G 3. AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

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1 Geosystems G 3 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Nitrogen isotopes in thermal fluids of a forearc region (Jalisco Block, Mexico): Evidence for heavy nitrogen from continental crust S. Inguaggiato Istituto Nazionale di Geofisica e Vulcanologia Seizione di Palermo, Via Ugo La Malfa 153, Palermo 90146, Italy (s.inguaggiato@pa.ingv.it) Y. Taran Instituto de Geofisica, UNAM, Coyoacan, Mexico, D.F F. Grassa, G. Capasso, and R. Favara Istituto Nazionale di Geofisica e Vulcanologia Seizione di Palermo, Via Ugo La Malfa 153, Palermo 90146, Italy N. Varley Facultad de Ciencias, Universidad de Colima, Colima, Mexico E. Faber Federal Institute for Geosciences and Natural Resources, Stilleweg 2, Hannover, Germany Article Volume 5, Number 12 8 December 2004 Q12003, doi: /2004gc ISSN: [1] The Jalisco Block (JB) is a geologically and tectonically complex part of northwestern Mexico characterized by active subduction-type volcanism, rifting, and old stable structures. Thermal springs and groups of springs are widely distributed over JB. Bubbling gas from seven thermal springs located within different tectonic environments of the JB was analyzed for He, 20 Ne, and N 2 concentrations and d 15 N ratios. All gases are N 2 -dominant (>84%) with the exception of one sample (Rió Purificación), which has a significant CH 4 content (about 50%). All collected gas samples are relatively high in He, up to 1500 ppm vol and with 3 He/ 4 He values ranging from 0.6 to 4.5 Ra. All measured nitrogen isotope ratios are heavier than air with d 15 N values ranging from 0.5 to 5.0%. The relative N 2 excess with respect to air-saturated water computed on the basis of N 2 and 20 Ne contents indicates the contribution of a nonatmospheric N 2 source. All the samples show a good correlation between d 15 N and the relative excess of N 2 with d 15 N +5.3% for the maximum N 2 excess of 100%. Due to a presumed lack of seafloor sediment involved in the subduction process, such a d 15 N positive value seems to reflect the addition to the fluids of a heavy nitrogen originating from metamorphism processes of rocks occurring within the overlying continental crust. Components: 3999 words, 5 figures, 1 table. Keywords: bubbling gases; forearc region; Jalisco Block-Mexico; nitrogen isotopes; subduction-related volcanism. Index Terms: 1040 Geochemistry: Isotopic composition/chemistry; 3040 Marine Geology and : Plate tectonics (8150, 8155, 8157, 8158); 8424 Volcanology: Hydrothermal systems (8135). Received 31 May 2004; Revised 2 September 2004; Accepted 20 September 2004; Published 8 December Inguaggiato, S., Y. Taran, F. Grassa, G. Capasso, R. Favara, N. Varley, and E. Faber (2004), Nitrogen isotopes in thermal fluids of a forearc region (Jalisco Block, Mexico): Evidence for heavy nitrogen from continental crust, Geochem. Geophys. Geosyst., 5, Q12003, doi: /2004gc Copyright 2004 by the American Geophysical Union 1 of 9

2 1. Introduction [2] The Jalisco Block (JB) is a distinct crustal unit bounded toward the mainland by rifting and toward the Pacific Ocean by the NW section of the Middle America trench, a contact between the subducting Rivera plate and the continent. Therefore JB can be considered as a large geologically and tectonically complex forearc region. In this area there are numerous hydrothermal systems linked to active volcanism and also to tectonic discontinuities. Thermal discharges, whose temperatures range from 32 to 260 C [Taran et al., 2002b], are located in four main regions (Figure 1): along the northern margin (Tepic-Zacoalco Rift), the inner part of the Jalisco Block, the Colima Rift and along the Pacific coast [Taran et al., 2002a, 2002b]. [3] Bubbling gas from six groups of thermal springs located within the JB are enriched in molecular nitrogen (N 2 > 84% Vol.) except for one sample where nitrogen and methane show almost equal contents. All the investigated sites show N 2 / 20 Ne values higher than air (N 2 / 20 Ne AIR = ) with some samples also elevated relative to air-saturated water (N 2 / 20 Ne ASW = ) thus suggesting a nonatmospheric contribution of nitrogen (atmosphere d 15 N air = 0%). Other possible known reservoirs of nitrogen are (1) sediments from continental crust and/or subducted oceanic slab [d 15 N sediment =+7%, Sano et al., 2001] and (2) mantle-derived nitrogen including unaltered ocean crust and N 2 degassed from the upper mantle (d 15 N mantle = 5% [Marty and Humbert, 1997; Sano et al., 2001; Snyder et al., 2003]). [4] Differences in the isotopic signature among these three main potential sources of nitrogen make this volatile constituent a useful tracer in order to assess deep and shallow geochemical processes. [5] An excess in nitrogen is quite common in natural hydrocarbon gases and subduction-related hydrothermal manifestations [Matsuo et al., 1978; Jenden et al., 1988; Giggenbach, 1992; Kita et al., 1993]. At convergent plate boundaries, the main process responsible for the contribution of nonatmospheric nitrogen enriched in 15 N into geothermal systems seems to be the recycling of subducted sedimentary material (oceanic sediments) and sedimentary material from the continental crust. Sources for the isotopically light nitrogen could be organic sediments of a low degree of maturation [Prasolov et al., 1990; Zhu et al., 2000] as well as the upper mantle [Marty and Humbert, 1997]. [6] In this work, N 2 and 20 Ne contents, d 15 N values and 3 He/ 4 He ratios have been used to investigate the sources of excess nitrogen in thermal waters from the Jalisco Block. 2. Sampling and Analytical Procedures [7] Bubbling gas have been collected by means of an upside down funnel with a PET tube connected at the end. The samples have been pumped with a syringe and a three-ways manual valve into a 30 ml Pyrex flask having two gastight stopcocks. Chemical analyses of O 2, N 2, CO 2 and CH 4 were carried out by a Perkin- Elmer gas chromatograph, equipped with a Carbosieve II column, two detectors (HWD and FID), using Ar as carrier. The errors of measurements are 5% for each gas component. Sulfur compounds were not determined. [8] He and Ne contents (1s = 1.7% and 2.06%, respectively [Inguaggiato and Rizzo, 2004]) were determined by a VG Quartz quadrupole mass spectrometer, after sample purification into an ultra-high vacuum line, consisting of a liquid nitrogen trap, two SAES Getter pumps and a cryogenic charcoal trap at 40 K. [9] Nitrogen isotopes analyses were carried out by means a gas chromatograph coupled with a Finnigan Delta Plus isotope ratio mass spectrometer, as better described by Eberhard et al. [1994]. Nitrogen isotope ratios are expressed in delta per mil assuming air as reference. Error of measurements (1s) is in the range %. 3. Gas Geochemistry [10] Chemical and isotopic compositions of bubbling gases from seven thermal discharges in Jalisco Block (JB) are reported in Table 1. Collected samples are enriched in nitrogen (up to 99.2%) with high helium contents (>195 ppm vol) and very low oxygen concentrations (<1.6% vol.). N 2 /He ratios, in the range , are typical for gases from volcanic arcs [Giggenbach, 1996; Fischer et al., 1998; Sano et al., 2001]. Volatile carbon compounds are generally minor components: the carbon dioxide contents in all the samples was below 2.5% vol., with the exception of Agua Caliente spring, which had the highest concentration (about 15% vol.); the CH 4 contents varied from 2 ppm to 9% vol. However, methane is the major gas only at Rió 2of9

3 Geosystems G 3 inguaggiato et al.: nitrogen isotopes in thermal fluids /2004GC Figure 1. Map showing the location of collected thermal springs together with the main volcanic structure (Volcán de Colima) and the geodynamic context of the Jalisco Block. Purificación spring (about 50% vol.) with similar amount of nitrogen. All the collected geothermal gases showed N 2 / 20 Ne ratios varying from to , higher than air (N 2 / 20 Ne AIR = ) or air-saturated water (N 2 / 20 Ne ASW = ) and nitrogen isotope ratios heavier than air (d 15 N from 0.5 to 5.0%). [11] OntheN 2 -CO 2 -CH 4 ternarydiagram(figure2a) the bubbling gas have been plotted together with the gas sample collected from a high temperature fumarole (820 C) at Volcán de Colima [Taran et al., 2002b; Varley and Taran, 2003] as representative point of the main volcanic system of Jalisco Block. The arrangement of the data points suggests the 3of9

