HELIUM-ISOTOPES AS TRACERS OF TRAPPED HYDROTHERMAL FLUIDS IN OCEAN-FLOOR SULFIDES

Helium isotopes have been measured in fluids trapped in minerals from three mid-ocean ridge hydrothermal systems at bare-rock sites and at t he sediment-covered site at Middle Valley, northern Juan de Fuca Ridge . Fluid-inclusion He-3/He-4 ratios from the bare-rock sites are indist inguishable from those of the vent fluids and are close to the local b asalt values. Values for deltaS-34 of the host sulfide are consistent with variable contributions of seawater sulfate S to the basalt-derive d S in the hydrothermal fluids. This does not affect inclusion-hosted He-3/He-4 because of the high He concentration of the hydrothermal flu ids relative to seawater. Fluid-inclusion He-3/He-4 ratios from Middle Valley are a mixture of basalt-derived He and radiogenic He acquired from pore fluids in the overlying Pleistocene turbidite sediments afte r seawater-basalt interaction. The radiogenic He accompanies isotopica lly heavy S, demonstrating that He isotopes may be used to distinguish pore fluids from seawater in hydrothermal systems.