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Analytical Chemistry

Diamond Illumines Carbon Cycling

Geological Sciences: Preserved mineral inclusions yield new insights on deep-mantle geologic processes

by Elizabeth K. Wilson
September 19, 2011 | A version of this story appeared in Volume 89, Issue 38

Credit: Science/AAAS (c)2011
Tiny diamonds contain preserved ancient minerals.
A micro diamond from Juina, Brazil, with oceanic crust inclusions inside.
Credit: Science/AAAS (c)2011
Tiny diamonds contain preserved ancient minerals.

A new examination of tiny diamonds carried to the surface of Earth from the deep mantle helps confirm that Earth’s carbon cycle runs to great depths. This confirmation, researchers say, could lead to a better understanding of the oxidation state, volatile content, and geologic history of the lower mantle.

Oceanic crust—the outer surface layer of Earth found in ocean basins—descends into the upper mantle by subduction, a process caused by tectonic plate movements. The crustal material can return to the surface via mantle upwellings. Some scientists suspected that these processes also extend to the deep mantle, more than 400 miles below the surface.

But that possibility had not been unequivocally confirmed because samples of diamonds from the upper mantle, less than 200 miles below the surface, are common, but those from the deep mantle are extremely rare. Additionally, some researchers believed oceanic crust might simply remain deep in the mantle rather than participate in the carbon cycle.

Now, earth sciences professor Michael J. Walter and researcher Simon C. Kohn at England’s University of Bristol and their colleagues have found what they say is the smoking gun geochemists have been looking for to verify growing evidence of deep carbon cycling (Science, DOI: 10.1126/science.1209300).

The group found microdiamonds from the deep mantle in the well-studied Juina region of Brazil. They probed tiny mineral inclusions inside the diamonds, a phenomenon they liken to that of prehistoric bugs trapped in amber.

The inclusions match the mineral profile of the ancient ocean floor, suggesting that subducted surface crust material was trapped in the deep mantle during the diamond-forming phase. The diamonds then made their way back to the surface in upwellings. The carbon isotopic ratios of the deep-mantle diamonds also imply that they were formed from crustal material.

“This is a very important” study, says Ben Harte, a geosciences professor at the University of Edinburgh. This work and future discoveries will generate “a much clearer picture of the behavior of subducted slabs,” he says.



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