Geochemistry and geophysics of the lower mantle

Seismic tomography has revealed two regions of low seismic wave speed in the deepest mantle beneath Africa and the Central Pacific, called Large Low S-Wave Velocity Provinces (LLSVPs). They have sharp perimeters in map-view and rise hundreds of km above the core-mantle boundary. The lower wave speeds suggest they are hot while sharp lateral velocity gradients suggest they are compositionally distinct. Seismology has also revealed smaller, thin (10s of km) regions of stronger S-wave velocity reduction, known as Ultra-Low Velocity Zones (ULVZs), that lie directly atop the core-mantle boundary, may be melts, and are often associated with LLSVPs. Much remains unknown about both these features, including their density, composition, and their geodynamic evolution. Oceanic island basalts (OIB) produced by mantle plumes rising from the deep mantle provide a means of chemically investigating the lower mantle. Radiogenic and stable isotope ratios in OIB reveal that plumes contain a component that must have once been at or near the Earth's surface. Many OIB also have rare gas isotopic compositions and W isotope anomalies indicating they also contain a component of material that has remained isolated for most of Earth's history. While mantle plumes are spatially associated with the LLSVPs and in some cases with ULVZs, it is premature to assume from that they are derived from LLSVPs, as this spatial association may simply reflect the historical pattern of subduction concentrating both LLSVPs away from downwelling regions. Future seismic and geodynamic studies are needed to confirm the relationship between plumes and deep mantle structure.