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Published September 8, 1967 | public
Journal Article

Phase Changes in the Upper Mantle


The C-region of the upper mantle has two transition regions 75 to 90 kilometers thick. In western North America these start at depths of 365 kilometers and 620 kilometers and involve velocity increases of about 9 to 10 percent. The locations of these transition regions, their general shape, and their thicknesses are consistent with, first, the transformation of magnesium-rich olivine to a spinel structure and, then, a further collapse of a material having approximately the properties of the component oxides. The velocity increases associated with each transition region are slightly less than predicted for the appropriate phase change. This can be interpreted in terms of an increasing fayalite content with depth. The location of the transition regions and the seismic velocities in their vicinity supply new information regarding the composition and temperature of the upper mantle. The depths of the transition regions are consistent with temperatures near 1500°C at 365 kilometers and 1900°C at 620 kilometers.

Additional Information

© 1967 American Association for the Advancement of Science. Received 24 July 1967. Supported in part by a Sloan Foundation Fellowship and by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under AFOSR contract AF-49(638)-1337. I thank L. Johnson, C. Archambeau, E. A. Flinn, T. J. Ahrens, Y. Syono, and H. Kanamori for preprints of their manuscripts, and B. Kamb, L. Silver, and A. Albee for discussion. Contribution No. 1467.

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