The Eurelia diamonds are the latest ultra-deep diamonds to have been found (Image: R Tappert)

Talk about deep, dark secrets. Rare “ultra-deep” diamonds are valuable – not because they look good twinkling on a newlywed’s finger – but because of what they can tell us about conditions far below the Earth’s crust.

Now a find of these unusual gems in Australia has provided new clues to how they were formed.

The diamonds, which are white and a few millimetres across, were found by a mineral exploration company just outside the village of Eurelia, some 300 kilometres north of Adelaide, in southern Australia. From there, they were sent to Ralf Tappert, a diamond expert at the University of Adelaide.


Tappert and colleagues say minerals found trapped inside the Eurelia diamonds could only have formed more than 670 kilometres (416 miles) beneath the surface of the Earth – a distance greater than that between Boston and Washington, DC.

Clues from the deep

“The vast majority of diamonds worldwide form at depths between 150 km and 250 km, within the mantle roots of ancient continental plates,” says Tappert. “These diamonds formed in the Earth’s lower mantle at depths greater than 670 km, which is much deeper than ‘normal’ diamonds.”

Fewer than a dozen ultra-deep diamonds have been found in various corners of the globe since the 1990s. Sites range from Canada and Brazil to Africa – and now Australia.

“Deep diamonds are important because they are the only natural samples that we have from the lower mantle,” says Catherine McCammon, a geologist at the University of Bayreuth in Germany. “This makes them an invaluable set of samples – much like the lunar rocks are to our studies of the moon.”

The Eurelia gems contain information about the carbon they were made from. Their heavy carbon isotope signatures suggest the carbon was once contained in marine carbonates lying on the ocean floor.

‘Oddball’ gems

Location, though, provides researchers with a common thread for the Brazilian, African and Australian deep diamonds, which could explain how they were born. All six groups of diamonds were found in areas that would once have lined the edge of the ancient supercontinent Gondwana.

“Deep diamonds have always been treated like oddball diamonds,” says Tappert. “We don’t really know what their origin is. With the discovery of the ones in Australia we start to get a pattern.”

Their geographic spread suggests that all these ultra-deep diamonds were formed in the same way: as the oceanic crust dived down beneath Gondwana – a process known as subduction – it would have dragged carbon down to the lower mantle, transforming it into graphite and then diamond along the way.

Eventually, kimberlites – volcanic rocks named after the town of Kimberley in South Africa – are propelled to the surface during rapid eruptions, bringing the gems up to the surface.

Surprisingly young

According to John Ludden of the British Geological Survey, if the theory were proven true, it would mean the Eurelia diamonds are much younger than most diamonds are thought to be.

“Many of the world’s diamonds are thought to have been sampled from subducted crust in the very early Earth, 3 billion years ago,” says Ludden.

Yet Tappert’s theory suggests these diamonds would have been formed about 300 million years ago. “This may well result in a revision of exploration models for kimberlites and the diamonds they host, as to date exploration has focused on very old rock units of the early Earth,” Ludden told New Scientist.

McCammon says Tappert’s theory is “plausible” but just “one among possible models”. She says not all deep diamonds fit the Gondwana model, but adds that the new gems “proved a concrete idea that can be tested by others in the community”.

Journal reference: Geology (vol 37, p 43)