Chemistry of Thermal Waters and Gases in Iceland * Introduction

Transcription

1 Chemistry of Thermal Waters and Gases in Iceland * G, E. SIGVALDASON University Research Institute, Reykjavik Introduction Thermal activity in Iceland has been classified by BODVARSSON (1961) into two groups on the basis of temperature conditions at depth in the geothermal areas. In the low temperature areas the maximum temperature at the base of water circulation is 150 C. Geothermal areas where temperature is higher than 150"C are classified as high temperature areas. This division finds a logical background in the geological surroundings of the thermal areas. Iceland is entirely built up by volcanic rocks ranging in age from Tertiary to Recent. Fig. I gives a geological map of Iceland showing the Tertiary plateau basalt series in the East, West and North. This is cut by a broad belt of Pleistocene and Holocene volcanics running from Southwest to Northeast, often referred to as the Neovolcanic Zone. During this century the frequency of volcanic eruptions has been one every fifth year on the average. The high temperature thermal areas are all situated within the Neovolcanic Zone, but the low temperature areas are mostly found in the Tertiary plateau basalts in the West and North. The present paper describes the chemical character of the thermal waters and gases, and is based on analytical material which has accumulated through the past 15 years. In order to check the quality of older data several new analyses have been run from a number of thermal areas. These new analyses have generally shown good agreement with the older material, also indicating the constancy of chemical composition of hot spring water over extended periods of time. The gas analyses reported in this paper are mostly new, the latest ~" Paper read at the IAV International Symposium on Volcanology (New Zealand), scientific session of Nov. 30, 1965.

2 were run with a combined Orsat and gaschromatographic technique. Since the major interest in utilization of geothermal energy in Iceland is for domestic and greenhouse heating, the greater part of the analytical material is from the low temperature areas. The high tern-,,, ~.th),x',,\\\~,~ NliO-VO LGANIC, ZONE k,.,t/-".,~ ~ ICELAND ".,//,~," x MAGNITUDE Z O gr~'suvik J ~ ~ ~"* t'~'=~oovm LOW" TEIM~.RATURE LINES FiG. I - Geological map of Iceland, showing major thermal areas (BSDVARSSON, 1961). perature fields are, with few exceptions, too remote from populated areas to be of immediate value for such utilization and less attention has therefore ben given to their investigation. The Low Temperature Areas For a detailed description of thermal areas in Iceland the reader is referred to BARTH (1950). The number of thermal springs in the low temperature areas is approximately 600 and water analysis are now available from all of those. The hot springs are mostly concentrated in valleys of the Tertiary plateau basalts, often with a linear alignment along dikes. The temperature of these springs spans the range from slightly above normal temperature of surrounding coldwater springs to 100 C. The highest temperature measured directly in a drill hole in Reykjavik is 146 C at a depth of 2200 m, but a temperature of 140 C had already been reached at 1200 m (B~DVARSSON and PALMASON, 1961).

3 591 The chemistry of the low temperature waters varies with regard to some elements whereas others appear to be relatively constant. In table 1 analyses of hot spring waters from Eyjafj6rdur in North Iceland are presented. In this group of analyses the chemical variation is small except for silica, which spans the range from 70 to 123 ppm SiOv TABLE 1 - Chemical analyses of hot spring water from Evjafj6rdur (New analyses. Not used in compilation of Fig. 5) SiO ,2 Ca Mg l Na K HCO~ CO~ OH CI SO.; F e Diss. solids ph ~ 9.94 T"C I. Laugaland, H6rg~u'dalur, 21.6, Laugaland, Ev.jafjOrdur, Kristnes, Hrafnagil, Brt~nalaug, 23.7, Botn, Glerardalur, 21, Svalbardseyri, A common maximum value [or silica in the low temperature waters is 180 ppm and this will vary downwards to 50 ppm or even less, within the same thermal area. Generally the spring water containing the highest amount of silica is either boiling or of higher temperature than springs with lower silica content. This temperature

4 dependence of the amount of dissolved silica led BSDVARSSON and PALMASON (1961) to use the silica content as a rough indicator of the subsurface temperature. Mean values of silica content within temperature groups gave a straight correlation TB + 25 = ppm SiOz, where TB is the highest temperature at the base of water circulation or base temperature. This correlation was interpreted as indicating the solubility of basaltic rock in water. T~ 7O 6~.jl SO.o! 3O to /0 ao JO ao ~0 /o0 Si 0. moll //0 /~0 /30 / FI Dissolved silica in thermal water of two low temperature areas plotted against temperature of respective hot springs. From studies on hydrothermal alteration at depth in Icelandic geothermal areas (SIavALDASON, 1963) it is apparent, that the amount of silica in deep thermal waters is controlled by solution equilibrium with quarz, and the temperature of surface springs may have a very indirect relation to the silica content of the deep water. To illustrate this point silica content has been plotted against temperature of surface springs from two thermal areas in northern and western Iceland (Fig. 2). A linear relation does exist between silica content and temperature in both cases but the points are scattered within two parallel fields. This relation does probably not represent solution equilibria but fits well to a mechanism of mixing hot water with cold, silicapoor surface water. The separation of the two fields would indicate a difference in the silica content of the deep water from the individual areas.

