James G. Blencoe

Experimental determination of the partitioning behavior of rare earth and high field strength elements between pargasitic amphibole and natural silicate melts

Abstract The primary goal of this investigation was to derive a set of expressions that can be used to calculate the amphibole-melt partitioning behavior of the rare earth elements (REE) and the high field strength elements (HFSE) in natural systems. To supplement the existing data set on basaltic systems, we conducted experiments on systems where amphibole was in equilibrium with dacitic, tonalitic and low Si rhyolitic melts. These experiments, doped with La, Sm, Gd, Lu, Ta, Nb, Y, Zr, and Hf, were run at pressures of 2 and 5 kbar, temperatures between 900°C and 945°C, and oxidation conditions ranging from QFM-1 to NiNiO+1. The partitioning data obtained in this study were combined with published data to calculate two sets of expressions describing trace element partitioning. The first set models the partitioning of trace elements into amphibole using temperature, pressure and several compositional parameters, including the compositionally-compensated partition coefficients of Ti, Al, Caand SiO2, and the exchange of Fe and Mg between the crystal and the melt (DMg/DFe). The second set of expressions are slightly less precise, but require no specific knowledge of P, T, or fO2 and, for application to natural systems, can be constructed solely on the basis of information available from standard electron microprobe analyses. These expressions predict amphibole-melt partition coefficients for REE and HFSE within an internal precision of 14–40% (relative) for alkali basalt to low Si rhyolite, from 850°C to 1100°C, 2–20 kbar and oxygen fugacity from QFM-1 to NiNiO+1. Partition coefficients calculated from the expressions derived in this study were used to model the partial melting and fractional crystallization of a hypothetical amphibolite and hydrous melt, respectively. Fractionation and/or melting in amphibole-bearing systems produces a magma with a convex upward REE pattern, a characteristic common to many hornblende-bearing dacites. However, the removal or addition of an amphibole component cannot produce the strong HFSE depletion relative to the REE observed in many arc magmas.

The CO2-H2O system: III. A new experimental method for determining liquid-vapor equilibria at high subcritical temperatures

Abstract A highly precise and accurate vibrating U-tube technique was developed to determine the upper baric stabilities of liquid-vapor assemblages in the CO2-H2O system at high subcritical temperatures (∼275-360 °C). The first step is to create an isobaric-isothermal, physically isolated and chemically homogeneous sample of “high-pressure” CO2-H2O fluid of known composition. Fluid pressure (P) is then lowered slowly at constant temperature. Pressure readings and matching values for τ (the period of vibration of the U-tube) are recorded at 0.1 or 0.2 MPa intervals. When the fluid begins to separate into two phases (liquid + vapor), a distinct inflection is observed in the trend of P vs. τ. Performing such experiments for fluid compositions at 0.05 mole fraction CO2 (XCO2) intervals in the range 0.05 ≤ XCO2 ≤ 0.40 at 300 °C produced a complete high-P liquid-vapor boundary curve for the CO2-H2O system at that temperature. Agreement with corresponding curves determined in previous studies ranges from poor to excellent.

The paragonite-muscovite solvus: II. Numerical geothermometers for natural, quasibinary paragonite-muscovite pairs☆

Three parametric (nonthermodynamic) equations have been developed to calculate final equilibration temperatures for natural, quasibinary paragonite-muscovite (Pg-Ms) pairs (mole fraction margarite in Pg < 0.05, Si/formula unit ≤ 6.2 and/or Σ(Mg + Fe2+ + Fe3+) ≤ 0.35 for coexistent Ms). The first two equations are paragonite-based and muscovite-based, because they yield equilibration temperatures that depend on the Na-K compositions of K-saturated Pg and Na-saturated Ms, respectively. The third equation is closure-based, because it yields equilibration temperatures that vary with the width of the gap between the Na-K compositions of coexisting Pg and Ms (i.e., the degree of solvus closure). Equilibration temperatures can be estimated, using just one of the geothermometers, or by averaging the temperatures obtained from any combination of the three equations. Comparison of the three calculated temperatures yields information on the mutual consistencies of the Na-K compositions of coexisting Pg and Ms with respect to their utility in Pg-Ms solvus thermometry. Geothermometric inconsistencies in the Na-K compositions of quasibinary Pg-Ms pairs can be caused by (1) inaccuracies in determinations of the Na-K compositions of the coexisting micas, (2) H3O+ → K+ and vacancy → K+ substitution in muscovite and/or paragonite (3) crystallization and preservation of quasibinary Pg-Ms pairs with metastable Na-K compositions. Due to the effects of pressure on the stability relations of Pg-Ms pairs and the K/Na ratios of Nasaturated muscovites, practical applications of Pg-Ms solvus thermometry are restricted to quasibinary Pg-Ms pairs that equilibrated at pressures between approximately 2 and 8 kbar. Within this pressure range, the utility of Pg-Ms solvus thermometry is limited thermally to approximately 300 ≤ T ≤ 700°C by (1) the decomposition of K-saturated paragonite at temperatures between approximately 580 and 700°C, and (2) steepening of the solvus limbs at temperatures below 300°C. Graphical and calculated Pg-Ms solvi based on solvus data for synthetic, binary Pg-Ms micas are inconsistent with solvus data for natural, quasibinary Pg-Ms micas. Consequently, these solvi should not be used to estimate equilibration temperatures for natural, quasibinary Pg-Ms pairs.

