Nickel May be Magic Ingredient in Planetary Core Formation

Knowledge of the composition of Earth’s core and the cores of other planetary bodies offer clues to how the solar system formed and to Earth’s early history. Scientists believe that Earth’s core is a dense ball of iron, nickel, and other lighter elements, but the identity and proportions of those elements are still up for debate.

While investigating lighter element candidates, DCO Extreme Physics and Chemistry Community member Anat Shahar and postdoctoral associate Stephen Elardo (both of the Carnegie Institution of Washington, USA) made an interesting discovery about the role of nickel in the formation of cores in planetary bodies. Through high-temperature and pressure experiments simulating core formation, the researchers found that nickel can affect how iron isotopes separate into the mantle and core. Adding more nickel causes heavy iron isotopes, which are iron atoms with extra neutrons, to enter the planet’s metal core, while lighter iron atoms stay in the surrounding mantle. This discovery may offer an explanation for the varying iron isotope signatures found on Earth, the Moon, Mars, and the asteroid Vesta. Shahar and Elardo describe their findings in a new paper in the journal Nature Geoscience [1].

Iron-containing samples from Mars, Vesta, and from meteorites called chondrites, have very similar isotope profiles, while Earth and Moon basalts, which form from melted mantle rocks, have an unusually large percentage of heavier isotopes. Scientists have proposed explanations for the differences, including the idea that during a planet’s initial formation from chondrite-like materials, light iron isotopes left with other volatile elements in the solar nebula, while the planet trapped the heavier isotopes. Others have proposed that the Moon-forming Giant Impact drove off light iron isotopes from the Earth-Moon system, but no model has accounted for the separation of iron isotopes into different planetary layers during core formation.

“We know all planetary cores have nickel in them because there is nickel in the meteorites that made them, so the effects of nickel on iron isotope fractionation during core formation are relevant to all planetary formation,” said Elardo. “We think our model can explain the different isotope ratios observed for small planets across the solar system.”

Shahar and Elardo simulated iron isotope fractionation between core and mantle formation by mixing iron and varying amounts of nickel with peridotite, the dominant mantle rock. They placed the mixture into a piston cylinder, a large hydraulic ram that squeezes the sample to bring it up to pressure, and passed an electrical current around the sample to generate heat. The samples reached 1850 degrees Celsius and 1gigapascal of pressure, recreating the high temperature and pressure of core formation for the smaller planets and asteroids.

The researchers observed that the more nickel present in these reactions, the greater the amount of heavy iron that went into the core, leaving lighter iron in mantle materials. Previous experiments by Shahar et al. suggested that sulfur has a similar effect. Coupled with mantle melting processes that concentrate heavy iron in the magma that spills out at mid-ocean ridges, these findings have the potential to explain a lot of the variation in iron isotopes observed across the solar system.

The researchers proffer the caveat that Earth formed at a higher temperature and pressure than used in the experiment, so these findings may have limited application to Earth.

Currently, Elardo and Shahar are exploring the effects of silicon in iron isotope fractionation and core formation. Silicon is another top candidate for the lighter elements that may be found in the cores of planetary bodies.

For more background on core formation in planets and how the work of Elardo and Shahar, and other DCO researchers fits into the picture, read the News and Views article accompanying their paper in Nature Geosciences [2].

A scanning electron microscope image of the core formation experiment: The dark grey silicate mineral, which represents the mantle, surrounds a light grey metal blob, which acts like a planet’s core. This image is featured on the cover of the April 2017 print issue of Nature Geoscience. Credit: Stephen Elardo.

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