ICELAND DEEP DRILLING PROJECT (IDDP): STABLE ISOTOPES AS A TRACER OF FLUID SOURCE AND EVOLUTION IN THE KRAFLA GEOTHERMAL SYSTEM

The Krafla geothermal system, located within the active rift zone of Iceland, was host to the first Iceland Deep Drilling Project drillhole (IDDP-1), which drilled into magma at a depth of 2104m in June, 2009. To fully understand the source and composition of this melt, as well as further potential for drilling to the supercritical region in the Krafla system, a detailed understanding of the hydrogeologic history of the system is necessary. We use oxygen and hydrogen stable isotopes in hydrothermal minerals to resolve the spatial and temporal complexities evident in the source and evolution of Krafla geothermal fluids.

Elemental and isotope chemistry studies of geothermal fluids from Krafla present evidence of a local, but widely varying, meteoric fluid source. Additionally, previously measured oxygen isotope compositions of present-day geothermal fluids (δ¹⁸O ~ -11.8‰, δD ~ -89‰) are not significantly more positive than local groundwater (δ¹⁸O = -12.3‰, δD = -87‰), indicating either limited fluid/rock interaction or an extremely high water to rock ratio. Our analyzed hydrogen isotope values of hydrothermal epidote in the Krafla geothermal system are between -108 and -127‰ in wells K-17, -20, -25, -26, -32, -34 and -39. Only epidotes from wells K-17, -20, and -32 are in hydrogen isotopic equilibrium with the average Krafla geothermal fluid composition based on published fractionation curves. Oxygen isotope compositions of epidote in three of these wells are -9.7 to -13.0‰ (K-20), -9.6 to -12.3‰ (K-34) and -11.1 to -11.7‰ (K-26). With the exception of two outliers, none of the geothermal epidote is in oxygen isotope equilibrium with the geothermal fluids.

The observed variations in stable isotope properties of epidote from Krafla likely reflect complex hydrologic features that characterize this geothermal system. This includes multiple hydrothermal fluid sources, mixing of fluids between upper and lower aquifers in the northwestern portion of the geothermal system and between northwest and southeast geothermal fields, the effects of boiling and condensation, and finally, the likely input of magmatic volatiles. Further analysis of the quenched magma and of alteration minerals in several wells throughout the Krafla system will improve our understanding of how these factors influence the evolution of hydrothermal fluids.