Japan Geoscience Union Meeting 2018

Presentation information

[EE] Oral

S (Solid Earth Sciences) » S-IT Science of the Earth's Interior & Tectonophysics

[S-IT18] Planetary cores: Structure, formation, and evolution

Mon. May 21, 2018 1:45 PM - 3:15 PM A11 (Tokyo Bay Makuhari Hall)

convener:Hidenori Terasaki(Graduate School of Science, Osaka University), Eiji Ohtani(Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University), William F McDonough (共同), George Helffrich(Earth-Life Science Institute, Tokyo Institute of Technology), Chairperson:Terasaki Hidenori, McDonough William(University of Maryland / Tohoku University)

2:45 PM - 3:00 PM

[SIT18-05] The Fe-Fe2P and Ni-Ni2P phase diagrams at 6 GPa

Daniil Minin1, Anton Shatskiy1, *Konstantin Litasov1, Hiroaki Ohfuji2 (1.V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia, 2.Geodynamic Research Center, Ehime University, Matsuyama, Japan)

Keywords:iron phosphide, nickel phosphide, high-pressure experiment, planetary core , meteorite

Fe-P and Ni-P are among the basic phase diagrams for understanding the core formation in asteroids and planets. Recent finding of high-pressure minerals in iron meteorites (Holtstam et al., 2003; Litasov and Podgornykh, 2017) and abundance of complex Fe-Ni-P-S quench textures with unusual bulk compositions indicate great importance of the study of related systems at elevated pressures. Although high-pressure modifications of meteorite appear under shock impact conditions for the short time, static high-pressure experiments are more relevant for interpretation of these processes relative to shock wave experiments, where duration of shock is too short to model meteorite impact. We have determined the Fe-Fe2P and Ni-Ni2P phase diagrams at 6 GPa and 900-1600 oC. Experiments have been conducted in ceramic (3 MgO + 4 SiO2) capsules using multianvil technique.
The Fe-Fe2P system has two stable phosphide compounds: Fe3P and Fe2P. The Fe-Fe3P eutectic is established at 1075 oC and 16 mol% P. The Fe3P compound melts incongruently at 1250 oC to produce Fe2P and liquid containing 23 mol% P. The Fe2P compound melts congruently at 1575 oC. In whole studied temperature range, metallic iron dissolved measurable amounts of P suggesting an existence of limited solid solutions of P in Fe. The maximum P content in Fe, 4.2-5.2 mol%, is established at 1100-1200 oC. X-ray diffraction study indicates that Fe2P corresponds to barringerite structure, whereas Fe3P corresponds to schreibersite. Thus, quenched phases at 6 GPa do not corresponds to high-pressure polymorphs, such as (Fe,Ni)2P allabogdanite (Britvin et al., 2002), which was synthesized previously at 8 GPa (Dera et al., 2008).
The Ni-Ni2P system has three stable phosphide compounds: Ni3P, Ni3-xP, where x = 0.4-0.7 and Ni2P. The Ni-Ni3P eutectic locates at 975 oC and 20 mol% P. The Ni3-xP solid solution field narrows to the Ni2.3P composition as temperature increases to 1175 oC, where Ni2.3P melts incongruently to Ni2P and liquid containing 29 mol% P. The Ni2P compound melts congruently at 1250 oC. Ni also forms limited solid solutions with P. Similarly to Fe-P system, Ni2P and Ni3P compounds corresponds to barringerite and nickelphosphide crystal structures at 6 GPa. The intermediate compound Ni3-xP has variable composition, which may correspond to Ni5P2 or Ni12P5 observed at 1 atm.
The work is supported by Russian Science Foundation (No 17-17-01177).

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Dera, P., Lavina, B., Borkowski, L.A., et al. (2008) Geophys. Res. Lett., 35: doi: 10.1029/2008GL033867.
Holtstam, D., Broman, C., Soderhielm, J., Zetterqvist, A. (2003) Meteorit. Planet. Sci., 38: 1579-1583.
Litasov, K.D., Podgornykh, N.M. (2017) J. Raman Spect., 48: 1518-1527.