Spin Transition of Fe3+ in δ-(Al,Fe)OOH and Implication for Mid-Lower Mantle Seismic Heterogeneities

δ-(Al,Fe)OOH is an important water carrier and plays a critical role on Earth's deep water cycle. Lattice parameters of δ-(Al0.89Fe0.11)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 KT and VΦ of δ-(Al0.89Fe0.11)OOH occur significant elastic softening, with maximum reductions of 50% on KT and 29% on VΦ at 33 GPa and 300 K to 37% on KT and 23% on VΦ 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.