Mixing properties in the Earth's mantle: Effects of the viscosity stratification and of oceanic crust segregation



[1] Although a large quantity of geochemical data on oceanic basalts has been collected, it is not sufficient to characterize the recycling processes between the Earth's mantle, on the one hand, and the crust and the lithosphere, on the other hand. In particular, it remains unclear why mid-oceanic ridge basalts (MORBs) are relatively homogeneous throughout the world, while oceanic island basalts (OIBs) present a wide spectrum of heterogeneities. Assuming whole mantle convection, it is often argued that the increase of viscosity with depth could be responsible for some stratification of the mixing properties of the mantle and consequently could generate the observed geochemical and isotopic differences between MORBs and OIBs. In this study we test this assumption by means of two-dimensional circulation models where passive tracers are advected. First, we quantify the ability of the upper and lower mantles to erase heterogeneities. Second, we simulate the evolutions of U, 4He, and 3He concentrations, taking into account the magmatic processes at ridges (differentiation, outgassing, and recycling). We conclude that the viscosity layering does not induce any vertical stratification of the mixing properties of the mantle. On the contrary, the partial segregation of the oceanic crust in the D″ layer would explain the generation of large zones of high 3He/4He ratios on top of D″. In our simulations these zones have a primitive He signature, although they have already been processed at ridges. Trapping oceanic crust in D″ would not only explain the presence of recycled components in hotspots and particularly in the high μ type hotspots [Hofmann and White, 1982] but would also suggest that the high 3He/4He ratios of volcanoes like Loihi could have their origin in ancient subducted lithosphere.