Impact of Varied Slab Thermal Structure on Subduction-Related Fluxes and Source Regions of Northern Cascadia Basalt Magmas

University Of Alabama Tuscaloosa, Tuscaloosa AL

Investigators

Abstract

Green EAR-0106900 Subduction zone magmas have geochemical signatures atypical of mantle-derived melts. The source of their distinctive geochemistry remains controversial, as considerable uncertainty exists about the nature of processes that transport elements from the subducted plate to the overlying mantle. A central issue is determining how much of the characteristic geochemical signature results from transport processes and how much is due simply to the composition of subducted materials. Another factor is the intrinsic characteristics of the mantle wedge. Correctly identifying the importance of these different factors is critical in attempting mass balance of elements at the subduction zone and hence, in determining the flux of continental material recycled into the mantle. Basalts of the northern Cascadia subduction system share tectonic and geochemical relationships with many convergent margin volcanic suites, but occur within a geophysically well-constrained "hot" subduction environment for which it can be shown that (1) the thermal state of the subducted plate varies along the convergent margin; (2) the nature of the subduction component added to the mantle wedge and/or the state of mantle depletion likely changes along strike of the volcanic front; and (3) the subduction signature of the basaltic lavas probably reflects processes operating primarily within the forearc region. Because slab melting may occur under 'hot' subduction conditions, these volcanic rocks provide a unique opportunity to evaluate the extent and nature of fluid versus slab melt (sediment and/or basaltic crust) contributions to the chemical budget of arc source regions in the overlying mantle. This project will utilize major-element, trace-element, and isotopic compositions of primitive to near-primitive basalts to: (1) detail the nature of arc-parallel geochemical variations in the lavas and implications for melting processes within the mantle wedge; (2) evaluate the degree of mantle source depletion and heterogeneity prior to introduction of slab-derived fluxes; (3) quantify subduction fluxes (fluids or melts) recorded in the basalts along the volcanic front; and, (4) establish the influence of slab thermal structure on the extent and nature of mantle-slab interactions. The results will provide crucial information for identifying different factors controlling the nature and origin of subduction-related fluxes (in particular, sediment melts), and will place important constraints on proposed relationships between generation of intermediate-depth earthquakes and metamorphic reactions in the subducted plate benearth convergent margins.