Magma can store far more oxygen than scientists thought, study finds

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

Oxygen is one of the most abundant elements inside Earth, accounting for more than half of the atoms in the planet's silicate interior. Scientists have traditionally assumed that magma can hold only a specific amount of oxygen, fixed by the quantities and electrical charge states of other elements in the molten rock. Water and carbon dioxide are treated as volatile components that dissolve in magma and escape as gases when pressure falls, but oxygen has generally not been viewed that way.

To test the assumption, the team heated a synthetic version of mid-ocean ridge basalt, a type of volcanic rock commonly produced where tectonic plates move apart beneath the oceans, at pressures from normal atmospheric conditions to 7 gigapascals, about 69,000 times the pressure at Earth's surface, and temperatures of 1200 to 1800 °C. Platinum oxide was added to create an oxygen-rich environment. Rapidly cooled samples preserved as glass showed almost no excess oxygen at normal pressure, but at 7 gigapascals excess oxygen accounted for more than 5% of the sample's weight. Comparable experiments without platinum oxide produced only 0.2% to 0.5%.

Raman spectroscopy found no detectable signature of oxygen gas, indicating the oxygen had become part of the glass structure rather than being trapped as bubbles. The measurements may even underestimate magma's capacity, since some oxygen could have escaped while the samples cooled.

Prof. Niu led computer simulations using deep neural network models with accuracy approaching ab initio calculations, which describe atomic behavior from quantum mechanics. The simulations showed additional oxygen dissolving into the melt until equilibrium, reproducing the pattern that storage capacity rises with pressure. As more oxygen entered, the electrical charges of its atoms became more varied and a small proportion formed peroxide-like bonds.

The researchers propose that magma formed in oxidized regions of the mantle could absorb substantial excess oxygen under immense pressure, then release some of it as it rises and the pressure falls, potentially increasing the oxidation of the upper mantle, crust and eventually the atmosphere. They note that between roughly 2.5 billion and 2.0 billion years ago, when atmospheric oxygen levels underwent major increases, some of Earth's largest large igneous provinces were forming. Their preliminary calculation suggests that if magma erupted in that period released excess oxygen equal to just 0.27% of its weight, the total could theoretically match all the oxygen in Earth's atmosphere today. The calculation does not establish that volcanic activity actually supplied that much atmospheric oxygen.

The findings could change how scientists understand ancient magma oceans and introduce a possible new factor to volcanic degassing, the process through which volatile substances escape molten rock and interact with a planet's atmosphere. The researchers propose that oxygen should be recognized as an independent component of silicate melts. Further research is needed to establish how much oxygen natural magmas carry and how much reaches the atmosphere, and understanding how oxygen moves between the interiors and atmospheres of rocky planets could help researchers investigate the conditions that allow potentially habitable environments to develop.