The viscosity of hydrous phonolites and trachytes

The 1-atm viscosities of hydrated synthetic iron-free phonolite and trachyte melts are reported between 108.4 and 1013.1 Pa s for water contents between 0 and 5 wt.%. These show a very strong reduction with increasing water content, particularly at low contents. Empirical formulae are derived for the dependence of viscosity on temperature and water content. At magmatic temperatures of about 1100 K and water contents of 5 wt.%, viscosities are about 910 Pa s for the phonolite and 1120 Pa s for the trachyte. Comparisons with data on peralkaline rhyolites show that the effects of dissolved water are similar, even though the viscosities of the different compositions vary. This indicates that the mechanism by which water reduces the viscosities of partially depolymerized aluminosilicate liquids is independent of the alkali/alkaline earth element ratio. In addition, no major change in this pattern is observed whether water dissolves primarily as OH– or molecular H2O. This suggests that a third water-dissolution mechanism may be important in these compositions, in addition to Si–O–Si bond-breaking and dissolution of molecular water.