Estimation of heat flow in Tuva from data on helium isotopes in thermal mineral springs

Concentrations of helium isotopes were measured in gas and water samples from 28 thermal mineral springs in Tuva and adjacent regions of Buryatia and Gorny Altai. It is shown that fluids from 16 springs are rich in mantle helium (4–35%). With regard to the air contamination of the samples, the corrected ratios of helium isotopes (R_cor = ³He/⁴He) in these springs vary from 5.3 × 10^–8 to 422 × 10^–8. Using these R_cor values, we estimated the heat flow; these estimates were then applied to calculate the deep-level temperatures and thickness of thermal lithosphere. According to these parameters, the Tuva region is divided into two parts. Eastern Tuva (from ~96° E to the boundary with Buryatia) is characterized by abnormal helium isotope ratios and heat flow indicating the intense heating of the Earth’s crust in eastern Tuva: At a depth of 50 km, a temperature reaches 1000–1200 °C, and the thickness of thermal lithosphere is reduced to 70–50 km. This testifies to a rift process west (probably, up to 96° E) of the Baikal Rift Zone. In western Tuva, the average heat flow is much lower, ~45–50 mW/m², which is commensurate with that in the Altai–Sayan folded area as a whole. The deep-level temperatures here are twice lower, and the lithosphere thickness increases to 150 km.     

Characteristics of the geothermal water in Changbai Mountain volcanic region,northeast of China

Geothermal water is plentiful in Changbai Mountain region, northeastern China, due to the volcanic activities and widespread faults. For the exploration of geothermal resources, this study uses quartz and cation geothermometer to estimate the temperatures of the geothermal reservoir and uses the tubular models to evaluate the thermal gradient. The hydrogeochemical characteristics of the geothermal resources were also evaluated by hydrogeochemical analysis. The results showed that the geothermal reservoir temperatures of the four major thermal springs in Changbai Mountain region range from 72 to 169 °C. The average geothermal reservoir temperatures of Jinjiang hot springs, Changbai hot springs I, Xianrenqiao hot springs, and Changbai hot springs II are 129.25, 169, 89, and 73.67 °C, respectively. The geothermal gradient values of the four major thermal springs have different characteristics. The geothermal gradient values of Jinjiang hot springs and Changbai hot springs I are 4.6 and 3.1 °C/100 m, respectively. The geothermal gradient values of Xianrenqiao thermal springs and Changbai thermal springs II are both lower than 1.5 °C/100 m, with the values of 1.1 and 1.4 °C/100 m. And the geothermal gradients are influenced by Changbai Mountain Tianchi volcano. In addition, the water chemical analyses showed that the geothermal water types are HCO₃-Na with higher concentrations of Na⁺, Cl^?, SO₄ ^2?, TDS, and HCO₃ ^? than the non-thermal waters, which suggested a deep and long water cycle of the thermal water, and therefore a sufficient water-rock interaction.     

Oxygen and hydrogen isotope geochemistry of thermal springs of the Western Cape, South Africa: Recharge at high altitude?

A number of thermal springs with temperature up to 64°C are found in the Western Cape Province of South Africa. The average δ¹³C value of gas (CO₂+CH₄) released at three springs is −22, which is consistent with an entirely biogenic origin for the C and supports previous investigations which showed that the springs are not associated with recent or nascent volcanic activity. Most springs issue from rocks of the Table Mountain Group, where faulted and highly jointed quartzites and sandstones of the Cape Fold Belt act as the main deep aquifer. The δD and δ¹⁸O values of the springs range from −46 to −18 and from −7.3 to −3.9, respectively. Although the thermal springs have isotope compositions that plot close to the local meteoric water line, their δD and δ¹⁸O values are significantly lower than ambient meteoric water or groundwater. It is, therefore, suggested that the recharge of most of the thermal springs is at a significantly higher altitude than the spring itself. The isotope ratios decrease wuth increasing distance from the west coast of South Africa, which is in part related to the continental effect. However, a negative correlation between the spring water temperature and the δ¹⁸O value in the thermal springs closest to the west coast indicates a progressive in increase in the average altitude of recharge away from the coast.     

