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

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 herein focus on three subduction-related mantle sections: an intra-slab serpentinized shear zone (Monviso, Italian Alps), 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 (δ¹¹B) show systematic variations at several hundred meters scale approaching the main structural boundaries for each locality. A decrease is observed in the Monviso and Urals localities (from c. 25‰  to c. 7‰, and from c. 16‰ to c. 0‰, respectively), while the Zagros section shows an increase from c. 1‰, up to c. 9‰ in the most sheared and serpentinized samples. These variations reflect complex fluid-rock interactions processes including B loss or B addition associated with protolith and fluid variability. This demonstrates that major shear zones exert a first-order control on the serpentinite boron isotopic signature. 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. These combined 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^-19 to 10^-18 m².