In 2008, Canadian researchers led by McGill PhD student Jonathan O’Neil said they’d found the world’s oldest rocks, formed 4.3 billion years ago in what is now northwestern Quebec. Such rocks would give scientists an unprecedented glimpse into Earth’s early history during its very first eon, the Hadean, just a few hundred million years after the Earth formed 4.5 billion years ago.
But the discovery was controversial, and other scientists argued that the rocks were simply mixtures of older and younger material, unable to really tell us what the world was like at that time.
Now, after more than a decade of hard work, O’Neil and his team have done a new analysis of rocks from the Nuvvuagittuq Greenstone Belt (NGB), a rock formation located in Quebec’s Nunavik region, about 40 kilometres south of Inukjuak, close to the eastern shore of Hudson Bay.
The newly studied rocks, which formed later than the original rocks analyzed, are at least 4.16 billion years old, reports a paper published today in Science. That confirms that even they are from the Hadean, and since the original rocks are even older, and the rock formation includes “the oldest rocks preserved on Earth,” the study says.
O’Neil, now a professor of earth sciences at the University of Ottawa, said that to geologists, rocks are like books, full of chemical records of when they were formed and the environment at that time – offering clues about when the oceans formed, when life began, and when plate tectonics started creating the continents. With such old rocks, he added, “We have the opportunity to open a new window on a time when we have almost no record.”
When the Earth first formed, it was a ball of molten lava. And originally, scientists considered the Earth’s first eon, the Hadean, to have ended when the first rocks formed. What’s known as the “golden spike,” marking the end of the Hadean, is also located in Canada, at the Acasta Gneiss formation in the Northwest Territories, which is 4.03 billion years old.
Scientists agree on that date because the Acasta Gneiss contains zircons, minerals that give very reliable ages for rocks fairly easily using a technique called isotopic dating. The technique relies on the constant rate of decay or radioactive materials and uses those as a clock.
While scientists used to think there were no rocks during the Hadean, O’Neil said, they’ve changed their minds as a result of more and more evidence uncovered in the past 20 years, including zircons formed 4.4 billion years ago in Australia. (These zircons, which are tiny sand grains embedded in sedimentary rocks, are too small to be considered rocks themselves.)
Such tiny mineral grains can’t give nearly as much information as an entire rock — only something equivalent to “maybe a page,” O’Neil said. With an entire rock, “we may have a chapter or [an entire book],” O’Neil said.
In the past decade his team has found intriguing evidence that the NGB rocks were formed on the ocean floor and may show some early traces of life and possible evidence of plate tectonics — but the “unfortunate” age controversy, as O’Neill describes it, has left them unable to say for sure that these are things that happened during the Hadean.
The Nuvvuagittuq Greenstone Belt is made of an unusually pale basalt, a type of rock often formed on the ocean floor. Unfortunately, basalt doesn’t contain zircons, so geologists can’t tell its age using the most reliable isotopic technique available.
Instead, O’Neil and his team turned to a technique called samarium-neodynium dating, suitable for rocks older than four billion years old.
“This has been applied on rocks from the moon and on rocks from Mars — it’s just there are no rocks on Earth that are old enough to use that technique, except perhaps from the rocks from northern Quebec,” O’Neil said.
In addition, dates from two different isotopic “clocks” didn’t agree in that earlier study. O’Neil thought that was because one of the two clocks was more vulnerable to events that happened to the rocks long after they formed, giving a younger date.
But other scientists thought it was a sign that the rock was a mixture of older and younger material.
Graham Pearson, a University of Alberta professor who has been dating and tracing some of Canada’s oldest rocks for around the past 15 years, said, “It’s really easy to get the wrong date on rocks using any approach.”
But he added that in the case of O’Neil’s original study, which he wasn’t involved in, some assumptions were made between the relationships of different rocks that allowed for two ways of interpreting the data.
To resolve the controversy, O’Neil’s team analyzed a piece of neighbouring rock from the NGB. The newly analyzed rock is an intrusion, or liquid magma that had squeezed its way between the cracks of the original rock at a later date and then solidified. By definition, that makes it younger than the original rock.
In the new analysis, funded by the Natural Sciences and Engineering Research Council of Canada and the government of Ontario, both isotopic clocks agree, O’Neil said, “giving the same exact age at 4.16 billion years old” for that newer rock.
Martin Bizzarro, a Canadian professor at the University of Copenhagen, was among those who thought mixing was behind the 2008 results. He acknowledged that the agreement of clocks in the new study is “rare.” But he told CBC News in an email, “I don’t think [the data] conclusively prove the rocks are Hadean” as there are other explanations “given the complexity of early Earth processes.”
Meanwhile, Pearson said he is convinced by the new data, given the variety of techniques O’Neil and his team use and their “really careful study.”
O’Neil hopes the new data will give other scientists confidence that the rocks are Hadean, and that’s the time period that pertains to signs of ocean chemistry, plate tectonics and life that his colleagues are finding in these same rocks. “That’s why the age of these rocks is so crucial.”
