Geochimie et teneurs isotopiques des systèmes saisonniers de dissolution de la calcite dans un sol sur craie

In a soil developed on the Cretaceous chalk of the Eastern Paris basin, calcite dissolution begins at the surface. The soil water is rapidly saturated in calcite. Calcite dissolution follows two different pathways according to seasonal pedoclimatic conditions.During winter: the soil is only partly saturated in water and the CO₂ partial pressure is low (Ca 10^?3 atm.). As a consequence total inorganic dissolved carbon (TIDC) is a hundred times the carbon content of the gaseous phase. Equilibrium is usually observed between the two phases. It is a closed system. The measured carbon 14 activity (87,5%) and ¹³C content ($\text{δ}{}_{\mathrm{tidc}}{}^{13}\text{C = ?12,2\%0}$) of the drainage water are very close to theoretical values calculated for an ideal mixing system between gaseous and mineral phases (respectively characterized by the following isotopic values: $\text{δ}{}_{\mathrm{G}}{}^{13}\text{C = ?21,5\%0}$; $\text{A}{}_{\mathrm{G}}{}^{14}\text{C = 118\%}$; $\text{δ}{}_{\mathrm{M}}{}^{13}\text{C = +2,9\%0}$; $\text{A}{}_{\mathrm{M}}{}^{14}\text{C = 28\%}$).During spring and summer: the soil moisture decreases, the input of biogenic CO₂ induces an increase of the soil CO₂ partial pressure (Ca from 3.10^?3 atm to 7.10^?3 atm). The carbon content of the gaseous phase is higher by an order of magnitude compared to winter conditions. Therefore the aqueous phase is undersaturated in CO₂ with respect to the latter. This disequilibrium occurs as a result of unbalanced rates of CO₂ dissolution and CO₂ effusion toward atmosphère. It is an open system. The carbon isotopic ratio of the aqueous phase is regulated by that of the gaseous phase, as demonstrated by the agreement between measured and calculated isotopic compositions (respectively δ_{L mes} = from ?9,4%0 to ?11,5%0, δ_{l calc} = from ?9,8%0 to ?13,9%0 A_{L mes} = 119%, A_{L calc} = from 119% to 125%).The solutions originating from both systems (open and closed) move downwards without significant mixing together. It has also been observed that no significant variation of the TIDC isotopic composition occurs during precipitation of secondary calcite.     

Experimental observations on carbon isotope exchange in carbonate-water systems

This study presents data from experiments investigating carbon isotope exchange between carbonate solution and solid calcite using carbon-13 as a tracer. All experiments were done with calcite saturated solutions and results show that a two-step adsorption-recrystallization reaction takes place. Isotope effects are caused by exchange by carbonate on the solid surface with carbon in the aqueous phase. Adsorption reactions are characterized by a maximum isotopic exchange capacity (IEC) on crystal surfaces of about 10¹¹ reaction sites per cm², following a second order rate law with respect to ¹³C concentration in solution ($\text{constant}\text{kex ? 10}{}^6\text{cm}{}^5\text{mole}{}^{\mathrm{?1}}\text{s}{}^{\mathrm{?1}}$ and $\text{half-life}\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 700}\text{s}$). The adsorption reaction was followed by a first order recrystallization which is characterized by a rate constant of the order of 10^?8 s^?1 and a $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ of 10⁷ s. Negative isotopic gradient experiments and runs with calcite crystals in Mg²⁺ spiked solutions provided the preliminary basis for the characterization of the mechanisms of both proposed reactions.     

Kinetic and thermodynamic factors controlling the distribution of SO32− and Na+ in calcites and selected aragonites

Significant amounts of SO₄^2?, Na⁺, and OH^? are incorporated in marine biogenic calcites. Biogenic high Mg-calcites average about 1 mole percent SO₄^2?. Aragonites and most biogenic low Mg-calcites contain significant amounts of Na⁺, but very low concentrations of SO₄^2?. The SO₄^2? content of non-biogenic calcites and aragonites investigated was below 100 ppm. The presence of Na⁺ and SO₄^2? increases the unit cell size of calcites. The solid-solutions show a solubility minimum at about 0.5 mole percent SO₄^2? beyond which the solubility rapidly increases. The solubility product of calcites containing 3 mole percent SO₄^2? is the same as that of aragonite. Na⁺ appears to have very little effect on the solubility product of calcites. The amounts of Na⁺ and SO₄^2? incorporated in calcites vary as a function of the rate of crystal growth. The variation of the distribution coefficient ($\text{D}$) of SO₄^2? in calcite at 25.0°C and 0.50 molal NaCl is described by the equation $\text{D = k}{}_0\text{+ k}{}_1\text{R}$ where $\text{k}{}_0$ and $\text{k}{}_1$ are constants equal to $\text{6.16 × 10}{}^{\mathrm{?6}}$ and $\text{3.941 × 10}{}^{\mathrm{?6}}$, respectively, and $\text{R}$ is the rate of crystal growth of calcite in mg·min^?1·g^?1 of seed. The data on Na⁺ are consistent with the hypothesis that a significant amount of Na⁺ occupies interstitial positions in the calcite structure. The distribution of Na⁺ follows a Freundlich isotherm and not the Berthelot-Nernst distribution law. The numerical value of the Na⁺ distribution coefficient in calcite is probably dependent on the number of defects in the calcite structure. The Na⁺ contents of calcites are not very accurate indicators of environmental salinities.     

A systematic deviation from Na-K-Ca geothermometer below 75°C and above 10−4 atm Pco2

If the temperature of ground water is below 75°C and the partial pressure of CO₂ in the aquifer is above 10^?4 atm, a chemical steady-state between water and felsic rocks (rather than chemical equilibrium) may be maintained. The temperature of water in the aquifer may be estimated using a modified form of the Na-K-Ca geothermometer from, . where the departure of the steady-state from equilibrium, $\text{I}$, is a function of : .     

Sedimentary iron monosulfides: Kinetics and mechanism of formation

The reaction between hydrous iron oxides and aqueous sulfide species was studied at estuarine conditions of pH, total sulfide, and ionic strength to determine the kinetics and formation mechanism of the initial iron sulfide. Total, dissolved and acid extractable sulfide, thiosulfate, sulfate, and elemental sulfur were determined by spectrophotometric methods. Polysulfides, S₄^2? and S₅^2?, were determined from ultraviolet absorbance measurements and equilibrium calculations, while product hydroxyl ion was determined from pH measurements and solution buffer capacity.Elemental sulfur, as free and polysulfide sulfur, was 86% of the sulfide oxidation products; the remainder was thiosulfate. Rate expressions for the reduction and precipitation reactions were determined from analysis of electron balance and acid extractable iron monosulfide vs time, respectively, by the initial rate method. The rate of iron reduction in moles/liter/minute was given by $\text{d(}\text{reduction}\text{Fe)}\text{dt}\text{= kS}{}_{\mathrm{t}}{}^{0.5}\text{(J}{}^{\mathrm{+}}\text{)}{}^{0.5}\text{A}{}_{\mathrm{FeOOH}}{}^1$ where S_t was the total dissolved sulfide concentration, (H⁺) the hydrogen ion activity, both in moles/ liter; and A_FeOOH the goethite specific surface area in square meters/liter. The rate constant, k, was 0.017 ± 0.002m^?2 min^?1. The rate of reduction was apparently determined by the rate of dissolution of the surface layer of ferrous hydroxide. The rate expression for the precipitation reaction was $\text{d(FeS)}\text{dt}\text{= kS}{}_{\mathrm{t}}{}^1\text{(H}{}^{\mathrm{+}}\text{)}{}^1\text{A}{}_{\mathrm{FeOOH}}{}^1$ where $\text{d(FeS)}\text{dt}$ was the rate of precipitation of acid extractable iron monosulfide in moles/liter/minute, and k = 82 ± 18 mol^?1l²m^?2 min^?1.A model is proposed with the following steps: protonation of goethite surface layer; exchange of bisulfide for hydroxide in the mobile layer; reduction of surface ferric ions of goethite by dissolved bisulfide species which produces ferrous hydroxide surface layer elemental sulfur and thiosulfate; dissolution of surface layer of ferrous hydroxide; and precipitation of dissolved ferrous specie and aqueous bisulfide ion.     

