July 17, 2013
Earth’s Origins May Be Explained Through Mantle’s ‘Hidden Flux’
Brett Smith for redOrbit.com - Your Universe Online
One of the more popular theories surrounding the formation of the planets involves the countless collisions of smaller objects in orbit around the sun 4.5 billion years ago. However, proponents of that theory are missing one thing: the Earth's chemical composition is distinctly different from the meteors that are currently striking the planet.
Scientists have found that the lead-uranium ratio of meteors is much different than that of the Earth's mantle, but a new study from an MIT researcher has found a "hidden flux" of material in the mantle that could explain the difference.
As tectonic plates in the Earth's crust slide against each other, one is often pushed down deep into the mantle. Simultaneously, liquid, hot magma from the mantle rises to the surface via volcanic eruptions.
According to the MIT scientist's study, published in Earth and Planetary Science Letters, as much as 70 percent of this rising magma turns into dense rock and drops back into the mantle. When calculated into the entire composition of the mantle, this lead-heavy flux of material puts Earth's composition much closer to that of meteorites.
"Now that we know the composition of this flux, we can calculate that there's tons of this stuff dropping down from the base of the crust into the mantle, so it is likely an important reservoir," said co-author Oliver Jagoutz, an assistant professor of geology at MIT. "This has a lot of implications for understanding how the Earth evolved through history."
To reach their conclusion, the scientists examined an area of northern Pakistan called the Kohistan arc, which was once a group of islands that were slowly smashed as India collided into Asia 40 million years ago.
"When India came in, it slammed into the arc, and the arc extended and rotated itself," Jagoutz said. "Because of that, we now have a cross-section of the mantle-to-crust transition. This is the only place on Earth where this exists."
Between 2000 and 2007, Jagoutz traveled the region collecting various rock samples representing the crust and mantle. An analysis of these rock samples revealed that some of them were "density-unstable" -- meaning they could potentially sink into the mantle and create a hidden lead reservoir. A further analysis showed that the amount of lead that the samples represented would be enough to explain the uranium-lead ratio discrepancy between Earth and meteors.
Bruce Buffett, a professor of earth and planetary science at the University of California at Berkeley, said the hidden reservoir theory supported by the study is "interesting and plausible," but he noted that there are other valid theories.
"There are a large variety of options on the table to explain the complex structure we detect seismically at the bottom of the mantle," said Buffett, who was not involved in the study. "The attractive aspect of [Jagoutz's] idea is that it has testable consequences. This is how progress is made."
"If we are right, one of the questions we have is: Why is the Earth capable of hiding something from us? Why is there never a volcano that brings up these rocks?" Jagoutz added. "You'd think it'd come back up, but it doesn't. It's actually interesting."