Tinker, D.1, Charles E. Lesher1, G M Baxter1, Takayuki Uchida2, and Yanbin Wang2
1Department of Geology, UC Davis, Davis, CA 95616
2Consortium for Advance Radiation Sources, University of Chicago, Chicago, Ill 60637
lesher@geology.ucdavis.edu
APS-GSECARS
Rheological studies of silicate melts are critical for understanding melt generation and migration in the Earth?s mantle. We are pursuing such studies by real-time X-ray radiography of falling marker spheres using the T-25 multianvil apparatus on the 13-ID-D beamline at the GSECARS - APS. We present initial results for silicic melt (67 wt. % SiO2) at pressures between 1.6 and 7 GPa. Video records of Pt spheres (90-300 µm in diameter) settling through molten silicate melt allow unambiguous determinations of terminal velocities. Viscosities are recovered from Stoke's law, modified for the Faxen correction. Melt viscosity is found to decrease from 44.5 ± 5.3 Pa s at 2.3 GPa to 7.8 ± 0.9 Pa s at 5 GPa at 1800 K. At 1900 K, melt viscosity decreases from 7.3 ± 0.9 Pa s at 3.9 GPa to 2.9 ± 0.3 Pa s at 6 GPa. The negative pressure dependence of viscosity results in an activation volume of 8.1 ± 0.6 cm3/mol at 1800 K and 7.1 ± 1.0 cm3/mol at 1900 K. We compare these viscosity measurements to predictions based on the Eyring equation and oxygen self-diffusion coefficients determined by Tinker and Lesher (2001, Am. Min., 86) for the same melt composition. Viscosities derived from the Erying equation agree well with measured viscosities below 5 GPa. At higher pressures, deviations from measured and predicted viscosities indicate that oxygen self-diffusion occurs more slowly than the structural rearrangements permitting viscous flow. The apparent changes in the mechanisms for oxygen diffusion and viscous flow on compression are not adequately accounted by Eyring relations and emphasizes the need for both self-diffusion and direct viscosity measurements in order to better understand the links between transport and structural properties of melts at high pressures.