5 BORGARFJO'RDUR S~ Ot mg/l f S t olot o48 Fla. 3 - Distribution of silica in hot springs of Borgarfj6rdur. (Scale 1: ). 'DJSP I I / / FIG. 4- Distribution of silica in hot springs of Isafjardardjtip. (Scale 1: ). 38

6 J EYdAtr#ORIPUR FLdOT $10z Bf /L FIG. 5- Distribution of silica in hot springs of EyjafjSrdur. (Scale 1: ).,PlNGEYJARS(*SLA S~ Ot rng /L TJd#NES o 4 FIG. 6 - Distribution of silica in hot springs of Thingeyjars~sla. (Scale 1: ).

7 As silica is the largest component contributing to the dissolved solids of the thermal water, and the difference between the silica content of the thermal water and cold surface water is marked, the dilution effect will show up most strongly for this element. \ SKAGAFJO'RDUR S;O a mo/l I \\\xx\ \ ~o _~.5o I I n59 ~. 60 /?2"--.o I 1 I 73 ~ 70 Fl(;. 7 - Distribution of silica in hot springs of Skagai'j~Srdur. (Scale 1: ). In order to study the field relation between thermal waters with different silica contents the silica values are plotted on a map of the thermal area (Figs. 3-7). In all cases a distinct pattern in observed showing one or more silica maxima with regularly decreasing silica values away from the maxima. The water with the highest silica content in any thermal area probably bears the closest resemblance to the deep water of that area and also indicates the zone of major upflow of deep water. The regular pattern of decreasing silica or dilution might be interpreted as flow of thermal water away from the zones of major upflow along

8 shallow layers. Since the involved distances of flow are considerable one must bear in mind the stratigraphic regularity of the Tertiary plateau basalt series, where individual lava flows can be traced over SKAGAFJORDUR % I o O 6it =8 41' 9 FIG. 8 - Distribution of sulfate in hot springs of Skagafj6rdur. (Scale 1: ). tens of kilometers. Structural discontinuities such as dikes and faults must, however, be less effective in directing the main water flow than hitherto believed, although many hot springs are aligned on such structures. Sulfate shows a similar pattern of distribution as the silica (Fig. 8). Increasing solubility of calcium sulfate with decreasing temperature and oxidation of H2S work against the dilution effect by increasing the amount of sulfate. Other components do not show any pattern of distribution sim-

9 liar to that of the silica or sulfate. This is partly caused by the fact, that the concentration differences between the cold surface water and the thermal water are too small to make the dilution effect apparent and partly because the effect is masked by reaction with wall rock. The concentration of chloride is of the order of ppm BORGARFJORDUR CI mg/l i ~ 7/ il \.3,; i I g ~,g'~, ~.~."-~'.--~ H(ssate:/ / ~,.o-.:j go 70 llj, t / /,,/," 1 I ~Li ". l / / i ]~.~,.L_unclur, / / /;,'y-: /,,,,, Fit;. 9 - Distribution of chloride in hot springs of Borgar[j6ordur. (Scale 1: ). in the low temperature waters, and the amount of chloride in the precipitation is of the same order of magnitude. In a few instances, as in the Borgarfj6rdur thermal area (Fig. 9), the chloride reaches higher values, but here the thermal water has flowed through marine sediments, which thicken towards the coast. The information on the amount and distribution of alkalimetals and alkaline earths is too incomplete to give any general picture. The amount of sodium is about ppm. Potassium amounts to 2-3 ppm and rubidium 0,01 ppm. There are some indications of depletion of K and Rb with distance from the zones of major upflow, due to adsorption on montmorillonite in the near surface layers. Calcium and magnesium are usually in the order of 2 ppm and 0,5 ppm respectively.

