Albite Melt Research Articles

Granitic melt viscosities; empirical and configurational entropy models for their calculation

Newly measured viscosities of a low H20-content granitic melt and a low H20-content albitic melt have been combined with viscosity data from the literature to create two new models for the calculation of granitic melt viscosities at crustal pressures and temperatures between 700 and 900 °C; one model is purely empirical, and the other is based on config- urational entropy theory. When the molecular weight of the anhydrous melt is calculated on the basis of eight 0 atoms, or assumed to be 260 gram formula weight (gfw), and the total H20 concentration is expressed by its mole fraction, the viscosity, '1/,of per- and metaluminous granitic melts, -70-76 wt% SiOl, can be calculated from the following empirical equations: For X~~o ::s 0.25, log '1/= (0.0292649T - 53.198903)X~~o- 0.0129207T + 24.2973 and for X~~o> 0.25, log '1/= - 2.2977826X~:o - 0.0095llOT + 15.6293 where viscosity is in pascal-seconds, and temperature, T, is in kelvins. The first equation expresses the effects of H20 addition on melt viscosity at low H20 contents where the predominant H20 species in the melt is OH- (as demonstrated in previous studies) and small additions of H20 have large effects on melt viscosity; above X~:o ' 0.25, molecular H20 is the dominant species of H20, the addition of which has only modest effects on melt viscosities. This empirical model is applicable to granitic melts with total H20 con- tents between 0.3 and 12.3 wt%. Following configurational entropy theory, viscosities of the same hydrous granitic melts can be calculated by modeling the viscosities as ternary mixtures of various mole fractions of molecular H20, X H20,OH ~, XOH-,and silicate melt, Xsilieatemelt: In '1/ = -15.398 + (181.622/{T(Sconf(Xsilieate melt) - R(CH2oXH201n XH20 + CowXowln Xow + CsilicatemeltXsilicatemeltln Xsilicatemelt))}), where Seonf, the configurational en- tropy calculated from the viscosity measurements, is 4.567 x 10-3 J/(mol. K); R is the gas constant, 8.314 J/(mol. K); CH20is -1.899 x 10-3; COH- is -1.531 x 10-3; and Csilicatemelt is 4.913 X 10-3. Mole fractions of H20 and OH- are estimated from previously published measurements of species abundances in quenched rhyolitic glasses, and the molecular weight of the silicate melt is 260 gfw. Application of this model is limited to melts with. a maximum of approximately 6 wt% total H20 because X H20-XOH- speciation information is not available at higher H20 contents. Numerical values in the above equation are not considered to have any physical or chemical significance. Calculated granitic melt viscos- ities deviate on average from measured viscosities by -0.03 log units for the empirical model and -0.2 log units for the configurational entropy model. At H20 concentrations below 4 wt% and above 7 wt%, these models for viscosity estimation are significantly better than previous models, which can be in error by one to two orders of magnitude. Using either of these new models, the viscosities of granitic melts with 1-3 wt% H20 are calculated to be up to two orders of magnitude lower than previously thought. This results in a one to two order-of-magnitude increase in the viscosities of melt extraction from source regions, transport of melts through the crust, and Stokes settling of crystals.

An associated solution model for albite-water melts

An associated solution model linking microscopic and macroscopic data is developed to describe the thermodynamic properties of NaAlSi 3O 8-H 2O melt mixture. The model is based upon the following two homogeneous melt speciation reactions: Na 0.75Al 0.75Si 2.25O 60.3Na 2Al 2SiO 6 + 0.15NaAlSi 2O 6 + 0.55Si 3O 6; Na 2Al 2SiO 6, + H 2ONa 2Al 2(OH) 2SiO 5, where the first reaction represents the formation of three-membered rings in albite melt and the second one represents hydrolysis of Al-O-Al linkages. The equilibrium constants of these two reactions are calibrated on the basis of available water speciation data, phase equilibrium data; thermodynamic data, solubility data, and P-V-T data. The calculated thermodynamic mixing properties show that relative to our standard states, albite and H 2O melt components exhibit strong negative deviation from ideality. The value for G ex for any given P and T reaches a minimum at approximately 33 mol% water, which coincides with the composition at which there is a significant change in many microscopic and macroscopic properties in this system. For all P, T conditions, it is calculated that hydroxyl-bearing species dominate over molecular water at low water contents but the latter becomes dominant after the total water content exceeds about 33 mol%. When the total water content in the albite melt increases, the abundance of molecular water increases and that of albite species decreases, while the abundances of both three-membered rings and hydroxyl-bearing species (Na 2Al 2(OH) 2SiO 5) first increase, then decrease. The calculated water solubility is positively correlated with P and negatively correlated with T for pressures lower than approximately 5 kb but positively correlated with T for pressures higher than this, consistent with the behavior suggested by previous workers. The water solubility has been readily modeled by hydrolysis of Al-O-Al linkages in spite of their very small abundance in these melts. The calculated hypersolidus phase relations are consistent with available experimental data. In addition, critical phenomena are predicted within the system, in agreement with the implications of available experimental data.

