Density calculations for silicate liquids. I. Revised method for aluminosilicate compositions

Equations are developed for calculating the density of aluminosilicate liquids as a function of composition and temperature. The mean molar volume at reference temperature T_r, is given by $\text{V}{}_{\mathrm{r}}\text{= ∑X}{}_{\mathrm{i}}\text{V}\text{?}{}^{\mathrm{o}}{}_{\mathrm{i}}\text{+ X}{}_{\mathrm{A}}\text{V}\text{?}{}^{\mathrm{o}}{}_{\mathrm{A}}$, 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 $\text{V}\text{?}{}^{\mathrm{o}}{}_{\mathrm{A}}$ is the composition-dependent apparent partial molar volume of Al₂O₃. The thermal expansion coefficient of aluminosilicate liquids is given by $\text{α = ∑X}{}_{\mathrm{i}}\text{}\text{?}\text{ga}{}_{\mathrm{i}}{}^{\mathrm{o}}\text{+ X}{}_{\mathrm{A}}\text{}\text{?}\text{ga}{}_{\mathrm{A}}{}^{\mathrm{o}}$, where $\text{}\text{?}\text{ga}{}_{\mathrm{i}}{}^{\mathrm{o}}$ terms are constants independent of temperature and composition, and $\text{}\text{?}\text{ga}{}^{\mathrm{o}}{}_{\mathrm{A}}$ 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) = V_rexp[α(T-T_r)], where T_r = 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 $\text{cc}\text{mole}$ and an average absolute difference of 0.90% for 248 experimental observations. The compositional dependence of $\text{V}\text{?}{}^{\mathrm{o}}{}_{\mathrm{A}}$ is discussed in terms of several possible interpretations of the structural role of Al³⁺ in aluminosilicate melts.     

On calculating activity coefficients and other excess functions from the intracrystalline exchange properties of a double-site phase

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 ΔG_E°(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, G_mix^EX, unambiguously. Conversely, for any fixed G_mix^EX 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, G_mix^EX 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, γ_A^T, can be calculated for any bulk composition, X_A^T, either from G_mix^EX directly or from more basic equations involving the interrelation of chemical potentials at equilibrium. A general form for γ_A^T is ln $\text{γ}{}_{\mathrm{A}}{}^{\mathrm{T}}\text{= 1n[}\text{2(X}{}_{\mathrm{A}}{}^{\mathrm{\alpha }}\text{X}{}_{\mathrm{A}}{}^{\mathrm{\beta }}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{/}\text{(X}{}_{\mathrm{A}}{}^{\mathrm{\alpha }}\text{+X}{}_{\mathrm{A}}{}^{\mathrm{\beta }}\text{)}\text{]+Y}$, where Xj^κ 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 γ_A^T 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, H_mix^N 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 $\text{H}{}_{\mathrm{mix}}{}^{\mathrm{N}}\text{= 2RT(1? X}{}_{\mathrm{A}}{}^{\mathrm{T}}\text{) ∝}\text{Y}\text{(1 ? X}{}_{\mathrm{A}}{}^{\mathrm{T}}\text{)}{}^2\text{dX}{}_{\mathrm{A}}{}^{\mathrm{T}}$.In those cases, G_mix^EX can also be determined exactly.     

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.     

Etude des inclusions fluides du système N2-CO2 de dolomites et de quartz de Tunisie septentrionale. Données de la microcryoscopie et de l'analyse à la microsonde à effet Raman

Optical and analytical studies were performed on 400 N₂ + CO₂ gas bearing inclusions in dolomites and quartz from Triassic outcrops in northern Tunisia. Other fluids present include brines (NaCl and KCl bearing inclusions) and rare liquid hydrocarbons. At the time of trapping, such fluids were heterogeneous gas + brine mixtures. In hydrocarbon free inclusions the $\text{N}{}_2\text{(N}{}_2\text{+ CO}{}_2\text{)}$ mole ratio was determined using two different non-destructive and punctual techniques: Raman microprobe analysis, and optical estimation of the volume ratios of the different phases selected at low temperatures. In the observed range of compositions, the two methods agree reasonably well.The N₂ + CO₂ inclusions are divided into three classes of composition: (a) $\text{N}{}_2\text{(N}{}_2\text{+ CO}{}_2\text{)}\text{> 0,57}$: Liquid nitrogen is always visible at very low temperature and homogenisation occurs in the range ?151°C to ? 147°C (nitrogen critical temperature) dry ice (solid CO₂) sublimates between ?75°C and ?60°C; (b) $\text{0,20 <}\text{N}{}_2\text{(N}{}_2\text{+ CO}{}_2\text{)}\text{? 0,57}$: liquid nitrogen is visible at very low temperature but dry ice melts on heating; liquid and gas CO₂ homogenise to liquid phase between ?51°C to ?22°C; (c) $\text{N}{}_2\text{(N}{}_2\text{+ CO}{}_2\text{)}\text{? 0,20}$: liquid nitrogen is not visible even at very low temperature (?195°C) and liquid and gas CO₂ homogenise to liquid phase between ?22°C and ?15°C. The observed phases changes are used to propose a preliminary phase diagram for the system CO₂-N₂ at low temperatures.Assuming additivity of partial pressures, isochores for the CO₂-N₂ inclusions have been computed. The intersection of these isochores with those for brine inclusions in the same samples may give the P and T of trapping of the fluids.     

