Jökull


Jökull - 01.12.1987, Page 44

Jökull - 01.12.1987, Page 44
(mmoles/kg) densation according to Zb in Table 4 it was asssumed that the cold water was at 5°C and the steam at 100°C. How- ever, calculated values of Zb are relatively insensitive to the temperatures of the water and steam at the point of mixing particularly when the temperature of the parent water is high (>300 °C). If it was erronously assumed that condensation occurred at 100°C and that it actually occurred at higher temperatures too high Zb values would be obtained (Fig. 7B). However, if there is much condensation, causing N2 to approach 10 mmoles/kg, or more, the effects of the steam and water temperatures are negligible. If separation of steam and water occurs at elevated pressure in the upflow steam condensation, using equa- tions (3a) and (14), will be overestimated. Such sep- aration will cause the N2 concentrations in the steam to be high compared with its concentrations in steam formed by adiabatic boiling to atmospheric pressure. SUBSURFACE TEMPEREATURES The gas geothermometers of Arnórsson and Gunn- laugsson (1985), D'Amore and Panichi (1980) and Nehr- ing and D’Amore (1984) have been applied to estimate subsurface temperatures in the Krísuvík field using the fumarole steam compositions (Table 4). In addition C02/N2 ratios have been applied as a gas geothermom- eter using equation (2) in this contribution. Evaluation of condensation in the upflow, discussed in the previous section, helps interpreting geothermometry results which are based on gas concentrations (C02, H2S and H2) in the steam. Thus, values given in parenthesis for the C02 geothermometer in Table 4 were derived by taking into account the effect of condensation according to Zb (last column in Table 4) on the C02 concentrations in the steam. Similarily H2S- and H2-temperatures could be corrected. In calibrating their gas geothermometers Arnórsson and Gunnlaugsson (1985) assumed adiabatic boiling from the temperature of the equilibrated reservoir water to atmospheric pressure. In an earlier section it was stated that it was difficult to provide direct evidence in support of this assumption. However, the N2 concentra- tions in the steam in most of the samples from the Krísuvík field (1-3 mmoles/kg) indicate that the model of adiabatic boiling is a reasonable approximation. To generate these N2 concentrations in the fumarole steam, boiling without separation of the phases over a temper- ature range of 50°C, or more, is required depending on the temperature of the parent water (Fig. 8). Alterna- tive explanations would be (a) that the parent water contained less than 0.71 mmoles/kg of N2 (the concentra- tion in 5°C water in equilibrium with the atmosphere) and that the steam was separated from the water at elevated pressure or (b) that condensation in the upflow counteracted the low N2 concentrations in the parent water to give the observed concentrations. Both these alternative explanations involve the counteracting effect of two processes to give rather similar N2 concentrations for the larger part of the Krísuvík samples whereas the first explanation requires a single process only and is, therefore, preferred. When applying the gas geothermometers of Nehring and D ’Amore (1984) it was assumed that a single liquid phase existed in the reservoir where equilibrium was attained and that boiling was adiabatic in the upflow. The Sveifluháls Area In this area the CO, geothermometer indicates sub- surface temperatures in the range 290-300°C for the majority of the samples. By correcting the analysed C02 concentrations for condensation according to Zb and Zc Fig. 8. N2 concentrations in steam formed by adiabatic boiling of water over the temperature interval At and initially at 200°C, 250°C, 300°C and 350°C as indicat- ed. The N2 content of the parent water was taken to be 0.71 mmoles/kg. — Styrkur N2 í gufu sem myndast hefur við innræna suðu yfir hitabilið At. Upphafshitastig vatnsins eru sýnd á myndinni. Gert var ráð fyrir að styrkur N2 í upphafsvatninu vœri 0,71 mmól/kg. 42
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