The viscosity of hydrous phonolites and trachytes

The I-arm viscosities of hydrated synthetic iron-free phonolite and trachyt e melts are reported between 10(8.4) and 10(13.1) Pa s for water contents b etween 0 and 5 wt.%. These show a very strong reduction with increasing wat er content, particularly at low contents. Empirical formulae are derived fo r 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 a for the phonolite and 1120 Pa s for the trachyte. Compariso ns with data on peralkaline rhyolites show that the effects of dissolved wa ter are similar, even though the viscosities of the different compositions vary. This indicates that the mechanism by which water reduces the viscosit ies of partially depolymerized aluminosilicate liquids is independent of th e 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 importan t in these compositions, in addition to Si-O-Si bond-breaking and dissoluti on of molecular water. (C) 2001 Elsevier Science B.V. All rights reserved.