Diamonds unravel deep mysteries

Geology

New research into rare diamonds is helping University of Adelaide geologists to better understand the origins and structure of the earth before the world's continents were formed.

The research, being led by Professor John Foden and Dr Ralf Tappert in the School of Earth & Environmental Sciences, is being conducted in conjunction with Flinders Mines Ltd under an Australian Research Council (ARC) Linkage grant.

The project is studying the composition of diamonds found at Eurelia in South Australia's mid north. What makes these diamonds unique is that they contain traces of unusual minerals (magnesium-perovskite and ferropericlase) that are only stable together at great depths beneath the earth's crust.

"Most diamonds that are in the bulk of commercial production are often in what is considered to be the shallower part of the mantle beneath the crust, down to 200-250 kms. A lot of the diamonds that Flinders Mines are dealing with are from those sorts of depths," Professor Foden said.

"But there is a small group of diamonds, and they're very rare internationally, that seem to come from great depths - more than 650 kms beneath the crust. There's very little that we see at the Earth's surface that comes from those depths normally, unless it's in some changed state, so these diamonds are an interesting window into the deep earth.

"They're very interesting in terms of the science of where some diamonds originate and how they rise from deep in the earth," he said.

The ultradeep diamonds have been discovered in intrusions of an igneous rock called kimberlite, which has helped to bring the diamonds to the surface. Because the diamonds contain traces of minerals, their commercial value is extremely limited. However, as these are among only a handful of such diamonds in the world, their scientific value is greatly increased.

It's ironic, then, that in order to fully examine the trace minerals - known as "inclusions" - the researchers need to crush and burn the diamond samples in a laboratory.

Professor Foden, Dr Tappert and colleagues believe these diamonds were formed underneath the moving plates of the supercontinent Gondwana several hundreds of millions of years ago.

"The composition of the diamonds provides evidence that they grew deep beneath the Gondwanan supercontinent in oceanic rocks trapped in the collapsed Pacific plate," Professor Foden said.

"As the Pacific plate moved deep into the Earth's mantle under the Gondwanan supercontinent, it took with it these rocks, which over time hosted diamond growth. When the diamonds formed, they did so around other minerals that are normally only stable at the great pressures found at depths of 650 kms or more."

Professor Foden said these ultradeep diamonds, and others discovered in deposits in southern Africa and South America, were all formed along the rim of Gondwana.

"There are only about 100 deep diamonds known in the world, and these are unique in Australia," he said. "These diamonds are also found in higher abundance in South Australia than anywhere else in the world, which makes them perfect for us to research."

The research into ultradeep diamonds and their connections with Gondwana's plate movements will be published next month in Geology, the journal of the Geological Society of America.

Story by David Ellis