image: Excess oxygen in silicate glass increases with pressure. Circles and diamonds show experiments on basalt that was fully molten or partly molten, respectively; open symbols mark control runs without an added oxygen source; squares show molecular dynamics results for MgSiO3 melt. Colors indicate temperature. The dashed line marks the starting composition.
Credit: ©Science China Press
The research team, led by Dr. Yanhao Lin and Dr. Ho-kwang Mao of the Center for High Pressure Science and Technology Advanced Research (HPSTAR), together with Prof. Haiyang Niu of Northwestern Polytechnical University and Prof. Wim van Westrenen of Vrije Universiteit Amsterdam, found that molten rock can carry more oxygen than its chemical composition would allow. The study was published in National Science Review.
Oxygen comprises more than half of the atoms in Earth’s silicate interior. Geoscientists have long assumed that the amount of oxygen in magma is fixed by the amounts and charge states of the other elements present, such as silicon, magnesium and iron. Unlike water and carbon dioxide, oxygen has therefore not been considered a volatile component that can dissolve in magma. However, whether silicate melts themselves can store oxygen as an independent volatile component, remains largely unexplored.
The research team melted a synthetic mid-ocean ridge basalt at pressures from 1 atmosphere to 7 gigapascals and temperatures of 1200-1800 °C, using piston-cylinder and cubic presses. Platinum oxide (PtO2) was added to create an oxygen-rich environment. The researchers then measured elements composition, water and carbon dioxide contents in the quenched glass, and subtracting the oxygen needed to balance all other elements and the oxygen in water and carbon dioxide, they defined the residual oxygen as “excess oxygen.”
They found that the quenched silicate glass can store substantial excess oxygen, and that the amount rises with pressure: it is close to zero at 1 atmosphere but reaches more than 5 wt.% at 7 gigapascals. Control experiments run at the same pressures and temperatures without added PtO2 yielded only 0.2-0.5 wt.%. Raman spectroscopy detected no signature of oxygen gas in the glass, indicating that the excess oxygen is incorporated into the glass structure rather than trapped as bubbles. Because some oxygen may have escaped as the samples quenching, the measured values are likely a lower limit of what the melt can store.
These observations are supported by theoretical simulations, led by Prof. Niu. By using deep neural network (DNN) interatomic potentials with ab initio accuracy, they found that oxygen dissolved into the melt until equilibrium was reached. Because the simulations used a simplified magnesium-silicon-oxygen system, the values are lower than in the experiments, but the same rise with pressure was observed. They also showed that, as the melt becomes richer in oxygen, the electric charges on oxygen atoms spread over a wider range, and a small fraction of oxygen atoms form peroxide-like bonds. The increase of oxygen content with pressure resembles how water and carbon dioxide dissolve more readily in magma at higher pressure, and the authors suggest that oxygen should be considered a separate component of silicate melts.
The findings suggest a possible way for carrying oxidizing power from deep Earth to the surface. Magma formed in oxidized mantle regions could carry substantial excess oxygen. As the magma rises and the pressure drops, it would gradually release this oxygen. This could oxidize the shallow mantle, the crust and the atmosphere. This idea may explain why major rises in atmospheric oxygen roughly 2.5 to 2.0 billion years ago overlap in time with the formation of some of the largest large igneous provinces (LIPs) formed. A rough estimation suggests that magma erupted during this period, release of just 0.27 wt.% excess oxygen would be enough to supply all the oxygen in today’s atmosphere.
This research provides a new perspective on assessing magma ocean evolution, volatile degassing, the evolution of Earth's atmosphere and the long-term habitability on other terrestrial planets.