The other ‘Antarctic ice’
ARC-funded research has revealed a new occurrence of one of Earth’s most precious rocks in the heart of its most pristine wilderness.
Diamonds are not only the quintessential precious stone of the jewellery industry, but are also of interest to geologists as one of the rare minerals that occur toward the very deep reaches of the earth’s interior, forming only at depths over about 140km.
The peculiar hardness of diamond is due to the intense pressures from overlying rock found at these depths which compress the diamond’s constituent carbon atoms into a tight cubic structure.
Diamonds are rare in surface rocks because an enormous volcanic eruption has to occur, transporting them upwards from their deep womb in a violent stream of lava—and for diamonds to survive this hot wild journey to the surface, they have to be transported quickly, as the depressurisation can easily cause them to transform into worthless forms of graphite. When the lava eventually cools the rare rocks formed by this activity are called kimberlites, and were first described for their occurrence at the famous diamond mining town of Kimberley in South Africa. About 10% of South African kimberlites are found to yield economic diamond deposits, so the recent discovery of the same kind of kimberlite in the last pristine continent on earth—Antarctica—has ignited much speculation as to what this might mean for the current international ban on mining in the continent.
“I would protest in the streets against mining in Antarctica in any form,” said Dr Greg Yaxley, lead author of the first paper to identify the Antarctic kimberlites, published in the December 2013 edition of Nature Communications
Professor Yaxley, from the Australian National University, has recently completed a four-year ARC Future Fellowship and is the recipient of a 2014 Discovery Projects grant to study the Earth’s deep carbon cycle. His research team has analysed the Antarctic kimberlites to determine their origins as part of a vast province which formed when all the southern continents and India formed the ancient supercontinent, Gondwana.
This is the same kimberlite province which produced the rich diamond fields of South Africa, and although no diamonds have been found in the three Antarctic samples analysed so far “only a few tenths of a carat per ton of rock might be required for a deposit to be economic,” Dr Yaxley said.
These samples date from the rifting of the Indian subcontinent from Gondwana, as it began to drift northwards towards the rest of Asia about 120 million years ago. Although the exact processes are still poorly understood, such major tectonic upheavals produce rifts which extend deep into the earth, igniting the distinctive kimberlite volcanoes which can convey diamonds to the surface.
“The volcanoes that produce kimberlites are like nothing presently seen on Earth, fortunately…as they must have been incredibly violent and destructive.”
The evidence of this ancient burst of extreme volcanism is now scattered over four continents. On Antarctica the evidence occurs in rocks exposed on a remote mountain beside the world’s largest glacier—the Lambert glacier—but knowledge of the true surface extent of these kimberlites in Antarctica is still vague.
“Co-author Dr Geoff Nichols originally collected these rocks when he was still studying for his PhD in the late 1980s. He thought they looked interesting and so he sent them to us for analysis,” Dr Yaxley said.
“They then sat in a drawer for a while before we got to look at them properly and were able to positively identify them as a kimberlite. But to my knowledge no Australian geologists have been back to the site for some years.”
Dr Yaxley says that Russian scientists have expressed interest in further examining the samples to see if they contain any diamonds.
“The chances of this being an economic grade deposit are remote...but still the interest from the press has been phenomenal. Following publication of the article in Nature Communications, I have spoken on ABC Radio, and the article has been featured by the BBC, the London-based Financial Times and even Der Spiegel. The hits on Google ran to eight pages... in all my years of research I have never seen anything like the interest generated by this story!”
This Australian study demonstrates the power of research to drive public discussion and fire imagination—for our collective consciousness is easily captured by the revelation that there may be two kinds of ice in Antarctica.
For more information please contact Dr Greg Yaxley.
Image: Geoff Nichols at the site of the samples, looking from the slopes of Mt Meredith out across the Lambert Glacier. Image courtesy: Dr Greg Yaxley.