Geochemistry and isotope geochemistry of the Monfalcone thermal waters (northern Italy): inference on the deep geothermal reservoir

Geochemical investigations were carried out to define the origin of the low- to moderate-temperature thermal waters feeding the Monfalcone springs in northern Italy. Chemical data indicate that waters approach the composition of seawater. Mixing processes with cold low-salinity waters are highlighted. The δ¹⁸O and δD values are in the range ?5.0 to ?6.4 ‰, and ?33 to ?40 ‰, respectively, suggesting the dilution of the saline reservoir by karst-type freshwaters. A surplus of Ca²⁺ and Sr²⁺ ions with respect to a conservative mixing is ascribed to diagenetic reactions of the thermal waters with Cretaceous carbonates at depth. The measured Sr isotopic composition (⁸⁷Sr/⁸⁶Sr ratio) ranges between 0.70803 and 0.70814; after correction for the surplus Sr, a ⁸⁷Sr/⁸⁶Sr ratio indicating Miocene paleo-seawater is obtained. The dissolved gases indicate long-lasting gas–water interactions with a deep-originated gas phase of crustal origin, dominated by CO₂ and marked by a water TDIC isotopic composition in the range ?5.9 to?8.8 and helium signature with 0.08?<?R/Ra?<?0.27, which is a typical range for the crust. A possible scenario for the Monfalcone thermal reservoir consists of Miocene marine paleowaters which infiltrated through the karstic voids formed within the prevalently Cretaceous carbonates during the upper Eocene emersion of the platform, and which were entrapped by the progressive burial by terrigenous sediments.     

A multi-isotope approach (O,H, C,S, B and Sr) to understand the source of water and solutes in some the thermal springs from West Coast geothermal area,India

The West Coast belt, consisting of nearly 60 thermal springs, is one of the most diversified geothermal fields in India. The present work describes the multi-isotopic (O, H, C, S, B and Sr) characterization of thermal waters carried out in the Tural-Rajwadi geothermal field, situated in southern sector of the west coast geothermal area. The aim of this study is to delineate the origin of thermal water as well as to ascertain the sources of carbon, sulphur, boron and strontium dissolved in those thermal springs. The stable isotopes (δ²H and δ¹⁸O) and tritium data indicate that these thermal springs are not recently recharged rain water rather, it contains very old component of water. Oxygen-18 shift is observed due to rock-water interaction over a long period of time. Carbon isotopic composition of DIC points out to the silicate weathering with soil CO₂ coming from C₃ type of plants whereas δ³⁴S of dissolved sulphate confirms the marine origin of sulphate. This marine signature is basically derived from paleo-seawater possibly entrapped within the flows. Boron isotopic data reveals that both the seawater and rock dissolution are the sources of boron in the thermal waters whereas high ⁸⁷Sr/⁸⁶Sr ratios (0.7220–0.7512) of the thermal waters conclusively establishes that archean granitic basement is the predominant rock source of strontium, not the Deccan flood basalts. In addition, like strontium, concentrations of lithium, rubidium and caesium are also governed by the rock-water interaction. Thus, the combined use of this multi-isotope technique coupled with trace element concentrations proves to be an effective tool to establish the sources of solutes in the thermal water.     

Hydrogeochemistry and stable isotopes of thermal springs: earthquake-related chemical changes along Belice Fault (Western Sicily)

