Hydrous minerals within the subducting oceanic slab are important hosts for water. Clarification of the stability field of hydrous minerals helps to understand transport and distribution of water from the surface to the Earth’s interior. We investigated the stability of brucite, a prototype of hydrous minerals, by means of electrical conductivity measurements in both open and closed systems at 3 GPa and temperatures up to 1300 K. Dramatic increase of conductivity in association with characteristic impedance spectra suggests that partial dehydration of single-crystal brucite in the open system with a low water fugacity occurs at 950 K, which is about 300 K lower than those previously defined by phase equilibrium experiments in the closed system. By contrast, brucite completely dehydrates at 1300 K in the closed system, consistent with previous studies. Partial dehydration may generate a highly defective structure but does not lead to the breakdown of brucite to periclase and water immediately. Water activity plays a key role in the stability of hydrous minerals. Low water activity (aH₂O) caused by the high wetting behavior of the subducted oceanic slab at the transition zone depth may cause the partial dehydration of the dense hydrous magnesium silicates (DHMSs), which significantly reduces the temperature stability of DHMS (this mechanism has been confirmed by previous study on super hydrous phase B). As a result, the transition zone may serve as a ‘dead zone’ for DHMSs, and most water will be stored in wadsleyite and ringwoodite in the transition zone.