MELT EXTRACTION FROM THE MANTLE BENEATH SPREADING CENTERS

Melt migration beneath a mid-oceanic spreading center must be rapid and it must focus melt to the narrow ridge axis. Porous flow models that neglect compaction of the matrix result in vertical melt flow, and can not account for melt focussing. We show that dilation, or decompaction, of the porous matrix creates a high-porosity boundary layer, beneath an impermeable cap of cooler, solid mantle. The thickness of the layer, 200-300 m, is determined by a balance between the buoyancy of the melt and the viscous stresses resisting decompaction. The parameters that control the layer thickness include the upwelling velocity, mantle viscosity and density difference between the melt and solid. This layer exists near the solidus where melt is being frozen, and therefore deepens with distance from the ridge axis. The along-layer component of gravity drives melt in this highly permeable layer toward the ridge axis. We incorporate the effects of the high-porosity layer into a simple two-dimensional flow and thermal model of a spreading center, and calculate the velocity of melt and the porosity in the layer as a function of distance from the ridge axis. From this model we estimate that, for a reasonable range of physical parameters, the fraction of melt that is extracted by flow along the decompaction boundary layer is sufficient to produce the observed thickness of the oceanic crust.