Total individual ion activity coefficients of calcium and carbonate in seawater at 25°C and 35%. salinity,and implications to the agreement between apparent and thermodynamic constants of calcite and aragonite

We have calculated the total individual ion activity coefficients of carbonate and calcium, and , in seawater. Using the ratios of stoichiometric and thermodynamic constants of carbonic acid dissociation and total mean activity coefficient data measured in seawater, we have obtained values which differ significantly from those widely accepted in the literature. In seawater at 25°C and 35%. salinity the (molal) values of and are $\text{0.038 ± 0.002}$ and $\text{0.173 ± 0.010}$, respectively. These values of and are independent of liquid junction errors and internally consistent with the value . By defining and on a common scale (), the product is independent of the assigned value of and may be determined directly from thermodynamic measurements in seawater. Using the value and new thermodynamic equilibrium constants for calcite and aragonite, we show that the apparent constants of calcite and aragonite are consistent with the thermodynamic equilibrium constants at 25°C and 35%. salinity. The demonstrated consistency between thermodynamic and apparent constants of calcite and aragonite does not support a hypothesis of stable Mg-calcite coatings on calcite or aragonite surfaces in seawater, and suggests that the calcite critical carbonate ion curve of Broecker and Takahashi (1978, Deep-Sea Research25, 65–95) defines the calcite equilibrium boundary in the oceans, within the uncertainty of the data.     

Partial molal volume calculations for the dissolution of aged amorphous silica in salt water and seawater at 0–2°C

Measurement of solubility as a function of pressure allows calculation of $\text{3}\text{V}\text{?}{}_1$. Using this experimental approach, the best estimate of $\text{3}\text{V}\text{?}{}_1$ for the dissolution of aged amorphous silica in salt water or seawater at 0–2°C is ?9.9 cm³ mol^?1 (standard error = 0.4 cm³ mol^?1). This gives , which compares well with other published values of .     

A fundamental equation for calcite dissolution kinetics

A fundamental rate equation for the dissolution of calcite in a pure 0.7 M KC1 solution has been determined. Between pH 8.0 and 10.1 the kinetics of the dissolution reaction can be expressed by the equation

$\text{d[Ca}{}^{\mathrm{2+}}\text{]/dt = kA(C-[Ca}{}^{\mathrm{2+}}\text{]}\text{1}\text{2}\text{[CO}{}_3{}^{\mathrm{2?}}\text{]}\text{1}\text{2}\text{)}$

, where d[Ca²⁺]/dt is the rate in mole cm^?3s^?1, k is the apparent rate constant in s^?1 cm^?2, A is the calcite surface area and C is the square root of the calcite solubility constant. The apparent rate constant at 20°C is 9.5 × 10^?6s^?1cm^?2. The apparent activation energy for the reaction between 5 and 50°C is 8.4 kcal mole^?1.The reaction rate is pH independent above pH = 7.5. At pH values less than 8, [CO₃^2?] becomes negligible, and the rate becomes fast and should be dependent on the calcite surface area alone, if there is no change in mechanism.The stirring coefficient between 2.8 and 11.1 rev s^?1 is 0.33. This, together with the relatively high activation energy, indicates that the reaction is mainly chemically controlled.Interpolation of the experimental results into seawater systems gives a computed rate several magnitudes greater than the observed rate, but considerably less than that calculated for a diffusion-controlled reaction.     

The significance of metamorphic fluorite in the Adirondacks

Thermodynamic calculations for selected silicate-oxide-fluorite assemblages indicate that several commonly occurring fluorite-bearing assemblages are restricted to relatively narrow fields at constant P?T. The presence of fayalite-ferrohedenbergite-fluorite-quartz ± magnetite and ferrosalite-fluorite-quartz-magnetite assemblages in orthogneisses from Au Sable Forks, Wanakena and Lake Pleasant, New York, buffered fluorine and oxygen fugacities during the granulite facies metamorphism in the Adirondack Highlands. These buffering assemblages restrict$\text{?}{}_{\mathrm{F}}{}_2$ to 10^{?29 ± 1} bar and $\text{?}{}_0{}_2$ to 10^{?16 ± 1} bar at the estimated metamorphic temperature of 1000K and pressure of 7 kbar. The assemblage biotite-magnetite-ilmenite-K-feldspar, found in the same Au Sable Forks outcrop as the fayalite-fluorite-ferrohedenbergite-quartz-magnetitie assemblage, restricts H₂O fugacities to less than 10^3·3 bar. These fugacities limit H₂ and HF fugacities to less than 10¹ bar for the Au Sable outcrop. The data indicate that relative to H₂O, O₂, H₂, F₂ and HF are not major species in the fluid equilibrated with Adirondack orthogneisses. The calculated F₂ fugacilies are similar to the upper limits possible for plagioclase-bearing rocks and probably represent the upper $\text{?}{}_{\mathrm{F}}{}_2$ limit for metamorphism in the Adirondacks and in other granulite facies terranes.     

