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A small leak in the Earth's core

The Earth’s core is not as isolated from the rest of the globe as previously believed. An international research group headed by Aarhus University has discovered that tiny amounts of iron from the core 2,900 km beneath us are seeping into the mantle and all the way up to volcanic islands on the Earth's crust.

2020.05.04 | Peter F. Gammelby

Orange coloured fountains of hot lava on a black background.

Lava fountains from the Holuhraun fissure eruption in Iceland in September 2014. The composition of iron isotopes in the basalt reflects a leak in the Earth's core. Photo: Joschenbacher - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/ w/index.php?curid=35599563

Graphic showing the layers of the Earth.

Schematic cross-section of the Earth's interior. 1. Earth crust (continental) 2. Earth crust (seabed) 3. Upper mantle 4. Lower mantle 5. Outer core (liquid) 6. Inner core (solid) A: The crust-mantle boundary (also called Mohorovičić discontinuity) B: The core-mantle boundary (also called Gutenberg discontinuity). This is where the heavy iron isotopes leak into the Earth’s mantle according to the new study. C: The boundary between the inner and outer core. Graphic: Dake, CC BY-SA 2.5.

It has long been considered impossible that material or chemical signals from the Earth's core, located 2,900 km beneath us, can reach the surface of the Earth.

However, in recent years, research into the element tungsten has demonstrated that material from the core could perhaps reach the surface of the earth via so-called hotspots that form volcanic islands such as Iceland and Hawaii.

And now, researchers from Denmark, the US and Canada have used iron to prove that material from the core can actually be transported up through the mantle to the surface. Or rather: they have used different versions of iron, i.e. isotopes (iron atoms with a different number of neutrons).

The researchers have based their research on the special isotope signatures – i.e. the distribution of various tungsten and iron isotopes – found in the volcanic rock basalt on ocean islands such as Iceland and Hawaii.

Heavy iron flows upwards

"We've found an explanation for this. We’ve demonstrated that the isotope composition of iron in the basalt on some ocean islands may be a reflection of what is happening between the Earth's core and mantle. The Earth’s core consists of 90% iron, and some of it leaks into the mantle," says Charles E. Lesher, who is a professor at Earth System Petrology at Aarhus University and head of the research team behind the study, which has been published in Nature Geoscience.

The reason for this is that the layer between the Earth's core and mantle consists of a thin layer (nobody knows exactly how thick the layer is, but it is less than 100 km), where the temperature drops dramatically by approx. 1,500 degrees Celsius. When the metallic iron crosses this layer, it changes its chemistry. In a process called thermal diffusion, the slightly heavier iron isotopes migrate towards the lower temperature (i.e. away from the core), while the lighter iron isotopes move back down towards the core. Therefore, the iron that reaches the Earth's mantle contains a relatively higher proportion of heavier iron isotopes (see fact box).

There are four naturally occurring and stable iron isotopes, 54Fe, 56Fe (by far the most common), 57Fe and 58Fe. The numbers show the atomic mass. When liquid iron from the core infiltrates the core-mantle boundary, the composition changes such that the slightly heavier iron isotopes, 57Fe and 58Fe, migrate up towards the Earth's mantle, and the lighter isotopes, 54Fe and 56Fe, return towards the Earth’s core.

Lesher’s team has discovered this by simulating the process in experiments under high pressure and high temperatures. The experiments showed that the core material crossing the core-mantle boundary, constantly has a clear disproportionately higher amount of heavy iron isotopes, and this points towards the isotope signature found in basalt from the volcanic islands.

What other discoveries are possible?

The researchers' computer modelling also shows that the separation of iron isotopes can develop over a relatively short geological time scale, and that the modified core material can then be transported up through the mantle of hot ascending material – through a process known as pluming and which is best illustrated by the classic lava lamp – and finally reach the surface of the Earth via lava. 

"What’s new and surprising about this discovery is that this indicates that there’s contact from the core all the way up to the crust. We’ve developed a model that can be used to investigate signals that may come from the Earth’s core. Now we’ve seen that such signals from iron in the core can rise to the surface of the Earth. The question is now, what else can we learn about our planet? More than 95% of the Earth's precious metals (such as gold and platinum) are in the core. These elements have enormous financial and strategic importance," says Charles E. Lesher.


Further information

You can find the scientific article in Nature Geoscience here: Iron isotope fractionation at the core–mantle boundary by thermodiffusion

Contact

Professor Charles E. Lesher
Department of Geoscience
Aarhus University
Mail: lesher@geo.au.dk
Mobile: +45 2382 3082    

Department of Geoscience