Evidence for SiO2‐NaCl complexing in H2O‐NaCl solutions at high pressure and temperature

Experimental studies reveal complex dissolution behavior of quartz in aqueous NaCl solutions at high temperature and pressure, involving variation from salting‐in to salting‐out that changes with temperature, pressure, and salt concentration. The behavior is not explainable by traditional electrostatic theory. An alternative hypothesis appeals to complexing of SiO₂ with NaCl and can explain the observations. However, the hypothesis of complexing, as previously applied, is inadequate in several respects: it neglects polymerization of solute silica, regards the SiO₂‐NaCl hybrid complex(es) as anhydrous, which seems unlikely, and invokes an incorrect stoichiometry of the hydrated silica monomer, now known to be Si(OH)₄?2H₂O. These neglected features can be incorporated into the complexing model in a revised formulation based on a simple thermodynamic analysis using existing quartz solubility data. The analysis leads to a quasi‐ideal solution model with silica monomers, dimers, and two distinct hydrous SiO₂‐NaCl hybrid complexes with overall NaCl:H₂O = 1:6, one Na‐bearing and one Cl‐bearing. Their (equal) molar concentrations (X_hc) are governed by a pressure‐ and temperature‐dependent equilibrium constant, , where a_Nacl and are the respective activities of the solvent components. The stability of the hybrid complexes (i.e., their concentration) is very sensitive to H₂O activity. The entire set of experimental quartz‐solubility data at 700°C, 1–15 kbar, is reproduced with high fidelity by the expression (P is pressure in kbar), including the transition from low‐pressure salting‐in to high pressure salting‐out. The results indicate that hybrid SiO₂‐NaCl complexes are the main hosts for dissolved silica at NaCl concentrations greater than 6 wt%, which are likely common in crustal fluids. At higher temperatures, approaching the critical end point in the system SiO₂‐H₂O, the model becomes progressively inaccurate, probably because polymers higher than the dimer become significant as SiO₂ concentration increases.     

Diffusion and kinetic control of weathering layer development

The formation of porous weathering rinds (layers of chemical alteration) on the exterior of rocks is a consequence of dissolution and precipitation of minerals occurring at the mineral–fluid interface within the pores. The speed at which the developed rind advances is controlled by both kinetic reaction rates and the transport of reaction products away from the pore spaces into the outside fluid. We show, using both reaction‐diffusion theory and numerics, that under diffusion limitations, the weathering rate depends on the size and curvature of the sample. This leads to a relationship between rind thickness, δ, and age, t. As the rind thickens, the result in three dimensions differs substantially from the one‐dimensional result of . We describe the conditions under which the one‐dimensional and diffusion‐limited approximations apply and how they evolve as the rock weathers. Under chemical kinetic limitations, the rind advances at a constant rate, dδ/dt = v. We defend the application of a spherical approximation to irregular non‐spherical rocks and apply our results to field observations reported in the literature to show consistency with established methods. Finally, we argue that the variability in size, as well as in mineralogy, over ensembles of grains contributes to heterogeneous weathering rates. We demonstrate that this heterogeneity can contribute to the aging, or gradual decrease with time, of weathering rates previously observed in laboratory and field measurements.     

Fluid inclusion and stable isotope constraints on the genesis of the Jian copper deposit,Sanandaj–Sirjan metamorphic zone,Iran

