Water diffusion in basalt and andesite melts under high pressures

Water diffusion is one of the most important characteristics of many processes dealt with in magmatic geochemistry, petrology, and volcanology. We have experimentally examined water diffusion in Fe-free andesite and basalt melts (stoichiometric mixtures of the minerals albite + diopside + wollastonite) at 3 and 100 MPa, 1300×C, up to approximately 4 wt % water in the melts, and a total (lithostatic) pressure of 100 MPa on a high gas pressure apparatus equipped with a unique internal device. The experiments were conducted simultaneously with the use of two different methods: diffusion hydration and couples. Water solubility in the melts and water concentrations along the diffusion profiles (C _{H 2 O}) were determined by quantitative IR microspectroscopy, using the Beer-Lambert law. A structural chemical model is proposed for calculating and predicting the concentration dependence of the molar absorption coefficient of the hydroxyl group and water molecules in andesite and basalt glasses. The diffusion coefficients of water (D _{H 2 O}) are derived by the mathematical analysis of concentration profiles and the analytical solution of the second Fick diffusion law. Preliminary results indicated D _{H 2 O} is roughly one order of magnitude higher in basaltic melts than in andesitic ones (at the same temperatures and P _{H 2 O}) and significantly (exponentially) increases with increasing water concentrations in andesitic and basaltic melts. The newly obtained experimental data are proved to be fully consistent with the results obtained on the D _{H 2 O} dependence on C _{H 2 O} in melts of acid rocks (rhyolite and obsidian). The derived quantitative dependence between D _{H 2 O} and melt viscosity is used to develope principles of a new method for predicting and calculating the temperature, concentration, and pressure dependences of D _{H 2 O} in magmatic melts of the of acid-basic series (up to 3 wt % C _{H 2 O}) at crustal T, P parameters.     

Concentration dependence of water diffusion in obsidian and dacitic melts at high-pressures

The diffusion of water in natural obsidian and model dacitic melts (Ab90Di8Wo2, mol %) has been studied at water vapor pressure up to 170 MPa, temperatures of 1200°C, H₂O contents in melts up to ～6 wt % using a high gas pressure apparatus equipped with a unique internal device. The experiments were carried out by a new low-gradient technique with application of diffusion hydration of a melt from fluid phase. The water solubility in the melts and its concentration along $C_{H_2 O} $ diffusion profiles were determined using IR microspectrometry by application of the modified Bouguer-Beer-Lambert equation. A structural-chemical model was proposed to calculate and predict the concentration dependence of molar absorption coefficients of the hydroxyl groups (OH^?) and water molecules (H₂O) in acid polymerized glasses (quenched melts) in the obsidian-dacite series. The water diffusion coefficients $D_{H_2 O} $ were obtained by the mathematical analysis of concentration profiles and the analytical solution of the second Fick diffusion law using the Boltzman-Matano method. It was shown experimentally that $D_{H_2 O} $ exponentially increases with a growth of water concentration in the obsidian and dacitic melts within the entire range of diffusion profiles. Based on the established quantitative correlation between $D_{H_2 O} $ and viscosity of such melts, a new method was developed to predict and calculate the concentration, temperature, and pressure dependences of $D_{H_2 O} $ in acid magmatic melts (obsidian, rhyolite, albite, granite, dacite) at crustal T, P parameters and water concentrations up to 6 wt %.     

H<Subscript>2</Subscript>O diffusion in peralkaline to peraluminous rhyolitic melts

The diffusion of water in a peralkaline and a peraluminous rhyolitic melt was investigated at temperatures of 714–1,493 K and pressures of 100 and 500 MPa. At temperatures below 923 K dehydration experiments were performed on glasses containing about 2 wt% H₂O _t in cold seal pressure vessels. At high temperatures diffusion couples of water-poor (<0.5 wt% H₂O _t ) and water-rich (~2 wt% H₂O _t ) melts were run in an internally heated gas pressure vessel. Argon was the pressure medium in both cases. Concentration profiles of hydrous species (OH groups and H₂O molecules) were measured along the diffusion direction using near-infrared (NIR) microspectroscopy. The bulk water diffusivity () was derived from profiles of total water () using a modified Boltzmann-Matano method as well as using fittings assuming a functional relationship between and Both methods consistently indicate that is proportional to in this range of water contents for both bulk compositions, in agreement with previous work on metaluminous rhyolite. The water diffusivity in the peraluminous melts agrees very well with data for metaluminous rhyolites implying that an excess of Al₂O₃ with respect to alkalis does not affect water diffusion. On the other hand, water diffusion is faster by roughly a factor of two in the peralkaline melt compared to the metaluminous melt. The following expression for the water diffusivity in the peralkaline rhyolite as a function of temperature and pressure was obtained by least-squares fitting:

