Newfound rocks may be progeny of primordial crust

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 of Canada’s Hudson Bay, the researchers found 15 granitic rocks that exhibit Hadean-like ¹⁴²Nd deficits despite being just 2.7 billion years old. Based on the samples’ isotopic profiles, O’Neil and Carlson effectively ruled out every possible explanation except one: Somehow, Hadean material survived for more than 1 billion years—many times a rock’s life expectancy—before being recycled to form the granitic rocks. The discovery may help researchers pinpoint elusive events in the planet’s geological time line, such as the emergence of tectonics and the formation of oceans. (J. O’Neil, R. W. Carlson, Science 355, 1199, 2017, doi:10.1126/science.aah3823 .)