The Jian copper deposit, located on the eastern edge of the Sanandaj–Sirjan metamorphic zone, southwest of Iran, is contained within the Surian Permo‐Triassic volcano‐sedimentary complex. Retrograde metamorphism resulted in three stages of mineralization (quartz ± sulfide veins) during exhumation of the Surian metamorphic complex (Middle Jurassic time; 159–167 Ma), and after the peak of the metamorphism (Middle to Late Triassic time; approximately 187 Ma). The early stage of mineralization (stage 1) is related to a homogeneous H₂O–CO₂ (XCO₂ > 0.1) fluid characterized by moderate salinity (<10 wt.% NaCl equivalent) at high temperature and pressure (>370°C, >3 kbar). Early quartz was followed by small amounts of disseminated fine‐grained pyrite and chalcopyrite. Most of the main‐ore‐stage (stage 2) minerals, including chalcopyrite, pyrite and minor sphalerite, pyrrhotite, and galena, precipitated from an aqueous‐carbonic fluid (8–18 wt.% NaCl equivalent) at temperatures ranging between 241 and 388°C during fluid unmixing process (CO₂ effervescence). Fluid unmixing in the primary carbonaceous fluid at pressures of 1.5–3 kbar produced a high XCO₂ (>0.05) and a low XCO₂ (<0.01) aqueous fluid in ore‐bearing quartz veins. Oxygen and hydrogen isotope compositions suggest mineralization by fluids derived from metamorphic dehydration (δ¹⁸O_fluid = +7.6 to +10.7‰ and δD = ?33.1 to ?38.5‰) during stage 2. The late stage (stage 3) is related to a distinct low salinity (1.5–8 wt.% NaCl equivalent) and temperatures of (120–230°C) aqueous fluid at pressures below 1.5 kbar and the deposition of post‐ore barren quartz veins. These fluids probably derived from meteoric waters, which circulated through the metamorphic pile at sufficiently high temperatures and acquire the characteristics of metamorphic fluids (δ¹⁸O_fluid = +4.7 to +5.1‰ and δD = ?52.3 to ?53.9‰) during waning stages of the postearly Cimmerian orogeny in Surian complex. The sulfide‐bearing quartz veins are interpreted as a small‐scale example of redistribution of mineral deposits by metamorphic fluids. This study suggests that mineralization at the Jian deposit is metamorphogenic in style, probably related to a deep‐seated mesothermal system.     

Migration of metamorphic CO2 into a coal seam: a natural analog study to assess the long‐term fate of CO2 in Coal Bed Carbon Capture,Utilization, and Storage Projects

This study assesses the displacement of coalbed methane by CO₂ migration along a fault into the coal seam in the Yaojie coalfield. Coal and gas samples were collected continuously at various distances in NO.2 coal seam from F19 fault. Vitrinite reflectance, maceral, and pore distributions and proximate analysis of fourteen coal samples were performed. Gas components, concentrations, carbon isotopes of 28 gas samples were determined. We examined the coal–gas trace characteristics of coalbed methane displaced away from the fault by CO₂ injection after geological ages. From east to west, away from the F19 fault, the CO₂ concentration decreased, whereas the CH₄ concentration increased gradually. The δ¹³C values for CO₂ varied between ?9.94‰ and 1.12‰, suggesting a metamorphic origin. A wider range of values (from ?9.94‰ to 20‰) was associated with the mixing of microbial carbon dioxide, isotopic fractionation during CO₂ migration through the microporous structures of coals, and/or carbon isotope fractionation during gas–water exchange and dissolution of CO₂. Away from the F19 fault, the volumes of micropores, mesopores and macropores decrease gradually. The Dubinin–Radushkevich (DR) micropore volume decreased from 0.0059 to 0.0037 cm³ g^‐1, and the mesopore and macropore volumes decreased from 0.066 to 0.026 cm³ g^‐1. The CO₂ injection can mobilize aromatic hydrocarbons and mineral matter from coal matrix, resulting in the decrease in the absorption peak intensity for coal samples after supercritical CO₂ treatment, which indicates that chemical reactions occur between coal and CO₂, not only physical adsorption.     

Indications of formation water flow and compartmentalization on the flank of a salt structure derived from salinity and seismic data

Vertical and lateral variations in lithology, salinity, temperature, and pressure determined from wireline LAS logs, produced water samples, and seismic data on the south flank of a salt structure on the continental shelf, offshore Louisiana indicate three hydrogeologic zones in the study area: a shallow region from 0 to 1.1 km depth with hydrostatically pressured, shale‐dominated Pleistocene age sediments containing pore waters with sea water (35 g l^?1) or slightly above sea water salinity; a middle region from 1.1 to 3.2 km depth with near hydrostatically pressured, sand‐dominated Pliocene age sediments that contain pore waters that range from seawater salinity to up to 5 times sea water salinity (180 g l^?1); and a deep section below 3.2 km depth with geopressured, shale‐dominated Miocene age sediments containing pore waters that range from sea water salinity to 125 g l^?1. Salt dissolution has generated dense, saline waters that appear to be migrating down dip preferentially through the thick Pliocene sandy section. Sand layers that come in contact with salt contain pore waters with high salinity. Isolated sands have near sea water salinity. Salinity information in conjunction with seismic data is used to infer fluid compartmentalization. Both vertical and lateral lithologic barriers to fluid flow at tens to hundreds of meters scale are observed. Fluid compartmentalization is also evident across a supradomal normal fault. Offset of salinity contours are consistent with the throw of the fault, which suggests that saline fluids migrated before fault formation.     

