Where does oxygen come from on Earth? A new study points to an unexpected source

Where does oxygen come from on Earth? A new study points to an unexpected source

Granitoid rocks 2750 to 2670 million years old collected from the largest preserved Archean continent may help unravel the story of the origin of oxygen on Earth. Credit: Dylan McKevitt, author

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 – back in the Neoarchean 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 Nature Geoscienceadds a tantalizing new possibility: that at least some of Earth’s early oxygen came from a tectonic source via movement and destruction of the Earth’s crust.

Archean Earth

The Archaean Eon represents one third of our planet’s history, from 2.5 billion years ago to four billion years ago.

This alien Earth was a water world, covered green oceansenveloped a methane haze and multicellular life is completely absent. Another alien 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 the Earth beneath the ocean—sinks into the Earth’s mantle (the area between the Earth’s crust and its core) at points of convergence called subduction zones. . However, there is considerable debate about whether plate tectonics was at work as early as the Archean era.

One of the characteristics of modern subduction zones is their connection with oxidized magma. These magmas are formed when sediments and bottom water – the cold, dense water nearby – are oxidized ocean floor— are brought into the Earth’s mantle. This produces magma with high oxygen and water content.

Our research aimed to verify whether the absence of oxidized materials in the waters and sediments of the Archean floor could prevent the formation of oxidized magmas. The identification of such magma in Neoarchean igneous rocks could provide evidence that subduction and plate tectonics occurred 2.7 billion years ago.

Where does oxygen come from on Earth?  A new study points to an unexpected source

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

An experiment

We collected samples of 2750 to 2670 Ma granitoid rocks from across the Abitibi-Wawa subprovince of the Superior Province—the largest preserved Archean continent that stretches over 2000 km from Winnipeg, Manitoba, to distant Quebec. This allowed us to investigate the level of oxidation of the magma that formed during the Neoarchean era.

Measurement of the oxidation state of these igneous rocks – formed by cooling and crystallization magma or a lion – represents a challenge. Post-crystallization events may have modified these rocks through subsequent deformation, burial, or heating.

So we decided to take a look mineral apatite which is present in zircon crystals in these rocks. Zircon crystals can withstand the intense temperatures and pressures of post-crystallization events. They retain clues about the environment in which they were originally formed and give a precise age for the rocks themselves.

Small apatite crystals less than 30 microns wide – the size of a human skin cell – are trapped in the zircon crystals. They contain sulfur. By measuring the amount of sulfur in apatite, we can determine whether the apatite originated from oxidized magma.

We have successfully measured oxygen fugacity 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 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 around zero, increased to 2000 parts per million around 2705 million years ago. This indicated that the magmas became richer in sulfur. In addition, the predominance of S6+—a type of sulfur ion—in apatite suggests that the sulfur is from an oxidized source, which fits data from zircon parent crystals.

These new discoveries show that oxidized magmas were 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 oxidized sulfur-rich 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 implications of these oxidized magmas go beyond understanding early Earth geodynamics. It was previously thought unlikely that Archean magmas could be oxidized, when it was ocean water and rocks or sediments of the ocean floor they weren’t.

Although the exact mechanism is unclear, the occurrence of these magmas suggests that the process of subduction, where ocean water is transported hundreds of kilometers into our planet, creates free oxygen. This then oxidizes the top layer.

Our research shows that Archean subduction may have been a vital, unforeseen factor in early Earth oxygenation breaths of oxygen 2.7 billion years ago and also The Great Oxidation Event, which marked a two percent increase in atmospheric oxygen 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 study may partially explain the deficiency oxygen and, finally, life on other rocky planets in the future.

More information:
Xuyang Meng et al, Formation of oxidized sulfur-rich magmas in Neoarchean subduction zones, Nature Geoscience (2022). DOI: 10.1038/s41561-022-01071-5


This article was republished by Conversation under Creative Commons license. Read it original article.Conversation

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