HR: 0800h
AN: V41A-0390    [Abstracts]
TI: The Iceland Deep Drilling Project (IDDP): (II) Fluid Origin and Evolution in the Reykjanes Geothermal System - A Stable Isotope Study of Hydrothermal Epidote
AU: * Pope, E C
EM: ecpope@stanford.edu
AF: Dept. of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Bldg. 320, Stanford, CA 94305, United States
AU: Bird, D K
EM: bird@pangea.stanford.edu
AF: Dept. of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Bldg. 320, Stanford, CA 94305, United States
AU: Arnórsson, S
EM: stefanar@raunvis.hi.is
AF: Science Institute, University of Iceland, Dunhagi 3, Reykjavik, 107, Iceland
AU: Fridriksson, T
EM: Thrainn.Fridriksson@isor.is
AF: ISOR, Iceland Geosurvey, Grensasvegur 9, Reykjavik, 108, Iceland
AU: Elders, W A
EM: wilfred.elders@ucr.edu
AF: Department of Earth Sciences, University of California, Riverside, Riverside, CA 92521, United States
AU: Fridleifsson, G \
EM: Gudmundur.O.Fridleifsson@isor.is
AF: Hitaveita Sudurnesja Ltd., Brekkustig 36, Reykjanesbaer, 260, Iceland
AB: The Reykjanes geothermal system, located on the landward extension of the Mid-Atlantic Ridge in southeast Iceland, provides an on-land proxy to the hydrothermal systems of oceanic spreading centers and is a candidate for future deep drilling into the supercritical zone. In preparation for study of supercritical fluids from this region, an understanding of hydrothermal processes at shallower levels is necessary. Previous studies of elemental composition and salinity have shown that Reykjanes geothermal fluids are likely hydrothermally modified seawater. However, hydrogen isotope properties of these fluids indicate a significant component of meteoric water, with δD_FLUID values as low as -23‰. Here we constrain the origin of hydrothermal solutions by analysis of hydrogen and oxygen isotope compositions of geothermal epidote from drilling wells within the Reykjanes system at depths between 1 and 3 km. δD_EPIDOTE values range from -64 to - 70‰ in well RN-10 between 1.0 and 2.1 km depths, from -63 to -78‰ in well RN-17 at .95 to 3.0 km depth, and between -61‰ and -63‰ in well RN-9 between 1.0 and 1.3 km depth. Published δD_EPIDOTE values from well RN-8 at 1.6 km depth are -48‰. At the same depths, δ¹⁸O_EPIDOTE range from 1.8 to -0.4‰ in well RN-10, from 2.3 to -0.1‰ in well RN-17, and from 0.2 to -3.0‰ in well RN-9. δD values of epidote progressively increase moving away from the geothermal upflow zone at well RN-10, whereas δ¹⁸O values decrease. For comparative analysis, the Nesjavellir and Krafla geothermal systems, which are dominated by meteoric water and have a δD_FLUID of approximately -79‰ and -89‰ respectively, have a δD_EPIDOTE of -115‰ and -125‰. However, δD_EPIDOTE from the mixed meteoric-seawater Svartsengi geothermal system is -68‰; comparable to δD_EPIDOTE from well RN-10. Stable isotope compositions of geothermal fluids are computed based upon the measured isotope composition of Reykjanes epidotes and temperatures approximated from the boiling point curve with depth, and are compared to the published temperature dependent isotope fractionation curves of epidote. Calculated δD and δ¹⁸O of geothermal fluids are less than 0‰, suggesting that fluids of meteoric origin are an important component of the hydrothermal solutions. Additionally, variations between wells suggest a heterogeneous evolution of fluid flow or fluid source within the system. These results, in conjunction with the hydrogen and oxygen isotope composition and elemental chemistry of modern geothermal fluids, allow evaluation of the relative influence that fluid source, mixing, rock-fluid interaction and boiling have had on the geochemical evolution of the Reykjanes geothermal system.
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 3653 Fluid flow
DE: 9325 Atlantic Ocean
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting