Argon and CO₂ on the race track in silicate melts: A tool for the development of a CO₂ speciation and diffusion model     

Marcus Nowak  Dominik Schreen 《Geochimica et cosmochimica acta》2004,68(24):5127-5138

We have analysed the kinetics of Argon and CO₂ diffusion in simplified iron free rhyolitic to hawaiitic melts using the diffusion couple technique. The concentration distance profiles of Ar and CO₂ were measured with electron microprobe analysis and Fourier Transform Infrared Spectroscopy, respectively. Error functions were fitted to the symmetrical concentration distance profiles to extract the diffusion coefficients.In the temperature range 1373 to 1773 K the activation energies for Ar diffusion range from 169 ± 20 to 257 ± 62 kJ mol⁻¹. Ar diffusivity increases exponentially with the degree of depolymerisation. In contrast, the mobility of total CO₂, that is identical to Ar mobility in rhyolitic melt, keeps constant with changing bulk composition from rhyolite to hawaiite. CO₂ speciation at 1623 K and 500 MPa was modeled for the range of compositions studied using the diffusion data of Ar and total CO₂ in combination with network former diffusion calculated from viscosity data. Within error this model is in excellent agreement with CO₂ speciation data extrapolated from temperatures near the glass transition temperature for dacitic melt composition. This model shows that even in highly depolymerised hawaiitic and tholeiitic melts molecular CO₂ is a stable species and contributes 70 to 80% to the total CO₂ diffusion, respectively.  相似文献   

Carbon dioxide and argon diffusion in silicate melts: Insights into the CO₂ speciation in magmas     

K. Spickenbom  M. Sierralta 《Geochimica et cosmochimica acta》2010,74(22):6541-6564

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 CO₃^2- 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   

CO₂ solubility and speciation in intermediate (andesitic) melts: the role of H₂O and composition     

P.L. King  J.R. Holloway 《Geochimica et cosmochimica acta》2002,66(9):1627-1640

We determined total CO₂ solubilities in andesite melts with a range of compositions. Melts were equilibrated with excess C-O(-H) fluid at 1 GPa and 1300°C then quenched to glasses. Samples were analyzed using an electron microprobe for major elements, ion microprobe for C-O-H volatiles, and Fourier transform infrared spectroscopy for molecular H₂O, OH⁻, molecular CO₂, and CO₃²⁻. CO₂ solubility was determined in hydrous andesite glasses and we found that H₂O content has a strong influence on C-O speciation and total CO₂ solubility. In anhydrous andesite melts with ∼60 wt.% SiO₂, total CO₂ solubility is ∼0.3 wt.% at 1300°C and 1 GPa and total CO₂ solubility increases by about 0.06 wt.% per wt.% of total H₂O. As total H₂O increases from ∼0 to ∼3.4 wt.%, molecular CO₂ decreases (from 0.07 ± 0.01 wt.% to ∼0.01 wt.%) and CO₃²⁻ increases (from 0.24 ± 0.04 wt.% to 0.57 ± 0.09 wt.%). Molecular CO₂ increases as the calculated mole fraction of CO₂ in the fluid increases, showing Henrian behavior. In contrast, CO₃²⁻ decreases as the calculated mole fraction of CO₂ in the fluid increases, indicating that CO₃²⁻ solubility is strongly dependent on the availability of reactive oxygens in the melt. These findings have implications for CO₂ degassing. If substantial H₂O is present, total CO₂ solubility is higher and CO₂ will degas at relatively shallow levels compared to a drier melt. Total CO₂ solubility was also examined in andesitic glasses with additional Ca, K, or Mg and low H₂O contents (<1 wt.%). We found that total CO₂ solubility is negatively correlated with (Si + Al) cation mole fraction and positively correlated with cations with large Gibbs free energy of decarbonation or high charge-to-radius ratios (e.g., Ca). Combining our andesite data with data from the literature, we find that molecular CO₂ is more abundant in highly polymerized melts with high ionic porosities (>∼48.3%), and low nonbridging oxygen/tetrahedral oxygen (<∼0.3). Carbonate dominates most silicate melts and is most abundant in depolymerized melts with low ionic porosities, high nonbridging oxygen/tetrahedral oxygen (>∼0.3), and abundant cations with large Gibbs free energy of decarbonation or high charge-to-radius ratio. In natural silicate melt, the oxygens in the carbonate are likely associated with tetrahedral and network-modifying cations (including Ca, H, or H-bonds) or a combinations of those cations.  相似文献   

