RELATIONSHIP BETWEEN OXYGEN SHIFT AND WATER TEMPERATURE Like aqueous B concentrations oxygen isotope shift in geothermal waters from the best

O 0

+ NW-Peninsula 4 Hreppar

o Biskupstungur D Selfoss

o SL cold water

^

O

Thjorsardalur *

* o ° o o^ o n n r^ o « n° ß*

J3 * ^Drt o° °° P, n

jSf J*^*!»^ °

< **^^° ++ + D + D

—— , —— , —— , —— , —— ; —— , —— , —— , —— , —— , —— : —— , —— , —— rl-, —— , —— , —— . —— , —— , —— , —— , —— . —— , —— . —— 1 —— . —— , —— , —— . —— , —— , —— , —— 1 —— F——

0 20 40 60 80 100 120 140

Temperature °C

160 180

Fig. 8. Relationship between temperature of waters from the NW-Peninsula and the Southern Lowlands and the concentration of aqueous B derived from the dissolving rock.

data from the Southern Lowlands are limited but both alkalic and tholeiitic basalts are present.

The boron present in the rock is, as deduced from [18, 19] partly on a soluble form and partly within the structure of silicates. Relatively old basalts, which have undergone some alteration may have lost a large part of their soluble B. As a result waters percolating through such rocks may gain relatively little B for a certain amount of heating and rock dissolution. In waters from the Hreppar area in the Southern Lowlands B concentrations increase by about 0.4 ppm per 100°C (see Fig. 8). Taking the B content of the rock to be 3.8 ppm which represents the average value in alkali-olivine basalts in Iceland [13] the increase in B concentrations with temperature implies that the amount of rock dissolved by 100 °C water is almost 1/10 kg by every kg of water. In the NW-Peninsula the increase in aqueous B concentrations with temperature is only about 0.08 ppm for temperature increase of 100 °C (Fig. 8). Using a B concentration value for the rock of 1.2 ppm (average for tholeiitic basalt) gives a dissolved rock/water ratio of 1/15 for temperature increase of 100 °C.

7. RELATIONSHIP BETWEEN OXYGEN SHIFT AND WATER TEMPERATURE Like aqueous B concentrations oxygen isotope shift in geothermal waters from the best fit line through the 6 ²H - ô ¹⁸O cold water data is a measure of the amount of rock the water has reacted with. There is a general increase in the magnitude of the oxygen shift, not only for the geothermal waters of the present study areas but also of other geothermal waters in Iceland (Fig. 4). It is difficult to assess the increase in the oxygen shift with temperature due to the large scatter of the high-temperature data which are from [20] but it seems to be of the

order of one 6¹⁸O%o unit for every 100 °C, or a little higher. The amount of the shift depends not only on the amount of rock reacted but also on the initial ô ¹⁸O of the meteoric parent water and on the amount of a seawater component in the geothermal water. The ¹⁸O values of basalts in Iceland lie in the range +3.5 to +5.7 %o [21]. Selecting a value of +5 %o and taking the initial 0¹⁸O value of the geothermal water to be -8 %o (corresponds to 5²H of -56 %o) an oxygen shift of 1 %o corresponds to a ratio of 1/12 for dissolved rock/water.

This ratio is in reasonable agreement with the estimated dissolved rock/water ratios derived from B data. The highest oxygen shift values in Fig. 4 of about -5 %o correspond to almost 1/1 dissolved rock/water ratios.

8. CONCLUSIONS

The source of supply of Cl and B in cold and geothermal waters from the NW-Peninsula and the Southern Lowlands of Iceland are. (1) the atmosphere, i.e. seawater spray and aerosols of marine origin; (2) the rock being dissolved by the water; and (3) marine groundwater. In cold waters, supply from the atmospheric source dominates. In geothermal waters, on the other hand, contribution from the dissolving rock is more important and accounts for practically all the aqueous B to temperatures as low as 30-40 °C. Supply from the dissolving rock increases with temperature indicating that the amount of rock that dissolves per unit mass of water is proportional to the water temperature. In some areas close to the coast there may be a significant supply of B, and especially Cl, to the geothermal water from marine groundwater. In the case of the Southern Lowlands this marine water is considered to have entered the bedrock during the déglaciation period some 10,000 years ago when the area was submerged. On the other hand it is considered that infiltration of seawater under present-day hydrological conditions accounts for the marine groundwater component in some of the geothermal waters from the NW-Peninsula. In waters not containing a significant marine groundwater component the calculated amount of Cl derived from the atmosphere in the geothermal waters is similar to the Cl concentrations in local cold groundwaters, i.e. local precipitation. This is taken to indicate that the geothermal waters represent local precipitation. This interpretation agrees with previous interpretation of 0²H data in the case of the NW-Peninsula, namely that the geothermal waters represent local precipitation but not with previous interpretation of such data from the Southern Lowlands.

