Super-deep diamonds shine on ancient rocks from Earth’s birth

  Last updated August 22, 2019 at 12:18 pm


A first-of-its-kind analysis on diamonds from hundreds of kilometres below the Earth’s surface has revealed hints of a primordial magma.

diamonds_deep earth_earth mantle

These super-deep diamonds provide a window into deep Earth. Credit: Graham Pearson

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

In fact, 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.

Lead researcher Suzette Timmerman, who conducted her research at ANU, says that for the first time researchers analysed the helium isotopes that are contained in the tiny microscopic bubbles inside the diamonds.

Diamonds form a perfect window into deep Earth

Most diamonds form at depths of 150 to 200 kilometres, says Timmerman, but, the diamonds from the Juina area of western Brazil are different.

“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_deep earth_earth mantle

These tiny diamonds are shedding new light on deep Earth. Credit: Graham Pearson.

“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.”

Mantle rocks that never mixed with the rest of the crust

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 formedmantle 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.”

The oldest remaining, undisturbed material on Earth

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.


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About the Author

Richard Lovett
Richard A. Lovett is a Portland, Oregon-based science writer and science fiction author. He is a frequent contributor to Cosmos Magazine.

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