Three geothermal systems, Montevago, Castellammare-Alcamo and Sciacca, are located along the main seismogenetic structures in Western Sicily. Concentrations of dissolved species including the gases CO₂, N₂, He and the results of stable isotope measurements δ¹⁸O, δD and δ¹³C_TDIC in water samples collected from six thermal springs and 28 cold discharges were used to characterise their feeder aquifers and to reveal the relationships between water chemistry and regional seismicity. The Sciacca thermal springs differ chemically and isotopically from those of Montevago and the Castellammare-Alcamo areas. The inferred deep end-members of the thermal waters of Montevago and Castellammare-Alcamo are almost similar, suggesting that both systems are fed by carbonate waters and selenite waters. A slight contribution (1–3%) of seawater, during groundwater ascent it is also present. The Sciacca thermal springs are fed by a deep reservoir comprising a mixture of 50% carbonate water and 50% seawater. During ascent towards the surface, these waters interact with NaCl-rich evaporite layers. By combination of published and present data significant temporal variations of temperature and some chemical parameters in the thermal waters of Western Sicily have been recorded. These variations were mostly between 1966 and 1969. Although the data are discontinuous it is still possible to reveal a direct link between physico–chemical changes in the Acqua Pia and Terme Selinuntine springs and the 1968 Belice Valley earthquake. Within the studied springs, two kinds of geochemical behaviour have been recognised. The chemistry of the Montevago thermal springs was permanently changed in response to changes in the groundwater system. Water temperature and dissolved SO₄, Cl, Na, and TDS showed minimum values before the earthquake and maximum values after the event. Almost constant values substantially higher than before, were recorded after the seismic event. Conversely, the temporal variations observed in the waters of the Terme Selinuntine spring, from 1965 to 1991, exhibit a transient increase most probably caused by a temporary contribution of deep CO₂-rich fluids caused by the strain release during the 1968 earthquake.     

Water and gas geochemistry of the Euganean and Berician thermal district (Italy)

Between 1987 and 1995 more than 100 chemical and isotopic analyses were carried out on the thermal fluids discharged at surface from wells and springs of the Euganean and Berician thermal district. Results for δD and δ¹⁸O in waters, δ¹³C in CO₂ and in C₁–C₄ n-alkanes, δD in CH₄, ³He/⁴He and ⁴⁰Ar/³⁶Ar ratios in natural gases were coupled with chemical analyses in an attempt to determine the main characteristics and evolutionary trends of thermal fluids emerging in the region. The isotopic and chemical composition of thermal waters has led to the postulation of a meteoric origin of discharged thermal fluids and of a “maturation” trend as water moves from the peripheral manifestations of the Berici Hills towards those of the Battaglia, Montegrotto and Abano springs in the inner part of the geothermal field. Numerical simulation suggested that the observed evolutionary path is consistent with differentiation due to processes of water–rock interaction.The results of bulk analyses have shown that the gases are made up mainly of N₂ (65–95 vol%), CO₂ (0.5–20.5 vol%) and CH₄ (up to 10 vol%), with relatively high Ar and He contents (up to 1.5 vol% and 0.16 vol%, respectively) and detectable amounts of C₂–C₆ saturated hydrocarbons. The chemical and isotopic composition of the gases suggests that both the meteoric and crustal contributions to the natural discharges are significant, while any significant magmatic contribution, possibly related to vestiges of the volcanic activity that occurred in the Abano area during the Tertiary age, can be ruled out.     

Carbon-Isotope Characteristics of CO2 and CH4 in Geothermal Springs from the Central Andes

Carbon stable-isotope compositions of coexisting carbon dioxide and methane from geothermal springs across the Central Andes of northern Chile and Bolivia are reported. A total of 60 samples were analyzed for δ¹³C_CO2 and, of these, 10 were selected for δ¹³C_CH4 analyses. The Central Andes are characterized by an active volcanic arc and an unusually thick (up to 75 km) continental crust behind the arc, beneath the high plateau region of the Altiplano. Furthermore, helium-isotope evidence suggests active mantle degassing in a 350-km-wide zone beneath the thick continental crust in the Central Andes (Hoke et al., 1994).

The present results show a wide range of δ¹³C_CO2 (-14.9 to -0.6‰) and a surprisingly heavy δ¹³C_CH4 (?20.9 to ?12.3‰). The difference between δ¹³C_CO2 and δ¹³C_CH4 (δ¹³C_CO2-CH4 ) for individual samples varies between 1.5‰ and 13.5‰. The δ¹³C_CO2 results show wide and overlapping ranges in the samples collected from the Precordillera, the Volcanic Arc (or Western Cordillera), the Altiplano, and the Eastern Cordillera. The widest ranges occur in the Eastern Cordillera (?15.0 to ?4.8‰) and the Altiplano (?20 to ?6‰). The δ¹³C_CO2 results for geothermal samples from the Volcanic Arc range between ?8.0‰ (Surire) and ?0.6‰ (Abra de Nappa), whereas δ¹³C_CO2 measured in gases collected from geothermal springs in the Precordillera range from ?10 to ?5‰.

