By Lloyd Jones

The Earth’s oldest known asteroid impact crater has been found in the outback and dated to three billion years ago after clever detective work by scientists.

Curtin University researchers investigated rock formations at the North Pole Dome in the Pilbara region of Western Australia, a site long suspected to be an ancient asteroid impact structure.

Using advanced mineral dating techniques, the team found evidence the impact occurred around three billion years ago.

Lead researcher Chris Kirkland said the impact left a “mineral clock” behind and by dating minerals in the damaged rocks it was possible to pin down when the impact happened.

“The key evidence comes from zircon, a tiny but extraordinarily resilient mineral that can keep geological time for billions of years,” he said.

Some of the examined zircon had unusual branching, interpreted as crystals modified during the intense heating caused by the impact.

“These zircon crystals record an event at about three billion years ago, which we believe is the best estimate for the impact.”

To confirm the result, the team analysed a second mineral, apatite, which formed as hot fluids moved through the shock-damaged rocks, and this independent dating method produced the same age.

“The agreement between two different mineral systems gives us confidence that we are seeing the signature of a single major event – a meteorite impact,” Professor Kirkland said.

The dating places the North Pole Dome structure as Earth’s oldest known impact crater and the only recognised example from the Archean eon, a time when the planet’s earliest continents were forming.

The asteroid is believed to have impacted on what was then ocean, Prof Kirkland told reporters.

Scientists believe there were early life forms on Earth at that time in the form of stromatolites, layered sedimentary rocks built by ancient micro-organisms.

Previously the oldest-known impact crater, also in Western Australia, was dated at 2.2 billion years old.

Scientists have yet to determine the size of the North Pole Dome asteroid or of the crater.

Because the Earth recycles itself with plate tectonics and erosion, it does not show a cratered landscape like the Moon.

Western Australia was one of the few places on Earth where deep time capsules in the rocks allowed scientists to peer into the processes that formed the surface of the planet, Prof Kirkland said.

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