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

Abstract

Abstract Boron is an effective tracer of fluid processes in subduction zones. High B and δ11B in arc magmas require efficient B transfer from the slab to magma source regions. The Higashi-akaishi 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 µg/g B, enriched compared to typical mantle olivine, and δ11B of −10‰ to −1‰, consistent with incorporation of fluids from dehydrating slab at ~90–120 km depth. Rims of primary mantle olivine as well as olivine neoblasts have even higher B (5–20 µg/g) and higher δ11B (−8‰ to +2‰) due to incorporating slab fluids at depths of ~70–100 km. Antigorite, formed below 650 °C, shows comparable δ11B 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 mélange diapirs, and limited porous fluid flow.