Jökull - 01.12.1987, Page 36

heat source is compatible with the many isolated occur-
rences of thermal manifestations and the short lifetime
of some of them, like Hverinn Eini (no. 23 in Fig. 1). At
the end of the last century this was an extremely strong
fumarole (Thoroddsen, 1925). Today it is almost extinct.
GAS COMPOSITIONS
The concentrations of C02 in the fumarole steam is
most often in the range 200-300 mmoles/kg and this gas
constitutes 80-90% of the total gas (Tables 1 and 2). The
remainder is mostly H2S and H2 except in samples which
are significantly air contaminated and contain, there-
fore, rather high N2. CH4 is very low in most samples,
less than 0.1 mmoles per kg of steam. H2S and H2 show
much larger variation than C02 in both the eastern
(Sveifluháls area) and the western (Vesturháls area)
parts of the Krísuvík field. However, their concentra-
tions are an order of magnitude lower around Trölla-
dyngja in the latter area. Drillhole data indicate that the
reservoir water around Trölladyngja is more saline
(>1000 ppm Cl') than that under Sveifluháls (500 ppm
Cl') (Arnórsson etal. 1975a). Arnórsson and Gunnlaugs-
son (1985) showed that water above 200°C and with less
than about 500 ppm Cl' equilibrates with the buffer
epidote + pyrite + pyrrhotite whereas saltier waters
equilibrate with epidote + pyrite + magnetite or chlo-
rite, the latter mineral buffer fixing H2S and H2 at lower
concentrations at any particular temperature. It seems
likely that H2S and H2 in the reservoir under Trölla-
dyngja, and probably also by Köldunámur, equilibrate
with the mineral buffer including magnetite/chlorite
whereas equilibrium with the pyrrhotite containing buff-
er is attained in the south, under Sveifluháls and Hve-
rinn Eini (Fig. 1). However, it cannot be excluded that
reactions with atmospheric oxygen contribute signifi-
cantly to the low H2 and H2S concentrations in the
fumarole steam at Trölladyngja and Köldunámur due to
the high permeability of the surface formations.
In the Sveifluháls area the H2 concentrations in the
steam are low relative to H2S when compared with other
studied geothermal fields in Iceland with low chloride
reservoir waters (Arnórsson and Gunnlaugsson, 1985).
In gas samples from hot pools of steam heated water
in the Sveifluháls area C02/H2S ratios are on the whole
A
o
o
z
t
STEAM HEATED POOLS
-► CO^/H^S
B
co2/h2
Fig. 2. Distribution of C02/H2S and C02/H2 ratios in fumarole steam and in gas from hot pools of steam heated
water in the Sveifluháls area. — Dreifing C02/H2S og C02IH2 hlutfalla í gufuaugum og í gasi í hverum með
gufuhituðu vatni á Sveifluhálssvœðinu.
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