Saturday, March 29, 2008

Hawaii scientists find seafloor rock 2 billion years old.

All known life on Earth lives on the planet's solid crust, but below that, there is a roiling mass of molten rock known as the mantle.
Scientists have long considered the mantle to be a well-blended mix of rock—but a team of University of Hawai'i researchers recently found something new and different in rock three miles deep under the Arctic ice, near the North Pole.
There, beneath a feature called the Gakkel Ridge, they found some rock that had not mixed well, and which still retained the mineral signatures of two billion years ago.
UH geologist Eric Hellebrand was co-leader of an expedition aboard the German research icebreaker Polarstern. UH postdoctoral researcher Anette von der Handt also participated. Their research findings were printed in the March 20, 2008, edition of the journal Nature.
In the language of the geologists, when moltten mantle rock finds its way through the crust and erupts to the surface, it is expected to be well-mixed or homogeneous. What they found when they brought up samples from the ridge on the Arctic ocean floor was rock that was, instead, heterogeneous—not like everything else.
It's like biting into a plain cookie and coming across a chocolate chip. And now, scientists are wondering whether they'll find more surprises if they keep looking.
“The unprecedented discovery of these rocks indicates that mantle heterogeneity may turn out to be more widespread in mid-ocean ridge settings than inferred from the more commonly studied erupted lavas,” said a UH press release announcing their find. The results of the Arctic research are published in the March 20th edition of the prestigious journal Nature.
When they put their rocks under a microscope, Hellebrand and von der Handt—who were part of an international research team-- what they found was unexpected and “as rare and fascinating as moon rocks.”
It was rock that had not been significantly altered by mixing, and studies of isotopes of the rare metal osmium indicated the rock was 2 billion years old.
“We can’t exaggerate how important these rocks are – they’re a window into that deep part of the earth,” said paper co-author Jonathan Snow, an assistant professor of geosciences at the University of Houston.
They may, in fact, be a kind of ancient library, a technique for looking into the early geology of our planet.
That's not to say that there isn't any diversity within the molten rock of the mantle. There is.
“Compared to the varied and ancient continental crust, most geologists think of the mantle as a relatively homogeneous region of the deep Earth”, Hellebrand said.
“Some variations in composition, or heterogeneities, are well known, and are seen in volcanic rocks from Hawaii and Iceland.”
But to find rocks that hadn't participated in the vast global mixing—that was special, the researchers said.
© 2008 Jan W. TenBruggencate