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Published March 1984 | Published
Journal Article Open

Composition of the upper mantle: Geophysical tests of two petrological models


The elastic properties of candidate mantle phases are used to test the viability of olivine-rich (pyrolitic) and CaO + Al_2O_3-rich (eclogitic) assemblages for the mantle. High temperature adiabats for each phase of interest are constructed and compared to mantle seismic properties. Both pyrolitic and eclogitic assemblages satisfy the seismic properties between ∼ 200 and 400 km. Between 400 and 670 km depth an eclogitic assemblage yields a superior match to velocities and velocity gradients. The 400 km seismic discontinuity may represent a chemical boundary between pyrolite and picritic eclogite ("piclogite") or phase transformations in the olivine + orthopyroxene components of a piclogitic assemblage containing about 16% olivine. High velocity gradients in the transition zone may be explained by the transformation of Ca-rich cpx to majorite garnet. Seismic properties at the top of the lower mantle are consistent with pyrolite, piclogite or perovskite, implying that the 670 km discontinuity may be a chemical boundary.

Additional Information

© 1984 American Geophysical Union. Received December 15, 1983; accepted January 10, 1984. Paper number 4L0072. We thank D. Weidner and H. Sawamoto for preprints, and B. Hager, and T. Ahrens for helpful comments. This research was supported by NSF Grant No. EAR811-5236. Contribution No. 4000, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 92115.

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