A new study reveals the unique seismological signature of an electron spin crossover in the deep earth

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Most people know that electrons are negatively charged particles that surround the nucleus of atoms and that their behavior determines chemical interactions. However, it is less common knowledge that there are two different types of electrons: spin-up and spin-down. And the tendency towards pairing between up- and down-spin electrons, which form “dance partners” with one another, is one of the most important behaviors that affect the electron clouds that control nature’s chemistry. Under pressure like deep inside the earth, the paths in which the electrons move are squeezed, and the “dance floor” changes. Electron pairs are sometimes forced to change their dance pattern and the way they connect with each other, resulting in what is known as an “electron-spin pairing crossover” (“spin crossover” is often used as a shorthand).

It was long predicted that such a spin crossover would occur at increased pressures of the middle mantle (~ 1500 km deep) in a mineral called “feropericlase”, which is believed to be the second most abundant material in the earth’s rocky mantle. Such predictions for a ferropericlass spin crossover have been largely confirmed both by high pressure laboratory experiments and by quantum mechanical computer models. However, the predicted effects of this spin crossover escaped seismological discovery, leaving researchers in the deep-sea countries wondering whether the predictions were in error or whether conditions in the mantle are suppressing seismic expression.

A new research paper published in Nature communication by an international research team that includes Professor John W. Hernlund (Tokyo Institute of Technology) of the Earth-Life Science Institute (ELSI) and specially appointed ELSI Assistant Professor Christine Houser, proposes a unique seismological signature of this spin crossover in Ferropericlas . The team’s detection method is based on the different behavior of the spin crossover for P-waves and S-waves, two different types of seismic waves that travel through the earth. Seismologists use these two waves (created by earthquakes and recorded at global seismographic stations) to create tomographic images of the mantle in a process somewhat similar to a medical CT scan. The images show material that propagates these two types of seismic waves faster or slower than average.

Seismic images of high wave velocity features taken at intermediate mantle depths show that the P-wave signatures of prominent seismic fast-moving features are attenuated compared to their S-wave counterparts. As it turns out, precisely this behavior is expected for rocks with plausible amounts of ferropericlass under medium mantle conditions and is due to a combination of the reduction in the induced volume of the spin crossover and its broadening over a larger pressure range. causes higher temperatures.

Encouraged by this possible association, the research team hypothesized that if the spin crossover explains this behavior in fast seismic structures in the middle mantle, then, due to the properties of the spin transition at high temperatures, it should also occur for slow structures at greater depths. When they searched for this signature in slow features, they again found evidence of a weakening of the P-wave features compared to S-wave counterparts at the greater depths they predicted.

The research team then had to rule out the possibility that these signals in P-wave and S-wave seismic images were not simply due to resolution artifacts such as differences in the behavior of these waves and the construction of the corresponding images. They used a variety of seismic images created by different research groups, most of them with different imaging techniques, and then compared the features that everyone agreed on. This “vote map” method was originally developed by lead author Grace Shephard at the University of Oslo. When they calculated and graphed profiles of the frequency of fast and slow S-wave and P-wave features, feature muting, consistent with spin crossover, was widespread and unmistakable in P-wave models.

When asked which of the pieces of evidence appeared to provide the strongest support for spinning crossover detection, co-author Christine Houser said that all evidence must be considered together. Houser added that “the relative muting of P-wave signals at two different depths for fast and slow anomalies is difficult to explain as a result of artifacts. While not impossible, it would be an unlikely coincidence for models assembled with different data and methods to consistently display the same seismic signals as in spin crossover. “

While the detection of the seismic signal of the iron spin crossover reveals regions in which oceanic plates rise and fall in the deep mantle, a blatant problem remains the lack of the predicted signal in globally averaged seismic profiles of the earth’s mantle. Members of the same team previously found that this cannot be explained by averaging the same types of materials at different temperatures. Instead, large-scale changes in chemical composition may be required, such as the presence of regions in the middle mantle that contain rocks that are low in ferropericlase (and therefore no visible signature of a spin crossover). An earlier study, involving several members of this research team, suggested the presence of such features in the Earth’s middle mantle that anchor the pattern of deep mantle convection and, due to their high strength, persist for billions of years. Calling these “Bridgmanite Enriched Ancient Mantle Structures” or BEAMS (Bridgmanite is the most abundant mineral on Earth and is also considered the most powerful), they speculated that they might have some basic control over the pattern of tectonic plate movements over history .

The spin crossover detection in the fastest and slowest wave speed regions of the mantle underlines another critical geophysical effect. Fast regions consist of former sea bedrock that dips through the mantle on its journey to the core-mantle boundary. In contrast, slow regions consist of rocks that are heated by contact with the molten iron core and rise to the surface like a lava lamp. This convection process recycles rocks between the surface and the interior and powers plate tectonics. The identification of the pronounced seismological signature of the spin crossover in ferropericlase in the earth’s mantle shows that building a bridge between material physics and geophysics is crucial for understanding the interior of the earth and the planet. The unique seismic signature allows us to determine which parts of the deep mantle contain more or less the mineral ferropericlase, effectively creating 4D geological maps and revealing the history of the earth over the vast expanses of deep interior and deep time.

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Materials provided by Tokyo Institute of Technology. Note: the content is editable in terms of style and length.


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