1,2,3,1,2- 1State Key Laboratory of Lithospheric and Environmental Coevolution, University of Science and Technology of China, Hefei 230026, China
- 2Deep Space Exploration Laboratory, Hefei 230026, China
- 3Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
Mantle-derived fluids are increasingly recognized as key contributors to hydrocarbon and natural hydrogen resources, yet their phase relations, compositions, and evolutionary pathways remain poorly constrained. Petrological observations and experiments suggest that deep hydrogen reacts with carbon-bearing materials to form hydrocarbons such as methane, resulting in the coexistence of H2 and CH4 in the upper mantle. In contrast, surface natural hydrogen accumulations commonly contain >90% H2. This disparity points to significant fluid fractionation during ascent, potentially driven by phase separation. However, a lack of data at high pressure and temperature has prevented clear constraints on the phase behavior and thermodynamic properties of H2-CH4 systems under upper mantle conditions.
In this study, we investigate the structure and thermodynamic properties of the H2–CH4 system under upper mantle conditions using first-principles molecular dynamics simulations integrated with available experimental constraints. Simulations were performed for pure H2, pure CH4 and H2–CH4 mixtures over a wide range of compositions under upper mantle conditions. Long-range interactions were treated using the SCAN+rVV10 functional. The resulting simulation data were used to construct P–V–T equations of state and to develop a thermodynamic model for the H2–CH4 binary system. Our results show that at high hydrogen concentrations, H2 and CH4 exhibit fluid immiscibility, leading to the segregation of hydrogen-rich fluids. This immiscibility becomes more pronounced with decreasing pressure and temperature, consistent with conditions expected during fluid ascent. Radial distribution function analyses indicate that both components remain molecular, with no evidence for additional species formation under the investigated conditions. Large-scale simulations involving up to 1012 atoms reproduce the same immiscibility behavior, confirming the robustness of the results.
These findings place new constraints on the phase behavior of H2–CH4 fluids in the upper mantle and provide a plausible mechanism for the generation of hydrogen-rich fluids observed at Earth’s surface. The thermodynamic models developed here offer a quantitative framework for future studies of deep hydrogen cycling and mantle hydrocarbon systems.
How to cite: Gao, H., Li, Y., Zhang, Z., and Ni, H.: Phase Behavior and Immiscibility of H2–CH4 Fluids Under Upper Mantle Conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4772, https://doi.org/10.5194/egusphere-egu26-4772, 2026.