4 Table 1. Chemical and Isotopic Composition of Bubbling Gases a Sample T vent He O 2 N 2 CH 4 CO 2 20 Ne d 15 N N 2 excess A S M 15 N c 1 Rió Purificación Juchitlan Guamuchil Jamurca San Juan de Arriba Las Tunas Agua Caliente Volcán de Colima b c d d 72.1 d 9.6 d 5.6 Air ASW 4.6E E Sediments 7.0 d Mantle 5.0 d a Temperature (T vent ) is expressed in C; chemical compositions are expressed in % vol, with the exception of helium and 20 Ne contents, expressed in ppm vol. Composition of ASW is expressed in cc/l STP. The nitrogen isotope composition (d 15 N) is reported with respect to air (d 15 N = 0). Based on the observed N2/20Ne ratios, the mixing proportions among air (A), crustal sediments (S), and upper mantle (M) components are also reported. d 15 N c stands for nitrogen isotope composition corrected for air contamination and computed on the basis of relative S and M percentages. ASW, Air-Saturated Water. b Varley and Taran [2003]. c Taran et al. [2002b] d Sano et al. [2001]. occurrence of at least three geochemical processes that act jointly or separately. Starting from a CO 2 - dominant end-member (Colima fumarole), all the collected gases are progressively enriched in Nitrogen due to (1) the addition of molecular nitrogen as a consequence of a mixing with air and/or with a nonatmospheric N-rich source; (2) a removal of CO 2 due to a selective dissolution during interaction with local groundwater increasing the residual gas in less soluble components such as N 2 [Taran et al., 2002b]. Moreover, Rió Purificación, S.J. de Arriba and Jamurca samples lie along the N 2 -CH 4 alignment probably as a result of (3) a mixing process between modified magmatic gases (N 2 -enriched gases) and hydrocarbons (CH 4 -rich component) originating from shallow reservoirs. [12] The N 2-20 Ne-He triangle diagram (Figure 2b) highlights the enrichment in He with respect to atmospheric component (air and air-saturated water (ASW)) linked to a marked contribution of mantlederived and crustal helium [Taran et al., 2002b]. On the same diagram, four bubbling gas sample (Las Tunas, Rió Purificación, S.J. de Arriba and Jamurca) as well as the fumarolic gas sample plot to the right of the mixing line between ASW and the He-vertex suggesting for these gases that the N 2 excess is related to the contribution of a nonatmospheric component. [13] A good linear correlation (R 2 = 0.98) has been found between 20 Ne content and d 15 N values with an increasing of nitrogen isotope composition as the 20 Ne content decreases (Figure 3). This is a reflection of different degrees of atmospheric contamination, since the 20 Ne content in crustal and mantle-derived fluids can be assumed to be negligible [Allègre et al., 1986] with respect to 20 Ne concentration in air (16.4 ppm vol). On the basis of the observed N 2 and 20 Ne contents, we have computed the relative excess in nitrogen with respect to atmospheric contamination as follows: N 2excess ð% Þ ¼ N 2 observed N 2atmospheric 100 ð1þ N 2observed N 2atmospheric ¼ 20 Ne observed N 2 : ð2þ 20 Ne ATM The atmospheric-derived nitrogen component (N 2atmospheric ) can be expressed in terms of N 2 / 20 Ne ratios in air or in air-saturated water (ASW). In our calculations we have used a [N 2 / 20 Ne] ATM value of averaged between the same ratio in air and in ASW due to a combined effect of nitrogen contamination from both sources. The average value is required because some gas samples show N 2 / 20 Ne ratios between air and ASW (Figure 2b) and the use of the ASW as an end-member would result in an over-estimation of the atmospheric contamination. The values obtained plotted versus nitrogen isotope composition highlight the contribution of nitrogen from a reservoir enriched in heavy isotopes (Figure 4). The extrapolated limit d 15 N representing a maximum N 2 excess of 100% is close +5.3%, that 4of9

5 and the mantle wedge; and (3) sediments including subducted oceanic deposits and continental crust. Because each of these reservoirs has distinct d 15 N values, the nitrogen isotope composition can be used as a tracer of gas. Atmospheric nitrogen has been conventionally taken as international reference (d 15 N = 0%) being at least 65% of total molecular N in the Earth reservoirs (data from Snyder et al. [2003]). The upper mantle, investigated through analyses on mid-ocean ridge basalts (MORB) and diamonds [Marty and Humbert, 1997; Cartigny et al., 1997] has revealed the presence of a light nitrogen component with typical d 15 N values in the range 3% to 8%. The d 15 N values in sediments are generally enriched relative to the atmosphere. Metamorphic rocks show d 15 N values from +2% to +7.7% [Mingram and Brauer, 2001] while Sano at al. [2001] used in their calculations a d 