Canadian Rocks Stake a Claim as Earth's Oldest
Scientists may have unearthed vestiges of rock nearly as old as Earth itself.
A unique chemical signature suggests volcanic rocks recently collected on Baffin Island in the Canadian Arctic survived untouched for 4.5 billion years from before the melted mantle began firing up magma to form Earth's crust and plate tectonics then mixed that crust back into the mantle below.
"This may be the original stuff," said lead researcher Matthew Jackson of Boston University. "We may have discovered the oldest piece of the Earth."
Jackson and his colleagues think the newfound material somehow remained protected from the inner Earth's vigorous churning and recycling by hiding out in an ancient mantle reservoir. (The mantle is the layer of viscous, super-hot rock that roils below the thin, solid crust of the planet's surface.)
The researchers' novel finding could offer insights into the Earth's original composition, as well as the dynamics at play in the Earth's interior.
Of course, to label a piece of rock as primitive is not simple. Certain chemical requirements need to have been met.
Previous research had accounted for the first of three necessary conditions: Volcanic rocks on Baffin Island and nearby Greenland were found to contain a high concentration of one isotope of helium, called helium-3, compared with another, helium-4. (An isotope is a variation of a chemical element, with a differing number of neutrons in the atom's core.) Helium-3 is extremely rare most of the mantle's original supply has been released into space through volcanic eruptions while helium-4 is constantly replenished within the Earth.
"Enrichment in helium-3 is thought to reflect a very ancient signature in the Earth," Jackson noted. "But high helium-3 lavas have never had the correct lead isotope composition that would also suggest a primordial origin until now."
Jackson and his colleagues measured the isotopic compositions of lead in the Baffin Island lavas and found that they did, in fact, back up the helium's suggestion of ancient origin.
However, there was still the third hurdle, one that had long blocked geologists from identifying ancient mantle remnants.
Many researchers have thought the Earth was made of a special kind of meteorite called chondrite, which has led them on the hunt for what is called a chondritic neodymium isotope, a telltale signature of this origin. The hunt has been futile. But a recent discovery called this assumption into question. And based on the new theory, the non-chondritic neodymium isotopes that Jackson and his team identified in the Baffin Island lavas can be considered primitive.
"Finding the original material that continents, ocean crust and mountains formed from has been elusive," Jackson said. "But I think it's been because we've been looking under the wrong rock."
In the Baffin rocks, the primordial isotopes of helium, lead and neodymium had been found together for the first time; all three constraints appeared to be met.
"This might really be the 'Eve' reservoir," said Jackson, "from which all other mantle reservoirs were ultimately born."
The study is detailed in the Aug. 11 edition of the journal Nature.
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