The relationships between 3He/4He, δ¹³C_CO2 , and δ¹³C_CH4 are used to distinguish between crustal and mantle origins. The wide (21‰) range in the is interpreted to reflect contributions from different CO₂ sources that include organic and inorganic crustal and mantle carbon. Assuming isotopic equilibrium between coexisting methane and carbon dioxide, Δ¹³C_CO2-CH4 suggests very high equilibrium temperatures, in excess of 530°C, for some geothermal systems that also are characterized by a high (up to 63%) mantle-derived helium component.

δ¹³C_CH4 results suggest that methane has not formed by bacteriogenic processes or by thermal decomposition of organic matter, but rather abiogenically through the high-temperature reaction between H₂ and CO₂. The δ¹³C_CH4 results for the samples from the Volcanic Arc and from two CO₂-rich geothermal springs in the Altiplano (Coipasa-2 and Belen de Andamarca) are similar to those reported from hydrothermal fluids emitted from the East Pacific Rise (Welhan, 1988) and White Island, New Zealand (Hulston and McCabe, 1962), suggesting a mantle-derived carbon component in the methane.     

He,Ne, Ar,and C isotope systematics of geothermal emanations in the Lesser Antilles Islands Arc

We present He, Ne, Ar, and C isotope analyses of hydrothermal brines and gases from fumaroles, hot springs, mofettes and hydrothermal exploration drillings on the major islands of the Lesser Antilles Arc. The origin of hydrothermal brines, which have been analyzed also for O and H isotopes, is essentially meteoric-hydrothermal. Air-corrected isotope compositions of helium (2.2 Rc/Ra < ³He/⁴He < 8.6 Rc/Ra) and carbon (−20 < δ¹³C_PDB < +0.5) are variable and require a variety of crustal and magmatic sources. The diversity of δ¹³C_PDB and ³He/CO₂ ratios within individual volcanic centres suggests that crustal sources (e.g., limestone) contaminate magmatic CO₂ en route from high-level magma reservoirs (depth < 15 km) to the surface. A similar contamination may be found for magmatic helium on distal springs. The ³He/⁴He signature of summit fumaroles, thought to reflect the ³He/⁴He signature of high-level magmas, shows a remarkable systematic variation along the arc. In addition, there is a correlation throughout the arc between published Sr, Pb, and Nd isotope signatures of lavas and the ³He/⁴He signatures of summit fumaroles. On the northern islands (Nevis, Montserrat, Guadeloupe, and Dominica) summit fumaroles have the N-MORB signature (³He/⁴He = 8 ± 1 R/Ra), and the isotope signature of lavas is not dissimilar from comparable intra-oceanic arc tholeiites elsewhere. Variable enrichments in radiogenic Sr and Pb have been reported for lavas of individual volcanic centres of the Southern Islands (Martinique, St.Lucia, and Grenada), and summit fumaroles on these centres match these variations by variable radiogenic He-enrichments, i.e., lower ³He/⁴He ratios. This correlation suggests that radiogenic Sr and Pb enrichments of lavas and low ³He/⁴He signatures on summit fumaroles have a common origin, i.e., a terrigenous contaminant derived from the Orinoco depositionary fan. Crustal assimilation is thought to decouple the He isotope system from any other radiogenic isotope system and, therefore, we argue that the observed correlation of He, Sr, Pb, and Nd isotope systems is related to a terrigenous contaminant derived from subducted sediments. Support for this scenario also comes from the matching of low ³He/⁴He ratios and tectonic features of the forearc thought to favor the subduction of forearc sediments.The present study offers a first clue that, under suitable conditions, crustal helium from oceanic sediments might be subducted to the depth of arc magma sources and, possibly, even recycled into the deeper mantle.     

