Adsorption of Helium and Argon on the (001) Surface of Periclase: A First-Principles Study

The distribution of rare gases within the Earth’s interior has caught the attention of scientists for the past few years. The inertness and volatility of noble gases make them excellent tracers for understanding the chemical evolution of Earth’s mantle and atmosphere. Previous studies indicate that noble gases can be found associated with clathrates, form their own oxides, or, in some cases, noble gases such as helium and xenon can even bond with Fe under extreme pressure (p) - temperature (T) conditions like those in Earth’s core. However, the ability of lower mantle mineral phases to house rare gases remains poorly understood, leaving important gaps in knowledge. Helium and argon are noble gases of interest in this study. The isotopes ⁴He and ⁴⁰Ar are produced from the radioactive decay of ²³⁸U and ⁴⁰K within the Earth’s interior, while ³He and ³⁶Ar are regarded as primordial, introduced during the accretion of Earth. Dong et al. (2022) revealed that noble gases can become reactive under mantle pressure conditions. Still, their ability to be incorporated into mantle minerals via adsorption needs to be thoroughly studied, as there are many limitations in the experiments conducted to measure the solubility of noble gases in minerals under mantle p-T conditions. In this study, we investigated the adsorption behavior of helium and argon on the (001) plane of periclase (MgO) by employing first-principles density functional theory (DFT) calculations.

Adsorption energies were estimated across pressures ranging from 0 to 125 GPa, representative of conditions throughout Earth’s interior, i.e., approximately up to the Core Mantle Boundary (CMB). At ambient pressure, both helium and argon showed negative adsorption energies, indicating stable adsorption relative to isolated species (MgO, Ar, He). These energies became increasingly negative with pressure, becoming notably negative beyond 75 GPa which corresponds to lower mantle pressures. This may be due to the accelerated reactivity of noble gases at extreme pressure conditions, as reported in previous studies. Additionally, under all pressure conditions argon exhibited stronger adsorption than helium, indicating enhanced argon retention in lower mantle conditions. However, further investigations into the mechanical and dynamical stability of these adsorbed structures are required to completely understand the mechanisms governing noble gas occurrence in the Earth’s lower mantle.