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Most diamonds are made from carbon recycled over and over between the Earth’s surface and its crust. But diamonds with the deepest origins, like the famous Hope Diamond, are made of carbon from a separate source: a newly discovered ancient deposit hidden in the Earth’s lower mantle, scientists report September 10 in Nature.
Chemical clues within these super-deep diamonds suggest that there is a previously unknown limit to the depth of Earth’s carbon cycle. Understanding this part of the carbon cycle – how and where carbon from within the planet enters and leaves – can help scientists understand changes in the planet’s climate over eons, the researchers say.
Diamonds are formed at different depths before reaching the surface where they are dug up. “Most of the diamonds that people are familiar with are from the top 250 kilometers on the planet,” says Margo Regier, a geochemist at the University of Alberta in Edmonton. “Superdeep” diamonds are found at least 250 kilometers underground and “are quite rare,” says Regier. But the rarest of all are diamonds that form up to 700 kilometers downward, within the lower mantle.
“Often times, those are some of the biggest to be found, like the Hope Diamond,” says Regier. These deeper and more precious diamonds are also scientifically invaluable and offer a rare window into the lower mantle. For example, the tiny imperfections preserved in some of the diamonds contain geological treasures: the deepest form of water known within the Earth, or even some of the oldest preserved materials on the planet (SN: 3/8/18; SN: 8/15 / 19).
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The source of the carbon in these deeper diamonds has been a mystery, but scientists wondered if it came from the subduction of Earth’s tectonic plates. When one plate slides under another and sinks into the mantle, it carries carbon from the surface to the interior, a key part of the carbon cycle. Some of the carbon eventually returns to the surface, through erupting volcanoes or as diamonds, while another part is sequestered 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 some 2.3 billion years ago (SN: 6 / 2/17).
Diamonds and their inclusions, tiny rock chips that become embedded in crystal structures as diamonds form, provide brilliant clues to the environments in which they were formed. So Regier and his colleagues examined the diamonds that formed in the crust, the upper mantle and the lower mantle, looking for the chemical traces of the subduced crust. To do this, the team analyzed isotopes, different forms of an element, of carbon and nitrogen in diamonds, as well as isotopes of oxygen in inclusions.
The relative amounts of these elemental forms indicate the chemical composition of the magma in which the diamonds crystallized. For example, diamonds that formed in the crust and upper mantle had oxygen-18 enriched inclusions, suggesting that the gemstones crystallized from magma formed from subduced oceanic crust.
“All isotopes tell the same story in a different way,” says Regier. “Carbon, nitrogen and oxygen all say that subducting slabs can transport carbon and similar elements to a similar depth in the mantle. But between 500 and 600 kilometers deep, most of that carbon is lost through magma ”that returns to the surface, he says. “After that, the slabs are relatively carbon depleted.”
The chemical composition of diamonds deeper than 660 kilometers was markedly different from that of shallower diamonds. Those “are formed in a different way, from carbon that is already stored within the mantle,” says Regier. “The deepest samples must have been (made of) primordial carbon that never escaped the planet.”
The finding also suggests a limit to how deep carbon from the surface can be buried inside the planet. One implication of this, Regier says, is that it questions whether subduction was able to bury the carbon deeply and long enough to be a driving force behind the Great Oxidation Event.
But subduction plates don’t need to transport carbon down to the lower mantle to sequester it, or have a profound impact on Earth’s climate, says Megan Duncan, a petrologist at Virginia Tech in Blacksburg. “Carbon doesn’t need to go that low,” says Duncan. “It just needs to be removed from the surface to have that oxygen-boosting effect.”
The link between subduction and increased oxygen on ancient Earth remains an open question, Regier acknowledges. “The Earth is complex … (and) the fact that we have samples that inform us about this carbon cycle deep in the planet is exciting,” he adds. “He says there are a lot of things we don’t understand about our planet.”
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