The interior of the Earth is a mystery, especially at greater depths (> 660 km). Researchers have only seismic tomographic images of this region and in order to interpret them they need to calculate seismic (acoustic) velocities in minerals at high pressures and temperatures. Using these calculations, they can create 3D velocity maps and determine the mineralogy and temperature of the observed regions. When there is a phase transition in a mineral, such as a change in the structure of a crystal under pressure, scientists notice a change in velocity, usually a sharp break in seismic velocity.
In 2003, scientists in the laboratory noticed a new type of phase change in minerals – the spin change of iron in ferropericlase, the second most widespread component of the Earth’s lower mantle. Centrifuge change or centrifuge crossover can occur in minerals such as ferropericlase under an external stimulus, such as pressure or temperature. Over the next few years, experimental and theoretical groups confirmed this phase change in both ferropericlase and bridgmanite, the most common phase of the lower mantle. But no one was quite sure why and where it was happening.
Columbia Engineering professor Renata Ventzcovitch published her first paper on ferropericlase in 2006, providing a theory of spin crossings in this mineral. Her theory suggests that it happened a thousand kilometers in the lower mantle. Since then, Ventzcovitch, who is a professor in the Department of Applied Physics and Applied Mathematics, Earth and Environmental Sciences, and the Lamont-Doherty Earth Observatory at Columbia University, has published 13 papers with her group on the subject, exploring speeds in every possible the situation of spin crossover in ferropericlase and bridgmanite and predicting the properties of these minerals during this crossover. In 2014, Venzcovitch, whose research focuses on computational quantum-mechanical studies of materials in extreme conditions, especially planetary materials, predicted how this spin change phenomenon could be detected on seismic tomography, but seismologists have not yet been able to see it.
Working with a multidisciplinary team from Columbia Engineering, the University of Oslo, the Tokyo Institute of Technology and Intel Co., Venzcovitch’s latest work details how they have now identified a ferropericyclase crossover signal, a quantum phase transition deep in the Earth’s lower mantle. This has been achieved by observing specific regions in the Earth’s mantle where ferropericlase is expected to be abundant. The study was published on October 8, 2021 Nature Communications.
“This exciting discovery, which confirms my earlier predictions, illustrates the importance of the material that physicists and geophysicists work together to learn more about what is happening deep in the Earth,” Ventzcovitch said.
Spin transition is commonly used in materials such as those used for magnetic recording. If you stretch or compress only a few nanometer layers of magnetic material, you can change the magnetic properties of the layer and improve the recording properties of the media. A new study by Wentzcowitch shows that the same phenomenon occurs thousands of kilometers inland, taking this from nano- to macro-scale.
“Moreover, geodynamic simulations have shown that spin-crossover enhances convection in the Earth’s mantle and the movement of tectonic plates. Therefore, we think that this quantum phenomenon also increases the frequency of tectonic events such as earthquakes and volcanic eruptions,” notes Ventzcovitch.
There are still many areas of the mantle that researchers do not understand, and the change in the state of the spin is critical for understanding speeds, phase stability, etc. Ventzcovitch continues to interpret seismic tomographic maps using the seismic velocities predicted From the beginning calculations based on density functionality theory. It also develops and applies more precise material simulation techniques to predict seismic velocities and transport properties, especially in regions rich in iron, molten or near-melting temperatures.
“What’s particularly exciting is that our material simulation methods are applicable to highly bonded materials – multiferroic, ferroelectric and high temperatures in general,” says Ventzcowitch. “We will be able to improve our analysis of 3D tomographic images of the Earth and learn more about how the earthquake pressures of the Earth’s interior indirectly affect our lives above, on the Earth’s surface.”