Iceland Deep Drilling Project: (VI) Fluid-rock interactions in the Reykjanes geothermal system as indicated by alteration mineralogy and sulfur isotopes.
Naomi Marks1, Peter Schiffman1, Robert Zierenberg1, W. A. Elders2 and G. O. Fridleifsson3
1Department of Geology, University of California-Davis, One Shields Avenue, Davis, Ca, 95616, USA; E-mail: marks@geology.ucdavis.edu; Telephone: 530-754-8760; Fax: 530-752-0951
2 Dept. of Earth Sciences, University of California, Riverside, California 92521, USA
3 Iceland Geosurvey, ISOR, Grensasvegur 9, IS-108, Iceland
The composition and salinity of geothermal fluids at Reykjanes resemble evolved seawater, suggesting that subsurface conditions at Reykjanes may be analogous to sea-floor black smokers. The high temperature reaction zone that is presumed to control the composition of the hydrothermal fluid is interpreted to occur deeper than the present depth of drilling, which reached just over 3 km in the well RN-17, during the initial stage of the Iceland Deep Drilling Project. The geothermal fluids deposit massive sulfide scale in production pipes with sulfur isotope values ranging from 2.0‰ to 4.4‰, similar to black smoker sulfide deposits. Sulfur isotope values from altered basalt in drill cuttings range from 4.3‰ to 10.5‰, suggesting a larger component of reduced seawater sulfate in the shallow up-flow zone relative to the H2S in the high temperature hydrothermal fluid. Minor element distributions in the samples suggest the presence of two or more lava series with varying degrees of differentiation. The cutting samples are primarily composed of glassy hyaloclastites, holocrystalline basalt flows, and hypabyssal diabasic intrusives. An assemblage of greenschist facies alteration minerals, including actinolite, prehnite and epidote, that implies temperatures reached at least 250°C, is found at depths as shallow as 350 m. This requires hydrostatic pressures that exceed the boiling point to depth curve, and therefore it must record alteration at higher fluid pressures when Reykjanes was covered by a Pleistocene ice sheet. These alteration phases are presumed to have formed from meteoric waters, rather than from the presently active seawater-recharged system. There is a profound disparity in the intensity of alteration within the two dominant rock types even at greenschist grades. The holocrystalline basalts/intrusives have undergone only limited alteration: plagioclase is mostly unalbitized and uralitization of clinopyroxene is very limited. In contrast, the hyaloclastites show intense alteration with calc-silicate alteration assemblages comprising calcic plagioclase, grandite garnet, prehnite, epidote, hydrothermal clinopyroxe, and titanite. These assemblages indicate higher water-rock ratios, and much higher aCa++/aH+2 in accompanying hydrothermal fluids than typically encountered during normal seafloor alteration, possibly reflecting the higher glass content of the basaltic rocks in Reykjanes relative to normal seafloor crust. Future core drilling at depths up to 5 km, into the high temperature reaction zone, will provide the opportunity to trace the evolution of the dominant fluids in the system from meteoric water to seawater.
