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The researchers painstakingly counted individual lead atoms within the oldest-known zircon with a recently developed technique called atom-probe tomography.
Inside the zircon, lead atoms clustered together in damage zones just a few nanometers wide.
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The ancient Australian crystals date back to just 165 million years after Earth formed, and have survived tumbling trips down rivers, burial deep in the crust, heating, squeezing and a tectonic ride back to the surface.
The Australian zircons, from the Jack Hills, aren't the oldest rocks on Earth — those are in Canada — but about 3 billion years ago, the minerals eroded out some of Earth's first continental crust and became part of a riverbed.
Their extreme age always makes the dates suspect, because of possible radiation damage.
By zapping single atoms of lead in a tiny zircon crystal from Australia, researchers have confirmed the crystal is the oldest rock fragment ever found on Earth — 4.375 billion years old, plus or minus 6 million years. Confirmation of the zircon age holds enormous implications for models of early Earth.
"We've proved that the chemical record inside these zircons is trustworthy," said John Valley, lead study author and a geochemist at the University of Wisconsin, Madison. Trace elements in the oldest zircons from Australia's Jack Hills range suggest they came from water-rich, granite-like rocks such as granodiorite or tonalite, other studies have reported.
But as the uranium kicks out lead atoms, the radioactive decay releases alpha particles, which can damage the crystals, creating defects.
These defects mean fluids and outside elements can infiltrate the crystals, casting doubt on any conclusions about early Earth based on the zircons.