Global variations in H_2O/Ce: 2. Relationships to arc magma geochemistry and volatile fluxes
We compiled a data set of 100 primitive arc magma compositions from melt inclusion and whole rock analyses to compare volatile contents, slab tracers, and calculated subduction component compositions between 18 subduction zone segments spanning the global range in slab thermal structure. The average primitive magma H_2O content in our data set is 3.3 ± 1.2 wt.% (1 s.d.) for melts erupted within 50 km of the volcanic front. While there is a wide range of volatile contents in magmas within individual arcs, the highest values occur in magmas erupted from vents along the volcanic front, where the subducting slab is located 104 ± 29 km (avg ± 1 s.d.) beneath the surface. This observation, coupled with positive correlations between H_2O, Cl, S, and B contents and predictions from geodynamic models, provides strong evidence for the active supply of volatile-rich slab-derived components from the subducting oceanic plate beneath volcanic arcs. We also show that temperature-sensitive ratios (e.g., H_2O/Ce) for both primitive arc magmas and calculated subduction components are similar and display monotonic behavior with slab thermal parameter. Furthermore, calculated subduction component compositions have higher trace element to H_2O ratios in arcs with hotter slabs (lower thermal parameter), suggesting that hydrous melts of differing compositions are added beneath different arcs. Finally, we present new volatile outflux estimates for Central Cascades magmatism and then compare these to estimates for the Central American and Kamchatka-Kurile arcs to create a combined data set spanning a large range in slab thermal parameter.
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