Diffusion of ions in sea water and in deep-sea sediments

The tracer-diffusion coefficient of ions in water, D_j⁰, and in sea water, $\text{D}{}_{\mathrm{j}}{}^1$, differ by no more than zero to 8 per cent. When sea water diffuses into a dilute solution of water, in order to maintain the electro-neutrality, the average diffusion coefficients of major cations become greater but of major anions smaller than their respective $\text{D}{}_{\mathrm{j}}{}^1$ or D_j⁰ values. The tracer diffusion coefficients of ions in deep-sea sediments, D_j,sed., can be related to $\text{D}{}_{\mathrm{j}}{}^1$ by $\text{D}{}_{\mathrm{j,sed.}}\text{= D}{}_{\mathrm{j}}{}^1\text{·}\text{α}\text{θ}{}^2$, where θ is the tortuosity of the bulk sediment and a a constant close to one.     

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:

     

7.

The system pargasite-H₂O-CO₂: a model for melting of a hydrous mineral with a mixed-volatile fluid—I. Experimental results to 8 kbar     

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 ($\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$) ranged from 1.0 to 0.2.At P < 4 kbar the stability temperature (T) of pargasite decreases with decreasing $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$ at constant P. Above P ? 4 kbar stability T increases as $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$ is decreased below one, passes through a T maximum and then decreases with a further decrease in $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$. This behavior is due to a decrease in the H₂O content of the silicate liquid as $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$ decreases. The magnitude of the T maximum increases from about 10°C (relative to the stability T for $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}\text{= 1}$) at P = 5 kbar to about 30°C at P = 8 kbar, and the position of the maximum shifts from $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}\text{? 0.6}$ at P = 5 kbar to $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}\text{? 0.4}$ 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 $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$ at high P and T, hornblende can be an important phase in igneous processes even at relatively low H₂O fugacities.     

8.

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

L. Dever  R. Durand  J.Ch. Fontes  P. Vachier 《Geochimica et cosmochimica acta》1982,46(10):1947-1956

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.     

9.

Finite and incremental deformations recorded by planar subfabrics in S-tectonites     

Richard J. Dzis 《Tectonophysics》1976,36(4):367-393

Analysis of relative componental movements in foliated rocks is formulated in terms of space-continuous deformations assuming that a portion of the strain recorded by planar subfabrics results from differential movements on closely spaced shear surfaces (i.e. fiducial planes). Continuous and discontinuous velocity boundary conditions controlling deformation patterns within subdomains of folded layers are analyzed by combining the spatial velocity and finite deformation gradients. Within each subdomain internal rotations cause material elements oblique to the principal strain rate directions to undergo a series of complex strain transfers resulting in their compensatory lengthening and shortening during finite intervals. Equations are derived which continuously monitor successive variations in the logarithmic strain rates, ?(N,t), for fabrics whose rotation axes are parallel to an intermediate principal axis. Values of ?(N,t) at an angle N to the shear plane are numerically equal at time t, to the magnitude of the Hencky strain rate vector ($\text{dh}{}_{\mathrm{i}}\text{dt}$) referred to natural strain coordinates and used in conjunction with $\text{e}\text{?}\text{(N,t)}$ and the finite stretch, evaluate contemporary strain profiles for groups of planar fabrics replacing passive material planes. Applications to rectilinear shearing modes reveal that the most significant changes in local extensional rates are located between the maximum shearing and principal stretching directions. Assuming sectional continuity and constant material properties of the subfabrics and their matrix, these variations are correlated with systematic spacings between boudin structures suggesting that recognition of multiple orders of boudinage with respect to a potential shear surface in natural S-tectonites can be useful in deciphering local finite and incremental deformation coefficients as well as differentiate continuous ($\text{dh}{}_{\mathrm{i}}\text{dt}\text{= ?(t)}$ or constant) from pulsatory ($\text{dh}{}_{\mathrm{i}}\text{dt}$ is undefined at t) overprinting of the subdomain.     