CO₂ solubility in Martian basalts and Martian atmospheric evolution     

Ben D. Stanley  Marc M. Hirschmann  Anthony C. Withers 《Geochimica et cosmochimica acta》2011,75(20):5987-6003

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

Na₂O solubility in CaO-MgO-SiO₂ melts     

R. Mathieu  G. Libourel  E. Deloule  C. Rapin 《Geochimica et cosmochimica acta》2011,75(2):608-628

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

Synthesis, characterization, and thermochemistry of K-Na-H₃O jarosites     

Christophe Drouet 《Geochimica et cosmochimica acta》2003,67(11):2063-2076

The thermochemistry of well-characterized synthetic K-H₃O, Na-H₃O and K-Na-H₃O jarosites was investigated. These phases are solid solutions that obey Vegard’s law. Electron probe microanalyses indicated lower alkali and iron contents than predicted from the theoretical end-member compositions, in agreement with thermal analyses, suggesting the presence of hydronium and “additional” water. The standard enthalpies of formation (ΔH°_f) of K-H₃O, Na-H₃O and K-Na-H₃O jarosites were determined by high-temperature oxide melt solution calorimetry. These enthalpies vary linearly with the K/H₃O, Na/H₃O and K/Na ratio, respectively. The enthalpy of formation of pure hydronium jarosite was also determined experimentally (ΔH°_f = −3741.6 ± 8.3 kJ.mol⁻¹), and it was used to evaluate ΔH°_f for the end-members KFe₃(SO₄)₂(OH)₆ (ΔH°_f = −3829.6 ± 8.3 kJ.mol⁻¹) and NaFe₃(SO₄)₂(OH)₆ (ΔH°_f = −3783.4 ± 8.3 kJ.mol⁻¹). Finally, enthalpies of dehydration (loss of the “additional” water) of some jarosites were determined and found to be near the enthalpy of vaporization of water, suggesting that the “additional” water is weakly bonded in the structure.  相似文献   

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

James D. Prikryl 《Geochimica et cosmochimica acta》2008,72(18):4508-4520

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

Thermochemistry of yavapaiite KFe(SO₄)₂: Formation and decomposition     

Ferenc Lázár Forray  Alexandra Navrotsky 《Geochimica et cosmochimica acta》2005,69(8):2133-2140

Yavapaiite, KFe(SO₄)₂, is a rare mineral in nature, but its structure is considered as a reference for many synthetic compounds in the alum supergroup. Several authors mention the formation of yavapaiite by heating potassium jarosite above ca. 400°C. To understand the thermal decomposition of jarosite, thermodynamic data for phases in the K-Fe-S-O-(H) system, including yavapaiite, are needed. A synthetic sample of yavapaiite was characterized in this work by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and thermal analysis. Based on X-ray diffraction pattern refinement, the unit cell dimensions for this sample were found to be a = 8.152 ± 0.001 Å, b = 5.151 ± 0.001 Å, c = 7.875 ± 0.001 Å, and β = 94.80°. Thermal decomposition indicates that the final breakdown of the yavapaiite structure takes place at 700°C (first major endothermic peak), but the decomposition starts earlier, around 500°C. The enthalpy of formation from the elements of yavapaiite, KFe(SO₄)₂, ΔH°_f = −2042.8 ± 6.2 kJ/mol, was determined by high-temperature oxide melt solution calorimetry. Using literature data for hematite, corundum, and Fe/Al sulfates, the standard entropy and Gibbs free energy of formation of yavapaiite at 25°C (298 K) were calculated as S°(yavapaiite) = 224.7 ± 2.0 J.mol⁻¹.K⁻¹ and ΔG°_f = −1818.8 ± 6.4 kJ/mol. The equilibrium decomposition curve for the reaction jarosite = yavapaiite + Fe₂O₃ + H₂O has been calculated, at pH₂O = 1 atm, the phase boundary lies at 219 ± 2°C.  相似文献   

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

Alasdair P. LeeBrian L. Phillips  William H. Casey 《Geochimica et cosmochimica acta》2002,66(4):577-587

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

Self-diffusion of Si and O in diopside-anorthite melt at high pressures     

David Tinker  Charles E Lesher 《Geochimica et cosmochimica acta》2003,67(1):133-142

