Structure of the Core- Mantle Transition Zone: A Chemical and Thermal Boundary Layer

The core-mantle boundary (CMB) sepa­ rates dynamic systems with remarkable con­ trasts in composition and material properties (Table 1), comparable in magnitude to varia­ tions in properties across the surface of the solid Earth. A natural consequence of this pronounced stratification is the development of thermal and chemical boundary layers ad­ jacent to these interfaces, which thereby con­ stitute the major transition zones in the plan­ et. The importance of the surface and CMB boundary layers for both dynamic and chemi­ cal fractionation processes throughout the history of the Earth has long been appreciat­ ed, but only recently has a concerted effort been made to study the boundary layer at the base of the mantle. Whereas the near-surface regime is accessible for a broad range of anal­ yses, all investigations of the core-mantle transition zone must be conducted using geo­ physical remote sensing procedures. As a re­ sult, progress in characterizing this region has been slow and controversial; however, recent advances in several geophysical disciplines have accelerated our understanding of and interest in the processes occurring near the major internal discontinuity within the Earth.