Individual diamonds dated for the first time

VU Earth Scientists were able to date precisely, for the first time, a characteristic type of diamond using individual garnet mineral inclusions to show that half of them were billions of years younger than previously thought.

09/21/2017 | 2:21 PM

Janne Koornneef, Michael Gress and Gareth Davies publish this today in their paper ‘Archaean and Proterozoic diamond growth from contrasting styles of large-scale magmatism’ in Nature Communications.

Diamonds are categorised by the mineral inclusions trapped within the carbon crystal structure that give clues about the conditions under which they formed. Harzburgitic diamonds, the subject of the study, form associated with very large amounts of melting of the deep Earth.

Processes still poorly understood
Despite decades of research the processes that control the movement of large amount of carbon in the Earth’s interior are poorly understood. We know that carbon travels from the interior in gases that escape from volcanoes. We also know that carbon is returned to the interior of the Earth when plates collide and sediments and other rocks are transported to great depth.

However, how these processes have changed over Earth’s history associated with the cooling of the Earth’s interior is unknown, as are the exact processes that lead to diamond formation. “We do not even know how often diamonds are formed in one part of the Earth, nor the scale over which diamonds are formed in a single event, m3 or km3 or 100 km3”, says first author of the study Janne Koornneef, who was recently awarded an ERC Starting Grant. “By dating diamonds and studying the carbon they contain, we have the potential to study how the Earth’s carbon cycle has changed over geological time.”

Diamonds less ancient than previously assumed
This new research reports the ability to date precisely for the first time parts of an individual diamond with a characteristic type of mineral inclusion. The VU scientists demonstrated that earth scientists, who previously used 100’s of diamonds to try and understand when and how diamonds were formed, had misinterpreted their data. Professor Gareth Davies: “Their large diamond samples must have mixed different generations. By working on individual diamonds we were able to distinguish multiple diamond forming events, 2 of which we dated.”

The VU researchers demonstrate that these characteristic diamonds from one mine formed in events separated by billions of years (3,000 million and 1,150 million). Previously they were assumed to be primarily older than 3 billion years. The researchers also show that the two generations of diamonds formed in totally different geological environments suggesting that, as the Earth has cooled, the younger diamonds can only be formed under very special geological conditions where hot mantle from the interior rises towards the surface.

Better study the interior of the Earth
With the potential to date individual diamonds it is possible in the future to study how the sources of carbon in the interior of the Earth have changed over time. This is expected to help researchers better understand the evolution of the early Earth and perhaps the geological and biological processes that led to our present day habitable planet.

The different ages and geological environments indicated for the diamond formation mean that we have to change our ideas of when and how diamonds are formed. Koornneef: “This may mean that industry decides to modify their exploration strategies in the search for new diamond mines. Although today there is a ready supply of diamonds for industry and the jewellery markets, current resources only have a life time of a few decades.”

Davies: “Previously the harzburgitic diamonds were assumed to be very ancient (> 3 billion years) and so the exploration companies looked for ancient geological events as the cause of diamond formation. Our work shows that they need to include an extra 2 billion years of Earth history. That’s almost half the age of the Earth!”