10.

Geochemistry of Archean metasedimentary rocks from West Greenland     

Scott M McLennan  S.R Taylor  V.R McGregor 《Geochimica et cosmochimica acta》1984,48(1):1-13

Archean metasedimentary rocks occur as components of the Isua supracrustals, Akilia association and Malene supracrustals of southern West Greenland. Primary structures in these rocks have been destroyed by metamorphism and deformation. Their chemistry and mineralogy is consistent with a sedimentary origin, but other possible parents (e.g. acid volcanics, altered pyroclastic rocks) cannot be excluded for some of them. There is little difference in the composition of metasedimentary rocks from the early Archean Isua supracrustals and probable correlative Akilia association. Both have a wide range in rare earth element (REE) patterns with $\text{La}{}_{\mathrm{<mtext>N</mtext>}}\text{Yb}{}_{\mathrm{<mtext>N</mtext>}}$ ranging from 0.61?5.8. The REE pattern of one Akilia sample, with low $\text{La}{}_{\mathrm{<mtext>N</mtext>}}\text{Yb}{}_{\mathrm{<mtext>N</mtext>}}$, compares favourably with that of associated tholeiites and it is likely that such samples were derived almost exclusively from basaltic sources. Other samples with very steep REE patterns are similar to felsic volcanic boulders found in a conglomeratic unit in the Isua supracrustals. Samples with intermediate REE patterns are best explained by mixing of basaltic and felsic end members. Metasedimentary rocks from the Malene supracrustals can be divided into low silica (≤55% SiO₂) and high silica (>77% SiO₂) varieties. These rocks also show much variation in $\text{La}{}_{\mathrm{<mtext>N</mtext>}}\text{Yb}{}_{\mathrm{<mtext>N</mtext>}}$ (0.46?14.0) and their origin is explained by derivation from a mixture of mafic volcanics and felsic igneous rocks. The wide range in trace element characteristics of these metasedimentary rocks argues for inefficient mixing of the various source lithologies during sedimentation. Accordingly, these data do not rigorously test models of early Archean crustal composition and evolution. The systematic variability in trace element geochemistry provides evidence for the bimodal nature of the early Archean crust.     

11.

The thermodynamics of the carbonate system in seawater     

Frank J Millero 《Geochimica et cosmochimica acta》1979,43(10):1651-1661

The apparent constants (K'_i) for the ionization of carbonic acid in seawater at various salinities (S,%.) have been fit to equations of the form ln $\text{K'}{}_{\mathrm{i}}\text{= ln K}{}_{\mathrm{i}}\text{+ A}{}_{\mathrm{i}}\text{S}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{+ B}{}_{\mathrm{i}}\text{S}$whereK_i is the thermodynamic ionization constant in water, A_i, and B_i are adjustable parameters. The temperature dependence (TK) of K_i, A_i and B_i were of the form, a₀ + a₁/T + a₃ ln T. Equations of similar forms have been used to analyze the ionization constants for water and boric acid and the solubility product of calcite in seawater. The effect of pressure on the apparent constants ($\text{K}{}^{\mathrm{p}}{}_{\mathrm{i}}\text{K}{}^{\mathrm{o}}{}_{\mathrm{i}}$) have been fit to equations of the form ln $\text{(}\text{K}{}^{\mathrm{p}}{}_{\mathrm{i}}\text{K}{}^{\mathrm{o}}{}_{\mathrm{i}}\text{) = ? (ΔVP + 0.5 ΔK P}{}^2\text{)/RT}$ where the volume (ΔV) and compressibility (ΔK) changes are polynomial functions of temperature. The equations generated for various açids in seawater have been used to examine the carbonate system in seawater. Equations relating the NBS and Tris pH scales have been derived as well as equations of pH as a function of temperature and pressure. The equations from Hansson (1972, Ph.D. Thesis, University of Göteborg, Sweden) and Mehrbachet al. (1973, Limnol. Oceanogr.18, 897–907) have been used to examine the components of the carbonate system. At a fixed total alkalinity and total carbon dioxide, differences of ±0.01 m-equiv kg^?1 in HCO^?₃ and CO^2?₃ were found; however, the [CO₂] and P_co2 are nearly the same. The contribution of borate ion, B(OH)^?₄ determined from the equations of Hansson (1972, Ph.D. Thesis, University of Göteborg, Sweden) and Lyman (1957, Ph.D. Thesis, University of California, Los Angeles) differ by ±0.01 m-equiv kg^?1 for waters with the same salinity and temperature.     

