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Water bound to various hydrous minerals can be transported deep into the mantle by slab subduction. Serpentine is one of most important hydrous minerals in the crust and shallow upper mantle. A partially serpentinized slab mantle limits the amount of water that can enter deep into the mantle. The partitioning of water between hydrous minerals and nominally anhydrous minerals (NAMs) is a crucial factor in controlling the physical properties and dynamics of slabs. Recent experiments on water partitioning have revealed that water strongly partitions to coexisting hydrous minerals. NAMs, such as olivine and its high-pressure polymorphs, have limited water content in water-undersaturated wet slabs. Metastable olivine wedges are not a feature of dry slabs, but can be of wet slabs that are not saturated with water. The transformation kinetics of the dry state, which generates deep-focus earthquakes and produces significant deformation in the slab, can work under wet slabs. Water bound to hydrous minerals is transported by the slab to the mantle transition zone and lower mantle. Hydrous minerals in stagnant slabs over 660 km depth release water as the slab warms, producing locally hydrated mantle transition zones and dense water-bearing magmas at the base of the upper mantle, and generating intraplate volcanism, which are referred to as the big mantle wedge model. Seismic scatterers are observed in the lower mantle at depths from 700 to 1900 km. These scatterers may be caused by water release at the top of the lower mantle by dehydration of hydrous minerals such as dense hydrous magnesium silicates. The shear instability due to the second order phase transformation from stishovite to CaCl2-type phase in hydrous aluminous SiO2 also causes the depth variation of seismic scatterers in the lower mantle. The high-pressure polymorphs of aluminous SiO2 contain a large amount of water more than 1 wt%, which can be important water carriers under lower mantle conditions. [ABSTRACT FROM AUTHOR]

The stishovite to post‐stishovite phase transition may modify the scattering of seismic waves by stishovite‐bearing rocks in the Earth's lower mantle. A series of continuous compression experiments on sintered polycrystalline stishovite was performed to study the effect of stress on the phase transition. The experimental results show that the phase transition shifts to lower pressures as the magnitude of deviatoric stress increases. Our results further show that the bulk modulus of sintered polycrystalline stishovite differs from that derived from single crystal measurements and decreases at the phase transition. In cold regions, such as subducted slabs, stresses may accumulate and shift the phase transition to a shallower depth. In hot regions with less stress, such as rising plumes, the phase transition is shifted to a greater depth. In addition, the phase transition may have varying seismic signatures depending on the behavior of the grain boundaries in mantle rocks and the micro‐stresses present in neighboring grains. Plain Language Summary: Stishovite and post‐stishovite are high‐pressure polymorphs of SiO2 that are stable in the Earth's lower mantle. The stishovite to post‐stishovite phase transition may affect the way seismic waves travel through mantle rocks. To detect SiO2 phases in the deep mantle, it is essential to investigate the depth of the phase transition and its effects on seismic wave propagation. Experiments were conducted to simulate the high‐pressure environment of the lower mantle and to study the phase transition of sintered polycrystalline stishovite aggregates, in which crystals are firmly connected to each other. Our study finds that the phase transition is sensitive to deviatoric stress, which is a measure of the directional stress difference. An increase in deviatoric stresses shifts the phase transition to shallower depths in the mantle. We further find that at pressures below the phase transition, sintered polycrystalline stishovite aggregates exhibit lower compressibility compared to stishovite powder and single crystals, but the compressibility increases abruptly at the phase transition, probably caused by grain‐grain interactions in the sintered samples. Therefore, the behavior of seismic waves will be different when passing through stishovite‐bearing rocks with different degree of grain‐grain interactions, particularly under conditions close to the phase transition boundary. Key Points: Continuous compression experiments were performed to study the post‐stishovite phase transition under different stress conditionsDeviatoric stresses shift the phase transition to a shallower depth in the lower mantleThe bulk modulus of sintered polycrystalline stishovite differs from that of stishovite powder and drops at the phase transition [ABSTRACT FROM AUTHOR]