A geochemical and geophysical approach to derive a conceptual circulation model of CO2-rich mineral waters: A case study of Vilarelho da Raia, northern Portugal

The Vilarelho da Raia-Chaves region, located in northern Portugal adjacent to the Spanish border, is characterized by both hot and cold CO₂-rich mineral waters issuing from springs and drilled wells. The present paper updates the conceptual circulation model of the Vilarelho da Raia cold CO₂-rich mineral waters. Vilarelho da Raia mineral waters, dominated by Na and HCO₃ ions, have formed mainly by interaction with CO₂ of deep-seated mantle origin. The δ ¹⁸O, δ ²H and ³H values indicate that these waters are the result of meteoric waters infiltrating into Larouco Mountain, NW of Vilarelho da Raia, circulating at shallow depths in granitic rocks and moving into Vilarelho da Raia area. The conceptual geochemical and geophysical circulation model indicates that the hot and cold CO₂-rich mineral waters of Chaves (76 °C) and Vilarelho da Raia (17 °C) should be considered manifestations of similar but not the same geohydrological systems. Electronic Publication     

Hydrogeochemical properties of CO2-rich thermal–mineral waters in Kayseri (Central Anatolia), Turkey

The present study highlights the hydrogeological and hydrogeochemical characteristics of the CO₂-rich thermal–mineral waters in Kayseri, Turkey. These waters of Dokuzpınar cold spring (DPS) (12–13°C), Yeşilhisar mineral spring (YMS) (13–16°C), Acısu mineral spring (ACMS) (20–22.5°C), Tekgöz thermal spring (TGS) (40–41°C), and Bayramhacı thermal-mineral spring (BTMS) (45–46.5°C) have different physical and chemical compositions. The waters are located within the Erciyes basin in the Central Anatolian Crystalline complex consisting of three main rock units. Metamorphic/crystalline rocks occur as the basement, sedimentary rocks of Upper Cretaceous-Quaternary age form the cover, and volcanosedimentary rocks Miocene-Quaternary in age represent the extrusive products of magmatism acting in that period. All these units are covered unconformably by terrace and alluvial deposits, and travertine occurrences have variable permeability. Dokuzpinar cold spring, YMS and ACMS localized mainly along the faults within the region have higher Na⁺ and Cl⁻ contents whereas TGS and BTMS have higher amounts of Ca²⁺ and HCO ₃ ⁻ . The high concentrations of Ca²⁺ and HCO ₃ ⁻ are mainly related to the high CO₂ contents resulting from interactions with carbonate rocks. Whereas the high Na⁺ content is derived from the alkaline rocks, such as syenite, tuff and basalts, the Cl⁻ is generally connected to the dissolution of the evaporitic sequences. These waters are of meteoric-type. BTMS deviates from meteoric water line. The content is related to the increases in the δ¹⁸O compositions due to mineral–water interaction (re-equilibrium) process. CO₂-dominated YMS and ACMS with low temperatures have higher mineralizations. Yeşilhisar mineral spring, ACMS, TGS and BTMS are oversaturated in terms of calcite, aragonite, dolomite, goethite and hematite, and undersaturated with respect to gypsum, halite and anhydrite. Yeşilhisar mineral spring, ACMS and BTMS are also characterized by recent travertine precipitation. Dokuzpınar cold spring is undersaturated in terms of the above minerals. The higher ratios of Ca/Mg and Cl/HCO₃, and lower ratios of SO₄/Cl in BTMS than TGS suggest that TGS has shallow circulation compared to BTMS, and/or has much more heat-loss enroute the surface. The sequence of hydrogeochemical and isotopic compositions of the waters is in an order of DPS>YMS>ACMS>TGS>BTMS and this suggests a transition period from a shallow circulation to a deep circulation path.     

