The solubility of molybdenum dioxide and trioxide in HCl-bearing water vapour at 350 °C and pressures up to 160 bars

The solubility and speciation of the assemblage MoO₂-MoO₃ in water vapour were investigated in experiments conducted at 350 °C, P_total from 59 to 160 bar and fHCl from 0 to 3.4 bar (0-2.0 mol%). Measured solubility at these conditions ranges from 22 to 2500 ppm (∑fMo from 4.4 × 10⁻⁴ to 6.5 × 10⁻² bar). The concentration of Mo in the vapour at fHCl below 0.1 bar is similar to that in pure water vapour, but increases by two orders of magnitude at fHCl above 0.1 bar. The fugacity of gaseous Mo species is independent of chloride concentration at fHCl below 0.1 bar, but increases with increasing fHCl above this pressure. The dominant Mo species at fHCl below 0.1 bar is interpreted to be the same as it is in pure water vapour, and to form as a result of the reaction

(A1)     

The uptake and solubility of water in quartz at elevated pressure and temperature

The uptake of water in quartz at 1.5 GPa total pressure, 1173 K and high water fugacity, over times up to 24 h, has been investigated using a newly developed assembly to prevent microcracking. It is found that the uptake is small, and below the detectability of the presently used technique of infrared spectroscopy and serial sectioning. This observation reflects either a low value for the diffusivity or the solubility or a combination of both, and is in agreement with the observations of Kronenberg et al. (1986) and Rovetta et al. (1986). It brings into question the interpretation of the early experiments on water weakening by Griggs and Blacic (1964) and the recent estimates of the solubility and diffusivity by Mackwell and Paterson (1985). Rults of a combined T.E.M., light-scattering and infrared-spectroscopy investigation of ‘wet’ synthetic quartz before and after heating at 0.1, 300 and 1500 MPa total pressure and 1173 K, strongly suggest that the water in ‘wet’ quartz is mainly in the form of H₂O in inclusions, consistent with the solubility being low, possibly less than 100 H/10⁶Si. From these observations, water-containing inclusions appear to play a major role in the plasticity of quartz, while any role of water in solid solution remains to be clarified.     

Oxygen chemical diffusion in three basaltic liquids at elevated temperatures and pressures

The diffusivity of oxygen has been measured in three basaltic liquids from 1280 to 1450°C and 4 to 21 kilobars using a solid media piston-cylinder apparatus. The measurements were done by monitoring the reduction of ferric iron in previously oxidized spheres of basalt melt. The compositions studied were olivine nephelinite, alkali basalt, and 1921 Kilauea tholeiite.The isobaric temperature dependence of oxygen diffusivity is adequately described by Arrhenius relationships for the three liquids studied. Arrhenius activation energies were determined at 12 kilobars for olivine nephelinite (62± 6 kcal/mole) and tholeiite (51 ± 4 kcal/mole) and at 4, 12, and 20 kilobars for alkali basalt (70 ± 7, 86 ± 6, and 71 ± 14 kcal/mole, respectively). The Arrhenius parameters for the three compositions define a compensation law which is indistinguishable from those for oxygen diffusion in simple silicate melts (DUNN, 1982) and for divalent cation diffusion in basaltic melts (Hofmann, 1980). These results suggest that the principal species contributing to the total diffusivity of oxygen is the oxide anion (O^2?).The isothermal pressure dependence of oxygen diffusion is complex and quite different from that observed for cationic diffusion in silicate melts. All three compositions show a sharp decrease in oxygen diffusivity at approximately the same pressure as the change in the liquidus phase from olivine to pyroxene, but otherwise the pressure dependence can be described by Arrhenius type equations. The equations yield negative activation volumes for the olivine nehpelinite and the alkali basalt. The activation volumes determined for the tholeiite are near zero at low pressure and positive at high pressure. A negative activation volume represents a decrease in the average size of the principal diffusing species.The results of this study are consistent with a melt model which includes both continuous changes in the relative proportions of the various anionic species in the melt with pressure and the occurrence of anionic disproportionation reactions within narrow pressure ranges.     