Water speciation and diffusion in haploandesitic melts at 743-873 K and 100 MPa

Water is an important volatile component in andesitic eruptions and deep-seated andesitic magma chambers. We report an investigation of H₂O speciation and diffusion by dehydrating haploandesitic melts containing ?2.5 wt.% water at 743-873 K and 100 MPa in cold-seal pressure vessels. FTIR microspectroscopy was utilized to measure species [molecular H₂O (H₂O_m) and hydroxyl group (OH)] and total H₂O (H₂O_t) concentration profiles on the quenched glasses from the dehydration experiments. The equilibrium constant of the H₂O speciation reaction H₂O_m+O?2OH, K = (X_OH)²/(X_H2OmX_O) where X means mole fraction on a single oxygen basis, in this Fe-free andesite varies with temperature as ln K = 1.547-2453/T where T is in K. Comparison with previous speciation data on rhyolitic and dacitic melts indicates that, for a given water concentration, Fe-free andesitic melt contains more hydroxyl groups. Water diffusivity at the experimental conditions increases rapidly with H₂O concentration, contrary to previous H₂O diffusion data in an andesitic melt at 1608-1848 K. The diffusion profiles are consistent with the model that molecular H₂O is the diffusion species. Based on the above speciation model, H₂O_m and H₂O_t diffusivity (in m²/s) in haploandesite at 743-873 K, 100 MPa, and H₂O_t ? 2.5 wt.% can be formulated as

     

Water diffusion in dacitic melt

H₂O diffusion in dacitic melt was investigated at 0.48-0.95 GPa and 786-893 K in a piston-cylinder apparatus. The diffusion couple design was used, in which a nominally dry dacitic glass makes one half and is juxtaposed with a hydrous dacitic glass containing up to ∼8 wt.% total water (H₂O_t). H₂O concentration profiles were measured on quenched glasses with infrared microspectroscopy. The H₂O diffusivity in dacite increases rapidly with water content under experimental conditions, similar to previous measurements at the same temperature but at pressure <0.15 GPa. However, compared with the low-pressure data, H₂O diffusion at high pressure is systematically slower. H₂O diffusion profiles in dacite can be modeled by assuming molecular H₂O (H₂O_m) is the diffusing species. Total H₂O diffusivity D_H2Ot within 786-1798 K, 0-1 GPa, and 0-8 wt.% H₂O_t can be expressed as: where D_H2Ot is in m²/s, T is temperature in K, P is pressure in GPa, K = exp(1.49 − 2634/T) is the equilibrium constant of speciation reaction (H₂O_m+O?2OH) in the melt, X = C/18.015/[C/18.015 + (100 − C)/33.82], C is wt.% of H₂O_t, and 18.015 and 33.82 g/mol correspond to the molar masses of H₂O and anhydrous dacite on a single oxygen basis. Compared to H₂O diffusion in rhyolite, diffusivity in dacite is lower at intermediate temperatures but higher at superliquidus temperatures. This general H₂O diffusivity expression can be applied to a broad range of geological conditions, including both magma chamber processes and volcanic eruption dynamics from conduit to the surface.     

H2O diffusion models in rhyolitic melt with new high pressure data

Water diffusion in silicate melts is important for understanding bubble growth in magma, magma degassing and eruption dynamics of volcanos. Previous studies have made significant progress on water diffusion in silicate melts, especially rhyolitic melt. However, the pressure dependence of H₂O diffusion is not constrained satisfactorily. We investigated H₂O diffusion in rhyolitic melt at 0.95–1.9 GPa and 407–1629 °C, and 0.2–5.2 wt.% total water (H₂O_t) content with the diffusion-couple method in a piston-cylinder apparatus. Compared to previous data at 0.1–500 MPa, H₂O diffusivity is smaller at higher pressures, indicating a negative pressure effect. This pressure effect is more pronounced at low temperatures. Assuming H₂O diffusion in rhyolitic melt is controlled by the mobility of molecular H₂O (H₂O_m), the diffusivity of H₂O_m (D_H2Om) at H₂O_t ≤ 7.7 wt.%, 403–1629 °C, and ≤ 1.9 GPa is given by

