Talk about deep, dark secrets. Rare 鈥渦ltra-deep鈥� diamonds are valuable 鈥� not because they look good twinkling on a newlywed鈥檚 finger 鈥� but because of what they can tell us about conditions far below the Earth鈥檚 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 , some 300 kilometres north of Adelaide, in southern Australia. From there, they were sent to , a diamond expert at the University of Adelaide.
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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 鈥� a distance greater than that between Boston and Washington, DC.
Clues from the deep
鈥淭he vast majority of diamonds worldwide form at depths between 150 km and 250 km, within the mantle roots of ancient continental plates,鈥� says Tappert. 鈥淭hese diamonds formed in the Earth鈥檚 lower mantle at depths greater than 670 km, which is much deeper than 鈥榥ormal鈥� diamonds.鈥�
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 鈥� and now Australia.
鈥淒eep diamonds are important because they are the only natural samples that we have from the lower mantle,鈥� says , a geologist at the University of Bayreuth in Germany. 鈥淭his makes them an invaluable set of samples 鈥� much like the lunar rocks are to our studies of the moon.鈥�
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.
鈥極ddball鈥� 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.
鈥淒eep diamonds have always been treated like oddball diamonds,鈥� says Tappert. 鈥淲e don鈥檛 really know what their origin is. With the discovery of the ones in Australia we start to get a pattern.鈥�
Their geographic spread suggests that all these ultra-deep diamonds were formed in the same way: as the oceanic crust dived down beneath Gondwana 鈥� a process known as subduction 鈥� it would have dragged carbon down to the lower mantle, transforming it into graphite and then diamond along the way.
Eventually, kimberlites 鈥� volcanic rocks named after the town of Kimberley in South Africa 鈥� are propelled to the surface during rapid eruptions, bringing the gems up to the surface.
Surprisingly young
According to John Ludden of the , if the theory were proven true, it would mean the Eurelia diamonds are much younger than most diamonds are thought to be.
鈥淢any of the world鈥檚 diamonds are thought to have been sampled from subducted crust in the very early Earth, 3 billion years ago,鈥� says Ludden.
Yet Tappert鈥檚 theory suggests these diamonds would have been formed about 300 million years ago. 鈥淭his 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,鈥� Ludden told New 杏吧原创.
McCammon says Tappert鈥檚 theory is 鈥減lausible鈥� but just 鈥渙ne among possible models鈥�. She says not all deep diamonds fit the Gondwana model, but adds that the new gems 鈥減roved a concrete idea that can be tested by others in the community鈥�.
Journal reference: (vol 37, p 43)
