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1.
The Mt. Amiata volcano in central Italy is intimately related to the post-orogenic magmatic activity which started in Pliocene times. Major, trace elements, and isotopic composition of thermal and cold spring waters and gas manifestations indicate the occurrence of three main reservoir of the thermal and cold waters in the Mt. Amiata region. The deepest one is located in an extensive carbonate reservoir buried by thick sequences of low-permeability allochthonous and neo-autochthonous formations. Thermal spring waters discharging from this aquifer have a neutral Ca-SO4 composition due to the presence of anhydrite layers at the base of the carbonate series and, possibly, to absorption of deep-derived H2S with subsequent oxidation to SO42− in a system where pH is buffered by the calcite–anhydrite pair (Marini and Chiodini, 1994). Isotopic signature of these springs and N2-rich composition of associated gas phases suggest a clear local meteoric origin of the feeding waters, and atmospheric O2 may be responsible for the oxidation of H2S. The two shallower aquifers have different chemical features. One is Ca-HCO3 in composition and located in several sedimentary formations above the Mesozoic carbonates. The other one has a Na-Cl composition and is hosted in marine sediments filling many post-orogenic NW–SE-trending basins. Strontium, Ba, F, and Br contents have been used to group waters associated with each aquifer. Although circulating to some extent in the same carbonate reservoir, the deep geothermal fluids at Latera and Mt. Amiata and thermal springs discharging from their outcropping areas have different composition: Na-Cl and Ca-SO4 type, respectively. Considering the high permeability of the reservoir rock, the meteoric origin of thermal springs and the two different composition of the thermal waters, self-sealed barriers must be present at the boundaries of the geothermal systems. The complex hydrology of the reservoir rocks greatly affects the reliability of geothermometers in liquid phase, which understimate the real temperatures of the discovered geothermal fields. More reliable temperatures are envisaged by using gas composition-based geothermometers. Bulk composition of the 67 gas samples studied seems to be the result of a continuous mixing between a N2-rich component of meteoric origin related to the Ca-SO4 aquifer and a deep CO2-rich component rising largely along the boundaries of the geothermal systems. Nitrogen-rich gas samples have nearly atmospheric N2/Ar (=83) and

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(δ=0‰) ratios whereas CO2-rich samples show anomalously high

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values (up to +6.13 ‰), likely related to N2 from metamorphic schists lying below the carbonate formations. On the basis of average

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isotopic ratio (

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around 0‰), CO2 seems to originate mainly from thermometamorphic reactions in the carbonate reservoir and/or in carbonate layers embedded in the underlying metamorphic basement. Distribution of

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isotopic ratios indicates a radiogenic origin of helium in a tectonic environment that, in spite of the presence of many post-orogenic basins and mantle-derived magmatics, can presently be considered in a compressive phase.  相似文献   

2.
Thermal and cold waters from Castellammare–Alcamo (Western Sicily-Italy) were collected between May 1994 and May 1995 and studied for their chemical and isotopic composition. During the same period, mean monthly samples of meteoric water were also collected and measured for their isotopic composition. The main purpose of this study was the characterization of the acquifers and, if possible, of their recharge areas. According to the results obtained, the acquifers were divided into three main groups: (a) selenitic waters, (b) cold carbonatic waters, and (c) deep thermal waters resulting from the mixing of the other two types. Besides a mixing process between carbonatic and selenitic waters, contamination processes of thermal waters by seawater take place during their ascent. The water temperature of the acquifer feeding the thermal springs was estimated by means of various geothermometers to range between 60°C and 97°C. Isotope data on rainwater samples show a wide seasonal variation of both δ and δD values. The fairly constant values of thermal waters through time and the lack of an apparent correlation with the isotopic values of rainwater suggest the existence of a deep circuit determining an almost complete homogenisation of the seasonal variations of the isotopic values.  相似文献   

