Yi-Gang Zhang

Enstatite-forsterite-water equilibria at elevated temperatures and pressures

Abstract The compositions of aqueous fluids in equilibrium with enstatite + forsterite were investigated at temperatures from 900 to 1200 °C and pressures from 1.0 to 2.0 GPa. Experiments, performed in a piston-cylinder apparatus, involved the location of phase boundaries between the stability fields of enstatite and enstatite + forsterite, and enstatite + forsterite and forsterite. The intersection of these two phase-boundaries near the H2O apex was used to define the fluid composition. The results indicated a systematic increase in the concentration of silica in the fluid phase with increasing temperature. The experiments indicated that the concentration of dissolved MgO was below 0.3 mol% and not resolvable using our techniques. This finding was corroborated by microprobe analyses of quench precipitates from the fluid phase, which gave on average 0.2 mol% MgO. Because of the low MgO concentrations, the mean values of the intercepts with SiO2-H2O binary of the fitted lines representing, respectively, the phase boundaries between the enstatite and enstatite + forsterite and between the enstatite + forsterite and forsterite stability fields were taken to represent the concentrations of dissolved silica at the various temperatures and pressures of the present study. The concentrations increased from 0.6 mol% at 900 °C to 3.9 mol% at 1200 °C at 1.0 GPa. The pressure effect from 1.0 to 2.0 GPa at 1000 °C appeared to be minor and not resolvable using our techniques. At 1.0 GPa, the base 10 logarithm of the molal concentration of dissolved silica in equilibrium with enstatite + forsterite was obtained combining data from the present study with those from Nakamura and Kushiro (1978) and Manning and Boettcher (1994): logmSiO2(aq)En-Fo = 6.869 - 1.335 × 104 / T(K) + 5.544 × 106 / T(K)2. Comparison with studies of the solubility of quartz (Manning 1994) indicated that thermodynamic properties of aqueous silica derived from silica-saturated systems may not be applicable to calculations in silica-deficient systems at high pressure.

Experimental determination of the compositional limits of immiscibility in the system CaCl2H2OCO2 at high temperatures and pressures using synthetic fluid inclusions

Abstract Using the synthetic fluid-inclusion method, the temperature, pressure, and compositional limits of fluid immiscibility in the binary system CaCl 2 H 2 O have been determined at 600° and 700°C at pressures between 1 and 2 kbar. The limits of the immiscible regions were determined by observation of two different inclusion types in the samples and by measurements of the depression of ice melting temperatures. The region of immiscibility in this system extends to much higher pressures than that of the NaClH 2 O system with similar compositional ranges. A similar study was done on the CaCl 2 H 2 OCO 2 system at 500°, 600° and 700°C at pressures of 1, 1.5, 2 and 3 kbar. In the ternary system, the presence of two different inclusion types and the measurement of clathrate melting temperatures were used to delineate the compositional regions of immiscibility. The presence of CO 2 extends the regions of immiscibility shown in the binary system study to much higher pressures. Tie-lines in the two-phase regions were determined by measuring the homogenization temperatures of CO 2 vapor and liquid phases and the volume ratios of inclusion bubble to total inclusion in vapor-rich type inclusions. The locations of these tie-lines indicate that the CaCl 2 is heavily partitioned towards the more liquid-rich phase. The experimental results show that with even relatively small amounts of a divalent salt such as CaCl 2 , immiscibility can exist to very high temperatures and pressures. From these results immiscibility should be a very common phenomenon in geologic processes ranging from sedimentary to quite high-grade metamorphic environments, and that it should have a very important effect on reactions between hydrothermal fluids and rocks and the resulting mineral assemblages.

Can the dodecahedral water cluster naturally form in methane aqueous solutions? A molecular dynamics study on the hydrate nucleation mechanisms

