Earth’s rarest diamonds formed in pockets of liquid metal

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A giant diamond

Some of the most prized diamonds on Earth are unusually clear, exceedingly rare, and often extraordinarily large. Researchers have long wondered how such gems formed, but they’ve been hard to study because they’ve typically ended up on ring fingers rather than under a microscope. Now, a new analysis of imperfections trapped within the diamonds provides the first direct evidence that they were forged within blobs of liquid metal hundreds of kilometers below Earth’s surface.

Previous analyses had suggested that scenario, says Graham Pearson, a geochemist at the University of Alberta in Edmonton, Canada, who was not involved in the new study. But they weren’t definitive. The new studies, he says, “go a long way toward providing an explanation of where these diamonds form.”

The gems the team studied are a subset of so-called type II diamonds. They have a low nitrogen content, which makes them very clear. Scientists rarely get access to large numbers of such diamonds. But Evan Smith, a geologist at the Gemological Institute of America (GIA) in New York City and co-author of the new study, had an inside track, as GIA often processes thousands of gems each day—including the large, valuable stones. He and his colleagues analyzed 53 of these diamonds, particularly their inclusions, small chunks of material trapped inside. Many of those tiny blobs were long thought to be bits of graphite—like diamond, another form of pure carbon—and were thus cut away and discarded by jewelers. But inclusions in 38 of the 53 diamonds, or about 72%, were a graphite-coated mix of metal-rich minerals that also contained an alloy of iron and nickel. Other substances in those inclusions, including hydrogen and methane, suggest that the imperfections were once a molten mix of iron, nickel, carbon, sulfur, and various trace elements, the researchers report today in Science.

Inclusions within the other 15 diamonds contained silicate minerals such as garnet. That’s likely a sign that those gems formed at depths between 360 and 750 kilometers, in particular because at pressures higher than those found at 750 kilometers garnet minerals aren’t stable. Later, those gems were carried to the surface in sudden eruptions via processes scientists don’t yet fully understand. Those eruptions leave behind tubular deposits, called kimberlites, which are the ultimate source of most of Earth’s diamonds.

Because the inclusions were trapped within the diamond as it formed and have been physically and chemically isolated since then, they are a window into the environment in which the gem crystallized. “The diamonds have delivered these well-preserved materials to us at the surface,” says study co-author Steven Shirey, a geochemist at the Carnegie Institution for Science in Washington, D.C. “They’re a classic example of how the tiniest bits of material can tell us big things about our planet.”

For example, the presence of hydrogen and methane are clues that the chemical environment of the fluid in which the diamond crystallized was one in which the metal atoms could easily gain electrons and disengage from carbon atoms. That, in turn, generated molten metal and free carbon that could then crystallize to form diamond. Such reactions are likely taking place in many regions at depths between 410 and 660 kilometers, the team suggests. Besides having the right mix of ingredients and pressures, this range defines a well-known transition zone within the mantle, the 2900-kilometer-or-so-thick layer of slowly circulating material that lies between Earth’s crust and its outer core of molten iron.

Although the new findings most directly apply to a large subset of type II diamonds, they may provide insights into how diamonds in general are created. Indeed, says Smith, the mantle is full of iron-rich minerals under high pressure—and large swaths of it are likely peppered with diamonds just waiting to be blasted toward the surface in gem-studded eruptions. And though the team’s study won’t make diamonds any cheaper or easier to find, they offer an interesting tale better delivered with drink in hand at a cocktail party rather than from one knee.

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