Stefán Arnórsson

The chemistry of geothermal waters in Iceland. III. Chemical geothermometry in geothermal investigations

Abstract New data from geothermal wells in Iceland have permitted empirical calibration of the chalcedony and Na K geothermometers in the range of 25–180°C and 25–250°C respectively. The temperature functions are: t°C= 1112 4.91− log SiO 2 −273.15 t°C= 933 0.993+ log Na/K −273.15 Concentrations are expressed in ppm. These temperature functions correspond well with the chalcedony solubility data of Fournier (1973) and the thermodynamic data for low-albite/microcline/solution equilibria of Heloeson (1969). A new CO2 geothermometer is proposed which is considered to be useful in estimating underground temperatures in fumarolic geothermal fields. Its application involves analysis of CO2 concentrations in the fumarole steam. The temperature function which applies in the range 180−300°C is: logCO2 = 37.43 + 73192/T- 11829· 103/T2 + 0.18923T- 86.187·logT where T is in °K and CO2 in moles per kg of steam.

The chemistry of geothermal waters in Iceland. I. Calculation of aqueous speciation from 0° to 370°C

Abstract A computer programme has been developed to calculate the composition and aqueous speciation of geothermal reservoir waters including pH, redox potential and gas partial pressures. The programme is specifically suited to handle geochemical data from wet-steam wells, hot-water wells and boiling hot springs, but it may also be used for non-thermal waters. Solubility data for selected geothermal minerals are incorporated to facilitate the study of solution mineral equilibria. The programme may also be used to study chemical changes in water chemistry accompanying boiling, variable degassing and cooling, and how these changes disturb solution mineral equilibria.

Amorphous silica solubility and the thermodynamic properties of H4SiO°4 in the range of 0° to 350°C at Psat

Abstract The solubility of amorphous silica was determined in the temperature range 8° to 310°C at 1 bar below 100°C and at Psat at higher temperatures. Our results are consistent with previous experiments between 100° and 200°C, but at higher temperatures they indicate lower solubility. Below 100°C our result are lower than the results of some researchers, but in good agreement with others. Our solubility data have been combined with previously reported data to retrieve a temperature equation describing amorphous silica solubility. Quartz solubility data have also been assessed. The solubility equations for the reaction SiO2,s + 2H2O = H4SiO°4 are: logK am.silica = −8.476 − 485.24 × T −1 − 2.268 × 10 −6 × T 2 + 3.068 × logT logK quartz = −34.188 + 197.47 × T −1 − 5.851 × 10 −6 × T 2 + 12.245 × logT where T is in K. They are valid in the temperature range 0° to 350°C at 1 bar below 100°C and at Psat at higher temperatures. From the quartz solubility equation and the thermodynamic properties of quartz and liquid water, the standard partial molal Gibbs energy of formation and the third law entropy of H4SiO°4 were calculated as −1,309,181 J/mole and 178.85 J/mole/K at 25°C. The difference in the standard apparent Gibbs energy of H4SiO°4 as calculated from quartz solubility, on one hand, and amorphous silica solubility, on the other, is about the same over the temperature range 0° to 350°C indicating that the solubility temperature equations obtained for the two solids in this study are internally consistent. This indicates that the quartz solubility data of Rimstidt (1997) , which were used in this study to retrieve the quartz solubility equation, are valid and also our data on ΔḠ°f and S° for H4SiO°4 at 25°C and 1 bar as well as the ΔḠ° temperature equation presented for this species. These new Gibbs energy values for H4SiO°4 indicate that all silicate minerals are considerably more soluble under Earth’s surface conditions than generally accepted to date, or by about 0.6 log K units at 0°C per silicon atom in the unit formula.

The chemistry of geothermal waters in Iceland. II. Mineral equilibria and independent variables controlling water compositions

Abstract The major element chemistry of Icelandic geothermal waters is predictable provided two parameters are known. This follows from an attainment of, or a close approach to, an overall chemical equilibrium in the geothermal systems at temperatures as low as 50°C. It is considered that the geothermal system composition, temperature and kinetic factors determine which alteration minerals form. The system composition is not so much fixed by rock composition as by the rate of leaching of the various constituents from the fresh rock and the composition of inflowing water. The water chemistry is determined by the system composition and the external variables acting on the system. They include temperature and the mobility of chloride. Pressure, which theoretically should be regarded as an external variable, has insignificant effect on water compositions in the range (1–200 bars) occurring in the geothermal systems.

