Soil CO2 emission and stable isotopes (13C/12C, 18O/16O) of CO2 and calcites reveal the fluid origin and thermal energy in the supercritical geothermal system of Krafla, Iceland

Abstract The Krafla geothermal system is located within a volcanic center that periodically emit basaltic lavas, and has recently attracted an economic interest due to supercritical fluids forming nearby a shallow magma intrusion (~ 2 km depth). Here, we discuss new soil CO 2 flux and stable isotope data of the CO 2 efflux (δ 13 C) and hydrothermal calcites (δ 13 C, δ 18 O) of drill cuttings, to estimate both the current magmatic outgassing from soils and the thermal flows in the geothermal system. Soil CO 2 emission is controlled by the tectonic, following the NNE-SSW fissure swarm direction and a WSW-ENE trend, and accounts for ~ 62.5 t d –1 . While the δ 18 O of the H 2 O in equilibrium with deep calcites is predominantly meteoric, both the δ 13 C of the soil CO 2 efflux and of the fluids from which calcite precipitated have a clear magmatic origin, overlapping the δ 13 C estimated for the Icelandic mantle (–2.5 +/- 1.1 ‰). Estimates based on the soil CO 2 emission from the southern part of the system show that these fluxes might be sustained by the ascent and depressurization of supercritical fluids with a thermal energy of ~800 MW. Such significant amount of energy might reach 1.5 GW, if supercritical conditions extended below the whole investigated area. Finally, we report an increase in the soil CO 2 emission of about 3 times with respect to 14 years ago, likely due to recent changes in the fluid extracted for power production or magmatic activity. Pairing the soil CO 2 emission with stable isotopes of the efflux and calcite samples has important implications for both volcano monitoring and geothermal exploration, since it can help us to track magmatic fluid upflows and the associated thermal energy.