An analysis of isotopes in Arctic basalts indicates that the rocks may originate from a reservoir of ancient mantle that has avoided being recycled in the planet's active interior since Earth's infancy. The survival of such primitive samples from shortly after the formation of Earth, some 4.5 billion years ago, could provide important clues to the planet's composition and early geophysical history.

"We think we've found pretty strong evidence for the survival of a reservoir in the Earth's mantle that is pretty close to the age of the Earth," says geochemist Matthew Jackson of Boston University, lead author of the new study in the August 12 Nature. (Scientific American is part of Nature Publishing Group.)

Jackson and his colleagues based their study on an analysis of helium, lead and neodymium isotopes—atoms of the same element with different numbers of neutrons—in volcanic basalts from West Greenland and Baffin Island in Canada. Basalts from those areas had already been shown to have high ratios of helium 3 to helium 4, indicating a relatively primitive composition. "Helium 3 is a primordial isotope," Jackson says. "It's not produced inside the Earth." It is, however, found in the solar wind and is thought to have been a part of Earth's original chemical inventory that has since been escaping as gas during volcanic activity and geologic reprocessing. So rocks that retain a great deal of helium 3 relative to helium 4, such as the Arctic basalts, may have largely escaped geologic recycling since their formation.

But until recently, Jackson says, geochemists assumed that the neodymium isotopic ratios of the rocks, which did not match with that of primitive solar system material in chondritic (granule-containing) meteorites, implied that the samples had undergone melt processing and hence did not represent pristine terrestrial material. But a 2005 study in Science showed that early differentiation—the separation of a planetary body into layers such as crust and mantle—less than 30 million years after Earth had formed could account for ancient rocks with nonchondritic compositions. "The 2005 paper gives a hint of what the neodymium content should be like, and that's what we've found," Jackson says.

Alternatively, Earth's bulk composition could simply be different from that of chondrites. Either way, truly primitive rocks would be dated by lead isotopes, which are extremely sensitive to contamination by reprocessed material, to roughly 4.5 billion years ago, right around the time of Earth's formation. But none of the samples to date had the right lead composition to match an ancient reservoir of Earth mantle, Jackson says—until now.

"For the first time we find lavas that have the high helium 3–to–helium 4 ratio, the right neodymium based on this discovery in 2005, and they plot on the geochron," Jackson says, referring to a geochronological dating scheme based on lead isotope ratios. The implication is that the lavas, which erupted about 60 million years ago—relatively recently in geologic time—were unmixed, unprocessed mantle at the time of eruption. "We know that this mantle survived 98.5 percent of Earth's history," he adds.

The linking of helium and lead is a key component of the new study, says geochemist Erik Hauri of the Carnegie Institution for Science's Department of Terrestrial Magnetism in Washington, D.C., who did not contribute to the new research. Both elements have isotopes that emerge from the decay of the same parent element, uranium, but geologic processes can upset the balance between helium and lead. "When mantle reservoirs cycle through the Earth and melt near the surface and mix with other reservoirs, they can have their helium and lead systems decouple from each other," he says. The fact that both isotopic measurements are in agreement bodes well for the authors' contention that the basalts represent a long-unsullied pocket of mantle.

"This is really the first time that we've identified primitive helium and [primitive] lead together in the same reservoir," Hauri says. "That's what makes it a really unique find."