Jökull


Jökull - 01.12.1984, Side 42

Jökull - 01.12.1984, Side 42
ppm (see Ellis and Mahon 1977) and in Icelandic geothermal waters (NEA data and Table 3) only saline or C02- waters have magnesium concentra- tions exceeding 0.1 ppm. We consider the com- parison with low-temperature C02-water from Lýsuhóll (40-60 °C, 20 ppm Mg) as done by Steinthórsson and Óskarsson (1983) to be mis- leading. Even carbonate-water from high temper- ature areas would have considerably lower Mg content as the solubility of Mg minerals increases with lower temperatures. Therefore, we do not believe that mixing of meltwater and geothermal water can explain the observed magnesium con- centration in jökulhlaups. We suggest instead the interaction between water and highly reactive volcanic glass to be a more likely explanation of the observed Mg concentrations in the water from Skeidará during jökulhlaups. The magne- sium was leached from the volcanic glass at temperatures not exceeding 40 °C, and the water has not been exposed to higher temperatures in the lake for any length of time. The Na/K ratio has been successfully used as a thermometer of geothermal water by many geochemists (Ellis 1979, Fournier 1981, Arnórs- son et al. 1983). However, the use of the Na/K geothermometer for highly diluted geothermal water is generally considered questionable (see Ellis 1979, Fournier 1981, and Benjamin et al. 1983). For waters originally at temperatures below 100 °C it is applicable only in special cases (Kristmannsdóttir and Johnsen 1982, Krist- mannsdóttir 1983). Steinthórsson and Óskarsson (1983) suggested that the Na/K ratio of water from jökulhlaups could be used to calculate the reservoir temperature in the Grímsvötn geoth- ermal system. The mean value of the Na/K ratio in samples from the jökulhlaup in February 1982 was 24±2 (standard deviation), in December 1983 24±6 and in 1976 the ratio was 19±4. Sam- ples of normal water in Skeidará in 1982 and 1983 show a mean value of 24 ±6 for the Na/K ratio. The variations of this ratio are greater from one month to another in 1982 and 1983 than between the jökulhlaups in 1976 and 1983. According to Steinthórsson and Óskarsson (1983) this ratio was 20 in samples from the jökulhlaup in 1982, 50 in samples from the jökulhlaup in 1972 but it was 40 in their „normal” water (see their table 3, p. 78). For comparison, the average ratio of Na/K in the rivers in SW-Iceland ranged from 17 to 40 in the years 1972/1973 (Ármannsson et al. 1972, Rist 1974) and the variation in samples from a single river was in the same range. In the river Súla the Na/K ratio in normal water varies from 14 to 65 and from 17 to 45 in the river Djúpá. In light of the described circumstances the use of a Na/K geothermometer on water in the jökulhlaups from Grímsvötn (Steinthórsson and Óskarsson 1983) appears questionable. Substances whose concentrations are not changed by reactions in the lake. Some substances are not expected to have leached out of basaltic glass in the Grímsvötn lake. First because the glass does not contain the substances in appreciable quantities. This applies for carbonate, fluoride and chloride. Secondly, the substances are poorly soluble in cold water. That applies for silica. Carbonate is not a major component of basal- tic glass and thus would not be leached in the lake. The concentration of total carbonate in the jökulhlaups is the result of plain mixing of the geothermal component and the meltwater. The concentration of carbonate (as C02) in normal river water is 10-30 mg/kg as shown in Fig. 8. The concentration for the meltwater component in Grímsvötn is probably lower and an estimate of 20 mg/kg (as C02) seems to be an upper limit. It is difficult to estimate the carbonate concentra- tion of the geothermal component. This con- centration is variable from one high-temperature geothermal area to another as shown in Table 3. The concentration of C1 and F cannot be signi- ficantly affected by water/rock interaction in the lake but may be slightly changed by increased volcanic activity as HCl and HF are components in volcanic gases. Silica is poorly soluble in cold water. Hence, no significant leaching is expected in the lake. Therefore, the observed silica concentration of the jökulhlaups reflects the original concentra- tion of the fluid that enters the lake. The discus- sion of silica will be continued separately in the next section. GEOTHERMAL MASS FRACTION AND THE BALANCES OF MASS AND ENERGY. Energy and mass balance equations have so far been used to describe the heat and mass flow in the Grímsvötn area (Björnsson 1974, 1983). Now 40 JÖKULL 34. ÁR
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