O,H,and Sr isotope evidence for origin and mixing processes of the Gudui geothermal system,Himalayas,China

Tho Gudui geothermal field records the highest temperature at equivalent borehole depths among the lainland hydrothermal systems in mainland China.Located about 150 km southeast of Lhasa City,the capital of Tibet,the Gudui geothermal field belongs to the Sangri-Cuona rift belt,also known as the Sangri-Cuona geothermal belt,and is representative of the non-volcanic geothermal systems in the Himalayas.In this study,oxygen-18 and deuterium isotope compositions as well as ~(87)Sr/~(86)Sr ratios of water samples collected from the Gudui geothermal field were characterized to understand the origin and mixing processes of the geothermal fluids at Gudui.Hydrogen and oxygen isotope plots show both,deep and shallow reservoirs in the Gudui geothermal field.Deep geothermal fluids are the mixing product of magmatic and infiltrating snow-melt water.Calculations show that the magma fluid component of the deep geothermal fluids account for about 21.10%-24.04%;magma fluids lay also be a contributing source of lithium.The linear relationship of the ~(87)Sr/~(86)Sr isotopic ratio versus the 1/Sr plot indicates that shallow geothermal fluids form from the mixing of deep geothermal fluids with cold groundwater.Using a binary mixing model with deep geothermal fluid and cold groundwater as two end-members,the nixing ratios of the latter in most surface hot springs samples were calculated to be between 5% and 10%.Combined with basic geological characteristics,hydrogen and oxygen isotope characteristics,strontium concentration,~(87)Sr/(86)Sr ratios,and the binary mixing model,we infer the 6 th-Class Reservoirs Evolution Conceptual Model(6-CRECM) for the Gudui geothermal system.This model represents an idealized summary of the characteristics of the Gudui geothermal field based on our comprehensive understanding of the origin and mixing processes of the geothermal fluid in Gudui.This study may aid in identifying the geothermal and geochemical origin of the Gudui high-temperature hydrothermal systems in remote Tibet of China,whose potential for geothermal development and utilization is enormous and untapped.     

西藏羊八井地热田硼同位素地球化学特征初步研究

基于国外大量硼同位素示踪的地热研究实例,在先期建立的MC-ICP-MS测定水中硼同位素分析方法基础上,以羊八井地热田为研究对象,进行了热田地热流体的硼同位素地球化学初步研究。研究表明,羊八井地热田区热储流体的硼同位素值为 -10.5‰～-9.1‰,为非海相来源; 结合区域地质特征,研究认为热储内的硼组分来源于蚀变花岗岩围岩,并且蚀变花岗岩的硼同位素特征可能与深层地热流体相似。研究表明羊八井浅层热储内硼元素的地球化学行为并非完全是保守元素,存在着一定的硼同位素分馏。在端元硼同位素特征差异较小的羊八井热田,为硼同位素二元混合模型示踪水体混合过程带来了相当的复杂性。本文从热田开采过程与采样时间、水-岩相互作用、气-液相分离以及结垢过程等方面分析了浅层热储内引起硼同位素分馏的可能因素,其中从整体来看,气-液相分离过程的影响相对较大;而在个别井位水-岩相互作用与结垢的影响可能相对较为显著。     

Mixing of geothermal and non-geothermal fluids in shallow aquifers in the Germencik-Nazilli area,Büyük Menderes Basin (SW Turkey)

