共查询到20条相似文献，搜索用时 31 毫秒
1.
Roger A. Burke James M. Brooks William M. Sackett 《Geochimica et cosmochimica acta》1981,45(5):627-634
Light hydrocarbon (C1-C3) concentrations in the water from four Red Sea brine basins (Atlantis II, Suakin, Nereus and Valdivia Deeps) and in sediment pore waters from two of these areas (Atlantis II and Suakin Deeps) are reported. The hydrocarbon gases in the Suakin Deep brine (T = ~ 25°C, , ) are apparently of biogenic origin as evidenced by ) ratios of ~ 1000. Methane concentrations (6–8 μl/l) in Suakin Deep sediments are nearly equal to those in the brine, suggesting sedimentary interstitial waters may be the source of the brine and associated methane.The Atlantis II Deep has two brine layers with significantly different light hydrocarbon concentrations indicating separate sources. The upper brine (T = ~ 50°C, , CH4 = ~ 155 μl/l) gas seems to be of biogenic origin [], whereas the lower brine (T = ~ 61°C, , CH4 = ~ 120μl/l) gas is apparently of thermogenic origin []. The thermogenic gas resulting from thermal cracking of organic matter in the sedimentary column apparently migrates into the basin with the brine, whereas the biogenic gas is produced in situ or at the seawater-brine interface. Methane concentrations in Atlantis II interstitial waters underlying the lower brine are about one half brine concentrations; this difference possibly reflects the known temporal variations of hydrothermal activity in the basin. 相似文献
2.
Diopside-melt and forsterite-melt rare earth (REE) and Ni partition coefficients have been determined as a function of bulk compositions of the melt. Available Raman spectroscopic data have been used to determine the structures of the melts coexisting with diopside and forsterite. The compositional dependence of the partition coefficients is then related to the structural changes of the melt.The melts in all experiments have a ratio of nonbridging oxygens to tetrahedral cations () between 1 and 0. The quenched melts consist of structural units that have, on the average, 2 (chain), 1 (sheet) and 0 (three-dimensional network) nonbridging oxygens per tetrahedral cation. The proportions of these structural units in the melts, as well as the overall , change as a function of the bulk composition of the melt.It has been found that Ce, Sm, Tm and Ni crystal-liquid partition coefficients () decrease linearly with increasing . The values of the individual REE crystal-liquid trace element partition coefficients have different functional relations to , so that the degree of light REE enrichment of the melts would depend on their .The solution mechanisms of minor oxides such as CO2, H2O, TiO2, P2O5 and Fe2O3 in silicate melts are known. These data have been recast as changes of of the melts with regard to the type of oxide and its concentration in the melt. From such data the dependence of crystal-liquid partition coefficients on concentration and type of minor oxide in melt solution has been calculated. 相似文献
3.
Equations are developed for calculating the density of aluminosilicate liquids as a function of composition and temperature. The mean molar volume at reference temperature Tr, is given by , where the summation is taken over all oxide components except A12O3, X stands for mole fraction, terms are constants derived independently from an analysis of volume-composition relations in alumina-free silicate liquids, and is the composition-dependent apparent partial molar volume of Al2O3. The thermal expansion coefficient of aluminosilicate liquids is given by , where terms are constants independent of temperature and composition, and is a composition-dependent term representing the effect of Al2O3 on the thermal expansion. Parameters necessary to calculate the volume of silicate liquids at any temperature T according to V(T) = Vrexp[α(T-Tr)], where Tr = 1400°C have been evaluated by least-square analysis of selected density measurements in aluminosilicate melts. Mean molar volumes of aluminosilicate liquids calculated according to the model equation conform to experimentally measured volumes with a root mean square difference of 0.28 and an average absolute difference of 0.90% for 248 experimental observations. The compositional dependence of is discussed in terms of several possible interpretations of the structural role of Al3+ in aluminosilicate melts. 相似文献
4.
J.R Holloway 《Geochimica et cosmochimica acta》1973,37(3):651-666
The stability of the amphibole pargasite [NaCa2Mg4Al(Al2Si6))O22(OH)2] in the melting range has been determined at total pressures (P) of 1.2 to 8 kbar. The activity of H2O was controlled independently of P by using mixtures of H2O + CO2 in the fluid phase. The mole fraction of H2O in the fluid () ranged from 1.0 to 0.2.At P < 4 kbar the stability temperature (T) of pargasite decreases with decreasing at constant P. Above P ? 4 kbar stability T increases as is decreased below one, passes through a T maximum and then decreases with a further decrease in . This behavior is due to a decrease in the H2O content of the silicate liquid as decreases. The magnitude of the T maximum increases from about 10°C (relative to the stability T for ) at P = 5 kbar to about 30°C at P = 8 kbar, and the position of the maximum shifts from at P = 5 kbar to at P = 8 kbar.The H2O content of liquid coexisting with pargasite has been estimated as a function of at 5 and 8 kbar P, and can be used to estimate the H2O content of magmas. Because pargasite is stable at low values of at high P and T, hornblende can be an important phase in igneous processes even at relatively low H2O fugacities. 相似文献
5.
Variations in the chemical composition of sedimentary rocks and the nature of kerogen through geologic time were investigated in order to obtain information on biological and environmental evolution during the pre-Phanerozoic eon. Rock samples differing in lithology, depositional environment, and age were pulverized, pre-extracted with organic solvents, and analyzed for total nitrogen (N), phosphorus (P) and organic carbon (org. C or CT). Variations in the molecular structure of kerogen were measured by determining the ratio of org. C content after pyrolysis (CR) to org. C content before pyrolysis (CT), the ratio being considered an index of the degree of condensed-aromatic (as opposed to aliphatic) character. The rocks included mudstones (Early Archean (> 3 · 109 years old) to Miocene), carbonate rocks (mid-Proterozoic (1.3 · 109 years old) to Eocene), cherts (Early Archean (> 3 · 109 years old) to Late Proterozoic (0.8 · 109 years old)), and coal (Archean (> 2.7 · 109 years old) to Early Proterozoic (～1.8 · 109 years old)).The mudstones and carbonates showed progressive increase in org. C content with decreasing age, as reported by other investigators, but the cherts unexpectedly showed a decrease in org. C content with decreasing age. In all samples, a simple inverse correlation between ratio and org. C was observed, each rock type forming its own trend separate from but parallel to those of the other rock types. Thus, the older cherts tend to be richer in org. C and have lower ratios, but the older carbonates and mudstones are poorer in org. C and have higher ratios. For a given org. C concentration, chert has the highest ratio and carbonate rock the lowest, mudstone being intermediate; this may mean that chert is relatively ineffective as a catalyst for the thermal cracking of kerogen or that it inhibits cracking. N appears to be correlated with org. C. The relationship between ratio and org. C or N suggests that the concentrations of org. C and N in sedimentary rocks are largely determined by selective elimination of labile aliphatic and nitrogenous groups of kerogen during post-depositional maturation, although the nature, abundance and depositional environment of the organic source material must be taken into consideration as well. The observed secular variations of org. C, N and ratio may be ascribed to several possible causes, including age-dependent post-depositional alteration of kerogen, secular decrease in the ratio of the atmosphere and hydrosphere during pre-Phanerozoic time, secular increase in rates of accumulation of organic matter in sediments and evolutionary changes in the composition of the biological source material. The secular variations of the carbonates and mudstones could be accounted for by age-dependent cumulative effects of post-depositional alteration alone, whereas the secular variations of the cherts probably reflect changes in the nature of the biological source material and the composition of the atmosphere and hydrosphere. The available evidence suggests that primary characteristics of kerogen are better preserved in chert than in the other types of sediment.The ratios of the carbonates and cherts correlate negatively with the absorbance ratios of “humic matter” extracted from the same rock samples with benzene—methanol. Thus, the greater the degree of condensed-aromatic character of the kerogen, the greater the degree of condensed-aromatic character of