Abnormal Elasticity of Fe‐Bearing Bridgmanite in the Earth's Lower Mantle

Abstract

Abstract We measured the effect of pressure on the compressional and shear wave velocity ( V P , V S ) as well as density of Fe‐bearing bridgmanite, Mg 0.96(1) Fe 2+ 0.036(5) Fe 3+ 0.014(5) Si 0.99(1) O 3 , using impulsive stimulated light scattering, Brillouin light scattering, and X‐ray diffraction, respectively, in diamond anvil cells up to 70 GPa at 300 K. A drastic softening of V P by ~6(±1)% is observed between 42.6 and 58 GPa, while V S increases continuously with increasing pressure. A significant reduction in Poisson's ratio from 0.24 to 0.16 occurs at ~42.6–58 GPa, while V S increases by ~3(±1)% above ~40 GPa compared to MgSiO 3 ‐bridgmanite. Thermoelastic modeling of the experimental results shows that the observed elastic anomaly of Fe‐bearing bridgmanite is consistent with a spin transition of octahedrally coordinated Fe 3+ in bridgmanite. These results challenge traditional views that Fe enrichment will reduce seismic velocities, suggesting that seismic heterogeneities in the mid‐lower mantle may be due to a spin transition of Fe in Fe‐bearing bridgmanite.