In that area the ô ²H of the geothermal waters is lower (more negative) than that of local cold waters. It is considered that the geothermal waters in the Southern Lowlands largely represent local precipitation. The lower 6 ²H-values, as compared with the local cold waters, are accounted for the presence of an "ice-age" water component that was precipitated when the climate was considerably colder than that of today and, therefore, isotopically lighter. The relationship between aqueous B concentrations and ô ¹⁸O with temperature indicate that the amount of rock dissolved by the water is roughly proportional to the water temperature and amounts to some 1/10 to 1/15 kg of rock per kg of water for every 100 °C increase in temperature.

References

[1] ARNASON, B., Groundwater systems in Iceland traced by deuterium, Societas Scientiarum Islandica 42 (1976) 236 pp.

[2] ARNASON, B., Hydrothermal systems in Iceland traced by deuterium, Geothermics 5 (1977) 125-151.

[3] HENLEY, R.W., et al., Fluid-mineral equilibria in hydrothermal systems, Reviews in Economic Geology 1, Society of Economic Geologists (1984) 267pp.

[4] FRITZ, P., FONTES, J.C., Handbook of Environmental Isotope Geochemistry, Elsevier, Cambridge (1980).

[5] NUTI, S., Isotopic techniques in geothermal studies, Application of Geochemistry in Geothermal Reservoir Development, UNITAR-UNDP publ., Rome (1991) 215-251.

[6] GIGGENBACH, W.F., Isotopic composition of geothermal water and steam discharges, Application of Geochemistry in Geothermal Reservoir Development, UNITAR-UNDP publ., Rome (1991) 253-273.

[7] MCDOUGALL, L, et al., Magnetostratigraphy and geochronology of northwest Iceland, J. Geophys. Res. 89 (1984) 7029-7060.

[8] JOHNSEN, S., et al., The origin of Arctic precipitation under present and glacial conditions, Tellus 41B (1989) 452-468.

[9] MERLIVAT, L., JOUZEL, J., Global climatic interpretion of the deuterium-oxygen 18 relationship for precipitation, J. Geophys. Res., 84 (1979) 5029-5033.

[10] WEDEPOHL H.K. (Ed.), Handbook of Geochemistry, Springer-Verlag (1969).

[11] AIRO, J., The chemistry of groundwater in the LangjUkull-Thingvellir drainage area, Nordic Volcanological Institute report, Reykjavik (1982) 22 pp.

[12] SIGURDSSON F., EINARSSON, K., Groundwater resources of Iceland - availability and demand, Jökull 38 (1988) 35-53.

[13] ARNÔRSSON, S., ANDRÉSDÔTTIR, A., Processes controlling the distribution of B and Cl in natural waters in Iceland, in preparation.

[14] BJÖRNSSON, A., et al., The nature of hot spring systems in Iceland, Nâttùru-fraedingurinn 60 (1990) 15-38. (In Icelandic with an English summary).

[15] ARNORSSON, S., GÎSLASON, S.R., On the origin of low-temperature geothermal activity in Iceland, Nättürufraedingurinn 60 (1990) 39-56. (In Icelandic with an English summary).

[16] ARNÔRSSON, S., ANDRÉSDÔTTIR A., these Proceedings.

[17] ARNÔRSSON, S., et al., The distribution of Cl, B, ôD and ô'⁸O in natural waters in the Southern Lowlands of Iceland, Geofluids '93 Contributions to an International Conference on fluid evolution, migration and interaction in rocks, British Gas (1993) 313-318.

[18] ELLIS, A.J., MAHON, W.A.J., Natural hydrothermal systems and experimental hot water-rock interactions. Part II, Geochim. Cosmochim. Acta 31 (1967) 519-538.

[l 9] HARDER, H., Boron content of sediments as a tool in faciès analysis, Sediment. Geol.

4 (1970) 153.

[20] ARNÖRSSON, S., The use of mixing models and chemical geothermometers for estimating underground temperatures in geothermal systems, J. Vole. Geothermal Res., 23 (1985) 299-335.

[21] HÉMOND, C., et al., Thorium, strontium and oxygen isotope geochemistry in recent tholeiites from Iceland: Crustal influence on mantle-derived magmas, Earth Planet. Sei.

Lett. 87 (1988) 273-285.

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