Self-diffusion coefficients for Si and O in Di₅₈An₄₂ liquid were measured from 1 to 4 GPa and temperatures from 1510 to 1764°C. Glass starting powders enriched in ¹⁸O and ²⁸Si were mated to isotopically normal glass powders to form simple diffusion couples, and self-diffusion experiments were conducted in the piston cylinder device (1 and 2 GPa) and in the multianvil apparatus (3.5 and 4 GPa). Profiles of ¹⁸O/¹⁶O and ^29,30Si/²⁸Si were measured using secondary ion mass spectrometry. Self-diffusion coefficients for O (D(O)) are slightly greater than self-diffusion coefficients for Si (D(Si)) and are often the same within error. For example, D(O) = 4.20 ± 0.42 × 10⁻¹¹ m²/s and D(Si) = 3.65 ± 0.37 × 10⁻¹¹ m²/s at 1 GPa and 1662°C. Activation energies for self-diffusion are 215 ± 13 kJ/mol for O and 227 ± 13 kJ/mol for Si. Activation volumes for self-diffusion are −2.1 ± 0.4 cm³/mol and −2.3 ± 0.4 cm³/mol for O and Si, respectively. The similar self-diffusion coefficients for Si and O, similar activation energies, and small, negative activation volumes are consistent with Si and O transport by a cooperative diffusion mechanism, most likely involving the formation and disassociation of a high-coordinated intermediate species. The small absolute magnitudes of the activation volumes imply that Di₅₈An₄₂ liquid is close to a transition from negative to positive activation volume, and Adam-Gibbs theory suggests that this transition is linked to the existence of a critical fraction (∼0.6) of bridging oxygen.  相似文献   

A model for silicate melt viscosity in the system CaMgSi₂O₆-CaAl₂Si₂O₈-NaAlSi₃O₈     

James K. Russell  Daniele Giordano 《Geochimica et cosmochimica acta》2005,69(22):5333-5349

Five hundred eighty-five viscosity measurements on 40 melt compositions from the ternary system CaMgSi₂O₆ (Di)-CaAl₂Si₂O₈ (An)-NaAlSi₃O₈ (Ab) have been compiled to create an experimental database spanning a wide range of temperatures (660-2175°C). The melts within this ternary system show near-Arrhenian to strongly non-Arrhenian properties, and in this regard are comparable to natural melts. The database is used to produce a chemical model for the compositional and temperature dependence of melt viscosity in the Di-An-Ab system. We use the Vogel-Fulcher-Tammann equation (VFT: log η = A + B/(T − C)) to account for the temperature dependence of melt viscosity. We also assume that all silicate melts converge to a common viscosity at high temperature. Thus, A is independent of composition, and all compositional dependence resides in the parameters B and C. The best estimate for A is −5.06, which implies a high-temperature limit to viscosity of 10^-5.06 Pa s. The compositional dependence of B and C is expressed by 12 coefficients (b_i=1,2.6, c_j=1,2..6) representing linear (e.g., b_i=1:3) and higher order, nonlinear (e.g., b_i=4:6) contributions. Our results suggest a near-linear compositional dependence for B (<10% nonlinear) and C (<7% nonlinear). We use the model to predict model VFT functions and to demonstrate the systematic variations in viscosity due to changes in melt composition. Despite the near linear compositional dependence of B and C, the model reproduces the pronounced nonlinearities shown by the original data, including the crossing of VFT functions for different melt compositions. We also calculate values of T_g for melts across the Di-An-Ab ternary system and show that intermediate melt compositions have T_g values that are depressed by up to 100°C relative to the end-members Di-An-Ab. Our non-Arrhenian viscosity model accurately reproduces the original database, allows for continuous variations in rheological properties, and has a demonstrated capacity for extrapolation beyond the original data.  相似文献   

Thermodynamic modeling of melts in the system Na₂O-NaAlO₂-SiO₂-F₂O₋₁     

David Dolejš  Don R. Baker 《Geochimica et cosmochimica acta》2005,69(23):5537-5556

  相似文献   

Interaction between sulphide and H₂O in silicate melts     

Jan Stelling  Harald Behrens  Jörg Göttlicher 《Geochimica et cosmochimica acta》2011,75(12):3542-3557

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   

The combined effects of ƒ_O2 and melt composition on SnO₂ solubility and tin diffusivity in haplogranitic melts     

Robert L. Linnen  Michel Pichavant  Franois Holtz 《Geochimica et cosmochimica acta》1996,60(24):4965-4976