High‐temperature magmatic fluid exsolved from magma at the Duobuza porphyry copper–gold deposit,Northern Tibet

The Duobuza porphyry copper–gold deposit (proven Cu resources of 2.7 Mt, 0.94% Cu and 13 t gold, 0.21 g t^?1 Au) is located at the northern margin of the Bangong‐Nujiang suture zone separating the Qiangtang and Lhasa Terranes. The major ore‐bearing porphyry consists of granodiorite. The alteration zone extends from silicification and potassic alteration close to the porphyry stock to moderate argillic alteration and propylitization further out. Phyllic alteration is not well developed. Sericite‐quartz veins only occur locally. High‐temperature, high‐salinity fluid inclusions were observed in quartz phenocrysts and various quartz veins. These fluid inclusions are characterized by sylvite dissolution between 180 and 360°C and halite dissolution between 240 and 540°C, followed by homogenization through vapor disappearance between 620 and 960°C. Daughter minerals were identified by SEM as chalcopyrite, halite, sylvite, rutile, K–feldspar, and Fe–Mn‐chloride. They indicate that the fluid is rich in ore‐forming elements and of high oxidation state. The fluid belongs to a complex hydrothermal system containing H₂O – NaCl – KCl ± FeCl₂ ± CaCl₂ ± MnCl₂. With decreasing homogenization temperature, the fluid salinity tends to increase from 34 to 82 wt% NaCl equiv., possibly suggesting a pressure or Cl/H₂O increase in the original magma. No coexisting vapor‐rich fluid inclusions with similar homogenization temperatures were found, so the brines are interpreted to have formed by direct exsolution from magma rather than trough boiling off of a low‐salinity vapor. Estimated minimum pressure of 160 MPa imply approximately 7‐km depth. This indicates that the deposit represents an orthomagmatic end member of the porphyry copper deposit continuum. Two key factors are proposed for the fluid evolution responsible for the large size of the gold‐rich porphyry copper deposit of Duobuza: (i) ore‐forming fluids separated early from the magma, and (ii) the hydrothermal fluid system was of magmatic origin and highly oxidized.     

Petroleum type determination through homogenization temperature and vapour volume fraction measurements in fluid inclusions

Physical parameters of petroleum‐bearing fluid inclusions such as bulk density (ρ), molar volume (V_m), vapour volume fraction (?_vap) and homogenization temperature (T_h) are essential information to model petroleum composition (x) in inclusions and to reconstruct palaeotemperature and palaeopressure of trapping. For the main petroleum types contained in a fluid inclusion, we can follow how ?_vap and T_h are simultaneously influenced by a change of bulk density in a ?_vap versus T_h projection. We have correlated T_h and ?_vap for different petroleum compositions for a large range of bulk density values. However, postentrapment events under new pressure (P) and temperature (T) conditions can greatly modify the initial fingerprints of physical conditions and chemical composition of fluid inclusions. Re‐equilibration is frequent, especially in the case of fragile minerals. Stretching and leakage phenomenon have been simulated using the Petroleum Inclusion Thermodynamics (pit ) software, from virtual petroleum inclusions with known hydrocarbon composition. The aim of these simulations is to understand how ?_vap and T_h evolve with these re‐equilibration phenomena, with respect to the oil composition. Results of stretching simulations show a characteristic increase of T_h and ?_vap along correlation curves, with the curve shape dependent on petroleum composition. Leakage simulations show an increase of T_h and a smaller increase or even a decrease in ?_vap. Consequently, the better preserved inclusions in a given population can be presumed to be those that have the lowest T_h. Applications of T_h and ?_vap measurements of natural inclusions in calcite and in quartz showed that the fragility of the host mineral is a key factor allowing the recording of post‐entrapment events. Inclusions that have stretched or leaked are identified and the best preserved inclusions selected for evaluation of P–T–x trapping conditions. Moreover, petroleum types trapped in inclusions can be identified from ?_vap and T_h measurements without compositional modelling.     

