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

New data from geothermal wells in Iceland have permitted empirical calibration of the chalcedony and $\text{Na}\text{K}$ geothermometers in the range of 25–180°C and 25–250°C respectively. The temperature functions are:

$\text{t°C=}\text{1112}\text{4.91?}\text{log SiO}{}_2\text{?273.15}$

$\text{t°C=}\text{933}\text{0.993+}\text{log Na/K}\text{?273.15}$

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.     

Classical and integrated multicomponent geothermometry at the Tengchong geothermal field,Southwestern China

To reconstruct deep fluid chemical composition and increase the confidence in estimated reservoir temperatures, a more integral geothermometry method was compared to other classical geothermometers. Here, we apply the integrated multicomponent geothermometry (IMG) method using the GeoT code to estimate reservoir temperatures at the Tengchong geothermal field in Southwestern China. Results show reservoir temperatures calculated using the quartz geothermometer are closest to those estimated with the IMG method. The concentrations of Al and Mg, as well as selected minerals for geothermometry computations, are key factors for successfully using the IMG. Using the IMG method together with classical geothermometers can significantly increase confidence in reservoir temperature estimations. The methods presented and simulation program used here may be useful for analysis of other geothermal fields under similar conditions.     

Germanium in Icelandic geothermal systems

Germanium concentrations in geothermal waters in Iceland lie mostly in the range 2–30 ppb. There is an overall positive relation between the germanium content of the water and its temperature. Most of the germanium occurs as Ge(OH)^?₅in solution but Ge(OH)₄ may also be present in significant amounts in saline waters when above 200°C. Evidence indicates that aqueous germanium concentrations are controlled by exchange reactions where it substitutes for silica in silicates and iron in sulphides. It is the rate of dissolution and the relative abundance of the alteration minerals which take up germanium to a variable extent that ultimately fix Ge(OH)₄ concentrations in the water. This, together with water pH, fixes total dissolved germanium. It is mostly the primary rock composition that dictates the relative abundance of the alteration minerals. Conductive cooling in upflow zones favours removal of germanium from solution. During the initial stages of boiling of rising hot water dissolution is enhanced but precipitation at later stages.Thermodynamic data of various aqueous germanium species and several minerals are summarized and dissociation constants and solubilities estimated at elevated temperatures using available predictive methods.     

Chemical control of Al-solubility in orthopyroxene and its implications on pyroxene geothermometry

Two suites of spinel peridotites of the Westeifel/West Germany were found to have equilibrated in a narrow temperature interval each. Temperatures calculated from the CaO-solubility in orthopyroxene using experimental data of Lindsley and Dixon (1976) are 945° C to 980° C for the amphibole bearing Ia-suite and 1,150° C to 1,165° C for the Ib-suite. From the study of phase equilibria in both series it appears that Al-solubility in orthopyroxene is controlled by the composition of the coexisting spinel. Temperatures calculated from Al-solubility in orthopyroxene using an equation of Fujii (1976) derived from the univariant reaction en+spfo+Al-en in the MgO-Al₂O₃-SiO₂ system show a clear dependence on the FeCr₂O₄ component in the spinel phase, although ideal solution correction allowing for additional components was made. Temperatures obtained for the most chromite rich spinel — orthopyroxene pairs are 1,370° C and 1,240° C, respectively. These temperatures are by 220° C higher than those from Cr-poor pairs and those obtained from Ca solution in orthopyroxene.For practical purposes of geothermometry in spinel peridotites, an empirical equation allowing for the mole fraction of FeCr₂O₄ in spinel has been derived by a linear least squares fit to the orthopyroxene — spinel compositional data and temperatures calculated from the CaO-solubility of orthopyroxene in the present paper.     

Heat extraction from aquifer geothermal systems

This study aims to model temperature distributions in an aquifer thermal extraction (ATE) system that contains a single extraction well in a thin confined aquifer. The aquifer is bounded by hot dry rocks with different thermomechanical properties and thicknesses. Based on the heat convection–conduction equation, a mathematical model is developed to describe the spatial and temporal temperature distributions of the ATE systems. The mechanisms of heat transfer in the model involve horizontal convection and thermal conduction in the aquifer, and vertical thermal conduction in both rocks. A semi‐analytical solution in dimensionless form is developed using the Laplace transform technique and its corresponding time‐domain result is computed by the modified Crump method. In addition, the steady‐state solution is obtained by applying the final value theorem. The simulation results from the semi‐analytical solution indicate that the aquifer temperature distributions are affected by aquifer thickness, the thermomechanical properties of the aquifer and rocks, geothermal gradient, outer boundary temperatures of the rocks, extraction rate, and operating time. The present solution can be used as a preliminary tool for assessing heat extraction efficiency in ATE systems. Copyright © 2010 John Wiley & Sons, Ltd.     

