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Earth's Oldest Cranny Explored
11 August 2010 5:24 pm
Despite billions of years of churning and melting beneath the tectonic plates, a pocket of deep mantle rock that formed just as Earth was first solidifying may have survived intact. And this old fogey still has kick: It may have spewed lavas on Canada's Baffin Island a mere 62 million years ago.
The Baffin Island rocks had showed signs of being the product of primordial stuff. They have the highest proportion of the isotope helium-3 relative to helium-4 of any rocks known. This suggests that the rocks came from a "primitive" region of Earth, as, unlike helium-4, helium-3 can't be replenished and thus must have come from the original building blocks of the planet. What's more, the ratio of two isotopes of the element neodymium match what geochemists would expect for a residue from Earth's early ocean of molten magma. That residue was proposed to have formed 4.5 billion years ago and sunk to the bottom of the mantle.
Now Matthew Jackson of Boston University and his colleagues have dated Baffin Island lavas using isotopes of lead. The age is about 4.50 billion years, they report in the 12 August issue of Nature. All three geochemical signatures are consistent with the idea that the eruption that produced the Baffin Island lavas tapped into a pocket of mantle rock undisturbed since a few tens of millions of years after Earth first came together. The consistency "doesn't prove we're right," says Jackson, "but it becomes pretty difficult to argue it's all serendipitous."
If Jackson and colleagues are indeed correct, they have in the Baffin Island lavas a sample of earliest Earth. How it would have gotten there is not certain. The rock formed from a huge volcanic eruption 62 million years ago where the continents were splitting apart to form the North Atlantic Ocean, about where Iceland is today. Many researchers think such eruptions are fed by towering plumes of hot rock rising from the bottom of the mantle hard against the molten iron core. In a scenario favored by co-author Richard Carlson of the Carnegie Institution of Washington's Department of Terrestrial Magnetism in Washington, D.C., the bottom of the mantle is also where rock that separated from primordial crust 4.5 billion years ago would have ended up.
Geochemical theories are notoriously hard to prove, so some scientists are hedging their bets about the new result. "I'm not going to commit myself," says geochemist Albrecht Hofmann of the Max Planck Institute for Chemistry in Mainz, Germany. "It's an exciting story, a possibility that has to be explored." Geochemist David Graham of Oregon State University, Corvallis, is willing to go further. "Taken together," he says, "it's reasonably strong evidence" of the oldest surviving glob of mantle rock in the planet.