Scientists still have doubts over the theory that Earth arose from the collision of asteroids, as its composition does not resemble that of meteoroids — the small particles that break off from asteroids.
The Earth’s mantle is missing an amount of lead found in meteorites whose composition has been analyzed following impact with the Earth.
Much of the Earth is composed of rocks with a high ratio of uranium to lead (uranium naturally decays to lead over time). However, according to standard theories of planetary evolution, the Earth should harbour a reservoir of mantle somewhere in its interior that has a low ratio of uranium to lead, to match the composition of meteorites. But such a reservoir has yet to be discovered — a detail that leaves Earth’s origins hazy.
Now researchers in MIT’s Department of Earth, Atmospheric and Planetary Sciences have identified a “hidden flux” of material in the Earth’s mantle that would make the planet’s overall composition much more similar to that of meteorites. This reservoir likely takes the form of extremely dense, lead-laden rocks that crystallize beneath island arcs, strings of volcanoes that rise up at the boundary of tectonic plates.
As plates subduct, material is pushed from the crust down into the mantle. At the same time, molten material from the mantle rises up to the crust, and is ejected via volcanoes onto the Earth’s surface.
According to the MIT researchers’ observations and calculations, however, up to 70 per cent of this rising magma crystallizes into dense rock-dropping, lead-like, back into the mantle, where it remains relatively undisturbed. The lead-heavy flux, they say, puts the composition of the Earth’s mantle on a par with that of meteorites.
“This has a lot of implications for understanding how the Earth evolved through history,” said Oliver Jagoutz, an assistant professor of geology at MIT.