Water-induced bridgmanite coarsening may produce highly viscous lower mantle seismic anomalies.

Two seismic anomalies, known as large low velocity provinces (LLVPs), are detected seismically at the bottom of the Earth's lower mantle. Acting as the source of mantle plumes and as barriers to mantle convection, LLVPs have stayed in the lower mantle over a long duration and significantly influence the dynamics of the Earth. However, it is unclear how the LLVPs could be preserved in the lower mantle because the elevated temperature and water content within LLVPs should reduce their rheological strength, leading to a rapid mixing by mantle convection. Here, through high-pressure experiments, we demonstrate that the grain growth of bridgmanite, the dominant mineral in the lower mantle, is significantly enhanced by water incorporation. The relatively water-rich conditions of LLVPs and consequent larger grain size may cause their viscosity 1.4 orders of magnitude higher than the surrounding mantle despite the negatively thermo-hydrous effect on mineral strength, providing a reliable mechanism for their longevity.