Complexing of silica by iron(III) in natural waters

Determination of amorphous silica solubility in acidified ferric nitrate solutions confirms the presence of ferric silicate complexing. A dissociation constant for the reaction:

$\text{FeH}{}_3\text{SiO}{}_4{}^{\mathrm{2+}}\text{→}\text{Fe}{}^{\mathrm{3+}}\text{+}\text{H}{}_3\text{SiO}{}_4{}^{\mathrm{?}}$

of 10^?9.8 ± 0.3 pK units at room temperature (22 ± 3°C) is obtained, in close agreement with reported values at 25°C corrected to zero ionic strength of 10^?9.9 by Weber and Stumm and 10^?9.5 by Olson and O'Melia. Iron-silicate complexing may be of significance to the mobilization of silica in acid waters associated with oxidizing sulphide deposits and coal strip mining and the precipitation of secondary silicate mineral phases.     

Thermodynamics of brine-salt equilibria—I. The systems NaCl-KCl-MgCl2-CaCl2-H2O and NaCl-MgSO4-H2O at 25°C

A thermodynamic model for concentrated brines has been developed which is capable of predicting the solubilities of many of the common evaporite minerals in chloro-sulfate brines at 25°C and 1 atm. The model assumes that the behaviour of the mean stoichiometric ionic activity coefficient in mixtures of aqueous electrolytes can be described by the Scatchard deviation function and Harned's Rule. In solutions consisting of one salt and H₂O, the activity coefficient is described by the expression where $\text{a}\text{?}$ and B? salt specific parameters obtained from data regression. In a mixture of n electrolytes and H₂O, B? for the ith component is given by $\text{B}{}^{\mathrm{<mtext>i</mtext><mtext>?</mtext>}}{}_{\mathrm{i}}\text{=B}\text{i}\text{?}{}_{\mathrm{i}}\text{+σ α}{}_{\mathrm{ij}}\text{y}{}_{\mathrm{j}}$ where α_ij is a (constant) mixing parameter characterizing the interaction of the i and j components and y_j is the ionic strength fraction of the jth component. The activity of H₂O is obtained from a Gibbs-Duhem integration and does not require any additional parameters or assumptions. In this study, parameters have been obtained for the systems NaCl-KCl-MgCl₂-CaCl₂-H₂O and NaCl-MgSO₄-H₂O at 25°C and 1 atm. Computed solubility curves and solution compositions predicted for invariant points in these systems agree well with the experimental data. The model is flexible and easily extended to other systems and to higher temperatures.     

Rates of reaction of pyrite and marcasite with ferric iron at pH 2

The relative reactivities of pulverized samples (100–200 mesh) of 3 marcasite and 7 pyrite specimens from various sources were determined at 25°C and pH 2.0 in ferric chloride solutions with initial ferric iron concentrations of 10^?3 molal. The rate of the reaction:

$\text{FeS}{}_2\text{+ 14}\text{Fe}{}^{\mathrm{3+}}\text{+ 8}\text{H}{}_2\text{O}\text{= 15}\text{Fe}{}^{\mathrm{2+}}\text{+ 2}\text{SO}{}^{\mathrm{2?}}{}_4\text{+ 16}\text{H}{}^{\mathrm{+}}$

was determined by calculating the rate of reduction of aqueous ferric ion from measured oxidation-reduction potentials. The reaction follows the rate law:

where m_Fe³⁺ is the molal concentration of uncomplexed ferric iron, k is the rate constant and $\text{A}\text{M}$ is the surface area of reacting solid to mass of solution ratio. The measured rate constants, k, range from 1.0 × 10^?4 to 2.7 × 10^?4 sec^?1 ± 5%, with lower-temperature/early diagenetic pyrite having the smallest rate constants, marcasite intermediate, and pyrite of higher-temperature hydrothermal and metamorphic origin having the greatest rate constants. Geologically, these small relative differences between the rate constants are not significant, so the fundamental reactivities of marcasite and pyrite are not appreciably different.The activation energy of the reaction for a hydrothermal pyrite in the temperature interval of 25 to 50°C is 92 kJ mol^?1. This relatively high activation energy indicates that a surface reaction controls the rate over this temperature range. The BET-measured specific surface area for lower-temperature/early diagenetic pyrite is an order of magnitude greater than that for pyrite of higher-temperature origin. Consequently, since the lower-temperature types have a much greater $\text{A}\text{M}$ ratio, they appear to be more reactive per unit mass than the higher temperature types.     

