Experiments challenge models about the deep Earth

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Experiments challenge models about the deep Earth - Carnegie Institution News

In the first experiments able to mimic the crushing, searing conditions found in Earth’s lower mantle, and simultaneously probe tell-tale properties of iron, scientists* have discovered that material there behaves very differently than predicted by models.The research also points to the likelihood of a new zone deep in the Earth. The work is published in the September 21, 2007, issue of Science.

Surface phenomena such as volcanoes and earthquakes are generated by what goes on in Earth’s interior. To understand some of these surface dynamics, scientists have to probe deep into the planet. The lower mantle is between 400 and 1,740 miles deep (650 km- 2,800 km) and sits atop the outer core.

 

Coauthor of the paper, Viktor Struzhkin of the Carnegie Institution’s Geophysical Laboratory explains: “The deeper you go, the higher the pressures and temperatures become. Under these extreme conditions, the atoms and electrons of the rocks become squeezed so close together that they interact very peculiarly. In fact, spinning electrons in iron, which is prevalent throughout the inner Earth, are forced to pair up. When this spin state changes from unpaired electrons—called a high-spin state—to paired electrons—a low-spin state—thedensity, sound velocities, conductivity,and other propertiesof the materials can change. Understanding these conditions helps scientists piece together the complex puzzle of the interior/surface interactions.”

 

The pressures in the lower mantle are brutal, ranging from about 230,000 times the atmospheric pressure at sea level (23 GPa), to almost 1.35 million times sea-level pressure (135 GPa). The heat is equally extreme—from about 2,800 to 6,700 degrees Fahrenheit (1800 K–4000 K).

 

Using a laser-heated diamond anvil cell to heat and compress the samples, the scientists subjected ferropericlase to almost 940,000 atmospheres and 3,140 °F. They analyzed it using so-called X-ray emission spectroscopy. As its name suggests, ferropericlase is iron-laden. It is also the second most prevalent material found in the lower mantle.Previous to this study, ferropericlase has been subjected to high pressures, but only to room temperatures. The new experiments are the highest pressures and temperatures attained to probe the spin state of iron in the mineral at lower-mantle conditions.