Stable isotopes evidence of recycled subduction fluids in the hydrothermal/volcanic activity across Nicaragua and Costa Rica

Abstract The Central America volcanic front provides a unique opportunity to study hydrothermal inputs and their interaction and mixing with modern meteoric waters. The objectives of this study were to: a) characterize the isotopic composition (δ 18 O, δ 2 H, d -excess, and lc-excess) of hydrothermal/volcanic systems, b) analyze the influence of kinetic fractionation and meteoric water inputs in the isotopic composition of hydrothermal waters, and c) estimate the ‘andesitic water’ contribution (recycled subduction fluids) within the volcanic front of Nicaragua and Costa Rica. Hydrothermal evaporation lines are described as: δ 2 H = 4.7·δ 18 O − 13.0 (Costa Rica) and δ 2 H = 2.7·δ 18 O − 31.6 (Nicaragua). These regressions are significantly ( p 2 H = 7.6·δ 18 O + 7.4 (Costa Rica) and δ 2 H = 7.4·δ 18 O + 5.2 (Nicaragua). The greater rainfall inputs in Costa Rica with respect to Nicaragua, resulted in the attenuation of the evaporative effect as observed in the strong bimodal distribution of the hydrothermal waters, which can be divided in fluids: a) isotopically-close to meteoric conditions and b) isotopically-altered by the interaction with recycled subduction fluids and kinetic fractionation. The latter is clearly depicted in the significantly ( p d -excess and lc-excess median values between Costa Rica (+ 5.10‰, − 5.25‰) and Nicaragua (− 2.42‰, − 10.65‰), respectively. Poor correlations between δ 18 O/δ 2 H and the elevation gradient emphasize that the contribution of recycled subduction fluids and subsequent surface kinetic fractionation are the main drivers of the isotopic departure from the orographic distillation trend captured in the rainfall isoscapes. End-member mixing calculations resulted in a significant difference ( p 2 /He contributions across the volcanic front of Nicaragua and Costa Rica.