major gas constituent in a variety of basin environments [Jenden et al., 1988; Welhan, 1981] as well as forearc regions [Giggenbach et al., 1993; Wakita et al., 1987; Motyka et al., 1989], particularly those with a known potential for commercial gas production. A study of global nitrogen fluxes must be able to discern between potential overprinting of the subducted component by accreted and depositional sources in the shallow crust. This investigation includes the use of 129I as an age-dependent tracer of organic matter to determine the source of nitrogen in geothermal systems. The high nitrogen and methane concentrations in the gases are often accompanied by high iodine concentrations in groundwater [Fehn et al., 1992, 1994; Muramatsu et al., 2001; Motyka et al., 1989] (U. Fehn and G. T. Snyder, Origin of volatile elements in subduction zones: Iodine and 129I in volcanic fluids from North Island of New Zealand, submitted to Economic Geology, 2002, hereinafter referred to as Fehn and Snyder, submitted manuscript, 2002). Assuming that all three are produced during the diagenesis of sedimentary organic matter, the stable isotopic composition of nitrogen and methane, combined with the age-dependent 129I isotopic composition, is useful in revealing both the timescales and processes involved in the release of nitrogen and methane.

[5] The Central American volcanic arc was chosen as the focus of this study, because of the availability of four geothermal production fields (Ahuachapán and Berlín in El Salvador, Momotombo in Nicaragua, and Miravalles in Costa Rica) distributed along the arc (Figure 1), as well as the known variability of geophysical parameters along the arc which influence the input of subducted sediments [Protti et al., 1995; Leeman and Carr, 1995]. The geothermal fields all have wells with temperatures over 200°C, and depths ranging from 500 to over 2000 m, a situation, which greatly reduces the degree of surface contamination, caused by Recent, atmospheric or dissolved meteoric components. These high-temperature fluids also provide the link between volatile (e.g., N2 and CH4) and soluble (e.g., I, 129I, and Cl) components, which are generally absent in fumaroles where subsurface phase separation has occurred. Central America is also one of two focus sites for the MARGINS Subduction Factory Initiative (M. Hirschman, D. Weins, and S. Peacock, Revised Subduction Factory Science Plan, available at http://www.ldeo.columbia.edu/margins/SF_Sci_Plan_revised.pdf), and the results of this investigation will thus complement a wide variety of ongoing geophysical and geochemical investigations that are currently underway.

image

Figure 1. Locality map for Central American geothermal samples. 1 = Poás Crater Lake, 2 = Cañas hot spring, 3 = Miravalles Geothermal, 4 = Mombacho/Mecatepe fumarole and hot spring, 5 = Momotombo Geothermal, 6 = El Carol/Ñajo hot springs, 7 = Casitas fumaroles, 8 = Chinameca fumaroles, 9 = Berlín Geothermal, 10 = San Vicente fumaroles and boiling springs, 11 = Ahuachapán Geothermal.

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1.1. The Nitrogen System

[6] Biological fixation and surface runoff provide the major influx of nitrogen in the oceans (Figure 2). As atmospheric nitrogen is incorporated into amino acids in marine biomass, an appreciable amount makes its way to the ocean floor in organic matter. Decomposition of organic matter releases ammonia, a portion of which is trapped in clay minerals. During metamorphism, ammonia may substitute for potassium in the crystal lattices of micas and feldspars [Boyd, 2001]. Because the amount of nitrogen that is actually incorporated into sediments and the altered oceanic crust is poorly quantified, estimates for each of these reservoirs vary by nearly an order of magnitude (Figure 3). If there were no return of nitrogen from marine sediments, the atmosphere would be depleted of nitrogen in 285 Ma, given the present sedimentary flux of nitrogen [Jaffe, 1992]. Denitrification in anoxic sedimentary environments transforms NO3 into gaseous N2 and N2O, which are returned to the atmosphere. Nitrogen profiles from sediment cores [Waples and Sloan, 1980] suggest that only 25% of the organic nitrogen is retained in sediments. Were denitrification the only process governing the release of sedimentary nitrogen, the atmosphere would still be depleted of nitrogen after 1000 Ma [Boyd, 2001].

image

Figure 2. Principal fluxes of nitrogen (108 mol N2/yr). Sano et al. [2001] (1); Bebout [1995] (2); Jaffe [1992] (3); Zhang and Zindler [1993] (4); Marty and Zimmermann [1999] (5).

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image

Figure 3. Global nitrogen reservoirs, expressed as 1017 mol N2. Jaffee [1992] (1); Sediment mass from the study of Muramatsu and Wedepohl [1998] and concentrations from the study of Waples and Sloan [1980] (2). Mass from the study of Muramatsu and Wedepohl [1998] and concentrations from the study of Bebout [1995] (3). Bebout [1995] (4); Zhang and Zindler [1993] (5).

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[7] In order to determine the long-term consequences of nitrogen loss to marine sediments, the fate of nitrogen at convergent plate boundaries must be quantified. These regions provide important sites for nitrogen remobilization, since all of the sediments arriving at the ocean trenches must either lose nitrogen during dewatering in the forearc region, temporarily store nitrogen during accretion and forearc shortening, or directly contribute nitrogen to the subducted volatile inventory of the mantle wedge. Comparisons of nitrogen and argon depletion in the mantle (88% and 99.7%, respectively) suggest that some return of sedimentary nitrogen into the depleted mantle does occur, but not to the same degree as for CO2 [Zhang and Zindler, 1993].