This study observes groundwater hydrochemical characteristics during mixing between geothermal and non-geothermal fluids in Germencik–Nazilli area in the Büyük Menderes Basin (SW Turkey). Hydrogeochemical features of 32 non-geothermal, geothermal and surface samples were studied. The mean temperatures of the geothermal reservoirs are calculated to be 150–240 °C in Germencik field, based on Na-K-Mg geothermometry. Hydrochemical characteristics of Germencik geothermal fluid differ from non-geothermal fluids, mainly Na-Cl-HCO₃-type geothermal fluid, while non-geothermal fluid is mostly Ca-Mg-HCO₃-SO₄ type. High contents of some minor elements in geothermal fluids are most likely sourced from prolonged water-rock interaction, reflecting the signals of flow paths and residence times. A mass-balance approach was used to calculate mixing ratios between geothermal and non-geothermal fluids based on B, Cl and Na concentrations. Germencik field is considerably characterised by rising geothermal fluids and overlying non-geothermal fluids. The amount of water stored in the Quaternary aquifer evolved from a deep thermal source is low in Germencik (.5–40% geothermal fluid in non-geothermal wells). Mixing between geothermal and non-geothermal fluids has been caused by groundwater utilisation practices and is increased close to active faults. Irrigation of the shallow groundwater composition is considered as influx of low-temperature geothermal fluid.     

西藏羊八井地热田热水的化学组成

羊八井地热田深、浅层热水都是Cl-Na类型,具有相同的B/Cl比值,说明深层热水在上升通道中与冷水相混合形成了浅层热水。浅层流体自西北向东南流动,温度逐渐降低。浅层热储内普遍存在着水岩交换反应,对热水的化学组成有一定的影响。石英和玉髓地热温度计分别适用于计算深、浅层的热储温度。纳木错(湖)不是羊八井地热田的补给区。深层热水在井筒内绝热汽化时不会出现SiO₂结垢,CaCO₃是否会在井筒壁沉淀需要放喷较长时间来检验。文中还阐述了对热水的化学组分进行监测的必要性。     

Spatial characteristics of groundwater temperature in the Ishikari Lowland, Hokkaido, northern Japan: analytical and numerical applications

In porous sediments of the Ishikari Lowland, there is a gradual increase in the background geothermal gradient from the Ishikari River (3–4 °C 100 m^–1) to the southwest highland area (10 °C 100 m^–1). However, the geothermal gradient at shallow depths differs in detail from the background distribution. In spite of convective heat-flow loss generally associated with groundwater flow, heat flow remains high (100 mW m^–2) in the recharge area in the southwestern part of the Ishikari basin, which is part of an active geothermal field. In the northeastern part of the lowland, heat flow locally reaches 140 mW m^–2, probably due to upward water flow from the deep geothermal field. Between the two areas the heat flow is much lower. To examine the role of hydraulic flow in the distortion of the isotherms in this area, thermal gradient vs. temperature analyses were made, and they helped to define the major components of the groundwater-flow system of the region. Two-dimensional simulation modeling aided in understanding not only the cause of horizontal heat-flow variations in this field but also the contrast between thermal properties of shallow and deep groundwater reservoirs. Electronic Publication     

The use of O,H, B,Sr and S isotopes for tracing the origin of dissolved boron in groundwater in Central Macedonia,Greece

The groundwater B concentration in Mesozoic karst, Neogene and alluvial aquifers in the West part of Chalkidiki province in Central Macedonia, Greece reaches 6.45 mg L⁻¹, which exceeds the limit of 1 mg L⁻¹, set by the European Union for drinking water. The high B contents have been detected in this area, not only near the shoreline, where seawater intrusion occurs, but also in the inland part of the basin. Multi isotope (²H, ¹⁸O, ³⁴S, ¹⁸O_(SO4), ¹¹B, ⁸⁷Sr/⁸⁶Sr) data from borehole and thermal water springs allow identification of the possible B sources. The B dissolved in groundwater in the Chalkidiki area is mainly geogenic. The low δ¹¹B values, 0–1‰, similar to those of thermal fluids from continental geothermal fields, and the low Cl/B ratio compared to seawater both indicate a geothermal origin for B and reflect deep circulation and interaction with igneous rocks. The ⁸⁷Sr/⁸⁶Sr ratio also indicates that the deep-aquifer granodiorite is the predominant rock source of Sr, while the shallow limestone unit has negligible effects on the dissolved Sr budget in these thermal karst waters which O and H isotopes show to be of meteoric origin. The main source of high B in borehole water is mainly mixing with B-rich geothermal water. The mixing between geothermal water and water from the Neogene aquifer is also reflected by isotopic contents of SO₄.     