12.

Karibibite,a new FeAs mineral from South West Africa     

Oleg V Knorring  Th.G Sahama  Pentti Rehtijärvi 《Lithos》1973,6(3):265-271

Karibibite (ideally, Fe₂As₄O₉) occurs in vugs in massive loellingite of the Karibib pegmatite area, South West Africa. It is brownish yellow and finely fibrous. The thickness of the soft, single fibers is less than 1 micron, unsuitable for single-crystal X-ray study. Electron diffraction and X-ray powder pattern indicate that the mineral is orthorhombic, with $\text{a}{}_0\text{= 27.91}\text{A}\text{?}\text{, b}{}_0\text{= 6.53}\text{A}\text{?}\text{and c}{}_0\text{(}\text{fiber axis}\text{) = 7.20}\text{A}\text{?}$. The space group cannot be given. The mineral is paramagnetic with yellow fluorescence and is pleochroic with γ > 2.10, α = 1.96, 2V_α large, d = 4.07. It is soluble in acids and alkali hydroxide. Decomposition starts around 320 °C. The infra-red absorption spectrum indicates absence of AsO₄ groups. The mineral is classified tentatively as an oxide or arsenite.     

13.

Extension of the empirical Alyavdin equation for representing batch grinding data     

L.G Austin  K Shoji  V.K Bhatia  F.F Aplan 《International Journal of Mineral Processing》1974,1(2):107-123

The Alyavdin equation for batch grinding data is:

$\text{1 ? P(χ, t) = [1 ? P(χ, 0)]}\text{exp}\text{?}\text{c(x)t}{}^{\mathrm{p}}\text{]}$

where P(χ,t) is the weight fraction less than size χ after grinding time t, c (χ) is constant with t and p is a constant close to one. It is shown that this equation is illogical (except for a single size of feed) unless c (χ) varies with P(χ,0), which makes the equation of little utility. A new empirical equation is developed for finite size intervals:

which reduces to the Alyavdin equation for a single size of feed, and which gives consistent computations for any feed size distribution. Techniques are given for determining K_i, γ values from sets of batch grinding data. The values are then used to predict size distributions for other times and other feed size distributions. The equation was quite successful in predicting size distributions in batch milling: (a) providing the feed size distribution was not un-natural, that is, not truncated or (b) if a truncated feed was used, the values of K_i and γ are determined from size distributions of grinding of the same type of feed. Thus, K_i, γ are not, unfortunately, completely independent of the starting feed size distribution.     

14.

Light hydrocarbons in Red Sea brines and sediments     

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, $\text{Cl}{}^{\mathrm{?}}\text{= ~ 85‰}$, $\text{CH}{}_4\text{=}\text{~ 71}\text{1}$) are apparently of biogenic origin as evidenced by $\text{C}{}_1\text{(C}{}_2\text{+ C}{}_3$) 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, $\text{Cl}{}^{\mathrm{?}}\text{= ~ 73‰}$, CH₄ = ~ 155 μl/l) gas seems to be of biogenic origin [$\text{C}{}_1\text{(C}{}_2\text{+ C}{}_3\text{) = ~1100}$], whereas the lower brine (T = ~ 61°C, $\text{Cl}{}^{\mathrm{?}}\text{= ~ 155‰}$, CH₄ = ~ 120μl/l) gas is apparently of thermogenic origin [$\text{C}{}_1\text{(C}{}_2\text{+ C}{}_3\text{) = ~ 50}$]. 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.     

15.

Methane-producing bacteria: natural fractionations of the stable carbon isotopes   总被引：1，自引：0，他引：1  

Larry M. Games  J.M. HayesRobert  P. Gunsalus 《Geochimica et cosmochimica acta》1978,42(8):1295-1297

The ${}^{13}\text{C}{}^{12}\text{C}$ fractionation factors ($\text{CO}{}_2\text{CH}{}_4$) 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 ($\text{CO}{}_2\text{H}\text{CH}{}_3$) approximating the observed $\text{CO}{}_2\text{CH}{}_4$ fractionation.     

