Chemical controls on the solubility, speciation and mobility of lanthanum at near surface conditions: A geochemical modeling study

Recent experimental determinations of the solubility products of common rare earth minerals such as monazite and xenotime and stability constants for chloride, sulfate, carbonate and hydroxide complexes provide a basis to model quantitatively the solubility, and therefore the mobility, of rare earth elements (REE) at near surface conditions. Data on the mobility of REE and stabilities of REE complexes at near-neutral conditions are of importance to safe nuclear waste disposal, and environmental monitoring. The aim of this study is to understand REE speciation and solubility of a given REE in natural environments. In this study, a series of formation constants for La aqueous complexes are recommended by using the specific interaction theory (SIT) for extrapolation to infinite dilution. Then, a thermodynamic model has been employed for calculation of the solubility and speciation of La in soil solutions reacted with the La end-member of mineral monazite (LaPO₄), and other La-bearing solid phases including amorphous lanthanum hydroxide (La(OH)₃, am) and different La carbonates, as a function of various inorganic and organic ligand concentrations. Calculations were carried out at near-neutral pH (pH 5.5–8.5) and 25 °C at atmospheric CO₂ partial pressure. The model takes account of the species: La³⁺, LaCl²⁺, , , , , , , , , La(OH)²⁺, LaOx⁺, , LaAc²⁺ and (where Ox²⁻ = oxalate and Ac⁻ = acetate).The calculations indicate that the La species that dominate at pH 5.5–8.5 in the baseline model soil solution (BMSS) include La³⁺, LaOx⁺, , and in order of increasing importance as pH rises. The solubility of monazite in the BMSS remains less than 3 × 10⁻⁹ M, exhibiting a minimum of 2 × 10⁻¹² M at pH 7.5. The calculations quantitatively demonstrate that the concentrations of La controlled by the solubility of other La-bearing solid phases are many orders of magnitude higher than those controlled by monazite in the pH range from 5.5 to 8.5, suggesting that monazite is likely to be the solubility-controlling phase at this pH range. The calculations also suggest that significant mobility of La (and other REE) is unlikely because high water–rock ratios on the order of at least 10⁴ (mass ratio) are required to move 50% of the La from a soil. An increase in concentration of oxalate by one order of magnitude from that of the baseline model solution results in the dominance of LaOx⁺ at pH 5.5–7.5. Similarly, the increase in concentration of by one order of magnitude makes the dominant species at pH 5.5–7.5. Above pH 7.5, carbonate complexes are important. The increase in oxalate or concentrations by one order of magnitude can enhance the solubility of monazite by a factor of up to about 6 below neutral pH, in comparison with that in the baseline model soil solution. From pH 7.0 to 8.5, the solubility of monazite in the soil solutions with higher concentrations of oxalate or is similar, or almost identical, to that in the BMSS.     

Environmental isotopes (N, S, C, O, D) to determine natural attenuation processes in nitrate contaminated waters: Example of Osona (NE Spain)

Nitrate-contaminated groundwater from an aquifer in the Osona region (NE Spain) was chemically and isotopically (δ¹⁵N_NO3,δ¹⁸O_NO3,δ³⁴S_SO4,δ¹⁸O_SO4, δD, δ¹⁸O_H2O and δ¹³C_DIC) characterized. Diffuse- contamination reached values of 366 mg/L. Nearly 75% of the 37 sampled sites had higher concentrations than the 50 mg/L in limit for drinking water. To identify the source of pollution δ¹⁵N_NO3 and δ¹⁸O_NO3 were used with results, for most samples, in the range of pig manure . Nitrification processes were evaluated by means of the δ¹⁸O of and water. Isotopic data suggested that natural attenuation of was taking place. This process was confirmed using the δ¹⁸O_NO3 coupled with the ratio, avoiding the influence of continuous inputs. A further insight on denitrification processes was obtained by analyzing the ions involved in denitrification reactions. Although the role of organic matter oxidation could neither be confirmed nor discarded, this approach showed a link between denitrification and pyrite oxidation. Therefore, in areas with no adequate infrastructure (e.g. multipiezometers), such as the one studied, this approach could be useful for implementing better water management.     

