The use of mixing models and chemical geothermometers for estimating underground temperatures in geothermal systems

Journal of Volcanology and Geothermal Research, 23 (1985) 299--335 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

299

T H E USE O F M I X I N G M O D E L S A N D C H E M I C A L G E O T H E R M O M E T E R S F O R ESTIMATING U N D E R G R O U N D T E M P E R A T U R E S IN G E O T H E R M A L SYSTEMS

STEFAN ARNORSSON Science Institute, University of Iceland, Dunhagi 3, 107 ReykjavCk (Iceland) (Received August 11, 1983; revised and accepted August 7, 1984)

ABSTRACT ArnSrsson, S., 1985. The use of mixing models and chemical geothermometers for estimating underground temperatures in geothermal systems. J. Volcanol. Geotherm. Res., 23: 299--335. Application of various chemical geothermometers and mixing models indicate underground temperatures of 260°C, 280°C and 265°C in the Geysir, Hveravellir and Landmannalaugar geothermal fields in Iceland, respectively. Mixing of the hot water with cold water occurs in the upflow zones of all these geothermal systems. Linear relations between chloride, boron and 61sO constitute the main evidence for mixing, which is further substantiated by chloride, silica and sulphate relations in the Geysir and Hveravellir fields. A new carbonate-silica mixing model is proposed which is useful in distinguishing boiled and non-boiled geothermal waters. This model can also be used to estimate under ground temperatures using data from warm springs. This model, as well as the chlorideenthalpy model and the Na-Li, and CO~-gas geothermometers, invariably yield similar results as the quartz geothermometer sometimes also does. By contrast, the Na-K and the Na-K-Ca geothermometers yield low values in the case of boiling hot springs, largely due to loss of potassium from solution in the upflow. The results of these geothermometers are unreliable for mixed waters due to leaching subsequent to mixing.

INTRODUCTION G e o t h e r m a l w a t e r w h i c h ascends f r o m g e o t h e r m a l reservoirs and emerges at the surface in h o t springs m a y cool o n t h e w a y , either b y c o n d u c t i o n , boiling, or mixing with shallow cold water, or b y a n y c o m b i n a t i o n o f these three processes. T h e c h e m i c a l c o m p o s i t i o n o f this w a t e r in surface discharge, as well as the gas c o n t e n t in associated f u m a r o l e s , m a y be used t o evaluate u n d e r g r o u n d t e m p e r a t u r e s , a p p l y i n g various chemical g e o t h e r m o m e t e r s and mixing m o d e l s ( A r n 6 r s s o n , 1 9 7 0 a , b, 1 9 7 5 , 1 9 8 3 ; A r n 6 r s s o n et al., 1 9 8 3 a ; Cusicanqui et al., 1 9 7 5 ; D ' A m o r e and Panichi, 1 9 8 0 ; Ellis, 1 9 7 0 , 1 9 7 9 ; Fouillac and Michard, 1 9 8 1 ; F o u r n i e r , 1 9 7 3 , 1 9 7 7 , 1 9 7 9 ; F o u r n i e r and R o w e , 1 9 6 6 ; F o u r n i e r and Truesdell, 1 9 7 3 ; F o u r n i e r et al., 1 9 7 4 , 1 9 7 5 , 1 9 7 9 ; M a h o n , 1 9 7 5 ; Truesdell, 1 9 7 5 ; Truesdell and F o u r n i e r , 1 9 7 5 , 1 9 7 7 ; White, 1970). 0377-0273/85/$03.30

© 1985 Elsevier Science Publishers B.V.

300 The present article focuses on the interpretation of chemical data of hot spring water and fumarole steam from three geothermal fields in Iceland, at Geysir, Landmannalaugar and Hveravellir (Figs. 1, 3 and 4) and special emphasis is put on assessing evidence for mixing processes in upflow zones, and on evaluating chemical reactions between water and rock which set in as a consequence of the mixing process. Comparison is made between the outcome of the various chemical geothermometers (quartz, chalcedony, Na-K, Na-K-Ca, Na-Li and COs) and mixing models. The sources of discrepancy are discussed specifically and the overall results assessed with respect to underground temperatures. A new mixing model is proposed, which is based on the relation between the silica and carbon dioxide content of equilibrated reservoir water. This model is useful in evaluating whether the geothermal water has become degassed, i.e. if it has boiled, but this model may also be used independently to estimate underground temperatures where mixed non-boiled waters emerge in springs. A brief description is given of the geology of the areas studied, as such background information is considered to be essential for the reader's own assessment of the interpretation given here. Chemical analyses of waters from all three geothermal fields are given in Tables 1 to 3. ANALYTICAL For the silica-carbonate mixing model proposed in this contribution, the method of sample collection and analysis for total carbonate is of major importance. Alkalinity cannot be taken to represent carbonate. For hot springs with a separate gas phase containing CO2 there is probably no way to rigorously evaluate the carbonate content of the total discharge. For the present study only the aqueous phase was collected. A funnel connected to silicone tubing was submerged into the water. The tubing was fitted to a cooling coil of stainless steel placed in a bucket of cold water. The water from the spring was sucked up through the tubing and the cooling coil, making sure that the far end of the coil was at a lower level than the water level in the spring so that gravity flow would be established. A gas sampling bulb was connected to the cooling coil with silicone tubing and filled with cooled water (about 20°C) and the flow maintained through the tube for a few minutes so that the water in the bulb, which was to be analyzed, should not come into contact with the atmosphere. Total carbonate and pH were analyzed in the sample in the gas bulb at the end of each field day. In determining total carbonate, pH was adjusted to 8.2 (practically all carbonate occurs as HCO~ at this pH) with NaOH- or HCl-solution, as appropriate, and the sample titrated with 0.1 N HC1 to a pH of 3.8 (all carbonate occurs as CO2 at this pH). Correction for interference from weak acids other than CO2 (silica, sulphide, boron, water) was made by calculation following analysis of the respective components.

301 GEOLOGY AND THERMAL MANIFESTATIONS The three geothermal fields considered in the present study have been classified as high-temperature (Arn6rsson, 1970a; P~lmason and Saemundsson, 1974). Two of them (Landmannalaugar and Hveravellir) are located within the active volcanic belts and one (the Geysir field) close to the eastern boundary of the main volcanic belt in southern Iceland (Figs. 1, 3 and 4). The most prominent thermal manifestations at Hveravellir and in the Geysir field include geysers, boiling hot springs and considerable silica sinter deposits. In the Landmannalaugar field fumaroles are, on the other hand, the most prominent feature of surface thermal activity.

The Geysir field The Geysir geothermal field is located in the uppermost part of the southern lowlands of Iceland at an elevation of about 120 m. Most of the geothermal activity occurs in an area which is only a few hundred metres across, b u t thermal manifestations, mostly in the form o f warm springs, are spread over an area of approximately 6 by 0.5 km, which is elongated in the prevailing tectonic direction within this region (Fig. 1). The rocks of the area within which the Geysir field is located belong to the Brunhes magnetic epoch (less than 700.000 years). They consist of interglacial basalt lavas, pillow lava and hyaloclastite formations, also of basaltic composition, which have erupted subglacially, and four domes of spherulitic rhyolite. The basaltic hyaloclastites are often rich in tachylyte and palagonite. Pitchstone and tillite-like breccia are abundant in one of the rhyolite domes, probably due to its subglacial extrusion. High-temperature geothermal fields in Iceland are typically associated with so-called central volcanoes (P~lmason and Saemundsson, 1974). One of the characteristics of these volcanoes is relatively abundant acid volcanics, b u t also intrusions and caldera structures, the latter often being represented b y a dip of the lava flows towards the core of the volcano (Walker, 1963). The area of the Geysir field may well represent a central volcano, although the evidence for this is, admittedly, rather meager. The only evidence of intrusives is a gravity high south of the main hot spring area (P~lmason, unpublished data) and rather anomalously steeply dipping lavas (towards the northeast) b y Stakks~ (Fig. 1). Most of this area was submerged at the end of the last glaciation and thick near-shore clastic sediments from that period cover the bedrock in the southern part of the area (Fig. 1), forming extensive plains. All the boiling hot springs within this field are situated within intensely altered basalts just east of the rhyolite d o m e of Laugarfell (Fig. 1). Tepid springs occur at the foot of the cliffs on the west side of Laugarfell and on the alluvial plain to the south, as far as 4 km from the main hot spring area. Warm and hot springs also occur in Haukadalur about 2 km to the north.

302

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303 Acid pools and small mud pots are found within the main hot spring area. The higher ground on the lower slopes of the Laugarfell rhyolite d o m e is intensely altered b y acid surface leaching. Here inconspicuous fumaroles appear. The high fluoride content of the thermal waters indicates contact with acid rocks (Arn6rsson et al., 1983b). This, and the distribution of the thermal springs, suggests that the major upflow occurs either along permeable contacts between the Laugarfell rhyolite d o m e and the enveloping rocks, or that the water ascends along permeable cracks within the rhyolite itself. Many records exist of changes in the surface thermal manifestations during earthquake episodes (Thoroddsen, 1925). This indicates rejuvenation of permeability in the upflow caused b y tectonic movements. One drillhole has been sunk within the Geysir field by the farm Nedridalur. It is 850 m deep and struck an aquifer at 386 m depth with a temperature of 68°C. The flow rate is 5 dm a s -I. B o t t o m hole temperature is 173°C and there is a constant gradient below the producing aquifer equivalent to 220°C km -1 (Fig. 2). As will be discussed m the following section, there is strong evidence that the warm and tepid waters have formed by ~ixing of geothermal water with Temperature °C 40

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304 cold groundwater. It appears particularly likely that mixing is prone to occur when hot water emerges from the bedrock and ascends into unconsolidated post-glacial sediments. Total flow from the boiling hot springs is only 13 dm 3 s -1 and the flow from all the warm springs is also rather low. Table 1 gives flow rates for those springs analyzed for the present study. If reservoir temperatures are in the range of 260°C (see later sections), adiabatic boiling in the upflow would convert some 30% of the reservoir water into steam and, accordingly, a steam discharge of 6 kg s -1 is expected to accompany the measured water discharge of 13 dm 3 s -1 . The impression is that the steam flow is less, suggesting that conductive heat loss in the upflow is significant. Cooling by conduction evidently occurs in pipes feeding some of the hot springs and geysers, as shown by studies of the geyser activity of the Great Geysir (Einarsson, 1937, 1949; Sigurgeirsson, 1949). These studies indicate surface cooling and convection within the pipe below the geyser basin such that the water at 20 m depth is about 130°C, but between 70°C and 90°C at the surface, depending on weather conditions. No boiling occurs and the flow from the spring is 2.2 dm 3 s -~ (Table 1).

