28 February 2014

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.