Thermodynamic modeling of non-ideal mineral-fluid equilibria in the system Si-Al-Fe-Mg-Ca-Na-K-H-O-Cl at elevated temperatures and pressures: Implications for hydrothermal mass transfer in granitic rocks

We present the results of thermodynamic modeling of fluid-rock interaction in the system Si-Al-Fe-Mg-Ca-Na-H-O-Cl using the GEM-Selektor Gibbs free energy minimization code. Combination of non-ideal mixing properties in solids with multicomponent aqueous fluids represents a substantial improvement and it provides increased accuracy over existing modeling strategies. Application to the 10-component system allows us to link fluid composition and speciation with whole-rock mineralogy, mass and volume changes. We have simulated granite-fluid interaction over a wide range of conditions (200-600 °C, 100 MPa, 0-5 m Cl and fluid/rock ratios of 10⁻²-10⁴) in order to explore composition of magmatic fluids of variable salinity, temperature effects on fluid composition and speciation and to simulate several paths of alteration zoning. At low fluid/rock ratios (f/r) the fluid composition is buffered by the silicate-oxide assemblage and remains close to invariant. This behavior extends to a f/r of 0.1 which exceeds the amount of exsolved magmatic fluids controlled by water solubility in silicate melts. With increasing peraluminosity of the parental granite, the Na-, K- and Fe-bearing fluids become more acidic and the oxidation state increases as a consequence of hydrogen and ferrous iron transfer to the fluid. With decreasing temperature, saline fluids become more Ca- and Na-rich, change from weakly acidic to alkaline, and become significantly more oxidizing. Large variations in Ca/Fe and Ca/Mg ratios in the fluid are a potential geothermometer. The mineral assemblage changes from cordierite-biotite granites through two-mica granites to chlorite-, epidote- and zeolite-bearing rocks. We have carried out three rock-titration simulations: (1) reaction with the 2 m NaCl fluid leads to albitization, chloritization and desilication, reproducing essential features observed in episyenites, (2) infiltration of a high-temperature fluid into the granite at 400 °C leads to hydrolytic alteration commencing with alkali-feldspar breakdown and leading to potassic, phyllic and argillic assemblages; this is associated with reduction and iron metasomatism as observed in nature and (3) interaction with a multicomponent fluid at 600 °C produces sodic-calcic metasomatism. Na, Ca and Fe are the most mobile elements whereas immobility of Al is limited by f/r ∼ 400. All simulations predict a volume decrease by 3.4-5.4%, i.e., porosity formation at f/r < 30. At higher fluid/rock ratios simulation (2) produces a substantial volume increase (59%) due to mineral precipitation, whereas simulation (3) predicts a volume decrease by 49% at the advanced albitization-desilication stage. Volume changes closely correlate with mass changes of SiO₂ and are related to silica solubility in fluids. The combined effects of oxygen fugacity, fluid acidity and pH for breakdown of aqueous metal complexes and precipitation of ore minerals were evaluated by means of reduced activity products. Sharp increases in saturation indexes for oxidative breakdown occur at each alteration zone whereas reductive breakdown or involvement of other chloride complexes favor precipitation at high fluid/rock ratios only. Calculations of multicomponent aqueous-solid equilibria at high temperatures and pressures are able to accurately predict rock mineralogy and fluid chemistry and are applicable to diverse reactive flow processes in the Earth’s crust.     

