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Published March 10, 1975 | Published
Journal Article Open

Interpretation of Velocity Profiles of the Mantle


The purpose of this note is to demonstrate that recent V_p and V_s profiles are consistent with finite strain extrapolations of reasonable mineralogies. The difference in the size of the discontinuities in the V_p and V_s profiles is due to the relative pressure insensitivity of shear velocity in spinel structures. Most of the structure in the upper mantle can be attributed to the collapse of forsterite rich olivine to the β spinel and then the γ spinel structures. The 640-km discontinuity on the basis of recent work appears to be due to the disproportionation of γ spinel to a mixture of magnesiowustite (Mg, Fe)O and stishovite. The present results indicate that this postspinel phase in the lower mantle is enriched in SiO_2 relative to the upper mantle. The 640-km discontinuity is therefore a chemical as well as a phase boundary.

Additional Information

© 1975 by the American Geophysical Union. Received June 27, 1974; revised September 13, 1974; accepted October 25, 1974. We are grateful to D. Helmberger and G. Engen for an advance copy of their paper. Conversations with D. Helmberger have been most helpful. This research was supported by the Advanced Research Projects Agency of the Department of Defense and was monitored by the Air Force Office of Scientific Research under contract F 44620-72-C-0078 and National Science Foundation grant GA 40752. This paper is contribution 2506 of the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California.

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