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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, Mg0.96(1)Fe0.036(5)2+Fe0.014(5)3+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 V-P by similar to 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 similar to 42.6-58GPa, while V-S increases by similar to 3(+/- 1)% above similar to 40GPa 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.

Plain Language Summary Seismic heterogeneities in the Earth's lower mantle have been attributed to thermal and/or chemical variations of constituent minerals. Bridgmanite is the most abundant lower-mantle mineral and contains Fe and Al in its structure. Knowing the effect of Fe on compressional and shear wave velocities (V-P, V-S) and density of bridgmanite at relevant pressure-temperature conditions can help to understand seismic heterogeneities in the region. However, experimental studies on both V-P and V-S of Fe-bearing bridgmanite have been limited to pressures below 40GPa. In this study, V-P and V-S of Fe-bearing bridgmanite were measured up to 70GPa in the diamond anvil cell. We observed drastic softening of V-P by similar to 6(+/- 1)% at 42.6-58GPa and increased V-S at pressures above 40GPa. We interpret these observations as due to a spin transition of Fe3+. These observations are different to previous views on the effect of Fe on seismic velocities of bridgmanite. We propose that the abnormal sound velocities of Fe-bearing bridgmanite could help to explain the seismically observed low correlation between V-P and V-S in the mid-lower mantle. Our results challenge existing models of Fe enrichment to explain the origin of Large Low Shear Velocity provinces in the lowermost mantle.