A proper determination of the lower-mantle viscosity profile is fundamental to understanding Earth geodynamics. Based on results coming from different sources, several models have been proposed to constrain the variations of viscosity as a function of pressure, stress and temperature. While some models have proposed a relatively modest viscosity variation across the lower mantle, others have proposed variations of several orders of magnitude. Here, we have determined the viscosity of ferropericlase, a major mantle mineral, and explored the role of the iron high-to-low spin transition. Viscosity was described within the elastic strain energy model, in which the activation parameters are obtained from the bulk and shear wave velocities. Those velocities were computed combining first principles total energy calculations and the quasi-harmonic approximation. As a result of a strong elasticity softening across the spin transition, there is a large reduction in the activation free energies of the materials creep properties, leading to viscosity undulations. These results suggest that the variations of the viscosity across the lower mantle, resulting from geoid inversion and postglacial rebound studies, may be caused by the iron spin transition in mantle minerals. Implications of the undulated lower mantle viscosity profile exist for both, down- and up-wellings in the mantle. We find that a viscosity profile characterized by an activation free energy of G*(z0)~300-400kJ/mol based on diffusion creep and dilation factor δ=0.5 better fits the observed high velocity layer at mid mantle depths, which can be explained by the stagnation and mixing of mantle material. Our model also accounts for the growth of mantle plume heads up to the size necessary to explain the Large Igneous Provinces that characterize the start of most plume tracks.
Bibliographical noteFunding Information:
This research was partially supported by grants NSF/EAR 0635990 , NSF/ITR 0428774 , and NSF/DMR 0325218 (ITAMIT) and the geochemistry and geophysics programs of the National Science Foundation . Computations were performed at the Minnesota Supercomputing Institute and on the Big Red Cluster at Indiana University. JFJ acknowledges partial support from Brazilian agencies FAPESP (grant number 2009/14082-3 ) and CNPq (grant number 473307/2013-8 ). GM acknowledges Louisiana Board of Regents – Research Competitiveness Subprogram LEQSF Grant (2014-17)-RD-A-14 .
- Lower mantle
- Mantle plume
- Spin transition