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Arctic rocks shed light on ancient Earth's inner workings

The rocks found on Canada's Baffin Island originated in the Earth's mantle and are nearly as old as the planet itself, geochemists say.

August 11, 2010|By Amina Khan, Los Angeles Times

Geochemists studying Arctic rocks say they have found evidence of ancient rock from the interior of the Earth that is nearly as old as the planet itself.

Such material gives scientists an idea of what the mineral structure of the inner Earth used to look like billions of years ago and may force them to adjust their theories about the evolution of the planet's structure over the eons, said Matthew Jackson, a geochemist at Boston University and lead author of the paper.

The findings are published in Thursday's issue of the journal Nature.

Some scientists had said it was unlikely to find rock this old from the mantle, the solid part of the Earth's interior. Covered by the 5-to-25-mile-thick crust, the mantle, which makes up four-fifths of the planet's volume, is hard to get to and study.

And even when samples are found, they are usually relatively young. That is because the mantle rock is constantly recycled, emerging at the surface as molten volcanic material and being replenished by parts of the Earth's crust that are pushed back down into it and melt.

Because of all the mixing, no uncontaminated examples of what the Earth's mantle looked like at the beginning of its formation had been found before now.

The rocks in the Nature study were spewed up to the surface of the Earth by an eruption about 60 million years ago, said Carnegie Institution for Science geochemist Richard Carlson, one of the authors of the study.

They were obtained from a site on Baffin Island, in the Canadian Arctic, that was known to be volcanically active. Earlier isotope analysis of the rocks there had indicated that the mantle from which they were formed was very ancient, and therefore must not have been exposed to the same mixing as the rest of the mantle.

In the new study, the geochemists extended that analysis, examining a variety of isotopes to assess the age of the rocks. (Elements can occur in various forms, known as isotopes, and over time ratios of these isotopes gradually change, allowing researchers to estimate rocks' age.)

Using ratios of lead isotopes, the scientists were able to date the rocks to about 4.5 billion years ago, just shy of the Earth's own age.

Scientists are not sure how certain parts of the ancient mantle managed to escape the mixing that occurs deep underneath the crust.

"People that are making models of how the Earth's interior behaves … have to consider now the possibility that this signature [of ancient mantle] isn't completely obliterated. Some of it has survived," said David Graham, an isotopic geochemist at Oregon State University who wrote a commentary on the paper in Nature.

But regardless of how it happened, this ancient sample of the planet's internal makeup will provide important information to geologists trying to piece together the early history of the Earth and its inner workings, Graham said.

amina.khan@latimes.com

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