Published online 25 September 2008 | Nature | doi:10.1038/news.2008.1128

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Record-breaking rocks?

Radiometric dating suggests Canadian outcrop contains the world's oldest rock.

The latest contender for the title of 'world's oldest rock' juts unassumingly out of the Canadian tundra, where data indicate it could have formed 4.28 billion years ago. If so, the beige outcropping bests the previous oldest rocks by 250 million years, and could be the closest thing to Earth's original crust in existence today.

"It opens a whole new window on the early Earth," says lead author Jonathan O'Neil, a graduate student in geology at McGill University in Montreal, Canada. The work is published in Science1.

The chemical make-up of the Canadian rocks are illustrative of their great age.AAAS/SCIENCE

Looking for a date

Until now, the oldest known rock formation was the Acasta gneiss in Canada's Northwest Territories, clocking in at 4.03 billion years old.2 O'Neil's rocks are in the Nuvvuagittuq greenstone belt, exposed bedrock on the eastern edge of Hudson Bay in Quebec. Ancient rocks are rare because most of Earth's primordial crust has been recycled by plate tectonics.

Scientists frequently use zircon crystals to date old rocks, but O'Neil's samples didn't have any zircon. Instead he focused on the isotope neodymium-142, which he found in unusually low levels in his rocks. Neodymium-142 is formed when samarium-146 decays, and the terrestrial concentration of neodymium-142 became fixed around 4.1 billion years ago when the last of Earth's samarium-146 finished decaying. When crust forms, it preferentially incorporates neodymium, but not samarium. The low neodymium levels, then, suggest that O'Neil's rocks formed before the samarium-146 had all decayed.

The samarium/neodymium chronometer has been used to date meteorites, but is less common for terrestrial samples. As samarium-146 no longer exists, the scientists had to estimate the samarium levels in the early Solar System. With that assumption and their samples, they calculated that the oldest Nuvvuagittuq rock was 4.28 billion years old, give or take around 50 million years.

"A very plausible interpretation of the data is that it means the rocks are actually 4.28 billion years old," says Vickie Bennett, a geochemist at the Australian National University in Canberra. "These were very careful measurements by a well-respected lab."

Confirmation needed

O'Neil continues to search the greenbelt for zircon, knowing that it would be more convincing to many geologists. "It would just nail the coffin, actually," he says. An alternative explanation, O'Neil adds, is that the rocks could be low-neodymium counterparts to 3.85-billion-year-old rocks in Greenland that contain higher-than-average amounts of the isotope3. That kind of variance suggests that the mantle was incompletely mixed when the rocks formed.

The Nuvvuagittuq greenstone belt is not just ancient, it is distinctive in its make-up. Greenstone formations usually contain hornblende, a dark-green or black compound. In its place, Nuvvuagittuq hosts mainly brownish cummingtonite, making it more of a beigestone belt. Thus, O'Neil thinks that perhaps conditions on Earth were different when these rocks formed.

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The rocks might also show evidence of the Late Heavy Bombardment, a time between 3.8 billion and 4.1 billion years ago when meteorites smashed into the young planets of the early Solar System, creating many of the craters on the Moon. "These rocks were presumably around when that occurred," O'Neil says. "I'd like to try to find traces of that event in these rocks." 

  • References

    1. O'Neil, J., Carlson, R. W., Francis, D. & Stevenson, R. K. Science 321, 1828–1831 (2008).
    2. Bowring, S. A. et al. Contrib. Mineral. Petrol. 134, 3-16 (1999). | Article | ISI | ChemPort |
    3. Bennett, V. C. et al. Science 318, 1907–1910 (2007).
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