Published August 12, 2010
Giving new meaning to the phrase "older than dirt," scientists have found evidence of an underground rock reservoir left over from the days when Earth was a ball of magma, a new study says.
Rocks recently found on Canada's Baffin Island erupted as lava from a deep rock reservoir that formed 4.5 billion years ago, when the solar system was new, the study says. The reservoir may hold the world's oldest rock, according to the authors.
(Related: "Oldest Rocks on Earth Discovered?" .)
Because Earth recycles its building blocks so thoroughly—particularly in the mantle, the layer just below the planet's relatively thin outer crust—no purely primordial material has been found before (overview of Earth's layers).
"We've been looking for pieces of the mantle that might have survived the chaotic mixing and churning within the deep Earth," said Boston University geochemist Matthew Jackson.
"But until now we haven't seen anything that looks like the original stuff that Earth formed from," added Jackson, who co-authored the study, published today in the journal Nature.
Even better, the newly analyzed rocks—which reached the surface some 62 million years ago—suggest a whole reservoir of the primordial rock could still lie somewhere beneath the Arctic, the study says.
The Mother of All Earth Rock?
Scientists discovered the rock's primordial origins by zeroing in on certain isotopes—atoms of the same element but with different numbers of neutrons.
One helium isotope, helium-3, is a signature of the early solar system. The isotope is rare in Earth's interior, because most helium-3 has been expelled by volcanic eruptions over the eons.
The Baffin Island rocks, though, were found to be high in helium-3, suggesting they came from a mantle area that had been relatively unmixed since just after the dawn of the solar system.
Lead isotopes in the rock seemed to confirm this theory, as they allowed the researchers to date the newly detected mantle reservoir to some 4.55 to 4.45 billion years ago—just a bit younger than Earth itself.
This type of primitive mantle material may be the mother of all the different rock and magma types we know today, the study suggests. The material probably existed even before the Earth's crust was extracted from the mantle below, and before the later mixing that recycled that crust back into mantle.
"If someone could live for tens of millions of years," Jackson said, he or she "would see the mantle flowing as a very dynamic, chaotic, mixing, and churning environment"—an environment that somehow left the primordial rock reservoir alone.
Why the reservoir survived remains a mystery. "Until now," he said, "we didn't think anything had escaped mixing."
Recent supercomputer models, though, have suggested that some reservoirs may be preserved in calm "eddies" in the flowing mantle or protected by their locations in the deepest mantle, where the convective motions of the Earth are thought to be less intense, Jackson said.
"The bottom line is that we really don't know how early reservoirs might be preserved (yet)," Jackson said via email. "But you can be sure that there are a lot of folks out there trying to figure this out!"
Whatever it's cause, the apparent primordial mantle deposit may still be fueling volcanic activity, according to Oregon State University geochemist David Graham, who wasn't involved in the study.
The deposit likely "fueled the mantle plume that gave rise to volcanic activity on Baffin Island and Greenland, and that is now responsible for volcanism in Iceland," due to the movement of Earth's tectonic plates, Graham wrote in an accompanying commentary published in Nature.
Rocky Time Capsules
The entire primordial magma reservoir could have been tapped out 62 million years ago, when it birthed the study rocks, study co-author Jackson added.
Even if that's the case, the deep-Earth reservoir still lasted for nearly 99 percent of Earth's geological history—preserving much yet-to-be-analyzed information about the infant Earth.
The rock's properties will likely lead scientists to tweak theories about what Earth's mantle chemistry was like in the planet's earliest days—and perhaps to rethink exactly how our planet evolved into our current home sweet home.
"Having a piece of the early Earth—it's like a time capsule," Jackson said. "This offers us the best possibility yet to understand the Earth's original composition."
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