HR: 0800h
AN: DI41A-1257    [Abstracts]
TI: The Effect of Water on the Structure and Sharpness of the Olivine to Wadsleyite Transformation.
AU: * Frost, D J
EM: dan.frost@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universitaet Bayreuth, Bayreuth, D-95440 Germany
AB: It is generally accepted that the 410 km discontinuity is caused by the transformation of (Mg,Fe)₂SiO₄ olivine to wadsleyite. Seismology provides evidence that the depth and sharpness of this discontinuity may vary between regions of the earth. There are a number of factors that may cause this e.g. temperature and water concentration. If these effects can be experimentally calibrated it may be possible to draw firm conclusions about the nature of the mantle on a regional scale from local estimates of the discontinuity width, position and velocity jump. The effect of H₂O on the olivine to wadsleyite transformation has been investigated in the Mg₂SiO₄-H₂O system. Platinum capsules containing forsterite and enstatite with 10 wt % water are placed inside outer platinum capsules of dry forsterite and ReO₂ to ensure oxidizing conditions. Below this capsule a multi chamber sample holder with four (Mg,Fe)₂SiO₄ compositions of varying Fe/(Fe+Mg) ratios is placed. These samples are ground with metallic Fe to ensure reducing conditions. Both capsules are run in a large 25 mm edge length multianvil assembly in a 5000 tonne press at conditions of the 410 discontinuity between 1200 and 1600^oC. Recovered experiments are analyzed to bracket the pressure at which the Mg₂SiO₄ forsterite to wadsleyite transition occurs under water saturated conditions. Pressure is determined using the divariant phase relations from the (Mg,Fe)₂SiO₄ samples. Results show that at 1200²C hydrous wadsleylite forms at pressures that are approximately 0.8 GPa lower than in the dry system. At 1400^oC this reduces to 0.5 GPa and at 1600^oC to less than 0.5 GPa. These measurements are combined with a thermodynamic model for the Fe-Mg phase relations to calculate the effect of water on the 410 km discontinuity. The presence of H₂O can broaden the 410 by up to 15 km at normal mantle temperatures. Mantle water contents of less than 300 ppm have negligible influence on the 410.
DE: 3611 Thermodynamics (0766, 1011, 8411)
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3621 Mantle processes (1038)
DE: 3625 Petrography, microstructures, and textures
DE: 3924 High-pressure behavior
SC: Study of Earth's Deep Interior [DI]
MN: Fall Meeting 2005