Jökull - 01.12.1987, Page 47

atures obtained from the gas geothermometers of Nehr-
ing and D’Amore (1984). The geothermometers of
Nehring and D’Amore (1984) constitute a product of
C02 and H2 on one hand and C02 and H2S on the other.
By reference to theoretical considerations of chemical
equilibria in open systems, like geothermal systems, one
would expect that the concentrations of each of the three
gases considered would depend on temperature alone
(Arnórsson etal., 1983) as the Cerro Prieto data indeed
show.
The C02/H2 ratios are lowest in the northeasternmost
part of the Sveifluháls area, around the southern shore
of Lake Kleifarvatn (Fig. 1). In the only fumarole sam-
ple from this part of the Sveifluháls area (No. 8 in Table
1) this is due to elevated H2 concentration. Presumably
this is also the case for gas samples from hot pools and
from Lake Kleifarvatn. The high H2 concentrations in
the steam in this part of the Sveifluháls area are indica-
tive of a primary upflow zone. An outflow of boiling
water from that zone could generate the steam emerging
around the ridge of Sveifluháls to the west. Such an
outflow, if it exists, must be at close to 1000 m depth as
deduced from the subsurface temperatures for the fuma-
role steam at Sveifluháls. A drillhole has been sunk to
about 800 m depth at the southern shore of Lake Kleifar-
vatn. Its bottom temperature is 153°C. Apparently, this
hole is not sufficiently deep to reach the presumed lat-
eral flow of boiling water.
The Vesturháls Area
Relatively few samples are available from this area
(Tables 1 and 2) which is due to difficult sampling condi-
tions rather than scarcity of fumaroles. Around Trölla-
dyngja subsurface temperatures obtained by all the gas
geothermometers lie in the range 195-297°C. The lowest
value of 195°C, which is from a gas sample from Sog (no.
45, Table 5) and based on the C02/N2 geothermometer,
is probably low because of atmospheric contamination
in the sample. Eliminating this value, the average of all
the geothermometry temperatures around Trölladyngja
in the Vesturháls area is 256°C. A temperature of 262°C
has been measured in 830 m deep drillhole at Trölla-
dyngja (Arnórsson etal., 1975a). The geothermometers
of D’Amore and Panichi (1980) and Nehring and D’A-
more (1984) give similar temperatures for the Trölla-
dyngja area samples as the geothermometers of Arnórs-
son and Gunnlaugsson (1985). As previously mentioned
the water salinity in the well at Trölladyngja suggests
that the mineral buffer controlling H2S and H2 concen-
trations in the reservoir include magnetite and pyrite.
This explains why all the gas geothermometers give
temperatures within a reasonable range.
The fumarole at Hverinn Eini contains the highest
concentrations of C02 and H2S of all samples collected
from the Krísuvík field (Table 1). At the turn of the last
century this was a very strong fumarole and could be
seen from Reykjavík at a distance of about 30 km (Tho-
roddsen 1925). Now the fumarole is almost extinct. As
deduced from the elevated N2 concentrations and the
high N2/Ar ratio the sample is atmoshperically contam-
inated making it difficult to estimate with confidence the
amount of steam condensation in the upflow. However,
bringing N2/Ar ratios to 57.2 (sample 23) would only
raise N2 to a total of some 5 mmoles/kg. Thus, it seems
evident that much condensation has occurred although
the values reported in Table 4 are regarded too high.
It is not known what the salinity of the water is in the
reservoir below Hverinn Eini. The figures for the H2S,
H2 and C02/H2 geothermometers in Table 4 assumed
dilute waters. Assuming, on the other hand, saline wa-
ters gives temperatures which are higher by 35°, 49° and
108°C, respectively. Using the Zb value for condensation
in Table 4 (98%) and the geothermometry functions
corresponding with dilute waters gives C02, H2S, and H2
temperatures as follows: 195°, 187° and 182°C. This cor-
responds well with the COz/H2 temperature. The limited
and apparently inconsistent data for Hverinn Eini can be
explained equally well in two ways: 1) Condensation is
very extensive in the upflow, the reservoir water is dilute
and its temperature is 180-200°C. 2) Condensation is not
so extreme in the upflow. The reservoir water is suffi-
ciently saline to equilibrate with magnetite rather than
pyrrhotite and its temperature is around 300°C.
SUMMARY AND CONCLUSIONS
The concentrations of C02, H2S, H2, CH4, N2 and Ar
in fumarole steam all provide useful information to de-
lineate the characteristics of the Krísuvík geothermal
reservoir including:
1) Subsurface temperatures.
2) Steam condensation in the upflow.
3) Boiling and partial degassing at high temper-
atures.
4) Presence of equilibrium steam in the reservoir.
The Krísuvík geothermal field probably represents
two hydrothermal systems, one underlying Sveifluháls
and the other underlying Vesturháls. In the Sveifluháls
area gas geothermometry indicates temperatures of at
least 280°C but they seem to be lower, may be about
260°C, around Trölladyngja in the Vesturháls area. At
Hverinn Eini, also in the Vesturháls area, subsurface
temperatures might be as high as 300°C. Boiling is exten-
sive in the upflow and steam condensation relatively
limited (0-30%).
45