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Olivine polymorphs are considered the most abundant minerals in Earth and vital to governing its dynamics. Seismic discontinuities near 410 and 660 km depth are attributed to phase transitions of olivine polymorphs and have long been in reference Earth models. However, the significance of the 520 km discontinuity (520) and its causative phase transition are debated. To address its prevalence and properties, receiver functions from >2,000 seismographs across the U.S. were inverted using parameterizations with and without the 520. A 520 is required for 84% of the area at 95% confidence. The 520s depths andS‐velocity contrasts nearly match predictions from the pyrolite model, as expected for a widespread feature that dominantly reflects the wadsleyite to ringwoodite transition.

The mantle transition zone connects two major layers of Earth’s interior that may be compositionally distinct: the upper mantle and the lower mantle. Wadsleyite is a major mineral in the upper mantle transition zone. Here, we measure the single-crystal elastic properties of hydrous Fe-bearing wadsleyite at high pressure-temperature conditions by Brillouin spectroscopy. Our results are then used to model the global distribution of wadsleyite proportion, temperature, and water content in the upper mantle transition zone by integrating mineral physics data with global seismic observations. Our models show that the upper mantle transition zone near subducted slabs is relatively cold, enriched in wadsleyite, and slightly more hydrated compared to regions where plumes are expected. This study provides direct evidence for the thermochemical heterogeneities in the upper mantle transition zone which is important for understanding the material exchange processes between the upper and lower mantle.

Measurement of anisotropy advances our understanding of mantle dynamics by linking remote seismic observations to local deformation state through constraints from mineral physics. The Pacific Northwest records the largest depth‐integrated anisotropic signals across the western United States but the depths contributing to the total signal are unclear. We used the amplitudes of orthogonally polarized P‐to‐S converted phases from the mantle transition zone boundaries to identify anisotropy within the ∼400–700 km deep layer. Significant anisotropy is found near slab gaps imaged by prior tomography. Focusing of mantle flow through slab gaps may lead to locally elevated stress that enhances lattice preferred orientation of anisotropic minerals within the transition zone, such as wadsleyite.