Jacques Roux

Water and the viscosity of andesite melts

The viscosity of a synthetic andesite-like melt has been measured between 1010 and 1014 P for water contents in the range 0–3.5 wt%. The very slow kinetics of water exsolution over this viscosity range allowed the measurements to be made at 1 bar with a high precision. After a steep viscosity decrease of > 5 orders of magnitude for 1 wt% H2O, an additional 2.5 wt% H2O causes a further viscosity decrease of only 2 orders of magnitude. These viscosity decreases are qualitatively similar to those observed previously for more silica-rich compositions. The new data join smoothly with available high-temperature measurements made at high pressures on water-bearing andesite melts. Because the intrinsic effects of pressure are as small for water-bearing as for water-free samples, the depressing effect of water on the viscosity of natural andesite melts can be estimated. Changes in water speciation as a function of either temperature or pressure do not seem to have marked effects on the viscosity. Although quantitative applications are not yet possible, the configurational entropy theory accounts qualitatively for these features.

Experimental study of arsenic speciation in vapor phase to 500°C: implications for As transport and fractionation in low-density crustal fluids and volcanic gases

Abstract The stoichiometry and stability of arsenic gaseous complexes were determined in the system As-H2O ± NaCl ± HCl ± H2S at temperatures up to 500°C and pressures up to 600 bar, from both measurements of As(III) and As(V) vapor–liquid and vapor–solid partitioning, and X-ray absorption fine structure (XAFS) spectroscopic study of As(III)-bearing aqueous fluids. Vapor–aqueous solution partitioning for As(III) was measured from 250 to 450°C at the saturated vapor pressure of the system (Psat) with a special titanium reactor that allows in situ sampling of the vapor phase. The values of partition coefficients for arsenious acid (H3AsO3) between an aqueous solution (pure H2O) and its saturated vapor (K = mAsvapor /mAsliquid) were found to be independent of As(III) solution concentrations (up to ∼1 to 2 mol As/kg) and equal to 0.012 ± 0.003, 0.063 ± 0.023, and 0.145 ± 0.020 at 250, 300, and 350°C, respectively. These results are interpreted by the formation, in the vapor phase, of As(OH)3(gas), similar to the aqueous As hydroxide complex dominant in the liquid phase. Arsenic chloride or sulfide gaseous complexes were found to be negligible in the presence of HCl or H2S (up to ∼0.5 mol/kg of vapor). XAFS spectroscopic measurements carried out on As(III)-H2O (±NaCl) solutions up to 500°C demonstrate that the As(OH)3 complex dominates As speciation both in dense H2O-NaCl fluids and low-density supercritical vapor. Vapor–liquid partition coefficients for As(III) measured in the H2O-NaCl system up to 450°C are consistent with the As speciation derived from these spectroscopic measurements and can be described by a simple relationship as a function of the vapor-to-liquid density ratio and temperature. Arsenic(III) partitioning between vapor and As-concentrated solutions (>2 mol As/kg) or As2O3 solid is consistent with the formation, in the vapor phase, of both As4O6 and As(OH)3. Arsenic(V) (arsenic acid, H3AsO4) vapor–liquid partitioning at 350°C for dilute aqueous solution was interpreted by the formation of AsO(OH)3 in the vapor phase. The results obtained were combined with the corresponding properties for the aqueous As(III) hydroxide species to generate As(OH)3(gas) thermodynamic parameters. Equilibrium calculations carried out by using these data indicate that As(OH)3(gas) is by far the most dominant As complex in both volcanic gases and boiling hydrothermal systems. This species is likely to be responsible for the preferential partition of arsenic into the vapor phase as observed in fluid inclusions from high-temperature (400 to 700°C) Au-Cu (-Sn, -W) magmatic-hydrothermal ore deposits. The results of this study imply that hydrolysis and hydration could be also important for other metals and metalloids in the H2O-vapor phase. These processes should be taken into account to accurately model element fractionation and chemical equilibria during magma degassing and fluid boiling.

