The Iron Spin Transitions in Hydrous Fe³⁺‐Bearing Bridgmanite and Its Geophysical Properties in the Lower Mantle

Abstract Hydrous Fe 3+ ‐bearing bridgmanite (Bdg) is potentially a critical water host in the lowermost mantle. The spin transition behaviors of such materials are pivotal for understanding geophysical heterogeneity in the deep Earth but are poorly understood. Here, we investigated the spin transition and related geophysical properties of Fe 3+ with associated H defects [Fe 3+ ‐H] at high P‐T conditions using first‐principles simulations. Our calculations predict that the presence of hydrogen reduces the onset pressure of the spin transition of Fe 3+ in Bdg, leading to higher fractions of low spin Fe 3+ . Along standard geotherms, spin transition is predicted to remain incomplete even at the core‐mantle boundary (CMB), and lateral temperature variations would significantly affect the proportions of high and low spin Fe and related properties like elasticity. The thermoelastic property of hydrous Fe 3+ ‐bearing bridgmanite exhibit stronger softening anomalies at the lower mantle conditions compared to dry system, which potentially enhancing the seismic detectability of the hydrous Bdg in the deep earth. Density profile of hydrous Fe 3+ ‐bearing bridgmanite indicates that the [Fe 3+ ‐H] defect modestly increases the system's density, but much less than that caused by incorporating an equivalent amount of iron alone. This is crucial for understanding regions like Large Low Shear Velocity Provinces (LLSVPs), which exhibits large velocity drops but only minor density changes. The co‐adsorption of Fe and H allows for the introduction of Fe to induce velocity drops without the concomitant sharp increase in density, as pure iron would, thus enabling Fe‐H enrichment as a potential source of LLSVPs.