High-pressure experiments and computer simulations suggest that molten rock deep inside Earth can hold substantially more oxygen than conventional chemical models predict. The finding, reported by researchers from the High Pressure Science and Technology Advanced Research Center (HPSTAR), Northwestern Institute of Technology and Vrije Universiteit Amsterdam in National Science Review, points to a possible connection between Earth’s interior, volcanic activity and the rise of an oxygen-rich atmosphere billions of years ago.
Rethinking oxygen in magma
Oxygen is one of the most abundant elements inside Earth, accounting for more than half of the atoms in the planet’s silicates. These minerals and molten materials, made primarily of silicon and oxygen along with other elements, form most of Earth’s mantle and crust.
Scientists have traditionally assumed that magma contains only the amount of oxygen required by its chemical composition. Under that view, oxygen content is determined by the quantity and charge state of elements such as silicon, magnesium and iron.
Water and carbon dioxide are treated differently. They can dissolve in magma and escape as gases when surrounding pressure decreases. These substances are known as volatile components, and their release is an important part of volcanic activity.
Oxygen, however, has generally not been considered an independent volatile substance that molten rock can absorb and release. Whether silicate melts can retain additional oxygen beyond the amount required by their usual chemical structure has remained largely unexplored.
The new study was led by Dr. Yanhao Lin and Dr. Ho-kwang Mao of HPSTAR, in collaboration with Professor Haiyang Niu of Northwestern Institute of Technology and Professor Wim van Westrenen of Vrije Universiteit Amsterdam.
Extreme conditions reveal hidden oxygen
The researchers used high-pressure equipment to recreate temperatures and pressures found deep inside Earth. They heated a synthetic form of mid-ocean ridge basalt, a 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 of 1200 to 1800 °C. A pressure of 7 gigapascals is approximately 69,000 times the atmospheric pressure at Earth’s surface.
A piston-cylinder apparatus and a cube press maintained these extreme conditions. The team also added platinum oxide (PtO2) to create an oxygen-rich environment and test how much additional oxygen the molten rock could absorb.
After the experiments, the researchers rapidly cooled the molten samples, producing glass that preserved key properties of the melt. They analyzed the glass’s elemental composition and measured the water and carbon dioxide it contained.
They then calculated how much oxygen was needed to balance the other elements, in addition to the oxygen already present in water and carbon dioxide. The remaining amount was designated “excess oxygen.”
The results showed a strong relationship between pressure and oxygen storage. At normal atmospheric pressure, the glass contained very little excess oxygen. At 7 gigapascals, however, excess oxygen accounted for more than 5% of the sample’s weight.
For comparison, samples produced at similar pressures and temperatures without platinum oxide contained only 0.2 to 0.5 wt% excess oxygen.
The scientists also tested whether the oxygen was genuinely incorporated into the glass or merely trapped in tiny bubbles. Raman spectroscopy, which identifies materials by measuring how they interact with light, detected no oxygen gas in the glass. That result indicates that the additional oxygen became part of the glass structure.
The measured concentrations could even be lower than the melt’s actual storage capacity. Some oxygen may have escaped as the molten sample cooled, meaning the original melt may have contained more.
Simulations show how the melt accommodates oxygen
To investigate the process at the atomic level, Professor Niu led theoretical computer simulations. The team used a deep neural network (DNN) model to simulate atomic interactions with an accuracy approaching that of ab initio calculations, which describe atomic behavior using fundamental principles of quantum mechanics.
The simulations indicated that additional oxygen can dissolve into molten material until the system reaches equilibrium, when the movement of oxygen into and out of the melt is balanced.
The model contained only magnesium, silicon and oxygen, making it much simpler than the actual basalt. Even so, it reproduced the main experimental pattern: the melt’s ability to hold excess oxygen increased as pressure rose.
The simulated concentrations were lower than those measured in the laboratory. The researchers attributed the difference to the model’s simplified chemistry compared with the more complex composition of natural basalt.
The calculations also showed that the behavior of oxygen atoms changes as more oxygen enters the melt. Their charges become more varied, and some form peroxide-like bonds in which oxygen atoms bond to one another.
These changes provide a possible explanation for how molten rock can accommodate more oxygen than traditional chemical models predict. The pressure-dependent behavior also resembles that of water and carbon dioxide, which become more soluble in magma as pressure increases.
On the basis of these findings, the researchers propose that oxygen should be treated as an independent component of silicate melts rather than considered only in relation to other elements.
Could ancient volcanism have affected atmospheric oxygen?
The discovery raises the possibility that magma transported substantial amounts of oxygen from Earth’s interior to its surface. The researchers describe a potential mechanism involving magma generated in oxidized regions of the mantle.
At great depth, immense pressure could allow this magma to absorb significant quantities of excess oxygen. As the magma rises, pressure decreases and the amount of oxygen it can retain is reduced. Some of the oxygen could then be released, promoting chemical reactions that increase the oxidation of the upper mantle, crust and, ultimately, the atmosphere.
Oxidation is a chemical process involving the transfer of electrons. In this context, oxygen carried by magma could increase the oxidizing capacity of surrounding materials and influence the planet’s overall oxygen balance.
The proposed mechanism may be relevant to a major transition in Earth’s geological history. Between about 2.5 billion and 2 billion years ago, atmospheric oxygen concentrations increased significantly. This interval also coincided with the formation of some of Earth’s largest large igneous provinces (LIPs), regions where exceptionally large volumes of magma produced enormous formations of volcanic rock.
The researchers suggest that oxygen released by these ancient volcanic systems may have contributed to changes in atmospheric composition.
Their preliminary calculations illustrate the potential scale of the process. If magma erupted during this period released excess oxygen equivalent to only 0.27% of its weight, the total amount could theoretically equal all the oxygen currently present in Earth’s atmosphere.
This estimate does not demonstrate that volcanic activity supplied that much atmospheric oxygen. Instead, it indicates that small amounts of oxygen released from vast quantities of magma could have significant planetary effects.
Implications for planetary science
The findings could affect models of ancient magma oceans, vast bodies of molten rock thought to have existed early in the histories of Earth and other rocky planets.
They may also introduce a previously overlooked factor into models of volcanic degassing, the process through which gases and other volatile substances escape from lava and interact with an atmosphere.
If magma can store and release oxygen independently of its other chemical components, models of Earth’s atmospheric evolution may need to include this process. Understanding how oxygen moves between the interiors and atmospheres of rocky planets could also help scientists investigate how potentially habitable environments form and persist over billions of years.
More research is needed to determine how much oxygen natural magma carries and how much ultimately reaches the atmosphere. For now, the study points to a possibility that some processes involved in creating Earth’s oxygen-rich environment may have begun far below the planet’s surface.
Source: www.sciencedaily.com
