Study of the Properties of Iron under Conditions Existing at the Earth's Core

Successful interpretation of available geophysical data requires both experimental and theoretical information on the elasticity of solids under the physical conditions of the Earth's interior where there are pressures up to 360 GPa and temperatures over several thousand degrees. Since iron is considered as the major component in the Earth's core, elastic properties of iron at high pressures and temperatures are very important for modelling its composition and dynamics. We have determined the thermal equation of state (EOS) and the Debye-Waller temperature factors and calculated aggregate sound velocities and Grüneisen parameter of -iron for the first time using a new approach which is based on Rietveld refinement at high pressures and temperatures. We used new in situ X-ray diffraction data on -iron at static pressures up to 300 GPa and temperatures to 1200 K, and showed that the inner core compressional (V_p) and shear (V_s) sound velocities could match properties of pure iron without any contributions from low-velocity components.

The experiments were performed on beamline ID30. In our experiments, powder diffraction data were collected with a fine incident X-ray beam of approximately rectangular shape (8*9 µm²) of 0.3738 Å wavelength on the FastScan imaging plate. The collected images were integrated in order to obtain a conventional diffraction spectrum.

We heated the samples externally in a Mao-Bell type diamond anvil cell (Figure 85). The external electrical heating assemblage employs flexible graphite foils and makes it possible to reach temperatures over 1400 K at multimegabar pressure range. A procedure of high temperature (800 K) synthesis of the -Fe from a submicrometre iron powder was developed, this allowed us to overcome problems associated with a preferred orientation, stresses and recrystallisation, and to collect reliable diffraction data to over 300 GPa. In total, 188 data points were collected over the pressure range of 18 to 305 GPa at temperatures between 300 to 1300 K to determine the EOS of the -iron. As a result, a thermal expansion of the -Fe was measured for the first time in the static compression experiments at conditions close to those at the Earth's Inner-Outer Core boundary (Figure 86). Our results generally support earlier shock wave data [1], except for the fact that the difference in density of -Fe and Preliminary Reference Earth Model (PREM) [2] (Dziewonski and Anderson, 1981) density at the inner core conditions is 2 to 4 times less than the earlier estimate and is just 2.5 - 5%.

Accurate Rietveld refinement and temperature dependence of intensities of different reflections of -iron at various pressures give enough information for determination of the Debye-Waller temperature factors as a function of P and T. Using the theory of lattice dynamics and experimental data on the density, bulk modulus and Debye-Waller parameters, we have found aggregate sound velocities and the acoustic Grüneisen parameter of -Fe at pressures up to 300 GPa and temperatures over 1200 K (Figure 86). For the first time it was demonstrated that shear sound velocities of -Fe at the Earth's Inner Core conditions are reasonably close to the PREM values (Figure 86).

References
[1] J.M. Brown, R.G. McQeen, J. Geophys. Res., 91, 7485-7494 (1986).
[2] A.M. Dziewonski, D.L. Anderson, Phys. Earth Planet. Inter., 25, 297-356 (1981).

Principal Publication and Authors
L.S. Dubrovinsky, S.K. Saxena, F. Tutti, S. Rekhi, D. Le Beller, T. Le Bihan, Phys. Rev. Lett., 84, 1720-1723 (2000).
Institute of Earth Sciences, Uppsala University (Sweden).