Regional variations in volatile composition: Isotopic evidence for carbonate recycling in the Central American volcanic arc

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 3He/4He ratios are 6.5 ± 0.7 Ra 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 CO2/3He 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 CO2/3He. 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 CO2 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 δ13C and CO2/3He 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.