Bina, C. R., Mantle discontinuities, Reviews of Geophysics, 29, International Union of Geodesy and Geophysics Supplement, 783-793, doi:10.1029/91RG00805, 1991.
Introduction. The nature of the discontinuities evident in seismic velocity profiles of the Earth's mantle has been a topic of active research since the work of Birch in the 1950s. The chemical, mineralogical, and thermal properties of these features have important implications outside the fields of seismology and mineral physics, constraining as they do the overall dynamics of the Earth's interior, the deep return flow for plate tectonics, and the geochemical signatures of source regions of igneous rocks.
In the period 1987-1990, great strides have been made in identifying and characterizing mantle discontinuities from a seismological standpoint. Advances in detailed observation of reflected and converted phases, analysis of multiple ScS reverberations, stacking of global digital data, application of seismometer arrays, and broadband data collection have permitted improved determination of the location, magnitude, and sharpness (depth-extent) of rapid seismic velocity changes in the mantle. Notable among such advances are the global stacking study of Shearer  and the SH-polarized mantle reverberation studies of Revenaugh and Jordan [1987, 1989, 1990a, 1990b] which systematically investigate the fine structure of seismic velocity variations in the mantle.
Important progress has also been made in understanding the physical and chemical properties of mantle discontinuities by better constraining the effects of pressure, temperature, and compositional variations upon the crystal structures and elastic moduli of the mantle's constituent minerals. Advances in multi-anvil press and diamond-anvil cell high-pressure technology, calorimetric study of mineral thermochemical properties, synchrotron radiation analysis of crystal structures, and first-principles computation of crystal properties have allowed better constraints upon both the stability fields and thermoelastic properties of mantle mineral assemblages. Notable among these advances are the detailed experimental study of post-spinel phase transformations by Ito and Takahashi  and the subsequent thermodynamic analysis by Wood [1989, 1990].
Taken together, these advances allow more meaningful correlation of seismological features with mineralogical changes. While this permits the placing of stronger constraints upon the composition, structure, and dynamics of the interior, there remain significant uncertainties and ambiguities. Here I review fundamental advances in both the seismological and mineralogical understanding of mantle discontinuities which have taken place during the period 1987-1990, as well as briefly discussing some implications for mantle thermal structure and dynamics. While I have endeavored to compile as complete a reference list as possible, the lesser number of references directly discussed in the text necessarily reflects my own interests and familiarity with the literature. I conclude by outlining the picture of mantle structure and composition which is emerging from these efforts while indicating the remaining major uncertainties.Copyright © 1991 American Geophysical Union