Geochemical evidence of seawater intrusion into a coastal geothermal field of central Greece: example of the Thermopylae system

Thermal water of Thermopylae and from other geothermal fields located in the southern part of the Sperchios basin (central Greece) are characterized by high salinity (total dissolved salts, or TDS, range from 1.2 to 30.3 g L⁻¹) associated with a degassing of CO₂. To determine the mineralization processes, geochemical and isotopic investigations (major elements, ¹⁸O, ²H and ¹³C) have been carried out upon 17 thermal waters from springs and boreholes. This study emphasizes that all the thermal waters result from the mixing of a seawater end-member, several fresh water components depending on the field location, and a mantle-derived CO₂ rising upward through an E–W fault system. The seawater identified in the thermal mixture is likely to be evolved Aegean seawater (ASW). Once intruded into the basin sediments, the trapped seawater has its chemical content modified by both water–rock interactions and massive dissolution of the deep CO₂ (pCO₂ of 10^0.5 atm). The modelling performed with PHREEQC indicates that the anomalous major ion ratios measured in the so-called evolved ASW are explained by the dissolution of calcite and dolomitization process associated to precipitation of gypsum within the thermal aquifer.     

Source and origin of active and fossil thermal spring systems,northern Upper Rhine Graben,Germany

Thermal water samples and related young and fossil mineralization from a geothermal system at the northern margin of the Upper Rhine Graben have been investigated by combining hydrochemistry with stable and Sr isotope geochemistry. Actively discharging thermal springs and mineralization are present in a structural zone that extends over at least 60 km along strike, with two of the main centers of hydrothermal activity being Wiesbaden and Bad Nauheim. This setting provides the rare opportunity to link the chemistry and isotopic signatures of modern thermal waters directly with fossil mineralization dating back to at least 500–800 ka. The fossil thermal spring mineralization can be classified into two major types: barite-(pyrite) fracture filling associated with laterally-extensive silicification; and barite, goethite and silica impregnation mineralization in Tertiary sediments. Additionally, carbonatic sinters occur around active springs. Strontium isotope and trace element data suggest that mixing of a hot (>100 °C), deep-sourced thermal water with cooler groundwater from shallow aquifers is responsible for present-day thermal spring discharge and fossil mineralization. The correlation between both Sr and S isotope ratios and the elevation of the barite mineralization relative to the present-day water table in Wiesbaden is explained by mixing of deep-sourced thermal water having high ⁸⁷Sr/⁸⁶Sr and low δ³⁴S with shallow groundwater of lower ⁸⁷Sr/⁸⁶Sr and higher δ³⁴S. The Sr isotope data demonstrate that the hot thermal waters originate from an aquifer in the Variscan crystalline basement at depths of 3–5 km. The S isotope data show that impregnation-type mineralization is strongly influenced by mixing with SO₄ that has high δ³⁴S values. The fracture style mineralization formed by cooling of the thermal waters, whereas impregnation-type mineralization precipitated by mixing with SO₄-rich groundwater percolating through the sediments.     

深部物质运动的气体地球化学特征

根据氦同位素地球化学资料讨论了中国东部和云南腾冲地区上地幔的脱气。尽管地球脱气作用主要发生在地球形成时的十亿年间，但是后期的脱气作用仍是影响大气圈演化的主要因素。在两种力学性质不同的构造带──中国东部大陆裂谷和位于欧亚板块与印度板块缝合带的腾冲火山区，采集了天然气样，并分析了气体组分和氦同位素组成，较高的３Ｈｅ／４Ｈｅ值和地质、地球物理资料表明天然气和温泉气中的氦相当一部分是来自上地幔。来自上地幔的氦和其他气体自第三纪以来不断在气藏中聚集或向大气中逃逸。伴有源于上地幔的岩浆活动的地幔脱气是深部物质运动的具体表现形式，它对新生代气候演变可能有直接影响。     

