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1.
The diffusion of water in dacitic and andesitic melts was investigated at temperatures of 1458 to 1858 K and pressures between 0.5 and 1.5 GPa using the diffusion couple technique. Pairs of nominally dry glasses and hydrous glasses containing between 1.5 and 6.3 wt.% dissolved H 2O were heated for 60 to 480 s in a piston cylinder apparatus. Concentration profiles of hydrous species (OH groups and H 2O molecules) and total water ( CH2Ot = sum of OH and H 2O) were measured along the cylindrical axis of the diffusion sample using IR microspectroscopy. Electron microprobe traverses show no significant change in relative proportions of anhydrous components along H 2O profiles, indicating that our data can be treated as effective binary interdiffusion between H 2O and the rest of the silicate melt. Bulk water diffusivity ( DH2Ot) was derived from profiles of total water using a modified Boltzmann-Matano method as well as using fittings assuming a functional relationship between DH2Ot and CH2Ot. In dacitic melts DH2Ot is proportional to CH2Ot up to 6 wt.%. In andesitic melts the dependence of DH2Ot on CH2Ot is less pronounced. A pressure effect on water diffusivity could not be resolved for either dacitic or andesitic melt in the range 0.5 to 1.5 GPa. Combining our results with previous studies on water diffusion in rhyolite and basalt show that for a given water content DH2Ot increases monotonically with increasing melt depolymerization at temperatures >1500 K. Assuming an Arrhenian behavior in the whole compositional range, the following formulation was derived to estimate DH2Ot (m 2/s) at 1 wt.% H 2O t in melts with rhyolitic to andesitic composition as a function of T (K), P (MPa) and S (wt.% SiO 2):
2.
Solubility mechanisms of water in depolymerized silicate melts quenched from high temperature (1000°-1300°C) at high pressure (0.8-2.0 GPa) have been examined in peralkaline melts in the system Na 2O-SiO 2-H 2O with Raman and NMR spectroscopy. The Na/Si ratio of the melts ranged from 0.25 to 1. Water contents were varied from ∼3 mol% and ∼40 mol% (based on O = 1). Solution of water results in melt depolymerization where the rate of depolymerization with water content, ∂(NBO/Si)/∂X H2O, decreases with increasing total water content. At low water contents, the influence of H 2O on the melt structure resembles that of adding alkali oxide. In water-rich melts, alkali oxides are more efficient melt depolymerizers than water. In highly polymerized melts, Si-OH bonds are formed by water reacting with bridging oxygen in Q 4-species to form Q 3 and Q 2 species. In less polymerized melts, Si-OH bonds are formed when bridging oxygen in Q 3-species react with water to form Q 2-species. In addition, the presence of Na-OH complexes is inferred. Their importance appears to increase with Na/Si. This apparent increase in importance of Na-OH complexes with increasing Na/Si (which causes increasing degree of depolymerization of the anhydrous silicate melt) suggests that water is a less efficient depolymerizer of silicate melts, the more depolymerized the melt. This conclusion is consistent with recently published 1H and 29Si MAS NMR and 1H- 29Si cross polarization NMR data. 相似文献
3.
The solubility behavior of H 2O in melts in the system Na 2O-SiO 2-H 2O was determined by locating the univariant phase boundary, melt = melt + vapor in the 0.8-2 GPa and 1000°-1300°C pressure and temperature range, respectively. The NBO/Si-range of the melts (0.25-1) was chosen to cover that of most natural magmatic liquids. The H 2O solubility in melts in the system Na 2O-SiO 2-H 2O (X H2O) ranges between 18 and 45 mol% (O = 1) with (∂X H2O/∂P) T∼14-18 mol% H 2O/GPa. The (∂X H2O/∂P) T is negatively correlated with NBO/Si (= Na/Si) of the melt. The (∂X H2O/∂T) P is in the −0.03 to +0.05 mol% H 2O/°C range, and is negatively correlated with NBO/Si. The [∂X H2O/∂(NBO/Si)] P,T is in the −3 to −8 mol% H 2O/(NBO/Si) range. Melts with NBO/Si similar to basaltic liquids (∼0.6-∼1.0) show (∂X H2O/∂T) P<0, whereas more polymerized melts exhibit (∂X H2O/∂T) P>0. Complete miscibility between hydrous melt and aqueous fluid occurs in the 0.8-2 GPa pressure range for melts with NBO/Si ≤0.5 at T >1100°C. Miscibility occurs at lower pressure the more polymerized the melt. 相似文献
4.
