Mantle xenoliths from the Quaternary Big Pine volcanic field of eastern California provide direct evidence for the presence of isotopically enriched mantle beneath the Sierran Province of the western United States. Sr and Nd isotopic compositions determined on five bulk rock and five clinopyroxene separates are negatively correlated; ϵNd values range from +13.8 to -3.4, and 87Sr/86Sr ratios range from 0.7022 to 0.7065. The measured Sr and Nd contents and Ce/Yb ratios of mantle xenoliths in the Big Pine volcanic field are well correlated with Sr and Nd isotopic compositions, and these xenoliths define an 820-m.y.-old Sm-Nd isochron. The Big Pine mantle xenoliths have similar textures and mineral compositions, and we interpret the large range in isotopic and trace element compositions to be a result of cryptic mantle metasomatism. The timing of this mantle metasomatic event is unclear but a maximum Late Proterozoic age can be inferred from the 820 Ma Sm-Nd isochron. If the Sm-Nd isochron is interpreted as a result of recent metasomatism then the metasomatising agent was similar in composition to the Big Pine basalts. If the isochron is interpreted to have age signficance then this mantle metasomatism was associated with late Proterozoic rifting of the western margin of North America. Although production of isotopically enriched mantle is generally attributed to ancient subduction, these data are inconsisten with such an interpretation. ¿ American Geophysical Union 1995