3.
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.  相似文献   

4.
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5.
Thermal waters hosted by Menderes metamorphic rocks emerge along fault lineaments in the Simav geothermal area. Thermal springs and drilled wells are located in the Eynal, Çitgöl and Na a locations, which are part of the Simav geothermal field. Studies were carried out to obtain the main chemical and physical characteristics of thermal waters. These waters are used for heating of residences and greenhouses and for balneological purposes. Bottom temperatures of the drilled wells reach 163°C with total dissolved solids around 2225 mg/kg. Surface temperatures of thermal springs vary between 51°C and 90°C. All the thermal waters belong to Na–HCO3–SO4 facies. The cold groundwaters are Ca–Mg–HCO3 type. Dissolution of host rock and ion-exchange reactions in the reservoir of the geothermal system shift the Ca–Mg–HCO3 type cold groundwaters to the Na–HCO3–SO4 type thermal waters. Thermal waters are oversaturated at discharge temperatures for aragonite, calcite, quartz, chalcedony, magnesite and dolomite minerals giving rise to a carbonate-rich scale. Gypsum and anhydrite minerals are undersaturated with all of the thermal waters. Boiling during ascent of the thermal fluids produces steam and liquid waters resulting in an increase of the concentrations of the constituents in discharge waters. Steam fraction, y, of the thermal waters of which temperatures are above 100°C is between 0.075 and 0.119. Reservoir pH is much lower than pH measured in the liquid phase separated at atmospheric conditions, since the latter experienced heavy loss of acid gases, mainly CO2. Assessment of the various empirical chemical geothermometers and geochemical modelling suggest that reservoir temperatures vary between 175°C and 200°C.  相似文献   

6.
Studies of the geology, geochemistry of thermal waters, and of one exploratory geothermal well show that two related hot spring systems discharge in Canõn de San Diego at Soda Dam (48°C) and Jemez Springs (72°C). The hot springs discharge from separate strands of the Jemez fault zone which trends northeastward towards the center of Valles Caldera. Exploration drilling to Precambrian basement beneath Jemez Springs encountered a hot aquifer (68°C) at the top of Paleozoic limestone of appropriate temperature and composition to be the local source of the fluids in the surface hot springs at Jemez Springs. Comparisons of the soluble elements Na, Li, Cl, and B, arguments based on isotopic evidence, and chemical geothermometry indicate that the hot spring fluids are derivatives of the deep geothermal fluid within Valles Caldera. No hot aquifer was discovered in or on top of Precambrian basement. It appears that low- to moderate-temperature geothermal reservoirs (< 100°C) of small volume are localized along the Jemez fault zone between Jemez Springs and the margin of Valles Caldera.  相似文献   

7.
Results of a chemical study of the fluids from drill holes and hot springs of Puga and Chumatang areas in the northwestern part of the Himalaya are presented and discussed in this paper. The thermal waters of Puga and Chumatang are of Na-HCO3-Cl and Na-HCO3 types, respectively. A comparison between these waters, their chemical classification and activity studies suggest a flow path within a quartzitic-schistose basement, containing quartz, K-feldspar and illite, and in clayey terrains containing montmorillonite and illite.The chemistry of thermal waters also indicate their association with magmatic activity. The chemical geothermometers indicate the possible existence of a geothermal reservoir at Puga with temperature ≈250°C. The Chumatang area has a comparatively cooler reservoir with a temperature of 150–180°C.  相似文献   

