Heterogeneous Accretion and the Moderately Volatile Element Budget of Earth

Earth's Silver Lining The age of the oldest rocks on Earth's surface is controversial, but, even if they are at their oldest estimate, hundreds of millions of years in our planet's earliest history are still missing. However, in some rocks that until relatively recently resided in the mantle, the isotopic signature from the time of Earth's formation is still preserved. Schönbächler et al. (p. 884 ) exploited this preservation to constrain models that describe the early material that assembled together to form Earth. Because the isotopic profile of silver in these rocks is nearly identical to that measured in a class of primitive meteorites, the earliest material probably had high volatile content. However, the fractionation of other isotopes suggests that the volatile content probably decreased over time in subsequent accretion events. With these isotopic model constraints, it is possible that one of the last major collisions—the Moon-forming giant impact—added considerable amounts of water and other volatile elements to Earth.