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From islands to estuaries, everything we see is a thin layer of ever-evolving dirt called the Earth’s crust. What you view on a daily basis is relatively young — approximately 600 million years old; the original crust is lost to time, as shifting tectonic plates bury it into the Earth’s molten mantle.
Or so we thought until today, when scientists revealed that remnants of the planet’s earliest crust are still accessible right in our backyard.
A group of geochemists ventured to Quebec to examine the age and composition of rocks with geological tracers, special chemicals — composed of neodymium and samarium — that are found in ancient stones. This area near the Hudson Bay in northern Canada is the center of the North American Craton, a part of the Earth’s surface that is relatively inactive.
This stability makes the region a perfect snapshot for ancient geology. The team unearthed rocks from approximately 2.7 billion years ago, the Archean period of Earth’s geological history.
A natural recycling process, whereby shifting plates swallowed up pieces of the Earth and spit them back out, created these ancient boulders. This recycling happens about every 250 million years, but the scientists found the original rocks were a lot older than that number would suggest.
“What we have found and confirmed is that the parent rocks are much, much older, about 1.5 billion years older [or 4.2 billion years old overall],” said Jonathan O’Neil, a geochemist at the University of Ottawa and lead researcher of the study published Thursday in the journal Science.
Reminder: The Earth is 4.5 billion years old.
Close-up of 2.7 billion years old continental crust from Nunavik, northern Quebec showing acomplex history of re-melting of oceanic-type rocks that were older than 4.2 billion years. Photo by Martin Simard
This assessment would have been impossible with traditional methods of dating the rocks. Every elemental isotope has a half-life, the amount of time that it takes to decay. More energetic tiers of the periodic table have shorter half-lives, while the stable elements have longer ones. Because of their shorter half-lives, the original isotopes are no longer found on Earth, but remnants of these extinct elements — like neodymium — are still present in the crust and can tell the true age of the rocks.
“Even if these rocks are 2.7 billion years old, because they have an anomalous amount of neodymium-142, it means that their parent rocks had to be formed prior to four billion years ago,” O’Neil said.
The boulders are much closer to oceanic crust than continental crust in terms of geologic composition. This means primitive oceanic crusts survived for much longer than they do today, O’Neil said, and it may provide clues into the geodynamics and plate tectonics on the early Earth.
Researchers Jonathan O’Neil and Don Francis returning to camp with backpacks full of rock samples. Photo by Alexandre Jean
“This is very timely and very exciting. Many scientists have been wondering about this kind of crust reworking,” said Yale University geophysicist Jun Korenaga, who was not involved in the study. “This is not a new question but they [the scientists] found a very clever way to demonstrate and show that a fraction of Archean rocks are actually made from older crust.”
O’Neil plans to head back into the field and collect a wider sampling of rocks. Given that the original study focused on 25 square miles, and the North American Craton is a little more than 3 million square miles, there’s a whole continent of information out there just waiting to be studied.
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