The origin of methane in serpentinite-hosted hyperalkaline hot spring at Hakuba Happo, Japan: Radiocarbon, methane isotopologue and noble gas isotope approaches

Methane (CH4) associated with marine and terrestrial sites of serpentinization has been proposed to be abiotic in origin. However, the source carbon depth temperature CH4 synthesis often remain inconclusive. We measured radiocarbon (14C), bulk stable isotope ratios (δ13C δD) isotopologue abundance (Δ13CH3D) CH4, noble gas composition samples from two hyperalkaline geothermal wells (Happo #1 Happo #3) at Hakuba Happo, Japan, constrain generation processes. The both were nearly 14C-free, whereas carbonate precipitates inside well contained 14C corresponding 51 62 percent modern carbon, indicating that majority was not generated reduction dissolved hot spring water. #3 yielded Δ13CH3D-based apparent temperatures 206−40+52 °C 323−85+143 °C, respectively, which are much higher than water (∼50 °C). Therefore, clumped suggest below where shallow meteoric circulated. 3He/4He 4.10 4.47 Ratm for #3, respectively. 4He/20Ne revealed approximately 50% He mantle origin, suggesting received volatiles, including CO2, mantle. observed δ13CCH4 values (approximately −35‰) significantly lower those typical mantle-derived (–5‰), implying CO2 is major formation. High concentrations fluids, compared springs area serpentinization, 13C-depleted fossil sources serpentinization-derived H2 high (>200 °C), subsequently entrained into cooler circulating system.