Phase transitions in natural C-O-H-N-S fluid inclusions - implications for gas mixtures and the behavior of solid H 2 S at low temperatures

C–O–H–N–S-bearing fluids are known as one of the most challenging geochemical systems due to scarcity of available experimental data. H 2 S-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 CH 4 –H 2 S–CO 2 –N 2 –H 2 O 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 H 2 S (α ↔ β ↔ γ) transitions, complex clathrates and structural transformations of solid state H 2 S 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 H 2 S-rich planetary systems owing to their low temperature environments. Different structures and solid states of H 2 S at low temperatures, pertaining to extraterrestrial environments, were measured in natural, mineral-hosted, micro-scale fluid inclusions containing complex CH 4 -H 2 S-CO 2 -N 2 gas mixtures.