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 distant past – as far back as the Neoarchaic era 2.8 to 2.5 billion years ago – this oxygen was almost absent.
So, how did Earth’s atmosphere become oxygenated?
Our research, published in Natural Geosciencesadds a tantalizing new possibility: that at least some of Earth’s early oxygen came from a tectonic source via the movement and destruction of the Earth’s crust.
The Archaic Earth
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 with green oceans, shrouded in a methane haze, and completely lacking in multicellular 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, in which oceanic crust — the outermost layer of the Earth below the oceans — sinks into the Earth’s mantle (the region between the Earth’s crust and core) at convergence points called subduction zones. . However, there is much debate about whether plate tectonics operated in the Archean era.
A hallmark of modern subduction zones is their association with oxidized magmas. These magmas are formed when oxidized sediments and bottom water — cold, dense water near the ocean floor — are introduced into Earth’s mantle. This produces magma with a high oxygen and water content.
Our study aimed to test whether the absence of oxidized materials in archaic bottom waters and sediments could prevent the formation of oxidized magmas. The identification of such magmas in neoarchaic magmatic rocks could provide evidence that subduction and plate tectonics occurred 2.7 billion years ago.
We collected samples of 2,750 to 2,670 million-year-old granitic rocks from the Superior Province’s Abitibi-Wawa sub-province — the largest preserved Archean continent stretching 2,000 km from Winnipeg, Manitoba, to far eastern Quebec. This allowed us to examine the oxidation level of magmas generated in the Neoarchaic era.
Measuring the oxidation state of these magmatic rocks – formed by cooling and crystallization of magma or lava – is challenging. Post-crystallization events may have modified these rocks through subsequent deformation, burial, or heating.
So we decided to look at the mineral apatite, which is present in the zircon crystals in these rocks. Zircon crystals can withstand the intense temperatures and pressures of post-crystallization events. They preserve clues about the environment in which they originally formed and give precise ages for the rocks themselves.
Tiny apatite crystals less than 30 microns across – the size of a human skin cell – are trapped within the zircon crystals. They contain sulfur. By measuring the amount of sulfur in apatite, we can determine whether the apatite originated from oxidized magma.
We were able to successfully measure the oxygen volatility 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) at the Advanced Photon Source synchrotron at Argonne National Laboratory in Illinois.
Making oxygen from water?
We found that the magma sulfur content, which was initially about zero, increased to 2000 parts per million around 2705 million years. This indicated that the magma had become more sulphurous. In addition, the predominance of S6+ — a type of sulfur ion — in the apatite suggested that the sulfur came from an oxidized source, consistent with data from the host zircon crystals.
These new findings indicate that oxidized magmas formed 2.7 billion years ago in the Neoarchaic era. The data shows that the lack of dissolved oxygen in the archaic ocean reservoirs did not prevent the formation of sulphur-rich, oxidized magmas in the subduction zones. The oxygen in these magmas must have come from some other source and eventually entered the atmosphere during volcanic eruptions.
We found that the occurrence of these oxidized magmas correlates with major gold mineralization events in the Superior Province and Yilgarn Craton (Western Australia), demonstrating a link between these oxygenated sources and world-class ore deposits worldwide.
The implications of these oxidized magmas extend beyond understanding Earth’s early geodynamics. Previously, it was thought unlikely that archaic magmas could be oxidized if the ocean water and rocks or sediments on the ocean floor were not.
While the exact mechanism is unclear, the formation of these magmas suggests that the process of subduction, which funnels ocean water hundreds of miles toward our planet, generates free oxygen. This then oxidizes the overlying mantle.
Our study shows that Archean subduction may have been a vital, unforeseen factor in the oxygenation of the Earth, the early oxygen vortexes 2.7 billion years ago and also the Great Oxidation event, which caused an increase in atmospheric oxygen by 2, 45 to 2.32 percent. 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.
This article was originally published on The Conversation by David Mole at Laurentian University, and Adam Charles Simon, and Xuyang Meng at the University of Michigan. Read the original article here.