Influences of the buoyancy of partially molten rock on 3-D plume patterns and melt productivity above retreating slabs

Using 3-D petrological-thermo-mechanical subduction models, we investigate how the buoyancy of partially molten rock affects the development of thermal-chemical plumes and melt productivity in the mantle wedge. As a first order approximation we limit the positive buoyancy of partially molten rock (compared to non-molten rock), which can be decreased due to rapid melt extraction and removal to the surface. Our simulations show that a large to moderate density contrast (Δρ) of >200kg/m³ between non-molten (η_n-m) and partially molten rock (η_p-m). (i.e. low to moderate degree of melt removal from rock) promotes the development of three distinct patterns of plumes (finger-like, ridge-like and wave-like). In contrast, a low density contrast (Δρ) of 0-50kg/m³ (i.e. high to complete melt removal) suppresses pronounced plumes and is associated with low-amplitude (50-100km wide and 10-15km high) domal structures developing atop the slab due to the chemical buoyancy of subducted hydrated non-molten rock types (oceanic crust, sediments, serpentinites). Variation in partially molten rock viscosity (η_p-m) also notably affects plume patterns and lateral dimensions: wave-like plumes are most pronounced at higher (η_p-m=10¹⁹Pas) viscosity, which also favors the development of larger plumes compared to models with lower (η_p-m=10¹⁸Pas) viscosity. Integrated melt productivity above the slab is notably higher for cases with pronounced hydrated thermal-chemical plumes developed in the mantle wedge. Indeed, all models are characterized by periodic (5-10Myr long episodes of enhanced productivity), spatially clustered (30-50km distance between productivity maxima) melt production, which may explain the periodicity and clustering of volcanic activity observed in magmatic arcs such as in North-East Japan and New Zealand.