Jökull - 01.12.1987, Page 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