Surface and groundwater quality in the northeastern region of Buenos Aires Province, Argentina

This work studies the water quality of the Pergamino–Arrecifes River zone in the Rolling Pampa, northeast Buenos Aires Province, Argentina. Temperature, pH, specific conductivity, Na, K, Mg, Ca, , Cl⁻, , , Si, Ag, Al, As, B, Ba, Be, Br, Cd, Co, Cr, Cu, Fe, Hg, Li, Mn, Mo, Ni, P, Pb, Se, Tl, U, V, Zn, and the environmental stable δ¹⁸O and δ²H isotope ratios were determined in 18 sampling stations. Natural and anthropogenic features influence surface and groundwater quality. Point pollution sources (septic wells and other domestic and farming effluents) increase the nitrate concentration. The values of pH, , Al, As, B, Fe, and Mn exceed the respective Argentine reference thresholds in different sampling stations for human drinking water; B, Mo, U, and V for irrigation; and V and Zn for cattle consumption.     

Structural and geochronological constraints on the tectono-thermal evolution of the Danba domal terrane, eastern margin of the Tibetan plateau

The Songpan-Ganze terrane of the Tibetan plateau is underlain by Neoproterozoic crystalline basement rocks of the Yangtze block. These basement rocks are exposed as a series of extensional tectonic domes that form a nearly north–south trending extensional belt more than 1000 km long in the eastern margin of the Tibetan plateau. In the Danba area, detachment faults separate the basement core complexes (e.g., the Gezong and Gongcai complexes) from the Paleozoic strata which have been thinned or removed completely. The cover sequences have undergone upper greenschist to lower amphibolite facies metamorphism to form the Danba schist and are overlain by the Triassic Xikang Group, a thick flysch sequence. Both the basement rocks and the Paleozoic rocks have undergone multiple stages of deformation and thus provide an excellent opportunity to study the tectono-thermal evolution of the eastern margin of the Tibetan plateau. Two stages of deformation, corresponding to three generations of foliation (S₁, , and ), have been recognized on the basis of structural and microscopic observations. We selected amphibole and biotite separates associated with distinct generations of foliation for ⁴⁰Ar/³⁹Ar dating using laser microprobe incremental heating technique to place numerical constraints on the major tectono-thermal events within the Danba area. The geochronogical results reveal an earliest metamorphic event at 258.6 ± 0.5 Ma (S₁ biotite) and 263.6 ± 0.8 Ma (S₁ amphibole), coinciding temporally with the mantle plume that produced the voluminous Emeishan flood basalts. The second event was a progressive extensional deformation first occurred at 159–166 Ma ( amphibole) responsible for the earlier tectonic doming of the crystalline basement, and then the final tectono-thermal overprint recorded by foliation and metamorphism locally in the core complexes at 47–58 Ma for the Gezong complex and 64–81 Ma for the Gongcai complex. This major post-orogenic extensional event is believed to be a consequence of collision between the North China and South China blocks. The apparent discrepancy of the ⁴⁰Ar/³⁹Ar ages observed between localities suggests a slow cooling process associated with progressive uplift.     

Solubility and dissolution rate of silica in acid fluoride solutions

We performed 57 batch reactor experiments in acidic fluoride solutions to measure the dissolution rate of quartz. These rate data along with rate data from published studies were fit using multiple linear regression to produce the following non-unique rate law for quartz

where 10^−5.13 < a_HF < 10^1.60, −0.28 < pH < 7.18, and 298 < T < 373 K. Similarly, 97 amorphous silica dissolution rate data from published studies were fit by multiple linear regression to produce the following non-unique rate law for amorphous silica

where 10^−2.37 < a_HF < 10^1.61, −0.32 < pH < 4.76 and 296 < T < 343 K. Regression of the rates versus other combinations of solution species, e.g.  + H⁺, F⁻ + H⁺, HF + , HF + F⁻, or  + F⁻, produced equally good fits. Any of these rate laws can be interpreted to mean that the rate-determining step for silica dissolution in fluoride solutions involves a coordinated attack of a Lewis acid, on the bridging O atom and a Lewis base on the Si atom. This allows a redistribution of electrons from the Si–O bond to form a O–H group and a Si–FH group.     