Carbon isotopes and geochemical processes in CO2-rich cold mineral water,N-Portugal

This paper summarizes a new outlook on the conceptual model of Melgaço–Messegães CO₂-rich cold (≈18 °C) mineral water systems, issuing in N of Portugal, based on their isotopic (²H, ³H, ¹³C, ¹⁴C and ¹⁸O) and geochemical features. Stable isotopes indicate the meteoric origin of these CO₂-rich mineral waters. Based on the isotopic fractionation with the altitude, a recharge altitude between 513 up to 740 m a.s.l. was estimated, corroborating the tritium results. The lowest ³H content (0 TU) is found in the groundwater samples with the highest mineralization. The mineral waters circulation are mainly related to a granitic and granodioritic environment inducing two different groundwater types (Ca/Na–HCO₃ and Na/Ca–HCO₃), indicating different underground flow paths. Calcium dissolution is controlled by hydrolysis of rock-matrix silicate minerals (e.g. Ca-plagioclases) and not associated to anthropogenic sources. The shallow dilute groundwaters exhibit signatures of anthropogenic origins (e.g. NO₃) and higher Na/Ca ratios. The stable isotopes together with the geochemistry provided no indication of mixing between the regional shallow cold dilute groundwater and mineral water systems. The heavy isotopic signatures identified in the δ¹³C data (δ¹³C = 4.7 ‰, performed on the total dissolved inorganic carbon (TDIC) of CO₂-rich mineral waters) could be derived from a deep-seated (upper mantle) source or associated to methanogenesis (CH₄ source). The negligible ¹⁴C content (≈2 pmC) determined in the TDIC of the mineral waters, corroborates the hypothesis of a mantle-derived carbon source to the mineral groundwater systems or dissolution of carbonate layers at depth.     

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‰).     

Stable Carbon Isotope Biogeochemistry and Anthropogenic Impacts on Karst Ground Water, Zunyi, Southwest China

Natural and anthropogenic impacts on karst ground water, Zunyi, Southwest China, are discussed using the stable isotope composition of dissolved inorganic carbon and particulate organic carbon, together with carbon species contents and water chemistry. The waters can be mainly characterized as HCO₃–Ca type, HCO₃ · SO₄–Ca type, or HCO₃ · SO₄–Ca · Mg type, according to mass balance considerations. It is found that the average δ¹³C_DIC values of ground waters are higher in winter (low-flow season) than in summer (high-flow season). Lower contents of dissolved inorganic carbon (DIC) and lower values of δ¹³C_DIC in summer than in winter, indicate that local rain events in summer and a longer residence time of water in winter play an important role in the evolution of ground water carbon in karst flow systems; therefore, soil CO₂ makes a larger contribution to the DIC in summer than in winter. The range of δ¹³C_DIC values indicate that dissolved inorganic carbon is mainly controlled by the rate of carbonate dissolution. The concentrations of dissolved organic carbon and particulate organic carbon in most ground water samples are lower than 2.0 mg C L⁻¹ and 0.5 mg C L⁻¹, respectively, but some waters have slightly higher contents of organic carbon. The waters with high organic carbon contents are generally located in the urban area where lower δ¹³C_DIC values suggest that urbanization has had an effect on the ground water biogeochemistry and might threaten the water quality.     

Radiocarbon application in dating “complex” hot and cold CO2-rich mineral water systems: A review of case studies ascribed to the northern Portugal

The use of radioactive isotopes plays a very important role in dating groundwater, providing an apparent age of the systems in the framework of the aquifers conceptual modelling making available important features about the water fluxes, such as recharge, horizontal flow rates and discharge. In this paper, special emphasis has been put on isotopic constraints in the use of δ¹³C and ¹⁴C content as a dating tool in some hot (76 °C) and cold (17 °C) CO₂-rich mineral waters discharging in the Vilarelho da Raia–Pedras Salgadas region (N-Portugal). The radiocarbon content determined in these CO₂-rich mineral waters (¹⁴C activity from 4.3 up to 9.9 pmc) is incompatible with the systematic presence of ³H (from 1.7 to 7.9 TU). The δ¹³C values of the studied CO₂-rich mineral waters indicate that the total C in the recharge waters is being masked by larger quantities of CO₂ (¹⁴C-free) introduced from deep-seated (upper mantle) sources. This paper demonstrates that a good knowledge of mineral water systems is essential to allow hydrologists to make sound conclusions on the use of C isotopic data in each particular situation.     

A Geochemical study of the groundwater in the Misli basin and environmental implications

The aim of this study was to determine geochemical properties of groundwater and thermal water in the Misli Basin and to assess thermal water intrusion into shallow groundwater due to over-extraction. According to isotope and hydrochemical analyses results, sampled waters can be divided into three groups: cold, thermal, and mixed waters. Only a few waters reach water–rock chemical equilibrium. Thermal waters in the area are characterized by Na⁺–Cl⁻–HCO₃⁻, while the cold waters by CaHCO₃ facies. On the basis of isotope results, thermal waters in the Misli basin are meteoric origin. In particular, δ¹⁸O and δ²H values of shallow groundwater vary from −10.2 to −12.2‰ and −71.2 to −82‰, while those of thermal waters range from −7.8 to −10.1‰ and from −67 to −74‰, respectively. The tritium values of shallow groundwater having short circulation as young waters coming from wells that range from 30 to 70 m in depth vary from 10 to 14 TU. The average tritium activity of groundwater in depths more than 100 m is 1.59 ± 1.16, which indicates long circulation. The rapid infiltration of the precipitation, the recycling of the evaporated irrigation water, the influence of thermal fluids and the heterogeneity of the aquifer make it difficult to determine groundwater quality changes in the Misli Basin. Obtained results show that further lowering of the groundwater table by over-consumption will cause further intrusion of thermal water which resulted in high mineral content into the fresh groundwater aquifer. Because of this phenomenon, the concentrations of some chemical components which impairs water quality in terms of irrigation purposes in shallow groundwaters, such as Na⁺, B, and Cl⁻, are highy probably expected to increase in time.     

Hydrogeochemical study of the Terme and Karakurt thermal and mineralized waters from Kirşehir Area,central Turkey

The Terme and Karakurt thermal resorts are located in the center of Kirşehir city in central Anatolia. Thermal waters with temperatures of 44–60°C are used for central heating and balneologic purposes. Paleozoic rocks of the Kirşehir Massif are the oldest units in the study area. The basement of the Massif comprises Paleozoic metamorphic schist and marbles which partly contain white quartzite layers of a few tens of cm thickness. The metamorphic schists which are cut by granites of Paleocene age are overlain by horizontally bedded conglomerate, sandstone, claystone, and limestone of upper Paleocene-Eocene age. Among the thermal and cold waters collected from the areas of Terme and Karakurt, those from thermal waters are enriched with Ca–HCO₃ and cold waters are of Ca–Mg–HCO₃ type waters. The pH values of samples are 6.31–7.04 for the thermal well waters, 6.41 for thermal spring, 7.25 and 7.29 for the cold waters, and 7.52 for the Hirla lake water. EC values are 917–2,295 μS/cm for the thermal well waters, 2,078 μS/cm for thermal spring, and 471 and 820 μS/cm for the cold springs. The lowest TDS content is from water of T10 thermal well in the Terme area (740.6 mg/l). The hot and cold waters of Terme show very similar ion contents while the Karakurt hot waters at western most parts are characterized by distinct chemical compositions. There is ion exchange in thermal waters from the T5 (5), T6 (6), T12 (7), and T1 (8) wells in the Terme area. The thermal waters show low concentrations of Fe, Mn, Ni, Al, As, Pb, Zn and Cu. Waters in the study area are of meteoric origin, and rainwater percolated downwards through faults and fractures, and are heated by the geothermal gradient, later rising to the surface along permeable zones. δ¹³C_VPDB values measured on dissolved inorganic carbon in samples range from −1.65 to +5.61‰ for thermal waters and from −11.81 to −10.15‰ for cold waters. Carbon in thermal waters is derived from marine carbonates or CO₂ of metamorphic origin while carbon in cold waters originates from freshwater carbonates.     

Carbon and oxygen isotope constraints on fluid sources and fluid–wallrock interaction in regional alteration and iron-oxide–copper–gold mineralisation, eastern Mt Isa Block, Australia

The source of metasomatic fluids in iron-oxide–copper–gold districts is contentious with models for magmatic and other fluid sources having been proposed. For this study, δ ¹⁸O and δ ¹³C ratios were measured from carbonate mineral separates in the Proterozoic eastern Mt Isa Block of Northwest Queensland, Australia. Isotopic analyses are supported by petrography, mineral chemistry and cathodoluminescence imagery. Marine meta-carbonate rocks (ca. 20.5‰ δ ¹⁸O and 0.5‰ δ ¹³C calcite) and graphitic meta-sedimentary rocks (ca. 14‰ δ ¹⁸O and −18‰ δ ¹³C calcite) are the main supracrustal reservoirs of carbon and oxygen in the district. The isotopic ratios for calcite from the cores of Na–(Ca) alteration systems strongly cluster around 11‰ δ ¹⁸O and −7‰ δ ¹³C, with shifts towards higher δ ¹⁸O values and higher and lower δ ¹³C values, reflecting interaction with different hostrocks. Na–(Ca)-rich assemblages are out of isotopic equilibrium with their metamorphic hostrocks, and isotopic values are consistent with fluids derived from or equilibrated with igneous rocks. However, igneous rocks in the eastern Mt Isa Block contain negligible carbon and are incapable of buffering the δ ¹³C signatures of CO₂-rich metasomatic fluids associated with Na–(Ca) alteration. In contrast, plutons in the eastern Mt Isa Block have been documented as having exsolved saline CO₂-rich fluids and represent the most probable fluid source for Na–(Ca) alteration. Intrusion-proximal, skarn-like Cu–Au orebodies that lack significant K and Fe enrichment (e.g. Mt Elliott) display isotopic ratios that cluster around values of 11‰ δ ¹⁸O and −7‰ δ ¹³C (calcite), indicating an isotopically similar fluid source as for Na–(Ca) alteration and that significant fluid–wallrock interaction was not required in the genesis of these deposits. In contrast, K- and Fe-rich, intrusion-distal deposits (e.g. Ernest Henry) record significant shifts in δ ¹⁸O and δ ¹³C towards values characteristic of the broader hostrocks to the deposits, reflecting fluid–wallrock equilibration before mineralisation. Low temperature, low salinity, low δ ¹⁸O (<10‰ calcite) and CO₂-poor fluids are documented in retrograde metasomatic assemblages, but these fluids are paragenetically late and have not contributed significantly to the mass budgets of Cu–Au mineralisation.     

Groundwater problems resulting from CO2 pollution and overexploitation in Alto Guadalentín aquifer (Murcia,Spain)

 The Alto Guadalentín detrital aquifer is both overexploited and polluted. Water conductivity ranges between 1200 and 4900 μS cm^–1, HCO₃ ^– between 1000 and 1990 mg l^–1, and PCO₂ between 0.041 and 1.497 bars. The temperature and chemical composition of the water show a positive thermal anomaly directly attributable to the neotectonic activity in the area. The high CO₂ content has caused the abandonment of numerous wells due to water corrosiveness which attacks pumping equipment. Received: 10 October 1995 · Accepted: 14 November 1995     

