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Flawed Diamonds Deliver Precious Details about Early Earth's Tectonics

Volcanoes have coughed up ancient diamonds from hundreds of kilometers beneath Earth's surface. Their trace impurities provide some priceless information about the first shifts in plate tectonics
diamond flaws for early plate tectonics research



iStockphoto/Jezperklauzen

Girls and the rest of us aside, diamonds can be a geologist's best friend—especially if that geologist has a mass spectrometer and is looking for clues about what Earth looked like billions of years ago.

These precious rocks occasionally contain impurities trapped inside during formation billions of years ago. And with the right tools, scientists can mine these traces for date details and chemical composition to get a rare snapshot of early Earth. From such miniscule grains—sulfides and silicate in a new analysis—a pair of researchers is now proposing a date for the beginning of the modern plate tectonic cycle: 3 billion years ago.

Formed under ancient intense pressure deep in the mantle, these diamonds were occasionally spouted to the surface via volcanic eruptions. The cargo carried inside these marred diamonds started to look different starting around 3 billion years ago, containing traces of a rock, eclogite, that would have been more common with shallow melting of basalt. And that scenario is likely during the emergence of thick, moving continents like the ones we have today, assert the researchers in a paper published July 22 in Science.

With the tiny fragments of rock gleaned from these rare minerals, "we are seeing the beginning of a major period of slab subduction that is fundamentally different," says Steven Shirey, of the Carnegie Institution of Washington's Department of Terrestrial Magnetism and co-author of the new study.

Diamonds are for…billions of years

In the dynamic history of Earth, precise dates can be hard to come by—especially when they might extend back for billions of years.

To gather rough dates, "we've always had the rock record" from the surface, Shirey says. But continents and seafloors are in a constant state of recycling via weathering and plate tectonics, leaving very few masses safe from the forces of time.

Some extremely old chunks of continent do exist, however. Known as cratons, these masses have deep mantle roots that can reach down some 200 kilometers below the surface. And they contain diamonds that were formed by subsurface high pressures billions of years ago but have been protected by the relatively low temperatures there.

For the first half of its existence, Earth's surface was more of a fluid place, with bits of crust being formed here and there from rising hot mantle. But at some point, as the planet cooled, larger masses started to form and the cycle of supercontinents and plate tectonics as we know it today got underway.

"One of the key questions is: How far can we extend the current knowledge of processes that shaped the surface of the earth?" Shirey says.

From the outside, someone proposes to use tiny fragments of compounds trapped in rare diamonds "to draw conclusions about how the entire planet was operating 3 billion years ago, and it sounds a bit cheeky," says John Platt, a professor of Earth Sciences at the University of Southern California who was not involved in the new research. "But there's not a lot else that you can do," and diamonds are so hard that it makes them "very robust" samples for attempting such seemingly absurd dating attempts.

Dissecting diamonds
Diamond-based research can be a difficult endeavor. "Money matters here," Shirey says. He and his co-author, Stephen Richardson of the University of Cape Town's Department of Geological Sciences, both of whom have been studying diamond inclusions for years, must either purchase the diamonds—which usually range from 1/2 to 1 carat—or count on diamond distributors to donate them, "so we don't have total control over what we get," Shirey notes.

Although a flawless diamond might be hard to come by in the marketplace, ones with the kind of inclusions useful in geological analysis are perhaps even more rare. For example, to assemble a collection of 10 diamonds worth studying "it took us literally three years of De Beers setting the diamonds aside," Shirey says. For the new paper, he and Richardson reviewed previously published analyses from the past 25 years on some 100 sulfide inclusions and more than 4,000 silicate inclusions.

Some inclusions can be spotted with the naked eye, such as one smack in the center of the 80-carat Oppenheimer yellow diamond at the Smithsonian. But diamonds destined for Shirey's and Richardon's labs do not emerge unscathed. After verifying the inclusions with a scanning electron microscope, researchers have to slice into the diamonds with a laser and extract the tiny particles for analysis via mass spectrometer. The team compared results from both rhenium-osmium and samarium-neodymium radiometric dating of samples to arrive at estimates of when the minerals were locked inside of the diamonds. And that helps scientists to figure out when rock composition—and geologic—changes started to occur.

Scientific rifts
The new data fit in well with what many researchers have come to think about the shifting dynamics of the Earth's mid-history based on other rock and chemical evidence: there was a major shift some 3 billion years ago. As a jumping-off point, scientists like Shirey and Richardson are searching for clues in the past based on what we know about how Earth works today.

But, says Platt, "3 billion years ago, our planet might have been fundamentally different, and we simply don't have enough data or imagination" to conjure it up yet. He points out that "you could still have had processes that go on on an active planet even if they don't fit in with the processes we have today." He argues that traces of eclogite cropping up in diamonds about 3 billion years ago do not necessarily mean that a more modern supercontinent cycle (also known as the Wilson Cycle) had to have started at that time. He points out that eclogite has been known to crystallize some 80 kilometers below the surface, not having necessarily needed the deeper subduction implicated in the Wilson Cycle.

Don Anderson, a professor emeritus of California Institute of Technology's Division of Geological and Planetary Sciences agrees that finding eclogite in itself might not be enough to indicate the emergence of full-blown continental plates. "True plate tectonics may have started later," he says.

Nevertheless, Platt says, Shirey and Richardson's methods look sound: "It's still really interesting data, and it clearly does mean something." And even if Platt is not ready to fully accept the premise put forth in the new paper, for now, he says, "I can't come up with a better one."

Deep answers
Shirey is now turning his focus to hunting down diamonds from other areas of the planet to see how their inclusions compare to those he and Richardson have already found. One ancient formation, the Zimbabwe craton, is of interest because "it looks like it was formed in a different way" from others, such as that in Australia (Pilbara) or South Africa (Kaapvaal).

And another frontier is deep Earth itself. Nonetheless, "it's very hard to look deep into the mantle," where clues about early geologic dynamics might linger, Shirey says. "We think diamonds are forming in the mantle all the time. They just never make it up because there are reactions going on, and they get reabsorbed."

So-called deep-mantle geodynamics is "a whole new area of research," Shirey says. Below continents lurks the lithosphere, some 220 to 225 kilometers below the surface, which is separated from the mantle by the transition zone (400-700 km deep). Even the upper portions of the mantle are some 700 to 1,000 km below the surface—a generally solid but conductive part of the planet. Any intact rocks from that depth would be a proverbial goldmine for geologists. "If we can get minerals—like we can get diamonds—that haven't reacted, all the better," Shirey says. "Whatever they carry with them in their lattice is going to be frozen information." Information that could help advance scientific understanding of Earth's earliest continents—as well as, Platt points out, the geology of exoplanets.

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