Diffusion of silicon and gallium (as an analogue for aluminum) network-forming cations and their relationship to viscosity in albite melt

Self-diffusion of Si and tracer diffusion of Ga have been measured in an albitic melt, Na0.97Al0.99Si3.02O8, at 1 atmosphere between 1160 and 1558°C. Diffusion of these network-forming elements was studied for comparison with viscous transport and Nuclear Magnetic Resonance (NMR) relaxation parameters measured in albite melts and glasses. Gallium was used as an analogue for Al and estimated diffusion parameters for both Ga and Al are presumed to be equivalent. Diffusion can be described by Arrhenius lines for both Si and Ga (with 1-standard errors): +6.58 × 10−3Dsi = 1.37 × 10−4exp(−337.4 ± 52.6/RT)−1.35 × 10−4 +7.97 DAl ∼ DGa = 3.55 × 10−1 exp (−424.6 ± 42.5 /RT)−3.39 × 10−1, where diffusivities are in m2s−1 and activation energies are in kJ/mol. Comparison of Si and Ga diffusivities with Si/Al diffusivities in crystalline albite indicates relatively small differences between activation energies for diffusion of 3+ and 4+ network-forming cations in crystalline and in molten albite. Apparently, the energy barrier to diffusion of these network-forming cations is influenced significantly only by nearest neighbours, and is not strongly affected by changing the dominant structure from four-membered rings of (Si, Al)O4 tetrahedra in crystalline albite to three-, four-, and six-membered rings of tetrahedra in molten albite. Comparison of Na diffusion in albite crystals and glass demonstrates significant differences in activation energies for diffusion. The behaviour of oxygen diffusion appears to be intermediate between Na and network-forming cations, however, this observation is based upon limited oxygen diffusion data for rhyolitic, not albitic, melts. Activation energies for Ga diffusion in albite melt are similar to those measured in rhyolite melt at 1.0 GPa. The silicon activation energy for diffusion is three times higher in albite melt than in rhyolite melt at 1.0 GPa and does not correlate with the activation energy for the inverse of the NMR spin-lattice relaxation. The differences between Si activation energies in albite and rhyolite melts are suggested to be due to the presence of a mixed alkali-like effect in the latter melt. Calculated diffusivities based upon measured viscosities for albite melt and the Eyring equation agree almost within error with Ga diffusivities, but are typically 3–10 times greater than Si diffusivities. This correlation suggests that diffusion of Al in albite melt controls viscous transport. The activation energy for oxygen diffusion in crystalline albite and the estimated activation energy for oxygen diffusion in the melt are much lower than the activation energy for viscous transport and argue against a relationship between oxygen diffusion and viscous transport in this aluminosilicate melt. The more rapid diffusion of Ga and A1 compared to Si in albite melt is proposed to be due to the more frequent formation of five- or six-coordinated Ga-O and Al-O transition state complexes than similar Si-O complexes. The previously observed relationship between viscosity and chemical diffusivities during interdiffusion (Baker, 1992b) suggests that diffusion of Al also controls chemical diffusion in basaltic to rhyolitic melts.

Re-examination of Water Solubility in Albite Melt.

Most of the previously determined water solubilities in silicate melts are overestimated because of an experimental quenching problem. Hydration of run product glass with coexisting aqueous bubbles causes the overestimation. Due to relatively slow quenching rate in conventional IHPVs, hydration of the glass by trapped aqueous bubbles inevitably takes place at a significant speed. In this paper, a new method for the solubility determination is proposed by which the hydration problem can be avoided. Using a new IHPV with rapid quenching device and utilizing intensive mapping analysis of water for the experimental charges with SIMS, solubility of water was newly determined for albite melt at 2, 000bar. The solubility obtained was 10-30% lower than those previously reported. Because the water solubility data for albite melt have been used as standard reference in silicate melt literatures, the present observation has pivotal importance.

