Jökull - 01.12.1987, Page 7

trations in the waters below Landmannalaugar are thus
attributed to dissolution from the acid volcanics over-
lying the presumed basalt sheet intrusion heat source.
As for other geothermal reservoir waters in Iceland
associated with acid volcanics, fluoride concentrations
are relatively high in the sodium-chloride type waters
(5-25 ppm) and seem to be controlled by fluorite solu-
bility in the reservoir (Arnórsson et al. 1983).
Compositional features of the sodium-chloride waters
emerging in boiling hot springs are, apart from chloride
and fluoride, similar to those of waters discharged from
drillholes in basaltic terrain in different parts of Iceland.
Arnórsson etal. (1983) have shown that the composition
of the drillhole waters is governed by chemical equilib-
num between solutes and alteration minerals.
The mixed waters containing a sodium-chloride com-
ponent differ from the boiled waters in many respects.
They are relatively high in total carbonate, most likely
because the mixing process has prevented boiling and,
therefore, degassing of the hot water. Further the mixed
water is relatively high in calcium and magnesium and
has low Na/K ratios which is explained by leaching sub-
sequent to mixing (Arnórsson 1985). The mixed waters
are devoid of H2S, most likely due to oxidation.
In contrast to the mixed waters, waters in boiling hot
springs have relatively high Na/K ratios. Na-K geother-
rnometry temperatures for the boiling hot spring waters
nre significantly below those of quartz. The cause is
considered to be relatively effective removal of potassi-
ntn from the boiling water in the upflow either by precip-
itation of K-feldspar or adsorption on clay minerals.
INTERPRETATION OF MAJOR GAS
CHEMISTRY
Samples of fumarole steam are available from the
Landmannalaugar and Hrafntinnusker areas and a few
samples from Reykjadalir (Fig. 1, Table 2). There is
considerable variation in the gas compositions from the
different areas and an almost complete overlap for the
different gaseous constituents.
The fumarole steam generally contains 0.2-0.4% to-
tal gas by volume. Carbon dioxide always dominates
(>70%) except for a few samples high in nitrogen and,
therefore, taken to be atmospherically contaminated.
Hydrogen sulphide accounts for 2-8% of the gas in most
samples. Hydrogen concentrations are similar but more
variable, most of the samples fall in the range 0-10%.
Methane is below 0.6% except in one sample. Values
below 0.2% are most common. Almost all the samples
with methane in the range 0.2-0.6% are from the Hrafn-
tinnusker area.
Gas compositions comparable with those found in the
Torfajökull field are known in some other high-temper-
ature fields in Iceland (Arnórsson and Gunnlaugsson
1985). The concentrations of C02, H2S and H2 in the
fumarole steam are considered to be governed by tem-
perature dependent mineral-solution equilibria in a liq-
uid dominated reservoir at depth and boiling, reactions
and condensation in the upflow. The similarity of the gas
composition at Torfajökull with those in some other
fields in Iceland located in basaltic terrain suggests that
the same secondary mineral assemblages control the gas
TABLE 2.
Analysis of representative samples of fumarole steam from
Sample co2 H2S h2
no.
11 52.8 3.76 0.075
12 137.0 11.22 16.56
20 74.9 5.83 13.71
22 1133.5 52.78 198.2
29 177.9 11.38 20.29
31 62.6 5.95 1.86
35 170.0 6.87 4.61
36 1655.3 4.75 2.18
the Torfajökull field (mmoles/kg steam)
ch4 o2 n2 Ar
0.010 0.00 5.92 0.222
0.342 0.02 1.20 0.082
0.163 0.14 1.80 0.062
0.954 0.36 6.22 0.209
0.747 1.46 20.33 0.821
0.209 2.22 16.88 0.553
0.346 0.02 2.18 0.136
0.179 0.15 3.05 0.137
Sample locations are shown in Fig. 1. The first 4 analyses arefrom Arnórsson and Gunnlaugsson (1985), nos. 098,
100,110 and 120 in their Table 6.
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