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1.
Hydration of rhyolitic glass during weathering as characterized by IR microspectroscopy 总被引：1，自引：0，他引：1
Tadashi Yokoyama Satoshi Okumura Satoru Nakashima 《Geochimica et cosmochimica acta》2008,72(1):117-125
The mechanism and rate of hydration of rhyolitic glass during weathering were studied. Doubly polished thin sections of two rhyolites with different duration of weathering (Ohsawa lava: 26,000 yr, Awanomikoto lava: 52,000 yr) were prepared. Optical microscope observation showed that altered layers had developed along the glass surfaces. IR spectral line profile analysis was conducted on the glass sections from the surface to the interior for a length of 250 μm and the contents of molecular H2O (H2Om), OH species (OH) and total water (H2Ot) were determined. The diffusion profile of H2Om in Ohsawa lava extends beyond the layer observed by optical microscope. The content of H2Om in the hydrated region is much higher than that of OH species. Thus, the reaction from H2Om to OH appears to be little and H2Om is the dominant water species moving into the glass during weathering. Based on the concentration profiles, the diffusion coefficients of H2Om(DH2Om) and H2Ot(DH2Ot) were determined to be 2.8 × 10−10 and 3.4 × 10−10 μm2 s−1 for Ohsawa lava, and 5.2 × 10−11 and 4.1 × 10−11 μm2 s−1 for Awanomikoto lava, respectively. The obtained DH2Om during weathering are more than 2-3 orders of magnitude larger than the diffusion coefficient at ∼20 °C that is extrapolated from the diffusivity data for >400 °C. This might suggest that the mechanism of water transport is different at weathering conditions and >400 °C. 相似文献
2.
Diffusion parameters for HTO, 36Cl−, and 125I− were determined on Upper Toarcian argillite samples from the Tournemire Underground Research Laboratory (Aveyron, France) using the through diffusion technique. The direction of diffusion was parallel to the bedding plane. The purpose of the present study was 3-fold; it was intended (i) to confirm the I− interaction with Upper Toarcian argillite and to verify the effects of initial I− concentration on this affinity, as previously observed by means of radial diffusion experiments, (ii) to highlight any discrepancy between Cl− and I− diffusivity, and (iii) to investigate the effect of an increase of the ionic strength of the solution on the anionic tracers’ diffusive behaviour. The results show that the effective diffusion coefficient (De) and diffusion accessible porosity (εa) values obtained with an ionic strength (I.S.) synthetic pore water of 0.01 eq L−1 are: De = 2.35–2.50 × 10−11 m2 s−1 and εa = 12.0–15.0% for HTO, and De = 14.5–15.5 × 10−13 m2 s−1 and εa = 2.5–2.9% for 36Cl−. Because of anionic exclusion effects, anions diffuse slower and exhibit smaller diffusion accessible porosities than HTO, taken as a water tracer. The associated effective diffusion coefficient (De) and rock capacity factor (α) obtained for 125I are: De = 7.00–8.60 × 10−13 m2 s−1 and α = 4.3–7.2%. Such values make it possible to calculate low 125I distribution ratios (0.0057 < RD < 0.0192 mL g−1) which confirm the trend indicating that the 125I rock capacity factor increases with the decrease of the initial I− concentration. Additional through-diffusion experiments were carried out with a higher ionic strength synthetic pore water (I.S. = 0.11 eq L−1). No evolution of HTO diffusion parameters was observed. The anionic tracers’ effective diffusion coefficient increased by a factor of two but no clear evolution of their accessible porosity was observed. Such a paradox could be related to the particularly small mean pore size of the Upper Toarcian argillite of Tournemire. The most significant finding of this study is the large discrepancy (factor of two) between the values of the effective diffusion coefficient for 125I and 36Cl. Whatever the ionic strength of the synthetic solution used, 125I exhibited De values two times lower than those of 36Cl. A detailed explanation for this difference cannot be given at present even if a hypothesis based on ion-pairing or on steric-exclusion cannot be excluded. This makes questionable the assumption usually made for quantifying 125I sorption and postulating that 36Cl and 125I would diffuse in the same porosity. In other terms, at Tournemire, 125I sorption could be more pronounced than previously indicated. 相似文献
3.
