AIR CONTAMINATION OF BASALTIC MAGMAS - IMPLICATIONS FOR HIGH HE-3-HE-4 MANTLE AR ISOTOPIC COMPOSITION

Combined mass spectrometric and manometric techniques permit simultane ous analysis of noble gases and major volatiles from the same sample e xtraction. He, Ar, and CO2 data of inclusion- and vesicle- trapped flu ids from two high He-3/He-4 islands (Reunion and Iceland) presented he re show that air contamination is probably present in all trapped flui ds and that the Ar-36 content of high He-3/He-4 mantle volatiles is lo w. The least contaminated fluid compositions indicates a Ar-36 abundan ce in these fluids (CO2/Ar-36 = 2.7 x 10(8)) indistinguishable from pu blished mid-ocean ridge basalt (MORB) values. Although the high Ar-40/ Ar-36 ratio of the uncontaminated fluids (approximate to 2000) may be due to incorporation of radiogenic Ar (derived from MORB volatiles or the overlying crust), it is likely that high Ar-40/Ar-36 ratios are ch aracteristic of the high He-3/He-4 mantle source region. Degassing of primordial Ar-36 from the source region must have occurred while retai ning a primordial He component. Helium isotope ratios, Ar-40/Ar-36, an d CO2/Ar-36 of olivines entrained in Icelandic picrite glasses support petrographic evidence for their origin as lower crustal material entr ained by the picrite magma. It is probable that olivine ''phenocrysts' ' from other oceanic islands (e.g., Reunion) are similarly unrelated t o their host lavas and that cumulate or xenolith phases may represent more suitable ''bottles'' of mantle volatiles.