Sound Velocities of Iron-Nickel and Iron-Nickel-Silicon Alloys at High Pressure

Abstract:

Seismological and cosmochemical studies suggest Earth's core is primarily composed of iron with ~5 to 10 wt% nickel and some light elements [e.g. 1]. To date, the concentration of nickel and the amount and identity of light elements remain poorly constrained due in part to the difficulty of conducting experimental measurements at core conditions. The vibrational properties of a variety iron alloys paired with seismic observations can help better constrain the composition of the core.

We directly measured the partial phonon density of states of bcc- and hcp-structured Fe_0.9Ni_0.1 and Fe_0.85Ni_0.1Si_0.05 at high pressures. The samples were compressed using a panoramic diamond anvil cell. A subset of the experiments were conducted using neon as a pressure transmitting medium. Measurements of high statistical quality were performed with nuclear resonant inelastic x-ray scattering (NRIXS) at sector 3-ID-B of the Advanced Photon Source [2, 3, 4]. The unit cell volume of each sample was determined at each compression point with in-situ x-ray diffraction at sector 3-ID-B before and after each NRIXS measurement. The Debye, compressional, and shear sound velocities were determined from the low energy region of the partial phonon density of states paired with the volume measurements. We will present partial phonon density of states and sound velocities for Fe_0.9Ni_0.1 and Fe_0.85Ni_0.1Si_0.05 at high-pressure and compare with those of pure iron.

References:

[1] McDonough, W.F. (2004): Compositional Model for the Earth's Core. Elsevier Ltd., Oxford.

[2] Murphy, C.A., J.M. Jackson, W. Sturhahn, and B. Chen (2011): Melting and thermal pressure of hcp-Fe from the phonon density of states, Phys. Earth Planet. Int., doi:10.1016/j.pepi.2011.07.001.

[3] Murphy, C.A., J.M. Jackson, W. Sturhahn, and B. Chen (2011): Grüneisen parameter of hcp-Fe to 171 GPa, Geophys. Res. Lett., doi:10.1029/2011GL049531.

[4] Murphy, C.A., J.M. Jackson, and W. Sturhahn (2013): Experimental constraints on the thermodynamics and sound velocities of hcp-Fe to core pressures, J. Geophys. Res., doi:10.1002/jgrb.50166.