The purpose of this study is to determine how much isotopic variability in basalts arises in the mantle, and how much is a result of crustal interaction. It will consist of (1) an isotopic survey of upper mantle and crustal basalt-borne xenoliths, (2) detailed geochemical studies at volcanoes that show large variability in isotope ratios, (3) textural and microprobe studies of textural disequilibrium, in order to test for the mixing predicted by isotope studies, and (4) experimental studies of interaction between basalt magma and crustal mantle xenoliths. A successful outcome of this research should shed considerable light on the chemical nature of the mantle of the southwest United States.
