Jökull - 01.12.1983, Side 82

attributed to the initial scouring of altered bedrock
beneath the glacier.
'I’he composition of the discharge peak (No. 3,
Table 3) is high in alkalis, alkaline earths, silica,
carbonate, and sulphate indicating only moderate
dilution of the reservoir. Therefore it seems feasible
to try and restore the composition of the geothermal
component in order to define its thermal energy and
its contribution to the Grímsvötn system. The
calculations are based on the following assuptions:
a) The element ratios of the geothermal fluid are
preserved in the cold (4°C) reservoir, and the
precipitation of opaline silica in the reservoir is
prevented by mixing with melt water produced by
thermal equilibration of the geothermal fluid.
b) A chemical contribution from the precipit-
ation in the Grímsvötn drainage basin is neglected.
Analysis of chloride and sodium in samples from the
Bárdarbunga ice core (C1 = Na = ca. 1 ppm,
Science Inst., unpublished analyses) indicate high
altitude precipitation of no chemical importance to
the subsequent calculations.
c) Reactions of the cold water with the rock sur-
face of the caldera are assumed to be of no
significance.
The restoration of the reservoir water is made by
subtracting the components of normal Skeidará
water from those of the appropriate jökulhlaup
analyses. The restored compositions of the 1972 and
1982 bursts are shown in Table 3 (Nos. 5 and 6).
Applying chemical geothermometers to mixed
geothermal systems requires a knowledge of the
relevant end members. In the case of Grímsvötn
that problem is reduced to dilution of the geo-
thermal fluid with pure water. The selection of
chemical equilibria in the restoration of the geo-
thermal fluid is straight forward: we seek geo-
thermometers with the highest degree of internal
consistency. The calculation proceeds as follows
(see Table 4).
Table 4. Calculated temperatures and dilution factors of Grimsvötn thermal waters.
Tafla 4. Reiknuð hitastig og þynning jarðhitavatns í Grímsvötnum.
Year of jökulhlaup Alkali temperature1' Na-Ca-K temperature2* Restored silicia3' pH4) Pco25) Dilution factor
1972 104°C 120°C 89.7 ppm 5.9 1.44 atm 2.8
1982 192°C 178°C 244.3 ppm 5.7 1.7 atm 3.76
1) t°C = ------—-------------- 273.15 (Amórsson 1980)
log(Na/K) + 0.993
2) 1.6473 X 103/T°K — 2.2405 = log(Na/K) + í/3logCvCa/Na) (Foumier and Truesdell 1973)
3) Equilibrium with chalcedony (104°C, 1972-hlaup):
t°C = -------—--------------273.15 (Fournier 1977)
4.69 - logH4Si04
Equilibrium with quartz (192°C, 1982-hlauþ)
t°C = -------—--------------273.15 (Fournier 1977)
5.19 - logH4Si04
4) pH calculated at the appropriate alkali temperature assuming ionic balance and carbonate in
equilibrium with calcite:
(H+)2 = (Ca2+) ■ KHCQf ' Kh;co3 (co2)liq
K cc
where (Ca2+) is molality ofcalcium in the solution, KH2co3 and KHco3- are the lst and 2nd dissociation
constants of carbonic acid, Kcc is the solubility product of calcite, and (C02)hq molality of dissolved
C02 in the solution. (Data from Amórsson et al. 1982).
5) Partial pressure of C02(gas) in equilibrium with that dissolved in the solution. (Data from Arnórsson et
al. 1982).
80 JÖKULL 33. ÁR