Boron isotopes unravel cryptic fluid-rock interactions in sheared subduction zone serpentinites

• Serpentinite Boron isotopic signature is evaluated along three subduction shear zones. • Evidence for strong alteration of δ 11 B signature in several 100′s m-thick sheared domains. • Large-scale structures and multiple fluid-rock interaction events control boron isotopic signature. • Brittle deformation makes mantle wedge serpentinites relatively permeable (up to 10 –18 m²). The migration of fluids released during slab-dehydration in deep subduction environments is strongly controlled by deformation and lithological discontinuities such as serpentinized shear zones. However, the geological meaning of geochemical fingerprints and how they relate to transport mechanisms and spatial scales of fluid flux in deformed serpentinite at depth remain poorly understood. We focus on three subduction-related mantle sections: an intra-slab serpentinized shear zone (Monviso meta-ophiolite, Italy), an underplated ultramafic sliver (Zagros suture zone, Iran) and the former base of a mantle wedge (Polar Urals, Russia). Most major and trace element signatures of serpentinites appear rather homogeneous along the transects. In contrast, boron isotopic signatures (δ 11 B) show systematic variations at a scale of several hundred meters toward the main structural boundaries for each locality. A decrease in δ 11 B is observed in the most sheared and serpentinized samples of the Monviso and Urals localities (from c. 25 ‰ to c. 7 ‰ , and from c. 16 ‰ to c. 0 ‰ , respectively), whereas the Zagros section shows an increase from c. 1 ‰ , up to c. 9 ‰ . The spatial trends demonstrate that major shear zones exert a first-order control on the serpentinite boron isotopic signature. Isotopic variations reflect complex fluid-rock interactions processes, including overall loss or gain of B, as well as locally contrasting protolith and fluid composition. We combine boron isotopic data with Darcy-based flux models to quantify the volume of rock influenced by paleo-fluid fluxes in deep ultramafic settings. Integrating petrostructural and isotopic constraints highlight the importance of fracture-controlled fluid flow in slab-top serpentinites, and yield a time-integrated permeability of the (partly) serpentinized base of the mantle wedge in the range of 10 –20 to 10 –18 m².