By performing a large scale of molecular dynamics simulations, we analyze 60 x 10(6) hydration shells of methane to examine whether the dodecahedral water cluster (DWC) can naturally form in methane aqueous solutions--a fundamental question relevant to the nucleation mechanisms of methane hydrate. The analyzing method is based on identifying the incomplete cages (ICs) from the hydration shells and quantifying their cagelike degrees (zetaC=0-1). Here, the zetaC is calculated according to the H-bond topological network of IC and reflects how the IC resembles the complete polyhedral cage. In this study, we obtain the zetaC distributions of ICs in methane solutions and find the occurrence probabilities of ICs reduce with zetaC very rapidly. The ICs with zetaC>or=0.65 are studied, which can be regarded as the acceptable cagelike structures in appearance. Both increasing the methane concentration and lowering the temperature can increase their occurrence probabilities through slowing down the water molecules. Their shapes, cage-maker numbers, and average radii are also discussed. About 13-14 of these ICs are face saturated, meaning that every edges are shared by two faces. The face-saturated ICs have the potential to act as precursors of hydrate nucleus because they can prevent the encaged methane from directly contacting other dissolved methane when an event of methane aggregation occurs. The complete cages, i.e., the ICs with zetaC=1, form only in the solutions with high methane concentration, and their occurrence probabilities are about 10(-6). Most of their shapes are different from the known hydrate cages, but we indeed observe a standard 5(12)6(2) hydrate cage. We do not find the expected DWC, and its occurrence probability is estimated to be far less than 10(-7). Additionally, the IC analysis proposed in this work is also very useful in other studies not only on the formation, dissociation, and structural transition of hydrates but also on the hydrophobic hydration of apolar solutes.

Analysis of fluid inclusions by X-ray fluorescence using synchrotron radiation☆

Abstract The compositions of fluid inclusions in natural minerals provide a valuable source of information from which the conditions of formation of rock systems can be interpreted. The concentrations of trace elements and ionic complexes in the fluids within individual inclusions are especially important. Analysis by X-ray fluorescence using an X-ray microprobe combined with synchroton radiation has proved to be a useful tool for this purpose. Using multilayer mirrors in conjunction with the Kirkpatrick-Baez mirror geometry, a 10-keV synchrotron X-ray beam can be focussed to a spot 5–10 μm in diameter within an individual inclusion. The resulting fluorescence can then be used to determine the numbers of moles and concentrations of the elements contained in the fluid. Due to absorption, the intensity of the fluorescent radiation varies with the element analyzed, the composition of the host mineral, and the depth of the inclusion. To test the method, quartz crystals containing synthetic inclusions with known fluid compositions were analyzed. X-ray scans were made across inclusions containing calcium chloride, manganese chloride and zinc chloride. Accurate determination of the number of picograms contained within an inclusion can be made using thin glass films of known composition as standards. Concentrations, however, are dependent on knowing the volume of the inclusions and are thus less accurate.

Lifetimes of cagelike water clusters immersed in bulk liquid water: A molecular dynamics study on gas hydrate nucleation mechanisms

Molecular dynamics simulations were performed to observe the evolution of cagelike water clusters immersed in bulk liquid water at 250 and 230 K. Totally, we considered four types of clusters--dodecahedral (5(12)) and tetrakaidecahedral (5(12)6(2)) cagelike water clusters filled with or without a methane molecule, respectively. The lifetimes of these clusters were calculated according to their Lindemann index (delta) using the criterion of delta> or =0.07. The lifetimes of the clusters at 230 K are longer than that at 250 K, and their ratios are the same as the ratio of structure relaxation times of bulk water at these temperatures. For both the filled and empty clusters, the lifetimes of 5(12)6(2) cagelike clusters are similar to that of 5(12) cagelike clusters. Although the methane molecules indeed make the filled cagelike water clusters live longer than the empty ones, the empty cagelike water clusters still have the chance of being long lived. These observations support the cluster nucleation hypothesis for the formation mechanisms of gas hydrates.

Equilibrium molecular dynamics calculation of the bulk viscosity of liquid water

Twenty independent equilibrium molecular dynamics simulations were performed in NVE ensemble to calculate the bulk viscosity of water at a temperature of 303 K and a density of 0.999 gcm−3. The energy of each simulation with a production time of 200ps was conserved within 1 part in 104. By stopping the velocity-scaling procedure at a proper step, the energies of independent simulations were specified precisely. This caused the simulations of different start configurations to sample the same NVE ensemble. The shear viscosity of SPC/E water obtained in the present study was 6.5±0.4 × 10−4 Pas, which is in close agreement with a previous calculation in the NVT ensemble (Balasubramanian, S., Mundy, C. J., and Klein, M. L., 1996, J. clzern. Phys., 105, 11 190). The bulk viscosity was 15.5 ± 1.6 × 10−4 Pas, which is 27% smaller than the experimental value. Thus, like its behaviour in predicting the shear viscosity, the SPC/E model also underestimates the bulk viscosity of real water.

