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1 Department of Geology, University of Manchester, Manchester M13 9PL, UK
2 Isotope Geology Unit, SURRC, East Kilbride G75 0QU, UK
3 Department of Geology, University of Wales, Cardiff CF1 3YE, UK
4 IFREMER, BP70 29280 Plouzane, Brest Cedex, France
Noble gas isotopes have been measured in fluid inclusions in sulphides spanning 25 000 years of hydrothermal activity at 13°N on the East Pacific Rise. The 3He/4He ratios are typical of mid-ocean ridge hydrothermal fluids, albeit slightly higher than contemporary vent waters, and reveal no temporal variation or correlation with the
34S of the host sulphide. The absence of radiogenic He in fluids from the 25 000 year old mineralization on the SE Seamount suggests that the hydrothermal circulation occurred within an active magmatic system and not within the underlying 130 ka oceanic crust. This implies that seamount volcanism and hydrothermal activity occurred simultaneously off-ridge, and that magmatic activity shifted approximately 5 km off-ridge at this time. Helium concentrations in fluid inclusions from three samples are significantly greater than the end-member hydrothermal fluids at mid-ocean ridges. Small excesses of 40Ar in the included fluids demonstrate that mantle-derived 40Ar has been degassed along with primordial helium. Both are consistent with the direct addition of magmatic volatiles into the hydrothermal system at times during the history of hydrothermal activity at the site.
