Origin of methane in serpentinite-hosted hydrothermal systems: The CH4–H2–H2O hydrogen isotope systematics of the Hakuba Happo hot spring

Serpentinite-hosted hydrothermal systems have attracted considerable attention as sites of abiotic organic synthesis and as habitats for the earliest microbial communities. Here, we report a systematic isotopic study of a new serpentinite-hosted system: the Hakuba Happo hot spring in the Shiroumadake area, Japan ( 36 ° 42 ′ N , 137 ° 48 ′ E ). We collected water directly from the hot spring from two drilling wells more than 500 m deep; all water samples were strongly alkaline ( pH > 10 ) and rich in H2 (201–664 μmol/L) and CH4 (124–201 μmol/L). Despite the relatively low temperatures (50–60 °C), thermodynamic calculations suggest that the H2 was likely derived from serpentinization reactions. Hydrogen isotope compositions for Happo #1 (Happo #3) were found to be as follows: δ D-H 2 = − 700 ‰ ( − 710 ‰ ), δ D-CH 4 = − 210 ‰ ( − 300 ‰ ), and δ D-H 2 O = − 85 ‰ ( − 84 ‰ ). The carbon isotope compositions of methane from Happo #1 and #3 were found to be δ C 13 = − 34.5 ‰ and − 33.9 ‰ , respectively. The CH4–H2–H2O hydrogen isotope systematics indicate that at least two different mechanisms were responsible for methane formation. Happo #1 has a similar hydrogen isotope compositions to other serpentinite-hosted systems reported previously. The elevated δ D-CH 4 (with respect to the equilibrium relationship) suggests that the hydrogen of the Happo #1 methane was not sourced from molecular hydrogen but was derived directly from water. This implies that the methane may not have been produced via the Fischer–Tropsch-type (FTT) synthesis but possibly by the hydration of olivine. Conversely, the depleted δ D-CH 4 (with respect to the equilibrium relationship) in Happo #3 suggests the incorporation of biological methane. Based on a comparison of the hydrogen isotope systematics of our results with those of other serpentinite-hosted hydrothermal systems, we suggest that abiotic CH4 production directly from H2O (without mediation by H2) may be more common in serpentinite-hosted systems. Hydration of olivine may play a more significant role in abiotic methane production than previously thought.