18. Chemistry of the Lower Mantle

This chapter summarizes the arguments for and against large-scale chemical differentiation of the mantle and examines the variations in mineral chemistry that need to be accounted for when placing constraints on the compositional and thermal structure of the lower mantle using seismic velocity observations. Various lines of evidence have been used to argue that the lower mantle is chemically distinct in major elements from the upper mantle. One of the few approaches that may clarify this would be to compare observed seismic velocities with estimates computed from mineral physical models. Building a realistic model for the effects of composition on the seismic velocities of lower mantle minerals remains a key goal in mineral physics. The dominant lower mantle minerals are all solid solutions. An important aspect of measured bridgmanite Fe3+ contents is that they appear to be nominally independent of oxygen fugacity.