16.

Theoretical prediction of phase relations among aqueous solutions and minerals: Salton Sea geothermal system     

Dennis K. Bird  Denis L. Norton 《Geochimica et cosmochimica acta》1981,45(9):1479-1494

     

17.

Evaluation of deep temperatures of hydrothermal systems by a new gas geothermometer     

Franco D&#x;Amore  Costanzo Panichi 《Geochimica et cosmochimica acta》1980,44(3):549-556

The chemical composition of gas mixtures emerging in thermal areas can be used to evaluate the deep thermal temperatures. Chemical analyses of the gas compositions for 34 thermal systems were considered and an empirical relationship developed between the relative concentrations of H₂S, H₂, CH₄ and CO₂ and the reservoir temperature. The evaluated temperatures can be expressed by: $\text{t°C =}\text{24775}\text{α + β + 36.05}\text{?273}$ where $\text{α = 2 log}\text{CH}{}_4\text{CO}{}_2\text{?log}\text{H}{}_2\text{CO}{}_2\text{?3 log}\text{H}{}_2\text{S}\text{CO}{}_2$ (concentrations in % by volume) and β = 7 logP_co2     

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.

Liquidus relationships in the system CaCO₃Ca(OH)₂CaS and the solubility of sulfur in carbonatite magmas     

George R. Helz  Peter J. Wyllie 《Geochimica et cosmochimica acta》1979,43(2):259-265

CaCO₃Ca(OH)₂CaS serves as a model system for sulfide solubility in carbonatite magmas. Experiments at 1 kbar delineate fields for primary crystallization of CaCO₃, Ca(OH)₂ and CaS. The three fields meet at a ternary eutectic at 652°C with liquid composition (wt%): CaCO₃ = 46.1%, Ca(OH)₂ = 51.9%, CaS = 2.0%. Two crystallization sequences are possible for liquids that precipitate calcite, depending upon whether the liquid is on the low-CaS side, or the high-CaS side of the line connecting CaCO₃ to the eutectic liquid. Low-CaS liquids precipitate no sulfide until the eutectic temperature is reached leading to sulfide enrichment. The higher-CaS liquids precipitate some sulfide above the eutectic temperature, but the sulfide content of the melt is not greatly depleted as the eutectic temperature is approached. Theoretical considerations indicate that sulfide solubility in carbonate melts will be directly proportional to and inversely proportional to ; it also is likely to be directly proportional to melt basicity, defined here by . A strong similarity exists in the processes which control sulfide solubility in carbonate and in silicate melts. By analogy with silicates, ferrous iron, which was absent in our experiments, may also exert an important influence on sulfide solubility in natural carbonatite magmas.     

20.

Viscosity and configurational entropy of silicate melts     

Pascal Richet 《Geochimica et cosmochimica acta》1984,48(3):471-483

With the configurational entropy theory of relaxation processes of Adam and Gibbs (1965), one predicts that the viscosity depends on temperature according to log $\text{η = A}{}_{\mathrm{e}}\text{+}\text{B}{}_{\mathrm{e}}\text{TS}{}_{\mathrm{<mtext>conf</mtext>}}$, where S_conf is the configurational entropy of the liquid. Thermochemical calculations of S_conf performed for some mineral compositions show the importance of non-configurational contributions to the entropy differences between amorphous and crystalline phases. Except for the case of SiO₂, the available thermodynamic data indicate that the above equation for viscosity accounts quantitatively for the experimentally determined temperature dependence of the viscosity of silicate melts. The Adam and Gibbs theory also provides a simple rationale for the non linear variation of the logarithmic viscosity with composition in mixed alkali silicate liquids at low temperatures, the minimum of viscosity resulting from the contribution of the entropy of mixing to S_conf.     

	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

The system pargasite-H2O-CO2: a model for melting of a hydrous mineral with a mixed-volatile fluid—I. Experimental results to 8 kbar

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 ($\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$) ranged from 1.0 to 0.2.At P < 4 kbar the stability temperature (T) of pargasite decreases with decreasing $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$ at constant P. Above P ? 4 kbar stability T increases as $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$ is decreased below one, passes through a T maximum and then decreases with a further decrease in $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$. This behavior is due to a decrease in the H₂O content of the silicate liquid as $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$ decreases. The magnitude of the T maximum increases from about 10°C (relative to the stability T for $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}\text{= 1}$) at P = 5 kbar to about 30°C at P = 8 kbar, and the position of the maximum shifts from $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}\text{? 0.6}$ at P = 5 kbar to $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}\text{? 0.4}$ 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 $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$ at high P and T, hornblende can be an important phase in igneous processes even at relatively low H₂O fugacities.     