[8] The isotopic composition of nitrogen has also been used as a tracer of nitrogen recycling in island arc systems [Sano et al., 2001]. The upper mantle and the oceanic crust yield isotopically light δ15N values of −6‰ relative to air standard, while some surface reservoirs have positive δ15N values (Figure 4). The fact that volcanic gases nearly always present positive δ15N values, while back arc basin volcanic glasses often have slightly negative δ15N values, has been cited as evidence that fumaroles release recycled nitrogen derived predominantly from subducted sources. Values of δ15N for a variety of arc gases and fluids range from +0.1 to +4.6 [Sano et al., 2001]. It must be noted, however, that systematic variations in δ15N values may also be a result of the release of crustal nitrogen, which has undergone varying degrees of metamorphic alteration. The degree of metamorphism on sedimentary rocks has a strong effect on δ15N due to the preferential release of 14N-rich ammonium during low-grade metamorphism. Slates and other low-grade metasediments typically have δ15N values of +1 to +3, mica schists from +2 to +5, and gneisses from +5 to +11 [Mingram and Bräuer, 2001]. Molecular nitrogen in natural gas fields also displays large variations with increasing thermal maturity of organic matter. Gases derived from thermally immature organic matter have δ15N values as low as −18‰, while postmature sediments which have been exposed to magmatic heat present δ15N values as high as +18‰ [Zhu et al., 2000].

image

Figure 4. Representative values of δ15N for nitrogen in crustal and upper mantle reservoirs. Atmospheric standard (1); Zhu et al. [2000] (2); Mingram and Bräuer [2001] (3); Bickert [2000] (4); Boyd [2001] (5); Sano et al. [2001] (6).

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1.2. Methane in Geothermal Systems

[9] The major component of methane in subduction zone geothermal systems is through thermocatalytic reactions within organic matter in the crust [e.g., Poreda et al., 1988; Jenden et al., 1988; Schoell, 1988]. Typical values of δ13C for this thermogenic methane production range from −46‰ to −32‰ and CH4/3He ratios range from 1 × 108 to 1 × 1012 [Poreda et al., 1988]. As organic matter is progressively depleted through the thermogenic release of hydrocarbons, CH4/3He ratios drop and the isotopic ratio of methane in the residual portion approaches that of the magmatic end-member [Giggenbach, 1997]. In theory, the presence of biogenically derived methane could also be readily detected in geothermal systems, since methane production associated with CO2 reduction in marine environments generally yields δ13CCH4 values ranging from −110‰ to −60‰, and methane derived from acetate fermentation in freshwater environments yields δ13CCH4 values between −65‰ and −50‰ [Whiticar et al., 1986]. The absence of isotopically light carbon in geothermal systems, however, precludes the importance of CO2 reduction and acetate fermentation in high-temperature systems.

[10] Abiogenic methane is present only in trace amounts in sediment-free mid-ocean ridge environments, where it is produced through inorganic reactions such as [Giggenbach, 1997]:

  • equation image

or

  • equation image

Values of δ13CCH4 for methane equilibrated with carbon dioxide frequently range from −18 to −15‰ [Welhan and Craig, 1983; Schoell, 1988; Welhan, 1988]. Ratios of CH4/3He for abiogenic methane range from 1 × 105 to 1 × 106, and summit fumaroles, which have a low likelihood of crustal contamination, suggest that the magmatic end-member in subduction zones is less than 1 × 106 [Poreda et al., 1988]. In general, if methane is present in geothermal systems, the measured ratios of CH4/3He are high enough so that the inorganic contributions from the magmatic end-member derived from either (1) or (2) is negligible.

[11] A variety of geothermal systems contain gases with both high methane and high nitrogen concentrations [Jenden et al., 1988; Motyka et al., 1989; Zhu et al., 2000]. The relationship between these two gases is not always clear, however, since other systems show a bimodal distribution of high-nitrogen gases with low methane content, and high-methane wells with low nitrogen (e.g., New Zealand) [Giggenbach et al., 1993]. Because nitrogen has several sources (the atmosphere, devolatilizing magma bodies, and nitrogenous matter in the shallow crust), it is important to combine nitrogen systematics with other gas and isotopic systems in order to determine the sources of this gas.

1.3. Iodine in Geothermal Systems

[12] The iodine system is, in several respects, similar to the nitrogen system. Iodine is organophilic, and, like nitrogen, is scavenged from the ocean and deposited in organic-rich marine sediments. Both elements are highly depleted in the upper mantle [Déruelle et al., 1992; Zhang and Zindler, 1993] and form incompatible volatiles in magmatic systems. In both cases, the marine sediment reservoir is an order of magnitude higher than the seawater reservoir [Snyder and Fehn, 2002] and exceeds the igneous crust reservoir as well. Both elements exist in a number of oxidation states in nature and are released during early stages of diagenesis of organic matter. Most of the Earth's crustal iodine presently resides in marine sediments [Muramatsu and Wedepohl, 1998] where it exists as organic iodine, or as iodide or iodate in pore fluids [Cook et al., 2000; Kennedy and Elderfield, 1987]. This is obviously different from nitrogen, where the largest surface reservoir is in the atmosphere. The two elements are also related because nitrate and iodide are both released during shallow diagenesis where they can react with each other, forming molecular nitrogen and molecular iodine. While N2 may be lost back into the ocean, the I2 most likely reacts with organic matter to form iodinated organic molecules, which are subsequently buried [Anschutz et al., 2000]. In this sense, iodine promotes nitrification in shallow marine organic sediments and then is preferentially retained and transported to tectonic margins.

[13] The cosmogenic 129I system, with its long half-life (t1/2 = 15.7 Ma) can potentially determine the source of organic constituents in island arc systems, and therefore shed light on ongoing questions of whether nitrogen and other gases in arc systems are derived primarily form subducted sources or through metamorphic reactions in the crust. The 129I system and its relation to Central American volcanic systems in this study are described in detail by Snyder and Fehn [2002]. A similar approach to dating subducted iodine has been applied to other volcanic systems in New Zealand (Fehn and Snyder, submitted manuscript, 2002), Japan [Snyder et al., 2002], and Argentina [Fehn et al., 2002]. Iodine is also frequently associated with methane and other hydrocarbons, and a variety of 129I studies have been recently carried out to determine the age and migration of brines in forearc methane fields [Muramatsu et al., 2001], offshore methane hydrates [Fehn et al., 2000], hydrocarbon brines [Moran et al., 1995], coalbed methane deposits (G. T. Snyder et al., Origin and history of waters associated with coal-bed methane: 129I, 36Cl and stable isotope results from the Fruitland Formation, CO and NM, submitted to Geochimica et Cosmochimica Acta, 2002), and shallow biogenic methane deposits [Ridgley et al., 2001].

