ABUNDANCE AND ISOTOPIC COMPOSITION OF HELIUM IN HYDROTHERMAL SULFIDESFROM THE EAST PACIFIC RISE AT 13-DEGREES-N

The helium isotopes composition of fluids trapped in hydrothermal sulf ides from the East Pacific Rise (13 degrees N) were measured. Two extr action techniques, i.e., crushing and heating, were tested and sulfide s of different mineralogical types were studied. The comparison betwee n the two extraction methods suggests that both should give similar re sults, 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 c oncentration is of the order of 5.5 X 10(-5) cm(3) STP per gram of tra pped fluid. Comparison with the helium isotope composition of the hydr othermal vent fluids themselves shows that the mineral phases formed a t the highest temperature (i.e., with less dilution by the ambient sea water) retain the helium information most effectively, with values alm ost indistinguishable from the endmember vent fluid.