Conditions of basaltic magma generation at Mount Baker Volcanic Field, North Cascades

Significant unresolved questions remain on the processes of mantle melting throughout the wide range of thermal conditions encompassed by subduction zones. For example, subducting slabs in "hot" arc settings are thought to dehydrate at relatively shallow depths, yet volcanoes develop in locations indistinguishable from those in "cold" arcs. The northern Cascade arc is considered a classic end-member example of a "hot" subduction zone because the subducting crust is extremely young, 6-10 Ma at the trench [1], with a thick layer of insulating sediment and a relatively low convergence rate [2]. The most magmatically productive volcanic center of the northern Cascades is the Mt. Baker volcanic field (MBVF) [3], and here we glean information from the most primitive MBVF lavas to develop a petrogenetic model for basalt generation in a "hot" arc setting. Whole-rock geochemical data and the compositions of coexisting minerals are used to establish the initial water contents and redox states of the magmas, and the temperatures and pressures of segregation from the mantle. Melt silica activities indicate the MBVF magmas segregated from their residual mantle source assemblages at depths ranging from 60 to 40 km, corresponding to a few km shallower than the hot core of the mantle wedge [4] to the base of the crust. Plagioclase core compositions indicate that the initial water contents of the magmas ranged from 1.7 to 2.3 wt. % H₂O, and show a good inverse correlation with segregation depths. Fe-Ti oxide pairs and spinel inclusions in olivine phenocrysts indicate redox states slightly more oxidizing than the quartz-fayalite-magnetite buffer. Segregation depths are also strongly correlated with temperatures calculated from olivine-liquid equilibria, which range from 1286°C to 1350°C. Coupled with the most recent thermal model for the subducting slab in northern Cascadia [4], we use petrologic phase equilibria for the P-T stability of mineral assemblages in the mantle and subducting sediment and altered oceanic crust to develop a mantle melting model for MBVF predicting that melting is initiated by dehydration melting of amphibole peridotite at ~90 km and 1030°C. However, essentially all of the water in the subducted slab is released at fore-arc depths and not directly beneath MBVF. Therefore, mantle initially hydrated in the serpentine and/or chlorite stability fields must be down-dragged, by coupling with the subducting slab, to the region of amphibole stability for mantle melting to occur. Initial melt fractions are small and contain high water contents, likely near the saturation limit, and residual garnet is possible. However, the geochemical characteristics of the erupted basalts indicate much lower water contents, shallower pressures, higher temperatures, and no residual garnet. We conclude that substantial anhydrous melt fractions must be added to the initial melts as they ascend through the hotter core of the convecting mantle wedge. [1] Wilson D (2002) USGS open-file report 02-328; [2] Oleskevich D et al. (1999) JGR 104, 14965; [3] Hildreth W (2007) USGS Prof. Paper 1744; [4] Syracuse E et al. (2010) Phys. Earth Planet. Int. 183, 73.