3D numerical modeling of hydrothermal processes during the lifetime of a deep geothermal reservoir

Understanding hydrothermal processes during production is critical to optimal geothermal reservoir management and sustainable utilization. This study addresses the hydrothermal (HT) processes in a geothermal research doublet consisting of the injection well E GrSk3/90 and production well Gt GrSk4/05 at the deep geothermal reservoir of Groß Schönebeck (north of Berlin, Germany) during geothermal power production. The reservoir is located between ?4050 to ?4250 m depth in the Lower Permian of the Northeast German Basin. Operational activities such as hydraulic stimulation, production (T = 150°C; Q = ?75 m³ h^?1; C = 265 g l^?1) and injection (T = 70°C; Q = 75 m³ h^?1; C = 265 g l^?1) change the HT conditions of the geothermal reservoir. The most significant changes affect temperature, mass concentration and pore pressure. These changes influence fluid density and viscosity as well as rock properties such as porosity, permeability, thermal conductivity and heat capacity. In addition, the geometry and hydraulic properties of hydraulically induced fractures vary during the lifetime of the reservoir. A three‐dimensional reservoir model was developed based on a structural geological model to simulate and understand the complex interaction of such processes. This model includes a full HT coupling of various petrophysical parameters. Specifically, temperature‐dependent thermal conductivity and heat capacity as well as the pressure‐, temperature‐ and mass concentration‐dependent fluid density and viscosity are considered. These parameters were determined by laboratory and field experiments. The effective pressure dependence of matrix permeability is less than 2.3% at our reservoir conditions and therefore can be neglected. The results of a three‐dimensional thermohaline finite‐element simulation of the life cycle performance of this geothermal well doublet indicate the beginning of thermal breakthrough after 3.6 years of utilization. This result is crucial for optimizing reservoir management. Geofluids (2010) 10 , 406–421     

Monitoring fluid chemistry in iron oxide–copper–gold‐related metasomatic processes,eastern Mt Isa Block,Australia

Most researchers in the Proterozoic eastern Mt Isa Block, NW Queensland, Australia, favour magmatic fluid and salt sources for sodic‐(calcic) alteration and iron oxide–copper–gold mineralization. Here we compare spatial, mineralogic and stable isotope data from regional alteration assemblages with magmatic and magmatic‐hydrothermal interface rocks in order to track chemical and isotopic variations in fluid composition away from inferred fluid sources. Tightly clustered δ¹⁸O values for magnetite, quartz, feldspar and actinolite for igneous‐hosted samples reflect high temperature equilibration in the magmatic‐hydrothermal environment. In contrast, these minerals record predominantly higher δ¹⁸O values in regional alteration and Cu–Au mineralization. This dichotomy reflects partial equilibration with isotopically heavier wallrocks and slightly lower temperatures. Increases in Si concentrations of metasomatic amphiboles relative to igneous amphiboles in part reflect cooling of metasomatic fluids away from igneous rocks. Variations in X_Mg for metasomatic amphiboles indicate local wallrock controls on amphibole chemistry, while variations in X_Cl/X_OH ratios for amphiboles (at constant X_Mg) indicate variable aH₂O/aHCl ratios for metasomatic fluids. Biotite geochemistry also reflects cooling and both increases and decreases in aH₂O/aHCl for fluids away from plutonic rocks. Decreased aH₂O/aHCl ratios for metasomatic fluids reflect in part scavenging of chlorine out of meta‐evaporite sequences, although this process requires already saline fluids. Local increases in aH₂O/aHCl ratios, as well as local decreases in δ¹⁸O values for some minerals (most notably haematite and epithermal‐textured quartz), may indicate ingress of low salinity, low δ¹⁸O fluids of possible meteoric origin late in the hydrothermal history of the region. Taken together, our observations are most consistent with predominantly magmatic sources for metasomatic fluids in the eastern Mt Isa Block, but record chemical and isotopic variations along fluid flow paths that may be important in explaining some of the diversity in alteration and mineralization styles in the district.     

Density and isothermal compressibility of supercritical H2O–NaCl fluid: molecular dynamics study from 673 to 2000 K, 0.2 to 2 GPa,and 0 to 22 wt% NaCl concentrations

We report on molecular dynamics (MD) simulations for predicting the density and isothermal compressibility of an H₂O–NaCl fluid as a function of temperature (673–2000 K), pressure (0.2–2.0 GPa), and salt concentration (0.0–21.9 wt%). The atomistic behavior was analyzed via the hydration number of ions and number of ion pairs. Hydration numbers of Na⁺ and Cl^? increased with increasing pressure and decreasing temperature. Conversely, the fraction of Na–Cl ion pairs increased with decreasing pressure and increasing temperature. This hydration and association behavior is consistent with the low dielectric constant of H₂O under these conditions. The presence of polynuclear clusters of Na–Cl was confirmed at high temperatures, low pressures, and high salt concentrations. We propose a purely empirical equation of state (EoS) for H₂O–NaCl fluids under high temperatures and pressures that should be useful for estimating the fluid distribution in Earth's crust and upper mantle in relation to effects on earthquakes and volcanic eruptions.     