The partial molal volume of silicic acid in 0.725 m NaCl

The partial molal volume ($\text{V}\text{?}$) of silicic acid in 0.725 m NaCl at 20°C has been calculated from (1) direct volume changes due to the dissolution of anhydrous sodium silicate and (2) some literature values for the partial molal volumes of NaOH and water. , unconnected for electrostriction effects, was found to be 53 ± 2 ml mole^?1. , corrected for volume changes due to solvent electrostriction by charged Si species, was estimated to be in the range 58–62 ml mole^?1; this range is 7–11 ml mole^?1 greater than the calculated from Willey's (Mar. Chem. 2, 239–250, 1974) solubility data obtained from the dissolution, in seawater, of amorphous silica subjected to hydrostatic pressure. Our does, however, agree within experimental error with the calculated from Jones and Pytkowicz's (Bull. Soc. Roy. Sci. Liege 42, 118–120, 1973) data for the solubility of amorphous silica in seawater at high pressure and is nearly in agreement with Willey's (Ph.D. thesis, Dalhousie University, 1975) solubility data for amorphous silica in 0.6 m NaCl.     

Carbonate equilibria in hydrothermal systems: First ionization of carbonic acid in NaCl media to 300°C

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 $\text{Δ}\text{V}\text{?}{}^{\mathrm{o}}$ and $\text{Δ}\text{C}\text{?}{}^{\mathrm{o}}{}_{\mathrm{p}}$ 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 $\text{Δ}\text{C}\text{?}{}^{\mathrm{o}}{}_{\mathrm{p}}$ and $\text{Δ}\text{V}\text{?}{}^{\mathrm{o}}$ with temperature are small and, in addition, $\text{Δ}\text{C}\text{?}{}_{\mathrm{p}}$ and $\text{Δ}\text{V}\text{?}$ 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 P_CO2 of 12 bars increases from 5.1 to 5.5 between 100° and 300°C.     

Solubility product of galena at 298°K: A possible explanation for apparent supersaturation in nature

The synthetic chelating agent ethylenediaminetetraacetic acid (EDTA) has been used to evaluate the stoichiometric solubility product of galena (PbS) at 298°K: $\text{K}{}_{\mathrm{s2}}\text{=}{}^{\mathrm{a}}\text{Pb}{}^{\mathrm{2+}}{}^{\mathrm{a}}\text{HS}{}^{\mathrm{?}}{}^{\mathrm{a}}\text{H}{}^{\mathrm{+}}$ This method circumvents the possible uncertainties in the stoichiometry and stability of lead sulfide complexes. At infinite dilution, $\text{Log K}{}_{\mathrm{s2}}\text{= ?12.25 ±0.17}$, and at an ionic strength corresponding to seawater ($\text{I = 0.7 M}$), $\text{Log K}{}_{\mathrm{s2}}\text{= ?11.73 ± 0.05}$. Using the value of $\text{K}{}_{\mathrm{s2}}$ at infinite dilution, and the free energies of formation of HS^? and Pb²⁺ at 298°K (literature values), the free energy of formation of PbS at 298°K is computed to be $\text{?79.1 ± 0.8 KJ/mol (?18.9 Kcal/mol)}$. Galena is shown to be more than two orders of magnitude more soluble than indicated by calculations based on previous thermodynamic data.     

