Science Daily: Scientists have been puzzled by the fact that Earth’s mantle appears to have less lead than predicted by standard theories of planetary evolution. It has long been assumed that the planet formed from meteoritic material ejected from asteroids that smashed into each other, and thus the amount of lead in Earth’s mantle should be comparable to that found in meteorites. Yet until now, such a reservoir has gone undetected. To look for that hidden cache, researchers at MIT have been collecting rock samples from a region of northern Pakistan called the Kohistan arc; a collision of two massive tectonic plates there some 40 million years ago exposed some of Earth’s mantle. An analysis of those rocks revealed that some were much denser than the mantle and contained more lead. Based on that finding, the researchers calculated that roughly 70% of the magma that rises from the mantle during subduction events is so heavy with lead that it crystallizes into dense rock and drops back down into the mantle, where it collects and remains undisturbed. Their results could help further the study of how Earth has evolved.