D_H2Om=D₀exp(aX),

Platinum solubility in a haplobasaltic melt at 1250°C and 0.2 GPa: The effect of water content and oxygen fugacity

Haplobasaltic melts with a 101 kPa dry eutectic composition (An₄₂Di₅₈) and varying water contents were equilibrated with their platinum capsule at 1523 K and 200 MPa in an internally heated pressure vessel (IHPV) equipped with a rapid quench device. Experimental products were inclusion-free glasses representative of the Pt-saturated silicate melts at the experimental conditions. Platinum concentrations were determined using an isotope dilution multicollector inductively coupled plasma mass spectrometer and water contents and distribution by Karl Fischer titration and Fourier transform infrared spectroscopy, respectively.The water content of the melt has no intrinsic effect on platinum solubility, for concentrations between 0.9 wt.% and 4.4 wt.% H₂O (saturation). Platinum solubility increases with increasing water content, but this effect is an indirect effect because increasing water content at fixed f_H2 (imposed by the IHPV) increases the oxygen fugacity of the experiment.The positive oxygen fugacity dependence of Pt solubility in a hydrous silicate melt at 200 MPa is identical to that in anhydrous melts of the same composition determined in previous studies at 101 kPa. This study extends the range of platinum solubilities to oxygen fugacities lower than was previously possible. Combining the data of this and previous studies, Pt solubility is related to oxygen fugacity (in bar) at 1523 K by the equation:

[Pt]_total(ppb)=1389×f_O2+7531×(f_O2)^1/2     

Solution behavior of C-O-H volatiles in FeO-Na2O-Al2O3-SiO2 melts in equilibrium with liquid iron alloy and graphite at 4 GPa and 1550°C

In order to elucidate the solution behavior of carbon and hydrogen in iron-bearing magmatic melts in equilibrium with a metallic iron phase and graphite at oxygen fugacity (fO₂) values 2–5 orders of magnitude below the iron-wustite buffer equilibrium, fO₂ (IW), experiments were carried out at 4 GPa and 1550°C with melts of FeO-Na₂O-SiO₂-Al₂O₃ compositions. Melt reduction in response to an fO₂ decrease was accompanied by a decrease in FeO content. The values of fO₂ in the experiments were determined on the basis of equilibrium between Fe-C-Si alloy and silicate liquid. Infrared and Raman spectroscopy showed that carbon compounds are formed in FeO-Na₂O-SiO₂-Al₂O₃ melts: CH₄ molecules, CH₃ complexes (Si-O-CH₃), and complexes with double C=O bonds. The content of CO₂ molecules and carbonate ions (CO ₃ ^2? ) is very low. In addition to carbon-bearing compounds, dissolved hydrogen occurs in melt as H₂ and H₂O molecules and OH^? groups. The spectral characteristics of FeO-Na₂O-SiO₂-Al₂O₃ glasses indicate the occurrence of redox reactions in the melt, which are accompanied at decreasing fO₂ by a significant decrease in H₂O and OH^?, a slight decrease in H₂, and a significant concomitant increase in CH₄ content. The content of species with the double C=O bond increases considerably at decreasing fO₂ and reaches a maximum at ΔlogfO₂(IW) = ?3. According to the obtained IR spectra, the total water content (OH^? + H₂O) in the glasses is 1.2–5.8 wt % and decreases with decreasing fO₂. The high H₂O contents are due largely to oxygen release related to FeO reduction in the melt. The total carbon content at high H₂O (4.9–5.8 wt %) is approximately 0.4 wt %. The carbon content in liquid iron alloys depends on silicon content and, probably, oxygen solubility and ranges from 0.3 to 3.65 wt %. Low carbon contents were observed at a significant increase in Si content in liquid iron alloy, which may be as high as ～13 wt % at fO₂ values 4–5 orders of magnitude below fO₂(IW).     

