Sources of nitrogen and methane in Central American geothermal settings:: Noble gas and 129I evidence for crustal and magmatic volatile components -: art. no. 9001

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