Iron puzzles of the Earth Mantle solved

High-pressure experiments reveal unexpected electron structure in the Earth's interior

Looking through a microscope into a diamond die cell. In the center, the sample chamber with a mineral grain to be examined can be recognized. The diameter of the grain is 0.02 mm. © University of Bayreuth
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Recent Bayreuth experiments have revealed a profound transformation of iron-bearing minerals in the deep Earth's interior. The new research results explain earlier controversial observations that have remained puzzling to scientists until now.

Iron, according to the new study in minerals under high pressure, can assume an unusual electronic structure that is apparently stable almost everywhere in the deep mantle. These findings challenge some of the previous ideas about lower mantle properties - 660 to 2, 990 kilometers deep. The scientists of the University of Bayreuth report on the discovery not foretold by theorists in the journal Nature Geoscience.

Earth's interior not accessible for direct investigations

Most of the Earth's interior is inaccessible to direct investigation due to the high temperatures and pressures prevailing there. Scientists are therefore applying indirect methods based on geophysical data, such as earthquake events, and experimental results to understand the conditions and processes in the Earth's interior.

Such models reveal not only how temperature and chemical composition change depending on the depth of the earth, but also how processes in the Earth's interior affect processes on the Earth's surface.

The most widely used mineral in the earth is a magnesium- and iron-containing silicate with perovskite structure. Iron, as a transition element under altered environmental conditions, can change its electronic structure and affect physical properties such as density and conductivity. Transitions to other spin states - the number of electron pairs in the atomic shell - are caused, for example, by the fact that the ratio of paired and unpaired electrons in the atomic shells changes. display

Spin state of iron in the lower mantle investigated

Transitions in the spin state of iron in perovskites in the lower mantle have been studied for several years, but so far each study has come to different conclusions. The spin state of iron in the lower mantle has remained a mystery - until now.

Catherine McCammon from the Bavarian Geoinstitut convinces herself of the readiness of a diamond stamp cell. This small high-pressure device simulates the pressure conditions that prevail in the deep Earth's interior. University of Bayreuth

For researchers of the Bavarian Geoinstitut of the University of Bayreuth have now found, together with scientists from Grenoble / France and Chicago / USA, clear evidence on Spin berg nge at lower pressures in this mineral. For their high-pressure and high-temperature experiments, they used a Bayreuth diamond stamp cell with a miniature heater that can generate pressures up to 110 gigapascals (GPa). This corresponds to 1.1 million times the atmospheric pressure, which is almost equivalent to the pressure prevailing in the earth's core. For the examinations extremely strong X-rays were used, which are only available in a few large research institutes.

Adaptation of previous models needed

The international research group found that iron in these perovskites exists in an unusual spin state over almost the entire depth range of the lower mantle, in which the electrons are only partially mated. The iron behaves chemically and physically completely differently than it is used to under normal conditions.

The new results require an adaptation of existing models. How big this must be, according to the researchers, will be decided by further high-pressure experiments. It is always exciting to find out something new about the Earth's interior, especially if it means that our previous concepts need to be considered. Who can tell what further surprises are coming our way, "explains Catherine McCammon, principal author of the study.

(idw - University of Bayreuth, 23.09.2008 - DLO)