Extinct radioactive clocks and the first 500 million years of Earth

Earth formed about 4.57 billion years ago, yet almost no rocks survive from its first ~500 million years. This is the interval in which the planet segregated a metallic core, cooled from a global magma ocean and formed its first crust, setting the initial conditions for its thermal and chemical evolution. With so little direct evidence, how can this period be studied? One answer lies in short‑lived radionuclides: isotopes that were present when the Solar System formed but whose half‑lives are so short that they have long since decayed away. Because the parent nuclide existed only during the first tens to hundreds of millions of years, any variation in the abundance of its daughter isotope must have been created within that window, and can then be carried forward in rocks of much younger age. Two such systems are particularly powerful: 146 Sm→ 142 Nd (half‑life 92 million years), which records melting and crystallization within the rocky mantle, and 182 Hf→ 182 W (half‑life 8.9 million years), which records the separation of metal from silicate. The remaining signals are tiny (differences of a few parts per million in isotope ratios) and resolving them requires mass spectrometry at the limit of achievable precision.

In this seminar, I will introduce these isotopic clocks and present recent results. High‑precision 142 Nd measurements in rocks spanning a wide range of ages constrain when Earth’s mantle first differentiated chemically, an event that may be linked to the crystallization of the last global magma ocean or to the formation of the first crust. I will also discuss 182 W variations in volcanic rocks whose sources may reside in the deep mantle. These point to chemical exchange between Earth’s core and mantle, which could be facilitated by secular core cooling, grain‑boundary diffusion, or chemical disequilibrium at the core–mantle boundary. Together, these observations show that signatures of events in Earth’s first few hundred million years can survive billions of years of mantle convection, far longer than previously recognized. Fundamental questions about the timing, mechanisms and consequences of early differentiation nevertheless remain open. Answering them is essential for constraining when Earth became habitable, and for establishing a baseline for comparison with other terrestrial planets and rocky exoplanets.