2. Field Sampling and Analytical Techniques

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Field Sampling and Analytical Techniques
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[14] Sampling was carried out between July 1997 and August 2000 in Costa Rica, Nicaragua, and El Salvador. Methods of separation of gas and steam fractions, including field and laboratory techniques are described in detail by Snyder et al. [2001], while preparation of the fluid samples for 129I and anion analyses are described by Snyder and Fehn [2002]. The primary focus was on geothermal production wells, although hot springs, mud pots, and flank fumaroles were also sampled in order to provide a broader coverage. An advantage of sampling geothermal wells on the flanks of active systems is that they provide high-temperature samples from depths of 0.5–2 km, where there is generally much less of an effect of fractionation between gases as is seen in boiling springs and low-temperature fumaroles which often present atmospheric contamination. Fluid and gas fractions were collected from either the production separators or by connecting a Webber separator directly on the steam lines of the geothermal wells in production. Gas was streamed from the separator into evacuated 50-ml 1720 aluminosilicate glass flasks for subsequent analyses of noble gas composition and stable isotopic composition. Gas was also streamed through evacuated 250-ml Giggenbach flasks [Giggenbach, 1975] containing 2-M NaOH in order to concentrate methane and nitrogen for stable isotopic analyses. Wellhead pressure, and separation pressures were recorded at the time of sampling, for the subsequent calculation of steam fraction and reservoir concentrations [Arnórsson, 1985].

[15] Gas concentrations and noble gas isotopic composition were determined at the University of Rochester Rare Gas Facility [Poreda and Farley, 1992; Giggenbach and Poreda, 1993], iodine concentrations were determined using a VG Plasmaquad II at the ICP-MS Laboratory [Schnetger and Muramatsu, 1996], and iodine was extracted for 129I analysis at the Cosmogenic Isotope Laboratory [Snyder and Fehn, 2002; Fehn et al., 1992]. Gas determinations from fumaroles and hot springs in Nicaragua [CNE, 2001] were also carried out at the Rochester Rare Gas Facility, and are included in this study to provide additional coverage along the Nicaraguan Depression. Nitrogen and carbon isotopic composition was determined at the Environmental Isotope Laboratory, University of Waterloo. Accelerator mass spectrometry was used to determine 129I/I ratios at the Purdue Rare Isotope Measurement (PRIME) Laboratory, Purdue University [Sharma et al., 2000].

3. Results

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Field Sampling and Analytical Techniques
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

3.1. Gas Composition

[16] Because the presence of atmospheric nitrogen is common in geothermal fluids, nitrogen values typically are presented as “excess nitrogen,” i.e., the nitrogen contributed from sources other than atmospheric. Excess nitrogen can be calculated by comparing nitrogen to argon concentrations since, because the measured 40Ar/36Ar ratios are all less than <310, and total argon in the Central American geothermal systems is >90% atmospheric. Excess nitrogen (in ppm) is calculated as:

  • equation image

The air nitrogen component may be taken as either an atmospheric ratio, (N2/Ar)air = 84, or as the ratio of air-saturated water (N2/Ar)asw = 39 [Giggenbach and Poreda, 1993]. For consistency's sake, excess nitrogen is calculated here from air-saturated water, keeping in mind that small amounts of entrained air can further increase excess nitrogen to a small degree, particularly when sampling hot springs or other surface geothermal manifestations. Ratios of N2/Ar are, for the most part, between that of air and air-saturated water in the Central American geothermal fields (Table 1). The geothermal wells of Momotombo are an exception, where the highest N2/Ar ratios are more than an order of magnitude greater than air-saturated water (Figure 5). The excess nitrogen may also be normalized to 3He which is purely magmatic, in order to sort out mantle and crustal signatures:

  • equation image

The Momotombo field also stands out as having the highest CH4/3He ratios in Central America (Figure 6). The high ratios in Nicaragua are not simply a result of low 3He and Ar content, since absolute concentrations of excess nitrogen and methane (in mol/kg fluid) are highest in the Nicaraguan geothermal reservoir, and are positively correlated (Figure 7). In the rest of the arc system, both nitrogen and argon are >90% atmospheric, based not only on the N2/Ar ratios, but also the 40Ar/36Ar ratios which are less than 310 and the fact that the argon reservoir concentrations are close to atmospheric saturation (Table 1).

image

Figure 5. Ratios of N2/Ar along the arc, showing high excess nitrogen in the Momotombo field of Nicaragua, relative to air and air-saturated water (ASW). Distances are in km from the Guatemala–Mexico border. Symbols are the same as those used in Figure 1.

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image

Figure 6. Ratios of CH4/3He show similar trends to N2/Ar ratios along the arc.

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image

Figure 7. Absolute concentrations of CH4 and N2 show that the two systems are generally correlated. Concentrations have been corrected to reflect reservoir conditions prior to phase separation between fluids and gas.

