Early in the formation of the Earth, some forms of the element chromium separated and disappeared deep into the planet's core, a new study by UC Davis geologists shows.

The finding, to be published online by the journal Science Feb. 24, will help scientists understand the early stages of planet formation, said Qing-Zhu Yin, professor of geology at UC Davis and co-author of the paper.

Using specialized equipment at UC Davis, Yin, and his colleagues made very exact measurements of chromium isotopes in meteorites and compared these to rocks from the Earth's crust. With modern, high-performance computers, they then used the data to generate a simulation of Earth's early environment.

The team studied a class of meteorites called chondrites, which are leftovers from the formation of the solar system more than 4.5 billion years ago.

As well as adding shiny, rust-proof surfaces to metalwork, chromium adds color to emeralds and rubies. It exists as four stable (nonradioactive) isotopes with atomic masses of 50, 52, 53 and 54.

It has been known for decades that chromium is relatively underrepresented in the Earth's mantle and crust, Yin said. That could either be because it evaporated into space, or got sucked into the Earth's deep core at some point.

By making very accurate measurements of chromium isotopes in the meteorites compared to Earth rocks and comparing them to theoretical predictions, the researchers were able to show for the first time that lighter isotopes of chromium preferentially go into the core. From this the team inferred that some 65 percent of the missing chromium is most likely in the Earth's core.

Furthermore, the separation must have happened early in the planet-building process, probably in the multiple smaller bodies that assembled into the Earth or when the Earth was still molten but smaller than today.

Yin's co-authors are Frederic Moynier, a former postdoctoral scholar at UC Davis who is now assistant professor in the Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences at Washington University in St Louis, and Edwin Schauble of the UCLA Department and Earth and Space Sciences.

The work was funded by grants from NASA and the National Science Foundation.