10 TABLE 2 - Chemical analyses of water from zones of major upflow within the low temperature areas. I 1 _ 2 SiO Na K Ca Mg el F SO A 48, HC0~ C OH 8.0 Diss, solids ToC I ph Laugavatn, Reykjavik, drillhole G-4, , Borgarfj6rdur, Reykholt, driuhole, Sr ; Rb: 0.03; Br: 0.09; B: 2.3; As: SkagafjSrdur, Varmahlid, Flj6t, Akrar, Thingeyjars3~sla, Yztihver, On the basis of the above evidence it is suggested that the major part of thermal springs within the low temperature areas in Iceland are derived from waters of a deep source by dilution with cold water and reaction with wall rock, which is funneled towards the surface in relatively few zones of upflow. Table 2 is a comparison of analyses of water from the zones of upflow of individual thermal areas. The chemistry of these waters from widely separated localities is very uniform, indicating a similar origin in all cases. If one regards the concentration of individual elements as representing solution equilibria with the basaltic rocks at depth, the analyses indicate a rather similar base temperature for all low temperature thermal areas. Using the solution curve for quartz at different temperatures (KENNEDY, 1950,

11 cited from ELLIS and MAHON, 1964), a temperature of approximately 170 C would be needed to bring 180 ppm SiO2 into solution. 'This is somewhat higher than the upper temperature limit for the low temperature areas given by B(JDVARSSON (1961). TABLE 3- Analyses of thermal gases from the BorgarfjSrdur low temperature area. July Vol % CO2 02 N2 H~ CH~ Reykholt Nordurreykir Kleppjfirnsreykir Deildartunga Varmaland t The composition of gases from the low temperature areas is characterized by high content of nitrogen, which in most cases amounts to more than 95 percent. Carbon dioxide is present in some springs, especially in zones of major upflow, but others are a simple mixture of nitrogen and argon with traces of methane. Analyses from the Borgarfj6rdur area are given in Table 3. In BorgarfjSrdur a similar distribution pattern is observed for carbon dioxide as was the case for dissolved silica in the thermal waters, in the zone of major upflo,w the CO2 content is 13,1 percent, but decreases regularly with increasing distance from the upflow zone to 0,2 percent CO2 some 12 km away. Argon is in the order of two percent and the NJA ratio is approximately what would be expected from the solubility of these gases in cold surface water. A trace of hydrogen has been found in a few samples especially from zones of major upflow. The High Temperature Areas Relatively few analyses are available from the high temperature areas. Table 4 lists four analyses, which are representative of as many major thermal areas. As compared to waters from the low temperature areas these waters contain about three times as much dissolved solids. The highest temperature measured in drill holes in Hveragerdi

12 is 230 C (Bt~DVARSSON and P~.LMASON, 1961) and the larger amount of dissolved solids can at least in part be explained as resulting from higher solubilities at higher temperatures. At 230 C the water should contain 400 ppm SiO2 in equilibrium with quartz, which is lower than the amount found in most high temperature waters. The actual value of ppm SiO2 would indicate a base temperature of C. TABLE 4 - Analyses of thermal water from high temperature areas Si02 I Na ; K Ca Mg El ~ F ,4 3.3 SO, ; HCO CO ~8.6 OH '3.5 ph Diss. solids ~I H2S T=C I Geysir, Haukadalur, Hveragerdi, drillhole G-8, Krisuvik, drillhole, Hveravellir, The thermal water from Krisuvik (Table 4, col. 3) contains abnormally high sodium chloride as compared to the other high temperature waters in Iceland. The hydrography of the surrounding area is not well understood and the possibility of admixture of some sea water into the system cannot be excluded. The composition of gases from the high temperature areas is quite variable. Most of the differences found in the composition of

13 gases from natural fumaroles within the same thermal area are, however, believed to result from near surface alteration due to reaction with wall rock and the atmosphere. Gas samples obtained from drill holes give more uniform results, but major differences may, however, be found within the same area. Gas analyses from four thermal areas are listed in Table 5. TABLE 5 - Analyses of thermal gases from high temperature areas. CO: H,S H, N~+A CH~ ! 3.' I. Hveragerdi, drillhole, Krisuvlk, drillhole N~imaskard, driilholc, Kverkfj611, fumarole The analyses in the three first columns are from drill holes, but the last is a gas from a natural fumarole. The gases, especially from N,Smaskard and Kverkfj61I, contain high amounts of hydrogen, but in Hveragerdi and Krisuvik hydrogen sulfide is in excess of the hydrogen content. This high hydrogen content is a unique feature of the chemistry of thermal gases, and at the present time no explanation can be offered. It might, however, be pointed out, that the thermal areas in question m-e associated with very recent volcanic activity, and gas samples from the presently active volcanic island Surtsey off the Icelandic south coast contain similarly high amounts of hydrogcn although the chemistry is otherwise different. Discussion EINARSSON (1942) suggested that the chemical components of thermal water in Iceland could be adequately accounted for on the

