Identifying the processes that govern the hydrogen isotopic composition of H2 and CH4 in fumarolic emissions: a case study from Nisyros, Greece

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Previous investigations on discharges from geothermal wells revealed that hydrogen isotope partitioning between H2, CH4 and H2O can reflect aquifer reservoir temperature at depth. However, almost nothing is known about the isotopic equilibration behavior between H2, CH4 and H2O in natural gases ascending through the crust. We seek to analyze the bulk isotopic composition of H2, CH4 and H2O and the clumped isotopic composition of CH4 in volcanic-hydrothermal emissions from Nisyros volcano, Greece, to identify the processes exerting control on the apparent H2-H2O and CH4-H2O hydrogen isotope fractionations and to determine to what extent the bulk and clumped isotopic compositions of hydrothermal CH4 and H2 are reset during their ascent from the reservoir to the venting surface. Such information is required to be able to assess the origin of methane by means of hydrogen and clumped isotope data and to determine the state of volcanic activity by monitoring variations in the chemical and isotopic composition of the gases discharging at the surface.