We used boron (B) isotope systematics of co-existing olivine and serpentine to study deep fluid flow in subduction zones. Metamorphic olivine produced by serpentine dehydration at sub-arc conditions from high pressure ophiolites in the Western Alps contains significant concentrations of B (2–30 μg/g) with a high δ11B values (+9 to +28 ‰), whilst co-existing serpentine has 2–50 μg/g B with δ11B = +6 to +24 ‰. Boron isotope fractionation between olivine and its precursor serpentine (Δ11Bol-srp = δ11Bol – δ11Bsrp) is highly variable, which indicates significant isotopic disequilibrium between these minerals. Importantly, samples with B-enriched olivine have low Δ11Bol-srp (down to −9 ‰), evidence that olivine grew in the presence of a mixture of serpentine-derived fluids and external fluids with δ11B of ca. +6 to +15 ‰. The composition of these external fluids is consistent with those from subducting sediments and altered oceanic crust at 50–80 km depth, and at least 15–45 % fluid addition. Our work shows that large scale slab fluid infiltration and fluid-mobile element transport accompanies serpentinite dehydration in subduction zones.