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

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.

Continental Arc Plutonism in a Juvenile Crust: The Neoproterozoic Metagabbro-Diorite Complexes of Sinai, Northern Arabian-Nubian Shield

“…This water is liberated into the overlaying mantle wedge, where melting commences and hydrous magma forms [132,134]. Consequently, a subduction zone is the proper setting to generate these magmas, where further hydration for the magma can arise through metasomatism of the mantle wedge by the slab-derived fluid [132,133,135,136].…”