Experimental determination of hydrogen isotopic equilibrium in the system H₂O(l)-H₂(g) from 3 to 90 °C

Molecular hydrogen (H₂) is found in a variety of settings on and in the Earth from low-temperature sediments to hydrothermal vents, and is actively being considered as an energy resource for the transition to a green energy future. The hydrogen isotopic composition of H₂, given as D/H ratios or δD, varies in nature by hundreds of per mil from ∼−800 ‰ in hydrothermal and sedimentary systems to ∼+450 ‰ in the stratosphere. This range reflects a variety of processes, including kinetic isotope effects associated with formation and destruction and equilibration with water, the latter proceeding at fast (order year) timescales at low temperatures (<100 °C). At isotopic equilibrium, the D/H fractionation factor between liquid water and hydrogen (^Dα_H2O(l)-H2(g)) is a function of temperature and can thus be used as a geothermometer for H₂ formation or re-equilibration temperatures. Multiple studies have produced theoretical calculations for hydrogen isotopic equilibrium between H₂ and water vapor. However, only three published experimental calibrations used in geochemistry exist for the H₂O-H₂ system: two between 51 and 742 °C for H₂O_(g)-H_2(g) (Suess, 1949, Cerrai et al., 1954), and one in the H₂O_(l)-H_2(g) system for temperatures <100 °C (Rolston et al., 1976). Despite these calibrations existing, there is uncertainty on their accuracy at low temperatures (<100 °C; e.g., Horibe and Craig, 1995).