15 N=+7% as representative value for nitrogen isotope composition of oceanic seafloor sediments. The isotope signature of nitrogen in N 2 -rich natural gas accumulations varies in a wider range from strongly depleted ( 19%) to extremely enriched (+18%) values depending on the stage of organic matter maturation [Zhu et al., 2000]. Figure 2. (a) CO 2 -N 2 -CH 4 diagram: The relative chemical compositions of collected gases show two distinct processes: (1) the addition of nonatmospheric molecular nitrogen and/or selective dissolution of pristine magmatic CO 2 -rich gas and (2) mixing with shallow hydrocarbon gases. (b) He-Ne-N 2 diagram: All the samples show the excess of helium and nitrogen with respect to the atmospheric component (AIR and ASW). can be assumed as the local nonatmospheric nitrogen component. 4. Origin of Excess Nitrogen [14] Three main reservoirs have been considered as potential sources of nitrogen in the geothermal gases of the Jalisco Block: (1) atmosphere; (2) mantle both unaltered subducting oceanic crust [15] As previously described, on the basis of the maximum N 2 excess value of 100%, a heavy nitrogen component with a d 15 N of about +5.3% has been inferred as the local nonatmospheric nitrogen. Previous works [Giggenbach, 1992; Fischer et al., 1998, 2002; Snyder et al., 2003] have demonstrated that the occurrence of d 15 N values greater than air has to be attributed to the recycling of subducted seafloor sediments at convergent plate margins where accretionary prisms are absent. The discrepancy from a typical d 15 N value of +7% [Sano et al., 2001] might indicate that the prevalent source of Nitrogen in the gases from the JB is the subducting oceanic sediments and that only a much lower contribution of nitrogen originates from a lighter component such as from the degassing mantle wedge. [16] The combined N 2 / 20 Ne ratios and the d 15 N values (Figure 5) appear to indicate the nitrogen in geothermal gases collected within the JB as a result of mixing between air/air-saturated water (A), and crustal sediments (S). Only little amounts of mantle (M) component are also present. Hence, according to Sano et al. [2001], we have evaluated the mixing proportions of each source of nitrogen using the N 2 / 20 Ne and d 15 N values. The results obtained using atmospheric (d 15 N = 0% and 5of9

6 Figure 3. The d 15 N versus 20 Ne content. The good inverse correlation reflects different degrees of air contamination. N 2 / 20 Ne = ), sedimentary (d 15 N=+7% and N 2 / 20 Ne = ) and upper mantle (d 15 N = 5% and N 2 / 20 Ne = ) endmembers are reported in Table 1. [17] In all the hydrothermal gases the atmospheric contribution is quite variable (4.9 90%). It was observed that the sedimentary component (up to 79.4%) is always higher than that of the upper mantle (up to 16.3%). At Juchitlan, Guamuchil, S.J. de Arriba and Agua Caliente springs the nitrogen supply is dominated by the atmospheric component with a significant contribution (up to 27.7%) derived from sediments. While at Jamurca, Las Tunas and Rió Purificación the contribution of a sediment-derived heavy-isotopic nitrogen component ( %) dominates. [18] Following Fischer et al. [2002] the nitrogen isotopic composition was corrected (d 15 N c ) for atmospheric contamination as follows: d 15 N c ¼ d 15 N sed f þ d 15 N man ð1 f Þ; ð3þ where d 15 N sed =+7%, d 15 N man = 5% and f is the relative fraction of sediment-derived nitrogen in a binary sediment-mantle mixture. The obtained values fit into a narrow range from 4.5 to 5.5%. Figure 4. The d 15 N versus relative N 2 excess computed with respect to the atmospheric component highlights the contribution of nitrogen from a reservoir enriched in 15 N. The extrapolated limit d 15 N representing a maximum N 2 excess of 100% is +5.3%. 6of9