Fluid inclusion geothermometry

Fluid inclusions trapped within crystals either during growth or at a later time provide many clues to the histories of rocks and ores. Estimates of fluid-inclusion homogenization temperature and density can be obtained using a petrographic microscope with thin sections, and they can be refined using heating and freezing stages. Fluid inclusion studies, used in conjunction with paragenetic studies, can provide direct data on the time and space variations of parameters such as temperature, pressure, density, and composition of fluids in geologic environments. Changes in these parameters directly affect the fugacity, composition, and pH of fluids, thus directly influencing localization of ore metals.

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Zusammenfassung Flüssigkeiten in Kristallen können entweder während oder nach der Kristallisation eingeschlossen worden sein und geben viele Hinweise auf die Entstehung von Gesteinen und Erzen. An einem Dünnschliff kann unter dem Mikroskop die Homogenisierungstemperatur und die Dichte der Flüssigkeit geschätzt werden; zu deren genauer Bestimmung wird ein heiz- bzw. kühlbarer Mikroskoptisch benötigt. Daten aus der Untersuchung von Flüssigkeitseinschlüssen zusammen mit paragenetischen Untersuchungen können direkte Informationen über die zeitliche Variation der Temperatur, des Druckes, der Dichte und der chemischen Zusammensetzung von Lösungen bei geologischen Vorgängen geben. Änderungen dieser Parameter beeinflussen die Fugazität, den pH-Wert und die Zusammensetzung von Lösungen; hierdurch wiederum wird direkt die Bildung von Erzen gesteuert.

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Résumé Des inclusions liquides dans les cristaux ont pu se former soit pendant, soit après la période de cristallisation et fournissent beaucoup d'information sur la formation des roches et des minerais. Une plaque mince placée sous un microscope permet une évaluation de la température d'homogénéisation et de la densité du liquide. Pour déterminer celles-ci exactement il faut adapter au microscope une platine chauffante et réfrigérente. Les données provenant des études sur les inclusions liquides, associées aux recherches paragénétiques, peuvent fournir des informations directes sur la variation dans le temps de la température, de la pression, de la densité et de la composition chimique de solutions pendant les processus géologiques. Les changements de ces paramètres influent sur la pression partielle d'oxygène, le pH et la composition de solutions; de ce fait la formation de minerais est directement réglée.

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, ; . , ; . , , . , pH ; .

     

Enstatite-diopside solvus and geothermometry

The enstatite-diopside solvus presents certain interesting thermodynamic and crystal-structural problems. The solvus may be considered as parts of two solvi one with the ortho-structure and the other with clino-structure. By assuming the standard free energy change for the two reactions (MgMgSi₂O₆)opx ? (MgMgSi₂O₆)cpx and (CaMgSi₂O₆) opx ? (CaMgSi₂O₆) cpx as 500 and 1 000 to 3 000 cal/mol respectively, it is possible to calculate the regular solution parameter W for orthopyroxene and clinopyroxene. These W's essentially refer to mixing on M2 sites. The expression for the equilibrium constant by assuming ideal mixing for Fe-Mg, Fe-Ca and non-ideal mixing for Ca-Mg on binary M1 and ternary M2 sites is given by 1 $$K_a = \frac{{X_{{\text{Mg - cpx}}}^{{\text{M1}}} X_{{\text{Mg - cpx}}}^{{\text{M2}}} \exp \left[ {\frac{{W_{{\text{cpx}}} }}{{RT}}\left\{ {X_{{\text{Ca - cpx}}}^{{\text{M2}}} \left( {X_{{\text{Ca - cpx}}}^{{\text{M2}}} + X_{{\text{Fe - cpx}}}^{{\text{M2}}} } \right)} \right\}} \right]}}{{X_{{\text{Mg - cpx}}}^{{\text{M1}}} X_{{\text{Mg - opx}}}^{{\text{M2}}} \exp \left[ {\frac{{W_{{\text{cpx}}} }}{{RT}}\left\{ {X_{{\text{Ca - opx}}}^{{\text{M2}}} \left( {X_{{\text{Ca - opx}}}^{{\text{M2}}} + X_{{\text{Fe - opx}}}^{{\text{M2}}} } \right)} \right\}} \right]}}$$ where X's are site occupancies, R is 1.987 and T is temperature in ^oK. Temperature of pyroxene crystallization may be estimated by substituting for T in the above equation until the equation ?RT In K _a=500 is satisfied. The shortcomings of this method are the incomplete standard free energy data on the end member components and the absence of site occupancy data in pyroxenes at high temperatures. The assumed free energy data do, however, show the possible extent of inaccuracy in temperature estimates resulting from the neglect of Mg-Ca non ideality.     

