Reflection of P'P' Seismic Waves from Discontinuities in the Mantle

Abstract

A systematic study of the travel times and apparent velocities of precursors of the seismic core phase PKPPKP indicate that these phases are reflections from the mantle. The strongest reflection is from a depth of 630 km. In order of confidence, other reflectors were found at depths of 280, 520, 940, 410 (very weak), and 1250 km (tentative). The weakness of the 410-km reflection was surprising in view of the large velocity increase at this depth indicated by refraction and Love-wave studies. This transition region must be broader than the others or must involve a smaller density jump. Reflections were observed that were possibly from the top and bottom of the low-velocity zone at depths of 50 and 130 km, respectively. The above reflections are interpreted in terms of the following solid-solid phase changes, in order of increasing depth: pyroxene-garnet solid solution, olivine → β spinel, β spinel → spinel and pyroxene → spinel + stishovite, spinel → post-spinel, and garnet → ilmenite or oxides. A spin-spin transition in Fe^(2+) may be responsible for one of the deeper discontinuities found by others.

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