Nitrogen-Helium-Argon and Nitrogen Isotope Relationships in Geothermal Fluids from the Central American Volcanic Arc: Mapping Subducted and Crustal Contributions to Volatile Output

Volcanic arcs are locations where elements are recycled from the subducted slab, the mantle wedge and the overlying arc crust to the atmosphere and hydrosphere. A fundamental aim of arc-related studies and the MARGINS initiative is to quantify this flux and compare it with subduction zone parameters, such as sediment compositions and subduction rates. As part of the Central American arc study, we report N₂, He, and Ar abundance relationships and N-isotope ratios for 7 volcanic centers in Guatemala to complement on-going studies in Costa Rica (see previous abstract). In Guatemala, the arc crust is thicker and older than in Costa Rica and the entire sedimentary sequence on the down-going plate is likely to contribute to the slab flux. The Guatemalan volcanic centers of Amatitlan-Pacaya, Fuego, Moyuta, Tecuamburro, Amatitlan, Zunil and San Marcos have N₂/He ratios ranging from 2200 to 8100, typical for arc-related fluids. N₂/Ar ratios (40 - 500) and the high N₂/He indicate addition of N₂ from subducted sediments or arc crust to a mantle derived component (N₂/He < 200). The high N₂/He ratios of Guatemala, are in contrast to the mantle-derived N₂/He ratios measured at Poas, Costa Rica. Nitrogen isotope ratios for the Guatemalan volcanic centers range from δ ¹⁵N = +1.0‰ for San Marcos to +5.8‰ for Fuego (δ ¹⁵N_air = 0.0‰ ), indicating a sedimentary nitrogen signature. The mantle-derived N₂/He ratio for Poas is consistent with a more mantle-like δ ¹⁵N of - 1.0‰ . In Guatemala, the highest ³He/⁴He ratios (7.6 for Pacaya and 7.3 RA for Fuego) correlate with the lowest N₂/He ratios (1500 and 2100) and high δ ¹⁵N values (+3.8‰ and +5.8‰ ). Lower ³He/⁴He ratios for Zunil (4.7 RA) and San Marcos (2.2 RA) correlate with N₂/He of 5000 and 6600, and lower δ ¹⁵N values of +2.3‰ and +1.0‰ , respectively. These N-He relationships suggest that the nitrogen at Pacaya and Fuego is primarily of subducted organic sedimentary origin, with only minor crustal contributions. Additional nitrogen is supplied by the arc crust at Zunil and San Marcos, resulting in elevated N₂/He ratios. This is in contrast to the situation in Costa Rica, where lower N₂/He ratios and lower δ ¹⁵N values suggest possible loss of sediment by underplating and where higher ³He/⁴He indicate only minor crustal contributions to the discharging volatiles.