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Published June 1979 | Published
Journal Article Open

The upper mantle transition region: Eclogite?


The upper mantle transition region is usually considered to be peridotite which undergoes a series of phase changes involving spinel and post‐spinel assemblages. There are difficulties associated with attempts to explain the 220, 400 and 670 km discontinuities in terms of phase changes in a peridotitic mantle. Moreover, in a differentiated earth there should be large quantities of eclogite in the upper mantle. Eclogite is denser than Al_2O_3‐poor mantle to depths of 670 km, but it stays in the garnet stability field to pressures in excess of those required to transform depleted mantle to denser phases such as ilmenite and perovskite. Eclogite, therefore, remains above 670 km. The seismic properties of the transition region are more consistent with eclogite than peridotite. Most of the mantle's inventory of incompatible trace elements may be in this layer, which is a potential source region for some basalt magmas. The radioactivity in this layer is the main source of mantle heat flow, 0.7 µcal/cm² sec, and drives upper mantle convection.

Additional Information

© 1979 American Geophysical Union. (Received April 19, 1979; accepted April 30, 1979.) Paper number 9L0700. I would like to acknowledge discussions with Leon Silver and Frank Richter. Anton Hales revised the manuscript. This research was supported by NASA NGL05-002-069.

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Published - Anderson-1979-Geophysical_Research_Letters.pdf


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August 19, 2023
October 19, 2023