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.     

Finite and incremental deformations recorded by planar subfabrics in S-tectonites

Analysis of relative componental movements in foliated rocks is formulated in terms of space-continuous deformations assuming that a portion of the strain recorded by planar subfabrics results from differential movements on closely spaced shear surfaces (i.e. fiducial planes). Continuous and discontinuous velocity boundary conditions controlling deformation patterns within subdomains of folded layers are analyzed by combining the spatial velocity and finite deformation gradients. Within each subdomain internal rotations cause material elements oblique to the principal strain rate directions to undergo a series of complex strain transfers resulting in their compensatory lengthening and shortening during finite intervals. Equations are derived which continuously monitor successive variations in the logarithmic strain rates, ?(N,t), for fabrics whose rotation axes are parallel to an intermediate principal axis. Values of ?(N,t) at an angle N to the shear plane are numerically equal at time t, to the magnitude of the Hencky strain rate vector ($\text{dh}{}_{\mathrm{i}}\text{dt}$) referred to natural strain coordinates and used in conjunction with $\text{e}\text{?}\text{(N,t)}$ and the finite stretch, evaluate contemporary strain profiles for groups of planar fabrics replacing passive material planes. Applications to rectilinear shearing modes reveal that the most significant changes in local extensional rates are located between the maximum shearing and principal stretching directions. Assuming sectional continuity and constant material properties of the subfabrics and their matrix, these variations are correlated with systematic spacings between boudin structures suggesting that recognition of multiple orders of boudinage with respect to a potential shear surface in natural S-tectonites can be useful in deciphering local finite and incremental deformation coefficients as well as differentiate continuous ($\text{dh}{}_{\mathrm{i}}\text{dt}\text{= ?(t)}$ or constant) from pulsatory ($\text{dh}{}_{\mathrm{i}}\text{dt}$ is undefined at t) overprinting of the subdomain.     

Geochemistry of Archean metasedimentary rocks from West Greenland

Archean metasedimentary rocks occur as components of the Isua supracrustals, Akilia association and Malene supracrustals of southern West Greenland. Primary structures in these rocks have been destroyed by metamorphism and deformation. Their chemistry and mineralogy is consistent with a sedimentary origin, but other possible parents (e.g. acid volcanics, altered pyroclastic rocks) cannot be excluded for some of them. There is little difference in the composition of metasedimentary rocks from the early Archean Isua supracrustals and probable correlative Akilia association. Both have a wide range in rare earth element (REE) patterns with $\text{La}{}_{\mathrm{<mtext>N</mtext>}}\text{Yb}{}_{\mathrm{<mtext>N</mtext>}}$ ranging from 0.61?5.8. The REE pattern of one Akilia sample, with low $\text{La}{}_{\mathrm{<mtext>N</mtext>}}\text{Yb}{}_{\mathrm{<mtext>N</mtext>}}$, compares favourably with that of associated tholeiites and it is likely that such samples were derived almost exclusively from basaltic sources. Other samples with very steep REE patterns are similar to felsic volcanic boulders found in a conglomeratic unit in the Isua supracrustals. Samples with intermediate REE patterns are best explained by mixing of basaltic and felsic end members. Metasedimentary rocks from the Malene supracrustals can be divided into low silica (≤55% SiO₂) and high silica (>77% SiO₂) varieties. These rocks also show much variation in $\text{La}{}_{\mathrm{<mtext>N</mtext>}}\text{Yb}{}_{\mathrm{<mtext>N</mtext>}}$ (0.46?14.0) and their origin is explained by derivation from a mixture of mafic volcanics and felsic igneous rocks. The wide range in trace element characteristics of these metasedimentary rocks argues for inefficient mixing of the various source lithologies during sedimentation. Accordingly, these data do not rigorously test models of early Archean crustal composition and evolution. The systematic variability in trace element geochemistry provides evidence for the bimodal nature of the early Archean crust.     

