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

Abstract

AbstractWe measured the effect of pressure on the compressional and shear wave velocity (VP, VS) as well as density of Fe‐bearing bridgmanite, Mg0.96(1)Fe2+0.036(5)Fe3+0.014(5)Si0.99(1)O3, 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 VP by ~6(±1)% is observed between 42.6 and 58 GPa, while VS 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 VS increases by ~3(±1)% above ~40 GPa compared to MgSiO3‐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 Fe3+ 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.