Abundance and isotopic composition of helium in hydrothermal sulfides from the East Pacific Rise at 13 degrees N

The helium isotopes composition of fluids trapped in hydrothermal sulfides from the East Pacific Rise (13 degrees N) were measured. Two extraction techniques, i.e., crushing and heating, were tested and sulfides of different mineralogical types were studied. The comparison between the two extraction methods suggests that both should give similar results, provided the crushing efficiency is sufficiently high. Sulfides He-3/He-4 ratio for crushed samples are in the range 6.85-8.10 times the atmospheric ratio (R(a) = 1.38 X 10(-6)) and the measured helium concentration is of the order of 5.5 X 10(-5) cm(3) STP per gram of trapped fluid. Comparison with the helium isotope composition of the hydrothermal vent fluids themselves shows that the mineral phases formed at the highest temperature (i.e., with less dilution by the ambient seawater) retain the helium information most effectively, with values almost indistinguishable from the endmember vent fluid.