Confirmed: Oldest Fragment of Early Earth is 4.4 Billion Years Old

Oldest zircon — Cathodoluminescence image of a 400-μm Jack Hills zircon.

(Image credit: John Valley, University of Wisconsin)

Ever heard this life advice? When solving a big problem seems impossible, break it into smaller steps.

Well, scientists just took one of geology's biggest controversies and shrunk it down to atomic size. 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.

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Confirmation of the zircon age holds enormous implications for models of early Earth. 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. That means Earth cooled quickly enough for surface water and continental-type rocks just 100 million years after the moon impact, the massive collision that formed the Earth-moon system. [How Was The Moon Formed?]

Zircons are one of the toughest minerals on the planet. 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.

Zircons hold minute amounts of two naturally occurring uranium isotopes — isotopes are atoms of the same element with different numbers of neutrons. Uranium radioactively decays to lead at a steady rate. Counting the number of lead isotopes is how scientists date the crystals. 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.

Valley and his co-authors hope to end the debate by showing that even though one of the oldest Jack Hills zircons suffered radiation damage, the lead atoms stayed in place. 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. Imagine cliques of teens during high school lunch — like teenagers, no lead atoms had left their zones.

"This careful piece of work should settle the debate because it shows that indeed there is some mobility of lead, which was hypothesized to result in dates that were too old, but the scale of mobility is nanometers," said Samuel Bowring, a geochemist at MIT, who was not involved in the study. "Even the smallest volumes analyzed with the ion probe average out the heterogeneities," or variations within the zircon.

Becky Oskin covers Earth science, climate change and space, as well as general science topics. Becky was a science reporter at Live Science and The Pasadena Star-News; she has freelanced for New Scientist and the American Institute of Physics. She earned a master's degree in geology from Caltech, a bachelor's degree from Washington State University, and a graduate certificate in science writing from the University of California, Santa Cruz.