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.     

Ba/Sr,Ca/Sr and 87Sr/86Sr ratios in soil water and groundwater: implications for relative contributions to stream water discharge

Barium/Sr and Ca/Sr ratios have been used to model the relative importance of different sources of stream water. Major and trace element concentrations together with ⁸⁷Sr/⁸⁶Sr ratios were measured in precipitation, soil water, groundwater and stream water in a small (9.4 km²) catchment in northern Sweden. The study catchment is drained by a first order stream and mainly covered with podzolized Quaternary till of granitic composition. It is underlain by a 1.8 Ga granite. A model with mixing equations used in an iterative mode was developed in order to separate the stream water into 3 subsurface components: soil water, shallow groundwater, and deep groundwater. Contributions from precipitation are thus not included in the model. This source may be significant for the stream water generation, but it does not interfere with the calculations of the relative contributions from the subsurface components. The results show that the deep groundwater constitutes between 5 and 20% of the subsurface water discharge into the stream water. The highest values of the deep groundwater fraction occur during base flow. Soil water dominates during snowmelt seasons, whereas during base flow it is the least important fraction. Soil water accounts for 10–100% of the subsurface water discharge into the stream water. Shallow groundwater accounts for up to 80% of the subsurface water discharge with the lowest values at peak discharge during snowmelt seasons and the highest values during base flow. The validity of the model was tested by comparing the measured ⁸⁷Sr/⁸⁶Sr ratios in the stream water with the ⁸⁷Sr/⁸⁶Sr ratios predicted by the model. There was a systematic difference between the measured and modelled ⁸⁷Sr/⁸⁶Sr ratios which suggests that the fraction of soil water is overestimated by the model, especially during spring flood. As a consequence of this overestimation of soil water the amount of shallow groundwater is probably underestimated during this period. However, it is concluded that the differences between measured and predicted values are relatively small, and that element ratios are potentially effective tracers for different subsurface water flowpaths in catchments.     

北京北部地区深层热水开发对浅层冷水的影响

北京北部有小汤山和沙河2个地热田,呈三角形展布,东部边界为黄庄-高丽营断裂,西部边界为南口-孙河断裂．北部边界为阿苏卫-小汤山断裂。热储层为蓟县系雾迷山组、铁岭组和寒武系-奥陶系碳酸盐岩岩溶裂隙含水层．热储盖层为青白口系页岩、石炭系-二叠系砂页岩和侏罗系火山岩隔水层。该区雨水、浅层基岩冷水和深层基岩热水的H、O同位素组成基本上都落在克雷格降水线上,表明区内冷水与热水均来源于大气降水。热水的^3H值表现出北高南低的特点．说明热水与冷水一样自北向南流动。重点分析了深层热水开采对浅层地震观测井中冷水动态的影响,以及这种影响在不同的水文地质条件、离开采井不同距离和不同测项方面表现出的差异。结果表明,北京北部深层热水开采对浅层冷水动态的影响距离约为5km．对位于导水断裂带附近的观测井的影响最为明显。     

Br/Cl ratio,Zn and radon constraints on the origin and fate of geothermal fluids in the coastal region of southeastern China

