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Antigorite is the main carrier of water in Earth’s subduction zones. The dehydration processes of antigorite were investigated by carrying out in situ phase transition experiments using a dynamic diamond anvil cell, with a time-resolved Raman scattering system, at 0.3–10 GPa and 396–1100 K. Three typical phase transformation reactions occurred within the <i>P</i><i>–T</i> range of this study, corresponding to three reaction products. At low pressures (<0.7 GPa), antigorite transfers to talc and forsterite; as the temperature increases, the talc disappears and a combination of forsterite and clinoenstatite occurs. At moderate pressures (1.8–7.5 GPa), antigorite dehydrates into forsterite and clinoenstatite as temperatures increase; with the continuous increase in pressure, the dehydration products become clinoenstatite and phase A. At high pressures (>8.6 GPa), the products of the dehydration phase transition of antigorite are consistently clinoenstatite and phase A. Compared with the previous studies carried out by large-volume presses (such as a multi anvil press and a piston-cylinder press), the reaction to produce phase A occurs at higher <i>P</i><i>–T</i> conditions, and the stable temperature region for talc as a dehydration product is narrower. Moreover, large quantities of pores with 5–10 μm in diameter formed in dehydration products, supporting the hypothesis that intermediate-depth earthquakes may result from dehydration embrittlement. The precise phase boundary determined by this in situ study provides a better understanding of the dehydration phase transition behavior and geological phenomena exhibited by antigorite under different pressure and temperature conditions.