Stability and solubility of arsenopyrite, FeAsS, in crustal fluids.

The stability and solubility of natural arsenopyrite (FeAsS) in pure water and moderately acid to slightly basic aqueous solutions buffered or not with H2 and/or H2S were studied at temperatures from 300 to 450°C and pressures from 100 to 1000 bar. The solubilities of FeAsS in pure water and dilute HCl/NaOH solutions without buffering are consistent with the formation of the As(OH)30(aq) species and precipitation of magnetite. At more acid pH (pH ≤2), arsenopyrite dissolves either stoichiometrically or with formation of the As-FeAsS assemblage. In H2S-rich and H2-rich aqueous solutions, arsenopyrite dissolution results in the formation of pyrrhotite (±pyrite) and iron arsenide(s), respectively, which form stable assemblages with arsenopyrite. Arsenic concentrations measured in equilibrium with FeAsS in slightly acid to neutral aqueous solutions with H2 and H2S fugacities buffered by the pyrite-pyrrhotite-magnetite assemblage are 0.0006 ± 0.0002, 0.0055 ± 0.0010, 0.07 ± 0.01, and 0.32 ± 0.03 mol/kg H2O at 300°C/400 bar, 350°C/500 bar, 400°C/500 bar, and 450°C/500 bar, respectively. These values were combined with the available thermodynamic data on As(OH)30(aq) (Pokrovski et al., 1996) to derive the Gibbs free energy of FeAsS at each corresponding temperature and pressure. Extrapolation of these values to 25°C and 1 bar, using the available heat capacity and entropy data for FeAsS (Pashinkin et al., 1989), yields a value of −141.6 ± 6.0 kJ/mol for the standard Gibbs free energy of formation of arsenopyrite. This value implies a higher stability of FeAsS in hydrothermal environments than was widely assumed. Calculations carried out using the new thermodynamic properties of FeAsS demonstrate that this mineral controls As transport and deposition by high-temperature (>not, vert, similar300°C) crustal fluids during the formation of magmatic-hydrothermal Sn-W-Cu-(Au) deposits. The equilibrium between As-bearing pyrite and the fluid is likely to account for the As concentrations measured in modern high- and moderate-temperature (150 ≤ T ≤ 350°C) hydrothermal systems. Calculations indicate that the local dissolution of arsenopyrite creates more reducing conditions than in the bulk fluid, which is likely to be an effective mechanism for precipitating gold from hydrothermal solutions. This could be a possible explanation for the gold-arsenopyrite association commonly observed in many hydrothermal gold deposits.

The influence of H 2 O on the viscosity of a haplogranitic melt

Abstract The viscosity η of dry and hydrous haplogranitic melts (anhydrous normative composition: QZ28Ab34Or38) has been determined between 3 and 10 kbar and 800 and 1400 °C using the falling-sphere method. The H2O content of the melt ranged from 0.03 to 8.21 wt%. Experiments were performed in internally heated pressure vessels (T = 900-1400 °C)and cold-seal pressure vessels (T = 800 °C). The viscosity decreases with increasing H2O content of the melt. The strongest decrease is observed at low H2O concentrations. The effect of H2O is smaller at high H2O concentrations in the melt, with an almost linear behavior between log η and H2O content expressed as weight percent H2O (decrease of 0.26 log units per weight percent H2O for H2O contents ≥4 wt% H2O). No pressure effect could be observed between 3 and 10 kbar at 900 °C for a melt containing 5.90 wt% H2O. In the investigated range the activation energy of the viscous flow decreases from 195 to 133 kJ/mol for melts with 1.05 to 8.21 wt% H2O. The effect of T and H2O content of the melt on viscosity can be calculated with a precision of ±2 log units with the use of the following expression: log η= -1.57 + [23.398 - 13.197(cH₂O)0.11] × 103 (1/T). Viscosities calculated using the model of Shaw (1972) show that, for the investigated composition, the model underestimates the influence of H2O for low H2O concentrations (≤4 wt% H2O, difference up to two orders of magnitude at 800 °C) and overestimates slightly the influence of H2O for high H2O concentrations (≥5 wt% H2O). In comparison with the model of Persikov et al. (1990), which takes into account the OH- and molecular H2O proportions, the experimental data at 800°C are in good agreement with the calculated viscosities (deviation ≤1 log unit) assuming that the proportions of OH- groups and molecular H2O are those found in an in situ spectroscopic investigation of the melt. However, at higher temperatures (1000-1300 °C)the viscosity is overestimated for the OH- and H2O proportions recalculated for the appropriate temperatures.

