Behavior of Fluid Mobile Elements Across hot Subduction Zone: Insights from Along-Strike B, As, Sb and Cs Systematics in Mafic Lavas from the Garibaldi Volcanic Belt, Northern Cascadia Subduction System.

We explore the consequences of "hot" subduction zone thermal structure for the recycling of fluid mobile elements across convergent margins. We selected the Late Cenozoic Garibaldi Volcanic Belt (GVB) of the Northern Cascadia Subduction System because it is associated with the subduction of extremely young (< 23 Ma) and "hot" oceanic lithosphere. The volcanic belt is ~ 15 km wide and consists of near primitive high-Al tholeiites and Mg-andesites (near Glacier Peak in NW Washington) through transitional basalts to alkali olivine basalts and basanites (near Bridge River- Salal Glacier area in SW British Columbia). The age of the subducted lithosphere decreases northward (latitude range 48 to 51 North) by almost 10 Ma, and thus the thermal structure of the convergent margin must be hotter in the northern part of GVB. The average B (3.3 ppm), As (0.2 ppm), Sb (0.1 ppm) and Cs (0.2 ppm) contents of the GVB mafic lavas are much lower compared to those reported from other convergent margins (Noll et al., 1996, Geochimica et Cosmochimica Acta, v. 60, No. 4, pp. 587-611). As expected the B/La (range = 0.003- 0.63; ave. ~ 0.2), Cs/La (range = 0-0.032; ave. ~ 0.01) and Sb/Ce (range = 0.001-0.013; ave. ~ 0.003) ratios show progressively lower values with transition from volcanic centers sampling old (19-23 Ma) (colder) slabs (Glacier Peak and Mount Baker areas) toward volcanic centers sampling young (15-17) (hot) slabs (Meager-Elaho, Helm Creek, Cheakamus, Bridge River and Salal Glacier areas). Surprisingly, the As/Ce (range = 0.001-0.015; ave. ~ 0.005) increases with decreasing slab age. Fluid mobile elements are extremely enriched in all members of the subducted slab inventory (to extremes in the serpentinized mantle wedge) and are depleted in slab melts (adakites), OIBs and MORBs. Their enrichment-depletion patterns could potentially be used to trace the extent of slab devolatilization and to test models about the mantle structure beneath volcanic arc fronts.