The thermodynamics of the carbonate system in seawater

The apparent constants (K'_i) for the ionization of carbonic acid in seawater at various salinities (S,%.) have been fit to equations of the form ln $\text{K'}{}_{\mathrm{i}}\text{= ln K}{}_{\mathrm{i}}\text{+ A}{}_{\mathrm{i}}\text{S}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{+ B}{}_{\mathrm{i}}\text{S}$whereK_i is the thermodynamic ionization constant in water, A_i, and B_i are adjustable parameters. The temperature dependence (TK) of K_i, A_i and B_i were of the form, a₀ + a₁/T + a₃ ln T. Equations of similar forms have been used to analyze the ionization constants for water and boric acid and the solubility product of calcite in seawater. The effect of pressure on the apparent constants ($\text{K}{}^{\mathrm{p}}{}_{\mathrm{i}}\text{K}{}^{\mathrm{o}}{}_{\mathrm{i}}$) have been fit to equations of the form ln $\text{(}\text{K}{}^{\mathrm{p}}{}_{\mathrm{i}}\text{K}{}^{\mathrm{o}}{}_{\mathrm{i}}\text{) = ? (ΔVP + 0.5 ΔK P}{}^2\text{)/RT}$ where the volume (ΔV) and compressibility (ΔK) changes are polynomial functions of temperature. The equations generated for various açids in seawater have been used to examine the carbonate system in seawater. Equations relating the NBS and Tris pH scales have been derived as well as equations of pH as a function of temperature and pressure. The equations from Hansson (1972, Ph.D. Thesis, University of Göteborg, Sweden) and Mehrbachet al. (1973, Limnol. Oceanogr.18, 897–907) have been used to examine the components of the carbonate system. At a fixed total alkalinity and total carbon dioxide, differences of ±0.01 m-equiv kg^?1 in HCO^?₃ and CO^2?₃ were found; however, the [CO₂] and P_co2 are nearly the same. The contribution of borate ion, B(OH)^?₄ determined from the equations of Hansson (1972, Ph.D. Thesis, University of Göteborg, Sweden) and Lyman (1957, Ph.D. Thesis, University of California, Los Angeles) differ by ±0.01 m-equiv kg^?1 for waters with the same salinity and temperature.     

Karibibite,a new FeAs mineral from South West Africa

Karibibite (ideally, Fe₂As₄O₉) occurs in vugs in massive loellingite of the Karibib pegmatite area, South West Africa. It is brownish yellow and finely fibrous. The thickness of the soft, single fibers is less than 1 micron, unsuitable for single-crystal X-ray study. Electron diffraction and X-ray powder pattern indicate that the mineral is orthorhombic, with $\text{a}{}_0\text{= 27.91}\text{A}\text{?}\text{, b}{}_0\text{= 6.53}\text{A}\text{?}\text{and c}{}_0\text{(}\text{fiber axis}\text{) = 7.20}\text{A}\text{?}$. The space group cannot be given. The mineral is paramagnetic with yellow fluorescence and is pleochroic with γ > 2.10, α = 1.96, 2V_α large, d = 4.07. It is soluble in acids and alkali hydroxide. Decomposition starts around 320 °C. The infra-red absorption spectrum indicates absence of AsO₄ groups. The mineral is classified tentatively as an oxide or arsenite.     

Extension of the empirical Alyavdin equation for representing batch grinding data

The Alyavdin equation for batch grinding data is:

$\text{1 ? P(χ, t) = [1 ? P(χ, 0)]}\text{exp}\text{?}\text{c(x)t}{}^{\mathrm{p}}\text{]}$

where P(χ,t) is the weight fraction less than size χ after grinding time t, c (χ) is constant with t and p is a constant close to one. It is shown that this equation is illogical (except for a single size of feed) unless c (χ) varies with P(χ,0), which makes the equation of little utility. A new empirical equation is developed for finite size intervals:

which reduces to the Alyavdin equation for a single size of feed, and which gives consistent computations for any feed size distribution. Techniques are given for determining K_i, γ values from sets of batch grinding data. The values are then used to predict size distributions for other times and other feed size distributions. The equation was quite successful in predicting size distributions in batch milling: (a) providing the feed size distribution was not un-natural, that is, not truncated or (b) if a truncated feed was used, the values of K_i and γ are determined from size distributions of grinding of the same type of feed. Thus, K_i, γ are not, unfortunately, completely independent of the starting feed size distribution.     

