Jökull


Jökull - 01.12.1987, Side 37

Jökull - 01.12.1987, Side 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
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