December 14, 2018

South African diamond provides new clues on Earth's internal processes

10 March 2018, 10:31 | Dale Webster

Diamond discovery under pressure


Earth's crust and the upper part of the mantle together form the lithosphere, whose thickness ranges up to 60 kilometers below the surface, but can extend up to 200 kilometers underground, which is where most diamonds form.

The diamonds analyzed in the study were actually the diamonds that surged up from deep inside the Earth and found in China, South Africa, and Botswana, showing that this is not a regional occurrence but a global one.

University of Alberta Department of Earth and Atmospheric Sciences professor Graham Pearson and colleagues from the University of British Columbia recently observed calcium silicate perovskite - one of five fundamental minerals that make up 99.99 per cent of the silicate mass of the Earth - in a diamond.

"The only possible way to keep this mineral at the Earth's surface is when he is locked in a container, such as a diamond", explained Pearson.

One of the most abundant minerals in the world - that had never before been seen on the Earth's surface - has finally been eyed by scientists, thanks to one of the world's most sought-after gems.

It is found deep within the Earth's mantle and is known to be too unstable to exist on the surface.

An earlier study, from December 2016, looked at fragments of some of the world's largest precious stones (the flakes are produced when rough stones are cut and polished) and based on the minerals trapped in them, concluded that they formed at depths corresponding to the deep mantle.

Pearson said the calcium silicate perovskite they viewed probably grew at the same rate as the diamond and, as diamonds are the most incompressible of all natural minerals, it effectively created a protective barrier. However, this remained only a theory up until now.

Found in abundance deep in the Earth, at about 700 km from the surface, it has never been seen on the surface.

The authors of the paper, who ran X-ray and spectroscopy tests to confirm their suspicions, believe the perovskite-containing diamond likely formed some 700 km deep in the Earth and must have sustained 24 billion pascals of pressure, equivalent to 240,000 atmospheres.

The frozen water, called Ice-VII, is crystallized which could've only happened under unbelievably great pressure.

The diamond's structure managed to protect the CaSiO3 and prevented its crystal lattice from being deformed while the diamond moved to the Earth's surface. "The specific composition of the perovskite inclusion in this particular diamond very clearly indicates the recycling of oceanic crust into Earth's lower mantle".

"Diamonds are really unique ways of seeing what's in the Earth", said Pearson.

What else do you think this rare Earth mineral would reveal about our planet?

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