Relative contributions of sulfate- and iron(III) reduction to organic matter mineralization and process controls in contrasting habitats of the Georgia saltmarsh

The objectives of this study were to partition out the predominant anaerobic respiration pathways coupled to C oxidation and to further elucidate the controls of anaerobic C respiration in three major saltmarsh habitats at Skidaway Island, GA; the short form of Spartina alterniflora (SS), the tall form of S. alterniflora (TS), and unvegetated, bioturbated creekbank (CB). Geochemical analysis of pore water and solid phase constituents revealed that the SS site experienced highly reducing conditions with two orders of magnitude higher pore water sulfide inventories (1.884 mmol m⁻²) than TS (0.003 mmol m⁻²) and CB (0.005 mmol m⁻²), respectively. Conversely, reactive Fe(III) inventories at TS (2208 mmol m⁻²) and CB (2881 mmol m⁻²) were up to 7–9 times higher than at SS (338 mmol m⁻²). Incubations and intact core experiments indicated that reduction accounted for 95% (SS), 37% (TS) and 66% (CB) of total anaerobic respiration. There was no detectable Fe(III) reduction at SS, while Fe(III) reduction accounted for up to 70% of C oxidation in the 3–6 cm depth interval at TS and 0–3 cm depth of CB, and on average, approximately 55% of C oxidation over two-thirds of marsh surface area. Laboratory manipulations provided further evidence for the importance of Fe(III) reduction as the accumulation rates of fermentation products were high when Fe(III) reduction was inhibited by removing the Fe(III) minerals from highly bioturbated CB sediments with higher Fe(III) mineral contents. Anaerobic C oxidation, - and Fe(III)-reduction rates appeared to be highest at the TS site during active plant growth in summer. Overall results suggest that bioturbation by macrofauna is the overriding factor in modulating the pathway of C mineralization in the saltmarsh, whereas availability of organic substrates from plants is a key factor in controlling the C oxidation rate.     

Clinopyroxene dissolution in basaltic melt

The history of magmatic systems may be inferred from reactions between mantle xenoliths and host basalt if the thermodynamics and kinetics of the reactions are quantified. To study diffusive and convective clinopyroxene dissolution in silicate melts, diffusive clinopyroxene dissolution experiments were conducted at 0.47–1.90 GPa and 1509–1790 K in a piston-cylinder apparatus. Clinopyroxene saturation is found to be roughly determined by MgO and CaO content. The effective binary diffusivities, D_MgO and D_CaO, and the interface melt saturation condition, , are extracted from the experiments. D_MgO and D_CaO show Arrhenian dependence on temperature. The pressure dependence is small and not resolved within 0.47–1.90 GPa. in the interface melt increases with increasing temperature, but decreases with increasing pressure. Convective clinopyroxene dissolution, where the convection is driven by the density difference between the crystal and melt, is modeled using the diffusivities and interface melt saturation condition. Previous studies showed that the convective dissolution rate depends on the thermodynamics, kinetics and fluid dynamics of the system. Comparing our results for clinopyroxene dissolution to results from a previous study on convective olivine dissolution shows that the kinetic and fluid dynamic aspects of the two minerals are quite similar. However, the thermodynamics of clinopyroxene dissolution depends more strongly on the degree of superheating and composition of the host melt than that of olivine dissolution. The models for clinopyroxene and olivine dissolution are tested against literature experiments on mineral–melt interaction. They are then applied to previously proposed reactions between Hawaii basalts and mantle minerals, mid-ocean ridge basalts and mantle minerals, and xenoliths digestion in a basalt at Kuandian, Northeast China.     

Hydration in solution is critical for stable oxygen isotope fractionation between carbonate ion and water

Various isotope studies require accurate fractionation factors (α’s) between different chemical compounds in thermodynamic equilibrium. Although numerous isotope systems involve aqueous solutions, the conventional theory is formulated for the gas-phase and predicts incorrect α’s for many compounds dissolved in water. Here I show that quantum-chemistry calculations, which take into account solute–water interactions, accurately predict, for instance, oxygen isotope fractionation between dissolved and H₂O (hereafter ). Simple force field and quantum-chemistry calculations for the ‘gas-phase’ ion predict (15‰) at 25 °C. However, based on -clusters with up to 22 H₂O molecules, I calculate a value of 25‰, which agrees with the experimental value of 24.5 ± 0.5‰. Effects of geometry and anharmonicity on the calculated α were also examined. The calculations reveal the critical role of hydration in solution, which is ignored in the gas-phase theory. The approach presented provides an adequate framework for calculating fractionation factors involving dissolved compounds; it may also be used to predict α’s that cannot (or have not yet been) determined experimentally.     

