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...