The structure of H 2O-saturated silicate melts, coexisting silicate-saturated aqueous solutions, and supercritical silicate liquids in the system Na 2O·4SiO 2–H 2O has been characterized with the sample at high temperature and pressure in a hydrothermal diamond anvil cell (HDAC). Structural information was obtained with confocal microRaman and with FTIR microscopy. Fluids and melts were examined along pressure-temperature trajectories defined by the isochores of H 2O at nominal densities, ρfluid, (from EOS of pure H 2O) of 0.90 and 0.78 g/cm 3. With ρfluid = 0.78 g/cm 3, water-saturated melt and silicate-saturated aqueous fluid coexist to the highest temperature (800 °C) and pressure (677 MPa), whereas with ρfluid = 0.90 g/cm 3, a homogeneous single-phase liquid phase exists through the temperature and pressure range (25–800 °C, 0.1–1033 MPa). Less than 5 vol% quartz precipitates near 650 °C in both experimental series, thus driving Na/Si-ratios of melt + fluid phase assemblages to higher values than that of the Na 2O·4SiO 2 starting material.Molecular H 2O (H 2O°) and structurally bonded OH groups were observed in coexisting melts and fluids as well as in supercritical liquids. Their OH/(H 2O)-ratio is positively correlated with temperature. The OH/(H 2O)° in melts is greater than in coexisting fluids. Structural units of Q 3, Q 2, Q 1, and Q 0 type are observed in all phases under all conditions. An expression of the form, 12Q 3 + 13H 2O2Q 2 + 6Q 1 + 4Q 0, describes the equilibrium among those structural units. This equilibrium shifts to the right with increasing pressure and temperature with a ΔH of the reaction near 425 kJ/mol. 相似文献
5.
The sodium solubility in silicate melts in the CaO-MgO-SiO 2 (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 Na 2O- xSiO 2 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., aNa 2O (sample) = aNa 2O (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., Na 2O 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 < SiO 2 < 100; in wt%), we found that Na 2O solubility is conveniently modeled as a linear function of the optical basicity ( Λ) calculated on a Na-free basis melt composition. In our experiments, γNa 2O (sample) ranges from 7 × 10 −7 to 5 × 10 −6, indicating a strongly non-ideal behavior of Na 2O solubility in the studied CMS melts (γNa 2O (sample) ? 1). In addition to showing the effect of sodium on phase relationships in the CMS system, this Na 2O solubility study brings valuable new constraints on how melt structure controls the solubility of Na in the CMS silicate melts. Our results suggest that Na 2O addition causes depolymerization of the melt by preferential breaking of Si-O-Si bonds of the most polymerized tetrahedral sites, mainly Q 4. 相似文献
6.
Armenite, ideal formula BaCa 2Al 6Si 9O 30·2H 2O, and its dehydrated analog BaCa 2Al 6Si 9O 30 and epididymite, ideal formula Na 2Be 2Si 6O 15·H 2O, and its dehydrated analog Na 2Be 2Si 6O 15 were studied by low-temperature relaxation calorimetry between 5 and 300 K to determine the heat capacity, Cp, behavior of their confined H 2O. Differential thermal analysis and thermogravimetry measurements, FTIR spectroscopy, electron microprobe analysis and powder Rietveld refinements were undertaken to characterize the phases and the local environment around the H 2O molecule.The determined structural formula for armenite is Ba 0.88(0.01)Ca 1.99(0.02)Na 0.04(0.01)Al 5.89(0.03)Si 9.12(0.02)O 30·2H 2O and for epididymite Na 1.88(0.03)K 0.05(0.004)Na 0.01(0.004)Be 2.02(0.008)Si 6.00(0.01)O 15·H 2O. The infrared (IR) spectra give information on the nature of the H 2O molecules in the natural phases via their H 2O stretching and bending vibrations, which in the case of epididymite only could be assigned. The powder X-ray diffraction data show that armenite and its dehydrated analog have similar structures, whereas in the case of epididymite there are structural differences between the natural and dehydrated phases. This is also reflected in the lattice IR mode behavior, as observed for the natural phases and the H 2O-free phases. The standard entropy at 298 K for armenite is S° = 795.7 ± 6.2 J/mol K and its dehydrated analog is S° = 737.0 ± 6.2 J/mol K. For