Oxygen “whiffs” play a role in the formation of ore deposits

Oxygen “whiffs” may have changed the earth’s mantle by contributing to increased oxidation of calc-alkaline magma, altering the composition of the continental crust, and leading to the formation of ore deposits.

This, according to new research by a team at the University of Portsmouth and the University of Montpellier, who investigated magmas formed in ancient subduction zones, where portions of earth’s crust sink back into the mantle, from a pivotal moment in the planet’s history—the Great Oxidation Event (GOE). This event, which is estimated to have happened between 2.1 and 2.4 billion years ago, was a period of time when oxygen levels in the atmosphere increased rapidly and transformed life and environments.

The new study, published in the journal Nature Geoscience, examined the role of plate tectonics—the process by which our planet’s outer shell moves and reshapes its surface—in cycling and exchanging elements between the atmosphere, earth’s surface, and the deep mantle. Until now, reliable methods to understand these interactions were elusive.

By examining magmas from before and after the GOE, the team found a shift from reduced to more oxidized magmas. This was a result of the deep subduction of oxidized sediments from mountains transformed into sediments during weathering and erosion that were then recycled into the mantle via subduction processes—revealing how sediment recycling provided atmospheric access to the mantle.

“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,” lead author, Hugo Moreira, said in a media statement.

The research team used the ID21 beamline at the European Synchrotron Radiation Facility in France to analyze the sulphur state in minerals found in two-billion-year-old zircon crystals from the Mineiro Belt in Brazil, which acted as time capsules, preserving their original composition. They discovered that minerals from magmas that crystallized before the GOE had a reduced sulphur state. However, after the GOE, these became more oxidized.

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

The scientist explained that sulphur speciation and magma fugacity are dynamic parameters that can change throughout a magma’s journey from formation to crystallization. He noted that although the research considered factors like pressure and temperature, further analyses are needed to trace the complete ‘fugacity path’ from magma generation to final crystallization.

“Our 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,” paper co-author Craig Storey said. “It challenges us to ponder questions about the evolution of magma types over time and the intricate interplay between plate tectonics and atmospheric cycles.”