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High Poisson's ratio of Earth's inner core explained by carbon alloying

Nature Geoscience volume 8, pages 220–223 (2015)Cite this article

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Abstract

Geochemical, cosmochemical, geophysical, and mineral physics data suggest that iron (or iron–nickel alloy) is the main component of the Earth’s core1,2,3. The inconsistency between the density of pure iron at pressure and temperature conditions of the Earth’s core and seismological observations can be explained by the presence of light elements1,4. However, the low shear wave velocity and high Poisson’s ratio of the Earth’s core remain enigmatic2. Here we experimentally investigate the effect of carbon on the elastic properties of iron at high pressures and temperatures and report a high-pressure orthorhombic phase of iron carbide, Fe7C3. We determined the crystal structure of the material at ambient conditions and investigated its stability and behaviour at pressures up to 205 GPa and temperatures above 3,700 K using single-crystal and powder X-ray diffraction, Mössbauer spectroscopy, and nuclear inelastic scattering. Estimated shear wave and compressional wave velocities show that Fe7C3 exhibits a lower shear wave velocity than pure iron and a Poisson’s ratio similar to that of the Earth’s inner core. We suggest that carbon alloying significantly modifies the properties of iron at extreme conditions to approach the elastic behaviour of rubber. Thus, the presence of carbon may explain the anomalous elastic properties of the Earth’s core.

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Figure 1: Volume–pressure data for o-Fe7C3 with the fitted third-order Birch–Murnaghan equation of state (K300 = 168(4) GPa, K′ = 6.1(1)).
Figure 2: Nuclear resonance measurements of o-Fe7C3 at high pressure.
Figure 3: Variation of Debye sound velocity vD, bulk sound velocity vΦ, shear wave velocity vS and compressional wave velocity vP of Fe7C3 with density.
Figure 4: Extrapolation of Poisson’s ratio to the densities estimated in the Earth’s inner core using Birch’s law.

References

  1. Birch, F. Elasticity and constitution of the Earth’s interior. J. Geophys. Res. 57, 227–286 (1952).

    Article  Google Scholar 

  2. Dziewonski, A. M. & Anderson, D. L. Preliminary reference Earth model. Phys. Earth Planet. Inter. 25, 297–356 (1981).

    Article  Google Scholar 

  3. Dubrovinsky, L. S., Saxena, S. K., Tutti, F., Rekhi, S. & LeBehan, T. In situ X-Ray study of thermal expansion and phase transition of iron at multimegabar pressure. Phys. Rev. Lett. 84, 1720–1723 (2000).

    Article  Google Scholar 

  4. Badro, J. et al. Effect of light elements on the sound velocities in solid iron: Implications for the composition of Earth’s core. Earth Planet. Sci. Lett. 254, 233–238 (2007).

    Article  Google Scholar 

  5. Anderson, O. L. & Isaak, D. G. Another look at the core density deficit of Earth’s outer core. Phys. Earth Planet. Inter. 131, 19–27 (2002).

    Article  Google Scholar 

  6. Dewaele, A. et al. Quasihydrostatic equation of state of iron above 2 Mbar. Phys. Rev. Lett. 97, 215504 (2006).

    Article  Google Scholar 

  7. Antonangeli, D. et al. Composition of the Earth’s inner core from high-pressure sound velocity measurements in Fe–Ni–Si alloys. Earth Planet. Sci. Lett. 295, 292–296 (2010).

    Article  Google Scholar 

  8. Mao, Z. et al. Sound velocities of Fe and Fe-Si alloy in the Earth’s core. Proc. Natl Acad. Sci. USA 109, 10239–10244 (2012).

    Article  Google Scholar 

  9. Gubbins, D., Sreenivasan, B., Mound, J. & Rost, S. Melting of the Earth’s inner core. Nature 473, 361–363 (2011).

    Article  Google Scholar 

  10. Mao, H. K. et al. Phonon density of states of iron up to 153 gigapascals. Science 292, 914–916 (2001).

    Article  Google Scholar 

  11. Lin, J-F. et al. Sound velocities of iron–nickel and iron–silicon alloys at high pressures. Geophys. Res. Lett. 30, 2112 (2003).

    Article  Google Scholar 

  12. Mao, W. L. et al. Nuclear resonant X-ray scattering of iron hydride at high pressure. Geophys. Res. Lett. 31, L15618 (2004).

    Article  Google Scholar 

  13. Murphy, C. A., Jackson, J. M. & Sturhahn, W. Experimental constraints on the thermodynamics and sound velocities of hcp-Fe to core pressures. J. Geophys. Res. B 118, 1999–2016 (2013).

    Article  Google Scholar 

  14. Shibazaki, Y. et al. Sound velocity measurements in dhcp-FeH up to 70 GPa with inelastic X-ray scattering: Implications for the composition of the Earth’s core. Earth Planet. Sci. Lett. 313–314, 79–85 (2012).

    Article  Google Scholar 

  15. Gao, L. et al. Pressure-induced magnetic transition and sound velocities of Fe3C: Implications for carbon in the Earth’s inner core. Geophys. Res. Lett. 35, 1–5 (2008).

    Google Scholar 

  16. Gao, L. et al. Effect of temperature on sound velocities of compressed Fe3C, a candidate component of the Earth’s inner core. Earth Planet. Sci. Lett. 309, 213–220 (2011).