Carbon and other light element contents in the Earth’s core based on first-principles molecular dynamics

Carbon (C) is one of the candidate light elements proposed to account for the density deficit of the Earth’s core. In addition, C significantly affects siderophile and chalcophile element partitioning between metal and silicate and thus the distribution of these elements in the Earth’s core and mantle. Derivation of the accretion and core–mantle segregation history of the Earth requires, therefore, an accurate knowledge of the C abundance in the Earth’s core. Previous estimates of the C content of the core differ by a factor of ∼20 due to differences in assumptions and methods, and because the metal–silicate partition coefficient of C was previously unknown. Here we use two-phase first-principles molecular dynamics to derive this partition coefficient of C between liquid iron and silicate melt. We calculate a value of 9 ± 3 at 3,200 K and 40 GPa. Using this partition coefficient and the most recent estimates of bulk Earth or mantle C contents, we infer that the Earth’s core contains 0.1–0.7 wt% of C. Carbon thus plays a moderate role in the density deficit of the core and in the distribution of siderophile and chalcophile elements during core–mantle segregation processes. The partition coefficients of nitrogen (N), hydrogen, helium, phosphorus, magnesium, oxygen, and silicon are also inferred and found to be in close agreement with experiments and other geochemical constraints. Contents of these elements in the core derived from applying these partition coefficients match those derived by using the cosmochemical volatility curve and geochemical mass balance arguments. N is an exception, indicating its retention in a mantle phase instead of in the core.

Hydrothermal reactions involving equilibrium between minerals and mixed volatiles: 2. Investigations of fluid properties in the CO2CH4H2O system using synthetic fluid inclusions

Abstract The properties of fluids in the CO 2 CH 4 H 2 O system including CO 2 H 2 O and CH 4 H 2 O binary and CO 2 CH 4 H 2 O ternary mixtures with total gas compositions varying from 5.5 to 16.5 mole% were studied at temperatures between 400° and 600°C and pressures between 1000 and 3000 bar. Gas-loading techniques described in Part 1 combined with synthetic fluid inclusion methods were utilized to produce fluid inclusions of known composition. Densities, isochores, liquid-vapor curves and clathrate melting relations were determined for the binary and ternary compositions using measurements of the melting temperature of solid carbon dioxide, the homogenization temperature of carbon dioxide, the clathrate melting temperature and the homogenization temperature of the bubble. The liquid-vapor curves for the CH 4 H 2 O binary were found to be in excellent agreement with previously published results of H. Welsch while significant differences exist for the CO 2 H 2 O binary when compared to those of S. Takenouchi and G.C. Kennedy. Isochores resulting from this study vary significantly from theoretical calculations particularly in the case of low CO 2 concentrations in the CO 2 H 2 O binary.

Hydrothermal reactions involving equilibrium between minerals and mixed volatiles: 1. Techniques for experimentally loading and analyzing gases and their application to synthetic fluid inclusions

Abstract One of the principal difficulties in investigating mixed-volatile reactions under hydrothermal conditions has been in loading the experimental charges with gas mixtures of known composition. A new technique has been developed based on the fact that solid compounds with pore structures adsorb gases at low temperatures. A gas pipetting apparatus was constructed whereby a known quantity of a gas or gas mixture can be drawn into the experimental capsule by placing it in a low-temperature bath. A solid, such as a zeolite or a gel, is included in the experimental charge. Due to the adsorption of gas by these materials, the resulting vapor pressures are normally

Finite-size effect at both high and low temperatures in molecular dynamics calculations of the self-diffusion coefficient and viscosity of liquid silica

Molecular dynamics simulations are performed at 6543 and 3310 K to investigate how the viscosity and self-diffusion coefficient scale with system size in the liquid BKS silica system. We find that at high temperature the finite-size effect on shear viscosity is negligible. However, the size effect on the diffusion coefficient still exists and scales linearly with 1/N1/3, where N is the total number of particles in the system. At low temperature, the size effect on the viscosity becomes stronger than that on diffusion, and the logarithm of the viscosity and the diffusion coefficient scale linearly with 1/N. These results are consistent with previous theoretical developments, and demonstrate that the finite-size effect should be considered in both high- and low-temperature molecular dynamics simulations of liquid silica.