Interdiffusion of hydrous dacitic and rhyolitic melts and the efficacy of rhyolite contamination of dacitic enclaves

Effective binary diffusion coefficients of Si during the interdiffusion of hydrous, 3 and 6% H₂O, dacitic and rhyolitic melts have been determined at 1.0 GPa, 1100°–1400°C. Water is shown to enhance diffusivities by one to two orders of magnitude above dry Si diffusivities in the same compositional system for SiO₂ compositions 65–75wt%. The effect of silica content on diffusion is small and typically within experimental error. With 3% H₂O in the melts the Arrhenius equation for Si diffusion at 70% SiO₂ is: $${\text{D = }}2.583\operatorname{x} 10^{ - {\text{ }}8} {\text{ }}\exp ( - 126.5/R{\text{T}})$$ where D is the diffusivity in m²/s, the activation energy (126.5) is in kJ/mol, R is in J/mol and T in Kelvin. Although less-well constrained, the Si diffusivity at 70% SiO₂ with 6% H₂O in the melts can be described by: $${\text{D = }}2.692\operatorname{x} 10^{ - {\text{ }}7} {\text{ }}\exp ( - 131.4/R{\text{T}})$$ The activation energies for diffusion are substantially below the activation energy of 236.4kJ/mol measured during anhydrous interdiffusion in the same system (Baker 1990). The decrease in activation energy with the initial addition of 3% water and the relative insensitivity of the activation energy to the additional water is related to the abundance of OH species in the melt, and the reduction of (Si,Al)-O bond strengths due to the interaction of hydroxyls with the (Si,Al)-O network. Changes in the pre-exponential factor of Arrhenius equations are attributed to the abundance of H₂O species in the melts. No decoupling of non-alkalies from SiO₂ during interdiffusion of the two melts was observed, although alkalies diffuse much more rapidly than non-alkalies (but were not measured quantitatively in this study) and can become decoupled. Interdiffusion of Si and all non-alkalies is demonstrated to be predictable, at least to within a factor of ten, by the Eyring equation. Using the diffusion data of this study for nonalkalies and of other studies for alkalies and Sr isotopes the contamination of a host rhyolitic magma by dacitic enclaves, 5 and 50 cm radius, has been modeled for temperatures of 1000°, 900°, and 800° C with water contents of 3 and 6%. Even when the effects of phenocrysts on diffusion in the dacitic enclaves are estimated the results of the modeling demonstrate that significant contamination is possible in the case of small enclaves, and even large enclaves have the potential to affect the composition of their host magma in geologically short times.     

Viscosity of andesitic melts—new experimental data and a revised calculation model

The viscosity of a synthetic andesite-like melt was measured in the low viscosity range (10¹-10⁶ Pa s) using the falling sphere(s) method and in the high viscosity range (10⁸-10¹³ Pa s) using parallel-plate viscometry. Falling sphere experiments with melts containing 2.3 and 5.6 wt.% H₂O were carried out in an internally heated gas pressure vessel (IHPV) at 500 MPa confining pressure. The sinking velocity of Pt and Pd spheres and in one case of a corundum sphere was used to measure the melt viscosity. In addition, a creep experiment was performed at ambient pressure using a glass containing 2.73 wt.% H₂O . A more water-rich glass (5.6 wt.% H₂O ) was investigated with a high pressure parallel-plate viscometer at 400 MPa confining pressure in an IPHV. By combining our new data with previous results for a similar melt composition we derived the following expression to describe the viscosity η (in Pa s) as a function of temperature T (in K) and water content w (in wt.%)

     

Partitioning of F between H2O and CO2 fluids and topaz rhyolite melt

Fluid/melt distribution coefficients for F have been determined in experiments conducted with peraluminous topaz rhyolite melts and fluids consisting of H₂O and H₂O+CO₂ at pressures of 0.5 to 5 kbar, temperatures of 775°–1000°C, and concentrations of F in the melt ranging from 0.5 to 6.9 wt%. The major element, F, and Cl concentrations of the starting material and run product glasses were determined by electron microprobe, and the concentration of F in the fluid was calculated by mass balance. The H₂O concentrations of some run product glasses were determined by ion microprobe (SIMS). The solubility of melt in the fluid phase increases with increasing F in the system; the solubility of H₂O in the melt is independent of the F concentration of the system with up to 6.3 wt% F in the melt. No evidence of immiscible silica- and fluoriderich liquids was detected in the hydrous but water-undersaturated starting material glasses (8.5 wt% F in melt) or in the water-saturated run product glasses. F concentrates in topaz rhyolite melts relative to coexisting fluids at most conditions studied; however, D_F (wt% F in fluid/wt% F in melt) increases strongly with increasing F in the system. Maximum values of D_F in this study are significantly larger than those previously reported in the literature. Linear extrapolation of the data suggests that D_F is greater than one for water-saturated, peraluminous granitic melts containing 8 wt% F at 800° C and 2 kbar. D_F increases as temperature and as (H₂O/H₂O+CO₂) of the fluid increase. For topaz rhyolite melts containing 1 wt% F and with H₂O-rich fluids, D_F is independent of changes in pressure from 2 to 5 kbar at 800° C; for melts containing 1 wt% F and in equilibrium with CO₂-bearing fluids the concentrations of F in fluid increases with increasing pressure. F-and lithophile element-enriched granites may evolve to compositions containing extreme concentrations of F during the final stages of crystallization. If F in the melt exceeds 8 wt%, D_F is greater than one and the associated magmatic-hydrothermal fluid contains >4 molal F. Such F-enriched fluids may be important in the mass transport of ore constituents, i.e., F, Mo, W, Sn, Li, Be, Rb, Cs, U, Th, Nb, Ta, and B, from the magma.     

