The amount of oxygen in Earth’s atmosphere makes it a habitable planet.
21% of the atmosphere consists of this life-giving element. But in the distant past – up to the modern era, 2.8 to 2.5 billion years ago – This oxygen was almost absent.
So how does the Earth’s atmosphere oxidize?
Our researchPosted in Natural Sciences of the EarthIt adds a tantalizing new possibility: that at least some of Earth’s primary oxygen comes from a tectonic source through crustal movement and destruction.
Archean land
The Archean eon accounts for a third of our planet’s history, from 2.5 billion years ago to 4 billion years ago.
This strange land was a watery, overcast 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 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 the core) at meeting points called subduction zones.
However, there is considerable debate as to whether plate tectonics worked 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 verify whether the absence of oxidants in bottom waters and paleontological sediments can prevent the formation of oxidized magmas.
The identification of such magmas in the Newarse igneous rocks could provide evidence that subduction and plate tectonics occurred 2.7 billion years ago.
Experience
We collected samples of granite rocks dating between 2,750 and 2,670 million years ago in the Abitibi Wawa subdistrict of the Upper District, the largest preserved Archean continent stretching 2,000 kilometers (1,243 miles) from Winnipeg, Manitoba to the extreme Orient. Quebec.
This allowed us to study the oxidation level of the 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 a challenge. Post-crystallization events may have altered these rocks by subsequent deformation, burial, or heating.
So we decided to search The mineral apatite in which it is located zirconium 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 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 inside the zircon crystals. contain sulfur. By measuring the amount of sulfur in apatite, we can determine whether the apatite originated from oxidized magma.
We managed to measure escape of oxygen of the original Archean magma, which is basically the amount of free oxygen it contains, using a specialized technique called X-ray absorption spectroscopy near the edge structure (S-XANES) at the advanced synchrotron photonic source Argonne National Laboratory in Illinois.
Generate oxygen from water?
We found that the sulfur content in the magma, which was initially zero, increased to 2,000 ppm about 2,705 million years ago. This indicates that the magma has become richer in sulfur.
Also, the Predominance of S6+ – a type of sulfur ion – in apatite He suggested the sulfur came from an oxidizing source, quite the contrary It houses the data of the zirconium crystal.
These new discoveries indicate that oxidized magma formed in the new era, 2.7 billion years ago. The data indicate that the lack of dissolved oxygen in ancient ocean basins did not prevent the formation of sulphur-rich oxidized magmas in 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 oxidized magmas correlates with major gold mineralization events in Upper Province and Yilgarn Crater (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 the geodynamics of the early Earth. Previously, it was thought that Archean magma was unlikely to oxidize, when it was sea water And Ocean floor rocks or sediments has not been.
Although the exact mechanism is unclear, the presence of this magma indicates that the process of subduction, in which ocean water is carried hundreds of kilometers inland from 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 the oxygenation of the Earth, initially Oxygen was emitted 2.7 billion years ago also 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 partly explain the lack of oxygen and eventually life on other rocky planets in the future as well.
David MallPostdoctoral Fellow, Earth Sciences, Laurentian UniversityÛ 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 was republished by dialogue Licensed under Creative Commons. Read it The original article.
