The composition of the bulk silicate Earth (BSE) is the product of planetary accretion, core differentiation and Moon formation. By establishing the composition of the BSE, one can determine the composition of the bulk Earth and by subtraction, calculate the core’s composition. The BSE represents the bulk Earth minus the core, which in today’s terms equals the modern mantle, the continental crust, and the hydrosphere-atmosphere systems. The modern mantle can be framed in terms of two compositionally distinct components, an enriched and a depleted mantle, with the latter as the MORB (mid-ocean ridge basalt) source and the former as the OIB (ocean island basalt) source.

The spectrum of geochemical compositions of Oceanic Island Basalts (OIBs) and their systematic differences from Mid-Ocean Ridge Basalts (MORBs) reveal that the Earth’s mantle is chemically and isotopically heterogeneous. Two main processes, both related to plate tectonics, contribute to the creation of mantle heterogeneities: (1) partial melting generates melts enriched in incompatible elements and leaves a depleted residual rock; and (2) subduction of the oceanic lithosphere injects heterogeneous material at depth, in particular, altered oceanic crust and continental/oceanic sediments. Moreover, delamination and foundering of metasomatized subcontinental lithospheric mantle might have been important in the early Earth history, when plate tectonics did not operate as today. The fate of the subducted plate is still a matter of debate; presumably some of it is stirred by convection and some may segregate at the base of the mantle, in particular the oceanic crust, which is compositionally denser than the pyrolitic mantle. The view of the lower mantle as a “graveyard” of subducted crust prevailed for decades and was supported by the Hofmann and White ( 1982 ) observation that the geochemical fingerprint of most OIB reveals the presence of ancient recycled crust. However, recent geochemical data on short-lived systems ( e.g. 182 Hf→ 182 W has a half-life of 8.9 My) showed that some hotspots, namely Hawaii, Samoa, Iceland and Galápagos, have a negative µ 182 W anomaly. This discovery prompted a change in our view of the deep mantle because anomalies in short-lived systems require additional processes, which include, but are not limited to, the preservation of ‘pockets’ of melt from a primordial magma ocean, and/or chemical reactions between the metallic core and the silicate mantle. Exchanges at the core-mantle boundary would cause a negative µ 182 W anomaly, and might also add 3 He to mantle material later entrained by plumes. It is now clear that some plumes probe the deepest mantle and are highly heterogeneous, as revealed by isotope ratios from long-lived radiogenic systems, noble gases and short-lived isotope systems. Here I will focus on the dynamics of plumes carrying compositional and rheological heterogeneities. This contribution attempts to be pedagogic and multi-disciplinary, spanning from seismology to geochemistry and geodynamics.