Pressure dependence of melt viscosities on the join diopside-albite

Abstract

Abstract The pressure dependence of melt viscosities on the join diopside-albite has been studied using falling-sphere viscometry. The five melt compositions investigated are: diopside, Ab 25 Di 75 , Ab 50 Di 50 , Ab 75 Di 25 and albite. Experiments were performed at 1500° and 1600°C and at pressures of 5, 10, 15, 20 and 25 kbar. The positive and negative pressure dependence of the viscosity of diopside and albite, respectively, were confirmed. All intermediate compositions show an initial decrease in viscosity with increasing pressure; however, melt of Ab 25 Di 75 composition passes through a minimum viscosity at approximately 12 kbar and 1600°C. This behavior is analogous to the variation in the viscosity of water with pressure at low temperature. It is suggested that the three-dimensional, fully polymerized, albite structure dominates flow at low pressures. With increasing pressure, disruption of this structure and decrease in the average size of the flow units leads to domination by the diopside structure. The variation in viscosity with composition along the join at one atmosphere can be adequately modelled using the Adam and Gibbs (1965) configurational entropy model with an additional two-lattice configurational entropy of mixing term. The pressure dependence of viscosity in the diopside-albite system, however, cannot be predicted by the model, because there is an absence of information on the pressure dependence of the model parameters. It is probable that relatively polymerized magmas (e.g. rhyolites to SiO 2 -saturated basalts) show a negative pressure dependence of viscosity to depths where they originate in the lower crust or upper mantle. In contrast, the most depolymerized, naturally-occurring melts, such as strongly SiO 2 -undersaturated basalts and picrites, may exhibit a viscosity minimum. The viscosity of these melts may be sufficiently high at depths within the upper mantle to inhibit their segregation, rise and eventual eruption at the surface.