the solvent-extractable bituminous “humic matter” with which it is associated. In addition, the ratio of aliphatic to carbonyl-type groups () in the extractable “humic matter” of carbonates and cherts correlates with the non-extractable org. C content of the rocks, suggesting that the org. C data are related to the degree of aliphatic character of the kerogen. The chemical similarity between extractable “humic matter” and its associated kerogen is evidence that the “humic matter” is as old as its rock matrix and can be accepted as a valid chemical fossil. It also suggests that information obtainable from kerogen may be gotten more easily, rapidly and cheaply from solvent-extractable organic matter. The mudstones showed little or no relationship between ratio and ratio, or between ratio and org. C content. The data are consistent with the hypothesis that the kerogen in the carbonates and cherts is autochthonous, whereas the kerogen in the mudstones is partly allochthonous, implying the existence of soil humus and soil organisms in pre-Phanerozoic times. Moreover, the existence of coal in Archean sediments is consistent with the existence of very shallow-water and possibly terrestrial microfloras possessing adaptations for protection against ultraviolet solar radiation.The P content of the sediments showed a complicated zig-zag pattern of variation through geologic time. All the different suites of samples gave similar results, indicating that the variations represent phenomena whose effects were worldwide and independent of local environment. P levels are low in the early pre-Phanerozoic but rise with decreasing age until ～ 1 · 109 years B.P., then fall to a minimum at (～0.7–0.8) · 109 years B.P., and rise again to a lower Paleozoic (Ediacarian?) maximum, decline to a later Paleozoic minimum, and then rise again. The low P content of early pre-Phanerozoic sediments could be due to several factors, including high CO2 content of seawater, anaerobic conditions in the sea, absence of stable-shelf environments, and low rates of primary production. The minimum in the Late Proterozoic is tentatively attributed to the Late Proterozoic glaciations. The data are consistent with the theory that the glacial episode was of worldwide extent. 相似文献
6.
The fractionation factors () for the reduction of CO2 to CH4 by pure cultures of methane-producing bacteria are, for Methanosarcina barkeri at 40°C, 1.045 ± 0.002; for Methanobacterium strain M.o.H. at 40°C, 1.061 ± 0.002; and, for Methanobacterium thermoautotrophicum at 65°C, 1.025 ± 0.002. These observations suggest that the acetic acid used by acetate dissimilating bacteria, if they play an important role in natural methane production, must have an intramolecular isotopic fractionation () approximating the observed fractionation. 相似文献
7.
Bruce S. Hemingway Richard A. Robie James A. Kittrick 《Geochimica et cosmochimica acta》1978,42(10):1533-1543
Solution calorimetric measurements compared with solubility determinations from the literature for the same samples of gibbsite have provided a direct thermochemical cycle through which the Gibbs free energy of formation of [Al(OH)4 aq?] can be determined. The Gibbs free energy of formation of [Al(OH)4 aq?] at 298.15 K is ?1305 ± 1 kJ/mol. These heat-of-solution results show no significant difference in the thermodynamic properties of gibbsite particles in the range from 50 to 0.05 μm.The Gibbs free energies of formation at 298.15 K and 1 bar pressure of diaspore, boehmite and bayerite are ?9210 ± 5.0, ?918.4 ± 2.1 and ?1153 ± 2 kJ/mol based upon the Gibbs free energy of [A1(OH)4 aq?] calculated in this paper and the acceptance of ?1582.2 ± 1.3 and ?1154.9 ± 1.2 kJ/mol for the Gibbs free energy of formation of corundum and gibbsite, respectively.Values for the Gibbs free energy formation of [Al(OH)2 aq+] and [AlO2 aq?] were also calculated as ?914.2 ± 2.1 and ?830.9 ± 2.1 kJ/mol, respectively. The use of [AlC2 aq?] as a chemical species is discouraged.A revised Gibbs free energy of formation for [H4SiO4aq0] was recalculated from calorimetric data yielding a value of ?1307.5 ± 1.7 kJ/mol which is in good agreement with the results obtained from several solubility studies.Smoothed values for the thermodynamic functions CP0, (, , ST0 - S00, kaolinite are listed at integral temperatures between 298.15 and 800 K. The heat capacity of kaolinite at temperatures between 250 and 800 K may be calculated from the following equation: CP0 = 1430.26 ? 0.78850 T + 3.0340 × 10?4T2 ?1.85158 × 10?4T2.The thermodynamic properties of most of the geologically important Al-bearing phases have been referenced to the same reference state for Al, namely gibbsite. 相似文献
8.
