Geophysical and geochemical models of mantle convection : Successes and future challenges.

Monograph derived from Union Symposia at the 2003 General Assembly of the International Union of Geodesy and Geophysics (IUGG)Although more and more robust evidence of whole mantle convection comes from seismic tomography and geoid modeling, the rare gases and other isotopic or trace element signatures of ridge and hotspot basalts indicate the presence of various isolated geochemic al reservoirs in the mantle. We discuss this discrepancy between the fluid dynamic views of mantle convection and the chemical observations. We compare the standard model of geodynamicists where the mantle behaves as a fluid mostly heated from within with the findings of seismic tomography. We suggest that a significant part of the subducted oceanic crust transformed into dense eclogitic assemblages, partially segregates to form a D'' layer growing with time. We show how a two component marble cake mantle filling the whole mantle except D'' can account for the variability of OIBs and MORBs in rare gases. We then present the state of the art in thermochemical convection of the mantle and emphasize the numerical and conceptual progress that must be made to quantitatively test the geochemical hypothesis