Proceedings World Geothermal Congress 2010 Bali, Indonesia, 25-29 April 2010 1 Iceland Deep Drilling Project (IDDP): Stable Isotope Evidence of Fluid Evolution in Icelandic Geothermal Systems Emily C. Pope 1 *, Dennis K. Bird 1 , Stefán Arnórsson2, Thráinn Fridriksson 3 , Wilfred A. Elders 4 and Gudmundur Ó Fridleifsson 5 1 Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 2 Science Institute, University of Iceland, Dunhagi 3, 107 Reykjavik, Iceland 3 ISOR, Iceland GeoSurvey, Grensasvegur 9, 108 Reykjavik, Iceland 4 Department of Earth Sciences, University of California, Riverside, CA 5 HS-Orka hf, Brekkustigur 36, 260 Reykjanesbaer, Iceland *ecpope@stanford.edu Keywords: Stable Isotopes, Epidote, Iceland, Rock-Fluid Interaction ABSTRACT The Reykjanes and Krafla geothermal systems, located within the active rift zone of Iceland, are both sites that will be drilled to 4-5 km by the Iceland Deep Drilling Project (IDDP). To effectively characterize geochemical and hydrologic processes occurring at these depths, it is essential to establish the source, composition and evolution of geothermal fluids. We use oxygen and hydrogen stable isotopes in hydrothermal minerals to resolve the fluid history in these IDDP geothermal systems. Here we report the results from existing drillholes to depths of ≤ 3 km. The stable isotope composition of hydrothermal epidote in the Reykjanes geothermal system demonstrates a complex history of fluid source and fluid-rock interaction since at least the Pleistocene. The chlorine concentration of modern Reykjanes geothermal fluids indicate that they are hydrothermally modified seawater. However, measured hydrogen isotope values of these fluids are as low as -23‰. δD values of hydrothermal epidote from wells RN-8, -9, -10 and -17 collectively range from -60 to -78‰, and δ 18 O EPIDOTE in these wells are between -3.0 and 2.3‰. The analyzed epidotes are not in stable isotopic equilibrium with present-day geothermal fluids, but retain an isotopic signature of glacially derived fluids occurring early in the evolution of the geothermal system. Estimates of the water- rock ratio and modal abundance of hydrous alteration minerals in the geothermal system suggest that there is sufficient relict (Ice Age) hydrogen in the altered basaltic host rock to diffusionally exchange with modern 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. Additionally, oxygen isotope compositions of present-day geothermal fluids are not significantly more positive than local meteoric water, indicating either limited fluid/rock interaction or an extremely high water to rock ratio. Preliminary hydrogen isotope values of epidote in the Krafla geothermal system are between -116 and -125‰ in wells K-20, K-34, and K- 26, and are highly variable between wells. Oxygen isotope compositions of epidote in these wells are between -9.6 and -13.0‰. The variability observed spatially and with depth 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. 1. INTRODUCTION The active volcanic zone of Iceland is host to more than twenty high-temperature geothermal systems. These systems collectively provide 484MW of geothermal energy (Arnórsson et al., 2008) and represent an important natural resource in Iceland (Arnórsson, 1995). They also offer an opportunity to study, in situ, geochemical interaction between basaltic rocks (magmatic products of the Iceland mantle plume) and aqueous electrolyte solutions at elevated temperatures. Chemical mass transfer in these magma- hydrothermal environments is of fundamental importance because it affects the evolution of ocean chemistry resulting from hydrothermal alteration at spreading ridges (Muehlenbachs and Clayton, 1981) and is a basic process controlling the formation of massive sulfide deposits (e.g. Weissberg et al., 1979). The Iceland Deep Drilling Project, which intends to increase both the economic and scientific utility of these geothermal systems, will be drilling wells to a depth of 4-5 km within the Reykjanes and Krafla geothermal regions (Figure 1; Elders and Fridleifsson, 2010). Before we can fully understand the geochemical and hydrologic processes occurring at depth within these regions and thus maximize the benefit of this exploration, it is essential to establish the source, composition and evolution of geothermal fluids. Previous authors (e.g. Árnason, 1977; Darling and Ármannsson, 1989; Arnórsson, 1978 and 1995) used the major element chemistry and stable isotope composition of geothermal fluids to determine their source. This study undertakes a more critical look at fluid evolution in the Reykjanes and Krafla geothermal systems, using the stable isotope systematics of hydrothermal epidote. We aim to establish a more complex fluid history in both systems than is apparent from the chemical and isotopic characteristics of the geothermal fluids alone. 2. BACKGROUND 2.1 Reykjanes The Reykjanes geothermal system is located on the southwestern tip of the Reykjanes Peninsula, on the landward extension of the Mid-Atlantic Ridge (Figure 1a). Like other high-temperature systems in Iceland, it is composed of highly fractured basalt lavas and hyaloclastites