Earth's early continental crust formed from wet and oxidizing arc magmas.

Formation of continental crust has shaped the surface and interior of our planet and generated the land and mineral resources on which we rely. However, how the early continental crust of Earth formed is still debated. Modern continental crust is largely formed from wet and oxidizing arc magmas at subduction zones, in which oceanic lithosphere and water recycle into the mantle. The magmatic HO content and redox state of ancient rocks that constitute the early continental crust, however, are difficult to quantify owing to ubiquitous metamorphism. Here we combine two zircon oxybarometers to simultaneously determine magmatic oxygen fugacity (fO) and HO content of Archaean (4.0-2.5 billion years ago) granitoids that dominate the early continental crust. We show that most Archaean granitoid magmas were ≥1 log unit more oxidizing than Archaean ambient mantle-derived magmas and had high magmatic HO contents (6-10 wt%) and high HO/Ce ratios (>1,000), similar to modern arc magmas. We find that magmatic fO, HO contents and HO/Ce ratios of Archaean granitoids positively correlate with depth of magma formation, requiring transport of large amounts of HO to the lower crust and mantle. These observations can be readily explained by subduction but are difficult to reconcile with non-subduction models of crustal formation. We note an increase in magmatic fO and HO content between 4.0 and 3.6 billion years ago, probably indicating the onset of subduction during this period.