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1.
The hypothesized reaction mechanisms for U reduction by the dehydrogenation of hydroxyl groups and aliphatic hydrocarbonaceous moieties of lignite were verified by kinetic U reduction experiments using simple alcohols (1-octadecanol and 2-propanol) and aliphatic hydrocarbons (n-octacosane). The rate constants and activation energies for U reduction by these alcohols are similar to those obtained for U reduction by lignite. The rate-determining step for U reduction by both simple and natural organic matter is hypothesized to be controlled by oxygen diffusion through U oxides. The equilibria of the system lignite-aqueous uranyl have been used to calculate standard free energy changes ΔG° for lignite dehydrogenation. Their comparison with those for the dehydrogenation of simple organic molecules supports the proposed reactions thermodynamically.  相似文献   

2.
Characterization of the Panandhro lignite deposits from western Indian state of Gujarat, based on the geochemical and palynological evidences, has been performed to assess the floral composition, maturity and hydrocarbon potential of the sequence. Elementally, the lignites consist of moderate carbon, low hydrogen and moderate sulfur contents. The samples are characterized by high TOC contents (lignite: av. 46.43 wt.%, resin: 62.47 wt.%). The average HI values for the lignite is 136 mg HC/g TOC, and that of the associated resin is 671 mg HC/g TOC. The highest Tmax is recoded in lignite (422°C) and lowest in the resin (39°C) samples. The FTIR spectrum of lignite is characterized by highly intense OH stretching peak ~3350 cm-1, aliphatic CHx stretching peaks between 3000-2800 cm-1, aromatic C=O stretching and an aromatic C=C stretching. The spectrum of resin shows strongest absorption due to aliphatic CHx stretching between 2940-2915 cm-1 and 2870-2850 cm-1, and deformation by the medium peak between 1450 and 1650 cm-1. The recovered palynofloral assemblage indicates the dominance of angiosperm pollen grains with maximum abundance of Arecaceae family, and subdominant pteridophytic spores. Marine influence is indicated by the presence of abundant dinoflagellate cysts. The occurrence of flora from a variety of ecological niches suggests a luxuriant diverse vegetation pattern existed in the vicinity of depositional site under humid tropical conditions. The overall characteristics of the lignite deposits point towards their ability to generate (upon maturation) hydrocarbons as they have types III–II admixed kerogen (organic matters).  相似文献   

3.
High temperature infrared spectra of hydrous microcrystalline quartz   总被引：1，自引：0，他引：1  
A series of in-situ high temperature infrared (IR) measurements of water in an agate sample and in a milky quartz has been conducted in order to understand the nature of water in silica at high temperatures (50–700?°C) and the dehydration behavior. IR absorption bands of water molecules trapped in the milky quartz showed a systematic decrease in intensities and a shift from 3425?cm?1 at 50?°C toward 3590?cm?1 at 700?°C without any loss of water. This indicates a change in IR absorption coefficients corresponding to different polymeric states of water at different temperatures. The broad 3430?cm?1 band in the agate sample also showed a systematic decrease in IR intensity and a band shift toward higher frequency with increasing temperature (～700?°C). This indicates that the agate sample also contains fluid inclusion-like water. For this agate sample, a dehydration of loosely hydrogen-bonded molecular water occurred at lower temperatures (<200?°C). At higher temperatures (>400?°C), sharp bands around 3660 and 3725?cm?1 (3740?cm?1 at 50?°C) due to surface silanols, appeared. This indicates dehydration of H2O molecules that are hydrogen bonded to surface silanols. SiOH species in the agate are divided into three groups, namely SiOH group located at structural defects, surface silanols hydrogen bonded to each other and free surface silanols. Former two dehydrate below 700?°C and the dehydration rate of the SiOH at structural defects is faster than the other. IR spectra show that SiOH species decrease continuously even after the dehydration of most of H2O molecules. All these results provide realistic bases for the change in physicochemical states of different OH species in silica at high temperatures.  相似文献   

