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 degrees 42'N, 137 degrees 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 H-2 (201-664 mu mol/L) and CH4 (124-201 mu mol/L). Despite the relatively low temperatures (50-60 degrees C), thermodynamic calculations suggest that the H-2 was likely derived from serpentinization reactions. Hydrogen isotope compositions for Happo #1 (Happo #3) were found to be as follows: delta D-H-2 = -700 parts per thousand (-710 parts per thousand, delta D-CH4 = -210 parts per thousand (-300 parts per thousand), and delta D-H2O = -85 parts per thousand (-84 parts per thousand). The carbon isotope compositions of methane from Happo #1 and #3 were found to be delta C-13 = -34.5 parts per thousand and -33.9 parts per thousand, respectively. The CH4-H-2-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 delta D-CH4 (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 (FIT) synthesis but possibly by the hydration of olivine. Conversely, the depleted delta D-CH4 (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 H-2) may be more common in serpentinite-hosted systems. Hydration of olivine may play a more significant role in abiotic methane production than previously thought. (C) 2013 Elsevier B.V. All rights reserved.