A new experimental facility for investigating the formation and properties of gas hydrates under simulated seafloor conditions

A seafloor process simulator (SPS) has been developed for experimental investigations of the physical, geochemical, and microbiological processes affecting the formation and stability of methane and carbon dioxide hydrates at temperatures and pressures corresponding to ocean depths of 2 km. The SPS is a corrosion-resistant pressure vessel whose salient characteristics are: (i) an operating range suitable for study of methane and carbon dioxide hydrates; (ii) numerous access and observation ports, and (iii) a large (0.0722 m3) internal volume. Initial experiments have shown that the SPS can be used to produce large amounts of high-purity methane hydrate over a wide range of experimental conditions.

A vibrating-tube densimeter for fluids at high pressures and temperatures

A vibrating U-tube apparatus has been developed for determining the densities of pure fluids and fluid mixtures at 10-200 MPa and 323-773 K. Measured parameters areP,T, andr (period of vibration). Fluids are injected into the U-tube at constantP andT. Three or more reference fluids are used to calibrate the response of the instrument. Fluid mixtures are produced by pumping pure fluids into T-junctions on the upstream side of the U-tube using high accuracy syringe pumps. An automated syringe pump is used to maintainP at setpoint ±0.01 MPa.T is controlled to ±0.01 K using a closed-loop, electronic signal amplification/feedback system. For mixtures, a statistically significant number of measurements of r are obtained to account for the effects of small heterogeneities in fluid composition (generally <0.005X;). Typically, density data for 15 fluids can be obtained in a 6- to 8-h period. Considering all of the potential sources of error in the experimentation, conservative estimates of uncertainty are as follows:P, ±0.02 MPa;T, ±0.05 K;p (pure fluids), ±0.0005g.cm−3; andp (fluid mixtures), ±0.0005-0.0010g-cm−3.

The CO2-H2O system. II. calculated thermodynamic mixing properties for 400°C, 0–400 MPa

Abstract An excess molar volume (Vex)-explicit virial equation, and two empirical Vex expressions developed from experimentally determined densities, were used to calculate excess Gibbs free energies (Gex) and activity-composition (a-X) relations for CO2-H2O fluids at 400°C, 0–400 MPa. Excess Gibbs free energies are continuously positive and asymmetric toward H2O at all pressures up to 400 MPa, rising to peak values of approximately 1300, 1800, 2000 and 2100 J · mol−1 at 50, 100, 200 and 400 MPa, respectively. Calculated activities for H2O and CO2 vary correspondingly, increasing substantially from 0 to 100 MPa, moderately from 100 to 200 MPa, and slightly from 200 to 400 MPa. In addition, because Gex is asymmetric toward H2O, a-X relations for H2O are distinctly different from those for CO2. These results indicate that CO2-H2O fluids are strongly nonideal at 400°C and all pressures above ∼30 MPa, despite the fact that peak values for Vex decrease from ∼50 cm3 · mol−1 at 30 MPa to ∼1 cm3 · mol−1 at 200 MPa, and remain small to pressures at least as high as 500 MPa. Excess Gibbs free energies and a-X relations for CO2-H2O fluids at 400°C and pressures to 400 MPa calculated from modified Redlich-Kwong and Lee-Kesler equations of state generally suggest significantly smaller positive deviations from ideality.