14 basis of wallrock leaching, and that the generally accepted view of magmatic origin would need some revision. In their paper on experimental interaction between hot water and rocks ELLIS and MAHON (1964) concluded, that the dissolved components in thermal fluids could in most cases be explained on the basis of wallrock leaching. The components could be divided into two groups. Those which are governed by solution equilibria with minerals in the wallrock, and those, which cannot be accommodated in the structure of secondary minerals and become concentrated in the water phase. As would be expected one finds similar amounts of components of the first group in waters from thermal fields with similar base temperatures. Chemical components on the other hand, which are not governed by solution equilibria are in such low concentrations in the Icelandic waters are compared to thermal waters from other parts of the world, that this calls for a separate explanation. The components in question are among others chloride, bromide, boron and arsenide. In the Icelandic waters the amount of these elements is a whole order of magnitud less than in thermal waters from New Zealand, Japan or the U.S.A. The highest concentration of chloride in high temperature thermal water from Iceland is 600 ppm, but ppm is a more common figure. In New Zealand and the U.S.A. the thermal water contains ppm C1 and 4000 ppm CI has been reported from Japan, (WroTE, HEM and WARING, 1963). If we accept wallrock leaching as a primary mechanism in determining the chemical composition of thermal waters, two possible explanations of these differences might be suggested. The first concerns the availability of the elements in the rocks. There are some indications suggesting a very low boron content in Icelandic rocks as compared to volcanic rocks of strictly continental origin. The available data are, however, too few in order to give any conclusive evidence. The second and favoured explanation is a low rock/water ratio in the Icelandic thermal systems. This would indicate either a very small contact area between water and rock or a high rate of flow through permeable layers. In a young volcanic structure like Iceland, with recent fault systems, one would expect high permeability. In this case, and in order to maintain a high rock/water ratio, the rate of flow through the thermal system has to be considerable in order to explain the dilute character of the water. The age of the thermal area will also be a significant factor since the readily soluable elements would be

15 removed from the rock/water contact area in the early stages of the life of the system. Acknowledgements Thanks are especially given to Dr. Gunnar BODVARSSON, who initiated most of the chemical work, which has been done on the thermal waters and gases in Iceland. Gunnlaugur ELfSSON, chemist at the University Research Institute, has done most of the gas analyses, which are presented, and his skilled work is especially appreciated. Furthermore I would like to thank J6n J6NSSON, geologist, at the Geothermal Department of the State Electricity Authority, who has kept a file of the older data and made it accessible. Finally I would like to thank the Bauer Scientific Trust for a grant which made field trips for collecting water and gases in remote areas possible. References BarTH, T. F. W., Volcanic Geology Hot Springs and Geysers o/ Iceland. Carnegie Inst. Washington Publ BiJl~x:Arssor% G., Physical Characteristics of Natural Heat Resources in Iceland. J/Skull, vol. 11., and G. P~LMASON, Exploration of Subs,r/ace Temperature it~ Iceland. J6ku,ll, vol. 11. El:qarssoN, Tr., Ober das Wesert der heissett Qttellen Islands. Soc. Sci. Islandica, Reykjavik. ELI4S, A. J. and MMIO.~, Natural Hydrothermal Systems and Experimental Hot Water~Rock Interaction. Geochim. Cosmochim, Acta, vol. 28. KEYlXEO, G. C., A Portion o[ the System Silica-Water. Econ. Geol.. Vol. 45. SmvaLoaSOn, G. E., Epidote and Related Minerals in Two Deep Geothermal Drill Holes, Reykjavik and Hveragerdi, Iceland. U. S. Geol. Survey Prof. Paper 450-E, No WuIrE, D. E., J. D. Hv_x,~ and G. A. W.xr~m(;, Chemical Compositio, o[ Stlb.sllr- [ace Water. Data o[ Geochemistry. Sixth ed. U. S. Geol. Survey Prof. Paper 440-F. Discussion J. R. HULSTON: Although you do not have much methane in your gases I wonder if you have made any C" isotope measurements on CH~ and CO: which might give an indication of deep underground waters.

16 G. E. SIGVALDASON: This has not been made. A. J. ELLIS: In areas where the sulphate concentration in waters seems to be controlled by calcium sulphate solubility, do you find gypsum or anhydrite precipitating? Anhydrite should form, but there are suggestions that gypsum may form metastably in some thermal areas. G. E. SIGVALDASON: Gypsum or anhydrite has not been found in any of the drill cores from the thermal areas in Iceland. D. KEAR: Dr Walker showed areas of acidic volcanism in Iceland and showed areas of increased rock alteration to be associated with these. This appears to a world -- wide feature -- of greater alteration with acidic rather than with basic rocks. Do the Iceland spring water analyses reflect these trends by having greater concentrations or other differences in the areas of acidic rocks? G. E. SIGVALOASON: The high temperature area of Haukadalur with the Great Geysir is associated with a rhyolite intrusion of probable Quaternary age. The thermal water of this area is similar to waters in basaltic areas with the exception of fluoride, which is much higher in the water from Geysir. This high fluoride concentration appears to be consistently associated with rhyolite intrusions, also in the low temperature areas.