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Table 1. (a) Dry Gas Data for Central American Geothermal Fields and (b) Ratios and Reservoir Concentrations
(a) Dry Gas Data for Central American Geothermal Fields
LocalitySample IDDateH2 (%)aCH4 (%)aN2 (%)aCO2 (%)bO2 (ppm)aNe (ppb)a40Ar (ppm)a36Ar (ppb)a4He (ppm)b3He (ppb)a
El Salvador
Ahuachapán GWAH06Aug 19990.340.0495.9893.4113331996855 9.480.088
Ahuachapán GWAH16AAug 19990.200.24410.9787.9164421016043 5.900.051
Ahuachapán GWAH17Aug 19990.210.0151.7497.87114019439713393.160.030
Ahuachapán GWAH20Aug 19990.380.0382.5896.8497232267722934.410.042
Ahuachapán GWAH21Aug 19990.350.0702.5596.79189729644215104.930.048
Ahuachapán GWAH22Aug 19990.200.0363.0096.66684286367 8.280.079
Ahuachapán GWAH23Aug 19990.090.0131.4798.4023711375 5.500.054
Ahuachapán GWAH4BAug 19990.220.0562.5197.173392041976657.700.074
Berlín GWTR02Aug 19990.230.0304.2095.321648564510 4.830.037
Berlín GWTR4BAug 19990.310.0273.4896.08642573432 2.760.021
Berlín GWTR4CAug 19990.210.0936.4793.07715121183327575.970.044
Berlín GWTR09Aug 19990.200.02023.1972.0838,91048903169 5.390.034
San Vicente HSSV01Aug 19990.540.0193.4595.732222336.4360 7.020.062
San Vicente FCSV02Aug 19990.520.0683.3095.921566347277417.600.068
Chinameca FCLV01Aug 19990.370.02031.6951.76144,49881727617 6.740.025
Chinameca FCIF01Aug 19990.270.02011.0483.5543,8621967250484864.510.034
 
Nicaragua
Momotombo GWMT20Feb 19980.150.3054.1195.371472021816393.250.030
Momotombo GWMT27Feb 19980.240.2884.0395.0852870732475.020.039
Momotombo GWMT27Mar 19990.110.4847.9391.3295946450416843.510.032
Momotombo GWMT35Feb 19980.100.4244.2895.1417166682324.320.041
Momotombo GWMT35Mar 19990.110.2604.0895.5121964832763.510.041
Momotombo SATMT36-43Mar 19990.461.1824.1794.1718837481563.500.034
Momotombo GWMT38Feb 19980.140.2424.6994.7513151701675724.130.039
Momotombo GWMT38AMar 19990.090.4218.5790.04815593455618813.790.035
Momotombo GWMT40Feb 19980.110.3256.0893.2417542091806005.420.053
Momotombo GWMT40BMar 19990.070.4425.5493.902991401765874.190.041
Momotombo GWMT42Feb 19980.200.4363.9095.3940374752424.080.039
Momotombo GWMT42Mar 19990.180.4584.9594.2811252482137163.540.033
Momotombo GWMT43Mar 19990.781.3433.9893.8912031471513.840.036
El Carol, HSc1026May 19970.000.0106.9692.90 69035512004.570.047
Najo Norte, FCc1025May 19970.510.0915.9091.10 54061921005.260.051
Najo Norte, FCc1024May 19970.460.1834.9693.00 79050317005.700.052
Najo Sur, HS1023May 19970.380.1485.7692.70 110045815006.080.055
Mecatepe, HSc2May 20000.050.26714.1885.2921432871919652140.900.421
Casita, FCc4Apr 20000.680.0070.7298.595243381271.420.014
Mombacho, FCc3May 200019.310.1830.3780.13532023784.480.048
 
Costa Rica
Cañas HSCAS02Feb 19980.010.0020.5599.4121091892960.270.002
Miravalles GWPGM01Feb 19980.320.0251.3798.168981891896362.120.019
Miravalles GWPGM02Aug 19990.260.1503.1096.38629703550 1.930.018
Miravalles GWPGM03Mar 19980.200.0423.4596.245776136512432.530.022
Miravalles GWPGM03Aug 19990.090.0351.9397.8377141930110041.550.013
Miravalles GWPGM05Feb 19980.030.0250.8999.03911251193941.900.016
Miravalles GWPGM10Feb 19980.120.0281.0798.723741551294291.900.017
Miravalles GWPGM11Feb 19980.040.0160.3899.5511138451472.060.020
Miravalles GWPGM12Feb 19980.310.0401.1998.403282161434871.990.018
Miravalles GWPGM20Feb 19980.070.1446.2993.34378127773824971.990.016
Miravalles GWPGM21Feb 19980.080.0290.7799.08324185892983.760.035
Miravalles GWPGM29Feb 19980.000.0070.1899.80692218571.490.014
Miravalles GWPGM31Feb 19980.080.0191.0198.842342301394641.990.017
Miravalles GWPGM45Aug 19990.170.1074.3795.141460890705 1.440.011
Miravalles GWPGM46Aug 19990.060.0522.5397.092140531.4477 1.370.011
Poás Volcano CL,FCVPOCLMar 19990.050.0271.8397.43624416938813163.960.039
 