Isotope-Geochemical Features and Genesis of Gases in the East Carpathian Region

The paper presents data on the composition of a gas phase of underground fluids in the East Carpathian region, including ³He/⁴He, ⁴⁰Ar/³⁶Ar, and ⁴He/²⁰Ne ratios. The argon isotope composition of these gases was used to estimate the fractions of atmospheric Aratm and radiogenic ⁴⁰Ar_rad formed in the rocks, N₂/Aratm ratio, and to reveal the admixture of nonatmogenic (“excess”) nitrogen in most samples. The CO₂ content in gases positively correlates with the fraction of mantle component in fluid helium. At the same time, the CO₂ content shows a negative correlation with the total helium (and light ³He enriching mantle derivatives), thus excluding the simultaneous influx of CO₂ and helium from a common mantle source in the fluids. A wide spectrum of ³He/⁴He = R in gases of the region spanning three orders of magnitude confirms the concept of mixing of the crustal and mantle components in the helium. However, even gases with similar R values show a wide scatter of He concentrations. This is mainly caused by the additional influx of other gases: CH₄ formed during OM transformation or CO₂ released during the thermal metamorphism of carbonate sequences. Correlation of the CH₄/³He ratio and the helium isotope composition in the Carpathian gases indicates the crustal origin of hydrocarbons, which formed economic gas pools in the Ciscarpathian Trough and the adjacent part of the Folded Carpathians. Lateral chemical and isotope variations revealed in the underground fluids are related to the tectonic zoning of the region. The helium isotope variations are also consistent with the geodynamic setting of the region (thinning of the crust and lithosphere towards the Pannonian Basin, growth of the background conductive heat flow and corresponding ascent of isotherms). In combination with geothermal data, they reflect specifics of the mantle heat-and-mass flow discharge.     

Hydrogeochemical and isotopic characteristics of the Ilica geothermal system (Erzurum,Turkey)

In this paper, the hydrochemical isotopic characteristics of samples collected from geothermal springs in the Ilica geothermal field, Eastern Anatolia of Turkey, are examined and described. Low-temperature geothermal system of Ilica (Erzurum, Turkey) located along the Eastern Anatolian fault zone was investigated for hydrogeochemical and isotopic characteristics. The study of ionic and isotopic contents shows that the thermal water of Ilica is mainly, locally fed by groundwater, which changes chemically and isotopically during its circulation within the major fault zone reaching depths. The thermal spring has a temperature of 29–39 °C, with electrical conductivity ranging from 4,000 to 7,510 µS/cm and the thermal water is of Na–HCO₃–Cl water type. The chemical geothermometers applied in the Ilica geothermal waters yielded a maximum reservoir temperature of 142 °C according to the silica geothermometers. The thermal waters are undersaturated with respect to gypsum, anhydrite and halite, and oversaturated with respect to dolomite. The dolomite mineral possibly caused scaling when obtaining the thermal waters in the study area. According to the enthalpy chloride-mixing model, cold water to the thermal water-mixing ratio is changing between 69.8 and 75 %. The δ¹⁸O–δ²H compositions obviously indicate meteoric origin of the waters. Thermal water springs derived from continental precipitation falling on to higher elevations in the study area. The δ¹³C ratio for dissolved inorganic carbonate in the waters lies between 4.63 and 6.48 ‰. In low-temperature waters carbon is considered as originating from volcanic (mantle) CO₂.     

Major,trace element and stable isotope composition of water and muds precipitated from the hot springs of Bolivia: Are the waters of the spring's potential ore forming fluids?

The deposition of metal-rich black or reddish muds by many thermal springs in the Cordilleras and the Altiplano of Bolivia suggest that these geothermal waters may be related to those that once formed the world-class Bolivian tin, silver and gold mineralisation. The discharge temperatures of these springs are as high as 70 °C. According to δ¹⁸O, δD, tritium data and Ar/N₂ ratios these waters are predominantly of meteoric origin. Less than 10% of the discharging thermal water represents deep-seated metal-rich thermal brines of at least 530 °C according to carbon exchange between CO₂ and CH₄. These brines ascend along tectonic faults and mix with low-temperature meteoric water in surface-near aquifers. The meteoric component of the thermal water is recharged in the high Cordilleras with residence times exceeding 50 years. The chemical composition of the thermal water is dominated by the rather inefficient low-temperature leaching of the surface-near aquifer rocks by meteoric water. The small fraction of metal-rich hot deep-seated water is not able to increase the metal content of the water mix to a level sufficient to classify these thermal waters as ore-bearing. Surface-near leaching is supported, e.g., by the B/Li ratios of the spring water of the Western Cordillera and Caleras/Pulacayo in the Eastern Cordillera that correspond very closely to that of the easily leachable glassy inclusions of the outcropping andesitic lavas. The often remarkable metal content of the muds deposited by the springs originate from efficient scavenging of heavy metals by ferric oxyhydroxides. Under the given arid to semi-arid climate the muds are additionally enriched in metals by wind-transported dust. The present study does support a relation of the actual thermal waters with neither the classical subduction-related Upper Tertiary tin, silver and gold mineralisation nor the supposed younger Sb mineralisation of Bolivia.     

Ammonium in thermal waters of Yellowstone National Park: Processes affecting speciation and isotope fractionation

Dissolved inorganic nitrogen, largely in reduced form (), has been documented in thermal waters throughout Yellowstone National Park, with concentrations ranging from a few micromolar along the Firehole River to millimolar concentrations at Washburn Hot Springs. Indirect evidence from rock nitrogen analyses and previous work on organic compounds associated with Washburn Hot Springs and the Mirror Plateau indicate multiple sources for thermal water NH₄(T), including Mesozoic marine sedimentary rocks, Eocene lacustrine deposits, and glacial deposits. A positive correlation between NH₄(T) concentration and δ¹⁸O of thermal water indicates that boiling is an important mechanism for increasing concentrations of NH₄(T) and other solutes in some areas. The isotopic composition of dissolved NH₄(T) is highly variable (δ¹⁵N = −6‰ to +30‰) and is positively correlated with pH values. In comparison to likely δ¹⁵N values of nitrogen source materials (+1‰ to +7‰), high δ¹⁵N values in hot springs with pH >5 are attributed to isotope fractionation associated with loss by volatilization. NH₄(T) in springs with low pH typically is relatively unfractionated, except for some acid springs with negative δ¹⁵N values that are attributed to condensation. NH₄(T) concentration and isotopic variations were evident spatially (between springs) and temporally (in individual springs). These variations are likely to be reflected in biomass and sediments associated with the hot springs and outflows. Elevated NH₄(T) concentrations can persist for 10s to 1000s of meters in surface waters draining hot spring areas before being completely assimilated or oxidized.     

Hydrogeochemical and isotopic investigation of the Bursa-Oylat thermal waters, Turkey

The Oylat spa is located 80 km southeast of Bursa and 30 km south of Ineg?l in the Marmara region. With temperature of 40°C and discharge of 45 l/s, the Oylat main spring is the most important hot water spring of the area. Southeast of the spa the Forest Management spring has a temperature of 39.4°C and discharge of 2 l/s. The G?z spring 2 km north of the spa, which is used for therapy of eye disease, and cold waters of the Saadet village springs with an acidic character are the further important water sources of the area. EC values of Main spring and Forest Management hot spring (750–780 μS/cm) are lower than those of Saadet and G?z spring waters (2,070–1,280 μS/cm) and ionic abundances are Ca > Na + K > Mg and SO₄ > HCO₃ > Cl. The Oylat and Sızı springs have low Na and K contents but high Ca and HCO₃ concentrations. According to AIH classification, these are Ca–SO₄–HCO₃ waters. Based on the results of δ¹⁸O, ²H and ³H isotope analyses, the thermal waters have a meteoric origin. The meteoric water infiltrates along fractures and faults, gets heated, and then returns to surface through hydrothermal conduits. Oylat waters do not have high reservoir temperatures. They are deep, circulating recharge waters from higher enhanced elevations. δ¹³C_DIC values of the Main spring and Forest Management hot spring are −6.31 and −4.45‰, respectively, indicating that δ¹³C is derived from dissolution of limestones. The neutral pH thermal waters are about +18.7‰ in δ³⁴S while the sulfate in the cold waters is about +17‰ (practically identical to the value for the neutral pH thermal waters). However, the G?z and Saadet springs (acid sulfate waters) have much lower δ³⁴S values (~+4‰).     