Frank J Millero 《Geochimica et cosmochimica acta》1985,49(2):547-553
The effect of ionic interactions of the major components of natural waters on the oxidation of Cu(I) and Fe(II) has been examined. The various ion pairs of these metals have been shown to have different rates of oxidation. For Fe(II), the chloride and sulfate ion pairs are not easily oxidized. The measured decrease in the rate constant at a fixed pH in chloride and sulfate solutions agrees very well with the values predicted. The effect of pH (6 to 8) on the oxidation of Fe(II) in water and seawater have been shown to follow the rate equation where k1 and k2 are the pseudo first order rate constants, β1 and β2 are the hydrolysis constants for Fe(OH)+ and Fe(OH)0. The value of αFE is the fraction of free Fe2+. The value of k1 (2.0 ±0.5 min?1) in water and seawater are similar within experimental error. The value of k2 (1.2 × 105 min?1) in seawater is 28% of its value in water in reasonable agreement with predictions using an ion pairing model.For the oxidation of Cu(I) a rate equation of the form was found where k0 (14.1 sec?1) and k1 (3.9 sec?1) are the pseudo first order rate constants for the oxidation of Cu+ and CuCl0, β1 is the formation constant for CuCl0 and αCu is the fraction of free Cu+. Thus, unlike the results for Fe(II), Cu(I) chloride complexes have measurable rates of oxidation. 相似文献
9.
Polyunsaturated fatty acids (C18:2 and C18:3ω3 were analyzed in the upper 20m layer of a 200 m long sediment core taken from Lake Biwa. Concentration maxima occur in layers at depths of 0.2, 1–5, 11–12, and 16m. The vertical changes in the ratio appear to correlate with paleoclimatic condition suggested from palynological evidence. On the basis of ratios, it was suggested that it has been colder at 200, 1000–4000, 15,000 and 20,000 yr BP than at other times. 相似文献
10.
Frank J Millero 《Geochimica et cosmochimica acta》1977,41(2):215-223
The specific interaction model has been used to determine the partial molal volume of electrolytes in 0.725 m NaCl and 35‰ salinity seawater solutions at 25°C. The partial molal volumes of electrolytes (MX) were estimated at a given ionic strength (I) from , where SV is the Debye-Hückel limiting law slope, vi is the number of ions i formed when MX dissociated, [i] is the total molality of ion i and BMX is a specific interaction parameter that varies slowly with ionic strength. The values of estimated by using this equation were found to agree very well with experimental values in NaCl and seawater providing there are not strong interactions between M and X. For electrolytes that form ion pairs (i.e. MX°) corrections must be made. Methods are discussed for making these corrections. 相似文献
11.
John Geover 《Geochimica et cosmochimica acta》1974,38(10):1527-1548
For a phase at equilibrium in which two cation species are partitioned ideally between two sub-lattice sites, the excess functions of mixing (free energy, enthalpy and entropy) are directly related to the bulk composition of the phase and ΔGE°(T, P), the standard-state intra- crystalline exchange free energy. If the phase is not at equilibrium internally, an additional ordering parameter is necessary to fix the excess free energy of mixing, GmixEX, unambiguously. Conversely, for any fixed GmixEX there exists an infinity of possible intracrystalline cation dis- tributions, only one of which is the equilibrium distribution for the specified temperature and pressure. As ideal intraphase cation ordering becomes more pronounced, GmixEX decreases. In response, the total free energy of mixing for the phase decreases progressively for non-end member compositions, approaching, at the limits of ordering, values appropriate for stabilizing compounds of intermediate composition.The model-dependent activity coefficient for component A in the phase, γAT, can be calculated for any bulk composition, XAT, either from GmixEX directly or from more basic equations involving the interrelation of chemical potentials at equilibrium. A general form for γAT is ln , where Xjκ denotes the mole fraction of species j in site κ. The first term on the right-hand side of this equation is the contribution to γAT from ideal intracrystalline partitioning, and is common to the several theories lately presented to model intraphase cation partitioning. It can be shown rigorously that this term contributes to a negative deviation from ideality for the bulk phase. The second term is the contribution to the macroscopic activity coefficient from non-ideal intraphase partitioning, and is related to an enthalpy of mixing, HmixN in excess of that resulting from ideal inter-site cation ordering. While the expression represented by Y can take several functional forms, the additional enthalpy can be evaluated explicitly for specific non-ideal partitioning models from the relation .In those cases, GmixEX can also be determined exactly. 相似文献
12.
