Application of the Pitzer ion interaction model to isopiestic data for the Fe2(SO4)3-H2SO4-H2O system at 298.15 and 323.15 K

Recent isopiestic studies of the Fe₂(SO₄)₃-H₂SO₄-H₂O system at 298.15 K are represented with an extended version of Pitzer’s ion interaction model. The model represents osmotic coefficients for aqueous {(1 − y)Fe₂(SO₄)₃ + yH₂SO₄} mixtures from 0.45 to 3.0 m at 298.15 K and 0.0435 ? y ? 0.9370. In addition, a slightly less accurate representation of a more extended molality range to 5.47 m extends over the same y values, translating to a maximum ionic strength of 45 m. Recent isopiestic data for the system at 323.15 K are represented with the extended Pitzer model over a limited range in molality and solute fraction. These datasets are also represented with the usual “3-parameter” version of Pitzer’s model so that it may be incorporated in geochemical modeling software, but is a slightly less accurate representation of thermodynamic properties for this system. Comparisons made between our ion interaction model and available solubility data display partial agreement for rhomboclase and significant discrepancy for ferricopiapite. The comparisons highlight uncertainty remaining for solubility predictions in this system as well as the need for additional solubility measurements for Fe³⁺-bearing sulfate minerals. The resulting Pitzer ion interaction models provide an important step toward an accurate and comprehensive representation of thermodynamic properties in this geochemically important system.     

Uranophane dissolution and growth in CaCl2-SiO2(aq) test solutions

The dissolution and growth of uranophane [Ca(UO₂)₂(SiO₃OH)₂·5H₂O] have been examined in Ca- and Si-rich test solutions at low temperatures (20.5 ± 2.0 °C) and near-neutral pH (∼6.0). Uranium-bearing experimental solutions undersaturated and supersaturated with uranophane were prepared in matrices of ∼10⁻² M CaCl₂ and ∼10⁻³ M SiO₂(aq). The experimental solutions were reacted with synthetic uranophane and analyzed periodically over 10 weeks. Interpretation of the aqueous solution data permitted extraction of a solubility constant for the uranophane dissolution reaction and standard state Gibbs free energy of formation for uranophane ( kJ mol⁻¹).     

Residence times for protons bound to three oxygen sites in the AlO4Al12(OH)24(H2O)12 polyoxocation

Although, the kinetic reactivity of a mineral surface is determined, in part, by the rates of exchange of surface-bound oxygens and protons with bulk solution, there are no elementary rate data for minerals. However, such kinetic measurements can be made on dissolved polynuclear clusters, and here we report lifetimes for protons bound to three oxygen sites on the AlO₄Al₁₂(OH)₂₄(H₂O)₁₂⁷⁺ (Al₁₃) molecule, which is a model for aluminum-hydroxide solids in water. Proton lifetimes were measured using ¹H NMR at pH ∼ 5 in both aqueous and mixed solvents. The ¹H NMR peak for protons on bound waters (η-H₂O) lies near 8 ppm in a 2.5:1 mixture of H₂O/acetone-d₆ and broadens over the temperature range −20 to −5 °C. Extrapolated to 298 K, the lifetime of a proton on a η-H₂O is τ²⁹⁸ ∼ 0.0002 s, which is surprisingly close to the lifetime of an oxygen in the η-H₂O (∼0.0009 s), but in the same general range as lifetimes for protons on fully protonated monomer ions of trivalent metals (e.g., Al(H₂O)₆³⁺). The lifetime is reduced somewhat by acid addition, indicating that there is a contribution from the partly deprotonated Al₁₃ molecule in addition to the fully protonated Al₁₃ at self-buffered pH conditions. Proton lifetimes on the two distinct sets of hydroxyls bridging two Al(III) (μ₂-OH) differ substantially and are much shorter than the lifetime of an oxygen at these sites. The average lifetimes for hydroxyl protons were measured in a 2:1 mixture of H₂O/dmso-d₆ over the temperature range 3.7-95.2 °C. The lifetime of a hydrogen on one of the μ₂-OH was also measured in D₂O. The τ²⁹⁸ values are ∼0.013 and ∼0.2 s in the H₂O/dmso-d₆ solution and the τ²⁹⁸ value for the μ₂-OH detectable in D₂O is τ²⁹⁸ ∼ 0.013 s. The ¹H NMR peak for the more reactive μ₂-OH broadens slightly with acid addition, indicating a contribution from an exchange pathway that involves a proton or hydronium ion. These data indicate that surface protons on minerals will equilibrate with near-surface waters on the diffusional time scale.     

