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 remote past – already in the Neoarchean era, 2.8 to 2.5 billion years ago – that oxygen was almost absent🇧🇷
So how did Earth’s atmosphere become oxygenated?
our researchpublished in nature geoscienceadds a tantalizing new possibility: that at least some of Earth’s early oxygen came from a tectonic source through the movement and destruction of the Earth’s crust.
The Archean Earth
The Archean eon represents a third of our planet’s history, from 2.5 billion years to 4 billion years ago.
This alien Earth was a water world, covered in green oceanswrapped in a methane haze and completely without multicellular life. Another strange aspect of this world was the nature of its tectonic activity.
On modern Earth, the dominant tectonic activity is called plate tectonics, in which the oceanic crust – the Earth’s outermost layer beneath the oceans – sinks into the mantle (the area between the Earth’s crust and its core) at points of convergence called zones. of subduction. However, there is considerable debate over whether plate tectonics operated in the Archean era.
A feature of modern subduction 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 introduced into the Earth’s mantle🇧🇷 This produces magmas with high oxygen and water content.
Our research aimed to test whether the absence of oxidized materials in bottom Archean waters and sediments could prevent the formation of oxidized magmas. Identification of such magmas in Neoarchean magmatic rocks could provide evidence that subduction and plate tectonics occurred 2.7 billion years ago.
We collected samples of granite rocks from 2.750 to 2.670 billion years old throughout the Abitibi-Wawa sub-province of the Upper Province (Canada) – the largest preserved Archean continent, which extends over 2 thousand kilometers from Winnipeg, in the province of Manitoba, to the extreme east of the province of Quebec. This allowed us to investigate the level of oxidation of magmas generated throughout the Neoarchean era.
Measuring the oxidation state of these magmatic rocks – formed through the cooling and crystallization of magma or lava – is a challenge. Post-crystallization events may have modified these rocks through further deformation, burial or heating.
So we decided to look at the mineral apatite that is present in the zirconium crystals of these rocks. Zirconium 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 accurate ages for the rocks themselves.
Small apatite crystals less than 30 microns wide – the size of a human skin cell – are trapped in the zirconium crystals. They contain sulfur. By measuring the amount of sulfur in apatite, we can establish whether apatite grew from 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 specialized technique called X-ray absorption near-edge structure spectroscopy (S-XANES) in the advanced source synchrotron. photons at the Argonne National Laboratory in Illinois (USA).
Creating oxygen from water?
We found that the sulfur content of magma, which was initially around zero, increased to 2,000 parts per million around 2.705 billion years. This indicated that the magmas became richer in sulfur. In addition predominance of S6+ – a type of sulfur ion – in apatite suggested that the sulfur was from an oxidized source, combining with the data of the host zirconium crystals🇧🇷
These new discoveries 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 oceanic reservoirs did not prevent the formation of sulfur-rich oxidized magmas in the subduction zones. The oxygen in these magmas must have come from another source and was finally 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.
Mechanism not yet clear
The implications of these oxidized magmas go beyond understanding the geodynamics of the early Earth. Previously, it was thought that it was unlikely that Archean magmas could be oxidized when the ocean water and the rocks or sediments from the bottom of the ocean they were not.
While the exact mechanism is unclear, the occurrence of these magmas suggests that the process of subduction, in which ocean water is driven hundreds of kilometers into our planet, generates free oxygen. This then oxidizes the overlying mantle.
Our study shows that Archean subduction may have been a vital and unforeseen factor in Earth’s oxygenation, the first puffs of oxygen 2.7 billion years ago and also the Great Oxidation Event, which marked an increase in atmospheric oxygen by two percent from 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 could partially explain the lack of oxygen and ultimately life on the other rocky planets in the future as well.
* David Mole is a postdoctoral professor of earth sciences at Laurentian University (Canada); Adam Charles Simon is Professor of Earth and Environmental Sciences at the University of Michigan (USA); Xuyang Meng is a postdoctoral professor of Earth and Environmental Sciences at the University of Michigan.
🇧🇷 ANDthis article has been republished from the website The Conversation under a Creative Commons license🇧🇷 Read the original article here🇧🇷