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The Iron Spin Transition in the Earth's Lower Mantle Print

 

 

It is now known that the iron present in minerals of the lower mantle of the Earth undergoes a pressure-induced transition with pairing of the spins of its 3d electrons. A team from the University of California, Berkeley, Tel Aviv University, and Lawrence Livermore National Laboratory has used x-ray diffraction at very high pressure to investigate the effects of this transition on the elastic properties of magnesiowüstite (Mg1–xFex)O, the second most abundant mineral in the Earth’s lower mantle. The new results suggest that the effect of the spin-pairing transition on magnesiowüstite can be large enough to require a partial revision of the most accepted model of the lower mantle composition.

Pressure dependence of the density of (Mg1 –1 xFex)O (x = 0.17 and 0.2) shows an anomalous change in slope, modeled as a "mixed-state" region where iron with spin-paired and spin-unpaired 3d electron configurations coexist in the structure of magnesiowüstite.

Iron Under Pressure

The Earth's mantle is a rocky shell about 2,900 km thick that lies directly under the outer crust and above the iron-rich core. It occupies about 70% of Earth's volume. In the last few years, researchers have verified a 45-year-old proposal by Canadian geophysicist W.S. Fyfe that the effect of the high pressures found deep inside the Earth’s mantle may be able to collapse the atomic orbitals of iron, one of the Earth’s most important elements, dramatically altering the physical and chemical properties of iron-bearing minerals in the mantle. For example, variations of density and elastic moduli (tendency to be deformed when a force is applied) control the speed of seismic waves and play a crucial role in the interpretation of the seismological data used to study the Earth at depths not directly accessible from the surface.

To learn more about the effect of "squeezed iron," Speziale et al. have subjected one of the principal minerals in the mantle, a magnesium-iron-oxygen compound called magnesiowüstite, to a range of very high pressures in a special apparatus called a diamond-anvil cell while carrying out x-ray diffraction measurements of the density and elastic properties. Their findings suggest that the effect of the iron transition on magnesiowüstite as the pressure is increased can be large enough to require a partial revision of the most accepted model of the lower mantle composition.

Current knowledge of the structure of the deep interior of the Earth is based mainly on average geophysical observations, especially those from seismology. However, our picture of the lower mantle has been modified in recent times by results from physics measurements of candidate minerals in the laboratory as well new seismological observations. For example, recent experiments prove that the effect of high pressure deep inside the Earth's mantle is to collapse the atomic orbitals of iron from the high-spin to the low-spin state. This major change in the chemical-bonding character of one of the Earth's most important elements can dramatically alter physical and chemical properties of iron-bearing minerals at these depths.

In the standard mineralogical model, the lower mantle (the region between 670 and 2900 km below the surface) consists mainly of magnesium-rich (Mg,Fe)SiO3 perovskite, magnesium-rich magnesiowüstite, and CaSiO3 perovskite in a mass ratio 64:31:5, plus minor phases. Quantifying the effect of the spin-pairing transition on the elastic properties of the two major minerals of the lower mantle is one of the most fascinating research fronts of deep-Earth geophysics because variations of density (ρ) and of elastic moduli (incompressibility K and rigidity μ) control the speed of seismic waves and play a crucial role in the interpretation of seismological data.

Using synchrotron x-ray diffraction at Beamline 12.2.2 of the ALS and Beamlines 13ID-D (GSECARS) and 16ID-B (HPCAT) at the Advanced Photon Source (APS), the researchers determined the pressure dependence of density of magnesiowüstite with compositions (Mg0.80, Fe0.20) O and (Mg0.83,Fe0.17)O compressed in a diamond-anvil cell to pressures found in the lower mantle (24 to 135 GPa). In the pressure range at which the spin transition takes place (40 to 80 GPa corresponding to 1000 to 1900 km depth), the density–pressure curve shows an anomalous change in slope, modeled as a "mixed-state" region where iron with spin-paired and spin-unpaired 3d electron configurations coexist in the structure of magnesiowüstite.

Calculated pressure dependence of the bulk sound velocity of (Mg1 – xFex)O (x = 0.17 and 0.2). In the model, an empirical averaging weighted by the relative abundances of the spin-paired and unpaired states was used for the "mixed-state" region. The bulk sound velocity of (Mg0.94.Fe0.06)O is from J. Crowhurst et al., Science 319, 451 (2008).

The researchers put constraints on the pressure–density equations of state of both the low-pressure phase (containing Fe2+ with spin-unpaired configuration) and of the high-pressure phase (containing spin-paired Fe2+). They also modeled the density and incompressibility in the "mixed state"region and calculated the variation of bulk sound velocity vφ = (K/ρ)1/2 across the whole lower mantle pressure range. The high-pressure phase is systematically denser and more incompressible and has a higher bulk velocity than the low-pressure phase. Regarding the "mixed state," the x-ray diffraction results cannot unequivocally model the variation of bulk sound velocity, so the researchers proposed an empirical averaging weighted by the relative abundances of the spin-paired and unpaired states.

The effect of the transition, overlooked in the existing models of the lower mantle, is large enough to be visible even when it is weighted by the volume fraction of magnesiowüstite in the lower mantle. In order to fit the seismological velocity models, a correction either of the composition of the model or the temperatures in the lowermost mantle would be required.

 

 


 

 

Research conducted by S. Speziale (University of California, Berkeley; now at GeoForschungsZentrum Potsdam, Germany); V.E. Lee and R. Jeanloz (University of California, Berkeley); S.M. Clark (ALS); J.F. Lin (Lawrence Livermore National Laboratory); and M.P. Pasternak (Tel Aviv University, Israel).

Research Funding: University of California, U.S. National Science Foundation (NSF), Lawrence Livermore Fellowship, and the Miller Institute for basic Research in Science. Operation of the ALS and the APS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences (BES).

Publications about this research: S. Speziale, V.E. Lee, S.M. Clark, J.F. Lin, M.P. Pasternak, and R. Jeanloz, "Effect of Fe spin transition on the elasticity of (MgFe)O magnesiowüstite for the seismological properties of the Earth’s lower mantle," J. Geophys. Res. 112, B10212 (2007); S. Speziale, A. Milner, V.E. Lee, S.M. Clark, M.P. Pasternak, and R. Jeanloz, "Iron spin transition in Earth’s mantle," Proc. Natl. Acad. Sci. U.S.A. 102, 17918 (2005).