epididymite S° = 425.7 ± 4.1 J/mol K was obtained and its dehydrated analog has S° = 372.5 ± 5.0 J/mol K. The heat capacity and entropy of dehydration at 298 K are Δ = 3.4 J/mol K and Δ Srxn = 319.1 J/mol K and Δ = −14.3 J/mol K and Δ Srxn = 135.7 J/mol K for armenite and epididymite, respectively. The H 2O molecules in both phases appear to be ordered. They are held in place via an ion-dipole interaction between the H 2O molecule and a Ca cation in the case of armenite and a Na cation in epididymite and through hydrogen-bonding between the H 2O molecule and oxygen atoms of the respective silicate frameworks. Of the three different H 2O phases ice, liquid water and steam, the Cp behavior of confined H 2O in both armenite and epididymite is most similar to that of ice, but there are differences between the two silicates and from the Cp behavior of ice. Hydrogen-bonding behavior and its relation to the entropy of confined H 2O at 298 K is analyzed for various microporous silicates.The entropy of confined H 2O at 298 K in various silicates increases approximately linearly with increasing average wavenumber of the OH-stretching vibrations. The interpretation is that decreased hydrogen-bonding strength between a H 2O molecule and the silicate framework, as well as weak ion-dipole interactions, results in increased entropy of H 2O. This results in increased amplitudes of external H 2O vibrations, especially translations of the molecule, and they contribute strongly to the entropy of confined H 2O at T < 298 K. 相似文献
7.
We determined total CO 2 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 H 2O, OH −, molecular CO 2, and CO 32−. CO 2 solubility was determined in hydrous andesite glasses and we found that H 2O content has a strong influence on C-O speciation and total CO 2 solubility. In anhydrous andesite melts with ∼60 wt.% SiO 2, total CO 2 solubility is ∼0.3 wt.% at 1300°C and 1 GPa and total CO 2 solubility increases by about 0.06 wt.% per wt.% of total H 2O. As total H 2O increases from ∼0 to ∼3.4 wt.%, molecular CO 2 decreases (from 0.07 ± 0.01 wt.% to ∼0.01 wt.%) and CO 32− increases (from 0.24 ± 0.04 wt.% to 0.57 ± 0.09 wt.%). Molecular CO 2 increases as the calculated mole fraction of CO 2 in the fluid increases, showing Henrian behavior. In contrast, CO 32− decreases as the calculated mole fraction of CO 2 in the fluid increases, indicating that CO 32− solubility is strongly dependent on the availability of reactive oxygens in the melt. These findings have implications for CO 2 degassing. If substantial H 2O is present, total CO 2 solubility is higher and CO 2 will degas at relatively shallow levels compared to a drier melt. Total CO 2 solubility was also examined in andesitic glasses with additional Ca, K, or Mg and low H 2O contents (<1 wt.%). We found that total CO 2 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 CO 2 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
8.
Hydrogen isotope exchange between water and orthosilicic acid (H 4SiO 4) was modeled using B3LYP calculations and classical transition-state theory. Configurations of 1, 2, 3 and 7 water molecules and H 4SiO 4 were used to investigate energetically viable reaction pathways. An upper-bound of 71 kJ/mol was assumed for the zero-point energy corrected barrier (ZPECB) because this is the experimentally determined activation energy for Si-O bond breaking (Rimstidt and Barnes, 1980) and ZPECB is expected to be close to this value. Long range solvation forces were accounted for using the integral equation formalism polarized continuum model (IEFPCM; Cancès et al., 1997). Primary and secondary isotope effects were computed by exchanging hydrogen atoms with deuterium. Results show that reaction mechanisms involving 3 and 7 water molecules have ZPECB of 34 to 38 kJ/mol, whereas those involving 1 and 2 water molecules have ZPECB in excess of the set upper-bound. The lower range of ZPECB with 3 or 7 water molecules is reasonable to explain rapid hydrogen isotope exchange with silicates. Rate constant calculations accounting for tunneling, anharmonicity and scaling factors indicate that the reaction is fast and equilibrium can be assumed under most geologic conditions. 相似文献
9.