Hveravellir This geothermal field is located in central Iceland at an elevation of about 630 m between the icecaps of HofsjSkull and LangjSkull. The main hot springs emerge in glacial outwash plains by the northern edge of the postglacial lava field of Kjalhraun (a lava shield), but fumarolic activity occurs in the lava field (Fig. 3). Poor exposures on the glacial outwash plains indicate that the rocks in the vicinity of the thermal field are basaltic hyaloclastites. Rhyolitic rocks outcrop in several localities in the marginal mountains of LangjSkull some 5 km to the west. This geothermal field is very small. The main area of alkaline hot springs is only about 200 m across. Fumarolic activity extends about 1Ato 1 km to the south into the Kjalhraun lava field and hydrothermal clay formed by acid surface leaching has been found several hundred of metres northwest of the main hot springs. The total quantity of water discharged from the hot springs has been estimated to be some 15 dm 3 s -1 . The thermal activity at Hveravellir bears close resemblance to that in the Geysir field, that is, alkaline spring waters, geysers, extensive silica sinter deposits, some mud pools and fumaroles and clayey hydrothermal soil. The surface manifestations are spread on a north--south line coinciding with the active tectonic fracturing of the region.

The Landmannalaugar field The thermal activity in the vicinity of Landmannalaugar is a part of the TorfajSkull geothermal field in southern Iceland. This is the largest (areally)

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geothermal field in the country, covering some 200-300 km2. It is, however, only in the Landmannalaugar area that alkaline spring waters of the sodium chloride type are known and for that reason sampling for the present study was limited to the Landmannalaugar field, but the main aim of the study was to evaluate mixing processes and underground temperatures from the composition of hot spring waters. The Torfajijkull region constitutes the largest complex of acid volcanic rocks in Iceland. Within the Landmannalaugar field only acidic rocks outcrop and they include rhyolites, pitchstone and obsidian. It is considered that all of the rocks exposed have formed by subglacial eruptions in late Quaternary time, probably during the last glaciation (Saemundsson, 1972). A young lava has been extruded from a short fissure on the slopes of the rhyolitic dome of Brennisteinsalda. Tephrochronological evidence indicates

306

that this lava was erupted in the 14th century (G. Larsen, pets. commun., 1984). A large caldera or ring structure has been identified in the TorfajSkull region (Saemundsson, 1972), which envelopes practically all the geothermal manifestations and almost all the acid rocks. Fissure eruptions have simultaneously produced acid lavas within the caldera, basalt lavas outside it and mixed lava at the caldera rim. This has been taken as evidence for the existence of acid magma at shallow depths within the caldera. A distinct positive gravity anomaly in the TorfajSkull region has been taken as evidence for basaltic magma intrusions below the eruptive acid rocks {Walker, 1974). It appears thus that both acid and basic intrusives could constitute the heat source for the geothermal system. The terrain of the Landmannalaugar area is extremely rugged, as it is in fact for a rather large part of the TorfajSkull region, being characterized by steep-sided V-valleys and inassessable gorges. Valley bottoms, if there are any, are covered by coarse alluvial gravel. The thermal manifestations in the Landmannalaugar field differ greatly from those at Hveravellir and Geysir. Fumarolic activity and hot ground, altered by acid surface leaching, is much more prominent, especially on high ground. Hot springs with alkaline sodium chloride waters occur in and near the valley bottoms. They are widespread and numerous but the flow rate r. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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307

from individual springs is low (generally less than 1 dm 3 s-'). Sinter deposits, which most often show up as cementing of gravel, are not uncommon. Alkaline spring waters and strong steam vents sometimes occur at a distance of a few metres, suggesting that the two phases ascend together from the deep reservoir. Numerous warm springs with high flow rates issue from the edge of the Laugahraun lava. When this lava flowed it blocked the main stream course from the Vondugil area (Fig. 4). As discussed in the following section, there is evidence indicating that these waters are of mixed origin and the hydrological situation indicates that the mixing may occur below the lava, near its base or at deeper levels in the inferred alluvium deposits or in the bedrock. No estimate is available for the total heat o u t p u t from the Landmannalaugar field. EVIDENCE FOR MIXING Water formed by mixing of geothermal water and cold ground- or surface water possesses many chemical characteristics which serve to distinguish it from unmixed geothermal water. The reason is that the chemistry of geothermal waters is characterized by equilibrium conditions between solutes and alteration minerals, whereas the composition of cold waters appears mostly to be determined by the kinetics of the leaching process. The mixed waters tend to acquire characteristics which may be said to be intermediate between those just mentioned. It is, however, to be realized that the residence time in the bedrock after mixing and the temperature and salinity of the mixed water have an influence on the final chemical composition in spring discharges. Strong conductive cooling of geothermal waters in upflow zones and subsequent reaction with the rock may produce compositional affinities similar to those obtained by leaching subsequent to mixing. Since geothermal waters are often, but not always, much higher in dissolved solids than cold ground- and surface waters, the mixing process has often been referred to in the literature as dilution. The main chemical characteristics of mixed waters, which serve to distinguish them from equilibrated geothermal waters, include relatively high concentrations of silica in relation to the discharge temperature, low pH relative to the water salinity (often in the range of 6--7 for C1 concentrations of less than 100 ppm) and high total carbonate, at least if the mixing has prevented boiling and the temperature of the hot water component exceeds some 200°C. Further, like cold waters, mixed waters tend to be calcite undersaturated and with low calcium/proton activity ratios compared with geothermal waters. Figure 5 shows the state of calcite saturation for water from Hveravellir and the Geysir and Landmannalaugar fields as well as calcium/ proton activity ratios. As can be seen from this figure, some of the mixed waters show affinities towards cold waters, whereas others approach equilibrium conditions.

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Fig. 5. The state of calcite saturation and x/Ca~+/H ÷ activity ratios at discharge temperature in waters from Hveravellir (triangles) and the Geysir (circles) and Landmannalaugar (squares) geothermal fields. The broken curves indicate calcite saturation and x/Ca2÷/H ÷ activity relations for equilibrated waters according to Arn6rsson et al. (1983b). A m u c h s t r o n g e r case is m a d e f o r m i x i n g in u p f l o w z o n e s w h e n t h e m i x i n g can be q u a n t i f i e d b y considering a g r o u p o f spring w a t e r s r a t h e r t h a n relying o n t h e c h e m i c a l characteristics o f individual discharges. H e r e near-linear relat i o n s h i p s b e t w e e n c h l o r i d e c o n c e n t r a t i o n s o n t h e o n e h a n d and b o r o n conc e n t r a t i o n s and 5180 values o n t h e o t h e r are believed to be p a r t i c u l a r l y valuable. S o m e t i m e s a linear r e l a t i o n b e t w e e n chloride and silica and chloride and s u l p h a t e m a y also be useful. Chloride and b o r o n levels in cold w a t e r are, as a rule, low b u t m u c h higher in g e o t h e r m a l w a t e r s and, as these c o n s t i t u e n t s are n o t c o n s i d e r e d to be inc o r p o r a t e d in g e o t h e r m a l minerals, m i x i n g involves simple lowering o f conc e n t r a t i o n s w i t h o u t a f f e c t i n g t h e C1/B ratio. I f m i x i n g o f g e o t h e r m a l w a t e r w i t h cold w a t e r is r e s p o n s i b l e f o r variable c h l o r i d e c o n c e n t r a t i o n s , it is t o b e e x p e c t e d t h a t t h e i n t e r s e c t i o n at 0 p p m b o r o n o f a line t h r o u g h t h e d a t a p o i n t s is in the range o f 10 p p m chloride, as cold w a t e r ( a n d p r e c i p i t a t i o n ) c o n t a i n s chloride in t h a t range and less t h a n 0.01 p p m b o r o n . Figure 6 shows t h e c h l o r i d e - b o r o n r e l a t i o n f o r t h e w a t e r s f r o m Hveravellir and the G e y s i r and L a n d m a n n a l a u g a r fields. In every case t h e r e is a r a t h e r g o o d linear relation. F o r t h e Geysir and L a n d m a n n a l a u g a r fields t h e p o i n t o f i n t e r s e c t i o n c o r r e s p o n d i n g w i t h 0 p p m b o r o n is s o m e 10 p p m f o r chloride. T h u s t h e c h l o r i d e - b o r o n relationships o f w a t e r s f r o m t h e s e fields are t a k e n to p r e s e n t a strong evidence f o r m i x i n g o f g e o t h e r m a l w a t e r and cold g r o u n d -

309

water. For the Hveravellir waters the intersection of a regression line through the data points at 0 ppm boron corresponds with 17 ppm chloride, or somewhat higher than anticipated. This appears to be, at least partly, due to boiling prior to mixing. HVERAVELLIR

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310

with being largely local precipitation. One mixed water (82-018) is intermediate between the boiling hot spring waters and the meteoric line, the reason is considered to be that this water contains a rather large portion of the hot water component, as indicated by the chloride content. Sample 82-014 issues from the alluvial plain in the southern extreme of the field (Fig. 1). The cold water c o m p o n e n t in this water may not be local precipitation but that of the through-flowing rivers which originate at the southern edge of the LangjSkull ice-sheet, which would explain the relatively low 5 D and 5~sO values for this water. Sample 82-012, which is from a tepid spring some distance north from the main hot spring area, does not plot on the meteoric line {Fig. 7) and is high in 5D relative to 51sO. The reason for this is not known. '// 82-012e

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Figure 8 shows that there is a good relation between chloride concentrations and 51sO values for the Hveravellir and Landmannalaugar waters. This relation is considered to reflect mixing of geothermal water with cold water. The geothermal water has changed its 51sO value by reaction with the rock, but the hydrogen and oxygen isotope composition of the tepid water corresponds with that of the meteoric line (Fig. 7). A similar 8D/51sO pattern and interpretation has been presented for the geothermal system at Long Valley, California {Mariner and Willey, 1976; Fournier et al., 1979). Two points in Fig. 8A and 8B show high 51sO values relative to their chloride concentrations. A plausible explanation for the anomalous values is that steam containing CO2 has condensed in this water, b u t CO2 is much

311

enriched in the 1so isotope relative to water. The sample from the Geysir field with the high 5 l s o value contains also anomalously high sulphate which can be explained by the same process, namely oxidation of sulphide derived from condensed steam. LANDMANNALAUGAR

HVERAVELLIR

GEYSIR FIELD

i

i

[

I

i

FIELD

r

C

B

-8 I

-8

•

o

o

-10

-1C

I o

-12

40

80

120 C

ppm

-12

~

2'0

~.

