Research ArticleGEOPHYSICS

Constraints on Earth’s inner core composition inferred from measurements of the sound velocity of hcp-iron in extreme conditions

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Science Advances  26 Feb 2016:
Vol. 2, no. 2, e1500802
DOI: 10.1126/sciadv.1500802
  • Fig. 1 IXS spectrum of hcp-Fe at 163 GPa and 3000 K.

    The peak at zero energy is from elastic scattering. Curves are individual contributions [green, elastic scattering; red, longitudinal acoustic (LA) phonons of hcp-Fe; blue, transverse acoustic (TA) phonons of diamond; fitting the experimental data with Lorentzian functions]. a.u., arbitrary units.

  • Fig. 2 Compressional velocity of hcp-Fe at several temperatures as a function of density.

    (A) Results of the present study and of Ohtani et al. (12) based on IXS and shock compression experiments as reported by Brown and McQueen (19). Isothermal ρ-VP fitting lines are expressed as VP = [1.160ρ − 3.43] + [7.2 × 10−5 × (T − 300) × (ρ – 14.2)]. (B) Comparison of obtained isothermal ρ-VP fitting lines with previous studies of NIS reported by Mao et al. (10), diamonds; IXS reported by Antonangeli et al. (6), inverted triangles; ab initio calculation reported by Vočadlo et al. (20), open crosses; and report of Sha and Cohen (21), crosses.

  • Fig. 3 Comparison of the ρ-VP plot between hcp-Fe under ICB conditions and PREM.

    Stars indicate VP and ρ of hcp-Fe and PREM at 330 GPa. The difference in the ρ-VP plot between PREM and hcp-Fe at 5500 K (orange) shows a 4 to 5% core density deficit and a 4 to 10% core velocity deficit and that of 4200 K (blue) shows a 5 to 6% core density deficit and a 7 to 13% core velocity deficit.

  • Fig. 4 Comparison of the ρ-VP plot between hcp-Fe and Fe–light element compounds.

    The values of hcp-Fe and each Fe–light element compound [hcp-Fe92Ni8 (14, 24), purple; dhcp-FeH (11, 25), blue; hcp-Fe85Si15 (10, 26), aqua; Fe3S (13, 27), green; FeO (B1/rhombohedral phase) (7, 28), yellow-green; Fe3C (9, 29), pink; Fe7C3 (30), yellow] show distributions from the ICB to the COE. The obtained ρ-VP of hcp-Fe at 5500 K is indicated by orange, and the star is at the expected ICB conditions. The other stars denote ρ-VP of Fe–light element compounds under ICB conditions assuming that the temperature effects of these compounds are the same as those of hcp-Fe (the dashed lines are the temperature effect). Triangles connecting three stars (hcp-Fe–hcp-Fe92Ni8–Fe–light element compound) indicate the potential ρ-VP region given by mixing three components.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/2/e1500802/DC1

    Fig. S1. Typical temperature profiles across the laser-heated spot obtained in IXS69.

    Fig. S2. IXS spectra of hcp-Fe at 163 GPa and 3000 K.

    Fig. S3. XRD pattern at 163 GPa and 3000 K.

    Fig. S4. Comparison of dispersion curves determined by fitting with QMAX free and QMAX fixed.

    Fig. S5. Longitudinal acoustic phonon dispersion of hcp-Fe at different experimental conditions.

    Fig. S6. Comparison of dispersion curve between IXS101 and IXS102.

    Fig. S7. Comparison of experimental data with fit result.

    Fig. S8. Pole figures for hcp-Fe at 163 GPa and 3000 K.

    Fig. S9. P-wave velocity anisotropy of hcp-Fe at 163 GPa and 3000 K based on the diffraction pattern.

    Table S1. IXS experimental conditions.

    Table S2. Results of density and compressional sound velocity measurements.

    Table S3. Results of different fits to the data sets as described in the text.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Typical temperature profiles across the laser-heated spot obtained in IXS69.
    • Fig. S2. IXS spectra of hcp-Fe at 163 GPa and 3000 K.
    • Fig. S3. XRD pattern at 163 GPa and 3000 K.
    • Fig. S4. Comparison of dispersion curves determined by fitting with QMAX free and QMAX fixed.
    • Fig. S5. Longitudinal acoustic phonon dispersion of hcp-Fe at different experimental conditions.
    • Fig. S6. Comparison of dispersion curve between IXS101 and IXS102.
    • Fig. S7. Comparison of experimental data with fit result.
    • Fig. S8. Pole figures for hcp-Fe at 163 GPa and 3000 K.
    • Fig. S9. P-wave velocity anisotropy of hcp-Fe at 163 GPa and 3000 K based on the diffraction pattern.
    • Table S1. IXS experimental conditions.
    • Table S2. Results of density and compressional sound velocity measurements.
    • Table S3. Results of different fits to the data sets as described in the text.

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