Equilibrium and kinetic controls on molecular hydrogen abundance and hydrogen isotope fractionation in hydrothermal fluids

Abstract

Molecular hydrogen (H2) is among the main components in hydrothermal fluids and exerts a major role in geochemical and biological processes. Nevertheless, the understanding and quantification of factors controlling H2 abundance and isotope composition remain uncertain. Sub-aerial hydrothermal systems provide a unique opportunity to study sources and reactions of H2. Here, we present hydrogen fugacity (fH2) and δD values of H2 and H2O in hydrothermal fluids from such systems worldwide (Greece, Iceland, Kenya and New Zealand), representative of mid-ocean ridge and arc systems and sourced by seawater and meteoric water. The hydrothermal fluid temperatures are 226-359°C, and the fH2, δD-H2 and δD-H2O values are 0.002-3.3 bar, -646 to -391‰ and -94.1 to +11.3‰, respectively. Comparison of measured data with geochemical modeling reveals that H2 is predominantly sourced from the reduction of H2O, enabled through the oxidation of aqueous Fe+II to Fe+III and precipitation of Fe+III-bearing hydrothermal minerals (e.g., epidote). We argue that fH2 in hydrothermal fluids is controlled by metastable mineral-fluid and fluid-fluid equilibria upon progressive rock alteration and depend on temperature, rock-to-water ratio (r/w), source water composition and volcanic gas input. Consequently, fH2 in these systems is not fixed by a sole redox buffer. Our data demonstrate that δD-H2 values of reservoir fluids at depth are controlled by the δD value of the source water and the equilibrium isotope fractionation characteristic of the hydrothermal reservoir temperatures. Attainment of isotopic equilibrium at reservoir conditions is supported by fast isotopic equilibration times at T > 200°C, on the order of minutes to few hours, relative to hydrothermal fluid residence times within the reservoirs of years to decades or even longer. During its ascent to the surface, the isotopic composition of H2 can be modified due to isotopic re-equilibration with H2O. The extent of re-equilibration depends on the interplay between fluid flow velocity, cooling rate and kinetics of D-H exchange between H2 and H2O. For fast flowing well discharges (∼0.1 to >10 m/s), negligible changes occur, whereas for slowly flowing (<0.06 m/s) fumaroles, complete re-equilibration at surface temperatures (∼100°C) is sometimes observed. Based on the extent of hydrogen isotope disequilibrium relative to reservoir conditions, fluid reservoir-to-surface travel times are estimated to range from few minutes to <3 hrs for well fluids and few hours to days for fumarolic vapor.