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Paul Tackley, ETH Zurich (Switzerland)
Takashi Nakagawa, Kyushu University (Japan)
James Connolly, ETH Zurich (Switzerland)
Frederic Deschamps, ETH Zurich (Switzerland)
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Global numerical models of mantle convection and plate tectonics that account for melting and the resulting formation of oceanic crust generally find that slabs eventually reach the lower mantle, and that a substantial fraction of the subducted crust settles into a layer above the core-mantle boundary. Here we present numerical models of these processes in three-dimensional spherical geometry.
These models encompass the entire mantle depth, and include self-consistent generation of oceanic crust by melting. Plate tectonics is generated by plastic yielding of the lithosphere. Density contrasts, phase changes, and seismic velocities are calculated self-consistently from composition by minimization of free energy. It is generally found that slabs penetrate the lower mantle, often with some temporary inhibition at 660 km. A substantial fraction of the crustal component settles into a layer above the core-mantle boundary (CMB). Regional high-resolution models are used to study slab-CMB interaction in detail. The differential buoyancy off the crust and residue tend to rotate the slab such that the crustal side is lowermost. The part of the slab that is in contact with the CMB is rapidly heated and its components can separate. This typically induces plume heads, which are often depleted in composition but may be followed by plume tails that are enriched due to entrainment of crustal material. The residue, and some fraction of the crust, eventually re-enter the mantle circulation, where they are progressively stretched and folded, and fill it with heterogeneity at all lengthscales.
Composition-dependent phase transitions near 660 km depth cause partial compositional layering at this depth, with the transition zone enriched in crustal component and the region below this enriched in residue. Based on extrapolating current oceanic production rates into the past, at least 95% of the mantle has been processed through mid-ocean ridge melting. This estimate, when combined with these dynamical calculations, indicate that deep subduction of crust and its subsequent evolution is the key process in generating mantle chemical heterogeneity.
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