Formation of large low shear velocity provinces through the decomposition of oxidized mantle.

Large Low Shear Velocity Provinces (LLSVPs) in the lowermost mantle are key to understanding the chemical composition and thermal structure of the deep Earth, but their origins have long been debated. Bridgmanite, the most abundant lower-mantle mineral, can incorporate extensive amounts of iron (Fe) with effects on various geophysical properties. Here our high-pressure experiments and ab initio calculations reveal that a ferric-iron-rich bridgmanite coexists with an Fe-poor bridgmanite in the 90 mol% MgSiO-10 mol% FeO system, rather than forming a homogeneous single phase. The Fe-rich bridgmanite has substantially lower velocities and a higher V/V ratio than MgSiO bridgmanite under lowermost-mantle conditions. Our modeling shows that the enrichment of Fe-rich bridgmanite in a pyrolitic composition can explain the observed features of the LLSVPs. The presence of Fe-rich materials within LLSVPs may have profound effects on the deep reservoirs of redox-sensitive elements and their isotopes.