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Earth is unique in the solar system in having plate tectonics, which controls the growth of life-sustaining continents and the habitability of our planet. The oldest preserved continental crust comprises Eoarchaean (4.0–3.6 Ga) tonalite-trondhjemite (ETT) granitoids that formed through partial melting of thick Hadean (4.5–4.0 Ga) basaltic crust. However, the tectonic processes responsible for generating the earliest continental crust remain controversial. Partial melting of the thick basaltic crust may have occurred in primitive plate tectonic-like subduction zones at depths &gt;45 km. Alternatively, ETT could have been formed at shallower, intracrustal depths &lt;45 km, with plate tectonics starting much later in Earth’s history. Crucially, the composition of ETT requires that the partial melting left garnet and rutile as residual phases. Here, we report high-pressure–temperature melting experiments on a high-Fe, high-Ti oceanic plateau analogue for the early basaltic crust, which was chosen in order to maximize garnet and rutile stability. The experiments show that garnet and rutile are only stable in this starting composition at pressures ≥1.8 GPa (≈60 km). We conclude that magmas with the composition of Eoarchaean continental crust can only form at depths &gt;60 km in subduction-like environments, and that some form of primitive plate tectonics must have operated as early as the Eoarchaean.

Subduction Origin of the Nb Anomaly in Earth’s Oldest Continents