3:00 PM - 3:15 PM
[SCG54-12] Rheology of hcp-iron
Keywords:hcp-iron, inner core, rheology
Seismic observations have revealed presence of axi-symmetric anisotropy with respect to polar direction in the Earth’s inner core which consists of solid metal, where a P- wave propagating along the polar direction is ∼3% faster than that along the equatorial direction. Although many hypotheses have been proposed for origin of seismic anisotropy in the inner core, there is no general consensus for its origin. Lasbleis and Deguen (2015) discussed that dominant mechanism of the inner core dynamics depends on the inner core age and viscosity, and there are several candidate mechanisms which includes anisotropic growth model (Yoshida et al., 1996) and thermal convection model (Jeanloz and Wenk, 1988). Since the inner core is considered to consist of hexagonal close-packed iron(hcp-iron), information of viscosity of hcp-iron is a key for the understanding of the inner core dynamics. In this study, we have studied rheology of hcp-iron based on high-pressure and high-temperature deformation experiments.
Uniaxial deformation experiments were carried out using a D111-type apparatus installed on a beamline NE7A at PF-AR, KEK and a deformation-DIA apparatus installed on a beamline BL04B1 at SPring-8. Using polycrystalline bcc-iron as a starting material, deformation experiments were carried out at pressure of 16.9-22.6 GPa, temperature of 423-873 K, and strain rate of 0.2–5.2×10–5 s–1. Stress and strain during deformation were determined based on two-dimensional X-ray diffraction and X-radiography, respectively, using monochromatized synchrotron X-ray with energy of 60 keV.
A preliminary analysis of the observed data suggests that power-law dislocation creep with stress exponent of ~5 is dominant at >700 K whereas power-law breakdown occurs at lower temperatures. Extrapolation of the data based on homologous temperature scaling suggests that viscosity of hcp-iron at the inner core conditions is ~1021 Pa s or higher.
Uniaxial deformation experiments were carried out using a D111-type apparatus installed on a beamline NE7A at PF-AR, KEK and a deformation-DIA apparatus installed on a beamline BL04B1 at SPring-8. Using polycrystalline bcc-iron as a starting material, deformation experiments were carried out at pressure of 16.9-22.6 GPa, temperature of 423-873 K, and strain rate of 0.2–5.2×10–5 s–1. Stress and strain during deformation were determined based on two-dimensional X-ray diffraction and X-radiography, respectively, using monochromatized synchrotron X-ray with energy of 60 keV.
A preliminary analysis of the observed data suggests that power-law dislocation creep with stress exponent of ~5 is dominant at >700 K whereas power-law breakdown occurs at lower temperatures. Extrapolation of the data based on homologous temperature scaling suggests that viscosity of hcp-iron at the inner core conditions is ~1021 Pa s or higher.