Prof. Simon Redfern [University of Cambridge, UK]
Title: Earth’s Anisotropic Viscoelastic Inner Core: Light Element Defects and Zener Relaxations?
Time: 3:00 - 4:00 PM, Tuesday, July 12, 2016
Place: Auditorium Room 410, HPSTAR (Shanghai)
Host: Dr. Li Zhang
Earth's inner core is elastically anisotropic, with seismology showing faster wave propagation along the polar axis compared to the equatorial plane. Some inner core studies report anisotropic seismic attenuation. Attenuation of body-waves has, previously, been postulated to be due to scattering by anisotropic microstructure, but recent normal mode studies also show strong anisotropic attenuation (Mäkinen et al. 2014). This suggests that the anisotropic attenuation is a result of the intrinsic (and anisotropic) anelastic properties of the solid iron alloy forming Earth's inner core. Here, I consider the origins of inner core anisotropic attenuation. Possibilities include grain boundary relaxation, dislocation bowing/glide, or point defect (alloying element) relaxations. The inner core is an almost perfect environment for near-equilibrium crystallisation, with very low temperature gradients across the inner core, low gravity, and slow crystallisation rates. It is assumed that grain sizes may be of the order of hundreds of metres. This implies vanishingly small volumes of grain boundary, and insignificant grain boundary relaxation. The very high homologous temperature and the absence of obvious deviatoric stress, also leads one to conclude that dislocation densities are low. On the other hand, estimates for light element concentrations are of the order of a few % with O, Si, C and H at various times being suggested as candidate elements. Light element solutes in hcp metals contribute to intrinsic anelastic attenuation if they occur in sufficient concentrations to pair and form elastic dipoles. Switching of dipoles under the stress of a passing seismic wave will result in anelastic mechanical loss. Such attenuation has been measured in hcp metals in the lab, and is anisotropic due to the intrinsic elastic anisotropy of the host lattice. Such solute pair relaxations result in a "Zener effect", which is suggested here to be responsible for observed anisotropic seismic attenuation. Zener relaxation magnitude scales with solute concentration and is consistent with around 4% light element. Variations in attenuation are expected in a core with spatially varying concentrations of light element, and attenuation tomography of the inner core could, therefore, be employed to map chemical heterogeneity.
Biography of the Speaker:
Simon Redfern is a professor of Mineral Physics at the University of Cambridge, Department of Earth Sciences and (from October) incoming Head of Department. Before moving to Cambridge, Simon was Lecturer in Geochemical Spectroscopy in the Departments of Earth Sciences and Chemistry at the University of Manchester. His research covers a broad range of interests all linked by their relationship to the atomic-scale, nano-scale and microscopic structure of minerals. This includes relationships between structure, dynamics and properties of crystalline and amorphous materials from the Earth's core to the biosphere, and how these properties impact upon broader Earth and environmental processes. He is currently working on microstructure of biominerals and the physical properties of the structures that they form, the microstructure of rock-forming minerals and their seismic signatures (relevant to interpretations of deep Earth data), and the microstructures of manufactured materials based on mineral structures which may form the basis of new wasteforms for clean energy production. His work employs experimental methods in the lab and at national Synchrotron and Neutron radiation facilities.