Single-well circulation systems for geothermal energy transfer

As an alternative method of implementing environmentally friendly geothermal systems for space heating and/or cooling, groundwater has been increasingly used in open-loop ground-coupled heat-pump systems. Of various groundwater heat-pump configurations, the single-well circulation systems tend to provide the best geothermal exchange efficiency where the hydrogeological and thermogeological conditions allow for their installation. In comparison with the closed-loop systems consisting of geothermal boreholes, the single-well circulation systems may substantially reduce the number of boreholes needed, especially for large-scale geothermal applications. In small-scale applications of the single-well circulation system, the groundwater circulation supplies enough thermal energy underground that the temperature of the circulating water remains stable through the years. For district-scale applications, however, further studies are needed to fully understand their sustainability and the impacts on groundwater quality from operating ground-coupled heat-pump systems.     

Aquifer chemistry of four high-temperature geothermal systems in Iceland

The deep water feeding wet-steam wells in four high-temperature geothermal areas in Iceland have highly variable salinity as reflected in the chlorine concentrations which vary from 20 to 19000 ppm. Using available values for equilibrium constants, the activities of 26 chemical species involving the major components of the reservoir water have been calculated and quantitative evaluations of solute/ solute, mineral/solute chemical equilibria in these geothermal systems have been made.The unflashed reservoir water is just saturated with calcite. The saline geothermal waters, which represent heated sea-water, are just saturated with anhydrite, but the dilute waters, which are of meteoric origin, are somewhat undersaturated with this mineral. The fluoride mobility is thought to be limited by an ionic exchange reaction where F^? replaces some of the OH^? in the layered silicates. The pH of the unflashed reservoir water is governed by ionic exchange equilibrium in which all the major cations participitate. At a given temperature it seems likely that the activity of one cation fixes the activities of all the other major cations and hydrogen ion. If this is so and we take all the other chemical equilibria which have been demonstrated to exist for granted, it turns out that the major element composition of the unflashed high-temperature geothermal waters is controlled by two independent variables only. These variables are the temperature and the supply to the water of the incompatible element chlorine, incompatible indicating that this element is not incorporated in the geothermal minerals.     

阜阳地区地热水化学特征及同位素分析

为研究阜阳地区地热形成机制的控制作用,在分析研究区地热地质条件的基础上,利用已有的勘探井水质资料,对研究区地热水的化学特征进行了分析,得出地热流体在垂向、横向上的分布特征,揭示了太古宇五河群风化壳可以构成热储,其较低的总溶解固体(total dissoloved solids,TDS)不同于区域常见的新近系馆陶组、古近系界首组热储。同时,利用大气降水和地热水的稳定同位素资料,研究得出区内地热水的来源主要为大气降水,大气降水经侧向径流补给和蒸发/浓缩作用形成了高TDS地热流体; 利用地热水放射性同位素资料估算了研究区地热水年龄。通过地热水化学、同位素及控热构造等研究,建立了研究区地热资源形成的概念模型,为该地区地热开发利用提供了指导。     

Silicate solid solutions and geothermometry

The properties of the regular solution are described. A solid solution is assigned certain theoretical values of the heat of mixing and the results of the distribution of a component between two coexisting solutions are shown graphically on Roozeboom diagrams. Such representations may be useful in explaining the distribution of elements observed in natural mineral assemblages.     

