An extensive, thick MgO-rich primary crust underlain by highly residual mantle must have formed during the Archaean as a consequence of higher ambient mantle potential temperatures 1. However, the preserved volume of this crust is low suggesting much of it was recycled 2. Further, the tonalite-trondhjemite-granodiorites that dominate exposed Archaean crust cannot have been generated directly from MgO-rich primary crust since a hydrated low-MgO basalt source is required 3. Here we show that the thermodynamically stable mineral assemblages expected at the base of fully hydrated and anhydrous MgO-rich crust 45 km thick make it denser than the complementary underlying residual mantle. We use 2-D geodynamic models to explore the fate of this gravitationally unstable crust. Our results demonstrate that magmatically-overthickened MgO-rich crust, whether fully hydrated or anhydrous, could have delaminated by Rayleigh-Taylor instabilities for mantle potential temperatures > 1500-1550 °C, depending on rheology. The dripping instabilities generate return flow of asthenospheric mantle that melts adiabatically producing additional primary crust. Melting of overthickened and dripping MgO-rich crust and intracrustal fractionation of primary magmas both may produce the hydrated nature geoscience SUPPLEMENTARY INFORMATION

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