An experimental study of the stability of copper chloride complexes in water vapor at elevated temperatures and pressures

The solubility of copper chloride in liquid-undersaturated HCl-bearing water vapor was investigated experimentally at temperatures of 280 to 320°C and pressures up to 103 bars. Results of these experiments show that the solubility of copper in the vapor phase is significant and increases with increasing f_H2O, but is retrograde with respect to temperature. This solubility is attributed to the formation of hydrated copper-chloride gas species, interpreted to have a copper-chlorine ratio of 1:1 (e.g., CuCl, Cu₃Cl₃, etc.) and a hydration number varying from 7.6 at 320°C, to 6.0 at 300°C, and 6.1 at 280°C. Complex formation is proposed to have occurred through the reaction:

A1     

Prediction of oxygen solubility in pure water and brines up to high temperatures and pressures

A thermodynamic model is presented to calculate the oxygen solubility in pure water (273-600 K, 0-200 bar) and natural brines containing Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻, over a wide range of temperature, pressure and ionic strength with or close to experimental accuracy. This model is based on an accurate equation of state to calculate vapor phase chemical potential and a specific particle interaction model for liquid phase chemical potential. With this approach, the model can not only reproduce the existing experimental data, but also extrapolate beyond the data range from simple aqueous salt system to complicated brine systems including seawater. Compared with previous models, this model covers much wider temperature and pressure space in variable composition brine systems. A program for this model can be downloaded from the website: http://www.geochem-model.org.     

The distribution of strontium between coexisting silicate and carbonate liquids at elevated pressures and temperatures

Strontium is characteristically enriched in carbonatites and associated igneous rocks. Experiments between 600 and 800°C at 5 and 10 kb show that Sr fractionates strongly toward both liquid phases of a carbonate liquid-silicate liquid-solid assemblage, with the crystalline phases depleted in Sr. Considering that the alkalic igneous rocks associated with carbonatites are not significantly depleted in Sr compared with the carbonatites, it is concluded that these complexes are formed by a pair of immiscible magmas, rather than the carbonatites being late-stage fractionation products of the alkalic silicate magmas.     

The electrical conductivities at elevated temperatures and pressures of polycrystalline manganese,cobalt and nickel orthosilicates

Electrical conductivities of synthetic manganese, cobalt and nickel orthosilicates, also of mixed-cation orthosilicates containing together magnesium, iron and manganese, also magnesium, iron and nickel, have been studied as polycrystalline samples at temperatures up to 770°C and pressures up to 62 kbar.Electrical conductivities of these materials appear to depend on the presence of trivalent cations formed by partial oxidation; conductivities are less, by a factor of approximately 10⁴ than for ferrous orthosilicate.The mechanism of electronic semiconduction in these materials is discussed; it is suggested that the effective carrier centres are an association of bivalent cation vacancy and trivalent cations. An activation energy in the range 0.4–1.2 eV is required; its variation with pressure is examined. Ionic conduction is also present, particularly with the cobalt compound.It is concluded that small amounts of manganese or nickel are unlikely to enhance significantly the conductivities of ferromagnesian orthosilicates in the mantle of the Earth, and that a more significant factor is the extent of oxidation of bivalent iron cation.     

The solubility of gold in H2O-H2S vapour at elevated temperature and pressure

This experimental study sheds light on the complexation of gold in reduced sulphur-bearing vapour, specifically, in H₂O-H₂S gas mixtures. The solubility of gold was determined in experiments at temperatures of 300, 350 and 365 °C and reached 2.2, 6.6 and 6.3 μg/kg, respectively. The density of the vapour varied from 0.02 to 0.22 g/cm³, the mole fraction of H₂S varied from 0.03 to 0.96, and the pressure in the cell reached 263 bar. Statistically significant correlations of the amount of gold dissolved in the fluid with the fugacity of H₂O and H₂S permit the experimental data to be fitted to a solvation/hydration model. According to this model, the solubility of gold in H₂O-H₂S gas mixtures is controlled by the formation of sulphide or bisulphide species solvated by H₂S or H₂O molecules. Formation of gold sulphide species is favoured statistically over gold bisulphide species and thus the gold is interpreted to dissolve according to reactions of the form:

(A1)     

Spectra and coordination changes of transition metals in hydrothermal solutions: Implications for ore genesis

