The amount of oxygen in the Earth’s atmosphere makes the Earth a planet.
21 percent of the atmosphere consists of this life-giving substance. But in the deep past – from the Neoarchean period 2.8 to 2.5 billion years ago – this gas was almost non-existent.
So, how did Earth’s atmosphere become oxygenated?
Our research, published in Nature Geoscienceadds an intriguing new possibility: that some of Earth’s early gases came from a tectonic source through movement and erosion of the Earth’s crust.
The Archean world
The Archean eon represents one-third of Earth’s history, from 2.5 billion years ago to 4 billion years ago.
This alien world was a water world, covered in green seas, shrouded in methane gas, and devoid of multicellular life. Another unusual feature of the world was the nature of its tectonic activity.
In today’s world, the main tectonic activity is called plate tectonics, where the sliding of the ocean – the outside of the earth under the sea – sinks into the mantle of the earth (the area between the earth’s surface and its center) at a point of convergence called subduction zones. .
However, there is considerable debate as to whether plate tectonics was active during the Archean period.
One of the most common subduction zones is associated with oxidized magmas.
Magmas are formed when oxygen-rich sediments and groundwater – cold, frozen water near the ocean floor – are pumped into the Earth’s mantle. This creates magmas with high amounts of oxygen and water.
Our study aimed to test whether the absence of oxygen-rich elements in Archean groundwaters and sediments would prevent the formation of oxidized magmas.
The detection of such magmas in Neoarchean magmatic rocks may provide evidence that subduction and plate tectonics occurred 2.7 billion years ago.
We collected samples of 2750- to 2670-million-year-old granitoid rocks from the Abitibi-Wawa sub-region of the Superior Province – the largest preserved Archean continent located 2,000 km (1,243 miles) east of Winnipeg, Manitoba. Quebec.
This led us to investigate the oxygen content of magmas produced during the Neoarchean period.
Measuring the oxidation state of these magmatic rocks – which are formed through the cooling and crystallization of magma or lava – is difficult. Events after crystallization can alter these rocks through alteration, burial, or heat.
So, we decided to look at the mineral apatite that is present in zircons in these rocks.
Zircon crystals can withstand high temperatures and stress during post-crystallization events. They retain information about the place where they were made and give the exact age of the stones.
Small crystals of apatite that are less than 30 microns wide – the size of a human skin – are encased in zircon crystals. They have sulfur. By measuring the amount of sulfur in the apatite, we can determine whether the apatite grew from oxidized magma.
We were able to accurately measure the oxygen content of early Archean magma – which is essentially the amount of free oxygen in it – using a special technique called X-ray Absorption Near Edge Structure Spectroscopy (S-XANES) at the Advanced Photon Source synchrotron at Argonne National Laboratory in Illinois. .
Making carbon from water?
We found that the magma’s sulfur content, which was originally close to zero, rose to 2,000 parts per million in about 2705 million years. This indicates that the magmas were very rich in sulfur.
In addition, the prominence of S6 + – the type of sulfur ion – in the apatite said that the sulfur was from an oxygen source, similar to the data from the zircon curtains.
The new findings show that oxidized magmas formed in the Neoarchean period 2.7 billion years ago. The data show that the lack of dissolved oxygen in the Archean seawater did not prevent the formation of sulfur-rich magmas, oxidized in the reduction zones.
The oxygen contained in these magmas must have come from somewhere else and then been released into the atmosphere during volcanic eruptions.
We found that the presence of these oxygen magmas is associated with major gold mineralization events in the Superior Province and the Yilgarn Craton (Western Australia), indicating a connection between oxygen-rich sources and gold formation worldwide.
The effects of these oxygenated magmas extend the understanding of early Earth geodynamics. In the past, it was thought impossible that Archean magmas could be oxidized, while seawater and seabed rocks or sediments were not.
Although the exact process is not well known, the presence of magmas indicates that the reduction process, when sea water is transported hundreds of kilometers to our planet, creates free gas. This then oxidizes the top coat.
Our research shows that the Archean subduction would have been an important, unexpected factor in the atmosphere of the Earth, the first atmospheric atmosphere 2.7 billion years ago and the Great Oxidation Event, which showed an increase in atmospheric oxygen by two percent 2.45 to 2.32 billion years ago.
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 research may partially explain the lack of oxygen, and, ultimately, life on other rocky planets in the future.
David Mole, Postdoctoral fellow, Earth Sciences, Laurentian University; Adam Charles Simon, Arthur F. Thurnau Professor, Earth & Environmental Sciences, University of Michigan, and Xuyang Meng, Postdoctoral Fellow, Earth and Environmental Sciences, University of Michigan
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