Serpentinite dehydration in the subducted lithosphere produces no B isotopic fractionation

The fate of boron (B) and its isotopes during serpentinite dehydration is a matter of debate. To contribute to a better understanding of the B isotopic fractionation upon serpentinite dehydration, we present in situ δ11B analyses of antigorite and olivine from subducted high pressure serpentinites from the Western Alps (Zermatt-Saas and Erro-Tobbio). The different isotopic compositions of antigorite in different parts of the units (δ11B of −5 to +10 ‰ and +13 to +19 ‰ for Zermatt-Saas, and δ11B of up to +26 ‰ for Erro-Tobbio) are inherited from variable serpentinisation conditions on the sea floor. Serpentinite dehydration via the brucite-out reaction during subduction produces metamorphic olivine in oxygen (O) isotopic equilibrium with antigorite. This olivine shows near zero B isotopic fractionation with coexisting antigorite (Δ11BOl-Atg of −0.7 ± 3.4 ‰), which implies little B isotopic fractionation during serpentinite dehydration. In contrast, significant B isotopic disequilibrium (Δ11BOl-Atg of +25 ‰) is found between antigorite and olivine formed in shear bands, shear zones and veins, indicating influx of channelled external fluids, including serpentinite-derived fluids from a protolith with a different isotopic composition.