Revelations about the center of the Earth
Recently, seismologists have observed that the speed and direction of seismic waves in Earth鈥檚 lower mantle, between 400 and 1,800 miles below the surface, vary tremendously. "I think we may have discovered why the seismic waves travel so inconsistently there,"stated Jung-Fu Lin. Lin was with the Carnegie Institution鈥檚 Geophysical Laboratory at the time of the study and lead author of the paper published in the July 21, issue of Nature.
Image: As depth increases inside the Earth, so does the pressure and heat. The experiments conducted by Carnegie researchers on magnesiow眉stite mimicked pressures of the lower mantle--between 500,000 and 1 million times the pressure at sea level. Under those conditions, the electrons of iron in the mineral were forced to pair-up in orbits, which changed the elasticity of the magnesiow眉stite. This change may be the reason why seismic waves behave so peculiarly at those depths.( Image courtesy S. Jacobsen, M. Wysession, and G. Caras.)
鈥淯ntil this research, scientists have simplified the effects of iron on mantle materials. It is the most abundant transition metal in the planet and our results are not what scientists have predicted,鈥� he continued. 鈥淲e may have to reconsider what we think is going in that hidden zone. It鈥檚 much more complex than we imagined.鈥�
The crushing pressures in the lower mantle squeeze atoms and electrons so closely together that they interact differently from under normal conditions, even forcing spinning electrons to pair up in orbits. In theory, seismic-wave behavior at those depths may result from the vice-gripping pressure effect on the electron spin-state of iron in lower-mantle materials. Lin鈥檚 team performed ultra high-pressure experiments on the most abundant oxide material there, magnesiow眉stite (Mg,Fe)O, and found that the changing electron spin states of iron in that mineral drastically affect the elastic properties of magnesiow眉stite. The research may explain the complex seismic wave anomalies observed in the lowermost mantle.
As co-author of the study Viktor Struzhkin elaborated: 鈥淭his is the first study to demonstrate experimentally that the elasticity of magnesiow眉stite significantly changes under lower-mantle pressures ranging from over 500,000 to 1 million times the pressure at sea level (1 atmosphere). Magnesiow眉stite, containing 20% iron oxide and 80% magnesium oxide, is believed to constitute roughly 20% of the lower mantle by volume. We found that when subjected to pressures between 530,000 and 660,000 atmospheres the iron鈥檚 electron spins went from a high-spin state (unpaired) to a low-spin state (spin-paired). While monitoring the spin-state of iron, we also measured the rate-of-change in the volume (density) of magnesiow眉stite through the electronic transition. That information enabled us to determine how seismic velocities will vary across the transition.鈥�
鈥淪urprisingly, bulk seismic waves travel about 15% faster once the electrons of iron are spin-paired in the magnesium-iron oxide,鈥� commented co-author Steven Jacobsen. 鈥淭he measured velocity jump across the transition might, therefore, be detectable seismically in the deep mantle.鈥� The experiments were conducted inside a diamond-anvil pressure cell using the intense X-ray light source at the nation's third-generation synchrotron source, Argonne National Laboratory near Chicago.
鈥淭he mysterious lower mantle region can鈥檛 be sampled directly. So we have to rely on experimentation and theory. Since what happens in Earth鈥檚 interior affects the dynamics of the entire planet, it鈥檚 important for us to find out what is causing the unusual behavior of seismic waves in that region,鈥� stated Lin. 鈥淯p to now, earth scientists have understood Earth鈥檚 interior by only considering pure oxides and silicates. Our results simply point out that iron, the most abundant transition metal throughout the entire Earth, gives rise to very complex properties in that deep region. We look forward to our next experiments to see if we can refine our understanding of what is happening there,鈥� he concluded.
Source: Carnegie Institution of Washington