Solubility and solution mechanisms of H 2O in depolymerized melts in the system Na 2O-Al 2O 3-SiO 2 were deduced from spectroscopic data of glasses quenched from melts at 1100 °C at 0.8-2.0 GPa. Data were obtained along a join with fixed nominal NBO/T = 0.5 of the anhydrous materials [Na 2Si 4O 9-Na 2(NaAl) 4O 9] with Al/(Al+Si) = 0.00-0.25. The H 2O solubility was fitted to the expression, XH2O=0.20+0.0020 fH2O-0.7 XAl+0.9( XAl) 2, where XH2O is the mole fraction of H 2O (calculated with O = 1), fH2O the fugacity of H 2O, and XAl = Al/(Al+Si). Partial molar volume of H 2O in the melts, , calculated from the H 2O-solulbility data assuming ideal mixing of melt-H 2O solutions, is 12.5 cm 3/mol for Al-free melts and decreases linearly to 8.9 cm 3/mol for melts with Al/(Al+Si) ∼ 0.25. However, if recent suggestion that is composition-independent is applied to constrain activity-composition relations of the hydrous melts, the activity coefficient of H 2O, , increases with Al/(Al+Si).Solution mechanisms of H 2O were obtained by combining Raman and 29Si NMR spectroscopic data. Degree of melt depolymerization, NBO/T, increases with H 2O content. The rate of NBO/T-change with H 2O is negatively correlated with H 2O and positively correlated with Al/(Al+Si). The main depolymerization reaction involves breakage of oxygen bridges in Q 4-species to form Q 2 species. Steric hindrance appears to restrict bonding of H + with nonbridging oxygen in Q 3 species. The presence of Al 3+ does not affect the water solution mechanisms significantly. 相似文献
10.
Phase equilibria data in the systems SiO 2-P 2O 5, P 2O 5-M xO y, and P 2O 5-M xO y-SiO 2 are employed in conjunction with Chromatographic and spectral data to investigate the role of P 2O 5 in silicate melts. Such data indicate that the behavior of P 2O 5 is complex. P 2O 5 depolymerizes pure SiO 2 melts by entering the network as a four-fold coordinated cation, but polymerizes melts in which an additional metal cation other than silicon is present. The effect of this polymerization is apparent in the widening of the granite-ferrobasalt two-liquid solvus. In this complex system P 2O 5 acts to increase phase separation by further enrichment of the high charge density cations Ti, Fe, Mg, Mn, Ca, in the ferrobasaltic liquid. P 2O 5 also produces an increase in the ferrobasalt-granite REE liquid distribution coefficients. These distribution coefficients are close to 4 in P 2O 5-free melts, but close to 15 in P 2O 5-bearing melts.The dual behavior of P 2O 5 is explained in a model which requires complexing of phosphate anions (analogous to silicate anions) and metal cations in the melt. This interaction destroys Si-O-M-O-Si bonds polymerizing the melt. The higher concentration of Si-O-M-O-Si bond complexes in immiscible ferrobasaltic liquids relative to their conjugate immiscible granite liquids explains the partitioning of P 2O 5 into the ferrobasaltic liquid. 相似文献
11.
The magnitude of equilibrium iron isotope fractionation between Fe(H 2O) 63+ and Fe(H 2O) 62+ is calculated using density functional theory (DFT) and compared to prior theoretical and experimental results. DFT is a quantum chemical approach that permits a priori estimation of all vibrational modes and frequencies of these complexes and the effects of isotopic substitution. This information is used to calculate reduced partition function ratios of the complexes (10 3 · ln(β)), and hence, the equilibrium isotope fractionation factor (10 3 · ln(α)). Solvent effects are considered using the polarization continuum model (PCM). DFT calculations predict fractionations of several per mil in 56Fe/ 54Fe favoring partitioning of heavy isotopes in the ferric complex. Quantitatively, 10 3 · ln(α) predicted at 22°C, ∼ 3 ‰, agrees with experimental determinations but is roughly half the size predicted by prior theoretical results using the Modified Urey-Bradley Force Field (MUBFF) model. Similar comparisons are seen at other temperatures. MUBFF makes a number of simplifying assumptions about molecular geometry and requires as input IR spectroscopic data. The difference between DFT and MUBFF results is primarily due to the difference between the DFT-predicted frequency for the ν 4 mode (O-Fe-O deformation) of Fe(H 2O) 63+ and spectroscopic determinations of this frequency used as input for MUBFF models (185-190 cm −1 vs. 304 cm −1, respectively). Hence, DFT-PCM estimates of 10 3 · ln(β) for this complex are ∼ 20% smaller than MUBFF estimates. The DFT derived values can be used to refine predictions of equilibrium fractionation between ferric minerals and dissolved ferric iron, important for the interpretation of Fe isotope variations in ancient sediments. Our findings increase confidence in experimental determinations of the Fe(H 2O) 63+ − Fe(H 2O) 62+ fractionation factor and demonstrate the utility of DFT for applications in “heavy” stable isotope geochemistry. 相似文献
12.
