The impact of variability in the rheological activation parameters on lower-mantle viscosity stratification and its dynamics

The recently discovered spin-state crossover in iron in major mantle minerals at high pressures should exert dramatic influences on transport properties, such as the activation creep parameters in the deep mantle. Wentzcovitch et al. (2009) have computed the elastic parameter of ferropericlase that is affected by this crossover, the bulk modulus, at high realistic conditions, using first principles density functional theory plus Hubbard U. As a consequence of the spin crossover there is a significant softening of the bulk modulus and an attendant reduction of the activation energy in the creep law. Using a parameterized model capturing the basic physics, we have studied the dynamical consequences within the framework of a 2-D Cartesian convection model. Our models reveal a series of asthenospheres and are characterized first by a low-viscosity channel below the 670 km boundary ('second asthenosphere') caused by an increase of temperature and decrease in grain size reduction (superplascity) with the post-spinel transition, which results in partially layered convection. Further, the variability of the rheological parameters leads to the prevalent formation of viscosity minimum at a depth of about 1600 km ('third asthenosphere') caused by spin crossovers followed by a 'viscosity hill' in the bottom half of the lower mantle, the latter due to the increase of the activation energy in some low spin irons at the bottom of the mantle. Our numerical simulations reveal a tendency to the formation of small-scale convection below the 670 km boundary and bigger stabilized plumes - and/or plume clustering in the deep lower mantle. Other material properties at the bottom of the mantle (decrease of thermal expansivity, radiative thermal conductivity) also exert significant influence on the multi-scale lower-mantle plume dynamics.