5:15 PM - 6:30 PM
[SIT17-P01] Viscosity of silicate melts under high pressures
Keywords:Viscosity, Silicate melt, High pressure
I would like to review the viscosity measurements of silicate melts at high pressure. The viscosity of silicate melts controls their mobility, and gives us important knowledge about the timescale of volcanic activities and eruption styles in the present Earth. In addition, viscosity plays a central role in the dynamics of a terrestrial magma ocean, core formation, and chemical differentiation of the mantle in the Earth’s early history. For silicate melts, viscosity measurements at high pressure have been performed by a falling sphere method using a large volume press. The falling sphere method has been significantly advanced by combination with synchrotron-based X-ray radiography, which enables us to determine the melt viscosity more accurately. Because the viscosity of silicate melts is considered to be closely related to the network structure, simultaneous measurements of viscosity and structure at high pressure and high temperature significantly have enhanced our understanding of the relationship between viscosity and structure.
The ratio between the number of non-bridging oxygen anions (NBO) and that of tetrahedrally coordinated cations (T), NBO/T, is widely used as an indicator of the network structure of silicate melts. The degree of polymerization, which increases with decreasing NBO/T, has long been recognized as a fundamental structural factor influencing physical properties. Also, the viscosity of silicate melts is characterized by the degree of polymerization. Polymerized melts show higher viscosity at ambient pressure due to their strong three-dimensional network featuring oxygen bridges between tetrahedral cations. Depolymerized melts have lower tetrahedral connectivity and lower viscosity. The contrasting behavior between polymerized and depolymerized melts is also shown in the pressure dependence of viscosity. Although free-volume theory predicts that viscosity increases with pressure, isothermal viscosity of polymerized melts decreases with pressure up to 3-5 GPa, above which it turns over to positive pressure dependence.
The ratio between the number of non-bridging oxygen anions (NBO) and that of tetrahedrally coordinated cations (T), NBO/T, is widely used as an indicator of the network structure of silicate melts. The degree of polymerization, which increases with decreasing NBO/T, has long been recognized as a fundamental structural factor influencing physical properties. Also, the viscosity of silicate melts is characterized by the degree of polymerization. Polymerized melts show higher viscosity at ambient pressure due to their strong three-dimensional network featuring oxygen bridges between tetrahedral cations. Depolymerized melts have lower tetrahedral connectivity and lower viscosity. The contrasting behavior between polymerized and depolymerized melts is also shown in the pressure dependence of viscosity. Although free-volume theory predicts that viscosity increases with pressure, isothermal viscosity of polymerized melts decreases with pressure up to 3-5 GPa, above which it turns over to positive pressure dependence.