The evolution of the mantle wedge below the Colorado Plateau during low-angle subduction has been investigated by analysis of fragments from the Navajo Volcanic Field (NVF), most from serpentinized ultramafic microbreccia (SUM) diatremes. In most SUM-hosted olivine, concentrations of Al and V are < 1 ppm, and Cr, Ca, and Na concentrations also are unusually low: most temperatures from three olivine thermometers (Al, Cr, and V-based) are in the range 530°C to 650°C. The temperatures are consistent with the presence of chlorite in some inclusions, and they support the utility of olivine thermometry for diverse mineral assemblages in cool peridotite. Most pressures calculated for discrete diopside grains in SUM correspond to depths in the range 80 to 120 km. Diopside is relatively high in Li (~3.5 ppm), and two of five diopside grains have high Sr/Nd. SUM diatreme sources are inferred to be serpentine-rich mélange tectonically eroded from the forearc, transported above the Farallon slab, and incorporated into the lithosphere of the Plateau. Garnet peridotite xenoliths from minettes of the NVF record deeper depths in the range 120 to 150 km. These garnet peridotites also may be from forearc mantle emplaced during Farallon subduction. Calculated temperatures preclude the possibility that asthenosphere was in contact with that lithosphere at or near 150 km depth for tens of m.y. before NVF formation. Structures observed in seismic images of mantle to depths of 150 km below the central Colorado Plateau may be related to Farallon subduction, not inherited from Proterozoic lithosphere.