Characterization of H2 and hydrocarbons trapped in exhumed metamorphic rocks: origin and fluxes of energy sources in subduction zones

Methane (CH4) and molecular hydrogen (H2) significantly influence Earth’s habitability, providing essential life ingredients in the deep subsurface, potentially Earth's largest biosphere. However, hydrocarbons and H2 origin, migration, distribution, and impact on deep life at convergent margins remain largely unconstrained. This PhD project investigates metamorphic hydrocarbons and H2 origin and migration through spectroscopic and isotopic analysis of fluid inclusions (FIs) in exhumed metamorphic terranes across the globe. This research project addressed the analytical challenges associated with FIs isotope analysis by developing robust protocols for FIs mechanical extraction and δ13C-CH4 and δ13C-CO2 measurement through Cavity Ring-Down Spectroscopy. The protocols allowed replication of previously measured δ13C-CH4 values within 1 ‰ and enabled analysis of samples with CH4 and CO2 concentrations above 10 and 1000 ppm, respectively. Fluid inclusion analysis from the Belvidere Mountain Complex, Northern Vermont, revealed insights into deep energy fluxes within the Taconian subduction zone. Changes in CH4 concentration and δ13C-CH4 across different lithological reservoirs depict a CH4-H2 "catch and release" mechanism driven by multiple fluid-rock interaction events. Methane isotopic evolution, from epidote-amphibolite to greenschist facies metamorphic conditions, reflects increasing input of metasediment-derived CH4 and/or transformation to graphite/carbonate. The study of FIs in the Monte Maggiore ophiolite, Northern Corsica, demonstrated that subducted oceanic lithosphere can retain a diverse record of energy sources. Variations of CH4-C2H6 isotopes within populations of olivine-hosted FIs suggest changes in the settings of hydrocarbons and H2 genesis, from a mid-ocean ridge setting to high-pressure serpentinisation during the Alpine subduction. The complex isotopic patterns underscore our limited understanding of abiotic hydrocarbon formation through fluid-rock interactions. This research provides new insights into the role of fluid-rock interactions in deep energy sources production and migration at subduction zones, emphasizing the need for further FI isotope studies to better understand the deep CH4-H2 cycle and its influence on Earth's habitability.