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International team, ASU make 'shocking' discovery about Earth's lower mantle

Utilizing powerful shocks of pressure and energy, the team recreated inner-Earth conditions with the goal of exploring decades-old geological hypotheses

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ASU's Interdisciplinary Science and Technology Building 4, home of the School of Earth and Space Exploration, is pictured on the Tempe campus on Tuesday, March 30, 2021.

Researchers from ASU, in collaboration with the Laboratoire pour l’Utilisation des Lasers Intenses, SLAC National Accelerator Laboratory and Sorbonne Université, simulated the inner conditions of the Earth in their study published last month in Science Advances.

In order to do this, they exposed olivine, an igneous mineral created within the Earth's mantle, to an X-ray-free electron laser, shocking it with pressure and energy. This heats the mineral to temperatures between 3000 and 6000 kelvin, or between about 5000 and 10000 degrees Fahrenheit.

READ MORE: Final steps toward X-ray innovation: ASU's CXFEL to complete first phase this spring 

"This is a really innovative study, in that it's using new tech, a new type of technology to really get at physical properties (in those) extreme conditions," said Thomas Sharp, a geology professor at the School of Earth and Space Exploration who studies high-pressure minerals similar to the ones used in the study. "It's only been the last few years that that people have been combining laser-induced shock with high-speed X-ray diffraction measurements."

Using this technique, the multinational team of researchers were able to detect low-spin iron, a relic of Earth's violent past. Under the immense pressures and energies of the lower mantle, the iron electrons experience a change in spin, meaning its properties such as density or conductivity also change.

The dynamic nature of Earth’s surface adds to the significance of this detection since it serves as a link between the present and Earth’s history.

"Early in (the) Earth formation process, we have violent impacts that are common, large enough to melt the rock layer completely. And that's so-called the magma ocean stage," said Dan Shim, a professor at the School of Earth and Space Exploration and the lead author of the study.

"There has been a hypothesis or theory that at a certain depth, magma becomes denser than crystalline solid minerals," Shim said. "When magma froze from liquid state to solid state, you have an intermediate stage where you have liquid silicate magma and silicate minerals."

While initially proposed as far back as 50 years ago, the theory had not been properly tested until now, partially due to the flimsy nature of the sample under mantle-like conditions.

"(The) sample is only at high pressure for some number of femtoseconds," Sharp said. "It's a very, very short event, and the sample is destroyed."

An additional challenge lies in finding proper samples to test in the first place.

"Compare Earth with Mars," Shim said. "We have very little record about very ancient Earth. For example, finding a rock that age is around 4 billion years ago is extremely hard, because most of those rocks have been (destroyed) by weathering process. Whereas if you go to Mars, you have plenty of old rocks, because the geological activity like weathering has stopped a while ago."

The rarity of ancient geological samples drove the innovative nature of the project; by subjecting olivine to the pressures and energies of Earth's active layers, researchers can study the next closest thing to reality.

But also, Shim said this study provides an "atomic-scale" explanation of how the ancient magma sank and was preserved at the bottom of the mantle. 

"This technique opens up a whole world of study of liquids at very high pressure," Sharp said. "It's very difficult to do ... and measure their properties. This is a new avenue for doing that."

Edited by River Graziano, Jasmine Kabiri and Grace Copperthite.

Correction: An updated version of this article was published on Nov. 19, 2023, at 11:26 a.m., to properly attribute a quote. 

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