Geochemistry of H 2 - and CH 4 -enriched hydrothermal fluids of Socorro Island, Revillagigedo Archipelago, Mexico. Evidence for serpentinization and abiogenic methane Y. A. TARAN 1 , N. R. VARLEY 2 , S. INGUAGGIATO 3 AND E. CIENFUEGOS 4 1 Instituto de Geofı ´sica, Universidad Nacional Auto ´noma de Me ´xico, Coyoacan, Mexico D.F., Mexico; 2 Universidad De Colima, Colima, Mexico; 3 Istituto Nazionale di Geofisica e Vulcanologia, Palermo, Italy; 4 Instituto de Geologia, Universidad Nacional Autonoma de Mexico, Coyoacan, Mexico D.F., Mexico ABSTRACT 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 CO 2 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 H 2 (up to 20 mol% in dry gas) and CH 4 (up to 4 mol%). Both carbon and He isotopic ratios and abundances correspond to those in MORB flu- ids (d 13 C CO2  )5&; 3 He ⁄ 4 He = 7.6 R a , CO 2 ⁄ 3 He = (2–3) · 10 9 , where R a is the atmospheric ratio 3 He ⁄ 4 He of 1.4 · 10 )6 . Light hydrocarbons (CH 4 ,C 2 H 6 ,C 3 H 8 , and C 4 H 10 ) are characterized by a high C 1 ⁄ C 2+ ratio of approx. 1000. Methane is enriched in 13 C(d 13 C CH4 from )15 to )20&) and 2 H(d 2 H from )80 to )120&), and hydrocarbons show an inverse isotopic trend in both d 13 C and d 2 H (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 CO 2 to CH 4 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. Key words: abiogenic hydrocarbons, isotope geochemistry, spreading centers, steam-dominated hydrothermal systems, volcanic gas Received 28 February 2010; accepted 7 September 2010 Corresponding author: Yuri Taran, Institute of Geophysics, UNAM, Mexico D.F. 04510, Mexico. Email: taran@geofisica.unam.mx. Tel: +52 55 56224145. Fax: XXXXX 1 . Geofluids (2010) INTRODUCTION Methane-rich fluids discharging in environments where sedimentary matter is limited or almost absent may have methane and light hydrocarbons of abiotic origin, synthe- sized under reduced conditions in the Earth’s crust from the oxidized carbon (CO 2 , carbonates) and hydrogen. Such fluids have been found as cold seeps in ophiolites (Abrajano et al. 1990; 2 Lyon & Giggenbach 1990; Sano et al. 1993), in moderate- and high-temperature ultra- mafic-hosted spreading ridge vents (Charlou et al. 2002; Proskurowski et al. 2008; Konn et al., 2008) 3 , in gases associated with deep-seated waters of Precambrian shields (Sherwood Lollar et al. 2002, 2006, 2008), and in occluded gases from igneous alkaline rocks of Kola Penin- sula (Galimov & Petersilie 1967; Potter et al. 2004). These gases are characterized by high H 2 and CH 4 contents and sometimes by a rare ‘inverse’ trend in the carbon isotope composition of the light hydrocarbons, with ethane isoto- pically lighter than methane, in contrast to ‘thermogenic’ trend with the enrichment in 13 C of longer chains, com- mon for most natural hydrocarbons. Boiling-point fuma- roles of Socorro Island, a giant shield basaltic volcano built at the former spreading center on the ocean floor in the G F L 3 1 4 B Dispatch: 29.9.10 Journal: GFL CE: Balaji Prasad Journal Name Manuscript No. Author Received: No. of pages: 13 PE: Sangeetha Geofluids (2010) doi: 10.1111/j.1468-8123.2010.00314.x Ó 2010 Blackwell Publishing Ltd 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54