4.
《Applied Geochemistry》2003,18(8):1121-1135
Three samples of gouge from a U-mineralised fault, and two model samples, montmorillonite and muscovite, spiked with U, were heat-treated at a range of temperatures up to 1100 °C. Mineralogical changes were followed by thermal analysis, powder XRD and electron microscopy, and U extractability was measured by extraction with NH4+. Changes in U speciation in the montmorillonite sample were followed using EXAFS spectroscopy. On heating, the minerals progressively dehydrate, dehydroxylate and eventually decompose to form new phases in a glassy matrix. In the case of montmorillonite (90% of U extractable from unheated material), U extractability increased slightly on heating to temperatures around 400 °C. Almost 50% of U was extracted from unheated muscovite, and this increased slightly by 450 °C. Above 500–600 °C, U extractability from both montmorillonite and muscovite declined to very low levels, reflecting dehydration of the uranyl ion and trapping in the new phases and glassy matrix. Uranium extractability from the natural samples was much lower in all cases (0.25–5% of the total before heating). In 2 samples, a significant increase in U extraction was associated with dehydroxylation at around 600 °C, followed by a decrease to very low levels at higher temperatures. Uranium extraction from the third natural sample, which contained X-ray amorphous U minerals, decreased steadily on heating. The results show that changes in U extraction can be related to structural and morphological changes in sheet silicate minerals. Heat treatment has potential to fix U but only if temperatures above 800 °C are reached. If only lower temperatures, in the range 400–600 °C, are used, then U extraction may increase.  相似文献   

5.
Experimentally reversed corundum solubilities in pure water at 400° to 700°C and 0.7 to 3 kbar yield values of dissolved aluminum that range from 1–4 ppm Al. At constant pressure the solubility shows a sigmoidal behavior with a slight maximum at 500°C and minimum at 600°C. Corundum solubility increases with increasing pressure at constant temperature. The dissolved aluminum appears to form an uncharged, but polar species under these conditions probably of the form Al(OH)30.  相似文献   

6.
In-situ IR measurements of OH species in quartz at high temperatures   总被引：1，自引：0，他引：1  
The nature of OH species in natural clear quartz was investigated by means of in-situ IR measurements over the temperature range –185 to 1000 °C. Reversible thermal behavior of OH species was examined for a sample pre-heated to 1000 °C for 1 hour. At room temperature, the IR spectrum of the quartz sample examined includes an intense absorption peak at 3379 cm–1 which has been assigned to an OH stretching vibration associated with Al substituting for Si (OH(Al)). The major spectral changes of the OH(Al) bond involve a systematic shift of its peak position and a decrease in its integral absorbance with temperature. A quasi-linear increase of the peak position from –185 to 400 °C is interpreted to be due to the change in the vibrational frequency of OH(Al) with hydrogen bond (H bond) distance. At higher temperatures, the IR frequency shows only a slight change, indicating a small influence of the H bond. On the other hand, the gradual decrease of the integral absorbance of OH(Al) with temperature indicates a decrease of this defect’s molar absorptivity without any reduction in defect concentration. This is interpreted to result from a decrease in dipole moment of OH(Al) with temperature. A sudden shift of the vibrational frequency from 3396 to 3386 cm–1 between 550 and 560 °C and a constant value of the integral absorbance from 535 to 570 °C were considered to be related to the change in H bond distance during the structural transformation of α-quartz to its β-form. The local environment of OH(Al) begins to change at temperatures below 570 °C, where the crystallographic α–β transition occurs. Received: 18 February 1998/ Accepted: 10 July 1998  相似文献   

