Sources of nitrogen and methane in Central American geothermal settings: Noble gas and ¹²⁹I evidence for crustal and magmatic volatile components

Abstract

[1] Gases and fluids from four geothermal fields of Central America were analyzed for nitrogen, methane, and helium concentrations, isotopic composition, and 129 I/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 N 2 / 3 He are only slightly higher than average arc values of 1 Â 10 8 and the volcanic flux of N 2 for the Central American systems is estimated to be between 1.6 Â 10 8 and 3.2 Â 10 8 mol/yr. Analysis of 129 I/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 N 2 flux is estimated at 2.7 Â 10 9 to 5.4 Â 10 9 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 N 2 -CO 2 -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 N 2 / 3 He gases have essentially magmatic 3 He/ 4 He 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 Â 10 8 mol N 2 /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.

Geochemistry of fluid discharges from Peteroa volcano (Argentina-Chile) in 2010–2015: Insights into compositional changes related to the fluid source region(s)

“…do not allow to clearly discriminate between crustal and magmatic sources. Such a positive isotopic signature is indeed typical of 1) N 2 produced by thermal decomposition of organic material buried in sedimentary material (Jenden et al, 1988;Fischer et al, 2002;Snyder et al, 2003;Inguaggiato et al, 2004) likely involved in the subduction process, and/or 2) low-grade metamorphism affecting NH 4 fixed in the crystal lattices of K-rich minerals (micas and K-feldspars) of crystalline rocks (e.g., Honma and Itihara, 1981;Mingram and Brauer, 2001).…”

Tracing nitrogen in volcanic and geothermal volatiles from the Nicaraguan volcanic front

“…high N 2 /He) in Guatemalan volcanic and geothermal gases show a strong hemipelagic sedimentary signature, whereas Costa Rican gases have low N 2 /He and isotopically light nitrogen, thus supporting a predominant mantle wedge input for the nitrogen Zimmer et al, 2004). Snyder et al (2003) report N 2 /Ar ratios of geothermal wells and fumaroles ranging from $100 to >800 (air = 84) for Nicaragua, and significantly lower ratios ($40 to $100) for Costa Rica. Based on correlations between N 2 /Ar and CH 4 / 3 He ratios as well as 129 I ages, these authors argue that the elevated N 2 /Ar ratios in Nicaragua gases can be attributed to a significant crustal contribution.…”

“…The efficiency of nitrogen release from crustal sediments is a temperature-dependent process (Mariner et al, 2003;Snyder et al, 2003). Snyder et al (2003) suggested that the high nitrogen content of Momotombo well gases reflects contribution from a sedimentary basin characterized by nitrogen with positive d 15 N values, mantle 3 He/ 4 He ratios, and high heat flow.…”

“…Their work focused on geothermal well fluids and concluded that most of the nitrogen discharging in Nicaragua is crustal in origin, based on high N 2 / 3 He, magmatic 3 He/ 4 He ratios, and an inferred relationship between excess nitrogen and an iodine isotopic component that is much older than the subducted sediments. Snyder et al (2003), however, report nitrogen isotope values for only one locality in Nicaragua (Momotombo geothermal wells). In this communication, we expand this database and investigate the relative abundance and isotopic composition of nitrogen in a number of fumaroles, hot springs, and geothermal wells.…”

“…Snyder et al (2003) investigated the sources of nitrogen and methane in geothermal fluids along the Central American Arc. Their work focused on geothermal well fluids and concluded that most of the nitrogen discharging in Nicaragua is crustal in origin, based on high N 2 / 3 He, magmatic 3 He/ 4 He ratios, and an inferred relationship between excess nitrogen and an iodine isotopic component that is much older than the subducted sediments.…”