Spatial and seasonal variation of major ions in Himalayan snow and ice: A source consideration

The spatial and temporal variation of major ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, , , and Cl⁻) in Himalayan snow and ice is investigated by using two snow pits from the East Rongbuk glacier (28°01′N, 86°58′E, 6500 m a.s.l.), one snow pit from the Nangpai Gosum glacier (28°03′N, 86°39′E, 5700 m a.s.l.), one snow pit from the Gyabrag glacier (28°11′N, 86°38′E, 6303 m a.s.l.), and three ice cores from the Sentik (35°59′N, 75°58′E, 4908 m a.s.l.), Dasuopu (28°33′N, 85°44′E, 7000 m a.s.l.), and East Rongbuk (27°59′N, 86°55′E, 6450 m a.s.l.) glaciers, respectively. In general, the major ions show a significant seasonal variation, with high concentrations during the non-monsoon (pre-monsoon and post-monsoon) season and relatively low concentrations during the monsoon season. Monsoon precipitation with high local/regional dust loading related to summer circulation is possibly responsible for the high concentrations occurring sporadically during the monsoon season. The crest of the Himalayas is an effective barrier to the spatial distribution of Na⁺, Cl⁻ and concentrations, but not to the major ions associated with dust influx (e.g. Ca²⁺ and Mg²⁺). Atmospheric backward trajectories from the HYSPLIT_4 model used in identifying chemical species sourcing suggest that the major ions in the Himalayan snow and ice come mainly from the Thar Desert located in the North India, as well as West Asia, or even the distant Sahara Desert in the North Africa during the winter and spring seasons. This is different from the conventionally assumed arid and semi-arid regions of the central Asia. Factors, such as different vapor sources due to atmospheric circulation patterns and geographical features (e.g. altitude, topography), may contribute to the differences in major ionic concentrations between the western and eastern Himalayas.     

Isotopic composition of nitrate in five German rivers discharging into the North Sea

We determined concentrations and isotopic composition of nitrate in five German rivers (Rhine, Elbe, Weser, Ems, and Eider) that discharge into the North Sea. Samples were obtained on a biweekly to monthly basis and chemical and isotopic analyses were conducted for the period January 2006 to March 2007 at sampling stations situated before estuarine mixing with North Sea water. We observed maximum nitrate loads in winter and fall, when both discharge and concentration of nitrate are highest. Mean annual isotope values in nitrate ranged from 8.2‰ to 11.3‰ for and 0.4‰ to 2.2‰ for . The ranges of isotope values suggest that nitrate in these rivers derives from soil nitrification, sewage, and/or manure. These and published data on other rivers in northern Europe and northern America reveal a correlation between agricultural land use (>60% in the catchment areas of rivers examined) and values. The rivers Rhine, Elbe, Weser and Ems show similar seasonal patterns of the isotopic fractionation of nitrate with increasing values and simultaneously decreasing concentrations during summer months, indicating that assimilation of nitrate is the main fractionation process of riverine nitrate. Isotopic signals in winter are more depleted than the mean summer isotope values, attributed to less microbial activity and assimilative processes. Load weighted nitrate δ¹⁵N of the riverine input to the German Bight Coastal Water mass before estuarine mixing and processing is between 8‰ and 12‰. The high δ¹⁵N value of river nitrate is matched by high δ¹⁵N of nitrate in surface sediments in the German Bight.     

