Al, Si interdiffusion in Al-bearing bridgmanite under lower mantle conditions: Analytical TEM study

Diffusion studies in minerals are a fundamental tool for understanding the rheological properties of Earth’s mantle. Previous theoretical studies on deformational strain rates show that the key mechanism for deformation in bridgmanite (Brg) is diffusion-controlled creep of the slowest element, Si. The Al, Si interdiffusion rate in Brg is thus essential for understanding mantle dynamics. Brg diffusion couples were synthesised from 0-5 mol.% Al2O3-bearing MgSiO3 enstatite at 24 GPa and 2023 K using a conventional multianvil apparatus. A scanning transmission electron microscope (STEM) can provide a resolution down to the nanometre scale and is therefore ideal for studying slow diffusion, such as in this study. To achieve reliable results from STEM analysis, it is necessary to carry out sample preparation carefully. In this study, a focused ion beam was used for (S)TEM sample preparation. The Al substitution mechanisms in Al-bearing Brg diffusion couples in the recovered samples were analysed using an energy-dispersive X-ray spectrometer. The measured diffusion coefficient for Al, Si interdiffusion at 24 GPa and 2073K was Log10(D) = -20.1 ± 0.7 (based on units of m2/s). The result is two to three orders of magnitude slower compared to the previously reported Si self-diffusion coefficient Log10(DSi) = -18 ± 0.5 at 25 GPa and 2073 K. The slower diffusion rate places limits on deformational strain rates of Brg in the lower mantle.