Confirmed! Oldest rocks on Earth are nearly 4.4 billion years old

Tiny zircon crystals, collected from a sheep ranch in Western Australia, have been confirmed as the oldest rocks on Earth, dated at nearly 4.4 billion years old.

When University of Wisconsin geochemist John Valley first gathered samples of microscopic zircon crystals from the Jack Hills region of Western Australia, back in 2001, he and his colleagues found that they were an incredible 4.4 billion years old — the oldest pieces of the Earth ever discovered.

To date these crystals, the researchers used a method of 'radiometric dating' called uranium-lead dating. When the crystals first formed, atoms of two different isotopes of uranium (U-235 and U-238) got caught up in the crystal structure. As these two isotopes decayed radioactively, they formed two different isotopes of lead (Pb-207 and Pb-206, respectively). The half-life of uranium-235 decaying into lead-207 is around 700 million years, while the half-life of uranium-238 to lead-206 is roughly 4.5 billion years. Measuring the abundance of both isotopes of lead can not only give you a date, but the two half-lives also give an accurate 'cross-check' of the accuracy of that date.

One potential problem with the original dating, though, is that it's possible for the lead atoms to get shuffled around a bit inside the crystal. According to what Valley told LiveScience, that can throw off the dating technique, since areas of the crystal with more lead clustered together will appear older, while areas with less lead will appear younger. To overcome this problem, Valley and his team used a new technique called 'atom-probe tomography', to basically locate each individual lead atom and produce a map of where they are inside the crystal. They found that the crystals did move around a tiny bit, but it wasn't far enough to throw off the age measurements.

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The new, confirmed age of the crystals is 4.374 billion years old, within about 6 million years, either way. The previous age for the oldest rocks on Earth, taken from the Canadian Shield around Hudson's Bay, was 4.28 billion years, so this shows that Earth's crust cooled almost 100 million years earlier than previously thought. This also provides stronger evidence for the 'cool early Earth' theory, where our planet developed oceans and habitable conditions (at least to early life), much earlier than though as well.

"This confirms our view of how the Earth cooled and became habitable," Valley said in a University of Wisconsin news release. "This may also help us understand how other habitable planets would form."

(Photo courtesy: Jack Valley, University of Wisconsin)

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