First-principles study of helium migration in stishovite in the Earth's mantle

Abstract

The migration mechanisms of helium (He) in anhydrous and hydrous stishovite under Earth's mantle conditions are studied using density functional theory (DFT) and climbing image nudged elastic band (CI-NEB) transition state calculations. In anhydrous stishovite, He diffusion is constrained to the structurally unobstructed [001] channels, with diffusion activation energies along other directions significantly higher than [001], confirming that He migrates almost exclusively through these anisotropic pathways. Hydrous stishovite, however, hosts "one-dimensional superionic conduits", where the activation energy for He diffusion is less than 50% of that in anhydrous stishovite, leading to markedly enhanced He diffusion rate. Pressure exerts a similar effect on both stishovite types—increasing pressure elevates diffusion energy barriers and reduces He diffusion rates. However, hydrous stishovite exhibits weaker pressure sensitivity: a 50 GPa pressure increase causes a far smaller reduction in He diffusion rates compared to anhydrous stishovite. Temperature-dependent retention analysis further reveals that anhydrous stishovite exhibits delayed thermal responsiveness relative to hydrous stishovite, mirroring the trend observed between quartz and coesite—under equivalent He retention levels, quartz sustains higher temperatures than coesite. These findings underscore the decisive role of crystal structural features, specifically, the [001] channels in anhydrous stishovite and superionic conduits in hydrous stishovite are governing noble gas migration dynamics. By quantifying the modulation of He diffusion by pressure, temperature, and grain size, this work advances mechanistic models for subsurface noble gas cycling and deep Earth degassing processes.