Seminar Jintao Zhu

Water plays a critical role in Earth’s deep mantle by influencing geochemical cycles, slab dynamics, and deep-focus earthquakes. However, the stability and interactions of hydrous minerals with nominally anhydrous minerals (NAMs) under water-undersaturated conditions typical of subducting slabs remain poorly constrained. Here we conducted high-pressure and high-temperature experiments in MgO-SiO₂-H₂O systems (~2 wt% H₂O) simulating subducting ultramafic lithologies down to top-lower-mantle conditions. Our results reveal that hydrous minerals gradually dehydrate with increasing temperature in the mantle transition zone, progressively hydrating NAMs such as wadsleyite and ringwoodite, while the cold slab core remains dry. Rapid dehydration at the top of the lower mantle may generate fluids linked to the deepest earthquakes. These findings suggest that dry olivine transformations, rather than dehydration embrittlement, likely trigger deep-focus earthquakes near metastable olivine wedges, and that variations in the hydration state of NAMs affect slab deformation and stagnation above 660 km depth. The bulk Mg/Si ratio controls the stability of hydrous minerals in the mantle transition zone, indicating the harzburgite layer, with its relatively high Mg/Si ratio, allows more efficient water transport than the peridotite layer. Furthermore, other major components (e.g., CaO, Al₂O₃) in subducting slabs changes the species of hydrous minerals, and therefore affect water cycling in the deep mantle.