Fluid evolution at the Variscan front in the vicinity of the Aachen thrust

Quartz–carbonate–chlorite veins were studied in borehole samples of the RWTH-1 well in Aachen. Veins formed in Devonian rocks in the footwall of the Aachen thrust during Variscan deformation and associated fluid flow. Primary fluid inclusions indicate subsolvus unmixing of a homogenous H₂O–CO₂–CH₄–(N₂)–Na–(K)–Cl fluid into a H₂O–Na–(K)–Cl solution and a vapour-rich CO₂–(H₂O, CH₄, N₂) fluid. The aqueous end-member composition resembles that of metamorphic fluids of the Variscan front zone with salinities ranging from 4 to 7% NaCl equiv. and maximum homogenisation temperatures of close to 400°C. Pressure estimates indicate a burial depth between 4,500 and 8,000 m at geothermal gradients between 50 and 75°C/26 MPa, but pressure decrease to sublithostatic conditions is also indicated, probably as a consequence of fracture opening during episodic seismic activity. A second fluid system, mainly preserved in pseudo-secondary and secondary fluid inclusions, is characterised by fluid temperatures between 200 and 250°C and salinities of <5% NaCl equiv. Bulk stable isotope analyses of fluids released from vein quartz, calcite, and dolomite by decrepitation yielded δD_H2O values from −89 to −113 ‰, δ¹³C_CH4 from −26.9 to −28.9‰ (VPDB) and δ¹³C_CO2 from −12.8 to −23.3‰ (VPDB). The low δD and δ¹³C range of the fluids is considered to be due to interaction with cracked hydrocarbons. The second fluid influx caused partial isotope exchange and disequilibrium. It is envisaged that an initial short lived flux of hot metamorphic fluids expelled from the epizonal metamorphic domains of the Stavelot–Venn massif. The metamorphic fluid was focused along major thrust faults of the Variscan front zone such as the Aachen thrust. A second fluid influx was introduced from formation waters in the footwall of the Aachen thrust as a consequence of progressive deformation. Mixing of the cooler and lower salinity formation water with the hot metamorphic fluid during episodic fluid trapping resulted in an evolving range of physicochemical fluid inclusion characteristics.     

Controls on chemistry during fracture-hosted flow of cold CO2-bearing mineral waters,Daylesford, Victoria,Australia: Implications for resource protection

Mineral springs from Daylesford, Australia discharge at ambient temperatures, have high CO₂ contents, and effervesce naturally. Mineral waters have high HCO₃ and Na concentrations (up to 4110 and 750 mg/L, respectively) and CO₂ concentrations of 620–2520 mg/L. Calcium and Mg concentrations are 61–250 and 44–215 mg/L, respectively, and Si, Sr, Ba, and Li are the most abundant minor and trace elements. The high P_CO2 of these waters promotes mineral dissolution, while maintaining low pH values, and geochemical modelling indicates that the CO₂-rich mineral water must have interacted with both sediments and basalts. Amorphous silica concentrations and silica geothermometry indicate that these waters are unlikely to have been heated above ambient temperatures and therefore reflect shallow circulation on the order of several hundreds of metres. Variations in minor and trace element composition from closely adjacent spring discharges indicate that groundwater flows within relatively isolated fracture networks. The chemical consistency of individual spring discharges over at least 20 a indicates that flow within these fracture networks has remained isolated over long periods. The mineral water resource is at risk from mixing with potentially contaminated surface water and shallow groundwater in the discharge areas. Increased δ²H values and Cl concentrations, and lower Na concentrations indicate those springs that are most at risk from surface contamination and overpumping. Elevated NO₃ concentrations in a few springs indicate that these springs have already been contaminated during discharge.     

CO2-rich thermomineral groundwater in the Betic Cordilleras, southeastern Spain: Genesis and tectonic implications

The CO₂-rich thermal groundwater in the Betic Cordilleras in Spain has been studied with regard to the geological and hydrogeological setting, physical and chemical characteristics, and ¹³C-isotope content. The study area is about 60 km northeast of Almería city, in southeastern Spain. The thermomineral waters are plentiful and are related to regional geothermal anomalies. Temperatures of 20 −41°C, high bicarbonate concentrations (183–1824 mg/L), and high amounts of PCO₂ (<1.1 bar) characterize the groundwater. CO₂ spatial variations are related to proximity to the Carboneras, Palomares, and Guadalentín fault systems, which may be the surface representation of the zone of crustal thinning and magmatism. δ ¹³C values probably indicate a deep source for the CO₂, either the mantle or perhaps carbonate rocks in the metamorphic substratum. The high amount of CO₂ in the groundwater causes problems in wells and severely restricts water usage. The hydrothermal features of this area are probably related to neotectonic activity. Received, September 1998/Revised, June 1999, September 1999/Accepted, December 1999     

Origin of CO2-rich fluid inclusions in synorogenic veins from the Eastern Mount Isa Fold Belt, NW Queensland, and their implications for mineralization

