Monitoring hydrothermal fumaroles in the Azores archipelago - Applications and sources of analytical uncertainties

The Azores archipelago is located in the North Atlantic Ocean and is formed by nine volcanic islands. Present-day volcanic activity in the archipelago comprises not only seismic swarms and episodes of ground deformation in some volcanoes, but also hydrothermal gas manifestations. The main fumaroles are associated with central quiescent volcanoes and have been sampled in the past decade through the Giggenbach methodology. Analysis of the fumarolic fluids from four volcanic systems in São Miguel, Terceira, and Graciosa islands are displayed for the period between July 2015 and February 2023. No significant changes have been observed both on the selected gas ratios (CO₂/CH₄, H₂/CH₄, He/CH₄, He/CO₂, and H₂/Ar), and on the equilibrium temperatures estimated for the reservoirs feeding the fumaroles using the H₂/Ar gas geothermometer, what is in agreement with the dormant state of activity of the studied volcanic systems. Equilibrium temperatures ranging between 223 °C and 262 °C are estimated for Fogo and Furnas fumarolic fields (São Miguel Island), and an average equilibrium temperature of 254 °C is inferred for Terceira fumaroles. This study also suggests a procedure based on established guidelines to evaluate the analytical uncertainties of the methodologies associated with the Giggenbach sampling strategy. Comparison of the analytical uncertainties with the overall data variation (including additional natural variations and sampling uncertainty), shows that the analytical uncertainties are not the limiting factor for the interpretation of the survey results.