Magnetism discovered in the Earth’s mantle



image: The interior of the Earth illustrated graphically. The blue dotted lines show the magnetic field surrounding the Earth. Researchers squeezed and heated samples of iron oxide hematite found in the Earth’s mantle between two diamonds (right) to simulate the extreme conditions of the Earth’s mantle. They observed that iron oxide is magnetic under these conditions.
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Credit: Timofey Fedotenko

The huge magnetic field that surrounds the Earth, shielding it from radiation and charged particles from space – and which many animals even use for orientation purposes – is constantly changing, which is why geoscientists are watching it. permanently. The well-known ancient sources of the Earth’s magnetic field are the Earth’s core – up to 6,000 kilometers deep inside the Earth – and the Earth’s crust: in other words, the ground we stand on. The Earth’s mantle, on the other hand, extending 35 to 2,900 kilometers below the Earth’s surface, has so far been widely considered “magnetically dead”. An international team of researchers from Germany, France, Denmark and the United States have now demonstrated that a form of iron oxide, hematite, can retain its magnetic properties even deep in the earth’s mantle. This happens in relatively cool tectonic plates, called plates, which are mostly found under the western Pacific Ocean.

“This new knowledge about the Earth’s mantle and the strongly magnetic region of the Western Pacific could shed new light on any observation of the Earth’s magnetic field,” explains mineral physicist and first author Dr. Ilya Kupenko of the University of Münster ( Germany). The new findings could, for example, be relevant to any future observations of magnetic anomalies on Earth and other planets such as Mars. Indeed, Mars no longer has a dynamo and therefore no longer a source allowing the construction of a strong magnetic field coming from the core like that of the Earth. So it might be worth taking a closer look at his coat. The study was published in the “Nature“review.

Context and methods used:

Deep in the earth’s metallic core, it is a liquid iron alloy that triggers electrical flows. In the Earth’s outermost crust, rocks cause a magnetic signal. In deeper regions of the Earth’s interior, however, rocks were believed to lose their magnetic properties due to very high temperatures and pressures.

Researchers have now taken a closer look at the main potential sources of magnetism in the Earth’s mantle: iron oxides, which have a high critical temperature, that is, the temperature above which the material does not ‘is more magnetic. In the earth’s mantle, iron oxides are found in plates that are buried in the earth’s crust further into the mantle, as a result of tectonic changes, a process called subduction. They can reach a depth inside the Earth of between 410 and 660 kilometers – the so-called transition zone between the upper and lower mantle of the Earth. Previously, however, no one had succeeded in measuring the magnetic properties of iron oxides under the extreme pressure and temperature conditions encountered in this region.

Now scientists have combined two methods. Using a diamond anvil cell, they squeezed micrometer-sized iron oxide hematite samples between two diamonds and heated them with lasers to achieve pressures of up to 90 gigapascals and temperatures over 1000 ° C (1300 K). The researchers combined this method with so-called Mössbauer spectroscopy to probe the magnetic state of samples using synchrotron radiation. This part of the study was performed at the ESRF synchrotron in Grenoble, France, which observed the magnetic order changes in iron oxide.

The surprising result is that the hematite remained magnetic up to a temperature of about 925 ° C (1200 K) – the temperature prevailing in the subducted plates under the western part of the Pacific Ocean at the depth of the zone. transition of the Earth. “As a result, we are able to demonstrate that the Earth’s mantle is not as magnetically ‘dead’ as has been assumed so far,” explains Professor Carmen Sanchez-Valle of the Institute of Mineralogy of the University of Münster. “These results could justify other conclusions relating to the entire earth’s magnetic field,” she adds.

Relevance for investigations of the Earth’s magnetic field and pole movement

Using satellites and studying rocks, researchers observe the Earth’s magnetic field, as well as local and regional changes in magnetic force. Background: The geomagnetic poles of the Earth – not to be confused with the geographic poles – are in constant motion. As a result of this movement, they have actually changed position every 200,000 to 300,000 years in recent Earth history. The last pole reversal occurred 780,000 years ago, and scientists in recent decades have reported an acceleration in the movement of the Earth’s magnetic poles. The reversal of the magnetic poles would have a profound effect on modern human civilization. The factors that control the movements and reversal of the magnetic poles, as well as the directions they follow during the reversal are not yet understood.

One of the routes of the poles observed during the flips passes over the western Pacific, corresponding very appreciably to the electromagnetic sources proposed in the Earth’s mantle. The researchers therefore consider the possibility that the magnetic fields observed in the Pacific using rocky recordings do not represent the pole migration route measured at the Earth’s surface, but originate from the hitherto unknown electromagnetic source of rocks containing hematite in the Earth’s mantle under the western Pacific.

“What we know now – that there are magnetically ordered materials out there in the Earth’s mantle – should be factored into any future analysis of Earth’s magnetic field and pole motion,” says co-author, the Professor Leonid Dubrovnik. at the Bavarian Research Institute for Experimental Geochemistry and Geophysics at the University of Bayreuth.


Original edition:

I. Kupenko et al. (2019): Magnetism in cold subduction plates at the depths of the mantle transition zone. Nature; DOI: 10.1038 / s41586-019-1254-8

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