Viscosities of liquid albite (NaAlSi3O8,) 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 10102 Pa.s at 760 K to 1012,9 Pa.s at 658 K for granite containing 3.4 wt.% H2O. The leucogranite is the first naturally occurring liquid composition to be investigated over the wide range of T-X(H2O) conditions which may be encountered in both plutonic and volcanic settings. At typical magmatic temperatures of 750 degrees 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.% H2O. For the same temperature, the viscosity of liquid NaAlSi3O8 is reduced from loll I to 106 1 Pa.s by the addition of 1.9 wt.% H2O. Combined with published high-temperature viscosity data, these results confirm that water reduces the viscosity of NaAlSi3O8 liquids to a much greater degree than that of natural leucogranitic liquids. Furthermore, the viscosity of NaAlSi3O8 liquid becomes substantially non Arrhenian at water contents as low as I wt.% H2O, while that of the leucogranite appears to remain close to Arrhenian to at least 3 wt.% H2O, and viscosity-temperature relationships for hydrous leucogranites must be nearly Arrhenian over a wide range of temperature and viscosity. Therefore, the viscosity of hydrous NaAlSi3O8 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 NaAlSi3O8 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.
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