World's biggest diamonds may unlock mysteries of Earth's mantle, say geologists

The world's largest diamonds aren't just more valuable for jewelers: they also hold surprising clues about the composition of the Earth's mantle, says a new study.

Stefano Rellandini/Reuters
A high-quality diamond is seen in a jewelry shop in Milan on Oct. 18, 2016.

A new study of the world’s largest diamonds has shown that they're not only more valuable for jewelers, but also for geologists by offering an invaluable glimpse at the makeup of the Earth’s interior, hundreds of miles below the surface.

Led by the Gemological Institute of America (GIA) and published in the journal Science, the study drew on previous knowledge that larger diamonds are fundamentally different from their smaller counterparts in composition and structure, and explored how and where larger diamonds form that causes them to differ.

"Some of the world's largest and most valuable diamonds, like the Cullinan or Lesotho Promise, exhibit a distinct set of physical characteristics that have led many to regard them as separate from other, more common diamonds," Wuyi Wang, GIA's director of research and development, and an author of the study, said in a press release. "However, exactly how these diamonds form and what they tell us about the Earth has remained a mystery until now."

Diamonds are formed deep below the Earth’s crust in the mantle and are brought to the surface during volcanic eruptions, bringing with them tiny flecks of metal and minerals trapped inside. While these “inclusions” are cut out to sell the jewels, they offer scientists a unique look at the composition of the Earth’s interior.

"You really couldn't ask for a better vessel to store something in," Evan Smith, a diamond geologist at the GIA and an author of the study, told NPR. "Diamond is the ultimate Tupperware." 

The GIA procured eight fingernail-sized chunks of left-over diamond scraps, which the research team cut open and ground up to look at using microscopes, lasers, magnets, and electron beams.

They found that the inclusions contained a mixture of iron, nickel, carbon, and sulfur, encased in a thin layer of fluid methane and hydrogen. The metallic inclusions indicated that the diamonds were formed under extreme pressure, in oxygen-deprived patches of liquid metal.

Furthermore, some samples also contained mineral inclusions that suggested the large diamonds form at much greater depths than smaller ones – as deep as 200 to 500 miles below the surface, Dr. Smith told NPR, while smaller diamonds form at roughly 90 to 120 miles down.

Researchers once believed that the Earth's mantle was a pretty uniform mix of oxygen-rich rocks. But the study of these huge diamonds, born in oxygen-deprived patches, suggests that theory needs rethinking. 

"It further complicates things, but it makes us have to think more deeply about what's going on in the planet because ultimately this does affect what we see up on the surface," Kanani Lee, a mineral physicist at Yale University, told NPR.

The researchers say it is unclear if these pockets still exist, given the diamonds' age: They range from 100 million years old to approximately 1 billion years old. But they also have a broad geographic range, having been found on several continents, suggesting that the pockets are an important piece of geological history. 

"It tells you that we have to refine our thinking about how the planet – whether it's Earth or any other planet – evolves with time. And that our simple pictures may not be good enough anymore if we can't explain these features," Dr. Lee added.

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