the amount of oxygen in Earth’s atmosphere makes it a habitable planet.
Twenty-one percent of the atmosphere consists of this life-giving element. But in the deep past, already in the neo-archaic era, 2,800 to 2,500 million years ago, this oxygen was almost absent.
So how did Earth’s atmosphere become oxygenated?
our researchpublished in geoscience of natureadds a tantalizing new possibility: that at least some of Earth’s early oxygen came from a tectonic source through the movement and destruction of the Earth’s crust.
The Archaic Earth
The Archean eon represents a third of the history of our planet, from 2,500 million years ago to 4,000 million years ago.
This alien Earth was a water world covered in green oceanswrapped in a methane mist, and completely devoid of multicellular life. Another strange aspect of this world was the nature of its tectonic activity.
On modern Earth, the dominant tectonic activity is called plate tectonics, where the oceanic crust, the outermost layer of Earth beneath the oceans, sinks into the Earth’s mantle (the area between the Earth’s crust and its core) at convergence points called subduction zones. . However, there is considerable debate as to whether plate tectonics operated in the Archean era.
A characteristic of modern subduction zones is their association with oxidized magmas. These magmas form when oxidized sediments and bottom waters (cold, dense water near the ocean floor) are introduced into the earth’s mantle. This produces magma with high oxygen and water content.
Our research aimed to test whether the absence of oxidized materials in the Archean bottom waters and sediments could prevent the formation of oxidized magmas. Identification of such magmas in Neoarchean magmatic rocks could provide evidence that subduction and plate tectonics occurred 2.7 billion years ago.
We collected samples of 2.75 to 2.67 billion-year-old granitoid rocks from the entire Abitibi-Wawa subprovince of the Upper Province, the largest preserved Archean continent stretching 2,000 km from Winnipeg, Manitoba, to the tip of eastern Quebec. This allowed us to investigate the level of oxidation of the magmas generated throughout the Neoarchaic era.
Measuring the oxidation state of these magmatic rocks, formed through the cooling and crystallization of magma or lava, is challenging. Post-crystallization events may have modified these rocks through subsequent deformation, burial, or heating.
So we decided to look at the minerals apatitewho is present in the zircon crystals on these rocks. Zircon crystals can withstand the intense temperatures and pressures of post-crystallization events. They hold clues to the environments in which they originally formed and provide precise ages for the rocks themselves.
Tiny apatite crystals that are less than 30 microns across, the size of a human skin cell, become trapped in the zircon crystals. They contain sulfur. By measuring the amount of sulfur in the apatite, we can establish whether the apatite grew from oxidized magma.
We were able to successfully measure the oxygen fugacity of the original Archean magma, which is essentially the amount of free oxygen it contains, using a specialized technique called X-ray absorption near-edge structure spectroscopy (S-XANES) at the Advanced Photon Source Synchrotron Argonne National Laboratory in Illinois.
Create oxygen from water?
We found that the sulfur content of the magma, which was initially around zero, increased to 2,000 parts per million around 2,705 million years ago. This indicated that the magma had become richer in sulfur. Also, the predominance of S6+, a type of sulfur ion, in apatite suggested that the sulfur came from an oxidized source, agreeing the data of the host zircon crystals.
These new findings indicate that the oxidized magmas formed in the Neo-Archaic era 2.7 billion years ago. The data show that the lack of dissolved oxygen in Archean oceanic deposits did not prevent the formation of sulfur-rich oxidized magmas in subduction zones. The oxygen in these magmas must have come from another source and was eventually released into the atmosphere during volcanic eruptions.
We find that the occurrence of these oxidized magmas correlates with major gold mineralization events in the Upper Province and Yilgarn Craton (Western Australia), demonstrating a connection between these oxygen-rich sources and the global formation of gold ore deposits. world class.
The implications of these oxidized magmas go beyond understanding the geodynamics of the early Earth. Previously, it was thought unlikely that Archean magmas could oxidize when the ocean water Y rocks or sediment from the ocean floor they were not.
Although the exact mechanism is not clear, the appearance of these magmas suggests that the process of subduction, where ocean water is carried hundreds of kilometers towards our planet, generates free oxygen. This then oxidizes the overlying mantle.
Our study shows that Archean subduction could have been a vital and unexpected factor in the oxygenation of the Earth, the first puffs of oxygen 2.7 billion years ago and also him Great Oxidation Event, which marked an increase in atmospheric oxygen by two percent 2.45 to 2.32 billion years ago.
To the best of our knowledge, Earth is the only place in the solar system, past or present, with plate tectonics and active subduction. This suggests that this study could partially explain the lack of oxygen and ultimately also life on the other rocky planets in the future.