Scientists say they have identified Earth's oldest rocks. It could reveal an unknown chapter in our planet's history
A rocky outcrop in a remote corner of northern Quebec appears serene in its eerie isolation on the eastern shore of Canada's Hudson Bay.
But over the past two decades, this exposed remnant of ancient ocean floor, known as the Nuvvuagittuq Greenstone Belt, has been a heated scientific battleground in the quest to identify Earth's oldest rock.
New research suggests that the geological site harbors the oldest known surviving fragments of Earth's crust, dating back to 4.16 billion years ago. It's the only rock determined to be from the first of four geological eons in our planet's history: the Hadean, which began 4.6 billion years ago when the world was hot, turbulent and hell-like.
'Rocks are books for geologists … and right now we're missing the book (on the Hadean). The Nuvvuagittuq Greenstone Belt would be at least one page of that book, so that's why it's so important,' said geologist Jonathan O'Neil, author of the research published Thursday in the journal Science.
The Nuvvuagittuq Greenstone Belt has been dated several times by different research groups, with widely divergent results. Most agree the rock is at least 3.75 billion years old — but that wouldn't make it Earth's oldest.
The Acasta Gneiss Complex, a group of rocks exposed along a riverbank nearly 200 miles (300 kilometers) north of Yellowknife, in northwestern Canada, is more widely agreed to be the planet's oldest geological formation. These rocks are unambiguously dated at 4.03 billion years old, marking the boundary between the Hadean Eon and the next chapter in Earth's history: the Archean. (There are older rocks on the planet — but not from the planet — that aren't part of this debate: Some meteorites are 4.5 billion years old.)
A controversial 2008 paper coauthored by O' Neil, who has been studying the site since he was a doctoral student, argued Nuvvuagittuq Greenstone Belt was 4.3 billion years old; however, other geologists took issue with the limits of the dating techniques and how the data was interpreted. With this latest paper, O'Neil, now an associate professor at the University of Ottawa in the department of Earth and environmental sciences, aims to prove his critics wrong.
Dating rocks involves using radiometric techniques that harness the natural and spontaneous radioactive decay of certain elements in the rock, which acts as a type of clock.
O'Neil uses an hourglass analogy: Imagine counting grains of sand at the top (radioactive elements) and bottom (elements produced from radioactive decay). Knowing the speed of the flowing grains (which represents the decay rate), allows scientists to date rocks. Some of these radiometric clocks are robust and can withstand the high temperatures and pressures Earth's crust has endured over the eons, while others are more affected by these processes.
The gold standard and easiest way to date very old rock formations is with a very tough mineral known as a zircon. These tiny crystals incorporate a bit of uranium into their structure, and researchers can pinpoint their age by measuring the radioactive decay of uranium atoms, which turn into lead at a known rate.
However, the Nuvvuagittuq Greenstone Belt — which was mapped after a geological survey in the 1960s but first attracted scientific attention in the early 2000s — contains very few rocks bearing zircons as they rarely occur in specimens with lower levels of silicon, including ones that were once ancient ocean crust.
'We tried to find zircons. They're just not there, or formed at a later time during the metamorphism or cooking of the rocks,' O'Neil said. Metamorphic rock is that which has been transformed by heat, pressure or other natural forces.
Instead, for the new study, O'Neil turned to the rare earth element samarium, which decays into the element neodymium. It's a technique that has been used to date meteorites because the elements were only active more than 4 billion years ago.
'The controversy about the age is that some people believe the clock we use is not good or it was affected (by other geological processes),' he said.
'It's a debate about what exactly we are measuring in time because we can't use zircon, and some people in my field would only be convinced by zircons.'
O'Neil said the technique was valuable in this case because it's possible to measure the decay of two variants, or isotopes, of samarium into two distinct isotopes of neodymium — essentially getting two clocks for the price of one. The latest paper focused on a specific type of metamorphic ancient rock — metagabbroic intrusions — sampled from within the belt, and the two data points converged on the same age: 4.16 billion years old.
This age, the study concluded, meant that 'at least a small remnant' of Hadean crust was preserved in the Nuvvuagittuq Greenstone Belt, which would provide invaluable insights into Earth's origins and how life formed.
Nearby rocks from the same location may preserve various signatures of life from the eon, as well as microfossils, tiny filaments and tubes formed by bacteria, noted Dominic Papineau, a senior research scientist at the Institute of Deep-sea Science and Engineering at the Chinese Academy of Sciences. He wasn't involved in the latest research but has studied fossils from the site.
'The rocks that were newly dated come from the mantle, which is not thought to harbour life or be habitable for life,' said Papineau, who is also an honorary professor of Precambrian biogeochemistry and exobiology at the University College London.
'However, the adjacent sedimentary rocks are now confirmed to be at least 4,160 million years old, which is 'only' about 400 million years after the accretion of our planet and of the Solar System,' he added in an email.
'Evidence of very early life in these sedimentary rocks indicate that the origin of life can take place very quickly (relatively speaking), which increases the probability that life is common and widespread in the universe.'
It's not yet clear whether Nuvvuagittuq outcrops will become widely accepted as Earth's oldest rocks, according to other scientists who were not involved with the research.
Bernard Bourdon, a geochemist at the Lyon Geology Laboratory in France who had previously taken issue with the earliest dates for Nuvvuagittuq Greenstone Belt published by O'Neil, said he was 'more convinced' by the latest work, and it was 'well improved' on previous studies.
'What is better, compared to the 2008 paper, is the fact that the two techniques … they give the same age. That's good. That's where we criticized the first results,' Bourdon, who is also research director at French scientific research body CNRS, said.
'In the end, I think there's more credibility to the age,' he said, adding that he had some 'small doubts' and would like to investigate the data more in depth.
The age of the rocks 'remains an unsolved mystery,' according to Hugo Olierook, a geoscientist and senior research fellow at Curtin University in Australia.
'In the absence of 'easy' minerals to date, they have turned to whole-rock, which is fraught with problems as whole-rock samples have multiple minerals,' Olierook said via email.
'It only takes one of these minerals to have been altered and their age 'reset' to a younger age for the whole house of cards to fall over,' he added, noting that very high and low temperatures can naturally alter the crystallization age of minerals in rock.
Very little is definitive when dealing with rocks and minerals that have complex geological histories spanning more than 4 billion years, according to Jesse Reimink, the Rudy L. Slingerland Early Career Professor of Geoscience at Penn State University.
'Even if these rocks are 'only' 3.8 billion years old, it is quite amazing that they are preserved. This current work presents more compelling data, supporting an age of 4.15 billion years ago, than that which was previously produced, which was already compelling,' Reimink said.
'The timescales are so long, and the history of these rocks and minerals is so tortured, that gleaning any primary information from them at all is pretty amazing.'
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