•

~ 40'

'

6'0

J

CI,ppm

,

i

200

J

i

400

i

/

600 CI,ppm

Fig. 8. Plot o f chloride versus 6~sO in cold and t h e r m a l w a t e r f r o m Hveravellir and t h e Geysir and L a n d m a n n a l a u g a r g e o t h e r m a l fields. Lines a and b designate mixing w i t h cold w a t e r o f d i f f e r e n t 5180 c o n t e n t .

The waters from the Geysir field display a rather good linear relation between chloride and silica (Fig. 9). This relationship is not as clear for Hveravellir and very poor for the Landmannalaugar waters. If the dilution process is not followed by dissolution of silica from the rock, nor with precipitation, a linear silica-chloride relation is to be expected and the line through the data points should pass close to the point of origin in Fig. 9. If the unmixed geothermal water was in equilibrium with quartz and re-equilibrium with that phase occurred after mixing, considerable amounts of silica would have to precipitate from the water. The squares in Fig. 9 show the silica and chloride concentrations in the reservoir water as deduced from chloride-enthalpy mixing model diagrams discussed in a later section (see Fig. 17). The curves indicate silica/chloride relations that result from mixing if the silica levels are always controlled by quartz solubility. The broken and dotted lines indicate chloride-silica relations which result from mixing and steam loss respectively. For the Geysir field and Hveravellir the data points in Fig. 9 tend to plot below the dilution/boiling line, indicating that some precipitation is associated with those processes. The same holds for the Landmannalaugar field except for three samples (82-113, 82-115 and 82-117) of water from the edge of the Laugah-

312 raun lava which contain high silica relative to chloride. It is considered that leaching of silica from the rock after mixing is responsible. Data points which plot below the quartz re-equilibration curve must have lost silica from solution after mixing due to cooling by conduction or boiling. It is noteworthy that the waters from Landmannalaugar which contain highest chloride have lost the larger part of their original dissolved silica. GEYSIR FIELD r

i

I

HVERAVELLIR [

I

+

I

60(]

I

I

LANDMANNALAUGAR FIELD q

I

°:

60O

E o. @.

60(]

•

e~

4

0._

(£~1

O~ 40~

," • / ,/

Ze

•

40C •

.

+°

/

•

2413 2 117

/ ' ~2

2OO

/ // //"•82.018

/~4""V I

I 40

200

/

/ I

I I I 80 120 CI, p p m

/ ; /

2-115

•

•

e82127

/ • I

1

200 CI, p p m

I

I

I

1

400 6OO CI, p p m

Fig. 9. Plot of silica versus chloride in thermal and cold waters from the Geysir, Hveravellir and Landmannalaugar geothermal fields. The squares represent deep water silica and chloride concentrations as predicted by chloride-enthalpy mixing model plots (see Fig. 17). The broken and dotted lines indicate variations resulting from dilution and boiling respectively. The curves show silica-chloride relations resulting from mixing and attainment of quartz equilibrium. Practically all the data points plot below the mixing/boiling lines, indicating that some silica precipitation generally occurs subsequent to these processes. Data points with sample numbers are specifically discussed in text. Sulphate concentrations show a good relation with chloride in waters from Hveravellir and the Geysir field (Fig. 10): By contrast, no such relationship is observed for the Landmannalaugar waters. Yet, if waters issuing from the edge of the Laugahraun lava and those in other parts o f the field are considered separately, a positive and negative chloride-sulphate relation respectively is apparent (Fig. 10). It might be expected that the boiling hot spring waters, especially geysers and pools with large surface areas, had relatively high sulphate content as a result of sulphide oxidation by atmospheric oxygen. Indeed the 81sO value for the sulphate indicates sulphide oxidation subsequent to isotopic equilibration (TruesdeU and Arn6rsson, unpublished work). An explanation, which accounts for both the chloride-sulphate relationship and the 81sO values of

313

the sulphate, assumes that sulphide oxidation occurs at depth before mixing. It is conceivable that the waters emerging in the hot springs are mixed waters, in which case atmospheric oxygen in the cold water component was involved in converting the sulphide to sulphate. LANDMANNALAUGAR FIELD

HVERAVELLIR

GEYSIR FIELD

12(:

•

E 12(

~60

ffl

tN

•

o. '¢

00 8G

8(

40

4C

4c

2E

40

80

t20 CI. pprn

2b

4b

8b Cl.ppm

•

~o

45o 660 CI, pprn

Fig. 10. Plot of chloride versus sulphate in cold and thermal waters from Hveravellir and the Geysir and Landmannalaugar geothermal fields. For the Landmannalaugar field distinction is made between waters issuing from the edge of the Laugahraun lava (large dots) and data from other parts of the field (small dots).

i... "0 ~-6 (.,-

0 0 -8

I

°

oqw p

Temperature °C Fig. 11. The state of anhydrite saturation in waters discharged from boiling hot springs at Hveravellir (triangles) and in the Geysir (dots) and Landmannalaugar (squares) fields. Quartz equilibrium temperature was selected for reference, assuming adiabatic steam loss.

314 TABLE 1 C o m p o s i t i o n of w a t e r s f r o m t h e Geysir g e o t h e r m a l field, s o u t h e r n Iceland ( c o n c e n t r a tions in p p m ) Sample No.

Location

Temp. (°C)

Flow d m 3 s -'

pH/°C

SiO~

B

Na

K

79-004 79-005 79-006 82-005 82-006 82-007 82-008 82-009 82-010 82-011 82-012 82-013 82-014 82-015 82-016 82-017 82-018 82-019

Geysir Sisjbdandi Smidur Blesi Otherrishola Litli S t r o k k u r Litli Geysir Strokkur Marteinslaug Kaldilaekur

72 95 98 90 94 98 70 78 78 5 17 68 46 24 48 97 39 30

2.2 1 0.1 2 0 0 0 3 0.4

9.36 9.57 8.93 9.38 8.92 7.77 8.73 8.56 7.88 8.01 7.72 7.52 7.57 7.76 7.39 8.74 7.44 7.94

486.0 350.0 324.7 503.2 437.8 404.2 450.4 457.2 290.0 30.6 35.2 146.2 167.0 60.2 145.7 355.1 180.2 79.7

0.95 0.93 0.82 1.01 0.78 0.76 0.82 0.96 0.68 0.01 0.01 0.22 0.18 0.22 0.25 0.66 0.28 0.13

248.2 236.4 228.2 230.6 222.7 216.1 230.2 225.4 164.0 11.8 9.4 223.2 180.9 28.2 83.8 194.4 132.8 84.6

26.2 13.1 12.0 19.3 15.7 12.2 14.4 14.4 12.4 0.96 0.61 44.2 21.1 1.79 5.72 8.68 21.5 9.76

Nedridalur Mfili

Laugarfell Helludalur

2 5 2 2 1 1 0 5

'22 '22 '22 '20 '20 '20 '20 '20 '20 '20 '20 '20 '20 '20 '20 '20 '20 '20

a T o t a l c a r b o n a t e a n d t o t a l sulphide, respectively.

TABLE 2 C o m p o s i t i o n of waters f r o m t h e Hveravellir g e o t h e r m a l field, c e n t r a l Iceland ( c o n c e n t r a t i o n s in p p m ) Sample No.

Location

Temp. (°C)

Flow b pH/°C ( d m 3 s -1)

82-124 82-125 82-126 82-127 82-128 82-129 82-130 82-131 82-132 82-133

Braedrahver Fagrihver Eyvindarhola

96 93 97 97 92 35 40 65 89 20

1 0.1 0.3 2 0.5 2 0.1 0.5

9.75 9.59 9.26 9.48 9.60 8.73 8.80 9.12 9.41 7.34

a T o t a l c a r b o n a t e and t o t a l s u l p h i d e , respectively. bEstimated.

'10 '10 '10 '10 ~10 ¢20 ~20 f20 ~20 t20

SiO~

B

Na

K

606.5 595.5 478.8 196.9 564.5 42.3 78.4 280.9 384.8 50.6

0.51 0.54 0.46 0.34 0.51 0.10 0.14 0.26 0.38 0.10

169.6 159.6 147.5 115.8 158.0 45.5 74.7 105.3 132.6 45.9

16.7 15.7 8.56 4.81 16.2 2.32 3.00 8.86 10.4 1.76

315

Ca

Mg

Li

CO2a

SO,

H2S a

CI

F

5 D°/oo

6180o/00

0.76 0.72 0.68 0.86 1.47 0.85 0.71 0.73 4.48 3.22 4.93 7.72 15.5 12.7 4.00 1.19 10.2 2.90

0.002 0.002 0.006 0.010 0.015 0.012 0.010 0.011 0.120 0.461 0.980 0.611 4.08 1.58 0.530 0.018 3.11 0.540

0.340 0.201 0.194 0.348 0.296 0.236 0.308 0.285 0.262 0.001 0.001 0.017 0.016 0.008 0.006 0.210 0.044 0.013

136.8 109.6 92.1 99.0 126.1 180.2 146.8 159.0 180.0 16.6 36.5 462.5 399.6 73.8 94.9 160.5 243.2 147.8

117.0 101.5 150.2 103.5 93.8 80.2 104.8 104.6 61.1 0.5 0.3 28.3 34.8 9.8 22.4 79.3 47.8 13.2

0.86 1.56 0.59 1.34 1.99 2.61 0.28 0.25 0.72 <0.01 <0.01 0.04 0.07 <0.01 <0.01 1.81 <0.01 <0.01

132.4 122.9 115.0 132.2 124.4 118.2 131.3 132.2 86.6 7.4 10.6 46.4 36.0 18.9 43.6 105.5 64.5 26.4

8.66 12.18 10.90 9.90 11.00 13.20 13.10 12.90 5.06 0.07 0.13 5.88 1.92 1.24 7.93 15.20 6.02 2.53

-75.6 --82.5 --74.0 --77.9 --78.7 --81.4 --75.4 --77.4 /-80.3 --66.0 --46.2 --68.4 --78.1 --60.6 --64.3 --84.4 --71.6 --60.7

--7.60 --8.65 -6.14 8.46 --8.50 9.04 --7.81 --7.77 --10.18 --9.78 --8.50 --9.54 --10.90 --9.37 --9.39 --9.33 --8.88 --9.35

Ca

Mg

Li

CO2 a

SO,

H2S a

C1

F

5 D°/oo

~1~0°/oo

2.28 2.17 3.22 4.04 2.93 9.63 8.56 3.52 2.56 7.77

0.053 0.020 0.042 0.020 0.015 0.559 0.800 0.080 0.049 1.046

0.315 0.293 0.165 0.083 0.277 0.024 0.042 0.140 0.169 0.033

16.8 26.9 47.3 25.7 23.5 30.6 42.5 41.7 23.3 40.0

137.3 142.1 142.8 118.7 145.8 55.3 86.8 81.3 121.8 51.8

3.51 2.10 1.92 0.38 2.50 <0.01 <0.01 3.22 1.07 <0.01

70.0 73.0 67.2 54.5 74.0 29.0 38.6 39.6 59.0 26.4

3.12 3.12 2.20 1.34 3.12 0.33 0.50 1.58 2.39 0.40

--91.0 --91.1 --96.3 --91.4 --94.2 88.0 --88.8 --97.2

--11.00 --10.69 --9.49 --11.45 --10.60 --11.97 11.83 --12.21 --11.37 --11.97

--88.8

316 TABLE 3 Composition of waters from the Landmannalaugar geothermal field central south Iceland (concentrations in ppm) Sample no.

