Where does Earth’s oxygen come from? A new study hints at an unexpected source
The amount of oxygen in Earth’s atmosphere makes it a habitable planet.
Twenty-one percent of the atmosphere is made up of this life-giving element. 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.
So how did Earth’s atmosphere become saturated with oxygen?
Our researchpublished in Nature Geoscienceadds a tantalizing new possibility: that at least some of Earth’s early oxygen came from a tectonic source through the movement and destruction of Earth’s crust.
The Archean Earth
The Archean Eon represents a third of our planet’s history, from 2.5 billion years ago to four billion years ago.
On the modern Earth, the dominant tectonic activity is called plate tectonics, where the oceanic crust—the outermost layer of the Earth beneath the oceans—subducts into the Earth’s mantle (the region between the Earth’s crust and its core) at points of convergence called subduction zones. However, there is considerable debate as to whether plate tectonics was at work as early as the Archean era.
One feature of modern subduction zones is their association with oxidized magmas. These magmas form when oxidized sediments and bottom waters—cold, dense water near the bottom of the ocean—are introduced into the Earth’s mantle. This produces magma with high oxygen and water content.
Our study aimed to test whether the absence of oxidized materials in Archean bottom waters and sediments could prevent the formation of oxidized magmas. The identification of such magmas in Neoarchean igneous rocks may provide evidence that subduction and plate tectonics occurred 2.7 billion years ago.
We collected samples of granitoid rocks ranging in age from 2750 to 2670 Ma from across the Abitibi-Wawa Subprovince of the Superior Province—the largest preserved Archean continent, extending over 2000 km from Winnipeg, Manitoba, to far eastern Quebec. This allowed us to investigate the level of oxidation of the magmas generated during the Neoarchean era.
Measuring the degree of oxidation of these igneous rocks — formed by the cooling and crystallization of magma or lava — is challenging. Post-crystallization events may have altered these rocks through later deformation, burial, or heating.
So, we decided to take a look mineral apatitewhich is present in zircon crystals in these rocks. Zircon crystals can withstand the high temperatures and pressures of post-crystallization events. They preserve clues to the environment in which they were originally formed and provide an accurate age for the rocks themselves.
Tiny apatite crystals that are less than 30 microns wide—about the size of a human skin cell—are trapped within the zircon crystals. They contain sulfur. By measuring the amount of sulfur in the apatite, we can determine whether the apatite came from oxidized magma.
We were able to successfully measure oxygen volatility of the original Archean magma—which is essentially the amount of free oxygen in it—using a specialized technique called near-edge X-ray absorption structural spectroscopy (S-XANES) at the Advanced Photon Source synchrotron at Argonne National Laboratory in Illinois.
Creating oxygen from water?
We found that the sulfur content of the magma, which was initially about zero, increased to 2,000 parts per million about 2,705 million years ago. This indicates that the magma has become richer in sulfur. Furthermore, predominance of S6+ — a type of sulfur ion — in apatite suggests that the sulfur is from an oxidized source, a coincidence data from the host zircon crystals.
These new findings show that oxidized magmas formed in the Neoarchean era 2.7 billion years ago. The data show that the lack of dissolved oxygen in Archean oceanic reservoirs did not prevent the formation of sulphur-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 found that the occurrence of these oxidized magmas correlates with major gold mineralization events in the Superior Province and Yilgarn Craton (Western Australia), demonstrating a link between these oxygen-rich sources and the global formation of world-class ore deposits.
The consequences of these oxidized magmas go beyond the understanding of early Earth geodynamics. It was previously thought unlikely that Archean magmas could be oxidized when ocean water and rocks or sediments on the ocean floor they weren’t.
Although the exact mechanism is unclear, the appearance of these magmas suggests that the process of subduction, in which ocean water is carried hundreds of kilometers into our planet, generates free oxygen. This then oxidizes the upper mantle.
Our study shows that Archean subduction may have been a vital, unforeseen factor in Earth’s oxygen saturation in the early a whiff of oxygen 2.7 billion years ago and also of A major oxidation event that saw atmospheric oxygen increase by two percent 2.45 to 2.32 billion years ago.
As far as we know, Earth is the only place in the Solar System – past or present – with plate tectonics and active subduction. This suggests that this research may partially explain the lack of oxygen and ultimately life on other rocky planets in the future as well.