Iceland Deep Drilling Project (IDDP): Stable Isotope Evidence of Fluid Evolution in Icelandic Geothermal Systems


Emily C. Pope, Dennis K. Bird, Stefán Arnórsson, Thráinn Fridriksson, Wilfred A. Elders and Gudmundur Ó Fridleifsson

Key Words:

Stable Isotopes, Epidote, Iceland, Rock-Fluid Interaction

Geo Location:

Krafla, Iceland; Reykjanes, Iceland


World Geothermal Congress




39. Iceland Deep Drilling Project



Paper Number:


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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‰. D values of hydrothermal epidote from wells RN-8, -9, -10 and -17 collectively range from -60 to -78‰, and 18OEPIDOTE 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.

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