Origin of methane in serpentinite-hosted hydrothermal systems: The CH_4-H_2-H_2O hydrogen isotope systematics of the Hakuba Happo hot spring

摘要

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 (36o42'N, 137o48'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 μmol/L) and CH_4 (124-201 μmol/L). Despite the relatively low temperatures (50-60oC), 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: δD-H_2 = -700%o (-710%0), δD-CH_4 = -210%0 (-300%),and δD-H_2O = -85%0 (-84%.). The carbon isotope compositions of methane from Happo #1 and #3 were found to be δ~(13)C = -34.5%o and -33.9%.,respectively. The CH_4-H_2-H_2O 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 δD-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 δD-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 CH_4 production directly from H_2O (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.