Dissolution experiments on a serpentinite were performed at 70 °C, 0.1 MPa, in H2SO4 solution, in open and closed systems, in order to evaluate the overall dissolution rate of mineral components over different times (4, 9 and 24 h). In addition, the serpentinite powder was reacted with a NaCl-bearing aqueous solution and supercritical CO2 for 24 h at higher pressures (9-30 MPa) and temperatures (250-300 °C) either in a stirred reactor or in an externally-heated pressure vessel to assess both the dissolution rate of serpentinite minerals and the progress of the carbonation reaction. Results show that, at 0.1 MPa, MgO extraction from serpentinite ranges from 82% to 98% and dissolution rate varies from 8.5 × 10−10 mole m−2 s−1 to 4.2 × 10−9 mole m−2 s−1. Attempts to obtain carbonates from the Mg-rich solutions by increasing their pH failed since Mg- and NH4- bearing sulfates promptly precipitated. On the other hand, at higher pressures, significant crystallization (5.0-10.4 wt%) of Ca- and Fe-bearing magnesite was accomplished at 30 MPa and 300 °C using 100 g L−1 NaCl aqueous solutions. The corresponding amount of CO2 sequestered by crystallization of carbonates is 9.4-15.9 mole%. Dissolution rate (from 6.3 × 10−11 mole m−2 s−1 to 1.3 × 10−10 mole m−2 s−1) is lower than that obtained at 0.1 MPa and 70 °C but it is related to pH values much higher (3.3-4.4) than that (−0.65) calculated for the H2SO4 solution.Through a thorough review of previous experimental investigations on the dissolution kinetics of serpentine minerals the authors propose adopting: (i) the log rate [mole m−2 s−1] value of −12.08 ± 0.16 (1σ), as representative of the neutral dissolution mechanism at 25 °C and (ii) the following relationship for the acidic dissolution mechanism at 25 °C:
log rate=-0.45(±0.09)×pH-10.01(±0.30). 相似文献
4.
Solubility experiments were performed on nanocrystalline scorodite and amorphous ferric arsenate. Nanocrystalline scorodite occurs as stubby prismatic crystals measuring about 50 nm and having a specific surface area of 39.88 ± 0.07 m2/g whereas ferric arsenate is amorphous and occurs as aggregated clusters measuring about 50–100 nm with a specific surface area of 17.95 ± 0.19 m2/g. Similar to its crystalline counterpart, nanocrystalline scorodite has a solubility of about 0.25 mg/L at around pH 3–4 but has increased solubilities at low and high pH (i.e. <2 and >6). Nanocrystalline scorodite dissolves incongruently at about pH > 2.5 whereas ferric arsenate dissolution is incongruent at all the pH ranges tested (pH 2–5). It appears that the solubility of scorodite is not influenced by particle size. The dissolution rate of nanocrystalline scorodite is 2.64 × 10−10 mol m−2 s−1 at pH 1 and 3.25 × 10−11 mol m−2 s−1 at pH 2. These rates are 3–4 orders of magnitude slower than the oxidative dissolution of pyrite and 5 orders of magnitude slower than that of arsenopyrite. Ferric arsenate dissolution rates range from 6.14 × 10−9 mol m−2 s−1 at pH 2 to 1.66 × 10−9 mol m−2 s−1 at pH 5. Among the common As minerals, scorodite has the lowest solubility and dissolution rate. Whereas ferric arsenate is not a suitable compound for As control in mine effluents, nanocrystalline scorodite that can be easily precipitated at ambient pressure and temperature conditions would be satisfactory in meeting the regulatory guidelines at pH 3–4. 相似文献
5.
Mark E. Hodson 《Geochimica et cosmochimica acta》2006,70(7):1655-1667
Laboratory determined mineral weathering rates need to be normalised to allow their extrapolation to natural systems. The principle normalisation terms used in the literature are mass, and geometric- and BET specific surface area (SSA). The purpose of this study was to determine how dissolution rates normalised to these terms vary with grain size. Different size fractions of anorthite and biotite ranging from 180-150 to 20-10 μm were dissolved in pH 3, HCl at 25 °C in flow through reactors under far from equilibrium conditions. Steady state dissolution rates after 5376 h (anorthite) and 4992 h (biotite) were calculated from Si concentrations and were normalised to initial- and final- mass and geometric-, geometric edge- (biotite), and BET SSA. For anorthite, rates normalised to initial- and final-BET SSA ranged from 0.33 to 2.77 × 10−10 molfeldspar m−2 s−1, rates normalised to initial- and final-geometric SSA ranged from 5.74 to 8.88 × 10−10 molfeldspar m−2 s−1 and rates normalised to initial- and final-mass ranged from 0.11 to 1.65 molfeldspar g−1 s−1. For biotite, rates normalised to initial- and final-BET SSA ranged from 1.02 to 2.03 × 10−12 molbiotite m−2 s−1, rates normalised to initial- and final-geometric SSA ranged from 3.26 to 16.21 × 10−12 molbiotite m−2 s−1, rates normalised to initial- and final-geometric edge SSA ranged from 59.46 to 111.32 × 10−12 molbiotite m−2 s−1 and rates normalised to initial- and final-mass ranged from 0.81 to 6.93 × 10−12 molbiotite g−1 s−1. For all normalising terms rates varied significantly (p ? 0.05) with grain size. The normalising terms which gave least variation in dissolution rate between grain sizes for anorthite were initial BET SSA and initial- and final-geometric SSA. This is consistent with: (1) dissolution being dominated by the slower dissolving but area dominant non-etched surfaces of the grains and, (2) the walls of etch pits and other dissolution features being relatively unreactive. These steady state normalised dissolution rates are likely to be constant with time. Normalisation to final BET SSA did not give constant ratios across grain size due to a non-uniform distribution of dissolution features. After dissolution coarser grains had a greater density of dissolution features with BET-measurable but unreactive wall surface area than the finer grains. The normalising term which gave the least variation in dissolution rates between grain sizes for biotite was initial BET SSA. Initial- and final-geometric edge SSA and final BET SSA gave the next least varied rates. The basal surfaces dissolved sufficiently rapidly to influence bulk dissolution rate and prevent geometric edge SSA normalised dissolution rates showing the least variation. Simple modelling indicated that biotite grain edges dissolved 71-132 times faster than basal surfaces. In this experiment, initial BET SSA best integrated the different areas and reactivities of the edge and basal surfaces of biotite. Steady state dissolution rates are likely to vary with time as dissolution alters the ratio of edge to basal surface area. Therefore they would be more properly termed pseudo-steady state rates, only appearing constant because the time period over which they were measured (1512 h) was less than the time period over which they would change significantly. 相似文献
6.
