Generation of Archaean oxidizing and wet magmas from mafic crustal overthickening

Abstract

The geodynamic setting leading to the formation of 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, in the Eoarchaean era. 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. Instead, the apparent observed increase in oxygen fugacity and water contents during the Eoarchaean may indicate magmatic thickening and melting of overthickened crust with time, suggesting that this process may have contributed to the development of Earth’s first continents. The high oxygen fugacities and water contents recorded by zircons from Archaean granitoids can be explained by partial melting at the base of overthickened oceanic crust without requiring subduction, according to a phase equilibrium modelling study.