Scientists discover a hidden oxygen reservoir deep inside Earth

Scientists have discovered that molten rock deep inside Earth can store far more oxygen than previously believed. The finding challenges long-standing assumptions about magma and raises an intriguing possibility: oxygen released from Earth's interior may have helped transform the planet's atmosphere billions of years ago.

Through high-pressure experiments and advanced computer simulations, researchers found that magma can absorb additional oxygen beyond the amount normally required by its chemical composition. This hidden oxygen could provide a previously unrecognized connection between Earth's deep interior, volcanic activity, and the development of an oxygen-rich atmosphere.

The research was led by Dr. Yanhao Lin and Dr. Ho-kwang Mao of the Center for High Pressure Science and Technology Advanced Research (HPSTAR), working with Prof. Haiyang Niu of Northwestern Polytechnical University and Prof. Wim van Westrenen of Vrije Universiteit Amsterdam. Their findings were published in National Science Review.

Oxygen is one of the most abundant elements inside Earth, accounting for more than half of the atoms in the planet's silicate interior. Silicates are minerals and molten materials composed primarily of silicon and oxygen, along with other elements. They make up much of Earth's mantle and crust.

Despite oxygen's abundance, scientists have traditionally assumed that magma can contain only a specific amount of it. Under this understanding, the oxygen content is determined by the quantities and electrical charge states of other elements in the molten rock, including silicon, magnesium, and iron.

Water and carbon dioxide behave differently. Both can dissolve in magma and later escape as gases when the surrounding pressure decreases. These substances are known as volatile components, and their release plays an important role in volcanic activity.

Oxygen, however, has generally not been treated as a separate volatile substance that magma can absorb and release. Whether molten silicate rock could hold additional oxygen independently of its ordinary chemical composition remained largely unexplored.

To investigate, the scientists recreated conditions found deep within Earth using laboratory equipment capable of generating extraordinary pressures and temperatures.

They heated a synthetic version of mid-ocean ridge basalt, a type of volcanic rock commonly produced where tectonic plates move apart beneath the oceans. The samples were exposed to pressures ranging from normal atmospheric pressure to 7 gigapascals and temperatures between 1200 and 1800 °C.

The team used specialized piston-cylinder and cubic presses to maintain these extreme conditions. They also added platinum oxide (PtO2) to establish an oxygen-rich environment and test how much additional oxygen the molten rock could absorb.

After the experiments, the researchers rapidly cooled the molten material, producing glass that preserved important characteristics of the melt. They then analyzed its elemental composition and measured the amounts of water and carbon dioxide present.

By calculating how much oxygen was needed to balance the other elements, along with the oxygen already contained in water and carbon dioxide, the scientists could estimate how much remained unaccounted for.

At normal atmospheric pressure, the glass contained almost no excess oxygen. At 7 gigapascals, however, excess oxygen accounted for more than 5% of the sample's weight.

The researchers also wanted to determine whether the additional oxygen was truly incorporated into the material or merely trapped as tiny gas bubbles.

Using Raman spectroscopy, a technique that identifies substances by examining how they interact with light, they found no detectable signature of oxygen gas in the glass. This indicated that the excess oxygen had become part of the glass structure rather than accumulating in bubbles.

The measured concentrations may even underestimate magma's actual oxygen-storage capacity. Some oxygen could have escaped while the molten samples were cooling, meaning the original melts may have contained still greater amounts.

To better understand the discovery at the atomic level, Prof. Niu led a series of theoretical computer simulations.

The researchers used deep neural network (DNN) models to simulate interactions among atoms with accuracy approaching that of ab initio calculations, which describe atomic behavior using fundamental principles of quantum mechanics.

The simulations showed that additional oxygen could dissolve into molten material until the system reached equilibrium, a state in which oxygen entering and leaving the melt is balanced.

Although the computer models used a simplified system containing only magnesium, silicon, and oxygen, they reproduced the same overall pattern observed in the laboratory: magma's ability to hold excess oxygen increased with pressure.

The simulated oxygen concentrations were lower than the experimental measurements, which the scientists attributed to differences between the simplified model and the more complex chemical composition of actual basalt.

The calculations also revealed changes in how oxygen atoms behaved as more oxygen entered the melt. Their electrical charges became more varied, and a small proportion formed peroxide-like bonds, in which oxygen atoms bond to one another.

Importantly, the pressure-dependent behavior resembles that of water and carbon dioxide, which also become more soluble in magma as pressure increases.

Based on these findings, the researchers propose that oxygen should be recognized as an independent component of silicate melts, rather than being considered only in relation to other elements.

The discovery raises a much larger question about Earth's history: Could magma have transported significant amounts of oxygen from the planet's interior to its surface?

Under the immense pressures deep underground, this magma could absorb substantial quantities of excess oxygen. As it moves upward, the surrounding pressure decreases, reducing the amount of oxygen the molten rock can retain.

Some of that oxygen could then be released, potentially contributing to chemical reactions that increase the oxidation of the upper mantle, Earth's crust, and eventually the atmosphere.

Oxidation refers to chemical processes involving the transfer of electrons. In this context, oxygen carried by magma could increase the oxidizing capacity of surrounding materials and potentially influence the balance of oxygen in the atmosphere.

Between approximately 2.5 billion and 2.0 billion years ago, atmospheric oxygen levels underwent major increases. This interval overlaps with the formation of some of Earth's largest large igneous provinces (LIPs), enormous regions of volcanic rock created by exceptionally large outpourings of magma.

Their preliminary calculation highlights the potentially enormous scale of the process. They estimate that if magma erupted during this period released excess oxygen equal to just 0.27% of its weight, the total amount could theoretically match all the oxygen present in Earth's atmosphere today.

This does not establish that volcanic activity actually supplied that much atmospheric oxygen. Instead, it illustrates how even a relatively small release of oxygen from immense quantities of magma could have major planetary consequences.

For one, they may change how scientists understand ancient magma oceans, vast regions of molten rock believed to have existed during the early evolution of Earth and other rocky planets.

They also introduce a possible new factor in volcanic degassing, the process through which gases and other volatile substances escape from molten rock and interact with a planet's atmosphere.

If magma can store and release oxygen independently of its other chemical components, models of Earth's atmospheric development may need to account for this previously overlooked process.

The implications could extend beyond Earth. Understanding how oxygen moves between the interiors and atmospheres of rocky planets may help researchers investigate the conditions that allow potentially habitable environments to develop and persist over billions of years.

Although further research is needed to establish how much oxygen natural magmas carry and how much reaches the atmosphere, the discovery points to a surprising possibility: some of the processes that helped shape Earth's oxygen-rich environment may have originated far beneath the planet's surface.