Superdeep diamonds have a story to tell

Tiny imperfections in Brazilian diamonds have revealed a pocket of the Earth’s primordial past, deep in its interior. 

In fact, scientists say, these rocks appear to have survived largely undisturbed for 4.5 billion years, making them older than the Moon or anything on the Earth’s surface.

Diamonds form naturally only under high-pressure conditions existing deep beneath the Earth’s crust. That makes them messengers from the mantle, which then rise toward the surface via volcanic conduits, where miners ultimately find them. 

Most diamonds form at depths of 150 to 200 kilometres, says Suzette Timmerman, a Dutch geochemist who conducted her research at Australian National University. Diamonds from the Juina area of western Brazil are different, however. 

“The Juina area is special because more than 99% of the diamonds form between 410 and 660 kilometres in depth,” she says.

That’s important, because diamonds are notoriously durable. 

“Diamonds are the hardest, most indestructible natural substance known,” she says, “so they form a perfect window into the deep Earth.”

Timmerman’s study, published in the journal Science, focused on helium gas trapped in tiny bubbles of fluid in 23 of these diamonds. 

Helium comes in two forms: helium-3 and helium-4. The early Solar System had a mix of the two determined by the composition of the interstellar gas cloud from which it formed. But helium-4 continues to be formed as a byproduct of certain types of radioactive decay, particularly the decay of heavy elements such as uranium and thorium.

“If we have a lot of helium-4, it means it must have had quite a bit of time to form,” Timmerman says. “If we find a lot of helium-3, this must be because it’s ancient.”

It’s not quite that simple, of course, because geological processes when the Earth was young tended to move uranium and thorium (and their subsequent production of helium-4) out of the mantle into upper-level rocks. 

But when this is corrected for, Timmerman says, the helium isotope ratios in her diamonds prove that the helium trapped within them comes from regions very close in composition to the primordial matter from which the Earth initially formed – mantle rocks that, for whatever reason, never mixed with the rest of the mantle or with material descending from the crust. 

“In order to get the compositions we see today,” she says, “it mustn’t have interacted with the rest of the mantle at least since the core and mantle separated” – something that probably occurred in the aftermath of the giant impact that formed the Moon. “It’s definitely a part of the Earth that hasn’t been interacting with the crust, basically since the beginning of time.”

How much of this primordial matter remains is unclear, she says, but one place it apparently does exist is beneath the diamond mines of Brazil. And, she notes, “with this work we are beginning to home in on what is probably the oldest remaining, comparatively undisturbed, material on Earth”.

Other scientists are impressed. “This is an interesting result, with a lot of potential to ‘map out’ elevated helium-3/helium-4 domains in the Earth’s deep interior,” says Matthew Jackson, a geochemist at the University of California, Santa Barbara who was not part of the study team. 

It’s also intriguing because it comes only a year before the Japanese space agency hopes to return a sample of even more primordial material from asteroid 162173 Ryugu, and four years before NASA hopes to do the same for asteroid 101955 Bennu.

“So all connected,” Timmerman says.