Deep insights from diamonds

Gemstones reveal oxidation processes in the interior of the Earth

diamond from depths

A diamond from depths of around 500 kilometres. Each fragment of orange-coloured garnet in the foreground is around 200 micrometres (0.2 millimetres) in length. (Credit: Jeff W. Harris, University of Glasgow)

Diamonds have unparalleled value for geoscientific research. With measurements at DESY's X-ray source PETRA III and elsewhere, an international team including researchers from the University of Bayreuth recently discovered that inclusions of iron-containing garnet are highly oxidized in diamonds that originate at great depths. These tiny pieces of minerals help to explain chemical processes in the Earth’s interior. The scientists published their findings in Nature Geoscience. They suggest that redox reactions between iron and carbon take place in the transition zone and may play a crucial role in the Earth’s deep carbon cycle.

Amber is known to contain inclusions of animals and plants that give scientists a biological snapshot of earlier periods in the history of the Earth. In a similar way, diamonds often contain traces of minerals that provide geoscientists with insights into the depths of the Earth. When diamonds grow at such levels, minerals from their environment crystallize with the diamonds and become embedded. The chemical state of the minerals remains unchanged during transport, even if the diamonds make their way through the crust to the Earth’s surface – for example, as part of a volcanic eruption. Such minerals include various types of garnet.

Garnet inclusions were analysed in diamonds known to have originated at various depths between 260 and 500 kilometres below the Earth’s surface. Researchers at the University of Bayreuth’s Bavarian Research Institute of Experimental Geochemistry & Geophysics and the University of Oxford coordinated experiments involving X-ray crystallography and spectroscopy, many of which were carried out at DESY in Hamburg and the ESRF in Grenoble. They found that the oxidation state of iron in the garnet varied considerably: deeper diamonds showed more highly oxidized iron. The oxidized iron atoms have fewer electrons and therefore must have transferred electrons to their environment at great depths, around 500 kilometres.

“We have found strong evidence of redox reactions in the deep Earth where electrons are probably transferred from iron atoms to neighbouring carbon atoms. These processes lead to the production of new diamond crystals,” said Catherine McCammon of the Bavarian Research Institute of Experimental Geochemistry & Geophysics (BGI). “Through these observations, our data contribute to a better understanding of the Earth’s deep carbon cycle, which ranges from the Earth’s core up to the atmosphere. To a large extent there is only rudimentary information so far,” added Leonid Dubrovinsky of the BGI, who initiated the study published in Nature Geoscience.

(from DESY news)

Oxidized iron in garnets from the mantle transition zone; E S. Kiseeva, D. M. Vasiukov, B. J. Wood, C. McCammon, T. Stachel, M. Bykov, E. Bykova, A. Chumakov, V. Cerantola, J. W. Harris, L. Dubrovinsky; Nature Geoscience, 2018; DOI: 10.1038/s41561-017-0055-7