Isotopic fractionation of Mg(aq), Ca(aq), and Fe(aq) with carbonate minerals

Density-functional electronic structure calculations are used to compute the equilibrium constants for ²⁶Mg/²⁴Mg and ⁴⁴Ca/⁴⁰Ca isotope exchange between carbonate minerals and uncomplexed divalent aquo ions. The most reliable calculations at the B3LYP/6-311++G(2d,2p) level predict equilibrium constants K, reported as 10³ln (K) at 25 °C, of −5.3, −1.1, and +1.2 for ²⁶Mg/²⁴Mg exchange between calcite (CaCO₃), magnesite (MgCO₃), and dolomite (Ca_0.5Mg_0.5CO₃), respectively, and Mg²⁺(aq), with positive values indicating enrichment of the heavy isotope in the mineral phase. For ⁴⁴Ca/⁴⁰Ca exchange between calcite and Ca²⁺(aq) at 25 °C, the calculations predict values of +1.5 for Ca²⁺(aq) in 6-fold coordination and +4.1 for Ca²⁺(aq) in 7-fold coordination. We find that the reduced partition function ratios can be reliably computed from systems as small as and embedded in a set of fixed atoms representing the second-shell (and greater) coordination environment. We find that the aqueous cluster representing the aquo ion is much more sensitive to improvements in the basis set than the calculations on the mineral systems, and that fractionation factors should be computed using the best possible basis set for the aquo complex, even if the reduced partition function ratio calculated with the same basis set is not available for the mineral system. The new calculations show that the previous discrepancies between theory and experiment for Fe³⁺-hematite and Fe²⁺-siderite fractionations arise from an insufficiently accurate reduced partition function ratio for the Fe³⁺(aq) and Fe²⁺(aq) species.     

Oxygen isotope exchange between H2O and CO2 at elevated CO2 pressures: Implications for monitoring of geological CO2 storage

Traditionally, the application of stable isotopes in Carbon Capture and Storage (CCS) projects has focused on δ¹³C values of CO₂ to trace the migration of injected CO₂ in the subsurface. More recently the use of δ¹⁸O values of both CO₂ and reservoir fluids has been proposed as a method for quantifying in situ CO₂ reservoir saturations due to O isotope exchange between CO₂ and H₂O and subsequent changes in δ¹⁸O_H2O values in the presence of high concentrations of CO₂. To verify that O isotope exchange between CO₂ and H₂O reaches equilibrium within days, and that δ¹⁸O_H2O values indeed change predictably due to the presence of CO₂, a laboratory study was conducted during which the isotope composition of H₂O, CO₂, and dissolved inorganic C (DIC) was determined at representative reservoir conditions (50 °C and up to 19 MPa) and varying CO₂ pressures. Conditions typical for the Pembina Cardium CO₂ Monitoring Pilot in Alberta (Canada) were chosen for the experiments. Results obtained showed that δ¹⁸O values of CO₂ were on average 36.4 ± 2.2‰ (1σ, n = 15) higher than those of water at all pressures up to and including reservoir pressure (19 MPa), in excellent agreement with the theoretically predicted isotope enrichment factor of 35.5‰ for the experimental temperatures of 50 °C. By using ¹⁸O enriched water for the experiments it was demonstrated that changes in the δ¹⁸O values of water were predictably related to the fraction of O in the system sourced from CO₂ in excellent agreement with theoretical predictions. Since the fraction of O sourced from CO₂ is related to the total volumetric saturation of CO₂ and water as a fraction of the total volume of the system, it is concluded that changes in δ¹⁸O values of reservoir fluids can be used to calculate reservoir saturations of CO₂ in CCS settings given that the δ¹⁸O values of CO₂ and water are sufficiently distinct.     

The kinetics of oxygen exchange between the GeO4Al12(OH)24(OH2)12(aq) molecule and aqueous solutions

