Tracking molecular hydrogen migration along a subduction shear zone

Abstract This manuscript presents integrated field, petrographic, microstructural, and thermodynamic modeling results documenting the high-pressure channelization of H2 and the rheological impact of its reactivity with carbonate (dolomite and calcite)-rich rocks along a serpentinite-hosted shear zone from Alpine Corsica (France). Microstructures within the carbonates attest to the occurrence of a deformation continuum, evolving from initial brittle fracturing to strain localization by viscous deformation. Raman spectra of fluid inclusions within the carbonates reveal the circulation of H2 and CH4 at all stages of the microstructural evolution, the latter interpreted to be the result of H2-carbonate interactions. Thermodynamic models suggest that carbonate phase stability at pressure-temperature conditions representative of a subduction setting is modified by the presence of H2, with dolomite being progressively replaced by calcite + graphite + magnetite with increasing H2 in the system. An initial phase of overpressure created by H2-rich fluids led to the brecciation of dolomite, creating a fine-grained aggregate, which facilitated a switch to a semi-brittle mode of deformation and created high-permeability pathways for subsequent phases of H2 infiltration. Subsequent phases of infiltration of H2 were accompanied by transformation of dolomite to calcite, the degree of transformation dependent upon the efficiency of H2 percolation. Calcite, being rheologically weaker than dolomite at these temperatures, underwent viscous flow in domains of extensive dolomite reduction, whereas adjacent dolomite-rich domains contain minimal imprints of extensive plastic deformation. Our results demonstrate extensive fossilized H2-carbonate reactivity and show that the infiltration of H2-rich fluids strongly affects the rheology of carbonates by inducing reactivity and phase transitions.