The Effect of Water on the Solubility and Speciation of CO2 in Aluminosilicate Glasses Along the Join SiO2-NaAlO2

C O 2 and H20 are the most important volatiles in the majority of magmatic systems and although the C02 is usually subordinate in concentration, it is often the first component to reach saturation due to the much higher solubility of water in the melt. C02 vesiculation can therefore be the driving force in many eruptive situations and it is important to investigate quantitatively the effect that varying amounts of water have on CO2 saturation. A number of previous studies have suggested that the solubility of CO2 is greater in hydrous melts than in the anhydrous equivalents. This effect has been observed for a variety of polymerized and depolymerized compostions at high pressures (e.g. Brey and Green, 1975; Mysen, 1976). For albite and other simple sodium aluminosilicates, Mysen (1976) suggested that there is a maximum in the solubility as a function of the molar CO2/CO2 + H20 ratio in the capsule. In contrast, Blank et al. (1993), studying the solubility of CO2 in rhyolite at much lower pressures and temperatures found that the solubility of CO2 was not enhanced by the presence of water. The aim of this study is to examine the effect of water on the solubility and speciation of CO2 in albite and jadeite melts. The speciation information is important if changes in the physical properties of magmas under such conditions are to be predicted. Albite is the main composition studied as there is considerable data available on the solubility and speciation of the pure components (e.g. Stolper et al., 1987; Kohn et al., 1989; Silver and Stolper, 1989) as well as theoretical predictions of solubility mechanisms. In addition, our data will provide a direct test of the conclusions of Mysen (1976).

Noble gas solubilities in silicate melts and glasses: New experimental results for argon and the relationship between solubility and ionic porosity

New measurements of the solubility of Ar in basaltic, rhyolitic, orthoclasic, and albitic melts and glasses ar Ar pressures of 250–10,000 bar and temperatures of 400–1300°C are presented and combined with other solubility measurements for a wider range of melt compositions to parameterize the effects of pressure, temperature, and melt composition on Ar solubility. Argon solubility in melts and glasses is roughly linear with Ar pressure under these conditions. At near-liquidus temperatures, solubility in melts is approximately independent (within ∼ 10%) of temperature, while some results below 600–700°C show an increase in solubility with decreasing temperature, perhaps reflecting differences in the nature of Ar solubility in glasses and melts. There is also a positive, linear correlation between the “ionic porosity” of melt and the logarithm of Ar solubility. This correlation is better than previously noted correlations between inert gas solubility and melt density and volume, and provides a useful means of predicting how Ar solubility varies with melt composition. The solubilities of He, Ne, Kr, and Xe are also positively correlated with ionic porosity, but are increasingly sensitive to ionic porosity as the size of the gas atom increases, suggesting that with more efficient packing of the melt structure the availability of sites that can incorporate inert gas atoms decreases more rapidly for larger atoms than for smaller atoms. Comparison of inert gas solubilities with those of molecular CO 2 and molecular H 2O in rhyolitic melts shows that solubilities decrease in the order H 2O mol ⪢ He > Ne > Ar > CO 2,mol ∼ Kr > Xe. The much higher solubility of molecular H 2O compared to the other neutral gas species (and molecular CO 2) suggests that it is not merely passively occupying interstitial “holes” in the melt structure as is thought to be the case for the rare gases (and likely for molecular CO 2), but rather it is stabilized in the melt structure by chemical bonds (e.g., by hydrating cations or through hydrogen bonds).

Estimation of diffusion coefficients during interdiffusion of geologic melts: Application of transition state theory

Two equations for the prediction of non-alkali and alkali, Na and K, diffusivities during the interdiffusion of complex aluminosilicate silicate melts at crustal pressures are proposed. Both equations are derived from transition state theory. The Eyring equation is used to calculate diffusivities for Si from silicate melt viscosities (calculated by the Shaw method). Because previous studies of interdiffusion between silicate melts have reported that non-alkalies appear to chemically diffuse only as rapidly as Si and Al the Eyring equation also predicts the diffusivity of all non-alkali cations during interdiffusion. Alkali diffusivities are orders of magnitude greater than non-alkali diffusivities and therefore are decoupled from melt viscosity; a transition state theory equation which predicts diffusivities from jump distances during diffusion, ionic masses, the free volume of albite melt and the enthalpy of vaporization of Na 2O is used to predict alkali diffusivities. Both diffusivity calculations are demonstrated to typically reproduce measured diffusivities. The relative mean error between measured and predicted diffusivities for non-alkalies is approximately a factor of 2 and less than that for alkalies. The accuracy of these equations in predicting diffusivities during interdiffusion is sufficient to use diffusivities calculated by these equations to constrain and to model interdiffusional processes in melts during igneous petrogenesis.

Solubility and diffusion of noble gases in vitreous albite

Noble gas sorption experiments were performed on albite composition melts in an internally heated pressure vessel at 0.1–5 kbar pressure, in the temperature range 750°C <T < 1200°C. Solubility of Ne, Ar and Kr in quenched samples was determined by gas chromatography and Knudsen cell mass spectroscopy (KMS). The concentration of dissolved gas increases approximately linearly with pressure. Ar solubility in albite melt shows no temperature dependence for the investigated temperature interval. Modelling of noble gas solubility using Langmuir isotherms yields the following values for the number of sites (M) available for noble gas atoms: Ne: M = 1.5·1021 sites/cm3 melt at 1000°C; Ar: M = 1.8·1020 sites/cm3 melt at 1000°C; and Kr: M = 6.4·1019 sites/cm3 melt at 750°C. Henry’s law constant (K) can be calculated from low-pressure solubility at the indicated temperature as follows: Ne (1000°C), K = 1.5·10−3 cm3 STP g−1 bar−1; Ar (1000°C), K = 3.0·10−4 cm3 STP g−1 bar−1; Kr (750°C), K = 1.8·10−4 cm3 STP g−1 bar−1.Desorption kinetics were investigated by KMS at a constant heating rate of 4 K min−1. The temperature maxima of release spectra are linearly related to noble gas atomic diameter. The temperature dependence of diffusivities was obtained from Knudsen cell rate-heating and fitted to an Arrhenius equation D = D0exp(−Ea/RT), where D0 in cm2 s−1 and Ea in J mol−1, yielding DHe = 5.0·10−4 exp(−67000/RT), DNe = 2.2·10−2 exp−95000/RT), DAr = 9.0·10−4 exp(−133000/RT) and DKr = 4.3·10−3 exp(−168000/RT). The radius of diffusing noble gas shows a linear relationship with the square root of activation energy.

Water solubility in aluminosilicate melts of haplogranite composition at 2 kbar

The compositional dependence of H2O solubility was investigated at 2 kbar and 800°C in haplogranite melts (system SiO2-NaAlSi3O8-KAlSi3O8 or Qz-Ab-Or). The sixteen investigated compositions contained 25, 35 or 45 wt.% normative Qz and various Ab/(Ab+Or) ratios (0.15–0.92). Starting solid materials were anhydrous bubble-free glasses to which 10 wt.% H2O was added. The H2O contents of the isobarically quenched melts (glasses) were measured by Karl-Fischer titration.The results show that H2O solubility in aluminosilicate melts depends significantly upon anhydrous composition. The highest solubility values are obtained for the most Ab-rich melts. At a constant normative quartz content, the solubility of water decreases from 6.49 ± 0.20 wt.% H2O for a composition Qz35Ab60Or05 (normative composition expressed in wt.%) to 5.50 ± 0.15 wt.% H2O for a composition Qz35Ab10Or55. Along this join, the most significant changes are observed for Ab-rich melts whereas H2O solubility in Or-rich melts remains almost constant.The H2O solubility data imply that H2O is preferentially associated with the Ab component in aluminosilicate melts. Application of the results to natural granitic melts suggests that Na-rich, H2O-saturated melts may be significantly less viscous than H2O-saturated, K-rich melts.The temperature dependence of H2O solubility, investigated for composition Qz28Ab38Or34 at 2 kbar, is low. Increasing temperature from 750° to 1150°C only causes a decrease in H2O solubility from 6.00 to 5.41 wt.% H2O. These data are in agreement with previous data obtained for albite melts.

The system NaAlSi3O8?H2O?H2 (1200� C, 2 kbar): the solubility and interaction mechanism of fluid species with melt

The H2O and H2 solubilities in an albite melt at 1200° C and 2 kbar over the entire range of gas phase composition, from pure hydrogen to pure water were studied in gas-media pressure vessels. The water solubility initially increases with increasing hydrogen content until a maximum of 9.19 wt% H2O atXH 2 =0.1 is reached, withXH 2 >0.1 the water solubility decreases. The hydrogen solubility curve has a maximum atXH 2 =0.42 where the concentration reaches 0.206 wt% H2O. Over the entire compositional range1H NMR (nuclear magnetic resonance) spectra show distinct absorption lines due to protons bound to OH groups and to isolated firmly bound water molecules. In NMR and Raman spectra there were no bands attributable to the H−H vibrations of molecular hydrogen. The X-ray photo-electronic spectra of hydrogen-bearing glasses show the Si2p (99 eV) band which corresponds to the zero-valency silicon. The formation of OH groups and molecular water during interaction between hydrogen-bearing fluids and melts under reducing conditions has a qualitative effect, the same as for water dissolution. Another point of interest is that hydrogen-bearing melts undergo more depolymerization than do hydrous melts.

A multinuclear magnetic resonance study of the structure of hydrous albite glasses

The structures of a series of hydrous albite glasses quenched from melts at high pressures and temperatures have been studied using 29Si, 23Na, 27Al, and 1H nuclear magnetic resonance. Changes in the isotropic chemical shift, the chemical shift dispersion, and the mean nuclear quadrupole coupling constant for 23Na as a function of dissolved water concentration were deduced from spectra obtained at two different magnetic fields. Major changes in the sodium environment occur, but the spectra for 29Si and 27Al, and hence their structural environments, are similar throughout the range of water concentrations studied (0–67 mol%). No previous model is consistent with the results of this study. The data suggest the existence of the following structural features: i) exchange of H + for Na + as a charge-balancing cation; ii) formation of Na(OH) complexes; iii) incorporation of molecular water; iv) no octahedrally coordinated aluminium; v) no Al-OH or Si-OH. These features can be summarised in terms of the equilibrium NaAlSi 3 O 8 + H 2 O ⇋ HAlSi 3 O 8 + Na( OH). In contrast to all previous interpretations, we see no evidence for depolymerisation of the aluminosilicate framework, although an increase in the symmetry of the aluminium environments and decrease in the chemical shift dispersion of the sodium environments suggests a more ‘ordered’ structure than in the dry glass. If the structures of hydrous albite melts are the same as those of the glasses studied here the current understanding of the effect of dissolved water on the physical properties of felsic melts must be reassessed.

Water in Albitic Glasses

Infrared spectroscopy has been calibrated to provide a precise and accurate method for determining the concentrations of molecular water and hydroxyl groups in hydrous albitic glasses. At total water contents less than 4 wt.%, most of the water is dissolved as hydroxyl groups; at higher total water contents, molecular water becomes the dominant species. For total water contents above 4 wt.%, the amount of water dissolved as hydroxyl groups is nearly constant at about 2 wt.% and additional water is incorporated as molecular water. These trends in the concentrations of the H-bearing species are similar to those observed in other silicate glasses using infrared and NMR spectroscopies. The ratio of molecular water to hydroxyl groups at a given total water content is independent of the pressure and only weakly dependent on the temperature of equilibration. Molecular water and hydroxyl group concentrations in glasses provide constraints on the dissolution mechanisms of water in silicate liquids. Several mixing models involving homogeneous equilibria of the form H_2O+O^(2−) = 2OH^− among melt species in albitic melts have been developed. These models can account for the measured species concentrations if the effects of non-ideal behavior or mixing of polymerized units are included, or by allowing for several different anhydrous species. We used two thermodynamic models of hydrous albitic melts to calculate phase equilibria. The first assumes that Henry's law holds for molecular water in albitic liquids; i.e. that the activity of molecular water in the melt is proportional to its mole fraction as determined by infrared spectroscopy. The second describes the speciation and thermodynamics of hydrous albitic melts using the formalism of a strictly regular solution. These models can account reasonably well for the position of the vapor-saturated solidus of high albite and the pressure and temperature dependence of the solubility of water in albitic melts. To the extent that these models are successful, our approach provides a direct link between measured species concentrations in hydrous albitic glasses and the macroscopic thermodynamic properties of the NaAlSi_3O_8-H_2O system.

Structural change of albite (NaAlSi3O8) melt quenched at high pressure: density and Al K.BETA. X-ray emission.

