The Isotopic and Chemical Characteristics of Geothermal Fluids in Hengill Area, SW-Iceland (Hellisheidi, Hveragerdi and Nesjavellir Fields)

Abstract

Results of geochemical and isotopic investigations in the Hengill Geothermal area are presented. The area can be regarded as typical of Icelandic high temperature areas. It’s mainly built up of pillow lavas and hyaloclastites, which were piled up in sub glacial eruptions. Part of the area is transversed by a very active NE-SW trending fault zone about 5km broad, within which are several eruptive fissures of postglacial age. The volcanic rocks are basalts of various kinds, but minor occurrences of intermediate and rhyolitic rocks are also found.
The study was based on chemical and isotopic the analyses of fluid samples from Nesjavellir prior to production, and the results data collected in the years 2000-2007 from the exploited geothermal fields. In addition, new fluid samples were collected for the present from the Nesjavellir and Hellisheidi fields. These samples were analysed for chemistry and stable isotopes at the institute of Earth Sciences, University of Iceland. Grapher and Surfer were the main programs used to plot the data.
The results indicated that the Nesjavellir and Hellisheidi thermal fluids do not share the same origin. In Nesjavellir the water comes from a distant source the glacier Langjökull whereas in Hellisheidi the water is of local origin identical to the isotopic composition of the Hveragerdi thermal water. According to the deuterium isotope values well HE.1 in Hellisheidi is closer in origin to the Nesjavellir thermal fluid than the fluid circulating the Hellisheidi system. The Hellisheidi system is younger than the Nesjavellir system as suggested both by stable isotopes and chemistry of the thermal fluids (Cl-SO4-HCO3 plot). In Nesjavellir the fluid is richer in 18O and chemically more mature than in Hellisheidi, due to more intense water-rock interaction. Thermal fluids in Hveragerdi are of local origin as the Hellisheidi waters.