Melting Processes and Crust-Mantle Interaction in the Cascade Arc: Constraints from U-Series, Nd, Os, Pb and Sr Isotopic Data

University Of New Mexico, Albuquerque NM

Investigators

Abstract

The subduction zone system on Earth is where net material transfer from the mantle to the continental crust occurs, and where material that is created at spreading ridges is returned to the mantle. Unlike volcanism in mid-ocean ridges, where melting components are limited and melting mechanism are considerably simpler, volcanism in arcs is considerably more complex: components of melt are numerous, such as the down going slab, the mantle wedge, the overlying mantle and continental lithospheres, fluids and sediments. As a result, arc magmas exhibit rich diversity in elemental and isotopic composition over short distances. It is generally agreed that fluid addition plays an important role in melting in subduction zones, most likely by hydrating the mantle wedge and lowering the solidus temperature. The timing of fluid addition and the extent of fluid involvement in generation of magmas in certain arcs, such as the Cascade subduction zone, are not clear. The Cascade arc represents both a slow converging and warm end-member of the global arc system. It is also one of the best characterized arcs, given almost century long geophysical and geologic studies. A uranium-series based study in this arc is ideally suited to answer a number of the outstanding questions. These include, 1) the relative importance of flux melting and dynamic melting in arcs in general and the Cascades in particular, 2) the explanation for the paradox of the estimated high pre-eruptive water content (up to 8% H2O) inferred for some Cascade lavas and yet the lack of enrichment in key fluid indicating elements and isotopic ratios, 3) and the amount of crust component in Cascade lavas. The proposed study will utilize 230Th-238U, 226Ra-230Th, 231Pa-235U, Sr, Nd, Pb and Os isotopic data on young lavas and mineral separates to address the above and related questions. Although this project's aim is answering basic scientific questions, it will support the human development and infrastructural building efforts at the University of New Mexico. The project will serve as a venue for training two graduate students in analytically-based geochemistry and petrology. The lab is used not only by the PIs group but also by a variety of other groups, including students from small undergraduate colleges, such as Cornell College, Iowa, and University of Puget Sound, Washington. The University of New Mexico is a minority serving institution and the current proposal is coupled to an existing Sloan Foundation grant to the PI to support minority PhD students. The study area is a National area of concern for volcanic hazards, and results from this project may contribute towards understanding of volcanic eruption cycles.