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.