Whole-Mantle Convection and the Transition-Zone Water Filter D. Bercovici & S. Karato

In the transition-zone water-filter hypothesis, (see main text), water is assumed essentially trapped within the transition zone because of dehydration melting of ambient upwelling at 410km and exsolution of water from slabs at 660km. The assumption that recycling of water is closed allows us to develop a simple theoretical model with which to predict various important parameters such as the melt fraction of partially melted ambient upwelling (which determines how well incompatible elements are filtered out) and the average thickness of the melt layer. The model system is driven by a background circulation through the transition zone consisting of downwelling slabs and the resultant passive ambient upwelling. Super-saturation and melting occurs in ambient upwelling mantle after passing out of the transition zone. The resulting layer of heavy melt can reach a steady state in terms of its chemistry and thickness if all materials in the melt layer are removed by slab entrainment at the same rate they are injected by upwelling ambient mantle. Self-regulation of melt-layer growth (and thus a steady state) is attainable because the efficiency of slab entrainment increases as the melt-layer thickness (and hence the effective contact area between the slab and melt) increases. Here we examine the conditions for this steady state and predict the typical melt fraction of upwelling mantle, the melt-layer thickness, and chemistry (concentration of water and incompatible elements) of the transition zone and melt layer.