THE GENERATION OF A COMPOSITIONAL LITHOSPHERE BY MID-OCEAN RIDGE MELTING AND ITS EFFECT ON SUBSEQUENT OFF-AXIS HOTSPOT UPWELLING AND MELTING

Abstract

The generation of a ~ 50-500 times more viscous and 1-2% less dense depleted restite layer by melting and melt extraction beneath a mid-ocean ridge can be a significant barrier to subsequent off-axis upwelling and melting. Thiscompositional lithosphere is formed by melting at a mid-ocean ridge which results in the ''dewatering'' and increase in Mg# of olivine crystals in the restite residue from melt-extraction; effects which can produce up to a 1000-fold increase in the viscosity of the restite relative to upwelling asthenosphere. Compositional lithosphere may be the principle mechanical barrier to plume ascent and melting beneath young seafloor. The formation of compositional lithosphere at the ridge axis would also lead to uniform horizontal motion beneath the thickening thermal boundary layer -- this may be the physical reason underlying the excellent bathymetric fits of age-plate cooling models which assume this simple pattern of flow. The presence of a ~ 1021 Pa-s restite layer within and above the primary melting generation region of a mid-ocean ridge may also provide a mechanism for viscous pressure gradients to effectively focus migrating melt to a narrow region of axial volcanism and, simultaneously, to restrict ridge upwelling rates to the ''passive'' (plate separation) upwelling speeds implied by recent measurements of U-Th disequilibria in MORB. These last two effects critically depend on whether or not the presence of migrating melt will locally weaken the restite layer beneath the ridge-axis to an ''asthenosphere'' as opposed to a ''compositional lithosphere'' viscosity. Finally, melting at off-axis plumes that ceases at the base of a relatively uniform thickness compositional/thermal lithosphere would modulate different temperature plume inputs into a uniform temperature asthenosphere that reflects the solidus temperature near the base of the lithosphere. This may be the origin of the relatively uniform asthenosphere temperature that is sampled by melting and crustal production at the global mid-ocean ridge system.