More about the oldest rocks ever

Rocks formed from old mantle were found on Baffin Island in northwestern Canada. The rocks have been seen before, but they awaited dating methods that could distinguish them from the rest of the rocks on Earth. When lava solidifies, elements with their isotopes are frozen into it. We can use them to calculate the age of the rock.

The Earth’s mantle is the hot, stony layer between its crust and the nickel-iron core. Its upper layers can flow, however slowly, but further down it is compressed into a solid by the weight above it.

How can the mantle be modern or primitive? Isn’t it just a ball of molten ooze? Not exactly.

As the Earth formed, it melted from the energy of incoming material. When the loose material in the inner system was mostly accumulated, bombardment slowed and the molten Earth cooled off. In the molten crust, dense material sank to become the core and lighter material floated to the surface. Slow convection currents in the magma probably helped the continents to form in the cooler areas when less dense material was pushed aside by hot, rising plumes. Continents of lightweight silicon-aluminum rock, or sial, float on denser silicon-magnesium rock, or sima. Since the Earth’s crust cooled and solidified it has cracked into several large slabs or plates. The plates slide under or over each other, so that the upper plate is lifted into mountains, as in the U.S. Rockies. They ram into each other, and are compressed, buckling and folding into mountains, as in the Canadian Rockies.  The most spectacular example occurs where India collided with Asia, raising the Himalayas.

Over millions of years, wind, water, ice, and gravity erode the mountains back into rocks and particles, creating new sedimentary rock. However, particles in this rock still have the age signature of the original crust.  Discovery.com has the story:

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 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 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 a 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 that the 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.

It occurs to me that if there are longstanding downward currents of cooled magma in the mantle, there are places that are between those “downdrafts” and furthest from their influence. Those would be the most likely places for old magma to stay in place near the surface.

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