The CO2–H2O system: IV. Empirical, isothermal equations for representing vapor-liquid equilibria at 110–350 °C, P ≤ 150 MPa

Abstract Empirical formulae are presented for calculating vapor-liquid equilibria (VLE) in the CO2-H2O system at 10 temperatures between 110 and 350 °C. At each temperature, separate functions are used to represent the bubble- and dew-point boundary curves that: originate at the saturation vapor pressure of water (Psat;H₂O) at XCO₂ = 0; diverge with increasing pressure up to ~P(XmaxCO₂) where ∂P/∂XCO₂ = +∞ along the dew-point curve; then converge with increasing pressure above P(XmaxCO₂). At temperatures below 265 °C and pressures > P(XmaxCO₂), the compositions of coexisting liquid and vapor [XCO₂L(V) and XCO₂V(L)] do not converge completely with increasing pressure due to the absence of critical behavior. Thus, relatively simple functions suffice to accurately represent VLE at those temperatures. In contrast, at T > 265 °C, XCO₂L(V) and XCO₂L(V) converge rapidly as P approaches Pc (the critical pressure in the CO2-H2O system at a given temperature between 265 and 374 °C and P ≤ 215 MPa). For those temperatures, therefore, more complex VLE formulae are required to achieve close representation of phase relations. For dew-point equations, this includes adding an exponential “correction term” to ensure that ∂P/∂XCO₂ = 0 at the critical points indicated by corresponding bubble-point functions. Stable liquid-vapor coexistence in mixed-volatile systems requires ƒLi = ƒVI (isofugacity conditions) for all “i” (volatile components) in the two fluid phases. Thus, the equations presented in this paper specify numerous P-T-X conditions where ƒLH₂O = ƒVH₂O and ƒLCO₂ = ƒVCO₂ in the CO2-H2O system. These results have important applications in the ongoing effort to develop a more rigorous thermodynamic model for CO2-H2O fluids at geologically relevant temperatures and pressures.

Volumetric properties of CO2CH4N2 fluids at 200°C and 1000 bars: A comparison of equations of state and experimental data☆

Abstract Predictions of molar volume, excess molar volume, and isochoric P-T trajectories from thirteen published equations of state are compared with one another and with preliminary volumetric data for CO2CH4N2 fluids at 200°C and 1000 bars. The equations of state investigated represent a wide variety of empirical and semi-empirical approaches to the modeling of fluids. The experimental data indicate that excess volumes of CO2CH4N2 mixtures are small (

Dry melting of high albite

Abstract The properties of albitic melts are central to thermodynamic models for synthetic and natural granitic liquids. We have analyzed published phase-equilibrium and thermodynamic data for the dry fusion of high albite to develop a more accurate equation for the Gibbs free energy of this reaction to 30 kbar and 1400 °C. Strict criteria for reaction reversal were used to evaluate the phase-equilibrium data, and the thermodynamic properties of solid and liquid albite were evaluated using the published uncertainties in the original measurements. Results suggest that neither available phase-equilibrium experiments nor thermodynamic data tightly constrain the location of the reaction. Experimental solidus temperatures at 1 atm range from 1100 to 1120 °C. High-pressure experiments were not reversed completely and may have been affected by several sources of error, but the apparent inconsistencies among the results of the various experimentalists are eliminated when only half-reversal data are considered. Uncertainties in thermodynamic data yield large variations in permissible reaction slopes. Disparities between experimental and calculated melting curves are, therefore, largely attributable to these difficulties, and there is no fundamental disagreement between the available phase-equilibrium and thermodynamic data for the dry melting of albite. Consequently, complex speciation models for albitic melts, based on the assumption that these discrepancies represent a real characteristic of the system, are unjustified at this time.

Precise Measurement of the Activity/Composition Relations of H2O-N2 and H2O-CO2 Fluids at 500°C, 500 Bars

Abstract The activity/composition relations of H2O-N2 and H2O-CO2 fluids have been measured at 500°C, 500 bars. The results are more accurate, and much more precise, than any currently available, especially for H2O-poor compositions. Samples were reacted at fixed water activities (0.062 ≤ aH2O ≤ 0.777), using Cu3N as the source of N2, and Ag-oxalate as the source of CO2. After each experiment the masses of water and gas in the samples were analyzed manometrically. Results depend on the value used for the hydrogen fugacity for pure H2O in equilibrium with the oxide buffer, but using a newly measured value for Ni-NiO and fitting the data to a two-parameter Margules equation yields: for H2O-N2 fluids: WG,H2O = 3455.0 J/mol, WG,N2 = 1990.1 J/mol; and for H2O-CO2 fluids: WG,H2O = 3882.5 J/mol, WG,CO2 = 4902.6 J/mol. As uncertainties are not orthogonal, standard errors cannot be given. The data suggest that H2O-CO2 fluids exhibit large positive deviations from ideality at 500°C, 500 bars, in marked contrast to values predicted by available equations of state. These results indicate that more accurate models for both H2O-N2 and H2O-CO2 fluids are needed.