Diffusive fractionation of volatiles and their isotopes during bubble growth in magmas

Bubbles grow in decompressing magmas by simple expansion and by diffusive supply of volatiles to the bubble/melt interface. The latter phenomenon is of significant geochemical interest because diffusion can fractionate elements and isotopes (or isotopologues) of dissolved components. This raises the possibility that the character of volatile components in bubbles may not reflect that of volatiles dissolved in the host melt over the lifetime of a bubble—even in the absence of equilibrium vapor/melt isotopic fractionation. Recent experiments have confirmed the existence of an isotope mass effect on diffusion of the volatile element Cl in silicate melt [Fortin et al. (Isotopic fractionation of chlorine during chemical diffusion in a dacitic melt and its implications for isotope behavior during bubble growth (abstract), 2016 Fall AGU Meeting, 2016)], so there is a clear need to understand the efficacy of diffusive fractionation during bubble growth. In this study, numerical models of diffusion and mass redistribution during bubble growth were implemented for both “passive” volatiles—those whose concentrations are generally well below saturation levels—and “active” volatiles such as CO₂ and H₂O, whose elevated concentrations and limited solubilities are the cause of bubble nucleation and growth. Both diffusive and convective bubble-growth scenarios were explored. The magnitude of the isotope mass effect on passive volatiles partitioned into bubbles growing at a constant rate R in a static system depends upon R/D _L, K _d and D _H/D _L (K _d = bubble/melt partition coefficient; D _H/D _L = diffusivity ratio of the heavy and light isotopes). During convective bubble growth, the presence of a discrete (physical) melt boundary layer against the growing bubble (of width x _BL) simplifies outcomes because it leads to the quick onset of steady-state fractionation during growth, the magnitude of which depends mainly upon R?x _BL/D _L and D _H/D _L (bubble/melt fractionation is maximized at R?x _BL/D _L ≈0.1). Constant R is unrealistic for most real systems, so other scenarios were explored by including the solubility and EOS of an “active” volatile (e.g., CO₂) in the numerical simulations. For plausible decompression paths, R increases exponentially with time—leading, potentially, to larger isotopic fractionation of species partitioned into the growing bubble. For volatile species whose isotope mass effects on diffusion have been measured (Cl, Li), predicted isotope fractionation in the exsolved vapor can be as large as ?4‰ for Cl and ?25‰ for Li.     

The solution behavior of H2O in peralkaline aluminosilicate melts at high pressure with implications for properties of hydrous melts

Solubility and solution mechanisms of H₂O in depolymerized melts in the system Na₂O-Al₂O₃-SiO₂ were deduced from spectroscopic data of glasses quenched from melts at 1100 °C at 0.8-2.0 GPa. Data were obtained along a join with fixed nominal NBO/T = 0.5 of the anhydrous materials [Na₂Si₄O₉-Na₂(NaAl)₄O₉] with Al/(Al+Si) = 0.00-0.25. The H₂O solubility was fitted to the expression, X_H2O=0.20+0.0020f_H2O-0.7X_Al+0.9(X_Al)², where X_H2O is the mole fraction of H₂O (calculated with O = 1), f_H2O the fugacity of H₂O, and X_Al = Al/(Al+Si). Partial molar volume of H₂O in the melts, , calculated from the H₂O-solulbility data assuming ideal mixing of melt-H₂O solutions, is 12.5 cm³/mol for Al-free melts and decreases linearly to 8.9 cm³/mol for melts with Al/(Al+Si) ∼ 0.25. However, if recent suggestion that is composition-independent is applied to constrain activity-composition relations of the hydrous melts, the activity coefficient of H₂O, , increases with Al/(Al+Si).Solution mechanisms of H₂O were obtained by combining Raman and ²⁹Si NMR spectroscopic data. Degree of melt depolymerization, NBO/T, increases with H₂O content. The rate of NBO/T-change with H₂O is negatively correlated with H₂O and positively correlated with Al/(Al+Si). The main depolymerization reaction involves breakage of oxygen bridges in Q⁴-species to form Q² species. Steric hindrance appears to restrict bonding of H⁺ with nonbridging oxygen in Q³ species. The presence of Al³⁺ does not affect the water solution mechanisms significantly.     