Low-to-medium temperature geothermal fluids in the granite regions of southeastern China are an important renewable energy resource, but they are also a source of contamination containing highly toxic elements such as fluoride and arsenic. This study analyzed the origin of the geothermal fluids in a regional-scale hydrogeological unit in the city of Xiamen, China, based on isotope and hydrochemical analyses. The Br/Cl ratios suggested that the inland geothermal fluid is merely recharged by rainwater from the mountain edge, while the coastal geothermal fluid is originally recharged by the seawater and later mixed with rain-derived groundwater. The geothermal water featured high SiO₂ and detectable Zn concentrations. The former reflects the significant water–granite interaction along the flow path, and the latter indicates the active hydraulic connection between surface waters, shallow aquifers and deep geothermal fluids. High radon content was detected near the deep conductive fault adjacent to a geothermal well, demonstrating that the fault damage zone acts as a major conduit for upward transport of the deep geothermal fluid. As a result, the fault damage zones developed in the granite are necessary for the formation of geothermal water, which leads to the uneven distribution of geothermal water in the subsurface. High-temperature geothermal water can be found in those regions with fairly sparse fault damage zones. In contrast, in the region with high-density fault activities, the active communication between shallow cool water and deep geothermal fluids can decrease the water temperature.

     

Origin of groundwater salinity (current seawater vs. saline deep water) in a coastal karst aquifer based on Sr and Cl isotopes. Case study of the La Clape massif (southern France)

In this study a typical coastal karst aquifer, developed in lower Cretaceous limestones, on the western Mediterranean seashore (La Clape massif, southern France) was investigated. A combination of geochemical and isotopic approaches was used to investigate the origin of salinity in the aquifer. Water samples were collected between 2009 and 2011. Three groundwater groups (A, B and C) were identified based on the hydrogeological setting and on the Cl⁻ concentrations. Average and maximum Cl⁻ concentrations in the recharge waters were calculated (Cl_Ref. and Cl_Ref.Max) to be 0.51 and 2.85 mmol/L, respectively). Group A includes spring waters with Cl⁻ concentrations that are within the same order of magnitude as the Cl_Ref concentration. Group B includes groundwater with Cl⁻ concentrations that range between the Cl_Ref and Cl_Ref.Max concentrations. Group C includes brackish groundwater with Cl⁻ concentrations that are significantly greater than the Cl_Ref.Max concentration. Overall, the chemistry of the La Clape groundwater evolves from dominantly Ca–HCO₃ to NaCl type. On binary diagrams of the major ions vs. Cl, most of the La Clape waters plot along mixing lines. The mixing end-members include spring waters and a saline component (current seawater or fossil saline water). Based on the Br/Cl_molar ratio, the hypothesis of halite dissolution from Triassic evaporites is rejected to explain the origin of salinity in the brackish groundwater.Groundwaters display ⁸⁷Sr/⁸⁶Sr ratios intermediate between those of the limestone aquifer matrix and current Mediterranean seawater. On a Sr mixing diagram, most of the La Clape waters plot on a mixing line. The end-members include the La Clape spring waters and saline waters, which are similar to the deep geothermal waters that were identified at the nearby Balaruc site. The ³⁶Cl/Cl ratios of a few groundwater samples from group C are in agreement with the mixing hypothesis of local recharge water with deep saline water at secular equilibrium within a carbonate matrix. Finally, PHREEQC modelling was run based on calcite dissolution in an open system prior to mixing with the Balaruc type saline waters. Modelled data are consistent with the observed data that were obtained from the group C groundwater. Based on several tracers (i.e. concentrations and isotopic compositions of Cl and Sr), calculated ratios of deep saline water in the mixture are coherent and range from 3% to 16% and 0% to 3% for groundwater of groups C and B, respectively.With regard to the La Clape karst aquifer, the extension of a lithospheric fault in the study area may favour the rise of deep saline water. Such rises occur at the nearby geothermal Balaruc site along another lithospheric fault. At the regional scale, several coastal karst aquifers are located along the Gulf of Lion and occur in Mezosoic limestones of similar ages. The ⁸⁷Sr/⁸⁶Sr ratios of these aquifers tend toward values of 0.708557, which suggests a general mixing process of shallow karst waters with deep saline fossil waters. The occurrence of these fossil saline waters may be related to the introduction of seawater during and after the Flandrian transgression, when the highly karstified massifs invaded by seawater, formed islands and peninsulas along the Mediterranean coast.     