Andreas Birkefeld 《Geochimica et cosmochimica acta》2006,70(11):2726-2736
We report the application of an in situ method to obtain field dissolution rates of fine mineral particles in soils. Samples with different metal-containing mineral and slag particles (lead oxide, copper concentrate and copper slag) from the mining and smelting industry were buried in the topsoil of an acidic forest soil for up to 18 months. In addition we studied the dissolution of these particles in samples of the same soil, in a sand matrix and in acid solution under constant temperature and moisture conditions in the laboratory. Under field conditions the PbO particles dissolved quite rapidly (2.4 ± 0.7 × 10−10 mol Pb m−2 s−1), whereas the copper concentrate (<1 × 10−11 mol Cu m−2 s−1) and the copper slag particles (4.3 ± 0.8 × 10−11 mol Cu m−2 s−1) proved to be more resistant to weathering. In addition to qualitative information on dissolution features (SEM), the method yielded quantitative data on in situ dissolution rates. The dissolution rates followed the order: sand with acid percolation (pH 3.5; lab) < soil (lab) < soil (field) < acid solution (pH 3.5; lab). Dissolution rates in soil were found to be lower under laboratory than under field conditions. The faster field rates may in part be attributed to the higher biological activity in the field soil compared to the same soil in the laboratory. 相似文献
7.
To investigate the influence of temperature and composition on the diffusivities of dissolved carbon dioxide and argon in silicate melts, diffusion experiments were performed at magmatic pressure and temperature conditions in (a) albite melts with excess Na2O (0-8.6 wt%) and a constant Si/Al ratio of 3, and (b) albite70quartz30 to jadeite melts with decreasing SiO2 content and a constant Na/Al ratio of 1. We obtained diffusion coefficients at 500 MPa and 1323-1673 K. In the fully polymerized system Ab70Qz30 - Jd, the change in composition only has a weak effect on bulk CO2 diffusivity, but Ar diffusivity increases clearly with decreasing SiO2 content. In the system Ab + Na2O, bulk CO2 and Ar diffusivity increase significantly with gradual depolymerisation. The relatively small change in composition on molar basis in the depolymerized system leads to a significantly larger change in diffusivities compared to the fully polymerized Ab70Qz30-Jd join. Within error, activation energies for bulk CO2 and Ar diffusion in both systems are identical with decreasing silica content (Ab + Na2O: 159 ± 25 kJ mol−1 for bulk CO2 and 130 ± 8 kJ mol−1 for Ar; Ab70Qz30-Jd: 163 ± 16 kJ mol−1 for bulk CO2 and 148 ± 15 kJ mol−1 for Ar) even though this results in depolymerisation in one system and not the other.Although there is a variation in CO2 speciation with changing composition as observed in quenched glasses, it has previously established that this is not a true representation of the species present in the melt, with the ratio of molecular CO2 to carbonate decreasing during quenching. Thus, diffusion coefficients for the individual CO2 species cannot be directly derived by measuring molecular CO2 and CO32- concentration-distance profiles in the glasses. To obtain diffusivities of individual CO2 species, we have made two assumptions that (1) inert Ar can be used as a proxy for molecular CO2 diffusion characteristics as shown by our previous work and (2) the diffusivity of CO32− can be calculated assuming it is identical to network forming components (Si4+ and Al3+). This is derived from viscosity data (Eyring eqn.) and suggests that CO32− diffusion would be several orders of magnitude slower than molecular CO2 diffusion.The systematics of measured bulk CO2 diffusivity rates and comparison with the Ar proxy all suggest that the faster molecular CO2 species is much more dominant in melts than measurements on resulting quenched glasses would suggest. This study has confirmed an observation of surprisingly consistent bulk CO2 diffusivity across a range of natural compositions were Ar diffusivity significantly increases. This is consistent with an actual increase in molecular CO2 mobility (similar to Ar) that is combined with an increase in the proportion of the slower carbonate in the melt.These results demonstrate that the CO2 diffusion and speciation model provides an insight into the transport processes in the melt and is promising and an alternative tool to in situ speciation measurements at magmatic conditions, which at the moment are technically extremely difficult. We present the first high pressure high temperature in situ MIR spectra of a CO2 bearing albitic glass/melt suggesting that molecular CO2 is a stable species at high temperature, which is qualitatively consistent with the modelled CO2 speciation data. 相似文献
8.
