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Published 1991 | public
Book Section - Chapter

Chemical Boundaries in the Mantle


The seismic discontinuities at depths near 400 and 650 km are primarily due to phase changes, but they are not necessarily equilibrium phase boundaries in a homogeneous mantle. The jump in velocity near 400 km is too small to be the result of phase changes in an olivine or olivine-orthopyroxene rich material such as pyrolite. If this is an equilibrium phase boundary there must be substantially less olivine and orthopyroxene than is typical of mantle lherzolites or peridotites. The alternative is that the shallow mantle is olivine-rich, and the transition region is more eclogitic, with a high clinopyroxene/garnet ratio. Olivine-rich material, such as harzburgite, is buoyant relative to other mantle assemblages and may have accumulated in the shallow mantle during the various processes of mantle differentiation The seismic velocities in the transition region are less than calculated for the high-pressure phases of olivine, β- or γ-spinel. The transition region therefore is olivine-poor, less than about 50% olivine. The properties of the lower mantle are consistent with a "chondritic" Earth, with high FeO and SiO_2-contents compared to peridotites. Thus, there is evidence that the shallow mantle, the transition region and the lower mantle may differ in composition, in intrinsic density and in the depths of phase changes. In order to allow for this possibility, I propose that the shallow mantle (<400 km depth) be called the "perisphere" (peri- for around or nearby). This can also be called the peridotite shell. The transition region, or mesosphere, appears to be a garnetite, primarily garnet and majorite. There is no geophysical or geochemical evidence that there is any interchange of material between the mesosphere and the lower mantle, although it is likely that they are thermally coupled. The locations of hot regions in the upper mantle may be controlled by the locations of hot regions in the lower mantle.

Additional Information

© 1991 Kluwer Academic Publishers. This research was supported by grants from the National Science Foundation grant number EAR-88-14359. Contribution number 4911, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California.

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