Within the first few hundred million years of our planet’s formation, during the so-called Hadean eon, Earth’s first rocks had frozen out of the molten surface to form a primordial crust. Chemical fractionation endowed those rocks with overabundances of the lighter rare-earth elements; they were richer in neodymium-144, for instance, than they were in heavier samarium-146. As a result, the few remaining rocks from that time carry a distinct isotopic signature: They are depleted in 142Nd—the radioactive decay product of 146Sm—relative to 144Nd. Seemingly as a rule, rocks younger than about 3.3 billion years don’t exhibit that depletion; the repeated cycling of material between the mantle and crust washed out the primordial 142Nd signature.
Now Jonathan O’Neil (University of Ottawa) and Richard Carlson (Carnegie Institution for Science) have discovered more than a dozen counterexamples to that unwritten rule. Near the Nuvvuagittuq belt on the eastern shoreline...