Oxygen isotopic fractionation during inorganic calcite precipitation ― Effects of temperature, precipitation rate and pH

Stable oxygen isotopic fractionation during inorganic calcite precipitation was experimentally studied by spontaneous precipitation at various pH (8.3 < pH < 10.5), precipitation rates (1.8 < log R < 4.4 μmol m^− 2 h^− 1) and temperatures (5, 25, and 40 °C) using the CO₂ diffusion technique.The results show that the apparent stable oxygen isotopic fractionation factor between calcite and water (α_{calcite–water}) is affected by temperature, the pH of the solution, and the precipitation rate of calcite. Isotopic equilibrium is not maintained during spontaneous precipitation from the solution. Under isotopic non-equilibrium conditions, at a constant temperature and precipitation rate, apparent 1000lnα_{calcite–water} decreases with increasing pH of the solution. If the temperature and pH are held constant, apparent 1000lnα_{calcite–water} values decrease with elevated precipitation rates of calcite. At pH = 8.3, oxygen isotopic fractionation between inorganically precipitated calcite and water as a function of the precipitation rate (R) can be described by the expressions

at 5, 25, and 40 °C, respectively.The impact of precipitation rate on 1000lnα_{calcite–water} value in our experiments clearly indicates a kinetic effect on oxygen isotopic fractionation during calcite precipitation from aqueous solution, even if calcite precipitated slowly from aqueous solution at the given temperature range. Our results support Coplen's work [Coplen T. B. (2007) Calibration of the calcite–water oxygen isotope geothermometer at Devils Hole, Nevada, a natural laboratory. Geochim. Cosmochim. Acta 71, 3948–3957], which indicates that the equilibrium oxygen isotopic fractionation factor might be greater than the commonly accepted value.     

Gradients controlling natural attenuation of ammonium

Oxidation of reduced pollutants such as in groundwater often takes place at steep redox gradients where oxygenated water is being mixed into polluted water such as landfill leachate. In order to identify controlling parameters and quantify the influence of environmental factors for degradation, sensitivity analysis was performed by means of scenario specific numerical modelling. Geometrical factors such as aquifer thickness have been shown to be very influential on the capability of natural attenuation of pollutants in groundwater. The scenarios investigated here include biodegradation at redox gradients in groundwater, so called fringe processes, for (i) a partly contaminated aquifer with two reaction fronts, (ii) and a spatially variable aquifer thickness. In addition, (iii) the influence of groundwater recharge and (iv) restricted supply of O₂ to contaminated water by slow dispersion and diffusion across the capillary fringe are investigated. Contaminated aquifer thickness, zones of enhanced mixing due to flow focussing and diffusion/dispersion coefficients in the capillary fringe are identified qualitatively as controlling factors for natural attenuation under complex conditions, whereas predictive functions will require further research.     

Determination of diffusion coefficients of hydrogen in fused silica between 296 and 523 K by Raman spectroscopy and application of fused silica capillaries in studying redox reactions

Diffusion coefficients (D) of hydrogen in fused silica capillaries (FSC) were determined between 296 and 523 K by Raman spectroscopy using CO₂ as an internal standard. FSC capsules (3.25 × 10⁻⁴ m OD, 9.9 × 10⁻⁵ m ID, and 0.01 m long) containing CO₂ and H₂ were prepared and the initial relative concentrations of hydrogen in these capsules were derived from the Raman peak-height ratios between H₂ (near 587 cm⁻¹) and CO₂ (near 1387 cm⁻¹). The sample capsules were then heated at a fixed temperature (T) at one atmosphere to let H₂ diffuse out of the capsule, and the changes of hydrogen concentration were monitored by Raman spectroscopy after quench. This process was repeated using different heating durations at 296 (room T), 323, 375, 430, 473, and 523 K; the same sample capsule was used repeatedly at each temperature. The values of D (in m² s⁻¹) in FSC were obtained by fitting the observed changes of hydrogen concentration in the FSC capsule to an equation based on Fick’s law. Our D values are in good agreement with the more recent of the two previously reported experimental data sets, and both can be represented by:

where R is the gas constant (8.3145 J/mol K), T in Kelvin, and errors at 1σ level. The slope corresponds to an activation energy of 44.59 ± 0.14 kJ/mol.The D in FSC determined at 296 K is about an order of magnitude higher than that in platinum at 723 K, indicating that FSC is a suitable membrane for hydrogen at temperature between 673 K and room temperature, and has a great potential for studying redox reactions at these temperatures, especially for systems containing organic material and/or sulphur.     