We report rates of oxygen exchange with bulk solution for an aqueous complex, ^IVGeO₄Al₁₂(OH)₂₄(OH₂)₁₂⁸⁺(aq) (GeAl₁₂), that is similar in structure to both the ^IVAlO₄Al₁₂(OH)₂₄(OH₂)₁₂⁷⁺(aq) (Al₁₃) and ^IVGaO₄Al₁₂(OH)₂₄(OH₂)₁₂⁷⁺(aq) (GaAl₁₂) molecules studied previously. All of these molecules have ε-Keggin-like structures, but in the GeAl₁₂ molecule, occupancy of the central tetrahedral metal site by Ge(IV) results in a molecular charge of +8, rather than +7, as in the Al₁₃ and GaAl₁₂. Rates of exchange between oxygen sites in this molecule and bulk solution were measured over a temperature range of 274.5 to 289.5 K and 2.95 < pH < 4.58 using ¹⁷O-NMR.Apparent rate parameters for exchange of the bound water molecules (η-OH₂) are k_ex²⁹⁸ = 200 (±100) s⁻¹, ΔH^‡ = 46 (±8) kJ · mol⁻¹, and ΔS^‡ = −46 (±24) J · mol⁻¹ K⁻¹ and are similar to those we measured previously for the GaAl₁₂ and Al₁₃ complexes. In contrast to the Al₁₃ and GaAl₁₂ molecules, we observe a small but significant pH dependence on rates of solvolysis that is not yet fully constrained and that indicates a contribution from the partly deprotonated GeAl₁₂ species.The two topologically distinct μ₂-OH sites in the GeAl₁₂ molecule exchange at greatly differing rates. The more labile set of μ₂-OH sites in the GeAl₁₂ molecule exchange at a rate that is faster than can be measured by the ¹⁷O-NMR isotopic-equilibration technique. The second set of μ₂-OH sites have rate parameters of k_ex²⁹⁸ = 6.6 (±0.2) · 10⁻⁴ s⁻¹, ΔH^‡ = 82 (±2) kJ · mol⁻¹, and ΔS^‡ = −29 (±7) J · mol⁻¹ · K⁻¹, corresponding to exchanges ≈40 and ≈1550 times, respectively, more rapid than the less labile μ₂-OH sites in the Al₁₃ and GaAl₁₂ molecules. We find evidence of nearly first-order pH dependence on the rate of exchange of this μ₂-OH site with bulk solution for the GeAl₁₂ molecule, which contrasts with Al₁₃ and GaAl₁₂ molecules.     

Geochemistry of a continental saline aquifer for CO2 sequestration: The Guantao formation in the Bohai Bay Basin,North China

The Neogene Guantao formation in the Beitang sag in the Bohai Bay Basin (BBB) of North China, a Mesozoic–Cenozoic sedimentary basin of continental origin, has been chosen as a candidate for a pilot field test of CO₂ sequestration. Hydrogeological and geochemical investigations have been carried out to assess its suitability, taking advantage of many existing geothermal wells drilled to 2000 m or greater depths. Water samples from 25 wells and drill cores of three sections of the Guantao formation were collected for measurements of mineralogy, water chemistry and isotopes (δ¹⁸O, δD, δ¹³C, ¹⁴C). Formation temperature estimated by chemical geothermometry is in the range of 60–80 °C. Geochemical modeling of water–rock–CO₂ interaction predicts a strong geochemical response to CO₂ injection. Besides the elevated porosity (33.6–38.7%) and high permeability (1150–1980 mD) of the Ng-III formation and a favorable reservoir–caprock combination, it is also found that the formation contains carbonates that will react with CO₂ after injection. The low salinity (TDS < 1.6 g/L) offers high CO₂ solubility. The ¹⁴C age of the formation water indicates a quasi-closed saline aquifer system over large time scales, the lateral sealing mechanism for CO₂ sequestration requires further investigation. The CO₂ storage capacity of the Guantao formation within the Beitang sag is estimated to be 17.03 Mt, assuming pure solubility trapping.     

Rates of oxygen exchange between the Al2O8Al28(OH)56(H2O)26(aq) (Al30) molecule and aqueous solution

Rates of steady exchange of oxygens between bulk solution and the largest known aluminum polyoxocation: Al₂O₈Al₂₈(OH)₅₆(H₂O)₂₆¹⁸⁺(aq) (Al₃₀) are reported at pH≈4.7 and 32-40°C. The Al₃₀ molecule is a useful model for geochemists because it is ≈2 nm in length, comparable to the smallest colloidal solids, and it has structural complexity greater than the surfaces of most aluminum (hydr)oxide minerals. The Al₃₀ molecule has 15 distinct hydroxyl sites and eight symmetrically distinct bound waters. Among the hydroxyl bridges are two sets of μ₃-OH, which are not present in any of the other aluminum polyoxocations that have yet been studied by NMR methods. Rates of isotopic equilibration of the μ₂-OH and μ₃-OH hydroxyls and bound water molecules fall within the same range as we have determined for other aluminum solutes, although it is impossible to determine rate laws for exchange at the large number of individual oxygen sites. After injection of ¹⁷O-enriched water, growth of the ¹⁷O-NMR peak near 37 ppm, which is assigned to μ₂-OH and μ₃-OH hydroxyl bridges, indicates that these bridges equilibrate within two weeks at temperatures near 35°C. The peak at +22 ppm in the ¹⁷O-NMR spectra, assigned to bound water molecules (η-OH₂), varies in width with temperature in a similar fashion as for other aluminum solutes, suggesting that most of the η-OH₂ sites exchange with bulk solution at rates that fall within the range observed for other aluminum complexes. Signal from one anomalous group of four η-OH₂ sites is not observed, indicating that these sites exchange at least a factor of ten more rapidly than the other η-OH₂ sites on the Al₃₀.     

