Some Of The Earth's Original Crust Survives In Northern Canada

Jonathan O'Neil and Don Francis walk across nearly 3-billion-year-old rocks carrying samples formed from recycled material that dates from the very beginnings of the Earth's crust. Alexander Jean

The world has changed a lot in the almost 4.6 billion years since it formed. The vast majority of the crust that existed at that time has been swept into the mantle, losing all traces of its original composition. However, in northern Quebec, geologists have found traces of what they believe is original continental crust, dating back at least 4.2 billion years.

Australia, Canada, and Greenland have a bit of a competition going on for the world's oldest rocks. Other continents formed substantially more recently. Crystals known as zircons found in the Jack Hills of Western Australia date back 4.4 billion years, based on the decay of radioactive isotopes trapped inside.

These, however, are almost microscopic. In Science, University of Ottawa's Dr Jonathan O'Neil and Dr Richard Carlson of the Carnegie Institution report that parts of Canada, known as the Hudson Bay terrane, are composed of recycled, but much larger, rocks of similar age.

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These old rocks in Nunavik, northern Quebec, look quite pretty with water and hardy plants on top. Alexandre Jean

O'Neil and Carlson examined granite rocks aged 2.8 to 2.5 billion years old – impressive dates, but still long after the Earth's first dramatic years. However, the authors found that these rocks have a shortage of the isotope neodymium-142.

Neodymium-142 is a product of radioactive samarium-146, which gives up an alpha particle to form the metal O'Neil and Carlson sought. Samarium-146 has a half-life of 103 million years. Almost all of the small amount that the Earth started with had gone within the first 400 million years, leaving a legacy of Nd-142 behind. Rocks that formed after this time have consistent amounts of Nd-142, compared to its heavier sibling Nd-144.

On the other hand, rocks formed while there was still appreciable amounts of Sm-146 on Earth should have variable Nd-142:Nd-144 ratios depending on whether their origins are with rocks that incorporated or excluded samarium.

The authors argue that the low ratios of Nd-142 to Nd-144 seen in these samples could only have been produced during the Hadean era, the Earth's first geological period that ended 4 billion years ago. More than a billion years later, these rocks were melted and reformed as the granitoids we see today, but they held onto their original heavy metal composition.

Non-geologists might consider anything that has been remelted to not represent the original crustal material, but to geologists what matters is the elemental composition, which hasn't changed.

Such ancient rocks are only a small part of the Hudson terrane, most of which first emerged from the mantle less than 4 billion years ago.

Much of the information the original rocks might have conveyed to us has been lost in the recycling process, but in a video O'Neil said: “We can piece the puzzle together to try to understand how the oldest continents and the nucleus of our continets formed.”

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In the right place, you can reach out and touch the orgins of the Earth. Rick Carlson

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