Scientists find ‘smoking gun’ evidence of world’s oldest meteorite strike in Western Australia

A meteorite that struck Earth three billion years ago left behind a “smoking gun” – evidence of the world’s oldest impact crater in a remote part of Australia.Ancient rocks in Western Australia’s Pilbara region record the event, which occurred during the Archean eon, a period 4 to 2.5 billion years ago, when tectonic plates were beginning to form and early life emerging.To establish a precise date, Curtin University scientists analysed the age of rare geological features, called shatter cones, in an area known as the North Pole Dome crater, publishing their findings in Geology journal.Prof Chris Kirkland, a geologist from Curtin’s Timescales of Minerals Systems Group and lead author of the paper, said the well-preserved rock formations were an extremely rare and globally significant archive of geological time, offering “a rare glimpse of the violent processes that shaped the early Earth”.“There’s very few places that are these deep time capsules that let us peer into the formative processes on our planet. That’s why they’re quite special.”The findings revealed the North Pole Dome crater to be older than Yarrabubba, another crater in Western Australia, previously considered the oldest at 2.2 billion years old.Rare geological features were found in the Pilbara. Photograph: Chris Kirkland/Curtin UniversityThe researchers used two separate methods to determine when the meteorite strike occurred.First they analysed the age of “little lightning bolts” of zircon embedded in the basalt rock. Tiny zircon grains were recrystallised in the intense heat of the meteorite strike, forking into unusual skeletal patterns generally only found in impact craters on the moon.The age of these zircon crystals was measured using an Australian-designed instrument called the Sensitive High-Resolution Ion MicroProbe, determining that the shape-changing shock occurred about 3 billion years ago.Separately, scientists analysed the age of apatite – a calcium phosphate mineral which grew in rock fractures created by heat and hot fluids after the impact – with similar conclusions.Prof Chris Kirkland, a geologist from Curtin University’s Timescales of Minerals Systems Group and lead author of the paper published in Geology journal. Photograph: Curtin UniversityEarth was a vastly different place at the time the meteorite hit, Kirkland said. It was mostly a “water world” with few pieces of continental crust. The sun would have been dimmer and the moon closer, with early life present in the form of stromatolites (a type of cyanobacteria, like algae).Associate professor Bruce Schaefer, a geochemist at Macquarie University, said Earth was continuously “pummelled” by meteorites during the Archean – the impacts of which were still visible in craters on the moon’s surface – but have been mostly erased on land through the processes of erosion, subduction and plate tectonics.The age of ‘little lightning bolts’ of zircon embedded in the basalt rock were analysed by scientists. Photograph: Curtin University“To be able to find evidence of those same impact events on Earth is really exciting. We know it must have happened, but to actually see it, and put your hands on it, is very significant,” he said.Schaefer, who was not involved with the paper, said by relying on the recrystallisation of zircon and apatite growth that resulted from collision, the researchers had used a clever combination of innovative techniques to pick apart what happened.“It’s a real detective story,” he said. “The fact those two were reset at the same time is the really powerful evidence that this is the age of that event.“The apatite and the zircon together is what’s, if you like, the smoking gun.”