How did Earth’s early crust affect the gases released by volcanoes?

Earth’s early crust played a critical role in determining the chemical composition of the gases released by volcanoes through its level of oxidation. There is a direct relationship between the oxidation state of outgassed volatiles and the inorganic material they contact at volcanic temperatures.1

Geochemical evidence suggests that Earth’s early crust was much less oxidized than the modern crust, potentially containing metallic iron. If the earliest volcanic gases equilibrated with this metallic iron, the released products would have been more reduced: carbon would have been released as carbon monoxide, sulfur as hydrogen sulfide, and hydrogen as a mixture of molecular hydrogen and water vapour.1

In contrast, modern volcanic gases are oxidized because they interact with a modern crust that is already at least partly oxidized. As the early crust evolved and metallic iron was depleted, the outgassed products shifted toward more oxidized forms, such as carbon dioxide and water vapour. This process was fundamental to the evolution of the atmosphere, as these gases eventually contributed to the formation of the oceans and the early atmosphere.12