Solubility of Pt in sulphide mattes: Implications for the genesis of PGE-rich horizons in layered intrusions

The partitioning of Pt in sulphide melt (matte) has been studied as a function of fS₂ and fO₂ at 1200 and 1300 °C. The results show that the solubility of Pt in mattes increases strongly with increasing fS₂ and decreases weakly with increasing fO₂. The increase in Pt solubility with increasing fS₂ is attributed to Pt dissolving in the melt as a sulphide species and the weak inverse dependence of Pt solubility on fO₂ to the diluting effect of increasing O in the melt at high fO₂. These results, coupled with measurements of Pt solubility in silicate melts taken from the literature, allow the calculation of Pt matte/silicate-melt partition coefficients () for a range of conditions pertinent to the formation of Pt-rich horizons in layered intrusions. The calculated values range between 10⁷ and 10¹¹, depending on fO₂ and fS₂, several orders of magnitude higher than previously published values. Our preferred value for for conditions appropriate to the Merensky Reef is 10⁷ and for the Stillwater Pt-rich horizon 10⁸. The new results are consistent with the magmatic hypothesis for Pt-rich horizons in layered intrusions.     

A very hydrous mantle under the western Pacific region: Implications for formation of marginal basins and style of Archean plate tectonics

The western Pacific region has been refrigerated by the subducting cold oceanic plates since 450 Ma. However, the region is also characterized by the presence of many oceanic microplates less than 1300 km across, as well as active magmatism; the Philippine Sea plate is representative. We have compiled and examined petrochemical characters of drilled basalts of DSDP from the Philippine Sea plate, and conclude that the source mantle for oceanic basalts is rich in water ca. 0.2 wt.%, and is 50–60 °C lower than that for MORB. The extensive melting is due to the high water content in the source mantle.It is well known that some marginal basins apparently have greater depths than the major oceans. We calculated the age–depth correlation based on a model of transient half-space cooling at given parameters of temperatures of mantle and surface, 1280 and 0 °C, and the thermal diffusivity, 1 mm² s^− 1. The calculation shows the correlation of age-residual depth from a mid-oceanic ridge is 367 for the Philippine Sea, consistent with the bathymetric data. Moreover, the mid-oceanic ridge may be relatively deep because this region is underlain by the cooler mantle.Addition of water to the mantle peridotite lowers the solidus temperature and viscosity. Melting experiments of hydrous peridotite show that addition of 0.2 wt.% H₂O content lowers the solidus temperature by 150 °C. As a result, the mantle under the region may practically correspond to a ca. 90 °C hotter mantle than normal MORB-source mantle in terms of magmatism and rheology. Numerical simulation for a hotter mantle suggests that many small plates should be formed because of extensive heat release by active magmatism, consistent with many microplates in this region. The presence of many oceanic microplates may be analogous to Archean plate tectonics, characterized by a hotter mantle.     

A method for assessment of excavation damaged zone (EDZ) of a rock mass and its application to a dam foundation case

Excavation of slope is an unloading process, and also an energy releasing process when the stress in the rock mass is unloaded, leading to the rebounding deformation towards the free face. And then a zone, namely excavation damaged zone (EDZ), will be formed near the excavation boundary, where the rock is damaged or disturbed and its physical, mechanical and hydraulic properties are greatly changed. This paper is to put forward a new method to quantify the extent and damage degree of the EDZ according to the unloading strain energy which is released in the process of excavation. A simple relationship between unloading strain ε and accumulative opening displacement T of cracks, , is proposed to describe the damage degree of rock mass in the EDZ, where h is the depth variable in normal direction of the open cracks.This method is used to assess the EDZ of the dam foundation at Xiaowan hydropower station in southwestern China by the accumulative opening displacement curves of cracks along boreholes based on boreholes camera photos. We find that the EDZ can be naturally divided into two sub-zones, i.e. the excavation heavily disturbed sub-zone (EHDZ) and excavation slightly disturbed sub-zone (ESDZ), according to the variation of unloading strain ε. Two inflexions of the accumulative opening displacement curve indicate the lower limits of EHDZ and ESDZ respectively.     

