Secrets told by diamonds are a geologist’s best friend
Mags Pullen
The carbon trapped in diamonds that formed deep beneath the Earth’s surface brings new insight into how the planet works, geologists told the British Science Festival in Bradford today.
An international collaboration headed by Professor Michael Walters and Dr Simon Cohen from the University of Bristol has analysed diamonds formed 700km below the Earth’s surface and discovered that crust materials are cycled into much deeper layers than previously thought.
The Earth’s crust comprises a series of floating ‘tectonic plates’ moved by currents in the liquid mantle beneath. When an oceanic plate margin is pushed beneath a continental plate, the sea floor, rich in minerals and organic material, is forced down into the mantle. Here the high temperatures and pressures create conditions that form diamonds, later brought to the surface by volcanic activity.
A diamond’s composition is defined by its depth of formation and the conditions experienced during its uplift. Most form in the upper mantle, 150-450 km below the surface. Walter and Cohen needed crystals from the lower zone, which are rare.
Flaws in diamonds, known as ‘inclusions’, are caused by mineral-rich ‘dirt’ trapped in the crystals as they form. Coincidence brought Professor Walter into contact with a retired couple from the Rio Tinto ‘Junia 5’ diamond mine in Brazil, who persuaded the company to let the scientists look for ‘super-deep’ dirty diamonds amongst their thousands of reject stones.
‘Super-deep diamonds are really grungy, so they are fantastic for studying the inaccessible parts of the earth’ explained Walter. By comparing their diamonds to crystals created under laboratory conditions, Walter and Cohen determined the depth at which each formed and how quickly it travelled to the surface.
Analysis of the radioactive carbon isotopes in the inclusions then showed the origin of the starting material, which came from the ocean floor.
These techniques liberated the diamond’s ‘life story’ from the stone. Four of the six diamonds formed in the lower mantle from biologically-derived carbon, and their rapid journey to the surface proves the existence of upwellings known as mantle plumes.
‘Every batch of Junia diamonds proves something unexpected’ said Kohn. ‘As we investigate them in more detail, they continue to reveal their secrets’.
The carbon trapped in diamonds that formed deep beneath the Earth’s surface brings new insight into how the planet works, geologists told the British Science Festival in Bradford today.
An international collaboration headed by Professor Michael Walters and Dr Simon Cohen from the University of Bristol has analysed diamonds formed 700km below the Earth’s surface and discovered that crust materials are cycled into much deeper layers than previously thought.
The Earth’s crust comprises a series of floating ‘tectonic plates’ moved by currents in the liquid mantle beneath. When an oceanic plate margin is pushed beneath a continental plate, the sea floor, rich in minerals and organic material, is forced down into the mantle. Here the high temperatures and pressures create conditions that form diamonds, later brought to the surface by volcanic activity.
A diamond’s composition is defined by its depth of formation and the conditions experienced during its uplift. Most form in the upper mantle, 150-450 km below the surface. Walter and Cohen needed crystals from the lower zone, which are rare.
Flaws in diamonds, known as ‘inclusions’, are caused by mineral-rich ‘dirt’ trapped in the crystals as they form. Coincidence brought Professor Walter into contact with a retired couple from the Rio Tinto ‘Junia 5’ diamond mine in Brazil, who persuaded the company to let the scientists look for ‘super-deep’ dirty diamonds amongst their thousands of reject stones.
‘Super-deep diamonds are really grungy, so they are fantastic for studying the inaccessible parts of the earth’ explained Walter. By comparing their diamonds to crystals created under laboratory conditions, Walter and Cohen determined the depth at which each formed and how quickly it travelled to the surface.
Analysis of the radioactive carbon isotopes in the inclusions then showed the origin of the starting material, which came from the ocean floor.
These techniques liberated the diamond’s ‘life story’ from the stone. Four of the six diamonds formed in the lower mantle from biologically-derived carbon, and their rapid journey to the surface proves the existence of upwellings known as mantle plumes.
‘Every batch of Junia diamonds proves something unexpected’ said Kohn. ‘As we investigate them in more detail, they continue to reveal their secrets’.
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