Giant impacts and the origin and evolution of continents

Earth is the only planet known to have continents, although how they formed and evolved is not well understood. Using the oxygen isotope compositions (SIMS) of dated magmatic zircon, we show that the Pilbara Craton in Western Australia, Earth’s best-preserved Archaean (4.0–2.5 Ga) continental remnant, was built in three stages. Stage 1 zircons (3.6–3.4 Ga) form two age clusters with one-third recording submantle δ¹⁸O, indicating crystallization from evolved magmas derived from hydrothermally-altered basaltic crust similar to that in modern-day Iceland. Shallow melting is consistent with giant meteor impacts that typified the first billion years of Earth history. Giant impacts provide a mechanism for fracturing the crust and establishing prolonged hydrothermal alteration by interaction with the globally extensive ocean. A giant impact at around 3.6 Ga, coeval with the oldest low-δ¹⁸O zircon, would have triggered massive mantle melting to produce a thick mafic–ultramafic nucleus. A second low-δ¹⁸O zircon cluster at around 3.4 Ga is contemporaneous with spherule beds that provide the oldest material evidence for giant impacts on Earth. Stage 2 (3.4–3.0 Ga) zircons mostly have mantle-like δ¹⁸O and crystallized from parental magmas formed near the base of the evolving continental nucleus. Stage 3 (<3.0 Ga) zircons have above-mantle δ¹⁸O, indicating efficient recycling of supracrustal rocks. That the oldest felsic rocks formed at 3.9–3.5 Ga, towards the end of the so-called late heavy bombardment, seems unlikely to be a coincidence.