Deep mantle serpentinization in subduction zones: Insight from in situ B isotopes in slab and mantle wedge serpentinites

Abstract

Serpentinization occurs both within the seafloor, when seawater percolates through fractures to alter underlying peridotite, and in subduction zones, when aqueous fluids from the dehydration of the slab are released within the subduction channel. The main difference between these two environments is, therefore, the source of serpentinizing fluid. However, distinguishing between the various types of serpentinites encountered in suture zones is difficult, essentially because of the lack of significant differences in their mineralogy and major and trace element chemistry. Eight samples from Corsica and the Western Alps representing slab serpentinites (i.e., serpentinites embedded within the subducting slab, which are fragments of former abyssal serpentinites) and seven samples from Guatemala and Nicaragua representing mantle wedge serpentinites (i.e., serpentinites that form in situ above the subducting slab, by hydration of the overlying mantle wedge) have been selected to study their Fluid-Mobile Elements, REE concentrations, and boron (B) isotopic signatures. The trace element results do not allow a conclusion about the source of fluid(s), but the B isotopic signatures do permit identification of the different fluid sources responsible for the serpentinization.The δ11B values of slab serpentinites are significantly positive, ranging from +12 to +34‰, in the same range as present-day abyssal serpentinites (+5.5 to +40.5‰), which are altered by seawater. Therefore, these positive signatures indicate serpentinization by seawater-derived fluid. The B isotopic signature acquired during oceanic residency is therefore preserved even after subduction processes subjected the serpentinites to eclogite facies conditions. By contrast, δ11B in samples from the mantle wedge range from −14 to +10‰, supporting the interpretation of interactions with fluids derived from the progressive dehydration of the slab, which releases fluids with slightly positive and negative δ11B. It indicates that mantle wedge serpentinite isotopic signature is acquired via the infiltration of subducted crust-derived metamorphic fluids during the subduction processes.