RUI: Collaborative Research: Mantle Heterogeneities and Crustal Processes of the Cascade Arc: Small Scale Variations with Broad Implications

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Subduction zones are important sites of volcanism, continental crustal growth, and mass transfer from the mantle, through the crust, to the surface. The Cascade Arc in western North America is located above a subduction zone and encompasses some of the most active volcanic sites in the contiguous US. Previous investigations of the evolution of magmas in the Cascades focused on large-scale complexities in geochemistry, using individual primitive (or mantle-derived) basalts that erupt there to characterize the mantle. However, the geochemical array of primitive basalts erupted in close proximity to one another suggests that heterogeneous mantle domains may exist on a much smaller scale than previously recognized. Furthermore, many basalts that fit the definition of 'primitive' also have geochemical characteristics typical of some crustal or magmatic processing. The project will be conducted at two PUIs (University of Wisconsin Oshkosh and California State University, Chico) and will train 3-5 undergraduate students per year. The wide range of compositions and small area of the PLC will provide numerous small-scale research questions that can be answered with geochemical techniques, thus preparing undergraduate students for graduate study and careers in STEM. This work is transformative for its important implications for future work in the Cascades and other complex arcs where large-scale variations of a single volcanic center, across arc segments, and along entire arc systems are investigated.The goal of this project is to identify and characterize small-scale heterogeneities in the sub-Cascadian mantle using compositional variations in a suite of basalts in the Poison Lake chain (PLC) near the Lassen Volcanic Center (LVC) in California. Detailed study of the geochemical variations in the PLC will shed light on the nature of mantle diversity beneath the LVC, the Cascades and other complex subduction zone systems. Given the restricted time and space represented by PLC primitive basalts (~110-100 ka and 50km2), this work is unique because it concentrates solely on the variable of composition. Proposed work focuses on two working hypotheses: 1) Small-scale heterogeneities exist in the mantle and are represented by those PLC basalts that have not been processed; and 2) a variety of post mantle processes can be recognized in other PLC basalts, which do not represent direct mantle melt compositions. Geochemical and petrologic compositions of the most primitive PLC basalts will be used to characterize heterogeneous mantle domains and the scale of chemical variation there (Hypothesis 1). Compositions of modified (less primitive) samples from the PLC will provide insight into crustal processes that may modify mantle melts on their traverse from mantle to surface (Hypothesis 2). In this way, the project will result in models that reflect the scale and composition of mantle heterogeneity and subsequent intracrustal modification processes based on a very isolated section of the LVC.