Mantle xenoliths provide invaluable information on lithosphere composition and thus on the structure and thickness of compositional boundary layer (CBL). This chapter focuses on the composition of the lithospheric mantle constrained by petrologic studies of mantle xenoliths (the major reference books are Nixon (1987) and Pearson et al. (2003)), lithosphere density from petrologic and geophysical data, compositional variations in the continental lithospheric mantle constrained by various geophysical methods, the thickness of the chemical boundary layer, and its correlation with thermal boundary layer.

Tectosphere and CBL

The tectosphere hypothesis

A significant difference in seismic velocity structure of the subcontinental and suboceanic mantle had been recognized as early as the 1960s from the analysis of dispersion curves for Love and Rayleigh waves which revealed differences in phase velocities out to periods of 300 s (e.g. Dorman et al., 1960; Toksöz and Anderson, 1966; Kanamori, 1970; Dziewonski, 1971a). The early observations were used by MacDonald (1963) to argue against plate tectonics on the basis of the hypothesis that thermal and compositional differences between continents and oceans exist down to a ~500km depth.