Mantle wedge olivine modifies slab-derived fluids: Implications for fluid transport from slab to arc magma source

Boron is an effective tracer of fluid processes in subduction zones. High B and delta B-11 in arc magmas require efficient B transfer from the slab to magma source regions. The Higashiakaishi metaperidotite body in the Sanbagawa high-pressure belt, Japan, is composed of locally serpentinized mantle wedge peridotites exhumed in a subduction channel. Cores of coarse-grained primary mantle olivine have 1-4 mu g/g B, enriched compared to typical mantle olivine, and delta B-11 of -10 parts per thousand to -1 parts per thousand, consistent with incorporation of fluids from dehydrating slab at similar to 90-120 km depth. Rims of primary mantle olivine as well as olivine neoblasts have even higher B (5-20 mu g/g) and higher delta B-11 (-8 parts per thousand to +2 parts per thousand) due to incorporating slab fluids at depths of similar to 70-100 km. Antigorite, formed below 650 degrees C, shows comparable delta B-11 and B contents as olivine rims. The data show that olivine is capable of scavenging significant amounts of B from fluids by diffusion and recrystallization at sub-arc pressures and temperatures. Considering the large amount of olivine in the mantle wedge, transport of slab-derived material to magma sources requires processes with minimal interaction with mantle peridotite, such as intensely channelized fluid flow or ascent of melange diapirs, and limited porous fluid flow.