Density and Al Kβ X-ray emission band of the albite melts quenched at high pressure up to 8 GPa were measured. The density of the glass increases continuously from 2.473g/cm3 at 2 GPa to 2.743g/cm3 at 8 GPa. The rate of density increase however is not constant but relatively large in the pressure range of 2-6 GPa, suggesting that some structural change of the albite glass takes place in this pressure range. Measurement of Al Kβ emission bands of the albite glass revealed that the emission band shifts towards the higher energy side with increasing pressure. Present measurements of density and Al Kβ emission band of the albite glass are consistent with the results of the 27Al NMR spectrometry of the glass (Ohtani et al., 1985), suggesting a coordination change of Al3+ ions in the melt from 4-fold to 6-fold with increasing pressure.

Beta track autoradiography and infrared spectroscopy bearing on the solubility of CO2 in albite melt at 2 GPa and 1450� C

Albite glasses with 0.5–2.0 wt.% CO2 synthesized at 2 GPa and 1450° C, previously analyzed by beta track autoradiography (Tingle 1987), have been analyzed by infrared spectroscopy. Values of CO2 concentration determined by the beta track technique are 10–50% greater than the amount of CO2 added to the sample. Values determined by infrared spectroscopy match the loaded CO2 contents, substantiating the existence of a previously unrecognized error in the beta track technique. Concentrations, and hence solubilities, in albite determined by infrared spectroscopy are more accurate than those determined by beta track autoradiography. Values obtained by the beta track technique for diopside are 10% lower than the loaded CO2 contents (Tingle 1987) and suggest that the systematic error may be dependent on composition and/or density.

Stability of the assemblage albite plus forsterite at high temperatures and pressures with petrologic implications

The maximum limits of the assemblage albiteforsterite have been determined experimentally at high pressures and temperatures. At subsolidus temperatures, albite plus forsterite is replaced at high pressures by jadeitic clinopyroxene and enstatitic orthopyroxene. The boundary for this reaction lies within experimental uncertainity of that for jadeite=albite+nepheline. Melting of albite+forsterite at high pressures produces enstatite+liquid, which is different from the low-pressure eutectic behavior. Melting rates are very slow and several hundred hours are required to establish equilibrium near the solidus. The subsolidus boundary for albite plus forsterite lies near that for sanidine plus forsterite, but with a shallower slope which more closely matches that of anorthite plus forsterite. Both albite plus forsterite and anorthite plus forsterite are replaced at high pressures by an assemblage containing clinopyroxene plus orthopyroxene, unlike sanidine plus forsterite, which is replaced by a feldspathoid plus orthopyroxene. The presence of sodium enlarges the depth region over which plagioclase lherzolite can stably exist; it may also stabilize alkali feldspar plus olivine in crustal rocks.

Experimental studies of thermal grooving in the olivine and albite melt system

Thermal grooving of low angle tilt boundary of San Carlos olivine in the albite melt were experimentally investigated at 1200–1300°C in mixed CO 2 and H 2 gases for 1–20 h. The depth, d, of the thermal groove on (010) of olivine along the (100) sub-boundaries is in the function of time and temperatures as follows; d 4 = k o · t · exp(− 190 000/ RT), in which R is the gas constant, and k o is the material constant. The melt shape changes due to the thermal grooving driven by surface tension and deformation of the upper mantle. Compared with the time scales of these two counteracting mechanisms, it is inferred that the melt shape is unstable in the high temperature and low stress conditions, and that the melt shape takes a stable form during progressive deformation in the low temperature and high stress conditions.

Mixing Properties of NaAlSI3O8Melt-H2O: New Calorimetric Data and Some Geological Implications

We present the results of direct calorimetric determinations of the heat of mixing of $$NaAlSi_{3}O_{8}$$ and $$H_{2}O$$ to form hydrous melts and glasses. $$\Delta H_{m}$$ is small and exothermic throughout the P-T range investigated. At 298 K and 1 bar there is a minimum in $$\Delta H_{m} (-2.39 kcal/mole at X^{m}_{W} \approx 0.27$$). Results of high-pressure drop calorimetry on hydrous albite melts show that $$\Delta H_{m}$$ becomes more endothermic as T increases. The minimum in $$\Delta H_{m}$$ as a function of $$X^{m}_{W}$$ occurs in the compositional region where $$OH^{-}$$ is the dominant hydrogen-bearing species in the glass. The average partial molal heat capacity of $$H_{2}O$$ in hydrous albite melt (from 298 to 1306 K, 20 cal/mole/K) is similar to that of pure liquid water at ambient conditions. Reconnaissance studies in the system $$4Q-Ab-Or-H_{2}O$$ indicate a general similarity with the $$Ab-H_{2}O$$ system in that $$\Delta H_{m}$$ is small and exothermic. In detail, however, $$Ab-H_{2}O$$ ...