Hydrogeochemistry of thermal and mineralized waters in the Diyadin (Ağri) area,Eastern Turkey

The Diyadin Geothermal area, located in the eastern part of Anatolia (Turkey) where there has been recent volcanic activity, is favorable for the formation of geothermal systems. Indeed, the Diyadin geothermal system is located in an active geodynamic zone, where strike-slip faults and tensional cracks have developed due to N–S regional compression. The area is characterized by closely spaced thermal and mineralized springs, with temperatures in the range 30–64 °C, and flowrates 0.5–10 L/s. Thermal spring waters are mainly of Ca(Na)-HCO₃ and Ca(Mg)-SO₄ types, with high salinity, while cold groundwater is mostly of Ca(Na, Mg)-HCO₃ type, with lower salinity. High contents of some minor elements in thermal waters, such as F, B, Li, Rb, Sr and Cs probably derive from enhanced water–rock interaction.Thermal water samples collected from Diyadin are far from chemical equilibrium as the waters flow upward from reservoirs towards spring vents and possibly mix with cooler waters. The temperatures of the deep geothermal reservoirs are estimated to be between 92 and 156 °C in Diyadin field, based on quartz geothermometry, while slightly lower estimates are obtained using chalcedony geothermometers. The isotopic composition of thermal water (δ¹⁸O, δ²H, δ³H) indicates their deep-circulating meteoric origin. The waters are likely to have originated from the percolation of rainwater along fractures and faults to the deep hot reservoir. Subsequent heating by conduction due to the presence of an intrusive cupola associated with the Tendurek volcano, is followed by the ascent of deep waters to the surface along faults and fractures that act as hydrothermal conduits.Modeling of the geothermal fluids indicates that the fluid is oversaturated with calcite, aragonite and dolomite, which matches travertine precipitation in the discharge area. Likewise, the fluid is oversaturated with respect to quartz, and chalcedony indicating the possibility of siliceous precipitation near the discharge areas. A conceptual hydro-geochemical model of the Diyadin thermal waters based on the isotope and chemical analytical results, has been constructed.     

高温热田深部母地热流体的温度计算及其升流后经历的冷却过程: 以腾冲热海热田为例

高温地热系统中赋存着大量的地热能资源.为了进一步了解高温地热系统, 以腾冲热海热田为典型研究区, 利用热泉地球化学组成, 基于多种地球化学模型确定了热田深部母地热流体的温度, 并分析了其升流后经历的不同冷却过程.热海热田的硫磺塘水热区和热水塘水热区所排泄的热泉源自共同的深部热储, 该热储中母地热流体的Cl-质量浓度为265 mg/L, 温度为336 ℃.在热海热田, 母地热流体在经历绝热冷却过程后直接形成了泉口温度最高的大滚锅泉, 而其他中性泉均由母地热流体先与浅部地下冷水混合再经历绝热冷却形成.母地热流体的深部热储之上存在多个温度在200 ℃以上的热储, 这些热储的形成受控于热海地区发育的多组方向不同的断裂.      

Comparison of the Sr isotopic signatures in brines of the Canadian and Fennoscandian shields

A synthesis of Sr isotope data from shallow and deep groundwaters, and brines from the Fennoscandian and Canadian Shields is presented. A salinity gradient is evident in the water with concentrations varying from approximately 1–75 g L⁻¹ below 1500 m depth in the Fennoscandian Shield and from 10 up to 300 g L⁻¹ below 650 m depth in the Canadian Shield. Strontium isotope ratios were measured to assess the origin of the salinity and evaluate the degree of water–rock interaction in the systems. In both shields, the Sr concentrations are enriched relative to Cl, defining a positive trend parallel to the seawater dilution line and indicative of Sr addition through weathering processes. The depth distribution for Sr concentration increases strongly with increasing depth in both shields although the variation in Sr-isotope composition does not mirror that of Sr concentrations. Strontium-isotope compositions are presented for surface waters, and groundwaters in several sites in the Fennoscandian and Canadian Shields. Numerous mixing lines can be drawn reflecting water–rock interaction. A series of calculated lines links the surface end-members (surface water and shallow groundwater) and the deep brines; these mixing lines define a range of ⁸⁷Sr/⁸⁶Sr ratios for the deep brines in different selected sites. All sites show a specific ⁸⁷Sr/⁸⁶Sr signature and the occurrence of large ⁸⁷Sr/⁸⁶Sr variations is site specific in both shields. In Canadian Shield brines, the Sr isotope ratios clearly highlight large water rock interaction that increases the ⁸⁷Sr/⁸⁶Sr ratio from water that could have been of marine origin. In contrast to the Canadian Shield, groundwater does not occur in closed pockets in the Fennoscandian, and the well-constrained ⁸⁷Sr/⁸⁶Sr signatures in deep brines should correspond to a large, well-mixed and homogeneous water reservoir, whose Sr isotope signature results from water–rock interaction.     