8.
 The purpose of this work was to study jointly the volcanic-hydrothermal system of the high-risk volcano La Soufrière, in the southern part of Basse-Terre, and the geothermal area of Bouillante, on its western coast, to derive an all-embracing and coherent conceptual geochemical model that provides the necessary basis for adequate volcanic surveillance and further geothermal exploration. The active andesitic dome of La Soufrière has erupted eight times since 1660, most recently in 1976–1977. All these historic eruptions have been phreatic. High-salinity, Na–Cl geothermal liquids circulate in the Bouillante geothermal reservoir, at temperatures close to 250  °C. These Na–Cl solutions rise toward the surface, undergo boiling and mixing with groundwater and/or seawater, and feed most Na–Cl thermal springs in the central Bouillante area. The Na–Cl thermal springs are surrounded by Na–HCO3 thermal springs and by the Na–Cl thermal spring of Anse à la Barque (a groundwater slightly mixed with seawater), which are all heated through conductive transfer. The two main fumarolic fields of La Soufrière area discharge vapors formed through boiling of hydrothermal aqueous solutions at temperatures of 190–215  °C below the "Ty" fault area and close to 260  °C below the dome summit. The boiling liquid producing the vapors of the Ty fault area has δD and δ18O values relatively similar to those of the Na–Cl liquids of the Bouillante geothermal reservoir, whereas the liquid originating the vapors of the summit fumaroles is strongly enriched in 18O, due to input of magmatic fluids from below. This process is also responsible for the paucity of CH4 in the fumaroles. The thermal features around La Soufrière dome include: (a) Ca–SO4 springs, produced through absorption of hydrothermal vapors in shallow groundwaters; (b) conductively heated, Ca–Na–HCO3 springs; and (c) two Ca–Na–Cl springs produced through mixing of shallow Ca–SO4 waters and deep Na–Cl hydrothermal liquids. The geographical distribution of the different thermal features of La Soufrière area indicates the presence of: (a) a central zone dominated by the ascent of steam, which either discharges at the surface in the fumarolic fields or is absorbed in shallow groundwaters; and (b) an outer zone, where the shallow groundwaters are heated through conduction or addition of Na–Cl liquids coming from hydrothermal aquifer(s). Received: 9 November 1998 / Accepted: 15 July 1999  相似文献   

9.
The Sierra La Primavera, a late Pleistocene rhyolitic caldera complex in Jalisco, México, contains fumaroles and large-discharge 65°C hot springs that are associated with faults related to caldera collapse and to later magma insurgence. The nearly-neutral, sodium bicarbonate, hot springs occur at low elevations at the margins of the complex, whereas the water-rich fumaroles are high and central.The Comisión Federal de Electricidad de México (CFE) has recently drilled two deep holes at the center of the Sierra (PR-1 and Pr-2) and one deep hole at the western margin. Temperatures as high as 285°C were encountered at 1160 m in PR-1, which produced fluids with 820 to 865 mg/kg chloride after flashing to one atmosphere. Nearby, PR-2 encountered temperatures to 307°C at 2000 m and yielded fluids with chloride contents fluctuating between 1100 and 1560 mg/kg after flashing. Neither of the high-temperature wells produced steam in commercial quantities. The well at the western margin of the Sierra produced fluids similar to those from the hot springs. The temperature reached a maximum of 100°C near the surface and decreased to 80°C at 2000 m.Various geothermometers (quartz conductive, Na/K, Na-K-Ca, δ18O(SO4-H2O) and D/H (steam-water) all yield temperatures of 170 ± 20°C when applied to the hot spring waters, suggesting that these spring waters flow from a large shallow reservoir at this temperature. Because the hot springs are much less saline than the fluids recovered in PR-1 and PR-2, the mixed fluid in the shallow reservoir can contain no more than 10–20% deep fluid. This requires that most of the heat is transferred by steam. There is probably a thin vapor-dominated zone in the central part of the Sierra, through which steam and gases are transferred to the overlying shallow reservoir. Fluids from this reservoir cool from 170°C to 65°C by conduction during the 5–7 km of lateral flow to the hot springs.  相似文献   

