Laboratory experiments indicate that molten rock can absorb extra oxygen as a distinct volatile component when subjected to high pressures and temperatures, a finding that could change how scientists think about volcanic contributions to Earth's atmosphere.
New pathway for oxygen in magma
Researchers led by Dr. Yanhao Lin and Dr. Ho-kwang Mao at the Center for High Pressure Science and Technology Advanced Research (HPSTAR), with collaborators Prof. Haiyang Niu and Prof. Wim van Westrenen, report that synthetic mid-ocean ridge basalt can take up an additional supply of oxygen under experimental conditions. The work, published in National Science Review, shows that molten rock may store oxygen beyond the amounts predicted by standard chemical-balance calculations based on major element abundances.
Geoscientists typically estimate the oxygen content of silicate melts from the proportions and oxidation states of elements such as silicon, magnesium and iron. That approach assumes most oxygen is chemically bound into the melt structure. The new experiments explored whether a separate, mobile oxygen component — akin to the way water and carbon dioxide exist as volatiles in melt — could also be present.
Pressure drives oxygen uptake
To supply extra oxygen, the team added platinum oxide (PtO2) to laboratory charges of synthetic mid-ocean ridge basalt and melted them using piston-cylinder and cubic presses. Temperatures ranged from 1,200 to 1,800 °C and pressures from 1 atmosphere up to 7 gigapascals (roughly the pressure found about one kilometre beneath Earth's surface at the high end of the experimental range).
| Parameter | Experimental range |
|---|---|
| Temperature | 1,200–1,800 °C |
| Pressure | 1 atm–7 GPa |
Under the highest pressures tested, the melts incorporated more oxygen than would be expected from balancing the major-element chemistry alone. That suggests pressure enhances the ability of molten silicates to dissolve oxygen in a form that is not simply bound into the usual structural network.
- Mechanism: An unexpected volatile-like uptake of oxygen by melt, promoted by pressure.
- Experimental system: Synthetic mid-ocean ridge basalt with platinum oxide as an oxygen source.
- Implication: Rising magma might release additional oxygen as it decompresses during ascent, potentially affecting surface and atmospheric chemistry.
Implications for planetary history and volcanism
If real magmas in Earth’s interior behave as the experiments suggest, then ascending melts could release oxygen to shallow crustal levels or the atmosphere as pressure decreases. The authors note this process might have contributed to increases in atmospheric oxygen at times in Earth’s distant past, an idea that could influence interpretations of the evolution of Earth’s atmosphere and early surface environments.
The finding also reframes how researchers quantify volatile budgets in magmatic systems. Oxygen is already known to be abundant in Earth's silicate interior — occupying more than half of the atoms in many rocks — but the new work draws attention to the distinction between oxygen that is chemically bound and oxygen that might act like a volatile, mobile component during melting and ascent.
What remains uncertain
The experiments used synthetic basalt and a deliberately oxygen-rich environment created with platinum oxide. Translating laboratory results to natural systems requires caution: natural mantle and magma compositions, variable oxygen sources, and complex redox processes in the crust could alter how much oxygen is stored and subsequently released. Further study will be needed to determine the prevalence of this phenomenon in different tectonic settings and through geological time.
Nonetheless, the work adds a plausible mechanism by which volcanic processes could influence the redox state of Earth’s surface and atmosphere, complementing other pathways such as biological oxygen production and atmospheric escape processes.
As geoscientists continue to refine models of Earth’s volatile cycles, these high-pressure experiments underline how seemingly subtle behaviours of melts deep below our feet can have broad consequences for planetary evolution.
