Sound velocities and densities of Al-bearing superhydrous phase B (Al-bearing SuB) were investigated up to 24 GPa and 1300 K by synchrotron X-rays combined with ultrasonic interferometry techniques in a multi-anvil apparatus. We found that Al + H incorporation decreases the adiabatic bulk modulus and shear modulus of SuB. Our results however suggest that this effect is less important than that of temperature. The presence of hydrous pyrolite with similar to 10 wt.% Al-bearing SuB in cold subducting slabs could explain up to similar to 2% high velocity anomalies at the bottom of the mantle transition zone (500-660 km) while it turns into similar to 7.7% low velocity anomalies below 660 km. Al-bearing SuB is reportedly stable at mantle temperatures, where it could account for similar to 12.2% low velocity anomalies beneath subduction zones in the uppermost lower mantle.
Among dense hydrous magnesium silicates, Superhydrous phaseB (SuB) is believed to hold large amount of water and transport it to the deep mantle. Recent studies showed that Al can largely increase the thermal stability region of SuB, suggesting this phase may also be an important carrier of water in the mantle transition zone (MTZ) and uppermost lower mantle (ULM). Here we investigated the sound velocities of Al-bearing SuB up to 24 GPa and 1300 K using ultrasonic interferometry combined with synchrotron X-ray techniques in a multi-anvil apparatus. We found that Al + H incorporation decreases adiabatic bulk modulus and shear modulus of SuB. Our results however suggest that this effect is less important than that of temperature. Based on our elasticity data, we calculated velocity of a hydrated pyrolite composition, containing up to 10% of Al-SuB along temperature profiles corresponding to cold slab and hot mantle at the depths of the MTZ and the ULM. We found that the presence of hydrous pyrolite may explain high velocity anomalies up to 660 km depth at MTZ while low velocity anomalies appear below 660 km along cold slab geotherm. Nevertheless, low velocity anomalies are generated in the whole depth range along a normal geotherm.
We constrained the Al + H incorporation on sound velocities of superhydrous phase B (SuB) up to 24 GPa and 1300 K Al + H incorporation which promoted by increased temperature significantly affect velocities of SuB Accumulation of Al-bearing SuB can explain velocity contrast with various magnitude owing to the change of Al + H content with temperature