E.J. Reardon 《Chemical Geology》1976,18(4):309-325
Potentiometric measurements in dilute sodium borate solutions with added alkali earth chlordie salts yield the following expressions for the dissociation constants of alkali earth borate ion pairs from 10 to 50°C:where T is in °K. Enthalpies for the dissociation reactions at 25°C are less than 1 kcal./mole for all the alkali earth borate ion pairs.Values for pK(NaH2BO3°) from 5 to 55°C computed from the experimental data of Owen and King are in good agreement with those determined potentiometrically. The average value from both methods is 0.22 ± 0.1 at 25°C.Application to seawater of computed pK's for MgH2BO3+, CaH2BO3+ and NaH2BO30 yields an apparent dissociation constant for boric acid of 8.73 vs. 8.70 measured by Lyman, 8.68 by Buch and 8.73 by Byrne and Kester. 相似文献
13.
Calculations based on approximately 350 new measurements (CaT-PCO2) of the solubilities of calcite, aragonite and vaterite in CO2-H2O solutions between 0 and 90°C indicate the following values for the log of the equilibrium constants KC, KA, and KV respectively, for the reaction CaCO3(s) = Ca2+ + CO2?3: where T is in oK. At 25°C the logarithms of the equilibrium constants are ?8.480 ± 0.020, ?8.336 ± 0.020 and ?7.913 ± 0.020 for calcite, aragonite and vaterite, respectively.The equilibrium constants are internally consistent with an aqueous model that includes the CaHCO+3 and CaCO03 ion pairs, revised analytical expressions for CO2-H2O equilibria, and extended Debye-Hückel individual ion activity coefficients. Using this aqueous model, the equilibrium constant of aragonite shows no PCO2-dependence if the CaHCO+3 association constant is between 0 and 90°C, corresponding to the value logKCahco+3 = 1.11 ± 0.07 at 25°C. The CaCO03 association constant was measured potentiometrically to be between 5 and 80°C, yielding logKCaCO03 = 3.22 ± 0.14 at 25°C.The CO2-H2O equilibria have been critically evaluated and new empirical expressions for the temperature dependence of KH, K1 and K2 are , and logK2 = ?107.8871 ? 0.03252849T + 5151.79/T + 38.92561 logT ? 563713.9/T2 which may be used to at least 250°C. These expressions hold for 1 atm. total pressure between 0 and 100°C and follow the vapor pressure curve of water at higher temperatures.Extensive measurements of the pH of Ca-HCO3 solutions at 25°C and 0.956 atm PCO2 using different compositions of the reference electrode filling solution show that measured differences in pH are closely approximated by differences in liquid-junction potential as calculated by the Henderson equation. Liquid-junction corrected pH measurements agree with the calculated pH within 0.003-0.011 pH.Earlier arguments suggesting that the CaHCO+3 ion pair should not be included in the CaCO3-CO2-H2O aqueous model were based on less accurate calcite solubility data. The CaHCO+3 ion pair must be included in the aqueous model to account for the observed PCO2-dependence of aragonite solubility between 317 ppm CO2 and 100% CO2.Previous literature on the solubility of CaCO3 polymorphs have been critically evaluated using the aqueous model and the results are compared. 相似文献
14.
