ON THE FATE OF MANTLE PLUMES AT DENSITY INTERFACES

Abstract

The motion of plumes through compositionally layered systems is investigated by laboratory experiments in order to understand the ascent modes of deep mantle plumes. Experimental plumes entrain the ambient heavier fluid and the average density of the plume head ρP(t) increases with time. The degree of entrainment depends on the viscosity ratio of the matrix fluid μmatrix to the buoyant fluid μbuoyant, and the mass flux. Two types of plume penetration through the density interface are possible. The first, which can be characterized as a 'pass-through mode' (PTM), exhibits relatively little entrainment. Here the whole plume head passes directly through the interface because ρP(t) is smaller than the density of the upper layer ρu. In contrast, in the second type of interaction (named the 'rebirth mode', RBM), a large entrainment effect takes place. The plume is now greatly disturbed as it passes through the interface between the two fluid layers and entrained material remains at the density interface since ρP(t)>ρu. In addition, a new diapir emerges from the plume conduit and rises through the upper layer. The difference between these two types of interaction is reflected in the spatial and the temporal characteristics of geochemical data obtained from mantle plume products such as Ontong Java Plateau (OJP).