International Geology Review, Vol. 44, 2002, p. XXXXXX.
Copyright © 2002 by V. H. Winston & Son, Inc. All rights reserved.
The Case for Irreversible Chemical Stratification of the Mantle
DON L. ANDERSON
Seismology Laboratory, 252-21, California Institute of Technology
Pasadena, California 91125
Chemical differentiation of the Earth into a buoyant, olivine-rich upper mantle, along with proto-
crustal materials, a perovskite-rich deeper layer, and an iron-rich core occurred continuously during
accretion. Dense komatiitic liquids and eclogitic solids sank to mid-mantle depths. The large-ion
lithophile elements and primordial gases accumulated in the proto-upper mantle. During subse-
quent evolution, most of the crustal elements were sweated out of the upper mantle; the layer at the
base of the mantle collected light dross from the core and dense dregs from the mantle and reacted
with the core. This fractionation and gravitational sorting of primordial materials according to den-
sity, solubility, silicate compatibility, and melting point became irreversible as the planet grew
because of the effect of pressure on thermal expansion.
Chemical boundaries are hard to detect by seismic techniques, but evidence favors one such
boundary near 1000 km. Below this, the mantle is probably depleted in volatiles and the heat-pro-