On the decompression melting structure at volcanic arcs and back-arc spreading centers

Mantle dynamics can strongly affect melting processes beneath spreading centers and volcanic arcs. A 2‐D numerical model of the Tonga subduction zone, with the slab viscously coupled to the mantle beneath the brittle‐ductile transition but faulted above, shows that induced corner flow may cause asymmetric melting at the Lau back‐arc spreading center, 400 km away. The down‐going slab also entrains the high‐viscosity base of the overlying lithosphere, drawing hot, low‐viscosity asthenosphere upwards into the gap, triggering decompression melting in the wedge. Because the slab is decoupled from the brittle overlying plate, a cold upper corner develops, inhibiting melting where the slab is shallow. The cold corner is consistent with seismic attenuation and heat flow at arcs. Decompression melting may be a substantial fraction of magma production at some arcs, but less at others. Possibly more important, the shallow decompression melting structure may govern the pathways of melt extraction beneath volcanic arcs.