Air contamination of basaltic magmas: Implications for high 3 He/ 4 He mantle Ar isotopic composition

Combined mass spectrometric and manometric techniques permit simultaneous analysis of noble gases and major volatiles from the same sample extraction. He, Ar, and CO 2 data of inclusion‐and vesicle‐trapped fluids from two high 3 He/ 4 He islands (Réunion and Iceland) presented here show that air contamination is probably present in all trapped fluids and that the 36 Ar content of high 3 He/ 4 He mantle volatiles is low. The least contaminated fluid compositions indicates a 36Ar abundance in these fluids (CO 2 / 36 Ar = 2.7 × 10 8 ) indistinguishable from published mid‐ocean ridge basalt (MORB) values. Although the high 40 Ar/ 36 Ar 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 40 Ar/ 36 Ar ratios are characteristic of the high 3 He/ 4 He mantle source region. Degassing of primordial 36 Ar from the source region must have occurred while retaining a primordial He component. Helium isotope ratios, 40 Ar/ 36 Ar, and CO 2 / 36 Ar 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., Réunion) are similarly unrelated to their host lavas and that cumulate or xenolith phases may represent more suitable “bottles” of mantle volatiles.