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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.

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 $$ C_{{{\text{p}}\;{\text{H}}_{2} {\text{O}}}}^{\text{l}} $$ .

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.