Water Enhancement of Si Self‐Diffusion in Wadsleyite.

We investigated the H2O‐content dependence of Si self‐diffusion coefficient in Fe‐free wadsleyite using multi‐anvil experiments at pressures of 19–20 GPa, temperatures of 1573–1873 K, and H2O‐content ranging from ∼10 to 5,300 wt. ppm by the isotopic thin‐film diffusion‐couple method. The 29Si‐doped diffusion profiles were measured by nanoscale secondary ion mass spectrometry in the depth profiling mode. The H2O contents in the samples were analyzed by Fourier transformation infrared spectroscopy. The experimental results show a H2O enhancement of Si diffusion coefficient with a H2O content exponent of 0.8 ± 0.1. The activation enthalpy was found to be 270 ± 40 kJ/mol. The diffusion coefficients in the [100], [010], and [001] directions are indistinguishable. The temperature and H2O‐content dependences of Si diffusion indicate that H2O incorporation dramatically reduces the rheological strength of wadsleyite, whereas temperature has a relatively small effect. The viscosity in the mantle transition zone could be significantly reduced by H2O incorporation in wadsleyite. The viscosity contrast between mantle plumes and surroundings may control the evolution of plume shapes at 410–660 km depths. Plain Language Summary: Water incorporation in nominally anhydrous minerals may dramatically affect their physical and chemical properties by introducing additional point defects as hydroxyl in the crystal structures. In particular, the atomic diffusivity in minerals, which is controlled by the concentrations of point defects, could be enhanced, therefore, the diffusion‐controlled properties such as rheology could be affected by water incorporation. Because the mantle transition zone is considered to be water‐rich, we investigated the effect of water on silicon self‐diffusion rate in wadsleyite, which is the dominant mineral in the upper part of mantle transition zone at 410–520 km depth. The experimental results show that water‐rich (1.0 wt.% H2O) wadsleyite is rheologically more than 2.4 orders of magnitude weaker than water‐poor (less than 10 wt. ppm) conditions. Therefore, the mantle transition zone is significantly weakened by water incorporation. The viscosity contrast between plumes and surrounding mantle may control the evolution of plume shapes at mantle transition zone depths. Key Points: Silicon self‐diffusion in wadsleyite is enhanced by water incorporation with an exponent of 0.8 ± 0.1Silicon self‐diffusion in wadsleyite is nearly isotropic, and has an activation energy of 270 ± 40 kJ/molViscosity contrast between plume and surrounding mantle is controlled by water and temperature, which may affect the plume shape evolution [ABSTRACT FROM AUTHOR]