Interaction between subducting slabs and surrounding mantle near 660 km discontinuity is one of the key issues in the dynamic of deep Earth. Distinguishing phase transformations occur in the both major parts of the slab: basaltic crust and underlying peridotite. Post-spinel transformation in peridotite takes place at 660 km, whereas post-garnet transformation in basalt is considered to occur at 720--800 km creating density crossover. In this paper we determined the post-garnet phase boundary in Mid-Ocean Ridge basalt (representative for oceanic crust) by in situ x-ray diffraction studies. The phase boundary can be expressed as P(GPa) = 0.0041 T(K) + 17.92. The present results imply that the basaltic component of the cold slab transformed to perovskite assembly at lower pressure than the surrounding mantle and provide direct evidence for penetration of the oceanic crust into the lower mantle without gravitational separation from the peridotite body of the slab.