Pore waters associated with gas hydrates at Blake Ridge in the Atlantic Ocean were dated by measuring their iodine-129/iodine ratios. Samples collected from sediments with ages between 1.8 and 6 million years ago consistently yield ages around 55 million years ago. These ages, together with the strong iodine enrichment observed in the pore waters, suggest that the origin of iodine is related to organic material of early Tertiary age, which probably is also the source of the methane in the gas hydrates at this location.

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[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.

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[1] Abstract: Gas data were collected from geothermal production fields, fumaroles, and hot springs in Central America in order to investigate the relation between volatile output and spatial distribution of volcanic systems. The 3 He/ 4 He ratios are 6.5 ± 0.7 R a throughout the region, indicating that helium is predominantly of mantle origin and is largely independent of variations in the characteristics of the arc. Lower ratios produced by radiogenic production within the Chortis Block are restricted to the Berlín geothermal field and the region behind the volcanic front. Ratios of CO 2 / 3 He are inversely related to the distance between the volcanic system and the trench. In the southwestern portion of the arc, where the arc-trench gap is short and the subduction angle is shallow (Miravalles, Costa Rica), decarbonation is enhanced relative to the mantle helium flux resulting in higher CO 2 / 3 He. In the northwest, where the gap is greater and the subduction angle steeper (Ahuachapán, El Salvador), decarbonation decreases relative to the helium flux. While variations in the carbon isotopic signature have traditionally been linked to the composition of the subducted sediments, the Central American data provide evidence that other factors within the convergent plate boundary such as arc-trench gap, crustal thickness, and subduction angle play an important role in controlling the flux of CO 2 from the subducting slab. The Central American Volcanic Arc gases show no apparent contribution of carbon dioxide derived from subducted organic sediments. Shallow crustal processes, including partitioning and isotopic fractionation, account for the minor deviations from direct mixing of mantle and carbonate-derived end-members. Given that the Central American arc system is not unique in terms of the composition of the subducted sediments or the volcanic output, previous interpretations of global volcanic flux in terms of carbonate and sediment output should be reconsidered. Carbon-helium relationships in Central America require that only 0.3-3.3% of the subducted carbon is released in volcanic eruptions, while the rest is presumably reintroduced into the deeper mantle. This is generally an order of magnitude lower than global averages and is limited by the availability flux of mineral-bound water and the temperature of release. The d 13 C and CO 2 / 3 He ratios suggest that even though the amount of carbon that is released from the slab and subducted sediments is relatively low in Central America, it still makes up 86-98% of the total carbon released from arc volcanics.