An evaluation of garnet-clinopyroxene geothermometry in granulites

Four new formulations of the garnet-clinopyroxene geothermometer (Ellis and Green 1979; Ganguly 1979; Saxena 1979; Dahl 1980) have been evaluated in the Adirondacks and five other granulite terranes using results from 94 mineral pairs. The Saxena and Ganguly formulations give temperatures that are generally 100–150° C above those constrained by phase equilibria and other independent thermometry while the empirical calibration of Dahl gives widely scattered, erratic results. Despite some scatter in the data, the Ellis and Green calibration appears to be more accurate and precise than the others and is the most useful garnet-clinopyroxene thermometer currently available for quantitative thermometry in granulites. All four formulations are sensitive to large variations (>70–80° C) in temperature suggesting that problems with accuracy and precision can be improved with further refinement of model-based aspects of the thermometers.     

我国主要水热型地热系统形成机制与成因模式

我国地处环太平洋地热带与地中海-喜马拉雅地热带的交汇部位,且中、新生代沉积盆地发育,地热资源丰富。近年来,我国地热开发利用蓬勃发展,但由于没有进行整体性的地热系统分析,缺乏因地制宜的不同地区地热勘查开发方案,各地地热资源勘查开发工作存在一定的混乱现象。本文从系统论观点出发,提出了我国地热资源"同源共生-壳幔生热-构造聚热"的成因理论,并以我国主要水热型地热系统为例,建立了水热系统分析的系统论观点,将我国主要水热系统划分为沉积盆地古潜山型复合水热系统、沉积盆地深坳陷层控型水热系统、断陷盆地地压型水热系统、陆陆碰撞板缘型水热系统、板缘俯冲带热控构造型水热系统、隆起山地深循环型水热系统以及近代火山型水热系统等七种类型,并系统分析了典型水热系统的运移条件、热源机制,归纳阐述了不同类型的水热系统的成因模式,进一步完善了我国地热资源研究的基础理论和方法,为区域地热资源勘查开发提供了理论依据,对于进一步推进我国地热资源的科学开发利用具有一定的指导意义。     

Hydrogeochemistry of high-temperature geothermal systems in China: A review

As an important part of the Mediterranean-Himalayas geothermal belt, southern Tibet and western Yunnan are the regions of China where high-temperature hydrothermal systems are intensively distributed, of which Rehai, Yangbajing and Yangyi have been investigated systematically during the past several decades. Although much work has been undertaken at Rehai, Yangbajing and Yangyi to study the regional geology, hydrogeology, geothermal geology and geophysics, the emphasis of this review is on hydrogeochemical studies carried out in these geothermal fields. Understanding the geochemistry of geothermal fluids and their environmental impact is critical for sustainable exploitation of high-temperature hydrothermal resources in China. For comparison, the hydrogeochemistry of several similar high-temperature hydrothermal systems in other parts of the world are also included in this review.     

Two-feldspar geothermometry in granulite facies metamorphic rocks

A geothermometric technique based on equilibria between coexisting plagioclase and alkali feldspar was applied to quartzo-feldspathic granulites from Salvador, BA, Brazil. The conditions of metamorphism were determined to be in the range 750 ° C–800 ° C, 4–8 Kb, by comparison with experimental data on the stabilities of sapphirine, phlogopite and other minerals occurring in the associated rocks. Selected feldspar data gives temperatures near, but slightly below, this range. Several variants of the Wood and Banno model, as well as an empirical two-pyroxene geothermometer, were also tested and found to give temperatures which were apparently 50 °–100 ° high. The solubility of Al₂O₃ in orthopyroxene indicates temperatures which are about 200 ° to high, suggesting that Fe in the natural assemblages significantly changes relationships observed experimentally in MgO-Al₂O₃-SiO₂ systems.     

Spinel-olivine geothermometry in peridotites from ultramafic complexes

An empirical calibration of the spinel-olivine geothermometer (Evans and Frost, 1975) is attempted and applied to parageneses of lherzolitic rocks. In ultramafic complexes, most of the derived temperature estimates range between 700 ° and 850 °C, and appear generally lower than those given by other geothermometers, based upon Ca or Al contents of coexisting pyroxenes. A comparison of the different thermometric data in two well studied occurrences, the Ronda ultramafic complex and the San Carlos xenolith field, suggests that differential rates of diffusion and recrystalization may account for the large range of temperatures (between 700 ° and 1200 °C) determined by various mineral geothermometers in peridotites. The equilibrium exchange of Mg and Fe between spinel and olivine would be the fastest one and would continue effectively during the cooling down to relatively low temperatures, while other exchange reactions used as geothermometers in peridotites, like partitioning of Ca and Al in coexisting pyroxenes, are blocked at higher temperatures.     