The diffusion profile method has been employed to measure tin diffusion coefficients and SnO₂ solubility in water-saturated, peralkaline to peraluminous haplogranitic melts at 850°C, 2 kbar, and log ƒ_O2 conditions ranging from FMQ - 0.57 to FMQ + 3.49. At reduced conditions cassiterite is highly soluble and tin is present dominantly as a Sn²⁺ species, whereas at oxidized conditions SnO₂ is much less soluble, and tin is present dominantly as a Sn⁴⁺ species. There is a strong melt composition control on SnO₂ solubility; solubilities are at a minimum at the subaluminous composition, increase strongly with alkali content in peralkaline compositions and weakly with Al content in peraluminous compositions. In the case of the latter, this increase can only be distinguished at reduced conditions, e.g., at a log ƒ_O2 of FMQ - 0.57 cassiterite solubility increases from 2.78 to 4.11 wt% SnO₂ for melt with Al/(Na + K)compositions (A.S.I.) of 1.0 and 1.2, respectively. At oxidized conditions SnO₂ solubility is 500 ppm for both the A.S.I. 1.0 and 1.2 compositions. By comparison Sn0₂ solubilities in the most peralkaline composition investigated range from 3.94 wt% to -10 wt% Sn02, for the most oxidized to the most reduced conditions, respectively. Thermodynamic modelling of the data indicates that the Sn⁴⁺/ΣSn ratio in the melt is also at a minimum at the subaluminous composition, ranging from 0.4 at log ƒ_O2 of FMQ + 3.49 to 0.01 at FMQ - 0.57. Over the same log foZ range the Sn⁴⁺/ΣSn ratio for the A.S.I. 0.6 composition ranges from 0.98 to 0.4 and for the A.S.I. 1.25 composition, from 0.8 to 0.02.Tin diffusivity is dependent on both f_O2 and melt composition. The effective binary diffusion coefficient of tin at reduced conditions is approximately 10^−7.5 cm²/sec for the peraluminous compositions and 10^−8.2 cm²/sec for the peralkaline compositions. At oxidized conditions these values decrease to approximately 10^−8.2 and 10^−9.0 cm²/sec, respectively. These are interpreted to reflect relatively fast diffusion where Sn²⁺ is the dominant valence and tin in this case behaves similar to a network modifier and relatively slow diffusion where Sn⁴⁺ is dominant and tin likely has a lower coordination number. Alternatively, the coordination of Sn²⁺ and Sn⁴⁺ is the same, but the bond strengths are different. At fixed f_O2 the faster diffusivity in the peraluminous compositions reflects the lower Sn⁴⁺/Sn²⁺ ratio. The fact the Sn⁴⁺/Sn²⁺ ratio in melts varies greatly with ƒ_O2 at redox conditions near FMQ suggests that the partitioning behaviour of tin possibly changes during the evolution of an igneous suite in general and of a peraluminous granite suite in particular.  相似文献   

Formation enthalpy of ThSiO₄ and enthalpy of the thorite → huttonite phase transition     

Lena Mazeina  Alexandra Navrotsky  Lynn A. Boatner 《Geochimica et cosmochimica acta》2005,69(19):4675-4683

The standard enthalpy of formation of thorite and huttonite and the enthalpy of the phase transition between these polymorphs were determined using high-temperature oxide melt solution calorimetry and transposed temperature drop calorimetry. Standard enthalpies of formation of thorite and huttonite are reported for the first time and are −2117.6 ± 4.2 kJ/mol and −2110.9 ± 4.7 kJ/mol, respectively. Based on our measurements, thorite and huttonite are metastable relative to SiO₂ (quartz) and ThO₂ (thorianite) at standard conditions, but are presumably stabilized at high temperature by the entropy contribution. Based on the measured enthalpy of the thorite-huttonite phase transition of 6.7 ± 2.5 kJ/mol, a dP/dT slope for the transformation was calculated as −1.21 ± 0.45 MPa/K.  相似文献   

