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Thursday, October 28, 2021

Earth’s core attracts element called chromium

Chromium doesn’t get talked about much, but the element is all around us – we see it in glittering gemstones and stainless steel forks. UC Davis scientists are part of a team working to figure out how most of Earth’s chromium ended up in Earth’s molten core.

Researchers at UC Davis, and other universities, recently estimated that 65 percent of the chromium on Earth can be found in its core. Chromium, a chemical element, exists in depleted quantities in the Earth’s mantle and crust, and scientists wanted to find out where much of this transition metal had gone.

To study chromium, the scientists started out with acid and meteorites.

“We used acids to dissolve rocks and minerals extracted from meteorites in our ultra-clean lab at the UC Davis geology department,” said Qingzhu Yin, associate professor of geology at UC Davis.

They then dried out the meteorites and analyzed the elements using a technique called ion exchange chromatography. Ion exchange chromatography is a process that can separate elements, like chromium, based on positive or negative charges.

According to Yin, after this ion exchange chromatography, the team measured the isotopic composition with a tool called a spectrometer, which allows for the collection of different isotopes simultaneously. Through this collection, they were able to compare and discern very small differences in the chromium isotopic composition between the elemental building blocks of the Earth.

“Whatever we sample on the Earth is the left-over after metallic core materials were extracted from the original building block chemical composition,” Yin said.

He said that chromium is a siderophile – metal-loving – element, which is one reason it migrated to the Earth’s core during our planet’s early days.

“Lighter isotopes of chromium preferentially go into the core, heavier isotopes preferentially stay in the mantle,” Yin said. “It has to do with chemical bonding, which is isotopic mass-dependent. Based on such observation, we can do mass-balance calculations and figure out how much of chromium must have gone into the core, and how much is left in the silicate mantle of the Earth.”

In short, heavier isotopes stayed in the rocky parts of the Earth, and lighter isotopes traveled to the molten core.

Edwin Schauble, associate professor of the department of Earth and Space Sciences at UCLA, said that chromium is useful in everyday life.

“Chromium is one of the major components of stainless steel,” Schauble said.

He also said that chromium is responsible for giving rubies their colors -something Yin said can be attributed to the different oxidation states of chromium.

Like Yin, Schauble said that Earth’s crust and mantle are enriched in heavier isotopes, while meteorites are enriched in lighter isotopes – something that they were able to look at very precisely at a small scale.

“The idea of the study was to understand the behavior of chromium during the formation of the Earth,” Schauble said.

He said that in the solar nebula – before Earth’s formation – chromium could have stayed in the gas phase until the temperature dropped to about 1000 degrees Celsius; however, some of the chromium might have stayed in the vapor phase rather than condensing into materials that formed the Earth, and some may have evaporated away after condensing.

In the 2009 observation, scientists were able to detect chromium in an intergalactic space, a first for an element usually found in the Milky Way and other galaxies; showing that much about chromium is still being discovered.

ERIC C. LIPSKY can be reached at science@theaggie.org.

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