A fundamental assumption underlies our understanding of Earth’s composition: The abundance of elements and isotopes on Earth must reflect the primordial dust, rock, and colliding planetesimals that accumulated to form the planet. Primitive meteorites known as chondrites are thought to best represent that raw material condensed from the solar nebula. Scientists naturally expected the compositions of such meteorites to mirror Earth’s, especially in neodymium and other rareearth elements that are refractory enough to avoid boiling off the planet during its formation.
In the early 1980s, comparisons between the abundance ratios of neodymium isotopes found in chondrites and those measured in mantle samples and magma squeezed from Earth’s midocean ridges seemed to bear out that assumption. Within the bounds of experimental error, 142Nd/144Nd measurements in chondrites and the bulk Earth appeared the same, except for a few anomalous outliers in data from the oldest rock found in Greenland....
