9706373 Asmerom Rifts are unique tectonic settings where magmas derived from both the deep mantle and the old lithospheric continental mantle occur. Chemical and isotopic studies of continental basalts erupted in rifts can provide us with information on the mantle composition under the continents. In contrast to long-lived isotope systems, short-lived nuclides, such as the U-series isotopes, allow us to trace dynamical and compositional aspects of the present-day crust-mantle system. The study area, the Rio Grande Rift and its cratonic margins, is one of the best characterized continental rift systems in the US. It contains contemporaneous young alkali and tholeiitic basalts of presumed asthenospheric and lithospheric mantle origin in close spatial proximity. The U-series isotopic data in conjunction with Pb, Sr and Nd isotopic compositions of these basalts will contribute to our understanding of: 1) controls on the variation of Th enrichment in continental basalts such as differences in degree and rate of melting, and depth (source) of mantle melting. Information on the variation of mantle sources with time provides constrains for models of continental rifting. 2) Th-U compositional variability of the mantle under continents, and in particular the variability in the 232Th/238U ratio (k-value) of the continental lithospheric mantle. Information on the k-value is important with respect to global mass balance and as indicator of processes such as recycling and metasomatism in the mantle. 3) the role of continental crust in modifying the chemistry of rift magmas.
