Rit (Vísindafélag Íslendinga) - 01.06.1976, Page 147

The water from H-8, with SD = — 22.5 %0, therefore might repre-
sent the deep thermal water in this area.
The deuterimn data obtained in the Reykjanes area therefore sug-
gest the following model. The uppermost part of the ground con-
tains a thin fresh cold water body of meteoric origin. Below this
fresh water the lavas are penetrated by ocean water, which is still
cold. Beneath this the geothermal water occurs. The geothermal
water, whose maximum deep circulating temperature is at least
286°C, emerges at the surface within the central geothermal zone.
This is in agreement with the model proposed by Björnsson (1971).
The geothermal water at Svartsengi has only shghtly lower ðD =
— 24%0, but significantly lower Cl" and Na+ than the Reykjanes
water, indicating appreciable amount of fresh water low in salt
content.
Since the Cl_ and Na+ content of the Reykjanes thermal brine
is nearly the same as that of sea water, the water has been believed
to be sea water (Björnsson et al. 1972 and Amórsson 1974). As
regards several other components such as for instance Mg++, Ca++,
Si02 and K+, the composition of the brine differs strongly from sea
water. The Si02 and Ca++ content is much higher in the thermal
brine, K+ is much lower, and the Mg++ is only a few mg/1 in the
brine but 1270 mg/1 in sea water.
No matter whether the water of the brine is pure sea water or
water of some other origin which has acquired its salt content in
some other way, the chemical composition at least shows that strong
chemical reactions have taken place between water and rocks.
The 8d = — 22.5%0 in the brine is, however, significantly lower
than in the sea water. Craig measured samples of ocean water col-
lected near the island Eldey, approximately 12 km south-west of
Reykjanes. These samples gave 8D = +1.5%0 for the sea water
(Ölafsson 1974). Thus if the Reykjanes brine is pure ocean water,
the water must somehow have been depleted in its deuterium con-
tent.
Clayton et al. (1966) studied the 8D-values of thermal brine water
from the Illinois, Michigan, Alberta and the Gulf Coast brines. The
brines contained variable amounts of dissolved solids, ranging from
concentrations less than in sea water up to concentrations 16 times
that of the sea water. In two cases they found no relation between
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