Optical absorption spectra of Fe(II), Co(II), Ni(II) and Cu(II) have been measured in aqueous solutions of up to 5 m NaCl at temperatures from 25°C to 300°C and at water-saturated vapor pressures. Ni and Co complexes exhibited a change from octahedral to tetrahedral coordination, this occurring at both higher temperatures and Cl^? concentrations. Similar transitions to lower coordination number are predicted for Cu and Fe but were not directly observed because of interference with water overtone bands. The coordination changes in response to ligand type and concentration, pressure and temperature. Formation of lower coordination complexes is attributed to the decreased dielectric constant of the solvent, the predominance of electrostatic forces and a decrease in the octahedral site preference energy at elevated temperatures. Our data suggest that lower coordination complexes with lower or neutral formal charge, will result in minerals having a higher solubility. The molecular properties and changes in coordination of these complexes are important in determining the transport and deposition of hydrothermal minerals.     

A spectrophotometric study of samarium (III) speciation in chloride solutions at elevated temperatures

The speciation of samarium (III) in chloride-bearing solutions was investigated spectrophotometrically at temperatures of 100-250 °C and a pressure of 100 bars. The simple hydrated ion, Sm³⁺, is predominant at ambient temperature, but chloride complexes are the dominant species at elevated temperatures. Cumulative formation constants for samarium chloride species were calculated for the following reactions:

     

The stability of hercynite at high pressures and temperatures

The stability of hercynite (FeAl₂O₄) has been investigated experimentally between 7 and 24 GPa and 900 and 1,700°C. Hercynite breaks down to its constituent oxides at 7–8.5 GPa and temperatures >1,000°C. The incorporation of a small magnetite component in the hercynite necessitated a small correction to fix the location of the endmember reaction: FeAl₂O₄  = Al₂O₃ + FeO in P–T space. After making this correction, the position of the phase boundary was used to evaluate thermodynamic data for hercynite. Our results support a relatively large S ₂₉₈^° for hercynite, on the order of 115 J mol⁻¹ K⁻¹. Experiments up to 24 GPa and 1,400°C failed to detect any high-pressure polymorph of FeAl₂O₄; only corundum + wüstite were detected. This behaviour contrasts with that observed for the analogous MgAl₂O₄ system where the constituent oxides recombine at high pressure to produce “post-spinel” phases with CaFe₂O₄-type and CaTi₂O₄-type structures.     

CO2 solubility and solubility mechanisms in silicate melts at high pressures

CO₂ solubility has a slight negative temperature dependence in olivine melilitite at 30 kb with 9% CO₂ dissolved at 1,450 °C, 8.5% at 1,550 °C and 8.3% at 1,650° C. CO₂ is dissolved as the carbonate molecule (CO ₃ ^2– ) only. Feldspar melts (albite-anorthite) dissolve much less CO₂ at 30 kb (around 2%) with a slight increase with increasing anorthite content. A CO₂ absorption peak in infrared spectra of albite-rich glasses diappears in favour of the CO ₃ ^2– peak with increasing anorthite content. It is inferred that CO₂ was present as CO ₃ ^2– in albite-rich melts also, but reverts to CO₂ during quenching because of bonding differences related to Ca²⁺ and Na⁺ in the melts.     

The ionization constants of aqueous MgCl2 at elevated temperatures and pressures—a revision

The ionization constants of aqueous MgCl₂ have been revised, in the light of recent measurements of electrical conductance, from the calculations reported previously by Frantz and Popp (1979).     

Dehydration Temperature of Serpentine at Elevated Temperatures and Pressures by Electrical Conductivity Method and Its Implications

Dehydration temperatures of serpentine were measured in the pressure range between 1.0GPa and 5.0GPa by using the electrical conductivity metod simultaneously at high temperatures and high pressures.The results show that with increasing pressure th dehydration temperature of antigorite increases slightly below 2.0GPa ,but drops markedly above2.0GPa .This strongly suggests that high pressure would favor the dehydration of serpentine minerals and the water released thereby would be an important source of fluids involved in magmatism in a subduction zone and mantle metasomatism,Meanwhile,the greatly enhanced electric conductivity in the presence of water may be one of the reasons underlying the occurrence of a high-conductivity zone in the lower crust.     