10.
This paper examines groundwater hydrochemical characteristics during mixing between thermal and non-thermal groundwater in low-to-medium temperature geothermal fields. A case study is made of Daying and Qicun geothermal fields in the Xinzhou basin of Shanxi province, China. The two geothermal fields have similar flow patterns, with recharge sourced from precipitation in mountain areas heated through a deep cycle, before flowing into overlying Quaternary porous aquifers via fractures. Hydrochemical features of 60 ground- and surface water samples were examined in the context of hydrogeologic information. The average temperatures of the deep geothermal reservoirs are estimated to be 125 °C in Daying field, and 159 °C in Qicun field, based on Na–K–Mg geothermometry, while slightly lower estimates are obtained using silica geothermometers. Hydrochemical features of thermal water are distinct from cold water. Thermal groundwater is mainly Cl·SO4–Na type, with high TDS, while non-thermal groundwater is mostly HCO3–Ca·Mg and HCO3–Ca type in the Daying and Qicun regions, respectively. Hydrogeochemical processes are characterized by analyzing ion ratios in various waters. Higher contents of some minor elements in thermal waters, such as F, Si, B and Sr, are probably derived from extended water–rock interaction, and these elements can be regarded as indicators of flow paths and residence times. Mixing ratios between cold and thermal waters were estimated with Cl, Na, and B concentrations, using a mass balance approach. Mixing between ascending thermal waters and overlying cold waters is extensive. The proportion of water in the Quaternary aquifer derived from a deep thermal source is lower in Daying geothermal field than in Qicun field (5.3–7.3% vs. 6.3–49.3%). Mixing between thermal and non-thermal groundwater has been accelerated by groundwater exploitation practices and is enhanced near faults. Shallow groundwater composition has also been affected by irrigation with low-temperature thermal water.  相似文献   

11.
The Platanares geothermal area, Departamento de Copán, Honduras, is located within a graben that is complexly faulted. The graben is bounded on the north by a highland composed of Paleozoic (?) metamorphic rocks in contact with Cretaceous - Tertiary redbeds of unknown thickness. These are unconformably overlain by Tertiary andesitic lavas, rhyolitic ignimbrites, and associated sedimentary rocks. The volcanic rocks are mostly older than 14 Ma, and thus are too old to represent the surface expression of an active crustal magma body. Thermal fluids that discharge in the area are heated during deep circulation of meteoric water along faults in a region of somewhat elevated heat flow. Geothermometry based upon the chemical composition of thermal fluids from hot springs and from geothermal gradient coreholes suggests that the reservoir equilibrated at temperatures as high as 225 to 240°C, within the Cretaceous redbed sequence. Three continuously cored geothermal gradient holes have been drilled; fluids of about 165°C have been produced from two drilled along a NW-trending fault zone, from depths of 250 to 680 m. A conductive thermal gradient of 139°C/km, at a depth of 400 m, was determined from the third well, drilled 0.6 km west of that fault zone. These data indicate that the Platanares geothermal area holds considerable promise for electrical generation by moderate- to hightemperature geothermal fluids.  相似文献   

12.
Ground water can facilitate earthquake development and respond physically and chemically to tectonism. Thus, an understanding of ground water circulation in seismically active regions is important for earthquake prediction. To investigate the roles of ground water in the development and prediction of earthquakes, geological and hydrogeological monitoring was conducted in a seismogenic area in the Yanhuai Basin, China. This study used isotopic and hydrogeochemical methods to characterize ground water samples from six hot springs and two cold springs. The hydrochemical data and associated geological and geophysical data were used to identify possible relations between ground water circulation and seismically active structural features. The data for delta18O, deltaD, tritium, and 14C indicate ground water from hot springs is of meteoric origin with subsurface residence times of 50 to 30,320 years. The reservoir temperature and circulation depths of the hot ground water are 57 degrees C to 160 degrees C and 1600 to 5000 m, respectively, as estimated by quartz and chalcedony geothermometers and the geothermal gradient. Various possible origins of noble gases dissolved in the ground water also were evaluated, indicating mantle and deep crust sources consistent with tectonically active segments. A hard intercalated stratum, where small to moderate earthquakes frequently originate, is present between a deep (10 to 20 km), high-electrical conductivity layer and the zone of active ground water circulation. The ground water anomalies are closely related to the structural peculiarity of each monitoring point. These results could have implications for ground water and seismic studies in other seismogenic areas.  相似文献   