The oxygenation kinetics of nanomolar concentrations of Fe(II) in aqueous solution have been studied in the absence and presence of millimolar concentrations of phosphate over the pH range 6.0-7.8. At each phosphate concentration investigated, the overall oxidation rate constant varied linearly with pH, and increased with increasing phosphate concentration. A model based on equilibrium speciation of Fe(II) was found to satisfactorily explain the results obtained. From this model, the rate constants for oxygenation of the Fe(II)-phosphate species FeH2PO4+, FeHPO4 and FePO4− have been determined for the first time. FePO4− was found to be the most kinetically reactive species at circumneutral pH with an estimated oxygenation rate constant of (2.2 ± 0.2) × 10 M−1 s−1. FeH2PO4+ and FeHPO4 were found to be less reactive with oxygen, with rate constants of (3.2 ± 2) × 10−2 M−1 s−1 and (1.2 ± 0.8) × 10−1 M−1 s−1, respectively. 相似文献
9.
We have performed holographic interferometry measurements of the dissolution of the (0 1 0) plane of a cleaved gypsum single crystal in pure water. These experiments have provided the value of the dissolution rate constant k of gypsum in water and the value of the interdiffusion coefficient D of its aqueous species in water. D is 1.0 × 10−9 m2 s−1, a value close to the theoretical value generally used in dissolution studies. k is 4 × 10−5 mol m−2 s−1. It directly characterizes the microscopic transfer rate at the solid-liquid interface, and is not an averaged value deduced from quantities measured far from the surface as in macroscopic dissolution experiments. It is found to be two times lower than the value obtained from macroscopic experiments. 相似文献
10.
Stepped heating and crushing experiments have been used to investigate the noble gas and halogen degassing behaviour of quartz in detail. Samples with diverse character were selected from the Eloise and Osborne, Iron Oxide Copper Gold (IOCG) ore deposits, and the Railway Fault, 13 km south of the Mt Isa Mine, in the Proterozoic Mt Isa Inlier of northeast Australia. Quartz has been shown to have a bimodal degassing profile. The first degassing mode at temperatures of <700 °C is caused by thermally induced mechanical decrepitation of fluid inclusions. Changes in the Br/Cl, I/Cl, Ar/Cl and 40Ar/36Ar composition of gas released at different temperatures up to 700 °C can be related to the decrepitation of different types of fluid inclusion observed by microthermometry. These variations with temperature permit deconvolution of the complex fluid inclusion assemblages associated with the IOCG samples; the ultra high salinity, multi solid (MS) and liquid-vapour-daughter (LVD) fluid inclusions, with a predominantly primary origin, decrepitate at higher temperatures than lower salinity liquid-vapour (LV) and monophase (M) fluid inclusions that have a predominantly secondary origin. Three of the IOCG samples have primary MS and LVD fluid inclusions characterized by molar Br/Cl values of between 0.25 × 10−3 and 0.66 × 10−3, I/Cl between 0.37 × 10−6 and 5.0 × 10−6, 40Ar/36Ar values of <1000 and low 36Ar concentrations of 0.7-1.0 × 10−6 cm3 cm−3H2O. These low values are most easily explained by the involvement of halite dissolution water in IOCG genesis. One of the IOCG samples has Br/Cl of 1.3-2.0 × 10−3 and I/Cl of 10 × 10−6, similar to juvenile magmatic fluids in Phanerozoic Porphyry Copper Deposits. This sample also has a higher 36Ar concentration of 3.5 × 10−6 cm3 cm−3H2O and a slightly elevated 40Ar/36Ar of 2236. Step heating reveals limited and non-systematic variation within the more homogenous population of LV fluid inclusions from the Railway Fault. The samples have mean values of 8.1 × 10−3 for Br/Cl; 9.4-12 × 10−6 for I/Cl; <2000 for 40Ar/36Ar; and 4.7-4.8 × 10−6 cm3 cm−3H2O for 36Ar concentration. The Br/Cl values are similar to those previously reported for basinal brines present in silicic alteration at the Mt Isa Mine and the additional data can be explained by interaction of such a bittern brine with fine grained sedimentary rocks in the sub-surface. The second mode of quartz degassing occurs between 1200 and 1450 °C and releases a greater volume of gas than the first degassing mode. Several lines of evidence, including microscope observations, indicate that the gas released at high temperature is also from the fluid inclusion reservoir. However, its release may be triggered by a metastable phase transition of quartz (∼1200 °C) and caution is required in interpretation of the fluid compositions obtained at these temperatures. The data provide an improved understanding of fluid inclusion decrepitation behaviour that is different to that obtained in lower temperatures experiments designed by other workers to investigate H-isotope fractionation. 相似文献
11.