大兴安岭中北段古中公路钼矿床形成时代与矿床成因

古中公路钼矿床含矿岩体为碎裂硅化片麻状二长花岗岩体,蚀变呈带状分布,成矿过程可划分为钾长石+石英、石英+辉钼矿及石英+黄铁矿3个阶段。2件辉钼矿样品的Re-Os同位素模式年龄分别为(142.4±2.9)Ma和(142.3±2.0)Ma,表明矿床为早白垩世构造-岩浆活动的产物;辉钼矿中w(Re)含量分别为11.894×10~(-6)和11.584×10~(-6),暗示成矿物质主要为地壳来源,可能有地幔物质的参与。流体包裹体研究表明:主成矿阶段的石英中主要以气液两相包裹体为主,其次为含子晶三相包裹体,CO_2三相包裹体最少;不同类型包裹体的均一温度范围为196.3℃~450℃,盐度可分为0.18%~12.62%和45.33%~53.26%两个不同区间。因此,成矿流体为中高温、高盐度的Na Cl-H2O-CO2体系,在热液成矿阶段可能发生过流体沸腾作用,且是钼矿的主要形成机制。综合成矿地质条件、矿床地质特征及成矿流体特征认为,该矿床成因上属斑岩型,形成于区域伸展的构造环境。     

The elastic solid solution model for minerals at high pressures and temperatures

Non-ideality in mineral solid solutions affects their elastic and thermodynamic properties, their thermobaric stability, and the equilibrium phase relations in multiphase assemblages. At a given composition and state of order, non-ideality in minerals is typically modelled via excesses in Gibbs free energy which are either constant or linear with respect to pressure and temperature. This approach has been extremely successful when modelling near-ideal solutions. However, when the lattice parameters of the solution endmembers differ significantly, extrapolations of thermodynamic properties to high pressures using these models may result in significant errors. In this paper, I investigate the effect of parameterising solution models in terms of the Helmholtz free energy, treating volume (or lattice parameters) rather than pressure as an independent variable. This approach has been previously applied to models of order–disorder, but the implications for the thermodynamics and elasticity of solid solutions have not been fully explored. Solid solution models based on the Helmholtz free energy are intuitive at a microscopic level, as they automatically include the energetic contribution from elastic deformation of the endmember lattices. A chemical contribution must also be included in such models, which arises from atomic exchange within the solution. Derivations are provided for the thermodynamic properties of n-endmember solutions. Examples of the use of the elastic model are presented for the alkali halides, pyroxene, garnet, and bridgmanite solid solutions. Elastic theory provides insights into the microscopic origins of non-ideality in a range of solutions, and can make accurate predictions of excess enthalpies, entropies, and volumes as a function of volume and temperature. In solutions where experimental data are sparse or contradictory, the Helmholtz free energy approach can be used to assess the magnitude of excess properties and their variation as a function of pressure and temperature. The formulation is expected to be useful for geochemical and geophysical studies of the Earth and other planetary bodies.     

河南围山城金银成矿带铅同位素地球化学及矿床成因

桐柏山区的围山城金银成矿带包括破山特大型银矿、银洞坡大型金矿、银洞岭大型银多金属矿床及一些矿点,所有矿床赋存于上元古界歪头山组地层,并具有层控特征。矿石矿物的铅同位素组成为²⁰⁶Pb/²⁰⁴Pb=16.753~17.216,²⁰⁷Pb/²⁰⁴Pb=15.417~15.638,²⁰⁸Pb/²⁰⁴Pb=38.251~39.050;与歪头山组地层的铅同位素组成一致,而与桐柏地区的其他地层、岩体差别较大,表明成矿物质来自赋矿地层歪头山组。围山城成矿带应属于典型的层控造山型金银成矿系统,它形成于中生代扬子与华北板块的陆陆碰撞造山过程,碰撞造山期间的下插板片变质脱水诱发了矿带内流体成矿系统的发育,强烈的流体-岩石相互作用使歪头山组内的成矿物质被萃取、迁移、聚集到碳质绢云片岩层。