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Journal of the Geological Society

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Water–rock interaction in Krafla and Reykjanes geothermal systems, Iceland

An extended abstract

Hydrothermal alteration in two contrasting geothermal systems in Iceland, Krafla and Reykjanes, has been studied. Both are high temperature areas (Bodvarsson 1961), where subsurface temperature exceeds 200°C at relatively shallow depth. The alteration patterns in these systems are quite similar, in spite of contrasting fluids. The Reykjanes hydrothermal system contains a highly saline (Cl = 19,260 ppm; Bjornsson et al. 1972) circulating fluid at depth due to influx of seawater, whereas at Krafla the fluid is non-saline meteoric water.

Oxygen isotope measurements carried out at I.G.S., London, demonstrate that extensive oxygen isotope exchange has occurred between the hydrothermally altered rock and the circulating fluid in the Krafla geothermal field. These exchange reactions have left the rock considerably lighter–about 10%c–but have hardly affected the isotopic composition of the fluid. Consequently, the water/rock ratio in the Krafla area is very high (atomic ratio 10–20). Oxygen and deuterium measurements on the hydrothermal fluid in the Krafla geothermal field give ¹⁸O = –11.9% and D = –86.8%, indicating that it is derived from the local precipitation. This contrasts with the prevailing view that the hydrothermal fluid in the Krafla reservoir is derived from Vatnajokull (Iceland's largest icecap), some 100 km S of the Krafla area (Arnason 1975). The oxygen isotope measurements of quartz (¹⁸O = –0.9 to –7.7%) and calcite (¹⁸O = –3.6 to –9.2%) from various depths in the Krafla system (hole KJ-7) indicate that both phases are close to equilibrium with the hydrothermal solution at present field temperature determined

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