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 unclear. Here using the oxygen isotope compositions of dated magmatic zircon, we show that the Pilbara Craton in Western Australia, Earth's best-preserved Archaean (4.0-2.5 billion years ago (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 δ 18 O, indicating crystallization from evolved magmas derived from hydrothermally altered basaltic crust like that in modern-day Iceland 1,2 . Shallow melting is consistent with giant impacts that typified the first billion years of Earth history 3-5 . Giant impacts provide a mechanism for fracturing the crust and establishing prolonged hydrothermal alteration by interaction with the globally extensive ocean 6-8 . A giant impact at around 3.6 Ga, coeval with the oldest low-δ 18 O zircon, would have triggered massive mantle melting to produce a thick mafic-ultramafic nucleus 9,10 . A second low-δ 18 O zircon cluster at around 3.4 Ga is contemporaneous with spherule beds that provide the oldest material evidence for giant impacts on Earth 11 . Stage 2 (3.4-3.0 Ga) zircons mostly have mantle-like δ 18 O and crystallized from parental magmas formed near the base of the evolving continental nucleus 12 . Stage 3 (<3.0 Ga) zircons have above-mantle δ 18 O, indicating efficient recycling of supracrustal rocks. That the oldest felsic rocks formed at 3.9-3.5 Ga (ref. 13 ), towards the end of the so-called late heavy bombardment 4 , is not a coincidence.