POSSIBLE DIFFUSIVE FRACTIONATION OF HELIUM ISOTOPES IN OLIVINE AND CLINOPYROXENE PHENOCRYSTS

Abstract

New helium isotope data (He-3/He-4) from a series of co-genetic Siberian lavas of Cenozoic age (0.6 - 3 Ma) provide possible evidence for mass-dependent helium isotope fractionation. These lavas, erupted around the Baikal rift, have sampled subcontinental lithospheric mantle (SCLM). He-3/He-4 ratios measured in clinopyroxene and olivine phenocrysts are between 0.9 - 8.3 R-a. The SCLM in this region is therefore characterised by the higher unmodified He-3/He-4 ratios, which are similar to the source region of mid-ocean ridge basalts (MORB). Helium isotope variation of this scale and order within local regional volcanic settings has previously been ascribed to purely, crustal contamination or radiogenic ingrowth of He-4. However, we argue that helium isotopes may also be affected by mass-dependent fractionation. This can occur during helium diffusion from the phenocrysts within subsolidus magmas.