The origin of Earth's first continents and the onset of plate tectonics

The growth and recycling of continental crust has resulted in the chemical and thermal modification of Earth's mantle, hydrosphere, atmosphere, and biosphere for similar to 4.0 b.y. However, knowledge of the protolith that gave rise to the first continents and whether the environment of formation was a subduction zone still remains unknown. Here, tonalite melts are formed in high P-T experiments in which primitive oceanic plateau starting material is used as an analogue for Eoarchean (3.6-4.0 Ga) oceanic crust generated at early spreading centers. The tonalites are produced at 1.6-2.2 GPa and 900-950 degrees C and are mixed with slab-derived aqueous fluids to generate melts that have compositions identical to that of Eoarchean continental crust. Our data support the idea that the first continents formed at ca. 4 Ga and subsequently, through the subduction and partial melting of similar to 30-45-km-thick Eoarchean oceanic crust, modified Earth's mantle and Eoarchean environments and ecosystems.