Geology and Oceans: "Cloudy" Diamonds and Ancient Sodium Chloride

Diamonds are known to be beautiful gems, formed under high pressure deep underground, and, of course, a girl's best friend. However recent findings suggest there is more to them than meets the eye, according to a release.

Microscopic, uglier, versions of this valuable stone found in Canada's Northwest Territories indicate that ancient seawater was involved in their making. This water, streaming into the deep roots of the continent, was transported by plate tectonics. 

An international team of scientists from Canada, the U.S. and the U.K. made findings regarding this recycling of surface materials into deeper parts of the Earth via plate tectonics, and they are adding that to last year's discovery of the rare mineral ringwoodite by the University of Alberta and its indication of a large amount of water trapped more than 500 kilometres underground. The recent research has been published in Nature.

"With the ringwoodite discovery, we showed there is a lot of water trapped in really deep parts of the Earth, which probably all came from recycling ocean water," Graham Pearson, professor in the U of A's Department of Earth and Atmospheric Sciences and Canada Excellence Research Chair in Arctic Resources, said in a statement. "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. From there it has pumped that brine into the bottom of the root beneath the Northwest Territories, and it's made the diamonds."

The so-called "low-quality" diamonds, found roughly 200 km beneath Earth's surface, are covered in a coat of cloudy material. While all diamonds are formed from fluids, only these less-attractive, coated stones still contain traces of their scientifically valuable source fluids, the release said. 

"[The fluids in the coats] are sky-high in sodium and potassium and chlorine, and it's very difficult to get that stuff from the Earth's normal mantle," explained Pearson in the release. "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."

Pearson further explained that the seawater captured in the rock's coating is likely to have been trapped in a large portion of the Earth's oceanic crust, which was then subducted beneath North America some hundreds of millions of years ago. Diamonds are assumed to have then crystalized from a chemically diverse range of fluids that were created by the interaction of these seawater brines with the overlying mantle rocks. They have since been brought back to the Earth's surface by an erupting host volcanic rock known as a kimberlite. 

"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," said Pearson in a statement. "It literally carries pristine bits of material from right where it came from, essentially unchanged."

The diamonds found by researchers appear to be only a few hundred million years old, which is significantly younger than high-quality gem diamonds. To explain this age difference, scientists suggest that the two types of diamonds actually formed through similar processes, and then the fluid-rich stones eventually transformed 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 in the release. "There are more processes that form diamonds at a whole range of different times than we thought possible."

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