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, Eastern 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 rhyolite domes. Low-temperature, boiling point, fumaroles discharge gas with high H-2 (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 fluids (delta C-13(CO2) approximate to 5 parts per thousand; He-3/He-4 = 7.6 R-a, CO2/He-3 = (2-3) x 10(9), where R-a is the atmospheric ratio He-3/He-4 of 1.4 x 10(-6)). Light hydrocarbons (CH4, C2H6, C3H8, and C4H10) are characterized by a high C-1/C2+ ratio of approx. 1000. Methane is enriched in C-13 (delta C-13(CH4) from -15 to -20 parts per thousand) and H-2 (delta H-2 from -80 to -120 parts per thousand), and hydrocarbons show an inverse isotopic trend in both delta C-13 and delta H-2 (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: H-2 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 cooling, condensation and mixing with meteoric water.