Unique Clinkers and Paralavas from a New Nyalga Combustion Metamorphic Complex in Central Mongolia: Mineralogy,Geochemistry, and Genesis

The paper presents mineralogical and geochemical data on clinkers and paralavas and on conditions under which they were formed at the Nyalga combustion metamorphic complex, which was recently discovered in Central Mongolia. Mineral and phase assemblages of the CM rocks do not have analogues in the world. The clinkers contain pyrogenically modified mudstone relics, acid silicate glass, partly molten quartz and feldspar grains, and newly formed indialite microlites (phenocrysts) with a ferroindialite marginal zone. In the paralava melts, spinel microlites with broadly varying Fe concentrations and anorthite–bytownite were the first to crystallize, and were followed by phenocrysts of Al-clinopyroxene ± melilite and Mg–Fe olivine. The next minerals to crystallize were Ca-fayalite, kirschsteinite, pyrrhotite, minerals of the rhönite–kuratite series, K–Ba feldspars (celsian, hyalophane, and Ba-orthoclase, Fe³⁺-hercynite ± (native iron, wüstite, Al-magnetite, and fresnoite), nepheline ± (kalsilite), and later calcite, siderite, barite, celestine, and gypsum. The paralavas contain rare minerals of the rhönite–kuratite series, a new end-member of the rhönite subgroup Ca₄Fe ₈ ²⁺ Fe ₄ ³⁺ O₄ [Si₈Al₄O₃₆], a tobermorite-like mineral Ca₅Si₅(Al,Fe)(OH)O₁₆ · 5H₂O, and high- Ba F-rich mica (K,Ba)(Mg,Fe)₃(Al,Si)₄O₁₀F₂. The paralavas host quenched relics of microemulsions of immiscible residual silicate melts with broadly varying Si, Al, Fe, Ca, K, Ba, and Sr concentrations, sulfide and calcitic melts, and water-rich silicate–iron ± (Mn) fluid media. The clinkers were formed less than 2 Ma ago in various parts of the Choir–Nyalga basin by melting Early Cretaceous mudstones with bulk composition varies from dacitic to andesitic. The pyrogenic transformations of the mudstones were nearly isochemical, except only for volatile components. The CM melt rocks of basaltic andesitic composition were formed via melting carbonate–silicate sediments at temperatures above 1450°C. The Ca- and Fe-enriched and silicaundersaturated paralavas crystallized near the surface at temperatures higher than 900–1100°C and oxygen fugacity \(f_{O_2 }\) between the IW and QFM buffers. In local melting domains of the carbonate–silicate sedimentary rocks and in isolations of the residual melts among the paralava matrix the fluid pressure was higher than the atmospheric one. The bulk composition, mineral and phase assemblages of CM rocks of the Nyalga complex are very diverse (dacitic, andesitic, basaltic andesitic, basaltic, and silica-undersaturated mafic) because the melts crystallized under unequilibrated conditions and were derived by the complete or partial melting of clayey and carbonate–silicate sediments during natural coal fires.     

Water diffusion in Mount Changbai peralkaline rhyolitic melt

Diffusion couple experiments with wet half (up to 4.6 wt%) and dry half were carried out at 789–1,516 K and 0.47–1.42 GPa to investigate water diffusion in a peralkaline rhyolitic melt with major oxide concentrations matching Mount Changbai rhyolite. Combining data from this work and a related study, total water diffusivity in peralkaline rhyolitic melt can be expressed as:

$ D_{{{\text{H}}_{ 2} {\text{O}}_{\text{t}} }} = D_{{{\text{H}}_{ 2} {\text{O}}_{\text{m}} }} \left( {1 - \frac{0.5 - X}{{\sqrt {[4\exp (3110/T - 1.876) - 1](X - X^{2} ) + 0.25} }}} \right), $

