Earth’s Rarest Diamonds Form From Primordial Carbon in the Mantle
upstart writes in with an IRC submission for nutherguy:
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.
[...] 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.
[*] Wikipedia entry for the Hope Diamond.
Journal Reference:
M. E. Regier, D. G. Pearson, T. Stachel, et al. The lithospheric-to-lower-mantle carbon cycle recorded in superdeep diamonds, Nature (DOI: 10.1038/s41586-020-2676-z)
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