Incipient continent formation by shallow melting of an altered mafic protocrust.

Abstract

Understanding how Earth's continental nuclei first formed in the Archean eon (4.0-2.5 Ga) underpins our notions of early Earth geodynamics. Yet, the nature of Earth's early protocrust and the primary mechanism for its transformation are poorly understood, as very ancient rocks preserving petrological evidence for these processes are incredibly rare. Here we report the discovery of a formerly melt-bearing amphibolite from the Sylvania Inlier of the Pilbara Craton in Western Australia. Radiometric dating of zircon and titanite in these rocks constrain the time of partial melting to 3565 Ma, providing evidence for a metamorphic event that predates most exposed rocks in the Pilbara Craton by ~30 million years. Low δ¹⁸O compositions and modelled melt compositions comparable to evolved Hadean rocks in the Acasta gneisses indicate Earth's oldest continental crust may have sourced rocks of a similar composition. Thermodynamic modelling suggests partial melting at temperatures of 680-720°C and pressures of 0.8-1.0 GPa, implying a maximum burial depth of ~30 km. These results support models of continental nuclei formation via shallow partial melting of hydrothermally altered mafic protocrust in high heat flow environments.