7.
Aquifer thermal energy storage in urban and industrial areas can lead to an increase in subsurface temperature to 70 °C and more. Besides its impacts on mineral and sorption equilibria and chemical reaction kinetics in an aquifer, temperature sensitively influences microbial activity and thus redox processes, such as sulphate reduction. Microorganism species can only operate within limited temperature ranges and their adaptability to temperature is a crucial point for the assessment of the environmental consequences of subsurface heat storage. Column experiments with aquifer sediment and tap water at 10, 25, 40, and 70 °C showed that under the constant addition of acetate sulphate reduction could be initiated after 26–63 pore volumes exchanged at all temperatures. Fastest initiation of sulphate reduction with the highest reduction rates was found at 40 °C. Maximum rate constants during experimental run-time were 0.56 h?1 at 40 °C and 0.33, 0.36, and 0.25 h?1 at 10 and, 25, and 70 °C, respectively. Hence, microbial activity was enhanced by a temperature increase to 40 °C but was significantly lowered at 70 °C. At 25 °C methane was found in solution, indicating the presence of fermenting organisms; at 10, 40, and 70 °C no methane production was observed. It could be shown that redox processes in an aquifer generally can adapt to temperatures significantly higher than in situ temperature and that the efficiency of the reduction process can be enhanced by temperature increase to a certain limit. Enhancement of sulphate reduction in an aquifer due to temperature increase could also allow enhanced degradation of organic ground water contaminants such as BTEX, where sulphate is an important electron acceptor.  相似文献   

8.
The selectivity of clinoptilolite toward Pb2+ has stimulated many studies aimed to evaluate the metal uptake. Conversely, the management of a Pb-bearing clinoptilolite has not received the same attention, although it can release a harmful metal. This work aims to evaluate the possibility to control, through thermal treatments, the release of lead from a Pb-clinoptilolite, prepared to simulate the condition of highest dangerousness of an exhausted exchanger. A zeolite-rich rock from Sardinia (Italy) has been processed, obtaining a powder with almost 90 % (wt.) of clinoptilolite. This material has been initially Na- and then Pb-exchanged, reaching a Pb2+ content of 2.28 meq/g. The lead release has been tested before and after 2-h heating at eight different temperatures from 200 to 900 °C. The unheated material releases 64 % of the lead. Heating weakly affects lead release up to 400 °C (54 %), but higher temperatures determine an abrupt reduction from 44 % at 500 °C to 1 % at 700 °C, when the zeolite breakdown occurs. At 800 °C the nucleation of Pb-feldspar and silica polymorphs begins. Basically, the material heated at 900 °C does not release lead (0.03 %), because the metal is trapped in the lead feldspar, whose content attains 42 % (wt.). This solid-state transformation does not involve the emission of lead vapors, another significant environmental aspect.  相似文献   

9.
The temperature dependence of diffusion is usually found to follow the Arrhenius law: D = D0e?E/RT Winchell (1969) showed that there is commonly an inter-dependence between D0 and E (for diffusion in silicate glasses), such that diffusion of different species show a positive correlation on a log D0 vs E plot. A similar effect was noted by Hofmann (1980) for cation diffusion in basalt. This implies that diffusion rates of different species tend to converge at a particular temperature; this effect is known as the ‘compensation effect’. I will show that this effect is also present for diffusion in feldspars and olivines. The equations for the compensation lines (with E given in kcal/mol) are: basalt—E = 50 + 7.5 log D0 feldspar—E = 50.7 + 3.4 log D0 olivine—E = 78.0 + 7.5 log D0 The convergence, or crossover, temperatures for diffusion in various materials are: obsidian—3400°C basalt—1370°C olivine—1360°C feldspar—460°C Compensation plots are useful for evaluating and comparing experimental diffusion data (though of limited usefulness in a predictive sense) and for understanding ‘closure temperatures’ for diffusion in petrogenetic processes (since closure temperature, the temperature at which natural diffusion processes are frozen in, is dependent on E, log d0, and cooling rate). I show that most diffusing species in feldspar have a closure-temperature close to the crossover or convergence temperature, implying that all species in feldspars can be expected to ‘freeze-in’ simultaneously at temperatures in the range 400–600°C (for cooling rates in the range 101–105°C/myr). Closure temperatures of various species in olivine, on the other hand, span a much larger range (800°C) for a similar range in cooling rates, implying that different elements in olivine will record different time-temperature stages in petrogenetic processes.  相似文献   

