What mantle plumes tell us about the Earth

The Earth is an active planet that has permanently evolved since its accretion. After core formation, the mantle and crust differentiated, and at some stage, continental crust started to cycle back into the mantle. All these first-order processes had and still have significant impact on the geochemistry of the mantle and they created the diversity of compositions presently observed in volcanic rocks. Mantle plumes sample material from deep in the mantle, potentially from the core-mantle boundary. Their geochemical compositions are diverse, with variable levels of depletion and/or enrichment. Hofmann and White in 1982 suggested that their compositions could be explained by recycling oceanic crust and melting it in ascending plumes, an interpretation that is now widely accepted and increasingly supported by observations from radiogenic isotopic systems. For example, the very high Sr isotopic ratios of some ocean island basalts (OIB) can only be produced in high Rb/Sr material such as detrital sediment. Because subducting plates include materials as diverse as oceanic basalt, OIB and all sorts of sediments, they evolve with time to the variety of isotopic compositions that characterize the mantle zoo (DMM, HIMU, EM1&2, FOZO, etc). Attributing the characteristics of OIB to the diversity of subducted and recycled materials was consistent with observations up to few years ago but recent discoveries in OIB of isotopic anomalies of decay products of extinct isotopic systems (142Nd and 182W) cast doubt on such an interpretation. The problem is that such anomalies can only exist in materials created in the first million years of Earth existence and the nature of such materials is unclear. They could be some sort of recycled material but other origins are also likely. Explaining the concomitant presence of Hadean material and recycled oceanic crust with variable ages in the source of OIB will be the next exciting challenge for geochemists and geodynamicists. Geochemists will need to explain how such material formed and how it is sampled, geophysicists will need to determine where it is stored in the deep mantle, and geodynamicists will have to model how such materials can remain preserved in an actively convective mantle for billions of years.