THE VIABILITY OF THREE SIMULTANEOUS MELTING MECHANISMS AT VOLCANIC ARCS

Since the advent of plate tectonics, three separate melting mechanisms have each been argued at times to generate the magmas for the observed volcanism at island arcs: decompression, wedge hydration, and slab melting. Despite recognized cases of anhydrous melts and somewhat ubiquitous geochemical sediment melt signatures (e.g., Th and Be) at arcs, the currently dominant view strongly favors wedge hydration with the other two mechanisms occurring only in atypical environments, such as near slab edges or in relation to arc-parallel extension. The notion of wedge hydration as the foremost melting mechanism is supported by two main lines of evidence: 1) Some arc lavas have extremely high water contents, and 2) geodynamic models do not straightforwardly predict the other two mechanisms (flow is overall downwards and slab surface temperatures tend to be fairly cold). However, as geodynamic models become more realistic, decompression and slab melting behaviors emerge. Inclusion of temperature-dependent viscosity triggers decompression melting as the viscous lower portion of the overriding plate is ablated and replaced by hot, upwelling asthenoshpere. Temperature-dependent viscosity also results in slab surface temperatures 100-200 degrees C warmer than isoviscous models. Recent developments to allow the fault zone and overriding plate to evolve to their preferred geometry rather than remain fixed with preset dimensions can further increase slab surface temperatures. More cold, lithospheric material is sequestered in the upper plate and restricted from entrainment by the down going slab, so the slab surface can exceed the sediment solidus with this model formulation. Inclusion of shear heating produced at the fault interface will only increase the amount of sediment melting. These results suggest that volcanic arcs could possibly be simultaneously produced from all three different melting mechanisms without requiring an atypical tectonic environment.