10.
Electrical resistivity and 57Fe Mössbauer spectra are reported for three calcic amphiboles with different Fe concentrations. AC measurements (20?Hz–1?MHz) were performed, applying impedance spectroscopy between 100 and 785?°C in an N2 gas atmosphere. It was found that up to three semiconducting charge transport processes can be distinguished, which in part changed slightly when several runs were carried out to higher temperatures. The extrapolated DC resistivity is much smaller for an amphibole with high Fe content than for the two with lower Fe concentrations. The derived activation energies are between ～0.48 and ～1.06?eV. For temperatures ≤600?°C the results are compatible with a charge transport mechanism due to electron hopping between Fe2+ and Fe3+. Above 600?°C, dehydrogenation and/or beginning amphibole decomposition obviously alter the conduction mechanism. From Mössbauer spectra it was established that in all amphibole samples Fe2+ and Fe3+ are simultaneously present. Mössbauer parameters were derived by fitting the observed spectra to models taking the occupation of various M sites into account.  相似文献   

11.
X-ray diffraction (XRD) studies on the radioactive ore samples from various parts of Rajasthan and Haryana have revealed the presence of several uranium and other atomic mineral occurrences in the albitite belt of western India. The primary uranium minerals (PUMs) are uraninite and brannerite, whereas, the secondary uranium minerals (SUMs) show considerable speciations: phosphate, silicate, hydrous oxide hydrate, and vanadate. Multiple oxides (MOs) are davidite, fergusonite, aeschynite-(Y), microlite, samarskite, euxenite, betafite, and columbite-tantalite. The thorium minerals are huttonite, thorite, uranoan-thorite, thorianite, thorutite, and brabantite. The yttrium and REE-bearing minerals are xenotime, britholite, allanite, chevkinite, tritomite, and monazite. It is noted that the measured unit cell dimension (a0) of the investigated uraninites ranges from 5.4110 Å to 5.4646 Å. The highest unit cell dimension (5.4646 Å) represents a composition (or oxidation grade) of UO2.05, whereas, the lowest one (5.4110 Å) corresponds to a composition of UO2.54. Furthermore, it is also apparent that, with increase in oxidation grade, there is a concomitant decrease in unit cell dimension. As most of the values of ao of uraninites from the albitite belt are high (> 5.45 Å), it may be inferred that the overall temperature of formation of uraninites of the albitite belt was higher (ca. 400°C). However, the low values of a0 in certain localities could be due to the prevalence of relatively low and fluctuating temperature regimes locally (ca. 400°–100° C). Numerous occurrences of refractory, multiple oxides, and REE minerals, in association with uranium mineralisation, also support a high-temperature origin for the investigated uraninites. Binary data plots of unit cell dimension (a0) versus oxidation grade/composition (UO2+x) of uraninites (n = 36) suggest that the gross uranium mineralisation in the albitite belt of western India is mainly linked to regional metamorphism, anatexis, granitic intrusion, metasomatism, and contact metamorphosed granite-pegmatite aureoles and granite-related vein type with hydrothermal overprints, including redistribution of intrinsic sedimentary uranium and its concentration along suitable structural locales. These interpretations are consistent with the known gross geologic features of the albitite belt. Furthermore, the presence of marialite (calcian) in many places in the albitite belt also supports such a contention, as this mineral is known to be restricted to metamorphic and metasomatic environments. The speciation of secondary uranium minerals could be due to the higher oxidation of U4+ to U6+ in surface to near-surface conditions and its (U6+) remobilisation as uranyl ions. The combination of moving uranyl ions with available cations and anions en route caused re-precipitation of U as diversified assemblages of low-temperature uranyl minerals under suitable physicochemical conditions.  相似文献   

