Thermal Elasticity of δ‐AlOOH and Its Implications for the Velocity Structure of Deep Mantle

Abstract δ‐AlOOH can remain stable down to lower mantle depth, making it a potential carrier for water transport into the deep mantle and a plausible candidate for influencing deep‐seated velocity structures. To elucidate its impacts on the velocity structure of the deep Earth and the global water cycle, we conducted synchrotron radiation ultrasonic experiments to measure the sound velocities of δ‐AlOOH under conditions up to 18.5 GPa and 873 K, and derived its elastic parameters, yielding: K S 0 = 231.6 (2) GPa , K S ' = 4.1 (1) , ∂K S /∂T = −0.025 (1) GPa/K , G 0 = 159.2 (3) GPa , G' = 1.7 (1) , ∂G/∂T = −0.031 (1) GPa/K. By integrating the elastic parameters of major minerals within the stability field of subducting sediments, we modeled the velocity structure of δ‐AlOOH‐bearing sedimentary layers. Combined with the thermal stability of δ‐AlOOH and coexisting minerals, we propose that δ‐AlOOH may contribute to elevated velocity gradients of subducting sediments in the transition zone. Furthermore, using first‐principles calculations, we have determined the seismic velocities of δ‐AlOOH up to 130 GPa. This helps to evaluate its contribution to the seismic properties of localized, slab‐derived hydrous lithologies. Meanwhile, this phase plays a significant role in shaping the heterogeneity along the periphery of Large Low‐Shear‐Velocity provinces and in the generation of Ultra‐Low‐Velocity zones.