Fluid-rock interactions in a geothermal Rotliegend/Permo-Carboniferous reservoir (North German Basin)

Comprehensive data on the chemical composition of reservoir rocks and geothermal brines from the geothermal well doublet Groβ Schönebeck (North German Basin) drilled into a Rotliegend sedimentary and Permo-Carboniferous volcanic rock reservoir were sampled over the past years. They were characterized with respect to their major and minor elemental composition including various isotope ratios. The study considered the impact of drilling and reservoir operations on fluid composition and aimed at determining fluid–rock interactions to gain information on fluid origin and hydraulic pathways.The highly saline fluids (up to 265 g/L TDS) show δ ¹⁸O and δD of water (2.7–5.6 and −3.1–15, respectively) as well as δ ³⁴S of sulfate (3.6–5), and ⁸⁷Sr/⁸⁶Sr ratios (0.715–0.716) that resemble Rotliegend brines from an area located around 200 km in the west (the Altmark). Halogen ratios indicated that brines developed predominantly by evaporation of meteoric water (primary brine) together with halite dissolution brine (secondary brine). Indication for mixing with Zechstein brine or with younger meteoric water was not found.No geochemical distinction was possible between fluids deriving from different rock formations (dacites or sedimentary rocks, respectively). This is due to the evolution of the sediments from the effusive rocks resulting in a similar mineralogical and chemical composition and due to a hydraulic connectivity between the two types of rock. This connection existed probably already before reservoir stimulation as indicated by a set of faults identified in the area that could connect the Rotliegend formation with both, the volcanic rocks and the lower units of the Zechstein. Additional geochemical indication for a hydraulic connectivity is given by (1) the very high heavy metal contents (mainly Cu and Pb) in fluids and scaling that derive from the volcanic rocks and were that were also found in increased amounts up at the Zechstein border (Kupferschiefer formation). (2) The ⁸⁷Sr/⁸⁶Sr isotope ratios of fluid samples correspond to the ratios determined for the sedimentary rocks indicating that initially the fluids developed in the sedimentary rocks and circulated later, when faults structures were created by tectonic events into the volcanic rocks.     

Mixing geochemistry of cold water around non-volcanic thermal springs in high-grade metamorphic terrain,Sri Lanka

A geochemical survey was conducted on thermal water and cold water around non-volcanic geothermal fields at Mahaoya and Marangala in Sri Lanka. One hundred forty-two samples were analyzed for fifteen selected irons to investigate geochemical relationships resulting from water-rock interactions and mixing. Based on measurements using a Na-K-Mg geothermometer, the maximum temperatures of thermal reservoirs were estimated to be 148 °C in Mahaoya and 191 °C in Marangala, which were higher values than those obtained using Na-Li and Li-Mg geothermometers. This suggests that the reservoirs extend from intermediate to deep levels. Hydrogeochemistry of thermal waters is distinct from that of cold water; higher contents of Na, K, Cl, F, SO_4, and TDS in thermal water are most likely due to the dissolution of feldspar, mica, and sulfide minerals in the granitic rocks. Conversely, lower values of Fe, Mg, Cu, Zn, and Pb imply less ferromagnesian minerals in the basement. Classification based on major ions reveals a Na-K-SO₄ type of thermal water for Mahaoya and Marangala. Cold water is dominated by the Na-K-HCO₃ type, which indicates deep groundwater influence by iron exchange. Non-mixing cold water indicates a Ca-HCO₃ type. In general, chemistry of cold water wells (<400 m) close to the thermal water changes significantly due to direct mixing of thermal water and cold water. In comparison, a contrasting action occurs with increasing distance from the geothermal field. Gradual decline of SO₄ with increasing distance from thermal water may indicate a trend of clear oxidation. However, the chemistry of more distant wells demarcates deep circulations through fractures and faults in the basement.     

