Strange Behavior of Iron Deep in the Earth's Interior

CDAC members Jung-Fu Lin, Viktor Struzhkin, and Steve Jacobsen and other colleagues from CDAC-Carnegie, HPCAT, and State University of New York at Stony Brook, recently reported new insights into the behavior of iron deep in the Earth's interior near the core in the July 21st issue of the journalNature. Under the extreme pressure and temperature conditions of the lower mantle (400–1,800 miles below the surface), iron undergoes an unusual electronic transition wherein the outermost valence electron, rather than remain un-paired in its own separate orbital, pairs-up with another electron of opposite spin in the same orbital forming a so-called low-spin state. Lin's team monitored the spin-state of iron in the most abundant non-silicate oxide material of the lower mantle, magnesiowüstite -(Mg,Fe)O, to over 100 GPa (one megabar), and discovered that the spin-state transition is associated with a rather drastic change in the material's elastic properties. The experiments were carried out inside a diamond-anvil pressure cell. While monitoring the spin-state of iron, the mineral's rate-of-change in volume upon compression was also measured using x-ray diffraction, which allowed them to estimate the density and bulk-sound velocity contrast across the electronic transition. Surprisingly, bulk-wave speeds increase about 15% across the transition, indicating that the phenomenon should be observable seismically in Earth's lower mantle [Lin, J. F. et al., Nature, 436, 377-380 (2005)].

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Figure credits: S. Jacobsen, M. Wysession, and G. Caras.