New helium isotope data (3He/4He) 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). 3He/4He ratios measured in clinopyroxene and olivine phenocrysts are between 0.9 - 8.3 Ra. The SCLM in this region is therefore characterised by the higher unmodified 3He/4He 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 4He. However, we argue that helium isotopes may also be affected by mass-dependent fractionation. This can occur during helium diffusion from the phenocrysts within sub-solidus magmas.