Fluid-mediated mineral transformations in sub duction zones play a pivotal role in the deep-Earth water cycle. Using a diamond anvil cell combined with in situ Raman spectroscopy, we investigated the hydration reac tion of talc that transforms into the 10 Å phase in both pure H₂O and NaCl-H₂O fluids under high-pressure, high-tem perature conditions relevant to cold subduction zones. Our experimental results indicate that talc transforms into the 10 Å phase at significantly lower P–T conditions (3.5 GPa and 400 °C) than previously reported. The reaction kinetics exhibit a strong positive dependence on both temperature and pressure. Additionally, the presence of NaCl promotes the transformation rate. Under equivalent P–T conditions, the 10 Å phase formed within a shorter time frame in saline fluids than in pure water environments. Complementary thermodynamic calculations indicate that the 10 Å phase forms at lower pressures in NaCl-H₂O fluids (at T < 550 °C) than in pure water environments. The thermodynamic condi tions and kinetic processes of the talc-10 Å phase transition under saline conditions have profound implications for the deep-Earth water cycle in cold subducting slabs. This study provides essential experimental constraints for modeling fluid-mediated mineral transformations in natural subduc tion environments.