DISSOLUTION PROCESSES

Comparison between the data in Table 1 and the calculated concentrations of Cl derived from a marine source shows that a relatively small part of the aqueous Cl concentrations, except in the hottest waters, comes from the dissolving rock. This is not so for B. Only a minor part of this element in the Hreppar-Land geothermal waters, even if onlyslightly thermal, is derived from a marine source. The percentage, B , of boron dissolved from the rock may be evaluated from the following equation:

(3) where

rm represents the Cl/B ratio in seawater = 4350 [15] and rw and rr the same ratio in a water sample and the rock, respectively. Taking r^r to be equal to 50, which appears appropriate for the Hreppar-Land waters, as discussed in section 4 above, one can see from equation (3) that for aqueous Cl/B ratios of 500 and less over about 90% of the aqueous B comes from the rock and less than about 10% from the marine source. All waters with temperatures in excess of 20° for which data are presented in Table 1 possess Cl/B ratios of less than about 500. It may, therefore, be concluded that aqueous concentrations of the mobile element boron can be quite accurately used as an indicator of the amount of rock dissolved by the Hreppar-Land waters.

The calculated net amounts (= total amount dissolved minus what has been precipitated) of the major elements (Na, K, Ca, Mg, Si, C, SO ⁴ and F) in the Hreppar-Land waters dissolved from the rock have been plotted in Figs. 6 and 7 against the calculated amount of B dissolved from the rock. The solid line in each figure corresponds to the respective element/boron ratio in the basaltic rock. The concentration of marine derived boron, Bm, was estimated for each sample with the aid of equation (1). Substraction of this value from the analysed B concentration in each sample gives the amount of B in the samples derived from the rock. The amount of the marine contribution of other elements was obtained from the same equation but using the appropritate seawater Cl/element ratio.

The data on cold and slightly thermal waters, displayed in Fig. 6 and 7, suggest that rock dissolution is near stoichiometric for all the major elements at these low temperatures 30

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Boron dissolved from rock (ppm)

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-0.1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Boron dissolved from rock {ppm)

Fig. 6. Relationship between the amount of boron and the net amounts of the major cations (Na, K, Ca and Mg) in natural waters of the Hreppar-Land area dissolved from the rock and soil (=total amount dissolved minus what has been precipitated). The lines on each diagram correspond to stoichiometric dissolution of the respective elements. In order to derive these lines the following elemental concentrations were assumed in the rock: B = 3.8 ppm, Na2<3 = 3.0%, K2O = 0.8%, CaO = 10.0% and MgO = 6.5%. These numbers are representative of those in alkaline basalts in Iceland and are based on data in [13] except for B which is based on [5]. The value for Ca in the Thjorsardalur water, which is very high (52.9 ppm) was omitted from the respective plot. Symbols have the same signature as in Fig. 3B.

except for Ca, Mg and carbonate. There is an excess carbonate in these waters relative to stoichiometric C/B dissolution from the basaltic rock. The reason is no doubt supply of carbonate from an organic source, the soil. Ca and Mg are depleted in the warm waters relative to cold waters suggesting their precipitation from solution to form weathering minerals, most likely smectites.

The geothermal waters with temperatures above about 40 °C (dots in Figs. 6 and 7) are depleted in all the major aqueous components, except sulphate, relative to boron assuming 31

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-0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Boron dissolved from rock (ppm)

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1--0.1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Boron dissolved from rock (ppm)

Fig. 7. Relationship between the amount of boron and the net amounts of aqueous silica, total carbonate, sulphate and fluoride in natural waters of the Hreppar-Land area dissolved from the rock and soil (=total amount dissolved minus what has been precipitated). The lines on each diagram correspond to stoichiometric dissolution of the respective elements. In order to derive these lines the following elemental concentrations were assumed in the rock: B = 3.8 ppm, SiO² = 48.0%, CO² = 800 ppm, S = 800 and 230 ppm, and F = 600 ppm. Data on SiO² and F are from [13], B from [5], S from [14] and C estimated from data presented in [15]. The higher number (800 ppm) for S corresponds to the sulphur content of undegassed basalts whereas the lower value is close the average S concentration in Icelandic basalts. The value for S04 in the Thjorsârdalur water, which is very high (302.3 ppm) was omitted from the respective plot. Symbols have the same signature as in Fig. 3B.

32

stoichiometric rock dissolution. The cause is considered to be precipitation of these components from solution to form hydrothermal minerals. For the major cations departure from the stoichiometric line is smallest for Na suggesting that this element possesses the highest mobility. It is worth observing that the values for Mg are all negative in waters above about 40°C. This implies that some of the Mg derived from the atmospheric source is incorporated into hydrothermal minerals. In other words the hydrothermal alteration of the basaltic rock is accompanied by incorporation of Mg derived from a marine source transported to the site via the atmosphere. Data for Fe and Al have not been plotted but they are comparable to those of Ca shown in Fig. 6, i.e. most of what is dissolved from the rock enters alteration minerals, even at ambient temperature.

As can be seen from Fig. 7 silica is, like Na, relatively mobile in the geothermal waters compared to the other major elements. Carbonate has minimum mobility in warm waters (40-60°C) and despite a large organic source for C from the cold water environment the aqueous carbonate/boron ratio (C/B) in waters above some 100°C is significantly lower than that dictated by stoichiometric C/B dissolution. It is clear from Fig. 7 that a significant amount of carbon incorporated into the basaltic rock during hydrothermal alteration at low temperatures must come from an organic source.

A significant sink is not observed for SO⁴ (Fig. 7) so this component probably acts as mobile in the Hreppar-Land waters. Indeed all the waters in the area, except for that in Thjorsardalur (not plotted in Fig. 7), are significantly undersaturated with respect to anhydrite, the only hydrothermal mineral expected to take up SO⁴. The sulphur content of basalts in Iceland is quite variable due to degassing during cooling whether the basalt forms lavas or small intrusions [14]. Undegassed basaltic glass contains about 800 ppm S but the average of Icelandic basalts is about 230 ppm [14]. The steeper line for SO⁴ in Fig. 7 corresponds to stoichiometric S/B dissolution for ungassed basalt but the line with the shallower slope to basalt of average S composition.

As deduced from the work of [16] one would expect some of the sulphur in basalt to occur as sulphide and some as sulphate. The SO⁴ replaces SiO⁴-tetrahedra in the plagioclases [16]. Apparently, most of the sulphur dissolved from the basalts by the geothermal waters is on the sulphate form or that oxidation of dissolved sulphide sulphur is practically complete except for the hottest waters, which contain aqueous sulphide in appreciable concentrations (Table 1).

Although fluoride bearing minerals are not known to form as hydrothermal alteration products in basalts, it is apparent from Fig. 7 that the larger part of the F released from the basalt by the geothermal waters is removed from solution. It is considered that the fluoride enters the structures of some hydroxyl-bearing hydrothermal minerals such as smectite where F- replaces OH-. This type of exchange reaction is considered to control F- mobility.