Chlorine isotope and Cl–Br fractionation in fluids of Poás volcano (Costa Rica): Insight into an active volcanic–hydrothermal system

Abstract

Halogen-rich volcanic fluids issued at the surface carry information on properties and processes operating in shallow hydrothermal systems . This paper reports a long-term record of Cl–Br concentrations and δ 37 Cl signatures of lake water and fumaroles from the active crater of Poás volcano (Costa Rica), where surface expressions of magmatic-hydrothermal activity have shown substantial periodic changes over the last decades. Both the hyperacid water of its crater lake (Laguna Caliente) and subaerial fumaroles show significant temporal variability in Cl–Br concentrations, Br/Cl ratios and δ 37 Cl, reflecting variations in the mode and magnitude of volatile transfer. The δ 37 Cl signatures of the lake, covering the period 1985–2012, show fluctuations between + 0.02 ± 0.06‰ and + 1.15 ± 0.09‰. Condensate samples from adjacent fumaroles on the southern shore, collected during the interval (2010–2012) with strong changes in gas temperature (107–763°C), display a much larger range from − 0.43 ± 0.09‰ to + 14.09 ± 0.08‰. Most of the variations in Cl isotope, Br/Cl and concentration signals can be attributed to interaction between magma-derived gas and liquid water in the volcanic–hydrothermal system below the crater. The δ 37 Cl were lowest and closest to magmatic values in (1) fumarolic gas that experienced little or no interaction with subsurface water and followed a relatively dry pathway, and (2) water that captured the bulk of magmatic halogen output so that no phase separation could induce fractionation. In contrast, elevated δ 37 Cl can be explained by partial scavenging and fractionation during subsurface gas–liquid interaction. Hence, strong Cl isotope fractionation leading to very high δ 37 Cl in Poás’ fumaroles indicates that they followed a wet pathway. Highest δ 37 Cl values in the lake water were found mostly in periods when it received a significant input from subaqueous fumaroles or when high temperatures and low pH caused HCl evaporation. It is concluded that combined monitoring of δ 37 Cl and Br/Cl in Laguna Caliente and adjacent fumaroles provides valuable information on activity in the subsurface hydrothermal system with significant relevance for volcanic surveillance of Poás. • Long-term record of Cl–Br and δ 37 Cl signatures of lake water and fumaroles from Poás volcano • Significant fluctuations reflect variations in the mode and magnitude of volatile transfer. • Phase separation and interaction with shallow waters modify both Br/Cl ratios and δ 37 Cl values.