Hydration at the Base of the Mantle Transition Zone by Ancient Subductions in Asia

Whether and how subduction results in water enrichment at the base of the mantle transition zone (MTZ) remain elusive. The major orogenic belts of the Asian continent, including the Central Asian, Tethyan, and Alpine–Himalayan belts, which record extensive subduction processes, offer an ideal target to address the hydration of the MTZ and its relationship with subduction. Here, we map water content at the MTZ base by combing mineral physics constraints on hydrous pyrolite and global seismic observations of velocity structure and 660-km discontinuity topography. Our results indicate an average global water content of approximately 0.13 wt%, with pronounced hydration anomalies in parts of Asia. Linking these anomalies with reconstructions of past subduction events since 410 Ma reveals extensive water delivery to the MTZ, particularly beneath the Baikal region and across northwestern China, Kyrgyzstan, Uzbekistan, Kazakhstan, Afghanistan, Turkmenistan, Iran, and western Pakistan, where water content exceeds 0.5 wt%. These results connect ancient subduction history to present-day mantle hydration, offering new insights into Earth’s deep water cycle and highlighting the MTZ as a key reservoir for water.