A research team from the Institute of Geochemistry of the Chinese Academy of Sciences, in collaboration with international partners, has found that the mantle transition zone may serve as a critical barrier to the large-scale transport of water into the lower mantle.

Published recently in Communications Earth & Environment, the findings indicate that under conditions of low water activity, dense hydrous magnesium silicates are unlikely to form extensively in subducting slabs—limiting their role as major carriers of water to greater depths.

Water plays a pivotal role in regulating the physical and chemical properties of Earth's deep interior. A long-standing question in deep Earth science is how water is stored in subducting oceanic slabs within the mantle transition zone—located between approximately 410 and 660 kilometers deep—and whether dense hydrous magnesium silicates can efficiently transport water into the lower mantle. While previous studies have often emphasized the importance of these hydrous minerals, most experimental constraints were obtained under water-saturated conditions, which differ significantly from the relatively dry environments expected in natural slabs.

To address this gap, the team conducted high-pressure and high-temperature experiments in the MgO-SiO₂-H₂O (MSH) system at 16 and 21.5 GPa and 1400 K, using starting materials with bulk water contents ranging from 0.1 to 5 weight percent (wt%) H₂O. By integrating phase equilibrium experiments, electron backscatter diffraction, micro-focused X-ray diffraction, NanoSIMS water-content measurements, and mass balance calculations, the researchers systematically assessed how water partitions among mantle minerals under transition-zone conditions.

The results show that when the bulk water content is below a critical threshold of approximately 1.22 wt%, water is stored primarily in wadsleyite and ringwoodite rather than in dense hydrous magnesium silicates. At 16 GPa, phase E forms only when the bulk water content exceeds 1.22 wt% H₂O. At 21.5 GPa, superhydrous phase B forms only after ringwoodite approaches water saturation, with a corresponding threshold near 1.30 wt% H₂O.

These observations demonstrate that water preferentially enters nominally anhydrous mantle minerals in the transition zone, and that hydrous magnesium silicates form only after these minerals become water-saturated.

In addition, the study underscores the importance of water activity in controlling the stability of hydrous minerals. The researchers noted that in natural mantle environments—where H₂O may coexist with CO₂ or hydrous melt—water activity can be significantly lower than in a pure-H₂O system. Under such conditions, the effective threshold for stabilizing dense hydrous magnesium silicates may be even higher, further reducing the likelihood of extensive formation of these phases in real subducting slabs.

The findings have important implications for the deep Earth water cycle. Even in relatively cold and water-rich subduction zones, the bulk water content retained in slabs is generally estimated to remain below the threshold required for the widespread stabilization of dense hydrous magnesium silicates. As a result, water in the mantle transition zone is more likely to be hosted primarily by hydrous wadsleyite and ringwoodite. When slabs descend beyond the 660-kilometer discontinuity, the breakdown of hydrous ringwoodite may release water and facilitate melt formation, while only a limited amount of water may continue into the lower mantle through highly stable hydrous phases such as Al-rich phase D, phase H, and hydrous stishovite.

This work challenges the traditional view that dense hydrous magnesium silicates are the primary carriers of large-scale water through the mantle transition zone. Instead, it suggests that large-scale water recycling may be largely restricted to depths shallower than approximately 660 kilometers, offering new insights into the storage and transport of water in Earth's interior.