The kinetics of silica-water reactions

A differential rate equation for silica-water reactions from 0–300°C has been derived based on stoichiometry and activities of the reactants in the reaction SiO₂(s) + 2H₂O(l) = H₄SiO₄(aq)

where ($\text{A}\text{M}$) = (the relative interfacial area between the solid and aqueous phases/the relative mass of water in the system), and k₊ and k_? are the rate constants for, respectively, dissolution and precipitation. The rate constant for precipitation of all silica phases is $\text{log k}{}_{\mathrm{?}}\text{= ? 0.707 ?}\text{2598}\text{T}\text{(T, K)}$ and E_act for this reaction is 49.8 kJ mol^?1. Corresponding equilibrium constants for this reaction with quartz, cristobalite, or amorphous silica were expressed as $\text{log K = a + bT +}\text{c}\text{T}$. Using $\text{K =}\text{k}{}_{\mathrm{+}}\text{k}{}_{\mathrm{?}}$, k was expressed as $\text{log k}{}_{\mathrm{+}}\text{= a + bT +}\text{c}\text{T}$ and a corresponding activation energy calculated:

     

17.

Distribution coefficients of Eu and Sr for plagioclase-liquid and clinopyroxene-liquid equilibria in oceanic ridge basalt: an experimental study     

Richard J. Williams 《Geochimica et cosmochimica acta》1974,38(9):1415-1433

The distribution coefficients of Eu and Sr for plagioclase-liquid and clinopyroxene-liquid pairs as a function of temperature and oxygen fugacity were experimentally investigated using an oceanic ridge basalt enriched with Eu and Sr as the starting material. Experiments were conducted between 1190° and 1140°C over a range of oxygen fugacities between 10^?8 and 10^?14 atm.The molar distribution coefficients are given by the equations: log^PL_Sr = 7320/T ? 4.62logK^CPX_Sr = 18020/T ? 13.10. Similarly, the weight fraction distribution coefficients are given by the equations: logD^PL_Sr = 6570/T ? 4.30logD^CPX_Sr = 18434/T ? 13.62.Although the mole fraction distribution coefficients have a smaller dependence on bulk composition than do the weight fraction distribution coefficients, they are not independent of bulk composition, thereby restricting the application of these experimental results to rocks similar to oceanic ridge basalts in bulk composition.Because the Sr distribution coefficients are independent of oxygen fugacity, they may be used as geothermometers. If the temperature can be determined independently — for example, with the Sr distribution coefficients, the Eu distribution coefficients may be used as oxygen geobarometers. Throughout the range of oxygen fugacities ascribed to terrestrial and lunar basalts, plagioclase concentrates Eu but clinopyroxene rejects Eu.     

18.

The role of CO₂ in the chemical modification of deep continental crust     

William E. Glassley 《Geochimica et cosmochimica acta》1983,47(3):597-616

Compositional differences between granulite facies rocks and equivalent amphibolite facies rocks and the observation of CO₂-rich fluid inclusions in granulites, have led to the suggestion that CO₂ must play a role in modifying the composition of deep continental crust. How CO₂ effects this change has remained unclear. Using the thermodynamic properties of aqueous ions in a fluid of evolving $\text{CO}{}_2\text{H}{}_2\text{O}$ ratio, it is possible to model the incongruent dissolution of feldspars under conditions appropriate for granulite facies metamorphism. The results demonstrate that dissolution will be strongly enhanced at high $\text{CO}{}_2\text{H}{}_2\text{O}$ ratios, with ion solubilities being Na⁺ >K⁺ ? Ca⁺⁺. This enhancement is compatible with the reported compositional contrasts between granulite and amphibolite facies rock, but requires large fluid volumes.To test the dissolution model, a detailed field and petrologic study was conducted in a well exposed granulite facies terrane in West Greenland. Strong correlation between fluid composition and bulk rock chemistry can be documented; CO₂-rich regions contain rocks which consistently have low ratios, while H₂O-rich regions consistently have high ratios. Magnetite rims on sulfide grains are ubiquitous in high regions and are absent in high regions, and they provide evidence that CO₂ was introduced into the region. These correlations and observations are predictable from the properties of the dissolution process. These considerations, along with observations regarding graphite petrogenesis, provide strong arguments that the total fluid volume interacting with the rock during metamorphism was very large, in some cases equaling or exceeding total rock volume. Such large fluid volumes can lead to significant compositional modification of the crust, and will mask the original protolith chemistry. Such processes should lead to Ca- and Al-enriched, Na-, K-, S- and Si-depleted residues in the deep crust.     

19.

