Spinel lherzolite and harzburgite xenoliths from the Basin and Range and Colorado Plateau Provinces in the southwestern United States (SWUS) show a broad range in incompatible element distributions from depleted to enriched, both within and between sites. The most fertile xenoliths occur in the Basin and Range Province and the most refractory in the Colorado Plateau or Transition Zone. Mass balance calculations indicate that up to 80% of the high field strength (Nb, Ta, Zr, Hf, Th, Ti) and rare earth elements (50% for Yb) in these xenoliths occur in phases along grain boundaries (or in microfractures). In lower crustal and eclogite xenoliths, up to 90% of these elements occur in grain-boundary phases. Nb/Ta, Zr/Hf, La/Sm and Nb/Th ratios and Nb-Ta anomalies in both types of xenoliths are also controlled by grain-boundary phases. To interpret these ratios in whole-rock analyses, it is critical to understand the timing and origin of the grain-boundary components in the xenoliths. Most of the enriched mantle xenoliths appear to have been enriched by metasomatic fluids related to silicate magmas and not to carbonatite magmas. Because of a complex multi-event history that affected the composition of the xenoliths, there is no simple geochemical relationship between magmatic or metamorphic history of the lower crust and upper mantle in the SWUS as reflected by the xenolith populations.