John D. Frantz

Determination of the homogenization temperatures and densities of supercritical fluids in the system NaClKClCaCl2H2O using synthetic fluid inclusions

Synthetic fluid inclusions were experimentally produced by equilibrating small fractured prisms of quartz with aqueous solutions at temperatures from 300° to 700°C and pressures of 1000, 2000 and 3000 bar. Solution compositions included: 0.500, 2.000 and 4.500 molal NaCl; 0.500, 2.000 and 4.500 molal KCl; 0.50 and 2.00 molal CaCl2; and H2O. The homogenization temperatures of the synthetic fluid inclusions were analyzed by microthermometry. Plots of homogenization temperatures as a function of experimental temperature and pressure indicate that lines of constant homogenization temperature are linear and intersect the liquid-vapor curve at the homogenization temperature. For each of the four chemical systems, a relatively simple function was developed by which the homogenization temperature can be related to the temperature (°C) and pressure (bar) of inclusion formation and the composition (m) of the trapped fluid: P=A1+A2T A1=6.100·10−3 + (2.385·10−1−a1)Th−(2.855·10−3+a2)T22−(A3Th+a4T2h)m A2=a1+a2Th+9.888·10−6T2h+(A3+A4Th) m where m is the molality; Th is the homogenization temperature; and a1, a2, a3 and a4 are constants fit to the data sets of each of the four chemical systems. Ten-parameter polynomial regressions are given for the densities of the solutions on their liquid-vapor surfaces as functions of temperature and composition. These functions combined with the above equations permit calculation of the density as a function of temperature, pressure, solute and solute concentration in the supercritical region. A function is also given which permits calculation of isochores for fluids containing more than one solute. The results of these experiments compare favorably to previously published ones.

Raman spectroscopy of silicate melts at magmatic temperatures: Na2O1bSiO2, K2O1bSiO2 and Li2O1bSiO2 binary compositions in the temperature range 25–1475°C☆

High-quality Raman spectra of silicate glasses, supercooled liquids and liquids have been obtained in situ to temperatures of 1475°C. The success of the spectroscopic technique is fundamentally dependent on the ability to avoid spectra degradation caused by black-body radiation from the furnace. This can be accomplished by focussing the diameter of the exciting laser beam to ∼ 1 mm, and to control the depth of focus in the sample to 6–40-μm depth. The samples can be viewed visually through a microscope throughout the process, which ensures that the distance between the sources of blackbody radiation (furnace wall and bubbles in glass and liquid) and focus is optimized. With this technique, more than 100 spectra of glasses, supercooled melts and melts in the systems Li2O1bSiO2, Na21bSiO2 and K2O1bSiO2 have been recorded at 25–1475°C in the frequency range most sensitive to the overall anionic structure of amorphous silicates (800–1300 cm−1). The coexisting structural units generally are SiO32− (Q2), Si2O52− (O3) and SiO2 (Q4) in all glasses and melts in the temperature range investigated. The abundance of the structural units appear sensitive to temperature. The temperature dependence is qualitatively consistent with a shift to the right of the reaction: Si2O52−(2Q3)⇌SiO32−(Q2)+SiO2(Q4 with increasing temperature. From intensity variations with temperature of relevant Si1bO stretch bands, the free energy for this reaction probably is sensitive to both bulk melt NBO/Si and to the electronic properties of the alkali metal.

An optical cell for Raman spectroscopic studies of supercritical fluids and its application to the study of water to 500°C and 2000 bar

A high-temperature, high-pressure optical cell has been developed for the study of aqueous solutions by Raman spectroscopy. The disk-shaped cell has a sample volume of < 1 ml and utilizes diamond or sapphire windows set at 90° to one-another. Temperatures to 700°C and pressures to 4000 bar have been attained as measured using an internal thermocouple and a strain gauge. The apparatus was employed in the study of water to 500°C and 2000 bar with spectra of the OH stretching mode being collected at intervals of 50°C and 250 bar. A low-frequency shoulder between 3250 and 3300 cm−1 was found to persist to the maximum temperatures to at least 450°C at pressures above those of the liquid vapor curve; its intensity decreased with increasing temperature and decreasing pressure. The frequency of the maximum intensity of the spectral envelope increased dramatically with temperature to above 250°C and was found to be linear with respect to density and independent of temperature at constant density above 250°C. Similar behavior is seen for the viscosity, dielectric constant and the limiting equivalent conductances of pure water. The data indicate the presence of intermolecular hydrogen bonding to temperatures to well above 300°C at densities above the critical density.

The compositional limits of fluid immiscibility in the system H2ONaClCO2 as determined with the use of synthetic fluid inclusions in conjunction with mass spectrometry

Abstract The compositional limits of fluid immiscibility in the system NaClH2OCO2 were investigated from 500° to 700°C at pressures of 1, 2 and 3 kbar. Synthetic fluid inclusions formed in quartz prisms were equilibrated with high-temperature, high-pressure fluids in hydrothermal pressure vessels. The inclusions were analyzed optically, by mass spectrometry, and by microthermometry. The fields of immiscibility were defined as a function of temperature and pressure. Mass spectrometry was used to validate the existence of the immiscible fluids and to determine the CO 2 H 2 O ratios of the two coexisting phases. Microthermometry, i.e. the measurement of the melting temperature of solid sodium chloride, was used in conjunction with the results of the mass spectrometry measurements to define the tie-lines within the two-phase fields. Isochores and the location of a portion of the isopleth were determined for composition NaCl8.9H2O76.1CO215.0.