Petroleum infiltration of high-grade basement, South Norway: Pressure-Temperature-time-composition (P–T–t–X) constraints

Fluid inclusion data provide pressure–temperature–time–composition (P–T–t–X) constraints for an episode of petroleum infiltration of the crystalline basement in South Norway. Petroleum inclusions associated with pyrobitumen occur in postmetamorphic quartz veins in the Modum Complex. Three groups of fluid compositions have been shown, ranging from CH₄ ± CO₂ to condensates with alkanes up to C₁₅. The range in fluid composition is a result of petroleum decomposition at high temperature. Globular and massive pyrobitumen occurs in the quartz veins or in associated vein systems. Reflectance (%Rm) measurements of 3.20–3.35 correspond to a maximum temperature of 207–214°C for the pyrobitumen associated with group II and III inclusions. Geothermometry of chlorites included in the quartz show results of 226–231°C. Pressure conditions of trapping for all three groups of inclusion fluids have been estimated to 520–985 bar at 220°C. The pressure range is probably a result of fluctuations caused by repeated fracture opening and sealing due to seismic activity coupled with mineral growth. A lack of systematic textural relationships between the three groups of inclusions and similar pressure–temperature estimates for all fluid types indicate trapping at similar times and a process of rapid change. Fluid migration in fractures from an overlying, overpressured sedimentary basin into a dry, crystalline basement best explains the observed P–T–t–X constraints.     

Boron isotope geochemistry of Na‐bicarbonate,Na‐chloride,and Ca‐chloride waters from the Northern Apennine Foredeep basin: other pieces of the sedimentary basin puzzle

The boron stable isotope ratio δ¹¹B of 12 water samples representative of three chemical facies (fresh Na‐bicarbonate, brackish Na‐chloride, saline, and brine Ca‐chloride) has been analyzed. Interpretation of the δ¹¹B data, along with the chemical compositions, reveals that Na‐carbonate waters from the Northern Apennine are of meteoric origin, with boron contributions from clay desorption and mixing with seawater‐derived fluids of Na‐chloride or Ca‐chloride compositions. The comparison of our new results with the literature data on other sedimentary basins of Mediterranean, and worldwide, confirms the contribution of Na‐bicarbonate waters to the genesis of mud volcano fluids. The Na‐chloride sample of Salvarola (SAL), which may represent the end‐member of the mud volcanoes, and the Ca‐chloride brine water from Salsomaggiore (SM) indicate boron release from clays compatible with the diagenetic process. The empirical equation: relating boron concentration and the stable isotope composition of the fluids observed in this study and the literature is proposed to trace the effect of diagenesis in sedimentary basins. A geothermometer associated to the diagenetic equation is also proposed: The application of this equation to obtain reservoir temperatures from δ¹¹B compositions of waters should be carefully evaluated against the results obtained from other chemical and isotopic geothermometers from other basins around the world.     

Transient fluid flow in and around a fault

We present the results of simple numerical experiments in which we study the evolution with time of fluid flow around and within a permeable fault embedded in a less permeable porous medium. Fluid movement is driven by an imposed vertical pressure gradient. The results show that fluid flow is controlled by two timescales: τ_f = Sl²/κ_F and τ_F = Sl²/κ_M, where S is the specific storage of the porous material, l the length of the fault, and κ_M and κ_F are the hydraulic conductivities of the porous material and the fault, respectively. Fluid flow and the associated fluid pressure field evolve through three temporal stages: an early phase [t < τ_f] during which the initial fluid pressure gradient within the fault is relaxed; a second transient stage [τ_f < t < τ_F] when fluid is rapidly expelled at one end of the fault and extracted from the surrounding rocks at the other end leading to a reduction in the pressure gradient in the intact rock; a third phase [t < τ_F] characterized by a steady‐state flow. From the numerical experiments we derive an expression for the steady‐state maximum fluid velocity in the fault and the values of the two timescales, τ_f and τ_F. A comparison indicates excellent agreement of our results with existing asymptotic solutions. For km‐scale faults, the model results suggest that steady‐state is unlikely to be reached over geological timescales. Thus, the current use of parameters such as the focusing ratio defined under the assumption of steady‐state conditions should be reconsidered.     