Effect of oxygen fugacity on the H2O storage capacity of forsterite in the carbon-saturated systems

High pressure experiments have been performed in the systems Mg₂SiO₄-C-O-H and Mg₂SiO₄-K₂CO₃-C at 6.3 GPa and 1200 to 1600 °C using a split-sphere multi-anvil apparatus. In the Mg₂SiO₄-C-O-H system the composition of fluid was modeled by adding different amounts of water and stearic acid. The fO₂ was controlled by the Mo-MoO₂ or Fe-FeO oxygen buffers. Several experiments in the Mg₂SiO₄-C-O-H system and all experiments in the Mg₂SiO₄-K₂CO₃-C system have been conducted without buffering the fO₂. Forsterite in the system Mg₂SiO₄-K₂CO₃-C does not reveal OH absorption bands in the IR spectra, while forsterite coexisting with carbon-bearing fluid and silicate melt at logfO₂ from FMQ-2 to FMQ-5 (from 2 to 5 log units below fayalite-magnetite-quartz oxygen buffer) contains 800-1850 wt. ppm H₂O. The maximum concentrations were detected at 1400 °C and FMQ-3.5. We observed an increase in the solidus temperature in the system Mg₂SiO₄-C-O-H from 1200 to above 1600 °C with log fO₂ decreasing from FMQ-2 to FMQ-5. The increase of the solidus temperature and the broadening of the stability field of the H₂O-H₂-CH₄ subsolidus fluid phase at 1400-1600 °C explain the high H₂O storage capacity of forsterite relative to that crystallized from carbon-free, oxidized, hydrous, silicic melt. At temperatures above 1400 °C liquidus forsterite precipitated along with diamond from oxidized (FMQ-1) carbonate-silicate melt and from silicate melt dissolving the moderately reduced C-O-H fluid (from FMQ-2 to FMQ-3.5). Formation of diamond was not detected under ultra-reduced conditions (FMQ-5) at 1200-1600 °C. Olivine co-precipitating with diamond from dry carbonate-silicate or hydrous-silicic fluid/melt can provide information on the H₂O contents and speciation of the diamond-forming media in the mantle. The conditions for minimum post-crystallization alteration of olivine and its hydrogen content are discussed.     

Determination of carbon fractionation factor between aqueous carbonate and CO2(g) in two-direction isotope equilibration

The experiments were conducted in the open CO₂ system to find out the equilibrium fractionation between the carbonate ion and CO₂(g). The existence of isotopic equilibrium was checked using the two-direction approach by passing the CO₂−N₂ gases with different δ¹³C compositions (− 1.5‰ and − 23‰) through the carbonate solution with δ¹³C = − 4.2‰. The Δ_{CO3T²⁻−CO2(g)} equilibrium fractionation is given as 6.03 ± 0.17‰ at 25 °C. Discussion is provided about the significance of carbonate complexing in determination of Δ_{CO3T²⁻−CO2(g)} and Δ_{HCO3T⁻−CO2(g)} fractionations. Finally, an isotope numerical model of flow and kinetics of hydration and dehydroxylation is built to predict the isotopic behaviour of the system with time.     

