This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: A research team has found that molten rock can carry more oxygen than its chemical composition would seem to allow. Their study is 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 1,200–1,800 °C, using piston-cylinder and cubic presses. Platinum oxide (PtO₂) was added to create an oxygen-rich environment.
The researchers then measured the elemental composition and the water and carbon dioxide contents of the quenched glass. After 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 were quenched, the measured values are likely a lower limit of what the melt can store. These observations are supported by theoretical simulations led by Niu.
Using deep neural network (DNN) interatomic potentials with ab initio accuracy, the researchers 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 in pressure was observed. The simulations 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.
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