Earth’s lower mantle may be harder than expected: Al, Si diffusion in bridgmanite

Diffusion of atoms in crystalline solids at high temperatures and pressures influences dynamic processes in Earth’s lower mantle. This study aims to better understand the physical behaviour of Earth’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 ± 1 GPa and 1800 °C of Log10(DSi) = -18 ± 0.5 (based on units of m2/s). Our study revealed a significant 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 °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 °C was Log10(DAl,Si) = -20.1 ± 0.7. The resulting data can be used to estimate deformational strain rates of Brg in the lower mantle based on different creep mechanisms controlling the viscosity.