(b) Ratios and Reservoir Concentrations
LocalitySample IDDate3He/4He (R/Rair)b3He/4He (Rc/Rair)bN2/AraN2excess/4He (103)aδ15NN2aCH4/3He (106)aδ13CCH4a40Ar/36Ara129I/I (10−15)d 
El Salvador
Ahuachapán GWAH06Aug 19996.657.0169.92.70n.d.5.6n.d. n.d.231 
Ahuachapán GWAH16AAug 19996.186.7718.2n.d.47.8n.d.n.d.253 
Ahuachapán GWAH17Aug 19996.897.0043.80.48−0.314.9n.d.297n.d. 
Ahuachapán GWAH20Aug 19996.806.9338.10.02n.d.9.1n.d.295842 
Ahuachapán GWAH21Aug 19996.977.0857.71.590.3114.5n.d.292257 
Ahuachapán GWAH22Aug 19996.806.8681.91.85n.d.4.6n.d.n.d.764 
Ahuachapán GWAH23Aug 19997.047.07196.02.13n.d.2.4n.d.n.d.571 
Ahuachapán GWAH4BAug 19996.866.90127.52.240.607.6n.d.296802 
Berlín GWTR02Aug 19995.445.5982.54.68n.d.8.0n.d.n.d.1069 
Berlín GWTR4BAug 19995.515.8080.56.66n.d.12.3n.d.n.d.257 
Berlín GWTR4CAug 19995.275.5477.75.54n.d.20.3n.d.302564 
Berlín GWTR09Aug 19994.485.7173.220.68n.d.4.7n.d.n.d.721 
San Vicente, HSSV01Aug 19996.276.3496.02.971.943.1n.d.n.d.n.d. 
San Vicente FCSV02Aug 19996.416.48119.23.01n.d.9.9n.d.299n.d. 
Chinameca FCLV01Aug 19992.613.4741.64.07n.d.6.1n.d.n.d.n.d. 
Chinameca FCIF01Aug 19995.335.9544.33.38−3.885.4n.d.295n.d. 
 
Nicaragua
Momotombo GWMT20Feb 19986.606.71227.410.420.57101.5n.d.283n.d. 
Momotombo GWMT27Feb 19986.726.75552.17.47n.d.62.8n.d.298810 
Momotombo GWMT27Mar 19996.586.80157.517.133.70149.8−24.2299730 
Momotombo GWMT35Feb 19986.736.75629.49.28n.d.104.2n.d.293590 
Momotombo GWMT35Mar 19996.736.76629.410.67n.d.104.2n.d.302567 
Momotombo GWMT36Aug 1997n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.85 
Momotombo SATMT36-43Mar 19996.946.96866.911.364.43347.6−23.9309200 
Momotombo GWMT38Feb 19986.676.74280.39.74n.d.62.8n.d.292630 
Momotombo GWMT38AMar 19996.627.05154.016.74n.d.119.8n.d.296n.d. 
Momotombo GWMT40Feb 19986.926.99338.29.892.8261.9−24.6300540 
Momotombo GWMT40BMar 19996.987.04314.411.54n.d.108.0n.d.300750 
Momotombo GWMT42Feb 19986.766.79522.88.83n.d.113.0n.d.308441 
Momotombo GWMT42Mar 19996.666.78232.211.57n.d.138.8n.d.298523 
Momotombo GWMT43Mar 19996.716.72848.69.883.28372.6−23.5310n.d. 
El Carol, HSc1026May 19977.407.69175.1912.11n.d.2.0n.d.297n.d. 
Najo Norte, FCc1025May 19976.867.04116.826.50n.d.18.0n.d.295n.d. 
Najo Norte, FCc1024May 19976.556.79128.165.17n.d.35.0n.d.296n.d. 
Najo Sur, HSc1023May 19976.496.79152.526.46n.d.26.8n.d.305n.d. 
Mecatepe, HSc2May 20007.357.50741.59n.d.6.3n.d.294n.d. 
Casita, FCc4Apr 20007.317.371924.00n.d.4.8n.d.295n.d. 
Mombacho, FCc3May 20007.707.721610.62n.d.37.9n.d.294n.d. 
 
Costa Rica
Cañas HSCAS02Feb 19984.014.3462.17.24n.d.13.2n.d.299n.d. 
Miravalles GWPGM01Feb 19986.376.5272.62.90n.d.13.2n.d.297500 
Miravalles GWPGM02Aug 19996.507.1556.35.23n.d.78.4n.d.n.d.n.d. 
Miravalles GWPGM03Mar 19986.136.6294.47.86n.d.19.3n.d.2941760 
Miravalles GWPGM03Feb 1998n.d.n.d.94.4n.d.0.4219.3−29.0n.d.n.d. 
Miravalles GWPGM03Aug 19996.216.6564.34.88n.d.24.5n.d.3001100 
Miravalles GWPGM05Feb 19986.136.2374.92.18−0.4615.3−24.1301690 
Miravalles GWPGM10Feb 19986.516.6483.12.92n.d.16.2n.d.300580 
Miravalles GWPGM11Feb 19986.836.8684.60.970.578.1−27.4306677 
Miravalles GWPGM12Feb 19986.326.4983.23.10n.d.22.7n.d.2941180 
Miravalles GWPGM20Feb 19985.626.6785.216.78n.d.92.0n.d.296965 
Miravalles GWPGM21Feb 19986.736.8186.31.100.128.2−29.9300950 
Miravalles GWPGM29Feb 19986.666.68102.90.740.895.0−22.8307264 
Miravalles GWPGM31Feb 19986.066.2472.52.27n.d.11.3n.d.301580 
Miravalles GWPGM45Feb 19985.656.6662.011.74n.d.80.7−16.6n.d.1050 
Miravalles GWPGM46Feb 19995.936.5553.05.23n.d.41.3n.d.n.d.1030 
Poás Volcano CL,FCVPOCLMar 19997.087.1547.10.70n.d.6.9n.d.2952620 

3.2. Isotopic Composition of C and N

[17] Were subduction the major source of nitrogen and argon, then the stable isotope data from the geothermal fields should form a mixing trend between mantle nitrogen and the residual nitrogen of high thermal maturity (Figure 8). Most of the samples with excess nitrogen plot between air and a nitrogen source of moderate to high thermal maturity. Another group, which shows strong atmospheric overprinting, plots between air-saturated water and air, and includes most of the samples from El Salvador and some from Costa Rica. Were the samples displaying high thermal maturity, including those from Momotombo in Nicaragua, representative of a subducted component, they should trend toward the MORB end-member rather than toward air. It is interesting to note that only one sample can be interpreted as trending in the direction of a low thermal maturity source, none trend in the direction of even moderate thermal maturity, and none of the samples with a predominantly atmospheric signature trend toward high thermal maturity as would be expected if air-saturated groundwater were to receive significant contributions of magmatic nitrogen.

image

Figure 8. Comparison of the isotopic composition of nitrogen in Central America with other systems. Ratios of N2/Ar have been inverted to provide linear mixing trends relative to δ15N and to emphasize trends toward the postmature end-member. Shaded regions include the California Great Valley [Jenden et al., 1988], the Alaskan forearc [Motyka et al., 1989], and the Yinggehai Basin, China [Zhu et al., 2000]. The postmature end-member is from the study of Zhu et al. [2000] and the MORB end-member from the study of Sano et al. [2001].

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[18] Values of δ13C for methane in the geothermal samples of Miravalles and Momotombo (Figure 9) fall into the range typical for thermogenic methane found in island arc systems, and in basin deposits that derive their methane from thermocatalytic reactions of organic matter. The δ13C values for carbon in the methane and the carbon dioxide suggest a nonequilibrated system where methane has been added from other sources. The equilibration temperatures between methane and carbon dioxide [Richet et al., 1977; Horita, 2001] are greater than the observed reservoir temperatures in all of the geothermal fields. The Miravalles geothermal wells, with one exception, have calculated carbon geotemperatures that are generally 25–50°C greater than the actual reservoir temperatures. The Momotombo field has carbon geotemperatures that are between 60°C and 150°C greater than the reservoir temperatures, due to the addition of isotopically heavy carbon from methane derived in the shallow crust.

image

Figure 9. Stable isotopic composition of carbon in methane. Ranges of δ13C and CH4/3He for different systems from the studies of Schoell [1988], Welhan [1988], Poreda et al. [1988], and Whiticar et al. [1986].

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[19] Ratios of CH4/3He for many of the wells in the Momotombo system are higher than the rest of the Central American geothermal systems, suggesting the addition of crustal methane to the magmatic volatile component. There is no evidence that any of the methane is derived through biological processes in the shallow crust, since the δ13C values for the methane are much heavier than those expected for biogenically derived methane [Whiticar et al., 1986].

3.3. Iodine and 129I Results

[20] Because iodine concentrations and 129I data in Central America are the focus of a separate paper [Snyder and Fehn, 2002], the presentation of these results here is limited to the aspects relevant for the N2 and CH4 systematics. Iodine concentrations in geothermal fluids along the Central American volcanic arc are variable, reflecting influences such as subsurface boiling of geothermal waters, water–rock interactions, and adsorption [Snyder and Fehn, 2002; Snyder et al., 2002]. While it is difficult to quantitatively relate iodine concentrations to subduction recycling, 129I/I ratios are largely preserved, due to the fact that the mass difference between 129I and 127I is too small to produce observable isotopic fractionation, and the surficial processes within geothermal reservoirs are too short to produce differences in isotopic ratios on timescales relevant for this system. Contributions from anthropogenic water, on the other hand, can potentially increase the 129I/I ratios dramatically, since Central American meteoric 129I/I values are currently around 260,000 × 10−15 [Fehn and Snyder, 2000], whereas preanthropogenic meteoric ratios were only 1500 × 10−15 [Moran et al., 1998]. A plot of the changes in 129I/I along the arc (Figure 10) reveals that all of the 129I/I ratios in the geothermal fluids are below anthropogenic values, and most are below preanthropogenic meteoric values. The expected magmatic 129I/I ratio, based on the subduction of a 25 Ma sediment column, is between 500 × 10−15 and 900 × 10−15, depending on whether iodine is derived from the oldest sediments or from the entire sediment column undergoing subduction [Snyder and Fehn, 2002]. The iodine ages corresponding to the observed 129I/I ratios (Table 1) are calculated from the decay of 129I (t1/2 = 15.7 Ma) using the input ratio of 1500 × 10−15 [Moran et al., 1998]. These ages are minimum values, since no correction is made for dilution by recent waters or from addition of 129I due to in situ production [Fabryka-Martin et al., 1989]. The majority of isotopic ratios in all of the geothermal sites are within a range that is compatible with derivation from subducted sediments (Figure 10). Significantly, however, all of the geothermal fields also have several wells where the 129I ages are much older than can be explained by mobilization from subducted sediments. In the case of Momotombo, fluids are as old as 65 Ma, while in other geothermal fields they are at least 40 Ma. These old iodine ages suggest a crustal, rather than subducted origin.

image

Figure 10. Variations in 129I along the arc. Iodine ages (right axis) based on initial 129I/I ratio of 1500 × 10−15 [Moran et al., 1998]. Range of 129I/I ratios for subducted sediments from the study of Snyder and Fehn [2002].

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4. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Field Sampling and Analytical Techniques
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

4.1. Excess Nitrogen and Helium in Island Arc Settings

[21] Results for the Central American geothermal fields need to be put into the framework of results obtained for other geologic situations. Figure 11 shows the relationship between N2/Ar ratios and CH4/3He ratios in the context of other island arc systems and of forearc regions where thermogenic methane and nitrogen are present. The bulk of the geothermal samples in El Salvador and Costa Rica show values of N2/Ar and CH4/3He, which are typical for a variety of island arc systems, including Ontake, Japan [Sano et al., 1998a, 1998b], the Philippines [Giggenbach and Poreda, 1993], and Japanese hot springs [Ono et al., 1993]. The island arc samples have slightly higher ratios than geothermal wells and hot springs in Iceland [Poreda et al, 1992], suggesting that subduction recycling does not enrich the volatile budget with molecular nitrogen relative to argon, or with methane relative to 3He by more than an order of magnitude. The Momotombo field, on the other hand, shows mixing between crustal and magmatic components, and bears some similarity with nitrogen-rich gases of the Alaskan forearc [Motyka et al., 1989] as well as thermogenic methane and nitrogen from the gas fields of the Sacramento Basin [Jenden et al., 1988]. Methane-rich samples from Indonesia [Poorter et al., 1991; Varekamp et al., 1992] also plot in the same region as the Momotombo samples. Samples show much more variability in N2/Ar than in CH4/3He due to the fact that absolute N2/Ar ratios are sensitive to different degrees of contamination from air-saturated water. Although the Momotombo samples show high methane and excess nitrogen, they all have CO2, CO2/3He, and 3He/4He ratios similar to those of the rest of the Central American volcanic arc, and to island arc systems in general.

image

Figure 11. Comparison of Central American CH4/3He and N2/Ar ratios with a range of typical values for island arc systems [Sano et al., 1998a, 1998b; Giggenbach and Poreda, 1993; Ono et al., 1993], Alaskan forearc [Motyka et al., 1989], and the California Great Valley [Jenden et al, 1988].

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[22] Typical N2excess/3He values for a wide variety of island arc settings are ∼1 × 108 [Sano et al., 2001; Jenden et al., 1988; Giggenbach and Poreda, 1993; Fischer et al., 1998; Poorter et al., 1991; Giggenbach et al., 1993] while N2excess/4He values are ∼1 × 103. In certain regions, such as northern Central America [Fischer et al., 2002], these ratios may be more than an order of magnitude greater than the typical arc values. In a recent global compilation of mean arc values, Hilton et al. [2002] determined the N2excess/3He ratio to be 2.1 × 108 and the N2excess/4He ratio to be 1.6 × 103. TheN2/3He ratios are 2 orders of magnitude greater than present estimates for the upper mantle, which range from 8.9 × 105 [Sano et al., 2001] to 3 × 106 [Zhang and Zindler, 1993]. This indicates that nitrogen is effectively extracted from the subducted slab and sediments in subarc conditions, and is not returned to the deep mantle. Sano et al. [2001] suggest that the difference between arc and MORB is at least in part due to the greater solubility of helium than of nitrogen in arc melts; however, if this were the case, elemental fractionation between nitrogen and helium should produce N2/3He ratios which are initially 5 times greater than the melt reservoir and decrease significantly during the course of magmatic degassing, resulting in large variations in N2excess/4He ratios between systems in different stages of degassing. Instead, N2excess/4He ratios from a diverse selection of volcanic localities agree within an order of magnitude.

[23] Given this background, the ratios of N2excess/4He, as well as the ratios of N2excess/3He for most of the fumaroles, hot springs and geothermal wells in Nicaragua, and for a few wells from Miravalles are among the highest recorded for active volcanic systems in the world (Figure 12). Although thermogenic sedimentary deposits such as the Sacramento Basin [Jenden et al., 1988] have similar enrichments in excess nitrogen, the 3He/4He ratios there are often strongly influenced by crustal 4He production from U and Th. The Central American fields preserve an essentially magmatic helium signature even though the nitrogen/helium ratios are much higher than both the mantle and typical island arc signatures. The only field in Central America, which shows a trend toward lower 3He/4He ratios with increasing nitrogen, is the Berlín field in El Salvador. White Island in New Zealand [Giggenbach et al., 1993] presents N2/4He ratios which are shifted somewhat in the direction of the Momotombo geothermal wells, although it also has lower helium ratios. Thermal waters associated with sedimentary rock formations such as those of central Italy [Minissale et al, 1997a, 1997b] generally fall between the Sacramento Basin and the Alaskan forearc region, although showing generally lower 3He/4He ratios. Areas of relatively high heat flow in sediment-hosted magmatic systems such as Cerro Prieto, Mexico [Welhan, 1981], and the Taranaki Basin, New Zealand [Hulston et al., 2001] mostly plot between the Alaskan forearc and arc volcanoes. In general, all of the Central American samples show typical arc-related helium signatures and have only sparing amounts of excess nitrogen, in good agreement with observations from other island arcs. The majority of the samples from Momotombo and a few from Miravalles do not follow these trends, indicating that, while the magmatic helium flux is large enough to effectively mask the crustal helium signature, the contributions of nitrogen from the crust may overprint the magmatic nitrogen signature.

image

Figure 12. Excess nitrogen versus helium ratios. Shaded fields represent high-nitrogen sources in the Alaskan forearc [Motyka et al., 1989; Poreda et al., 1988], the Sacramento Basin, California [Jenden et al., 1988], and White Island, New Zealand [Giggenbach et al., 1993]. The typical range for arc volcanics includes those of Giggenbach and Poreda [1993], Fischer et al. [1998], Poorter et al. [1991], and Sano et al. [2001].

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4.2. Relationship Between 129I, Nitrogen, and Methane

[24] While the excess nitrogen in Momotombo and the rest of Nicaragua is probably of crustal origin, the gas data alone cannot exclude the possibility that it is derived from anomalously high subduction recycling. If subduction recycling were the only source, however, it would have to favor nitrogen over the other volatiles, since noble gas, and stable isotope relations suggest that recycling of volatiles such as CO2 in Nicaragua is actually less than in other portions of the arc [Snyder et al., 2001]. The presence of a significant crustal component in Nicaragua is confirmed by 129I/I ratios and corresponding minimum iodine ages of 40–65 Ma. The crustal and subducted nitrogen components may be distinguished in a plot of excess nitrogen versus 129I/I ratios (Figure 13). Considerable variability exists in the amount of excess nitrogen in the crustal component, although this is not unexpected given the variations in organic content existing among sedimentary rocks. Assumed ranges of excess nitrogen for samples of low organic content and high organic content are shown for the purpose of illustration. Several of the Ahuachapán samples are typical of the low-organic end-member, while the Momotombo samples trend toward the high-organic end-member. Variability within the Momotombo samples is probably related to mixing of the crustal end-member with variable amounts of recycled island arc sediments and preanthropogenic meteoric water. Most of the Miravalles samples fall closely to the recycled subducted component, while several have elevated iodine ratios due to addition of anthropogenic (<50-year-old) water. The same trends and end-members are also visible in a plot of 129I/I versus CH4/3He (Figure 14), where the contribution of methane from crustal sources is accompanied by a decrease in 129I/I ratios. The Berlín samples generally plot across a wide range of 129I/I ratios, with very low CH4/3He ratios, either due to dilution, or possibly fissiogenic input of 129I.

image

Figure 13. Nitrogen versus iodine isotopic ratios. Variations in excess nitrogen in the crustal end-member are due to variations in organic content of the deposited sediments. Other variations are due primarily to different inputs from recycled sediments (magmatic) and dilution by preanthropogenic groundwaters.

