|Evidence for a Weak Iron Core at Earth's Center|
Seismic waves that pass through the center of the Earth travel faster going from pole to pole than along the equatorial plane—why? One theory is that the grains of iron that make up most of the solid inner core could be aligned in a way that transmits waves more efficiently in one direction than the other. Recent evidence for this "texturing" of iron grains in the Earth's inner core comes from x-ray spectroscopy and diffraction measurements performed at high pressure and utilizing, in part, ALS Beamline 12.2.2. When extrapolated to the high-temperature and -pressure conditions in the Earth's core, the results show that the strength of iron is lower than previously thought. This weakness could explain how the crystal structure in the Earth's core has transformed over geological time scales.
The extreme conditions at the Earth's core are very difficult to reproduce in a laboratory. The pressure rises above three million atmospheres (320–370 gigapascals), and the temperature is comparable to the surface of the Sun (over 5000° Celsius). Seismologists have learned about the core by studying seismic waves that travel directly through the Earth's center. One surprising discovery is that core-traversing seismic waves travel 3% faster along the polar axis as compared to those moving through the equatorial plane. It is assumed that this seismic-wave anisotropy is due to iron crystals aligning their lattice structures. Such alignment requires the iron to have a certain amount of "flow" through the solid core, and this has yet to be explained.
Strength, which is a material's resistance to flow, is characterized by the pressure at which the material begins to deform. Previous studies of the strength of iron have typically applied pressure in a nonuniform (or nonhydrostatic) way, whereas the pressure on the iron in Earth's core is uniform (hydrostatic), like the pressure on a submersible in the deep ocean. To reproduce the hydrostatic conditions of the Earth's interior, the researchers loaded samples of hexagonally close-packed (hcp) iron (generally accepted as the stable phase of iron at inner-core conditions) into a gasket filled with a pressure-transmitting medium of neon or helium gas. This gasket was then placed in a diamond-anvil cell, where pressures as high as 200 gigapascals could be applied and studied using a spectroscopic technique. From the data analysis, they derived the shear modulus—a measure of the rigidity of a material—and found it to be slightly lower than previous measurements.
These data were then combined with nonhydrostatic radial x-ray diffraction (rXRD) measurements collected at ALS Beamline 12.2.2. Macro-stresses extending across a large number of grains produce shifts in the position of diffraction lines—the magnitude of which is linked to the material's strength. Line shifts in rXRD data collected at pressure allow the measurement of deviatoric strain (deformation at constant volume), relative to the hydrostatic strain (changes in volume), at a particular crystallographic orientation for a given lattice plane. Surprisingly, the derived strength was 60% lower than previous estimates, making iron one of the weakest metals at high pressures. Comparison of the pressure-dependent strength of hcp iron with other materials with hcp, face-centered cubic (fcc), and body-centered cubic (bcc) structures shows that only gold trends lower than hcp iron.
The researchers estimated that iron's strength is around 1 gigapascal at the pressure and temperature at the Earth's core. This low value has implications for how the material in the core deforms, or "creeps," over time. Previous models assumed that this creep was a very slow process, based mostly on diffusion of atoms. However, a lower strength for iron means that creep could occur through the movement of defects, or "dislocations," in the crystal structure. This faster dislocation creep would imply that the observed seismic-wave anisotropy developed relatively early in the Earth's history.
Research conducted by: A.E. Gleason (Stanford University) and W.L. Mao (Stanford University and SLAC National Accelerator Laboratory).
Research funding: National Science Foundation and U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES). Work was also performed in part at the Advanced Photon Source (APS). Operation of the ALS and APS is supported by DOE BES.
Publication about this research: A.E. Gleason and W.L. Mao, "Strength of iron at core pressures and evidence for a weak Earth's inner core," Nature Geoscience 6, 571 (2013).
ALS Science Highlight #288