$ {\text{with}}\;D_{{{\text{H}}_{ 2} {\text{O}}_{\text{m}} }} = \exp \left[ { - 1 2. 7 8 9- \frac{13939}{T} - 1229.6\frac{P}{T} + ( - 27.867 + \frac{60559}{T})X} \right], $

where D is in m² s^?1, T is the temperature in K, P is the pressure in GPa, and X is the mole fraction of water and calculated as X = (C/18.015)/(C/18.015 + (100 ? C)/33.14), where C is water content in wt%. We recommend this equation in modeling bubble growth and volcanic eruption dynamics in peralkaline rhyolitic eruptions, such as the ~1,000-ad eruption of Mount Changbai in North East China. Water diffusivities in peralkaline and metaluminous rhyolitic melts are comparable within a factor of 2, in contrast with the 1.0–2.6 orders of magnitude difference in viscosities. The decoupling of diffusivity of neutral molecular species from melt viscosity, i.e., the deviation from the inversely proportional relationship predicted by the Stokes–Einstein equation, might be attributed to the small size of H₂O molecules. With distinct viscosities but similar diffusivity, bubble growth controlled by diffusion in peralkaline and metaluminous rhyolitic melts follows similar parabolic curves. However, at low confining pressure or low water content, viscosity plays a larger role and bubble growth rate in peralkaline rhyolitic melt is much faster than that in metaluminous rhyolite.

     

Peralkalinity, Al{rightleftharpoons}Si Substitution, and Solubility Mechanisms of H2O in Aluminosilicate Melts

The solubility mechanisms of H₂O in peralkaline sodium aluminosilicatequenched melts (anhydrous NBO/T = 0.5) have been studied withRaman spectroscopy as a function of Al/(Al + Si) (0–0–3)and H²O content (0–7.5 wt.%). The coexisting structuralunits in the anhydrous quenched melts are TO2 (Q4), T2O5(Q3),and TO3 (Q2). In Al-free Na₂Si₄O₉ (NS4) melt, H₂O forms complexes with Na⁺(Na–OH bonds) and with Si⁴⁺ (Si–OH bonds). MolecularH₂O is also detected. TO3 structural units are not detectedin this composition. In the H₂O concentration range between0 and 4 wt.%, there is an approximately 20% increase in NBO/Tresulting from the increased abundance ratio, T₂O₅/TO2. Withfurther increments in water activity, the NBO/T of hydrous NS4melt is reduced. The depolymerization results from hydroxylationof the silica tetrahedra, whereas polymerization is due to formationof complexes with Na–OH bonding. In Al-bearing compositions on the Na₂Si₄O₉–Na₂(NaAl)₄O₉–join, there is evidence for Al–OH bonding in additionto Na–OH and Si–OH bonds. Among these complexes,the relative abundance of those with Si–OH bonds diminisheswith increasing Al/(A1 + Si), whereas complexes with Al–OHand Na–OH bonds become more important. Complexes withNa–OH bonds dominate for H₂O4 wt.%, whereas complexeswith Al–OH dominate at higher water content. The threestructural units, TO₃, T₂O₅, and TO², were observed in bothanhydrous and hydrous peralkaline sodium aluminosilicate melts.Their abundance varies, however, with the H₂O concentrationin the melts. The NBO/T decreases to a minimum (a 30–50%lowering of NBO/T relative to anhydrous materials) for low H₂Ocontents (3–4 wt.% H₂O), and increases as the H₂O contentis increased further.     

The speciation of water in silicate melts

Previous models of water solubility in silicate melts generally assume essentially complete reaction of water molecules to hydroxyl groups. In this paper a new model is proposed that is based on the hypothesis that the observed concentrations of molecular water and hydroxyl groups in hydrous silicate glasses reflect those of the melts from which they were quenched. The new model relates the proportions of molecular water and hydroxyl groups in melts via the following reaction describing the homogeneous equilibrium between melt species: H₂O_molecular (melt) + oxygen (melt) = 2OH (melt). An equilibrium constant has been formulated for this reaction and species are assumed to mix ideally. Given an equilibrium constant for this reaction of 0.1–0.3, the proposed model can account for variations in the concentrations of molecular water and hydroxyl groups in melts as functions of the total dissolved water content that are similar to those observed in glasses. The solubility of molecular water in melt is described by the following reaction: H₂O (vapor) = H₂O_molecular (melt).These reactions describing the homogeneous and heterogeneous equilibria of hydrous silicate melts can account for the following observations: the linearity between f_H2O and the square of the mole fraction of dissolved water at low total water contents and deviations from linearity at high total water contents; the difference between the partial molar volume of water in melts at low total water contents and at high total water contents; the similarity between water contents of vapor-saturated melts of significantly different compositions at high pressures versus the dependence on melt composition of water solubility in silicate melts at low pressures; and the variations of viscosity, electrical conductivity, the diffusivity of “water,” the diffusivity of cesium, and phase relationships with the total dissolved water contents of melts.This model is thus consistent with available observations on hydrous melt systems and available data on the species concentrations of hydrous glasses and is easily tested, since measurements of the concentrations of molecular water and hydroxyl groups in silicate glasses quenched from melts equilibrated over a range of conditions and total dissolved water contents are readily obtainable.     

