Some people toughen under increased pressure. Some rocks do too. Squeezing a common, iron-bearing mineral at the hellish pressures deep within our planet makes the material much stiffer than geophysicists had expected. This response may explain why seismic waves travel particularly fast through some deep zones of rock, say the researchers who performed the experiment.
The lower mantle, a thick region of Earth’s inner structure, lies just outside the planet’s core. Using vibrations from earthquakes as probes, scientists have detected large volumes of lower-mantle material where seismic waves travel much faster than they do in other mantle areas. Many of those speedups appear at depths of around 2,900 kilometers, says Jung-Fu Lin, a geophysicist at the Lawrence Livermore (Calif.) National Laboratory.
Many researchers have proposed that variations in mantle density that result from changes in mineral composition or temperature create those seismic anomalies. Now, lab investigations by Lin and his colleagues suggest that another phenomenon may be at play as well. In those experiments, the researchers squeezed suspected mantle minerals in an anvil that can generate ultra-high pressures on samples placed between the instrument’s diamond jaws.
One material that the scientists placed in the anvil was magnesiowüstite, a translucent mineral composed of various proportions of iron oxide and magnesium oxide. That combination makes up as much as 20 percent of the lower mantle, says Lin. The results of tests on one particular composition—17 percent iron oxide and 83 percent magnesium oxide—grabbed the researchers’ attention.
When they cranked the anvil on that sample to pressures at and above 600,000 times as great as those at sea level, the physical characteristics of the mineral’s iron atoms changed. The pressure forced two electrons that normally are unpaired within the atom into a shared orbital. Despite this shifting of electrons, the density of the material didn’t change significantly.
The forced electron pairing in the iron atoms made the mineral about 35 percent stiffer than the scientists had expected from theoretical analyses and previous experiments at lower pressures, says Lin. In the deep mantle, seismic vibrations would travel about 15 percent faster through this stiffer form of magnesiowüstite, compared with previous estimates.
The change observed in the magnesiowüstite’s iron atoms at high pressure also rendered the material more opaque. That increased opacity could significantly affect heat flow from Earth’s core through the mantle, says team member Steven D. Jacobsen, a geophysicist at the Carnegie Institution of Washington (D.C.).
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Lin, Jacobsen, and their colleagues report their findings in the July 21 Nature.
Their research is “a great piece of work,” says William A. Bassett, a geophysicist at Cornell University. The new finding can’t by itself displace other theories about what may cause seismic anomalies in Earth’s lower mantle, but it does add an important factor to be considered in researchers’ models, he notes.