Partial Melting and Melt Segregation in a Convecting Mantle

Abstract

Various causes for mantle melting (decompression, heating or release of water) combined with current estimates of upper mantle temperatures and the state of stress in the lithosphere suggest that in many regions the asthenosphere might be partially molten, but melts may not always be able to rise to the surface. The governing equations describing melting, melt segregation, compaction and depletion in a deforming medium are discussed with emphasis on the physical processes involved. To combine these processes with a convecting upper mantle flow, a “Compaction Boussinesq Approximation” (CBA) is introduced and tested with known solutions. Driving forces include thermal, melt, depletion and enrichment buoyancy. The bulk viscosity and its dependence on porosity has a significant effect on the melt flow even for distances large compared to the compaction length. 1D and 2D solitary porosity waves are discussed with particular emphasis on a variable bulk viscosity, compaction, and dilatation of the matrix. Melting, segregation and solidification processes are studied in a self-consistent model of a variable viscosity plume head arriving at the base of the lithosphere. It is shown that melt buoyancy dominates segregation velocities. However, a variable bulk viscosity may still have some influence on the segregation velocities, while dynamic pressures may be neglected. In the absence of a mantle plume a partially molten undepleted asthenosphere may develop melting instabilities, driven by thermal, melt and depletion buoyancy. This instability propagates laterally with velocities of the order of several cm/a and has a length scale of about 2 times the thickness of the partially molten asthenosphere. Volcanism associated with this propagating instability might have a similar appearance as hot spot tracks suggesting that this instability might be an alternative mechanism to the plume hypothesis at least for some volcanic chains.