17:15 〜 18:30
[SIT15-P03] Self-diffusion of hcp-iron at high pressure
The size of the earth’s inner core of the earth is much smaller than that of mantle and hence geophysical observation of the inner core is difficult, for example, rheological properties of the inner core is poorly understood. The recent observations have suggested that the flow is not only an axial symmetrical component but also the possibility of flow in the horizontal plane. For the interpretation of the anisotropy of seismic waves formed in such a flow geometry, the flow mechanism and mobility of the constituent minerals (the viscosity) are key parameters. Because viscosity is strongly dependent on the diffusion of constituent elements, we determine the diffusion rate experimentally and predict the viscosity and deformation mechanism of the inner core. The inner core is composed of a solid iron alloy with the crystal structure of hcp, hcp-iron. The alloying is mainly done with Ni and other light elements, but primarily the rheological properties of pure iron are thought to represent the inner core properties. Therefore, in this study, we focused on the self-diffusion coefficient of iron in hcp-iron. Since hcp-iron is a stable phase only under high pressure, a diffusion experiment was conducted at a pressure of 45 GPa and a temperature of 1100-1300 by means of high pressure and temperature experiment combined with the isotope diffusion method. In the high-pressure experiments, Kawai-type assemblies composed with “binderless” tungsten carbide material of the second stage anvil to generate a pressure of 45 GPa were compressed in a DIA-type high pressure apparatus at Okayama University. Because the upper limit of temperature for the stability field of hcp-iron at 45 GPa is ~1300K, which is relatively low temperature to observe the thermally activated process, we needed diffusion duration more than 100 hours to obtain reliable diffusion length. The diffusion profile was obtained on the recovered specimen by analysis with the isotope microscope IMS1270 + SCAPS installed at Hokkaido University. In order to calibrate the irregular deformation of the specimen under high pressure and the spatial resolution of the analysis, a 30-minute diffusion sample was used as a reference. At 45 GPa and 1300 K, the diffusion coefficient was determined to be 10^-17.49 m^2/s. In order to extrapolate the diffusion coefficient obtained by the experiment to the inner core condition, it is necessary to accurately estimate the model parameters of temperature and pressure dependence. Therefore, we are trying to determine the diffusion coefficient in the pressure range of 15 GPa to 60 GPa.