Sound Velocity of (Mg0.91Fe0.09)2SiO4 Wadsleyite and Its Implications to Water Distribution in Mantle Transition Zone

Abstract

The current elasticities of major mantle transition zone minerals and their hydrous counterparts, such as wadsleyite, ringwoodite, and majorite garnet, have the velocity gradients several times lower than those of the Earth models. Here we use ultrasonic interferometry technique to measure the sound velocities of wadsleyite (Mg0.91Fe0.09)2SiO4 at pressures up to 14 GPa and room temperature. Compared with previous studies, we have obtained possibly the highest ∂VP/P and ∂KS/P value at transition zone pressure, which are still noticeably lower than the Earth models, such as the preliminary reference Earth model, ak135, iasp91, etc. The results indicate that the VP contrast of “dry” olivine-wadsleyite transition at 410 km is higher than previous thought, while that of wadsleyite-ringwoodite transition at 520 km is noticeably low. Using the current and previous data on “dry” and hydrous wadsleyite a water “gradient” model at upper mantle transition zone were proposed and discussed.