Study Finds Core Formation Not Responsible for Earth’s Iron Anomaly

Compared to other bodies in the solar system, Earth has an unusual iron isotope profile, with a large percentage of heavy iron atoms occurring in the mantle. Scientists have conceived of several explanations for this anomaly, such as the high-pressure, high-temperature conditions that formed Earth’s core; the ancient collision with another body that created the Moon; or the melting occurring within the mantle to generate new crust. Now, new research casts doubt on core formation as one of the possible explanations.

A group led by DCO Extreme Physics and Chemistry Community Member Jung-Fu Lin (University of Texas at Austin, USA), subjected mantle and core materials under high pressure to simulate planetary formation. They found that the process had little effect on how iron isotopes separate into different layers, suggesting core formation cannot explain Earth’s isotope profile. The researchers describe their findings in a new paper in Nature Communications.

Meteorites called chondrites are vestiges of materials that initially coalesced to form Earth and other planets in the solar system. The minerals that make up chondrites contain iron, with both heavy and light isotopes. The ratio of these iron isotopes matches the profiles of rocks from the mantles of Mars and the asteroid Vesta. But Earth’s mantle appears to be different. Mid-ocean ridge basalts, which form from melted mantle rocks that erupt and spread to create the ocean floor, have about 0.01 percent more heavy iron than chondrites. The observation led some researchers to suggest core formation in Earth caused heavier isotopes to concentrate in the mantle.

To investigate iron isotope fractionation, which is differential separation of heavy and light isotopes into the mantle and core, Lin and his colleagues placed basaltic glass, to simulate the mantle, and iron alloys, to stand in for core materials, into a diamond anvil cell. This high-pressure device generated pressures up to 130 GPa, more than 1.3 million times Earth’s atmospheric pressure, reproducing the pressure conditions at the core/mantle boundary.

The group used a powerful but sophisticated technique called nuclear resonance inelastic x-ray scattering (NRIXS) at Argonne National Laboratory to measure the force constant, which is the strength of the iron bonds for different isotopes under high-pressure conditions. They observed that iron bonds only became stronger with greater pressure. Using these measurements, the researchers calculated that under high-temperature, high-pressure conditions, only a tiny amount of fractionation occurs between the mantle and the core.

“Based on our study we suggest that Earth’s core formation is not the main contributor to the isotope anomaly,” said Lin.

The researchers also conducted the same simulation of conditions with nickel, carbon, silicon, and other light elements believed to be in the core in smaller quantities than is iron.

“We investigated whether or not light elements affect the force constant and iron fractionation factor under Earth’s core conditions. Our answer is that they do not. All light elements seem to have very negligible effects on iron fractionation,” said Lin.

Iron is just one element with unusual fractionation on Earth. In current and future works, Lin and his colleagues are investigating isotope anomalies in other elements, including silicon and magnesium, to gain greater insight into the properties of planetary materials under high-temperature and high-pressure conditions.

For more information on core formation in planets and how the work of Lin and other DCO researchers fits into the big picture, read the News and Views article in a recent issue of Nature Geoscience [2].


Images: Top: A new study by Lin and colleagues finds that core-forming processes in planets are not responsible for unusual iron isotope fractionation on Earth. Credit: Image courtesy of NASA. Middle: Caption: Jung-Fu Lin, a professor at the of the University of Texas at Austin, holds a diamond anvil, a device that can exert extremely high pressures on samples, to simulate conditions found in the Earth’s core. Credit: Image courtesy of the University of Texas at Austin Jackson School of Geosciences. Bottom: Credit: Image created by Laura Martin of the University of Texas at Austin, Jackson School of Geosciences, using images from NASA, JPL-Caltech and X-Science.

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