The Theistareykir Geothermal Field, NE Iceland. Isotopic Characteristics and Origin of Circulating Fluids

The Theistareykir high temperature field in NE Iceland seems to be complex in terms of both inflow and structure, as reflected in the division of the area into several subfields. Oxygen and hydrogen isotopes in water and steam condensate from wells are reported. Some differences can be seen between the Theistareykir well fields, but the recharge is in all cases non-local in origin. The isotopic composition of some of the thermal waters is anomalously depleted in 2H, by about 35‰, compared to precipitation anywhere in Iceland today. The isotopes therefore suggest that the thermal water contains a component of past precipitation under a colder climate. The oxygen shift due to water-rock interaction is up to 6.5 ‰. The isotopic signature of the Theistareykir thermal water is compared to that from nearby low temperature fields within the westernmost part of the Northern Neovolcanic Zone.