The olivine-ilmenite thermometer of Andersen and Lindsley (1979) was based on an incorrect formulation for the excess free energy of an asymmetric ternary solution. A valid formulation is derived and used to revise the parameters of the olivine-ilmenite thermometer. For olivine and ilmenite that have equilibrated above 700°C, temperature can be calculated from: with . The revised model gives Wil·gkG = 5767?3.09T + 0.011P and ΔGexch = 7301 ? 8.9T ? 0.047P (T in K, P in bars). Applications include Apollo 17 breccias and kimberlites. 相似文献
15.
The ionization quotients of aqueous carbon dioxide (carbonic acid) have been precisely determined in NaCl media to 5 m and from 50° to 300°C using potentiometric apparatus previously developed at Oak Ridge National Laboratory. The pressure coefficient was also determined to 250°C in the same media. These results have been combined with selected information in the literature and modeled in two ways to arrive at the best fits and to derive the thermodynamic parameters for the ionization reaction, including the equilibrium constant, activity coefficient quotients, and pressure coefficients. The variation with temperature of the two fundamental quantities and were examined along the saturation vapor pressure curve and at constant density. The results demonstrated again that for reactions with minimal electrostriction changes the magnitudes and variations of and with temperature are small and, in addition, and are approximately independent of salt concentration.The results have also been applied to an examination of the solubility of calcite as a function of pH (in a given NaCl medium) for the neutral to acidic region both for systems with fixed CO2 pressure and systems where the calcium ion concentration equals the concentration of carbon. The pH of saturated solutions of calcite with PCO2 of 12 bars increases from 5.1 to 5.5 between 100° and 300°C. 相似文献
16.
Distribution and isotopic composition (δ13C) of low molecular weight hydrocarbon gases were studied in Big Soda Lake (depth = 64 m), an alkaline, meromictic lake with permanently anoxic bottom waters. Methane increased with depth in the anoxic mixolimnion (depth = 20–35 m), reached uniform concentrations (55 μM/l) in the monimolimnion (35–64 m) and again increased with depth in monimolimnion bottom sediments (>400 μM/kg below 1 m sub-bottom depth). The μ13C[CH4] values in bottom sediment below 1 m sub-bottom depth (<?70 per mil) increased with vertical distance up the core (δ13C[CH4] = ?55 per mil at sediment surface). Monimolimnion δ13C[CH4] values (?55 to ?61 per mil) were greater than most δ13C[CH4] values found in the anoxic mixolimnion (92% of samples had δ13C[CH4] values between ?20 and ?48 per mil). No significant concentrations of ethylene or propylene were found in the lake. However ethane, propane, isobutane and n-butane concentrations all increased with water column depth, with respective maximum concentrations of 260, 80, 23 and 22 nM/l encountered between 50–60 m depth. Concentrations of ethane, propane and butanes decreased with depth in the bottom sediments. Ratios of were high (250–620) in the anoxic mixolimnion, decreased to ~161 in the monimolimnion and increased with depth in the sediment to values as high as 1736. We concluded that methane has a biogenic origin in both the sediments and the anoxic water column and that C2-C4 alkanes have biogenic origins in the monimolimnion water and shallow sediments. The changes observed in δ13C[CH4] and with depth in the water column and sediments are probably caused by bacteria] processes. These might include anaerobic methane oxidation and different rates of methanogenesis and C2 to C4 alkane production by microorganisms. 相似文献
17.
