Serpentine’s wet breakdown path and enhanced water flux in cold subduction zones

Serpentine stability dictates deep water cycling in subduction zones. We investigated pure serpentinites (≈ 13 wt.% H2O) using in-situ synchrotron X-ray diffraction and ultrasonic velocity measurements under two cold-slab P–T paths, varying excess water (0.9–31.5 wt.%). Here we report that under a cold core (or ultracold Moho) geotherm with >6.9 wt.% excess water, serpentinites undergo deep hydration to form the 3.65-Å phase and phase E above 9.3 GPa and 420 °C (ca. 280 km depth), increasing total water to ≈ 19.3 wt.%. With <6.9 wt.% excess water, serpentine transforms to phase E without water loss. Conversely, under a cold Moho geotherm, serpentinites dehydrate into phase E and clinoenstatite above 7.4 GPa and 585 °C (ca. 220 km depth), releasing ≈ 5.3 wt.% water. These divergent reactions highlight an underestimated water transport capacity of subducting slabs, generating lateral contrasts in hydration, density, and seismic velocity. Extra water in cold tectonic plates causes sinking rocks (serpentinites) to transform and absorb more water. This boosts deep water transport by cold slabs, creating lateral contrasts in hydration, density, and seismic velocity.