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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.

The effect of P2O5 on the viscosity of a haplogranitic (K2O-Na2O-Al2O3-SiO2) liquid has been determined at 1 atm pressure in the temperature interval of 700 - 1650°C. Viscosity measurements of a haplogranite, haplogranite + 5.1 wt.% P2O5 and haplogranite + 9.5 wt.% P2O5 have been performed using the concentric cylinder and micropenetration methods. The viscosity of haplogranite liquid decreases with the addition of P2O5 at all temperatures investigated. The viscosity decrease is nonlinear, with the strongest decrease exhibited at low P2O5 concentration. The temperature-dependence of the viscosity of all the investigated liquids is Arrhenian, as is the case for P2O5 liquid. The Arrhenian activation energy is slightly lower in the P2O5-bearing liquids than in the P2O5-free haplogranite with the result that the effect of P2O5 on viscosity is a (weak) function of temperature. At temperatures corresponding to the crystallization of phosphorus-rich granitic and pegmatitic systems the addition of 1 wt.% of P2O5 decreases the viscosity 0.2 log10 units. The effect of P2O5 on haplogranitic melt viscosity is much less than that for B2O3, F2O−1 on the same melt composition (Dingwell et al., 1992 and this study). This implies that P2O5 concentration gradients in high-silica melts during, for example, phosphate mineral growth or dissolution in granitic magmas, will not significantly influence melt viscosity.

The effect of B2O3 on the viscosity of a haplogranitic liquid (KrO-Na,O-AlrOr-SiO,) has been determined at I atm pressure in the temperature interval of 600-1600 °C. Viscosity measurementso f a haplogranite, haplogranite + 4.35 wt% B2O3 and haplogranite + 8.92 wt% B2O3 have been performed using the concentric cylinder and micropenetration methods. The viscosity of a B-enriched natural rhyolite obsidian, macusanite from Macusani, Peru, has also been determined. The viscosity of haplogranite liquid decreases with the addition of B2O3 at all temperatures investigated. The viscosity decrease is nonlinear, with the strongestd ecreasee xhibited at low B2O3 concentration. The temperature dependence of the viscosity of all the investigated liquids is Arrhenian, in strong contrast to the case for B2O3 Iiquid. The Arrhenian activation energy is much lower in the B2O3-bearing liquids than in the B2O3-free haplogranite, with the result that the effect of B2O3 on viscosity is a strong function of temperature. At temperatures corresponding to the crystallization of B-rich granitic and pegmatitic systems the addition of I wt% of B2O3 decreases the viscosity 2 orders of magnitude. The macusanite liquid exhibits a reduced viscosity compared with B-free rhyolite that is consistent with the synthetic liquid systematics. B must be considered as a fluxing agent in B-rich granitic and pegmatitlc systems.

The glass transition in silicate melts is a curve in time-temperature space marking the transition of the melt structure from an unrelaxed, disequilibrium glass to a relaxed, equilibrium liquid. Tracer diffusivity data obtained in glasses vs. liquids cannot be compared without consideration of the effects of this transition. For tracer diffusivity experiments, two time scales are important, the time duration of the experiment (τd) and the inverse of the jump frequency (τp) of the tracer. When the time duration of the experiments reaches the relaxation time-scale (τd = τs) of the melt a transition occurs from diffusion in an unrelaxed matrix (undergoing vibrational thermal expansion) to diffusion in a relaxed matrix (undergoing equilibrium, configurational and elastic, thermal expansion). At this transition, an inflection is observed in the temperature dependence of cationic tracer diffusivity. At temperatures below the inflection, the diffusivity is Arrhenian whereas at temperatures above the diffusivity is non-Arrhenian. At high temperatures the tracer diffusivities of the cations approach the value of diffusivity obtained from the Eyring relation (τp = τs). The contrasting, high-temperature, composition dependence of Na and Li vs. Co, Cs, Sr, Ba, Eu, Fe and C diffusivities can be explained in terms of the Eyring (network O and Si) diffusivity influencing the latter group. The contrasting high- vs. low-temperature, composition dependence of Ba and Sr diffusivities can be similarly explained. These latter observations indicate that all cationic diffusivities will be within a log10 unit of the Eyring oxygen diffusivity in melts with viscosities below 10 P.

The viscosity-temperature relationships of five melts on the join Na2Si2O2-Na4Al2O5 (5, 10, 20, 30 and 40 mole percent Na4Al2O5) have been measured in air, at 1 atm and 1000–1350°C with a concentric cylinder viscometer. All the melts on this join of constant bulk polymerization behave as Newtonian fluids, in the range of shear rates investigated, and the melts exhibit Arrhenian viscosity-temperature relationships. Isothermal viscosities on this join initially decrease and then increase with increasing mole percent Na4Al2O5. The minimum viscosity occurs near 20 mole percent Na4Al2O5 at 1000°C and moves to higher Na4Al2O5 content with increasing temperature. The observation of a viscosity minimum along the join Na2Si2-O5-Na4Al2O5 is not predicted based on earlier viscosity data for the system Na2O-Al2O3-SiO2 (RlEBLlNG, 1966) or based on calculation methods derived from this and other data (Bottinga and Weill, 1972). This unexpected behavior in melt viscosity-temperature relations emphasizes the need for a more complete data set in simple silicate systems. Previous spectroscopic investigation of melts on the join Na22Si2O5-Na4Al2O5 offer a structural explanation for the observed viscosity data in terms of a disproportionation reaction involving polyanionic units. Macroscopically, the viscosity data may be qualitatively reconciled with the configurational entropy model for viscous flow (Richet, 1984).

The effect of fluorine on melt viscosities of five compositions in the system Na2O-Al2O3- SiO2h as been investigateda t one atmospherea nd 1000-1600'Cb y concentric-cylinder viscometry. The compositions chosen were albite, jadeite and nepheline on the join NaAlOlSiO2 and two others of the join at 75 mole percent SiO2, one peralkaline and one peraluminous. All melt viscosities were independent of shear rate over two orders of magnitude, indicating Newtonian behavior. All viscosity-temperature relationships were Arrhenian within error. Fluorine reduces the viscosities and activation energies of all melts investigated. The viscosity-reducing power of fluorine increases with the SiO2 content of melts on the join NaAlO2-SiO2 and is a maximum at Na/Al (molar) = I for melts containing 75 mole percent SiO2. Fluorine and water have similar effects on aluminosilicate melt viscosities, probably due to depolymerization of these melts by replacement of Si-O-(Si, Al) bridges with Si-OH and Si-F bonds, respectively. Evidence from slag systems shows that fluorine also reduces the viscosity of depolymerized silicate melts. The viscous flow of phonolites, trachytes and rhyolites will be strongly afected by fluorine. It appears that fluorine contents of igneous rocks may be combined with water in calculation schemes for determining the viscosity of natural melts.