‘Ugly’ NWT diamonds suggest gems formed by ancient seawater

‘Ugly’ NWT diamonds suggest gems formed by ancient seawater
A new study on diamonds in the NWT indicates the gems contain elements thought to originate from ancient seawater trapped deep within the earth.Photo: Dave Brosha.

Diamond deposits in the Northwest Territories could have been formed as a result of ancient seawater streaming deep into the earth via plate tectonics, according to a new study by scientists in Canada, the U.S. and U.K.

The research, published last week in the Nature journal, builds off a previous study, which found water trapped more than 500 km underground to be a result of ocean water recycling.

“This new study really highlights that process. It clearly demonstrates that ocean water in this case has been subducted via an old oceanic slab into a slightly shallower but still very deep part of the Earth,” said Graham Pearson, a professor of Earth and Atmospheric Sciences at the University of Alberta and Canada Excellence Research Chair in Arctic Resources.

“From there it has pumped that brine into the bottom of the root beneath the Northwest Territories, and it’s made the diamonds.”

While all diamonds are known to be formed from fluids, it’s the “ugly” ones – the low-quality, cloudy gems – that make the link to ancient seawater.

According to Pearson, the less attractive stones with a cloudy coat are “sky-high in sodium and potassium and chlorine,” which is very difficult to get from the Earth’s mantle.

“It’s a big mystery: where does that come from? Well, we can show that maybe the most sensible place for it to come from is seawater, which is basically a sodium chloride solution,” he said.

Pearson said the seawater likely became trapped in a massive slab of oceanic crust that was subducted beneath the continent of North America hundreds of millions of years ago. The interaction of the water with the overlying mantle rocks then produced a chemically diverse range of fluids from which diamonds were born and could be carried back to the Earth’s surface via volcanic rock known as kimberlite, bringing evidence of deep underground fluids with them.

“The beauty of the diamond is that because it’s such a robust capsule, it protects the material that it trapped at that depth from any subsequent change,” Pearson said. “It literally carries pristine bits of material from right where it came from, essentially unchanged.”

While high-quality diamonds are estimated to have been formed three to 3.5 billion years ago, the lower quality, fluid-rich diamonds appear to be just a few hundred million years old by comparison. Pearson and his team plan to do further studies to test their theory that the two types of diamonds were formed by similar processes, but that fluid-rich stones turned into gem diamonds over time.

“What we appear to be finding more and more is that the standard model that used to be around – diamonds are only formed in very ancient times, 3.5 billion years ago, by a very specific process – is not true,” says Pearson. “There are more processes that form diamonds at a whole range of different times than we thought possible.”

Scientists believe a greater understanding of how diamonds form can be used to develop exploration models used to find future deposits, which could be of great value to the NWT, the primary source of diamonds in Canada.

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