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The Secret’s Out: Scientists Discover How Diamonds Formed In The Deep Roots Of Earth’s Ancient Continents

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Using a standard laptop computer and a seemingly insignificant piece of rock, retrieved from the discarded material of a diamond mine, two researchers from QUT have succeeded in resolving a long-standing geological puzzle concerning the formation of diamonds in the ancient continental roots of the earth.

The research findings, authored by QUT PhD student Carl Walsh, QUT Professor Balz Kamber, and Emma Tomlinson from Trinity College, Ireland, have been published in the esteemed academic journal Nature.

As part of his MSc research, Mr. Walsh employed computer modeling techniques to analyze a rock sample sourced from the African continent, which was extracted from the bottom of the lithosphere – the outer layer of the Earth lying between 30km and 250km beneath the surface. Mr. Walsh further explained that the most significant aspect of a continent was not visible to the naked eye.

“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,” he added.

“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.

“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.”

The objective of this research, according to Professor Kamber of QUT’s Faculty of Science, School of Earth and Atmospheric Sciences, was to utilize a computer model to investigate the mechanisms that led to the creation of these subterranean roots.

“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 added.

The rock fragment utilized for the advanced computer simulation was excavated between 1871 and 1914 and subsequently deposited in the “waste-pile” of the Kimberley diamond mine, famously known as “The Big Hole,” situated in Kimberley, Northern Cape, South Africa. The specimen subjected to the simulation, garnet harzburgite, was extracted from a kimberlite pipe and procured by Professor Kamber, an expert in petrology, which deals with the study of rocks and the conditions under which they originate. He carefully broke down the rock into a smaller size that could be conveniently transported.

“It contains a jumble of minerals that were entrained on the way up as they ripped through the base of the whole continent in a supersonic volcanic eruption – the likes of which we have never seen,” Professor Kamber added.

“The minerals in this rock sample are so badly hurt, they are screaming still today, they were absolutely smashed.

“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.”

According to Mr. Walsh, the research has successfully solved the puzzle of diamond formation and the corresponding temperatures required for this process, as diamonds can transform into graphite if subjected to excessive heat.

“But yet, when we look at the rocks that contain diamonds, they must have been heated to massive temperatures.

“So why is it that it is exactly those rocks that experienced the highest temperatures that ended up having diamonds?”

The research conducted by them presents a challenge to the prevalent explanation of the two-stage surface-level process of “melting and stacking.”

“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,” Walsh added.

“Our work suggests that actually this might not be the case, that diamonds are rare today – and were in fact always rare.”

“And that’s because we can for the first time know what is missing from the cradle of the diamond and we can go hunt for it at the surface.”

As per Professor Kamber’s statement, the current distribution of heat and temperature in the mantle of the Earth 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.

“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.”

Mr. Walsh has now traveled to Canberra to reproduce comparable rocks in the laboratory at the Australian National University’s Research School of Earth Sciences.

Source: 10.1038/s41586-022-05665-2

Image Credit: Getty

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