Linking rates of slab sinking to long-term lower mantle flow and mixing

Abstract

Numerical models of Earth's mantle dynamics that aim to comply with a variety of surface observations and/or modern mantle structure, still predict a widely varying vigour of mantle flow which governs the long-term evolution of mantle structure and mixing. A yet unexplored source of intrinsic information on mantle flow characteristics are the geologically reconstructed average slab sinking rates. Here we evaluate from numerical experiments how average slab sinking rates relate to the vigour of mantle convection and mixing. We use a simplified mantle convection model and show that long-term mantle flow velocity and mixing is strongly sensitive to slab sinking rates. Models tuned to match lower mantle average sinking rates of 10–15 mm/a, yield lower mantle convection rates of only several mm/a. Furthermore, they reveal large unmixed regions in the mid-mantle which preserve 25% of ‘primordial’ lower mantle material after 1000 Ma, which may explain geochemical observations from hotspot volcanoes.