Location

Temp. (°C)

Flow b pH/°C (dm3s -' )

SiO 2

B

Na

K

79-053 79-054 82-093 82-095 82-097 82-099 82-106 82-113 82-114 82-115 82-116 82-117 82-118 82-119

Sullur Landm. laugar

94 82 94 45 95 94 94 59 24 58 10 45 93 96

1 3 1 2 1 3 1 10 20 5 100 10 0.2 1

210.2 258.4 207.1 160.6 271.8 145.4 299.1 186.0 81.8 165.5 36.9 173.1 265.4 297.3

1.63 1.72 6.14 1.25 4.21 1.27 1.96 0.39 0.25 0.36 0.06 0.36 2.61 2.32

263.0 252.6 363.6 151.6 413.9 208.2 291.9 65.4 57.6 70.8 17.9 61.7 278.9 273.4

11.6 35.3 17.3 14.0 16.1 8.97 15.1 13.0 8.74 14.5 3.54 13.4 13.6 22.6

Graenagil Eyrarhver Vondugil Vondugil Landm. laugar Landm. laugar Landm. laugar Landm. laugar Landm. laugar Vondugil Vondugil

8.80 r24 6.10 ~23 9.27 r14 6.64 t14 9.96 /14 9.58 t14 9.93 '14 6.32 f14 6.57 f13 6.62 '13 7.34 ~13 7.91 r13 9.82 r14 9.91 ~14

aTotal carbonate and total sulphide, respectively. bEstimated. The Geysir and the Hveravellir waters are in general c o n s i d e r a b l y higher in sulphate t h a n waters f r o m t h e L a n d m a n n a l a u g a r field (Tables 1--3). C o m p u t e r analysis indicates a n h y d r i t e u n d e r s a t u r a t i o n at d e p t h for the waters feeding boiling h o t springs in all t h r e e fields (Fig. 11). The lack o f chloride-sulphate relationship f o r the L a n d m a n n a l a u g a r waters c a n n o t , t h e r e f o r e , be explained b y the c o n t r o l o f a n h y d r i t e solubility u p o n sulphate m o b i l i t y . It m a y be t h a t r e d o x equilibrium involving h y d r o g e n , sulphide and pH is t h e c o n t r o l l i n g f a c t o r : H~S + 4 H 2 0 = SO/, 2 + 4H2 + 2H + REDISTRIBUTION OF CATIONS In t h e m i x e d w a r m waters t h e m a j o r c a t i o n (Na, K, Ca, and Mg) c o n c e n trations d o n o t s h o w a n y regular variation w i t h t h o s e o f chloride, indicating t h a t t h e mixing process is f o l l o w e d b y r e d i s t r i b u t i o n o f these ions. Dissolut i o n f r o m the r o c k following c o n v e r s i o n o f CO2 t o HCO~ is m o s t i m p o r t a n t , b u t u p t a k e in alteration minerals m a y also be involved. T h e boiling h o t spring waters f r o m all t h r e e fields are l o w e r in c a l c i u m and m a g n e s i u m t h a n b o t h c o l d and m i x e d waters. Calcium is highest in t h e w a r m waters o f m i x e d origin. Magnesium levels also t e n d t o be highest in t h e mixed waters. Potassium c o n c e n t r a t i o n s are highly variable. F o r t h e Geysir and L a n d m a n n a l a u g a r fields, waters o f m i x e d origin m a y be higher or l o w e r in

317

Ca

Mg

Li

CO2 a

SO 4

H2S a

CI

F

6 D°/oo

81sO°/o,

1.39 10.86 12.3 15.8 0.83 1.61 1.17 9.94 6.14 10.5 6.00 10.1 1.67 1.80

0.076 2.28 0.059 5.17 0.077 0.039 0.006 4.30 2.23 4.68 2.33 4.53 0.124 0.036

0.222 0.604 0.114 0.097 0.228 0.199 0.381 0.074 0.386 0.057 0.034 0.046 0.719 0.775

106.2 280.3 11.4 99.1 68.7 68.6 38.3 153.4 19.9 183.9 56.3 127.1 52.3 24.5

54.7 70.0 18.2 37.7 26.2 58.8 37.8 5.0 36.2 4.7 13.7 5.2 41.2 40.9

16.3 0.07 4.58 <0.01 17.5 10.9 18.3 <0.01 <0.01 <0.01 <0.01 <0.01 23.2 25.1

200.5 307.0 534.7 192.0 416.0 149.8 264.4 51.8 79.3 49.8 13.6 45.0 280.3 274.6

19.7 7.15 9.63 4.51 25.2 15.1 21.4 3.75 2.20 4.07 1.87 3.83 21.0 21.9

--69.2 --68.6 --70.6 --73.7 --71.4 72.4 --67.5 --72.2 --76.4 --77.2 ~72.6 --76.8 P72.8 --74.9

--8.49 --10.52 --9.20 10.68 --9.61 -10.86 --10.99 --11.10 --10.89 --10.98 --9.81 --10.04

potassium than waters in the boiling hot springs. At Hveravellir the mixed waters are much lower in potassium than the boiling hot spring waters, but in this field mixing occurs subsequent to boiling. Sodium is the d o m i n a n t cation with the exception o f the cold waters and those waters which contain a very high fraction of the cold water component. Here calcium may contribute significantly to the total cation content. Sodium concentrations bear no relation to chloride. In those instances where mixing prevented boiling, the ratio of bicarbonate to total anions is much higher in the warm waters and sodium, being the d o m i n a n t cation, matches most of the anions in solution. Special description of the water discharged from the drillhole at Nedridalur in the Geysir field is illustrative for the general change caused by the leaching process after mixing when the hot water component has not been degassed by boiling prior to mixing. The water discharged from the hole contains 46.4 ppm chloride (Table 1) and, according to the results of the chloride-enthalpy mixing model in Fig. 17, the deep hot water contains 114 ppm chloride and has a temperature of 260°C. For the path of cooling and mixing in Fig. 17 this indicates that the hot water component in the drillhole water is 0.37 and that the temperature after mixing is estimated at 73°C (assuming that sample 82-011 represents the cold water component). The sodium content, 223 ppm, is similar to that of the boiling hot spring waters in the area but, calculated from the dilution factor of 0.37 indicated by the chloride, a sodium concentration of some 80 ppm, is expected. The

318 difference has been leached from the rock. Leaching of potassium is even more extensive, as the drillhole water has twice as high a potassium content as the boiling hot spring waters. Some potassium may, and is thought to have been lost from solution in the upflow below the boiling springs (see later section) which could reduce the calculated quantity of 33 ppm potassium leached from the rock after mixing. For calcium and magnesium it is calculated that the Nedridalur well water should contain 2.3 and 0.27 ppm respectively, which is to be contrasted with the analyzed concentrations of 7.7 and 0.6 ppm. The difference in these two sets of figures is a measure of the a m o u n t of calcium and magnesium leached after mixing. For these calculations it was assumed that the hot water component contained 1 ppm calcium and 0.01 ppm magnesium. In contrast with the major cations, lithium appears to be precipitated from solution subsequent to mixing, as indicated by the overall convex (downwards) relation between chloride and lithium concentrations in waters from each field. This fact is important for geothermometry purposes as discussed in the following section. Equilibrated waters in geothermal reservoirs, at least when their temperature is above some 200°C, will contain substantial concentrations of carbonate which is mostly as carbon dioxide (Arn6rsson et al., 1983b). Upon mixing, and if boiling does not occur, a slightly acid solution is produced. This is demonstrated in Fig. 12 for selected samples (82-013--82-117) from the Geysir and Landmannalaugar fields. The composition of the hot water component was derived from analysis of samples 82-005 and 82-093 (Table 3) and the results for chloride concentrations and temperatures of reservoir waters according to Fig. 17. The cold water component was taken to be represented by sample 82-011. As can be seen from the data points in Fig. 12, the initial diluted waters containing unboiled hot water components, depart strongly from equilibrium and protons must be lost from solution for attainment of equilibrium. The actual mixed waters have approached the equilibrium state to a considerable degree. As protons are lost from the mixed water during the change of the system towards equilibrium, the carbon dioxide will be partly, or mostly, converted into bicarbonate. The formation of the bicarbonate ion requires dissolution of cations from the rock for maintainance of electric neutrality. The final pH at equilibrium must satisfy the cation/proton equilibrium ratios in Fig. 12 at the respective temperature and the equivalent sum of cations must in turn satisfy electric neutrality where conversion of carbon dioxide into bicarbonate is affected by pH changes. Mixing does not lead to strong departure from equilibrium with respect to cation/proton ratios when the hot water component has been degassed (by boiling) before mixing, as demonstrated by sample 82-130 from Hveravellir in Fig. 12. Consequently, leaching does not m o d i f y the water composition nearly as much as in those instances when the hot water component has not boiled.

319

The Na/K ratios in cold water and in the warm mixed waters, especially when containing undegassed hot water components, tend to be similar to or lower than those in the boiling hot springs and similar to the ratio in c o m m o n basaltic and acid volcanic rocks. This suggests that the Na/K ratios of the mixed waters may be dominated by the leaching process and not by the composition of the hot water component. The Na/Li ratios in the warm and cold waters from the Geysir field and Hveravellir tend to be similar to those of basaltic rocks and much higher than in boiling hot spring waters. The difference cannot be explained by leaching of sodium from the rock and precipitation of lithium must be assumed. The situation is different for the Landmannalaugar waters. Broadly speaking, the Na/Li ratios show a similar range for mixed warm waters and boiling hot spring waters.

