The asthenosphere and mantle are not the same thing. The asthenosphere is the majority of the mantle but not the uppermost section which is part of the lithosphere. The distinction between the lithospheric mantle and the asthenosphere is not one of composition but to do with the way it is able to deform. The asthenosphere can flow by diffusion creep whereas the lithospheric mantle can only move by dislocation creep. The result is that the asthenosphere can (and does) undergo convection which keeps it's temperature more or less constant and the lithosphere has a sharp temperature gradient.
The asthenosphere and mantle are not the same thing. The asthenosphere is the majority of the mantle but not the uppermost section which is part of the lithosphere. The distinction between the lithospheric mantle and the asthenosphere is not one of composition but to do with the way it is able to deform. The asthenosphere can flow by diffusion creep whereas the lithospheric mantle can only move by dislocation creep. The result is that the asthenosphere can (and does) undergo convection which keeps it's temperature more or less constant and the lithosphere has a sharp temperature gradient.