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 modern era, 2.8 to 2.5 billion years ago – This oxygen was almost absent.
So how does Earth’s atmosphere get oxidised?
Our researchPublished in Natural Earth SciencesIt adds a tantalizing new possibility: that at least some of Earth’s primary oxygen came from a tectonic source through crustal movement and destruction.
Archean Earth
The Archean eon represents a third of our planet’s history, from 2.5 billion years ago to 4 billion years ago.
This strange land was a watery, covered world green oceanswrapped in methane haze, and completely devoid of multicellular life. Another strange aspect of this world is the nature of its tectonic activity.
On modern Earth, the dominant tectonic activity is called plate tectonics, where the oceanic crust – the outermost layer of the Earth beneath the oceans – sinks into the Earth’s mantle (the area between the Earth’s crust and core) at meeting points called subduction zones.
However, there is considerable debate as to whether plate tectonics functioned in ancient times.
One of the characteristics of modern subduction zones is their connectivity oxidized magma.
This magma forms when oxidized sediments and bottom waters — the cold, dense water near the ocean floor — form. inserted into the Earth’s mantle. This produces magma with a higher oxygen and water content.
Our research aims to test whether the absence of oxidants in the bottom waters and paleontological sediments can prevent the formation of oxidized magmas.
The identification of such magmas in Newarse igneous rocks could provide evidence that subduction and plate tectonics occurred 2.7 billion years ago.
Experience
We collected samples of granitic rocks dating between 2,750 and 2,670 million years ago across the Upper District’s Abitibi Wawa Sub-District – the largest preserved Archean continent stretching 2,000 kilometers (1,243 miles) from Winnipeg, Manitoba to the far east. Quebec.
This allowed us to study the level of oxidation of magma generated during the New Age.
Measuring the oxidation state of these igneous rocks — which form from the cooling and crystallization of magma or lava — is challenging. Post-crystallization events may have altered these rocks by subsequent deformation, burial, or heating.
So we decided to look The mineral apatite which is located in zirconia crystals in these rocks.
Zircon crystals can withstand extreme temperatures and stresses from post-crystallization events. They have evidence of the environments in which they were originally formed and provide accurate ages for the rocks themselves.
Tiny apatite crystals less than 30 microns wide—the size of a human skin cell—are trapped within the zircon crystals. contain sulfur. By measuring the amount of sulfur in the apatite, we can determine whether the apatite originated from an oxidized magma.
We managed to measure oxygen escape of the original Archean magma—which is basically how much free oxygen it contains—using a specialized technique called X-ray absorption spectroscopy near the rim structure (S-XANES) at the Synchrotron’s Advanced Photon Source Argonne National Laboratory in Illinois.
Generate oxygen from water?
We found that the sulfur content in the magma, which was initially about zero, increased to 2,000 ppm about 2,705 million years ago. This indicates that the magma has become richer in sulfur.
Moreover, the Predominance of S6+ – a type of sulfur ion – in apatite He suggested that the sulfur came from an oxidizing source, in contrast Host zircon crystal data.
These new discoveries indicate that oxidized magma formed in the new era, 2.7 billion years ago. The data indicate that a lack of dissolved oxygen in ancient ocean reservoirs did not prevent the formation of sulfur-rich, oxidized magmas at subduction zones.
The oxygen in this magma must have come from another source and was eventually released into the atmosphere during volcanic eruptions.
We found that the presence of these oxidised magmas correlates with major gold mineralization events in the Upper Province and Yilgarn Craton (Western Australia), demonstrating a link between these oxygen-rich sources and the formation of gold deposits. world class raw material.
The implications of this oxidized magma go beyond understanding early Earth’s geodynamics. Previously, it was thought that Archean magma was unlikely to oxidize, when it is sea water And Ocean floor rocks or sediments has not been.
Although the exact mechanism is not clear, the presence of this magma indicates that the process of subduction, in which ocean water is transported hundreds of kilometers inland of our planet, generates free oxygen. This then oxidizes the upper mantle.
Our study shows that Archean subduction may be a vital and unexpected factor in Earth’s oxygenation, initially Oxygen puffed out 2.7 billion years ago as well The Great Oxidation Event, which saw a 2% increase in atmospheric oxygen 2.45–2.32 billion 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 partially explain the lack of oxygen and eventually life on other rocky planets in the future as well.
David MallPostdoctoral Fellow, Earth Sciences, Laurentian University؛ Adam Charles SimonProfessor Arthur F. Thornau, Earth and Environmental Sciences, University of MichiganAnd Xuyang MengPostdoctoral Fellow, Earth and Environmental Sciences, University of Michigan
This article has been republished from dialogue Under Creative Commons Licence. Read it The original article.