Volatiles of the Central American volcanic arc: Source determination through iodine-129, noble gas, and stable isotope analyses

The sources of volatiles within the Central American volcanic arc were studied, using noble gas, stable isotope, and iodine-isotopic systematics as tracers of magmatic, crustal, and meteoric end-members. Geothermal production wells were sampled at the four major geothermal fields of Central America: Ahuachapán and Berlín in El Salvador, Momotombo in Nicaragua, and Miravalles in Costa Rica. Most ratios of 129I/I lie within a range (129I/I ≈ 500 × 10−15, age ≈ 25 Ma) demonstrating derivation from subducted marine sediments. The stable isotopic composition of carbon dioxide indicates that mantle and subducted carbonate are the predominant sources of carbon in the Central American arc. Ratios of CO₂/3He, as well as δ 13C and absolute CO₂ concentration, suggest that subduction recycling of marine carbonates is more efficient in the southern portion of the Cocos Plate, which is both younger and hotter. The mantle carbon component increases to the north, as the arc-trench gap increases.; A component with 129I/I ratios below that of subducted sediments was found throughout the region. Methane concentrations are also quite high in some instances, and the 129I signature indicates an age of at least 65 Ma. The methane-rich Nicaraguan segment of the arc is unique in that N₂/Ar and N₂/He ratios are among the highest recorded for active island arc systems, while the 3He/ 4He ratios remain consistent with arc mantle signatures. This is likely the result of the trenchward migration of the Nicaraguan volcanic arc, and the incorporation of nitrogen and methane through pyrolysis of crustal organics related to the adjacent Sandino Basin.; Island arc geometry was found to have an effect on the recycling of subducted marine carbonates. The Nicaraguan portion of the Central American arc demonstrates how reorganization of an island arc system results in the release of nitrogen, methane, and other volatiles through shallow metamorphism. In this case, the 129I system provides an important tool in distinguishing between crustal and subducted components. Changes in subduction geometry, as well as reorganization and lateral migration of island arc systems may significantly impact the volcanic flux of volatiles through the course of geologic time.