Imagine you suddenly discovered part of your umbilical cord was still attached. Scientists just did that for the planet Earth. What's been found is a clear sign that beneath the crust in northern Canada there is a chunk of pristine, undisturbed rock from the time when Earth was nothing but molten rock.
The evidence comes in the form of lava rocks that, themselves, are a mere 60 million years old. But these rocks contain an early Earth mixture of helium, lead and neodymium isotopes which suggest the mantle rock beneath the crust that yielded them is a virgin pocket of Earth's original material.
That pocket had survived for 4.5 billion years under Baffin Island without being mixed by plate tectonics or erupted onto the surface.
"I was surprised that any of the (original) mantle survived," said geoscientist Matthew Jackson of Boston University. He is the lead author on a paper announcing the discovery in this week's issue of the journal Nature. "Finding a piece of the original mantle has been a holy grail. The original Earth was a big ball of magma. That's our (planet's) original composition."
The discovery has surprised other researchers as well.
"Even if a vestige of such material remained, it seems unlikely that it would be found in any samples from Earth's surface or the shallow subsurface that are available to geologists," observed David Graham of Oregon State University in Corvallis, who wrote a commentary in the same issue of Nature. "Yet that is what (this) new evidence suggests."
One of the obstacles in finding rocks from such ancient mantle, up to now, has been that researchers had assumed an early Earth was composed of rocks with helium and lead isotope matching those of a type of ancient meteorite called a chondrite.
That may be true up to a point, said Jackson. Some recent research by scientists at the Carnegie Institution of Washington has suggested that the Earth's early mantle would also have tell-tale neodymium isotopes that are unlike chondrites.
"That turns out to be the same as we find in these lavas (from Baffin Island)," said Jackson.
The other signs of untouched ancient mantle material -- which has not before lost any of its material to Earth's surface or been otherwise tainted -- is large amount of the isotopes helium-3 relative to helium-4. There is also an very old lead-isotope signature. It was these three criteria -- the helium, lead and neodymium -- that led Jackson and his team to the conclusion Baffin Islands massive volcanic cliffs are made of the oldest material on the planet.
As for how much of this original mantle might be around, the only way to tell is to look at lava rocks and see if they came from such stuff, said Jackson.
"We have no idea how common it might be," Jackson told Discovery News. Models suggest that up to 10 percent of the early mantle might still be around. But the new discovery could change those models and their predictions. "It turns everything on its head."
Thursday, August 12, 2010
Chunk of Original Earth Found : Discovery News
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