Synorogenic veins from the Proterozoic Eastern Mount Isa Fold Belt contain three different types of fluid inclusions: CO₂-rich, aqueous two-phase and rare multiphase. Inclusions of CO₂ without a visible H₂O phase are particularly common. The close association of CO₂-rich inclusions with aqueous two-phase, and possibly multiphase inclusions suggests that phase separation of low- to -moderate salinity CO₂-rich hydrothermal fluids led to the selective entrapment of the CO₂. Microthermometric results indicate that CO₂-rich inclusions homogenize between –15.5 and +29.9 °C which corresponds to densities of 0.99 to 0.60 g.cm⁻³. The homogenization temperatures of the associated aqueous two-phase inclusions are 127–397 °C, with salinities of 0.5 to 18.1 wt.% NaCl equivalent. The rarely observed multiphase inclusions homogenize between 250 and 350 °C, and have salinities ranging from 34.6 to 41.5 wt.% NaCl equivalent. Evidence used to support the presence of fluid immiscibility in this study is mainly derived from observations of coexisting H₂O-rich and CO₂-rich inclusions in groups and along the same trail. In addition, these two presumably unmixed fluids are also found on adjacent fractures where monophase CO₂-rich inclusions are closely related to H₂O-rich inclusions. Similar CO₂-rich inclusions are widespread in mineral deposits in this region, which are simply metal-enriched synorogenic veins. Therefore, we argue that fluid immiscibility caused volatile species such as CO₂ and H₂S to be lost from liquid, thus triggering ore deposition by increasing the fluid pH and decreasing the availability of complexing ligands. Received: 28 April 1997 / Accepted: 4 January 1999     

Groundwater resources of the middle and upper Odra River basin, southwestern Poland

 The total amount of groundwater resources in the middle and upper Odra River basin is 5200×10³ m³/d, or about 7.7% of the disposable groundwater resources of Poland. The average modulus of groundwater resources is about 1.4 L/s/km². Of the 180 'Major Groundwater Basins' (MGWB) in Poland, 43 are partly or totally located within the study area. The MGWB in southwestern Poland have an average modulus of groundwater resources about 2.28 L/s/km² and thus have abundant water resources in comparison to MGWB from other parts of the country. Several types of mineral waters occur in the middle and upper Odra River basin. These waters are concentrated especially in the Sudety Mountains. Carbon-dioxide waters, with yields of 414 m³/h, are the most widespread of Sudetic mineral waters. The fresh waters of the crystalline basement have a low mineralization, commonly less than 100 mg/L; they are a HCO₃–Ca–Mg or SO₄–Ca–Mg type of water. Various hydrochemical compositions characterize the groundwater in sedimentary rocks. The shallow aquifers are under risk of atmospheric pollution and anthropogenic effects. To prevent the degradation of groundwater resources in the middle and upper Odra River basin, Critical Protection Areas have been designated within the MGWB. Received, January 1995 Revised, May 1996, August 1997 Accepted, August 1997     

Water quality assessment in Akpabuyo, Cross River basin, South-Eastern Nigeria

 A baseline study involving analyses of surface and subsurface water samples from the Akpabuyo area was carried out in order to assess their suitability for drinking, domestic and agricultural purposes. Study results show that the waters are acidic (3.41≤pH≤6.28), soft (hardness 2.09–10.68 mg/l as CaCO₃), fresh (conductivity <1400 μs/cm) and characterised by low sodium adsorption ratio, SAR (0.08–0.59). In addition, the mean values of the major cations (Ca²⁺, Mg²⁺, Na⁺, K⁺) and anions (SO₄ ^2–, Cl^–, HCO₃ ^–) are all within the World Health Organisation (WHO) standards. Taking all this into consideration (except pH), the waters may be regarded as excellent for drinking, domestic and agricultural purposes. On the basis of regression equations, the major cations (K, Na, Ca, Mg) correlate well with conductivity. Finally, results also show that four chemical facies are delineated. These include Ca-Cl, Na-Cl, Ca-SO₄ and Ca-HCO₃. Received: 19 June 1996 / Accepted: 15 April 1997     

A preliminary analysis of the formation of travertine and travertine cones in the Jifei hot spring,Yunnan, China