CO2 solubility in Martian basalts and Martian atmospheric evolution

To understand possible volcanogenic fluxes of CO₂ to the Martian atmosphere, we investigated experimentally carbonate solubility in a synthetic melt based on the Adirondack-class Humphrey basalt at 1-2.5 GPa and 1400-1625 °C. Starting materials included both oxidized and reduced compositions, allowing a test of the effect of iron oxidation state on CO₂ solubility. CO₂ contents in experimental glasses were determined using Fourier transform infrared spectroscopy (FTIR) and Fe³⁺/Fe^T was measured by Mössbauer spectroscopy. The CO₂ contents of glasses show no dependence on Fe³⁺/Fe^T and range from 0.34 to 2.12 wt.%. For Humphrey basalt, analysis of glasses with gravimetrically-determined CO₂ contents allowed calibration of an integrated molar absorptivity of 81,500 ± 1500 L mol⁻¹ cm⁻² for the integrated area under the carbonate doublet at 1430 and 1520 cm⁻¹. The experimentally determined CO₂ solubilities allow calibration of the thermodynamic parameters governing dissolution of CO₂ vapor as carbonate in silicate melt, K_II, (Stolper and Holloway, 1988) as follows: , ΔV⁰ = 20.85 ± 0.91 cm³ mol⁻¹, and ΔH⁰ = −17.96 ± 10.2 kJ mol⁻¹. This relation, combined with the known thermodynamics of graphite oxidation, facilitates calculation of the CO₂ dissolved in magmas derived from graphite-saturated Martian basalt source regions as a function of P, T, and f_O2. For the source region for Humphrey, constrained by phase equilibria to be near 1350 °C and 1.2 GPa, the resulting CO₂ contents are 51 ppm at the iron-wüstite buffer (IW), and 510 ppm at one order of magnitude above IW (IW + 1). However, solubilities are expected to be greater for depolymerized partial melts similar to primitive shergottite Yamato 980459 (Y 980459). This, combined with hotter source temperatures (1540 °C and 1.2 GPa) could allow hot plume-like magmas similar to Y 980459 to dissolve 240 ppm CO₂ at IW and 0.24 wt.% of CO₂ at IW + 1. For expected magmatic fluxes over the last 4.5 Ga of Martian history, magmas similar to Humphrey would only produce 0.03 and 0.26 bars from sources at IW and IW + 1, respectively. On the other hand, more primitive magmas like Y 980459 could plausibly produce 0.12 and 1.2 bars at IW and IW + 1, respectively. Thus, if typical Martian volcanic activity was reduced and the melting conditions cool, then degassing of CO₂ to the atmosphere may not be sufficient to create greenhouse conditions required by observations of liquid surface water. However, if a significant fraction of Martian magmas derive from hot and primitive sources, as may have been true during the formation of Tharsis in the late Noachian, that are also slightly oxidized (IW + 1.2), then significant contribution of volcanogenic CO₂ to an early Martian greenhouse is plausible.     

Kinetics of H2-O2-H2O redox equilibria and formation of metastable H2O2 under low temperature hydrothermal conditions

Hydrothermal experiments were conducted to evaluate the kinetics of H_2(aq) oxidation in the homogeneous H₂-O₂-H₂O system at conditions reflecting subsurface/near-seafloor hydrothermal environments (55-250 °C and 242-497 bar). The kinetics of the water-forming reaction that controls the fundamental equilibrium between dissolved H_2(aq) and O_2(aq), are expected to impose significant constraints on the redox gradients that develop when mixing occurs between oxygenated seawater and high-temperature anoxic vent fluid at near-seafloor conditions. Experimental data indicate that, indeed, the kinetics of H_2(aq)-O_2(aq) equilibrium become slower with decreasing temperature, allowing excess H_2(aq) to remain in solution. Sluggish reaction rates of H_2(aq) oxidation suggest that active microbial populations in near-seafloor and subsurface environments could potentially utilize both H_2(aq) and O_2(aq), even at temperatures lower than 40 °C due to H_2(aq) persistence in the seawater/vent fluid mixtures. For these H₂-O₂ disequilibrium conditions, redox gradients along the seawater/hydrothermal fluid mixing interface are not sharp and microbially-mediated H_2(aq) oxidation coupled with a lack of other electron acceptors (e.g. nitrate) could provide an important energy source available at low-temperature diffuse flow vent sites.More importantly, when H_2(aq)-O_2(aq) disequilibrium conditions apply, formation of metastable hydrogen peroxide is observed. The yield of H₂O_2(aq) synthesis appears to be enhanced under conditions of elevated H_2(aq)/O_2(aq) molar ratios that correspond to abundant H_2(aq) concentrations. Formation of metastable H₂O₂ is expected to affect the distribution of dissolved organic carbon (DOC) owing to the existence of an additional strong oxidizing agent. Oxidation of magnetite and/or Fe⁺⁺ by hydrogen peroxide could also induce formation of metastable hydroxyl radicals (•OH) through Fenton-type reactions, further broadening the implications of hydrogen peroxide in hydrothermal environments.     

