Methane and hydrogen in fluid inclusions of metamorphic and hydrothermal origin: Implications for natural hydrogen system analysis and exploration

Natural hydrogen is considered to be of utmost importance for the transition towards a low carbon energy system. As natural hydrogen systems can be hosted by a variety of geological settings, they are extremely versatile so that generation, migration, and accumulation processes are often insufficiently understood. This contribution presents the results of a case study that was carried out to determine the origin and evolution of CH₄- and H₂-rich fluids trapped in faulted, felsic granulite from the Bohemian Massif (Lower Austria). Samples were taken in an active quarry, where the granulite occurs spatially associated with tectonically incorporated, partly serpentinized, ultramafic lenses.

Fluid inclusions are found in quartz and garnet. While the inclusions in quartz are clearly of secondary origin, the ones in garnet are primary and therefore related to the high-grade granulite facies metamorphism. Raman spectroscopy measurements revealed a complex polyphase composition. The primary inclusions contain CH₄ and H₂ as well as several (hydrous) mineral phases. Three mineral parageneses can be distinguished: (i) granitic, (ii) Al-silicates, and (iii) hydrous Mg-silicates. The secondary inclusions generally contain no solid phases and mainly consist of CH₄, H₂, N₂, and H₂S (± H₂O). Based on microthermometry measurements combined with thermodynamic calculations, the secondary inclusions in quartz can be attributed to fluid migration during the latest stage of exhumation at conditions of approximately 200 °C and 60 MPa corresponding to a depth of 2 to 2.5 km. Serpentinization at low temperature is further evidenced by lizardite being the predominant serpentine polymorph. Whereas H₂, CH₄, and H₂S in secondary inclusions are related to low temperature serpentinization and accompanying carbon hydrogenation (Fischer-Tropsch type) reactions, the gas species in the primary inclusions are of deep-crustal or mantle origin. Observations during Raman spectroscopy measurements, however, may also indicate an impact of photocatalytic reactions triggered by the laser on fluid composition in the primary inclusions.