Chemical and isotopic composition of steam from fumaroles in Hengill, SW Iceland

The greater Hengill volcanic complex in SW Iceland is located on a triple junction between the rift zone of the Reykjanes peninsula, the Western rift zone of Iceland and the South Iceland seismic zone. It is divided into three volcanic systems; Hengill, Hrómundartindur and Grændalur, each of which has associated fissure swarms. The Hengill volcanic complex is host to several geothermal areas that are spread over most of its extent, with geothermal surface manifestations generally connected to the tectonic features.Three of the geothermal systems within the youngest volcanic system (that of Mt. Hengill) have been exploited by Reykjavík Energy which runs two geothermal power plants: Hellisheiði and Nesjavellir. The power plants produce a total of 423 MWe which is sold on the national grid and 500 MWth in the form of heated groundwater for direct use in the capital area.We report samples of steam, gas and condensate collected from 12 fumaroles in 11 geothermal subfields defined within the Mt. Hengill fissure swarm (Nesjavellir, Nesjalaugagil, Köldulaugagil, Hagavíkurlaugar, Fremstidalur, Miðdalur, Innstidalur, Sleggjubeinsdalur, Skarðsmýrarfjall, Skíðaskáli and Hverahlíð) in October 2019. The samples were analysed for concentrations of all major gases (CO2, H2S, H2, CH4, N2, O2), stable noble gases (He, Ne, Ar, Kr, Xe) as well as the 3He/4He isotopic ratio and the stable isotope ratios of carbon (δ13CCO2), hydrogen (δD) and oxygen (δ18O). This study is relevant for the chemical characterization of the key upflow zones in the volcano-tectonic system as well as the identification of promising target areas for the installation of advanced gas monitoring systems. Monitoring data can contribute to an improved understanding of processes in the subsurface related to volcanic and seismic activity, but also to geothermal reservoir operations.The steam has generally rather low gas concentrations, with a total gas content of less than 500 mmol/kg steam for all samples except the few that had lost steam to condensation before sampling. All samples have CO2 as the most abundant gas followed H2S and H2. Gas geothermometry suggests temperatures ranging from about 250°C to more than 320°C. The stable water isotope values of the most powerful fumaroles are in good agreement with those obtained for well fluids, but the isotope values for steam from weaker fumaroles show signs of subsurface condensation. Carbon isotopes suggest that CO2 has a magmatic origin, as is most common for Icelandic high-temperature geothermal areas. Helium isotopes show clear mantle signatures with 3He/4He as high as 16 Ra but mixing with atmosphere is also observed.