Geothermal systems in Iceland: Structure and conceptual models—I. High-temperature areas

Abstract There are 20 known high-temperature geothermal areas in Iceland and another eight potential areas. Surface manifestations are meagre in these eight areas and not conclusive, and no drilling has been carried out to prove or disprove the existence of high-temperature geothermal systems at depth. The high-temperature areas are located within the active volcanic belts or marginal to them. The heat source is considered to be magmatic, shallow level crustal magma chambers in the case of high-temperature systems associated with central volcanic complexes, but dyke swarms for the systems on the Reykjanes Peninsula where no central volcanoes have developed. Fossil high-temperature systems are abundant in Quaternary and Tertiary formations as witnessed by alteration of the basaltic eruptive rocks into lower-greenschist mineral assemblages. The fossil systems are typically associated with central volcanoes where intrusives account for 50% or more of the rock. The fossil systems are considered to have formed within the active volcanic belts but drifted out of them in conjunction with crustal accretion within these belts. In the process they may develop into low-temperature geothermal systems. Permeability is very variable within the drilled high-temperature areas, in the range 1–150 millidarcies. The best permeability generally appears to be associated with sub-vertical fractures and faults. Permeability is poorest when the reservoir rock consists dominantly of intrusives, such as at Krafla, northeastern Iceland. It appears that intrusives are most abundant in reservoirs associated with central complexes that have developed a caldera. Temperatures follow the boiling point curve with depth, at least to the level of the deepest wells, in some areas, but in others they are lower. The highest recorded downhole temperature is >380°C. Hydrological considerations and permeability data favour that convection is density driven and that the source water is shallow groundwater in the vicinity of these systems. This groundwater is in most cases of meteoric origin. However, in three areas on the Reykjanes Peninsula it is largely or solely marine. The deuterium content of geothermal waters of meteoric origin is often lower than that of local precipitation. This has been taken to indicate that the source of supply is precipitation that has fallen on higher ground inland. This may indeed be the case, but flow from the source area is considered to be shallow. In some cases the low δD-values may stem from the presence of a component of an old water, which is isotopically lighter than todays precipitation at any particular site because the climate in Iceland was colder in the past. The geothermal seawater at Reykjanes and Svartsengi, southwestern Iceland, is considerably lower in deuterium than seawater. The cause of this is not known. However, reaction between seawater and basaltic rocks at very low temperatures may contribute, as well as rising of H 2 gas from deep levels and its reaction at shallower levels in the geothermal system to form water, but H 2 gas is much more depleted in deuterium than the associated water. Degassing of the magma heat source appears to add chemical constituents to the geothermal waters, such as boron, carbon and sulphur. Sometimes there may also be addition of Cl and H 2 O during events of recharge of new magma into the magma chambers in the roots of the geothermal system such as has been observed in the Krafla area. The high-temperature geothermal waters are close to chemical equilibrium with alteration minerals for all major components, except Cl and B. The alteration minerals typically display depth zoning because many of them are stable only over a limited temperature range. At temperatures above about 250°C the alteration mineral assemblage is that of the greenschist metamorphic facies. Precipitation of carbon as calcite and sulphur as sulphides, where boiling occurs in upflow zones of high-temperature geothermal systems, leads to strong enrichment of carbon and sulphur in the altered rock.

New gas geothermometers for geothermal exploration—calibration and application

Calibration of five gas geothermometers is presented, three of which used CO2, H2S and H2 concentrations in fumarole steam, respectively. The remaining two use CO2H2 and H2SH2 ratios. The calibration is based on the relation between gas content of drillhole discharges and measured aquifer temperatures. After establishing the gas content in the aquifer, gas concentrations were calculated in steam formed by adiabatic boiling of this water to atmospheric pressure to obtain the gas geothermometry functions. It is shown that the concentrations of CO2, H2S and H2 in geothermal reservoir waters are fixed through equilibria with mineral buffers. At temperatures above 230°C epidote + prehnite + calcite + quartz are considered to buffer CO2. Two buffers are involved for H2S and H2 and two functions are, therefore, presented for the geothermometers involving these gases. For waters containing less than about 500 ppm chloride and in the range 230–300°C pyrite + pyrrholite + epidote + prehnite seem to be involved, but pyrite + epidote + prehnite + magnetite or chlorite for waters above 300°C and waters in the range 230–300°C, if containing over about 500 ppm. The gas geothermometers are useful for predicting subsurface temperatures in high-temperature geothermal systems. They are applicable to systems in basaltic to acidic rocks and in sediments with similar composition, but should be used with reservation for systems located in rocks which differ much in composition from the basaltic to acidic ones. The geothermometry results may be used to obtain information on steam condensation in upflow zones, or phase separation at elevated pressures. Measured aquifer temperatures in drillholes and gas geothermometry temperatures, based on data from nearby fumaroles, compare well in the five fields in Iceland considered specifically for the present study as well as in several fields in other countries for which data were inspected. The results of the gas geothermometers also compare well with the results of solute geothermometers and mixing models in three undrilled Icelandic fields.

