Mantle plumes pinched in the transition zone

We have employed an axisymmetric, spherical shell model with the two major phase transitions and variable viscosity to study the dynamical processes associated with both regular mantle plumes and superplumes, which can develop spontaneously in strongly time-dependent situations. This model is heated both from below and from within and the viscosity varies according to the temperature, pressure and the phase boundaries. We have studied the physical processes due to the interactions between the different types of plumes and superplumes with the transition zone, which can shed light on some new understanding of intraplate volcanism. The interaction between the plume and the two major phase transitions gives rise to the production of a reservoir of plume material, localized in the transition zone. The spinel to olivine phase transition induces the ejection of very fast plume material within a very thin vertical channel towards the surface. The spreading of this plume is controlled by the upper-mantle circulation. Stable plumes, which extend throughout the mantle for several. hundred million years, can produce volcanism with an age progression of the seamounts. Other plumes, which are bent and move relative to the global circulation, may generate chaotic surface volcanism with a very limited age progression of the seamounts. Plume-plume collision produces a superplume, growing from the core-mantle boundary (CMB). The interaction between superplumes and the perovskite to spinel phase transition generates a thick thermal boundary layer, in which the upper-mantle can serve as the site for the launching of new smaller plumes. Stress fields associated with the superplume have the largest magnitude at the CMB and just below the 670 km discontinuity. They may be associated with seismic anisotropy found there. (C) 2000 Published by Elsevier Science B.V. All rights reserved.