Tracer diffusion of Mg, Ca, Sr, and Ba in Na-aluminosilicate melts     

Knut Roselieb  Albert Jambon 《Geochimica et cosmochimica acta》2002,66(1):109-123

We employed the thin source technique to investigate tracer diffusion of Mg, Ca, Sr, and Ba in glasses and supercooled melts of albite (NaAlSi₃O₈) and jadeite (NaAlSi₂O₆) compositions. The experiments were conducted at 1 bar and at temperatures between 645 and 1025°C. Typical run durations ranged between 30 min and 35 days. The analysis of the diffusion profiles was performed with the electron microprobe. Diffusivities of Ca, Sr, and Ba were found to be independent of either duration t of the experiment or tracer concentration M, initially introduced into the sample. Mg exhibits a diffusivity depending on run time and concentration and tracer diffusivity is derived by extrapolation to M/√t = 0. Temperature dependence of the diffusivity D can be represented by an Arrhenius equation D = D_o exp(−E_a/RT), yielding the following least-squares fit parameters (with D in m²/s and E_a in kJ/mol): D_Mg = 1.8 · 10⁻⁵ exp(−234 ± 20/RT), D_Ca = 3.5 · 10⁻⁶ exp(−159 ± 6/RT), D_Sr = 3.6 · 10⁻⁶ exp(−160 ± 6/RT), and D_Ba = 6.0 · 10⁻⁶ exp(−188 ± 12/RT) for albite; and D_Mg = 8.3 · 10⁻⁶ exp(−207 ± 18/RT), D_Ca = 3.8 · 10⁻⁶ exp(−153 ± 4/RT), D_Sr = 2.3 · 10⁻⁶ exp(−150 ± 4/RT), and D_Ba = 3.7 · 10⁻⁵ exp(−198 ± 4/RT) for jadeite composition. Ca and Sr diffusivities agree within error in both compositions and exhibit the fastest diffusivities, whereas Mg reveals the lowest diffusivity. The relationship between activation energy and radius shows a minimum at Ca and Sr for albite and jadeite compositions extending the relationship already observed elsewhere for alkalies. With increasing substitution of Si by (Na + Al), diffusivities increase, whereas activation energies decrease. Furthermore, a simple model modified from that of Anderson and Stuart (Anderson O. L. and Stuart D. A., “Calculation of activation energy of ionic conductivity in silica glasses by classical methods,” J. Am. Ceram. Soc.37, 573-580, 1954) is discussed for calculating the activation energies.  相似文献   

Activation energy for diffusion of helium in water-saturated, compacted Na-montmorillonite     

Tomohiro Higashihara  Tamotsu Kozaki 《Engineering Geology》2005,81(3):365-370

It is important to clarify the migration behavior of hydrogen gas dissolved in water-saturated, compacted bentonite, which is a promising material for geologic disposal of high-level waste and TRU waste disposal. The diffusion coefficients of helium, which can be detected under extremely low background conditions, in water-saturated, compacted Na-montmorillonite were determined as a function of temperature by a transient diffusion method. The activation energies for diffusion of helium were then obtained. The activation energies were from 6.9 ± 4.8 to 19 ± 2.8 kJ mol^− 1 and were regarded to be independent of dry density. The activation energies of helium in water-saturated Na-montmorillonite were roughly equal to those in bulk water, 14.9 kJ mol^− 1, and in ice, from 11 to 13 kJ mol^− 1. It is possible that helium diffuses not only in pore water but also in interlayer water.  相似文献   

Laboratory determination of thermal diffusion constants for N₂/N₂ in air at temperatures from −60 to 0°C for reconstruction of magnitudes of abrupt climate changes using the ice core fossil-air paleothermometer     

Alexi M Grachev  Jeffrey P Severinghaus 《Geochimica et cosmochimica acta》2003,67(3):345-360

Rapid temperature change causes fractionation of isotopic gaseous species in air in firn (snow) by thermal diffusion, producing a signal that is preserved in trapped air bubbles as the snow forms ice. Using a model of heat penetration and gas diffusion in the firn, as well as the values of appropriate thermal diffusion constants, it is possible to reconstruct the magnitude of a particular paleoclimate change. Isotopic nitrogen in air serves as a convenient tracer for such paleoreconstruction, because the ratio ²⁹N₂/²⁸N₂ has stayed extremely constant in the atmosphere for ≥10⁶ years. However, prior to this work no data were available for thermal diffusion of ²⁹N₂/²⁸N₂ in air, but only in pure N₂. We devised a laboratory experiment allowing fractionation of gases by thermal diffusion in a small, tightly controlled temperature difference. A mass spectrometer was employed in measuring the resulting fractionations yielding measurement precision greater than was attainable by earlier thermal diffusion investigators.Our laboratory experiments indicate that the value of the thermal diffusion sensitivity (Ω) for ²⁹N₂/²⁸N₂ in air is +(14.7 ± 0.5) × 10⁻³ per mil/°C when the average temperature is -30.0°C. The corresponding value for ²⁹N₂/²⁸N₂ in pure N₂ that we find is +(15.3 ± 0.4) × 10⁻³ per mil/°C at -30.6°C, in agreement with the previously available literature data within their large range of uncertainty. We find that an empirical equation, Ω = (8.656/T_K − 1232/T K2) ± 3% per mil/°C, describes the slight variation of the sensitivity values for ²⁹N₂/²⁸N₂ in air with temperature in the range of -60 to 0°C. A separate set of experiments also described in this paper rules out adsorption as a candidate for producing additional temperature change-driven fractionation of ²⁹N₂/²⁸N₂ in the firn air. The combined newly obtained data constitute a calibration of the fossil-air paleothermometer with respect to isotopic nitrogen and will serve to improve the estimates of the magnitudes of past abrupt climate changes recorded in ice cores.  相似文献   

