A precious Antarctic window to the mantle

17 January 2014

Geologists have discovered the first Antarctic kimberlite, an innocuous-looking volcanic rock precious to scientists and jewelers alike.

"Kimberlites are the main host of diamonds," says Dr Greg Yaxley from the Research School of Earth Sciences.

"Almost all diamond mines are kimberlite mines, with the notable exception of the Argyle mine in North Western Australia."

"They are quite rare, but are distributed across the different continents. Until now, they hadn't been found in Antarctica."

In the late 80s, Yaxley's colleague, Dr Geoff Nicholls, was studying metamorphic rocks at the foot of the Prince Charles Mountains in Antarctica when he stumbled across an unusual patch of rocks adjacent to the Lambert glacier.

"They were different and interesting so Geoff sampled them. He didn't know what they were at the time. It wasn't until much later we realised they were kimberlite," says Yaxley, who helped to identify the rocks almost 25 years after they were found.

In addition to the gem-carrying capacity of these rocks, kimberlites are precious to geologists too.

"Kimberlites are samples of the deep mantle. They form by partial melting of the Earth's mantle at least 200 kilometres down, and are brought to the surface in a violent process driven by volatile release. This creates a diatreme or carrot-shaped body sticking down into the Earth's crust, with a very small, round surface expression, making them hard to find.

"If we understand the chemical composition and characteristics of kimberlites, and the processes that form them, we can learn what the mantle is like down that deep. Kimberlites are a window to the mantle."

Yaxley says not to expect a mining boom in environmentally-protected Antarctica any time soon - there is not always a diamond in the rough.

"If you find a kimberlite, you get excited about it but the chances of it being diamondiferous are actually quite small, only around 10 per cent."

"You can spend your whole life mining kimberlite and never see a diamond, quite easily."