Noble gas (He, Ne, and Ar) solubilities in high-pressure silicate melts calculated based on deep-potential modeling

Abstract

Noble gas solubility in silicate melts has provided important insights into how these gases are incorporated into the Earth's interior and how they can be used as tracers of geochemical processes. However, previous studies have shown conflicting results regarding the noble gas solubility in silicate melts at high pressures. Some suggest that a drastic drop in solubility occurs above 5 GPa, while others indicate that maximum solubility is reached at this pressure but the solubility keeps unchanged when the pressure further increases. To clarify this discrepancy and gain a better understanding of the underlying mechanisms, we conducted a solubility investigation using the thermodynamic integration (TI) method and molecular dynamics (MD) simulations based on a machine-learning based force-field called deep potential (DP), which was trained by density functional theory (DFT) calculations. The effects of temperature, pressure and melt composition were investigated up to ∼30 GPa, 2500 K for the melt of model basalt, olivine and enstatite. Our results showed that the solubility of all noble gases (He, Ne and Ar) increased with temperature, while they increased with pressure until maximum solubility was reached at around 5 GPa. Beyond this pressure, solubility gradually decreased as pressure continued to increase. The structural and dynamic analysis of silicate melts revealed an abnormal increase in the self-diffusivity of oxygen atoms in silicate melts from ambient pressure to ∼5–8 GPa, which can be attributed to the formation of Si/Al-O5-6 in the melts upon compression. Interestingly, the upper limit pressure (5–8 GPa) for this abnormal increase in oxygen diffusivity was also the pressure at which noble gas solubilities reached their maximum. Our findings suggest that the maximum noble gas solubility at around 5 GPa may be a universal phenomenon in silicate melts. However, the specific values and changing trends of solubility depend on the specific melt compositions and noble gases. The complexity of noble gas solubility in silicate melts at high temperatures and pressures suggests that there is significant potential for improving the high-pressure noble gas partition coefficients between minerals, silicate melts, and even the Earth's core. Such improvements would extend our understanding of the behavior of noble gases in the Earth's interior and their role in tracing geodynamic processes.