~8

~6

82.130

¢o 82-130

_o

2~

82-1/

~2 013

4b

~ 8~ Temperature°C

2 2'o

+-r L~

1=6

117 ~

(~2 013 ~o

~o

Temperature °C

4b

6b

8b

Temperature°C

~

2 130

4 2b

4'0

do

80

Temperature °C

Fig. 12. Cation/proton activity ratios in selected mixed waters from the Geysir, Hveravellir and Landmannalaugar geothermal fields. The curves indicate relations in equilibrated geothermal waters according to Arn6rsson et al. (1983b). Open circles refer to measured discharge temperatures and analyzed compositions and filled circles to calculated temperatures and calculated compositions of mixed waters (see text). These calculations are based on the chloride content of the mixed water and chloride levels and temperatures in the hot water component as deduced from chloride-enthalpy mixing model plots. Lines connect data points for individual samples and the figures refer to sample numbers (see Tables 1 to 3).

320 TABLE 4 G e o t h e r m o m e t r y results Sample no.

tmeas

a,b

a,b

b

tqt z

tqt z

tch a

tNa K

tNaKC a

tNaLi

72 95 98 90 94 98 70 78 78 5 17 68 46 24 48 97 39 30

236/273 210/234 204/226 239/278 227/260 221/251 230/263 231/265 --/215

227/255 205/223 200/217 230/258 220/244 214/236 222/247 223/248 --/208 81 c 87 --/160 T/168 --/111 --/160 206/225 --/173 --/125

209/227 184/197 179/190 211/230 200/217 194/210 203/220 204/221 --/181 52 c 57 --/132 --/141 82 --/132 1851198 --/146 96

201 142 137 178 162 143 152 153 168 175 c 154 276 211 153 159 125 257 210

224 185 182 206 190 182 191 191 178 43 23 237 193 43 162 165 210 193

267 221 220 277 264 244 264 259 284 76 86 72 79 156 210 243 150 103

96 93 97 97 92 35 40 65 89 20

255]304 253/301 235/268

244/283 243/280 226/253 169/179 239/273 95 --/124 190/205 211/232 --/103

227/250 225/248 207/226 146/152 221/242 65 96 169/179 191/205 74

193 193 145 120 197 135 117 178 172 114

200 199 165 144 199 59 74 178 182 55

301 300 246 206 295 183 186 264 259 204

173'184 --'199 172r183 --~166 188'203 153'160 194'210 --f176 --~126 --t168 89 --/171 187/201 194/210

150~157 --'172 149 '156 --'138 166'176 128'132 173'184 --'148 98 --/140 59 --/143 165/174 172/183

124 232 130 187 115 122 136 277 241 279 276 285 132 177

168 214 159 176 174 162 180 209 194 211 72 212 173 202

220 333 146 189 185 231 262 248 ? 216 305 209 343 357

Geysir field 79.004 79-005 79-006 82-005 82-006 82-007 82-008 82-009 82-010 82-011 82-012 82-013 82-014 82-015 82-016 82-017 82-018 82-019

211/236

Hve~vellir 82-124 82-125 82-126 82-127 82-128 82-129 82-130 82-131 82-132 82-133

249/293

1931211 217/245

Landmannalaugar field 79-053 79-054 82-093 82-095 82-097 82-099 82-106 82-113 82-114 82-115 82-116 82-117 82-118 82-119

94 82 94 45 95 94 94 59 24 58 10 45 93 96

172/184 --/203 171/183 190/208 198/218

189/206 197/217

321 UNDERGROUND TEMPERATURE ESTIMATES

C h e m i c a l g e o t h e r m o m e try

Table 4 shows the calculated results for the silica, Na-K, Na-K-Ca and NaLi geothermometers. Results for quartz are presented for both the newly presented solubility curves of Fournier and Potter (1982a, b) and RagnarsdSttir and Walther (1983). In the discussion that follows reference is always, if not otherwise specified, made to the curve of Ragnarsd6ttir and Walther (1983). This does not imply that the present author considers this curve better for geothermometry purposes. The difference between the two solubility curves is small. The curve of Fournier and Potter (1982a, b) indicates higher solubility, thus yielding lower quartz geothermometry temperatures. Solubility at 200 and 205°C is 262 ppm silica. Corresponding figures for 673 ppm silica are 300 and 322°C. For convenience of presentation and calculation of quartz equilibrium temperatures, new equations have been derived to describe the results of Ragnarsd6ttir and Walther (1983) and Fournier and Potter (1982a) and the form selected for the equations is that used b y the latter authors: t(°C) = K1 + K 2 C + K 3 C ~ + K 4 C 3 + K s log C

(1)

where C is silica (SiO2) in ppm. The constants for the respective functions are presented in Table 5 and they show quartz solubility at the vapor pressure of the solution and, after correction for steam loss, by adiabatic boiling to 100°C. In calculating the quartz and chalcedony temperatures total silica, i.e. analyzed silica, was taken to represent undissociated silica (H4SiO °) in the deep water. In many of the boiling hot spring waters the pH is very high so a large portion of the dissolved silica is ionized, but the solubility of the silica minerals refers to the activity of unionized silica in solution: SiO2 ,solid + 2H20 = H4SiO °

(2)

The reason for taking analyzed silica to represent H4SiO ° is that boiling in the upflow and the accompanying degassing is taken to be the cause of the high pH and below the level of first boiling the pH is expected to be low enough to prevent significant ionization of the dissolved silica. If quartz equilibrium is assumed at depth, a relatively accurate determination of reservoir water pH can be obtained from the temperature dependence of the log asa÷/aH+ ratio, as shown by Arn6rsson et al. (1983b). The calculation ina T h e first t w o c o l u m n s are based o n t h e r ( 1 9 8 3 ) a n d F o u r n i e r and P o t t e r b T h e figures a b o v e a n d b e l o w t h e cooling, respectively. CThis is surface w a t e r a n d is n o t mineral.

q u a r t z s o l u b i l i t y a c c o r d i n g to R a g n a r s d S t t i r and Wal( 1 9 8 2 a , b), respectively. solidus refer to a d i a b a t i c s t e a m loss a n d c o n d u c t i v e e x p e c t e d to have e q u i l i b r a t e d w i t h a n y g e o t h e r m a l

322 TABLE 5 Constants in eq. 1 describing quartz solubility Function

ia 2a 3b 4b

K~

-42.198 39.536 --53.500 121.627

K2

K3

0.28831 0.58127 0.11236 0.10483

-3.6686X -6.1713 X - 0.5559 X -0.4541 X

Ks

K4

10 -4 10-* 10 -4 10 -4

3.1665x 3.7499x 0.1772 ~ 0.0890 x

10 -7 10 -7 10 -7 10 -7

77.034 19.985 88.390 117.810

aQuartz solubility along the three phase boundary quartz + water + steam. Function 1 is that reported by Fournier and Potter (1982a). Function 2 was derived from equation 5 in Ragnarsd6ttir and Walther (1983). bSilica concentrations in water initially in equilibrium with quartz at the vapor pressure of the solution corrected for adiabatic steam loss to 100°C. Function 3 is based on the data of Fournier and Potter (1982a) and function 4 on quartz solubility according to Ragnarsd6ttir and Walther (1983).

volves s u b s t i t u t i o n of the a n a l y z e d s o d i u m c o n t e n t and t h e c o n s t a n t KNa for the s o d i u m / p r o t o n ratio into t h e equilibrium describing silica i o n i z a t i o n aH+aH,sio,- /an4sio0 =

KH,SiO,

(3)

and s i m u l t a n e o u s s o l u t i o n o f eqs. 2 and 3. T h e c o n s t a n t s in b o t h e q u a t i o n s are t e m p e r a t u r e d e p e n d e n t and the s i m u l t a n e o u s s o l u t i o n o f t h e t w o equations involves an iteration process. I n s p e c t i o n s h o w s t h a t f o r t e m p e r a t u r e s in excess o f 200°C ( c o r r e s p o n d i n g with a b o u t 300 p p m dissolved silica) an insignificant f r a c t i o n o f the silica is ionized for s o d i u m c o n c e n t r a t i o n s as low as 50 p p m . A l t h o u g h c h a l c e d o n y equilibrium t e m p e r a t u r e s are r e p o r t e d in Table 4, it is c o n s i d e r e d , in line with results f r o m drillhole d a t a ( A r n 6 r s s o n , 1975), t h a t q u a r t z is the phase-controlling silica activity in the u n m i x e d g e o t h e r m a l waters w h e n t e m p e r a t u r e s in excess o f 180°C are indicated. It seems, h o w ever, conceivable t h a t s o m e o f t h e m i x e d waters have equilibrated m e t a s t a b l y with c h a l c e d o n y . F o r boiling h o t spring waters t h e q u a r t z g e o t h e r m o m e t e r always yields higher t e m p e r a t u r e s t h a n the Na-K and Na-K-Ca g e o t h e r m o m e t e r s , even f o r t h e m o s t conservative case w h i c h assumes adiabatic steam loss and uses the solubility curve o f F o u r n i e r and P o t t e r ( 1 9 8 2 a , b). T h e relationship b e t w e e n chloride o n o n e h a n d and s o d i u m and p o t a s s i u m o n t h e o t h e r indicates t h a t t h e lower Na-K t e m p e r a t u r e s , c o m p a r e d with q u a r t z , are d u e to loss o f p o t a s s i u m f r o m s o l u t i o n in t h e u p f l o w , at least f o r the Geysir and Hveravellir fields (Fig. 13). F o r these fields t h e r e is a p p r o x i m a t e l y a linear relation bet w e e n s o d i u m and chloride and e x t r a p o l a t i o n passes close to t h e p o i n t o f origin (Fig. 13), suggesting t h a t the s o d i u m variations in the boiling spring waters are due to d i l u t i o n a n d / o r steam loss. P o t a s s i u m c o n c e n t r a t i o n s are lower t h a n w o u l d be e x p e c t e d if o n l y the processes o f d i l u t i o n / b o i l i n g were

323

operative. Extrapolation of the data points for potassium and chloride (Fig. 13) intersects the chloride axis (zero potassium concentrations) at rather high chloride concentrations. Potassium appears thus to have been lost from solution, perhaps by precipitation of K-feldspar, but absorption onto clays is also possible. GEYSIR FIELD

HVERAVELLIR E

.

.

.

.

E

.

