Jökull - 01.12.1983, Side 83

1) The alkali temperature of the geothermal
fluid is calculated.
2) A dilutiorr factor for the geothermal comp-
onent in the reservoir is calculated, assuming
equilibrium with quartz or chalcedony.
3) Dilution due to melting is calculated from the
energy released through the cooling of the geo-
thermal fluid to 4°C. The difference between (2)
and (3) is melt water of meteorological origin.
4) The composition of the geothermal fluid is
restored by means of the dilution factor.
5) A Na-Ca-K geothermometer is tested against
the selected Na/K thermometer. (The former type
of thermometers are only applicable to undiluted
solutions).
6) The ionic balance, i.e. the difTerence in molar
equivalent sums of the positive and negative ions,
was calculated for the analyses of the restored geo-
thermal fluid, showing about 50% preponderance
in anions. This was interpreted as excess in
carbonate, resulting from free C02-gas in the
hydrothermal fluid that was dissolved in the caldera
lake water upon emergence from the hydrothermal
system. Therefore, sufficient carbonate to balance
the analyses (Nos. 7 and 8, Table 3) is reported as
bicarbonate (HCOj) and the rest as excess C02 gas.
The partial pressure of C02 in bubbles of gas and
steam, giving rise to the calculated bicarbonate in
solution at the appropriate temperatures, is given in
Table 4.
7) Assuming that calcium in solution in the fluid
was in equilibrium with calcite the pH can be
calculated from equations of carbonate equilibria.
The calculated values are reported in Tables 3 and
4.
The results of the restoration are shown in Table
3 (No. 7 for 1972, No. 8 for 1982) and in Table 4.
SUMMARY OF CHEMICAL INDICES
1) The low calculated pH indicates that the
approximation behind the use of the silica geo-
thermometer, i.e. that the analyzed SiOa equals
H4Si04 in the geothermal fluid, is justified for the
calculation of dilution factor.
2) The relatively good correlation between the
Na/K and Na-Ca-K geothermometers appearing in
Table 4 suggests that the assumed preservation of
element ratios in the reservoir is valid.
3) The very different calcium concentrations
which are in accordance with almost similar pH at
different temperatures strongly suggest that the
assumed calcite control is valid.
4) If it is assumed that the sulphide of the geo-
thermal fluid is oxidized to sulphate in the caldera
lake due to excess of ice-trapped oxygen, and that it
is represented by the analyzed sulphate it follows
that the hotter fluid of the 1982 burst indicates
Fig. 4. Variation of some cations prior to and
during the 1972 Skeidará jökulhlaup. Chemical
monitoring would give a clear indication of an
impending hlaup a week before its advent.
Mynd 4. Breytingar í styrk nokkurra efna í Skeiðarár-
hlaupinu 1972. Yfiroofandi hlaup mundi koma fram í upp-
leystum efnum í ánni u.p.b. viku áður en vaxa tæki í henni.
JÖKULL 33. ÁR 81