Sound Velocities in FeSi at Lower Mantle Conditions and the Origin of Ultralow‐Velocity Zones. Issue 14 (21st July 2021) - Mergner, V.
Abstract: 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. Plain Language Summary: Seismic studies have revealed patches on the Earth's core‐mantle boundary (CMB), which have extremely low seismic velocities. However, the origin of these so‐called ultralow‐velocity zones (ULVZs) remains unclear. One plausible source could be FeSi, which is assumed to form from chemical reactions between material from the core and the mantle. Here, we test this hypothesis in an experimental study, where the elastic behavior of FeSi is measured while the sample is exposed to very high pressures and temperatures comparable to those in the Earth's lower mantle. From these measurements we derived the sound velocities of the high‐pressure phase of FeSi at the CMB. We found that the sound velocities of FeSi are significantly lower than those of the average lowermost mantle measured in seismic studies, and also lower than those of other mineral phases in this region, which are also considered as potential sources of ULVZs. FeSi is thus a plausible candidate to explain the origin of ULVZs. Our results show that less than 8.4% of FeSi by volume in the aggregate are enough to explain both the amplitude of the velocity reduction and the high density anomaly observed in a wide range of ULVZs. Key Points: Very low sound velocities in B2‐FeSi, a candidate phase at the CMB, measured up to 115 GPa and 1600 K by NIS Negligible anharmonic contributions to sound velocities validate the extrapolation of Birch's law to CMB conditions Less than 8.4 vol% B2‐FeSi produced by core‐mantle reactions are enough to explain the seismic anomalies associated with the ULVZs