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The main objective of this study is to investigate and model the viscosity of multicomponent natural silicate melts and constrain the compositional effects which affect such a parameter. The results of this study, relevant to all petrological and volcanological processes which involve some transport mechanism, will be applied to volcanic setting. An extensive experimental study was performed, which constituted the basis for the general modelling of Newtonian viscosity in terms of composition and temperature. Composition, viscosity and density of selected samples were investigated at different water contents. The experimental method involved measuring the viscosity of dry and hydrated melts under superliquidus and supercooled conditions. In the high temperature range (1050 – 1600 °C) viscosities from 10-0.5 to 105 Pa·s were obtained using a concentric cylinder apparatus. Measurements of both dry and hydrated samples in the low temperature (616-860 °C) - high viscosity (108.5 – 1012 Pa·s) interval, from glassy samples quenched after high temperature viscometry, were performed using the dilatometric method of micropenetration. Hydrated samples measured in the supercooled state were synthesized, using a piston cylinder apparatus, between 1100° and 1600° C at 10 kbar. Water contents were measured using the Karl Fischer Titration (KFT) method. Fourier-Transform Infrared (FTIR) spectroscopy was used before and after the experiments in order to check that the water content was homogeneously distributed in the samples and that water had not been lost. Major element compositions of the dry remelted samples were determined using an electron microprobe. Newtonian viscosities of silicate liquids were investigated in a range between 10-1 to 1011.6 Pa s and parameterised using the non-linear 3 parameter (ATVF, BTVF and T0) TVF equation. The data provided in this work are combined also with previous data from Whittington et al. (2000, 2001); Dingwell et al. (1996); Neuville et al. (1993). There are strong numerical correlations between parameters (ATVF, BTVF and T0) that mask the effect of composition. Wide ranges of ATVF, BTVF and T0 values can be used to describe individual datasets. This is true even when the data are numerous, well-measured and span a wide range of experimental conditions. In particular, “strong” liquids (liquids that are Arrhenian or slightly deviate from Arrhenian behaviour) place only minor restrictions on the absolute ranges of ATVF, BTVF and T0. Therefore, strategies for modelling the effects on compositions should be built around high-quality datasets collected on non-Arrhenian liquids. x The relationships between important quantities such as the fragility F, characterizing the deviation from Arrhenian rheological behaviour, are quantified in terms of the chemical, structure-related parameter NBO/T. Initial addition of network modifying elements to a fully polymerised liquid (i.e. NBO/T=0) results in a rapid increase in F. However, at NBO/T values above 0.4-0.5 further addition of a network modifier has little effect on fragility. This parameterisation indicates that this sharp change in the variation of fragility with NBO/T is due to a sudden change in the configurational properties and rheological regimes, owing to the addition of network modifying elements. The resulting TVF parameterisation has been also used to build up a predictive model for Arrhenian to non-Arrhenian melt viscosity. The model accommodates the effect of composition via an empirical parameter called here the “structure modifier” (SM). SM is the summation of molar oxides of Ca, Mg, Mn, half of the total iron Fetot, Na and K. This approach is validated by the highly predictive capability of the viscosity model. The model reproduces all the original data set with about 10%, of the measured values of logη over the entire range of composition in the temperature interval 700-1600 °C. The combination of calorimetric and viscosimetric data has enabled a simple expression to be used to predict shear viscosity at the glass transition, that is the temperature which defines the transition from a liquid-like to a solid-like rheological behaviour. The basis for this stems from the equivalence of the relaxation times for both enthalpy and shear stress relaxation in a wide range of silicate melt compositions (Gottsmann et al., 2002). A shift factor that relates cooling rate data with viscosity at the glass transition appears to be slightly dependent on the melt composition. Finally, the effect of water content on decreasing the viscosity of silicate melts has also been parameterised using a modified TVF expression (Giordano et al., 2000). This leads to an improvement in our knowledge of the non-Arrhenian behaviour of silicate melts over a wide compositional range from basaltic to rhyolitic and from trachytic to peralkaline phonolite compositions in the temperature interval pertaining to volcanic and subvolcanic processes. The viscosities of natural hydrous basaltic liquids are shown to be lower than those of hydrous phonolites, whereas thachytes show viscosity that are higher than those of phonolites and lower that those of rhyolites. This is consistent with the style of eruption associated with these compositions, with trachytes generating eruptions that are dominantly explosive (e.g. xi Phlegrean Fields volcano), compared to the highly explosive style of rhyolitic volcanoes, the mixed explosive-effusive style of phonolitic volcanoes (e.g. Vesuvius) and the dominantly effusive style of basalts. Variations in composition between the trachytes translate into differences in liquid viscosity of nearly two orders of magnitude in dry conditions, and less than one order of magnitude in hydrous conditions. These differences increase significantly when the estimated eruptive temperatures of different eruptions at Phlegrean Fields are taken into account. At temperatures close to those of natural magmas and in the case of low viscosity hydrous liquids the uncertainty of the calculations is large, although it cannot be quantified, due to a lack of measurements under these conditions.