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.

Mineral-solution equilibria—I. An experimental study of complexing and thermodynamic properties of aqueous MgCl2 in the system MgO-SiO2-H2O-HCl

Abstract Speciation of aqueous magnesium in the system MgO-SiO2-H2O-HCl in supercritical aqueous fluids has been investigated using standard rapid-quench hydrothermal techniques and a modification of the Ag + AgCl buffer method ( Frantz and Eugster , 1973. Am. J. Sci.267, 268–286). A concentric double-capsule charge was utilized. The outer gold capsule contained the assemblage talc + quartz + Ag + AgCl + H2O-MgCl2 fluid; the inner platinum capsule, Ag + AgCl + H2O-HCl fluid. During the experiments, ƒ H 2 and thus ƒ HCl equilibrated between the two capsules. After quenching, measurement of the chloride concentration in the fluid in the inner capsule and total magnesium in the fluid in the outer capsule defines the concentrations of HCl and Mg that coexist with talc + quartz in the outer capsule. Changes in the measured molality of HCl as a function of the total magnesium concentration at constant P and T were used to identify the predominant species of magnesium in the hydrothermal fluid. Experimental results showed that at 2000 bar, MgCl°2 is the predominant species above 550°C and Mg2+, below 400°C. Data at intermediate temperatures when combined with the dissociation constant for HCl were used to obtain the dissociation constant for MgCl°2. The results of these experiments were combined with results from experiments using Ag + AgCl in conjunction with the oxygen buffer, hematite-magnetite, to obtain the equilibrium constant for the reaction 1 3 Talc + 2HC1° H 2 O MgCl° 2 + 4 3 Quartz + 4 3 H 2 O from which the difference in Gibbs free energy of MgCl°2 and HC1° was obtained as a function of temperature at 1000, 1500 and 2000 bar pressure, Solubility constants for brucite. forsterite, chrysotile, and talc were calculated.

Mineral-solution equilibria—IV. Solubilities and the thermodynamic properties of FeCl20 in the system Fe2O3-H2-H2O-HCl

Abstract The solubility of hematite in chloride-bearing hydrothermal fluids was determined in the temperature range 400–600°C and at 1000 and 2000 bars using double-capsule, rapid-quench hydrothermal techniques and a modification of the Ag + AgCl buffer method ( Frantz and Popp , 1979). The changes in the molalities of associated hydrogen chloride ( m HC l 0 ) as a function of the molality of total iron in the fluid at constant temperature and pressure were used to identify the predominant species of iron in the hydrothermal fluid. The molality of associated HCl varied from 0.01 to 0.15. Associated FeCl20 was found to be the most abundant species in equilibrium with hematite. Determination of Cl/Fe in the fluid in equilibrium with hematite yields values approximately equal to 2.0 suggesting that ferrous iron is the dominant oxidation state. The equilibrium constant for the reaction Fe2O3 + 4HCl0 + H2 = 2FeCl20 + 3H2O was calculated and used to estimate the difference in Gibbs free energy between FeCl20 and HCl0 in the temperature range 400–600°C at 1000 and 2000 bars pressure.

Raman spectra of potassium carbonate and bicarbonate aqueous fluids at elevated temperatures and pressures: comparison with theoretical simulations

Abstract One-molal solutions of potassium carbonate and potassium bicarbonate were investigated by Raman spectroscopy from 22° to 550°C and from 1000 to 2000 bar. Experiments were performed in a special hydrothermal pressure vessel fitted with conical diamond windows. Frequencies and half-widths of the vibrational modes were measured for both carbonate and bicarbonate as a function of temperature and pressure. In the case of potassium carbonate solutions, dissolved bicarbonate appeared with its concentration dramatically increasing with increasing temperature and decreasing density. The opposite behavior occurred in the case of potassium bicarbonate: the relative concentration of dissolved carbonate increased relative to total bicarbonate. In addition, at temperatures above 300°C, dissolved aqueous carbon dioxide appeared with its concentration continuing to increase with increasing temperature and decreasing density. Simulations using theoretically-predicted mass action constants were used to compute changes in solution speciation for these solutions as they were subjected to increased temperature and pressure. Results resulting from these predictions matched those discovered in the spectral measurements.

Mineral-solution equilibria—V. Solubilities of rock-forming minerals in supercritical fluids

The solubility constants of sixty-nine rock-forming minerals have been computed for temperatures between 400 and 600°C at 1000 and 2000 bar pressure using the free-energy data for aqueous solutes presented in Parts I through IV of this series combined with the thermodynamic properties of minerals from Helgesonet al. (1978). An example describing solution compositions in equilibrium with a spilite is discussed. A computer program for calculating solution compositions in equilibrium with mineral assemblages is included as an appendix.

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.