Jökull

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Jökull - 01.12.1987, Qupperneq 3

Jökull - 01.12.1987, Qupperneq 3
Geothermal activity in the Torfajökull field, South Iceland: Summary of geochemical studies STEFÁN ARNÓRSSON Science Institute University of Iceland Dunhagi 3, 107 Reykjavík GRÉTAR ÍVARSSON Nordic Volcanological Institute University of Iceland Jarðfrœðihús, 101 Reykjavík KEVIN E. CUFF Hawaiian Institute of Geophysics University of Hawaii 2525 Correa Road, Honolulu Hawaii 96822 ABSTRACT The Torfajökull geothermal field is located within the Torfajökull central volcanic complex in South Iceland. This complex contains anomalously abundant acid vol- canics. A large ring structure, probably related to a calde- ra subsidence, encircles the complex. The geothermal manifestations cover an area of about 140 km2. They are almost entirely located within the ring structure. Natural heatoutput has been estimated to be equivalent to 190-930 kg/s of steam. Basaltic intrusions underlying the silicic rocks may be the heatsource to the geothermal reservoir. The geothermal manifestations are mostly steaming ground but steam heated waters of the bicarbonate and the acid sulphate types are common in the area. Sodium- chloride type waters with as much as 500 ppm CT, repre- senting boiled and variably mixed reservoir water occur in the northeastern part ofthe field around Landmanna- laugar. The fumarole steam contains generally 0.2-0.4% total gas by volume. C02 is always the dominant gas constituent (>70%) butH2S andH2amountto2-8% and 0-10%, respectively. Ground radon and mercury con- centrations are anomalous over upflow zones of geother- mal steam. Helium isotope ratios as high as 23.4 times atmospheric have been reported indicating a relatively KRISTJÁN SÆMUNDSSON National Energy Authority Grensásvegur 9,108 Reykjavík primitive undegassed mantle source. Hot spring chem- istry at Landmannalaugar indicates subsurface temper- atures of 265°C. Gas chemistry indicates even higher temperatures (>300°C) for this and other parts of the geothermal field. INTRODUCTION Twenty eight high-temperature geothermal fields are known within the zones of active volcanism in Iceland (Gudmundsson 1983). These fields are typically located within central volcanic complexes (Pálmason and Sœ- mundsson 1974). The Torfajökull volcanic complex in Central South Iceland encompasses one such geothermal field. This complex is anomalously abundant in acid volcanics compared with other such complexes in the country. A large ring structure, probably a caldera struc- ture, encircles the complex (Sœmundsson 1972) (see Fig. 1). Several post-glacial eruptions have occurred in the area but otherwise the exposed rocks were mostly erupt- ed during the last glaciation (Sœmundsson 1972). The geology and the petrology of the Torfajökull central volcano has been described by fvarsson et al. (1987). The geothermal manifestations in the Torfajökull JÖKULL No. 37,1987
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