On the anatomy of mantle plumes: effect of the viscosity ratio on entrainment and stirring

Laminar entrainment of starting plumes with compositional buoyancy has been explored in a series of laboratory experiments. Two types of starting plume have been identified within the range of viscosity ratio ϵas of 10.4–856 (defined as the ratio of the ambient to the buoyant fluid). The first type represents a vortex ring at small viscosity ratio (ϵas<11) in which the plumes entrain the ambient fluids and form a rotating multi-layered structure within their heads. This is similar to the well-known structure observed in the thermal starting plumes of fluids having a strong temperature-dependent viscosity. In contrast, the second type of starting plume represents a more chaotic stirring regime at higher viscosity ratio (104<ϵas<856). Here, in the earlier stages, the plume head forms a double-layered structure by entrainment, with the buoyant fluid as the upper layer and the entrained fluid as the lower one. Thereafter, viscous coupling at the interface between the layers produces an intermediate stirring layer. This stirring layer grows with time with the end result that all fluids in the plume head mingle together. Using the fact that the resulting internal structure of the plume head depends on the viscosity ratio ϵas (a layered structure preserving the initial compositions vs. chaotic stirring tending towards homogeneity) a new viewpoint of mantle plume dynamics can be derived which is particularly useful in the interpretation of both spatial and temporal variations of geochemical data obtained from mantle plume products.