Compared to the mantles of the Moon and perhaps Mars, the Earth's mantle is much less differentiated chemically. Both the Moon and Mars appear to have undergone a major differentiation accompanying planet formation. The only clear signature of a similar event on the Earth is core formation. While this might imply that the Earth did not experience extensive melting and differentiation during planet formation, the higher pressures and temperatures present in the Earth could have led to a distinctly different chemical evolution for this initial differentiation. The most significant potential outcome of early differentiation on the Earth's mantle is formation of chemically distinct upper and lower mantles distinguished by Mg/Si higher and lower than chondritic, respectively. Plate tectonics on Earth provides a continuing mechanism for planet differentiation that forms crust at the expense of chemical differentiation of the mantle.

Plate tectonics, however, also offers a mechanism to return the chemically distinct materials of the crust back into the mantle. Mixing of subducted crustal material into the mantle through the stirring provided by mantle convection can serve to negate the effects of crust formation on the chemical composition of the mantle. Similarly, mixing within the mantle could serve to destroy evidence of early differentiation, if such differentiation occurred on Earth. Completely efficient operation of the plate tectonic cycle would result in remixing of crust and differentiated mantle, with the end result being a homogenous mantle with composition identical to that of the bulk earth minus the materials segregated into the core. In part, this may explain the relatively undifferentiated nature of the Earth's mantle.

Plate tectonics has not been completely efficient on Earth, however. Both oceanic and continental crust exist, and there is widespread evidence for chemical variability in the mantle. At least four chemically and isotopically distinct components are observed in mantle-derived rocks. The nature of these components points to the importance of crust formation and recycling in determining the chemical variability of the mantle. Mapping of the surface expression of chemical heterogeneity in the mantle is providing new views of the chemical structure of the mantle and the geodynamic processes that operate in the Earth's interior. ¿ American Geophysical Union 1994