    Article  Google Scholar 

  17. Prescher, C. et al. Structurally hidden magnetic transitions in Fe3C at high pressures. Phys. Rev. B 85, 6–9 (2012).

    Article  Google Scholar 

  18. Wood, B. J., Li, J. & Shahar, A. Carbon in the core: Its influence on the properties of core and mantle. Rev. Mineral Geochem. 75, 231–250 (2013).

    Article  Google Scholar 

  19. Nakajima, Y., Takahashi, E., Suzuki, T. & Funakoshi, K. “Carbon in the core” revisited. Phys. Earth Planet. Inter. 174, 202–211 (2009).

    Article  Google Scholar 

  20. Lord, O. T., Walter, M. J., Dasgupta, R., Walker, D. & Clark, S. M. Melting in the Fe–C system to 70 GPa. Earth Planet. Sci. Lett. 284, 157–167 (2009).

    Article  Google Scholar 

  21. Mookherjee, M. et al. High-pressure behavior of iron carbide (Fe7C3) at inner core conditions. J. Geophys. Res. 116, B04201 (2011).

    Article  Google Scholar 

  22. Chen, B. et al. Magneto-elastic coupling in compressed Fe7C3 supports carbon in Earth’s inner core. Geophys. Res. Lett. 39, 2–5 (2012).

    Article  Google Scholar 

  23. Herbstein, F. H. & Snyman, J. A. Identification of Eckstrom–Adcock iron carbide as Fe7C3 . Inorg. Chem. 3, 894–896 (1964).

    Article  Google Scholar 

  24. Fruchart, R. & Rouault, A. Sur l’existence de macles dans les carbures orthorhombiques isomorphes Cr7C3, Mn7C3, Fe7C3 . Ann. Chim. 4, 143–145 (1969).

    Google Scholar 

  25. Fornasini, M. L. & Merlo, F. Ca7Au3 and Ca5Au4, two structures closely related to the Th7Fe3 and Pu5Rh4 types. J. Solid State Chem. 59, 65–70 (1985).

    Article  Google Scholar 

  26. Potapkin, V. et al. The 57Fe synchrotron Mössbauer source at the ESRF. J. Synchrotr. Radiat. 19, 559–569 (2012).

    Article  Google Scholar 

  27. Chumakov, A. & Rüffer, R. Nuclear inelastic scattering. Hyperf. Interact. 113, 59–79 (1998).

    Article  Google Scholar 

  28. Achterhold, K. et al. Vibrational dynamics of myoglobin determined by the phonon-assisted Mössbauer effect. Phys. Rev. E 65, 051916 (2002).

    Google Scholar 

  29. Lin, J-F et al. Sound velocities of hot dense iron: Birch’s law revisited. Science 308, 1892–1894 (2005).

    Article  Google Scholar 

  30. Glazyrin, K. et al. Importance of correlation effects in hcp iron revealed by a pressure-induced electronic topological transition. Phys. Rev. Lett. 110, 117206 (2013).

    Article  Google Scholar 

Download references

Acknowledgements

Partial financial support was provided through the German Science Foundation (DFG), the German Ministry of Education and Research (BMBF), and the PROCOPE exchange programme. We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities through both the normal user programme and a Long Term Project. Portions of this work were performed at GeoSoilEnviroCARS (Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation—Earth Sciences (EAR-1128799) and Department of Energy—Geosciences (DE-FG02-94ER14466). Use of the Advanced Photon Source was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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Authors and Affiliations

  1. Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany

    C. Prescher, L. Dubrovinsky, E. Bykova, I. Kupenko, K. Glazyrin, A. Kantor, C. McCammon, M. Mookherjee, Y. Nakajima, N. Miyajima, R. Sinmyo & V. Cerantola

  2. Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, USA

    C. Prescher & V. Prakapenka

  3. Laboratory of Crystallography, Material Physics and Technology at Extreme Conditions, University of Bayreuth, D-95440 Bayreuth, Germany

    E. Bykova & N. Dubrovinskaia

  4. European Synchrotron Radiation Facility (ESRF), F-38043 Grenoble Cedex, France

    I. Kupenko, A. Kantor, R. Rüffer, A. Chumakov & M. Hanfland

  5. Petra III, P02, Deutsches Elektronen Synchrotron, 22607 Hamburg, Germany

    K. Glazyrin

  6. Earth and Atmospheric Sciences, Cornell University, Ithaca, New York 14853, USA

    M. Mookherjee

  7. Materials Dynamics Laboratory, RIKEN Spring-8 Center, Hyogo 679-5148, Japan

    Y. Nakajima

  8. National Research Center “Kurchatov Institut”, Moscow 123182, Russia

    A. Chumakov

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  1. C. Prescher
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Contributions

M.M. and Y.N. synthesized the starting materials. E.B., K.G., N.D., L.D. and M.H. performed the SCXRD measurements. C.P., L.D., C.M., I.K., A.K., M.M., R.S., V.C., R.R. and A.C. performed the NIS measurements. C.P., L.D. and V.P. performed the high-pressure, high-temperature powder X-ray diffraction measurements. N.M. performed the TEM analysis of the recovered molten sample. C.P., L.D. K.G., I.K., A.K., A.C. and R.R. performed the SMS experiments. C.P., L.D., E.B. and C.M. performed the data analysis. C.P., L.D. and C.M. wrote the paper. All authors discussed the results and commented on the manuscript.

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Correspondence to C. Prescher.

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Prescher, C., Dubrovinsky, L., Bykova, E. et al. High Poisson's ratio of Earth's inner core explained by carbon alloying. Nature Geosci 8, 220–223 (2015). https://doi.org/10.1038/ngeo2370

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