Mixing processes in hydrothermal spring systems and implications for interpreting geochemical data: a case study in the Cappadocia region of Turkey

Mixing is a dominant hydrogeological process in the hydrothermal spring system in the Cappadocia region of Turkey. All springs emerge along faults, which have the potential to transmit waters rapidly from great depths. However, mixing with shallow meteoric waters within the flow system results in uncertainty in the interpretation of geochemical results. The chemical compositions of cold and warm springs and geothermal waters are varied, but overall there is a trend from Ca–HCO₃ dominated to Na–Cl dominated. There is little difference in the seasonal ionic compositions of the hot springs, suggesting the waters are sourced from a well-mixed reservoir. Based on δ¹⁸O and δ²H concentrations, all waters are of meteoric origin with evidence of temperature equilibration with carbonate rocks and evaporation. Seasonal isotopic variability indicates that only a small proportion of late spring and summer precipitation forms recharge and that fresh meteoric waters move rapidly into the flow system and mix with thermal waters at depth. ³H and percent modern carbon (pmC) values reflect progressively longer groundwater pathways from cold to geothermal waters; however, mixing processes and the very high dissolved inorganic carbon (DIC) of the water samples preclude the use of either isotope to gain any insight on actual groundwater ages.     

Karstification in Miocene gypsum: an example from Sivas, Turkey

In this study, karstification developed in the Miocene gypsum which covers a large area around Sivas, Turkey, the relation between regional tectonics and karstification, and hydrogeological features have been investigated. The karstic features in the gypsum have developed conforming to the fault zones and the general strikes of gypsum beds. In the study area, numerous dolines (sinkholes) and ponors (swallow holes) of different sizes are observed. Most of these karstic features are in the different-sized longitudinal depressions (troughs) which conform to the tectonic structures. These karstic features occur NE-SW along bedding planes and about NW-SE and NE-SW along fault zones. At the intersections, high-capacity (yield) karst springs (Göydün and Seyfe springs) are observed. The Göydün and Seyfe springs issue from the karstified gypsum aquifer, with an average discharge of 1.10 m³/s and 0.25 m³/s, respectively. In addition, there are some low-yield (a few l/s) springs in the same drainage area. The surface drainage area of the springs is 64 km², and the precipitation in this area cannot provide the total groundwater discharging from Göydün and Seyfe springs. Water budget calculations indicate that more than 70% of the water discharged by these two springs is provided from the adjacent basins. The Göydün and Seyfe springs are rich in dissolved solids and average electrical conductivities (EC) are about 13?000 μS/cm. The major cations in the waters are Ca and Na; major anions are SO₄ and Cl, and the waters are brackish. Because of these properties, especially in summer and autumn, the springs cause extreme salinization in K?z?l?rmak river.     

Seasonal variation in nature and chemical compositions of spring water in Cuihua Mountain,Shaanxi Province,central China

Two springs (Cuihua Spring, Shuiqiuchi Spring) in Cuihua Mountain of the Qinling Mountains were observed and sampled monthly during 2004 and 2005 to trace their physical properties and chemical compositions with seasons. Although both pH values and cation (Ca²⁺, Mg²⁺, K⁺, and Na⁺) contents of Cuihua Spring are higher than those of Shuiqiuchi Spring, seasonal variations in both springs are obvious. The pH values of both spring waters are between 5.69 and 6.98, lower than that of rainwater during summer and autumn. From January to November, the pH values of both springs similarly vary from high to low and then to high again. Variations in electric conductivities of two spring waters are contrary, although their electric conductivities are positively correlative with the cation content respectively. This can be attributed to different water sources of the two springs or different acidic rocks they passed. The contents of HCO₃ ⁻, Ca²⁺, Mg²⁺, K⁺, and Na⁺ are low, indicating a low silicate weathering that the strata in this district are mainly composed of granite and schist of quartz and mica. Differing from change in spring water in karst regions of South China where abundant precipitation and dilution of rainwater cause low pH and electric conductivity in summer and autumn, the seasonal variations in the pH values and the electric conductivities of two springs in Qinling Mountains are attributed to seasonal changes in CO₂ produced by microorganisms’ activity in soil within respective year, rather than rainfall. The microorganisms’ activity in soil produces more CO₂ during summer and autumn. Therefore, the water nature of springs in silicate regions chiefly reflects the seasonal changes of CO₂ produced by the microorganisms in soil.