Role of free volume in the viscous behavior of magma.

Abstract

In the present study, the effects of free volume on the viscous behavior of magmatic melts are discussed on the basis of free-volume theory. The most important conclusion is that the change of viscosity with temperature, pressure, and composition is related approximately to the change of free volume in the melts. It has been found that an increase of temperature and mafic components induces an increase of free volume in magmatic melts, and it is well known that they also tend to cause a decrease of viscosity. The relation between free volume and viscosity is interpreted as a reflection of the greater ease of movement of constituent units in melts that have more interstitial space. Pressure effects on viscosity are also interpreted as fundamentally based on the change of free volume. Melts having an abundance of free volume at 1 atm are depolymerized melts and show an increase of viscosity with pressure owing to a decrease of free volume. On the other hand, in polymerized melts having a small free volume at low pressures, the controlling process of viscous flow is not the change of free volume but making or breaking of bonds between constituent units of the melt. Sharma et al. (1979) suggested on the basis of Raman spectroscopy that the decrease of viscosity of polymerized melts at high pressures was caused by deformation of constituent unit accompanying the weakening of Si-O-Si bond. Our explanation of the behavior of polymerized melts is in harmony with their interpretation.