GENERATION OF ARCHAEAN (RELATIVELY) OXIDIZING AND WET MAGMAS FROM MAFIC CRUSTAL OVERTHICKENING

The geodynamic setting leading to the formation of the Earth’s first continents remains debated. Zircons preserved in Archaean granitoids record evidence of a relatively oxidizing and wet magmatic source. Subduction-related mechanisms for the formation of Archaean granitoids have been invoked to explain these signatures, suggesting an early initiation of subduction on Earth between 4.0 and 3.6 billion years ago (Eoarchaean). Here, I use forward petrological modelling and Monte-Carlo randomization models to show that relatively oxidizing and wet magmas resembling Archaean granitoids worldwide can occur from melts derived from the partial melting of an overthickened mafic crust in a non-subduction scenario. The formation of oxidizing and wet magmatic signatures is therefore not diagnostic of continental crust generation by subduction or of subduction initiation in the Eoarchaean. The apparent observed increase in oxygen fugacity and water contents during the Eoarchaean rather than representing subduction initiation, indicates magmatic thickening and melting of over-thicken crust with time.