Syntectonic infiltration by meteoric waters along the Sevier thrust front,southwest Montana

Structural, petrographic, and isotopic data for calcite veins and carbonate host‐rocks from the Sevier thrust front of SW Montana record syntectonic infiltration by H₂O‐rich fluids with meteoric oxygen isotope compositions. Multiple generations of calcite veins record protracted fluid flow associated with regional Cretaceous contraction and subsequent Eocene extension. Vein mineralization occurred during single and multiple mineralization events, at times under elevated fluid pressures. Low salinity (T_m = ?0.6°C to +3.6°C, as NaCl equivalent salinities) and low temperature (estimated 50–80°C for Cretaceous veins, 60–80°C for Eocene veins) fluids interacted with wall‐rock carbonates at shallow depths (3–4 km in the Cretaceous, 2–3 km in the Eocene) during deformation. Shear and extensional veins of all ages show significant intra‐ and inter‐vein variation in δ¹⁸O and δ¹³C. Carbonate host‐rocks have a mean δ¹⁸O_V‐SMOW value of +22.2 ± 3‰ (1σ), and both the Cretaceous veins and Eocene veins have δ¹⁸O ranging from values similar to those of the host‐rocks to as low as +5 to +6‰. The variation in vein δ¹³C_V‐PDB of ?1 to approximately +6‰ is attributed to original stratigraphic variation and C isotope exchange with hydrocarbons. Using the estimated temperature ranges for vein formation, fluid (as H₂O) δ¹⁸O calculated from Cretaceous vein compositions for the Tendoy and Four Eyes Canyon thrust sheets are ?18.5 to ?12.5‰. For the Eocene veins within the Four Eyes Canyon thrust sheet, calculated H₂O δ¹⁸O values are ?16.3 to ?13.5‰. Fluid–rock exchange was localized along fractures and was likely coincident with hydrocarbon migration. Paleotemperature determinations and stable isotope data for veins are consistent with the infiltration of the foreland thrust sheets by meteoric waters, throughout both Sevier orogenesis and subsequent orogenic collapse. The cessation of the Sevier orogeny was coincident with an evolving paleogeographic landscape associated with the retreat of the Western Interior Seaway and the emergence of the thrust front and foreland basin. Meteoric waters penetrated the foreland carbonate thrust sheets of the Sevier orogeny utilizing an evolving mesoscopic fracture network, which was kinematically related to regional thrust structures. The uncertainty in the temperature estimates for the Cretaceous and Eocene vein formation prevents a more detailed assessment of the temporal evolution in meteoric water δ¹⁸O related to changing paleogeography. Meteoric water‐influenced δ¹⁸O values calculated here for Cretaceous to Eocene vein‐forming fluids are similar to those previously proposed for surface waters in the Eocene, and those observed for modern‐day precipitation, in this part of the Idaho‐Montana thrust belt.     

Br/Cl signature of hydrothermal fluids: liquid–vapour fractionation of bromine revisited

Br/Cl ratios of hydrothermal fluids are widely used as geochemical tracers in marine hydrothermal systems to prove fluid phase separation processes. However, previous results of the liquid–vapour fractionation of bromine are ambiguous. Here we report new experimental results of the liquid–vapour fractionation of bromine in the system H₂O–NaCl–NaBr at 380–450°C and 22.9–41.7 MPa. Our data indicate that bromine is generally more enriched than chlorine in the liquid phase. Calculated exchange coefficients K_D(Br‐Cl)^{liquid‐vapour} for the reaction Br^vapour + Cl^liquid = Br^liquid + Cl^vapour are between 0.94 ± 0.08 and 1.66 ± 0.14 within the investigated P–T range. They correlate positively with D_Cl^{liquid‐vapour} and suggest increasing bromine–chlorine fractionation with increasing opening of the liquid–vapour solvus, i.e. increasing distance to the critical curve in the H₂O–NaCl system. An empirical fit of the form K_D(Br‐Cl)^{liquid‐vapour} = a*ln[b*(D_Cl^{liquid‐vapour}?1) + e^1/a] yields a = 0.349 and b = 1.697. Based on this empirical fit and the well‐constrained phase relations in the H₂O–NaCl system we calculated the effect of fluid phase separation on the Br/Cl signature of a hydrothermal fluid with initial seawater composition for closed and open adiabatic ascents along the 4.5 and 4.8 J g^?1 K^?1 isentropes. The calculations indicate that fluid phase separation can significantly alter the Br/Cl ratio in hydrothermal fluids. The predicted Br/Cl evolutions are in accord with the Br/Cl signatures in low‐salinity vent fluids from the 9 to 10°N East Pacific Rise.     

