Jökull - 01.12.1987, Page 9

200 *„ 300 oc 200 t 300 °r 200 t~~ 300 °r
lC02 lCC>2 ° lC02 u
JHO-JK 860C- KS/SA
87 08. 07I4 SA
Fig. 2. Comparison between the results of individual gas geothermometers. tb œ2 designates C02-temperatures
corretced for condensation according to Zb (see Table 3). — Samanburður milli niðurstaða einstakra gashita-
mœla. tb C02 táknar C02-hitastig þar sem leiðrétt hefur verið fyrir þéttingu samkvœmt Zb (sjá Töflu 3).
H2S and H2 concentrations in the steam and C02/H2
ratios tend to be lower than C02-temperatures and also
lower than C02-N2-temperatures, at least in samples not
containing elevated N2 concentrations. This is attributed
to reactions in the upflow tending to remove H2, and
especially H2S, from the steam (Arnórsson and Gunn-
laugsson 1985).
Steam condensation in the upflow has been estimated
for individual samples by the method described by Ar-
nórsson (1987) (Table 3). The extent of condensation is
quite variable, ranging between 0 and 80%. The rela-
tively low N2/Ar ratios of many of the samples high in N2
indicate that condensation involving mixing with cold
water predominates. Some of the samples with high N2
and low N2/Ar ratios contain considerable amount of 02.
The presence of 02 suggests that the cold water in which
the steam condensed partly must be surficial as 02 is
rapidly removed upon reaction with the rock.
Estimation of condensation in the upflow allows cor-
rection to be made for those geothermometers which
use gas concentrations in the steam. C02-temperatures
given in parenthesis in Table 3 have been corrected for
condensation by assuming mixing with cold water and
are on average 20°C lower than the uncorrected
C02 temperatures. The C02-N2 geothermometer as-
sumes that condensation occurs by conductive heat loss.
Similar corrections could, of course, be made for the
H2S and the H2-geothermometers.
The variations in the gas geothermometry temper-
atures within each area of the Torfajðkull field may be
that the fumarole steam has been derived from equili-
brated water occurring at different depths within the
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