Comparative Geochemical Signatures of Subduction Zone Volcanism Across Cascades and Andes

Thesis Abstract

Subduction zone volcanism exhibits complex geochemical signatures shaped by slab flux, mantle wedge processes, and crustal assimilation, yet comparative cross-regional patterns between the Cascades and the Andes remain insufficiently resolved, limiting generalizable models of arc geochemistry under varying subduction parameters. This study aims to elucidate how geochemical fingerprints—including major and trace element abundances, Sr-Nd-Pb-Hf isotopic systematics, and volatile contents—vary between the Cascade and Andean arc systems and to identify the controlling tectonic, sedimentary, and mantle processes that drive these patterns. The specific objectives are (1) to compile a harmonized geochemical database of volcanic rocks from representative central to southern segments of the Cascades and the Andes (n = 240 samples, with at least 60 per arc), including whole-rock major and trace elements, REEs, and isotopic ratios (Sr87/Sr86, Nd143/Nd144, Pb206/Pb204, Pb207/Pb204, Hf176/Hf177); (2) to evaluate geochemical discrimination among arc segments using multivariate statistical techniques (principal component analysis, hierarchical clustering) and regression-based modeling to relate signatures to subduction parameters (slab age, convergence rate, sediment flux); (3) to test competing geodynamic models (torus-like mantle wedge metasomatism vs. slab-derived devolatilization) by comparing isotopic and trace element trends with predictions from mantle wedge and slab dehydration theories, including the application of the Rayleigh fractionation and coupled melting–assimilation frameworks; (4) to assess temporal variability by incorporating Pleistocene to Holocene eruptive sequences and addressing potential crustal assimilation biases through isotopic versus geochemical decoupling analyses; and (5) to synthesize a cross-arc framework that links geochemical signatures to arc productivity, magma differentiation pathways, and volatile budgets. A stratified sampling approach will be used, selecting 8–12 well-characterized volcanic complexes from the Cascades (Crater Lake, Mount St. Helens, Mount Rainier, andesitic to rhyodacitic centers) and the Andean arc (Central and Southern Volcanic Zone, Andean Altiplano-Puna). Analytical work will be conducted at accredited facilities, employing X-ray fluorescence (XRF) for major elements, inductively coupled plasma mass spectrometry (ICP-MS) for trace elements and rare earths, and isotope-dratio mass spectrometry for Sr-Nd-Pb-Hf systems. For volatile contents, secondary ion mass spectrometry (SIMS) will quantify H2O, CO2, and S in melt inclusions where feasible. Data quality will be ensured through duplicate analyses, standard reference materials, and cross-laboratory calibration. Statistical analyses will include multivariate regression to link geochemical variables with subduction parameters, ANOVA to test arc-wide vs. segmental differences, and structural equation modeling to integrate geochemical indicators with tectonic controls. Isotopic mixing models will be applied to deconvolve slab versus mantle wedge contributions, with uncertainty propagation via Monte Carlo simulations. Expected findings include (i) distinct isotopic envelopes for the Cascades and Andes reflecting differential slab ages, sediment loads, and mantle wedge replenishment, with Casades showing relatively depleted Sr-Nd-Pb isotopic signatures and Andean samples exhibiting enriched imprints consistent with higher sediment-derived flux; (ii) systematic variations in trace element ratios (e.g., Sr/Y, La/Yb, Nb/U) correlating with slab thermal structure and crustal assimilation indicators; (iii) evidence for two end-member magmatic processes—fluid-dominated slab dehydration in the forearc and melt-dominated differentiation in the arc crust—producing divergent volatile budgets and silica-oversaturation tendencies; (iv) a robust cross-arc model linking geochemical signatures to subduction parameters, enabling predictive assessments of magma productivity and potential volcanic hazards. The study will contribute to knowledge by articulating a transferable cross-arc geochemical framework that ties subduction system parameters to magmatic differentiation and volatile transport, clarifying how mantle wedge processes differ between ocean-continent subduction zones of the Cascades and the Andean system. Conclusions will inform arc hazard assessment and resource implications for arc-related mineralization. Recommendations include expanding temporal coverage with high-precision zircon U-Pb ages integrated with geochemical data to resolve eruption-specific evolution and extending the framework to other global subduction zones to test its universality.