The effect of Fe3+ on the grain growth kinetics of bridgmanite and implications for the lower mantle seismic velocity anomalies

Two large low shear velocity provinces (LLSVPs) near the core-mantle boundary beneath the Pacific Ocean and Africa were discovered about 40 years ago and are characterized by tomographic images as reductions of shear and compression wave velocities by up to 3-4% and 1%, respectively, over an area of several thousand kilometers and a height of more than a thousand kilometers. It was recently proposed that the Fe³⁺ enrichment in bridgmanite could reduce its shear wave velocity and thus account for the formation of LLSVPs (Wang et al., 2021). However, the viscosity of the Fe³⁺-enriched bridgmanite is unknown, while it is critical for the long-term stability of LLSVPs at the base of the lower mantle against mantle convection. Considering the operation of diffusion creep in the lower mantle, viscosity will be positively correlated with grain size. Therefore, we measured the grain growth rate of bridgmanite under lower mantle conditions as a function of Fe³⁺ content. The experimental results show a significant enhancement of grain growth by Fe³⁺ incorporation. Thus, a larger grain size of bridgmanite is expected in the Fe³⁺-enriched LLSVPs than in the Fe³⁺-poor surrounding mantle, leading to highly viscous LLSVPs. As a result, they are stabilized at the base of the lower mantle over geological time.