A fundamental challenge for earth scientists is to understand the origin of volcanic activity not associated with plate boundaries and especially that interior to the continents. Volcanic rock chemistry provides one of the most powerful ways to probe the sources and melting mechanisms responsible for this activity and, at the same time, to gain insights into the present structure and dynamics of the continents. This grant supports an international collaboration that will produce new geochemical limits on magma generation beneath the southwestern U.S. where numerous young volcanoes sample the continent and its underlying mantle over a broad geographical region. Interpretations based on these results will be refined in the context of ongoing geophysics-based efforts such as Earthscope to arrive at internally consistent models for volcanic activity associated with the Colorado Plateau and its transition zones.
Despite efforts to understand the origin of continental basalts in the southwestern U.S., disparate observations and interpretations persist for how and where melting occurs. In many cases, existing data are too piecemeal or too general to determine if and where the mantle is actively upwelling, the depths at which melts segregrate, and the chemical and physical role that mantle lithosphere plays in melt generation. This project will result in a comprehensive chemical and Th-Hf-Nd isotope characterization of young basalts from the Colorado Plateau and its transition zones. Inferences based on paired trace element and isotopic characteristics and from inversion of major element data will delimit the depth and depth interval of melting and the relative contributions of enriched and depleted sources. These constraints, as well as limits on the sizes of melt fractions, will permit a better understanding of the relationship between the structure and evolution of lithosphere and melting dynamics in the southwestern U.S.