13.
Thermal waters of the Ömer–Gecek geothermal field, Turkey, with temperatures ranging from 32 to 92°C vary in chemical composition and TDS contents. They are generally enriched in Na–Cl–HCO3 and suggest deep water circulation. Silica and cation geothermometers applied to the Ömer–Gecek thermal waters yield reservoir temperatures of 75–155°C. The enthalpy–chloride mixing model, which approximates a reservoir temperature of 125°C for the Ömer–Gecek field, accounts for the diversity in the chemical composition and temperature of the waters by a combination of processes including boiling and conductive cooling of deep thermal water and mixing of the deep thermal water with cold water. It is also determined that the solubility of silica in most of the waters is controlled by the chalcedony phase. Equilibrium states of the Ömer–Gecek thermal waters studied by means of the Na–K–Mg triangular diagram, Na–K–Mg–Ca diagram, K–Mg–Ca geoindicator diagram, activity diagrams in the systems composed of Na2O–CaO–K2O–Al2O3–SiO2–CO2–H2O phases, log SI diagrams, and finally the alteration mineralogy indicate that most of the spring and low-temperature well waters in the area can be classified as shallow or mixed waters which are likely to be equilibrated with calcite, chalcedony and kaolinite at predicted temperature ranges similar to those calculated from the chemical geothermometers. It was also observed that mineral equilibrium in the Ömer–Gecek waters is largely controlled by CO2 concentrations.  相似文献   

14.
Discharge areas of hydrothermal springs are known to be inhabited by diverse types of microorganisms including archaea, prokaryotes and eukaryotes. A total of 11 hydrothermal samples from the Rio Grande rift and the Valles caldera in New Mexico were analyzed to investigate the correlation between chemical and microbiological parameters of hydrothermal waters. The sampled fluids are categorized into three chemical groups: (I) steam-condensing acid sulfate waters, (II) deep geothermal and derivative waters and (III) thermal meteoric waters. Analyses of the microbial phospholipid fatty acids and denaturing gradient gel electrophoresis of DNA show that acid sulfate waters were populated by thermoacidophilic organisms and had high biomass content. Mineralized deep geothermal and derivative waters exhibited a high degree of microbial diversity, but had low biomass content. Thermal meteoric waters are low in total dissolved solids, and exhibit very low biomass content and microbial diversity. DNA sequences from several previously unknown microbial species were detected. The results of this study support the hypothesis that microbes can be used as tracers for specific types of subsurface environments.  相似文献   

15.
The South Poroto–Rungwe geothermal field, in the northern part of the Malawi rift, Tanzania divides in two main areas. The relatively high altitude northern area around the main Ngozi, Rungwe, Tukuyu and Kyejo volcanoes, is characterised by cold and gas-rich springs. In contrast, hot springs occur in the southern and low-altitude area between the Kyela and Livingstone faults. The isotopic signature of the almost stagnant, cold springs of the Northern district is clearly influenced by H2O–CO2(g) exchange as evidenced from negative oxygen-shifts in the order of few deltas permil. In contrast, the isotopic signature of waters discharged from the hot springs of the Southern district is markedly less affected by the H2O–CO2(g) interaction. This evidence is interpreted as an effect of the large, permanent outflow of these springs, which supports the hypothesis of a regional-scale recharge of the major thermal springs. Measurements of carbon isotope variations of the dissolved inorganic carbon of waters and CO2(g) from the Northern and Southern springs support a model of CO2(g)-driven reactivity all over the investigated area. Our combined chemical and isotopic results show that the composition of hot springs is consistent with a mixing between (i) cold surface fresh (SFW) and (ii) Deep Hot Mineralised (DHMW) Water, indicating that the deep-originated fluids also supply most of the aqueous species dissolved in the surface waters used as local potable water. Based on geothermometric approaches, the temperature of the deep hydrothermal system has been estimated to be higher than 110 °C up to 185 °C, in agreement with the geological and thermal setting of the Malawi rift basin. Geochemical data point to (i) a major upflow zone of geothermal fluids mixed with shallow meteoric waters in the Southern part of the province, and (ii) gas absorption phenomena in the small, perched aquifers of the Northern volcanic highlands.  相似文献   

