Jökull - 01.12.1987, Síða 37

significantly higher than in steam from nearby fumaroles
(Fig. 2). This is attributed to partial dissolution and
subsequent oxidation of the H2S. By contrast, C02/H2
ratios are similar in the fumarole steam and in gas from
nearby steam heated pools.
Variations in C02/H2S ratios in the fumarole steam
can be related to topography in the Sveifluháls area.
C02 concentrations vary little over this area but H2S
tends to be lowest on the high slopes of the ridge of
Sveifluháls. It is considered that this reflects oxidation in
the upflow above the groundwater table.
Variable C02/H2S ratios within geothermal fields
have been used to infer about the direction of subsurface
flow of boiling water (see e.g. Ellis and Mahon, 1977),
the ratio decreasing “downstream” due to the higher
solubility of H2S in water. In the Sveifluháls area neither
multistage steam loss nor condensation can explain the
C02/H2S distribution, nor in fact the C02/H2 distribu-
tion (Fig. 3). If part of the fumarole steam was second-
ary and derived from variably boiled water, C02 would
decrease more rapidly with decreasing H,S (curve in Fig.
3A) than the data indicate and in the same way H, would
decrease more rapidly with decreasing CO, (curve in
Fig. 3B).
About half of the samples from the Krísuvík field have
N2/Ar ratios which lie in between that of the atmosphere
(84) and that of cold water (37) in equilibrium with the
atmosphere (Fig. 4.). The other samples have lower
ratios. The majority of the samples contain between 1
and 3 mmoles/kg of N2. Some samples with N2/Ar ratios
similar to that of cold water, even lower, may contain as
much as 14 mmoles/kg of N2. Such compositions are
considered to result from partial condensation of the
steam in water in the upflow (probably very close to the
surface) and simultaneous degassing of the steam heated
water. Samples with higher N2/Ar ratios and elevated N2
concentrations are probably atmospherically contam-
inated; suction of air from the soil occurred during sam-
pling. The solid curves in Fig. 4 show how N2/Ar ratios
change with N2 concentrations when steam mixes with
air, the former being formed by adiabatic boiling to
100°C of water initially at 250° and 300°C, respectively,
as indicated.
The ðD and ðO18 values of geothermal well water in
Fig. 3. Relation between C02, H2S and H2 concentrations in fumarole steam from the Sveifluháls area. The
curves show variations in gas concentrations in secondary steam formed from water previously boiled adiabat-
ically in one stage but to a different degree. The figures show how selected steam fractions (by weight), formed
during the primary boiling, relate to gas concentrations in secondary steam. The initial water compositions
selected for A and B were those giving fumarole composition by one stage adiabatic boiling corresponding with
the samples from Sveifluháls highest in H2S and H2, respectively. — Samband milli styrks á C02, H2S og H2 ígufu
á Sveifluhálssvœðinu. Ferlarnir sýna breytingar á styrk gastegunda í gufu sem myndast við suðu á vatni sem áður
hafði soðið innrœnt í einu þrepi, þó mismikið. Tölurnar sýna hvernig gufuhlutinn (miðað við þunga) sem
myndaðist ífyrra suðuþrepinu tengist styrk gastegunda ígufunni sem myndaðist íseinna suðuþrepinu. Tilað reikna
ferlana í A og B var valin samsetning á vatni sem gefur gufu við suðu í einu þrepi samsvarandi þeirri á
Sveifluhálssvœðinu sem hefur hæsta mœldan styrk á H2S og H2.
35