[1] Hydration and partial melting along subducting slabs can trigger Rayleigh-Taylor-like instabilities. We use 3-D petrological-thermomechanical numerical simulations to investigate small-scale convection and hydrous, partially molten, cold plumes formed in the mantle wedge in response to slab dehydration. The simulations were carried out with the I3ELVIS code, which is based on a multigrid approach combined with marker-in-cell methods and conservative finite difference schemes. Our numerical simulations show that three types of plumes occur above the slab-mantle interface: (1) finger-like plumes that form sheet-like structure parallel to the trench, (2) ridge-like structures perpendicular to the trench, and (3) flattened wavelike instabilities propagating upward along the upper surface of the slab and forming zigzag patterns parallel to the trench. The viscosity of the plume material is the main factor controlling the geometry of the plumes. Our results show that lower viscosity of the partially molten rocks facilitates the Rayleigh-Taylorlike instabilities with small wavelengths. In particular, in low-viscosity models (10 18 -10 19 Pa s) the typical spacing of finger-like plumes is about 30-45 km, while in high-viscosity models (10 20 -10 21 Pa s) plumes become rather sheet-like, and the spacing between them increases to 70-100 km. Water released from the slab forms a low-viscosity wedge with complex 3-D geometries. The computed spatial and temporal pattern of melt generation intensity above the slab is compared to the distribution and ages of volcanoes in the northeast Japan. Based on the similarity of the patterns we suggest that specific clustering of volcanic activity in this region could be potentially related to the activity of thermal-chemical plumes.

No abstract

For some volcanic arcs, the geochemistry of volcanic rocks erupting above subducted oceanic fracture zones is consistent with higher than normal fluid inputs to arc magma sources. Here we use enrichment of boron (B/Zr) in volcanic arc lavas as a proxy to evaluate relative along-strike inputs of slab-derived fluids in the Aleutian, Andean, Cascades and Trans-Mexican arcs. Significant B/Zr spikes coincide with subduction of prominent fracture zones in the relatively cool Aleutian and Andean subduction zones where fracture zone subduction locally enhances fluid introduction beneath volcanic arcs. Geodynamic models of subduction have not previously considered how fracture zones may influence the melt and fluid distribution above slabs. Using high-resolution three-dimensional coupled petrological-thermomechanical numerical simulations of subduction, we show that enhanced production of slab-derived fluids and mantle wedge melts concentrate in areas where fracture zones are subducted, resulting in significant along-arc variability in magma source compositions and processes.

No abstract

Subduction initiation at straight passive margins can be investigated with two-dimensional (2-D) numerical models, because the geometry is purely cylindrical. However, on Earth, straight margins rarely occur. The construction of 3-D models is therefore critical in the modeling of spontaneous subduction initiation at realistic, curved passive margins. Here we report on the results obtained from gravitationally driven, 3-D thermomechanical numerical models using a visco-plastic rheology and a passive margin with a single curved section in the middle. The models show that the curvature angle β can control subduction initiation: the greater β is, the more diffi cult subduction initiation becomes. The 3-D thermomechanical models provide an in-depth physical understanding of the processes. Specifi cally, we fi nd that pressure gradients, arising from density differences between oceanic and continental