A first principles study of the oxidation energetics and kinetics of realgar

Quantum-mechanical calculations allow resolving and quantifying in detail important aspects of reaction mechanisms such as spin transitions and oxygen dissociation that can be the major rate-limiting steps in redox processes on sulfide and oxide surfaces. In addition, this knowledge can help experimentalists in setting up the framework of rate equations that can be used to describe the kinetics of, e.g., oxidation processes. The unique molecular crystal structure of realgar, As₄S₄ clusters held together by van der Waals bonds, allows for a convenient quantum-mechanical (q.m.) cluster approach to investigate the thermodynamics and kinetic pathways of oxidation. The interaction of As₄S₄ clusters with oxygen and co-adsorbed ions provides a model system for understanding the molecular-scale processes that underpin empirically-derived rate expressions, and provides clues to the oxidation mechanisms of other sulfides and oxides. Two activated processes are shown to dominate the kinetics of oxidation by molecular oxygen: (i) a paramagnetic ³O₂ to diamagnetic ¹O₂ spin transition and (ii) oxygen dissociation on the surface, in that order. The activation energies for the spin transition and O₂ dissociation step were determined to be 1.1 eV (106 kJ/mol) and 0.9 eV (87 kJ/mol), respectively, if molecular oxygen is the only reactant on the surface. In the case of As₄S₄, q.m. calculations reveal that ³O₂ transfers its spin to the cluster and forms a low-spin, peroxo intermediate on the surface before dissociating. The adsorption of a hydroxide ion on the surface proximate to the ³O₂ adsorption site changes the adsorption mechanism by lowering the activation energy barriers for both the spin transition (0.30 eV/29 kJ/mol) and the O₂ dissociation step (0.72 eV/69 kJ/mol). Thus, while spin transition is rate limiting for oxidation with O₂ alone, dissociation becomes the rate-limiting step for oxidation with co-adsorption of OH⁻. First-principles, periodic calculations of the realgar surface show that the energetics and structural changes that accompany oxidation of As₄S₄ clusters on the surface are similar to those involving individual As₄S₄ clusters. Thus, assuming that an As₄S₄ cluster with an adsorbed hydroxyl group is a reasonable approximation of the surface of As₄S₄ at high pH, the theoretically calculated oxidation rate (∼1 × 10⁻¹⁰ mol m⁻² s⁻¹) is of the same order as empirically-derived rates from experiments at T = 298 K, pH = 8, and similar dissolved oxygen concentrations. In addition, the co-adsorption of other anions found in alkaline waters (i.e. carbonate, bicarbonate, sulfate, and sulfite) were shown to energetically promote the oxidation of As₄S₄ (on the order of 5-40 kJ/mol depending on the co-adsorbed anion, OH⁻, , , , or , and accounting for changes in the hydration of products and reactants). The effect of the co-adsorbate on the kinetics and thermodynamics of oxidation is due to each adsorbate modifying the electronic and structural environment of the other adsorption site.Activation-energy barriers due to spin transitions are rarely discussed in the literature as key factors for controlling oxidation rates of mineral surfaces, even though the magnitude of these barriers is enough to alter the kinetics significantly. The attenuation of the activation energy by co-adsorbed anions suggests the possibility of pH− or p(co-adsorbate)-dependent activation energies that can be used to refine oxidation rate laws for sulfide minerals and other, especially semiconducting minerals, such as oxides.     

The relationship between collision-related calcalkaline, and within-plate alkaline volcanism in the Karacadağ Area (Konya-Türkiye, Central Anatolia)