Hydrothermal experiments were conducted to evaluate the kinetics of H 2(aq) oxidation in the homogeneous H 2-O 2-H 2O system at conditions reflecting subsurface/near-seafloor hydrothermal environments (55-250 °C and 242-497 bar). The kinetics of the water-forming reaction that controls the fundamental equilibrium between dissolved H 2(aq) and O 2(aq), are expected to impose significant constraints on the redox gradients that develop when mixing occurs between oxygenated seawater and high-temperature anoxic vent fluid at near-seafloor conditions. Experimental data indicate that, indeed, the kinetics of H 2(aq)-O 2(aq) equilibrium become slower with decreasing temperature, allowing excess H 2(aq) to remain in solution. Sluggish reaction rates of H 2(aq) oxidation suggest that active microbial populations in near-seafloor and subsurface environments could potentially utilize both H 2(aq) and O 2(aq), even at temperatures lower than 40 °C due to H 2(aq) persistence in the seawater/vent fluid mixtures. For these H 2-O 2 disequilibrium conditions, redox gradients along the seawater/hydrothermal fluid mixing interface are not sharp and microbially-mediated H 2(aq) oxidation coupled with a lack of other electron acceptors (e.g. nitrate) could provide an important energy source available at low-temperature diffuse flow vent sites.More importantly, when H 2(aq)-O 2(aq) disequilibrium conditions apply, formation of metastable hydrogen peroxide is observed. The yield of H 2O 2(aq) synthesis appears to be enhanced under conditions of elevated H 2(aq)/O 2(aq) molar ratios that correspond to abundant H 2(aq) concentrations. Formation of metastable H 2O 2 is expected to affect the distribution of dissolved organic carbon (DOC) owing to the existence of an additional strong oxidizing agent. Oxidation of magnetite and/or Fe ++ by hydrogen peroxide could also induce formation of metastable hydroxyl radicals (•OH) through Fenton-type reactions, further broadening the implications of hydrogen peroxide in hydrothermal environments. 相似文献
13.
Polymerization of silicate and aluminate tetrahedra in glasses,melts and aqueous solutions—II. The network modifying effects of Mg2+ , K+, Na+, Li+, H+, OH−, F−, Cl−, H2O,CO2 and H3O+ on silicate polymers
The effect of the group IA and VIIA ions, as well as Mg 2+, and the molecules H 2O, CO 2, H 3O + and OH ? on the energy of the Si-O bond in a H 6Si 2O 7 cluster has been calculated using semiempirical molecular orbital calculations (CNDO/2). Three types of elementary processes, i.e. substitution, addition, and polymerization reactions have been used to interpret data on the dynamic viscosity, surface tension and surface charge, hydrolytic weakening, diffusivity, conductivity, freezing point depression, and degree of polymerization of silicates in melts, glasses, and aqueous solutions. As a test of our calculational procedure, observed X-ray emission spectra of binary alkali silicate glasses were compared with calculated electronic spectra. The well known bondlength variations between the bridging bond [Si-O(br)] and the non-bridging bond [Si-O(nbr)] in alkali silicates are shown to be due to the propagation of oscillating bond-energy patterns through the silica framework. A kinetic interpretation of some results of our calculations is given in terms of the Bell-Evans-Polanyi reaction principle. 相似文献
14.
As part of a study of the effect of geologically common network modifiers on polymerization in silicate melts, glasses, and silica-rich aqueous solutions, we have studied the energies, electronic structures, and inferred chemical properties of IVT-O- IVT linkages in the tetrahedral dimers H 6,Si 2O 7, H 6AlSiO 71?, and H 6Al 2O 72? using semi-empirical molecular orbital theory (CNDO/2). Our results indicate that the electron donating character of the bridging oxygen, O(br), linking two tetrahedra increases with increasing T-O(br) bond length but decreases with decreasing T-O(br)-T angles and increasing O-T-O(br) angles. This increase or decrease of the donor character of O(br) coincides with an increase or decrease of the affinity of O(br) for hard acceptors. The calculated electronic structure for the H 6Si 2O 7 molecule is compared with the observed X-ray emission, absorption, and photoelectron spectra of quartz and vitreous silica; the reasonable match between calculated and observed oxygen Kα emission spectra of vitreous silica supports our assertion that non-bonded O(br) electron density energetically at the top of the valence band controls the chemical reactivity of IVT-O- IVT linkages in polymerized tetrahedral environments. 相似文献
15.