Volatilization of sodium from silicate melt spheres and its application to the formation of chondrules     

Akira Tsuchiyama  Hiroko Nagahara  Ikuo Kushiro 《Geochimica et cosmochimica acta》1981,45(8):1357-1367

The rates of volatilization of Na from liquid spheres of chondrule compositions have been determined as functions of time, temperature, partial pressure of oxygen, and sizes of the spheres. The Na₂O content in the sphere is uniform in each run. but it decreases with time of the run, indicating that the rate of diffusion of Na in the liquid is greater than that of volatilization, and that the latter is the rate-controlling process. The rate of sodium volatilization becomes greater with increasing temperature and with decreasing P_O2 and size of the spheres. The relation of the Na₂O content in the liquid sphere with time and its size indicate that the amount of Na₂O volatilized from the liquid spheres within unit time is proportional to the surface area of the spheres and the concentration of Na₂O in the liquid. From these relations, the rate of volatilization of sodium can be obtained at constant temperature and P_o2. The rate of volatilization of sodium satisfies the Arrhenius relation within the temperature range from about 1450–1600 C at 10^?9,2 atm p_O2; the activation energy for the sodium volatilization is approximately 100 kcal-mole^?1. The rate is also approximately proportional to within the range of p_O2 from 10^?10.2 to 10^?5.0 atm at about 1500° C. Based on the present results and the Na₂O contents in chondrules. it is suggested that they experienced an instant heating with maximum temperature of 1400–2200° C followed by an immediate cooling.     

20.

Use of Pitzer's equations to determine the media effect on the formation of lead chloro complexes     

Frank.J. Millero  Robert H. Byrne 《Geochimica et cosmochimica acta》1984,48(5):1145-1150

The spectrophotometric measurements of chloro complexes of lead in aqueous HCl, NaCl, MgCl₂ and CaCl₂ solutions at 25°C have been analyzed using Pitzer's specific interaction equations. Parameters for activity coefficients of the complexes PbCl⁺, PbCl₂⁰ and PbCl₃^? have been determined for the various media. Values of $\text{K}{}_1\text{= 30.0 ± 0.6}$, $\text{K}{}_2\text{= 106.7 ± 2.1}$ and $\text{K}{}_3\text{= 73.0 ± 1.5}$ were obtained for the cumulative formation constants. [$\text{Pb}{}^{\mathrm{2+}}\text{+ nCl}{}^{\mathrm{?}}\text{→ PbCl}{}_{\mathrm{n}}{}^{\mathrm{2?n}}\text{)}$]. These values are in reasonable agreement with literature data. The Pitzer parameters for the PbCl ion pairs in various media were used to calculate the speciation of Pb²⁺ in an artificial seawater solution.     

	a	b	c	Eact(kJ mol -1)
Quarts	1.174	-2.028 x 103	-4158	67.4–76.6
α-Cristobalite	-0.739	0	-3586	68.7
β-Cristobalite	-0.936	0	-3392	65.0
Amorphous silica	-0.369	-7.890 x 10-4	3438	60.9–64.9

Distribution coefficients of Eu and Sr for plagioclase-liquid and clinopyroxene-liquid equilibria in oceanic ridge basalt: an experimental study

The distribution coefficients of Eu and Sr for plagioclase-liquid and clinopyroxene-liquid pairs as a function of temperature and oxygen fugacity were experimentally investigated using an oceanic ridge basalt enriched with Eu and Sr as the starting material. Experiments were conducted between 1190° and 1140°C over a range of oxygen fugacities between 10^?8 and 10^?14 atm.The molar distribution coefficients are given by the equations: log^PL_Sr = 7320/T ? 4.62logK^CPX_Sr = 18020/T ? 13.10. Similarly, the weight fraction distribution coefficients are given by the equations: logD^PL_Sr = 6570/T ? 4.30logD^CPX_Sr = 18434/T ? 13.62.Although the mole fraction distribution coefficients have a smaller dependence on bulk composition than do the weight fraction distribution coefficients, they are not independent of bulk composition, thereby restricting the application of these experimental results to rocks similar to oceanic ridge basalts in bulk composition.Because the Sr distribution coefficients are independent of oxygen fugacity, they may be used as geothermometers. If the temperature can be determined independently — for example, with the Sr distribution coefficients, the Eu distribution coefficients may be used as oxygen geobarometers. Throughout the range of oxygen fugacities ascribed to terrestrial and lunar basalts, plagioclase concentrates Eu but clinopyroxene rejects Eu.     

