Where did Earth’s oxygen come from? A new study hints at an unexpected source

Where did Earth's oxygen come from?  A new study hints at an unexpected source

Granitic rocks between 2,750 and 2,670 million years old collected from the largest preserved Archean continent may help reveal the origin story of Earth’s oxygen. Credit: Dylan McKevitt, provided author

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 – since the modern era 2.8 to 2.5 billion years ago – this oxygen was almost absent.

So how did Earth’s atmosphere become oxygenated?

Our research published in Natural Earth Sciencesadds a tantalizing new possibility: that at least some of Earth’s early oxygen came from a tectonic source through movement and destruction of the Earth’s crust.

Archean land

The Archaeological Era represents a third of our planet’s history, from 2.5 billion years ago to four billion years ago.

This strange land was a watery world, covered in green oceans, shrouded in methane haze and completely lacking in multicellular life. Another strange aspect of this world is 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 land beneath the oceans—sinks into the Earth’s mantle (the area between the Earth’s crust and core) at meeting points called subduction zones. However, there is considerable debate as to whether plate tectonics made a comeback in the Archean era.

One feature of recent subduction zones is their association with oxidized magmas. This magma forms when oxidized sediments and bottom waters — cold, dense water near the ocean floor — are introduced into the Earth’s mantle. This produces magma that has a high oxygen and water content.

Our research aims to test whether the absence of oxidants in Archean bottom waters and sediments can prevent the formation of oxidized magmas. The identification of such magma in new igneous rocks could provide evidence that subduction and plate tectonics occurred 2.7 billion years ago.

Where did Earth's oxygen come from?  A new study hints at an unexpected source

Map of the Upper Province extending from central Manitoba to eastern Quebec in Canada. Credit: Xuyang Meng, author provided

We collected samples of 2,750- to 2,670-million-year-old granite rocks from across the Upper Province’s Abetepe Wawa sub-province – the largest preserved Archean continent stretching 2,000 kilometers from Winnipeg, Manitoba, to far eastern Quebec. This allowed us to investigate the level of magma oxidation generated throughout the new age.

Measuring the oxidation state of these igneous 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.

Therefore, we decided to look at the metal apatite found in zircon crystals in these rocks. Zircon crystals can withstand extreme temperatures and pressures for post-crystallization events. They hold clues about the environments in which they were originally formed and provide accurate ages for the rocks themselves.

Tiny apatite crystals less than 30 microns wide—the size of a human skin cell—are trapped in the zircon crystals. contain sulfur. By measuring the amount of sulfur in the apatite, we can determine if the apatite has grown from oxidized magmas.

We were able to measure the oxygen intensity of the original magma—essentially the amount of free oxygen in it—using a specialized technique called edge structure spectroscopy (S-XANES) at the Advanced Photon Source. at the Argonne National Laboratory in Illinois.

Making oxygen from water?

We found that the magma’s sulfur content, which was initially about zero, increased to 2,000 ppm about 2,705 million years ago. This indicates that the magma has become rich in sulfur. In addition, the predominance of S6+—a type of sulfur ion—in the apatite indicated that the sulfur was from an oxidized source, matching data from the host zircon crystals.

These new findings indicate that oxidized magmas formed in the modern era, 2.7 billion years ago. The data show that a lack of dissolved oxygen in the Archaean reservoirs did not prevent the formation of sulfur-rich, oxidized magmas at subduction zones. The oxygen in this magma 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 Upper Province and Yilgarn Craton (Western Australia), demonstrating a link between these oxygen-rich sources and the formation of world-class ore deposits.

The implications of this oxidized magma go beyond understanding early Earth’s geodynamics. Previously, it was thought that Archean magma was less likely to oxidize, when ocean water and ocean floor rocks or sediments did not oxidize.

While the exact mechanism is not clear, the occurrence of this magma indicates that the process of subduction, in which ocean water is transported hundreds of kilometers into our planet, generates free oxygen. This then oxidizes the upper mantle.

Our study shows that the Archean subduction could have been an unexpectedly vital factor in Earth’s oxygenation, early oxygen jets 2.7 billion years ago and also the Great Oxidation Event, which recorded an increase in atmospheric oxygen of 2% from 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 active plate tectonics and subduction. This suggests that this study could partially explain the lack of oxygen and, eventually, life on other rocky planets in the future as well.

more information:
Xuyang Meng et al, Formation of sulfur-rich oxidised magmas in the Neoarchaean subduction zones, Natural Earth Sciences (2022). DOI: 10.1038/s41561-022-01071-5

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