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We present quasielastic neutron scattering results on hydrous silica, sodium aluminosilicate, and sodium trisilicate melts with 10 mol% total water content, studied at high temperature under high pressure. Combining neutron time-of-flight spectrometry with neutron backscattering, intrinsic, microscopic proton dynamics is investigated on a time scale from 0.2 ps up to 1 ns between 850 K and 1250 K. All three hydrous silicate melts exhibit a relatively slow proton dynamics, although the melt viscosity is drastically reduced upon water dissolution. The self-diffusion coefficient of proton in the hydrous sodium trisilicate melt is on the order of 10 −11 m 2 s −1 , two orders of magnitude slower than the sodium dynamics in the corresponding anhydrous melt. The proton dynamics in hydrous silica and albite is not faster than that time scale. We show that the transport mechanism involves not only –OH but also molecular water species. All protons are mobile during the transport of the water instead of diffusion of a specific water speciation. These characteristics of the proton structural relaxation in the melt can be attributed to a transport in a complex H-bonding environment.

In this manuscript, we focused on the properties of magmas and the effects of water on these properties using classical molecular dynamics simulations. After having pointed out the importance of water in geological processes and presented some basic principles of molecular dynamics simulations, we present a force field for dry silicate melts. We use this force field to study the equation of state, the structure and the transport properties (viscosity, electrical conductivity, diffusion coefficient) of silicate melts of terrestrial and extra-terrestrial compositions. These results are systematically compared with data of the literature. We then develop a force field for pure water compatible to the one proposed for silicates. The validity of this force field is investigated by comparison with experimental studies, ab-initio simulations and other MD simulations using classical force fields. At last we add a water-silicate interaction to the two former force fields in order to describe hydrous silicate melts. This force field is used to study the properties of water-silicate mixtures (solubility, surface tension). The structure and properties of hydrous silicate melts are evaluated as well as the speciation of water, the density, the viscosity, the electrical conductivity and the diffusion coefficients. The local structure around protonated species (OH-,H2O and H3O+) and their diffusion coefficients are also determined. All these results are compared with available data of the literature., Dans ce manuscrit, nous nous sommes intéressés aux propriétés des magmas ainsi qu’aux effets de l’eau sur celles-ci en utilisant la dynamique moléculaire classique. Après avoir mis en avant l’importance de l’eau dans les phénomènes géologiques et présenté quelques bases de dynamique moléculaire, nous présentons un champ de force pour les liquides silicatés anhydres. Nous utilisons ce champ de force pour étudier l’équation d’état, la structure et les propriétés de transport (viscosité, conductivité électrique, coefficient de diffusion) de liquides silicatés de composition aussi bien terrestre qu’extra-terrestre. Ces résultats sont systématiquement comparés avec les données de la littérature. Nous développons ensuite un champ de force pour l’eau pure compatible avec celui proposé pour les silicates. La validité de ce champ de force est étudiée par comparaison avec des résultats expérimentaux, de simulations ab-initio et d’autre champs de force classiques. Nous ajoutons enfin une interaction eau-silicate aux deux champs de forces précédents pour obtenir un champ de force décrivant les silicates hydratés. Celui-ci est utilisé pour étudier les propriétés des mélanges eau-silicate (solubilité, tension de surface). La structure et les propriétés des silicates hydratés sont ensuite étudiées, notamment la spéciation de l’eau, la densité, la viscosité, la conductivité électrique et les coefficients de diffusion. La structure locale autour des espèces protonées (OH-,H2O et H3O+) et leurs coefficients de diffusion sont également déterminés. Ces différents résultats sont là encore comparés aux données existantes dans la littérature.

International audience; The equilibrium molar volumes of four series of anhydrous and hydrous aluminosilicate glasses and liquids (0 to 11 mol% H2O) were determined at 1 bar between 300 and 1050 K. The anhydrous compositions range from highly polymerized NaAlSi3O8 to depolymerized synthetic iron-free analogs of tephrite and foidite magma compositions (NBO/T = 0.8 and 1.5, respectively). For each sample the volume was derived from the room-temperature density of the glass and the thermal expansivity of the glass and supercooled liquid from 300 K to a temperature about 50 K higher than the standard glass transition. The partial molar coefficient of thermal expansion of water in hydrous silicate glasses is about (6.2 ± 3.5) × 10− 5 K− 1, and in the melts ranges from 11 × 10− 5 to 36 × 10− 5 K− 1. The present molar volumes of hydrous supercooled liquids are reproduced with the model of Ochs and Lange (1999) to within 1.1%, except for the hydrous foidite series. This agreement confirms that the partial molar volume of water (View the MathML sourceV¯H2O) near the glass transition cannot depend strongly on the chemical composition of the silicate end-member, nor on water speciation. In order to reproduce the molar volumes of the foidite series, a combined model (using Lange, 1997 and Courtial and Dingwell, 1999 models and values derived from the new data) is used where an excess volume term between SiO2 and CaO is introduced. Finally, our experimental data are better fit if View the MathML sourceV¯H2O = 23.8 ± 0.5 cm3 mol− 1 at 1273 K, and View the MathML sourcedV¯H2OdT = 15.9 ± 1.6 cm3 mol− 1 K− 1. Contrasting trends are also observed for the configurational contributions to the expansivity with a positive slope of View the MathML sourcedViconfdT versus water for the most polymerized base compositions (NBO/T ≤ 0.21) and a negative slope for the two most depolymerized base compositions with NBO/T of 0.86 and 1.51.

