PILBARA REGION — A study published in the journal Geology dates the North Pole Dome crater in the Pilbara region of Western Australia to approximately 3 billion years ago. This dating establishes the North Pole Dome crater as older than the Yarrabubba impact structure, which is also located in Western Australia and was previously dated to 2.2 billion years old.

Researchers determined the age of the North Pole Dome crater impact by analyzing shatter cones. They used the Sensitive High-Resolution Ion MicroProbe to analyze the age of zircon crystals embedded in basalt rock and apatite, a calcium phosphate mineral, in rock fractures. The study also analyzed calcite and muscovite in shatter cones, as well as a shocked quartz vein from the crater. Zircon grains in the samples displayed skeletal patterns, which are associated with impact craters, and indicated impact-modified crystals that formed when older zircon was disrupted, partly recrystallized, and regrown during intense heating caused by the impact.

Professor Chris Kirkland, the lead author of the paper from Curtin University, stated: "While the site had previously been identified as an ancient impact structure, its exact age remained uncertain. The impact left a 'mineral clock' behind. By dating minerals that were remade or newly grown in the damaged rocks, we can now pin down when this extraordinary event happened." He added: "The key evidence comes from zircon, a tiny but extraordinarily resilient mineral that can keep geological time for billions of years. Some zircons at North Pole Dome have unusual branching, skeletal shapes. We interpret these as impact-modified crystals, formed when older zircon was disrupted, partly recrystallised, and in places regrown during the intense heating caused by the impact." Kirkland also said: "Ancient impact craters are incredibly difficult to date because over billions of years, rocks are altered by heat, pressure and fluids, which can obscure or reset the original impact signals. The new age places the North Pole Dome structure as Earth's oldest known impact crater and the only recognised example from the Archean eon [4 billion to 2.5 billion years ago], a time when the planet's earliest continents were forming."

Bruce Schaefer, an associate professor and geochemist at Macquarie University who was not involved in the study, said: "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 called the combined evidence from apatite and zircon the "smoking gun" for dating the event, stating: "It's a real detective story. 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." Schaefer also noted: "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."