Phase Relations of Iron and Iron-Nickel Alloys up to 3 Mbars

HR: 0800h
AN: DI31A-0262 [Abstracts]
TI: Phase Relations of Iron and Iron-Nickel Alloys up to 3 Mbars
AU: * Kuwayama, Y
AF: IFREE, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061, Japan
AU: * Kuwayama, Y
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo, 152-8551, Japan
AU: Hirose, K
AF: IFREE, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061, Japan
AU: Hirose, K
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo, 152-8551, Japan
AU: Sata, N
AF: IFREE, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061, Japan
AU: Ohishi, Y
AF: Japan Synchrotron Radiation Research Institute, 1-1-1, Kouto, Sayo-gun, Hyogo, 679-5198, Japan
AB: Iron is believed to be the major component of the Earth's core because it is the most abundant element that satisfies the observed seismic densities. Based on cosmochemical models and the studies of iron meteorites, it is generally accepted that the Earth's core also contains substantial amounts of nickel. Therefore, the high pressure behaviour of iron-nickel alloys is crucially important for interpreting and constraining geophysical and geochemical models of the Earth's core. The phase relation of iron at relatively low pressure has been well established. α-Fe with bcc structure at ambient condition transforms to γ-Fe at high temperature and to ε-Fe with hcp structure at above ~ 10 GPa. In contrast, the phase relation and the crystal structure at high pressure and temperature are still highly controversial. The phase relations of iron-nickel alloys were also studied in an externally-heated diamond-anvil cell (Huang et al. 1988, 1992) and in a laser-heated diamond-anvil cell (Lin et al. 2002, Mao et al. 2005, Dubrovinsky et al. 2007), but these experiments were limited to the pressure of 225 GPa. Applications of the previous results to the Earth's inner core conditions required significant extrapolations. In this study, we have investigated the phase relations of iron and a number of iron-nickel alloys in a wide range of pressures (>300 GPa), temperatures (>2000 K) and compositions (0-80 wt% Ni) using a laser-heated diamond-anvil cell with synchrotron x-ray diffraction. For iron, in-situ x-ray diffraction studies showed a wide range of stability of ε-Fe with an hcp structure up to 300 GPa and 2000 K and up to 343 GPa at room temperature. No evidence for the existence of phases other than ε-Fe, such as β-Fe with a dhcp structure (suggested by Dubrovinsky et al. 2000) or orthorhombic structure (suggested by Andrault et al. 1997), was observed. For iron-nickel alloys, high pressure and temperature experiments were conducted on Fe-18.4 wt% Ni, Fe-24.9 wt% Ni, Fe-35.7 wt% Ni, Fe-50.0 wt% Ni and Fe-80.0 wt% Ni up to 300 GPa. The experimental results indicate that the iron-nickel alloys strongly favour an fcc structure under multimegabar pressures. Our results can directly apply to the Earth’s inner core pressures and the phase relations of iron- nickel alloys may interpret seismically observed anisotropy and discontinuity in the Earth's inner core.
DE: 1015 Composition of the core
DE: 3924 High-pressure behavior
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
DE: 7207 Core (1212, 1213, 8124)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting