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

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, up to the Neoarchaic era, 2.8 to 2.5 billion 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 movement and destruction of Earth’s crust.

The Archaic Earth

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

This alien Earth was a covered water world green oceanssurrounded by methane fog, and completely lacks multicellular life. Another aspect alien to this world was the nature of its tectonic activity.

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, 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 as to whether plate tectonics was at work in the Archaean.

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 a high content of oxygen and water.

Our research aimed to test whether the absence of oxidized materials in Archaean bottom waters and sediments could prevent the formation of oxidized magmas. Identifying these magmas in Neoarchaean igneous rocks could provide evidence that subduction and plate tectonics occurred 2.7 billion years ago.

the experiment

We collected samples of granitoid rocks between 2750 and 2670 Ma from the entire Abitibi-Wawa Subprovince of the Superior Province, the largest preserved Archaean continent extending 2000 km from Winnipeg, Manitoba , to the eastern end of Quebec. This allowed us to investigate the level of oxidation of the magmas generated during the Neoarchaic era.

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.

So, we decided to look into it mineral apatitewhich 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 environments in which they were originally formed and provide precise ages for the rocks themselves.

Small apatite crystals that are less than 30 microns across (the size of a human skin cell) are 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 an oxidized magma.

We were able to measure successfully oxygen fugacity of the original Archean magma, which is essentially how much free oxygen is there, using a specialized technique called near-X-ray absorption structure spectroscopy (S-XANE) at the Advanced Photon Source synchrotron a Argonne National Laboratory in Illinois.

Creating oxygen from water?

We found that the magmatic sulfur content, which was initially around zero, increased to 2000 parts per million around 2705 million years. This indicated that the magma had become richer in sulfur. In addition, the predominance of S6+ — a type of sulfur ion — in apatite suggested that the sulfur was from an oxidized source, coincidentally the data of the host zircon crystals.

These new findings indicate that the oxidized magmas formed in the Neoarchean era 2.7 billion years ago. The data show that the lack of dissolved oxygen in Archean ocean 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 Upper Province and Yilgarn Craton (Western Australia), demonstrating a connection between these oxygen-rich sources and the global formation of world-class ore deposits.

The implications of these oxidized magmas go beyond the understanding of early Earth geodynamics. Previously, it was thought unlikely that Archaean magmas could oxidize when the ocean water i rocks or sediments from the ocean floor they weren’t

Although the exact mechanism is unclear, the appearance of these magmas suggests that the process of subduction, where ocean water is brought hundreds of kilometers to our planet, generates free oxygen. It then oxidizes the upper mantle.

Our study shows that Archaean subduction could have been a vital and unforeseen factor in Earth’s oxygenation, the first smells of oxygen 2.7 billion years ago and also the Great Oxidation Event, which marked a two percent increase in atmospheric oxygen between 2.45 and 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 partly explain the lack of oxygen and ultimately life on the other rocky planets in the future as well.

This article was originally published on the conversation by David Mole at Laurentian University and Adam Charles Simon, and Xuyang Meng at the University of Michigan. read the original article here.