Oxygen isotope exchange between H 2O and H 4SiO 4 was modeled with ab initio calculations on H 4SiO 4 + 7H 2O. Constrained optimizations were performed with the B3LYP/6-31+G(d,p) method to determine reactants, transition states, and intermediates. Long-range solvation was accounted for using self-consistent reaction field calculations. The mechanism for exchange involves two steps, a concerted proton transfer from H 4SiO 4 forming a 5-coordinated Si followed by a concerted proton transfer from the 5-coordinated Si forming another H 4SiO 4. The 5-coordinated Si intermediate is C2 symmetric. At 298K and with implicit solvation included, the Gibbs free energy of activation from transition state theory is 66 kJ/mol and the predicted rate constant is 16 s −1. Equilibrium calculations between 298K and 673K yield α H4SiO4-H2O that are uniformly less than, but similar to, α qtz-H2O, and therefore α qtz-H4SiO4 is expected to be relatively small in this temperature range. 相似文献
16.
Summary Phase fields intersected by three joins in the System CaO-MgO-SiO 2-CO 2-H 2O at 2 kbar were investigated experimentally to determine the melting relationships and the sequences of crystallization of liquids co-precipitating silicate minerals and carbonates. These joins connect SiO 2 to three mixtures of CaCO 3-MgCO 3-Mg(OH) 2 with compositions on the primary îield for calcite, between the composition CaCO 3 and the low-temperature (650°C eutectic liquid co-precipitating calcite, dolomite and periclase. In the pseudo-quaternary tetrahedron calcite-magnesite-brucite-diopside, two of the significant reactions found are: (1) a eutectic at 650°C, calcite + dolomite + periclase + forsterite + vapor = liquid, and (2) a peritectic at 1038°Cwhich is either calcite + åkermanite + forsterite + vapor = monticellite + liquid calcite + monticellite + forsterite + vapor = åkermanite + liquid. The eutectic liquid has high MgO/CaO and CO 2/H 2O and only 2–3% SiO 2 (estimated 15–20% MgCO 3, 35–40% CaCO 3, 40–45% Mg(OH) 2, and 5–6% Mg 2SiO 4). The composition joins intersect a thermal maximum for åkermanite + forsterite + vapor = liquid, which separates high-temperature liquids precipitating silicates together with a little calcite, from low-temperature liquids precipitating carbonates with a few percent of forsterite; there is no direct path between the silicate and synthetic carbonatite liquids on these joins, but it is possible that fractionating liquid paths diverging from the joins may connect them. More complex relationships involving the pprecipitatioon off monticellite and åkermanite are also outlined. Magnetite-magnesioferrite may replace periclase in natural magmatic systems. The results indicate that the assemblage calcite-dolomite-magnetite-forsterite represents the closing stages of crystallization of carbonatites, whereas assemblages such as calcite-magnetite-forsterite and dolomite-magnetite-forsterite span the whole range of carbonatite evolution in terms of temperature and composition, and provide the link between liquids precipitating silicates and those precipitating carbonates.