7 Figure 5. Plot of d 15 N versus N 2 / 20 Ne. Lines represent theoretical mixing curves among three end-members: atmospheric (A), sediments (S), and mantle (M). The atmospheric N 2 / 20 Ne ratio of is the average of the N 2 / 20 Ne ratio in air and in air-saturated water. The N 2 / 20 Ne ratio of in sediments has been calculated by 3 He/ 20 Ne ratio of [Allègre et al., 1986] and a N 2 / 3 He ratio of [Sano et al., 2001]. The N 2 / 20 Ne of in the mantle has been calculated by 3 He/ 20 Ne ratio of 1.58 [Allègre et al., 1986] and a N 2 / 3 He ratio of [Sano et al., 2001]. The nitrogen in geothermal gases collected within the JB is the result of mixing between atmospheric and crustal sediment components. Only little amounts of mantle-derived nitrogen are also present. They are consistent both with the inferred local nonatmospheric component (d 15 N = +5.3%) and with an air-contamination corrected d 15 N value of +5.6% from a Volcán de Colima crater fumarole (340 C) [Sano et al., 2001]. [19] Within the JB, there are no important rivers transporting large amounts of suspended materials derived from the continental erosion that can be deposited in the trench zone [Melbourne et al., 1997; Bandy et al., 1999]. Moreover, according to Luhr [1992] and Chavez and Olsen [2002] the Rivera plate is very young and hot, being its age in the range from 10 to 15 Ma. These insights suggest that nonetheless there is little to no accretionary prism linked to the subduction of the Rivera plate only a negligible amount of sediments could be subducted with respect to other convergent plate margins [Melbourne et al., 1997]. Differing from other active margins, the recycling seafloor sediment involved in the subduction process seems to be ruled out. [20] For these reasons, we have also considered as possible 15 N-rich source, a continental crustal origin. In this case, subducted seafloor sediments do not contribute to the N budget at the JB, but heavyisotope nitrogen in the hydrothermal gases within the JB may be originated from metamorphic reactions involving sedimentary materials within the overlying continental crust. [21] Ammonium ion formed during decomposition of organic matter can be included in the lattice of some clay minerals, feldspar and micas as substitute for K + [Krooss et al., 1995; Littke et al., 1995]. Metamorphism results in the release of N-rich fluids from rocks with a progressive increase in d 15 N values as the metamorphism progresses. As reported by many authors, d 15 N values are in the range from +2.2 ± 0.6% to +7.7 ± 2.0% from lowgrade metamorphic rocks to gneiss unit, respectively [Bebout and Fogel, 1992; Zhu et al., 2000; Mingram and Brauer, 2001]. [22] Further evidence for the addition of crustalderived gases related to subduction-related volcanism is represented by the wide range of 3 He/ 4 He ratios (from 0.66 to 4.5 R/Ra) found by Taran et al. [2002b] which are intermediate between a MORB end-member (8.0 R/Ra) and a typical radiogenicderived helium component (0.01 R/Ra). [23] However, another possible origin of nitrogen with isotopic composition higher than air could be found in the altered oceanic crust subducting below 7of9

8 Jalisco Block, where metamorphic reactions would also occur. As pointed out by Bandy et al. [1999], at a depth greater than 30 km, corresponding at a distance of about 100 km from the trench zone, a transition phase from basalts to eclogite has been recognized. At the present time, the d 15 N values of such a contribution are not well known. Suitable data on Gneiss/Eclogite unit of the European Variskan Belt [Mingram and Brauer, 2001] indicate a d 15 N value of +7.7 ± 2%. 5. Concluding Remarks [24] In this preliminary study, we reported the d 15 N values from some thermal springs collected within the Jalisco Block (NW Mexico) a forearc region linked to the subduction of the Rivera plate beneath the North American plate. In this area, thermal manifestations are related both to subduction-related active volcanism and to intraplate rifting structures. Gases released from thermal discharges in the studied area are characterized by high nitrogen and helium contents, with N 2 /He ratios typical for gases from volcanic arcs [Giggenbach, 1996; Fischer et al., 1998; Sano et al., 2001]. d 15 N values ranging from 0.5 to 5.0% and N 2 / 20 Ne ratios higher than air indicate the contribution of a nonatmospheric N 2 source (N 2 excess). Because of the marked differences in the isotopic signature of the main nonatmospheric nitrogen reservoirs (mantle and sediments), the nitrogen isotope composition has been used as tracer of gas provenance. [25] The good correlation between d 15 N and the excess of N 2 computed on the basis of N 2 / 20 Ne ratios led to the estimation of a N 2 -source with d 15 N about +5.3% for the maximum N 2 excess of 100%. On the basis of our observations, we suggest that the source of nitrogen in thermal springs from the Jalisco Block is dominated by a crustal component. 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