E. Curti K. Fujiwara J. Tits A. Kitamura W. Müller 《Geochimica et cosmochimica acta》2010,74(12):3553-5422
High-purity synthetic barite powder was added to pure water or aqueous solutions of soluble salts (BaCl2, Na2SO4, NaCl and NaHCO3) at 23 ± 2 °C and atmospheric pressure. After a short pre-equilibration time (4 h) the suspensions were spiked either with 133Ba or 226Ra and reacted under constant agitation during 120-406 days. The pH values ranged from 4 to 8 and solid to liquid (S/L) ratios varied from 0.01 to 5 g/l. The uptake of the radiotracers by barite was monitored through repeated sampling of the aqueous solutions and radiometric analysis. For both 133Ba and 226Ra, our data consistently showed a continuous, slow decrease of radioactivity in the aqueous phase.Mass balance calculations indicated that the removal of 133Ba activity from aqueous solution cannot be explained by surface adsorption only, as it largely exceeded the 100% monolayer coverage limit. This result was a strong argument in favor of recrystallization (driven by a dissolution-precipitation mechanism) as the main uptake mechanism. Because complete isotopic equilibration between aqueous solution and barite was approached or even reached in some experiments, we concluded that during the reaction all or substantial fractions of the initial solid had been replaced by newly formed barite.The 133Ba data could be successfully fitted assuming constant recrystallization rates and homogeneous distribution of the tracer into the newly formed barite. An alternative model based on partial equilibrium of 133Ba with the mineral surface (without internal isotopic equilibration of the solid) could not reproduce the measured activity data, unless multistage recrystallization kinetics was assumed. Calculated recrystallization rates in the salt solutions ranged from 2.8 × 10−11 to 1.9 × 10−10 mol m−2 s−1 (2.4-16 μmol m−2 d−1), with no specific trend related to solution composition. For the suspensions prepared in pure water, significantly higher rates (∼5.7 × 10−10 mol m−2 s−1 or ∼49 μmol m−2 d−1) were determined.Radium uptake by barite was determined by monitoring the decrease of 226Ra activity in the aqueous solution with alpha spectrometry, after filtration of the suspensions and sintering. The evaluation of the Ra uptake experiments, in conjunction with the recrystallization data, consistently indicated formation of non-ideal solid solutions, with moderately high Margules parameters (WAB = 3720-6200 J/mol, a0 = 1.5-2.5). These parameters are significantly larger than an estimated value from the literature (WAB = 1240 J/mol, a0 = 0.5).In conclusion, our results confirm that radium forms solid solutions with barite at fast kinetic rates and in complete thermodynamic equilibrium with the aqueous solutions. Moreover, this study provides quantitative thermodynamic data that can be used for the calculation of radium concentration limits in environmentally relevant systems, such as radioactive waste repositories and uranium mill tailings. 相似文献
12.
Matthijs C. van Soest Brian D. Monteleone Jeremy W. Boyce 《Geochimica et cosmochimica acta》2011,75(9):2409-161
Ultraviolet lasers coupled with sensitive mass spectrometers provide a useful way to measure laboratory-induced noble gas diffusion profiles in minerals, thus enabling the calculation of diffusion parameters. We illustrate this laser ablation depth profiling (LADP) technique for a previously well-studied mineral-isotopic system: 4He in Durango fluorapatite. LADP studies were conducted on oriented, polished slabs from a single crystal that were heated under vacuum to a variety of temperatures between 300 and 450 °C for variable times. The resolved 4He profiles exhibited error-function loss as predicted by previous bulk 4He diffusion studies. All of the slabs, regardless of crystallographic orientation, yielded modeled diffusivities that are statistically co-linear on an Arrhenius diagram, suggesting no diffusional anisotropy of 4He in this material. The data indicate an activation energy of 142.2 ± 5.0 (2σ) kJ/mol and diffusivity at infinite temperature - reported as ln(D0) - of −4.71 ± 0.94 (2σ) m2/s. These values imply a bulk closure temperature for 4He in Durango fluorapatite of 74 °C for a 50 μm radius grain, infinite cylinder geometry, and a cooling rate of 10 °C/Myr. 相似文献
13.
