HR: 0800h
AN: MR31B-0370    [Abstracts]
TI: Effect of Hydrogen and Carbon on the Melting Temperature of the Core
AU: * Nakajima, Y
EM: ynakajim@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo, 152-8551, Japan
AU: Sakamaki, K
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo, 152-8551, Japan
AU: Takahashi, E
EM: etakahas@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo, 152-8551, Japan
AU: Fukai, Y
AF: Professor Emeritus, Institute of Science and Institute of Science and Engineering, Chuo University, Kasuga, Bunkyo-ku, Tokyo, 112-8551, Japan
AU: Suzuki, T
AF: Institute for Research on Earth Evolution, Japan Marine-Earth Science and Technology, 2- 15, Natsushima-cho, Yokosuka, 237-0061, Japan
AU: Funakoshi, K
AF: Japan Synchrotron Radiation Research Institute, 1-1-1, Koto, Sayo-cho, Sayo-gun, Hyogo, 679-5198, Japan
AB: The temperature of the Earth's outer core has been discussed based on the melting temperature of Fe- O-S alloys (e.g., Boehler, 1996). Although hydrogen and carbon are the possible candidates of the core component, their effects on the melting temperature of iron at high-pressures are unclear. Using a Kawai-type multi-anvil apparatus at SPring-8 synchrotron, we carried out a series of melting experiments on FeH and Fe₃C up to 20 and 28 GPa, respectively. In the experiments on FeH, Fe sponge mixed with MgO was packed into a NaCl container with a hydrogen source, LiAlH₄ (e.g., Fukai et al., 1989). During heating under high-pressures, hydrogenation of iron was observed by volume change. The phase boundary between ε'-phase (low-temperature phase) and γ-phase (high-temperature phase) of iron-hydride was determined using both cooling and heating experiments. Hydrogen concentrations in the γ-FeH_x and ε'-FeH_x were calculated based on the excess volume data from that of pure iron. It is found that γ-FeH_x and ε'-FeH_x synthesized in our experiments at pressures between 10 and 20 GPa are nearly stoichiometric FeH. Melting temperature of the γ-FeH was determined by the abrupt change in the X-ray diffraction patterns (crystalline to amorphous). The melting temperatures were determined to be 1473, 1473, 1493, 1573 and 1593 K at 10, 11.5, 15, 18 and 20 GPa, respectively. In the experiments using Fe₃C, the synthesized Fe₃C powder was encapsulated in a MgO container. In the diffraction sequences during heating, the peaks of Fe₃C disappeared, and the new peaks identified as those of Fe₇C₃ were observed with halo caused by liquid. Finally, the Fe₇C₃ peaks disappeared, and only the halo pattern was observed. Based on these observations, the incongruent melting of Fe₃C to Fe₇C₃ and liquid is estimated to occur at 1823 and 1923 K at 19.7 and 27.0 GPa, respectively. The liquidus temperatures of the Fe₃C composition are found to be at 2098 and 2198 K at 19.5 and 26.8 GPa, respectively. The melting temperatures of Fe₃C determined by our experiments are >700 K lower than that of the previous estimation based on thermodynamic calculation (Wood, 1993). Our experimental results show a possibility that the hydrogen and carbon lower the melting temperature of iron (outer core) dramatically. The melting temperatures of γ-FeH and Fe₃C at 20 GPa are already 500 K lower than that of pure iron estimated by Anderson and Isaak (2000). Extrapolating our experimental melting curves for FeH and Fe₃C to core pressures using Lindemann's melting law, we obtained the melting temperatures to be ~2600 and ~2900 K at the core-mantle boundary (CMB), respectively. In the presence of both hydrogen and carbon, melting temperature of the Earth's outer core could be >1500 K lower than that of the previous estimates, implying that the temperature gap at CMB could be much smaller than the current estimates.
DE: 3900 MINERAL PHYSICS
DE: 3924 High-pressure behavior
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting