Si and O diffusion in (Mg,Fe)₂SiO₄ wadsleyite and ringwoodite and its implications for the rheology of the mantle transition zone

Si and O diffusion rates on polycrystalline (Mg,Fe)₂SiO₄ wadsleyite and ringwoodite have been determined at pressures between 16 and 22 GPa and temperatures between 1400 and 1600 °C using a Kawai-type multi-anvil high-pressure apparatus. Pre-synthesized polycrystalline wadsleyite or ringwoodite was used as starting materials. Diffusing sources of ²⁹Si and ¹⁸O enriched (Mg,Fe)₂SiO₄ thin film were deposited on the surface of wadsleyite and ringwoodite by pulsed laser deposition. The diffusion profiles were obtained using secondary ion mass spectrometry in the depth-profiling mode. All measured diffusion profiles were composed of volume and grain-boundary diffusion regimes. Arrhenius relations for volume and grain-boundary diffusion rates of Si and O in wadsleyite and ringwoodite have been determined. The results show that Si is the slowest diffusing element among the major elements in both wadsleyite and ringwoodite under mantle transition zone as well as slab conditions. Therefore, Si should be the rate-controlling species for high-temperature creep in wadsleyite and ringwoodite. Comparisons with Si and O diffusion rates obtained in this study and those in olivine and silicate perovskite suggest that Si and O diffusion rates are enhanced at both 410 and 660 km seismic discontinuities. The deformation mechanism maps constructed from Si diffusion data in wadsleyite and ringwoodite suggest that the dominant deformation mechanism operating in the mantle transition zone is dislocation creep, which may be the origin of the observed global seismic anisotropy. Under cold subduction zone conditions, grain-size sensitive diffusion creep becomes dominant when the grain size is reduced to less than 10-100 μm below the metastable olivine wedge.