Magali Bonifacie Vincent Busigny Pascal Philippot Nathalie Jendrzejewski Marc Javoy 《Geochimica et cosmochimica acta》2008,72(1):126-139
Bulk-rock chlorine content and isotopic composition (δ37Cl) were determined in oceanic serpentinites, high-pressure metaperidotites and metasediments in order to gain constraints on the global chlorine cycle associated with hydrothermal alteration and subduction of oceanic lithosphere. The distribution of insoluble chlorine in oceanic serpentinites was also investigated by electron microprobe. The hydrothermally-altered ultramafic samples were dredged along the South West Indian Ridge and the Mid-Atlantic Ridge. The high-pressure metamorphic samples were collected in the Western Alps: metaperidotites in the Erro-Tobbio unit and metasediments in the Schistes Lustrés nappe.Oceanic serpentinites show relatively large variations of bulk-rock Cl contents and δ37Cl values with mean values of 1105 ± 596 ppm and −0.7 ± 0.4‰, respectively (n = 8; 1σ). Serpentines formed after olivine (meshes) show lower Cl content than those formed after orthopyroxene (bastites). In bastites of two different samples, Cl is positively correlated with Al2O3 and negatively correlated with SiO2. These relationships are interpreted as reflecting preferential Cl-incorporation into the bastite structure distorted by Al (substituted for Si) rather than different alteration conditions between olivine and orthopyroxene minerals. High-pressure metaperidotites display relatively homogeneous Cl contents and δ37Cl values with mean values of 467 ± 88 ppm and −1.4 ± 0.1‰, respectively (n = 7; 1σ). A macroscopic high-pressure olivine-bearing vein, formed from partial devolatilization of serpentinites at ∼2.5 GPa and 500-600 °C, shows a Cl content and a δ37Cl value of 603 ppm and −1.6‰, respectively. Metasediments (n = 2) show low whole-rock Cl contents (<15 ppm Cl) that did not allow Cl isotope analyses to be obtained.The range of negative δ37Cl values observed in oceanic serpentinites is likely to result from water-rock interaction with fluids that have negative δ37Cl values. The homogeneity of δ37Cl values from the high-pressure olivine-bearing vein and the metaperidotite samples implies that progressive loss of Cl inherited from oceanic alteration throughout subduction did not significantly fractionate Cl isotopes. Chlorine recycled in subduction zones via metaperidotites should thus show a range of δ37Cl values similar to the range found in oceanic serpentinized peridotites. 相似文献
14.
In order to better investigate the compositions and the origins of fluids associated with diamond growth, we have carried-out combined noble gas (He and Ar), C and N isotope, K, Ca and halogen (Cl, Br, I) determinations on fragments of individual microinclusion-bearing diamonds from the Panda kimberlite, North West Territories, Canada. The fluid concentrations of halogens and noble gases in Panda diamonds are enriched by several orders of magnitude over typical upper mantle abundances. However, noble gas, C and N isotopic ratios (3He/4He = 4-6 Ra, 40Ar/36Ar = 20,000-30,000, δ13C = −4.5‰ to −6.9‰ and δ15N = −1.2‰ to −8.8‰) are within the worldwide range determined for fibrous diamonds and similar to the mid ocean ridge basalt (MORB) source value. The high 36Ar content of the diamonds (>1 × 10−9 cm3/g) is at least an order of magnitude higher than any previously reported mantle sample and enables the 36Ar content of the subcontinental lithospheric mantle to be estimated at ∼0.6 × 10−12 cm3/g, again similar to estimates for the MORB source. Three fluid types distinguished on the basis of Ca-K-Cl compositions are consistent with carbonatitic, silicic and saline end-members identified in previous studies of diamonds from worldwide sources. These fluid end-members also have distinct halogen ratios (Br/Cl and I/Cl). The role of subducted seawater-derived halogens, originally invoked to explain some of the halogen ratio variations in diamonds, is not considered an essential component in the formation of the fluids. In contrast, it is considered that large halogen fractionation of a primitive mantle ratio occurs during fluid-melt partitioning in forming silicic fluids, and during separation of an immiscible saline fluid. 相似文献
15.
Carbon dioxide degassing and thermal energy release in the Monte Amiata volcanic-geothermal area (Italy) 总被引：2，自引：0，他引：2
Francesco Frondini Stefano Caliro Carlo Cardellini Giovanni Chiodini Nicola Morgantini 《Applied Geochemistry》2009
The quaternary volcanic complex of Mount Amiata is located in southern Tuscany (Italy) and represents the most recent manifestation of the Tuscan Magmatic Province. The region is characterised by a large thermal anomaly and by the presence of numerous CO2-rich gas emissions and geothermal features, mainly located at the periphery of the volcanic complex. Two geothermal systems are located, at increasing depths, in the carbonate and metamorphic formations beneath the volcanic complex. The shallow volcanic aquifer is separated from the deep geothermal systems by a low permeability unit (Ligurian Unit). A measured CO2 discharge through soils of 1.8 × 109 mol a−1 shows that large amounts of CO2 move from the deep reservoir to the surface. A large range in δ13CTDIC (−21.07 to +3.65) characterises the waters circulating in the aquifers of the region and the mass and isotopic balance of TDIC allows distinguishing a discharge of 0.3 × 109 mol a−1 of deeply sourced CO2 in spring waters. The total natural CO2 discharge (2.1 × 109 mol a−1) is slightly less than minimum CO2 output estimated by an indirect method (2.8 × 109 mol a−1), but present-day release of 5.8 × 109 mol a−1 CO2 from deep geothermal wells may have reduced natural CO2 discharge. The heat transported by groundwater, computed considering the increase in temperature from the infiltration area to the discharge from springs, is of the same order of magnitude, or higher, than the regional conductive heat flow (>200 mW m−2) and reaches extremely high values (up to 2700 mW m−2) in the north-eastern part of the study area. Heat transfer occurs mainly by conductive heating in the volcanic aquifer and by uprising gas and vapor along fault zones and in those areas where low permeability cover is lacking. The comparison of CO2 flux, heat flow and geological setting shows that near surface geology and hydrogeological setting play a central role in determining CO2 degassing and heat transfer patterns. 相似文献
16.
T. Le Druillennec G. Ielsch O. Bour C. Tarits G. Tymen G. Alcalde L. Aquilina 《Applied Geochemistry》2010
To investigate the possible variations of Rn concentration in crystalline rocks as a function of flow conditions, a field study was carried out of a fractured aquifer in granite. The method is based on the in situ measurement of Rn in groundwater, aquifer tests for the determination of hydraulic characteristics of the aquifer and laboratory measurement of Rn exhalation rate from rocks. A simple crack model that simulates the Rn concentration in waters circulating in a fracture intersecting a borehole was also tested. The Rn concentrations in groundwaters from boreholes of the study site ranged from 192 to 1597 Bq L−1. The Rn exhalation rates of selected samples of granite and micaschist were determined from laboratory experiments. The results yielded fluxes varying from 0.5 to 1.3 mBq m−2 s−1 in granite and from 0.5 to 0.9 mBq m−2 s−1 in micaschists. Pumping tests were performed in the studied boreholes to estimate the transmissivity and calculate the equivalent hydraulic aperture of the fractures. Transmissivities ranged from 10−5 to 10−3 m2 s−1. Using the cubic law, hydraulic equivalent fracture apertures were calculated to be in the range of 0.5–2.3 mm. 相似文献
17.
Guillaume Desbois 《Geochimica et cosmochimica acta》2007,71(21):5233-5243
Hydrogen-deuterium exchange in tourmaline single crystals of elbaite composition from Nepal has been studied at 1 atm and at temperatures between 973 and 1073 K. H/D ratios were determined after each annealing experiment using micro FTIR-spectroscopy. Diffusion coefficients (10−16-10−15 m2 s−1) were determined by fitting the data using a 3D numerical simulation. The rate of diffusion is two to three times faster along the c direction than along directions parallel to the basal plane. The diffusion laws are, respectively:
18.
Reaction between dissolved water and sulphide was experimentally investigated in soda-lime-silicate (NCS) and sodium trisilicate (NS3) melts at temperatures from 1000 to 1200 °C and pressures of 100 or 200 MPa in internally heated gas pressure vessels. Diffusion couple experiments were conducted at water-undersaturated conditions with one half of the couple being doped with sulphide (added as FeS or Na2S; 1500-2000 ppm S by weight) and the other with H2O (∼3.0 wt.%). Additionally, two experiments were performed using a dry NCS glass cylinder and a free H2O fluid. Here, the melt was water-saturated at least at the melt/fluid interface. Profiling by electron microprobe (sulphur) and infrared microscopy (H2O) demonstrate that H2O diffusion in the melts is faster by 1.5-2.3 orders of magnitude than sulphur diffusion and, hence, H2O can be considered as a rapidly diffusing oxidant while sulphur is quasi immobile in these experiments.In Raman spectra a band at 2576 cm−1 appears in the sulphide - H2O transition zone which is attributed to fundamental S-H stretching vibrations. Formation of new IR absorption bands at 5025 cm−1 (on expense of the combination band of molecular H2O at 5225 cm−1) and at 3400 cm−1 was observed at the front of the in-diffusing water in the sulphide bearing melt. The appearance and intensity of these two IR bands is correlated with systematic changes in S K-edge XANES spectra. A pre-edge excitation at 2466.5 eV grows with increasing H2O concentration while the sulphide peak at 2474.0 eV decreases in intensity relative to the peak at 2477.0 eV and the feature at 2472.3 eV becomes more pronounced (all energies are relative to the sulphate excitation, calibrated to 2482.5 eV). The observations by Raman, IR and XANES spectroscopy indicate a well coordinated S2− - H2O complex which was probably formed in the glasses during cooling at the glass transition. No oxidation of sulphide was observed in any of the diffusion couple experiments. On the contrary, XANES spectra from experiments conducted with a free H2O fluid show complete transformation of sulphide to sulphate near the melt surface and coexistence of sulphate and sulphide in the center of the melt. This can be explained by a lower H2O activity in the diffusion couple experiments or by the need of a sink for hydrogen (e.g., a fluid which can dissolve high concentration of hydrogen) to promote oxidation of sulphide by H2O via the reaction S2− + 4H2O = SO42− + 4H2. Sulphite could not be detected in any of the XANES spectra implying that this species, if it exists in the melt, it is a subordinate or transient species only. 相似文献
19.
