The planet’s carbon cycle may not go as deep as scientists have thought

Earth’s rarest diamonds form from primordial carbon in the mantle

Most diamonds are made of carbon recycled over and over again between Earth’s surface and its crust. But diamonds with the deepest origins — such as the famed Hope Diamond — are made of carbon from a separate source: a newly discovered, ancient reservoir hidden in Earth’s lower mantle, scientists report Sept. 10 in Nature.

Chemical clues within these superdeep diamonds suggest that there’s a previously unknown limit to how deep Earth’s carbon cycle goes. Understanding this part of the carbon cycle — how and where carbon moves in and out of the planet’s interior — can help scientists understand changes to the planet’s climate over eons, the researchers say. 

Diamonds form at different depths before making their way to the surface where they are unearthed. “Most of the diamonds people are familiar with are from the upper 250 kilometers of the planet,” says Margo Regier, a geochemist at the University of Alberta in Edmonton. “Superdeep” diamonds are from at least 250 kilometers underground, and “they’re really quite rare,” Regier says. But rarest of all are diamonds that form as far as 700 kilometers down, within the lower mantle.

“Often those are some of the biggest you find, like the Hope Diamond,” Regier says. These deepest, highly prized diamonds are also priceless scientifically, offering a rare window into the lower mantle. For example, tiny imperfections preserved in some of the diamonds contain geologic treasures: the deepest form of water known inside Earth, or even some of the oldest preserved material on the planet (SN: 3/8/18; SN: 8/15/19).

The source of the carbon in these deepest diamonds has been a mystery, but scientists wondered whether it came from the subduction of Earth’s tectonic plates. As one plate slides beneath another and sinks into the mantle, it transports carbon from the surface to the interior, a key part of the carbon cycle. Some of the carbon eventually returns to the surface, via erupting volcanoes or as diamonds, while some gets sequestered away in the deep crust or upper mantle. Carbon sequestration by subduction may have played a key role in creating space for oxygen to accumulate in Earth’s atmosphere, paving the way for the Great Oxidation Event about 2.3 billion years ago (SN: 2/6/17).

Diamonds and their inclusions — tiny slivers of rock that become embedded in the crystal structures as the diamonds form — provide sparkling clues to the environments in which they formed. So Regier and colleagues examined diamonds that formed in the crust, upper mantle and lower mantle, hunting for the chemical traces of subducted crust. To do this, the team analyzed isotopes — different forms of an element — of carbon and nitrogen in the diamonds, as well as isotopes of oxygen in the inclusions.

The relative amounts of these elemental forms indicate the chemical makeup of the magma in which the diamonds crystallized. For example, diamonds that formed in the crust and upper mantle had inclusions enriched in oxygen-18 — suggesting that the gemstones crystallized out of magma formed from subducted oceanic crust.

“All the isotopes tell the same story in a different way,” Regier says. “The carbon, nitrogen and oxygen, they’re all saying that subducting slabs are able to transport carbon and similar elements to a similar depth in the mantle. But at around 500 to 600 kilometers deep, most of that carbon is lost through magma” that rises back to the surface, she says. “After that, the slabs are relatively depleted in carbon.”

The chemical makeup of diamonds from deeper than 660 kilometers was markedly different from that of the shallower diamonds. Those “form in a different way, from carbon already stored within the mantle,” Regier says. “The very deepest samples must have been [made of] primordial carbon that never escaped from the planet.”

The finding also suggests a limit to how deeply carbon from the surface can be buried within the planet’s interior. One implication of this, Regier says, is that it calls into question whether subduction was able to bury carbon deeply and for long enough to be a driving force behind the Great Oxidation Event.

But subducting slabs don’t need to carry carbon all the way to the lower mantle to sequester it, or to have a profound impact on Earth’s climate, says Megan Duncan, a petrologist at Virginia Tech in Blacksburg. “The carbon doesn’t need to make it that far down,” Duncan says. “It just needs to be removed from the surface to have that oxygen-rise effect.”

The link between subduction and the rise of oxygen on the ancient Earth is still an open question, Regier acknowledges. “Earth is complex … [and] the fact that we have samples that tell us about this carbon cycle deep in the planet is exciting,” she adds. “It says that there’s a lot we don’t understand about our planet.”

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