Sound Velocities of Al‐Bearing Phase D up to 22 GPa and 1300 K

Sound velocities of Al‐bearing phase D were investigated up to 22 GPa and 1300 K using in situ synchrotron X‐ray techniques combined with ultrasonic measurements in a multianvil apparatus. The isothermal bulk modulus of Al‐bearing phase D was found to be ~16.7% lower than that of Mg‐endmember, suggesting a strong effect of Al incorporation on the bulk modulus of phase D. Al‐bearing phase D has higher P‐ (VP) and S‐ (VS) wave velocities compared to other mantle transition zone minerals, up to 4.3% for VP and up to 9.6% for VS compared with hydrous iron‐bearing ringwoodite, which might engender reduction of velocity contrast between anhydrous mantle and hydrous slab components. The accumulation of hydrated slab components carrying Al‐phase D in the uppermost lower mantle can explain some local negative shear velocity perturbations (ΔVS) up to −1.5%, although ΔVP (−0.5%) is expected to remain below the detection limit of seismological techniques. Key Points We determined sound velocities of Al‐bearing phase D up to 22 GPa and 1300 K Al‐bearing phase D has higher P‐ and S‐wave velocities than other mantle transition zone minerals Al‐bearing phase D accumulation might explain low‐velocity anomalies in the uppermost lower mantle