[岭杉]

By Margaret Munro, Postmedia News January 18, 2012 12:08 PM

A Canadian-led team's discovery of a volcanic process that carries diamonds to the Earth's surface may one day help miners refine the search for the valuable gems.


A Canadian-led team has uncovered a volcanic process that carries diamonds to the Earth's surface like "clients" in a taxi.

The trip takes less than two days and brings the gems up from depths of 120 kilometres, says lead researcher Kelly Russell, a volcanologist at the University of British Columbia.

The findings, reported in the journal Nature on Thursday, may one day help miners hone the search for diamonds. But for now Russell is tickled to be able to explain how the ancient crystals make it to the surface.

"We're providing an understanding of the taxis that bring the diamonds to the Earth's surface," Russell said in an interview.

It is a rough ride.

His research indicates rocks containing diamonds, which formed deep in the Earth millions of years ago, are picked up by hot, foaming molten rock rising up inside the planet. They are carried along as the molten magma forces its way up and then explodes at the surface.

The magma can carry "seemingly prohibitive loads", says Russell, noting that the material reaching the surface can be 45 per cent "cargo." The boulders, which can contain diamonds, can be up to 70 centimetres across.

He and colleagues wanted to know "how in the heck" the rising magma could carry such big loads, and travel to the surface fast enough to preserve the diamonds.

Russell started to run what he calls "cook-and-look" experiments in his UBC lab. After heating rock samples to a red-hot 1,000 C, he sprinkled it with a silica-rich mineral he suspected might be helping drive the process underground.

"All of sudden there was this vigorous foaming," Russell recalls. "We all went, 'ah-hah.' "

Detailed experiments confirmed the reaction between rising magma and silica-rich minerals can produce enough gas and foam to enable magma to carry the loads of diamond-bearing rocks upwards.

"The foaming keeps it accelerating and moving toward the Earth's surface," says Russell.

He says the findings may eventually help miners refine the search for diamonds, which are found in geological structures called kimberlite pipes. The pipes, which can measure up to 150 metres across and 800 metres deep, form when the foaming magma blows through the Earth's crust. They can contain diamond-bearing rocks, depending on where and how quickly the magma travelled through Earth's upper layers.

"We may be able to say there are critical criteria that tell you whether the kimberlite came up fast enough to carry and preserve diamonds," says Russell. "Or, if there are signs it was slower ascending and don't even bother."

The kimberlite pipes mined for diamonds in Canada formed millions of years ago, but Russell says one site in Africa may be only 10,000 to 15,000 years old.

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