HR: 0800h
AN: T11B-1251    [Abstracts]
TI: Silicon diffusion in wadsleyite at high pressure and implications for rheology of the mantle transition zone
AU: * Shimojuku, A
EM: simojuku@ganko.tohoku.ac.jp
AF: Institute of Mineralogy, Petrology, and Economic Geology, Faculty of Science, Tohoku University, Aza Aoba Aramaki Aoba, Sendai, 980-8578 Japan
AU: Kubo, T
EM: kubotomo@geo.kyushu-u.ac.jp
AF: Department of Earth and Planetary Sciences, Kyushu University, 6-10-1 Hakozaki Higashi, Fukuoka, 812-8581 Japan
AU: Ohtani, E
EM: ohtani@mail.tains.tohoku.ac.jp
AF: Institute of Mineralogy, Petrology, and Economic Geology, Faculty of Science, Tohoku University, Aza Aoba Aramaki Aoba, Sendai, 980-8578 Japan
AU: Yurimoto, H
EM: yuri@geo.titech.ac.jp
AF: Tokyo Institute of Technology, Department of Earth and Planetary Sciences, 2-12-1 Ookayama Meguro, Tokyo, 152-8551 Japan
AB: Wadsleyite is one of the major constituent minerals in the mantle transition zone. In order to understand the rheological behavior of the mantle transition zone, it is essential to determine the diffusion rate of the slowest diffusing species which may control the plastic deformation in silicate minerals undergoing diffusion creep or dislocation climb. Although Si is the slowest diffusing species in many silicate minerals, diffusion data in wadsleyite are limited to Mg-Fe interdiffusion. In this study, we carried out Si diffusion experiments in wadsleyite to discuss the rheological properties in the mantle transition zone. High-pressure experiments were carried out using a Kawai-type multi-anvil apparatus installed at Tohoku University. Starting material of polycrystalline wadsleyite was synthesized from powdered forsterite at 18 GPa and 2003K. Surface of the polycrystalline wadsleyite was polished and then coated with a $^{29}$SiO$_{2}$ thin film. Diffusion annealing was conducted at 18 GPa and 1703-1903K. After the diffusion annealing, concentration profiles of $^{29}$Si were obtained by the depth profiling method using secondary ion mass spectrometry (SIMS). The obtained diffusion profiles were composed of two regions. Volume diffusion coefficient (Dv) was calculated from the region near the sample surface using the solution of thin film diffusion model, and the solution by LeClaire (1963) was used to obtain the grain-boundary diffusion coefficient ($\delta$Dgb) from the deeper region. Dv and $\delta$Dgb were determined to be Dv= 3.44$\times$10$^{-11}$ [m$^{2}$/s] exp (-299 [kJ/mol]/RT) and $\delta$Dgb= 1.14$\times$10$^{-17}$ [m$^{3}$/s] exp (-248 [kJ/mol]/RT), respectively. Si diffusion rates are about five orders of magnitude slower than Mg-Fe interdiffusion rates in wadsleyite. Despite oxygen diffusion data has never been reported, Si is a good candidate for the slowest diffusing species in wadsleyite as it is in olivine. Assuming that Si is the rate-controlling species in wadsleyite, geophysical model of the viscosity in the mantle transition zone can be explained by diffusion creep in wadsleyite for a grain size of about 0.5-5 mm. The subducting slabs become weaker than surrounding mantle if grain size of spinel is reduced to less than 1 $\mu$m after the olivine-spinel transformation.
DE: 8162 Rheology--mantle
DE: 5120 Plasticity, diffusion, and creep
DE: 5139 Transport properties
DE: 3902 Creep and deformation
DE: 3924 High-pressure behavior
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting