© Universe Today

It's generally assumed that the Earth's overall composition is similar to that of chondritic meteorites, the primitive, undifferentiated building blocks of the solar system. But a new study in Science Express led by Frederic Moynier, of the University of California at Davis, seems to suggest that Earth is a bit of an oddball.

Moynier and his colleagues analyzed the isotope signature of chromium in a variety of meteorites, and found that it differed from chromium's signature in the mantle.
Thin section of a chondritic meteorite.

"We show through high-precision measurements of Cr stable isotopes in a range of meteorites, which deviate by up to ~0.4‰ from the bulk silicate Earth, that Cr depletion resulted from its partitioning into Earth's core with a preferential enrichment in light isotopes," the authors write. "Ab-initio calculations suggest that the isotopic signature was established at mid-mantle magma ocean depth as Earth accreted planetary embryos and progressively became more oxidized."

© Science/AAAS
Chromium’s origins. New evidence suggests that, in the early solar nebula (A), chromium isotopes were divided into two components, one containing light isotopes, the other heavy isotopes. In the early Earth (B), these components formed a homogeneous mixture. During core partitioning (C), the core became enriched with lighter chromium isotopes, and the mantle with heavier isotopes.

The results point to a process known as "core partitioning," rather than an alternative process involving the volatilization of certain chromium isotopes so that they would have escaped from the Earth's mantle. Core partitioning took place early on Earth at high temperatures, when the core separated from the silicate earth, leaving the core with a distinct composition that is enriched with lighter chromium isotopes, notes William McDonough, from the University of Maryland at College Park, in an accompanying Perspective piece.

McDonough writes that chromium, Earth's 10th most abundant element, is named for the Greek word for color and "adds green to emeralds, red to rubies, brilliance to plated metals, and corrosion-proof quality to stainless steels." It is distributed roughly equally throughout the planet.

He says the new result "adds another investigative tool for understanding and documenting past and present planetary processes. For the cosmochemistry and meteoritics communities, the findings further bolster the view that the solar nebula was a heterogeneous mixture of different components."

Source: Science. The McDonough paper will be published online today by the journal Science, at the Science Express website.