Is The Source of The Iceland Plume Located In The Transition Zone Below The 660 Km Discontinuity

Hydrodynamic theory indicates that hotspot-producing plume-like upwellings arise from a basal boundary layer of a convecting fluid layer. In the mantle, such boundary layers can exist above the core-mantle boundary (CMB) and the 660 km discontinuity, therefore these two surfaces are the commonly accepted alternatives for the birth-place of plumes. Which of them actually emit plumes: this question has been the subject of many studies. The Iceland hotspot is one of the most suitable experimental areas for the solution of the plume problem. The observations pertinent to the depth extent of the Iceland plume are however contradictory. As Foulger and Pearson [2001, Geophys. J. Int. 145, F1-F5] discussed, seismic tomography suggests that the plume anomaly is restricted to the upper mantle above the 660 km discontinuity, while isotope geochemistry strongly favours mass transport originating in the lower mantle under Iceland, for which the classical explanation is a CMB plume. We propose a dynamical solution for the apparent discrepancy of the observations. This solution is based on the phenomenon of mid-mantle plumes as described by Cserepes and Yuen [2000, Earth Planet. Sci. Lett. 183, 61-71]. This is a kind of plumes which originate from below a leaky boundary layer at the 660 km discontinuity, therefore they have a seismic anomaly only in the upper mantle, and yet they transport lower-mantle material to the surface. The condition for the existence of mid-mantle plumes is that the 660 km discontinuity, i.e. the phase boundary due to an endothermic phase transition, acts as a strong, but not fully impenetrable barrier to vertical flow. The critical parameters of the mantle (Rayleigh number, phase transition characteristics) closely meet this condition. If the discontinuity leaks at small spots, flow from the lower mantle takes, in the upper mantle, the form of the usual mushroom-shaped plumes. These mid-mantle plumes are the upwelling equivalents of the downwelling avalanche events which occur episodically in a partially layered convective system.