The prevailing model for convergent margin magmatism involves mantle melting in response to additions of fluids produced by dehydration of subducting (underthrusting) plates of oceanic lithosphere. However, because the Cascade arc is associated with slow subduction of young oceanic lithosphere, it is one of the warmest subduction zones known. For this extreme end member case, it is likely that the subducting plate is extensively metamorphosed and dehydrated as it descends to subarc depths, that the supply of slab-derived fluids beneath the arc is low, and that temperatures may even be high enough to promote direct melting of the slab at those depths. In this case the familiar subduction zone 'flux melting' paradigm may not strictly apply. Yet, the Cascades arc is characterized by voluminous basaltic magmatism. To explain this enigma, alternative mechanisms and/or magma sources are seemingly required, and the solution to this paradox may shed new light on origins of primitive arc magmas worldwide.
Our project focuses on basaltic magmatism because such lavas are likely to carry relevant information concerning fundamental mantle processes underpinning volcanic arc magmatism. A basic question that we address concerns the extent to which slab-derived fluids contribute to magma generation in this setting. In the southern Washington Cascades, primitive basaltic lavas lacking slab-derived chemical signatures have erupted over the entire width of the arc. From this observation we infer that much of the mantle wedge has received negligible slab contributions. In contrast, in the northern California Cascades (e.g., Mt. Shasta area), primitive lavas appear to be significantly hydrated, and prevailing interpretations suggest that slab-derived fluids do contribute significantly to magma formation in this area.
We propose a comparative study to investigate the nature and extent of slab contributions beneath both areas, using sensitive geochemical tracers for slab-derived fluids - Be and B isotopes, fluid-mobile trace elements, and radiogenic isotopes (Sr, Pb, and Os). If slab contributions are significant in the latter region, we can better define the origin and composition of that signature with respect to these compositional parameters. If this is not the case, we will investigate other scenarios (e.g., decompression melting) to explain the characteristics and origins of primitive magmas in these settings. This study will better define the relative contributions between competing melting processes, and allow us to address how they are influenced by external forcing functions related to subduction zone dynamics.
Intellectual merit: This work will provide, for the Cascades, a deeper understanding of the causes for magmatic diversity, the relative contributions of different melting processes, the influence of compositional diversity within the mantle wedge, and ultimately the thermal structure and processes underpinning this magmatism. This knowledge may be difficult to extract from more typical, cooler subduction systems.
Broader impacts: Graduate and undergraduate students involved in this work will gain basic scientific training and experience. Collaboration between researchers at Rice University, University of Arizona, and Washington University at St. Louis, and the Istituto di Geoscienze e Georisorse in Pisa, Italy, will foster intellectual exchange and provide access to a broad range of analytical approaches. Transfer of this knowledge through participation at national and international meetings will contribute to the overall benefit of many scientists studying the dynamics of convergent margins.
