Contrib Mineral Petrol (1996) 124: 19—28  Springer-Verlag 1996 D.B. Dingwell · C. Romano · K.-U. Hess The effect of water on the viscosity of a haplogranitic melt under P-T-X conditions relevant to silicic volcanism Received: 17 August 1995/Accepted: 8 January 1996 Abstract The viscosities of hydrous haplogranitic melts synthesized by hydrothermal fusion at 2 kbar pressure and 800 to 1040° C have been measured at temperatures just above the glass transition and at a pressure of 1 bar using micropenetration techniques. The micropenetration viscometry has been performed in the viscosity range of 10 Pas to 10 Pa s. The samples ranged in water content from 0.4 to 3.5 wt%. For samples with up to 2.5 wt% H  O, the water con- tents have been determined using infrared spectroscopy obtained before and after each viscometry experiment to be constant over the duration of the measurements. Above this water content a measurable loss of water occurs during the viscometry. The viscosity data illustrate an extremely nonlinear decrease in viscosity with added water. The viscosity drops drastically with the addition of 0.5 wt% of water and then shallows out at water contents of 2 wt%. An additional viscosity datum point obtained from the analysis of ﬂuid inclusions in a water-saturated HPG8 conﬁrms a near invariance of the viscosity with the addition of water between 2 and 6 wt%. These measurements may be compared directly with the data of Hess et al. (1995, in press) for the eﬀects of excess alkali and alkaline earth oxides on the viscosity of HPG8 (also obtained at 1 bar). The viscosity of the melts, compared on an equivalent molar basis, in- creases in the order H  O((Li  O(Na  O( K  O(Rb  O,Cs  O(BaO(SrO(CaO( MgO( BeO). The extraordinary decrease in melt viscosity with added water is poorly reproduced by the calculation scheme of Shaw (1972) for the range of water contents investigated here. The speciation of water in the quen- D.B. Dingwell ( ) · C. Romano, K.-U. Hess Bayerisches Geoinstitut, Universita¨t Bayreuth, D-95440 Bayreuth, Germany Editorial responsibility: J. Hoefs ched glasses can be used to quantify the dependence of the viscosity on hydroxyl content. Considering only the hydroxyl groups as active ﬂuidizers in the hydrous melts the nonlinearity of the viscosity decrease and the diﬀerence with the eﬀects of the alkali oxides becomes larger. Consequences for degassing calcalkaline rhyolite are discussed. Introduction A serious gap in present knowledge of the properties of degassing subvolcanic silicic magmas is the description of melt viscosity at the P-¹-X conditions immediately prior to and during volcanic eruptions. Modeling of the dynamics of such systems is ﬂourish- ing at present and a further optimization of models for the kinetics of melt degassing, vesiculation and fragmentation would be aided greatly by reliable vis- cosity data obtained under the appropriate P-¹-X conditions. Equally important is the realization that fully gene- ralizable models for the P-¹-X dependence of melt viscosity will only be achievable when structure pro- perty relationships for the melts of interest, based on systematic studies of the variation of both structure and properties with P, ¹ and X, are available. Both of these considerations, together with the ob- servations from glass transition studies (Dingwell and Webb 1992) and recent viscometry (Lejeune et al. 1994; Schulze et al. in press) that present methods of estima- ting melt viscosity are not very accurate at low water contents, have led to the present investigation. We have chosen to study of the inﬂuence of minor amounts of water on the viscosity of a haplogranitic melt composition serving as a model for calcalkaline rhyolite. The results provide the most precise compari- son of the eﬀects of diﬀering network-modifying components on the melt viscosity hitherto possible and systematics emerge in the temperature and composition