Rock stars solve long-standing diamond conundrum

Posted on: 16 March 2023

Three researchers from Queensland University of Technology and Trinity College Dublin have used a standard laptop computer and a humble piece of ancient rock – from the “waste pile” of a diamond mine – to solve a long-held geological conundrum about how diamonds formed in the deep roots of the earth’s ancient continents. The paper has been published in the prestigious academic journal Nature.

The lead author, Mr Carl Walsh, said his MSc research involved computer modelling on a rock from the African continent recovered from the bottom of the lithosphere, the outer part of the Earth between about 30 km and 250 km below the surface.

Mr Walsh said the dominant part of a continent was the part that you never see. 

“If you think of an iceberg – the visible part – if you just had an iceberg floating on the ocean surface it would tip over like a boat. This is like the keel of an iceberg,” Mr Walsh said. “We basically had a known starting composition of a rock, which is representative of the earth's mantle at an early time in the history of the earth before all the continents were formed,” Mr Walsh added. “We took that starting composition and modelled what would happen to it if it was progressively melted, and what would be left over. And that material is what forms the bulk of the roots of ancient continents that are still around today.”

Professor Balz Kamber, from QUT’s Faculty of Science, School of Earth and Atmospheric Sciences, and formerly from Trinity, said the aim of this research was to use a computer model to see how these deep roots might have formed.

“The model essentially predicts which minerals and melts will be present as you change the temperature of the mantle. So, it's a predictive tool you can compare with the composition of actual minerals and rocks,” Prof Kamber said.

Dr Emma Tomlinson, from Trinity’s School of Natural Sciences, added:

“Critically, it predicts the composition of the mineral garnet, and shows that melting at high pressure can produce the garnets like those that we see in natural rocks and as inclusions in diamonds. This research challenges the existing two-step shallow ‘melting and stacking’ explanation for mantle root formation.”

The piece of rock used for the advanced computer modelling was mined sometime between 1871 and 1914 and ended up in the “waste-pile” of the legendary Kimberley diamond mine, best known as “The Big Hole” – a combination open-pit and underground mine – in Kimberley, Northern Cape in South Africa.

The piece of rock they have modelled, garnet harzburgite, was brought to the surface in a kimberlite pipe.

“It is so exciting to see this preserved, it is extremely old - 3.3 billion years old. Probably the oldest rock most people will ever hold in their hands,” Professor Kamber said.

Mr Walsh said the study solved the conundrum of diamonds and the temperatures in which they formed, given a diamond will turn into graphite if heated up too much.

“Previously, it was believed that most of the ancient deep roots of continents would have been host to diamonds, and that these diamonds were destroyed over time, because the base of the continent is continually invaded and eroded by volatile rich melts and fluids, but yet, when we look at the rocks that contain diamonds, they must have been heated to massive temperatures,” Mr Walsh said.

“So why is it that it is exactly those rocks that experienced the highest temperatures that ended up having diamonds? Our work suggests that the high temperatures prepared the ground for future diamond formation, by producing reducing conditions favourable for diamond growth” said Dr Tomlinson.

“Because we can, for the first time, know what is missing from the cradle of the diamond, we can go hunt for it at the surface,” said Mr Walsh.

Professor Kamber said on the present-day earth the heat and temperature distribution in the mantle is not uniform.

“We have areas of relatively uniform mantle temperature, and areas where the mantle is a lot hotter. These are known as mantle plumes. And we have expressions of these in Hawaii and Iceland,” Professor Kamber said.

“What we're studying is the effect of ancient plumes – when much hotter plumes than we have now would have hit the base of a growing continent.”

This work is part of an ongoing collaboration between Prof Kamber and Dr Tomlinson, involving an Australian Research Council (ARC) Discovery Project grant at QUT and a European Research Council (ERC) funded project at Trinity.

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