Sound velocity of hcp-Fe at high pressure: experimental constraints, extrapolations and comparison with seismic models

Abstract

Determining the sound velocity of iron under extreme thermodynamic conditions is essential for a proper interpretation of seismic observations of the Earth's core but is experimentally challenging. Here, we review techniques and methodologies used to measure sound velocities in metals at megabar pressures, with specific focus on the compressional sound velocity of hexagonal close-packed iron. A critical comparison of literature results, coherently analyzed using consistent metrology (pressure scale, equation of state), allows us to propose reference relations for the pressure and density dependence of the compressional velocity of hexagonal close-packed iron at ambient temperature. This provides a key base line upon which to add complexity, including high-temperature effects, pre-melting effects, effects of nickel and/or light element incorporation, necessary for an accurate comparison with seismic models, and ultimately to constrain Earth's inner core composition.

Sound velocities and density measurements of solid hcp-Fe and hcp-Fe–Si (9 wt.%) alloy at high pressure: Constraints on the Si abundance in the Earth's inner core

“…163The measured compressional and shear sound velocities as a function of density for 164 hcp-Fe are plotted inFigure 4. These new measurements of V P are in very good agreement 165 with the V P -ρ linear relationship recently proposed by fitting combined datasets derived from 166 multiple techniques[Antonangeli and Ohtani, 2015], extending the data coverage at extreme 167 pressures. We notice that the extrapolation of the established trend to higher density is in 168 remarkable agreement with ab initio calculations at 0 K[Vočadlo et al, 2009; Sha and Cohen, 169 2010], clearly supporting a linear dependence of V P on density.…”

High‐Pressure Geophysical Properties of Fcc Phase FeHX

“…Two methods were used for this calculation. The first method fit experimental iron hydride data only (this study; Mao et al, ; Shibazaki et al, ), holding the values of a and b fixed to −4,000(11) m s −1 and 1.206(11) m 4 kg −1 s −1 respectively, adopting values reported by Antonangeli and Ohtani () to describe the linear V P ‐ ρ relationship in pure iron which persists up to the pressure of Earth's inner core. In the second method, experimental iron hydride data (this study; Mao et al, ; Shibazaki et al, ), and hcp iron data (Antonangeli et al, ; Ohtani et al, ), were fitted together to the above equation, solving for all three parameters ( a , b , and c ).…”

“…Two methods were used for this calculation. The first method fit experimental iron hydride data only (this study; Mao et al, 2004;Shibazaki et al, 2012), holding the values of a and b fixed to 24,000(11) mÁs 21 and 1.206(11) m 4 Ákg 21 Ás 21 respectively, adopting values reported by Antonangeli and Ohtani (2015) to Figure 4. Trade-off between the influence of increased temperature on the compression wave velocity (V P ) of iron and the hydrogen content needed to match the V P of the inner core boundary (ICB) based on equation (6).…”

The Features of the Propagation of Elastic Waves in Isotropic Media at High and Ultra-High Pressures

“…This constraint is not adequate with both practically [2,12,13] and experimentally observed result [14,29,30,32,33] in large classes of problems of geology and geophysics. In particular, the distribution of physico-mechanical as well as elastic parameters of the Earth are obtained in the theoretical structural and dynamic models of the Earth [2, 12, 13, 15, 17, 22-25, 27, 28, 31, 34-36, 38] while violating the condition of smallness of the indicated correlation.…”

“…In case of the quadratic elastic potential in the formulae (14), (15) and (18)- (21), it is necessary to consider 0 a b c = = = .…”