The core-mantle boundary (CMB) is the interface between Earth’s iron-metallic core and the thick, rocky mantle layer just above the core. A world with extremes such as temperatures of thousands of degrees Fahrenheit and pressure millions of times greater than the pressure at the Earth’s surface. Although seemingly distant from our environment on Earth’s surface, clouds of material from the CMB can rise across the planet over tens of millions of years, influencing the chemistry, geology, and plate tectonics of the surface world we inhabit.

Although scientists can’t go to the center of the Earth to study the CMB, they can get clues about what’s beneath the planet’s surface by measuring earthquakes. Seismic waves travel at different speeds depending on the material they pass through, allowing researchers to use seismic traces to understand what’s deep below the surface. This is similar to how ultrasound uses sound waves to image the inside of the human body.

Recent research shows that the base of the Earth’s mantle is actually complex and heterogeneous, with mountainous regions in particular where seismic waves mysteriously slow down. These spots, called ultra-low velocity zones (ULVZ) and first discovered by Don Helmberger of the Caltech Institute, are tens of kilometers thick and located approximately 3,000 kilometers below our feet.

“Because we can’t go to the CMB and make measurements, there are a lot of open questions about a region that is so important to the evolution of our planet,” says Jennifer Jackson, the William E. Leonhard Professor of Mines. Physical. “Why do ULVZs exist and what are they made of? What do they teach us about how the Earth developed and the role region plays in Earth dynamics? “Are these droplets solid or molten under CMB’s extreme conditions?”

In 2010, Jackson and his team suggested that these droplets contained higher iron oxide content than the mantle surrounding them. Solid iron oxide slows down seismic waves, which may explain the low velocities measured as they pass through the drops. So can iron oxide be a solid even at the extreme temperatures and pressures of CMB?

Now, a new study from Jackson’s lab has made detailed measurements of the behavior of iron oxide at a range of temperatures and pressures similar to those found in CMB. The resulting phase diagram shows that, contrary to previous theories, iron oxide remains solid even at very high temperatures. This is the strongest evidence yet that iron-rich solid zones are a realistic explanation for ULVZs and may play a key role in the formation of deep clouds. These findings encourage future studies of iron-rich solid materials to better understand Earth’s deep interior.

An article describing the research was published in the journal Nature Communication 13 November.

At the atomic level, solid iron oxide consists of iron and oxygen atoms neatly arranged in regularly repeating structures. When a solid begins to melt, the atoms lose their solid ordered structure and begin to move smoothly. The new research, led by former Caltech graduate student Vasiliy Dobrosavlevich (Ph.D. ’22), aimed to experimentally determine the temperatures and pressures at which this transition occurs.

Reaching extreme temperatures and pressures in experiments has been possible for decades; however, the experiments require samples smaller than the width of the average human hair. It is difficult to determine exactly the temperature at which the material begins to transition from solid to liquid using such small samples. For more than a decade, Jackson and his colleagues have been developing a technique to detect high-pressure melting. The new study uses this sensitive technique, called Mössbauer spectroscopy, to observe the dynamic configuration of iron atoms.

“We use Mössbauer to answer questions about the dynamic motion of iron atoms,” says Dobrosavlevich. “Over a short period of time, about 100 nanoseconds, we want to know: are they moving too little, as in solids, or too much, as in liquids? Our new study uses Mössbauer spectroscopy, an independent X-ray diffraction method that allows the positions of all atoms in the sample to be observed.” He completes it with his method.

After dozens of experiments at different temperatures and pressures, the team found that under Earth’s CMB pressure, iron oxide melts at higher temperatures than previously thought: more than 4,000 Kelvin, which is equivalent to about 6,700 degrees Fahrenheit. The study also revealed an unexpected result regarding so-called atomic defects in iron materials.

Researchers know that at sea-level pressure, every sample of iron oxide has tiny, regularly spaced defects in its atomic structure. There are only about 95 iron atoms for every 100 oxygen atoms, so about five iron atoms are “missing.” Researchers are studying how these defects at the atomic level might affect the material on a larger scale, how it conducts electricity and heat, whether it deforms under pressure, etc. argued. These parameters are critical for understanding planetary interiors, where heat flow and material deformation drive planetary dynamics. However, the behavior of defects at high pressures and temperatures similar to those found in CMB was still unknown.

Dobrosavlevich and his team found that at temperatures a few hundred Kelvin below the melting point of iron oxide, tiny atomic defects begin to slip through the solid material and become “disordered.” This may explain why previous experiments suggested that iron oxide melts at lower temperatures: these experiments actually observed a shift in defects rather than melting of the entire crystal structure.

“We see that the defect structure transitions from ordered to disordered before the transition from solid crystal to liquid crystal,” he says. “We now want to know what impact this newly discovered transition has on the physical properties of iron-rich zones such as the ULVZ. How the defects affect heat transfer and what this means in terms of the formation and formation of plumes that rise to the surface. These questions need further research.” will show the way.”