Jökull


Jökull - 01.12.1984, Page 45

Jökull - 01.12.1984, Page 45
ture, and its concentration is used as a geoth- ermometer (see Ellis 1979, Fournier 1981, Arnórsson et al. 1983). When water is close to magma, self-sealing due to precipitation of silica puts an upper limit of 330 to 350 °C to the temperature of the fluid (Fournier 1983). With increasing temperature, quartz has a solubility maximum at constant pressure. When this max- imum is reached (at about 340 °C) precipitation of quartz deep in hydrothermal systems may decrease the permeability to such an extent that convecting meteoric water no longer can attain temperatures higher than that given by the quartz solubility maximum. Known reservoir tempera- tures in Icelandic high- temperature geothermal areas range from 240 to 350 °C. Table 3 shows chemical concentrations of well discharges for five liquid dominated geothermal areas in Ice- land. The first three areas have boiling reservoirs with dilute fluid of meteoric origin. The table shows concentrations for both total discharge and deep water. The two other areas have saline reservoir water. The table shows the deep water concentrations. Further, calculated concentra- tions are given for water boiled at 235 °C for all the areas. Grímsvötn is a high-temperature geothermal system and the reservoir temperature is presumably above 300 °C. The fluid is dilute and probably liquid dominated. Boiling would occur at 235-250 °C on the lake floor, depending on the height of the lake level. The concentration of silica in the deep reser- voir water may be Cgw = 700 mg/kg. If fluid of that concentration were discharged into Gríms- vötn, we would estimate the geothermal mass fraction k=0.13 from equation (8). According to the calculations, illustrated in Fig. 9, the mass and energy balances would require the steam mass fraction to be x= 0.45 when the fluid enters the lake. During upflow, however, deep water as well as condensed steam would equilibrate with the formation rocks at or above 235 °C (given a few hours or days, see Rimstidt and Barnes 1980). Hence, we estimate the silica concentration Cgw= 400-600 mg/kg in the water entering the Gríms- vötn lake (see Table 3 for comparison). Further, we estimate the geothermal mass fraction k= 0.14-0.16 and the energy balance requires steam mass fraction x=0.20-0.35 for the fluid dis- charged to the lake; the mass flow of geothermal water Mgw= 0.60-0.83T011 kg/yr and Mgv= 0.24-0.34T011 kg/yr of steam. The mass of ice melted in the lake is estimated to be M;= 4.0-4.2T011 kg/yr. Furthermore, we expect the total thermal power of the Grímsvötn system to be 4700-4900 MW, of which 2100-3000 MW are transported by steam and 1900-2600 MW by water (see Fig. 9). Calculations similar to those for silica are diffi- cult for carbonate. A plausible estimate is not available for the carbonate concentration of the geothermal component as it varies from one high- temperature area (>300 °C) to another (see Table 3). But if we assume k=0.15 and if the meltwater component contains 20 mg/kg carbon- ate (as C02), (and Cr=Ca=Q), we can calculate the concentration for the geothermal component that would be consistent with the measured con- centrations in the jökulhlaups. The calculations show variations from C=2000 to 4500 mg/kg (as C02) for the geothermal component. This is high but not unlikely in an active volcanic area. Direct interaction with magma has been observed in the geothermal systems in Krafla and Námafjall (Björnsson et al. 1979). The concentration of C02 in geothermal fluids in the Krafla area increased considerably during the recent volcanic events (Armannsson et al. 1982). The concentrations of fluoride and chloride may well be consistent with a geothermal mass fraction k=0.15 (see Table 3). VOLCANIC ACTIVITY DEDUCED FROM WATER CHEMISTRY The high concentrations of sulphate and iron (as well as carbonate) during the jökulhlaup in December 1983 suggest direct contact between magma and geothermal fluid. Sulphate (S04) in the Grímsvötn lake origin- ates from oxidation of H2S as well as from the S04 in the geothermal discharge. The contribu- tion from the meltwater is small as is evidenced by the glacier rivers when not influenced by jökulhlaups (Fig. 8). The concentration of sul- phate will be influenced by volcanic activity. We may even expect a sharper increase in sulphate than carbonate shortly after volcanic activity because H2S (and S02) is more soluble in water than C02. This may explain the very high con- centration of sulphate in the jökulhlaup of December 1983 as compared to those of 1972, 1976 and 1982. The reported concentration of sulfate in the jökulhlaup in 1965 was also very high (Sigvaldason 1965). JÖKULL 34. ÁR 43
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