The amount of oxygen in Earth’s atmosphere makes it habitable.
Twenty-one percent of the atmosphere consists of this life-giving element. But in the deep past – until Neoarchean times 2.8 to 2.5 billion years ago – this pain was almost absent.
So how did Earth’s air become oxygenated?
Our research, published in . Nature Geoscienceadds a tantalizing new possibility: that at least some of Earth’s primordial oxygen arose from a tectonic source through the movement and destruction of the Earth’s crust.
Arc of the Earth
The Archean represents a third of our planetary history, from 2.5 billion years ago to 4 billion years ago.
This foreign land was aquatic, covered in green oceans, shrouded in methane, and completely devoid of multi-cellular life. Another strange aspect of 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 under the ocean – descends into the Earth’s mantle (the area between the Earth’s crust and its core) in collisions called subduction zones. .
But there is a great debate whether plate tectonics worked back in the Archean era.
One feature of modern reduction zones is their association with oxidized magmas.
These magmas are formed when oxidized sediments and deep water — cold, dense water near the ocean floor — are pushed into the Earth’s mantle. This produces magmas with high oxygen and water content.
Our research aims to prove whether the absence of oxidized material in Archean deep waters and sediments could prevent the formation of oxidized magmas.
The punching of such magma in Neoarchean igneous rocks could provide evidence of subduction and plate tectonics that occurred 2.7 billion years ago.
An experiment
We collected samples of granitoid rocks from the Abitibi-Wawa region, the largest samples (a continuation of the Archaean continent preserved over 2,000 kilometers (1,243 miles)) from Winnipeg, Manitoba, to the far east. Quebec.
This allowed us to investigate the degree of oxidation of the magma generated during the Neoarchean era.
Measuring the oxidation state of these igneous rocks – formed by the cooling and crystallization of magma or lava – is challenging. Post-crystallization events may have modified these rocks through deformation, burial, or heating.
So we decided to look at the mineral apatite which is in the zircon crystals in these rocks.
Zircon crystals can withstand the intense temperatures and pressures of post-crystallization events. They keep the rocks close to the environment in which they were originally formed and provide the rocks themselves with accurate ages.
Small apatite crystals—less than 30 microns wide—the size of a human skin cell—are captured in zircon crystals. They contain sulphur. By reducing the amount of sulfur in the apatite, it is possible to increase whether the apatite has grown from the oxidized magma.
We were able to successfully measure the oxygen fugacity of the original Archean magma – which is in itself the amount of free oxygen in it – using a special technique called X-ray Absorption Near Edge Structure Spectroscopy (S-XANES) at the Advanced Synchrotron Source. at Argonne National Laboratory in Illinois.
Creating oxygen from water?
We found that the sulfur content of the magma, which was initially zero, increased to 2,000 parts per million about 2705 million years ago. This meant that the magmas became more sulfur rich.
Additionally, the predominance of S6+ – the type of sulfur ion – in apatite suggested sulfur from an oxidized source, consistent with data from host zircon crystals.
These new findings indicate that 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 sulfur-rich, oxidized magmas in subduction zones.
The pain in these magmas must come from somewhere else and is eventually released into the atmosphere during volcanic eruptions.
We found the occurrence of these oxidic magmas with major gold mineralization events in the Superior Province and Yilgarn Craton (Western Astragalus), showing a link between these oxygen-rich sources and the formation of world-class ore deposits.
The consequences of these oxidized magmas exceed our understanding of the geodynamics of the early Earth. Previously, it was thought unlikely that Archean magmas could be oxidized, since ocean and seawater were not rocks or sediments.
While the exact mechanism is hidden, the occurrence of these magmas suggests a subduction process, where ocean water is drawn hundreds of kilometers into our planet, generating free oxygen. This then oxidizes the upper coat.
Our study shows that Archean subduction could have been a vital, unexpected element in the oxygenation of the Earth, the first ears of oxygen 2.7 billion years ago and also the Great Oxidation Event, which marked an increase in atmospheric oxygen from two percent 2.45 to 2.32. billions of 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 study suggests that this could partially explain the lack of oxygen and, ultimately, life on other rocky planets in the future as well.
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 & Environmental Sciences, University of Michigan
This article is republished from The Conversation under the Creative Commons license. Read the original article.
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