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The team conducted advanced computer modelling on a piece of ancient rock in an attempt to better understand how the Earth’s temperature influenced its formation in the lithosphere.
The lithosphere is the scientific term for the outer part of the Earth that is located between about 30 km and 250 km below the surface.
The rock they used for the computer modelling was not a standard stone, however. It was an ancient rock from the waste pile of a diamond mine. Its scientific name is garnet harzburgite.
The team found that a diamond will turn into a graphite if it is heated up too much.
Prof Balz Kamber, from Queensland University of Technology’s (QUT) Faculty of Science, School of Earth and Atmospheric Sciences, who used to work with TCD, was one of the lead investigators.
Kamber explained that the computer model worked by predicting which minerals and melts are present as the temperature of the mantle is changed. “So, it’s a predictive tool you can compare with the composition of actual minerals and rocks.”
According to Carl Walsh, lead author of a paper informed by the study, the reason it is so challenging to find out how the rock was formed is because it was located so far deep down in the lithosphere.
He thinks the discovery could have a wider impact on geologists’ understanding of how certain rocks were formed.
“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,” he said.
Using a computer, Walsh said the team “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.”
Walsh and his colleagues from Trinity College Dublin (TCD) and Australia’s Queensland University of Technology (QUT) co-authored a paper based on their research that is published in the academic journal Nature.
“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 said.
“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?”
The rock they used was mined sometime between 1871 and 1914 and ended up in the “waste-pile” of a South African diamond mine known as the Kimberley diamond mine, or ‘The Big Hole’. It was given the Big Hole nickname because it is a combination of an open-pit and an underground mine. The garnet harzburgite was brought to the surface in a kimberlite pipe.
“It is so exciting to see this preserved, it is extremely old – 3.3bn years old. Probably the oldest rock most people will ever hold in their hands,” Kamber said.
According to Walsh, the discovery means geologists can now “know what is missing from the cradle of the diamond” and they can “go hunt for it at the surface”.
The project is part of an ongoing research collaboration between TCD and QUT. Dr Emma Tomlinson, from TCD’s School of Natural Sciences, who worked on the team, said, “Our work suggests that the high temperatures prepared the ground for future diamond formation, by producing reducing conditions favourable for diamond growth.”
Blathnaid O’Dea
This article originally appeared on www.siliconrepublic.com and can be found at: https://www.siliconrepublic.com/innovation/trinity-geological-mystery-graphite-diamonds
