Fluid and Metal Origins at Theistareykir Geothermal Field, Iceland

Abstract

The ultimate source of metals in magmatic-hydrothermal systems is a matter of debate. Volatile trace metals and noble gases are here used as a potential way of tracing sources of metals in geothermal systems and differentiating between a magmatic source and derivation from the leaching of country rocks. We are currently studying the hydrothermal system of Theistareykir (Iceland), set on the mid-Atlantic oceanic ridge. A new geothermal field has been developed with comprehensive access to geological materials and fluids, thanks to 18 geothermal wells drilled and numerous fumaroles and mud volcanoes in the surrounding areas. Volatile trace metals have been analyzed in rocks and fluids from the deep wells, surface fumaroles and mud pots. The current objective is to use noble gases to constrain the geothermal reservoir’s behavior. The second objective is to compare volatile trace elements from the deep geothermal wells with noble gases from the same wells. Since noble gases are highly volatile, we expect to see some trends between the two families of elements. The first results from noble gases show that the magma beneath Theistareykir has a dominant influence of the mantle, as sampled at the mid-ocean ridges, and a lesser influence of the mantle plume. The 3He/4He mean ratio is 11.45Ra (with Ra the atmospheric ratio of 1.384 x 10-6), when a typical MORB is 8±1Ra. Finally, preliminary results show that volatile metals can be linked to their source using noble gases. As an example, thallium shows a positive correlation with 3He, which is an indicator of magma degassing zones in the Theistareykir area.