Enhanced convection and fast plumes in the lower mantle induced by the spin transition in ferropericlase

Using a numerical model we explore the consequences of the intrinsic density change (Delta rho/rho approximate to 2-4%) caused by the Fe2+ spin transition in ferropericlase on the style and vigor of mantle convection. The effective Clapeyron slope of the transition from high to low spin is strongly positive in pressure-temperature space and broadens with high temperature. This introduces a net spin-state driving density difference for both upwellings and downwellings. In 2-D cylindrical geometry spin-buoyancy dominantly enhances the positive thermal buoyancy of plumes. Although the additional buoyancy does not fundamentally alter large-scale dynamics, the Nusselt number increases by 5-10%, and vertical velocities by 10-40% in the lower mantle. Advective heat transport is more effective and temperatures in the core-mantle boundary region are reduced by up to 12%. Our findings are relevant to the stability of lowermost mantle structures. Citation: Bower, D. J., M. Gurnis, J. M. Jackson, and W. Sturhahn (2009), Enhanced convection and fast plumes in the lower mantle induced by the spin transition in ferropericlase, Geophys. Res. Lett., 36, L10306, doi: 10.1029/2009GL037706.