The Jifei hot spring emerges in the form of a spring group in the Tibet–Yunnan geothermal zone, southwest of Yunnan Province, China. The temperatures of spring waters range from 35 to 81°C and are mainly of HCO₃–Na·Ca type. The total discharge of the hot spring is about 10 L/s. The spring is characterized by its huge travertine terrace with an area of about 4,000 m² and as many as 18 travertine cones of different sizes. The tallest travertine cone is as high as 7.1 m. The travertine formation and evolution can be divided into three periods: travertine terrace deposition period, travertine cone formation period and death period. The hydrochemical characteristics of the Jifei hot spring was analyzed and compared with a local non-travertine hot spring and six other famous travertine springs. The results indicate that the necessary hydrochemical conditions of travertine and travertine cones deposition in the Jifei area are (1) high concentration of HCO₃ ⁻ and CO₂; (2) about 52.9% deep source CO₂ with significantly high value; (3) very high milliequivalent percentage of HCO₃ ⁻ (97.4%) with not very high milliequivalent percentage of Ca²⁺ (24.4%); and (4) a large saturation index of calcite and aragonite of the hot water.     

High-grade metamorphism, dehydration and crustal melting: a reinvestigation based on new experiments in the silica-saturated portion of the system KAlO2–MgO–SiO2–H2O–CO2 at P≤ 1.5 GPa

The system KAlO₂–MgO–SiO₂–H₂O–CO₂ has long been used as a model for the processes of granulite-facies metamorphism and the development of orthopyroxene-bearing mineral assemblages through the breakdown of biotite-bearing assemblages. There has been considerable controversy regarding the role of carbon dioxide in metamorphism and partial melting. We performed new experiments in this system (at pressures of 342 to 1500 MPa with T between 710 and 1045 °C and X ^Fl _H2O between 0.05 and 1.00), accurately locating most of the dehydration and melting equilibria in P-T-X ^Fl _H2O space. The most important primary result is that the univariant reaction Phl + Qtz + Fl = En + Sa + melt must be almost coincident with the fluid-absent reaction (Phl + Qtz = En + Sa + melt) in the CO₂-free subsystem. In conjunction with the results of previous measurements of CO₂ solubility in silicate melts and phase equilibrium experiments, our theoretical analysis and experiments suggest that CO₂ cannot act as a flux for partial melting. Crustal melting in the presence of H₂O–CO₂ mixed fluids will always occur at temperatures higher than with pure H₂O fluid present. Magmas produced by such melting will be granitic (s.l.) in composition, with relatively high SiO₂ and low MgO contents, irrespective of the H₂O–CO₂ ratio in any coexisting fluid phase. We find no evidence that lamprophyric magmas could be generated by partial fusion of quartz-saturated crustal rocks. The granitic melts formed will not contain appreciable dissolved CO₂. The channelled passage of hot CO₂-rich fluids can cause local dehydration of the rocks through which they pass. In rock-dominated (as opposed to fluid-dominated) systems, minor partial melting can also occur in veins initially filled with CO₂-rich fluid, as dehydration and local disequilibrium drive the fluid towards H₂O-rich compositions. However, CO₂ is unlikely to be a significant agent in promoting regional granulite-grade metamorphism, melting, magma generation, metasomatism or long-range silicate mass transfer in Earth's crust. The most viable model for the development of granulite-facies rocks involves the processes of fluid-absent partial melting and withdrawal of the melt phase to higher crustal levels. Received: 28 November 1996 / Accepted: 25 June 1997     

Biogeochemistry of the Russian Arctic. Kara Sea: Research results under the SIRRO project, 1995–2003

