Viscosities of liquid albite (NaAlSi 3 O 8 ) and a Himalayan leucogranite were measured near the glass transition at a pressure of one atmosphere for water contents of 0, 2.8 and 3.4 wt.%. Measured viscosities range from 10 13.8 Pa.s at 935 K to 10 9.0 Pa.s at 1119 K for anhydrous granite, and from 10 10.2 Pa.s at 760 K to 10 12.9 Pa.s at 658 K for granite containing 3.4 wt.% H 2 O. The leucogranite is the first naturally occurring liquid composition to be investigated over the wide range of T-X(H 2 O) conditions which may be encountered in both plutonic and volcanic settings. At typical magmatic temperatures of 750°C, the viscosity of the leucogranite is 10 11.0 Pa.s for the anhydrous liquid, dropping to 10 6.5 Pa.s for a water content of 3 wt.% H 2 O. For the same temperature, the viscosity of liquid NaAlSi 3 O 8 is reduced from 10 12.2 to 10 6.3 Pa.s by the addition of 1.9 wt.% H 2 O. Combined with published high-temperature viscosity data, these results confirm that water reduces the viscosity of NaAlSi 3 O 8 liquids to a much greater degree than that of natural leucogranitic liquids. Furthermore, the viscosity of NaAlSi 3 O 8 liquid becomes substantially non- Arrhenian at water contents as low as 1 wt.% H 2 O, while that of the leucogranite appears to remain close to Arrhenian to at least 3 wt.% H 2 O, and viscosity-temperature relationships for hydrous leucogranites must be nearly Arrhenian over a wide range of temperature and viscosity. Therefore, the viscosity of hydrous NaAlSi 3 O 8 liquid does not provide a good model for natural granitic or rhyolitic liquids, especially at lower temperatures and water contents. Qualitatively, the differences can be explained in terms of configurational entropy theory because the addition of water should lead to higher entropies of mixing in simple model compositions than in complex natural compositions. This hypothesis also explains why the water reduces magma viscosity to a larger degree at low temperatures, and is consistent with published viscosity data for hydrous liquid compositions ranging from NaAlSi 3 O 8 and synthetic haplogranites to natural samples. Therefore, predictive models of magma viscosity need to account for compositional variations in more detail than via simple approximations of the degree of polymerisation of the melt structure.

Isobaric crystallisation paths obtained from phase equilibrium experiments show that, whereas in rhyolitic compositions melt fraction trends are distinctly eutectic, dacitic and more mafic compositions have their crystallinities linearly correlated with temperatures. As a consequence, the viscosities of the latter continuously increase on cooling, whereas for the former they remain constant or even decrease during 80% of the crystallisation interval, which opens new perspectives for the fluid dynamical modelling of felsic magma chambers. Given the typical dyke widths observed for basaltic magmas, results of analogue modelling predict that injection of mafic magmas into crystallising intermediate to silicic plutons under pre-eruption conditions cannot yield homogeneous composition. Homogenisation can occur, however, if injection takes place in the early stages of magmatic evolution (i.e. at near liquidus conditions) but only in magmas of dacitic or more mafic composition. More generally, the potential for efficient mixing between silicic and mafic magmas sharing large interfaces at upper crustal levels is greater for dry basalts than for wet ones. At the other extreme, small mafic enclaves found in many granitoids behave essentially as rigid objects during a substantial part of the crystallisation interval of the host magmas, which implies that finite strain analyses carried out on such markers can give only a minimum estimate of the total amount of strain experienced by the host pluton. Mafic enclaves carried by granitic magmas behave as passive markers only at near solidus conditions, typically when the host granitic magma shows near-solid behaviour. Thus they cannot be used as fossil indicators of direction of magmatic flow.