Source and character of syntaxial hydrothermal calcite veins in Paleoproterozoic crystalline rocks revealed by fine‐scale investigations

Calcite veins in Paleoproterozoic granitoids on the Baltic Shield are the focus of this study. These veins are distinguished by their monomineralic character, unusual thickness and closeness to Neoproterozoic dolerite dykes and therefore have drawn attention. The aim of this study was to define the source of these veins and to unravel their isotopic and chemical nature by carrying out fine‐scale studies. Seven calcite veins covering a depth interval of 50–420 m below the ground surface and composed of breccias or crack‐sealed fillings typically expressing syntaxial growth were sampled and analysed for a variety of physicochemical variables: homogenization temperature (T_h) and salinity of fluid inclusions, and stable isotopes (⁸⁷Sr/⁸⁶Sr, ¹³C/¹²C, ¹⁸O/¹⁶O), trace‐element concentrations (Fe, Mn, Mg, Sr, rare earth elements) and cathodoluminescence (CL) of the solid phase. The fluid‐inclusion data show that the calcites were precipitated mainly from relatively low‐temperature (T_h = 73–106°C) brines (13.4–24.5 wt.% CaCl₂), and the ⁸⁷Sr/⁸⁶Sr is more radiogenic than expected for Rb‐poor minerals precipitated from Neoproterozoic fluids. These features, together with the distribution of δ¹³C and δ¹⁸O values, provide evidence that the calcite veins are not genetic with the nearby Neoproterozoic dolerite dykes, but are of Paleozoic age and were precipitated from warm brines expressing a rather large variability in salinity. Whereas the isotopic and chemical variables express rather constant average values among the individual veins, they vary considerably on fine‐scale across individual veins. This has implications for understanding processes causing calcite‐rich veins to form and capture trace metals in crystalline bedrock settings.     

Geothermometry and geobarometry of overpressured environments in Qiongdongnan Basin,South China Sea*

We demonstrate the use of PVT fluid inclusion modelling in the calculation of palaeofluid formation pressures, using samples from the YC21‐1‐1 and YC21‐1‐4 wells in the YC21‐1 structural closure, Qiongdongnan Basin, South China Sea. Homogenisation temperatures and gas/liquid ratios were measured in aqueous fluid inclusions, and associated light hydrocarbon/CO₂‐bearing inclusions, and their compositions were determined using a crushing technique. The vtflinc software was used to construct P–T phase diagrams that enabled derivation of the minimum trapping pressure for each order of fluid inclusion. Through the projection of average homogenisation temperatures (155, 185.5 and 204.5°C) for three orders of fluid inclusion on the thermal‐burial history diagram of the Oligocene Yacheng and Lingshui formations, their trapping times were constrained at 4.3, 2.1 and 1.8 Ma, respectively. The formation pressure coefficient, the ratio of fluid pressure/hydrostatic pressure established by PVT modelling coupled with DST data, demonstrates that one and a half cycles of pressure increase–discharge developed in the Yacheng and Lingshui formations for about 4.3 Ma. In comparison, the residual formation pressure determined by 2D numerical modelling in the centre of LeDong depression shows two and a half pressure increase–discharge cycles for about 28 Ma. The two different methods suggest that a high fluid potential in the Oligocene reservoir of the YC21‐1 structure developed at two critical stages for regional oil and natural gas migration and accumulation (5.8 and 2.0 Ma, respectively). Natural gas exploration in this area is therefore not advisable.     