E

LANDMANNALAUGAR

FIELD E

:;:, z 30C

]0

z 15C

40C

20

•

15 3O(]

20C

•

10C

n

t 20C

10(]

10

;1

40

80

10

•

50 10C

120 CI ppm

20

40

60 CL p p m

200

4.00

600 CI ppm

Fig. 13. Plot of chloride versus sodium (dots) and potassium (squares) for waters from boiling hot springs at Hveravellir and in the Geysir and Landmannalaugar fields. The Na-K-Ca geothermometer usually gives slightly higher temperatures than the Na-K geothermometer for the boiling hot spring waters. Loss of potassium from solution will, of course, lower the Na-K-Ca temperatures, but this tends to be counteracted when boiling occurs by loss of calcium through calcite precipitation. The different results obtained by these two cation geothermometers are not necessarily due to the net effect of the two counteracting processes, loss of potassium and loss of calcium. Systematic deviation relating to their calibration may just as well be the cause. The CO2 geothermometer of ArnSrsson et al. (1983a) always gives higher temperatures than the Na-K and Na-K-Ca geothermometers, whereas the silica and the Na-Li geothermometers yield similar or higher temperatures (see Tables 4 and 6). The CO2 geothermometry results are not entirely comparable with those of the solute geothermometers as they are not based on the same fluid discharge. However, for the assumption on which the geothermometry is based, the CO2 and other geothermometers should yield comparable results for close-by hot springs and fumaroles. The results of the gas geothermometer of D'Amore and Panichi (1980) are given in Table 6. The results compare rather well with the CO2 geothermometer for some samples from the Landmannalaugar field but for the Geysir field and Hveravellir the gas geothermometer of D'Amore and Panichi (1980) yields considerably lower temperatures. This can be attributed to low H2S and H2 in the steam from these fields, the reason probably being that oxidation occurs in the upflow. This interpretation is in line with that of the 51sO content of sulphate discussed at the end of the previous section.

324 TABLE 6 Gas geothermometry results (°C) Sample no.

too ~

t~

Ad

267 270 270 276 259 261

_b _6 174 _b 185 185

82-007 82-005 82-005 82-005 82-005 79-004

248 239

221 175

82-124

Geysirfield 82-020 82-021 82-034 82-035 82-134 82-135

Hveravellir 82-136 82-137

Landmanna~ugar field 82-094 82-096 82-098 82-100 e 82-102 82-103 82-104 82-105 82-107 82-108 82-109 82-110 82-111 82~120 e 82-121 e

243 262 245 269 279 245 248 268 242 249 263 255 263 307 291

_c _c 232 353 329 340 326 334 179 391 318 371 395 372 410

82-093 82-106

82-119

aBased on the gas geothermometer of D'Amore and Panichi (1980). bHydrogen or methane were not detected in the sample. cSamples were air contaminated. dThis column gives sample no. for nearby springs. eAverage of the following duplicate samples: 100/101, 120/123, and 121/122.

The relation between the chemical geothermometry results differs for the warm springs (mixed waters) from that of boiling springs. The Na-K tempera t u r e s a r e e q u a l t o o r h i g h e r t h a n t h e q u a r t z e q u i l i b r i u m t e m p e r a t u r e s in t h e warm waters and they are also about equal to or higher than the Na-K temperatures of the boiling hot spring waters (Fig. 14). In view of the interp r e t a t i o n in t h e p r e v i o u s s e c t i o n t h a t t h e m a j o r c a t i o n c o n c e n t r a t i o n s in t h e w a r m , m i x e d w a t e r s a r e g o v e r n e d b y l e a c h i n g a f t e r m i x i n g , it is c o n c l u d e d t h a t t h e N a - K t e m p e r a t u r e s , as w e l l as t h e N a - K - C a t e m p e r a t u r e s , a r e u n r e a l i s t i c f o r t h e w a r m w a t e r s as t h e a s s u m p t i o n o f m i n e r a l / s o l u t e e q u i librium with respect to sodium and potassium does not seem to be valid. The

325 Na-K-Ca g e o t h e r m o m e t e r w o u l d be e x p e c t e d to yield m o r e reliable estim a t e s , as i n d e e d i n d i c a t e d f o r s o m e w a r m w a t e r s (Table 4). H o w e v e r , o t h e r s a m p l e s yield high and, it is believed, d u b i o u s t e m p e r a t u r e estimates. Like t h e Na-K-Ca g e o t h e r m o m e t e r , the Na-Li g e o t h e r m o m e t e r o f Fouillac and Michard ( 1 9 8 1 ) t e n d s to give less misleading results for w a t e r s o f m i x e d origin t h a n t h e N a - K g e o t h e r m o m e t e r , the r e a s o n p r o b a b l y being t h a t the s o d i u m / l i t h i u m ratios in basaltic a n d acidic r o c k s are c o m p a r a b l e with t h o s e o f e q u i l i b r a t e d w a r m waters. I/I/

[]

O[~5

/

0

s/"

[]

[]

Z

O

200

O []

0

/

//

o

/

/

//

•

/

,,"~1

o

//

,,'g

•

•e A

,]I", 100 /"

/

J SI ~J ~J ~J

/t /t

/"

/

/

/• eo

;do

26o tqtz °C

Fig. 14. Relation between quartz equilibrium and Na-K temperatures. The data used on quartz solubility are from Ragnarsd6ttir and Walther (1983), assuming total pressure to be the vapour pressure of the solution and adiabatic steam loss to 100°C. The function given by Arndrsson et al. (1983a) was used for the Na-K geothermometer. Open symbols indicate warm, mixed waters but filled symbols boiling hot springs discharges. Circles, triangles and squares represent data from Geysir, Hveravellir and Landmannalaugar, respectively. A n e w silica-carbonate mixing m o d e l A r n 6 r s s o n et al. ( 1 9 8 3 b ) f o u n d t h a t t h e c o n c e n t r a t i o n s o f c a r b o n d i o x i d e in w a t e r s in g e o t h e r m a l reservoirs w e r e o n l y d e p e n d e n t o n t h e t e m p e r a t u r e o f t h e s e waters. T h e y c o n c l u d e d t h a t this was t h e result o f overall s o l u t e / m i n e r a l e q u i l i b r a t i o n in t h e s e reservoirs. At t e m p e r a t u r e s a b o v e a b o u t 200°C, m o s t o f t h e dissolved t o t a l c a r b o n a t e is in t h e f o r m o f c a r b o n d i o x i d e , so it is a s a t i s f a c t o r y a p p r o x i m a t i o n t o t a k e a n a l y z e d c a r b o n a t e t o r e p r e s e n t c a r b o n d i o x i d e . It is well k n o w n t h a t silica levels in h i g h - t e m p e r a t u r e w a t e r s

326 are determined by quartz solubility. It follows, therefore, that it is a satisfactory approximation to assume a fixed relation between silica and total carbonate in high-temperature geothermal reservoir waters. Boiling of such waters will lead to drastic reduction in its carbonate content but mixing without boiling will, on the other hand, produce waters with high carbonate/silica ratios relative to the equilibrated waters, due to the curvature of the silica/carbonate relationship {Fig. 15). GEYSIR FIELD

LANDMANNALAUGAR FIELD

500

500

E 400

E 400

f~ o=

O.

u

~5 3OO

V

300

oolI.

/

200

200

e:

e,'

400

100

-

i

i

800

12'00 16'00

Total carbonate, ppm

40'0

8oo 1 oo 16'oo Total carbonate, ppm

Fig. 15. A plot of silica versus total carbonate (silica-carbonate mixing model) for cold and thermal waters from the Geysir and Landmannalaugar fields. The curves represent silica/carbon-dioxide relationship in equilibrated geothermal waters. The temperature dependence of silica was assumed to be controlled by quartz solubility according to the data of Ragnarsdbttir and Walther (1983). The temperature dependence of carbon dioxide was derived from the respective function in table 5 of Arnbrsson et al. (1983b). Data points plotting above the equilibrium curve represent degassed waters, whereas data points which plot below correspond with mixed waters. The broken lines indicate evaluated silica-carbonate relationship in mixed, undegassed waters and their intersection with the equilibrium curve is a measure of the temperature of the hot water component.

The silica-carbonate diagram in Fig. 15 may be used in two ways to aid geothermometry interpretation. Firstly, it serves to distinguish boiled waters from conductively cooled waters and mixed waters which contain an undegassed (and therefore unboiled) hot water component, assuming that the boiling occurs between the points of last equilibrium with quartz and sampling. Secondly, if there are sufficient data on warm waters containing an unboiled water component, the diagram may be used to evaluate the temperature of the hot water component. The diagram is useful as a supple-

327

m e n t to interpretation of the silica-enthalpy warm spring mixing model because a choice often needs to be made as to whether it should be assumed that boiling had occurred before mixing or not. Little steam formation suffices to deplete the original h o t water almost quantitatively in total carbonate. Relationship between chloride and total carbonate might also be used to aid distinguishing boiled and non-boiled hot spring waters, as was demonstrated by Fournier (1981) for the Geyser Hill and Black Sand waters of the Upper Basin, Yellowstone National Park. Estimation of underground temperatures by the silica-carbonate mixing model involves extrapolation of a line through the data points for mixed and undegassed warm waters and determination of the point of intersection with the silica-carbonate curve for equilibrated waters. From the silica concentration corresponding with this point, the temperature may be derived from the quartz geothermometer. I

r

I

311°C

. o~

f

E

(I)

o.. o.

f

0 A

262 °C

Ul

U

"D

"0 o

"o i-

•

°'///s

~ 0~t.... I A X" •

2o0 ..... /

~,"

2-013 ~)~v/ D ^~,~ / ( I )

I

I

400

I

800 Enthalpy, J/g

I

1200

I

Fig. 16. A p l o t of undissociated silica versus enthalpy of spring discharges. The dots, triangles and squares designate data from the Geysir, Hveravellir and Landmannalaugar fields respectively. The undissociated silica c o n c e n t r a t i o n s were calculated f r o m total silica, the measured pH reported in Tables 1 to 3 and the values for KI-I SiO 4 given by Arn6rsson et al. (1982). Quartz saturation curves are based on: (1) Fourn]er and Potter (1982a); and (2) Ragnarsd6ttir and Walther (1983). The broken line represents silicaenthalpy relationship for cold water and the water from the Nedridalur well in t h e Geysir field (these waters have a relatively low pH so undissociated silica equals total silica) and the intersection (open squares) with the quartz saturation curves represents the enthalpy of the hot water c o m p o n e n t .

328 The curve describing the silica-carbonate relationship can be drawn from the temperature functions for quartz solubility in Table 5 and the CO2temperature function in table 5 of Arn6rsson et al. (1983b). In the Geysir and Landmannalaugar geothermal fields the silica-carbonate relation indicates that the hot water component in the mixed waters has not boiled (Fig. 16), whereas the warm Hveravellir waters are a mixture of cold water and degassed hot water. In the Geysir field most of the cold and warm waters show a linear relation between silica and carbonate and an extrapolation of a line through the data points indicates that the hot water component is at some 260°C {Fig. 15). Two waters show relatively low carbonate (samples 82-010 and 82-018). As deduced from the temperature of the first water, near surface degassing may be responsible, and for the second sample the hot water component may be, at least partly, boiled because this sample is located relatively close to the main area of boiling hot springs. In the Landmannalaugar field the silica-carbonate relationship indicates a temperature of 231°C for the hot water component. A silica-carbonate plot is not presented for Hveravellir as all waters issued at the surface in this field have been degassed.