Environments around Al, Si, and Ca in aluminate and aluminosilicate melts by X-ray absorption spectroscopy at high temperature

Abstract Structural data on silicate, aluminate, and aluminosilicate melts are difficult to measure and understand at high temperature. X-ray absorption spectroscopy (XAS) performed in situ at high temperature has been used to probe the local environment of low-Z elements (Al, Si, and Ca). For fully tetrahedral network glasses, CaAl2Si2O8 (anorthite) and CaAl2O4, the modifications in the Al K-edge spectra with increasing temperature can be attributed to a structural rearrangement of the network or to an increase of fivefold-coordinated Al. For the Ca3Al2O6 composition, where Al is localized in a depolymerized tetrahedral site associated with non-bridging O atoms, XAS spectra at the Al K-edge are barely affected by temperature. Depending on the composition, Ca K-edge spectra investigated in these experiments allow us to follow changes in the distortion of the Ca sites in melts at high temperature. The structural modifications at both short and intermediate range upon melting are well shown by these XAS measurements.

The effects of silica and water on the viscosity of hydrous quartzofeldspathic melts

Abstract The viscosities of hydrous melts (0.65 to 2.8 wt% H2O) with quartzofeldspathic compositions corresponding to Ab, Ab74Qz26, and Ab48Qz52 (mole proportions calculated on the basis of eight oxygen atoms; Ab = NaAlSi3O8, Qz = Si4O8) have been determined between 980 and 1375 °C at pressures between 190 and 360 MPa using the falling sphere technique. The use of large bubble-free hydrous glass cylinders (placed in internally heated pressure vessels) previously prepared and already containing markers and platinum spheres allows falling distances up to several centimeters to be measured with a precision of ± 50 to 200 μm. This results in a precision of ± 15% relative or less for most viscosity data (± 10% relative or less if the temperature is known within ± 5 °C). For a water content of 2.8 wt% H2O, viscosity increases with increasing Qz content. In the investigated viscosity range, no significant deviation from Arrhenian behavior is observed and the activation energy of viscous flow increases slightly with decreasing water content of the melt (for Ab). Combining the experimental data obtained in this study with data for a haplogranitic composition investigated previously by Schulze et al. (1996) shows that the viscosities, and hence, the activation energies of viscous flow are similar for compositions with the same atom ratio (Si + Al)/(H + Na + K) (SA/HNK). Thus, melt viscosity is constant if Al, charge balanced by Na or K, is exchanged with Si + H (H incorporated as OH or H2O). The viscosities (in dPa·s) of all investigated hydrous haplogranite compositions with water contents ranging between 0.7 and 8.2 wt% H2O can be calculated to better than ±0.15 log units using the expression: logη = - 1.8 + [940 + 5598·(SA/HNK)0.3774]·1/T where T is expressed in Kelvin and varies from 1073 to 1650 K.

