Phase transitions in natural C-O-H-N-S fluid inclusions - implications for gas mixtures and the behavior of solid H₂S...

C-O-H-N-S-bearing fluids are known as one of the most challenging geochemical systems due to scarcity of available experimental data. H₂S-rich fluid systems were recognized in a wide array of world-class mineral deposits and hydrocarbon reservoirs. Here we report on a nature of low-temperature (T ≥ -192 °C) phase transitions observed in natural CH₄-H₂S-CO₂-N₂-H₂O fluid inclusions, which are modeled as closed thermodynamic systems and thus serve as natural micro-laboratories representative of the C-O-H-N-S system. For the first time, we document solid-solid H₂S (α ↔ β ↔ γ) transitions, complex clathrates and structural transformations of solid state H₂S in natural inclusion gas mixtures. The new data on Raman spectroscopic features and a complete sequence of phase transition temperatures in the gas mixtures contribute to scientific advancements in fluid geochemistry. Enhanced understanding of the phase equilibria in the C-O-H-N-S system is a prerequisite for conscientious estimation of P-T-V-X properties, necessary to model the geologic evolution of hydrocarbon and mineral systems. Our findings are a driver for the future research expeditions to extraterrestrial H₂S-rich planetary systems owing to their low temperature environments.