Gas geochemistry of hydrothermal fluids of the S. Miguel and Terceira Islands, Azores

Abstract

We present here for the first time a complete dataset of the chemical and isotopic compositions of fumarolic fluids collected on the S.Miguel (Fogo and Furnas volcanoes) and Terceira (Pico Alto volcano) Islands. The data are analyzed and discussed, to provide both a comprehensive picture of the thermodynamic conditions of the hydrothermal systems on these two islands, and to give new insight into the origins of these fluids, for a better understanding of the geodynamic context of the Azores archipelago. For S. Miguel Island, the gas equilibria in the H2–CO2–CO–CH4–H2O system suggest temperatures of the hydrothermal system reservoirs from 190 C to 280 C for the Fogo volcano fumaroles, which are 30–35 C higher than those measured for geothermal wells. The equilibrium temperatures estimated for the Furnas volcano hydrothermal system are from 200 C to 275 C. Equilibrium vapor at a temperature of 190 C is inferred for the fumarolic fluids discharged on Terceira Island. The He isotopic composition of 9.6 R/Ra measured in the fumaroles of Terceira suggests that a plume-like source, presumably from the lower mantle, feeds this hydrothermal system. The relatively low 3He/4He ratios (from 5.21 to 5.35 R/Ra) and higher CO2/3He ratios of S. Miguel fluids suggest an addition of 45% of radiogenic He and 30% of crustal CO2 to a plume-like composition as at Terceira. A mainly meteoric origin is inferred for the fumarolic water, whereas the unreactive gas species (He, Ar, N2) arise from mixing processes between an atmospheric-like component and a magmatic component. On S. Miguel Island, the estimated magmatic fluid composition in terms of the ratios of N2/40Ar (62 ± 6) suggests a plume-like mantle source. Deep derived N2 isotope compositions characterized by very negative d15N values (d15N 6 14&) are inferred for the fluids of the Terceira Island. Also S. Miguel fluids are compatible with the same source with a possible involvement of crustal-derived Nitrogen. These values are more negative than the typical compositions of both the upper mantle and plume-like mantle,