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Bina, C. R., and B. J. Wood, The 400 km seismic discontinuity and the proportion of olivine in the Earth's upper mantle, Nature, 324, 449-451, doi:10.1038/324449a0, 1986.

Abstract. The 400 km seismic discontinuity has traditionally been ascribed to the isochemical transformation of alpha-olivine to the beta-modified-spinel structure in a mantle of peridotitic bulk composition. It has recently been proposed that the observed seismic velocity increase at 400 km depth is too abrupt and too small to be due to a phase change in olivine but instead requires that the transition zone be chemically distinct in bulk composition from the uppermost mantle. By requiring phase relations in the Mg2SiO4-Fe2SiO4 system to be internally consistent thermodynamically, we find that the alpha -> beta transition in olivine of mantle (Mg0.9Fe0.1)2SiO4 composition is extremely sharp, occurring over a depth interval (isothermal) of about 6 km. The magnitude of the predicted velocity increase is in agreement with that observed seismically if the transition zone is composed of about 60-70% olivine. Our results thus indicate that seismic velocities across the 400 km discontinuity are consistent with a transition zone of homogeneous peridotitic composition and do not require chemical stratification.

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