A chip of zircon found in Western Australian rocks at a place called North Pole Dome revealed the age of Earth's oldest known impact crater. The team that found it was working on age-dating the crater, which is located in a region called the Pilbara Craton. They used mineral dating to pinpoint the exact time it was dug out by an impactor.
Team lead Chris Kirkland from the Timescales of Minerals Systems Group within Curtin University's School of Earth and Planetary Sciences, said the findings help resolve a longstanding question about the timing of the impact. The results of the team's analysis of several minerals at the site, along with zircon, indicated that the North Pole Dome impact occurred at 3.024 billion years ago (plus or minus a few million years). ? “While the site had previously been identified as an ancient impact structure, its exact age remained uncertain,” Kirkland said, noting that the impact created a sort of "mineral clock" which reset the minerals — particularly zircon — to "time zero" from the moment of impact. "By dating minerals that were remade or newly grown in the damaged rocks, we can now pin down when this extraordinary event happened."
The mineral zircon is extremely common in Earth's crust, and geologists use it to do radiometric dating of rocks. Zircon crystals embedded in igneous rocks from volcanic activity are a standard way of determining the age of our planet's crust. It existed as part of the planet's earliest surface, back in a period called the Archaean Eon. It's role in age dating rocks made it invaluable in dating the timing of the crater at North Pole Dome, according to Kirkland. "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. These zircon crystals record an event at about 3 billion years ago, which we believe is the best estimate for the impact.”
A sample of tiny zircon crystals under study in the lab. Credit: Chris Kirkland.
To back up the zircon-deduced age for the crater, the team also analyzed the mineral apatite. It's also useful for age dating. The apatite at the Western Australia site formed as molten rock fluids moved through shock-damaged rocks. Those shocks were produced when the incoming bit of space debris smashed into the ground. Dating the apatite produced the same age for the impact as the zircon did. “The agreement between two different mineral systems gives us confidence that we are seeing the signature of a single major event — a meteorite impact,” Kirkland said.
“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, a time when the planet’s earliest continents were forming."
The planet Earth when that ancient impact occurred on was very different from what we see today. At that period of time, called the Archaeon Eon (which lasted from roughly 4 billion to 2.5 billion years ago), the still newborn planet went through a lot of changes. It overlapped with the Late Heavy Bombardment (at about 3.8 billion years ago), when a huge influx of space rocks hammered the surface was hammered. In the early stages of that Eon, the atmosphere had no oxygen, except in the water. Instead, the "air" was made up of methane, ammonia, and other gases emitted as a part of the heavy volcanic activity that built the crust. Under the crust, plate tectonics were just starting to commence. Earth was largely an ocean world, and what geological information we have about it comes from the few rare places where its surface rocks still remain today.
*An artist's view of what the Archaean might have been like; oceans and volcanic activity sculpting the planet. Credit: Tim Bertelink, CC BY-SA 4.0*
Life hadn't begun to form yet in the early Archaean, but later on, the conditions ripened to allow life in the form of microbes, mostly herded together in mats called stromatolites. As the eon continued, early life forms began to produce oxygen and that introduced changes in the oceans and atmosphere.
The timing of the impact that created the crater at North Pole Dome appears to have happened when life was just beginning its toehold on the planet. Since that time, ongoing most early craters have vanished. “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. What we’ve been able to do here is separate the moment of impact from its long geological history," said Kirkland. “This discovery pushes Earth’s impact record deeper into geological time than any previously well-dated crater, offering a rare glimpse of the violent processes that shaped the early Earth.”
Stromatolites at Lake Thetis, Western Australia. These life forms began appearing during the Archaean Eon. Credit: Ruth Ellison, CC BY 2.0.
Prior to the discovery of the ancient crater at North Pole Dome, the oldest known crater was 2.2 billion years old. This new data now confirms the Australian crater as Earth's oldest. Now geologists wnat to know more about the effect such an impact had on Earth beyond blasting out a hole in the ground. According to Kirkland, the energy produced in the impact could have radically changed the surface. “Uncovering this impact and finding more from the same time period could explain a lot about how life may have got started, as impact craters created environments friendly to microbial life such as hot water pools,” he said.
“It also radically refines our understanding of crust formation: the tremendous amount of energy from this impact could have played a role in shaping early Earth’s crust by pushing one part of the Earth’s crust under another, or by forcing magma to rise from deep within the Earth’s mantle toward the surface. It may have even contributed to the formation of cratons, which are large, stable landmasses that became the foundation of continents.”
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