Diffusive dissolution in ternary systems: analysis with applications to quartz and quartzite dissolution in molten silicates

Exact solutions to equations governing isothermal diffusive dissolution of a crystalline slab in a ternary liquid were obtained to include the effect of coupled chemical diffusion in the liquid. These analytical results, supplemented by approximate solutions valid for slow dissolving, provide new insights into the characteristics of diffusive dissolution in ternary systems. Dissolution rate is proportional to square root of time in diffusive dissolution. The coefficient of proportionality is a function of diffusion coefficients, liquidus relation, melt composition at the crystal–melt interface, and compositions of the dissolving crystal and starting melt. In the limit of slow dissolving, the dissolution rate can be written in terms of three dimensionless parameters that are functions of the aforementioned parameters. Dissolution rate is proportional to the diffusion rate of the slow eigen component in the melt when the diffusion rate of the minor eigen component is much slower than the diffusion rate of the major eigen component.Laboratory experiments of diffusive dissolution of single crystals and polycrystalline aggregates of quartz in a haplodacitic melt (25 wt.% CaO, 15 wt.% Al₂O₃, and 60 wt.% SiO₂) were conducted at 1500°C and 1 GPa. Measured dissolution distances (X_b, in microns) are proportional to the square root of experimental run time (t, in seconds), X_b = −0.620 (±0.019) √t. Chemical concentration profiles measured from quenched melts are invariant with time when displayed against the distance (measured from the crystal–melt interface) normalized by the square root of time. The melt compositions at the crystal–melt interface, extrapolated from the measured diffusion profiles in the quenched melts, are within 0.2 wt.% of the independently measured quartz liquidus in the ternary CaO–Al₂O₃–SiO₂ at 1500°C and 1 GPa. These results suggest that crystal and melt are in chemical equilibrium at their interface shortly after the onset of dissolution. Diffusive dissolution of quartz and quartzite is characterized by slow dissolving. Using quartz liquidus as one of the boundary conditions, it has been shown that the calculated dissolution distances and concentration profiles are in good agreement with the experimentally measured ones. Coupled diffusion played an essential role in quartz and quartzite dissolution in haplodacitic to haplobasaltic melts, and is likely to play an important role in diffusion-limited kinetic processes such as crystal growth and dissolution in natural melts of basaltic–rhyolitic compositions.     

Thermodynamic controls on element partitioning between titanomagnetite and andesitic–dacitic silicate melts

Titanomagnetite–melt partitioning of Mg, Mn, Al, Ti, Sc, V, Co, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Hf and Ta was investigated experimentally as a function of oxygen fugacity (fO₂) and temperature (T) in an andesitic–dacitic bulk-chemical compositional range. In these bulk systems, at constant T, there are strong increases in the titanomagnetite–melt partitioning of the divalent cations (Mg²⁺, Mn²⁺, Co²⁺, Ni²⁺, Zn²⁺) and Cu²⁺/Cu⁺ with increasing fO₂ between 0.2 and 3.7 log units above the fayalite–magnetite–quartz buffer. This is attributed to a coupling between magnetite crystallisation and melt composition. Although melt structure has been invoked to explain the patterns of mineral–melt partitioning of divalent cations, a more rigorous justification of magnetite–melt partitioning can be derived from thermodynamic principles, which accounts for much of the supposed influence ascribed to melt structure. The presence of magnetite-rich spinel in equilibrium with melt over a range of fO₂ implies a reciprocal relationship between a(Fe²⁺O) and a(Fe³⁺O_1.5) in the melt. We show that this relationship accounts for the observed dependence of titanomagnetite–melt partitioning of divalent cations with fO₂ in magnetite-rich spinel. As a result of this, titanomagnetite–melt partitioning of divalent cations is indirectly sensitive to changes in fO₂ in silicic, but less so in mafic bulk systems.