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 similar to3.5 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 H-2 and CH4 contents (up to 14 and 3.5 vol.% in dry gas, respectively) and an enrichment of CH4 in C-13 (delta(13)C(CH4) = -18 parts per thousand) 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 He-3/He-4 ratios (6.0-6.6 R-a) 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-He-3 hot spot. The maximum of the Ar-40/Ar-36 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 Xe-132 (or depletion in 94 Kr) 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. (C) 2002 Elsevier Science B.V. All rights reserved.