A direct-sampling, mass-spectrometric technique has been used to measure simultaneously the solubilities of He, Ne, Ar, Kr, and Xe in fresh water and NaCl brine (0 to 5.2 molar) from 0° to 65 °C, and at 1 atm total pressure of moist air. The argon solubility in the most concentrated brines is 4 to 7 times less than in fresh water at 65 °C and 0°C, respectively. The salt effect is parameterized using the Setschenow equation. where M is NaCl moiarity, βio(T) and βi(T) the Bunsen solubility coefficients for gas i in fresh water and brine, and KiM(T) the empirical salting coefficient. Values of KiM(T) are calculated using volumetric concentration units for noble gas and NaCl content and are independent of NaCl molarity. Below about 40°C, temperature coefficients of all KiM are negative. The value of KHeM is a minimum at 40°C. KArM decreases from about 0.40 at 0°C to 0.28 at 65 °C. The absolute magnitudes of the differences in salting coefficients (relative to KArM) decrease from 0° to 65°C. Over the range of conditions studied, all noble gases are salted out, and KHeM ? KNeM < KArM < KKrM < KXeM.From the solubility data, we calculated , , and for the transfer of noble gases from fresh water to 1 molar NaCl solutions. At low temperatures , is positive, but decreases and becomes negative at temperatures ranging from about 25°C for He to 45°C for Xe. At low temperatures, the dissolved electrolyte apparently interferes with the formation of a cage of solvent molecules about the noble gas atom. At higher temperatures, the local environment of the gas atom in the brine appears to be slightly more ordered than in pure water, possibly reflecting the longer effective range of the ionic fields at higher temperature.The measured solubilities can be used to model noble gas partitioning in two-phase geothermal systems at low temperatures. The data can also be used to estimate the temperature and concentration dependence of the salt effect for other alkali halides. Extrapolation of the measured data is not possible due to the incompletely-characterized minima in the temperature dependence of the salting coefficients. The regularities in the data observed at low temperatures suggest relatively few high-temperature data will be required to model the behavior of noble gases in high-temperature geothermal brines. 相似文献
18.
19.
E.Bruce Watson 《Geochimica et cosmochimica acta》1979,43(3):313-322
Calcium-45 was used as a radiotracer to measure self-diffusion coefficients for Ca in a sodium-calcium-aluminosilicate melt (29% Na2O, 5% CaO, 10% Al2O3, 56% SiO2) at temperatures in the range 1100–1400°C and pressures to 30 kbar. Calcium diffusivity (DCa) was found to depend upon both temperature and pressure in a complex but systematic manner: is always negative and has a larger absolute value at lower temperatures; is positive and increases with increasing pressure. The overall dependence of DCa upon T and P is given approximately by . When expressed in terms of volume (Va) and energy (E) of activation, the results are as follows: Va ranges from 2.2 cm3/mole at 1400°C to 11.9 cm3/mole at 1100°C. and E ranges from 25.4 kcal/mole (1 kban to 49.8 kcal/mole (20 kbar).From the systematic dependence of DCa upon T and P, it is concluded that diffusion of Ca2+ in silicate melts does not take place by means of a vacant site mechanism, but is controlled instead by the amount and distribution of free volume in the melt structure.If it is assumed that the viscosity of the melt used in this study decreases with increasing pressure (Kushiro, 1976, J. Geophys. Res.81, 6351–6356) as DCa does, then the Stokes-Einstein inverse relation between viscosity and diffusivity is clearly violated, and its validity for silicate melts must be questioned. Thus, it appears that in silicate melts, unlike many liquids, viscous flow and diffusion are fundamentally different transport processes, involving different structural units.The effect of pressure on calcium diffusion is too small to invalidate kinetic models of upper mantle processes that have been based upon diffusivity values measured at 1 atm. Pressure may, however, induce significant reductions in the diffusion rates of large ions such as Rb+ or SiO4?4 in silicate melts. 相似文献
20.
The partitioning of germanium between forsterite (Fo) and liquids in the diopside-anorthiteforsterite join was investigated by electron microprobe analysis of Ge-doped samples equilibrated at 1300°–1450°C. Germanium is somewhat incompatible in Fo relative to the haplobasaltic melts, with a grand mean for all simple partition coefficients (DFo-lGe) of 0.68 ± 0.06. For the melt composition range studied, DFo-lGe is virtually constant in isothermal series of experiments, and shows only minor overall temperature dependence. The exchange reaction partition coefficient ] is near unity in all cases, with a grand mean of 0.93 ± 0.11. One exploratory run at 20 kbar yielded a distinctly lower partition coefficient (DFo-lGe = 0.54 ± 0.04), which confirms the negative pressure dependence predicted by the thermodynamics of Ge ai Si exchange.These new data indicate that absolute Ge enrichment must occur in terrestrial magmas undergoing olivine fractionation, while remains nearly constant. 相似文献