Inner core dynamics inferred from grain growth of ε-iron

Abstract:

The inner core is thought to be composed of Fe-Ni alloy with hcp structure based on the high pressure experiments (Tateno et al., 2012) and hence the physical properties of hcp iron (ε-iron) are keys for understanding the dynamics of the inner core. Recent seismic observations suggest the variation in grain size in the inner core (Monnereau et al., 2010). It is important to understand the variation in grain size for constraints of the dynamics of the inner core because grain size is controlled by the growth rate and growth rate gives us information on time scale of the inner core growth and/or translation (Alboussiere et al., 2010). In this study, we experimentally determine the grain growth rate of ε-iron to understand the dynamics of inner core. 

ε-iron is only stable at high pressure and it is unquenchable to an ambient condition. Therefore, in this study, we conduct in situ high pressure experiments to determine the grain growth rate of ε-iron. In the high pressure experiment, the starting materials was compressed in a Kawai-type high pressure apparatus equipped with sintered diamond anvils with 1.0 truncated edge length at BL04B1, SPring-8. At the pressure of ~55 GPa, sample was heated for several hours to determine the grain growth rates. Grain growth can be detected by the reduction of number of diffraction spots on the two-dimensional detector with monochromatic X-ray (Offerman et al., 2002) with annealing time.

In the experiments, we observed the reduction of the number of diffracted spots, meaning that grain growth occurs during annealing experiments. From the reduction rates of spots at temperatures ranged from 1200 to 1500 K, we determined the growth constant for grain growth at each temperature and then temperature dependency at ~55 GPa was obtained. By using the homologous temperature scaling to extrapolate the experimental to the inner core condition, we estimated the grain growth rate of ε-iron at the inner core condition. Our results suggests that it takes ~2.9 Gy translation period to explain the variation in grain size, which ranged from ~500 m in the western side to ~5 km in the eastern side of the inner core estimated from the seismic observation (Monnereau et al., 2010). The age of the inner core may be older than 2.9 Gy.