The author reviews the phase transformations in the earth's mantle, which have been determined by using latest multi-anvil technology. The composition and constitution of the mantle are discussed on the basis of these phase equilibrium data, together with other data sets from laboratory measurements and seismic observations. The phase transformations in a pyrolite composition satisfactorily explain the gross nature of the seismic velocity/density profiles throughout the mantle. Thus pyrolite is considered to be a representative mantle composition. Further detailed seismic velocity structures in the mantle are also discussed in terms of the phase transformations in other compositions comprising subducted slabs.
