Earth's Lower Mantle Predominated by Ferric Iron‐Rich Bridgmanite Inferred From High Pressure Elasticity Measurements

Abstract The absence of sound‐velocity data spanning the entire lower mantle pressures for (Fe, Al)‐bearing bridgmanite impedes direct comparisons with seismic wave observations, leaving the chemistry of the lower mantle unresolved. The present ultra‐high pressure sound‐velocity measurements of in situ synthesized (Fe, Al)‐bearing bridgmanite up to 130 GPa indicate that a substantial portion of iron in it originally synthesized from a glass with ferrous (Fe 2+ ) iron becomes ferric (Fe 3+ ). Furthermore, its shear wave velocity profile shows a considerable reduction of approximately 1.6%–2.2% on average when compared to MgSiO 3 across the lower mantle. These results indicate that the lower mantle highly enriched with ferric iron‐rich bridgmanite comprising with greater than 95 vol% of the lower mantle, implying that the lower mantle is significantly rich in silica with a Mg/Si ratio approaching 1, compared to the upper mantle (Mg/Si ∼ 1.3), which supports the layered convection model securing the primordial chemical distinction.