<p>Water plays a central role in controlling the physical and chemical properties of Earth’s deep interior. It remains uncertain how water is stored in subducting slabs within the mantle transition zone, between depths of about 410 and 660 kilometers, and whether dense hydrous magnesium silicates act as major water carriers to greater depths. Here we report high-pressure and high-temperature laboratory experiments on the Mg-Si-H system at pressures of 16 and 21.5 GPa and a temperature of 1400 K to evaluate hydrous phase stability under transition zone conditions. We find that when bulk water content is below 1.22 wt%, H₂O is predominantly incorporated into wadsleyite and ringwoodite rather than forming dense hydrous magnesium silicates. Because estimated water contents in subducted oceanic slabs are typically lower than one weight percent, formation of these silicates is unlikely, suggesting that the mantle transition zone may restrict large scale water transport into the lower mantle.</p>