BOUNDARY-LAYER MODEL OF MANTLE PLUMES WITH THERMAL AND CHEMICAL DIFFUSION AND BUOYANCY

We present a boundary-layer model for mantle plumes driven by thermal and chemical diffusion and buoyancy. The problem is solved for a Boussinesq, Newtonian fluid with infinite Prandtl number and constant physical properties. We focus on axisymmetric mantle plumes, but also solve 2-D plumes due to line-sources for comparison. The results show that chemical plumes are much thinner than thermal plumes because of small chemical diffusivity in the mantle. When pressure-release partial melting occurs in a thermal-chemical plume, at least two mantle components may be involved: one from the chemical plume and one from the ambient mantle. A buoyant chemical boundary layer in the plume source region tends to cause narrow and strong plumes. A dense chemical source would have the opposite effect. The effects of chemical buoyancy diminish as the Lewis number, the ratio of thermal to chemical diffusivity, increases. For fully developed mantle plumes, the effects of chemical buoyancy may be insignificant. The physical parameters of mantle plumes may be estimated using surface information deduced from swell models. The total heat input from the Hawaiian plume source is about 1.3 x 10(11) W, nearly 5-10 per cent of the total heat loss from the core. The depth of the Hawaiian plume source is constrained to be near the core-mantle boundary. Our results show that 2-D plumes are generally stronger than axisymmetric plumes.