Light hydrocarbons in Red Sea brines and sediments

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, $\text{Cl}{}^{\mathrm{?}}\text{= ~ 85‰}$, $\text{CH}{}_4\text{=}\text{~ 71}\text{1}$) are apparently of biogenic origin as evidenced by $\text{C}{}_1\text{(C}{}_2\text{+ C}{}_3$) 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, $\text{Cl}{}^{\mathrm{?}}\text{= ~ 73‰}$, CH₄ = ~ 155 μl/l) gas seems to be of biogenic origin [$\text{C}{}_1\text{(C}{}_2\text{+ C}{}_3\text{) = ~1100}$], whereas the lower brine (T = ~ 61°C, $\text{Cl}{}^{\mathrm{?}}\text{= ~ 155‰}$, CH₄ = ~ 120μl/l) gas is apparently of thermogenic origin [$\text{C}{}_1\text{(C}{}_2\text{+ C}{}_3\text{) = ~ 50}$]. 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.     

Methane-producing bacteria: natural fractionations of the stable carbon isotopes

The ${}^{13}\text{C}{}^{12}\text{C}$ fractionation factors ($\text{CO}{}_2\text{CH}{}_4$) 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 ($\text{CO}{}_2\text{H}\text{CH}{}_3$) approximating the observed $\text{CO}{}_2\text{CH}{}_4$ fractionation.     

Evaluation of deep temperatures of hydrothermal systems by a new gas geothermometer

The chemical composition of gas mixtures emerging in thermal areas can be used to evaluate the deep thermal temperatures. Chemical analyses of the gas compositions for 34 thermal systems were considered and an empirical relationship developed between the relative concentrations of H₂S, H₂, CH₄ and CO₂ and the reservoir temperature. The evaluated temperatures can be expressed by: $\text{t°C =}\text{24775}\text{α + β + 36.05}\text{?273}$ where $\text{α = 2 log}\text{CH}{}_4\text{CO}{}_2\text{?log}\text{H}{}_2\text{CO}{}_2\text{?3 log}\text{H}{}_2\text{S}\text{CO}{}_2$ (concentrations in % by volume) and β = 7 logP_co2     

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.     

Liquidus relationships in the system CaCO3Ca(OH)2CaS and the solubility of sulfur in carbonatite magmas

CaCO₃Ca(OH)₂CaS serves as a model system for sulfide solubility in carbonatite magmas. Experiments at 1 kbar delineate fields for primary crystallization of CaCO₃, Ca(OH)₂ and CaS. The three fields meet at a ternary eutectic at 652°C with liquid composition (wt%): CaCO₃ = 46.1%, Ca(OH)₂ = 51.9%, CaS = 2.0%. Two crystallization sequences are possible for liquids that precipitate calcite, depending upon whether the liquid is on the low-CaS side, or the high-CaS side of the line connecting CaCO₃ to the eutectic liquid. Low-CaS liquids precipitate no sulfide until the eutectic temperature is reached leading to sulfide enrichment. The higher-CaS liquids precipitate some sulfide above the eutectic temperature, but the sulfide content of the melt is not greatly depleted as the eutectic temperature is approached. Theoretical considerations indicate that sulfide solubility in carbonate melts will be directly proportional to and inversely proportional to ; it also is likely to be directly proportional to melt basicity, defined here by . A strong similarity exists in the processes which control sulfide solubility in carbonate and in silicate melts. By analogy with silicates, ferrous iron, which was absent in our experiments, may also exert an important influence on sulfide solubility in natural carbonatite magmas.     

Viscosity and configurational entropy of silicate melts

With the configurational entropy theory of relaxation processes of Adam and Gibbs (1965), one predicts that the viscosity depends on temperature according to log $\text{η = A}{}_{\mathrm{e}}\text{+}\text{B}{}_{\mathrm{e}}\text{TS}{}_{\mathrm{<mtext>conf</mtext>}}$, where S_conf is the configurational entropy of the liquid. Thermochemical calculations of S_conf performed for some mineral compositions show the importance of non-configurational contributions to the entropy differences between amorphous and crystalline phases. Except for the case of SiO₂, the available thermodynamic data indicate that the above equation for viscosity accounts quantitatively for the experimentally determined temperature dependence of the viscosity of silicate melts. The Adam and Gibbs theory also provides a simple rationale for the non linear variation of the logarithmic viscosity with composition in mixed alkali silicate liquids at low temperatures, the minimum of viscosity resulting from the contribution of the entropy of mixing to S_conf.