The amount of oxygen in Earth’s atmosphere makes it a habitable planet.
Twenty-one percent of the atmosphere is made up of this life-giving element. But in the distant past – as far back as the Neoarchean era 2.8 to 2.5 billion years ago – this oxygen was almost absent.
So how did the Earth’s atmosphere get oxygenated?
Our research, published in nature geoscienceadds a tantalizing new possibility: that at least some of Earth’s initial oxygen came from a tectonic source via the movement and destruction of the Earth’s crust.
The Archean eon represents a third of our planet’s history, from 2.5 billion years ago to four billion years ago.
This alien Earth was an aquatic world covered in green oceans, shrouded in a methane haze and devoid of any multicellular life. Another foreign aspect to this world was the nature of its tectonic activity.
On modern Earth, the dominant tectonic activity is called plate tectonics, where the oceanic crust – the Earth’s outermost layer below the oceans – sinks into the Earth’s mantle (the area between the Earth’s crust and its core ) at convergence points called subduction zones. However, there is considerable debate over whether plate tectonics was functioning in Archean times.
A feature of modern subduction zones is their association with oxidized magmas. These magmas form when oxidized sediments and bottom waters – cold, dense water near the ocean floor – are introduced into the Earth’s mantle. This produces a magma with a high oxygen and water content.
Our research aimed to test whether the absence of oxidized materials in bottom waters and Archean sediments could prevent the formation of oxidized magmas. Identifying such magmas in Neoarchean igneous rocks could provide evidence that subduction and plate tectonics occurred 2.7 billion years ago.
We collected samples of granitoid rocks between 2,750 and 2,670 million years old throughout the Abitibi-Wawa subprovince of the Superior Province — the largest preserved Archean continent spanning over 2,000 km from Winnipeg, Manitoba to the far east of Quebec. This allowed us to study the level of oxidation of the magmas generated during the Neoarchean era.
Measuring the oxidation state of these igneous rocks – formed by the cooling and crystallization of magma or lava – is a challenge. Post-crystallization events may have altered these rocks by subsequent deformation, burial, or heating.
So we decided to look at the mineral apatite, present in the zircon crystals of these rocks. Zircon crystals can withstand the intense temperatures and pressures of post-crystallization events. They hold clues to the environments in which they originally formed and provide precise ages for the rocks themselves.
Small apatite crystals less than 30 microns wide – the size of a human skin cell – are trapped within the zircon crystals. They contain sulfur. By measuring the amount of sulfur in the apatite, we can determine if the apatite developed from an oxidized magma.
We were able to successfully measure the oxygen fugacity of the original Archean magma – which is essentially how much free oxygen it contains – using a specialized technique called X-ray absorption spectroscopy near the structure of the edge (S-XANES) at the Advanced Photon Source synchrotron at Argonne National Laboratory in Illinois.
Create oxygen from water?
We found that the sulfur content of the magma, which was initially around zero, increased to 2000 parts per million around 2705 million years ago. This indicated that the magma had become richer in sulphur. Moreover, the predominance of S6+ – a type of sulfur ion – in the apatite suggests that the sulfur comes from an oxidized source, which is consistent with data from host zircon 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 ocean reservoirs did not prevent the formation of sulfur-rich oxidized magmas in subduction zones. The oxygen in these magmas must have come from another source and was eventually released into the atmosphere during volcanic eruptions.
We found that the presence of these oxidized magmas is correlated with major gold mineralization events in the Superior Province and the Yilgarn Craton (Western Australia), demonstrating a link between these oxygen-rich sources and the formation of ore deposits. world class.
The implications of these oxidized magmas go beyond understanding early Earth geodynamics. Previously, it was thought unlikely that Archean magmas could be oxidized while ocean water and ocean floor rocks or sediments were not.
Although the exact mechanism is unclear, the presence of these magmas suggests that the process of subduction, where ocean water is transported hundreds of kilometers into our planet, generates free oxygen. This then oxidizes the overlying mantle.
Our study shows that Archean subduction could have been a vital and unforeseen factor in the oxygenation of the Earth, the first bursts of oxygen 2.7 billion years ago and also the great oxidation event, which has marked an increase in atmospheric oxygen of 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 suggests that this study could partly explain the lack of oxygen and ultimately life on other rocky planets in the future as well.
This article was originally published on The Conversation by David Mole at Laurentian University, and Adam Charles Simon, and Xuyang Meng from the University of Michigan. Read the original article here.
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