Another definition of the LAB (lithosphere–asthenosphere boundary) involves differences in composition of the mantle at depth. Lithospheric mantle is ultramafic and has lost most of its volatile constituents, such as water, calcium, and aluminum. Knowledge of this depletion is based upon the composition of mantle xenoliths. The depth to the base of the CBL can be determined from the amount of forsterite within samples of olivine extracted from the mantle. This is because partial melting of primitive or asthenospheric mantle leaves behind a composition that is enriched in magnesium, with the depth at which the concentration of magnesium matches that of the primitive mantle being the base of the CBL.