The silica-enthalpy warm spring mixing model Silica concentrations versus discharge enthalpy show the same general pattern for waters from all three geothermal fields (Fig. 16). Cold and tepid waters are undersaturated with amorphous silica. Warm waters tend to be saturated or slightly undersaturated. Boiling spring waters at Hveravellir and in the Geysir field range from significant supersaturation to undersaturation, whereas waters in boiling springs in the Landmannalaugar field are always considerably undersaturated. When plotting the data for the boiling hot spring waters, which generally have a pH of more than 9, correction was made for the fraction in solution that is ionized. It may be that the solubility of an amorphous aluminium silicate sets an upper limit to silica levels in the springs at Landmannalaugar. Precipitates of this kind are known from geothermal waters in Iceland that are conducted through asbestos pipes in a few district-heating systems (ThSrhallsson et al., 1975). The Landmannalaugar waters issue either through alluvial gravel or altered rhyolite, so the water has good access to the aluminium in the rock. As all the cold waters are amorphous silica undersaturated and waters in high-temperature geothermal reservoirs axe k n o w n to equilibrate with quartz, it follows that mixing alone never produces amorphous silica-saturated water. For attainment of such saturation, cooling by boiling or conduction is necessary, or leaching of silica from the rock subsequent to mixing. Conductive cooling beyond amorphous silica saturation will lead to precipitation of this phase, which in turn will give low silica relative to chloride in the mixed water. Such is the case for samples 82-010 and 82-018 from the Geysir field, samples 79-054 and 82-095 from Landmannalaugar and sample 82-131 from HveraveUir.

329 In selecting data for estimation of the enthalpy of the hot c o m p o n e n t in mixed waters by the silica-enthalpy model it is useful to study silica-chloride relations (Fig. 5) and the state of amorphous silica saturation at the discharge temperature. For undegassed waters showing low silica relative to chloride, as well as amorphous silica saturation, it can be inferred that conductive cooling has occurred and the silica-enthalpy plot is liable to yield high estimates for the temperature of the hot water component. The water issuing from the Nedridalur well in the Geysir field may not have cooled significantly by conduction after mixing, as deduced from the relatively high discharge rate (5 dm 3 s -1) and the depth level of the producing aquifer (390 m), although cooling during ascent amounts to 5°C (Fig. 2). Taking sample 82-011 to represent the cold water component, a silica enthalpy plot intersects the quartz saturation curves of Fournier and Potter (1982a) and Ragnarsd6ttir and Walther (1983) at 1145 and 1408 J g-', respectively. These values correspond with a temperature of 262 ° and 311°C. If the temperature (73°C) in the well at the point of inflow had been selected, instead of the discharge temperature, to derive the enthalpy of the hot water component, the corresponding temperatures are 248 and 288°C. This shows how sensitive the temperature estimate is to the selection of the enthalpy value for the mixed water, the reason being the large extrapolation of the line through the data points onto the quartz solubility curve. From the chloride-enthalpy diagram in Fig. 17 it is deduced that water at 260°C in the Geysir field will contain 114 ppm C1. Using these values for temperature and chloride to represent the hot water component, as well as the indicated path of cooling and mixing in Fig. 17 and sample 82-011, to represent the cold water component, the initial temperature and the degree of conductive cooling of warm waters in the Geysir field has been evaluated from their chloride content. It is found that only samples 82-010, 82-018 and 82-016 have cooled significantly after mixing, or by 65, 66 and 20°C, respectively. Other warm spring waters are calculated to have cooled after mixing by less than 10°C. If water at a temperature of about 260°C or higher mixes to give water with a temperature in the range of 50°C, one would expect the mixing to occur at a depth of less than a few hundreds of metres, as the temperature gradient in the region of the Geysir field is probably 100--150°C km -1 (P~ilmason et al., 1979). The steam pressure exerted by water at 262--311°C (the temperature estimates from the silica-enthalpy plot Fig. 16) is 48--100 bar abs., so boiling of this water, if it exists, would be expected to begin at some 600--1200 m depth. The only real possibility for mixing would be that it either occurs in a series of steps or that the cold water encountered twophase flow where the total fluid discharge enthalpy was 1145--1408 J g-~ and that the steam was condensed in the process. The assumption generally made in using the silica-enthalpy mixing model may not always be valid, namely that the temperature and the silica content of the hot water c o m p o n e n t corresponds with quartz saturation. Cooling of

330 GEYSIR FIELD

HVERAVELLIR

1 /

1600

LANDMANNALAUGAR FIELD

352' 160(

160(

• 311 287

~_120C j:

(

")

8®l

400

i

=

~262

, /, /~

' //

•

! t

!

/

80C

.¢'

40C ,

c

k

40

80

120

160

~oo Clppm

20

40

60 CI, p p m

~!

"L(9 0 5 4

'

- -

CI p p m

Fig. 17. Chloride-enthalpy mixing model diagram. For boiling hot spring waters (filled symbols) the enthatpy was derived from the silica content, assuming equilibrium with quartz and adiabatic steam loss. For warm springs, represented by open symbols, the enthalpy was obtained from measured discharge temperatures. Large triangles represent enthalpy derived from the silica-enthalpy warm spring mixing model using the curves of Fournier and Potter (1982a) and Ragnarsd6ttir and Walther (1983), respectively (see Fig. 16 ). Half-filled symbols represent the boiling springs with the highest chloride in each field at the discharge temperature and at the quartz equilibrium temperature, assuming conductive heat loss. Broken, dotted and slim solid lines indicate mixing, adiabatic boiling and conductive cooling, respectively. The thick solid line shows the most likely evolutionary path for the deep hot water. The large circles represent the estimated chloride and enthalpy (corresponding temperature is reported) for the deep hot water in each field.

the water by conduction prior to mixing, without sufficient silica precipitation to restore equilibrium, may well have occurred as indicated schematically by the broken line on the chloride-enthalpy diagram in Fig. 17. From a hydrological point of view this mixing process is more acceptable than the one described above which involved mixing of cold water with rising hot water and steam. The silica-enthalpy mixing model cannot be applied with confidence to the data from HveraveUir and the Landmannalaugar field. For Hveravellir the only waters which are undersaturated with amorphous silica are the tepid ones and sample 82-127. The chloride-silica relationship (Fig. 9) indicates precipitation of silica from solution prior to, or subsequent to, mixing. At Landmannalaugar all the warm waters are close to amorphous silica saturation. For samples 82-113, 82-115 and 82-117 this saturation has evidently been attained by dissolution of silica from the rock, as indicated by the chloride-silica relationship in Fig. 9. For sample 79-054 conductive cooling may be important. The silica-carbonate relationship in Fig. 15 shows that saturation with amorphous silica cannot have resulted from mixing after boiling (which means degassing) of the hot water c o m p o n e n t for any of the warm waters.

331

The chloride-enthalpy mixing model This mixing model takes into account both mixing and boiling processes. Its application involves basically relating analyzed chloride levels to water enthalpy which can be derived in a number of ways, such as from measured discharge temperatures, geothermometry temperatures, and silica-enthalpy mixing model temperatures. The silica-enthalpy mixing model indicated that the temperatures of the hot water c o m p o n e n t in the mixed warm waters from the Geysir field was as high as 262°C if the quartz solubility curve of Fournier and Potter (1982b) was used and 311°C if the curve of RagnarsdSttir and Walther (1983) was used. By conventional use of the chloride-enthalpy diagram these temperature values imply, if there was a unique hot water phase with respect to temperature and chloride, that the unmixed hot water was as much as 352°C (Fig. 17) and that the hot water c o m p o n e n t in the mixed water had boiled to 311°C before mixing, whereas the boiling hot spring waters had cooled by mixing to some 260°C and then boiled. This model is not compatible with the present interpretation of the silica-carbonate relations from which it was concluded that the hot water c o m p o n e n t in the mixed water had not been degassed and had, therefore, not boiled. The discrepancy is thought to result from an erroneous conclusion of the silica-enthalpy mixing model of a hot water c o m p o n e n t at 311°C. An explanation which is compatible with the combined silica-chloride-carbonate-enthalpy relations and anticipated maximum depth of mixing is that the equilibrated hot water c o m p o n e n t has an enthalpy and chloride indicated by the large circles in Fig. 17, and that conductive cooling of this water followed by mixing produced the warm waters. Boiling, but also some conductive cooling and mixing, caused changes in the composition of the water in the upflow below the boiling hot springs. It seems logical that conductive cooling is more extensive if it is followed by mixing as one factor -- low permeability -- increases the probability for and the magnitude of both processes. Where permeability is highest the chances for hot water to reach the surface are at maximum and mixing at minimum. For Hveravellir the chloride-enthalpy mixing model yields underground temperatures similar to those of the quartz geothermometer (Fig. 17), indicating that the boiling hot spring waters, which are highest in chloride, have n o t mixed with surficial water, and that significant silica had not precipitated during cooling in the upflow. In the Landmannalaugar field chloride-enthalpy relations indicate substantial loss of silica from solution for the waters highest in chloride. Consequently, this mixing model yields higher underground temperatures than the quartz geothermometer and also higher than the Na-K and Na-K-Ca geothermometers. Comparison of Fig. 17 with the results in Tables 4 and 6 shows that the Na-Li and CO2 geothermometry results spread around the temperature values indicated by the chloride-enthalpy mixing model, both for Hveravellir and the Landmannalaugar field.

