The Iceland Deep Drilling Project: Stable Isotope Constraints of Fluid Source and Evolution in Icelandic Geothermal Systems

Abstract

The Reykjanes and Krafla geothermal systems are located along the active rift zone of Iceland, and are both sites that will be drilled to 4-5 km by the Iceland Deep Drilling Project (IDDP). We use oxygen and hydrogen stable isotopes in hydrothermal minerals to characterize the source, composition and evolution of hydrothermal fluids in these geothermal systems in order to better assess ongoing geochemical and hydrological processes. Stable isotope compositions of hydrothermal epidote in the Reykjanes geothermal system indicate a complex history of fluid source and fluid-rock interaction since at least the Pleistocene. Modern Reykjanes geothermal fluids have chloride concentrations consistent with hydrothermally modified seawater source. However, measured hydrogen isotope values of these fluids are as low as -23.1‰. ?D values of hydrothermal epidote analyzed from four Reykjanes wells range from -60 to -78‰. These values are not in isotopic equilibrium with present day geothermal fluids. Instead, they retain an isotopic signature of glacially derived fluids that occurred early in the evolution of the geothermal system. Estimates of the water-rock ratio and modal abundance of hydrous alteration minerals in the Reykjanes geothermal system suggest that there is enough relict (Ice Age) hydrogen in altered host rock basalt to diffusionally exchange with modern sea-water derived geothermal fluids and lower the fluid hydrogen isotope composition by as much as 20‰. Fluid elemental and isotope chemistry studies of geothermal fluids from Krafla present evidence of a local, meteoric fluid source that has undergone limited geochemical interaction with the basalt host rock. Preliminary epidote hydrogen isotope values from three wells in the Krafla geothermal system range from -108 and -125‰. Oxygen isotope compositions of epidote in these wells are between -9.7 and -13.0‰. Hydrothermal quartz from one well, KJ-20, is isotopically variable with depth, ranging from ?18O = -5.3 to 1.8‰. The three-dimensional variability observed in the Krafla system is likely due to complex subsurface hydrology and multiple potential fluid sources, including a significant input of magmatic fluids. Additional analysis of hydrothermal alteration minerals within the Krafla system is necessary to fully resolve the fluid evolution within this geothermal field