Combined mass spectrometric and manometric techniques permit simultaneous analysis of noble gases and major volatiles from the same sample extraction. He, Ar, and CO2 data of inclusion- and vesicle-trapped fluids from two high 3He/4He islands (R¿union and Iceland) presented here show that air contamination is probably present in all trapped fluids and that the 36Ar content of high 3He/4He mantle volatiles is low. The least contaminated fluid compositions indicates a 36Ar abundance in these fluids (CO2/36Ar= 2.7¿108) indistinguishable from published mid-ocean ridge basalt (MORB) values. Although the high 40Ar/36Ar ratio of the uncontaminated fluids (≈2000) may be due to incorporation of radiogenic Ar (derived from MORB volatiles or the overlying crust), it is likely that high 40Ar/36Ar ratios are characteristic of the high 3He/4He mantle source region. Degassing of primordial 36Ar from the source region must have occurred while retaining a primordial He component. Helium isotope ratios, 40Ar/36Ar, and CO2/36Ar of olivines entrained in Icelandic picrite glasses support petrographic evidence for their origin as lower crustal material entrained by the picrite magma. It is probable that olivine ''phenocrysts'' from other oceanic islands (e.g., R¿union) are similarly unrelated to their host lavas and that cumulate or xenolith phases may represent more suitable ''bottles'' of mantle volatiles. ¿ American Geophysical Union 1994