Die Beziehungen zwischen silikarischen Schmelzen und karbonatbildenden Schmelzen im System CaO-MgO-SiO2-CO2-H2O bei 2 kbar Zusammenfassung Phasenfelder, die durch den Schnitt von drei Verbindungslinien im System CaO-MgO-SiO2-CO2-H2Odefiniert werden, wurden experimentell bei 2 kbar untersucht, um die Schmelzbeziehungen und die Kristallisationsfolge von Schmelzen, die gleichzeitig silikatische und karbonatische Minerale ausscheiden, zu bestimmen. Diese Linien verbinden SiO2 mit drei Mischungen von CaCO3-M9CO3-Mg(OH)2 mit Zusammensetzungen im primären Calcitfeld, zwischen der Zusammensetzung CaCO3 und der tieftemperierten (650°C Calcit-, Dolomit- und Periklasbildenden eutektischen Schmelze. Zwei wichtige im ppseudo-quaternären Tetraeder Calcit-Magnetit-Brucit-Diopsid gefundene Reaktionen sind: (1) Ein Eutektikum bei 650°C Calcit + Dolomit + Periklas + Forsterit + Vapor = Liquid und (2) ein Peritektikum bei 1038°C mit entweder Calcit + Åkermanit + Forsterit + Vapor = Monticellit + Liquid oder Calcit + Monticellit + Forsterit + Vapo = Åkermanit + Liquid Die eutektische Schmelze zeigt hohe MgO/CaO und CCO2H2O Verhältnisse und nur 2–3% SiO2(geschätzter Anteil an MgCO315–20%, CaCO3 35–40%, Mg(OH)2 40–50% und Mg2SiO4 5–6%). Die Verbindungslinie schneidet ein thermisches Maximum von Åkermanit + Forsterit + Vapor = Liquid, das höher temperierte Schmelzen, die Silikate gemeinsam mit etwas Clacit ausscheiden, von tiefer temperierten Schmelzen trennt, aus denen sich Karbonate gemeinsam mit wenigen Prozenten Forsterit abscheiden. Es existiert keine direkte Verbindung zwischen silikatischen und synthetischen karbonatitischen Schmelzen entlang dieser Verbindungslinien, es wäre aber möglich, daß Fraktionierungspfade, die von diesen Verbindungslinien ausgehen, sie verbinden. Komplexere Beziehungen, die die Kristallisation von Monticellit und Åkermanit beinhalten, werden ebenfalls aufgezeigt. Magnetit-Magnesioferrit könntean die Stelle von Periklas in nnatürlichenmagmatischen Systemen treten. Die Ergebnisse weisen darauf bin, daß die Vergesellschaftung Calcit-Dolomit-Magnetit-Forsterit das Endstadium der Karbonatitkristallisation repräsentiert, während die Vergesellsschaftungen von Calcit-Magnetit-Forsterit bzw. Dolomit-Magnetit-Forsterit die gesamte Spannweite der Karbonatitevolution hinsichtlich Temperatur und Zusammensetzung umfassen und demnach ein Verbindungsglied zwischen silikat- und karbonatausscheidenden Schmelzen darstellen.
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17.
The solubility of CO 2 in dacitic melts equilibrated with H 2O-CO 2 fluids was experimentally investigated at 1250°C and 100 to 500 MPa. CO 2 is dissolved in dacitic glasses as molecular CO 2 and carbonate. The quantification of total CO 2 in the glasses by mid-infrared (MIR) spectroscopy is difficult because the weak carbonate bands at 1430 and 1530 cm −1 can not be reliably separated from background features in the spectra. Furthermore, the ratio of CO 2,mol/carbonate in the quenched glasses strongly decreases with increasing water content. Due to the difficulties in quantifying CO 2 species concentrations from the MIR spectra we have measured total CO 2 contents of dacitic glasses by secondary ion mass spectrometry (SIMS).At all pressures, the dependence of CO 2 solubility in dacitic melts on xfluidCO2,total shows a strong positive deviation from linearity with almost constant CO 2 solubility at xCO2fluid > 0.8 (maximum CO 2 solubility of 795 ± 41, 1376 ± 73 and 2949 ± 166 ppm at 100, 200 and 500 MPa, respectively), indicating that dissolved water strongly enhances the solubility of CO 2. A similar nonlinear variation of CO 2 solubility with xCO2fluid has been observed for rhyolitic melts in which carbon dioxide is incorporated exclusively as molecular CO 2 (Tamic et al., 2001). We infer that water species in the melt do not only stabilize carbonate groups as has been suggested earlier but also CO 2 molecules.A thermodynamic model describing the dependence of the CO 2 solubility in hydrous rhyolitic and dacitic melts on T, P, fCO2 and the mol fraction of water in the melt ( xwater) has been developed. An exponential variation of the equilibrium constant K 1 with xwater is proposed to account for the nonlinear dependence of xCO2,totalmelt on xCO2fluid. The model reproduces the CO 2 solubility data for dacitic melts within ±14% relative and the data for rhyolitic melts within 10% relative in the pressure range 100-500 MPa (except for six outliers at low xCO2fluid). Data obtained for rhyolitic melts at 75 MPa and 850°C show a stronger deviation from the model, suggesting a change in the solubility behavior of CO 2 at low pressures (a Henrian behavior of the CO 2 solubility is observed at low pressure and low H 2O concentrations in the melt). We recommend to use our model only in the pressure range 100-500 MPa and in the xCO2fluid range 0.1-0.95. The thermodynamic modeling indicates that the partial molar volume of total CO 2 is much lower in rhyolitic melts (31.7 cm 3/mol) than in dacitic melts (46.6 cm 3/mol). The dissolution enthalpy for CO 2 in hydrous rhyolitic melts was found to be negligible. This result suggests that temperature is of minor importance for CO 2 solubility in silicic melts. 相似文献
18.
