Potential Occurrence of Accessory Minerals in the Lower Mantle

In a seminal paper V.M. Goldschmidt pointed out that, in terms of volume of the constituent ions, Earth’s crust and mantle are basically a packing of negatively charged oxygen ions bound together by the volumetrically barely significant cations. Here, this statement is revisited using modern assessments of mantle composition and pressure-dependent ionic radii. It is found that the transition to the lower mantle marks a reduction in the O2− crystal ionic volume percentage from 86 to less than 80%, significant enough to suggest an overall reduced compatibility of less abundant elements within the first few hundred km of depth below that transition from lower-mantle to upper-mantle rock. An equivalent drop across both, the 410- and 670 km mantle discontinuities occurs for large polyhedral sites, which are the potential hosts for incompatible elements. Accordingly, most large ionic lithophiles and rare earth elements in the lower mantle are highly enriched in one minor phase, davemaoite. It is proposed that those minor and trace elements that are less compatible with this mineral, such as some of the high-field strength elements, are concentrated in yet unknown accessory minerals that potentially affect geochemical signatures of deep mantle-derived igneous rocks.