Sound velocities of MORB and absence of a basaltic layer in the mantle transition region

Compressional (Vp) and shear (Vs) wave velocities of mid-ocean-ridge basalt (MORB) was investigated at in situ high pressure and high temperature conditions of the mantle transition region by using a combination of ultrasonic and in situ x-ray measurements. Both Vp and Vs of MORB are lower than the previously predicted velocities of the major mantle mineral phases. We found that the Vp and Vs of MORB along a typical geotherm are lower by about 2 and 5%, respectively than those of seismological models, and do not match any global and regional seismological models in the deeper parts of the mantle transition region. Thus, the existence of a basaltic layer in this region is unlikely, suggesting that the oceanic crust materials are transported into the lower mantle.