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Melting of iron at the physical conditions of the Earth's core

Abstract

Seismological data can yield physical properties of the Earth's core, such as its size and seismic anisotropy1,2,3. A well-constrained iron phase diagram, however, is essential to determine the temperatures at core boundaries and the crystal structure of the solid inner core. To date, the iron phase diagram at high pressure has been investigated experimentally through both laser-heated diamond-anvil cell and shock-compression techniques, as well as through theoretical calculations4,5,6,7,8,9,10,11,12,13,14,15,16,17. Despite these contributions, a consensus on the melt line or the high-pressure, high-temperature phase of iron is lacking. Here we report new and re-analysed sound velocity measurements of shock-compressed iron at Earth-core conditions15. We show that melting starts at 225 ± 3 GPa (5,100 ± 500 K) and is complete at 260 ± 3 GPa (6,100 ± 500 K), both on the Hugoniot curve—the locus of shock-compressed states. This new melting pressure is lower than previously reported16, and we find no evidence for a previously reported solid–solid phase transition on the Hugoniot curve near 200 GPa (ref. 16).

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References

  1. 1

    Lehmann, I. P′. Publ. Bur. Cent. Seismol. Int. A 14, 87–115 (1936)

  2. 2

    Song, X. & Richards, P. G. Seismological evidence for differential rotation of the Earth's inner core. Nature 382, 221–224 (1996)

  3. 3

    Su, W., Dziewonski, A. M. & Jeanloz, R. Planet within a planet: rotation of the inner core of Earth. Science 274, 1883–1887 (1996)

  4. 4

    Williams, Q., Jeanloz, R., Bass, J., Svendsen, B. & Ahrens, T. J. The melting curve of iron to 250 gigapascals: a constraint on the temperature at Earth's center. Science 236, 181–182 (1987)

  5. 5

    Boehler, R. Temperatures in the Earth's core from melting-point measurements of iron at high static pressures. Nature 363, 534–536 (1993)

  6. 6

    Shen, G., Mao, H. K., Hemley, R. J., Duffy, T. S. & Rivers, M. L. Melting and crystal structure of iron at high pressures and temperatures. Geophys. Res. Lett. 25, 373–376 (1998)

  7. 7

    Saxena, S. K. & Dubrovinsky, L. S. Iron phases at high pressures and temperatures: phase transition and melting. Am. Mineral. 85, 372–375 (2000)

  8. 8

    Andrault, D., Fiquet, G., Charpin, T. & Le Bihan, T. Structure analysis and stability field of β-iron at high pressure and temperature. Am. Mineral. 85, 364–371 (2000)

  9. 9

    Yoo, C. S., Akella, J., Campbell, A. J., Mao, H. K. & Hemley, R. J. Phase diagram of iron by in situ x-ray diffraction: implications for Earth's core. Science 270, 1473–1475 (1995)

  10. 10

    Alfè, D., Price, G. D. & Gillan, M. J. Iron under Earth's core conditions: Liquid-state thermodynamics and high-pressure melting curve from ab initio calculations. Phys. Rev. B 65, 165118 (2002)

  11. 11

    Laio, A., Bernard, S., Chiarotti, G. L., Scandolo, S. & Tosatti, E. Physics of iron at Earth's core conditions. Science 287, 1027–1030 (2000)

  12. 12

    Ahrens, T. J., Bass, J. D. & Abelson, J. R. in Shock Compression of Condensed Matter—1989 (eds Schmidt, S. C., Johnson, L. W. & Davison, L. W.) (Elsevier Science, Amsterdam, 1990)

  13. 13

    Yoo, C. S., Holmes, N. C., Ross, M., Webb, D. J. & Pike, C. Shock temperatures and melting of iron at Earth core conditions. Phys. Rev. Lett. 70, 3931–3934 (1993)

  14. 14

    Benuzzi-Mounaix, A. et al. Absolute equation of state measurements of iron using laser driven shocks. Phys. Plasma 9, 2466–2469 (2002)