Surface simulation studies of the hydration of white rust Fe(OH)₂, goethite α-FeO(OH) and hematite α-Fe₂O₃     

Nora H. de Leeuw  Timothy G. Cooper 《Geochimica et cosmochimica acta》2007,71(7):1655-1673

Computer modelling techniques were used to elucidate the hydration behaviour of three iron (hydr)oxide minerals at the atomic level: white rust, goethite and hematite. A potential model was first adapted and tested against the bulk structures and properties of eight different iron oxides, oxyhydroxides and hydroxides, followed by surface simulations of Fe(OH)₂, α-FeO(OH) and α-Fe₂O₃. The major interaction between the adsorbing water molecules and the surface is through interaction of their oxygen ions with surface iron ions, followed by hydrogen-bonding to surface oxygen ions. The energies released upon the associative adsorption of water range from 1 to 17 kJ mol⁻¹ for Fe(OH)₂, 26 to 80 kJ mol⁻¹ for goethite and 40 to 85 kJ mol⁻¹ for hematite, reflecting the increasing oxidation of the iron mineral. Dissociative adsorption at goethite and hematite surfaces releases larger hydration energies, ranging from 120 to 208 kJ mol⁻¹ for goethite and 76 to 190 kJ mol⁻¹ for hematite.The thermodynamic morphologies of the minerals, based on the calculated surface energies, agree well with experimental morphologies, where these are available. When the partial pressures required for adsorption of water from the gas phase are plotted against temperature for the goethite and hematite surfaces, taking into account experimental entropies for water, it appears that these minerals may well be instrumental in the retention of water during the cyclic variations in the atmosphere of Mars.  相似文献   

Studies of equilibrium, structure, and dynamics in the aqueous Al(iii)-oxalate-fluoride system by potentiometry, C and F NMR spectroscopy     

Andrea Bodor  Istudor Bányai  Staffan Sjöberg 《Geochimica et cosmochimica acta》2003,67(15):2793-2803

The AlOx_1-3 (Ox = oxalate) species were identified in 0.6 M aqueous NaCl by ¹³C nuclear magnetic resonance (NMR). Rate constants and activation parameters for intramolecular cis/trans isomerization of the Werner-type AlOx₂⁻ complex (k(298 K) = 5 s⁻¹, ΔH^# = 67 ± 5 kJ mol⁻¹, ΔS^# = −6 ± 6 J mol⁻¹ K⁻¹, the rate determining step could be the breaking of the Al-O(C=O) bond) and a very slow intermolecular ligand exchange reaction of AlOx₃³⁻ complex and the free ligand (k₃₀(298 K) = 6.6 · 10⁻⁵ s⁻¹, ΔH^# = 164 ± 17 kJ mol⁻¹, ΔS^# = 225 ± 51 J mol⁻¹ K⁻¹, D/I_d mechanism) were determined by dynamic 1D and 2D ¹³C NMR measurements. Mixed complexes, AlFOx, AlFOx₂^2-, AlF₂Ox⁻, and AlF₂Ox₂^3-, with overall stability (logβ) of 11.53 ± 0.03, 15.67 ± 0.03, 15.74 ± 0.02, and 19.10 ± 0.04 were measured by potentiometry using pH- and fluoride-selective electrodes and confirmed by ¹³C and¹⁹F NMR. The role of these complexes in gibbsite dissolution was modeled. The mixed Al(III)-Ox^2--F⁻ complexes have to be considered as the chemical speciation of Al(III) in natural waters is discussed.  相似文献