Structural and electronic evolution of the As(OH)3 molecule in high temperature aqueous solutions: An x-ray absorption investigation

The geometrical and electronic structure of the arsenious acid molecule As(OH)(3) in aqueous solutions has been investigated by x-ray absorption spectroscopy (XAS) within extended x-ray absorption spectroscopy (EXAFS) and x-ray absorption near edge structure (XANES), using realistic first-principle calculations in the latter case. This investigation was performed on aqueous solutions of arsenious acid from ambient to supercritical conditions (P = 250 and 600 bars, T <or= 500 degrees C) using a new optical cell. The analysis of the XAS spectra is consistent with (1) a constant As-O distance, (2) an opening of the O-As-O angles within the C(3V) pyramidal structure in the range 30-200 degrees C. This structural evolution comes along with a small decrease of the partial charges of the atoms in the As(OH)(3) molecule. The explanation invoked for both structural and electronic modifications observed is the weakening of the interactions, through hydrogen bonds, between the As(OH)(3) complex and water molecules. This is a fingerprint of the similar weakening of hydrogen bonding interactions in the solvent itself.

Experimental and theoretical study of (Fe2+, Mg)(Al, Fe3+)2O4 spinels: Activity-composition relationships, miscibility gaps, vacancy contents

Abstract The compositions of (Fe2+, Mg)(Al, Fe3+)2O4 spinels equilibrated with a l M (Fe2+, Mg)Cl2 aqueous solution at 800°C, 4 kbars were determined. General considerations of reciprocal systems allow derivation of the exchange isotherm between a chloride aqueous solution and (Mg, Fe2+)Al2O4 spinels. They enable calculation of ΔG of the reaction: FeCl2 + MgAl2O4 = MgCl2 + FeAl2O4ΔG = 2.9 kcal at 800°C, 4 kbars and provide the activity-composition relationships for the binary join FeAl2O4-MgAl2O4, which shows a substantial positive deviation from ideality. Some tie-lines between coexisting aluminous and ferric spinels were also obtained in the (Fe2+, Mg)(Al, Fe3+)2O4 system. These experimental data are modeled by a Gibbs free energy formulation of the spinel solid solution (Lehmann and Roux, 1984), where the corrective function g2, necessary to reproduce the deviations from ideality, is artificially split into two parts: 1. (1) A homogeneous second degree polynomial in the composition variables, containing only the terms specific to the reciprocal nature of the system, whose coefficients are deduced from ΔG of the exchange reaction: MgAl2O4 + FeFe2O4 = MgFe2O4 + FeAl2O4ΔG = 4.5 kcal at 800°C, 4 kbars 2. (2) A homogeneous second degree polynomial in the site occupancy fractions, to model the non-ideal behavior of the (Fe2+, Mg)Al2O4 and (Fe2+, Mg)Fe2O4 spinels and the miscibility gap along the Fe(Al, Fe3+)2O4 join. A model of reciprocal spinel solution involving defect end-members is used to estimate the vacancy contents of the spinels in equilibrium with sesquioxides. In this case, the corrective function necessary to take into account the reciprocal nature of the system is no longer a second degree polynomial, but a rational fraction.

Hydrogen isotope fractionation in the system H2O-liquid NaAlSi 3O8: new data and comments on D/H fractionation in hydrothermal experiments

Measurements in the system H 2 O-NaAlSi 3 O 8 at P = 2 kbar and T = 870 and 1250°C indicate that the vapour is enriched in D relative to the silicate melt by 8 to 25‰. The large error margins of the data are due to the important changes in the isotopic compositions of the charges that are caused by the kinetic fractionations associated with hydrogen diffusion through platinum capsules in high-temperature, high- pressure experiments. A new interpretation of theseeffects is presented, and it is concluded that the equilibrium hydrogen fractionation should be at least 25‰ between 870 and 1250°C at 2-kbar pressure.

Magic angle spinning NMR observation of sodium site exchange in nepheline at 500° C

Dynamics in minerals at time scales from seconds to microseconds are important in understanding mechanisms of displacive phase transitions, diffusion, and conductivity. High resolution, magic-angle-spinning (MAS) NMR spectroscopy can directly show the rates of exchange among sites, potentially providing less model-dependent information than more traditional NMR relaxation time measurements. Here we use a newly developed high temperature MAS probe (Doty Scientific, Inc.) to observe the exchange of Na+ among the alkali sites in a high-Na nepheline at temperatures as high as 500° C. Observed exchange rates are consistent with correlation times derived from cation diffusivity.