12.
Diffusion modelling of growth-zoned garnet is used in combination with standard geothermometric and geobarometric techniques to estimate cooling and denudation rates from the mafic eclogites of the Red Cliff area, Great Caucasus, Russia. Euhedral garnet porphyroblasts exhibit different degrees of prograde growth zoning depending on the size of the grain (100 μm to several mm in diameter). Zoning patterns are mainly expressed in terms of Fe–Mg exchange, with 100*Mg/(Mg+Fe) increasing from 18–20 to 33–37 from core to rim. Geothermobarometry yields conditions of 680±40 °C and a minimum of 1.6±0.2 GPa and of 660±40 °C and 0.8±0.2 GPa for the high-pressure and retrograde stages of equilibration, respectively. A temperature of 600±40 °C has been recorded for the late-stage metamorphic overprint in the mica schists surrounding the eclogites. Relaxation of garnet zoning profiles was modelled for three different hypothetical P–T –t trajectories, all with an initial temperature of 680 °C and a pressure change of 0.8 GPa. The first two trajectories involve decompression associated with regular cooling down to 660 °C (near isothermal) and 600 °C. The third path is a two-step trajectory comprising near-isobaric cooling down to 600 °C followed by isothermal decompression to 0.8 GPa. These P–T trajectories cover as wide a range of pressure and temperature changes endured by the rocks as possible, thus representing extreme cases for calculating cooling and exhumation rates. Calculations indicate that the zoning pattern of the smallest garnet (i.e. garnet for which the zoning is most easily eliminated during post-growth processes) along the different paths can be preserved for the following average exhumation and cooling rates: path 1, 143 mm a?1 and 102 °C Ma?1; path 2, 60 mm a?1 and 171 °C Ma?1; path 3, 11–30 mm a?1 and 200–400 °C Ma?1. These results are discussed in light of theoretical P–T–t paths extracted from thermal models of regions of thickened crust, and from analogue models of accretionary wedge and continental lithosphere subduction.  相似文献   

13.
The Wyoming bentonite was saturated with alkali and alkaline earth cations and allowed to react hydrothermally for one month at 300°C, and one week and one month at 400°C. Generally, the rate of formation of collapsed layers for the alkali clays was inversely related to interlayer hydration energy. This pattern of reaction is expected if layer dehydration results from an attraction between negatively charged 2:1 layers and the positive interlayer cation. In this mechanism, the greater the hydration energy of the interlayer region, the greater the charge that must develop on the 2:1 layers to cause dehydration, and, therefore, the slower the reaction rate. Reaction rate for the alkaline earth clays was directly related to interlayer hydration energy. Clays saturated with cations of hydration energy equal to or greater than Na+ did not react at 300°C, and ceased to react at 50% expandable layers at 400°C. Those saturated with cations of lower hydration energy continued to form mica-like layers with increasing run time at both 300° and 400°C. Clays saturated with monovalent cations reacted by a process of gradual transformation of smectite layers into collapsed layers, whereas those with divalent cations reacted directly from the smectite structure to form a rectorite-like phase.  相似文献   

14.
The paper presents critical analysis of literature data on the stability constants of aqueous species in the system Zn-O-H-S?II-Cl. In order to more accurately determine the composition and stability of chloride Zn complexes, additional experiments were carried out to determine the solubility of sphalerite ZnSc in chloride-sulfide solutions at 175°C and the saturated vapor pressure of the solution. Having processed the data, we obtained the thermodynamic properties at 25°C and parameters of the HKF (Helgeson-Kirkham-Flowers) equation of state (EoS) for hydroxide, chloride and hydrosulfide Zn complexes. The constants of sphalerite dissolution reactions with the formation of hydrosulfide and, particularly, chloride complexes increase with increasing temperature. The predominant Zn transport species in high-temperatures (>250°C) chloride-sulfide hydrothermal solutions are chloride complexes, first of all, ZnCl 4 2? . As the temperature decreases, the concentrations of complexes with smaller numbers of Cl? ligands increase. The region of weakly acidic to alkaline pH is dominated by hydrosulfide Zn complexes, but their concentrations in equilibrium with sphalerite are relatively low (a few ppm at 400°C and S concentrations <0.1 mol kg?1) and decreases with a temperature decrease. In the region dominated by chloride complexes, the concentration of dissolved Zn can amount to a few fractions of a percent at near-neutral pH, 400°C, and m(NaCl) = 1.0 and increases if the fluid becomes more acidic. An extremely important factor controlling the concentrations of dissolved Zn is temperature: cooling leads to the effective precipitation of sphalerite, particularly in the region dominated by chloride complexes. The thermodynamic properties of the solid phases and parameters of the HKF model for aqueous species in the system Zn-O-H-S-II-Cl are presented in the on-line version of the FreeGC database (http://www-b.ga.gov.au/minerals/research/methodology/geofluids/thermo/calculator/search.jsp), which enables calculating the Gibbs energy values of components of the system and reaction constants involving these components at PT parameters up to 600°C and 3 kbar.  相似文献   

15.
《Applied Geochemistry》2000,15(6):777-784
In connection with the discovery of a new type of Pt deposit in low-rank brown coals and black shales, the interaction of Pt-bearing aqueous solutions with fractionated organic matter (asphaltenes and asphaltenic acids) was studied at 200–400°C and 1 kbar total pressure. It was found that chemical sorption onto the organic matter lowers Pt content in the aqueous solutions by about two orders of magnitude relative to organic-free systems. Thermal maturation of the asphaltenes leads to its aromatization and concomitant sorption of Pt from n×10−4 mPt (mol per kg of dry matter) at 200°C to n×10−2 mPt at 400°C. Thus, the Pt chemisorption on activated carbonized organic matter may be an effective mechanism of Pt accumulation in C-bearing rocks.  相似文献   

16.
Uranyl adsorption was measured from aqueous electrolyte solutions onto well-characterized goethite, amorphous ferric oxyhydroxide, and hematite sols at 25°C. Adsorption was studied at a total uranyl concentration of 10?5 M, (dissolved uranyl 10?5 to 10?8 M) as a function of solution pH, ionic strength and electrolyte concentrations, and of competing cations and carbonate complexing. Solution pHs ranged from 3 to 10 in 0.1 M NaNO3 solutions containing up to 0.01 M NaHCO3. All the iron oxide materials strongly adsorbed dissolved uranyl species at pHs above 5 to 6 with adsorption greatest onto amorphous ferric oxyhydroxide and least onto well crystallized specular hematite. The presence of Ca or Mg at the 10?3 M level did not significantly affect uranyl adsorption. However, uranyl carbonate and hydroxy-carbonate complexing severely inhibited adsorption. The uranyl adsorption data measured in carbonate-free solutions was accurately modeled with the surface complexation-site binding model of Davis et al. (1978), assuming adsorption was chiefly of the UO2OH+ and (UO2)3(OH)+5, aqueous complexes. In modeling it was assumed that these complexes formed a monodentate UO2OH+ surface complex, and a monodentate, bidentate or tridentate (UO2)3(OH)+5surface complex. Of the latter, the bidentate surface complex is the most likely, based on crystallographic arguments. Modeling was less successful predicting uranyl adsorption in the presence of significant uranyl carbonate and hydroxy-carbonate complexing. It was necessary to slightly vary the intrinsic constants for adsorption of the di- and tricarbonate complexes in order to fit the uranyl adsorption data at total carbonate concentrations of 10?2 and 10?3 M.  相似文献   

