But in the deep past – as far back as the Neoarchean era 2.8 to 2.5 billion years ago – this oxygen was almost absent.

These magmas are formed when oxidized sediments and bottom waters – cold, dense water near the ocean floor – are introduced into the Earth's mantle.

The identification of such magmas in Neoarchean magmatic rocks could provide evidence that subduction and plate tectonics occurred 2.7 billion years ago.

These new findings indicate that oxidized magmas did form in the Neoarchean era 2.7 billion years ago.

The data show that the lack of dissolved oxygen in the Archean ocean reservoirs did not prevent the formation of sulfur-rich, oxidized magmas in the subduction zones.

Previously, it was thought unlikely that Archean magmas could be oxidized, when the ocean water and ocean floor rocks or sediments were not.

While the exact mechanism is unclear, the occurrence of these magmas suggests that the process of subduction, where ocean water is taken hundreds of kilometers into our planet, generates free oxygen.

Our study shows that Archean subduction could have been a vital, unforeseen factor in the oxygenation of the Earth, the early whiffs of oxygen 2.7 billion years ago and also the Great Oxidation Event, which marked an increase in atmospheric oxygen by two percent 2.45 to 2.32 billion years ago.