Xenoliths from the southwestern U.S. will be analyzed to study the compositional and thermal evolution of mantle lithosphere and the relationships between mantle properties and crustal tectonics. Characterization of textures and compositional gradients will be combined with electron microprobe and PIXE analysis of thin sections and with isotopic analysis of mineral separates. Xenoliths from the Grand Canyon field on the Colorado Plateau will be studied in most detail. Compositions and thermal histories of the samples will be compared to those of xenoliths from the Basin and Range province, to learn how crustal uplift and extension are related to mantle dynamics. Textures and Sr- Nd systematics in the Grand Canyon xenoliths are unusual, and they will be characterized to understand metasomatic processes that may be subduction-related. A large collection from the Navajo field in the central Plateau has been recently acquired, and choice samples will be analyzed to determine depth and timing of mantle hydration and eclogite-facies recrystallization. In all parts of the study, particular effort will be made to elucidate transport processes and rates of mineral equilibration in the mantle, by careful analyses of gradients and by numerical simulations.
