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Deep-Earth diamonds may contain gassy relics from the early solar system

Scientists studying diamonds from deep within Earth’s mantle found evidence of a reservoir of rocks and gas that may be nearly as old as the planet itself.

ByKatherine J. WuNOVA NextNOVA Next
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Some diamonds from Juina, Brazil contain isotopes of helium that point to the existence of a primordial reservoir of rock left over from Earth’s early years. Image Credit: Suzette Timmerman

Even after 4.5 billion years of growing pains, Earth may still be young at heart.

Reporting today in the journal Science, an international team of researchers has unearthed a small trove of diamonds that may harbor gassy heirlooms from Earth’s early years. The helium atoms locked within these alluring crystals, which hail from beneath Brazil, hint at the existence of a subterranean reservoir of rocks and gas that could be nearly as old as the planet itself.

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Located at least 250 miles beneath Earth’s surface, this primordial cache can’t be accessed directly, and its true nature remains a topic of contention. But as hot rock shifts within our planet’s interior, it may deliver some of the contents of this reservoir to diamonds in the making, which in turn chauffeur the geologic goods to the surface. Understanding this rocky relay could give researchers a glimpse of Earth’s interior—and even unveil some of the conditions surrounding our planet’s cataclysmic conception.

“This is a very impressive study,” says Cornelia Class, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory who was not involved in the study. “By looking at these diamonds and their geochemical signatures...we can understand the overall evolution of Earth and its deep reservoirs.”

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Diamonds from Juina, Brazil. Diamonds can trap solids, liquids, and gases in pockets within their crystal structure, preserving them over time and space. Shown here are images from an electron microscope used to visualize the minerals. Image Credit: Suzette Timmerman

A lot has changed since the planets in our solar system first came together. In its infancy, Earth was little more than a roiling ball of magma, kept astir by volcanism and frequent collisions with other newborn space rocks. As it began to cool, our planet separated into a Russian nesting doll of concentric shells: the crust, the upper and lower mantle, and the outer and inner core. But even after its planetary pubescence, Earth continued to rejigger its internal architecture—particularly within its gooey mantle, which is home to a chaotic slew of chemical reactions.

Millennia of churning have erased almost all traces of Earth’s geological genesis. Still, many researchers have held out hope that our planet retains an echo of its molten fetal state—a sort of primordial goop that’s managed to survive the mantle’s tumultuous history.

Pinpointing this elusive reservoir isn’t easy. If it exists, it’s buried hundreds of miles beneath our planet’s surface, at depths researchers have no way to access (despite what Hollywood might want you to think, we aren’t equipped to journey to the center of the Earth).

One window into Earth’s interior comes from studying the chemical composition of rocks and minerals belched up from the mantle. Some samples of lava, for instance, have been found to contain unusually high amounts of helium-3—a rare isotope of helium that’s thought to be a chemical relic from the early solar system.

Many geochemists have taken this as an indication of an ancient wellspring of helium-3 lurking somewhere in the mantle. But lava data has to be taken with a grain of (ba)salt, because it’s difficult to determine the depth at which the lava’s starting material originates, says Rita Parai, a geochemist at Washington University in St. Louis who wasn’t involved in the study.

That’s where diamonds come in. As these minerals take shape, their near-indestructible crystal structure can trap solids, liquids, and gases from their surroundings in an exquisite state of preservation—the geologic equivalent of a cryogenic chamber. The chemicals locked within these crystalline cages can persist undisturbed for millennia, surviving even the bumpy ride diamonds take to the surface when deep-seated volcanic eruptions rustle them from their nurseries. Diamonds’ interior riches can clue researchers in to the exact conditions of their birth, as well as the approximate depths at which they were forged, making the minerals especially informative envoys from the deep.

In other words, diamonds are kind of forever—and the same can be said for whatever cargo they’re carrying.

“Diamonds are unique, powerful samples because they tell us so much about the deep Earth that we couldn’t get in any other way,” says Karen Smit, a geochemist and diamond expert at the Gemological Institute of America who was not involved in the study.

To see what these tiny time capsules had to say about our planet’s innards, a group of researchers led by Suzette Timmerman, a geochemist at the University of Münster in Germany, cracked open a collection of crystals mined from Juina, Brazil. Unlike most diamonds, which form at depths around 100 to 150 miles beneath Earth’s surface, the Brazilian stash was “super-deep,” with origins in the mantle transition zone—a region 250 to 400 miles down.

The team’s original intention had been to draw on the diamonds’ powers of preservation to better understand the mixing in Earth’s mantle. The crystals’ composition showed they were a chemically diverse bunch, suggesting they’d been forged in part from a mashup of material from different depths, probably within the last 500 million years, before jetting to the surface. But Timmerman was also surprised to find the diamonds were—relatively speaking—chock full of helium-3.

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A diamond from a site in Juina, Brazil, which produces an unusually large number of super-deep diamonds from 250 to 400 miles beneath Earth’s surface. Image Credit: Antony Burnham

This unexpected find suggested that the crystals had access to the mantle’s mythical primordial ooze—and set an upper limit for its location. “This provides a very important clue that constrains the depth of this reservoir,” Smit says. Helium-3 can only hitch a ride on a diamond that’s at its level or above, meaning it must originate from within or below the transition zone.

That’s still a lot of room to work with, though: The mantle is almost 2,000 miles thick. Gobs of primordial gunk could be bopping around the transition zone, where the super-deep diamonds was forged. Alternatively, helium-3 from deeper layers may get spiked into diamond territory by mantle plumes—upward gushes of hot material—and ensnared en route by crystals in the making.

It’s also unclear just how many of these reservoirs there are—or what else is in them (helium-3 probably isn’t the only ancient ingredient). But perhaps the biggest elephant in the room is how the reservoir has stayed intact all these years, given all the mixing that happens in the mantle.

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Diamonds mined from Juina, Brazil. The area is known for its super-deep diamonds. Image Credit: Graham Pearson

That sort of deep-Earth tranquility would be hard to accomplish, but not impossible, says Hanika Rizo, a geochemist at Carleton University in Canada who was not involved in the study. Some researchers have proposed that ancient rock reservoirs may remain above the fray by plunging below it: These primordial pockets may have chemical properties that make them denser than the rest of the mantle, forcing them inward toward Earth’s core like sinking stones.

This theory awaits confirmation. But none of the lingering uncertainty takes away from the study’s importance, Class says. “This is the first direct sample [of these isotopes] we have from as deep as the transition zone,” she says.

Much of the credit for that, of course, goes to the diamonds. These crystals are certainly pretty, Timmerman says. But their geological value far outshines their glimmer.

“A lot of people use them for jewels...but for scientists, they’re so much more,” she says. “It’s just so cool to hold one and know you have in your hands information about the history of the Earth.”

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