An experimental study of the solubility of baddeleyite (ZrO2) in fluoride-bearing solutions at elevated temperature

The solubility of baddeleyite (ZrO₂) and the speciation of zirconium have been investigated in HF-bearing aqueous solutions at temperatures up to 400 °C and pressures up to 700 bar. The data obtained suggest that in HF-bearing solutions zirconium is transported mainly in the form of the hydroxyfluoride species ZrF(OH)₃° and ZrF₂(OH)₂°. Formation constants determined for these species (Zr⁴⁺ + nF⁻ + mOH⁻ = ZrF_n(OH)_m°) range from 43.7 at 100 °C to 46.41 at 400 °C for ZrF(OH)₃°, and from 37.25 at 100 °C to 43.88 at 400 °C for ZrF₂(OH)₂°.Although the solubility of ZrO₂ is retrograde with respect to temperature, the measured concentrations of Zr are orders of magnitude higher than those predicted from theoretical extrapolations based on simple fluoride species (ZrF³⁺-ZrF₆²⁻). Model calculations performed for zircon show that zirconium can be transported by aqueous fluids in concentrations sufficient to account for the concentration of this metal at conditions commonly encountered in fluoride-rich natural hydrothermal systems.     

Na2O solubility in CaO-MgO-SiO2 melts

The sodium solubility in silicate melts in the CaO-MgO-SiO₂ (CMS) system at 1400 °C has been measured by using a closed thermochemical reactor designed to control alkali metal activity. In this reactor, Na_(g) evaporation from a Na₂O-xSiO₂ melt imposes an alkali metal vapor pressure in equilibrium with the molten silicate samples. Because of equilibrium conditions in the reactor, the activity of sodium-metal oxide in the molten samples is the same as that of the source, i.e., aNa₂O_(sample) = aNa₂O_(source). This design also allows to determine the sodium oxide activity coefficient in the samples. Thirty-three different CMS compositions were studied. The results show that the amount of sodium entering from the gas phase (i.e., Na₂O solubility) is strongly sensitive to silica content of the melt and, to a lesser extent, the relative amounts of CaO and MgO. Despite the large range of tested melt compositions (0 < CaO and MgO < 40; 40 < SiO₂ < 100; in wt%), we found that Na₂O solubility is conveniently modeled as a linear function of the optical basicity (Λ) calculated on a Na-free basis melt composition. In our experiments, γNa₂O_(sample) ranges from 7 × 10⁻⁷ to 5 × 10⁻⁶, indicating a strongly non-ideal behavior of Na₂O solubility in the studied CMS melts (γNa₂O_(sample) ? 1). In addition to showing the effect of sodium on phase relationships in the CMS system, this Na₂O solubility study brings valuable new constraints on how melt structure controls the solubility of Na in the CMS silicate melts. Our results suggest that Na₂O addition causes depolymerization of the melt by preferential breaking of Si-O-Si bonds of the most polymerized tetrahedral sites, mainly Q⁴.     

Coupled phase and aqueous species equilibrium of the H2O-CO2-NaCl-CaCO3 system from 0 to 250 °C, 1 to 1000 bar with NaCl concentrations up to saturation of halite

A model is developed for the calculation of coupled phase and aqueous species equilibrium in the H₂O-CO₂-NaCl-CaCO₃ system from 0 to 250 °C, 1 to 1000 bar with NaCl concentrations up to saturation of halite. The vapor-liquid-solid (calcite, halite) equilibrium together with the chemical equilibrium of H⁺, Na⁺, Ca²⁺, , Ca(OH)⁺, OH⁻, Cl⁻, , , CO_2(aq) and CaCO_3(aq) in the aqueous liquid phase as a function of temperature, pressure, NaCl concentrations, CO_2(aq) concentrations can be calculated, with accuracy close to those of experiments in the stated T-P-m range, hence calcite solubility, CO₂ gas solubility, alkalinity and pH values can be accurately calculated. The merit and advantage of this model is its predictability, the model was generally not constructed by fitting experimental data.One of the focuses of this study is to predict calcite solubility, with accuracy consistent with the works in previous experimental studies. The resulted model reproduces the following: (1) as temperature increases, the calcite solubility decreases. For example, when temperature increases from 273 to 373 K, calcite solubility decreases by about 50%; (2) with the increase of pressure, calcite solubility increases. For example, at 373 K changing pressure from 10 to 500 bar may increase calcite solubility by as much as 30%; (3) dissolved CO₂ can increase calcite solubility substantially; (4) increasing concentration of NaCl up to 2 m will increase calcite solubility, but further increasing NaCl solubility beyond 2 m will decrease its solubility.The functionality of pH value, alkalinity, CO₂ gas solubility, and the concentrations of many aqueous species with temperature, pressure and NaCl_(aq) concentrations can be found from the application of this model. Online calculation is made available on www.geochem-model.org/models/h2o_co2_nacl_caco3/calc.php.     

