He, Ne and Ar isotope compositions of fluid inclusions in hydrothermal sulfides from the TAG hydrothermal field, Mid-Atlantic Ridge

Helium, rieon and argon isotope compositions of fluid inclusions have been measured in hydrothermal sulfide samples from the TAG hydrothermal field at the Mid-Atlantic Ridge. Fluid-inclusion He-3/He-4 ratios are 2.2-13.3 time s the air value (Ra), and with a mean of 7.2 Ra. Comparison with the local vent fluids (He-3/He-4=7.5-8.2 Ra) and mid-ocean ridge basalt values (He-3/ He-4=6-11 Ra) shows that the variation range of He-3/He-4 ratios from sulfi de-hosted fluid inclusions is significantly large. Values for Ne-20/Ne-22 a re from 10.2 to 11.4, which are significantly higher than the atmospheric r atio (9.8). And fluid-inclusion Ar-40/Ar-36 ratios range from 287 to 359, w hich are close to the atmospheric values (295.5). These results indicate th at the noble gases of fluid inclusions in hydrothermal sulfides are a mixtu re of mantle- and seawater-derived noble gases; the partial mantle-derived components of trapped hydrothermal fluids may be from the lower mantle; the helium of fluid inclusions is mainly from upper mantle; and the Ne and Ar components are mainly from seawater.