Sound Velocity of Hematite and Its Implications for Seismic Scatterers in the Uppermost Lower Mantle

Seismological observations have detected seismic scatterers with up to -12% shear wave anomalies in subduction zones such as the Japan Sea, Mariana, and Izu-Bonin at the uppermost lower mantle. Proposed explanations—including MORB accumulation, the post-stishovite phase transition of alumina-bearing SiO₂, and plume-induced melting—fail to account for shear wave reductions exceeding 7%. We propose that banded iron formations (BIFs) embedded in subducted sediments may explain these anomalies.

While subduction at Izu-Bonin-Mariana began 50–60 Ma ago, the youngest BIFs were deposited during the Sturtian glaciation (ca. 770–500 Ma). To reconcile this, we suggest a “First Subduction–Exhumation–Double Parallel Subduction” model, supported by evidence of present-day double parallel subduction at Izu-Bonin-Ryukyu. Ancient BIFs, initially preserved in accretionary prisms, could have been later re-subducted to the deep mantle.

To test this hypothesis, we measured P- and S-wave velocities of polycrystalline hematite up to 13 GPa using ultrasonic interferometry. Velocity modeling suggests that even a small fraction of BIFs (up to 6 km thick) in subducted sediments could reduce shear wave velocity by ~12% relative to a pyrolitic mantle. These findings provide a viable alternative explanation for observed seismic scatterers in the uppermost lower mantle.