Initial behavior of granite in response to injection of CO2-saturated fluid

To understand the initial reactions of granite in a CO₂-saturated hydrothermal system, experiments were conducted using a batch-type autoclave over a temperature range of 100–350 °C at up to 250 bar and numerical computations of phase equilibria based on the experimental results were carried out. The experiments showed that the dissolution of granite and the deposition of secondary minerals were encouraged by the addition of CO₂. Solution chemistry and examination of the granite’s surface texture suggested that its initial dissolution is characterized by the release of Na and Ca (from the dissolution of plagioclase) and that initial precipitation occurs by deposition of some secondary minerals on to plagioclase and/or biotite in the CO₂-saturated system. However, the effect of CO₂ was small at 350 °C owing to the low activity of H₂CO₃. According to EDX analysis and numerical phase equilibrium calculations, the secondary minerals formed might be kaolinite, muscovite, smectite and calcite. That is, the granite as a whole might have the potential to take-up dissolved CO₂. The results suggest that the alteration of granite under CO₂-saturated hydrothermal conditions has the potential to capture CO₂ when it is injected at moderate temperatures (150–250 °C) into granite-hosted rock masses.     

Shear viscosities of CaO-Al2O3-SiO2 and MgO-Al2O3-SiO2 liquids: Implications for the structural role of aluminium and the degree of polymerisation of synthetic and natural aluminosilicate melts

The shear viscosity of 66 liquids in the systems CaO-Al₂O₃-SiO₂ (CAS) and MgO-Al₂O₃-SiO₂ (MAS) have been measured in the ranges 1-10⁴ Pa s and 10⁸-10¹² Pa s. Liquids belong to series, nominally at 50, 67, and 75 mol.% SiO₂, with atomic M²⁺/(M²⁺+2Al) typically in the range 0.60 to 0.40 for each isopleth. In the system CAS at 1600°C, viscosity passes through a maximum at all silica contents. The maxima are clearly centered in the peraluminous field, but the exact composition at which viscosity is a maximum is poorly defined. Similar features are observed at 900°C. In contrast, data for the system MAS at 1600°C show that viscosity decreases with decreasing Mg/(Mg + 2Al) at all silica contents, but that a maximum in viscosity must occur in the field where Mg/2Al >1. On the other hand, the viscosity at 850°C increases with decreasing Mg/(Mg + 2Al) and shows no sign of reaching a maximum, even for the most peraluminous composition studied. The data from both systems at 1600°C have been analysed assuming that shear viscosity is proportional to average bond strength and considering the equilibrium:

Al^[4]-(Mg,Ca)_0.5⇔(Mg,Ca)_0.5-NBO+Al_XS     

Caprock interaction with CO2: A laboratory study of reactivity of shale with supercritical CO2 and brine

Crushed rock from two caprock samples, a carbonate-rich shale and a clay-rich shale, were reacted with a mixture of brine and supercritical CO₂ (CO₂–brine) in a laboratory batch reactor, at different temperature and pressure conditions. The samples were cored from a proposed underground CO₂ storage site near the town of Longyearbyen in Svalbard. The reacting fluid was a mixture of 1 M NaCl solution and CO₂ (110 bar) and the water/rock ratio was 20:1. Carbon dioxide was injected into the reactors after the solution had been bubbled with N₂, in order to mimic O₂-depleted natural storage conditions. A control reaction was also run on the clay-rich shale sample, where the crushed rock was reacted with brine (CO₂-free brine) at the same experimental conditions. A total of 8 batch reaction experiments were run at temperatures ranging from 80 to 250 °C and total pressures of 110 bar (∼40 bar for the control experiment). The experiments lasted 1–5 weeks.Fluid analysis showed that the aqueous concentration of major elements (i.e. Ca, Mg, Fe, K, Al) and SiO₂ increased in all experiments. Release rates of Fe and SiO₂ were more pronounced in solutions reacted with CO₂–brine as compared to those reacted with CO₂-free brine. For samples reacted with the CO₂–brine, lower temperature reactions (80 °C) released much more Fe and SiO₂ than higher temperature reactions (150–250 °C). Analysis by SEM and XRD of reacted solids also revealed changes in mineralogical compositions. The carbonate-rich shale was more reactive at 250 °C, as revealed by the dissolution of plagioclase and clay minerals (illite and chlorite), dissolution and re-precipitation of carbonates, and the formation of smectite. Carbon dioxide was also permanently sequestered as calcite in the same sample. The clay-rich shale reacted with CO₂–brine did not show major mineralogical alteration. However, a significant amount of analcime was formed in the clay-rich shale reacted with CO₂-free brine; while no trace of analcime was observed in either of the samples reacted with CO₂–brine.     