16.
The chemical compositions of a total of 120 thermal water samples from four different tectonically distinct regions (Central, North, East and West Anatolia) of Turkey are presented and assessed in terms of geothermal energy potential of each region through the use of chemical geothermometers. Na–Ca–HCO3 type waters are the dominant water types in all the regions except that Na–Cl type waters are typical for the coastal areas of West Anatolia and for a few inland areas of West and Central Anatolia where deep water circulation exists. The discharge temperature of the springs ranges up to 100°C, and the bottom-hole temperatures in drilled wells up to 232°C. Geothermometry applications yield reservoir temperatures of about 125°C for Central Anatolia, 110°C for North Anatolia, 136°C for East Anatolia and 251°C for West Anatolia, the latter agreeing with some of the bottom hole temperatures measured in drilled wells. The results reveal that the highest geothermal energy potential in Turkey is associated with the West Anatolian extensional tectonics which provides a regional, deep-seated heat source and a widespread graben system allowing deep circulation of waters. The North Anatolian region, bounded to the south by the dextral North Anatolian Fault along which most of the geothermal sites are located, has the lowest energy potential, probably due to the restriction of the heat source to local magmatic activities confined to pull-apart basins. The East Anatolian region (undergoing contemporary compression) and the Central Anatolian region (where the compressional regime in the east is converted to the extensional regime in the west) have moderate energy potential. Although the recently active volcanoes suggest the presence, at depth, of still cooling magma chambers that are potential heat sources, the lack of well-developed fault systems is probably responsible for the comparatively low energy potential of these regions. Almost all the thermal waters of Turkey are saturated with respect to calcite and, hence, have a significant calcite scaling potential which is particularly high for West Anatolian waters.  相似文献   

17.
The Latera field (Vulsini volcanic complex, Latium, Italy) is one of the geothermal areas of the peri-Tyrrhenian belt along which a regional, high thermal anomaly has been detected. So far nine deep wells have been drilled within the Latera caldera and four of them have been productive. The geothermal reservoir is located within the fractured carbonatic rocks of the Tuscan nappe; the overlying volcanic units, sealed by hydrothermal minerals (mainly calcite and anhydrite), act as an impervious cover.The fluid produced by the wells comes from a deep aquifer (about 1000–1500 m depth) which at present is not connected with the shallow aquifer in the volcanoclastic units. Fluid temperatures range between 200 and 230°C; in-hole temperatures as high as 343°C at 2775 m depth have been measured in dry wells.The study of the newly formed mineral assemblages from both volcanic and sedimentary units as sampled from the geothermal wells can be used to reconstruct the thermal evolution of the geothermal field. The intrusion of a syenitic melt, up to a depth of about 2000 m, dated 0.86 Ma, represents the major thermal event for the units in the area and is assumed to represent the first step in the geothermal evolution of the Latera system.The above mentioned newly formed mineral assemblages can be divided into three groups: (a) “contact-metasomatic”: calcite, anhydrite, diopsidic pyroxene, grossularitic garnet, phlogopite, wollastonite or monticellite; (b) “high-temperature hydrothermal”: calcite, anhydrite, K-feldspar, vesuvianite, melanitic garnet, tourmaline, amphibole, epidote, sulphides; (c) “low-temperature hydrothermal”: calcite, anhydrite, K-feldspar, clay minerals, sulphides. Group (a) minerals are now relics. Part of (b) and all of (c) group are still in equilibrium with the existing conditions in different parts of the geothermal system.Thermodynamic calculations on the observed mineral assemblages permitted estimates of the P, T conditions and gas fugacities.  相似文献   