Processes controlling the distribution of boron and chlorine in natural waters in Iceland

Abstract The concentrations of B and Cl in natural waters in Iceland lie in the range 0.001–10 and 1–20,000 ppm, respectively. Lowest concentrations occur in surface and groundwaters in the central highlands. Highest Cl concentrations are found in high-temperature waters on the Reykjanes Peninsula and highest B concentrations in well waters from high-temperature geothermal systems in the axial zones of the active volcanic belts. The B and Cl contents of Icelandic basalts are 0.1–6.6 and 75–750 ppm, respectively. These large variations are considered to result from variable degassing during consolidation as well as concentration variations in the initial magma. Tholeftes are lowest in both elements (av. 1.2 ppm B and 170 ppm CI), transitional basalts intermediate (av. 2.1 ppm B and 240 ppm CI), and alkali-basalts highest (av. 3.8 ppm B and 340 ppm Cl). Cl B ratios in tholeiites are most often within 25–50, but somewhat lower in alkali basalts. Most geothermal waters contain B and Cl within 0.05–1 and 10–100 ppm, respectively. These rather low values are attributed to the low content of B and Cl in the basaltic rock. The B and Cl distribution in the Icelandic natural waters indicate that these elements act as essentially incompatible. Their sources of supply include (1) the atmosphere (seawater spray and aerosols), (2) the rock with which the water interacts, (3) seawater-groundwater, and (4) magma intrusions. In surface water and nonthermal groundwater the dominant source of supply of Cl is seawater spray and aerosols, but the soil and rock contribute often significantly to the B in these waters. Their Cl B ratio is similar to that of seawater (1330) or somewhat lower. Boron and Cl concentrations generally increase with water temperature. Warm waters ( Cl B ratios intermediate between those of seawater and basalt. With increasing temperature this ratio decreases and gradually approaches that of the rock. Some high-temperature geothermal waters have as low Cl B ratios as 1 and B concentrations as high as 10 ppm. The cause is either B degassing of the magmatic intrusive heat source or phase separation in the producing aquifer. Some low-temperature waters (

Chemical equilibria in icelandic geothermal systems: implications for chemical geothermometry investigations

Chemical geothermometry represents the most important tool for estimating reservoir temperatures in the exploration of geothermal resources. Chemical equilibria between alteration minerals and solution are generally attained in geothermal systems for all major components except chloride. For the interpretation of analyses of natural waters involving geothermometry major emphasis should be placed on assessing the overall water composition with respect to mineral equilibria, rather than attempting to distinguish geothermal waters from shallow waters by a classification involving the relative abundance of major anions and major cations. Generally, cold waters may be distinguished from geothermal waters by low chloride (< 10 ppm), in conjunction with relatively low pH (6–7) and low Na/K ratios (same as the associated rock), calcite undersaturation and low √Ca2+ H+ activity ratios.

Dissolution of primary basaltic minerals in natural waters: saturation state and kinetics

Abstract The state of saturation of olivine, orthopyroxene and plagioclase of variable composition has been assessed in various types of natural waters in Iceland including river waters, groundwater and geothermal waters with temperatures up to 250°C. The stability of olivine and orthopyroxene decreases with increasing Mg content. Similarly, the stability of plagioclase decreases with increasing anorthite content. The river waters which are representative of the weathering environment are always undersaturated with both olivine and orthopyroxene, the degree of undersaturation being dominated by the water pH. All river waters tend to dissolve olivine of the composition encountered in basalts according to the linear rate law and orthopyroxene when the pH is 200°C) are undersaturated with olivine, orthopyroxene and plagioclase, the first two particularly when Fe rich, and sufficient to cause olivine to dissolve according to the linear rate law. At intermediate temperatures (50–150°C) the geothermal waters are close to equilibrium with these minerals except for Mg-rich olivine. It seems likely that dissolution of glass from basalt, which is very reactive, will favor stability of the igneous minerals.

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

Abstract 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 δ18O 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 underground temperatures using data from warm springs. This model, as well as the chloride-enthalpy model and the Na-Li, and CO2-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.