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image

Figure 14. Methane normalized to 3He versus iodine isotopic ratios. Variations in methane are likely due to contributions from sediments containing variable amount of organic matter. Variations in 129I/I ratios are to some degree controlled by contributions from preanthropogenic meteoric water.

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4.3. Implications for the Global Fluxes Derived from Subduction Recycling

4.3.1. Nitrogen Flux

[25] 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. [2001] 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. [2001] 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.

[26] 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. [2001] 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. [2002] 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.

[27] 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.

image

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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[28] 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:

  • equation image

Similarly, the subducted component of the arc 4He flux may then be expressed as:

  • equation image

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):

  • equation image

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. [2002]4He,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.

[29] 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:

  • equation image

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. [2001] (6.4 × 108 mol N2/yr) and that of Hilton et al. [2002] (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.

image

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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[30] It is worth mentioning that Hilton et al. [2002] 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.

[31] Our determination of the erupted N2 flux in arc systems diverges from previously cited values. Sano et al. [2001] 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. [2002] 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

[32] 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).

[33] 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,cPercent Island Arc Sediment DevolatilizationPercent Arc Contribution from Subducted Sediment
CO2d89 × 10104 × 1010 to 37 × 1010150 × 10104.2–46%86–98%
N2e92 × 10827 × 108 to 54 × 10822 × 108 to 28 × 10829–58%∼100%
4Hee5.2 × 10427 × 1048.7 × 107∼100%20%
3Hee0.00732.5976∼100%0.3%

[34] 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 [1996], 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. [2001].

image

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

[35] 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. [2002] and Hilton et al. [2002] 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. [2002] 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).

[36] 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.

5. Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Field Sampling and Analytical Techniques
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[37] The data presented for the four Central American geothermal fields allow the following observations and conclusions:

  1. Nitrogen, helium, and carbon are subducted along the Middle American Trench, and are released beneath the Central American volcanic arc, helium more readily than nitrogen and carbon.
  2. This model assumes that 4He and N2 is derived from the subducted sediments, and not from the basaltic slab. The consistency between the Central American gas ratios and those from diverse island arc systems such as New Zealand, the Philippines, and Indonesia suggests that this process is widespread at convergent plate boundaries.
  3. The global flux of nitrogen from island arc systems is between 2.7 × 109 and 5.4 × 109 mol/yr, which is 29–58% of the subducted flux, and is comparable to the mid-ocean ridge flux [Marty and Zimmermann, 1999]. While this flux is high compared to other volatiles, still a substantial amount of the total subducted sediment flux is lost either behind the volcanic front or, more likely, in the forearc.
  4. Our estimates are consistent across the three gas systems: He, CO2, and N2. Although the relative subducted and upper mantle contributions vary greatly between these three gases, the volcanic flux can be tied to the composition of subducted material and varying efficiencies of devolatilization in the forearc and subarc regions.
  5. In the Nicaraguan Depression, the association between high excess nitrogen and methane suggests that both gases are derived from thermogenic reactions with organic matter in the crust, enhanced by volcanic activity. This is the first study which combines the 129I system with the gas systems, and while the higher 129I/I ratios indicate local contributions from meteoric water, the lower ratios found throughout the arc provide definitive evidence that the iodine released in Central America cannot be entirely magmatic. The iodine isotopic signature indicates a source of the organic material, which is at least 65 Ma and thus considerably older than presently subducted sediments (∼25 Ma). The high N2/3He ratios of Momotombo stand out from other island arc systems, and are indicative of a special case, where migration of the Nicaraguan segment of the volcanic arc during Eocene to Oligocene times has resulted in the release of volatiles from older crustal materials. This nitrogen release is significant, and the present release of crustal nitrogen along this segment of the Central American volcanic arc may be as large as the global release of magmatic nitrogen during arc volcanism. Because basins associated with continental margins are areas of deposition of organic matter, the thermogenic release of nitrogen and methane during reorganization of island arc systems may have a significant impact on the global flux of volcanic nitrogen.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Field Sampling and Analytical Techniques
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[38] Local assistance was graciously provided by the Central American institutions in charge of overseeing and carrying out geothermal operations, namely the Instituto Costarricense de Electricidad (ICE), the Empresa Nicaragüense de Electricidad (ENEL), and Geotérmica Salvadoreña (GESAL). Particular thanks goes to Guillermo Alvarado, Eddy Sánchez, Karla Miranda, Ernesto Martínez, José Tenorio Mejía, and Carlos Pullinger, without whose advice and logistical support we would not have been able to carry out our research. Stable isotope determinations were carried out by Robert Drimmie and the Environmental Isotope Laboratory, University of Waterloo, and 129I/I determinations by David Elmore and Pankaj Sharma, PRIMELab, Purdue University. G.S. is grateful for student grants from Sigma Xi and the Geological Society of America. This research was supported by NSF grant EAR9902919.

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  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References
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Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Field Sampling and Analytical Techniques
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information
FilenameFormatSizeDescription
ggge279-sup-0001-t01.txtplain text document10KTab-delimited Table 1.
ggge279-sup-0002-t02.txtplain text document1KTab-delimited Table 2.

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