The role of CO2 in the chemical modification of deep continental crust

Compositional differences between granulite facies rocks and equivalent amphibolite facies rocks and the observation of CO₂-rich fluid inclusions in granulites, have led to the suggestion that CO₂ must play a role in modifying the composition of deep continental crust. How CO₂ effects this change has remained unclear. Using the thermodynamic properties of aqueous ions in a fluid of evolving $\text{CO}{}_2\text{H}{}_2\text{O}$ ratio, it is possible to model the incongruent dissolution of feldspars under conditions appropriate for granulite facies metamorphism. The results demonstrate that dissolution will be strongly enhanced at high $\text{CO}{}_2\text{H}{}_2\text{O}$ ratios, with ion solubilities being Na⁺ >K⁺ ? Ca⁺⁺. This enhancement is compatible with the reported compositional contrasts between granulite and amphibolite facies rock, but requires large fluid volumes.To test the dissolution model, a detailed field and petrologic study was conducted in a well exposed granulite facies terrane in West Greenland. Strong correlation between fluid composition and bulk rock chemistry can be documented; CO₂-rich regions contain rocks which consistently have low ratios, while H₂O-rich regions consistently have high ratios. Magnetite rims on sulfide grains are ubiquitous in high regions and are absent in high regions, and they provide evidence that CO₂ was introduced into the region. These correlations and observations are predictable from the properties of the dissolution process. These considerations, along with observations regarding graphite petrogenesis, provide strong arguments that the total fluid volume interacting with the rock during metamorphism was very large, in some cases equaling or exceeding total rock volume. Such large fluid volumes can lead to significant compositional modification of the crust, and will mask the original protolith chemistry. Such processes should lead to Ca- and Al-enriched, Na-, K-, S- and Si-depleted residues in the deep crust.     

Volatilization of sodium from silicate melt spheres and its application to the formation of chondrules

The rates of volatilization of Na from liquid spheres of chondrule compositions have been determined as functions of time, temperature, partial pressure of oxygen, and sizes of the spheres. The Na₂O content in the sphere is uniform in each run. but it decreases with time of the run, indicating that the rate of diffusion of Na in the liquid is greater than that of volatilization, and that the latter is the rate-controlling process. The rate of sodium volatilization becomes greater with increasing temperature and with decreasing P_O2 and size of the spheres. The relation of the Na₂O content in the liquid sphere with time and its size indicate that the amount of Na₂O volatilized from the liquid spheres within unit time is proportional to the surface area of the spheres and the concentration of Na₂O in the liquid. From these relations, the rate of volatilization of sodium can be obtained at constant temperature and P_o2. The rate of volatilization of sodium satisfies the Arrhenius relation within the temperature range from about 1450–1600 C at 10^?9,2 atm p_O2; the activation energy for the sodium volatilization is approximately 100 kcal-mole^?1. The rate is also approximately proportional to within the range of p_O2 from 10^?10.2 to 10^?5.0 atm at about 1500° C. Based on the present results and the Na₂O contents in chondrules. it is suggested that they experienced an instant heating with maximum temperature of 1400–2200° C followed by an immediate cooling.     

Use of Pitzer's equations to determine the media effect on the formation of lead chloro complexes

The spectrophotometric measurements of chloro complexes of lead in aqueous HCl, NaCl, MgCl₂ and CaCl₂ solutions at 25°C have been analyzed using Pitzer's specific interaction equations. Parameters for activity coefficients of the complexes PbCl⁺, PbCl₂⁰ and PbCl₃^? have been determined for the various media. Values of $\text{K}{}_1\text{= 30.0 ± 0.6}$, $\text{K}{}_2\text{= 106.7 ± 2.1}$ and $\text{K}{}_3\text{= 73.0 ± 1.5}$ were obtained for the cumulative formation constants. [$\text{Pb}{}^{\mathrm{2+}}\text{+ nCl}{}^{\mathrm{?}}\text{→ PbCl}{}_{\mathrm{n}}{}^{\mathrm{2?n}}\text{)}$]. These values are in reasonable agreement with literature data. The Pitzer parameters for the PbCl ion pairs in various media were used to calculate the speciation of Pb²⁺ in an artificial seawater solution.