Geochemistry of hydrothermal fluids from an intraplate ocean island: Everman volcano, Socorro Island, Mexico

Socorro Island, Mexico, still volcanically active, is an alkaline and peralkaline volcanic island in the eastern Pacific Ocean located on a mid-ocean ridge spreading center that was abandoned at ∼3.5 Ma. Gas and water samples collected on the island in 1999 from the dome fumaroles and hot springs of Everman volcano have been analyzed for major components, H-O-C-S, He and Ar isotopic composition, and noble gas abundances. The boiling-point-temperature steam vents and low-salinity thermal springs discharge Cl-free meteoric water, and are typical surface manifestations of a high-temperature hydrothermal system. High H2 and CH4 contents (up to 14 and 3.5 vol.% in dry gas, respectively) and an enrichment of CH4 in 13C (δ13CCH4=−18‰) may suggest that these reduced gases were at least partially produced by the interaction of ultramafic rocks with water inside volcanic edifice due to serpentinization of olivine. The 3He/4He ratios (6.0–6.6 Ra) which are lower than the MORB-values, and the high He/Ne ratios (>4000) and He/Ar ratios (up to 4) indicate a contribution of radiogenic He from the oceanic crust or shallow magma chamber like at a low-3He hot spot. The maximum of the 40Ar/36Ar values (311) is close to the maximum value found in the Socorro volcanic rocks. Noble gas abundances are close to values for the partially degassed air-saturated water, however, with some excess Ne concentrations. An enrichment in 132Xe (or depletion in 84Kr) of the least air-contaminated gas sample (He/Ne>4000) relative to air-saturated water can be related to the contribution of noble gases from a non-atmospheric source, probably oceanic crustal component assimilated in a shallow magma reservoir.