LIMITATION ON THE MASS-EXCHANGE BETWEEN THE UPPER AND LOWER MANTLE - THE EVOLVING CONVECTION REGIME OF THE EARTH

It is increasingly accepted, through the use of geochemical arguments, that the Earth's mantle has a two-layer structure, comprising an uppe r and lower mantle. Estimates of the exchange of matter and energy bet ween these two layers are, thus, central to understanding the dynamics of the Earth. Geochemical mass balance permits us to estimate the ave rage flux of exchange between the lower and upper mantle. This estimat e can be made using two different classes of chemical tracers: the Lit isotopic systems and the rare gas isotopic system. The absolute value s and comparison between these tracers argues for: 1. a limited exchan ge rate between the lower and upper mantle, the average rate over 4.5 x 10(9) yr being less than 10% of the present-day subduction rate, S; 2. from comparison of these values with the seismic evidence of slab p enetration, two-layer convection having operated for most of the Earth 's history, but whole mantle convection having started less than 1 bil lion years ago. We are presently in a transitional stage between these two modes of operation. (C) 1997 Elsevier Science B.V.