Jökull - 01.12.1987, Síða 10

reservoir. It is quite possible that temperatures follow
the boiling point curve with depth in all the areas. The
highest subsurface temperatures are indicated by fuma-
roles near the mouth of Brandsgil in the Landmanna-
laugar area and by two of the four fumaroles sampled at
Reykjadalir. In both cases over 300°C is indicated. Tem-
peratures of 300°C would be expected at about 1000 m
depth if they follow the boiling point curve with depth.
The data presented in this contribution cover only a
small part of the Torfajökull geothermal field. They are
too limited to picture lateral variations in subsurface
temperatures that may exist across the field.
HELIUM ISOTOPES
Helium isotopes in fumarole steam from Torfajökull
have been determined by Poreda etal. (1984). Ratios as
high as 23.4 times atmospheric values have been en-
countered. These high ratios are taken to indicate a
relatively primitive undegassed mantle source and have
been correlated with the identified mantle plume under
the SE-part of central Iceland (Poreda et al. 1984). Com-
parable ratios occur in the geothermal fields in the vol-
canic zone to the northeast of Torfajökull. Rise of mag-
ma from a primitive source below Torfajökull and in-
jection along the fissure swarm could be the cause of
these high ratios. High helium isotope ratios (20 times
atmospheric) are also observed west of the Torfajökull
volcanic complex in the vicinity of the transform fault in
southern Iceland connecting the two volcanic zones
(Kononov et al. 1974, Poreda et al. 1984). Elsewhere in
Iceland helium isotope ratios in geothermal discharges
are lower with the exception of some localities in Terti-
ary formations on the Northwest-Peninsula (Kononov et
al. 1974). Basaltic rocks in the Torfajökull region have
helium isotope ratios comparable with those of the fu-
marole steam (Kurz etal. 1985, Condomines etal. 1983).
RADON AND MERCURY
The distribution of ground radon and mercury in soil
has been studied in some detail in the Hrafntinnusker
area (Fig. 1). The techniques used to detect the radon
and mercury emission are very similar to those currently
used in geothermal exploration (Varencamp and Buseck
1983, Whitehead 1984). Radon detection was achieved at
over 250 locations using integrated solid-state alpha par-
ticle detectors giving results reproducable to within
15%. Solid samples were collected at each of these loca-
tions, as well as samples from prominent spring orifices,
and analysed for mercury using a gold film mercury
detector which yields results reproducable to within
10%.
The results of the radon and mercury measurements
are depicted in Fig. 3. Individual measurements were
ratioed to background levels. For radon the background
values were empirically determined for each sample
location. Average background value for mercury was
calculated for specific areas in which measurements
were made.
As can be seen from Fig. 3 zones with elevated radon
and mercury levels are observed following two distinct
elongate trends. One is oriented in a NW-SE direction
and the other in a NE-SW direction. The latter coincides
with the present tectonic grain in the area but the former
with an earlier tectonic fabric. Although there are some
differences, the radon and mercury anomalies coincide
fairly well. The highest radon and mercury levels occur
where the linear anomalies intersect. Here, they are, at
least, five times background values and, in some cases
more than forty times background. Major thermal activ-
ity is seen to lie just outside well defined zones of anoma-
lous outgassing.
It is considered that the radon and mercury anomalies
are the manifestations of, and overly, permeability
zones associated with tectonic fractures. The shapes of
the anomalies of radon on one hand and mercury on the
other probably differ because they reflect vapour circu-
lation at different depths. For mercury circulation is at
shallower depth.
SUMMARY
Geothermal manifestations in the Torfajökull field
cover an area of about 140 km2 and they are almost
entirely located within the caldera structure. The nat-
ural heat output has been estimated to be equal to
190-930 kg/s of steam. The stored heat in the uppermost
3 km of the reservoir was estimated by Pálmason (1980)
as 281 • 1018 J.
The geothermal manifestations consist mostly of
steaming ground which is, as a rule, intensely altered by
acid surface leaching. Steam heated waters of the bicar-
bonate and the acid sulphate types are relatively com-
mon. In the northeastern part of the field, around Land-
mannalaugar, sodium-chloride type waters occur. They
represent boiled and variably mixed reservoir water.
This water is higher in chloride than geothermal water
associated with basaltic rocks in Iceland. The reason is
considered to be higher chloride content of the rhyolites
as compared with basaltic rocks. Fluoride concentra-
tions are relatively high (5-25 ppm) in the sodium-chlo-
ride type water as is the case for geothermal waters in
Iceland associated with acid volcanics. Fluoride mobility
is considered to be controlled by fluorite solubility. Oth-
8