Clinton Rissmann Bruce Christenson Cynthia Werner Matthew Leybourne Jim Cole Darren Gravley 《Applied Geochemistry》2012
Carbon dioxide emissions and heat flow have been determined from the Ohaaki hydrothermal field, Taupo Volcanic Zone (TVZ), New Zealand following 20 a of production (116 MWe). Soil CO2 degassing was quantified with 2663 CO2 flux measurements using the accumulation chamber method, and 2563 soil temperatures were measured and converted to equivalent heat flow (W m−2) using published soil temperature heat flow functions. Both CO2 flux and heat flow were analysed statistically and then modelled using 500 sequential Gaussian simulations. Forty subsoil CO2 gas samples were also analysed for stable C isotopes. Following 20 a of production, current CO2 emissions equated to 111 ± 6.7 T/d. Observed heat flow was 70 ± 6.4 MW, compared with a pre-production value of 122 MW. This 52 MW reduction in surface heat flow is due to production-induced drying up of all alkali–Cl outflows (61.5 MW) and steam-heated pools (8.6 MW) within the Ohaaki West thermal area (OHW). The drying up of all alkali–Cl outflows at Ohaaki means that the soil zone is now the major natural pathway of heat release from the high-temperature reservoir. On the other hand, a net gain in thermal ground heat flow of 18 MW (from 25 MW to 43.3 ± 5 MW) at OHW is associated with permeability increases resulting from surface unit fracturing by production-induced ground subsidence. The Ohaaki East (OHE) thermal area showed no change in distribution of shallow and deep soil temperature contours despite 20 a of production, with an observed heat flow of 26.7 ± 3 MW and a CO2 emission rate of 39 ± 3 T/d. The negligible change in the thermal status of the OHE thermal area is attributed to the low permeability of the reservoir beneath this area, which has limited production (mass extraction) and sheltered the area from the pressure decline within the main reservoir. Chemistry suggests that although alkali–Cl outflows once contributed significantly to the natural surface heat flow (∼50%) they contributed little (<1%) to pre-production CO2 emissions due to the loss of >99% of the original CO2 content due to depressurisation and boiling as the fluids ascended to the surface. Consequently, the soil has persisted as the major (99%) pathway of CO2 release to the atmosphere from the high temperature reservoir at Ohaaki. The CO2 flux and heat flow surveys indicate that despite 20 a of production the variability in location, spatial extent and magnitude of CO2 flux remains consistent with established geochemical and geophysical models of the Ohaaki Field. At both OHW and OHE carbon isotopic analyses of soil gas indicate a two-stage fractionation process for moderate-flux (>60 g m−2 d−1) sites; boiling during fluid ascent within the underlying reservoir and isotopic enrichment as CO2 diffuses through porous media of the soil zone. For high-flux sites (>300 g m−2 d−1), the δ13CO2 signature (−7.4 ± 0.3‰ OHW and −6.5 ± 0.6‰ OHE) is unaffected by near-surface (soil zone) fractionation processes and reflects the composition of the boiled magmatic CO2 source for each respective upflow. Flux thresholds of <30 g m−2 d−1 for purely diffusive gas transport, between 30 and 300 g m−2 d−1 for combined diffusive–advective transport, and ?300 g m−2 d−1 for purely advective gas transport at Ohaaki were assigned. δ13CO2 values and cumulative probability plots of CO2 flux data both identified a threshold of ∼15 g m−2 d−1 by which background (atmospheric and soil respired) CO2 may be differentiated from hydrothermal CO2. 相似文献
20.
Dramatic seasonal changes in water chemistry and precipitate mineralogy associated with acid-mine drainage (AMD) in the waterfall and creek sections of the Chinkuashih area, northern Taiwan were investigated. Special attention has been paid to the kinetic effects of seasonal temperature variation and waterfall aeration. Precipitation of schwertmannite associated with removal of metals and As are indicated by delicate growth microstructures on precipitate surfaces, X-ray diffraction data, and downstream reductions of metal and As concentrations. Geochemical modeling suggested a downstream increase of the degree of saturation/supersaturation with respect to schwertmannite in the waterfall section, which can be attributed to high Fe2+ oxidation rates. The waterfall section was characterized by high rates and model rate constants of Fe2+ oxidation (6.1–6.7 × 10−6 mol L−1 s−1 and 2.7–2.9 × 10−2 s−1) and Fe (schwertmannite) precipitation (1.7–2.1 × 10−6 mol L−1 s−1 and 3.5–4.1 × 10−7 mol L−1 s−1). A high As sorption rate (4.7–6.3 × 10−9 mol L−1 s−1) and low As distribution coefficient (7.9–11.8 × 10−9 mol−1 L) were observed. The creek section showed up to 1–2 orders of magnitude slower rates and lower rate constants than the waterfall section and had seasonal variations comparable to those in areas polluted by AMD elsewhere. The summer rates were 4–5 times higher than the winter rates in the creek section, and are largely attributed to a temperature effect. In contrast, the seasonal differences in rate and rate constant were small in the waterfall section. Several factors associated with the waterfall aeration in addition to elevated temperature and As concentration enhanced Fe and As attenuation in the waterfall section. The waterfall effects on Fe precipitation rate were enhanced when the flow rate was large in the winter. Despite the remarkable removal of metals and As by the rapid precipitation of As-bearing schwertmannite, large effluent loads of potentially hazardous contaminants including As, Cu and Zn discharged to the sea in the Chinkuashih area. 相似文献