Spin Transition of Fe ³⁺ in δ‐(Al,Fe)OOH and Implication for Mid‐Lower Mantle Seismic Heterogeneities

Abstract δ‐(Al,Fe)OOH is an important water carrier and plays a critical role on Earth's deep water cycle. Lattice parameters of δ‐(Al 0.89 Fe 0.11 )OOH were measured by synchrotron single‐crystal X‐ray diffraction at simultaneously high temperature and pressure up to 65 GPa and 800 K in diamond anvil cells. The results reveal that the spin crossover increases from 30 to 37 GPa at 300 K to 36–48 GPa at 700 K. Moreover, at the spin crossover, the K T and V Φ of δ‐(Al 0.89 Fe 0.11 )OOH occur significant elastic softening, with maximum reductions of 50% on K T and 29% on V Φ at 33 GPa and 300 K to 37% on K T and 23% on V Φ at 41 GPa and 700 K. The anomalous elastic properties of δ‐(Al,Fe)OOH at the spin crossover enhance our understanding of local seismic observations anomalies and help identify potential water‐rich regions in the mid‐lower mantle.