17.
This work presents new experimental results on the kinetics of mineral dissolution in near-critical and supercritical water in a temperature range (T) from 25 to 400℃ and a constant pressure of 23 MPa. Kinetic experiments were carried out by using a flow reactor (packed bed reactor) of an open system. The dissolution rates of albite and magnetite were measured under these experimental conditions. Na, Al and Si release rates for albite dissolution in water were measured as a function of the temperature and flow velocity in the reaction system. The maximum release rates of Na, AI and Si of albite dissolution in the hydrothermal flow systems under different flow velocities were always obtained at 300℃, that is to say, the maximum albite dissolution rates in the flow systems, regardless of different flow rates, were repeatedly measured at 300℃. Results indicate a wide fluctuation in albite dissolution rates occurring close to the critical point of water. The dissolution rates increased when the temperatu  相似文献   

18.
Phase relations in the system Pb-Sn-Fe-Sb-S were investigated through the diagrams of projecting plane 8x(PbS-SnS-SnS2)from the vertrex point Fe0.96Sb2.04S4.12by vacuum silica tube technique.Experimental results have shown that franckeite has a wide solid solution with substitution of Pb^2 by Sn^2 ,In franckeite s.s.the content of Sn^2 varies from 0 to 4.8 atoms (total metal atoms are 11 atoms per formula) at 500℃ and 0-4.0 atoms at 400℃,respectively,Meanwhile,the content of Sn^4 ranges from 1.3to 2.0 atoms at 500℃ and 1.5-2.1 atoms at 400℃ in franckeite s.s.These results are consistent well with analytic data on natural franckeite.The cylindrite solid solutiopn has a relatively small range with Sn^2 -1.8atoms and Sm^4 =3.2-4.2 atoms per formula at 500℃ and ,Sn^2 =0.5-1.7 atoms and Sm^4 =3.3-4.2 atoms at 400℃ which are comparable with natural cylindrite.The phases coexisting in equilibrium with franckeite s.s. are galena,boulangerite,robinsonite.teallite,SnS,cylindrite.s.s.and synthetic phase Ⅲ ss or I ss.The cylindrite s.s.coexists with SnS2 and the above mentioned phases,but not with galena.teallite and SnS,and probably not with boulangerite in this projecting plane.  相似文献   

19.
This study reports the kinetic experimental results of albite in water and in KCI solution at 22 MPa in the temperature range of 25 to 400℃. Kinetic experiments have been carried out in an open flow-through reaction system (packed bed reactor). Albite dissolution is always incongruent in water at most temperatures, but becomes congruent at 300℃ (close to the critical point 374℃). At temperatures from 25 to 300℃, the incongruent dissolution of albite is reflected by the fact that sodium and aluminum are easily dissolved into water; from 300 to 400℃ it is reflected by silicon being more easily dissolved in water than Al and Na. Maximum albite dissolution rates in the flow hydrothermal systems have been repeatedly observed at 300℃, independent of flow rates.The kinetic experiments of albite dissolution in a KCl aqueous solution (0.1 mol KCl) indicate that the dissolution rate of albite increases with increasing temperature. Maximum silicon release rates of albite have been observed at 400℃, while ma  相似文献   

20.
The trace element signatures of fluids were investigated by leaching experiments on natural samples of partly altered mafic igneous rocks recovered from the drilling site 1,256 of ODP Leg 206 on the Cocos plate (Central America). Experiments with ultrapure water were performed at 400 °C/0.4 GPa and 500 °C/0.7 GPa. Both fluids and residual solids were examined to obtain the partition coefficients (Dfluid/rock) of various trace elements. Element partition coefficients (Dfluid/rock) obtained at 500 °C/0.7 GPa are significantly lower compared to results obtained at 400 °C/0.4 GPa, which is in contrast to observations at higher pressures (2.2–6 GPa) and temperatures between 700 and 1,400 °C (Kessel et al. in Earth Planet Sci Lett 237: 873–892, 2005a; Spandler et al. in Chem Geol 239: 228–249, 2007). This finding may indicate a considerable pressure effect on the leaching processes and strongly divergent fluid–rock interactions in the upper part of a subduction zone at 0.4–0.7 GPa compared to deeper subduction areas with higher pressures. Furthermore, this may be interpreted as one of the earliest fractionation processes during the subduction of crustal material.  相似文献   

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