Origin of Earth’s Oxygen

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Key findings-

• 21 percent of the atmosphere consists of this life-giving element. But in the deep past, as far back as the Neoarchean era 2.8 to 2.5 billion years ago  this oxygen was almost absent.
• As per the new study at least some of the Earth’s early oxygen came from a tectonic source via the movement and destruction of the Earth’s crust.
• The Archean eon represents one third of our planet’s history, from 2.5 billion years ago to four billion years ago.
• This alien Earth was a water-world, covered in green oceans, shrouded in a methane haze and completely lacking multicellular life. Another alien aspect of this world was the nature of its tectonic activity.
• On modern Earth, the dominant tectonic activity is called plate tectonics, where oceanic crust — the outermost layer of the Earth under 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 over whether plate tectonics operated back in the Archean era.
• One feature of modern subduction zones is their association with oxidised magmas. 
• These magmas are formed when oxidised sediments and bottom waters — cold, dense water near the ocean floor are introduced into the Earth’s mantle. This produces magmas with high oxygen and water contents.
• This research aimed to test whether the absence of oxidised materials in Archean bottom waters and sediments could prevent the formation of oxidised magmas. 
• The identification of such magmas in Neoarchean magmatic rocks could provide evidence that subduction and plate tectonics occurred 2.7 billion years ago.
• Researchers collected samples of 2750- to 2670-million-year-old granitoid rocks from across the Abitibi-Wawa subprovince of the Superior Province — the largest preserved Archean continent stretching over 2000 km from Winnipeg, Manitoba to far-eastern Quebec.

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