  15. 15

    Nguyen, J. H. & Holmes, N. C. in Shock Compression of Condensed Matter—1999 (eds Furnish, M. D., Chhabildas, L. C. & Hixson, R. S.) 81–84 (American Institute of Physics, New York, 1999)

  16. 16

    Brown, J. M. & McQueen, R. G. Phase transitions, Grüneisen parameter, and elasticity for shocked iron between 77 GPa and 400 GPa. J. Geophys. Res. 91, 7485–7494 (1986)

  17. 17

    Ahrens, T. J., Holland, K. G. & Chen, C. Q. Phase diagram of iron, revised-core temperatures. Geophys. Res. Lett. 29, 54-1–54-4 (2002)

  18. 18

    Hixson, R. S., Boness, D. A., Shane, J. W. & Moriarty, J. A. Acoustic velocities and phase transitions in molybdenum under strong shock compression. Phys. Rev. Lett. 62, 637–640 (1989)

  19. 19

    Brown, J. M. & Shaner, J. W. in Shock Compression of Condensed Matter—1983 (eds Asay, J. R., Graham, R. A. & Straub, G. K.) 91–94 (North-Holland, Amsterdam, 1984)

  20. 20

    McQueen, R. G., Fritz, J. N. & Morris, C. E. in Shock Compression of Condensed Matter—1983 (eds Asay, J. R., Graham, R. A. & Straub, G. K.) 95–98 (North-Holland, Amsterdam, 1984)

  21. 21

    Shaner, J. W., Brown, J. M. & McQueen, R. G. Melting of metals above 100 GPa. Mater. Res. Soc. Symp. Proc. 22, 137–141 (1984)

  22. 22

    Zel'dovich, Y. B. & Raizer, Y. P. Physics of Shock Waves and High-Temperature Hydrodynamic Phenomena (Academic, New York, 1967)

  23. 23

    McQueen, R. G., Hopson, J. W. & Fritz, J. N. Optical technique for determining rarefaction wave velocities at very high pressures. Rev. Sci. Instrum. 53, 245–250 (1982)

  24. 24

    Brown, J. M., Fritz, J. N. & Hixson, R. S. Hugoniot data for iron. J. Appl. Phys. 88, 5496–5498 (2000)

  25. 25

    Wasserman, E., Stixrude, L. & Cohen, R. E. Thermal properties of iron at high pressures and temperatures. Phys. Rev. B 53, 8296–8309 (1996)

  26. 26

    Lou, S. N. & Ahrens, T. J. Superheating systematics of crystalline solids. Appl. Phys. Lett. 82, 1836–1838 (2003)

  27. 27

    Lou, S. N. & Ahrens, T. J. Application of shock-induced superheating to the melting of geophysically important minerals. Phys. Earth Planet. Inter. (in the press)

  28. 28

    Boehler, R. & Ross, M. Melting curve of aluminum in a diamond cell to 0.8 Mbar: implications for iron. Earth Planet. Sci. Lett. 153, 223–227 (1997)

  29. 29

    Wallace, D. C. Irreversible thermodynamics of flow in solids. Phys. Rev. B 22, 1477–1486 (1980)

  30. 30

    Boehler, R. Melting of the Fe-FeO and Fe-FeS systems at high-pressure—constraints on core temperatures. Earth Planet. Sci. Lett. 111, 217–227 (1992)

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Acknowledgements

We benefited from discussions with J. M. Brown, O. L. Anderson, M. Ross and R. Boehler. We acknowledge F. H. Streitz for the formulation of equations (2) and (3). We are grateful for the technical efforts of S. Caldwell, E. Ojala, L. Raper, K. Stickle. Work was performed by the University of California under the auspices of the US DOE by the Lawrence Livermore National Laboratory.

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Correspondence to Jeffrey H. Nguyen.

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Further reading

Figure 1: Iron phase diagram.
Figure 2: Sound velocity and overtake ratio.

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