Diamonds from the Juina region in Brazil. Most are Superdeep diamonds.

Graham Pearson

Tiny imperfections in Brazilian diamonds have exposed a pocket of the earth's ancient past deep inside.

In fact, these rocks seem to have survived largely undisturbed for 4.5 billion years, making them older than the moon or anything on the surface of the earth.

Of course, diamonds only form under high-pressure conditions that are deep below the earth's crust. This makes them messengers of the mantle, which then rise above volcanic pipelines to the surface, where they are finally found by miners.

Most diamonds form at depths of 150 to 200 kilometers, says Suzette Timmerman, a Dutch geochemist who has done research at the Australian National University. However, diamonds from the Juina region in western Brazil are different.

"The Juina area is special because more than 99% of the diamonds are between 410 and 660 kilometers deep," she says.

This is important because diamonds are notoriously durable.

"Diamonds are the hardest and most indestructible natural substance there is," she says. "They form a perfect window into the depths of the earth."

Timmermans study, published in the journal sciencefocused on helium gas trapped in tiny fluid bubbles in 23 of these diamonds.

Helium comes in two forms: helium-3 and helium-4. The early solar system had a mixture of the two, which was determined by the composition of the interstellar gas cloud from which it formed. However, helium-4 continues to be produced as a by-product of certain types of radioactive decay, in particular the decay of heavy elements such as uranium and thorium.


Very rare diamonds form in extreme depths

"If we have a lot of Helium-4, that means it has had a lot of time to form," says Timmerman. "If we find a lot of helium-3, that must be because it's ancient."

This, of course, is not so easy, as geological processes at a young age tended to promote uranium and thorium (and the subsequent production of helium-4) from the mantle into aboveground rocks.

But if this is corrected, Timmerman says, the helium isotope ratios in their diamonds prove that the helium trapped in them comes from regions that are very close in composition to the original matter from which the Earth was originally formed – mantle rocks For some reason, never mix with the rest of the mantle or with material derived from the crust.

"In order to preserve the compositions we see today," she says, "it must not have interacted with the rest of the mantle, at least since the core and cloak have separated" – something that probably happened after the huge impact formed the moon. "It's definitely part of Earth that has not interacted with the crust since the beginning of time."

How much of this original matter remains is unclear, she says, but one place she apparently exists in is among the diamond mines of Brazil. And she notes, "With this work, we begin to deal with the probably oldest remaining, relatively undisturbed material on earth."

Other scientists are impressed. "This is an interesting finding that offers a lot of potential for mapping elevated helium-3 / helium-4 domains deep inside the Erath," says Matthew Jackson, a geochemist at the University of California at Santa Barbara, part of the study team ,

It's also fascinating, because the Japanese space agency would like to return a sample of more of the original material from the 162173 Ryugu asteroid just four years later, and four years before NASA wants to do so for the 101955 Bennu asteroid.

"So all connected," says Timmerman.