Palaeo‐formation water evolution in the Latrobe aquifer,Gippsland Basin,south‐eastern Australia continental shelf

Offshore fresh or brackish groundwater has been observed around the globe and represents an interesting but unusual freshwater reserve. Formation waters in sedimentary basins evolve at geological time through fluid–rock interactions and water movements in aquifers. However, the mechanism and timing of freshwater displacing and mixing with pre‐existing formation water offshore under the seafloor has not been investigated in many cases. The growing need for developing freshwater resources in deeper parts of sedimentary basins that have not been economic or technically feasible in the past, may potentially lead to an increasing conflict with petroleum production or injection of carbon dioxide. For being able to assess and mitigate possible impacts of fluid production or injection on groundwater flow and quality, a better understanding of the natural history of the interaction between fresh meteoric water and deep basin formation water is necessary. A low‐salinity wedge of meteoric origin with less than 5000 ppm currently extends to about 20 km offshore in the confined Latrobe aquifer in the Gippsland Basin (Australia). The Latrobe aquifer is a freshwater resource in the onshore, hosts major petroleum reservoirs and has been considered for carbon dioxide storage in the offshore parts of the basin. The objective of this study is to constrain the evolution of formation water in the Latrobe aquifer by investigating the water naturally trapped in fluid inclusions during burial. The measured palaeo‐salinities from onshore and offshore rock samples have a minimum of about 12 500 ppm (NaCl equivalent) and a maximum of about 50 000 ppm. Most of the salinities are in the 32 000–35 000 ppm range. There is no evidence for freshwater in fluid inclusions and the variation in palaeo‐salinity across the basin is consistent with the palaeogeography of deposition of the sedimentary rocks. The current low‐salinity water wedge must have started to form recently after most of the diagenetic processes that led to the trapping of water in fluid inclusions happened. The minimum homogenisation temperatures (T_h) recorded are consistent with current formation temperature. However, they are generally higher than present day suggesting that hotter temperatures were attained in the past. The T_h and salinity data together suggest that the fluid inclusions record the diagenetic modification of connate water to higher salinities over a time period that was accompanied by an increase in temperature, consistent with a westward palaeo‐fluid flow from the deeper part of the basin through the aquifer. Subsequent pore‐water evolution from palaeo‐ to current day conditions is consistent with an influx of fresher and cooler meteoric water into the Latrobe Group. The meteoric recharge originates from the area of the Baragwanath anticline in the onshore part of the basin where the Latrobe Group subcrops at high elevations.     

Long‐term chemical evolution and modification of continental basement brines – a field study from the Schwarzwald,SW Germany

Highly saline, deep‐seated basement brines are of major importance for ore‐forming processes, but their genesis is controversial. Based on studies of fluid inclusions from hydrothermal veins of various ages, we reconstruct the temporal evolution of continental basement fluids from the Variscan Schwarzwald (Germany). During the Carboniferous (vein type i), quartz–tourmaline veins precipitated from low‐salinity (<4.5wt% NaCl + CaCl₂), high‐temperature (≤390°C) H₂O‐NaCl‐(CO₂‐CH₄) fluids with Cl/Br mass ratios = 50–146. In the Permian (vein type ii), cooling of H₂O‐NaCl‐(KCl‐CaCl₂) metamorphic fluids (T ≤ 310°C, 2–4.5wt% NaCl + CaCl₂, Cl/Br mass ratios = 90) leads to the precipitation of quartz‐Sb‐Au veins. Around the Triassic–Jurassic boundary (vein type iii), quartz–haematite veins formed from two distinct fluids: a low‐salinity fluid (similar to (ii)) and a high‐salinity fluid (T = 100–320°C, >20wt% NaCl + CaCl₂, Cl/Br mass ratios = 60–110). Both fluids types were present during vein formation but did not mix with each other (because of hydrogeological reasons). Jurassic–Cretaceous veins (vein type iv) record fluid mixing between an older bittern brine (Cl/Br mass ratios ~80) and a younger halite dissolution brine (Cl/Br mass ratios >1000) of similar salinity, resulting in a mixed H₂O‐NaCl‐CaCl₂ brine (50–140°C, 23–26wt% NaCl + CaCl₂, Cl/Br mass ratios = 80–520). During post‐Cretaceous times (vein type v), the opening of the Upper Rhine Graben and the concomitant juxtaposition of various aquifers, which enabled mixing of high‐ and low‐salinity fluids and resulted in vein formation (multicomponent fluid H₂O‐NaCl‐CaCl₂‐(SO₄‐HCO₃), 70–190°C, 5–25wt% NaCl‐CaCl₂ and Cl/Br mass ratios = 2–140). The first occurrence of highly saline brines is recorded in veins that formed shortly after deposition of halite in the Muschelkalk Ocean above the basement, suggesting an external source of the brine's salinity. Hence, today's brines in the European basement probably developed from inherited evaporitic bittern brines. These were afterwards extensively modified by fluid–rock interaction on their migration paths through the crystalline basement and later by mixing with younger meteoric fluids and halite dissolution brines.