Simulated garnet-clinopyroxene geothermometry of eclogites

The Fe-Mg thermometer widely used to infer the “equilibration” temperature of garnet-clinopyroxene assemblages of eclogites records the progressive blocking of diffusion-limited exchanges between coexisting mineral phases. It is argued that equilibrium is achieved through the fast grain boundary model in which Fe and Mg circulate in an interstitial medium fast enough for the rate-limiting step to be the volume diffusion in each mineral phase. A semi-analytic solution is found and the influence of the cooling history, grain size distribution, and rock composition on the temperature at which the Fe-Mg exchange between garnet and clinopyroxene is frozen in are quantitatively evaluated. In particular, the model simulates the temperatures that would be obtained from the concentration of Fe and Mg in the rim of adjacent garnet and clinopyroxene crystals such as those commonly obtained by electron probe. For eclogites, the simulations show that correct peak temperatures are retrieved as long as the temperature of the metamorphic climax does not exceed 650 °C. At higher climax temperatures, rim-rim temperatures underestimate the peak temperature and cluster around 650 °C. Fast cooling, however, strongly limits diffusive equilibration. In crustal-type eclogites, cooling rates of a few degrees per million years at 700 °C and of a few tens of degrees at 800 °C preserve the record of the peak temperature. It is shown that these results are largely independent of the chemical composition of the garnet and clinopyroxene. On the contrary, the mineral grain size and the respective proportions of garnet and clinopyroxene have a major effect on retrogressive diffusion. For eclogites formed at a temperature of 800 °C and under conditions of slow cooling, the record of the climax temperature is preserved by crystal rims as long as the volume of clinopyroxene is smaller than that of garnet and crystal size is at least millimetric. The choice of a particular set of experimental data for the diffusion coefficients is found to be immaterial. We suggest that the temperature of the metamorphic climax of eclogites is best estimated from the composition of clinopyroxene rims combined with that of the inner edge of the diffusion boundary layer fringing the coexisting garnet grains. Received: 15 January 1998 / Accepted: 6 November 1998     

Silicate solid solutions and geothermometry

Principal component analysis, using eigenvalues and eigenvectors, of the encountered variation in the chemistry of 153 garnets indicates the following: (1) In low grade metamorphic rocks, spessertite and almandine are distinctly isomorphous. The variability of pyrope content does not influence the binary Fe-Mn relationship. However there may be some influence of the grossularite content. (2) In high grade metamorphic rocks, pyrope and almandine are distinctly isomorphous. Variability of grossularite content does not affect the binary Mg-Fe relationship. However, variability in spessertite content may influence the linearity particularly when there is little pyrope.Similar statistical analysis of the chemical data on 119 samples of clinopyroxenes indicates that only significant changes in the concentrations of Al in octahedral and/or of Al in tetrahedral sites and Ti could cause some change in the FeMg ratio in the mineral.Principal component analysis of the data on coexisting garnet and clinopyroxene could be used to classify rocks into their petrogenetic types.Distribution coefficient calculated by assuming ideal binary solution of Mg and Fe members in pyroxene and garnet is useful to indicate the P-T of the formation of the rocks.     

Solute and gas geothermometry of geothermal wells: a geochemometrics study for evaluating the effectiveness of geothermometers to predict deep reservoir temperatures

Deep reservoir temperatures of 10 important geothermal systems of the world were estimated by applying 13 solute (Na/K) and 21 gas geothermometers. The predicted temperatures were comprehensively evaluated and compared with measured bottom-hole temperatures using geochemometric techniques. The present study reveals (1) high prediction performances in most of the Na/K geothermometers for the majority of the geothermal fields with liquid-dominated reservoirs, whereas low prediction performances were indicated for the geothermal fields with vapour-dominated and high-temperature reservoirs; (2) the gas geothermometers, in comparison to Na/K, are more successful in predicting the subsurface temperatures in high-temperature geothermal systems; (3) the geothermal systems for which Na/K geothermometers have indicated a high prediction performance, the gas geothermometers have specified a low prediction performances, and vice versa; (4) both Na/K and gas geothermometers, generally, overestimated the reservoir temperatures for the majority of the low-enthalpy geothermal fields and underestimated for the majority of the high-enthalpy geothermal fields; (5) the reservoir temperature predictions of gas geothermometers have more scatter than those temperatures inferred from Na/K geothermometers; and (6) in general, Na/K geothermometers seem to be a more successful geochemical tool in predicting reliable reservoir temperatures than gas geothermometers.