A model for the upper mantle: 2.

A model for the upper mantle, previously proposed by the author, is slightly modified in order to maintain consistency with recent experimental data. The importance of complex aluminous pyroxene solid solutions in the transition between plagioclase and garnet facies is emphasized. The constitution of the oceanic mantle is discussed in terms of recent seismic data. If seismic velocity should increase or remain constant in the layer between the M discontinuity and the beginning of the low-velocity zone then this layer must be inhomogeneous. Thus it could not be composed of peridotite (or eclogite). If the oceanic mantle consists of primitive pyrolite, however, such inhomogeneity may be explicable in terms of different metamorphic facies in which the pyrolite may crystallize.