Newly Discovered 'Hidden' Material Explains Missing-Mantle Mystery on Earth

Billions of years ago, Earth formed from a maelstrom of gas and dust circling the sun in the form of a massive disk--or at least that's the theory. For years, scientists have been trying to confirm this idea; yet something never added up. Although researchers should be able to find a large quantity of the lead found in meteorites in our planet's mantle, this material seems to be missing. Now, scientists may have found an explanation for this missing-mantle mystery.

The idea that lead should be present in the Earth's mantle comes from the general theory of our planet's formation. The giant disk surrounding the sun gathered into rocky clumps to form asteroids. These, in turn, created larger bodies of rubble which eventually cooled to form the planets. Yet if Earth arose from these collisions, its composition should resemble that of meteoroids, which contain lead. However, most of Earth consists of rocks with a high ratio of uranium.

In order to examine this particular mystery, researchers took a look at Earth itself. More specifically, they measured the composition of material that has dropped into the mantle. As two massive plates push against each other on Earth, one plate subducts under the other, pushing material from the crust and into the mantle. At the same time, molten material rises from the mantle and is ejected from volcanoes onto Earth's surface. By measuring the composition of this material, the researchers could find out what might have happened to the "missing" lead.

Yet measuring this composition is extremely difficult. Dense rocks form at a depth about 40 to 50 kilometers below the surface, which is why the scientists had to journey to a region of northern Pakistan called Kohistan arc. There, there's essentially a cross-section of the mantle-to-crust transition. The scientists collected several samples from the area and then analyzed the rocks' density and composition. Some of these were density-unstable, which meant that they could potentially sink into the mantle and create a hidden reservoir of material.

After examining these rocks, the researchers found that the denser rocks contained much more lead than uranium--exactly the ratio predicted for the "missing" material in the mantle. The researchers then performed a mass balance to determine how much dense rock drops into the mantle based on the composition of the region's crust, rocks and mantle. In the end, the scientists found enough evidence to indicate that there was enough material to suggest that Earth did indeed form from the collision of meteorites.

"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?" said Oliver Jagoutz, an assistant professor of geology at MIT, in a news release.  "You'd think it'd come back up, but it doesn't. It's actually interesting."

The findings are published in the journal Earth and Planetary Science Letters.