Jökull - 01.12.1982, Side 90

As numerous deep drillholes have been drilled in
the area, there is good opportunity to compare mea-
sured deep water temperature with that estimated
from chemical geothermometers (see Ellis and
Mahon 1977, Arnórsson et al. 1982) as chalcedony,
Na-K feldspar etc.
In Fig. 3 is shown measured temperature against
estimated chalcedony temperature of water from
drillholes. A few of the points plot below the line
connecting equal chalcedony and measured temp-
eratures. These points all represent water samples
from shallow drillholes (<1000 m deep). The rest
falls above or on the line. A concentration of points
defines a line displaced parallel by about 10°C with
that ofequal chalcedony and measured temperatur-
es. The various feldspar geothermometers (Foumier
and Truesdell 1973, White 1970, Ellis and Mahon 1977)
give very scattered and inconsistent numbers as
compared to measured deep temperatures and the
chalcedony temperature. A direct estimate from the
theoretical equilibrium (Helgeson 1969) of the feld-
spars low-albite and orthoclase gives more consist-
ent numbers. Graphs ofthose estimated “feldspar”
temperatures against measured temperature and
chalcedony temperature are shown in Figs. 4a and
b. A rather good correlation is obtained with mea-
sured temperature and a systematic displacement
relative to chalcedony temperature. Alkalifeldspars
are not found in the basalts and are notobserved to
be formed by such low-temperature geothermal act-
ivity (Kristmannsdóttir 1978, 1979). Why this esti-
mate works out so well has not yet been explained.
One would expect the Na/K ratio to be govemed by
exchange reactions between the fluid and clay min-
erals and/or zeolite.
Chloride content is low (around 10 ppm) in most
of the samples as would be expected in precipitation
far from the coast (see Pálmason et al. 1979). The
Hólsgerdi and Stóridalur springs in southem Eyja-
fjördur contain about 40 ppm C1 and the Mjadmár-
dalur spring yields water with 118 ppm Cl. The
Grýta and Gardsá springs also have relatively high
C1 contents of20-30 ppm.
The regional fluoride content of the geothermal
water is 0.3 —0.8 ppm. Exceptionsare the Hólsgerdi
spring and the Mjadmárdalur spring where the
contents are considerably higher. Water from the
spring in Stóridalur has fluoride contents in bet-
ween those two (0.95 ppm). The fluorine is mostly
found as F in low-temperature water. The cont-
ent of fluoride is much higher in geothermal water
which has reacted with acid volcanic rocks than
Fig.4. a) ,,Feldspar“ temperature against measur-
ed temperature in samples from drillholes in Eyja-
fjördur. The points are water sampled at well head
and crosses are samples from depth in the drillhole.
b) „Feldspar” temperature against chalcedony
temperature in samples from drillholes in Eyjafjörd-
ur. The points are water sampled at well head and
crosses are samples from depth in the drillhole.
Mynd 4. a) Alkalifeldspatahitastig á móti tmtldu hita-
stigi í sýnum af borholuvatni úr Eyjafirði. b) Alkalifelds-
patahitastig á móti kalsedonhitastigi í sýnum aj borholu-
vatni úr Eyjajirði. Punktar eru sýnifrá holutoppi, en krossar
eru djúpsýni úr borholum.
with basalt. Sedimentary rocks are also often richer
in fluoride than basalts and waters reacting with
them are consequently enriched in fluoride. The
high fluoride content in the waters in southern part
of the Eyjafjördur valley is due to the existence of
acid rocks in the underground. Regarding the
86 JÖKULL 32. ÁR