18.
The Aegean volcanic arc is the result of a lithosphere subduction process during the Quaternary time. Starting from the Soussaki area, from west to east, the arc proceeds through the islands of Egina, Methana, Milos, Santorini, the Columbus Bank, Kos and Nisyros. Volcano-tectonic activities are still pronounced at Santorini and Nisyros in form of seismic activity, craters of hydrothermal explosions, hot fumaroles and thermal springs. A significant number of cold water springs emerge in the vicinity of hot waters on these islands.Chemical and isotopic analyses were applied on water and fumaroles samples collected in different areas of the volcanic arc in order to attempt the assessment of these fluids. Stable isotopes of water and carbon have been used to evaluate the origin of cold and thermal water and CO2.Chemical solute concentrations and isotopic contents of waters show that the fluids emerging in Egina, Soussaki, Methana and Kos areas represent geothermal systems in their waning stage, while the fluids from Milos, Santorini and Nisyros proceed from active geothermal systems.The δ2H–δ18O–Cl? relationships suggest that the parent hydrothermal liquids of Nisyros and Milos are produced through mixing of seawater and Arc-Type Magmatic Water (ATMW), with negligible to nil contribution of local ground waters and with very high participation of the magmatic component, which is close to 70% in both sites. A very high magmatic contribution to the deep geothermal system could occur at Santorini as well, perhaps with a percentage similar to Nisyros and Milos, but it cannot be calculated because of steam condensation heavily affecting the fumarolic fluids of Nea Kameni before the surface discharge.The parent hydrothermal liquid at Methana originates through mixing of local groundwaters, seawater and ATMW, with a magmatic participation close to 19%. All in all, the contribution of ATMW is higher in the central–eastern part of the Aegean volcanic arc than in the western sector. This difference, which is spotted in the variable isotopic composition of the sampled fluids from west to east along the arc, is probably due to several causes, including the tectonic regime, the depth of the deep reservoir below sea level, the age of volcanic activity and in general the geomorphologic state of each island.  相似文献   

19.
In this study, representative samples from thermal wells and springs were chemically analyzed and geothermometers were used to calculate the deep temperatures of geothermal reservoirs on the basis of water–mineral equilibrium. In some cases, however, the chemical components are not in equilibrium with the minerals in the reservoir. Therefore, log(Q/K) diagrams are used to study the chemical equilibrium for the minerals that are likely to participate. The Na–K–Mg triangular diagram is also applied to evaluate the equilibrium of water with reservoir rocks. Standard curves at the reference temperatures are prepared to reveal which type of silica geothermometer is appropriate for the specified condition. This study shows that water samples from geothermal wells W9 and W12 are in equilibrium with the selective minerals, and chalcedony may control the fluid–silica equilibrium. It is estimated that there is an exploitable low-temperature reservoir with possible temperatures of 80–90°C in the Guanzhong basin.  相似文献   

20.
Detailed geochemistry supported by geologic mapping has been used to investigate Sulphur Springs, an acid-sulfate hot spring system that issues from the western flank of the resurgent dome inside Valles Caldera. The most intense activity occurs at the intersection of faults offsetting caldera-fill deposits and post-caldera rhyolites. Three geothermal wells in the area have encountered pressures <1 MPa and temperatures of 200°C at depths of 600 to 1000 m. Hot spring and fumarole fluids may discharge at boiling temperatures with pH 1.0 and SO4 8000 mg/l. These conditions cause argillic alterations throughout a large area.Non-condensible gases consist of roughly 99% CO2 with minor amounts of H2S, H2, and CH4. Empirical gas geothermometry suggests a deep reservoir temperature of 215 to 280°C. Comparison of 13C and 18O between CaCO3 from well cuttings and CO2 from fumarole steam indicates a fractionation temperature between 200 and 300°C by decarbonation of hydrothermally altered Paleozoic limestone and vein calcite in the reservoir rocks. Tritium concentrations obtained from steam condensed in a mudpot and deep reservoir fluids (Baca #13, 278°C) are 2.1 and 1.0 T.U. respectively, suggesting the steam originates from a reservoir whose water is mostly >50 yrs old. Deuterium contents of fumarole steam, deep reservoir fluid, and local meteoric water are practically identical even though 18O contents range through 4‰, thus, precipitation on the resurgent dome of the caldera could recharge the hydrothermal system by slow percolation. From analysis of D and 18O values between fumarol steam and deep reservoir fluid, steam reaches the surface either (1) by vaporizing relatively shallow groundwater at 200°C or (2) by means of a two-stage boiling process through an intermediate level reservoir at roughly 200°C.Although many characteristics of known vapor-dominated geothermal systems are found at Sulphur Springs, fundamental differences exist in temperature and pressure of our postulated vapor-zone. We propose that the reservoir beneath Sulphur Springs is too small or too poorly confined to sustain a “true” vapor-dominated system and that the Sulphur Springs system may be a “dying” vapor-dominated system that has practically boiled itself dry.  相似文献   

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