Isotopic evidence (Sr/Sr, δLi) for alteration of the oceanic crust at deep-rooted mud volcanoes in the Gulf of Cadiz, NE Atlantic Ocean

The chemical and isotopic composition of pore fluids is presented for five deep-rooted mud volcanoes aligned on a transect across the Gulf of Cadiz continental margin at water depths between 350 and 3860 m. Generally decreasing interstitial Li concentrations and ⁸⁷Sr/⁸⁶Sr ratios with increasing distance from shore are attributed to systematically changing fluid sources across the continental margin. Although highest Li concentrations at the near-shore mud volcanoes coincide with high salinities derived from dissolution of halite and late-stage evaporites, clayey, terrigenous sediments are identified as the ultimate Li source to all pore fluids investigated. Light δ⁷Li values, partly close to those of hydrothermal vent fluids (δ⁷Li: +11.9‰), indicate that Li has been mobilized during high-temperature fluid/sediment or fluid/rock interactions in the deep sub-surface. Intense leaching of terrigenous clay has led to radiogenic ⁸⁷Sr/⁸⁶Sr ratios (0.7106) in pore fluids of the near-shore mud volcanoes. In contrast, non-radiogenic ⁸⁷Sr/⁸⁶Sr ratios (0.7075) at the distal locations are attributed to admixing of a basement-derived fluid component, carrying an isotopic signature from interaction with the basaltic crust. This inference is substantiated by temperature constraints from Li isotope equilibrium calculations suggesting exchange processes at particularly high temperatures (>200 °C) for the least radiogenic pore fluids of the most distal location.Advective pore fluids in the off-shore reaches of the Gulf of Cadiz are influenced by successive exchange processes with both oceanic crust and terrigenous, fine-grained sediments, resulting in a chemical and isotopic signature similar to that of fluids in near-shore ridge flank hydrothermal systems. This suggests that deep-rooted mud volcanoes in the Gulf of Cadiz represent a fluid pathway intermediate between mid-ocean ridge hydrothermal vent and shallow, marginal cold seep. Due to the thicker sediment coverage and slower fluid advection rates, the overall geochemical signature is shifted towards the sediment-diagenetic signal compared to ridge flank hydrothermal environments.     

西藏羊八井热田地热流体成因及演化的惰性气体制约

地热流体中惰性气体的相对丰度和同位素组成，不仅可以揭露热田的热源性质，而且还能够揭示深，浅层地热流体的内在联系和演化过程等。在西藏羊八井热田的地热气体中，已检测出大量的^4He组分，3He/^4He值是大气的0．087－0．259倍，表明深部地壳物质的局部熔融为热田提供能量，浅层地热流体的3He/4He 值自西北向东南呈降低趋势，与热储温度的变化相一致，反映出侧向运移时补充了更多的壳源氦，热田北区深层地热流体具有稍高的3He/4He值，是浅层地热流体的母源，气体中氪和氙的相对丰度具有大气降水成因的特征，结合现有的实际资料，建立了热田地热流体的概念模型。     

Iodine budget in surface waters from Atacama: Natural and anthropogenic iodine sources revealed by halogen geochemistry and iodine-129 isotopes