The solubility of Ti- and P-rich accessory minerals has been examined as a function of pressure and K 2O/Na 2O ratio in two series of highly evolved silicate systems. These systems correspond to (a) alkaline, varying from alkaline to peralkaline with increasing K 2O/Na 2O ratio; and (b) strongly metaluminous (essentially trondhjemitic at the lowest K 2O/Na 2O ratio) and remaining metaluminous with increasing K 2O/Na 2O ratio (to 3). The experiments were conducted at a fixed temperature of 1000 °C, with water contents varying from 5 wt.% at low pressure (0.5 GPa), increasing through 5–10 wt.% at 1.5–2.5 GPa to 10 wt.% at 3.5 GPa. Pressure was extended outside the normal crustal range, so that the results may also be applied to derivation of hydrous silicic melts from subducted oceanic crust. For the alkaline composition series, the TiO2 content of the melt at Ti-rich mineral saturation decreases with increasing pressure but is unchanged with increasing K content (at fixed pressure). The P2O5 content of the alkaline melts at apatite saturation increases with increased pressure at 3.5 GPa only, but decreases with increasing K content (and peralkalinity). For the metaluminous composition series (termed as “trondhjemite-based series” (T series)), the TiO2 content of the melt at Ti-rich mineral saturation decreases with increasing pressure and with increasing K content (at fixed pressure). The P2O5 content of the T series melts at apatite saturation is unchanged with increasing pressure, but decreases with increasing K content. The contrasting results for P and Ti saturation levels, as a function of pressure in both compositions, point to contrasting behaviour of Ti and P in the structure of evolved silicate melts. Ti content at Ti-rich mineral saturation is lower in the alkaline compared with the T series at 0.5 GPa, but is similar at higher pressures, whereas P content at apatite saturation is lower in the T series at all pressures studied. The results have application to A-type granite suites that are alkaline to peralkaline, and to I-type metaluminous suites that frequently exhibit differing K2O/Na2O ratios from one suite to another. 相似文献
19.
Chemical equilibrium between sodium-aluminum silicate minerals and chloride bearing fluid has been experimentally determined in the range 500–700°C at 1 kbar, using rapid-quench hydrothermal methods and two modifications of the Ag + AgCl acid buffer technique. The temperature dependence of the thermodynamic equilibrium constant ( K) for the reaction Albite Andalusite Qtz. can be described by the following equation: log k = ?4.437 + 5205.6/ T( K) The data from this study are consistent with experimental results reported by Montoya and Hemley (1975) for lower temperature equilibria defined by the assemblages albite + paragonite + quartz + fluid and paragonite + andalusite + quartz + fluid. Values of the equilibrium constants for the above reactions were used to estimate the difference in Gibbs free energy of formation between NaCl o and HCl o in the range 400–700°C and 1–2 kbar. Similar calculations using data from phase equilibrium studies reported in the literature were made to determine the difference in Gibbs free energy of formation between KCl o and HCl o. These data permit modelling of the chemical interaction between muscovite + kspar + paragonite + albite + quartz assemblages and chloride-bearing hydrothermal fluids. 相似文献
20.
The effects of excess SiO 2 and CO 2 on the solubility of molybdenite in hydrous sodium disilicate melts were experimentally determined at 680 bars and 650°C. The molybdenite solubility decreases with increasing SiO 2 and CO 2. Under the experimental conditions, the MoS 2 content of the vapor-saturated liquid decreases from 10 wt.% to 2.5 wt.% at SiO 2 saturation. In the presence of CO 2, the solubility decreases to 4.6 wt.% MoS 2 and becomes negligible at high P CO2. These results are explained as deriving from the increased polymerization and hence decreased NBO/Si ratio of the melt with increasing SiO 2 content and CO 2, respectively. Sulfur dissolves principally as SO 4?2 at the relatively high fo 2 of the experiments. Consequently, the effect of sulfur is to lower the Mo solubility by effectively decreasing the NBO/Si ratio of the melt. Sulfur saturation is, therefore, likely to be a limiting factor in the Mo content of alkali silicate melts because of the chalcophile affinities of molybdenum. 相似文献
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