Al, Si diffusion in bridgmanite to estimate the Earth's lower mantle rheology

<p>The diffusion of atoms in minerals at high temperatures and pressures influences Earth&#8217;s lower mantle dynamic processes. This study aims to better understand the physical behaviour of Earth&#8217;s most abundant mineral with implications for lower mantle viscosity. Previous studies that measured Si-self diffusion coefficients in bridgmanite (Brg) showed a value at 25 &#177; 1 GPa and 1800 &#176;C of Log₁₀(D_Si) = -18 &#177; 0.5 (based on units of m²/s). Our study revealed a significantly slower diffusion coefficient that may challenge previous calculations of lower mantle viscosity. We investigated Al, Si interdiffusion in Brg experimentally at 24 GPa and 1750 to 2000 &#176;C using a multianvil apparatus using diffusion couples composed of bridgmanites that were pre-synthesised from 0-5 mol.% Al₂O₃-bearing MgSiO₃ enstatite. The Al diffusion profiles were analysed across the diffusion interface in the recovered samples using a scanning transmission electron microscope equipped with an energy-dispersive X-ray spectrometer. The obtained diffusion coefficient for interdiffusion (volume diffusion) at 24 GPa and 1800 &#176;C was Log₁₀(D_Al,Si) = -20.1 &#177; 0.7. The resulting data can be used to estimate deformational strain rates of Brg in the lower mantle from viscosity based on different creep mechanisms.</p>