Nodules of Devonian age in Colorado-Wyoming kimberlites include infertile, enriched, and ilmenite-bearing peridotites; garnet websterites and associated garnet pyroxenites; garnet-spinel websterites; plus megacrysts, eclogites, and granulites. Both enriched and infertile peridotites equilibrated in the spinel, spinel-garnet, and garnet facies, but enriched garnet peridotites were sampled only at depths of less than 100 km, whereas infertile garnet peridotites extend to depths of 200 km. Occurrence of infertile rocks and cool paleogeothermal gradient define the sampled mantle as lithosphere.

Infertile peridotites are interpreted to be residua from a Precambrian melting event involving the entire lithosphere; spinel and spinel-garnet peridotites are generally more infertile than garnet peridotites. The infertile lithosphere was subsequently enriched at shallow depths by igneous processes in two events: one produced Fe- and Ti-enriched ilmenite peridotites; the other, more widespread event was associated with websteritic rocks. A few composite nodules as well as geothermobarometry suggest that websterites formed a network of dikes, veins, and layers cutting spinel and spinel-garnet infertile peridotite, and that these were bordered by zones of enriched peridotite. Websterites are Na-, Ca-, and slightly Ti-enriched, but are magnesian, which requires that the protoliths were crystalline assemblages, not melts.

The ratio of collected infertile spinel and spinel-garnet peridotites to enriched peridotites and websterites indicates that enriched rocks occupied 80 percent of the lithosphere sampled between the crust and a depth of 100 km. This very high percentage, unusual among xenolith populations, suggests that ancient enriched rocks occurred along conduits subsequently exploited by kimberlite eruptions.