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Earth's ancient breath: Study links atmospheric oxygen and mantle chemistry

An international team of scientists has found a crucial link between the chemistry of Earth’s deep mantle and its early atmosphere. The study uncovers new insights into the evolution of life on our planet and the surge of atmospheric oxygen.

The scientists focused their investigation on magmas formed in ancient subduction zones, areas where portions of Earth’s crust sink back into the mantle. 

Focus of the study

The experts examined a critical juncture in Earth’s history known as the Great Oxidation Event (GOE), which occurred between 2.1 and 2.4 billion years ago. 

This event marked a rapid increase in Earth’s atmospheric oxygen levels, transforming life and environments on our planet. However, little research has been conducted on how these atmospheric changes impacted the Earth’s mantle.

The study, published in the journal Nature Geoscience, explored the role of plate tectonics – the movement and reshaping of the planet’s outer shell – in cycling and exchanging elements between the atmosphere, Earth’s surface, and the deep mantle. Previously, reliable methods to understand these interactions were lacking.

What the researchers learned 

By comparing magmas from before and after the GOE, the researchers found a transition from reduced to more oxidised magmas. 

The shift was due to the deep subduction of oxidized sediments from mountains, which were transformed into sediments during weathering and erosion and then recycled into the mantle through subduction processes. This revealed how sediment recycling enabled the atmosphere to access the mantle.

Study significance 

Study lead author Dr Hugo Moreira of the University of Montpellier is a visiting researcher at the University of Portsmouth. He explained the significance of the findings.

“With these findings, our understanding of Earth’s ancient ‘breath’ has taken a significant leap forward. Not only does it provide crucial insights into Earth’s geological evolution, but it also sheds light on how the deep Earth and its mantle are intimately connected to atmospheric changes. It provides us a better understanding of the relationship between Earth’s external and internal reservoirs,” said Dr. Moreira.

Far-reaching implications 

The implications of this discovery are far-reaching. They suggest that these “whiffs” of oxygen may have altered the mantle by contributing to the increased oxidation of calc-alkaline magma, changing the composition of the continental crust, and leading to the formation of ore deposits on Earth.

Furthermore, the discovery raises intriguing questions about the role of oxygen in shaping the planet’s history and the conditions that set the stage for life as we know it.

Mantle oxygen fugacity

To analyze the sulphur state in minerals found in two-billion-year-old zircon crystals from the Mineiro Belt in Brazil, the research team used the ID21 beamline at the European Synchrotron Radiation Facility in France. These crystals acted as time capsules, preserving their original composition. 

The experts discovered that minerals from magmas that crystallized before the GOE had a reduced sulphur state, but after the GOE, they became more oxidised.

“Mantle oxygen fugacity, in simple terms, is a measure of oxygen’s ability to drive chemical reactions in magmas and is critical for understanding volcanic activity and ore formation,” said Dr. Moreira.

“However, in the past, we lacked a reliable way to track changes in this parameter for ancient parts of Earth’s history – until now.”

Exciting new avenues of research

Study co-author Professor Craig Storey said the study opens exciting new avenues of research, offering a deeper understanding of the Earth’s ancient past and its profound connection to the development of our atmosphere. 

“It challenges us to ponder questions about the evolution of magma types over time and the intricate interplay between plate tectonics and atmospheric cycles,” noted Professor Storey.

Study implications 

The findings of this study not only provide a more comprehensive understanding of the Earth’s geological evolution but also open up new areas for research. 

“As we continue to probe the mysteries of Earth’s geological history, one thing is certain – there is much more to discover beneath the surface,” said Dr. Moreira.

The collaborative study involved researchers from the University of Portsmouth, the Universities of Brest, Montpellier, and the University of Sorbonne (France), the Federal University of Ouro Preto and University of São Paulo (Brazil), and the European Synchrotron Radiation Facility.

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