The noble gas geochemistry of natural CO2 gas reservoirs from the Colorado Plateau and Rocky Mountain provinces, USA

Identification of the source of CO₂ in natural reservoirs and development of physical models to account for the migration and interaction of this CO₂ with the groundwater is essential for developing a quantitative understanding of the long term storage potential of CO₂ in the subsurface. We present the results of 57 noble gas determinations in CO₂ rich fields (>82%) from three natural reservoirs to the east of the Colorado Plateau uplift province, USA (Bravo Dome, NM., Sheep Mountain, CO. and McCallum Dome, CO.), and from two reservoirs from within the uplift area (St. John’s Dome, AZ., and McElmo Dome, CO.). We demonstrate that all fields have CO₂/³He ratios consistent with a dominantly magmatic source. The most recent volcanics in the province date from 8 to 10 ka and are associated with the Bravo Dome field. The oldest magmatic activity dates from 42 to 70 Ma and is associated with the McElmo Dome field, located in the tectonically stable centre of the Colorado Plateau: CO₂ can be stored within the subsurface on a millennia timescale.The manner and extent of contact of the CO₂ phase with the groundwater system is a critical parameter in using these systems as natural analogues for geological storage of anthropogenic CO₂. We show that coherent fractionation of groundwater ²⁰Ne/³⁶Ar with crustal radiogenic noble gases (⁴He, ²¹Ne, ⁴⁰Ar) is explained by a two stage re-dissolution model: Stage 1: Magmatic CO₂ injection into the groundwater system strips dissolved air-derived noble gases (ASW) and accumulated crustal/radiogenic noble gas by CO₂/water phase partitioning. The CO₂ containing the groundwater stripped gases provides the first reservoir fluid charge. Subsequent charges of CO₂ provide no more ASW or crustal noble gases, and serve only to dilute the original ASW and crustal noble gas rich CO₂. Reservoir scale preservation of concentration gradients in ASW-derived noble gases thus provide CO₂ filling direction. This is seen in the Bravo Dome and St. John’s Dome fields. Stage 2: The noble gases re-dissolve into any available gas stripped groundwater. This is modeled as a Rayleigh distillation process and enables us to quantify for each sample: (1) the volume of groundwater originally ‘stripped’ on reservoir filling; and (2) the volume of groundwater involved in subsequent interaction. The original water volume that is gas stripped varies from as low as 0.0005 cm³ groundwater/cm³ gas (STP) in one Bravo Dome sample, to 2.56 cm³ groundwater/cm³ gas (STP) in a St. John’s Dome sample. Subsequent gas/groundwater equilibration varies within all fields, each showing a similar range, from zero to ∼100 cm³ water/cm³ gas (at reservoir pressure and temperature).     

The separate production of H2S from the thermal reaction of hydrocarbons with magnesium sulfate and sulfur: Implications for thermal sulfate reduction

The yields and stable C and H isotopic composition of gaseous products from the reactions of pure n-C₂₄ with (1) MgSO₄; and (2) elemental S in sealed Au-tubes at a series of temperatures over the range 220–600 °C were monitored to better resolve the reaction mechanisms. Hydrogen sulfide formation from thermochemical sulfate reduction (TSR) of n-C₂₄ with MgSO₄ was initiated at 431 °C, coincident with the evolution of C₂–C₅ hydrocarbons. Whereas the yields of H₂S increased progressively with pyrolysis temperature, the hydrocarbon yields decreased sharply above 490 °C due to subsequent S consumption. Ethane and propane were initially very ¹³C depleted, but became progressively heavier with pyrolysis temperature and were more ¹³C enriched than the values of a control treatment conducted on just n-C₂₄ above 475 °C. TSR of MgSO₄ also led to progressively higher concentrations of CO₂ showing relatively low δ¹³C values, possibly due to input of isotopically light CO₂ derived from gaseous hydrocarbon oxidation (e.g., more depleted CH₄).