332 SUMMARY AND CONCLUSIONS Interpretation of chemical analyses of hot spring waters and fumaroles with respect to predicting underground temperatures should integrate as m a n y parameters as possible into a single model rather than considering individual geothermometers and mixing models in isolation. In the present study use was made of almost all the chemical components for which results are presented in Tables 1--3. Relation of chloride concentrations to boron and 5180 appears to be particularly useful to evaluate mixing processes. If mixing has occurred, a linear relation between these parameters should be observed. Linear, or nearlinear, relations between chloride and silica and sulphate may also be useful to strengthen evidence for mixing. It was found unreliable to apply the cation geothermometers, except for Na-Li, to mixed waters. The reason is that leaching after mixing tends to dominate the relative cation concentrations. Na-K ratios become similar to those in the associated rocks. Mixing involving hot water relatively rich in carbon dioxide produces slightly acid solutions, but the carbon dioxide content of the hot water is determined by its temperature, if boiling has not occurred. The leaching process occurs mostly in response to increasing pH of the mixed water and conversion of carbon dioxide to bicarbonate. The effect of the leaching process on the validity of the Na-K and the Na-K-Ca geothermometers will, it is believed, depend on water salinity or, to be more precise, on the ratio of total cations to carbon dioxide. When carbon dioxide concentrations are low in relation to total cation concentrations, the leaching process will not greatly affect the relative amounts of cations in solution. The Na-K and the Na-K-Ca geothermometers give lower results for boiling hot spring waters than do the quartz and the Na-Li geothermometers and the mixing models. This results from loss of potassium from solution during cooling in the upflow. The Na-K-Ca temperatures tend to be somewhat higher than the Na-K temperatures, probably because the effect of potassium loss is to some extent counter-balanced by loss of calcium from solution through calcite precipitation. Usually some silica is precipitated from solution as a consequence of mixing, but occasionally it may be leached from the rock to an extent dictated by amorphous silica solubility. The Na-Li and CO2-gas geothermometry temperatures are similar to those obtained from chloride-enthalpy mixing models, at least if enthalpy values for warm spring waters and boiling hot spring waters are based on discharge temperatures and quartz geothermometry temperatures respectively. The quartz geothermometer also yields similar results for the Geysir and Hveravellir fields, but lower temperatures for the Landmannalaugar, the reason being precipitation in the upflow. The new carbonate-silica mixing model proposed here is useful to estimate underground temperatures if the mixing process prevents boiling and,

333

therefore, degassing. For the Geysir field this mixing model yields results comparable to those of the chloride-enthalpy mixing model and the Na-Li and CO2 gas geothermometers, but for the Landmannalaugar field these geothermometers tend to yield somewhat higher values. ACKNOWLEDGEMENTS

The writer wishes to acknowledge Dr. Valgardur Stef~nsson of the National Energy Authority, Reykjavik, for making available the downhole temperature data for the well at Nedridalur. Special thanks are due to Halld6r .~rmansson and Thorsteinn Thorsteinsson for providing facilities and assistance in carrying out the lithium analyses, and to my daughter, Harpa, for making all the drawings. Dr. Sigffls Johnson is also thanked especially for arranging the oxygen isotope analyses at the Geophysical Isotope Laboratorium in Copenhagen. The hydrogen isotope analyses were carried out at the International Atomic Energy Agency in Vienna under the supervision of Dr. Bryan Payne. Most of the analyses of waters from the Geysir geothermal field were carried out by Mr. Oliver Jordan as a part of his work at the United Nations University Geothermal Programme operated by the National Energy Authority. This study has been supported by the Icelandic Science Foundation.

REFERENCES .~rnason, B., 1976. Groundwater Systems in Iceland Traced by Deuterium. Publ. Societas Scientiarum Islandica, Reykjav~, 236 pp. ,~rnason, B., 1977. Hydrothermal systems in Iceland traced by deuterium. Geothermics, 5: 125--152. Arn6rsson, S., 1970a. Underground temperatures in hydrothermal areas in Iceland as deduced from the silica content of the thermal water. Geothermics, 2: 536--541. Arn6rsson, S., 1970b. Geochemical studies of thermal waters in the southern lowlands of Iceland. Geothermics, 2: 547--552. Arn6rsson, S., 1975. Application of the silica geothermometer in low-temperature hydrothermal areas in Iceland. Am. J. Sci., 275: 763--784. Arn6rsson, S., 1983. Chemical equilibria in Icelandic geothermal systems -- implications for chemical geothermometry investigations. Geothermics, 12: 119--128. Arn6rsson, S., Sigurdsson, S. and Svavarsson, H., 1982. The chemistry of geothermal waters in Iceland I. Calculation of aqueous speciation from 0° to 370°C. Geochim. Cosmochim. Acta, 46: 1513--1532. Arn6rsson, S., Gunnlaugsson, E. and Svavarsson, H., 1983a. The chemistry of geothermal waters in Iceland. III. Chemical geothermometry in geothermal investigations. Geochim. Cosmochim. Acta, 47: 567--577. Arn6rsson, S., Gunnlaugsson, E. and Svavarsson, H., 1983b. The chemistry of geothermal waters in Iceland. II. Mineral equilibria and independent variables controlling water compositions. Geochim. Cosmochim. Acta, 47: 547--566. Cusicanqui, H., Mahon, W.A.J. and Ellis, A.J., 1975. The geochemistry of the E1 Tatio geothermal field, northern Chile. Proceedings of the Second United Nations Symposium on the Development and Use of Geothermal Resources, San Francisco 20--29 May, 1975, 1: 703--711.

334 D'Amore, F. and Panichi, C., 1980. Evaluation of deep temperatures in hydrothermal systems by a new gas geothermometer. Geochim. Cosmochim. Acta, 44: 549--556. Einarsson, T., 1937. t~ber die neuen Eruptionen des Geysir in Haukadatur. Publ. Societas Scientiarum Islandica, Ser. 1, 2: 149---166. Einarsson, T., 1949. The eruptions of Geysir in Haukadalur. N~itt6rufraedingurinn, 19: 20--26 (in Icelandic). Ellis, A.J., 1970. Quantitative interpretation of chemical characteristics of hydrothermal systems. Geothermics, 2: 516--527. Ellis, A.J., 1979. Chemical geothermometry in geothermal systems. Chem. Geol., 25: 219--226. Fouillac, C. and Michard, G., 1981. Sodium]lithium ratio in water applied to geothermometry of geothermal reservoirs. Geothermics, 10: 55--70. Fournier, R.O., 1973. Silica in thermal waters: laboratory and field investigations. Pro L ceedings International Symposium on Hydrochemistry and Biochemistry, Japan, pp. 122--139. Fournier, R.O., 1977. Chemical geothermometers and mixing models for geothermal systems. Geothermics, 5: 41--50. Fournier, R.O., 1979. Geochemical and hydrologic considerations and the use of enthalpychloride diagrams in the prediction of underground conditions in hot-spring systems. J. Volcanol. Geotherm. Res., 5: 1--16. Fournier, R.O., 1981. Application of water chemistry to geothermal exploration and reservoir engineering. In: L. Rybach and L.J.P. Muffler (Editors), Geothermal Systems: Principles and Case Histories. Wiley, New York, N.Y., pp. 109--143. Fournier, R.O. and Potter, R.W. II, 1982a. A revised and expanded quartz geothermometer. Geotherm. Resour. Council Bull., Nov., 1982: 3--9. Fournier, R.O. and Potter, R.W. II., 1982b. An equation correlating the solubility of quartz in water from 25 ° to 900°C at pressures up to 10,000 bars. Geochim. Cosmochim. Acta, 46: 1969--1974. Fournier, R.O. and Rowe, J.J., 1966. Estimation of underground temperatures from the silica content of water from hot springs and wet-steam wells. Am. J. Sci., 264: 685---697. Fournier, R.O. and Truesdell, A.H., 1973. An empirical Na-K-Ca geothermometer for natural waters. Geochim. Cosmochim. Acta, 37: 1255--1275. Fournier, R.O., White, D.E. and Truesdell, A.H., 1974. Geochemical indicators of subsurface temperature, 1. Basic assumptions. J. Res. U.S. Geol. Surv., 2: 259--262. Fournier, R.O., White, D.E. and Truesdell, A.H., 1975. Convective heat flow in Yellowstone National Park. Proceedings Second United Nations Symposium on the Development and Use of Geothermal Resources, San Francisco, 1: 731--739. Fournier, R.O., Sorey, M.L,, Mariner, R.H. and Truesdell, A.H., 1979. Geochemical prediction of aquifer temperatures in the geothermal system at Long Valley, California. J. Volcanol. Geotherm. Res., 5: 17--34. Mahon, W.A.J., 1975. Review of hydrogeochemistry of geothermal systems--prospecting, development and use. Proceedings Second United Nations Symposium on the Development and Use of Geothermal Resources, San Francisco, 1: 775--783. Mariner, R.H. and Willey, L.M., 1976. Geochemistry of thermal waters in Long Valley, Mono Country, California. J. Geophys. Res., 81: 792--800. P~lmason, G. and Saemundsson, K., 1974. Iceland in relation to the Mid-Atlantic Ridge. Annu. Rev. Earth Planet. Sci., 2: 25--50. P~lmason, G., ArnSrsson, S., Fridleifsson, I.B., Kristmannsd6ttir, H., Saemundsson, K., Stef~insson, V., Steingrfmsson, B., T6masson, J. and Kristj~nsson, L., 1979. The Iceland crust: evidence from drillhole data on structure and processes. Ewing Series, AGU (1979), pp. 43--65. Ragnarsd6ttir, K.V. and Walther, J.W., 1983. Pressure sensitive "silica geothermometer" determined from quartz solubility experiments at 250°C. Geochim. Cosmochim. Acta, 47 : 941--946.

335 Saemundsson, K., 1972. On the geological features of the Torfajbkull region, N~tttirufraedingurinn, 42: 81--99. Sigurgeirsson, Th., 1949. Temperature measurements in Geysir. N~ttfirufraedingurinn, 1 9 : 2 7 - - 3 3 (in Icelandic). Th6rhallsson, S., Ragnars, K., Arn6rsson, S. and Kristmannsd6ttir, H., 1975. Rapid scaling of silica in two district heating systems. Proceedings Second United Nations Symposium on the Development and Use of Geothermal Resources, San Francisco, 2: 1445--1449. Thoroddsen, Th., 1925. Die Geschichte der isl~/ndischen Vulkane. Kgl. Danske Vidensk. Selsk. Skr., Ser. 8, 9 : 4 5 8 pp. Truesdell, A.H., 1975. Summary of section III -- geochemical techniques in exploration. Proceedings Second United Nations Symposium on the Development and Use of Geothermal Resources, San Francisco, 1: liii--lxiii. Truesdell, A.H. and Fournier, R.O., 1975. Calculation of deep temperatures in geo~ thermal systems from the chemistry of boiling spring waters of mixed origin. Proceedings Second United Nations Symposium on the Development and Use of Geothermal Resources, San Francisco, 1 : 837--844. Truesdell, A.H. and Fournier, R.O., 1977. Procedure for estimating the temperature of hot water component in a mixed water using a plot of dissolved silica vs enthalpy. J. Res., U.S. Geol. Surv., 5: 49--52. Walker, G.P.L., 1963. The Breiddalur central volcano, eastern Iceland. Q.J. Geol. Soc. London, 119: 29--63. Walker, G.P.L., 1974. Eruptive mechanisms in Iceland. In: Geodynamics of Iceland and the North-Atlantic Area. Reidel, Dordrecht, pp. 189--202. White, D.E., 1970. Geochemistry applied to the discovery, evaluation and exploitation of geothermal energy resources. Geothermics, 1: 58--80.