Continuous Release of Saline Fluids From a Permeable Subducting Slab

The mobility of aqueous fluids in subducting slabs fundamentally controls volatile and element cycling, yet the efficiency of fluid extraction from eclogitized oceanic crust remains poorly constrained. While NaCl is a key solute in subduction zone fluids, its specific role in controlling fluid connectivity in slab eclogite has not been quantified. Here, we present systematic measurements of dihedral angles ( θ ) in the eclogite–NaCl–H 2 O system at 2–15 GPa and 800–1200°C. The addition of NaCl reduces garnet–fluid–garnet, garnet–fluid–clinopyroxene, and clinopyroxene–fluid–clinopyroxene θ below the 60° interconnectivity threshold under all investigated conditions. Notably, fluid connectivity ( θ < 60°) is established with only 1 wt.% NaCl at 2 GPa and 800°C, whereas complete wetting ( θ ≈ 0°) occurs at pressures ≥9 GPa and 1200°C. These results demonstrate that saline fluids form interconnected networks within eclogitized slabs, enabling efficient and pervasive fluid release in global subduction zones. This mechanism provides a direct mechanism for continuous element fluxing, metasomatism, and flux melting in the mantle wedge. Consequently, our findings imply that subducting oceanic crust is unlikely to transport significant free water to the deep mantle; instead, the deep mantle water inventory may be governed by water stored in hydrous and/or nominally anhydrous minerals.