Geochemistry of H2- and CH4-enriched hydrothermal fluids of Socorro Island, Revillagigedo Archipelago, Mexico. Evidence for serpentinization and abiogenic methane

Socorro Island is the exposed part of an approx. 4000-m-high volcanic edifice rising from the oceanic floor to approx. 1000 m asl at the northern part of the Mathematician Ridge, Western Pacific. The volcano is active, with the most recent basaltic eruption in 1993. Moderate fumarolic activity and diffuse degassing with a total CO2 flux of approx. 20 total day)1 are concentrated in the summit region of the volcano composed of a group of rhy- olite domes. Low-temperature, boiling point, fumaroles discharge gas with high H2 (up to 20 mol% in dry gas) and CH4 (up to 4 mol%). Both carbon and He isotopic ratios and abundances correspond to those in MORB flu- ids (d13CCO2 )5&; 3He ⁄ 4He = 7.6 Ra, CO2 ⁄ 3He = (2–3) · 109, where Ra is the atmospheric ratio 3He ⁄ 4He of 1.4 · 10)6. Light hydrocarbons (CH4, C2H6, C3H8, and C4H10) are characterized by a high C1 ⁄C2+ ratio of approx. 1000. Methane is enriched in 13C (d13CCH4 from )15 to )20&) and 2H (d2H from )80 to )120&), and hydrocarbons show an inverse isotopic trend in both d13C and d2H (ethane is isotopically lighter than methane). These isotopic and concentration features of light hydrocarbons are similar to those recently discovered in fluids from ultramafic-hosted spreading ridge vents and may be related to the serpentinization processes: H2 generation and reduction of CO2 to CH4 within high-temperature zone of volcano-seawater hydrothermal system hosted in basaltic and ultramafic rocks beneath a volcano edifice. The thermodynamic analysis of this unusual composition of the Socorro fluids and the assessment of endmember compositions are complicated by the near-surface cool- ing, condensation and mixing with meteoric water