HR: 0800h
AN: V41A-1367    [Abstracts]
TI: Pressure Dependence of Viscosity of Hydrous Rhyolitic Melts
AU: * Hui, H
EM: huih@umich.edu
AF: The University of Michigan, Dept. of Geological Sciences 425 E. University Ave. 2534 C.C. Little Building, Ann Arbor, MI 48109-1063 United States
AU: Xu, Z
EM: zhengjiu@umich.edu
AF: The University of Michigan, Dept. of Geological Sciences 425 E. University Ave. 2534 C.C. Little Building, Ann Arbor, MI 48109-1063 United States
AU: Zhang, Y
EM: youxue@umich.edu
AF: The University of Michigan, Dept. of Geological Sciences 425 E. University Ave. 2534 C.C. Little Building, Ann Arbor, MI 48109-1063 United States
AB: Knowledge of viscosity of silicate melts is critical to the understanding of igneous processes, such as bubble growth, magma fragmentation, etc. Numerous measurements have been made on the viscosities of natural and synthetic silicate melts, but only a few investigations have been carried out on the pressure dependence of viscosity of silicate melts. A new method using the cooling-rate method based on the interconversion of two water species in silicate melts (Zhang et al., 2003) is applied to investigate the pressure dependence of viscosity of hydrous rhyolitic melt at near glass-transition temperatures. All the samples used so far are natural obsidian glass with about 0.8 % H$_{2}$O by weight. The experiments were at 3 GPa and 1 GPa. A prerequisite for viscosity estimation using the method of Zhang et al. (2003) is to know the temperature dependence of the equilibrium constant K of the interconversion reaction at a given pressure. Equilibrium experiments were verified by reversal experiments. Equilibrium constant changes with pressure nonlinearly. At 600 \deg C, ln(K) is almost constant from one bar (-1.69) to 1 GPa (-1.72), but increases from one bar to 3 GPa (-1.56). Cooling rates varied from 100 \deg C/s to 0.1 \deg C/s in the cooling-rate experiments. Viscosity (in Paús) at the apparent equilibrium temperature of the hydrous species reaction (i.e., glass transition temperature) is obtained as 10$^{11.45}$/q where q is cooling rate in K/s. So the range of viscosity inferred from this method can be 3 orders of magnitude. Data show that viscosity of hydrous rhyolitic melt increases with pressure nonlinearly. At 600 \deg C, the viscosity increases by 0.3 log units from 1 bar to 1 GPa, and by 1.5 log units from one bar to 3 GPa. It appears that in terms of the pressure dependence of viscosity, rhyolite with 0.8 wt% water at low temperature range behaves as a depolymerized melt such as diopside melt. The equilibrium and cooling rate data suggest a structure change occurring between 1 GPa to 3 GPa, leading to a sudden change in the equilibrium constant and viscosity.
DE: 8400 VOLCANOLOGY
DE: 8429 Lava rheology and morphology
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting