Sound Velocities in FeSi at Lower Mantle Conditions and the Origin of Ultralow-Velocity Zones

The origin of ultralow-velocity zones (ULVZs) remains an open question despite recent advances in mineral physics and seismology. Here, we examine the hypothesis that FeSi formed from core-mantle chemical reactions is a plausible source of ULVZs at the core-mantle boundary (CMB). The sound velocities of B2-structured FeSi were measured up to 115(2) GPa and 1600(200) K by nuclear inelastic scattering (NIS) in laser-heated diamond anvil cells (LH-DACs). Within uncertainties, the sound velocities of B2-FeSi display negligible anharmonicity, hence validating the extrapolation of velocity-density relations (Birch's law) to P-T conditions of the CMB. The sound velocities of B2-FeSi are significantly lower compared to other candidate phases in a lowermost mantle assemblage, and the Preliminary Reference Earth Model at CMB conditions. Less than 8.4 vol% of FeSi in the aggregate is thus sufficient to explain both the velocity decrements and the high density anomaly observed in a wide range of ULVZs.