Channeling of plume flow beneath mid-ocean ridges

We investigate the interaction of mantle plumes with mid-ocean ridges in 3-D numerical convection models. Using a finite-volume multigrid technique with local grid refinement, we are able to incorporate a much stronger temperature-dependence of viscosity than was possible in previous models. This allows lateral viscosity variations of more than six orders of magnitude, which is realistic in a ridge situation. In models of ridge-centered plumes with constant flux we determine how the shape and the along-axis width of the plume head depends on various model parameters. The plume material is significantly channeled along the axis of a slow-spreading ridge if the plume viscosity is in the order of 1017 Pa s. The plume material spreads radially below the ridge if the spreading rate is fast or the viscosity contrast between plume and ambient mantle is small. In time-dependent calculations we investigate the temporal evolution of short-term variations of the plume flux. At slow-spreading ridges these perturbations propagate in the form of pulses along the ridge with a speed much larger than the spreading rate. Our results support the hypothesis that pulsations of the Iceland plume are the cause for V-shaped topography and gravity anomalies at the Reykjanes Ridge.