The Karacadağ (Kulu-Konya) area is one of the main volcanic provinces in Central Anatolia. The Karacadağ volcanites are composed of large volumes of andesitic-dacitic lavas associated with pyroclastics and small volumes of alkali basalt, trachybasalt and trachyandesite lavas. Two groups of volcanic rocks can be distinguished: (1) calcalkaline rocks including andesites and dacites, and (2) alkaline rocks including basalts, trachybasalts and trachyandesites. ⁴⁰Ar/³⁹Ar ages show that the Karacadağ volcanites were erupted during Early Miocene (ca.18–19 Ma) and suggest that alkaline volcanites succeed shortly afterwards calcalkaline volcanites. Major oxides and trace elements plotted versus SiO₂ suggest fractionation of hornblende, Fe–Ti oxide and apatite for calcalkaline volcanic rocks and olivine, clinopyroxene and Fe–Ti oxide for alkaline volcanic rocks in the magmatic evolution. The incompatible trace element patterns of the calcalkaline volcanites show enrichment of LILEs (Sr, K, Rb, Ba and Th) and negative HFSEs (Nb, Ta) anomalies suggesting an enriched lithospheric source by a subduction-related process. On the other hand, alkaline volcanites show enrichment of both LILEs and HFSEs suggesting an enriched lithospheric source by small volume melts from the asthenosphere. The rocks also have moderately fractionated REE patterns with (La/Lu)_N ratios of 7–24 for calcalkaline and 6–17 for alkaline volcanites. Moreover, the volcanites have relatively low ⁸⁷Sr/⁸⁶Sr(t) ratios for between 0.703782 and 0.705129, and high ε_Nd(t) values between +2.25 and +4.49. Generally, the Sr–Nd isotopic compositions of the rocks range from the mantle array to bulk earth. All of these observations and findings suggest that the calcalkaline volcanites were formed in a subduction modified orogenic setting, and the alkaline volcanites in a within-plate setting.     

Tephras in lacustrine sediments of the Sarliève marsh (French Massif Central): age and preservation

The Sarliève marsh sediments (Massif Central, France) contain two tephras. The first tephra [, ca. 12 000 BP], regionally well known, enables to date the beginning of lacustrine infill to the Lateglacial. The second tephra, the ‘tephra de Sarliève’, the emitting volcano of which is unknown, would be dated to around the Early Subboreal from pollen data. This occurrence, after the discovery of the ‘tephra de Beaunit’, emphasizes that volcanic eruption(s) occurred in the ‘Chaîne des Puys’ or in the volcanic Cézallier more than 1000 years after the last known eruption (Pavin) in the ‘Chaîne des Puys’ at around 6.6/6.7 ka (5800/5900 BP). In the Sarliève piles, these tephras, well preserved in thick and more silicated deposits of deltas, were not observed in carbonated basin sediments where they were altered. The abundance of authigenic zeolites formed during the Lateglacial in restricted depocentre lacustrine waters allows us to detect initial CF1 tephra occurrence. To cite this article: A. Fourmont et al., C. R. Geoscience 338 (2006).     

Dawsonite synthesis and reevaluation of its thermodynamic properties from solubility measurements: Implications for mineral trapping of CO2

Over the last decade, a significant research effort has focused on determining the feasibility of sequestering large amounts of CO₂ in deep, permeable geologic formations to reduce carbon dioxide emissions to the atmosphere. Most models indicate that injection of CO₂ into deep sedimentary formations will lead to the formation of various carbonate minerals, including the common phases calcite (CaCO₃), dolomite (CaMg(CO₃)₂), magnesite (MgCO₃), siderite (FeCO₃), as well as the far less common mineral, dawsonite (NaAlCO₃(OH)₂). Nevertheless, the equilibrium and kinetics that control the precipitation of stable carbonate minerals are poorly understood and few experiments have been performed to validate computer codes that model CO₂ sequestration.In order to reduce this uncertainty we measured the solubility of synthetic dawsonite according to the equilibrium: , from under- and oversaturated solutions at 50-200 °C in basic media at 1.0 mol · kg⁻¹ NaCl. The solubility products (Q_s) obtained were extrapolated to infinite dilution to obtain the solubility constants (. Combining the fit of these values and fixing  at 25 °C, which was derived from the calorimetric data of Ferrante et al. [Ferrante, M.J., Stuve, J.M., and Richardson, D.W., 1976. Thermodynamic data for synthetic dawsonite. U.S. Bureau of Mines Report Investigation, 8129, Washington, D.C., 13p.], the following thermodynamic parameters for the dissolution of dawsonite were calculated at 25 °C: , and . Subsequently, we were able to derive values for the Gibbs energy of formation (, enthalpy of formation ( and entropy ( of dawsonite. These results are within the combined experimental uncertainties of the values reported by Ferrante et al. (1976). Predominance diagrams are presented for the dawsonite/boehmite and dawsonite/bayerite equilibria at 100 °C in the presence of a saline solution with and without silica-containing minerals.