Iodine enrichment in the Atacama Desert of northern Chile is widespread and varies significantly between reservoirs, including nitrate-rich “caliche” soils, supergene Cu deposits and marine sedimentary rocks. Recent studies have suggested that groundwater has played a key role in the remobilization, transport and deposition of iodine in Atacama over scales of millions-of-years. However, and considering that natural waters are also anomalously enriched in iodine in the region, the relative source contributions of iodine in the waters and its extent of mixing remain unconstrained. In this study we provide new halogen data and isotopic ratios of iodine (¹²⁹I/I) in shallow seawater, rivers, salt lakes, cold and thermal spring water, rainwater and groundwater that help to constrain the relative influence of meteoric, marine and crustal sources in the Atacama waters. Iodine concentrations in surface and ground waters range between 0.35 μM and 26 μM in the Tarapacá region and between 0.25 μM and 48 μM in the Antofagasta region, and show strong enrichment when compared with seawater concentrations (I = ∼0.4 μM). In contrast, no bromine enrichment is detected (1.3–45.7 μM for Tarapacá and 1.7–87.4 μM for Antofagasta) relative to seawater (Br = ∼600 μM). These data, coupled to the high I/Cl and low Br/Cl ratios are indicative of an organic-rich sedimentary source (related with an “initial” fluid) that interacted with meteoric water to produce a mixed fluid, and preclude an exclusively seawater origin for iodine in Atacama natural waters. Iodine isotopic ratios (¹²⁹I/I) are consistent with halogen chemistry and confirm that most of the iodine present in natural waters derives from a deep initial fluid source (i.e., groundwater which has interacted with Jurassic marine basement), with variable influence of at least one atmospheric or meteoric source. Samples with the lowest isotopic ratios (¹²⁹I/I from ∼215 to ∼1000 × 10⁻¹⁵) strongly suggest mixing between the groundwater and iodine storage in organic-rich rocks (with variable influence of volcanic fluids) and pre-anthropogenic meteoric water, while samples with higher values (∼2000–93,700 × 10⁻¹⁵) indicate the input of anthropogenic meteoric fluid. Taking into account the geological, hydrologic and climatic features of the Atacama region, we propose that the mean contribution of anthropogenic ¹²⁹I is associated with ¹²⁹I releases during nuclear weapon tests carried out in the central Pacific Ocean until the mid 1990's (¹²⁹I/I = ∼12,000 × 10⁻¹⁵). This source reflects rapid redistribution of this radioisotope on a global scale. Our results support the notion of a long-lived continental iodine cycle in the hyperarid margin of western South America, which is driven by local hydrological and climate conditions, and confirm that groundwater was a key agent for iodine remobilization and formation of the extensive iodine-rich soils of Atacama.     

Salinization of groundwater in the North German Basin: results from conjoint investigation of major, trace element and multi-isotope distribution

Conjoint consideration of distribution of major, rare earth elements (REE) and Y (combined to REY) and of H, O, C, S, Sr isotopes reveals that four types of groundwater are distinguishable by their chemical composition presented by spider patterns. REY patterns indicate thermo-saline deep water and two types of shallow saline groundwaters. Presence of connate waters is not detectable. Sr isotope ratios distinguish three sources of Sr: fast and slow weathering of biotite and K-feldspar in Pleistocene sediments, respectively, and dissolution of limestones. δ¹³C(DIC) indicate dissolution of limestone under closed and open system conditions. Numerous samples show δ¹³C(DIC) > 13‰ which is probably caused by incongruent dissolution of calcite and dolomite. The brines from below 1,000 m represent mixtures of pre-Pleistocene seawater or its evaporation brines and infiltrated post-Pleistocene precipitation. The shallow waters represent mixtures of Pleistocene and Recent precipitation salinized by dissolution of evaporites or by mixing with ascending brines. The distribution of water types is independent on geologic units and lithologies. Even the Tertiary Rupelian aquiclude does not prevent salinization of the upper aquifer.