Zircons studied by the research team, photographed using cathodoluminescence, a technique that allowed the team to visualize the interior of the crystals using a specialized scanning electron microscope. The dark circles on the zircons are the pits left by the laser that was used to analyze the age and chemistry of the zircons.
Michael Ackerson/Smithsonian
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Michael Ackerson/Smithsonian
Zircons studied by the research team, photographed using cathodoluminescence, a technique that allowed the team to visualize the interior of the crystals using a specialized scanning electron microscope. The dark circles on the zircons are the pits left by the laser that was used to analyze the age and chemistry of the zircons.
Michael Ackerson/Smithsonian
The oldest known material from Earth that remains on our planet’s surface is a mineral called the “Time Lord” because it is incredibly good at keeping geological time.
The mineral is zircon, and scientists have found chunks of it that formed 4.37 billion years ago, not long after proto-Earth’s epic collision with an object the size of Mars that spawned our moon.
Tiny zirconium crystals can look like sand or useless rubbish. But don’t be fooled. With a radioactive tick tock that marks the passage of billions of years, these small but powerful minerals offer us a peek into Earth’s early development.
“They really are the best markers of Earth time, or Earth history,” says Michael Ackerson, a geologist at the Smithsonian’s National Museum of Natural History.
A thin, polished slice of rock collected from Jack Hills, Western Australia, viewed through a special microscope fitted with a plasterboard that shows the rainbow spectrum of the quartz that makes up the rock. While the rocks at Jack Hills are over 99% quartz, the remaining 1% of the material includes precious zircons.
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Michael Ackerson/Smithsonian
A thin, polished slice of rock collected from Jack Hills, Western Australia, viewed through a special microscope fitted with a plasterboard that shows the rainbow spectrum of the quartz that makes up the rock. While the rocks at Jack Hills are over 99% quartz, the remaining 1% of the material includes precious zircons.
Michael Ackerson/Smithsonian
Zirconium crystals originate from cooling magmas in continental crusts, along with other minerals, says Ackerson. But these other minerals tend to disappear over time.
“Most minerals don’t survive,” says Ackerson. “So things like quartz, things like feldspar — they’re chemically or physically weathered and eroded to a point where they’re not quartz and feldspar anymore.”
The oldest Earth material ever dated by mankind is a nearly 4.4 billion-year-old zircon mineral from a sedimentary rock formation in the Jack Hills of western Australia, seen in this satellite image.
Universal History Archive/Universal Images Group via Getty Images
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Universal History Archive/Universal Images Group via Getty Images
The oldest Earth material ever dated by mankind is a nearly 4.4 billion-year-old zircon mineral from a sedimentary rock formation in the Jack Hills of western Australia, seen in this satellite image.
Universal History Archive/Universal Images Group via Getty Images
On the other hand, zirconium is very resilient – which is “one of the main reasons” why this mineral is so useful, he says.
In the face of high winds, crushing pressures or high heat, these tough crystals persist. And, eventually, they can end up incorporating other rocks that are still forming. That means scientists can crush Earth’s oldest rocks, sift through the debris, and find tiny grains of even older zirconium.
In the Jack Hills region of western Australia, for example, there is a rock that formed from a beach 3 billion years ago. The oldest zircons ever discovered came from this rock.
Ackerson once found a zircon that was 4.32 billion years old. Zircons that old “are extremely, extremely, extremely rare and are the only windows we have into the oldest Earth,” he says.
Today, to know the exact age of a zirconium, scientists can shoot it with a laser like the one at Penn State University’s geochronology lab. There, Joshua Garber shows how he can place a tiny crystal in a device that drills a small hole in it and rips out tiny pieces.
“And then I torture them in an argon plasma to break them down into their smallest constituents,” he says, explaining that a detector counts atoms of different chemical elements.
The most important are uranium and lead. Zirconium loves uranium and will absorb it as it grows, but zirconium hates lead. This means that if you find lead inside, it most likely came from the decay of uranium, which occurs at a steady pace, like the ticking of a clock.
“If you were to design a timer for Earth from scratch, you would basically design zircon and uranium dating,” says Jesse Reimink, a geologist at Penn State. “If you believed in a higher power, you’d say, ‘Oh, the higher power created this mineral with this particular system because it’s so perfect.’
But looking at the chemical composition of zirconium can do more than just reveal its age or the age of its associated rock. It could also give scientists clues about the conditions that existed when the zircon was originally created.
For example, Ackerson recently analyzed aluminum concentrations within ancient zircons to infer that plate tectonics may have started 3.6 billion years ago.
A zirconium crystal estimated to be around 3.6 billion years old taken from the Jack Hills metaconglomerate of Western Australia. The concentric rings record processes in the magma from which zirconium minerals crystallized billions of years ago. The black circles inside the crystal are the holes left by a laser beam that was used to determine the age and chemistry of the zircons.
Tim Gooding, Timothy Rose, Rob Wardell/Smithsonian Institution
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Tim Gooding, Timothy Rose, Rob Wardell/Smithsonian Institution
A zirconium crystal estimated to be around 3.6 billion years old taken from the Jack Hills metaconglomerate of Western Australia. The concentric rings record processes in the magma from which zirconium minerals crystallized billions of years ago. The black circles inside the crystal are the holes left by a laser beam that was used to determine the age and chemistry of the zircons.
Tim Gooding, Timothy Rose, Rob Wardell/Smithsonian Institution
And he says scientists used to think Earth was a hot, glowing hellhole for the first 500 million years. But the oldest zircons found on Earth show otherwise.
“We know just from this collection of zirconium crystals that Earth had continents, which we didn’t think was possible, that interacted with oceans of liquid water,” he says. “We are beginning to understand how and when continents emerged, how and when oceans emerged, and how this may have helped us lay the groundwork for the spread of life on our planet.”
These are big philosophical questions that require looking back in time, he says – which is only possible thanks to tiny zircons.
This story is part of NPR’s periodic science series “Finding Time – A Journey into the Fourth Dimension to Learn What Makes Us Move”.