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Published May 2013 | Published
Journal Article Open

Experimental constraints on the thermodynamics and sound velocities of hcp-Fe to core pressures

Abstract

We report the high-pressure thermoelastic and vibrational thermodynamic parameters for hexagonal close-packed iron (ε-Fe), based on nuclear resonant inelastic X-ray scattering and in situ X-ray diffraction experiments at 300 K. Long data collection times, high-energy resolution, and quasi-hydrostatic sample conditions produced a high-statistical quality data set that comprises the volume-dependent phonon density of states (DOS) of ε-Fe at eleven compression points. From the integrated phonon DOS, we determine the Lamb-Mössbauer factor (f_(LM)), average force constant (Φ), and vibrational entropy (S_(vib)) of ε-Fe to pressures relevant to Earth's outer core. We find f_(LM) = 0.923 ± 0.001 at 171 GPa, suggesting restricted thermal atomic motion at large compressions. We use Φ to approximate ε-Fe's pressure- and temperature-dependent reduced isotopic partition function ratios (β-factors), which provide information about the partitioning behavior of iron isotopes in equilibrium processes involving solid ε-Fe. In addition, we use the volume dependence of S_(vib) to determine the product of ε-Fe's vibrational thermal expansion coefficient and isothermal bulk modulus, which we find to be pressure-independent and equal to 5.70 ± 0.05 MPa/K at 300 K. Finally, from the low-energy region of each phonon DOS, we determine the Debye sound velocity (v_D), from which we derive the compressional (v_P) and shear (v_S) sound velocities of ε-Fe. We find v_D = 5.60 ± 0.06, v_P = 10.11 ± 0.12, and v_S = 4.99 ± 0.06 km/s at 171 GPa, thus providing a new tight constraint on the density dependence of ε-Fe's sound velocities to outer core pressures.

Additional Information

© 2013 American Geophysical Union. Received 27 November 2012; revised 7 March 2013; accepted 19 March 2013; published 20 May 2013. We would like to thank B. Chen, D. Zhang, H. Yavas, and J.K. Wicks for assistance during the experiments, and NSF CAREER-0956166 and Caltech for support of this research. We would also like to thank two anonymous reviewers and the editor, Michael Walter, for helpful comments and suggestions. Use of the Advanced Photon Source was supported by the U.S. D.O.E., O.S., O.B.E.S. (DE-AC02-06CH11357). Sector 3 operations and the GSE-CARS gas-loading facility are supported in part by COMPRES (NSF EAR 06-49658).

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August 19, 2023
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