The amount of oxygen the Earth’s atmosphere makes that planet 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.
The Archean represents a third of our planetary history, from 2.5 billion years ago to 4 billion years ago.
This was an alien Earth with green watery oceans, covered 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 beneath 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 around the ocean floor — are pushed into the Earth’s mantle. This produces magma with a high content of oxygen and water.
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.
We collected samples of granitoid rocks at 2670 million years old, from the Abitibi-Wawa subprovince of the Superior Province—the largest preserved Archaean continent over 2000 km from Winnipeg, Manitoba, to Quebec in the far east. This allowed us to investigate the degree of oxidation of the magma generated during the Neoarchean era.
It challenges the oxidation state of these igneous rocks — formed by the cooling and crystallization of magma or lava. Post-crystallization events may have modified these rocks through deformation, burial, or heating.
So. We decided to look at the mineral apatitewhich in these rocks contains zircon crystals. 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 that are less than 30 microns wide — about 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 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 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 2000 parts per million over about 2705 million years. This meant that the magma became more sulfur-rich. Additionally, the predominance of S6+—the type of sulfur ion—in the 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), demonstrating a link between these oxygen-rich sources and global, world-class ore deposit formation.
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 when ocean water and sea rocks and sediments were not present.
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.
This article was first published in Conversation by David Mole at Laurentian University, and Adam Charles Simon; and Xuyang Meng at the University of Michigan. Read the original article here.
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