The 400-km seismic discontinuity and the proportion of olivine in the Earth's upper mantle

Craig R. Bina*, Bernard J. Wood

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    20 Citations (Scopus)


    The 400-km seismic discontinuity has traditionally been ascribed to the isochemical transformation of α-olivine to the β-modified-spinel structure in a mantle of peridotitic bulk composition1-6. It has recently been proposed7,8 that the observed seismic velocity increase at 400km depth is too abrupt and too small to result from 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 α-β transition in olivine of mantle (Mg0.9Fe0.1) 2SiO4 composition is extremely sharp, occurring over a depth interval (isothermal) of ∼6 km. The magnitude of the predicted velocity increase is in agreement with that observed seismically9,10 if the transition zone is composed of ∼60-70% olivine. Thus, our results 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.

    Original languageEnglish
    Pages (from-to)449-451
    Number of pages3
    Issue number6096
    Publication statusPublished - 1986


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