VISCOSITY DATA FOR HYDROUS PERALUMINOUS GRANITIC MELTS - COMPARISON WITH A METALUMINOUS MODEL

We performed 27 viscosity determinations on dry and water-bearing pera luminous haplogranitic melts. The dry melt compositions cover the rang e of normative corundum to be expected in peraluminous granitic melts in nature. The compositions are based on addition of Al2O3 to a haplog ranitic melt (HPG8) whose composition is near that of the projection o f the 2 kbar H2O-saturated minimum melt composition into the system Na AlSi3O8-KAlSi3O8-SiO2. The H2O contents of the hydrous melts were anal yzed using Karl Fischer titration ranging from 1 to 3 wt%. The viscosi ty determinations were performed using a modified micropenetration met hod in the viscosity range of 10(10) to 10(11) Pa.s, at 1 atm pressure , and in the temperature ranges of 880-940 degrees C and 470-640 degre es C for the dry and wet melts, respectively. For the dry peraluminous melts in this high viscosity range, addition of the first few percent of normative corundum to a metaluminous granitic melt increases the v iscosity, which remains nearly constant despite further addition of Al 2O3. Thus a viscosity maximum is inferred for dry slightly peraluminou s granitic melts. The hydrous melt viscosity data were compared with t he recent calculational model of Hess and Dingwell (1996), which was b ased on and designed for metaluminous melt viscosities. That model is capable of describing the viscosities of hydrous peraluminous granitic melts within the uncertainties stated for its application in metalumi nous melts.