Influence of gradational compositional layering on plume entrainment and its implication for geochemistry

The influence of gradational compositional layering on the entrainment of a chemically dense layer by thermal plumes is studied, using two-dimensional axisymmetric numerical models. The gradational zone comprises five flat sub-layers initially and the chemical density gradient is expressed by assigning the five sub-layers with different chemical densities. The results can be summarized as follows. (1) For a homogeneous chemical layer, small extra density causes the whole chemical layer to be entrained by the thermal plumes while large extra density causes the plume to entrain only the upper part of the chemical layer as previous studies have shown. The chemically dense layer convects as a whole, and the convective vigor increases with increasing extra chemical density. (2) When chemical density increases with depth, thermal plumes tend to entrain only the less dense upper part of the chemical layer, while the denser lower part is stable and resists the chemical mixing in the layer as a whole. The larger the chemical density gradient, the more stable the lower part is. (3) Large thermal plumes result in large entrainment capacity, thus enhancing the capacity to entrain the denser lower part, and increasing viscosity contrast also increases the entrainment capacity as expected when having a small extra chemical density. (4) Reinjection might be needed for the less dense upper part in the gradational zone at the base of the mantle to survive the Earth's history. The long-term stability of the denser lower part and its resistance against entrainment together with the reinjection might be the key to reconciling the overestimate of the OIB ages from thermochemical numerical models with the observed ages and might account for the isotope difference and similarity of MORBs and OIBs.