Solubility of Water in Albite-Melt Determined by the Weight-Loss Method

A table of water solubility in albite melt at pressure up to 8 kbars is presented. These data have only been published previously in graph form; the weight-loss method was used to determine these solubilities. A recent paper by Dingwell and others critical of the weight-loss method is discussed.

Effects of water and fluorine on the viscosity of albite melt at high pressure: a preliminary investigation

The viscosities of fluorine- and water-bearing melts based on albite composition have been determined at 7.5, 15 and 22.5 kbar by the falling-sphere method. All melt viscosities decrease isothermally with increasing pressure. At 1200°C the viscosity of the fluorine-bearing melt (albite + 5.8 wt.% fluorine substituted for oxygen, denoted AbF 2O −1) decreases from5000 ± 750P at7.5kbar to1600 ± 240P at22.5kbar. At 1400°C the viscosity of this melt decreases from1300 ± 200P at7.5kbar to430 ± 65P at22.5kbar. At 1400°C the viscosity of albite + 2.79 wt.% water (denoted AbH 2O) decreases from650 ± 100P at7.5kbar to400 ± 60P at22.5kbar. Fluorine (as F 2O −1) and water strongly decrease the viscosity of albite melt over the entire range of investigated pressures. The ratio of the effects of 5.8 wt.% fluorine [F/(F + O)molar = 0.10] and 2.79 wt.% water [OH/(OH + O)molar = 0.10] on the log of melt viscosity [Δ log η(AbF 2O −1)/Δ log η(AbH 2O)] equals0.90 ± 0.05, 0.84 ± 0.05and0.97 ± 0.05at7.5, 15and22.5kbar, respectively. Comparison with available data on the high-pressure viscosity of albite melt indicates that both F 2O −1 and H 2O maintain their viscosity-reducing roles to lower crustal pressures. The difference between the viscosities of melts of albite, AbF 2O −1 and AbH 2O, may be explained in terms of the relatively depolymerized structures of AbF 2O −1 and AbH 2O melts. The depolymerization of albite melt by the addition of water results from the formation of Si OH bonds. The depolymerization of albite melt by F 2O −1 substitution results from the formation of non-bridging oxygens associated with network-modifying aluminum cations that are formed upon fluorine solution. The strong viscosity-reducing effects of water and fluorine in albite melt at pressures corresponding to the mid- to lower continental crust indicate that these two components will strongly influence the dynamic behavior of anatectic melts during initial magma coalescence and restite-melt segregation.

Chemical diffusion of fluorine in melts in the system Na 2O [sbnd]Al 2O 3[sbnd]SiO 2

The volatilization of fluorine from three melts in the system Na 2O Al 2O 3 SiO 2 has been investigated at 1 atm pressure and 1200–1400°C. The melts chosen have base compositions corresponding to albite, jadeite and a peraluminous melt with 75 mole % SiO 2. Melt spheres were suspended from platinum loops in a vertical tube furnace in a flow of oxygen gas, then quenched, sectioned and analysed by electron microprobe. The microprobe scans indicate that transport of fluorine to the melt-vapor interface is by binary, concentration-independent interdiffusion of fluorine and oxygen. F O interdiffusivity increases in the order albite < peraluminous < jadeite. There is no simple reciprocal relationship between F O interdiffusivity and melt viscosity. Comparison with data on high-pressure interdiffusivity of fluorine and oxygen in jadeite melt indicates that F O interdiffusivity increases with pressure from 0.001 to 10 kbar while the activation energy remains unchanged. Fluorine chemical diffusivity in albite melt is substantially lower than H 2O chemical diffusivity in obsidian melts suggesting that different diffusive mechanisms are responsible for the transport of F and H 2O in igneous melts. Fluorine diffuses in albite melt via an anionic exchange with oxygen whereas water probably diffuses in obsidian melt via an alkali exchange mechanism.