The specific heat capacity (C p) of six variably hydrated (~3.5 wt% H2O) iron-bearing Etna trachybasaltic glasses and liquids has been measured using differential scanning calorimetry from room temperature across the glass transition region. These data are compared to heat capacity measurements on thirteen melt compositions in the iron-free anorthite (An)–diopside (Di) system over a similar range of H2O contents. These data extend considerably the published C p measurements for hydrous melts and glasses. The results for the Etna trachybasalts show nonlinear variations in, both, the heat capacity of the glass at the onset of the glass transition (i.e., C p g ) and the fully relaxed liquid (i.e., C p l ) with increasing H2O content. Similarly, the “configurational heat capacity” (i.e., C p c = C p l − C p g ) varies nonlinearly with H2O content. The An–Di hydrous compositions investigated show similar trends, with C p values varying as a function of melt composition and H2O content. The results show that values in hydrous C p g , C p l and C p c in the depolymerized glasses and liquids are substantially different from those observed for more polymerized hydrous albitic, leucogranitic, trachytic and phonolitic multicomponent compositions previously investigated. Polymerized melts have lower C p l and C p c and higher C p g with respect to more depolymerized compositions. The covariation between C p values and the degree of polymerization in glasses and melts is well described in terms of SMhydrous and NBO/T hydrous. Values of C p c increase sharply with increasing depolymerization up to SMhydrous ~ 30–35 mol% (NBO/T hydrous ~ 0.5) and then stabilize to an almost constant value. The partial molar heat capacity of H2O for both glasses ( $$ C_{{{\text{p}}\;{\text{H}}_{2} {\text{O}}}}^{\text{g}} $$ ) and liquids ( $$ C_{{{\text{p}}\;{\text{H}}_{2} {\text{O}}}}^{\text{l}} $$ ) appears to be independent of composition and, assuming ideal mixing, we obtain a value for $$ C_{{{\text{p}}\;{\text{H}}_{2} {\text{O}}}}^{\text{l}} $$ of 79 J mol−1 K−1. However, we note that a range of values for $$ C_{{{\text{p}}\;{\text{H}}_{2} {\text{O}}}}^{\text{l}} $$ (i.e., ~78–87 J mol−1 K−1) proposed by previous workers will reproduce the extended data to within experimental uncertainty. Our analysis suggests that more data are required in order to ascribe a compositional dependence (i.e., nonideal mixing) to $$ C_{{{\text{p}}\;{\text{H}}_{2} {\text{O}}}}^{\text{l}} $$ .

The ratio of molecular water and SiOH groups in silicates has been discussed for a long time due to its relevance for volcanic processes. With the help of neutrons it could be shown that the ratio SiOH to H_2O does not govern the dynamic properties of the silicate. Statistic quantities like the neutron scattering structure factor can also be extracted from molecular dynamics computer simulations. A comparison of the structure factor to experimental data of the glass shows that the simulations are very close to reality. The simulations can then give access to the hydrogen diffusion mechanisms in the melt. They allow unknown diffusion mechanisms to be revealed. Das Verhältnis von molekularem Wasser und SiOH Gruppen wird in Silikaten seit Langem wegen der Auswirkungen auf vulkanische Prozesse diskutiert. Es konnte experimentell mit Hilfe von Neutronen gezeigt werden, dass das Verhältnis von SiOH zu H_2O sich nicht auf die dynamischen Eigenschaften des Silikatglases auswirkt. Statistische Größen wie der Neutronenstrukturfaktor können ebenfalls aus ab initio Molekulardynamiksimulationen extrahiert werden. Ein Vergleich des Strukturfaktors zu experimentellen Daten des Glases zeigt, dass die Simulationen sehr nahe an der Realität sind. Die Simulationen geben dann einen konkreten Zugang zu den Wasserstoffdiffusionsreaktionen in der Schmelze. Sie ermöglichen es bisher unbekannte Mechanismen für die Diffusion aufzudecken.

Sophisticated models of volcanic scenarios are increasingly sensitive to the accuracy of their input parameters and constitutive equations for magma properties. Viscosity is certainly one of the most important magma properties, but only recently systematic investigations on silicate liquids with natural compositions have started. We investigated the Newtonian viscosity of dry and hydrous phonolitic and trachytic melts from Vesuvius and Phlegrean Fields volcanic complexes, respectively. The analysed samples come from the deposits of the AD 1631 (Vesuvius) and ca. 4400 BP Agnano Monte Spina (AMS) (Phlegrean Fields) eruptions that are commonly taken as reference events for the most hazardous scenarios in case of reactivation of the two volcanoes. Samples were hydrated via piston cylinder synthesis at P= 10 kbar and T = 1600 °C. The dry high temperature and dry or hydrous low temperature viscosities were measured by a combination of micropenetration and concentric cylinder techniques, covering a total temperature range from about 400 to 1500 °C, water content range from virtually dry to 3.8 wt.%, and viscosity range from 102 to 1012 Pa s. The viscosity data for each composition were fitted by a modified Tamman–Vogel–Fulcher equation, allowing viscosity calculations at eruption temperatures and for dissolved water contents in the range of those examined. The viscosity data and model calculations were used for a comparison with other natural or synthetic phonolitic and trachytic melts, as well as with rhyolitic melts, for which viscosities had been measured. At water contents less than 1 wt.%, a trend of increasing viscosity from phonolitic to trachytic to rhyolitic melts is found. At water contents larger than 1 wt.%, the viscosity of trachytic melts is close to that of rhyolitic melts, while the viscosity of phonolitic melts is one to two orders of magnitude lower. A compositional parameter given by the (Na +K+ H)/(Si + Al) molar ratio is found to be linearly related to the low-T hydrous viscosity of the trachytic and phonolitic melts considered, either analysed in this work or taken from literature. Differently, the rhyolitic melt shows significant variations from the trend found for phonolitic and trachytic melts.