Numerical investigation of plume-lithosphere-interaction beneath mid-ocean ridges

The interaction of mantle plumes with mid-ocean ridges is investigated in 3-D numerical convection models. A new numerical method for solving convection problems with variable viscosity in Cartesian geometry is presented. It is based on a finite-volume discretization in combination with a multigrid method. A local mesh refinement technique, which is more efficient and flexible than previously used refinement methods, makes a simple implementation of complex grid structures feasible. Comparisons with other numerical methods reveal that accurate results are obtained even when viscosity varies strongly. Using this numerical method, a much stronger temperature-dependence of viscosity can be considered than was possible in previous models. For the first time, numerical calculations of plume-ridge-interaction with a realistic viscosity law, which allow lateral viscosity variations of more than 6 orders of magnitude, are presented.In models with constant plume flux the influence of various model parameters on the interaction of plumes with mid-ocean ridges is determined. It turns out that the dynamics of plume-ridge-interaction depends more strongly on the plume viscosity than previously assumed. The plume material is significantly channeled along the axis of a slow-spreading ridge if the plume viscosity is of order 10E17 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. A scaling law is derived from the numerical calculations in order to describe the along-axis plume width dependence on the model parameters.In time-dependent calculations the temporal evolution of short-term variations of the plume flux is investigated. If the plume viscosity is small and the spreading rate is slow, these perturbations propagate in the form of pulses along the ridge with a speed much larger than the spreading rate of the ridge. The concentration of the perturbation on the ridge axis becomes smaller with increasing spreading rate and decreasing viscosity contrast between plume material and ambient mantle. Pulsations of the Iceland mantle plume have been suggested as the cause of V-shaped topography and gravity anomalies at the Reykjanes Ridge. The model results are discussed in this context