Diamonds form from ancient, underground seawater, study suggests

Microscopic, ugly diamonds from the Northwest Territories are illuminating how diamonds are made — and pointing to an unexpected helper in the process.

Diamonds from Northwest Territories preserve droplets for hundreds of millions of years

This diamond has a gem-quality core and fluid-rich 'coat.' The coat is studded with millions of tiny droplets of saltwater that have been trapped inside for hundreds of millions of years. (Anetta Banas)

Microscopic, ugly diamonds from the Northwest Territories are illuminating how diamonds are made — and pointing to an unexpected helper in the process. A new study suggests they formed from ancient seawater trapped deep below the surface of the Earth.

Most of us know that diamonds form deep underground under high pressure, but beyond that scientists have been fuzzy on the details.

They do know that diamonds form like other crystals — they crystallize from a fluid, like rock candy from a solution of sugar dissolved in water.

But they were never sure what that fluid was.

University of Alberta geochemist Graham Pearson holds a diamond used in a previous study that found oceans' worth of water deep inside the Earth. He was a co-author of the new study that provides more evidence of how water is recycled deep within the Earth. (University of Alberta)

Now, a group of U.S., U.K. and Canadian scientists led by Yaakov Weiss at Columbia University in New York think they have good evidence that a key component of the fluid that produced at least some kinds of diamonds was ancient seawater, trapped 200 kilometres beneath the surface of the Earth.

"I think it really helped to get the diamond forming reaction going," says Graham Pearson, a University of Alberta geochemist who co-authored the paper, published today in the journal Nature."We would argue having some seawater and brine helps formation because it's a very reactive fluid."

He acknowledged that the idea is a "fairly bold conclusion" but that researchers have lots of evidence to back it up.

That evidence comes from 11 diamonds from the Ekati mine in the Northwest Territories.

The diamonds provided to the researchers by Dominion Diamonds aren't fit for a ring by any stretch of the imagination — they're far less than a millimetre wide and "fibrous" or cloudy.  Scattered throughout the crystal are droplets of fluid — millions of them, in some cases.

'Caught in the act'

Fluids typically get trapped in minerals when they crystallize very quickly from solution, usually when they cool suddenly. The liquid trapped inside tends to be the solution that they crystallized from. Sudden cooling doesn't just trap impurities — it also produces much smaller crystals than gradual cooling. (This applies to making rock candy also.)

"It's really diamond formation caught in the act," Pearson told CBC News in a phone interview from Greenland, where he was on a research trip.

The researchers used a technique called spectroscopy that involved shining infrared light into the pockets and looking for the chemical signature of different substances. They found strong signals for carbonate (not surprising, since diamonds are made of carbon) and water.

The scientists then used lasers to vapourize the diamond, including everything trapped in the bubbles, and separated out and analyzed all the trace elements and isotopes in the sample.

A solid impurity inside a diamond contains minerals that grew from fluid trapped in the diamond when the diamond was cooled. (Yaakov Weiss)

What they found was very high concentrations of sodium and chlorine — the main components of the salt dissolved in seawater. And when they looked at the pattern of the kinds of strontium in the sample, it was "very similar to dissolved strontium in ancient seawater several hundred million years ago," Pearson said.

The researchers couldn't come up with any other explanation for all the salt, he added.

"There's nowhere really in the deepest parts of the earth that are obvious sources of all that sodium and chlorine."

Recycled water

That led the researchers to conclude that the source of the fluid was ancient seawater pushed under the Northwest Territories with nearby tectonic plates, whose edges are constantly subducting or moving into the Earth's crust beneath other plates. The seawater would have interacted with carbon-containing rocks to generate diamonds.

"The broader significance is that it provides now another key element in the picture of how water is cycled and carbon is cycled in the Earth," said Pearson. "This is really more evidence of recycling of fluid of water within the Earth."

While some diamonds contain fluids with other substances in them besides salt, they all appear to have started off as a brine solution that reacted with other minerals, Pearson said.

What remains a mystery is the relationship between the small, dirty diamonds that crystallize from seawater and the big, shiny gem-quality diamonds prized for earrings and engagement rings.

Pearson said that one possibility is that gem quality diamonds form gradually through the slow metamorphosis of fluid-rich diamonds. Some evidence for this is the fact that there is a gem-quality centre inside many fluid-rich diamonds.

Steven Shirey, a research scientist at the Carnegie Institution of Washington who has been studying diamonds for nearly two decades, but wasn't involved in the new study, says he thinks the findings will change the way people think about the way fibrous diamonds grow.

He added that many scientists now think that gem-quality diamonds grow from the same fluid, but more slowly.

Shirey spoke to CBC News from a geochemistry conference where he said the new study was causing quite a stir.

Daniel Howell is a research associate at the University of Bristol who has done research on diamond formation but wasn't involved in the new study either. He said it builds a strong case for diamond-forming fluids starting off very salty and thinks the conclusions about diamond formation make sense for the Canadian diamonds used in the study.

But whether they apply to other kinds of diamonds in other parts of the world "remains to be seen," he said in an email to CBC News.

"Extensive work is still required to understand how these fluids are involved in mobilizing carbon in the deep Earth, and how diamonds are directly recording this."


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