Jökull - 01.12.1987, Síða 44

(mmoles/kg)
densation according to Zb in Table 4 it was asssumed that
the cold water was at 5°C and the steam at 100°C. How-
ever, calculated values of Zb are relatively insensitive to
the temperatures of the water and steam at the point of
mixing particularly when the temperature of the parent
water is high (>300 °C). If it was erronously assumed
that condensation occurred at 100°C and that it actually
occurred at higher temperatures too high Zb values
would be obtained (Fig. 7B). However, if there is much
condensation, causing N2 to approach 10 mmoles/kg, or
more, the effects of the steam and water temperatures
are negligible.
If separation of steam and water occurs at elevated
pressure in the upflow steam condensation, using equa-
tions (3a) and (14), will be overestimated. Such sep-
aration will cause the N2 concentrations in the steam to
be high compared with its concentrations in steam
formed by adiabatic boiling to atmospheric pressure.
SUBSURFACE TEMPEREATURES
The gas geothermometers of Arnórsson and Gunn-
laugsson (1985), D'Amore and Panichi (1980) and Nehr-
ing and D’Amore (1984) have been applied to estimate
subsurface temperatures in the Krísuvík field using the
fumarole steam compositions (Table 4). In addition
C02/N2 ratios have been applied as a gas geothermom-
eter using equation (2) in this contribution. Evaluation
of condensation in the upflow, discussed in the previous
section, helps interpreting geothermometry results
which are based on gas concentrations (C02, H2S and
H2) in the steam. Thus, values given in parenthesis for
the C02 geothermometer in Table 4 were derived by
taking into account the effect of condensation according
to Zb (last column in Table 4) on the C02 concentrations
in the steam. Similarily H2S- and H2-temperatures could
be corrected.
In calibrating their gas geothermometers Arnórsson
and Gunnlaugsson (1985) assumed adiabatic boiling
from the temperature of the equilibrated reservoir water
to atmospheric pressure. In an earlier section it was
stated that it was difficult to provide direct evidence in
support of this assumption. However, the N2 concentra-
tions in the steam in most of the samples from the
Krísuvík field (1-3 mmoles/kg) indicate that the model
of adiabatic boiling is a reasonable approximation. To
generate these N2 concentrations in the fumarole steam,
boiling without separation of the phases over a temper-
ature range of 50°C, or more, is required depending on
the temperature of the parent water (Fig. 8). Alterna-
tive explanations would be (a) that the parent water
contained less than 0.71 mmoles/kg of N2 (the concentra-
tion in 5°C water in equilibrium with the atmosphere)
and that the steam was separated from the water at
elevated pressure or (b) that condensation in the upflow
counteracted the low N2 concentrations in the parent
water to give the observed concentrations. Both these
alternative explanations involve the counteracting effect
of two processes to give rather similar N2 concentrations
for the larger part of the Krísuvík samples whereas the
first explanation requires a single process only and is,
therefore, preferred.
When applying the gas geothermometers of Nehring
and D ’Amore (1984) it was assumed that a single liquid
phase existed in the reservoir where equilibrium was
attained and that boiling was adiabatic in the upflow.
The Sveifluháls Area
In this area the CO, geothermometer indicates sub-
surface temperatures in the range 290-300°C for the
majority of the samples. By correcting the analysed C02
concentrations for condensation according to Zb and Zc
Fig. 8. N2 concentrations in steam formed
by adiabatic boiling of water over the
temperature interval At and initially at
200°C, 250°C, 300°C and 350°C as indicat-
ed. The N2 content of the parent water was
taken to be 0.71 mmoles/kg. — Styrkur N2 í
gufu sem myndast hefur við innræna suðu
yfir hitabilið At. Upphafshitastig vatnsins
eru sýnd á myndinni. Gert var ráð fyrir að
styrkur N2 í upphafsvatninu vœri 0,71
mmól/kg.
42