The Kara Sea is an area uniquely suitable for studying processes in the river-sea system. This is a shallow sea into which two great Siberian rivers, Yenisei and Ob, flow. From 1995 to 2003, the sea was studied by six international expeditions aboard the R/V Akademik Boris Petrov. This publication summarizes the results obtained, within the framework of this project, at the Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. Various hydrogeochemical parameters, concentrations and isotopic composition of organic and carbonate carbon of the sediments, plankton, particulate organic matter, hydrocarbons, and dissolved CO₂ were examined throughout the whole sea area at more than 200 sites. The δ¹³C varies from −22 and −24‰ where Atlantic waters enter the Kara Sea and in the North-eastern part of the water area to −27‰ in the Yenisei and Ob estuaries. The value of δ¹³C of the plankton is only weakly correlated with the δ¹³C of the organic matter from the sediments and is lower by as much as 3–4‰. The paper presents the results obtained from a number of meridional river-sea profiles. It was determined from the relations between the isotopic compositions of plankton and particulate matter that the riverwaters carry material consisting of 70% detrital-humus matter and 30% planktonogenic material in the river part, and the material contained in the off-shore waters consists of 30% terrigenous components, with the contribution of bioproducers amounting to 70%. The carbon isotopic composition of the plankton ranges from −29 to −35‰ in the riverine part, from −28 to −27‰ in the estuaries, and from −27.0 to −25% in the marine part. The relative lightness of the carbon isotopic composition of plankton in Arctic waters is explained by the temperature effect, elevated CO₂ concentrations, and long-distance CO₂ supply to the sea with riverwaters. The data obtained on the isotopic composition of CO₂ in the surface waters of the Kara Sea were used to map the distribution of δ¹³C_{CO 2}. The complex of hydrocarbon gases extracted from the waters included methane, C₂–C₅, and unsaturated C₂₌–C₄₌ hydrocarbons, for which variations in the concentrations in the waters were studied along river-estuary-sea profiles. The geochemistry of hydrocarbon gases in surface fresh waters is characterized by comparable concentrations of methane (0.3–5 μl/l) and heavier hydrocarbons, including unsaturated ones. Microbiological methane with δ¹³C from −105 to −90‰ first occurs in the sediments at depths of 40–200 cm. The sediments practically everywhere display traces of methane oxidation in the form of a shift of the δ¹³C of methane toward higher values and the occurrence of autogenic carbonate material, including ikaite, enriched in the light isotope. Ikaite (δ¹³C from −25 to −60‰) was found and examined in several profiles. The redox conditions in the sediments varied from normal in the southern part of the sea to highly oxidized along the Novaya Zemlya Trough. Vertical sections through the sediments of the latter exemplify the complete suppression of the biochemical activity of microorganisms. Our data provide insight into the biogeochemistry of the Kara Sea and make it possible to specify the background values needed for ecological control during the future exploration operations and extraction of hydrocarbons in the Kara Sea. Original Russian Text ? E.M. Galimov, L.A. Kodina, O.V. Stepanets, G.S. Korobeinik, 2006, published in Geokhimiya, 2006, No. 11, pp. 1139–1191.     

Sodic(–calcic) alteration in Fe-oxide–Cu–Au districts: an origin via unmixing of magmatic H2O–CO2–NaCl ± CaCl2–KCl fluids

Iron-oxide–Cu–Au deposits, particularly those formed in deeper level (plutonic) environments, are commonly characterized by regional scale sodic(–calcic) alteration, which typically formed pre- or syn-Cu–Au mineralization. The sodic(–calcic) assemblages include albite, scapolite, pyroxene, actinolite, apatite, titanite, epidote and calcite. The consistent presence of coexisting hypersaline aqueous and CO₂-rich fluids in minerals from sodic(–calcic) alteration and associated Fe-oxide–Cu–Au deposits is the result of unmixing of H₂O–CO₂–NaCl ± CaCl₂–KCl magmatic fluids. Experimental evidence indicates that the Na/(Na + K) ratio of fluids in equilibrium with two alkali feldspars in CO₃ ²⁻-bearing parent fluids would be significantly higher than in unmixed chloride-bearing aqueous fluids. Therefore, fluid unmixing caused by decreases in temperature and/or pressure, will result in albitization of wall rocks, as is observed in most deeper level Fe-oxide–Cu–Au deposits. This alteration style may be succeeded by K-feldspathization with decreasing temperature because of the increase in equilibrium Na/(Na + K) in chloride-bearing fluids buffered by alkali feldspars. Received: 26 May 1999 / Accepted: 8 June 2000