Iron oxide may explain mysterious anomalies beneath Earth’s surface

Just below Earth’s surface, just above the planet’s iron-metallic core, lies the core-mantle boundary (CMB). This extreme region separates the planet’s outer core from a thick layer of molten rock mantle and lies about 1,800 miles below our feet.

Although it appears far below, the CMB has an impact on our world above. Over tens of thousands of years, material from the CMP rises upward through Earth’s core, shaping the volcanic activity, chemistry, and plate tectonics we experience on the surface.

By measuring seismic waves, scientists have discovered that there are many strange regions in and around the CMB where waves travel much slower than expected. For some time, scientists have speculated that these areas may be particularly rich in iron oxide, which explains the wave’s speed. But can iron oxide solids exist even at such extreme temperatures and pressures? According to a new study published in Natural communicationLooks like an opportunity.

Ancient Protoplanet Under Earth’s Surface?

In early November, geophysicists published a paper Nature Two strange „bubbles” found at the very bottom of Earth may be the remnants of a hypothesized protoplanet that collided with a young Earth. The paper, which focuses on two large low-velocity provinces (LLVPs) located beneath the Pacific Ocean and the African continent, suggests that fragments of an iron-rich protoplanet may have formed after a collision with Earth, trapped beneath Earth’s surface. These LLVPs.

In addition to these LLVPs, there are two large ultra-low velocity zones (ULVZs) that can be seen in the CMB, one just west of Hawaii and one near Samoa. As with LLVPs, scientists hypothesize that the slow speed of seismic waves through ULVZs is due to the large amount of iron oxides in these regions. But very little is known about these ULVZs.

„Because we can’t go below the CMP and take measurements, there are many open questions about a region that is so important to the evolution of our planet.” said Jennifer JacksonWilliam E. Leonhardt is a professor of inorganic physics at the California Institute of Technology (Caltech) and the new senior author. Natural communication study „Why do ULVZs exist and what are they made of? What do they teach us about how the Earth formed and what role the region plays in Earth dynamics? Are the bubbles solid or molten under extreme conditions in the CMB?”

Curious whether iron oxides could exist in solid form at such extremes, Jackson’s lab conducted detailed measurements of the behavior of iron oxide at the range of temperatures and pressures found in the CMB. From this, the researchers were able to create a phase diagram demonstrating that iron oxide remains solid even at very high temperatures.

This is the strongest evidence to date that solid iron-rich regions can be a realistic explanation for the peculiar behavior observed in ULVZs. Such iron-rich regions may play an important role in explaining volcanic activity, such as deep-seated mantle plume formation.

Novel spectroscopy method probes iron under harsh conditions

For their study, Jackson’s team used an innovative spectroscopic technique developed by the team to monitor the changing configuration of atoms over very short timescales.

A technique called Mössbauer spectroscopy can take samples smaller than the average width of a human hair and detect the precise temperature at which a substance begins to change from a solid to a liquid, based on the motion of atoms in the sample. Their stiffening breaks out of their repetitive solid structures and becomes more fluid.

„We use Mössbauer to answer questions about the dynamical motion of iron atoms.” said Former Caltech graduate student Vasilije Dobrosavljevic, first author of the study. „On a short timescale of about 100 nanoseconds, we want to know: Do they move as little as in a solid, or more like a liquid? Our new study complements Mössbauer spectroscopy with an independent method, X-ray diffraction, that allows us to observe the positions of all the atoms in the sample. .”

After dozens of experiments, Dobrosavljevic, Jackson, and colleagues discovered that iron oxide melts at 4,000 Kelvin (approximately 6,700 degrees Fahrenheit) at pressures similar to those found in the CMB. This is higher than previously estimated, but importantly, higher than recent temperature estimates of the current CMB, meaning that solid iron oxides are a real possibility in explaining the composition of ULVZs.

An unexpected discovery

The study also provided another, more unexpected, result. At atmospheric pressure, iron oxide samples have atomic-scale defects. For every 100 oxygen atoms, there are only about 95 iron atoms in the same site, meaning 5 iron atoms are „missing”. Materials scientists are interested in such matters because these defects can have a significant impact on the properties of materials on a large scale. For example, such defects can affect how a material deforms under pressure or how it conducts heat.

Such defects are of interest to geophysicists, who can use defect information to understand planetary interiors by studying where heat flow and material deformation drive various planetary dynamics. The behavior of such defects at the types of pressures and temperatures found in the CMB has not been known until now.

With their new method, Dobrosavljevic and his team were able to observe how tiny atomic defects in the material alternated and „disordered” within the sample at temperatures several hundred Kelvin below the melting point of iron oxide.

The researchers believe this may explain previous low estimates for the melting point of iron oxide — rather than detecting melting of the entire crystal structure, those tests looked at defect transitions.

„Before the solid crystal transitions to liquid, we see the defect structure change to a disordered state,” explained Dobrosavljevic. „We now want to know what effect this newly discovered change has on the physical properties of iron-rich regions such as ULVZ? How do defects affect heat transport, and what does this mean for its formation and production. Will it reach the surface? These questions will guide further research.”

Note: Dobrosavljevic VV, Zhang D, Sturhahn W, et al. Melting and defect transitions in FeO up to the pressure of the Earth’s core-mantle boundary. Not Common. 2023;14(1):7336. doi: 10.1038/s41467-023-43154-v

This article is a rework Press release Published by the California Institute of Technology. Material edited for length and content.