Chemical inhomogeneity of mantle above 670 km transition

What chemical composition the mantle has and on what scale mantle convection takes place are related questions which require data from both seismology and petrology to answer. The mantle, which represents about 68 per cent of the Earth's mass, is usually considered, especially by petrologists, to be mostly homogeneous in composition. Discontinuities within it are generally assumed to represent isochemical phase changes in an olivine-rich material rather than chemical boundaries. Thus, the 400 km and 670 km boundaries detected by seismic means are taken to represent the 'olivine-spinel' and 'spinel-post-spinel' phase changes, respectively. The 670 km discontinuity is the more significant, for seismic velocities increase greatly at this depth. This boundary must be very sharp as it is both a good reflector of short-period seismic waves and is the lower boundary of earthquake activity - no reliably-located earthquake has ever been found below it. These observations can be used to argue that the '670' is a chemical interface that provides a barrier to convection