The chemistry of the mantle has already entered this presentation implicitly and explicitly. It is explicit in the discussions of compositional differences and radioactive heating, and it is implicit in that the material properties we have called upon depend of course on the composition of the relevant materials. The geochemistry of the mantle is divided for convenience into major elements, trace elements and isotopes. Trace elements and their isotopes give us some key information, but in order to interpret them properly we need to consider the major elements as well.

Trace elements and isotopes give us some important, though indirect, information about the structure of the mantle, and the isotopes give us some time information as well. These are very important kinds of information that are not available from other sources, so we should take advantage of them if we possibly can. It is turning out that the interpretation of these geochemical observations requires a careful consideration of both geochemical and geophysical processes.