Jökull


Jökull - 01.12.1987, Page 10

Jökull - 01.12.1987, Page 10
reservoir. It is quite possible that temperatures follow the boiling point curve with depth in all the areas. The highest subsurface temperatures are indicated by fuma- roles near the mouth of Brandsgil in the Landmanna- laugar area and by two of the four fumaroles sampled at Reykjadalir. In both cases over 300°C is indicated. Tem- peratures of 300°C would be expected at about 1000 m depth if they follow the boiling point curve with depth. The data presented in this contribution cover only a small part of the Torfajökull geothermal field. They are too limited to picture lateral variations in subsurface temperatures that may exist across the field. HELIUM ISOTOPES Helium isotopes in fumarole steam from Torfajökull have been determined by Poreda etal. (1984). Ratios as high as 23.4 times atmospheric values have been en- countered. These high ratios are taken to indicate a relatively primitive undegassed mantle source and have been correlated with the identified mantle plume under the SE-part of central Iceland (Poreda et al. 1984). Com- parable ratios occur in the geothermal fields in the vol- canic zone to the northeast of Torfajökull. Rise of mag- ma from a primitive source below Torfajökull and in- jection along the fissure swarm could be the cause of these high ratios. High helium isotope ratios (20 times atmospheric) are also observed west of the Torfajökull volcanic complex in the vicinity of the transform fault in southern Iceland connecting the two volcanic zones (Kononov et al. 1974, Poreda et al. 1984). Elsewhere in Iceland helium isotope ratios in geothermal discharges are lower with the exception of some localities in Terti- ary formations on the Northwest-Peninsula (Kononov et al. 1974). Basaltic rocks in the Torfajökull region have helium isotope ratios comparable with those of the fu- marole steam (Kurz etal. 1985, Condomines etal. 1983). RADON AND MERCURY The distribution of ground radon and mercury in soil has been studied in some detail in the Hrafntinnusker area (Fig. 1). The techniques used to detect the radon and mercury emission are very similar to those currently used in geothermal exploration (Varencamp and Buseck 1983, Whitehead 1984). Radon detection was achieved at over 250 locations using integrated solid-state alpha par- ticle detectors giving results reproducable to within 15%. Solid samples were collected at each of these loca- tions, as well as samples from prominent spring orifices, and analysed for mercury using a gold film mercury detector which yields results reproducable to within 10%. The results of the radon and mercury measurements are depicted in Fig. 3. Individual measurements were ratioed to background levels. For radon the background values were empirically determined for each sample location. Average background value for mercury was calculated for specific areas in which measurements were made. As can be seen from Fig. 3 zones with elevated radon and mercury levels are observed following two distinct elongate trends. One is oriented in a NW-SE direction and the other in a NE-SW direction. The latter coincides with the present tectonic grain in the area but the former with an earlier tectonic fabric. Although there are some differences, the radon and mercury anomalies coincide fairly well. The highest radon and mercury levels occur where the linear anomalies intersect. Here, they are, at least, five times background values and, in some cases more than forty times background. Major thermal activ- ity is seen to lie just outside well defined zones of anoma- lous outgassing. It is considered that the radon and mercury anomalies are the manifestations of, and overly, permeability zones associated with tectonic fractures. The shapes of the anomalies of radon on one hand and mercury on the other probably differ because they reflect vapour circu- lation at different depths. For mercury circulation is at shallower depth. SUMMARY Geothermal manifestations in the Torfajökull field cover an area of about 140 km2 and they are almost entirely located within the caldera structure. The nat- ural heat output has been estimated to be equal to 190-930 kg/s of steam. The stored heat in the uppermost 3 km of the reservoir was estimated by Pálmason (1980) as 281 • 1018 J. The geothermal manifestations consist mostly of steaming ground which is, as a rule, intensely altered by acid surface leaching. Steam heated waters of the bicar- bonate and the acid sulphate types are relatively com- mon. In the northeastern part of the field, around Land- mannalaugar, sodium-chloride type waters occur. They represent boiled and variably mixed reservoir water. This water is higher in chloride than geothermal water associated with basaltic rocks in Iceland. The reason is considered to be higher chloride content of the rhyolites as compared with basaltic rocks. Fluoride concentra- tions are relatively high (5-25 ppm) in the sodium-chlo- ride type water as is the case for geothermal waters in Iceland associated with acid volcanics. Fluoride mobility is considered to be controlled by fluorite solubility. Oth- 8
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