Geothermal activity in the Torfajökull field, south Iceland: Summary of geochemical results

Abstract

The Torfajokull geothermal field is located within the Torfajokull central volcanic complex in South Iceland. This complex contains anomalously abundant acid volcanics. A large ring structure, probably related to a caldera subsidence, encircles the complex. The geothermal manifestations cover an area of about 140 km'. They are almost entirely located within the ring structure. Natural heat output has been estimated to be equivalent to 190-930 kgls of steam. Basaltic intrusions underlying the silicic rocks may be the heat source to the geothermal reservoir. The geothermal manifestations are mostly steaming ground but steam heated waters of the bicarbonate and the acid sulphate types are common in the area. Sodiumchloride type waters with as much as 500 ppm Cl", representing boiled and variably mixed reservoir water occur in the northeastern part of the field around Landmannalaugar. The fumarole steam contains generally 0.2-0.4% total gas by volume. CO, is always the dominant gas constituent (>70%) but H2S and H2 amount to 2-8% and 0-10%, respectively. Ground radon and mercury concentrations are anomalous over upflow zones of geothermal steam. Helium isotope ratios as high as 23.4 times atmospheric have been reported indicating a relatively P\LIVal\ ay. -3t1 primitive undegassed mantle source. Hot spring chemistry at Landmannalaugar indicates subsurface temperatures of 265°C. Gas chemistry indicates even higher temperatures (>300°C) for this and other parts of the geothermal field.