Study quantifies deep reaction behind ‘superdeep’ diamonds

New research led by Feng Zhu, who is now a postdoctoral researcher at the Hawaii Institute of Geophysics and Planetology (HIGP), shows deep inside the Earth, fast reactions between subducted tectonic plates and the mantle at specific depths may be responsible for generating the most valuable diamonds.

The diamonds mined most often around the world are formed in the Earth’s mantle at depths of around 150-250 kilometers (93-155 miles). They are created by extreme pressure and temperature of at least 1050 degrees Celsius (1922 degrees Fahrenheit). Only a small amount of these diamonds make it to mineable regions since most are destroyed in the process of reaching the Earth’s crust via deep source volcanic eruptions.

But a tiny portion of mined diamonds, called sub-lithospheric or superdeep diamonds, are formed at much deeper depths than others, mostly in two rich zones at depths of 250-450 kilometers (155-279 miles) and 600-800 kilometers (372-497 miles). These diamonds stand out from others due to their compositions, which occasionally include materials from the deep Earth like majorite garnet, ferropericlase and bridgmanite.

“Although only composing 1 percent of the total mined diamonds, it seems lots of large and high-purity diamonds are superdeep diamonds, so they have good value as gems,” said Zhu, who was a post-doctoral geology researcher at the University of Michigan when he performed the research.

No previous theory has completely explained the reason why very few diamonds have been found near the surface from the area at depths of 450-600 kilometers (372-497 miles) – the region between the zones where most superdeep diamonds are formed.

High temperatures promote reactions which form diamonds, but pressure does the opposite. At depths roughly 475 kilometers (295 miles) below the surface, the pressure increases, and the reactions slow down drastically, the authors said. That’s why few diamonds are found near the Earth surface coming from between 450-600 kilometers (372-497 miles).

“When your pressure reaches the diamond stable region, it will form. But when you increase pressure it will form at lower rates. You have a trade off there,” Zhu said.

One exception to this rule is in the deeper region of 600-800 kilometers (372-497 miles) beneath the surface. In this region, accumulation of carbonate due to the stagnation of tectonic slabs pushing downwards makes up for the overdose in pressure. So while the reactions slow down, higher temperatures and an abundance of carbonate makes for a diamond-rich region.

Zhu said the new study adds to scientists’ understanding of the Earth’s mantle, about which relatively little is known for sure.

“Superdeep diamond inclusions bring us the only mineral samples from the Earth’s deep mantle,” he said. “Seeing is believing, and these inclusions provide a solid ground for the studies on the inaccessible mantle.”