Inner-core anisotropy: Bridging our understanding between seismology and mineral physics
Abstract
Mineral physics has evolved over the past two decades to bridge gaps between many disciplines, including petrology, crystallography, chemistry and seismology. Coupling seismological data with experiments and theoretical work, geophysicists have been able to shed light on the structure, chemical composition, thermal history and dynamics of the Earth's deep interior. In 1986, using travel time and free oscillation data, Morelli et al. (1986) and Woodhouse et al. (1986) determined that the Earth's inner core is seismically anisotropic. Subsequent studies, using higher quality seismic data with increased coverage, have mapped out a 3-dimensional structure of Earth's inner core. The combined studies show a very complex inner core, with variation in structure both hemispherically and radially displaying a general increase of velocity and attenuation anisotropy as a function of depth. The evolving picture of Earth's inner core has created opportunities for mineral physicists to explain the forces and mechanisms involved in the creation of such a complex and dynamic structure. Models created to account for these features have invoked solid-state convection, solidification texturing, magnetic stresses, equatorial to polar mass flows and translation of the inner core constrained by temperature gradients, heat flow, mass variations in the mantle and the super rotation of the inner core. Using recent data on the evolution of microstructures in metals at very low stresses and new diffusion measurements in Fe-Ni alloys, we provide a predictive mechanism for the structure and deformation of Earth's inner core as a function of differential stress, effectively discriminating between models invoking the processes listed above by requiring self-consistent strain rates, grain sizes, and viscosities. We find that the radial structure of the inner core and depth dependence of velocity and attenuation anisotropy can be explained by a shift in deformation mechanism from an isotropic, diffusion-controlled mechanism at the surface of the inner core to a dislocation-controlled mechanism in the interior. Our calculations constrain the solid-state viscosity of the inner core between 10^20 and 10^22 Pa s with differential stresses of 10^3 - 10^4 Pa. We discuss the implications of these results in light of the current understanding of Earth's inner core coupled with past and future considerations.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMDI12A..02R
- Keywords:
-
- 3902 MINERAL PHYSICS / Creep and deformation