Geochemical observations and one layer mantle convection
Résumé
Rare gas systematics are at the heart of the discrepancy between geophysical and geochemical models. Since more and more robust evidence of whole mantle convection comes from seismic tomography and geoid modeling, the interpretation of high 3He/4He in some oceanic island basalts as being primitive has to be revisited. A time dependent model with five reservoirs (bulk mantle, continental crust, atmosphere, residual deep mantle and D″) is studied for Rb/Sr and U/Pb/He systems. The dynamics of this model correspond to whole mantle convection in which subducted oceanic crust, transformed into dense assemblages, partially segregates to form a D″ layer growing with time as in Christensen and Hofmann [J. Geophys. Res. 99 (1994) 19867-19884]. A complementary cold and depleted harzburgitic lithosphere remains above D'. We assume that hotspots arise from the deep thermal boundary layer and tap, in variable proportions, material from both the residual deep mantle and D'. The difference between HIMU and Hawaiian basalts is attributed to HIMU being mostly from strongly degassed oceanic crust, though enriched in incompatibles (D'), while Hawaii is mostly from MORB source residuals that are variably degassed and depleted. We suggest that a significant part of the Earth's radioactive elements (~1/3) is trapped in the D″ layer.