Scientists find oxidised iron deep within the Earth's interior

Scientists digging deep into the Earth's mantle recently made an unexpected discovery.

Diamonds with garnet inclusions can form at depths down to 550 kilometres below the surface. Image: Jeff W. Harris, University of Glasgow

Five hundred and fifty kilometres below the Earth's surface, they found highly oxidised iron, similar to the rust we see on our planet's surface, within garnets found within diamonds.

The result surprised geoscientists around the globe because there is little opportunity for iron to become so highly oxidised deep below the Earth's surface.

Surprising discovery
"On Earth's surface, where oxygen is plentiful, iron will oxidise to rust," explains Thomas Stachel, professor in the Department of Earth and Atmospheric Sciences at the University of Alberta, who co-authored the study. "In the Earth's deep mantle, we should find iron in its less oxidised form, known as ferrous iron, or in its metal form. But what we found was the exact opposite--the deeper we go, the more oxidized iron we found."

This discovery suggests that something oxidised the rocks in which the superdeep diamonds were founds. The scientists suspect that it was molten carbonate, carried to these great depths in sinking slabs of ancient sea floor.

"It's exciting to find evidence of such profound oxidation taking place deep inside the Earth," said Stachel, Canada Research Chair in diamonds.

Carbon cycle
The study also has implications for understanding the global carbon cycle that involves the transport of surface carbon back into the Earth's mantle.

"We know lots about the carbon cycle on Earth's surface, but what about in the mantle?" explained Stachel. "Our study suggests that surface carbon goes down as carbonates to at least 550 kilometres below the surface. There, these carbonates may melt and react with the surrounding rocks, eventually crystallising into diamonds. Diamonds can then be taken down even deeper in the mantle."

The study shows that the carbon cycle extends deep into mantle, possibly all the way down to the core-mantle boundary, with billion year storage times.